Description:FIELD OF INVENTION
[0001] The present invention relates to the novel and effective processing method of waste materials to recover materials of value. Particularly, the present invention relates to an improved process for the recovery of valuables in the form of graphitic carbon fractions, cryolite, sodium fluoride, sodium carbonate/sulphate, and mixed hydroxides from spent anode dust and first-cut spent pot lining materials obtained from electrolytic reduction cells which are used to produce aluminium metal.

BACKGROUND OF INVENTION
[0002] The electrically conductive carbon based anode and cathode of any electrolytic reduction cell frequently degrade as they are subjected to extremely high temperatures and electric current. The molten electrolyte reacts with the electrode and causes the structural degradation through chemical reactions. This usually leads to lowering of the overall cell efficiency and mandates removal and replacement of electrodes as a general practice. This act of removal leads to excessive generation of carbonaceous waste. The anode is subjected to shot blasting operation to remove the degraded areas with the generation of shot blast dust majorly comprising of carbon with other electrolytic contaminants. The cathode is dismantled and classified as first cut (carbon-rich part) and second cut (refractory-rich part). Both the shot blast dust (SBD) and first cut spent pot lining (SPL) are rich in carbon (about 60-65% by weight) in addition to the presence of, but not limited to a mixture of cryolite, sodium fluoride, alumina and aluminium fluoride physically mixed or fused with each other. First-cut SPL is classified as a hazardous material by central pollution control board (CPCB), India due to the presence of soluble inorganic cyanide and fluoride whereas SBD is not classified as a toxic material due to relatively low and acceptable limits of only fluoride. There exists a necessity to detoxify the hazardous SPL by such treatment methods as laid down by CPCB (India) preferably by subjecting SPL to a method of heat treatment. However, the complete detoxification method as laid down by CPCB (India) consists of destruction of cyanide and fixing of fluoride into an insoluble calcium fluoride by addition of lime. This although helps in removal of cyanide, and renders the fluoride non-usable.
[0003] The current invention thus intends to inherit/adopt only the cyanide destruction step as laid down by CPCB, followed by which the SPL is converted into a cyanide free mass, preferably referred to as detoxified first cut SPL from the perspective of absence of cyanide alone. It is worthwhile to note here that the mass will still be rendered toxic only if stockpiled/allowed to be stored unutilized due to the presence of leachable fluoride.
[0004] Xiaoming Li et. al., Metallurgical and Materials Transactions B, 50, 914–923 (2019) discloses a three-step process was employed to separate cryolite from used carbon cathodes, known as spent pot lining (SPL), and obtain valuable carbon. The process comprises two-step leaching process: (1) NaF is leached by water from the imbedded electrolyte, then (2) Na3AlF6, CaF2 and NaAl11O17 are leached using acidic anodizing wastewater (H2SO4 solution). Then the electrolyte components are precipitated from the mixed filtrates of steps (1) and (2). Most of the NaF in the SPL was dissolved in step (1); the residual electrolyte was mainly cryolite (with ~0.95% NaF). The purity of the carbon recovered was about 95.5% under (80°C; L/S = 8 L/kg; 300 rpm; 3 h). The cryolite recovery from the mixed filtrate at (75°C; 4 h; pH 9; F/Al ratio of 6:1) was 98.4% while the Na2SO4 crystals purity was 92.0%.
[0005] SHI Zhong-ning et. al., Trans. Nonferrous Met. Soc. China, 22, 222-227 (2012) discloses recovery of carbon and cryolite from spent pot lining of aluminium reduction cells by dual chemical leaching process wherein the SPL sample is initially grinded in the range of 100-165 µm. Further it is leached with 2.5 mol/L NaOH solution to obtained solid carbon containing NaAl11O17 and CaF2. The carbon obtained from the alkaline leaching was further leached in the acidic solution of 9.7mol/L HCl at 100 °C for 3 h. Their results showed a recovery of 65.0% of soluble Na3AlF6 and Al2O3 compounds starting with NaOH leaching. However, they recovered 96.2% of the CaF2 and NaAl11O17 compounds in the following HCl leaching step. By combining the acidic and alkaline leaching solutions, 95.6% of the cryolite precipitates (at pH = 9, T = 70°C, and time = 2 h) with a 96.4% purity.
[0006] Canadian patent, CA2327878C discloses a process for the treatment of spent pot lining material from electrolyte reduction cells wherein the said spent material having a substantial content of cryolite, which comprises (a) leaching said spent material with water at a dilution and a time sufficient to dissolve substantially all water soluble fluorides, thereafter separating the solid residue from the liquid and (b) further subjecting the solid residue obtained to a caustic leach with an aqueous sodium hydroxide solution containing about 20 to 50 g/L of NaOH and thereafter separating the solid residue from the liquid. A process, wherein the solid residue from the caustic leach is treated with a dilute aqueous acid solution and thereafter the separate solid residue is further leached with an aqueous sodium hydroxide solution to obtain fluorides.
[0007] Chinese patent, CN114438329B discloses a comprehensive recovery method of waste lithium-containing aluminum electrolyte. Firstly, crushing and finely grinding waste lithium-containing aluminum electrolyte into electrolyte powder; preparing acid liquor, and adding a leaching enhancer; then adding electrolyte powder into acid liquor for leaching and filtering to obtain filter residue A and filtrate A; washing and drying the filter residue A to obtain a cryolite product; adding alkali liquor into the filtrate A for adjustment to generate crystal precipitate, and filtering to obtain filter residue B and filtrate B; drying and calcining the filter residue B to obtain an aluminum fluoride product; adding sulfate into the filtrate B for reaction to generate precipitate, and filtering to obtain filtrate C; and evaporating and concentrating the filtrate C to separate out solid substances, filtering to obtain a sodium salt and a concentrated solution D, adding carbonate into the concentrated solution D for reaction, filtering and drying after the reaction to obtain a lithium carbonate product.
[0008] European patent application, EP3868907A1 discloses a process for the preparation of spent pot lining for use in recycling is characterized by that, the spent pot lining is first coarsely ground and poured into a reactor to form a packed bed column, further the packed bed column of the spent pot lining is first washed with water, then the same packed bed column is leached with a sodium hydroxide solution and finally washed again with water. In both steps, the washing with water and the leaching with a sodium hydroxide solution, hydrogen peroxide is added concurrently.
[0009] The processing methods, available in the art, utilizes huge amount of hazardous chemicals for recovering the materials of value from shot blast dust (SBD) and first cut spent pot lining (SPL) which possess serious environmental issues. Therefore, there is an unmet need in the art to provide an improved process for recovering the materials of value from shot blast dust (SBD) and first cut spent pot lining (SPL) that utilizes low chemicals, is safe and economically viable and yields high pure materials of value.

