GB2189235A - Extraction of sulphur from iron minerals treated by oxidising lixiviation - Google Patents

Description

SPECIFICATION Improvement in the extraction of sulphur from iron minerals treated by oxidising lixiviation (leaching) The present invention relates to improvements in a process of recovery of sulphur in the elementary state from pyrrhotic minerals. It concerns more particularly a process in which the mineral is treated by oxidising lixiviation in order to liberate the sulphur from the iron sulphide which contains it. The invention also relates to extraction of non-ferrous and noble metals which can be found in the sulphided mineral treated.

Prior processes are known using the oxidising lixiviation of pyrrhotic iron minerals, that is to say ones more or less rich in sulphides approximating to the composition FeS. Some of them, particularly those dealt with in US Patents 2898197 and 3034864, comprise fusion of the sulphur precipitated by the lixiviation and agglomeration of the liquid into granules to which remain fixed after solidification particles of the lixiviation pulp. As these particles can contain metals such as Au and Ag, their separation becomes difficult, as also is the production of sufficiently pure sulphur. In a general manner, the prior art indicated above is subject to separation difficulties, in particular to awkward filtrations, of the powders or pulps with the particles or granules of sulphur.

The present invention provides a substantial improvement in the extraction of sulphur from pyrrhotic minerals in that it makes effective and facilitates separation of the sulphur from the other constituents of the mineral, particularly by simple filtration, decantation or centrifugation.

Also, it makes the operations more economical, permitting the use of solvents for sulphur with considerably reduced losses.

The process according to the invention consists in subjecting a finely ground iron sulphide mineral to oxidising lixiviation in the hot, followed or accompanied by fusion of the sulphur thus liberated, with agglomeration of the sulphur into granules which are then solidified by cooling and are physically separated from the lixiviation pulp; it is characterised in that the granular sulphur separated from the pulp is dissolved in the hot in an organic solvent, the solution obtained being freed from all lixiviation residues and then subjected to crystallisation of the sulphur.

Thus, in accordance with the invention, a double separation is carried out between the elementary sulphur obtained from the lixiviation and the pulp from the latter step; quite unexpectedly, the sum of these two operations is easier and leads to a much better separation than occurs with the prior art processes.

When the iron mineral treated contains non ferrous metals, these can be extracted in known manner from the solids and the solution which accompany them in the lixiviation pulp. Thus, Cu and Zn can be recovered from the aqueous solution separated from the pulp solids. Noble metals, particularly Au and Ag can be obtained from the solid of this pulp, for example by cyanidation.

As sulphided minerals usually contain arsenic, it is recommendable to eliminate this before crystallisation of the sulphur; in the process according to the invention, the preferred method of dearsenification consists in contacting the organic solution of the sulphur with a relatively small volume of an aqueous solution of a base, particularly alkaline, alkaline earth or ammoniacal, which retains the As. Crystallisation from the thus purified organic solution provides very pure sulphur.

When the initial mineral contains iron sulphides richer in S than the pyrrhotines, in particular FeS2, it is of interest to subject it to a preliminary heating in a non-oxidising atmosphere, preferably between 600 and 900"C, in order to volatilise the sulphur and convert the persulphided minerals into pyrrhotine. It is then preferable to unite the volatilised sulphur with that from the lixiviation, in order to carry out dearsenification on all the sulphur derived from the two operations.

As oxidising lixiviation in an acid medium is known in the art, it is not necessary to describe it here. As regards the fusion and agglomeration of the liquid sulphur derived from this operation, it is preferably carried out around 1300C, under such agitation that the sulphur forms granules having a diameter of about 0.3 to 3 mm, most preferably of the order of 1 mm; after cooling below 1200C, these solid granules can be separated from the lixiviation pulp by simple screening, the solids from the pulp generally having a particle size below 100 ij.

The granules of sulphur thus separated are generally formed of 60 to 80% of S, the remainder being formed mainly by minerals not affected during the lixiviation, particularly blende, chalcopyrite, pyrrhotine etc. which are moistenable by liquid sulphur. These residual materials separate easily from the organic solution of the sulphur and can be recycled to the initial material, that is to the lixiviation.

It is of interest to note that in the process of the invention the major part of the lixiviation pulp, which comprises a gangue with the goethite obtained from attack of the FeS, is not moistened by the melted sulphur and separates well; there is thus no or very little goethite in contact with the organic solvent in the granular sulphur dissolution stage, giving considerable reduction in the losses of solvent by absorption in the goethite, an advantage of the process mentioned above.

As regards the organic solvents for sulphur utilisable in the present process, they can be such as liquid hydrocarbons, halogenated hydrocarbons, thioalkanes etc. By way of non-limitative examples, reference can be made to compounds such as benzene, toluene, ethyl benzene, xylene, dichlorobenzene, dichloromethane, trichloromethane, dichloroethylene, trichloroethylene, perchloroethylene, carbon disulphide, diethyl sulphide, dipropyl sulphide etc.

The whole of the process is illustrated by the scheme in the accompanying single figure, where references 1 to 16 represent the following operations.

