FIELD OF THE INVENTION
The present invention relates to the recycling process of electronic waste resources to recover metals of value. More particularly, it relates to a hydrometallurgical process to recover metal values from chips of waste printed circuit boards (hereinafter PCBs). The process and method comprises eco-friendly yet efficient procedures for recovery 5 of metals of value.
BACKGROUND OF THE INVENTION
Waste PCBs have an implicit value due to abundance of precious metals contained therein. Hence, the recovery of precious metals become necessary for an effective 10 waste management or recycling process and the same has become area of interest for the innovators and waste management/recycling industries.
PCBs contain three main types of recyclable materials that can be further retrieved when PCBs are transformed into a waste product, they are: 1) recyclable metals, such as copper, aluminum, tin, lead and precious metals (gold, silver and platinum). 15 Boards that have been produced recently may not have lead in their composition, but may contain other metals such as bismuth or silver; 2) recyclable polymeric materials, from which energy can be recovered by combustion and incineration; and 3) ceramic materials, which can be reused or disposed of more appropriately if they are free of metals, polymers or other contaminants. 20
At present, domestic and foreign resources for PCBs scrap processing methods can be divided into mechanical methods, Pyrometallurgical methods, biological methods and hydrometallurgy. Mechanical treatment method leads to less secondary pollution, but require further separation and purification of metals, usually as a pre-wet and other processing methods. Pyrometallurgical processing leads to high 25 recovery of metals, but during incineration process, large amount of toxic gases, dioxins and furans cause serious environmental pollution. Biological leaching from
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waste printed circuit boards and other precious metals is a simple, low cost, and easy to operate but have some drawbacks like extended leaching time, low leaching rate, unavailability of considerable strains, difficulties in bacterial cultivation and the like.
The state of the art technology is not readily available for industrial applications as while scaling up basic metal recovery process, harmful chemicals get leached into the 5 environment. The recovery of metals is neither economical nor of quality.
Hydrometallurgical process can deliver excellent results by providing high metal recovery; short recycling cycle; wet technology eventually obtain high-purity metal values.
Therefore, future research trends in this technology should be considered in the 10 overall recovery process for recycling precious metal values by the use of environment-friendly agents as leaching agents, precipitants, and solvents.
OBJECT OF THE INVENTION
The main object of the present invention is to provide an industrial method and 15 process of recovery metal values from chips of waste PCBs.
Yet another object of the present invention is to provide an industrial method and process of recovering metals by hydrometallurgical process.
Yet another object of the present invention is to provide an improved method to separate out precious metal values using selective precipitation process. 20
Yet another object of the present invention is to provide an industrial method and process with low chemical consumption in the metal recovery process.
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Yet another object of the present invention is to provide an improved method and process for recovery of gold and copper present in e-wastes preferably in chips of waste PCBs.
Yet another object of the present invention is to provide a convenient and eco-friendly approach for e-waste management. 5
Yet another object of the present invention it to provide a metal recovery process ends with minimum solid waste and with no liquid discharge.
Yet another object of the present invention is to provide an efficient metal recovery method with high extraction rate of metal values.
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SUMMARY OF THE INVENTION
The present invention relates to an improved process and method of recovering precious metal values from chips of waste PCBs. The invention resides in providing the possibility of reutilization of metals abundant in chips of waste PCBs. The method is specific to metal recovery from waste PCBs especially from mother boards 15 of computers, laptops and other electronic equipments equipped with printed circuit boards.
In a preferred embodiment of the present invention, the method of recovering precious metals from chips of waste printed circuit boards comprises the following major steps of: 20
a) Pulverization of chips of waste PCBs using a ball mill.
b) Sieving of pulverized chips obtained in step a) with mesh of pore size 300 microns.
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c) Leaching of gold from chip powder of below 300 microns size obtained in step b. in a mixture of nitric acid and hydrochloric acid in the molar ratio of 1:3 at a temperature ranging from 70-80 0C.
d) Filtration of above slurry to separate gold leach liquor from the slurry.
e) Selective Precipitation of gold from the leach liquor of step (d) with urea 5 and sodium metabisulphite.
f) Purification of precipitated gold powder by re-leaching with mixture of nitric acid and hydrochloric acid in the molar ratio of 1:3 at a temperature ranging from 70-80 0C, followed by precipitation with urea and sodium metabisulphite. 10
g) Recovery of Copper from pulverized chips of above 300 microns size of step (b), by electro-refining method.
h) Residue obtained from electro-refining process after copper removal, is also added to gold leaching in step (c).
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BRIEF DESCRIPTION OF DRAWINGS
Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:
FIG. 1 is a process flow diagram elucidating the steps involved during hydrometallurgical process of metal recovery. 20
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DETAILED DESCRIPTION OF THE INVENTION
Reference will now be made in detail to the present embodiment of the present invention, examples of which are illustrated in the description below. The embodiments are described below in order to explain the present invention.