OBJECTIVE OF THE INVENTION
[0010] It is an objective of the present invention to provide a unified and economically viable process for treating carbon waste material, SBD and detoxified first cut SPL for the recovery of valuables in the form of graphitic carbon fractions, cryolite, sodium fluoride, sodium carbonate/sulphate, and mixed hydroxides in which the difficulties associated with the separation of fluorides and carbon are considerably reduced.
[0011] It is another objective of the present invention to provide a process for co-removal of water soluble sodium salts by selective solubilisation during the removal of carbon rich concentrate from carbon poor tailing by froth flotation of SBD and detoxified first cut SPL.
[0012] It is another objective of the present invention to provide a process for solid-liquid separation and evaporation-crystallization of the solubilized sodium salts.
[0013] It is yet another objective of the present invention to provide a process for leaching the reconstituted and separated froth-flotation streams and utilizing the leach liquors to precipitate sodium and aluminium salts in the form of cryolite and the leach residue for purifying and economically recovering carbon fractions.
[0014] It is yet another objective of the present invention to provide a process of recovering graphite and cryolite from spent anode dust and first-cut spent pot lining materials that can be reused for various applications.
[0015] It is a further object of the present invention to provide a solution to minimize stockpiling of the SBD and first cut SPL from the electrolytic reduction cell for promotion of greener technology development.

SUMMARY OF THE INVENTION
[0016] The present invention relates to an improved process for the recovery of valuables in the form of graphitic carbon fractions, cryolite, sodium fluoride, sodium carbonate/sulphate, and mixed hydroxides from spent anode dust and first-cut spent pot lining materials obtained from electrolytic reduction cells using a combination of froth flotation process and alkali-acid leaching.
[0017] The present invention utilizes a froth flotation method before alkali and acid leaching process to obtain wash water, concentrates and tailings. The single-step froth flotation process serves both water washing and separation of feed material into carbon rich concentrate and carbon poor tailings, and it completely removes the necessity of two different unit operations, which leads to a significant reduction in operational costs. The size reduction after flotation will also aid in better liberation of mixed phases. Because of the above-said operational advantage, the composition of alkali and acid leach liquors will differ, leading to an optimal stoichiometric ratio of ingredients to arrive at cryolite precipitation quickly. The duration of alkali and acid leaching is also significantly reduced because of prior water washing and separation of carbon rich concentrate and carbon poor tailing fractions by improved liberation and separation. Final mixing of the alkali and acid leach liquors leads to recovering cryolite by precipitation. In addition to the technical advantage, complete utilization of the entire waste material by preparing several value-added products in downstream is economically beneficial for commercial exploitation.
[0018] In an aspect, the present invention provides a process for the recovery of valuable materials from a carbon waste material comprising the steps of:
(a) subjecting the cyanide free carbon waste material to plurality of froth flotation process using a frother and collector to obtain a wash water, a concentrate and a tailing;
(b) subjecting separately the wash water obtained in step (a) to evaporative crystallization to obtain water soluble products;
(c) subjecting separately the concentrate and tailing obtained in step (a) to alkali leaching using a base to obtain an alkali leached residue and a leach liquor;
(d) subjecting both the alkali leached residues to acid leaching using an inorganic acid to obtain an acid leached residue and a leach liquor;
(e) drying the acid leached residue to obtain a valuable material, graphite; and
(f) mixing both the leach liquors obtained in step (c) and (d) and adjusting the pH to a range of 8-9 using base or acid to obtain valuable materials, cryolite, sodium salts and mixed hydroxides.
[0019] In another aspect of the present invention, the carbon waste material is shot blast dust and first cut spent pot lining.
[0020] In another aspect of the present invention, the frother used in the present process are natural frothers such as methyl isobutyl carbinol, pine oil, cresols, and so on and the collector used in the present process are diesel oil, oleic acid, xanthates such as sodium diethyl xanthate and dithiophosphates such as sodium diethyl dithiophosphate and so on.
[0021] In another aspect of the present invention, the base used in step (c) and (f) of the present process is sodium hydroxide and the inorganic acid used in step (d) and (f) of the present process is hydrochloric acid.
[0022] In another aspect of the present invention, obtained products in the present process include the iron rich residue and the sodium salt such as sodium fluoride, sodium sulphate or sodium carbonate.
[0023] In another aspect of the present invention, the mixed hydroxides obtained in the present process are aluminium hydroxide, calcium hydroxide, sodium hydroxide and iron hydroxide.
[0024] In another aspect of the present invention, the alkali leaching of the present process is carried out with liquid to solid ratio of 1:5 for a time period in a range of 1 to 5 hours at a temperature in a range of 90?C to 110?C.
[0025] In another aspect of the present invention, the acid leaching of the present process is carried out with liquid to solid ratio of 1:5 for a time period in a range of 1 to 5 hours at a temperature in a range of 90?C to 110?C.
[0026] In another aspect of the present invention, the drying of the acid leached residue is carried out at a temperature in a range of 110?C to 130?C for a time period in a range of 2 to 4 hours.
[0027] In yet another aspect, the present invention relates to graphite obtained by the process of the present invention.
[0028] In yet another aspect, the present invention relates to cryolite obtained by the process of the present invention.
[0029] In another aspect of the present invention, the graphite obtained by the present process is characterized by at least one of an XRD pattern having peaks at 26.12 and 45.01 degrees two-theta ±0.2 degrees two theta and one ICP analysis having purity greater than 99.6% ± 0.2%.
[0030] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments.

