Abstract:
A method of recovering precious metals from metal alloys produced aluminothermically from base metal smelter slag, includes treating the metal alloy with aqueous sulphuric acid solution having a pH of not greater than about 2 to cause dissolution of nickel and/or cobalt and iron and to cause formation of hydrogen sulfide which reacts with copper in the metal alloy to form a copper sulfide precipitate containing at least one precious metal, and separating the copper sulphide precipitate containing at least one precious metal from the remaining solution containing cobalt sulphate and/or nickel sulphate and ferrous sulphate.

Description:
This invention relates to the recovery of precious metals from metal alloys produced aluminothermically from base metal smelter slag. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 5,865,872 (Krofchak et al) describes a method of recovering metals and producing a secondary slag from base metal smelter slag by means of an aluminothermic reaction. In addition to the secondary slag, the reaction produces a metal alloy containing copper, nickel and/or cobalt, iron and sulphur. The contents of the aforementioned U.S. patent is hereby incorporated herein by reference. A base metal smelter slag may include (by weight) the following: 
     from about 10% to about 50% SiO 2 , 
     from about 0.5% to about 10% Al 2 O 3 , 
     from about 25% to about 50% Fe, 
     from about 0.5% to about 20% CaO, 
     from about 0.5% to about 15% MgO, 
     from about 0.1% to about 15% Cu, 
     from about 0.1% to about 15% Ni, 
     from about 0.1% to about 7% Co, 
     from about 0.1% to about 3% S 
     A metal alloy produced in accordance with the method described in the aforementioned patent may include (by weight): 
     from about 2.5% to about 99% Fe. 
     from about 0.1% to about 10% Si, 
     from about 0.1% to about 25% Ni, 
     from about 0.2% to about 25% Cu. 
     from about 0.1% to about 10% Co, 
     from about 0.1% to about 25% S 
     A base metal smelter slag may also include precious metals as follows (by weight): 
     from about 0.01 oz/ton to about 5 oz/ton Ag, 
     from about 0.01 oz/ton to about 3 oz/ton Pt, 
     from about 0.01 oz/ton to about 3 oz/ton Pd, 
     from about 0.01 oz/ton to about 3 oz/ton Au, 
     from about 0.01 oz/ton to about 3 oz/ton Rh 
     Accordingly, a metal alloy produced from a base metal smelter slag containing precious metals in accordance with the method described in the aforementioned patent may also contain precious metals (by weight) as follows: 
     from about 0.02 oz/ton to about 10 oz/ton Ag, 
     from about 0.02 oz/ton to about 6 oz/ton Pt, 
     from about 0.02 oz/ton to about 6 oz/ton Pd. 
     from about 0.02 oz/ton to about 6 oz/ton Au, 
     from about 0.02 oz/ton to about 6oz/ton Rh 
     It is therefore an object of the invention to provide a method of recovering at least one precious metal from a metal alloy produced aluminothermically from a base metal smelter slag. 
     SUMMARY OF INVENTION 
     The preset invention is based on the discovery that, if such a metal alloy is treated with aqueous sulphuric acid solution having a pH which does not exceed about 2, nickel and/or cobalt and iron is dissolved with the formation of hydrogen sulphide which reacts with copper in the metal alloy to form a copper sulphide precipitate which contains at least one precious metal, so that the precipitate can be easily separated from the remaining solution which contains cobalt sulphate and/or nickel sulphate and ferrous sulphate. The copper sulphide precipitate containing the at least one precious metal can then be treated to recover the precious metal. 
     The remaining solution may be subjected to an evaporation step to cause crystallizing of ferrous sulphate as a hydrate, and removing the crystallized ferrous sulphate therefrom to produce a further remaining solution containing cobalt sulphate and/nickel sulphate and any remaining ferrous sulphate. 
     The pH of the further remaining solution may be raised to about 3.5 and converting the ferrous iron to ferric iron to cause precipitation of iron as ferric hydroxide and removing the precipitated ferric hydroxide from the cobalt sulphate and/or nickel sulphate containing solution. 
