Description:TECHNICAL FIELD
The present disclosure relates to the field of metal recovery and recycling, more specifically to purification of gold from gold scrap. Particularly, the present disclosure provides a method and a system to facilitate gold purification from gold scrap with reduced lead time for purification and reduced risk of human exposure to hazardous acids and temperatures.

BACKGROUND OF THE DISCLOSURE
Gold refining process deals with two major hazards namely highly corrosive and hazardous acids (Hydro metallurgical process) and high temperature (1100°C) furnace operations (Pyro Metallurgical process). The handling of acids in conventional processes is usually a manual process, thus carrying a chance for manual error during acid addition and direct human exposure to highly hazardous acids.

In addition to the above, the process of gold dissolution in the highly corrosive and hazardous acid to form gold solution is usually carried out based on a fixed time for each batch, irrespective of the weight and purity of the input impure gold grains, with no method to determine the process end point, therefore leading to the tendency of unnecessary consumption of resources such as energy, time and man hours.

Conventionally, some of the systems employed to purify gold from gold scrap included complex operating parameters and numerous valves for operation. Such operation of complex systems and timely opening and closing of various valves required skilled operators. Hence, such systems were prone to human errors such as mismanagement, errors in operation of system management and the like which lead to reduced efficiency and system downtime. There was also the risk of exposure to harmful chemical during the purification process itself which was a work hazard for many operators.

In view of the safety challenges and lack of methods for monitoring process completion and hence efficiency, there is clearly a need in the art for a gold recovery process that minimizes the process lead times and increases process efficiency without compromising on the quality of output (99.99% Pure gold).

STATEMENT OF THE DISCLOSURE
Addressing shortcomings of conventional methods of gold recovery as mentioned above, the present disclosure provides a method for purification of gold from gold scrap comprising –
a) Dissolving the gold scrap in aqua regia to obtain a leachate;
b) Subjecting the leachate of step (a) to filtration to obtain a filtrate;
c) Subjecting the filtrate from step (b) to precipitation by addition of a precipitation agent till a redox potential of about 380mV to about 420mV is reached to obtain a gold precipitate and a supernatant;
d) Subjecting the gold precipitate to melting to obtain pure gold.

In some embodiments, the gold scrap has purity level ranging from about 75% to about 90%; and/or wherein the gold scrap is subjected to melting and graining prior to step (a).

In some embodiments, the melting of the gold scrap is performed at a temperature of about 1000°C to about 1100°C, preferably about 1100°C; and/or wherein the graining of the melted gold scrap is performed by atomization at a water pressure of about 2 bar to about 3 bar and air pressure of about 5 bar to about 6 bar.

In some embodiments, the aqua regia comprises nitric acid and hydrochloric acid at a ratio of about 1:3 to about 1:4; and wherein the ratio between the gold scrap, the Nitric acid and the Hydrochloric acid ranges from about 1:1:3 to about 1:1:4.

In some embodiments, wherein the concentration of nitric acid is about 52-55% and concentration of hydrochloric acid is about 30-33%; wherein the ratio between the nitric acid and the hydrochloric acid in the aqua regia is about 1:3.8; and wherein the ratio between the gold scrap, the nitric acid and the hydrochloric acid is about 1:1:3.8.

In some embodiments, the dissolution in step (a) is performed by sequential addition of the nitric acid and the hydrochloric acid to the gold scrap.

In some embodiments, the sequential addition of the nitric acid and the hydrochloric acid to the gold scrap comprises addition of the nitric acid followed by the hydrochloric acid to the gold scrap.

In some embodiments, the dissolution in step (a) is performed by addition of the aqua regia to the gold scrap till temperature of NOx fumes arising from the process reaches a peak value of about 47°C to about 52°C, decreases and shows saturation in the range of about 33°C to about 35.75°C.

In some embodiments, the filtration in step (b) comprises filtering the leachate through one or more filter papers of mesh size 5 micron, one or more filter bag(s) having mesh size of about 1 micron to about 5 micron and one or more filter cartridge(s) having mesh size of about 0.2 to about 0.5 micron.

In some embodiments, the filtration in step (b) is through a filter paper of mesh size 5 micron, a filter bag having mesh size of about 1 micron and 4 filter cartridge(s) having mesh size of about 0.2 micron.

In some embodiments, the precipitation agent in step (c) is selected from a group comprising as Sodium bisulfite (SBS) and sulphur dioxide (SO2).

In some embodiments, wherein the precipitation in step (c) comprises adding the precipitation agent to the filtrate at a ratio of about 1:3.25 to about 1:4.

In some embodiments, the supernatant obtained in step(c) is separated from the precipitate by filtration and is further subjected to cementation using sodium borohydride (NaBH4) as the cementation agent.

In some embodiments, the ratio between the supernatant obtained in step (c) and the NaBH4 ranges from about 1:0.0002 to about 1:0.000225 (Supernatant in litres:NaBH4 powder in Kg).
In some embodiments, the ratio between the supernatant obtained in step (c) and the NaBH4 is about 1:0.0002.

In some embodiments, the melting of the gold precipitate in step(d) is performed at a temperature of about 1000°C to about 1100°C, preferably about 1100°C.

In some embodiments, the melted gold of step (d) is converted into a solid form such as gold granules; and wherein the said solid form of the gold is subjected to cleaning with about 5% to about 6%, preferably about 5% Isopropyl Alcohol (IPA).

In some embodiments, the method of purification of gold from gold scrap comprises –
a) Melting the gold scrap;
b) Subjecting the melted gold scrap of step (a) to graining by atomization;
c) Dissolving the gold scrap in aqua regia comprising nitric acid and hydrochloric acid at a ratio of about 1:3.8 by addition of the aqua regia to the gold scrap till temperature of NOx fumes arising from the process reaches a peak value, decreases and shows saturation, to obtain a leachate;
d) Subjecting the leachate of step (c) to filtration through a filter paper having a mesh size of 5 micron, filter bag having mesh size of about 1 micron and 4 filter cartridge(s) having mesh size of about 0.2 micron to obtain a filtrate;
e) Subjecting the filtrate from step (d) to precipitation by addition of sodium bisulfite (SBS) till a redox potential of about 380mV to about 420mV is reached to obtain a gold precipitate and supernatant;
f) Subjecting the gold precipitate of step (e) to melting to obtain pure gold having purity of at least 99.99%;
g) Subjecting the supernatant of step (e) to cementation by adding sodium borohydride (NaBH4) as cementation agent;
h) Recovering gold from the cementation step (g).

In some embodiments, the method is automated for handling of bulk batches of gold refining with highly corrosive/hazardous aqua-regia acid.

In some embodiments, the method reduces lead time of the gold purification by about 65% to about 70% as compared to a conventional gold recovery process.

Further provided in the present disclosure is a system (500) for purification of gold from gold scrap, the system comprising:
at least one tumbler reactor (10) configured to receive gold scrap;
at least one primary acid tank (30, 30’) fluidly connectable to at least one secondary acid tank (20, 20’), wherein the secondary acid tank is fluidly connected to the at least one tumbler reactor for dissolution of the gold scrap to gold solution;
at least one precipitation tank (50) fluidly connected to the at least one tumbler reactor, wherein the at least one precipitation tank receives the gold solution;
at least one secondary precipitation tank (60) is connected to the at least one precipitation tank, wherein the at least one secondary precipitation tank receives decanted and filtrate solution and generates residues;
at least one cementation tank (70’, 70’’, 70’’’) is connected to the at least one secondary precipitation tank, wherein the at least one cementation tank receives filtrate solution and generates residues;
a control unit configured to:
determine level of fluid in the at least one primary acid tank and the secondary acid tank;
actuate at least one valve to initiate fluid flow from the at least one primary acid tank and the at least one secondary acid tank to the at least one tumbler reactor;
actuate the at least one tumbler reactor for dissolution of the gold scrap to the gold solution.

In some embodiments, the at least one tumbler reactor (10) receives HCl and HNO3 solution from the at least one primary tank (30, 30’) and the secondary tank (20, 20’).

In some embodiments, the at least one valve member is configured to the control unit to allow metered volume of fluid.

In some embodiments, the system comprises at least a disk filter (11) connectable to the at least one tumbler reactor (10) and the at least one precipitation tank (50) for filtration.

In some embodiments, the system comprises a disk filter (11), at least one bag filter (12) and at least one cartridge filter (13) fluidly connectable to the at least one tumbler reactor for filtration of the gold solution from the tumbler reactor into the precipitation tank.

In some embodiments, the tumbler is fluidly connectable to a chimney through a series scrubbers comprising but not limited to a pre-stage scrubber, a primary scrubber and a secondary scrubber to allow exit of the scrubbed NOx fumes.

In some embodiments, the system comprises at least one collection tank fluidly connectable to the at least one cementation tank (70’, 70’’, 70’’’) to collect effluent generated from the at least one cementation tank for further treatment.

In some embodiments, the system comprises at least one induction furnace (100’) to receive and melt the gold scrap.

In some embodiments, the melted gold scrap is transferred to a graining machine to form gold grains for dissolution in the at least one tumbler reactor.

In some embodiments, the system comprises an SOx scrubber fluidly connected to the at least the at least one precipitation tank, the at least one cementation tank, the at least one collection tank and the at least one neutralization tank for neutralization of SOx gas generated.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
In order that the invention may be readily understood and put into practical effect, reference will now be made to exemplary embodiments as illustrated with reference to the accompanying figures. The figures together with a detailed description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, in accordance with the present invention wherein:
Figure 1 depicts (a-h) different components that enable automation of the process according to an embodiment of the present disclosure.
Figure 2 (a-b) depicts the different filtration steps employed between the dissolution and precipitation steps in the process according to an embodiment of the present disclosure.
Figure 3 depicts the process improvement in terms of process lead time between conventional processes and the automated process according to an embodiment of the present disclosure.
Figure 4 depicts the relationship between dissolution end point and temperature of NOx fumes generated during the dissolution in the automated process according to an embodiment of the present disclosure.
Figure 5 illustrates a schematic of a system for refining gold from gold scrap, in accordance with an embodiment of the present disclosure.
Figure 6 depicts a flow chart representing the different steps of the automated according to an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE
In view of the drawbacks associated, and to remedy the need for reducing the human exposure to hazardous chemical agents and for devising a method to determine the process end point for improving process efficiency, the present disclosure provides such a process as well as a system for carrying out the said process.

