GB2049734A - Extracting precious metals from mattes - Google Patents

Abstract

A method of extracting and concentrating small quantities of precious metals such as gold, silver and platinum contained in high sulphur content base metal sulphide ore residues comprises mixing the ore residue with extra copper or other suitable base metal, e.g. by pelletising with powdered copper, smelting and casting the mixture to form a two phase block having an upper lighter layer rich in sulphur and a lower denser metal layer rich in the precious metals and then treating the lower layer for the recovery of a concentrate containing the precious metals. If the correct amount of copper or other base metal be added a relatively small lower layer is formed so that the precious metals are not too dispersed and a sharp line of demarcation if formed between the two layers. <IMAGE>

Description

SPECIFICATION Recovering precious metals from residue This invention relates to the recovery of platinum and other precious metals such as gold and silver from high sulphur content base metal sulphide ore residues.

In known processes for the recovery of precious metals from base ore residues containing trace quantities of the precious metals, various metallurgical treatment routes are employed to segregate the precious metals into a concentrate. For example during electro-refining the precious metals migrate to the anode slimes and are concentrated there whilst during electrowinning the precious metals are concentrated in the leach residues.

It has been found that these concentrates and leach residues containing the precious metals necessarily vary considerably in composition, the precious metal and minor element components being heavily dependent on the nature of the parent ore. However, it can be said that because of the refractory nature of the precious metals, all the normal known extraction methods for these elements concentrate upon the removal of the other elements with a concurrent extraction of the precious metals which are left concentrated in a final residue. As the bulk of these other elements are base metal sulphides the various known or conventional routes available for precious metal recovery can be classified according to the means adopted to remove the metal sulphides.Thus at present there are the following known examples of alternative methods for removing base metals and sulphur from base ore residues: 1. Roasting off the sulphur and acid leaching of the base metal oxides and sulphate formed.

2. Direct electrolysis of sulphide-rich anodes cast from the base ore residues.

3. Pressure leaching using air or oxygen in conjunction with an acid or ammoniacal leaching agent.

4. Slow cooling of the molten base ore residues to give relatively coarse dispersed phases of sulphides and metallics with the precious metals tending to concentrate in the metallic fraction.

In our investigations of the problem of extracting and recovering precious metals we have been using residues from an Outukumpu-Oy leaching cementation process. We found on analysis that the residues contained about 10% Ni, 60% Cu and 20% S and we also found that all of the above extraction methods required either complex equipment or precise control for their satisfactory operation.

It is an object of the present invention to provide an improved method for the recovery of precious metals from base ore residues.

According to the present invention there is provided a method for the treatment of a high sulphur content base metal sulphide ore residue containing small amounts of precious metals in order to separate out a concentrate rich in precious metals, comprising the steps of mixing the ore residue with extra copper or other suitable base metal, smelting and casting an upper lighter sulphur-rich layer and a lower denser metal rich layer containing the precious metals and treating the lower layer for the recovery of a concentrate containing the precious metals.

In one way of carrying out the method an ore residue filter cake from a filter, after leaching out base metal in a base metal refinery, is mixed with powdered copper and the mixture is pelletized in a pelletizer and drier, the dried pellets then being smelted and cast as above. Alternatively the ore residue may be peiletized and dried and the pellets so formed may be mixed with powdered copper and then further treated as above.

We have therefore discovered that a smelt of a pelletized ore residue if mixed with additional copper will give a straight forward two-layer separation of an upper matte phase and a lower phase rich in precious metals.

These phases separate easily e.g. with the aid of a few strokes of a hammer and by a suitable selection of moulds an anode-shaped casting can be formed from the lower metallic phase from which the base metals can be removed by electrolysis leaving an anode slime rich in precious metal values suitable for sale or final refining.

Alternatively the base metals in the metallic phase can be removed by using the metallic phase as a feed-stock for a conventional base metal refining route. One suitable process is the leach-cementation route developed by Outukumpu-Oy in Finland.

We have found that the key to the control of the formation of the metallic phase is the loss of sulphur from the molten material, where the percentage of sulphur is expressed by taking only nickel, copper and sulphur as the constituents of the melt. When these figures are plotted on an appropriate phase diagram as in Figure 1 the reason for the emergence or formation of the two separate phases can be seen.

In figure 1 the upper line represents the stoichiometric sulphur content for the corresponding Ni3S2 and Cu2S mixtures. The lower line, separating the three phase region from the two phase region follows closely, but not exactly, the sulphur content for the corresponding amount of Cu2S.

