GB2189234A

GB2189234A - Process for the production of elementary sulphur from an iron sulphide mineral

Info

Publication number: GB2189234A
Application number: GB08709046A
Authority: GB
Inventors: Didier Anglerot
Original assignee: Societe National Elf Aquitaine
Current assignee: Societe National Elf Aquitaine
Priority date: 1986-04-17
Filing date: 1987-04-15
Publication date: 1987-10-21
Also published as: FR2597466B1; MA20943A1; FR2597466A1; AU7172587A; SE8701604D0; SE8701604L; PT84710A; IT8720141A0; FI871714A0; GB8709046D0; NO871607L; NO871607D0; PT84710B; DE3713098A1; IT1203906B; FI871714A; ES2003254A6

Description

SPECIFICATION Process for the production of elementary sulphur from an iron sulphide mineral The invention relates to a process for the preparation of elementary sulphur from minerals containing iron sulphides, particularly pyrites and pyrrhotine; it relates to a sequence of chemical operations for liberation of the sulphur from these sulphides. Such minerals can in general contain other minerals, in particular compounds of Cu, Zn, Pb, Au, Ag, As etc., so that the process of the invention applies to the recovery of these useful elements at the same time as the sulphur.

The extraction of sulphur in the elementary state from sulphided minerals and in particular from pyrites is known and has given rise to a number of studies during the last three decades.

Reference can be made to the prior art in US Patent 2898197, which describes the oxidizing lixiviation of minerals containing pyrrhotic sulphides and the subsequent recovery of the sulphur liberated, as well as the non-ferrous metals. However, the known technique leaves much to be desired from the standpoint of economics and also from practical execution. In fact, as the oxidising lixiviation leads to the formation of a finely divided solid phase containing the sulphur, in suspension in a liquid phase, which contains the non-ferrous metals in solution, it is difficult to separate these metals from the divided solid, because of the difficulties of filtration of the latter.

The present invention relates to a marked improvement in effecting the recovery of sulphur and the other useful elements by oxidising lixiviation, which allows this recovery to be carried out more conveniently than in the past and with better yields.

The process according to the invention is particularly well suited to the recovery of sulphur in arseniferous minerals and also it makes more economical the recovery of the metals copper and zinc. Moreover, the sulphur obtained by this process is of excellent quality.

The process according to the invention applies to various minerals principally containing the sulphides of iron of the pyrites type FeS2 and/or pyrrhotine FeS, which can contain various other minerals as indicated above. Generally, the mineral contains 30 to 40% of Fe, 30 to 53% of S and can contain variable quantities of the other elements.

When the mineral. contains pyrites FeS2, the process comprises first a calcination of the crushed mineral in a generally non-oxidising atmosphere and recovery of the sulphur volatilised by this heating step. This operation is based on the decomposition of the pyrites present according to the reaction: FeS2-~FeS+1/2 Soft.

It is not useful when the mineral os entirely pyrrhotic and practically free from FeS2.

For minerals containing carbonates or oxides capable of being reduced by the sulphur vapour, the calcination gives rise to a certain proportion of SO2 which is employed for the production of sulphuric acid which is then used in one or more of the subsequent stages of the process.

When the mineral does not contain compounds reducible by the sulphur vapour and thus does not theoretically allow the formation of SO2 during the calcination, it is possible nevertheless to produce the desired quantity of SO2 by carrying out this calcination in the presence of a controlled quantity of oxygen. The reaction FeS2+02- - FeS+SO2 is exothermic which also allows a substantial improvement in the thermal balance of the calcination of the mineral.

The powder resulting from the calcination which no longer contains pyrrhotine (FeS) nor iron sulphide and possibly the oxides, is put into suspension in acidic water and heated in an autoclave under oxygen or air pressure until there is no longer formation of elementary sulphur.

The principal reaction in this lixiviation can be written diagrammatically.