As used herein the term ‘process end point’ refers to the point where the gold dissolution process may be terminated to provide the desired results. Determination of process end point provides real-time information about the process being done.

The term ‘gold scrap’ as used throughout the present disclosure refers to scrap material comprising gold at a purity of at least about 75%. Reference to gold scrap includes but is not limited to reject jewellery, excess gold obtained in the making of jewellery, impure gold etc.

The term ‘pure gold’ as used in the present disclosure refers to gold having purity of 99.99%.

As used throughout the present disclosure, ranges are a shorthand for describing each and every value within the range. Any value within the range can be selected as the terminus of the range.
With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. The use of the expression ‘at least’ or ‘at least one’ suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results. The use of the expression ‘about’ refers to values ±20% of the values defined immediately following said term. Throughout this specification, the word “comprise”, or variations such as “comprises” or “comprising” wherever used, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Addressing the need in the art for safe and energy and time efficient processes for the purification and recovery of gold from gold scrap, the present disclosure provides a method for purification of gold scrap specifically characterized by a reduced lead time as well as low risk of human exposure to hazardous reagents.

Accordingly, the present disclosure provides a method for purification of gold scrap comprising–
a) Dissolving the gold scrap in aqua regia to obtain a leachate;
b) Subjecting the leachate of step (a) to filtration to obtain a filtrate;
c) Subjecting the filtrate from step (b) to precipitation by addition of a precipitation agent till a redox potential of about 380mV to about 420mV is reached to obtain a gold precipitate and a supernatant; and
d) Subjecting the gold precipitate to melting to obtain pure gold.

In some embodiments, the gold scrap has purity level ranging from about 50% to about 90%.
In exemplary embodiments, the gold scrap has purity level ranging from about 75% to about 90%.

In some embodiments, examples of such gold scrap include but are not limited to used or reject jewellery, excess gold during jewellery making and impure gold.

In some embodiments, the gold scrap is subjected to melting and graining prior to step (a).
In some embodiments, the melting of the gold scrap is performed at a temperature of about 1000°C to about 1100°C.

In exemplary embodiments, the melting of the gold scrap is performed at a temperature of about 1100°C.

The melted gold, thereafter, is subjected to graining. In some embodiments, the graining of the melted gold scrap is performed by any method known in the art for the graining of metals, more particularly gold.
In some embodiments, the graining is performed by methods such as but not limited to atomization. In a non-limiting embodiment, the atomization of the melted gold is conducted at a water pressure of about 2 bar to about 3 bar and air pressure of about 5 bar to about 6 bar.

Accordingly, in some embodiments, the method for purification of gold scrap comprises –
a) Melting the gold scrap;
b) Graining the melted gold scrap to obtain impure gold grains;
c) Dissolving the impure gold grains in aqua regia to obtain a leachate;
d) Subjecting the leachate of step (c) to filtration to obtain a filtrate;
e) Subjecting the filtrate from step (d) to precipitation by addition of a precipitation agent till a redox potential of about 380mV to about 420mV is reached to obtain a gold precipitate and a supernatant; and
f) Subjecting the gold precipitate to melting to obtain pure gold.

In some embodiments, the method for purification of gold scrap comprises –
a) Melting the gold scrap at a temperature of about 1000°C to about 1100°C.;
b) Subjecting the melted gold scrap to atomization at a water pressure of about 2 bar to about 3 bar and air pressure of about 5 bar to about 6 bar obtain impure gold grains;
c) Dissolving the impure gold grains in aqua regia to obtain a leachate;
d) Subjecting the leachate of step (c) to filtration to obtain a filtrate;
e) Subjecting the filtrate from step (d) to precipitation by addition of a precipitation agent till a redox potential of about 380mV to about 420mV is reached to obtain a gold precipitate and a supernatant; and
f) Subjecting the gold precipitate to melting to obtain pure gold.

Aqua regia is a mixture of nitric acid and hydrochloric acid. The dissolution of the gold scrap in aqua regia yields gold chloride. The chemical reaction of the Aqua Regia Dissolution Process is as follows -
Au (Solid) + HNO3+3HCl→ AuCl3+ NOX↑+ 2H2O
In some embodiments, the concentration of nitric acid is about 52-55%. In some embodiments, the concentration of hydrochloric acid is about 30-34%.
In exemplary embodiments, the concentration of nitric acid is about 55% and the concentration of hydrochloric acid is about 33%.
In some embodiments of the present disclosure, the aqua regia comprises nitric acid and hydrochloric acid at a ratio of about 1:3 to about 1:4.

In some embodiments of the present disclosure, the aqua regia comprises nitric acid and hydrochloric acid at a ratio of about 1:3, about 1:3.1, about 1:3.2, about 1:3.3, about 1:3.4, about 1:3.5, about 1:3.6, about 1:3.7, about 1:3.8, about 1:3.9 or about 1:4.

In exemplary embodiments of the present disclosure, the ratio between the nitric acid and the hydrochloric acid in the aqua regia is about 1:3.8. Without intending to be limited by theory, the excess 0.8 addition of hydrochloric acid for the preparation of Aqua regia (over and above the standard 1:3 ratio between HNO3 and HCl) makes sure that the dissolved silver in the ionic form is completely converted into solid silver chloride. Once the silver is converted into solid silver chloride, the established system of filtration can be used for effective capturing of the silver chloride to make sure there is no solid silver chloride escaping into the precipitation tank along with the gold solution.

In some embodiments, the ratio between the gold scrap in the form of grains, the nitric acid and the hydrochloric acid ranges from about 1:1:3 to about 1:1:4.

In an exemplary embodiment, the ratio between the gold scrap, the nitric acid and the hydrochloric acid is about 1:1:3.8.

In exemplary embodiments, the concentration of nitric acid is about 52-55% and the concentration of hydrochloric acid is about 30-33% and, the ratio between the gold scrap in the form of grains, the nitric acid and the hydrochloric acid in step (a) is about 1:1:3.8.

In some embodiments, the concentration of nitric acid is about 55% and concentration of hydrochloric acid is about 33%; wherein the ratio between the nitric acid and the hydrochloric acid in the aqua regia is about 1:3.8; and wherein the ratio between the gold scrap, the nitric acid and the hydrochloric acid is about 1:1:3.8.

In some embodiments, the dissolution in step (a) is performed by sequential addition of the Nitric acid and the Hydrochloric acid to the gold scrap. In preferred embodiments, the dissolution in step (a) is performed by sequential addition of the nitric acid and the hydrochloric acid to the gold scrap.

In an exemplary embodiment, the sequential addition of the nitric acid and the hydrochloric acid to the gold scrap comprises addition of the nitric acid followed by the hydrochloric acid to the gold scrap.

In some embodiments, the dissolution is performed in a reactor. In an exemplary embodiment, the reactor is a tumbler reactor. In some embodiments, the tumbler reactor contains means for agitation of the contents of the reactor. In some embodiments, the tumbler reactor facilitates mixing and dissolution of the impure gold grains by tumbling action in one or both of clockwise and anti-clockwise direction.

Without intending to be limited by theory, dissolution of gold does not start till the addition of nitric acid. Accordingly, in order to avoid idle time in the reactor, the nitric acid is added first, followed by the hydrochloric acid, therefore decreasing the time required for dissolution of gold inside the reactor.

In some embodiments, the required volume of each acid i.e. the nitric acid and the hydrochloric acid are added as a single dose or as multiple doses of each acid over a span of time, preferably at fixed intervals.

In a non-limiting embodiment, complete addition of the acids into the reactor takes about 30 minutes to about 60 minutes.

In another non-limiting embodiment, complete addition of the acids takes about 30 minutes, about 35 minutes, about 40 minutes, about 45 minutes, about 50 minutes, about 55 minutes or about 60 minutes.

In an exemplary embodiment, the complete addition of acid takes about 40 minutes.

In some embodiments, the dissolution in step (a) is performed by addition of the aqua-regia to the impure gold, preferably in the form of gold grains. Once the dissolution starts, the temperature of NOx fumes arising from the process starts to increase and reaches a peak value wherein the maximum amount of dissolution takes place followed by decrease in temperature and saturation of the temperature which denotes the complete conversion of all solid impure gold grains into gold chloride solution.

In some embodiments, the dissolution in step (a) is performed by addition of the aqua regia to the impure gold grains till temperature of NOx fumes arising from the process reaches a peak value of about 47°C to about 52°C, decreases and shows saturation in the range of about 33°C to about 35.75°C. In a non-limiting embodiment, once the saturation temperature is reached, the agitation is stopped and the next step i.e. the filtration is allowed to commence.

In some embodiments, the dissolution in step (a) is performed by the addition of aqua regia to the impure gold grains in a tumbler reactor. The process is carried out till the temperature of NOx fumes arising from the process reaches a peak value of about 47°C, about 48°C, about 49°C, about 50°C, about 51°C or about 52°C, decreases and shows saturation in the range of about 33°C, about 34°C, about 35°C or about 35.75°C. Once the saturation temperature is reached, the tumbling action of the reactor is stopped and the contents of the reactor are subjected to filtration.

The above monitoring of the temperature of the NOx fumes enables carrying out the process based on the impurity levels in the input rather than fixed time (for example, contrary to flat 5 hours for dissolution of gold, as per conventional methods). This ensures that incomplete dissolution of the impure gold is avoided, while also making sure that extra energy is not spent unnecessarily after completion of dissolution.

In some embodiments, during the precipitation in step (c), impurity in the form of silver chloride (formed as a by-product during the gold dissolution with aqua regia in tumbler reactor) may have a tendency to mix with the pure gold powder thereby hindering the formation of Ultra-Pure 99.99% of pure gold (<100ppm impurities) to yield a pure gold having 99.90% purity (which may have nearly 1000 ppm impurities). The present disclosure, therefore, employs a filtration step prior to the precipitation step. This allows achieving the required 99.99% of pure gold with single step since all the silver chloride gets captured and separated by the filtration and pure gold powder can be precipitated using suitable precipitation agents.

