GB2189236A - Extraction of sulphur from pyritic minerals - Google Patents

Description

GB2189236A 1

SPECIFICATION

Process for extraction of sulphur from pyritic minerals The invention relates to a process for obtaining elemental sulphur starting from minerals contain- 5 ing iron sulphides, notably pyrites and pyrrhotine; it comprises a sequence of chemical oper ations for liberation of sulphur from these sulphides. As such minerals can in general contain other minerals, in particular compounds of Cu, Zn, Pb, Au, Ag, As etc., the process of the invention applies to the recovery of these elements at the same time as sulphur.

The extraction of sulphur in the elemental state, starting from sulphurbearing minerals, and in 10 particular starting from pyrites, is known and has given rise to a certain number of studies during the last three decades. This prior technique is known from the patent US 2 898 197 which describes the oxidising lixiviation of minerals containing pyrrhotic sulphides and the subsequent recovery of the liberated sulphur, as well as non-ferrous metals. However, the known technique leaves much to be desired both from the economic point of view and from that of 15 practical execution. In practice, the oxidising Iixiviation leads to the formation of a sulphur containing finely divided solid phase in suspension in a liquid phase which contains the non ferrous metals in solution: it is difficult to separate these finely divided solid metals because of difficulties of filtration. Besides, the lixiviation requires severe operating conditions, blende and chalcopyrite being more difficult to oxidise than pyrrhotine. It is also to be noted that the 20 oxidised lixiviation must in general be preceded by an energy-consuming calcination, in particular when the mineral contains carbonates or other compounds which decompose endothermically.

The present invention overcomes the disadvantages mentioned; it brings about a marked improvement in carrying-out recovery of sulphur and non-ferrous elements. It comprises a suc cession of operations allowing this recovery to be effected more conveniently than in the past 25 and with better yields.

The process according to the invention can be applied to different minerals containing princi pally iron sulphides of the type pyrites FeS, and/or pyrrhotine FeS, which can contain different other minerals, as indicated above. Generally, the mineral contains 30 to 47% of Fe, 30 to 53% of S and can contain variable quantities of other elements. 30 The invention allows one to obtain pyrites free from compounds, notably carbonates, which consume energy at the time of calcination. It makes possible the application of milder conditions to oxidising lixiviation. Besides, the process according to the invention does not involve the treatment, after lixiviation, of an aqueous solution, and thus avoids more or less difficult filtra tions. 35 The process according to the invention, which comprises forming elemental sulphur starting from an iron sulphide-containing mineral by oxidising lixiviation, then extraction of the sulphur formed by means of an organic compound, is characterised in that the lixiviation is preceded by concentration, by flotation, of the iron sulphides.

Thus, in the present case, where the iron sulphides are associated with those of Cu, Pb, Zn, 40 one separates first the sulphides of these non-ferrous heavy metals by flotation then the gangue, to isolate a concentrate of iron sulphides, and one subjects to oxidising lixiviation the concen trate of iron sulphide thus obtained. When this concentrate contains pyrites FeS, besides pyrrhotine FeS, it is to be recommended to submit it to a thermal treatment allowing liberation of sulphur, the vapour of which is condensed and recovered whereas the mineral, thus calcined, 45 is subjected to the lixiviation. It can be seen that the process of the invention gives a marked improvement in the lixiviation operation, since the latter is carried out on a concentrate which contains neither sterile materials nor sulphides more difficult Iixiviate, such as those of copper and zinc.

The thermal treatment indicated above is in general effected between 600 deg C and 800 deg 50 C, preferably at about 750 deg C, in a non-oxidising atmosphere; it effects the reaction FeS2.FeS+1/2 S2.

The preliminary flotation is realised in the manner known per se, that is on a pulp, containing generally 20 to 25% of mineral in fine powder form of 10 to 200 urn, preferably 20 to 100 urn, in the presence of 50 to 200 g of each of foaming agent and collector per tonne of mineral. 55 Various foaming agents known in the art can be employed, for example methyl isobutyl carbinol ("MIBC"), 4-methyl pentan-2-ol, methyl ethers of propylene glycol, various terpenes, creosote etc. As collectors one can use, for example, a xanthate of the type ROCSSNa, a dithiophosphate (R0)2PSSNa, monothiocarbamate Ri-IN-CS-OR, mercaptans RSH, polysulphides R'S,,R, salts of thioacids such as R'SCH2CH2COOH or thioamides, R being preferably a C2 to C, alkyl and R' a C, 60 to C, alky]. When the mineral contains zinc sulphides, the pulp contains additionally, in apporpri ate quantities, the usual depressants and activants for these sulphides.