BRIEF DESCRIPTION OF THE FIGURES
[0031] Figure 1 depicts the flow chart of process for recovery of sodium salts, iron rich residue, cryolite and carbon fraction from SBD.
[0032] Figure 2 depicts the flow chart of process for recovery of sodium salts, cryolite and carbon fraction from detoxified first cut SPL.
[0033] Figure 3 depicts the XRD pattern of crystallized sodium salts recovered from SBD flotation wash water and Sodium Fluoride from SPL flotation wash water.
[0034] Figure 4 depicts the XRD pattern of the graphite recovered from the SPL and the XRD pattern of the graphite recovered from the SBD.
[0035] Figure 5 depicts the XRD pattern of precipitated iron fraction from SBD.
[0036] Figure 6 depicts the XRD pattern of precipitated cryolite.
[0037] Figure 7 depicts the Raman spectrum of recovered graphite from SBD and SPL.

DETAILED DESCRIPTION OF THE INVENTION
[0038] The following is a detailed description of embodiments of the disclosure. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0039] Unless the context requires otherwise, throughout the specification which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.”
[0040] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
[0041] As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.
[0042] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable.
[0043] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein.
[0044] All processes described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0045] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.
[0046] The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.
[0047] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
[0048] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description that follows, and the embodiments described herein, is provided by way of illustration of an example, or examples, of particular embodiments of the principles and aspects of the present disclosure. These examples are provided for the purposes of explanation, and not of limitation, of those principles and of the disclosure.
[0049] It should also be appreciated that the present invention can be implemented in numerous ways, including as a system, a method or a device. In this specification, these implementations, or any other form that the invention may take, may be referred to as processes. In general, the order of the steps of the disclosed processes may be altered within the scope of the invention.
[0050] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[0051] The term “carbon waste materials” as used herein refers to a waste that is generated in smelters as an integral part of the electrolytic cell. The carbon waste materials essentially made of carbon in the form of graphite. Due to repeated cycles of electrolysis, the electrodes undergo oxidation which results in producing a lot of unreacted carbon/graphite which is loosely bound as dust. This spent anode/cathode is scrubbed out from the electrode surface by shot blasting and discarded as carbon waste materials.
[0052] The term “first cut spent pot lining” as used herein refers to waste materials removed from the carbon cathode that deteriorates after a certain period of operation in the aluminium smelters.
[0053] The term “shot blast dust” as used herein refers to waste materials removed from the carbon anode that deteriorates after a certain period of operation in the aluminium smelters.
[0054] The term “froth flotation process” and “froth flotation” as used herein refers to a process that selectively separates the hydrophobic concentrate graphitic part and other hydrophilic tailings or non-graphitic parts through the use of frother, collectors, surfactants and wetting agents.
[0055] The present invention relates to an improved process for the recovery of valuables in the form of graphitic carbon fractions, cryolite, sodium fluoride, sodium carbonate/sulphate, and mixed hydroxides from spent anode dust and first-cut spent pot lining materials obtained from electrolytic reduction cells using a combination of froth flotation process and alkali-acid leaching.
[0056] In an embodiment, the present invention relates to a process for the recovery of valuable materials from a carbon waste material comprising the steps of:
a) subjecting the cyanide free carbon waste material to plurality of froth flotation process using a frother and collector to obtain a wash water, a concentrate and a tailing;
b) subjecting separately the wash water obtained in step (a) to evaporative crystallization to obtain water soluble products
c) subjecting separately the concentrate and tailing obtained in step (a) to alkali leaching using a base to obtain an alkali leached residue and a leach liquor;
d) subjecting both the alkali leached residues to acid leaching using an inorganic acid to obtain an acid leached residue and a leach liquor;
e) drying the acid leached residue to obtain a valuable material, graphite; and
f) mixing both the leach liquors obtained in step (c) and (d) adjusting the pH to a range of 8-9 using base or acid to obtain a valuable material, cryolite, sodium salts and mixed hydroxides.
[0057] In another embodiment of the present invention, the carbon waste material is subjected to size reduction prior to froth flotation to obtain the carbon waste material in a particle size ranges from 1 µm to 300 µm. Preferably, in a ranges from 1 µm to 45 µm, 1 µm to 75 µm, 1 µm to 110 µm, 1 µm to 250 µm, or 1 µm to 275 µm. More preferably, in a range of 1 µm to 45 µm, 1 µm to 75 µm, 1 µm to 110 µm, 1 µm to 250 µm.
[0058] In another embodiment of the present invention, the carbon waste material is shot blast dust and first cut spent pot lining.
[0059] In another aspect of the present invention, the frother used in the present process are natural frothers such as methyl isobutyl carbinol, pine oil, cresols, and so on and the collector used in the present process are diesel oil, oleic acid, xanthates such as sodium diethyl xanthate and dithiophosphates such as sodium diethyl dithiophosphate and so on. Preferably, the frother used in the present invention is methyl isobutyl carbinol and the collector used in the present invention is diesel oil.
[0060] In another embodiment of the present invention, the froth flotation steps, whether single or multiple steps, combined to serve both the purpose of co-removal by solubilizing water soluble sodium salts and simultaneously achieving the separation of lighter carbon in the form of carbon rich concentrate and heavier bath materials in the form of carbon-poor tailings.