In some embodiments, the filtration in step (b) comprises filtering the leachate through one or more filter papers having mesh size of about 5 micron to about 7 micron, one or more bag filter(s) having mesh size of about 1 micron to about 5 micron and/or one or more filter cartridge(s) having mesh size of about 0.2 to about 0.5 micron. In a non-limiting embodiment, the one or more filter papers having mesh size of about 5 micron to about 7 micron may be comprised in a disk filter.

In some embodiments, the filtration in step (b) comprises filtering the leachate through one or more filter papers having mesh size of about 5 micron, about 6 micron or about 7 micron, one or more bag filter(s) having mesh size of about 1 micron, about 2 micron, about 3 micron, about 4 micron or about 5 micron and/or four or more filter cartridge(s) having mesh size of about 0.2 micron, about 0.3 micron, about 0.4 micron or about 0.5 micron.

In some embodiments, the filtration in step (b) is through 1-2 filter paper(s) having mesh size of about 5 micron, 1-2 bag filter(s) having mesh size of about 1 micron and/or 4-5 filter cartridges having mesh size of about 0.2 micron.

Satisfactory results may be obtained employing a single filter paper and a single filter bag along with 4 or more filter cartridges. Accordingly, in an exemplary embodiment, the filtration in step (b) is through a filter paper having mesh size of about 5 micron, a bag filter having mesh size of about 1 micron and 4 filter cartridges having mesh size of about 0.2 micron.

Without intending to be limited by theory, in some embodiments, the filtration unit is designed in such a way that initially the bigger size particles (greater than 5 micron and 1 micron in size) are captured in the filter paper placed in the disk filter and bag filter unit, respectively. All the other particles of size around 0.2 microns are captured in the series of cartridges. Thereby all the micron level particles are captured by the said arrangement.

In some embodiments, the method for purification of gold from gold scrap comprises –
a) Dissolving the gold scrap in aqua regia to obtain a leachate;
b) Subjecting the leachate of step (a) to filtration through 1-2 filter paper(s) having mesh size of about 5 micron, 1-2 filter bag(s) having mesh size of about 1 micron and/or 4-5 filter cartridges having mesh size of about 0.2 micron, to obtain a filtrate;
c) Subjecting the filtrate from step (b) to precipitation by addition of a precipitation agent till a redox potential of about 380mV to about 420mV is reached to obtain a gold precipitate and a supernatant; and
d) Subjecting the gold precipitate to melting to obtain pure gold.

In some embodiments, the method for purification of gold from gold scrap comprises –
a) Dissolving the gold scrap in aqua regia to obtain a leachate;
b) Subjecting the leachate of step (a) to filtration through a filter paper having mesh size of about 5 micron, a filter bag having mesh size of about 1 micron and 4 filter cartridges having mesh size of about 0.2 micron, to obtain a filtrate;
c) Subjecting the filtrate from step (b) to precipitation by addition of a precipitation agent till a redox potential of about 380mV to about 420mV is reached to obtain a gold precipitate and a supernatant; and
d) Subjecting the gold precipitate to melting to obtain pure gold.

In some embodiments, the method for purification of gold from gold scrap comprises –
a) Melting the gold scrap;
b) Graining the melted gold scrap to obtain impure gold grains;
c) Dissolving the gold scrap in aqua regia to obtain a leachate;
d) Subjecting the leachate of step (c) to filtration through a filter paper having mesh size of about 5 micron, a filter bag having mesh size of about 1 micron and 4 filter cartridges having mesh size of about 0.2 micron, to obtain a filtrate;
e) Subjecting the filtrate from step (d) to precipitation by addition of a precipitation agent till a redox potential of about 380mV to about 420mV is reached to obtain a gold precipitate and a supernatant; and
f) Subjecting the gold precipitate to melting to obtain pure gold.

In some embodiments, the method for purification of gold from gold scrap comprises –
a) Melting the gold scrap at a temperature of about 1000°C to about 1100°C;
b) Graining the melted gold scrap by atomization at a water pressure of about 2 bar to about 3 bar and air pressure of about 5 bar to about 6 bar to obtain impure gold grains;
c) Dissolving the gold scrap in aqua regia to obtain a leachate;
d) Subjecting the leachate of step (c) to filtration through a filter paper having mesh size of about 5 micron, a filter bag having mesh size of about 1 micron and 4 filter cartridges having mesh size of about 0.2 micron, to obtain a filtrate;
e) Subjecting the filtrate from step (d) to precipitation by addition of a precipitation agent such as but not limited to SBS till a redox potential of about 380mV to about 420mV is reached to obtain a gold precipitate and a supernatant; and
f) Subjecting the gold precipitate to melting to obtain pure gold.

In some embodiments, the filtrate from the filtration of the leachate is treated with urea to neutralize any excess nitric acid present in filtrate, prior to precipitation.

In some embodiments, the ratio between the filtrate of step (b) and the urea ranges from about 1:0.135 to about 1:0.146 (Filtrate in liters: Urea crystals in Kg).

In some embodiments, the precipitation agent in step (c) is selected from a group comprising as Sodium bisulfite (SBS) and sulphur dioxide (SO2).

In an exemplary embodiment, the precipitation agent is Sodium bisulfite (SBS).

Without intending to be limited by theory, the precipitation occurs as a result of the following reaction -

In the above reaction, the reactant side has AuCl4- which has high tendency for getting reduced and therefore its Oxidation Reduction Potential (ORP), in a non-limiting embodiment, ranges from about 680mV to about 700mV. In some embodiments, after precipitation with SBS, AuCl4- gets converted into Au and the ORP value is reduced to about 380mV to about 420mV.

In some embodiments, when the precipitation agent is SBS, it is employed as a solution in water. In some embodiments, when the precipitation agent is SBS, it is added to the filtrate derived from the dissolution step in the form of a solution comprising Sodium Metabisulphite (SMBS) dissolved in water, preferably demineralized water. In a non-limiting embodiment, the ratio between the SMBS (in Kg) and the water (in liters) is about 1.2:2 to about 1:2.

In an exemplary embodiment, the ratio between the SBS and the filtrate in step (c) ranges from about 1:3.25 to about 1:4.

In some embodiments, the ratio between the SBS and the filtrate in step (c) is about 1:3.25, about 1:3.3, about 1:3.4, about 1:3.5, about 1:3.6, about 1:3.7, about 1:3.8, about 1:3.9 or about 1:4.

The precipitation following the filtration of the leachate from the dissolution step yields a precipitate and a supernatant, wherein the precipitate is pure gold.

In some embodiments, the precipitation reaction is stopped when the redox potential of the system comprising the precipitate and the supernatant reaches a value ranging from about 380mV to about 420mV.

In some embodiments, the precipitation reaction is stopped when the redox potential of the system comprising the precipitate and the supernatant reaches a value of about 380mV, about 385mV, about 390mV, about 395mV, about 400mV, about 405mV, about 410mV, about 415mV, or about 420mV.

The precipitate obtained in step (c), in some embodiments, is separated from the supernatant by method(s) such as but not limited to filtration and/or decantation. Said precipitate, in some embodiments, is allowed to air dry for 5 minutes to about 10 minutes to yield a pure gold powder, before being subjected to melting.

The pure gold powder obtained post the precipitation step, in some embodiments, is further subjected to melting. In some embodiments, said melting is performed at a temperature of about 1000°C to about 1100°C, preferably about 1100°C.

The pure gold powder obtained post the precipitation step, in some embodiments, is subjected to melting at a temperature of about 1000°C, about 1010°C, about 1020°C, about 1030°C, about 1040°C, about 1050°C, about 1060°C, about 1070°C, about 1080°C, about 1090°C or about 1100°C.

In some embodiments, the melted pure gold is converted into a solid form such as but not limited to granules and subjected to cleaning.

In some embodiments, the melted pure gold is converted into a solid form such as but not limited to granules by pouring the molten metal into a bath consisting of 5-6% of Isopropyl Alcohol.for the purpose of granulation and cleaning. In some embodiments, the IPA is about 5% to about 6% IPA, preferably about 5% IPA. Agents such as IPA, act as good cleaning reagents to help remove microbial stains from the surface of the gold granules. Said cleaning therefore confers a bright surface finish to the gold granules.

In some embodiments, the supernatant obtained in the precipitation step is separated from the precipitate by method(s) such as but not limited to filtration and is further subjected to cementation using cementation agent(s) such as but not limited to sodium borohydride (NaBH4).

Optionally, the supernatant obtained in the precipitation step is separated from the precipitate by method(s) such as but not limited to filtration and subjected to one or more further rounds of precipitation as per the above embodiments and the supernatant obtained in the final precipitation step is separated from the precipitate by method(s) such as but not limited to filtration and subsequently subjected to cementation using cementation agent(s) such as but not limited to sodium borohydride (NaBH4).

Optionally, the supernatant obtained in the precipitation step is separated from the precipitate by method(s) such as but not limited to filtration and subjected to a further round of precipitation as per the above embodiments and the supernatant obtained in the further precipitation step is separated from the precipitate by method(s) such as but not limited to filtration and is subsequently subjected to cementation using cementation agent(s) such as but not limited to sodium borohydride (NaBH4).

The sodium borohydride reacts with gold chloride to settle any residual gold in the supernatant obtained after the precipitation step, as per the following reaction:
AuCl3(liq.) + NaBH4(solid) → Au↓+ NaCl + BH3 + H2
Cementation by the NaBH4 is an exothermic reaction. Therefore, it is only used in a small concentration, as a cementing agent, since bulk addition can cause heat up (up to 80°C) of the solution leading to overflow from the tank.

In some embodiments, the ratio between the supernatant obtained the precipitation step and the NaBH4 ranges from about 1:0.0002 to about 1:0.000225.

In an exemplary embodiment, the ratio between the supernatant obtained in the precipitation step and the NaBH4 is about 1:0.0002.

The said cementation step recovers any residual gold present in the supernatant from the precipitation step, to minimize any gold loss in the process effluent. Optionally, in order to try and avoid loss of any residual gold in the process effluent, the cementation step is repeated about 1 times to about 4 times, preferably about 3 times, wherein the contents of the cementation tank are subjected to filtration and the filtrate from each step is subjected to the subsequent round of cementation.

In a non-limiting embodiment, residual gold is settled by the cementation agent and is recovered by filtration, in the form of filtration residues at the end of the cementation step(s). In a non-limiting embodiment, the gold recovered from the cementation step(s) may contain additional impurities such as but not limited to Cu, Zn and Ag along with the recovered gold. Accordingly, in some embodiments, the gold recovered from the cementation step is mixed with the gold scrap to be subjected to the next round of purification.