The flotation is carried out in a manner to separate as well as possible non-ferrous heavy metal (principally Cu and Zn) sulphides from pyrites and pyrrohtine, that is to say from the iron sulphides. The latter, separated by flotation from their ganque, are subjected, when the propor- 65 2 GB 2 189.236A 2 tion of pyrites FeS2 is considerable, to the calcination mentioned above, before proceeding to Iixiviation.

The oxidising lixiviation is effected by heating a pulp formed from 1 part by weight of mineral concentrate and 1 to 2 parts of water, particularly 1. 1 to 1.3 parts of acidified water in an autoclave, in general between 100 deg C and 120 deg C. Preferably, the acidity of the water, 5 expressed in acid equivalents, is from 0.7 to 1.5 per litre, preferably 0. 9 to 1.1 equivalents/litre.

Air is injected into the autoclave under a pressure of about 1 to 15 bars, instead of 5 to 30 bars which are necessary if one has not already separated the non-ferrous metal sulphide minerals. After 2 to 6 hours of such lixiviation, the contents of the autoclave are cooled to below 100 deg C and the pulp is mixed with an organic solvent for sulphur. 10 The solvents used for this purpose can be, for example, ones such as benzene, toluene, ethyl benzene, xylene, etc.

After the dissolution of the sulphur at around the boiling point of the solvent, the organic solution is separated, by simple decantation, from the aqueous suspension of non-extractable solids. It is preferable to pass the sulphur solution over lime, in an appropriate column, to 15 absorb arsenic present. One can also eliminate the arsenic present in the sulphur solution bypassing the solution through a bed of absorbants such as silica, clay or alumina which is regenerable by elution with the aid of a caustic soda solution or also by washing the sulphur solution with the aid of an aqueous alkaline solution such as milk of lime or a dilute solution of caustic soda or ammonia. 20 The cooling of this solution produces crystalisation of sulphur which can be separated by centrifuging, the solvent being re-utilised for a new extraction.

Within the scope of the process of the invention, it is possible to recover gold or silver which can be found in the mineral treated. For this, it is convenient to neutralise the solid residues remaining and to subject them to cyanidation, in order to disolve Au and Ag which one recovers 25 afterwards from their alkaline cyanide solution in the known manner.

In a variation, the elemental sulphur contained in the pulp from the oxidising Iixiviation is melted and put in the form of agglomerates which are separated from the pulp, then the agglomerates are dissolved in an organic solvent for sulphur whilst the pulp free from sulphur is treated to extract precious minerals which it can contain. 30 The operation of fusion and of agglomeration of liquid sulphur resulting from this fusion is - preferably carried out at about 130 deg C, with agitation such that the sulphur forms granules of a diameter of about 0.3 to 3 mm, preferably of the order of 1 mm; after cooling to below 120 deg C these solid granules can be separated from the Iixiviation pulp by simple sieving, the solids of the pulp having in general a granule size below 100 pm. 35 The sulphur granules, thus separated, are in general formed from 60 to 80% S, the remainder being constituted mainly by minerals not attacked in the course of the lixiviation. These residual materials separate easily from the organic sulphur solution and can be recycled to the starting minerals, that is to say, to the fixiviation.

It is interesting to note that, in the process of the invention, the bulk of the Iixiviation pulp, 40 which comprises a gangue with goethite resulting from attack of FeS, is not wetted by the melted sulphur and separates itself well from it; one thus has no, or very little, geothite in contact with organic solvent, at the granular sulphur dissolution giving considerable reduction of loss of solvent by absorbtion into the goethite.

The recovery of sulphur starting from its solution in the organic solvent as well as the 45 extraction of precious metals contained in the pulp obtained after separation of the sulphur can be realised as indicated above.

The following non-limiting example is given to illustrate the invention.

A mineral having the following composition was treated:

50 Fe 40.8% Zn 1. 12% S 32.4% Ag 16 g/t Cu 0.56% Au 1.28 g/t Pb 0.21 % As 0.11% 55 After crushing to a fine powder of which 80% has dimensions not greater than 35 urn, an aqueous dispersion of 24% of this powder was prepared, then three successive flotations carried out, namely of copper sulphide, of zinc sulphide then of iron sulphide, whilst regulating the pH by lime to a value between 10 and 11 and using, per tonne of mineral 400 9 of a thiocarbamate, 150 g of potassium amyixanthate, 100 g of Flotol B (terpene) and 500 g of 60 copper sulphate.