     The pH of the nickel sulphate and/or cobalt sulphate containing solution may be raised to at least 9 with sodium carbonate and/or nickel sulphate containing solution to at least about 9 with sodium carbonate to precipitate cobalt sulphate and/or nickel sulphate as cobalt carbonate and/or nickel carbonate, and removing the precipitated cobalt carbonate and/or nickel carbonate from the still remaining solution. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     One embodiment of the invention will now be described with reference to the accompanying drawing which is a flow chart showing base metal smelter slag being subjected to an aluminothermic reaction, with the resultant metal alloy being treated in accordance with the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring to the drawing, base metal smelter slag containing copper, nickel, cobalt, iron and sulphur as well as previous metals is subjected to all aluminothermic reaction step  10  as described in aforementioned U.S. Pat. No. 5,865,872 to produce a secondary slag and a metal alloy containing copper, nickel, cobalt, iron, sulphur and precious metals. The secondary slag is separated from the metal alloy and utilized in a cement plant. The metal alloy is passed through a dissolution step  12  where the metal alloy is digested in sulphuric acid solution with a pH not greater than about 2, with some air and heat being supplied. 
     In the solution step  12 , iron, nickel and cobalt go into solution and hydrogen sulphide is formed. Copper in the metal allow reacts with the hydrogen sulphide to form a copper sulphide precipitate which also contains precious metals. Excess hydrogen sulphide is expelled as a gas which is collected and subjected to a treatment step  14  to produce elemental sulphur and water. 
     The copper sulphide precipitate containing precious metals is separated from the nickel cobalt and iron containing solution in a separating step  16 , and is treated in a refinery for recovery of the precious metals. The nickel cobalt and iron containing solution is passed to a crystallization step  18  where water is evaporated to cause crystallization of iron sulphate as ferrous sulphate heptahydrate, which is separated from the remaining solution in a separation step  20 . The remaining solution containing, nickel sulphate, cobalt sulphate and some remaining ferrous sulphate is passed to an iron removal step  20  where remaining iron is precipitated as ferric hydroxide by addition of lime to raise the pH to about 3.5 and by addition of oxygen to convert ferrous iron to ferric iron by a redox process. Precipitated ferric hydroxide and calcium sulphate are separated from the remaining cobalt sulphate and nickel sulphate in a separation step  22  as a sludge for disposal. 
     The remaining cobalt sulphate and nickel sulphate solution is passed to a nickel and cobalt precipitation step  24  where the cobalt and nickel are precipitated as cobalt carbonate and nickel carbonate by addition of sodium carbonate to raise the pH to at least about 9. The cobalt carbonate and nickel carbonate precipitate is recovered in a separation step  26 , and the remaining water is passed to a waste water treatment plant. 
     Examples of the invention will now be described. 
     EXAMPLE 1 
     A sample of rich base metal smelter slag was obtained, which has been segregated from a slag heap that has been accumulated over 50 years from one of the largest nickel producers located in Sudbury, Ontario, Canada. 800 grams of the sample were crushed to 1 mm (100% passing 18 mesh sieve) and mixed with 80 gams of A 1  metal (100% passing 20 mesh sieve) and 160 g of CaO (¼″ chips). The charge was put in a crucible lined with alumina and crucible inside a propane fired drum furnace. The furnace was then heated for 1 hour to about 1300° C. After 1 hour the A 1  reacted with the metal oxides contained in the slag and the aluminothermic reaction lasted a few minutes. The material was then poured into a cast iron mold to allow cooling and separation of the new two phases, the metal alloy and the new slag, that has been identified as obsidian. Samples of the original slag, metal alloy and obsidian were sent for analysis. 
     The results were as follows: 
     
       
         
               
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                   
                 Slag 
                 Al 
                 CaO 
                 Crucible 
                 Total 
                 Total 
                 Metal Alloy 
                 Obsidian 
               