Thus, in some embodiments of the present disclosure, the method for purification of gold from gold scrap comprises –
a) Dissolving the gold scrap in aqua regia to obtain a leachate;
b) Subjecting the leachate of step (a) to filtration to obtain a filtrate;
c) Subjecting the filtrate from step (b) to precipitation by addition of a precipitation agent till a redox potential of about 380mV to about 420mV is reached to obtain a gold precipitate and a supernatant;
d) Subjecting the gold precipitate of step (c) to melting to obtain pure gold;
e) Subjecting the supernatant of step (c) to cementation; and
f) Recovering gold from the cementation step.

In some embodiments of the present disclosure, the method for purification of gold from gold scrap comprises –
a) Dissolving the gold scrap in aqua regia to obtain a leachate;
b) Subjecting the leachate of step (a) to filtration to obtain a filtrate;
c) Subjecting the filtrate from step (b) to precipitation by addition of a precipitation agent till a redox potential of about 380mV to about 420mV is reached to obtain a gold precipitate and a supernatant;
d) Subjecting the gold precipitate of step (c) to melting to obtain pure gold
e) Subjecting the supernatant of step (c) to cementation by adding sodium borohydride (NaBH4) as cementation agent; and
f) Recovering gold from the cementation step (g), wherein the said recovered gold is mixed with gold scrap for the next round of purification.

In some embodiments, the processing of the precipitate and the supernatant arising from step (c), in steps (d) and (e), respectively are performed simultaneously or at different times.
In an exemplary embodiment, the processing of the precipitate and the supernatant arising from step (c), in steps (d) and (e), respectively are performed simultaneously.
In some embodiments of the present disclosure, the method of purification of gold from gold scrap comprises -
a) Melting the gold scrap;
b) Subjecting the melted gold scrap of step (a) to graining by atomization to obtain impure gold grains;
c) Dissolving the impure gold grains in aqua regia comprising Nitric acid and Hydrochloric acid at a ratio of about 1:3.8 by addition of the aqua regia to the gold scrap till temperature of NOx fumes arising from the process reaches a peak value, decreases and shows saturation, to obtain a leachate;
d) Subjecting the leachate of step (c) to filtration through a filter paper having a mesh size of 5 micron, filter bag having mesh size of about 1 micron and 4 filter cartridge(s) having mesh size of about 0.2 micron to obtain a filtrate;
e) Subjecting the filtrate from step (d) to precipitation by addition of sodium bisulfite (SBS) till a redox potential of about 380mV to about 420mV is reached to obtain a gold precipitate and supernatant;
f) Subjecting the gold precipitate of step(e) to melting to obtain pure gold having purity of at least 99.99%;
g) Subjecting the supernatant of step(e) to cementation by adding sodium borohydride (NaBH4) as cementation agent; and
h) Recovering gold from the cementation step (g).

In some embodiments of the present disclosure, the method of purification of gold from gold scrap consists of -
a) Melting the gold scrap at a temperature of about 1000°C to about 1100°C;
b) Graining the melted gold scrap by atomization at a water pressure of about 2 bar to about 3 bar and air pressure of about 5 bar to about 6 bar to obtain impure gold grains;
c) Dissolving the impure gold grains in aqua regia comprising Nitric acid and Hydrochloric acid at a ratio of about 1:3.8 by addition of the aqua regia to the gold scrap till temperature of NOx fumes arising from the process reaches a peak value, decreases and shows saturation, to obtain a leachate;
d) Subjecting the leachate of step (c) to filtration through a filter paper having a mesh size of 5 micron, a filter bag having mesh size of about 1 micron and 4 filter cartridges having mesh size of about 0.2 micron to obtain a filtrate;
e) Subjecting the filtrate from step (d) to precipitation by addition of sodium bisulfite (SBS) till a redox potential of about 380mV to about 420mV is reached to obtain a gold precipitate and supernatant;
f) Subjecting the gold precipitate to melting to obtain pure gold having purity of at least 99.99%;
g) Subjecting the supernatant of step(e) to cementation by adding sodium borohydride (NaBH4) as cementation agent; and
h) Recovering gold from the cementation step (g), wherein the said recovered gold is mixed with gold scrap for the next batch of purification.

Aligning with one of the objectives of the present disclosure, in some embodiments, the method of the present disclosure is completely automated to facilitate safe handling of highly corrosive/hazardous aqua-regia acid in refining or purification of bulk batches of gold scrap.
In some embodiments, the method of the present disclosure reduces lead time of the gold purification by at least about 60% as compared to a conventional gold recovery process.
In some embodiments, the method of the present disclosure reduces lead time of the gold purification by about 65% to about 70% as compared to a conventional gold recovery process.
In some embodiments, the method of the present disclosure reduces lead time of the gold purification by about 65%, about 66%, about 67%, about 68%, about 69% or about 70% as compared to a conventional gold recovery process.

Further provided in the present disclosure is a system (500) for purification of gold from gold scrap, the system comprising: at least one tumbler reactor (10) which may be coupled to an actuator [not shown in figures] in order to carry out tumbling function of the tumbler reactor (10). The at least one tumbler reactor (10) may be connectable to a graining machine (200) which is in turn fluidly connected to an induction melting furnace (100). In an embodiment, the induction melting furnace (200) receives raw materials such as impure gold scraps like old jewellery, defective products etc. which are melted at a temperature ranging from about 1000 ℃ to about 1100℃. The melted gold scrap is further processed in the graining machine (200). Here the melted gold scraps are atomized in order to form impure gold grains. Further, the at least one tumbler reactor (10) is fluidly connected to at least one secondary tank (20, 20’) wherein at least one primary tank (30, 30’) is connected to the secondary tank (30). The at least one tumbler reactor (10) is manually loaded with impure gold grains. The tumbler reactor receives nitric acid and hydrochloric acid at a ratio of about 1:3.8 which leads to the formation of aqua regia inside the reactor. Temperature of NOx fumes arising from the process reaches a peak value, decreases and shows saturation.

The gold solution or leachate is then subjected to filtration through a filter paper (11) having a mesh size of 5 micron, filter bag (12) having mesh size of about 1 micron and 4 filter cartridge(s) (13) having mesh size of about 0.2 micron to obtain a filtrate. In an embodiment, the filtration may be carried out by a disk filter (11) fluidly connectable to the at least one tumbler reactor (10). Further, filtration may subsequently be carried out by a bag filter (12) and finally the gold solution may be filtered by a series of cartridge filters (13).

In an embodiment, NOx released from the gold filtration process emanating out from the at least one tumbler reactor (10) is scrubbed by a 3 stage scrubber (40).

Further, the gold solution that is filtered from the above-mentioned steps is controlled and allowed into at least one primary precipitation tank (50). The at least one primary precipitation tank (50) is fluidly connected to the at least one tumbler reactor (10) in order to receive the gold solution. In an embodiment, at least one valve (14) is provided in-between the at least one tumbler reactor (10) and the at least one primary precipitation tank (50). At least one secondary precipitation tank (60) is connected to the at least one primary precipitation tank (50) through a disk filter in order to receive controlled flow of the filtered solution from the at least one primary precipitation tank. This residues in the disk filter i.e. the precipitate generated in the primary precipitation tank, which is nothing but pure gold is further subjected to melting in the induction melting furnace (100’). Once the pure gold powder is melted, the melted fluid is subjected to granule formation in a granulating machine (200’).

Further referring to figure 5, at least one cementation tank (70) is connected to the at least one secondary precipitation tank (60), where the at least one cementation tank (70) receives filtrate solution and generated residues. In an embodiment, the at least one cementation tank (70) may be two or more cementation tanks that are laid out in series in order to cement the filtrate solution. As an example, and referring to figure 5, the system (500) includes at least three cementation tanks (70’, 70’’, 70’’’) all laid out in series. The filtration solution is first passed through the cementation tank (70’) and the filtrate solution is filtered through a bag filter (12’). Similarly, the filtrate solution is then circulated to the second cementation tank (70’’) and further filtered through a bag filter (12’’). Finally, the filtrate solution passes through the third cementation tank (70’’’) and through the bag filter (12’’’) before the same is collected in storage tanks (80) for further processing such as processing through an ion exchange resin.

In an embodiment, the operation of the system is carried out by a control unit [not shown in figure]. More specifically, the control unit operates the tumbler reactor in order to facilitate dissolution of the impure gold. The control unit is configured to selectively operate the at least one valve for allowing the flow of aqua regia acids into the tumbler reactor, wherein the control unit determines the level of fluid in the at least one primary tank and the at least one secondary tank. Once the fluid level is determined, it initiates actuation of the at least one valve of the at least one primary tank (30) and the at least one secondary tank (20) and into the at least one tumbler reactor (10). The control unit further actuates the at least one tumbler reactor (10) for dissolution of the gold scrap to the gold solution. In an embodiment, the control unit may control and be programmed to automate other operating parameters such as valves, actuators in order to automate the operation of the system.
Accordingly, in some embodiments, provided herein is an automated method for purification of gold from gold scrap comprising-
a) Dissolving the gold scrap in aqua regia to obtain a leachate;
b) Subjecting the leachate of step (a) to filtration to obtain a filtrate;
c) Subjecting the filtrate from step (b) to precipitation by addition of a precipitation agent till a redox potential of about 380mV to about 420mV is reached to obtain a gold precipitate and a supernatant;
d) Subjecting the gold precipitate to melting to obtain pure gold.

In some embodiments, the automated method for purification of gold from gold scrap comprises-
a) Melting the gold scrap at a temperature ranging from about 1000°C to about 1100°C;
b) Subjecting the melted gold scrap of step (a) to graining by atomization to obtain impure gold grains;
c) Dissolving the impure gold grains in aqua regia comprising Nitric acid and Hydrochloric acid at a ratio of about 1:3.8 by addition of the aqua regia to the gold scrap till temperature of NOx fumes arising from the process reaches a peak value, decreases and shows saturation, to obtain a leachate;
d) Subjecting the leachate of step (c) to filtration through a filter paper having a mesh size of 5 micron, filter bag having mesh size of about 1 micron and 4 filter cartridge(s) having mesh size of about 0.2 micron to obtain a filtrate;
e) Subjecting the filtrate from step (d) to precipitation by addition of sodium bisulfite (SBS) till a redox potential of about 380mV to about 420mV is reached to obtain a gold precipitate and supernatant;
f) Subjecting the gold precipitate to melting to obtain pure gold having purity of at least 99.99%;
g) Subjecting the supernatant of step (e) to cementation by adding sodium borohydride (NaBH4) as cementation agent; and
h) Recovering gold from the cementation step (g), wherein the recovered gold of step (g) is mixed with gold scrap for the next batch of purification.