1 3 FeS+ H2O + O2~S + FeOOH (goethite) 2 4 The non ferrous metals, particularly Cu and Zn pass into aqueous solution, while the noble metals Au, Ag remain in the solid phase.

When the calcination of the mineral leads to the formation of a certain quantity of SO2, which is transformed into sulphuric acid, the water necessary for lixiviation of the calcined mineral can advantageously consist of a solution at the desired concentration of at least a part of the sulphuric acid so obtained.

The product resulting from the lixiviation consists of a pulp of a solid which contains the sulphur and if present in the mineral the precious metals such as Au, Ag and arsenic; the aqueous phase of the pulp contains in solution metals such as Cu and Zn. This pulp is subjected to a pressure substantially equal to the atmospheric pressure, and then undergoes a treatment for separation of the sulphur and the metals which it contains. As the expansion pressure cools the pulp, this is under a temperature low than that of the lixiviation, generally below 100 C.

The treatment according to the invention is characterised in that the solids are separated from the liquids which accompanies them in the pulp and then the sulphur is extracted from these solids by means of a suitable organic solvent.

The separation of the solids can be carried out by any known means and more particularly by filtration or centrifugation; the latter allows reduction to a minimum of the humidity of the solids.

The important step according to the invention, dissolution of the sulphur freed by the lixiviation, is effected by means of an organic solvent capable of dissolving sulphur hot. In contrast to the procedure where the aqueous pulp as such as treated by a solvent which must have a density higher than 1, to avoid difficulties with the subsequent filtration, because of entry of goethite into suspension in the aqueous phase, the present process can be carried out with solvents of any density, lower or higher than 1. Thus, use can be made of liquid hydrocarbons, halogenated hydrocarbons, thioalkanes etc. By way of non-limitative Examples reference is made to compounds such as benzene, toluene, ethyl-benzene, xylene, dichlorobenzene, dichloromethane, trichloromethane, dichloroethylene, trichloroethylene, perchloroethylene, carbon sulphide, diethyl sulphide, dipropyl sulphide etc.

Extraction of the sulphur from the solids of the pulp lixiviated takes place in the hot, preferably around the boiling point of the solvent, at this boiling point or even at a higher temperature under pressure. The solution obtained is separated from the solids of the pulp by means known in the art, particularly decantation and/or filtration or even by centrifugation; the subsequent cooling of this solution allows crystallisation of the sulphur which is recovered, while the solvent is recycled for a further extraction step.

When the sulphur extracted from the mineral in this way contains arsenic, the process according to the invention comprises the dearsenification of the organic solution, before crystallisation of the sulphur. A practical means consists in passing this solution over the hot solid or over an adsorbent such as silica, clay, alumina or the like or of agitating it with a substance capable of fixing As. This substance can then be regenerated by elution with an aqueous alkaline solution. Another means for dearsenification comprises treatment of the organic solution with an aqueous alkaline solution or suspension, for example milk of lime or a solution of NaOH or NH4OH.

When the mineral is subjected to preliminary calcination, mentioned above, with volatilisation of the sulphur and the latter contains arsenic, the best procedure consists in reuniting the volatilised sulphur with the organic solution before the dearsenification of the latter.

Parallel to the organic solution of sulphur in the process according to the invention, the solid derived from the lixiviation pulp is treated, which is left after treatment with the organic solvent.

This solid can contain gold and silver; if this is the case, it is agitated with an alkaline cyanide solution after neutralisation with milk or lime; from the cyanide solution obtained Ag and Au are recovered in known manner, while the solid residue is cleansed with hydrogen peroxide before being discarded.

The aqueous solution which has been separated from the pulp before undergoing lixiviation has a pH of about 2 to 3 and generally contains compounds of useful non-ferrous metals, particularly Cu and Zn as indicated above. With a view to extraction of these metals, it is preferable first to reduce to the state of Fetid the trivalent iron which this solution contains. Cu is precipitated by cementation, particularly with Fe powder, after which the Zn can be recovered, for example by precipitation with H2S.