During cooling of a sulphur-rich high copper low nickel material such as is found in the leach base ore residue of the Outukumpu-Oy leach concentration process Cu2S freezes out first so that the copper present has first choice of the sulphur (point A on the phase diagram). If the sulphur is insufficient (point B on the phase diagram) then two phases are left with a marked specific gravity difference between them namely Cu2S at 5,6 and a copper nickel alloy at 8,9 so that the alloy separates cleanly from the remainder of the material, giving the two phases.

On the other hand a typical matte for thn slow cooling process mentioned in process 4 above runs at 50% Ni, 30% Cu and 20% S (point C on the phase diagram). Thus both the higher Ni and S tend to position the composition firmly in the three phase section of the diagram. In these circumstances the metal, after separation of solid Cu2S, still has a high S content and hence a fairly low specific gravity about 6,6. The metal alloy that begins to freeze out at a temperature of about 7000C thus does so in matrix of Cu2S. As cooling continues, nickel sulphide precipitates at the low ternary eutectic temperature of 5750C, and there are further important solid state segregations of dissolved copper from these nickel sulphides at even lower temperatures.This leads to a complex structure, and hence the need for slow cooling to obtain a grain size suitable for physical separation.

The control of the sulphur level in carrying out the present invention is important in the two phase separation area, as too great a sulphur loss will give an inconveniently large metallic fraction with a dilute precious metal content. On the other hand too low a sulphur loss gives a poorly defined metallic fraction that tends to be high in sulphur.

We have found that the addition of a small amount of copper powder to the residue in accordance with the invention allows control of the separation. This powder, which may arise from the electrolysis of the metallic phase, has the effect of bringing the composition of the mixture over to the left of the phase diagram. Here a reduced sulphur loss is required for the phase separation, allowing the production of a satisfactory metallic component to occur without air or oxygen blowing.

In the preferred form of the invention the base ore residue arises from an Outukumpu-Oy leachcementation process, the residue is mixed with a powdered base metal, preferably copper, and the mixture is pelletised. The pellets are then fed to a melting furnace together with suitable fluxing agents and an ingot or cake in two phases is formed.

The bottom phase may be used as an anode for use in an electrolytic process in which the base metals plate out on the cathode or go into solution and the anodes are enclosed in bags in which a residue concentrate is deposited containing upgrade precious metal values.

Alternatively the bottom phase can be crushed or granulated, and then used as a 'white metal' feedstock to the Outukumpu-Oy leach-cementation base metal refining route.

The residue from the electrolysis br leaching step may be sold as such or may be further upgraded by roasting followed by leaching of base metals. The 'tops' from this process can be conveniently returned to the converters at a base metal smelter for recycling and recovery of the contained nickel and copper.

The process diagram given in Figure 2 shows a flow sheet of an example of a process according to the invention. (Numbers in parentheses refer to flows in the accompanying mass balance schedule).

In the process diagram a filter cake (1) of base ore residues from a filter 19, after leaching of base metal in a base metal refinery (in this case for the recovering of copper and nickel via the Outukumpu Oy leach-cementation route) is mixed with powdered copper and the mixture is pelletized in a pelletizercum-drier 20. Strong green pellets are formed and are allowed to cure for about a week on a drying floor 25. The pellets are charged direct to an arc furnace 21 to which is added a certain amount of recycled slag plus some fluxes. The furnace 21 is tapped periodically into moulds 22. After cooling the moulds are tipped and a block of slag, matte and metal is ejected. The bottoms of metal are knocked off and the slag and matte separated. Some of the slag is recycled to the furnace 21 while the balance and the matte (2) may be returned to a smelter.

The bottoms (3) are employed as anodes which are used in an electrolysis cell 23 encased in suitable bags.

A loose powdery copper deposit (6) forms on the cathodes and some of this may be recovered for recycling through the pelletiser 20. The electro!yte (5) picks up the nickel component of the metal bottoms and this is returned to the refinery. The anode bags retain a precious metal-rich sludge.

Alternatively nickel and copper may be removed from the bottoms by, for example, a leachcementation process. Copper for recycle then arises from the electrowinning step in this process.

The dried sludge may be sold as such or it may be roasted and leached to remove base metals in the sludge and to provide an enriched sludge (8) and a leach solution (7).