In some embodiments, the tumbler reactor (10) is made of material such as but not limited to polypropylene homo-polymer (PPH). The tumbler reactor provides agitation to the contents of the reactor by way of a tumbling action. In some embodiments, the tumbling action takes place in both clock-wise and anti-clockwise direction for efficient agitation of the impure gold grains take place, thereby promoting quick and complete dissolution of the impure gold in the tumbler reactor.

In a non-limiting embodiment, the tumbler reactor is powered by a variable-frequency drive (VFD) motor.

In some embodiments, the tumbler (10) receives the gold scrap in the form of impure gold grains.

Accordingly, in some embodiments, the system (500) comprises at least one induction furnace (100) to receive and melt the gold scrap. In some embodiments, the melted gold scrap is transferred to a graining machine (200) to form impure gold grains for dissolution in the at least one tumbler reactor.

In some embodiments, the at least one tumbler reactor (10) receives HCl and HNO3 solution from the at least one primary acid tank (30, 30’) routed through at least one secondary acid tank (20, 20’).

In some embodiments, the at least one tumbler reactor receives HCl and HNO3 solution from one primary acid tank each for HCl (30) and HNO3 (30’) routed through at least one secondary acid tank each for HCl (20) and HNO3 (20’).

In some embodiments, the at least one valve member is configured to the control unit to allow metered volume of fluid from the primary acid tank to the secondary acid tank.

In a non-limiting embodiment, the secondary acid tank (20, 20’) is a transparent glass vessel with level indicator, which can be used for monitoring the amount of acid being added into the tumbler reactor (10).

While the acid may be added to the reactor (10) directly from the primary acid tank (30, 30’), this could potentially lead to inaccurate dosing. For instance, due to bulging effect in the primary acid tank there could be a variation in the amount of acid being added into the secondary acid tank. Therefore, the secondary acid tank serves the purpose of calibration of acid dosage to avoid improper acid addition into the reactor.

In some embodiments, the tumbler reactor (10) is fitted with a means to detect temperature of the NOx fumes arising from the reaction of gold with aqua regia.

In some embodiments, the tumbler reactor is fitted with a thermocouple near the NOx fumes outlet to detect temperature of the NOx fumes arising from the reaction of gold with aqua regia.
In some embodiments, the contents in the tumbler are allowed to react till temperature of NOx fumes arising from the process reaches a peak value of about 47°C, about 48°C, about 49°C, about 50°C, about 51°C or about 52°C, decreases and shows saturation in the range of about 33°C, about 34°C, about 35°C or about 35.75°C. In a non-limiting embodiment, once the saturation temperature is reached, the tumbling action of the reactor is stopped and the next step i.e. the filtration of the contents of the reactor is allowed to commence.

In some embodiments, the tumbler is connectable to a chimney to allow exit of the NOx fumes through a 3 stage scrubber system.

In some embodiments, the tumbler is fluidly connectable to a chimney through a series scrubbers comprising but not limited to a pre-stage scrubber, a primary scrubber and a secondary scrubber to allow exit of the scrubbed NOx fumes.

In some embodiments, the system further comprises a disk filter connectable to the at least one tumbler reactor (10) and the at least one precipitation tank (50) for filtration.

In some embodiments, the system comprises at least one disk filter unit having filter paper(s), at least one bag filter and at least one cartridge between the at least one tumbler reactor (10) and the at least one precipitation tank (50) for filtration of the gold solution from the tumbler reactor (10) to the at least one precipitation tank (50).

In some embodiments, the system comprises at least one disk filter unit having filter paper mesh size of about 5 micron to about 7 micron, at least one bag filter(s) having mesh size of about 1 micron to about 5 micron and/or at least one filter cartridge(s) having mesh size of about 0.2 to about 0.5 micron between the at least one tumbler reactor and the at least one precipitation tank for filtration of the gold solution.

In some embodiments, the system comprises at least one disk filter unit having 1-2 filter paper(s) having mesh size of about 5 micron, 1-2 bag filter(s) having mesh size of about 1 micron and/or 4-5 filter cartridges having mesh size of about 0.2 micron between the at least one tumbler reactor and the at least one precipitation tank for filtration of the gold solution.

In some embodiments, the system comprises at least one disk filter unit having a filter paper having mesh size of about 5 micron, a bag filter having mesh size of about 1 micron and 4 filter cartridges having mesh size of about 0.2 micron between the at least one tumbler reactor and the at least one precipitation tank for filtration of the gold solution.

In a non-limiting embodiment, the system is designed such that the flow of filtered leachate from the disk filter unit to the bag filter(s); and from the bag filter(s) to the filter cartridge(s) is regulated by diaphragm pumps.

In some embodiments, the system is designed to enable addition of urea crystals to the precipitation tank for the purpose of removal of excess HNO3 acid present in the solution in order to inhibit premature gold precipitation. Accordingly, in some embodiments, the system further comprises a urea tank fluidly connectable with the at least precipitation tank.

In some embodiments, the precipitation agent such as but not limited to SBS, is prepared and stored in a separate tank, from where it is pumped into the at least one precipitation tank (50) for the precipitation of the gold from the gold solution.

In an exemplary embodiment, the contents of the at least one precipitation tank (50) are monitored to measure the redox potential after addition of the precipitation agent. Said monitoring, in some embodiments, is facilitated by a redox meter that is either a permanent fixture in the precipitation tank in the form of a device or a sensor, or is introduced into the precipitation tank post addition of the precipitation agent, through designated opening in the precipitation tank. The redox meter value after complete precipitation of the gold powder ranges from about 380 mV to about 420 mV. In some embodiments, dosing of the precipitation agent is stopped once the redox meter shows a reading of 380 mV to about 420 mV.
In some embodiments, the precipitation tank is connected to a disk filter to separate the pure gold precipitate from the supernatant formed after precipitation.

In some embodiments, the at least one precipitation tank (50) is connected to an induction melting furnace (100’) and the at least one cementation tank through the disk filter such that pure gold precipitate obtained as residue in disk filter is sent to the induction melting furnace (100’) and the supernatant obtained as filtrate after filtration through the disk filter is sent to the at least one cementation tank.

In some embodiments, the system comprises a tool specially designed for the purpose of lifting hot crucibles such as those in the induction melting furnace, specially characterized by customized pick & place mechanism with the help of pneumatic actuators. This allows easy replacement of hot crucible without any waiting time and therefore reduces the process lead time by about 50 minutes to about 1 hour.

In some embodiments, in order to increase the system capacity, the system comprises two tumbler reactors and two precipitation tanks, wherein the two tumbler reactors each receive a batch of impure gold grains and are individually dosed with aqua regia acids from the secondary HCl and HNO3 acid tanks to carry out gold dissolution of two batches of impure gold grains in parallel. In some embodiments, the two tumbler reactors are each connected to two precipitation tanks through separate filtration units each having at least one disk filter unit having filter paper, at least one bag filter and at least one cartridge for filtration of the gold solution from the tumbler reactors.

In some embodiments, the system comprises at least one induction melting furnace (100’) to receive and melt the pure gold precipitate from the precipitation tank(s). In some embodiments, the melted pure gold is transferred to a granulation-machine to form pure gold granules.

In some embodiments, operations like tilting of furnace, stirring of molten metal in the furnace is automated with the help of hydraulic and pneumatic units. This allows elimination of direct human-high temperature interface.

In some embodiments, the system comprises a secondary precipitation tank (60) that is connected to the precipitation tank to obtain the filtrate through the disk filter.
In some embodiments, the secondary precipitation tank (60) is designed to conduct a second round of precipitation and therefore, is connected to the SBS storage tank to enable addition of the said precipitation agent for precipitation of any residual gold in the filtrate from the precipitation tank.

In some embodiments, the secondary filtration tank is further connected to the at least one cementation tank (70’, 70’’, 70’’’) through a disk filter, such that the filtrate flows into the at least one cementation tank(70’, 70’’, 70’’’).

In some embodiments, the at least one cementation (70’, 70’’, 70’’’) tank receives cementation agent such as but not limited to NaBH4 from a precipitation agent storage tank.

In some embodiments, the at least one cementation tank comprises a series of 2 or more cementation tank.

In an exemplary embodiment, the at least one cementation tank comprises a series of 3 cementation tanks (70’, 70’’, 70’’’), each connected to the subsequent cementation tank through a bag filter (12’, 12’’, 12’’’).

In an exemplary embodiment, the at least one cementation tank comprises a series of 3 cementation tanks, each connected to the subsequent cementation tank through a bag filter such that the filtrate from the first cementation tank flows to the second cementation tank and the filtrate from the second filtration tank flows to the third.

In some embodiments, the residues from the filters connected to the secondary precipitation tank and the at least one cementation tank are optionally mixed with fresh gold scrap and subjected to melting and grain formation to obtain impure gold grains to be subjected to further rounds of purification.

In some embodiments, the system further comprises at least one storage tank (80) to receive the filtrate from the final cementation tank.

In some embodiments, the at least one storage tank (80) is connected to first collection tank through at least one ion exchange resin.
In some embodiments, the first collection tank is further connected to at least one neutralization tank that is dosed with one or more neutralization agent(s) from neutralization agent(s) storage tank(s). In some embodiments, the neutralization agent(s) includes but is not limited to sodium hydroxide i.e. NaOH.

In some embodiments, the at least one neutralization tank comprises a first neutralization tank and a second neutralization tank, connected though a filter press.

In some embodiments, the at least one neutralization tank is connected to a second collection tank though a filter press, wherein the second collection tank receives a clear solution with no solids from the at least one neutralization tank through the filter press.

In some embodiments, the system comprises a SOx scrubber fluidly connected to the at least the at least one precipitation tank, the at least one cementation tank, the at least one collection tank and the at-least one neutralization tank for neutralization of SOx.