[0061] In another embodiment of the present invention, the froth flotation process results in stream 1 (concentrates) and stream 2 (tailings) both of which are subjected to solid-liquid separation, evaporation-crystallization of filtrate which ends in recovery of water soluble sodium salts such as, but not limited to, iron rich fraction, sodium fluoride, sodium sulphate and sodium carbonate and left with froth flotation residues of stream 1 (concentrate) and stream 2 (tailing) which are dried and left for further processing.
[0062] In another embodiment of the present invention, the dried concentrate and the dried tailings of the froth flotation process are subjected separately to further size reduction for achieving better liberation in each fraction; followed by separately subjecting the fractions to caustic leaching; followed and solid-liquid separation to collect alkali leach liquor and alkali leached residues of both concentrate and tailings; followed by mixing the dried alkali leached residues together and subjecting the mixture to acid leaching, which ends in solid-liquid separation to recover carbon rich residue and acid leach liquor which is further processed to obtain acid and alkali soluble materials.
[0063] In another embodiment of the present invention, the base used in step (c) and (f) of the present process is sodium hydroxide or sodium carbonate and the inorganic acid used in the present process is hydrochloric acid or sulphuric acid.
[0064] In another embodiment of the present invention, the sodium salt obtained in the present process is sodium fluoride, sodium sulphate and sodium carbonate.
[0065] In another embodiment of the present invention, the mixed hydroxides obtained in the present process are aluminium hydroxide, calcium hydroxide, sodium hydroxide and iron hydroxide.
[0066] In another embodiment of the present invention, the alkali leaching of the present process is carried out with liquid to solid ratio of 1:5 for a time period in a range of 1 to 5 hours at a temperature in a range of 90?C to 110?C.
[0067] In another aspect of the present invention, the acid leaching of the present process is carried out with liquid to solid ratio of 1:5 for a time period in a range of 1 to 5 hours at a temperature in a range of 90?C to 110?C.
[0068] In another aspect of the present invention, the drying of the acid leached residue is carried out at a temperature in a range of 110?C to 130?C for a time period in a range of 2 to 4 hours.
[0069] In another embodiment of the present invention, the carbon rich concentrate and carbon poor tailings are also subjected to further size reduction before leaching in the alkali solution for primarily achieving better liberation and as a direct consequence, leading to quicker and a higher degree of solubilisation of alkali soluble compounds. Similarly, the size reduction and liberation also leads to quicker and a higher degree of solubilisation of acid soluble compounds.
[0070] In yet another embodiment of the present invention, the alkali treatment of concentrates and tailings from froth flotation are carried out separately in different conditions to maximise solubilisation of alkali soluble compounds in each fraction and arrive at similar composition at the end of the alkali leaching, thereby enabling the possibility of mixing of the alkali leach residues for subsequent acid leaching.
[0071] In another embodiment of the present invention, the caustic and acid leaching filtrate is subjected to pH adjustment to remove aluminium, calcium, sodium and iron as mixed hydroxides, and the pH adjusted caustic and acidic filtrates are mixed together by slow addition and stirring leading to the precipitation of cryolite under controlled pH and temperature.
[0072] In yet another embodiment of the present invention, the effluent generated during the process such a water leaching, alkali leaching and acid leaching was utilised for the precipitation of the other valuable products in addition to recovering water.
[0073] In an embodiment, the present invention relates to a process for the recovery of valuable materials from first cut spent pot lining (SPL) comprising the steps of:
(a) size reduction, screening and detoxifying the SPL using a heat treatment at a temperature in a range of 100?C to 500?C to obtain a cyanide free SPL;
(b) subjecting the cyanide free carbon waste material to further size reduction and screening, followed by a plurality of froth flotation process using a frother and collector to obtain a wash water, a concentrate and a tailing;
(c) subjecting the wash water obtained in step (b) to evaporative crystallization of water soluble products, while the evaporated vapours are condensed to recover water.
(d) subjecting separately the concentrate and tailing obtained in step (b) to alkali leaching using a base to obtain alkali leached residue and an alkali leach liquor;
(e) subjecting both the alkali leached residues to acid leaching using an inorganic acid to obtain an acid leached residue and an acid leach liquor;
(f) drying the acid leached residue to obtain the valuable material, graphite; and
(g) mixing and adjusting pH of the leach liquors obtained in step (d) and (e) to a range of 8-9 using base or acid to obtain a precipitate of valuable material, cryolite, and subjecting the filtrate to further pH adjustment followed by evaporative crystallization to obtain sodium salts and mixed hydroxides, while the evaporated vapours are condensed to recover water.
[0074] In another embodiment, the process of present invention includes size reduction of the heat treated first cut SPL to the SPL in a particle size ranges from 1 µm to 150 µm. Preferably, in a particle size of less than 45 µm, less than 75 µm and less than 106 µm.
[0075] In an embodiment, the present invention relates to a process for the recovery of valuable materials from a shot blast dust (SBD) which is primarily cyanide free from the source of collection, comprising the steps of:
a) subjecting the cyanide free carbon waste material to size reduction and screening, followed by a plurality of froth flotation process using a frother and collector to obtain a wash water, a concentrate and a tailing;
b) subjecting separately the wash water obtained in step (a) to pH adjustment and precipitation of iron, followed by solid-liquid separation to obtain iron rich residue and filtrate.
c) subjecting separately the filtrate obtained in step (b) to evaporative crystallization of water soluble products, while the evaporated vapours are condensed to recover water.