The present invention relates to an improved process and method of recovering 5 precious metal values from chips of waste PCBs. The invention resides in providing the possibility of reutilization of metals abundant in chips of waste PCBs. The method is specific to metal recovery from waste PCBs especially from mother boards of computers, laptops and other electronic equipments equipped with printed circuit boards. 10
In an embodiment of the invention, the hydrometallurgical method is provided to separate out metal values from chips of waste PCBs. In this method, the source material is pulverized using a size reduction apparatus preferably a ball mill followed by sieving through specific mesh size (300 microns) that leads to separation of Metal impurities, polymeric matrix from precious metals and silicious matrix. 15
The chip powder of below 300 microns is then subjected to hydrometallurgical process. The process then transfers the precious metals into solution by leaching process followed by selective precipitation, purification and final recovery of metals.
FIG. 1 represents a hydrometallurgical process for metals recovery according to an embodiment of the present invention, the metals recovery process comprises the 20 steps of:
a) Pulverization of chips of waste PCBs using a ball mill.
b) Sieving of pulverized chips obtained in step a) with mesh of pore size 300 microns.
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c) Leaching of gold from chip powder of below 300 microns size obtained in step b. in a mixture of nitric acid and hydrochloric acid in the molar ratio of 1:3 at a temperature ranging from 70-80 0C.
d) Filtration of above slurry to separate gold leach liquor from the slurry.
e) Selective Precipitation of gold from the leach liquor of step (d) with urea 5 and sodium metabisulphite.
f) Purification of precipitated gold powder by re-leaching with mixture of nitric acid and hydrochloric acid in the molar ratio of 1:3 at a temperature ranging from 70-80 0C, followed by precipitation with urea and sodium metabisulphite. 10
g) Recovery of Copper from pulverized chips of above 300 microns size of step (b), by electrorefining method.
h) Residue obtained from electrorefining process after copper removal, is also added to gold leaching in step (c).
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In preferred embodiment of the present invention, the waste PCBs scrap is initially crushed by any crusher, preferably a ball mill. Sieving is then carried out using a sieve of pore size 300μ. The chip powder having size less than 300μ is then subjected to initial leaching process with a mixture of nitric acid and hydrochloric acid in the molar ratio of 1:3 at a temperature ranging from 70-20 80ºC. The slurry is then filtered to separate gold leach liquor from the slurry. The chip powder having size more than 300μ is kept separately for the purpose of copper recovery by electro-refining method.
Selective precipitation of gold from gold rich liquor is then carried out using Urea and sodium meta-bisulphite. The gold precipitate is then purified through re-25 leaching process with the mixture containing nitric acid and hydrochloric acid in
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the molar ration of 1:3, followed by further precipitation with urea and sodium metabisulphite.
In parallel, the chip powder having size more than 300μ is subjected to electro-refining process for copper recovery. The residue obtained in this step after copper recovery is also added to the step c of the metal recovery process to leach 5 out gold present in the residue.
The invention will be described in view of foregoing examples:
Example 1
In a 21.32 kg batch (Batch 01) of waste printed circuit boards were taken for the 10 study. Initially, the sample was pulverized in a ball mill containing eight iron balls weighing each 18 kg for 6 hours at a speed of 40 rpm and finally passed through a sieve having 50 mess size. The chemical analysis of the sieved material is shown in table 1.
About 20 Kg sample of the sieved material was taken and agitated with about 100 15 liters of the mixture containing nitric acid and hydrochloric acid in the molar ratio of 1:3 at a temperature ranging from 70-80 ºC for atleast four hours. The slurry was filtered to separate cake and leach liquor. The cake was washed out with water and wash liquor was added to the leach liquor. The analysis of leach liquor is shown in table 2. 20
About 100 litre of urea solution (25% w/v) is added to leach liquor under agitation followed by addition of sodium metabisulphite (about 200g) under agitation continued for atleast four hours. After a predetermined interval of time, the slurry was filtered to separate residue and filtrate.
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In the subsequent step, the residue obtained was re-dissolved in about 8 litres of mixture containing nitric acid and hydrochloric acid (molar ratio 1:3) by agitating it at a temperature ranging from 70-80 ºC it for atleast four hours. The slurry was filtered to obtain filtrate. To the filtrate, about 8 litre of urea solution was added under agitation followed by addition of about 150 g of sodium metabisulphite 5 under agitation continued for atleast two hrs for completion of gold precipitation. The precipitated gold powder was collected by filtration of the above slurry. The collected gold powder was dried and melted at 1100 ºC. The dry weight of the gold was found to be 94.58g.
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Example 2
In another batch (Batch 02) of 21.35 kg waste printed circuit boards were taken for the study. Initially, the sample was pulverized in a ball mill containing eight iron balls weighing each 18 kg for 6 hours at a speed of 40 rpm and finally passed through a sieve having 50 mess size. The chemical analysis of the sieved material 15 is shown in table 1.
