PROCESS FOR RECOVERY OF ZINC FROM SPHALERITE CONTAINING ORES OR CONCENTRATES

Abstract: The invention concerns a process for recovery of zinc from sphalerite containing ores or concentrates, whereby, in a first step, these ores or concentrates are submitted to a thermal treatment to convert sphalerite to a substance that is more readily attacked in hydrometallurgical media and, in a second step, said substance is leached, whereby said thermal treatment consists essentially of a heating cycle performed in such conditions that sphalerite is, at least partly, transformed to wurtzite, and of a rapid cooling cycle, performed in such conditions as to retain, at least partly, this wurtzite at low temperature.

Description: Process for recovery of zinc from sphalerite containing ores or concentrates. The invention concerns a process for recovery of zinc from sphalerite containing ores or concentrates, whereby, in a first step, these ores or concentrates are submitted to a thermal treatment to convert sphalerite to a substance that is more readily attacked in hydrometallurgical media and, in a second step, said substance is leached. According to the state of the art, sphalerite containing ores or concentrates are submitted to a roasting step wherein the ore or concentrate is oxidized at an elevated temperature. In this roasting step, zinc sulfide is converted to zinc oxide while liberating SO 2 and heat. Subsequently zinc oxide is leached to bring it into solution. This traditional flowsheet has several disadvantages: the SO 2 has to be further processed and results in a high amount of sulfuric acid. Also, the leaching which is carried out in sulfuric acid, normally necessitates two steps, whereby in a first step the ore or concentrate is submitted to neutral leaching and, in a second step, the remaining zinc ferrites are conducted to strong acid leaching. The iron which has dissolved during the strong acid leaching is precipitated, depending upon the method, as jarosite, goethite or hematite. The invention has as an aim, among others, to present a process which renders it possible to generate much less acid than in the process according to the state of the art and to carry out the leaching in a single step instead of two. For this purpose, according to the invention, said thermal treatment consists essentially of a heating cycle performed in such conditions that sphalerite is, at least partly, transformed to wurtzite, and of a rapid cooling cycle, performed in such conditions as to retain, at least partly, this wurtzite at low temperature. In a preferred embodiment of the process according to the invention, said thermal treatment is performed in non-oxidizing conditions. Particularly, said thermal treatment has to be performed at a temperature above the inversion temperature of sphalerite to wurtzite. Usually, a temperature above 900 °C and preferably above 1020 C° is adequate to perform said inversion. It is presumed that this inversion temperature is function of the amount of impurities in the sphalerite, in such a way that, the larger the amount of iron dissolved in sphalerite, the lower said inversion temperature would be. According to a preferred embodiment of the invention, the said rapid cooling cycle is carried out by quenching in water or in said hydrometallurgical media. Leaching in the second step can take place in all kinds of acid, neutral or basic hydrometallurgical media capable of leaching zinc out of wurtzite. Although, according to the state of the art, hydrometallurgical media used for leaching usually contain sulfuric acid, it is also possible to use solutions containing HC1, NaOH, NH ions (e.g. NH 4 CI solutions) or others; HC1 and or H 2 SO 4 are used in a preferred embodiment of the invention. Leaching can be performed under non-oxidizing conditions or under oxidizing conditions, for instance with O 2 or Cl 2 as oxidizing agent. After leaching, the zinc containing solution is purified and subsequently zinc is recovered from this solution by electrolysis. Other characteristics and advantages of the invention will result from the following description of a specific embodiments of the process for treating ores or bulk concentrates according to the invention; this description is given for the sake of example only, without limiting the scope of the invention. The following comparative experiment on two samples of a zinc sulfide concentrate from Brunswick Mining and Smelting is given as an example. The approximate wt% analysis of this concentrate is: 52 Zn, 10 Fe, 2.0 Pb, 0.4 Cu, 33.6 S and 2 wt % of typical impurities including SiO 2 , MgO, Al 2 O 3 and CaO. One sample has been heated to a temperature of about 1100° C and was cooled down quickly. Subsequently, from each sample 100 g was leached in 1 liter of a 6 N HC1 solution at 90°C. For the heat treated sample a residue of only 4 g remained, while for the non-treated sample a residue of 50 g remained. When heating the concentrate to the temperature of 1100 °C, the sphalerite of the sample transformed to wurtzite, which was retained by rapid cooling by means of a quench in water. Consequently, it has been proven that the non-oxidizing leaching of ZnS, according to the reaction ZnS + 2 T → Zn ++ + H 2 S, can be promoted by transforming sphalerite into wurtzite by a combination of heat treatment and rapid cooling. In this experiment, the wurtzite leaching reaction produced ZnCl 2 and H 2 S. Zinc can be recovered from the solution by electrolysis, after the necessary purification is performed. The electrolysis generates zinc at the cathode and Cl 2 at the anode. Particularly, this Cl 2 can be reacted with the H 2 S generated by the leaching reaction, thus producing elementary sulfur, which is easier to stockpile than sulfuric acid, and HC1, which can be recycled to the leaching step. Alternatively, the H 2 S and or Cl 2 can be used directly as a reagent in a large number of other hydrometallurgical operations. The invention is of course not limited to the example and description of the embodiments of the process given hereinabove.

Claims: Claims 1. Process for recovery of zinc from sphalerite containing ores or concentrates, whereby, in a first step, these ores or concentrates are submitted to a thermal treatment to convert sphalerite to a substance that is more readily attacked in hydrometallurgical media and, in a second step, said substance is leached, characterized in that said thermal treatment consists essentially of a heating cycle performed in such conditions that sphalerite is, at least partly, transformed to wurtzite, and of a rapid cooling cycle, performed in such conditions as to retain, at least partly, this wurtzite at low temperature. 2. Process according to claim 1, characterized in that said thermal treatment is performed under non-oxidizing conditions. 3. Process according to claims 1 or 2, characterized in that said thermal treatment is performed above 900 °C , preferably above 1020°C. 4. Process according to any one of claims 1 to 3, characterized in that the said rapid cooling cycle is carried out by quenching in water or in said hydrometallurgical media. 5. Process according to any one of claims 1 to 4, characterized in that, the second step comprises leaching in an acid solution. 6. Process according to claim 5, characterized in that said acid solution contains hydrochloric acid and/or sulfuric acid. 7. Process according to any one of claims 1 to 6, characterized in that the leaching is performed under non-oxidizing conditions. 8. Process according to any one of claims 1 to 7, characterized in that the H 2 S generated in the leaching is recovered and used in any other chemical unit operation. 9. Process according to any one of claims 1 to 8, characterized in that the leaching is performed under oxidizing conditions e.g. with O 2 or Cl 2 as oxidizing agent. 10. Process according to any one of claims 1 to 9, characterized in that the leaching is performed in presence of NH + ions. 11. Process according to any one of claims 1 to 4 and 7 to 10, characterized in that the leaching is performed in a neutral or alkaline solution. 12. Process according to any one of claims 1 to 1 1, characterized in that the solution obtained in the second step is purified. 13. Process according to any one of claims 1 to 12, characterized in that zinc is recovered from the solution by electrolysis.

Inventors: BROUWER SYBOLT, VANHOUTTE DIRK, BROUWER, SYBOLT, VANHOUTTE, DIRK
Applicants: BROUWER SYBOLT, UNION MINIERE SA, VANHOUTTE DIRK, BROUWER, SYBOLT, N.V. UNION MINIERE S.A., VANHOUTTE, DIRK
Application year: 1997
Main classification: C22B19
IPC codes: C22B1, C22B3, C22B19, C25C1