d) subjecting separately the concentrate and tailing obtained in step (a) to alkali leaching using a base to obtain alkali leached residue and an alkali leach liquor;
e) subjecting the alkali leached residue to acid leaching using an inorganic acid to obtain an acid leached residue and an acid leach liquor;
f) drying the acid leached residue to obtain the valuable material, graphite; and
g) mixing and adjusting pH of the leach liquors obtained in step (d) and (e) to a range of 8-9 using base or acid to obtain a precipitate of the valuable material, cryolite, subjecting the filtrate to further pH adjustment followed by evaporative crystallization to obtain sodium salts and mixed hydroxides, while the evaporated vapours are condensed to recover water.
[0076] In another embodiment, the process of present invention includes size reduction of the SBD to a particle size ranges from 1 µm to 300 µm. Preferably, in a particle size of less than 250 µm, less than 200 µm, less than 150 µm, less than 100 µm and less than 75 µm.
[0077] In an embodiment, the process of present invention provides graphite having a purity of greater than 99%.
[0078] In another embodiment of the present invention, the graphite obtained from the process of present invention can be utilized for various applications.
[0079] In yet another embodiment of the present invention, the recovered graphite and cryolite can be further used in the aluminium industry.
EXAMPLES
[0080] The present invention is further explained in the form of following examples. However, it is to be understood that the example is merely illustrative and is not to be taken as limitations upon the scope of the invention.
Detoxification step:
[0081] Heat treatment is a suitable way to detoxify or decompose the cyanide present in the SPL. The cyanide decomposition started around 350°C to 110°C by breaking and reforming with releasing flue gases such as N2, NO, NO2, and CO2.
4NaCN + 9O2 ? Na2O + 2NO2 + 2CO2 (1)
2NaCN + 4O2 ? Na2O + N2O3 + 2CO2 (2)
Detoxification Procedure:
[0082] The samples with less than 31 mm particle size were placed in a furnace at 500°C for 1 hour with a heating rate of 10 degrees/min. After 1h at 500°C, the samples were allowed to cool down to room temperature. The mass difference before and after the removal of the furnace was recorded. The weight loss after heat treatment suggests the successful removal of total cyanide from the SPL samples.
Flotation, Leaching Solubilisation and recovery:
[0083] A series of experiments were carried out to arrive at the process objectives of the present invention. These experiments help in developing the process flow (Figures 1 and 2) as described in the objects of the invention.
[0084] Flotation is a process of separating the hydrophobic concentrate graphitic carbon part and other hydrophilic tailings or non-graphitic parts.
[0085] During the experiments, a mass of 500g to 1Kg of SBD and detoxified first cut SPL were taken and subjected to froth flotation separately, to separate constituents of the respective materials by differences in density and wetting behaviour. While about 200g to 450g of concentrates and about 200g to 400g of tailings are collected by solid liquid separation, the water solubilized mass of about 50g to 200g, namely, but not limited to, Sodium fluoride, Sodium carbonate, Sodium sulphate, which are desirable products and are separately collected and dried by selective evaporation-crystallization by exploiting the evaporation and crystallization rates. The entire mass of carbon rich concentrates and carbon poor tailings of froth flotation experiments are then subjected size reduction to a suitable particle size and then subjected to alkaline leaching at about 60 to 180oC preferably 150oC at about 8-35 moles per Kg, preferably 20 moles per Kg of the tailing or concentrate for about 1 to 4 hours, preferably 3 hours, to further separate carbon from the remaining alkali soluble compounds in each of the fraction. The remaining mass of about 130g to 320g of the froth flotation concentrate residue and 20g to 280g of the froth flotation tailing residue are mixed together and subjected washing in water to remove any soluble materials from the solid fraction, followed by subjecting to acid leaching at about 60 to 150oC preferably 120oC at about 9-50 moles per Kg, preferably 25 moles per Kg of the mixed residue for about 1 to 4 hours, preferable 3 hours, to further separate carbon from the remaining acid soluble compounds. The remaining solid mass is a product which is free from soluble materials and enhanced in carbon mass percentage. The alkali and acid leach liquors are then mixed together and pH is adjusted above 8 to precipitate cryolite, which is another product, which is free from carbon materials. Both the products are dried and stored for further use. The remaining filtrate is subjected to pH adjustment and precipitation of another product rich in hydroxides of aluminium, calcium, iron and sodium.
EXAMPLE
[0086] SPL (after heat treatment and grinding to say for example less than 106 µm, less than 75 µm and less than 45 µm) were taken as feed sample for the flotation experiment to ascertain the effect of size reduction on liberation and as a consequence, optimal separation. The energy spent increases with decrease in particle size. However, during grinding, particles may get liberated better. Three cycles of flotation experiments were carried out: rougher, cleaner and scavenger leads to increasing the carbon percentage by efficient separation using MIBC for example as frother and Diesel for example as collector. The total time taken for the 3 cycles of flotation was about 1 h.
[0087] In a similar fashion, crushed, ground and sieved SBD samples were taken as a feed for flotation. A flotation experiment was carried out to simultaneously separate the concentrate and tailing part of the SBD samples and solubilize the water soluble components in the feed sample.
[0088] SBD (after grinding to say for example less than 250 µm and less than 75 µm) were taken as feed sample for the flotation experiment to ascertain the effect of size reduction on liberation and as a consequence, optimal separation. The energy spent increases with a decrease in particle size. However, during grinding, particles may get liberated better. Three cycles of flotation experiments were carried out, such as rougher, cleaner and scavenger leading to increasing the carbon mass percentage by efficient separation, for example, using MIBC as frother and for example using diesel oil as collector. The total time taken for the 3 cycles of Flotation was about 1 h.