The three fractions of floated minerals, thus obtained, constituted the concentrates of interest, and a residue, essentially of silicates and of iron carbonate, that was rejected, remained.

obtained:

19.3 kg of copper concentrate with 20% coppper, 65 3 GB 2 189.236A 3 12.2 kg of zinc concentrate with 50% zinc, 573 kg of iron sulphide concentrate with 48% sulphur and 395.5 kg of sterile.

The iron sulphide concentrate was then submitted to thermal treatment under a neutral atmos phere, at 750 deg C, for twenty minutes. In the vapour given off, 119 kg of sulphur and 180 9 5 of arsenic were recovered.

The solid was then placed in an autoclave, into which were also introduced 650 kg of water, that is to say 1.13 times the weight the of third concentrate, and 35 kg of sulphuric acid. The mixture was agitated for three hours at 110 deg C under an air pressure of air corresponding to 2 bars of oxygen. This pressure was sufficient, whereas more than 5 bars is necessary if the 10 initial mineral is treated without prior separation of Cu and Zn. The mixture was cooled to 95 deg C. The pulp was mixed with 600 litres of toluene (519 kg) at 95 deg C. After complete dissolution of sulphur, the hot solution of sulphur in toluene and the aqueous suspension of nonextractable solids were separated by decantation.

The 119 kg of sulphur provided by the thermal decomposition were added to the hot solution, 15 the temperature being maintained between 95 deg C and 100 deg C. This solution was then passed through a column charged with lime which quantitatively fixed the arsenic. The solution was then cooled to 50 deg C: the sulphur crystalised and was separated by simple centrifuging, whilst the toluene thus separated was re-used. The solids of the aqueous suspension were neutralised to pH 10 with lime and mixed with 400 litres of an aqueous solution of 3 9/1 of 20 NaCn. 84% of the gold and 34.5% of the silver contained in the solids were recovered in the cyanide solution.

The residues, after separation of the liquor, were treated with oxygenated water, to eliminate the cyanide ions, then rejected.

The yield of sulphur represented 91.8% of the total sulphur in the iron sulphide concentrate. 25

Claims (11)

1. Process of extraction of sulphur from pyritic minerals, which comprises the oxidising lixiviation of pulverised mineral, followed by extraction by means of an organic solvant, charac terised in that the lixiviation is preceded by the separation of non- ferrous metal sulphides by 30 flotation.

2. Process according to claim 1, characterised in that the pulverised starting mineral is first subjected to flotation of copper sulphide compounds, in order to separate a copper concentrate.

3. Process according to claim 1 or 2, characterised in that, after the separation of copper concentrate, the remaining pulp is subjected to flotation of compounds of lead sulphide com- 35 pounds, if they are present, to separate from it a lead concentrate, then to flotation of zinc sulphide compounds to separate a zinc concentrate.

4. Process according to claims 1 to 3, characterised in that, after the separation of zinc concentrate, the remaining pulp is subjected to flotation of iron sulphide, to separate an iron concentrate from sterile residues. 40

5. Process according to claim 1 or 4, characterised in that before the lixiviation of iron concentrate, the latter is subjected to a thermal non-oxidising treatment, preferably between 600 deg C and 800 deg C, with recovery of volatilised sulphur.

6. Process according to claim 4 or 5, characterised in that the iron concentrate, as such or calcined, is subjected to the oxidising lixiviation in admixture with acidified water, between 100 45 deg C and 120 deg C, under a pressure of air of 1 to 15 bars.

7. Process according to claim 6, in which the pulp arising from the lixiviation is mixed hot with an organic solvent, capable of dissolving sulphur, after which the sulphur is recovered by cooling of its solution in the solvent.

8. Process according to claim 6, characterised in that the elemental sulphur contained in the 50 pulp resulting from the lixiviation is melted and put into the form of agglomorates which are separated from the pulp and in that the agglomorates of sulphur are dissolved hot in an organic solvent for sulphur, after which the sulphur is recovered by cooling of the solution in the solvent.

9. Process according to claim 7 or 8, characterised in that the organic solvent for sulphur is 55 treated with lime, in a manner to absorb the arsenic present in the sulphur.

10. Process according to claim 7 or 8, characterised in that the residual solids constituting the sulphur-free pulp are subjected to cyanidation to extract gold and silver.

11. A process according to claim 1, substantially as described with reference to the forego- ing Examples. 60 Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd, Dd 8991685, 1987. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 'I AY, from which copies may be obtained.