Further provided herein is gold obtained by the method of the present disclosure, wherein the gold has purity of at least 99.99% with single stage gold purification.

While the present disclosure is susceptible to various modifications and alternative forms, specific aspects thereof have been shown by way of examples and drawings and are described in detail below. However, it should be understood that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and the scope of the invention as defined above.

EXAMPLES

EXAMPLE 1: GOLD PURIFICATION AS PER THE METHOD OF THE PRESENT DISCLOSURE

The gold purification process was performed in a system (Figure 1, Figure 5) that enables automation of the process of the present disclosure as outlined in the above embodiments and in Figure 6.

About 25 Kg of gold scrap having about 84.66 % of average gold batch purity was taken for the purpose of purification. Said gold was first subjected to melting in a crucible at a temperature of about 1100°C and then sent for graining in an atomizer operating at a water pressure of about 2 bar to about 3 bar and air pressure of about 5 bar to about 6 bar.

The weight of gold grains obtained from the atomizer was about 25 Kg. After loading the impure gold grains into the reactor, weight of the same was entered into the control panel to initiate the process. At first the program started to add Nitric acid and then Hydrochloric acid into the tumbler reactor at a ratio of 1:3.8 employing Nitric acid at a concentration of about 52 - 55% and Hydrochloric Acid at a concentration of about 30 - 33%, respectively.

HNO3 was first added into the reactor which took about 9 minutes for the addition of about 25 litres of HNO3 for about 25 Kg impure gold grains. This was followed by the addition of HCl which took about 20 minutes for addition of 95 litres of HCl for 25 Kg impure gold.

Tumbling action was made to take place in both clockwise and anti-clockwise direction for the dissolution of impure gold. The temperature of the NOx fumes generated in the reactor was measured, wherein the temperature was found to first steadily increase, then reach a peak value of about 47.75°C, followed by saturation at a temperature of about 35.75°C (figure 4(f)). The point of temperature saturation as indicated in the figure denotes the complete dissolution of the impure gold grains inside the reactor.

In order to effectively capture all the silver chloride particles, the gold chloride solution was passed through a 5 micron filter paper, followed by a 1 micron filter bag and subsequently, four filter cartridges consisting of 0.2 micron mesh size (figure 2).

Urea was added to the filtrate at a ratio of about 1:0.135 to about 1:0.146 (Filtrate in liters: Urea crystals in Kg), to neutralize excess nitric acid.

Subsequently, pure gold powder was precipitated out of the obtained filtrate using sodium Bi-sulphite (SBS) solution. The ratio between the filtrate and the SBS solution was maintained at about 1:3.25. This allowed obtaining the required 99.99% of pure gold with single step since all the silver chloride was captured at the filtration stage.
The pure gold was obtained in the form of a precipitate allowed to air dry, yielding pure gold in crystalline/amorphous powder form. Table 1 provides the elemental analysis for a sample obtained after establishment of the filter series as given in the figure 2. This result is in accordance with the ASTM B-562 -95(2005) standard specification for refined pure gold.

Table 1: Spark OES Analysis Result for 99.99% pure gold (Au and Ag in % and rest of the elements are denoted in ppm)
Au (%) Ag (%) Cu (ppm) Zn (ppm) Cd (ppm) Fe (ppm) Ni (ppm) Pb (ppm) Al (ppm)
1 99.997644 0.002 <0.0000 0.74 <0.0000 2.39 0.50 0.24 <0.0000
2 99.997568 0.002 <0.0000 0.64 <0.0000 3.16 0.64 0.26 <0.0000
Rep 99.997606 0.002 <0.0000 0.69 <0.0000 2.78 0.57 0.25 <0.0000
Mean 100.005306 0.002 <-0.0005 0.69 <-0.057 2.78 0.57 0.25 <-0.14
As (ppm) Bi (ppm) Ca (ppm) Co (ppm) Cr (ppm) Mg (ppm) Mn (ppm) Pt (ppm) Rh (ppm)
1 0.98 <0.0000 <0.0000 -- 0.56 <0.0000 <0.0000 <0.0000 <0.0000
2 1.79 <0.0000 <0.0000 -- 0.55 <0.0000 <0.0000 <0.0000 <0.0000
Rep 1.38 <0.0000 <0.0000 -- 0.56 <0.0000 <0.0000 <0.0000 <0.0000
Mean 1.38 <-0.62 <-0.65 -- 0.56 <-0.30 <-0.022 <-45.85 <-0.26
Sb (ppm) Se (ppm) Si (ppm) Sn (ppm) Te (ppm) Bg (ppm) Total. Imp
1 <0.0000 <0.0000 <0.0000 <0.0000 <0.0000 -- 0.002
2 <0.0000 <0.0000 <0.0000 0.14 <0.0000 -- 0.002
Rep <0.0000 <0.0000 <0.0000 0.068 <0.0000 -- 0.002
Mean <-1.23 <-9.45 <-8.53 <-0.39 <-3.95 -- -0.005

Post precipitation, the supernatant and the precipitate were separated by filtration. The said filtrate obtained from the precipitation stage was treated with Sodium Borohydrate (NaBH4) to achieve complete settling of un-precipitated or escaped ionic gold and to ensure the Zero PPM level of gold in the processed effluents. The NaBH4 was added to the filtrate at a ratio of about 0.0002:1 (NaBH4 in Kg: Effluent in litres) and the residual gold in the filtrate was allowed to settle. The settled gold sediment was recycled back into the gold scrap for further rounds of purification with the next batch of gold scraps.
The pure gold powder of 42.33 Kg (approx.21.165 Kg from Tumbler reactor 1 and 21.165 Kg from Tumbler reactor 2) obtained at the end of the precipitation step, further, was subjected to melting at a temperature of about 1100°C in an induction furnace and the molten metal was poured into a gold granule making machine which consisted of a circulating 5% Isopropyl Alcohol (IPA) bath for the purpose of cleaning and improving surface brightness of the solid pure gold granules.
With the combination of all the automated process steps stated above, the overall refining lead time got reduced to about 5 hours to process per batch of about 50 Kg of impure gold from about 16.7 hours to process per batch of about 25kg of impure gold (Figure 3). The process design also broke both the human-acid interface and human-high temperature interface, thereby increasing process safety.

EXAMPLE 2: SIGNIFICANCE OF MULTI-STAGE FILTRATION
The experiment of Example 1 was repeated, but the multi-stage filtration was replaced by a single stage filtration employing a 5 micron filter paper alone.
Minute silver chloride nano-particles escaping through the first stage of filtration of gold chloride solution hindered the achievement of 99.99% of pure gold. Therefore, post precipitation, only 99.9% of pure gold was produced with silver as main impurity with some trace impurities as denoted in the Spark OES analysis result in Table 2.
Table 2: Spark OES Analysis Result for 99.9% pure gold (Au and Ag in % and rest of the elements are denoted in PPM)
Au (%) Ag (%) Cu (ppm) Zn (ppm) Cd (ppm) Fe (ppm) Ni (ppm) Pb (ppm) Ir (ppm)
1 99.943849 0.049 0.004 <0.0000 0.96 18.12 1.30 1.19 <0.0000
2 99.943412 0.049 0.004 <0.0000 0.94 21.11 1.30 1.21 <0.0000
Rep 99.943630 0.049 0.004 <0.0000 0.95 19.62 1.30 1.20 <0.0000
Mean 100.050144 0.049 0.004 <-1.40 0.95 19.62 1.30 1.20 <-42.15
Os (ppm) Ru (ppm) Pd (ppm) Al (ppm) As (ppm) Bi (ppm) Ca (ppm) Co (ppm) Cr (ppm)
1 <0.0000 <0.0000 <0.0000 0.35 1.73 1.01 0.77 <0.0000 0.75
2 <0.0000 <0.0000 <0.0000 0.53 2.47 1.32 1.28 <0.0000 0.91
Rep <0.0000 <0.0000 <0.0000 0.44 2.10 1.16 1.02 <0.0000 0.83
Mean <-530 <-126 <-284 0.44 2.10 1.16 1.02 <-2.23 0.83
Mg (ppm) Mn (ppm) Pt (ppm) Rh (ppm) Sb (ppm) Se (ppm) Si (ppm) Sn (ppm) Te (ppm)
1 <0.0000 0.24 <0.0000 0.29 0.34 2.05 <0.0000 <0.0000 0.64
2 <0.0000 0.21 <0.0000 0.24 0.37 6.14 <0.0000 <0.0000 0.59
Rep <0.0000 0.22 <0.0000 0.26 0.35 4.09 <0.0000 <0.0000 0.61
Mean <-1.00 0.22 <-61.1 0.26 0.35 4.09 <-13.40 <-3.98 0.61
Bg (ppm) Total.Imp
1 -- 0.056
2 -- 0.057
Rep -- 0.056
Mean -- 0.048

EXAMPLE 3: SIGNIFICANCE OF CEMENTATION
The experiment of Example 1 was repeated, without performing the cementation step.

It was found by performing Atomic Absorption Spectroscopy that post precipitation, the supernatant that was separated from the precipitate had a residual amount of gold of about 1.69 ppm, as opposed to the zero ppm level in the final effluent arising from Example 1.

EXAMPLE 4: SIGNIFICANCE OF GOLD DISSOLUTION END TIME
The process of Example 1 was repeated, with the following samples –

Table 3: Relationship between purity of input gold and time taken for complete dissolution of the gold
Input gold grains weight Purity of the input gold grains Time taken for complete dissolution
Sample 1 10382 g 89.05 % 65 minutes
Sample 2 20032 g 88.93 % 95 minutes
Sample 3 15184 g 80.55 %
90 minutes
Sample 4 23010 g 82.77 % 130 minutes

From the observations it can derived that the amount of time taken for dissolution of gold grains is directly proportional to the weight of the input gold grains.
Though the weight of gold taken for dissolution in case of sample 3 was lower than the one taken in case of sample 2, still the amount of time taken for dissolution was almost at par with the other samples. The major factor contributing to the increase in the dissolution time therefore was found to be the purity of the gold. Therefore, it can be interpreted that lower the purity of the input gold scrap, longer the time taken for the dissolution process.
From the four plots generated based on the above (Figures 4(a)-4(d)), a generic plot was derived for the process of dissolution of gold as given in Figure 4(e) which can be divided into different stages in the plot. Ending the process before the attainment of temperature saturation was found to lead to improper dissolution of gold.
Based on the above, it was concluded that the dissolution done based on the input (different weight and purities) rather than fixed time (5 hours for dissolution of gold) had higher efficiency.