Table 1: Weight of the concentrate and tailings collected after 3 flotation cycles including recovery data indicating the effect of particle size reduction on the yield and grade of recovered concentrates
Sample name Feed sample wt. (g)
(FC %) Concentrate (g)
(FC%)
Recovery % Tailings (g)
(FC%)
SPL less than 106 µm
(after 3 cycles) 1000
(41.90%) 451
(53.93%)
58% recovery 432
(38.56%)
SPL less than 75 µm
(after 3 cycles) 1000
(41.99%) 340
(49.22%)
40% recovery 647
(35.22%)
SPL less than 45 µm
(after 3 cycles) 1000
(41.89%) 384
(55.96%)
51% recovery 607
(32.29%)

SBD less than 250 µm
(after 3 cycles) 1000
(53.73%) 538
(87.47%)
88% recovery 429
(25.20%)
SBD less than 75 µm
(after 3 cycles) 1000
(53.70%) 317
(85.01%)
50% recovery 690
(32.76%)

[0089] It is clearly observed from the data in Table 1 that reducing the particle size of SPL any further below 106 µm neither leads to better yield nor better separation. Similarly, reducing the particle size of SBD any further below 250 µm neither leads to better yield nor separation. So the particle size of less than 106 µm for SPL and less than 250 µm for SBD would be ideal in terms of requiring fewer volumes of alkali and acid in leaching steps further leading to savings.
[0090] The collected filtration of the flotation of SPL and SBD samples were kept in a reflux condenser and heated at 150°C to make the solution super-saturated, followed by evaporation of water. The super-saturated solution was dried and collected for further analysis. Also, on the other hand the pure water was collected through a condenser. Figure 3 depicts the XRD pattern of crystallized NaF. The XRD analysis of the crystallized samples was carried out to analyze the phase of the sample. The highly intense and sharp peaks at 2? = 38.83 and others at 2? = 56.09 and 70.31 are the signature peaks of the NaF. From XRD analysis, it was confirmed that the crystallized product is NaF.
[0091] After the flotation, the concentrate and tailings samples from SPL and SBD were taken separately for alkali leaching to dissolve the alkali-soluble part from the feed materials. The following alkali leaching experiments were carried out by increasing the leaching duration to check the efficiency of this leaching.
Na3AlF6+ 4NaOH = NaAl(OH)4+ 6NaF (3)
Al2O3+ 2NaOH + 3H2O = 2NaAl(OH)4 (4)
Al(OH)4 ? + 2OH- =Al(OH)6 3- (5)
2Al(OH)4- + 2OH- = Al2(OH)104- (6)
[0092] Procedure: 3M Sodium Hydroxide (NaOH) solution was prepared and added the SPL sample (Concentrate and tailings separately) was in such a way as to maintain the 1:5 liquid-to-solid ratio (L/S). The solution was then continually stirred at 100°C with changing the duration. (Refer to table: 2). During leaching the 3M NaOH solution was added to the solution to maintain the L/S ratio. After alkali leaching, the leached liquor was filtered and washed several times with DI to maintain the pH of the sample. The residue was dried and the filtrate was collected for further use. The weight loss of the samples due to the removal of alkali-soluble materials from the SPL and SBD samples were recorded.

Table 2: Examples of conditions for alkali leaching
Example of conditions for Alkali leaching
Sample name Leaching duration Leaching temperature L/S ratio
Experiment :1
SPL less than 106 µm – Concentrate 1h 100°C 1:5
SPL less than 106 µm – Tailings 2h 100°C 1:5
Experiment:2
SPL less than 106 µm – Concentrate 2h 100°C 1:5
SPL less than 106 µm – Tailings 4h 100°C 1:5
Experiment: 3
SBD less than 250 µm – Concentrate 1h 100°C 1:5
SBD less than 250 µm – Tailings 2h 100°C 1:5
Experiment: 4
SBD less than 250 µm – Concentrate 2h 100°C 1:5
SBD less than 250 µm – Tailings 4h 100°C 1:5
Experiment: 5
SBD less than 75 µm – Concentrate 2h 100°C 1:5
SBD less than 75 µm – Tailings 3h 100°C 1:5

[0093] The alkali-leached residue was used for acid leaching for the removal of the acid-soluble product of the SBD sample. Hydrochloric acid (HCl) was used for acid leaching. The L/S is the same as alkali leaching, that is 1:5. Hydrochloric acid was added dropwise during leaching to maintain the L/S ratio. The solution was then continually stirred at 100°C with changing the duration. (Refer to table: 3)
NaAl11O17 + 34HCl = NaCl + 11 AlCl3+ 17 H2O (7)
CaF2+2HCl=CaCl2 + 2HF ? (8)
[0094] An additional set of alkali-acid leaching similar to the previous steps were performed to further enhance the purity of the carbon rich fraction. Table 4 and Table 5 depict the results of the performed experiments.

Table 3: Example of conditions for acid leaching
Example of conditions for Acid leaching
Sample name Leaching duration Leaching temperature L/S ratio
Experiment :1
SPL less than 106 µm – Concentrate 1h 100°C 1:5
SPL less than 106 µm – Tailings 2h 100°C 1:5
Experiment:2
SPL less than 106 µm – Concentrate 2h 100°C 1:5
SPL less than 106 µm – Tailings 4h 100°C 1:5
Experiment: 3
SBD less than 250 µm – Concentrate 1h 100°C 1:5
SBD less than 250 µm – Tailings 2h 100°C 1:5
Experiment: 4
SBD less than 250 µm – Concentrate 2h 100°C 1:5
SBD less than 250 µm – Tailings 4h 100°C 1:5
Experiment: 5
SBD less than 75 µm – Concentrate 2h 100°C 1:5
SBD less than 75 µm – Tailings 3h 100°C 1:5

Table 4: Analysis data of SPL and SBD sample after acid leaching
Fixed carbon data of acid and alkali-leached samples
Sl. No. Sample name Sample type % of FC after alkali leaching % of FC after acid leaching
01 SPL less than 106 µm Concentrate 71.17 84.26
02 Tailings 42.47 57.04
03 SBD less than 250 µm Concentrate 86.02 94.21
04 Tailings 30.08 54.22

Another set of leaching was performed to improve the purity of recovered carbon fractions.