Mass Balance Schedule

Stream No. (1) (2) (3) (4) (5) (6) (7) (8) (Dry) Matte Anode Copper Leach Leach Material Residue and Slag 'Bottoms' Residue Electrolyte Powder Solution Residue Ni kg/day 1650 1088 562 2 549 11 2 0,02 Cu " 8250 6900 1350 120 1 1299 119 1,14 S " 3000 2600 150 100 - Tr - Tr "Insols" " 2070 2059 11 11 - - - 11 Au " 1,500 0,450 1,050 1,037 Tr 0,013 0,006 1,031 Pt " 1,800 0,540 1,260 1.254 Tr 0,006 Tr 1,254 Pd " 3,300 0,990 2,310 2,297 Tr 0,013 0,025 2,272 Ag " 22,500 15,083 7,417 7,385 Tr 0,032 6,750 0,635 T.P.D. (Dry) 15 12,670 2,080 0,240 - 1,240 - 0,023 m /Day Soln. 4,07 0,690 % Ni 11,00 8,6 27,0 0,8 135 g/l 0,8 1,0 g/l 0,10 % Cu 55,00 54,5 64,9 48,9 250 ppm 94 170 g/l 6,57 % S 20,00 20,4 7,2 40,0 - 0,1 - Tr Au ppm 100 34,6 504 4233 Tr 10 9 59417 Pt ppm 120 41,5 606 5200 Tr 5 Tr 72268 Pd ppm 220 76,1 1111 9377 Tr 10 36 130936 Ag ppm 1500 1172,6 3570 30145 Tr 25 10 g/l 36595

Claims (14)

1. A method for the treatment of a high sulphur content base metal sulphide ore residue containing small quantities of precious metals in order to separate out a concentrate rich in the precious metals comprising the steps of mixing the ore residue with extra copper or other suitable base metal, smelting and casting the mixture to form a two phase metal block having an upper lighter sulphur rich layer and a lower denser metal rich layer containing the precious metals and treating the lower layer for the recovery of a concentrate containing the precious metals.

2. A method according to claim 1 characterized in that the ore residue is mixed with powdered copper and the mixture is pelletised and dried.

3. A method according to claim 1 characterized in that the ore residue is pelletized and dried and the pellets so formed are mixed with powdered copper.

4. A method according to claim 1 or 2 or 3 wherein the casting step is carried out in an anode shaped mould.

5. A method according to any of claims 1 to 4 wherein the base metals are removed from the lower layer by electrolysis leaving an anode'slime rich in precious metal values.

6. A method according to any of claims 1 to 4 wherein the base metals in the metallic phase are removed by a conventional base metal refining process.

7. A method according to any of the preceding claims wherein the sulphur level in the ore residue is controlled by the addition of a predetermined amount of base metal such as copper to give a relatively small metallic phase rich in precious metals and a well defined demarcation between the two phases.

8. A method according to claim 1 wherein the ore residue is rich in silver, gold and/or platinum group metals and is derived from the treatment of a nickel-copper matte by the Outukumpu-Oy leachcementation process.

9. A method according to claim 1 wherein the'ore residue material has a sulphur content of 1 5-25%, a nickel content of 1030% and a copper content of 4070%.

1 0. A method according to claim 1 wherein the copper used to adjust the composition of the charge to the furnace arises from the electrolysis of the furnace product at a later stage in the method.

11. A method according to claim 1 wherein the upper lighter sulphur-rich phase is returned to a base metal smelter for recycling to a base metals refinery.

12. A method according to claim 1 wherein the lower metallic phase is cryshed or granulated to form a feedstock for an Outukumpu-Oy leach-cementation-base metal refining process.

1 3. A method according to claim 1 wherein the lower metallic phase is cast to form a flat shape suitable as an anode in an electroyltic cell.

14. A method according to claim 1 3 wherein the anode is surrounded by a bag of acid-resistant material wherein the anode residues accumulate.

1 5. A method according to claim 1 5 wherein the anode residue is roasted at a temperature not exceeding 5000C followed by leaching in dilute sulphuric acid, and the roast-leach cycle is repeated until a concentrate containing a significant percentage of gold, silver and platinum group metals has been formed.

1 6. A method according to claim 12 wherein the feedstock arising from the treatment is roasted at a temperature not exceeding 5000C followed by leaching in dilute sulphuric acid until a concentrate containing a significant percentage of gold, silver and/or platinum metals has been formed.