1. Oxidising lixiviation of a fine powder of the mineral, generally having a particle size below 3 mm, preferable about 20 to 100 it; with 1.2 to 2 parts by weight of acidified water per part of powder, the acidity being 0.7 to 1.5 equivalents/litre; heating between 100" and 120"C, preferably 105 to 115"C, under an air pressure of 5 to 30 bars for 2 to 6 hours.

2. Agglomeration of the liquid sulphur with agitation, preferably with formation of granules of 0.3 to 3 mm diameter.

3. Standing at atmospheric pressure with consequent cooling, giving solidification of the sulphur granules.

4. Separation of the granular sulphur from the pulp obtained in 1; effected by screening.

5. Dissolution of the sulphur using the granules obtained in 4, by means of an organic solvent recycled from step 8; in general between 40 and 120"C, depending upon the nature of the solvent.

6. Separation by decantation, filtration or centrifugation of the solution of sulphur in the organic solvent with the solids entrained by the sulphur during the agglomeration step 2.

7. Dearsenification of the sulphur carried out on the organic solution.

8. Cooling of the organic solution, crystallisation of the sulphur and collection of the solvent for recycling to 5.

9. Filtration of the lixiviation pulp separated in 4 from the sulphur agglomerates.

10. Reception of the aqueous acid solution separated in 9 from the lixiviated pulp and reduction of Fe+++ to Fe++ in this solution.

11. Precipitation of Cu, particularly by Fe powder and separation of the precipitated metal or extraction of Cu by electrolysis.

12. Precipitation of ZnS, separation and discharge of the remaining solution.

13. Reception of the solids of the lixiviation pulp, separated from the solution in 9.

14. Cyanidation of the solids from 9 with an aqueous NaCN solution, separation of Ag and Au and recycling of the resultant mud to 11.

15. Destruction of the CN- ions remaining, particularly by means of H202 or NaOCI.

16. Discharge to waste of the residual mud, cleansed in 15.

While the process of the invention applies to minerals having variable contents of the various components, it is particularly of interest for iron minerals of the following compositions: Fe - 35 -47 % Pb O - 0.6 % S - 30 -53 As 0.1- 0.3 % C - 0 - 3 Ag 10 -50 ppm Cu - 0.4- 1 Au 1 - 2 ppm Zn - 0.5- 3 The Examples which follow illustrate the invention without limiting it.

EXAMPLE 1 (The operative steps of the scheme are indicated in parentheses).

A pyritic mineral is treated as a powder having a particle size below 80 ,a, containing: Fe 38.7 % 500 ppm As S 45.1 12 ppm Ag Zn 1.11 1.9 ppm Au Cu 0.58 1200 g of this powder is subjected to calcination under a nitrogen atmosphere at 800"C for 2 hour. The gases evolved during the heating are condensed; thus 239 g of sulphur is recovered having an arsenic content of 1300 ppm. After cooling the 959 g of powder which has undergone this thermal treatment, 90 g is set aside in order to be used in a reduction reaction in a later stage, while the remaining 869 g is mixed with 1500 g of water and 76 g of 66" Be sulphuric acid, namely 96% by weight.

This suspension is put into an autoclave, taken to llO"C and air is introduced under a pressure of 20 bars. The powder is thus left to react with the sulphuric solution with agitation for 3 hours. The suspension is then taken to 130"C, which produces fusion to the elementary sulphur formed during the lixiviation. This is agitated in order to agglomerate the sulphur into granules of about 1 mm diameter (scheme, box 2), after which the material is cooled by standing to below 120"C and placed on a screen. The granules of sulphur are thus separated (4) from the pulp derived from the lixiviation.

The granular sulphur is heated to 105"C with 2500 ml of toluene for 1 h, namely until complete dissolution. The organic solution obtained is filtered, for separation (6) of the solids not attacked during the lixiviation which have been entrained by the melted sulphur. These solids, principally blende, chalcopyrite, pyrrhotine and gangue, are recycled to the initial mineral for a new operation.

The toluene solution of S is then dearsenified (7) by treatment with 500 ml of 0.1 N caustic soda for 1 hour, after which the sulphur is obtained by crystallisation (8) on cooling, while the toluene mother liquor is recycled (5) for a new dissolution of the agglomerates or granules of S.

90.5% of the total sulphur of the mineral is recovered.

The pulp, separated (4) from the agglomerated sulphur, is filtered or centrifuged (9) in its turn.

Two products are then available: A-the cake (13) derived from the filtration (9), namely the solids remaining after the lixiviation (1), and B-the aqueous solution (10) separated from the solids A, which contains the metals Cu and Zn.

Treatment of cake A These solids mainly comprising goethite can include noble metals.

They are treated with milk of lime and then with an aqueous solution of 3 g NaCN per litre.

This extracts in the form of an aqueous solution 85.6% of the Au and 33.8% of the Ag from the minerals used. The solid residue is treated with hydrogen peroxide in order to destroy the CN ions before being discarded.

Treatment of the solution B As this solution (10) contains trivalent iron which disturbs subsequent precipitation of copper and zinc, 90 g of the calcine set aside previously is added to it, the mixture is agitated and then heated to 80"C for 1 hour.

