MXPA00004288A - Two-stage bioleaching of sulphidic materials containing metal values and arsenic - Google Patents

Abstract

A method of leaching sulphidic material that contains metals and arsenic under oxidising conditions and with the aid of microorganisms is characterised by mixing the material with an acid aqueous solution to form a pulp, and in that the material is leached in a first leaching stage at a pH of below 2 and at a temperature that ranges from room temperature to about 55 DEG C in the presence of an active quantity of microorganisms of the mesophilic and/or moderately thermophilic type, wherewith the major part of the arsenic content of the material and possibly also part of its metal content is/are leached-out and the arsenic leached from said material is oxidised successively to a pentavalent state. The remaining leachable metal content of the material is leached-out in a following leaching stage in the presence of an active quantity of an extremely thermophilic micro-organism and at a temperature within the active range of said micro-organism, said temperature being higher than the temperature in the first stage. The pH is suitably raised in the second stage to a level at which the pentavalent arsenic present in solution is caused to transform to a solid state and precipitate out as a metal-containing arsenate which is separate from the leaching solution together with unleached material in the pulp after termination of the leaching process.

Description

BIOLIXIVIATION OF SULFID MATERIAL DESCRIPTION OF THE INVENTION The present invention relates to a method for leaching sulfidic material containing metals and arsenic under oxidizing conditions, and with the help of microorganisms. It has been known for a long time to lixiviate sulfidic material, such as minerals and mineral concentrates, in the presence of microorganisms in the form of different types of bacteria capable of favoring the oxidation of both sulfur and iron and other metals in materials, with the object of extracting the valuable metal content from the materials. This type of leaching is also called biolixiviació. For example, valuable metals can be extracted by leaching and placed in solution, which is then treated for the selective extraction of valuable metals, such as copper, nickel, cobalt, uranium and zinc. The content of noble or precious metals that can not be directly recovered by leaching in this way, for example the content of noble metal or precious metal of refractory materials such as iron pyrite and arsenopyrite, can be recovered by first dissolving the adjoining metal sulfides. to release the precious or noble metals, and after that treat the bioleaching residue hydrometallurgically in a conventional manner, to extract by leaching the precious or noble metals, for example by treating them with cyanide. Bioleaching processes provide certain advantages over other possible hydrometallurgical processes for treating metal sulphide material, for example pressure leaching, by virtue of the fact that the bacteria favor the oxidation of both the sulphide sulfur and the elemental sulfur to form a sulfate, Oxidation of Fe (II) to Fe (III), as well as from As (III) to As (V) is also favored. The material leached with bacteria can then be leached later in subsequent stages, for example in a process of recovery of precious metals, without the risk of problems caused by the presence of elemental sulfur. However, a serious disadvantage with bioleaching is that very long leaching times at room temperature are required to achieve sufficiently high metal yields. Consequently, it is necessary to work at high temperatures, and with this accelerate the leaching process, so that the leaching can be carried out within periods of reasonable duration. The bioleaching of different types of sulfidic materials with the help of various types of microorganisms is described in our previous publication US-A-5, 397, 380, while the basic antecedent technique in this field can be found in AU-A-11201 / 92, CA- - 1, 023, 947 and US -A-4, 571, 387, for example. To accelerate the leaching process and thereby improve the efficiency of the metal extraction process during reasonable leaching times, it is thus necessary to leach at high temperatures, with the help of special thermotolerant (thermophilic) bacterial cultures, such as those proposed in WO 92/16669, which describes the leaching of refractory sulfide material. With respect to their ability to withstand high temperatures, the bacterial cultures concerned can be divided into three groups, ie mesophilic bacteria, for example Thi oba ci ll ferroxy dans, which have a range of use of up to 40 ° C to at most, moderate thermophilic bacteria (thermotolerant) that have a range of use up to about 50-55 ° C, and extremely thermophilic bacteria, some of which can be used up to a temperature of about 90 ° C, although the Most can only be used effectively at temperatures of 65-70 ° C. Several investigations, in which thermotolerant cultures have been used to bioleer different sulfidic materials have been presented in the scientific literature in recent times. For example, E. B. Lindstrom et al. : J. Ind. Microbiol. (1990) 5: 375-382, describes experiments in relation to the leaching of arsenopyrite with the help of extreme thermophilic Sulfolobus cultures, O. H. Tuovinen et al .: Appl. Environ. Microbiol. (1994) 60: 3268-3274, describes experiments regarding the leaching of arsenopyrite with mesophilic and moderate thermotolerant bacteria, A. Sandstrom et al.; Hydrometallurgy (1997) 46: 181-190, which discusses the bioleaching of sulfidic minerals with the same type of bacteria as the previous reference, and K. B. Hallberg et al .: Appl. Microbiol. Biotechnol. (1996) 45: 212-216; which discusses the toxicity of arsenic with respect to the high-temperature bioleaching of arsenopyrite containing gold. During experiments carried out with the use of extremely thermophilic microorganisms, for example of the Sulfolobus metallicus type, it has been established, in several of the articles mentioned above, among others, that the possibility of using bioleaching at elevated temperatures is restricted by the presence of arsenic in the material, since arsenic tends to have a toxic effect on extremely thermophilic bacterial cultures, although not at the same high degree on moderate mesophilic and thermophilic cultures, and that this toxicity increases with arsenic concentrations more high in the material. In this regard, As (III) is particularly toxic and, unfortunately, even As (V) exhibits a toxicity that prevents it from being tolerated in large quantities. This toxicity is manifested by the inability of the bacteria to reproduce during the leaching process, which would otherwise take place under favorable conditions for reproduction, and is therefore never effective. Consequently, to make possible such bioleaching of arsenic-containing material, it is necessary to dilute the pulp concentration excessively, which can also be expressed as pulp density, that is, the ratio of material quantity to volume of leaching solution should be maintained low, so that it falls below the toxic limit of the crop concerned with respect to arsenic. It will be understood that this problem has, unfortunately, a highly negative effect on the economics of the bioleaching process with respect to the treatment of arsenic-containing materials. It is an