As regards preliminary production of Fez++ mentioned above, it should be noted that this can be realised by any means known in the art, particularly by the addition of a suitable reducing agent which is economically viable, such as for example SO2, SH2 or an organic substance.

However, a preferred form of operation which is particularly interesting as regards the cost of the process consists in utilising as the reducing agent iron monosulphide, notably Fe S. This compound can be taken in the form of natural pyrrhotine, that is a mineral not containing FeS2 in practice. It can also be consistuted by a portion of the calcined mineral powder which no longer contains pyrrhotine, obtained in the first stage of the process explained above. A characteristic of the invention thus consists in utilising a purely pyrrhotic mineral or in first taking a portion of the calcined power in the first stage of the process and mixing and heating it with the filtered solution after the oxidising lixiviation, which forms the second stage of the process. The quantity of the calcined powder to be used is thus calculated on the basis of the content of ferric compounds in the solution resulting from the oxidising lixiviation.

The steps in the process are illustrated by the scheme in the single accompanying figure, in which Sections 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 and preferably from about 20 to 100 ; with 1.2 to 2 parts by weight of acidified water per part of powder, the acidity being from 0.7 to 1.5 equivalents per litre; heating between 100" and 1200C, preferably 105 to 115"C, under an air pressure of 5 to 30 bars for 2 to 6 hours.

2. Standing at atmospheric pressure with cooling so produced.

3. Separation of the pulp obtained into solids and solution; carried out by filtration, drying or centrifugation.

4. Extraction of the sulphur from the solid obtained in 3, by means of the organic solvent recycled from step 8; in general between 40 and 1200C, depending upon the nature of the solvent.

5. Separation, by decantation, filtration or centrifugation of the solution of sulphur in the organic solvent, with the solids of the pulp treated in 4.

6. Reception of the hot sulphur solution into the organic solvent.

7. Dearsenification of the sulphur, effected on the organic solution.

8. Cooling of the organic solution, crystallisation of the sulphur and taking up in the solvent for recycling to 4.

9. Reception of the solids separated in 5, from the sulphur solution in the organic solvent.

10. Cyaniding of the solids from 9 by means of an aqueous NaCN solution, separation of Ag and Au and evacuation of the remaining solids to 11.

11. Destruction of the remaining CN- ions, particularly by means of H202.

12. Evacuation to waste of the remaining material cleansed in 11.

13. Reception of the aqueous acid solution separated in 3 from the pulp lixiviated in 1.

14. Reduction of Fe+++ to Fe++ in the solution of 13.

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

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

While the process of the invention applies to minerals having variable contents of the various compositions, it is of particular interest for iron minerals of the following compositions: Fe - 35 - 47% Pb 0 - 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 A pyritic mineral in the form of a powder having a particle size below 80,u is treated, 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 are subjected to calcination under a nitrogen atmosphere at 800"C for 2 hour. The gases released during this heating are condensed; thus 239 g of sulphur is recovered having an arsenic content of 1300 ppm. After cooling 959 g of powder which has undergone this thermal treatment, 90 g is taken in order to be utilised in a reducing reaction in a subsequent stage, while the remaining 869 g is mixed with 1500 g of water and 70 g of sulphuric acid of 66" Be, namely 96% by weight.

This suspension is placed in an autoclave, taken to 110"C and then air is introduced under a pressure of 20 bars; the powder is then allowed to react with the sulphuric solution with agitation of 3 h. The suspension is then left which reduces the temperature to about 90"C, when it is filtered (step 3 in the scheme). Two products are then present; A-the cake (4) derived from the filtration (3), namely the remaining solids after the lixiviation (1), and B-the aqueous solution (13), separated from the solids A, which contains the metals Cu and Zn.