Table 5: Analysis of SPL and SBD samples after 2nd set of leaching
Fixed carbon data of 2nd set of acid and alkali-leached samples
Sl. No. Sample name Sample type % of FC after alkali leaching % of FC after acid leaching
01 SPL less than 106 µm Concentrate 91.34 98.42
02 Tailings 77.92 92.79
03 SBD less than 250 µm Concentrate 97.22 98.71
04 Tailings 76.89 94.77

[0095] Ball milling operation to during size reduction step can contaminate the samples with iron. Iron is leached out and can be precipitated as goethite in alkaline conditions. Sodium Hydroxide pellets were added to the acid-leached filtrate to precipitate the Iron oxide at higher pH (~ 8.2 to 9.8). The precipitate was filtered and dried between 90 and 110°C for 2h. Hydrochloric acid was added to the alkali filtrate collected after separating the iron oxide to maintain the pH at 2. At pH value 2, the alkali-leached filtrate was added to the solution to maintain the pH at 9. The solution was then continually stirred for 1h anywhere between 50°C and 110°C more preferably at 85°C for precipitation of the cryolite.
Al(OH)4 + 4H+ +3Na+ +6F- = Na3AlF6?+ 4H2O (12)
[0096] The obtained cryolite from the above-mentioned precipitation method may contain traces of NaCl, which can be easily separated from the cryolite by washing it in water.

EFFICIENCY OF THE PROCESS
[0097] The examples and analysis provides the overall process efficiency in recovering highly concentrated graphite from the SPL. After crushing and milling operation, the fixed carbon of the raw sample of the SPL was found to be about 41.99%. This fixed carbon content was increased in flotation by separating the non-graphitic content of SPL. Furthermore, the remaining impurities were leached out from the SPL sample followed by alkali and acid leaching. The remaining mass left after the leaching was pure graphite with minor impurities. The carbon content can be further increased up to about 99% by additional leaching circuits. After crushing and milling operation, the fixed carbon of the raw sample of the SBD was found to be about 53.71%. This fixed carbon content was increased in Flotation by separating the non-graphitic content of SBD. Furthermore, the remaining impurities were leached out from the SBD sample followed by alkali and acid leaching. The remaining mass left over after both the leaching was pure graphite with minor impurities. Leaching can further increase the fixed carbon content up to 99% while the recovery % also increases.
INDUCTIVELY COUPLED PLASMA (ICP) ANALYSIS
[0098] 1 g of acid-leached SPL sample after washing and drying was taken and dissolved at aqua regia for 3h at 250°C and collected the filtrate by filtration. The filtrate solution was then diluted for ICP analysis. The data of the ICP analysis of the samples are mentioned in table 6. The purity of the samples was determined by elimination of the element weight percentage from the total mass of samples taken.

Table 6: ICP Data
Elements SPL Graphite SBD Graphite
PPM Element in% PPM Element in%
Al 29.07 0.2907 7.514 0.07514
Fe 1.224 0.01224 4.448 0.04448
Na 6.61 0.0661 3.674 0.03674
Ca 5.488 0.05488 3.93 0.0393
K 4.5 0.045 4.03 0.0403
Mg 0.06 0.0006 0.01 0.0001
Si 4.018 0.04018 2.644 0.02644
Mn 2.986 0.02986 1.8212 0.018212
Ni 0 0 3.04 0.0304
V 0.064 0.00064 0.068 0.00068
P 0.374 0.00374 0.418 0.00418
Purity: 99.45606% Purity: 99.684028%

XRD ANALYSIS OF RECOVERED PRODUCTS INCLUDING GRAPHITE
[0099] The structural analysis of all the recovered products was carried out using the X-ray diffraction technique. Figure 3a depicts the XRD peaks corresponding to Sodium carbonate sulfate recovered from the flotation wash water from SBD; Figure 3b depicts the XRD peaks of highly crystalline and phase pure Sodium fluoride recovered by evaporative crystallization of wash water from the flotation of SPL samples. Figure 4a depicts XRD peaks at 2? = 26.12° and 45.01°, the signature XRD peaks of graphite recovered form SPL samples. The presence of the highly shape and intense peaks confirms the presence of highly crystalline graphite. Figure 4b depicts the XRD peaks at 2? = 26.23° and 45.11°, the signature XRD peaks of graphite recovered from SBD samples. Figure 5 depicts the XRD pattern of iron rich precipitate recovered by pH adjustment of wash water from flotation of SBD samples. This indicates that the iron from the steel balls used in shot blasting operation is separated from the sample mass. Figure 6 depicts the XRD typical XRD pattern of cryolite which was recovered by precipitation of reconstituted wash waters (from removal of soluble sodium salts in the previous unit operations) from acid and alkali leaching streams. The indexed planes of all the samples indicate high purity levels of the recovered materials.

RAMAN ANALYSIS OF RECOVERED GRAPHITE
[0100] The Raman spectrum of the recovered graphite sample from SPL (Figure 7b) was analysed to identify the C-C bonding of the SPL leached sample. The peaks at 1347 cm-1, 1581 cm-1 and 2695 cm-1 well known as D, G, and 2D, are the signature Raman shifts of graphite. The highly sharp and intense peak at 1581cm-1 indicates the presence of highly crystalline graphite. The peaks at 1347 cm-1 indicate the presence of defects in the graphitic lattices. The broad peaks at 2690 cm-1 indicate the presence of the multiple graphitic layers of the sample. The Raman spectrum of the recovered graphite sample from SBD (Figure 7a) was analysed to identify the C-C bonding of the SBD leached sample. The peaks at 1357 cm-1, 1588 cm-1 and 2658 cm-1 well known as D, G, and 2D, are the signature Raman shifts of graphite. The peak at 1588 cm-1 indicates the presence of graphitic carbon. The peaks at 1357 cm-1 indicate the presence of defect in the graphitic lattices; the highly intense and broad D peak indicates the presence of the amorphous carbon content or defects in the graphitic lattices due to oxidation. The broad peaks at 2658 cm-1 indicate the presence of the multiple graphitic layers of the sample.
[0101] The numerical values mentioned for the various physical parameters, quantities or dimensions are only indicative and it is envisaged that the variations in the values, whether higher or lower than the values assigned to the parameters, quantities or dimensions fall within the scope of the disclosure, unless there is a contrary statement in the specification.
[0102] While sufficient emphasis has been placed on the specific features of the preferred embodiment, may additional features can be added and that many changes can be made in the preferred embodiment without departing from the principles of the invention disclosed herein. Such and other such changes in the preferred embodiment of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive subject matter is to be interpreted solely as an illustration of the disclosure and not as a limitation.