object of the present invention to eliminate the problems that arsenic toxicity creates with respect to extremely thermophilic bacterial cultures, so that an economically attractive process can be provided to bio-excrete arsenic-containing high-purity minerals, and concentrates of such minerals. This object is achieved by the present invention, which comprises the aspects and steps specified in the appended claims. According to the invention, the material is first mixed with an aqueous solution acid to form a pulp, and most of the arsenic contained in the material is extracted in a first stage of leaching, possibly together with a part of the metal content of the material. The leaching is carried out under oxidizing conditions, at a pulp pH below 2. In the presence of active amounts of microorganisms of the mesophilic and / or moderate thermophilic type, and at a temperature that is in the range from room temperature to 60. ° C - the leached arsenic of the material is successively oxidized from a trivalent to a pentavalent state. During this first stage, the pulp toxicity will decrease successively, at the rate at which the Ad (III) / As (V) ratio decreases with the increasing degree of oxidation of the pulp. The remaining leachable metal content of the material is leached from it in a subsequent leaching stage, under conditions that are favorable for the growth of extremely thermophilic bacterial cultures, the leaching process is thus carried out in the presence of an amount of an extremely thermophilic microorganism, preferably of the type Sul fol obus me tal li cus, after raising the temperature to a level within the active range of the thermophilic microorganism. As a result of raising the temperature in the second stage, the As (V) will tend to separate by precipitation progressively in the form of different metal arsenates, for example iron arsenates. The equilibrium that determines the residual content of As (III) in the solution will be displaced with it to the right, that is, in a direction towards As (V), and the content of As (III) highly toxic will decrease even more, in accordance with the amount of arsenate that is separated by precipitation, resulting in a significant reduction in the total toxicity of the pulp. Compared to a corresponding one-stage process, the two-stage process according to the invention allows a higher pulp density, in practice a much higher pulp density, to be used from the very beginning of the process. The present invention is based, inter alia, on the understanding that extremely thermophilic microorganisms will survive the presence of moderate mesophilic and thermophilic microorganisms, and hence may already be present, albeit inactive, in the first leaching stage, and then ee will cause an effective population to reproduce during the second stage, in which favorable conditions are selected for such growth. Pulp toxicity can be further reduced in the second stage of leaching, by appropriately raising the pH of the pulp to a level at which the pentavalent arsenic formed previously present in solution is caused to transform to a solid state and it is separated by precipitation as a metallic arsenate, which is separated from the leaching solution together with the unleached material with the completion of the leaching process. The elevation of the pH in the second stage of the leaching in addition to the increase in temperature mentioned above, for example an elevation at pH >; 1.5, accelerates the precipitation of arsates and consequently the equilibrium As (III) E > As (V) will be moved further to the right, to obtain a total arsenic content that can be tolerated by the extremely thermophilic culture in the second stage of leaching. The first stage is preferably carried out at a temperature of 45-55 ° C, and at a pH of 1.0-1.3, these intervals allow an optimum rate of leaching and oxidation of the arsenic to be maintained. The second stage of the leaching process is preferably carried out at a temperature of 65-70 ° C, and at a pH of 1.5-2.0, thereby maintaining the highest possible oxidation rate of another sulfide mineral in the material. The duration of the first stage is suitably selected so that a concentration of arsenic which is non-toxic to the selected thermophilic microorganism will be obtained in the first stage. The volume of leaching solution is suitably adapted in the process to result in a pulp density within the range of 10 to 25%. When arsenic-sulfur pyrites containing refractory gold, or concentrates of such minerals, are being biolixed, the waste separated from the leaching can be conveniently treated with cyanide at a subsequent stage, to recover the gold and other precious metals thereof, without a harmful effect to any elemental sulfur present. The invention will now be further described with reference to a flow diagram illustrating an example of a suitable process and an example embodiment. The flow diagram illustrates the bioleaching of materials that have a high arsenic content, which include different sulfide minerals or concentrates of such minerals that have values of metals that can be extracted either in the form of sulfides in a simple form and / or complex, or precious metals in refractory minerals, that is, minerals that are conred difficult to treat with the intention of extracting their content of precious metals, such as arsenopyrite (FeAsS) or pyrrhotite (Fen-? S). In the first stage of the bioleaching, the material is leached at a temperature of 45-50 ° C in the presence of active amounts of moderate mesophilic or thermophilic bacteria with an aqueous acidic solution, for example diluted sulfuric acid, to form a pulp that has a pH of 1-1.3. During the leaching process, arsenic is primarily separated by dissolving its sulfur mineral to form trivalent arsenic in solution, and is successively oxidized, by atmospheric oxygen supplied to the pulp, in coaction with the bacteria present in the pentavalent arsenic pulp , which begins to separate pro precipitation in the form of different metal arsenates. The leaching of the material in the first stage is continued while the arsenic content of the pula is higher than the value at which the arsenic is toxic to the extremely thermophilic bacterium that is already present in the first stage, although in an inactive state, but that can be reproduced at active quantities in the second stage of leaching, provided that the arsenic content in this stage is non-toxic. The sampling is carried out continuously, to determine an appropriate time in which the second stage of leaching can be initiated. The cultivation of extremely thermophilic bacteria in the pulp is activated, causing it to reproduce by raising the temperature to 60-65 ° C, and raising the pH to about 1.5 at the same time, successively cultivating a population of the bacterial culture that is active regarding the process. Other metals that can be leached present in the pulp are extracted by leaching at this stage of leaching, at the same time that the oxidation of As (III) to As (V) continues, and with this the arsenic is caused to separate by precipitation primarily as non-readily dissolved arsenates, such as iron arsenate and complex arsenate / iron compounds. Because the concentration of trivalent arsenic will fall rapidly as a result of this process, the toxic effect of the pulp on the extremely thermophilic bacteria will also fall at the same rate.