Treatment of the cake A These solids which contain about 40% moisture are mixed (4) with toluene at the rate of 2600 ml of the latter per 1000 g of cake and the mixture is maintained at 105"C for 1 h in order to effect dissolution of the elementary sulphur present. The solution of S in the toluene is then decanted (5) and partly recovered (6), to which sulphur liberated is added by the initial calcination at 800"C mentioned above. The total sulphur contains 550 ppm of arsenic. In order to eliminate this, the toluene solution is agitated with 2 g of Ca(OH)2 suspended in 500 ml of water, which is then separated by decantation from the aqueous layer which contains all the As of the sulphur.The organic solution is then cooled (8) which gives crystallisation of the sulphur free from As, which is then recovered in a yield of 92.8% with respect tothe initial S of the mineral; the solvent is recycled (to 4) for a new sulphur extraction.

The solids (9) separated from the toluene solution are treated with milk of lime and then with an aqueous solution of 3 g NaCN per litre. This extracts in the form of the aqueous solution 86% of the Au and 34% of the Ag from the mineral 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 (13) contains trivalent iron, which affects the subsequent precipitation of copper and zinc, there is added to it (14) 90 g of the calcine used in the preceding step and it is then agitated and heated to 80"C for 1 h.

After decantation, the solution recovered (15) is free from trivalent iron and 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, 5.25 g of copper is recovered or a yield of copper of 75% on the initial mineral.

The solution remaining after separation of the copper and thus having a pH of about 2 is treated with H25 (16) derived from the reaction of a part of the calcine with the sulphuric acid.

Thus, 16.8 g of ZnS is recovered, which represents a zinc recovery of 84.5% 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 calculated distribution of the iron compounds amounts to FeS2 50.3% FeS 15.4% CeCO3 22.2% By calcination 9% of the initial sulphur is transformed into SO2 because of the presence of the carbonate and this SO2 is utilised to produce the sulphuric acid. The major part of this acid is employed in the lixiviation stage of the calcined mineral and the remainder serve to form the H2S necessary for precipitation of the ZnS by reaction with a part of the calcined mineral. Also, the sulphur liberated from the calcination is introduced into the suspension of the calcined mineral entering the autoclave and is not added to the toluene solution of the sulphur obtained after filtration of a pulp at 3 (see the scheme); as a result, the solution of the sulphur produced does not contain more than 30 ppm of arsenic with respect to the sulphur.

In this Example, the sulphur has been obtained with a yield of 82.8% with respect to the total sulphur of the mineral.

EXAMPLE 3 The operations of Example 1 are repeated, but the filtration cake from the lixiviated pulp (3) is treated in (4) with 2500 ml of perchlorethylene in place of toluene. The results are the same as in Example 1.

Claims

1. Process of preparation of elementary sulphur from an iron sulphide mineral, which comprises the oxidising acid lixiviation of the pulverised mineral with a view to extraction of the sulphur, the pulp obtained from the lixiviation being subjected to separation of its solids (4) from the solution (13) which contains them, characterised in that the solids are treated in the hot with an organic solvent for sulphur, which is then recovered by crystallisation after cooling (8) while the solution (13) is treated for extraction of the 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 are reduced which are present in the aqueous solution (13) obtained from the lixiviation, before subjecting this solution to extraction of the non-ferrous metals (15,16).

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

5. Process according to claim 4, characterised in that the calcination of the mineral releases SO2, which is converted to sulphuric acid, a part of this acid being utilised 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 the 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 the precipitation of the ZnS is realised with the aid of H2S formed by treatment of a part of the powder of the calcined mineral according to claim 4 or of a pyrrhotic mineral by means of sulphuric acid.

10. Process according to claim 9, characterised in that the sulphuric acid utilised for production of the H2S necessary for the 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 that the organic phase consisting of a solution of the sulphur in the organic solvent is subjected to treatment for dearsenification 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 vaporised 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 vaporised 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 consists in washing the organic phase constituted by 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 from the extraction of the sulphur with the organic solvent contains precious metals, particularly gold and silver, characterised in that the solid is treated with milk of lime and then with an aqueous solution of alkaline cyanide for extraction of these metals.

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

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