EXAMPLE 5: SIGNIFICANCE OF MONITORING OXIDOREDUCTION POTENTIAL
Example 1 was repeated, varying the ORP value at which the precipitation step was stopped. The ORP values were deviated to below 380mV and above 420 mV.

Experiment 1 – Precipitation stopped at ORP 370mV
About 19389 g of impure gold grains having purity of about 76.89 % were subjected to dissolution in the tumbler reactor. Post dissolution, filtration, precipitation, and cementation were performed as per Example 1. The pure gold output was about 14921 g. Said output was slightly higher than the expected output of about 14908 g.
SPARK OES analysis of the output was performed to analyse the quality of the output.
Table 4: SPARK OES Result

Fineness of Gold (Au) (%) Ag (%) Silver Cu (%) Copper Fe (Iron) PPM
99.956 0.0194 0.0226 6.63

The above table shows the SPARK OES result for the obtained gold. Since the ORP value dropped below 380 mv, copper also precipitated along with gold. Due to this, the purity of the obtained gold got reduced to 99.9560 % with high level of copper due to which the required purity of 99.99% was not achieved. Also, the weight of pure gold was higher than the expected weight because of copper precipitation. Therefore, stopping the precipitation at an ORP value below 380 mv was not successful in acquiring high purity gold having purity >99.99%.
Deviation of ORP value from the mentioned range was therefore found to lead to either failure in achieving the 99.99% purity of gold or incomplete precipitation of gold.
Experiment 2 – Precipitation stopped at ORP 430mV
About 24202 g of impure gold grains having purity of about 89.27% was subjected to dissolution in the tumbler reactor. Post dissolution, filtration, precipitation, and cementation were performed as per Example 1. The pure gold output was about 19853 g. Said output was slightly lower than the expected output of about 21607 g.
It was found that about 1754 g was left un-precipitated in the solution.
Table 5: SPARK OES Result
Fineness of Gold (Au) (%) Ag (%) Silver Cu (%) Copper Fe (Iron) PPM
99.997 0.0024 0 4.39

The above table shows the SPARK OES result for the obtained gold. Even though the purity of the gold obtained was found to be 99.997%, the amount of gold left in the solution un-precipitated was a significantly large amount (i.e. about 1754 g) and recovering it in the later stages by performing further rounds of precipitation or cementation using other suitable chemical agents are time consuming and tedious. This leads to improper mass balancing and accounting. Therefore, stopping the precipitation at an ORP value above 380 mV was not successful in view of the amount of un-precipitated gold in the solution.
The above experiments show that in order to achieve complete precipitation of the gold having purity of 99.99%, the precipitation must be done by making sure that the ORP Value reaches a value in the range of 380 mv to 420 mv.
EXAMPLE 6: SIGNIFANCE OF RATIO OF ACIDS IN AQUA REGIA
Example 1 was repeated in different batches while varying the ratio HNO3:HCl ratio in
The trials began with the standard ratio of Aqua-regia. The objective of the experiment was to obtain less undissolved grains with shortest dissolution time. The input purity was about 80% to about 82% on an average. And on an average, about 10Kg of the samples were taken for trial per batch till SI.No.8 in Table 6, thereafter 20-25 Kg of sample was taken per batch. The results are summarized in the table below –
Table 6: Impact of varying acid ratio in aqua regia
SI. No. Acid Ratio (HNO3:HCl) No. of Batches Processed Cumulative Weight of Gold Processed
(Kg) Average Purity of Input Gold
(%) Pure Gold Output
(Kg) Average Weight of Undissolved Gold Grains per batch
(Kg) Dissolution Time
(Hours)
1 1:3 3 35.667 85.02 30.32 0.984 6
2 1:4 5 53.4 82.596 44.11 0.1134 6
3 1:4 10 92.559 78.93 73.06 0.192 6
4 1.4:4.5 3 36.729 81.5 29.93 0.015 7
5 1.44:5.18 14 148.99 81.26 121.07 0.8599 6
6 1.5:5 5 52.009 81.074 42.17 0.1027 5.2
7 1.8:5.8 3 30.117 80.71 24.31 0.0586 6
8 2:6 17 181.2012 82.11 148.78 0.0085 4
9 0.8:3.6 89 1705.3 86.83 1444.03 0.1-0.25 5 to 5.5
10 1:3.8 114 2720.4 87.03 2196.04 0 – 0.1 4 to 5

Starting with the ratio of about 1:3, the amount of un-dissolved grains obtained was almost 1kg per batch with 6 hours required just for dissolution per batch. Whereas at the ratio of about 1:4, the amount of un-dissolved grains obtained were low as compared to the earlier one but still the lead time for dissolution was high. At a ratio of about 1.4:4.5, the amount of un-dissolved grains decreased further but the time taken for dissolution was 7 hours.
At a ratio of about 0.8:3.6 , with about 20-25 Kgs of impure gold grains per batch, the amount of un-dissolved grains was about 100 to 250 grams per batch and the time taken for dissolution was about 5 to 5.5 hours. Even though the un-dissolved grains were more than the earlier scenario, this acid ratio and time was much more suitable in terms of cost efficiency and long-term production.
The above experiments repeated with aqua regia having an HNO3:HCl ratio of about 1:3.8 per batch of about 20-25 Kgs of impure gold grains. The amount of undissolved grains was about 0 to 100 grams per batch and the time taken for dissolution was about 4 to 5 hours.
Therefore, a key takeaway from all these trials was that with the increase in acid ratios, the amount of acid consumption and the effluent also increased which was not cost efficient and feasible for long term production.

EXAMPLE 7: Significance of choice of cementation agent
Example 1 was repeated in 2 batches, one employing NaBH4 as cementation agent and one employing Zinc as the cementation agent.
One experiment was a replica of Example 1, employing NaBH4 as the cementation agent. In the second experiment performed for comparison, about 0.06 Kg of Zinc was added as the cementation agent to about 5 litres of the supernatant of the precipitation step as an alternative to NaBH4.
In case of use of NaBH4 as cementation agent, the solution collected after the cementation showed presence of gold at a level below possible quantification. The Atomic absorption spectroscopy test result of the solution collected after the cementation of the solution with Zinc in this experiment showed presence of gold at around 1.15 ppm level. From these results, it was very evident that proper cementing of the gold particles did not take place with Zinc.
From the above-described results of the experiments, it was evident that efficient cementing of gold particles can been achieved by NaBH4 than Zinc.

EXAMPLE 8: Significance of sequence of addition of acids HCl and HNO3
In all the scenarios listed below a batch of 25 Kg of impure gold grains with an average purity of 80 – 85 % was considered for the dissolution process.
Scenario 1: Manual addition of Acid into Tumbler reactor (Acid Ratio: 0.8:3.6)
In the first Scenario, after loading of impure grains into the tumbler reactor, acid addition into the reactor was done by manually opening the valves. HCl was added into the reactor followed by the addition of HNO3. The sequence of acid addition is listed below in Table 7.
Table 7: Sequence of Acid Addition in First Batch
Step No. Acid Added Amount Of Acid Added
(Litres) Time Taken for Addition
(Minutes) Interval Time
(Minutes)
1 HCl 14 5 5
2 HCl 14 5 5
3 HCl 14 5 5
4 HCl 14 5 5
5 HCl 14 5 5
6 HCl 14 5 5
7 HCl 6 5 5
8 HNO3 7 5 10
9 HNO3 7 5 10
10 HNO3 6 5 240 (Tumbling time)
Total = 110 50 295

Total Time Taken for Dissolution was calculated by the following formula –
Total Time Taken for Dissolution (minutes) = Total time taken for acid addition + Total Interval time (Intermediate Tumbling) + Final Tumbling time = 50+55+240 = 345 minutes
Scenario 2: Manual addition of Acid into Tumbler reactor with changed sequence (Acid Ratio: 1:3.8)
The dissolution of gold did not start till the addition of nitric acid in the earlier scenario in view of which there was a huge idle time in the reactor which was not utilized since the amount and time taken for the addition of HCl was large as per the set ratio for Aqua Regia preparation. Due to this, additional tumbling time was required. In order to reduce the tumbling time required for complete dissolution of gold grains, in the next scenario, first HNO3 was added into the reactor followed by the addition of HCl to speed up the reaction. The sequence of acid addition is listed below in Table 8.
Table 8: Sequence of Acid Addition in Second Scenario
Step No. Acid Added Amount Of Acid Added
(Litres) Time Taken for Addition
(Minutes) Interval Time
(Minutes)
1 HNO3 9 5 10
2 HNO3 9 5 10
3 HNO3 7 5 10
4 HCl 15 5 5
5 HCl 15 5 5
6 HCl 15 5 5
7 HCl 15 5 5
8 HCl 15 5 5
9 HCl 15 5 5
10 HCl 5 5 160 (Tumbling Time)
Total = 120 50 220

Total Time Taken for Dissolution (minutes) = Total time taken for acid addition + Total Interval time (Intermediate Tumbling) + Final Tumbling time = 50+60+160 = 270 minutes
Scenario 3: Addition of Acid into Tumbler reactor with automation and reduced Interval Time (Acid Ratio: 1:3.8)
In this scenario the acid addition was done reducing interval time. In this experiment, the interval time was reduced to 1 minute to check the feasibility of the process to run with low interval time. The sequence of acid addition is listed below in Table 9.
Table 9: Sequence of Acid Addition in Second Batch
Step No. Acid Added Amount Of Acid Added
(Litres) Time Taken for Addition
(Minutes) Interval Time
(Minutes)
1 HNO3 9 3 0
2 HNO3 9 3 0
3 HNO3 7 3 0
4 HCl 15 2 1
5 HCl 15 2 1
6 HCl 15 2 1
7 HCl 15 2 1
8 HCl 15 2 1
9 HCl 15 2 1
10 HCl 5 2 90 (Tumbling Time)
Total = 120 23 96