APPLICATION OF THE RECOVERED GRAPHITE AS ESD COATINGS
[0103] The recovered graphite was found to have high electrical conductivity. Owing to this observed property, the recovered graphite was evaluated for possible utilization in Electrostatic Charge dissipative coatings. International Electrotechnical Commission (IEC) has established standards for electrostatic dissipative materials used in electronic equipment manufacturing, which include specific electrical conductivity requirements. At about 3% by weight as an additive, the resistance in the static dissipative coatings was found to be in the few kilo ohms range and this will easily meet International Standards of the same. Also this Graphite powder can be a very good candidate material for Conductive ink and Anode Material in Li-Battery. This serves as an excellent performance indicator of the recovered graphite and provides a highest value addition for graphite recovered from waste materials reported so far. This also indicates the quality of the recovered graphite and the possibility of direct utilization of the recovered graphite in above applications.

ADVANTAGES OF THE PRESENT INVENTION
[0104] The process of the present invention has several operational and economic advantage such as consolidation of otherwise stored or discarded materials.
[0105] The process of the present invention utilizes traditional unit operations such as froth flotation to serve the process of co-removal of water soluble compounds as well as achieve density based separation of materials in one go, thereby reducing the following in the leaching steps,
i. reduced requirement of acid and alkali for leaching,
ii. reduced leaching duration,
iii. reduced energy requirement,
iv. improved recovery and yield of valuable materials, and
v. multiple product recovery.
[0106] The process of the present invention provides a means of efficiently recovering carbon values in a reusable or saleable form from shot blast dust and first cut spent pot linings generated by aluminium smelter plants.
[0107] The process of the present invention provides a means of efficiently removing sodium in a reusable or saleable form such as sodium fluoride, sodium sulphate, sodium carbonate, cryolite using wet methods and does not require use of special equipment such as high temperature furnace.
[0108] The process of the present invention is environmental friendly as the residue is free from fluorides; acid and alkali are consumed in the process by contributing to the product formation chemistry; and water is recovered during evaporation and can be reused in process streams.
[0109] The process of the present invention resulted in the high pure graphite from the SPL and SBD.
[0110] The process of the present invention provides recovery of materials of value from SPL and SBD with zero waste.
, Claims:1. A process for the recovery of valuable materials from a carbon waste material comprising the steps of:
a) subjecting the cyanide free carbon waste material to plurality of froth flotation process using a frother and collector to obtain a wash water, a concentrate and a tailing;
b) subjecting separately the wash water obtained in step (a) to evaporative crystallization to obtain water soluble products;
c) subjecting separately the concentrate and tailing obtained in step (a) to alkali leaching using a base to obtain an alkali leached residue and a leach liquor;
d) subjecting both the alkali leached residues to acid leaching using an inorganic acid to obtain an acid leached residue and a leach liquor;
e) drying the acid leached residue to obtain a valuable material, graphite; and
f) mixing both the leach liquors obtained in step (c) and (d) and adjusting the pH to a range of 8-9 using base or acid to obtain valuable materials, cryolite, sodium salts and mixed hydroxides.

2. The process as claimed in claim 1, wherein the carbon waste material is subjected to size reduction prior to froth flotation to obtain the carbon waste material in a particle size ranges from 1 µm to 300 µm.

3. The process as claimed in claim 1, wherein size reduction before and after froth flotation aids in improved liberation and better solubility of compounds in both alkali and acid respectively.

4. The process as claimed in claim 1, wherein the size reduction and subsequent liberation also reduces the demand for leaching duration to achieve desirable solubility.

5. The process as claimed in claim 1, wherein the waste material is shot blast dust and first cut spent pot lining.

6. The process as claimed in claim 1, wherein the frother is methyl isobutyl carbinol, pine oil, or cresols; and the collector is diesel oil, oleic acid, xanthates, or dithiophosphates.

7. The process as claimed in claim 1, wherein the base in step (c) and (f) is sodium hydroxide, or sodium carbonate and the inorganic acid is hydrochloric acid or sulphuric acid.

8. The process as claimed in claim 1, wherein the sodium salt is sodium fluoride, sodium sulphate or sodium carbonate.

9. The process as claimed in claim 1, wherein the mixed hydroxides are selected from aluminium hydroxide, calcium hydroxide, sodium hydroxide and iron hydroxide.

10. The process as claimed in claim 1, wherein the alkali leaching is carried out with liquid to solid ratio of 1:5 for a time period in a range of 1 to 5 hours at a temperature in a range of 90?C to 110?C.

11. The process as claimed in claim 1, wherein the acid leaching is carried out with liquid to solid ratio of 1:5 for a time period in a range of 1 to 5 hours at a temperature in a range of 90?C to 110?C.

12. The process as claimed in claim 1, wherein the drying of the acid leached residue is carried out at a temperature in a range of 110?C to 130?C for a time period in a range of 2 to 4 hours.
13. The process as claimed in claim 1, wherein the process further comprises detoxifying the first cut spent pot lining using a heat treatment at a temperature in a range of 100?C to 500?C to obtain a cyanide free carbon waste material.

14. A graphite obtained by a process as claimed in claim 1, wherein the graphite is characterized by at least one of an XRD pattern having peaks at 26.12 and 45.01 degrees two-theta±0.2 degrees two theta and one ICP analysis having purity greater than 99.6% ±0.2%.