               
                   
               
               
                 Before 
                 800 
                 80 
                 160 
                 1090.9 
                 2130.9 
               
               
                 After 
                   
                   
                   
                   
                   
                 2127.4 
                 417.3 
                 619.2 
               
               
                   
                   
                   
                   
                   
                   
                 99.8% 
               
               
                   
               
             
          
           
               
                 MASS BALANCE 
               
             
          
           
               
                   
                 Input 
                   
                 Output (%) 
                 Mass Out (g) 
                   
               
             
          
           
               
                   
                 Slag 
                 Al 
                 CaO 
                 Mass In (g) 
                 Metal 
                 Obsidian 
                 Metal 
                 Obsidian 
                   
               
             
          
           
               
                   
                 Weight (g) 
                 Distribution (%) 
               
             
          
           
               
                   
                 800 
                 80 
                 160 
                 1040 
                 417.3 
                 619.2 
                 417.3 
                 619.2 
                 Metal 
                 Obsidian 
               
               
                   
               
               
                 Ni 
                 6.586 
                   
                   
                 52.686 
                 12.976 
                 0.016 
                 54.149 
                 0.099 
                 99.812 
                 0.188 
               
               
                 Cu 
                 6.759 
                   
                   
                 54.069 
                 11.626 
                 0.023 
                 48.515 
                 0.142 
                 99.737 
                 0.263 
               
               
                 Co 
                 0.395 
                   
                   
                 3.163 
                 0.766 
                 0.003 
                 3.197 
                 0.020 
                 99.374 
                 0.626 
               
               
                 S 
                 8.024 
                   
                   
                 64.194 
                 11.710 
                 0.544 
                 48.866 
                 3.368 
                 94.753 
                 5.247 
               
               
                 Fe 
                   
                   
                   
                   
                 57.301 
                   
                 239.117 
                   
                 69.116 
               
               
                 FeO 
                 45.518 
                   
                   
                 364.147 
                   
                 6.022 
                   
                 37.287 
                   
                 10.240 
               
               
                 Si 
                   
                   
                   
                   
                 0.721 
                   
                 3.009 
                   
                 0.878 
               
               
                 SiO 2   
                 26.085 
                   
                   
                 208.678 
                   
                 33.068 
                   
                 204.757 
                   
                 98.121 
               
               
                 Al 
                   
                 80 
                   
                 80.000 
                 0.269 
                   
                 1.121 
                   
                 2.635 
               
               
                 Al 2 O 3   
                 3.782 
                   
                   
                 30.259 
                   
                 27.850 
                   
                 172.447 
                   
                 95.019 
               
               
                 Ca 
                   
                   
                   
                   
                 0.157 
                   
                 0.653 
                   
                 7.506 
               
               
                 CaO 
                 2.138 
                   
                 160 
                 177.103 
                   
                 25.591 
                   
                 158.459 
                   
                 89.473 
               
               
                 Mg 
                   
                   
                   
                   
                 0.015 
                   
                 0.061 
                   
                 −9.892 
               
               
                 MgO 
                 1.691 
                   
                   
                 13.527 
                   
                 2.543 
                   
                 15.746 
                   
                 116.404 
               
               
                   
               
             
          
         
       
     
     This example shows that the aluminothermic process is all efficient way of metal recovery, i.e. over 99% of Ni, Cu, and Co in the original slag reported to the metal alloy. Also, the new slag, identified as obsidian, is an environmentally clean glass material, which is an excellent supplementary cementing material. 
     Substantially all the precious metals in the original base metal smelter slag reported to the metal alloy. 
     Virtually no precious metals reported to the new slag (Pt.0.0042 oz/ton; Pd,0.0021 oz/ton; Rh,0.001 oz/ton; Ag-0.01.01 oz/ton). 
     EXAMPLE 2 
     200 g of the metal alloy from Example 1 were crushed to 1 mm (100% passing 18 mesh sieve) and dissolved in a beaker with 6,000 g sulfuric acid @ 10% solution (equal to 600 g sulfuric acid and 5,400 g of water). After the reaction was completed it was noted that an undissolved residue (U.R.) reported to the bottom of the beaker. The solution was then filtered and the filtrate washed, dried and weighed. Samples of the metal alloy, undissolved residue and solution were sent for analysis. 
     The results were as follows: 
     