After decantation, the solution recovered (11) is free from trivalent iron and it can thus be subjected to separation of the copper and the zinc. By the addition of 4.7 g of iron powder to this solution, 5.1 g of copper is recovered for a yield of copper of 74% based on the initial mineral.

The solution remaining after separation of the copper and thus having a pH of about 2 is treated with H25 (12) derived from the reaction of a part of the calcine with sulphuric acid. Thus 17.1 g of ZnS is recovered, which represents a zinc recovery of 86% based on the initial mineral.

EXAMPLE 2 The operations described in Example 1 have been applied to a mineral which contains: Fe 44% Pb 0.48% S 34 As 0.23% C 2.3 Ag 34 ppm Cu 0.62 Au 1.38 ppm Zn 2.24 The distribution calculated for the iron compounds shows: FeS2 50.3% FeS 15.4% FeCO3 22.2% In the calcination, 9% of the initial sulphur is converted to SO2 due to the presence of carbonate, this SO2 being utilised to produce sulphuric acid. The main part of this acid is used in the stage of lixiviation of the calcined mineral and the remainder serves to form the H25 necessary for precipitation of the ZnS by reaction with a part of the calcined mineral. Also, the sulphur liberated by the calcination is introduced into the suspension of the calcined mineral introduced into the autoclave and is not added to the toluene solution of the sulphur (5); as a result, the solution of the sulphur produced contains no more than 30 ppm of arsenic with respect to the sulphur.

in this Example, the sulphur was obtained in a yield of 82% with respect to the total sulphur of the mineral.

EXAMPLE 3 The operations of Example 1 are repeated, but the agglomerates of sulphur (5) are treated in (5) by 2500 ml of perchlorethylene in place of toluene. The results are the same as in Example 1.

Claims (17)

1. Process of production of elementary sulphur from an iron sulphide mineral, which comprises the oxidising acid lixiviation of the pulverised mineral for extraction of the sulphur, the pulp obtained from the lixiviation being subjected to separation of its solids (13) from the solution (10) containing them, characterised in that before this separation, the elementary sulphur formed by the lixiviation is melted and put into the form of agglomerates which are separated from the pulp, then the agglomerates are dissolved in an organic solvent for sulphur, which is then recovered by crystallisation with cooling (8), while the solution (10) is treated for the extraction of non-ferrous metals.

2. Process according to claim 1, characterised in that the solvent for the sulphur is an aromatic hydrocarbon or a chlorinated hydrocarbon.

3. Process according to claim 1 or 2, characterised in that the trivalent iron compounds present in the aqueous solution (10) formed by the lixiviation are reduced, before this solution is subjected to extraction of the non-ferrous metals (11, 12).

4. Process according to any of claims 1 to 3, characterised in that before the lixiviation the mineral powder is calcined, to convert the FeS2 which it can contain into pyrrhotine, and the vapourised sulphur is recovered.

5. Process according to claim 4, characterised in that the calcination of the mineral liberates SO2, which is converted into sulphuric acid, a part of this acid being used in the lixiviation stage.

6. Process according to claim 3, characterised in that reduction of the trivalent iron is effected by heating the solution obtained from the lixiviation with a powder containing FeS, particularly with a pyrrhotic mineral or with the powder calcined according to claim 4.

7. Process according to any of claims 3 to 6, characterised in that the reduced aqueous solution is treated with iron powder to precipitate copper.

8. Process according to claim 7, characterised in that the solution remaining after separation of the copper is subjected to precipitation of the zinc in the form of the sulphide.

9. Process according to claim 8, characterised in that precipitation of ZnS is effected with H25 formed by treatment of a part of the mineral powder calcined according to claim 4 or a pyrrhotic mineral by means of sulphuric acid.

10. Process according to claim 9, characterised in that the sulphuric acid used for production of the H25 necessary for precipitation of the ZnS is derived from a part of the sulphuric acid obtained according to claim 5.

11. Process according to any of claims 1 to 10, characterised in that the mineral contains arsenic and in that the organic phase consisting of a solution of sulphur in the organic solvent is subjected to dearsenification treatment before separating the sulphur by crystallisation.

12. Process according to claim 11, characterised in that the mineral is calcined according to claim 4 and the sulphur vapourised is dissolved in the organic phase before subjecting the latter to the dearsenification treatment.

13. Process according to claim 11, characterised in that the mineral is calcined according to claim 4 and the sulphur vapourised during this operation is added to the calcined mineral subjected to lixiviation.

14. Process according to any of claims 11 to 13, characterised in that the dearsenification treatment comprises washing the organic phase comprising the solution of sulphur in the organic solvent by means of an aqueous alkaline solution and in particular a dilute aqueous alkaline solution or milk of lime.

15. Process according to any of claims 1 to 14, in which the solid residue (13) contains precious metals, particularly gold and silver, characterised in that the solid is treated with milk or lime, then with an aqueous alkaline cyanide solution for extraction of these metals.

16. A process according to claim 1, substantially as described with reference to the foregoing Examples.

17. Sulphur, when produced by a process according to any preceding claim.