Total Time Taken for Dissolution (minutes) = Total time taken for acid addition + Total Interval time (Intermediate Tumbling) + Final tumbling time = 23+6+90 = 119 minutes
Scenario 4: With all the above mentioned featured (Inclusive of Automation, New Sequence and monitoring of complete dissolution) (Acid Ratio: 1:3.8)
This scenario is in accordance with the method described in Example 1. The sequence of acid addition is listed below in Table 10. The highlighted part shows the short lead time achievement using Thermogram system.
Table 10: Sequence of Acid Addition in Second Batch
Step No. Acid Added Amount Of Acid Added
(Litres) Time Taken for Addition
(Minutes) Interval Time
(Minutes)
1 HNO3 9 3 0
2 HNO3 9 3 0
3 HNO3 7 3 0
4 HCl 15 2 1
5 HCl 15 2 1
6 HCl 15 2 1
7 HCl 15 2 1
8 HCl 15 2 1
9 HCl 15 2 1
10 HCl 5 2 64
Total = 120 23 70

Total Time Taken for Dissolution (minutes) = Total time taken for acid addition + Total Interval time (Intermediate Tumbling) + Time taken for temperature saturation for complete dissolution = 23+6+70 = 93 minutes.
The observations in all of the above scenarios are summarized below -
Table 11: Summary of stepwise improvement (Input = 25 Kg; Purity = 80- 85%)
Sl.No. Continuous Improvement Acid Addition + Intermediate Tumbling Time (Minutes) Tumbling Time (Minutes) Total Time Taken for Dissolution (Minutes)
Scenario 1 Manual addition
1- HCl
2- HNO3 105 240 345
Scenario 2 Manual addition
1- HNO3
2- HCl 110 160 270
Scenario 3 With Tumbler Automation and Acid Addition interval cycle as 1 minute 29 90 119
Scenario 4 With Thermogram system 29 64 93

This shows that a method characterized by the features of the present invention significantly reduces process lead time as compared to conventional methods and/or methods lacking features of the present invention.
Additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based on the description provided herein. The embodiments herein provide various features and advantageous details thereof in the description. Descriptions of well-known/conventional methods and techniques are omitted so as to not unnecessarily obscure the embodiments herein.
The foregoing description fully reveals the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments in this disclosure have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein, without departing from the principles of the disclosure.
Any discussion of documents, acts, materials, devices, articles and the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.
, Claims:1. A method for purification of gold from gold scrap comprising –
a) Dissolving the gold scrap in aqua regia to obtain a leachate;
b) Subjecting the leachate of step (a) to filtration to obtain a filtrate;
c) Subjecting the filtrate from step (b) to precipitation by addition of a precipitation agent till a redox potential of about 380mV to about 420mV is reached to obtain a gold precipitate and a supernatant;
d) Subjecting the gold precipitate to melting to obtain pure gold.
2. The method as claimed in claim 1, wherein the gold scrap has purity level ranging from about 75% to about 90%; and/or wherein the gold scrap is subjected to melting and graining prior to step (a).
3. The method as claimed in claim 2, wherein the melting of the gold scrap is performed at a temperature of about 1000°C to about 1100°C, preferably about 1100°C; and/or wherein the graining of the melted gold scrap is performed by atomization at a water pressure of about 2 bar to about 3 bar and air pressure of about 5 bar to about 6 bar.
4. The method as claimed in claim 1, wherein the aqua regia comprises nitric acid and hydrochloric acid at a ratio of about 1:3 to about 1:4; and wherein the ratio between the gold scrap, the Nitric acid and the Hydrochloric acid ranges from about 1:1:3 to about 1:1:4.
5. The method as claimed in claim 4, wherein the concentration of nitric acid is about 52-55% and concentration of hydrochloric acid is about 30-33%; wherein the ratio between the nitric acid and the hydrochloric acid in the aqua regia is about 1:3.8; and wherein the ratio between the gold scrap, the nitric acid and the hydrochloric acid is about 1:1:3.8.
6. The method as claimed in claim 1, wherein the dissolution in step (a) is performed by sequential addition of the nitric acid and the hydrochloric acid to the gold scrap.
7. The method as claimed in claim 6, wherein the sequential addition of the nitric acid and the hydrochloric acid to the gold scrap comprises addition of the nitric acid followed by the hydrochloric acid to the gold scrap.

8. The method as claimed in claim 1, wherein the dissolution in step (a) is performed by addition of the aqua regia to the gold scrap till temperature of NOx fumes arising from the process reaches a peak value of about 47°C to about 52°C, decreases and shows saturation in the range of about 33°C to about 35.75°C.
9. The method as claimed in claim 1, wherein the filtration in step (b) comprises filtering the leachate through one or more filter papers of mesh size 5 micron, one or more filter bag(s) having mesh size of about 1 micron to about 5 micron and one or more filter cartridge(s) having mesh size of about 0.2 to about 0.5 micron.
10. The method as claimed in claim 9, wherein the filtration in step (b) is through a filter paper of mesh size 5 micron, a filter bag having mesh size of about 1 micron and 4 filter cartridge(s) having mesh size of about 0.2 micron.
11. The method as claimed in claim 1, wherein the precipitation agent in step (c) is selected from a group comprising as Sodium bisulfite (SBS) and sulphur dioxide (SO2).
12. The method as claimed in claim 1, wherein the precipitation in step (c) comprises adding the precipitation agent to the filtrate at a ratio of about 1:3.25 to about 1:4.
13. The method as claimed in claim 1, wherein the supernatant obtained in step(c) is separated from the precipitate by filtration and is further subjected to cementation using sodium borohydride (NaBH4) as the cementation agent.
14. The method as claimed in claim 13, wherein the ratio between the supernatant obtained in step (c) and the NaBH4 ranges from about 1:0.0002 to about 1:0.000225.
15. The method as claimed in claim 14, wherein the ratio between the supernatant obtained in step (c) and the NaBH4 is about 1:0.0002.
16. The method as claimed in claim 1, wherein the melting of the gold precipitate in step(d) is performed at a temperature of about 1000°C to about 1100°C, preferably about 1100°C.
17. The method as claimed in claim 1, wherein the melted gold of step (d) is converted into a solid form such as gold granules; and wherein the said solid form of the gold is subjected to cleaning with about 5% to about 6%, preferably about 5% Isopropyl Alcohol (IPA).
18. The method of purification of gold from gold scrap as claimed in claim 1 comprising –
a) Melting the gold scrap;
b) Subjecting the melted gold scrap of step (a) to graining by atomization;

c) Dissolving the gold scrap in aqua regia comprising nitric acid and hydrochloric acid at a ratio of about 1:3.8 by addition of the aqua regia to the gold scrap till temperature of NOx fumes arising from the process reaches a peak value, decreases and shows saturation, to obtain a leachate;
d) Subjecting the leachate of step (c) to filtration through a filter paper having a mesh size of 5 micron, filter bag having mesh size of about 1 micron and 4 filter cartridge(s) having mesh size of about 0.2 micron to obtain a filtrate;
e) Subjecting the filtrate from step (d) to precipitation by addition of sodium bisulfite (SBS) till a redox potential of about 380mV to about 420mV is reached to obtain a gold precipitate and supernatant;
f) Subjecting the gold precipitate of step (e) to melting to obtain pure gold having purity of at least 99.99%;
g) Subjecting the supernatant of step (e) to cementation by adding sodium borohydride (NaBH4) as cementation agent;
h) Recovering gold from the cementation step (g).
19. The method as claimed in claim 1, wherein the method is automated for handling of bulk batches of gold refining with highly corrosive/hazardous aqua-regia acid.
20. The method as claimed in claim 1, wherein the method reduces lead time of the gold purification by about 65% to about 70% as compared to a conventional gold recovery process.
21. A system (500) for purification of gold from gold scrap, the system comprising:
at least one tumbler reactor (10) configured to receive gold scrap;
at least one primary acid tank (30, 30’) fluidly connectable to at least one secondary acid tank (20, 20’), wherein the secondary acid tank is fluidly connected to the at least one tumbler reactor for dissolution of the gold scrap to gold solution;
at least one precipitation tank (50) fluidly connected to the at least one tumbler reactor, wherein the at least one precipitation tank receives the gold solution;
at least one secondary precipitation tank (60) is connected to the at least one precipitation tank, wherein the at least one secondary precipitation tank receives decanted and filtrate solution and generates residues;
at least one cementation tank (70’, 70’’, 70’’’) is connected to the at least one secondary precipitation tank, wherein the at least one cementation tank receives filtrate solution and generates residues;
a control unit configured to:
determine level of fluid in the at least one primary acid tank and the secondary acid tank;
actuate at least one valve to initiate fluid flow from the at least one primary acid tank and the at least one secondary acid tank to the at least one tumbler reactor;
actuate the at least one tumbler reactor for dissolution of the gold scrap to the gold solution.
22. The system as claimed in claim 21, wherein the at least one tumbler reactor (10) receives HCl and HNO3 solution from the at least one primary tank (30, 30’) and the secondary tank (20, 20’).
23. The system as claimed in claim 21, wherein the at least one valve member is configured to the control unit to allow metered volume of fluid.
24. The system as claimed in claim 21, comprising at least a disk filter (11) connectable to the at least one tumbler reactor (10) and the at least one precipitation tank (50) for filtration.
25. The system as claimed in claim 21, comprising a disk filter (11), at least one bag filter (12) and at least one cartridge filter (13) fluidly connectable to the at least one tumbler reactor for filtration of the gold solution from the tumbler reactor into the precipitation tank.
26. The system as claimed in claim 21, wherein the tumbler is fluidly connectable to a chimney through a series scrubbers comprising a pre-stage scrubber, a primary scrubber and a secondary scrubber to allow exit of the scrubbed NOx fumes.
27. The system as claimed in claim 21, comprising at least one collection tank fluidly connectable to the at least one cementation tank (70’, 70’’, 70’’’) to collect effluent generated from the at least one cementation tank for further treatment.
28. The system as claimed in claim 21, comprising at least one induction furnace (100’) to receive and melt the gold scrap.
29. The system as claimed in claim 28, wherein the melted gold scrap is transferred to a graining machine to form gold grains for dissolution in the at least one tumbler reactor.
30. The system as claimed in claim 21, comprising a SOx scrubber fluidly connected to the at least one precipitation tank, the at least one cementation tank, the at least one collection tank and the at least one neutralization tank for neutralization of SOx.