       
         
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                   
                 Metal Alloy 
                 H 2 SO 4   
                 Water 
                 Total 
                 Total 
                 U.R. 
                 Solution 
               
               
                   
               
               
                 Before 
                 200 
                 600 
                 5400 
                 6200 
               
               
                 After 
                   
                   
                   
                   
                 6200 
                 40.8 
                 6159.2 
               
               
                   
                   
                   
                   
                   
                   
                 20.4% 
               
               
                   
               
             
          
           
               
                 MASS BALANCE 
               
             
          
           
               
                   
                 Input 
                   
                 Output (%) 
                 Mass Out (g) 
                   
               
             
          
           
               
                   
                 Metal 
                 H 2 SO 4   
                 Water 
                 Mass In (g) 
                 U.R. 
                 SOL (g/l) 
                 U.R. 
                 Solution 
                   
               
             
          
           
               
                   
                 Weight (g) 
                 Distribution (%) 
               
             
          
           
               
                   
                 200 
                 600 
                 5400 
                 6200 
                 40.8 
                 1000 
                 40.8 
                 1000 
                 U.R. 
                 Solution 
               
               
                   
               
               
                 Ni 
                 12.976 
                   
                   
                 25.952 
                 5.486 
                 20.936 
                 2.238 
                 20.936 
                 8.625 
                 91.375 
               
               
                 Cu 
                 11.626 
                   
                   
                 23.252 
                 57.347 
                 0.002 
                 23.398 
                 0.002 
                 100.626 
                 −0.626 
               
               
                 Co 
                 0.766 
                   
                   
                 1.532 
                 0.146 
                 1.269 
                 0.060 
                 1.269 
                 3.888 
                 96.112 
               
               
                 S 
                 11.710 
                 181.4 
                   
                 204.820 
                 16.993 
                 140.300 
                 6.933 
                 140.300 
                 3.385 
                 96.615 
               
               
                 Fe 
                 57.301 
                   
                   
                 114.602 
                 4.851 
                 92.151 
                 1.979 
                 92.151 
                 1.727 
                 98.273 
               
               
                 Si 
                 0.721 
                   
                   
                 1.442 
                 0.923 
                 0.663 
                 0.377 
                 0.663 
                 26.115 
                 73.885 
               
               
                 Al 
                 0.269 
                   
                   
                 0.537 
                 1.428 
                 0.074 
                 0.583 
                 0.074 
                 108.415 
                 −8.415 
               
               
                 Ca 
                 0.157 
                   
                   
                 0.313 
                 0.085 
                 0.009 
                 0.035 
                 0.009 
                 11.073 
                 88.927 
               
               
                 Mg 
                 0.015 
                   
                   
                 0.029 
                 0.006 
                 0.010 
                 0.002 
                 0.010 
                 8.327 
                 91.673 
               
               
                   
                 oz/ton 
                   
                   
                   
                 oz/ton 
               
               
                 Pt 
                 0.219 
                   
                   
                 0.0150 
                 1.017 
                   
                 0.0142 
                   
                 94.734 
               
               
                 Pd 
                 0.195 
                   
                   
                 0.0134 
                 0.922 
                   
                 0.0129 
                   
                 96.455 
               
               
                 Rh 
                 0.021 
                   
                   
                 0.0014 
                 0.099 
                   
                 0.0014 
                   
                 96.171 
               
               
                 Au 
                 0.057 
                   
                   
                 0.0039 
                 0.269 
                   
                 0.0038 
                   
                 96.274 
               
               
                 Ag 
                 1.610 
                   
                   
                 0.1104 
                 7.58 
                   
                 0.1060 
                   
                 96.045 
               
               
                 TPM 
                 0.470 
                   
                   
                 0.0322 
                 2.208 
                   
                 0.0309 
                   
                 95.837 
               
               
                 (Pt + Pd + Au) 
               