About 20 Kg sample of the sieved material was taken and agitated with about 100 liters of the mixture containing nitric acid and hydrochloric acid in the molar ratio of 1:3 at a temperature ranging from 70-80 ºC for atleast four hours. The slurry was filtered to separate cake and leach liquor. The cake was washed out 20 with wash liquor and the wash liquor was added to the leach liquor. The analysis of leach liquor is shown in table 2.
About 100 litre of urea solution (25% w/v) is added to leach liquor under agitation followed by addition of sodium metabisulphite (about 200g) under agitation continued for atleast four hours. After a predetermined interval of time, 25 the slurry was filtered to separate residue and filtrate.
In the subsequent step, the residue obtained was re-dissolved in about 8 litres of the mixture containing nitric acid and hydrochloric acid (Molar ratio 1:3) by
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agitating at a temperature ranging from 70-80 ºC it for atleast four hours. The slurry was filtered to obtain filtrate. To the filtrate, about 8 litre of urea solution was added under agitation followed by addition of about 150 g of sodium metabisulphite under agitation continued for atleast two hrs for completion of gold precipitation. The precipitated gold powder was collected by filtration of the 5 above slurry. The collected gold powder was dried and melted at 1100 ºC. The dry weight of the gold was found to be 93.52g.
Table 1: Chemical analysis of the sieved PCB’s
Batch
Size
(in kg)
Element
%
01
20
Au
0.489
02
20
Au
0.483
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Table 2: Analysis after leaching
Batch
Sample
Quantity
Unit
Element
Analysis value
01
Dry cake
12.4
Kg
Au
0.01 %
Leach liquor
12.0
Liter
Au
0.8 g/l
02
Dry cake
12.1
Kg
Au
0.01 %
Leach liquor
12.3
Liter
Au
0.77 g/l
Table 3: Analysis of Gold metal Recovered
Sample description
Batch no
Element
Cu
Ag
Pb
Ni
Cr
Mn
Zn
Fe
Al
Cd
Co
Gold purity
Gold purity
01
%
0.14
0.15
0.12
BDL
BDL
BDL
BDL
0.02
0.01
BDL
BDL
99.57
Gold purity
02
%
0.05
0.019
0.08
BDL
BDL
BDL
0.02
BDL
0.01
BDL
BDL
99.64
BDL=BELOW DETECTION LIMIT
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The method provides an economical and eco-friendly approach for e-waste management in the form of metal recovery from chips of waste PCBs.
The method also provides selective precipitation approach to precipitate out precious metals including gold and Copper abundant in chips of waste PCBs.
In yet another embodiment, the pulverized chip powder of above 300 microns 5 size is can be taken for recovery of other valuable metals preferably, copper using electro-refining process. Copper free powder is further used to recover gold to provide an efficient gold recovery process.
In an embodiment of the invention, the method provides an efficient method leaving minimum solid and no liquid wastes after completion of the recovery 10 process.
The process provides an overall average recovery of precious metallic fraction of above 99 percent along with a metal leaching efficiency of above 98 percent.

CLAIMS
We claim:
1. An industrial process for recovering metal values from chips of waste printed circuit board comprising the steps of: 5
a) pulverizing and sieving the chips to get particles of desired size;
b) agitating the sieved material of step a) in a mixture of nitric acid and hydrochloric acid at a temperature ranging from 70-80 ºC for atleast 3-4 hours to obtain a slurry;
c) filtering the slurry of step b) to obtain a leach liquor and a cake; 10
d) washing the cake of step c) with water and wash liquor is added to the leach liquor of step c);
e) treating the leach liquor of step d) with 25 % w/v urea solution under agitation to obtain a slurry;
f) treating the slurry of step e) with sodium metabisulphite under agitation for 15 atleast 3-4 hours to obtain a metal rich slurry;
g) filtering the slurry of step f) to obtain a metal rich cake and a filtrate wherein the metal rich cake is re-dissolved in the mixture of nitric acid and hydrochloric acid by agitating it at a temperature ranging from 70-80 ºC for 3-4 hours to further obtain a slurry; 20
h) filtering the slurry of step g) to obtain a precious metal rich filtrate and a residue;
i) treating the metal rich filtrate of step h) with sodium metabisulphite to obtain a mixture of precious metal containing precipitate and supernatant; and 25
j) filtering the mixture of step i) to separate precious metal containing precipitate followed by drying and melting the precipitate at a temperature of 1000-1100 ºC to obtain pure precious metal;
wherein,
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the process results in minimum solid waste and with no liquid discharge; and the process has metal leaching efficiency of more than 98%.
2. The process of recovering valuable metals as claimed in claim 1, wherein the desired size of particles is less than 0.3 mm. 5
3. The process of recovering valuable metals as claimed in claim 1, wherein the precious metal is gold.
4. The process of recovering valuable metals as claimed in claim 1, wherein 10 the precious metal recovered in step j) has metal impurity level below 0.36 %.
5. The process of recovering valuable metals as claimed in claim 1, wherein the process is economical and environment friendly.