               
                   
               
             
          
         
       
     
     This example shows that previous metals can be successfully recovered through acid dissolution in an undissolved residue composed mainly of copper sulfide, leaving in solution iron, nickel and cobalt sulfate. 
     EXAMPLE 3 
     1000 g of solution from Example 2 were heated in order to increase the concentration of ferrous sulfate. When it reached about ¼ of its original volume, it was cooled very rapidly to allow crystallization of ferrous sulfate heptahydrated (FeSO 4 , 7H 2 O), leaving the nickel and the cobalt in solution. Then residual iron was removed through redox reaction of ferrous (Fe II ) to ferric (Fe III ) ions and precipitated out with PH increased to 3.5 with milk of lime—CaO. After iron removal, a solution of sodium carbonate was added to allow precipitation of nickel and cobalt carbonate. Samples of the solution, ferrous sulfate heptahydrated and nickel and cobalt carbonate were sent for analysis. 
     The results were as follows: 
     
       
         
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                   
                 Solution 
                 CaO 
                 Na2CO3 
                 FeSO 4 .7H 2 O 
                 (Ni, Co)CO 3   
                 Solution 
               
               
                   
                   
               
               
                   
                 Before 
                 1000 g 
                 excess 
                 excess 
               
               
                   
                 After 
                   
                   
                   
                 98 g 
                 235 g 
                 250 g 
               
               
                   
                   
               
             
          
           
               
                 MASS BALANCE 
               
             
          
           
               
                   
                 Input 
                   
                 Output (%) 
                 Mass Out (g) 
                   
               
             
          
           
               
                   
                 Sol. 
                 Mass In 
                 Ferrous 
                 (Ni,Co) 
                   
                 Ferrous 
                 (Ni,Co) 
                   
                   
               
               
                   
                 (g/l) 
                 (g) 
                 Sulfate 
                 Carbonite 
                 Sol. (g/l) 
                 Sulfate 
                 Carbonate 
                 Sol. 
                 Distribution (%) 
               
             
          
           
               
                   
                 Weight (g) 
                 Ferrous 
                 (Ni,Co) 
                   
               
             
          
           
               
                   
                 1000 
                 1000 
                 98 
                 235 
                 250 
                 98 
                 235 
                 250 
                 Sulfate 
                 Carbonate 
                 Sol. 
               
               
                   
               
               
                 Ni 
                 20.936 
                 20.936 
                 1.973 
                 7.639 
                 0.0680 
                 1.934 
                 17.952 
                 0.272 
                 9.235 
                 90.765 
                 1.299 
               
               
                 Cu 
                 0.002 
                 0.002 
                 0.031 
                 0.026 
                 0.0015 
                 0.030 
                 0.061 
                 0.006 
               
               
                 Co 
                 1.269 
                 1.269 
                 0.203 
                 0.321 
                 0.0060 
                 0.199 
                 0.754 
                 0.024 
                 15.677 
                 84.323 
                 1.891 
               
               
                 S 
                 140.300 
                 140.300 
                 11.440 
                 0.220 
                 5.5460 
                 11.211 
                 0.517 
                 22.184 
                 7.991 
                 0.368 
                 15.846 
               
               
                 Fe 
                 92.151 
                 92.151 
                 17.537 
                 0.203 
                 0.0082 
                 17.186 
                 0.477 
                 0.084 
                 18.650 
                 0.518 
                 0.091 
               
               
                 Si 
                 0.663 
                 0.663 
                 0.014 
                 0.017 
                 0.0062 
                 0.003 
                 0.040 
                 0.064 
               
               
                 Al 
                 0.074 
                 0.074 
                 0.011 
                 0.017 
                 0.0095 
                 0.011 
                 0.040 
                 0.096 
               
               
                 Ca 
                 0.009 
                 0.009 
                 0.067 
                 23.057 
                 0.0094 
                 0.066 
                 54.184 
                 0.096 
               
               
                 Mg 
                 0.010 
                 0.010 
                 0.005 
                 2.287 
                 0.3431 
                 0.006 
                 5.374 
                 1.372 
               
               
                   
                   
                   
                 oz/ton 
                 oz/ton 
               
               
                 Pt 
                   
                   
                 0.0032 
                 0.0041 
               
               
                 Pd 
                   
                   
                 0.0016 
                 0.0020 
               
               
                 Rh 
                   
                   
                 0.0016 
                 0.0020 
               
               
                 Au 
                   
                   
                 0.0008 
                 0.0010 
               
               
                 Ag 
                   
                   
                 0.0100 
                 0.0004 
               
               
                 TPM 
                   
                   
                 0.0056 
                 0.0071 
               
               
                 (Pt + Pd + Au) 
               
               
                   
               
             
          
         
       
     
     This method shows that nickel and cobalt can be successfully separated from iron producing valuable products, i.e. ferrous sulfate heptahydrate and nickel/cobalt carbonate. No precious metals were detected (already recovered in the undissolved residue of Example 2). 
     EXAMPLE 4 
     600 g of new slag from Example 1 were ground to 45 micron (100% passing −325 mesh sieve) and ASTM/CSA standard cubes were made using various ratio new slag:normal Portland cement type 10 to be test the uniaxial strength at 1, 3, 7, 28 and 90 days. 
     The results were as follows: 
     
       
         
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Mix Proportion in gm 
                   
               
             
          
           
               
                   
                 Cement 
                   
                   
                   
                   
               
               
                   
                 Type 
                 Silica 
                 New 
                   
                 Compressive Strength in psi 
               
             
          
           
               
                   
                 10 
                 Sand 
                 Slag 
                 Water 
                 1 day 
                 3 day 
                 7 day 
                 28 day 
                 90 day 
               
               
                   
                   
               
             
          
           
               
                 Control 
                 500 
                 1375 
                 — 
                 242.5 
                 1021 
                 3920 
                 4474 
                 6181 
                 7143 
               
               
                 #1 
                 425 
                 1375 
                 75 
                 242.5 
                 1018 
                 3768 
                 4386 
                 6065 
                 7256 
               
               
                 #2 
                 375 
                 1375 
                 125 
                 242.5 
                 880 
                 2877 
                 3707 
                 7291 
                 7489 
               
               
                 #3 
                 325 
                 1375 
                 175 
                 242.5 
                 604 
                 2912 
                 4513 
                 7535 
                 8535 
               
               
                 #4 
                 250 
                 1375 
                 250 
                 242.5 
                 470 
                 2198 
                 4132 
                 7401 
                 8033 
               
               
                   
               
             
          
         
       
     
     From these results it is clear that the cubes made with the blend of the ground new slag normal Portland cement exceeded the control. 
     Besides, the Pozzolanic Strength Activity Index (P.S.A.I.) at 28 day has to be at least 75% of a control sample. The P.S.A.I. with Portland cement according to the ASTM Standards is calculated in the following manner: 
     
       
         Pozzolanic Strength Activity Index with Portland cement=( A/B )×100 
       
     
     Where: 
     A=average compressive strength of test mix cubes made with the blended cement 
     B=average compressive strength of control mix cubes 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 #1 
                 #2 
                 #3 
                 #4 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 PSAI 
                 98% 
                 118% 
                 122% 
                 120 
               
               
                   
                   
               
             
          
         
       
     
     This example shows that the new slag after metal recovery produces an excellent supplementary cementing material. 
     Other embodiments and examples of the invention will now be readily apparent to those skilled in the art, the scope of the invention being defined in the appended claims.