With the completion of the second stage of leaching, the pulp is separated into leaching residues and leaching solution, whereby the leaching residues can be discharged when essentially all of their valuable metal content has been extracted by leaching. With respect to refractory minerals containing a precious metal, this metal can be extracted in a subsequent stage of cyanide leaching. The leaching solution is then cleaned of its remaining arsenic content by adding lime, thereby allowing the resulting gypsum precipitate containing arsenic and any available iron to be discharged. The valuable metal content of the leach solution containing metal purified in this way can then be treated in some suitable manner depending on its composition, for example by electrolysis or selective precipitation.

EXAMPLE A flotation concentrate that originated from Petknás, and that contained arsenopyrite was leached in a first leaching series. The bioleaching was carried out in a leaching vessel A with a moderate thermophilic culture at 45 ° C, while the bioleaching was carried out in a leaching vessel B with a Sul folus culture at 65 ° C. The concentrate had the following composition (% by weight) Cu Zn Pb S As Fe 0.6 2.8 0.6 34.7 12.1 36.2 The bioleaching process was initiated discontinuously in the two containers, with a pulp consistency or density of 4% (p (v). started after 13 calendar days, whereby material was pumped from container B into a collection container C, and after that the same volume was pumped from container A into container B, and finally fresh suspension of ore was pumped from a M ore tank to leach vessel A. The pumping rate was adapted to D-50 L "1 at a pulp density of 12% solids in the first phase, and at a pulp density of 15% in the second phase It was taken n samples once or twice per calendar day, for analysis of Fe (tot), Fe (sup), (ie Fe in solution), As (tot), As (sup), Fe (II), pH and redox . The tests were continued for a total of 45 calendar days. The results of the tests could be interpreted and summarized as showing that the cultivation of Sul fol obus survived the presence of moderate thermophilic culture. The arsenic content of the concentrate was extracted by quantitative leaching already at 45 ° C. The arsenic content was reduced to a level that was non-toxic to the Sulfol obus culture, as a result of the arsenate precipitation achieved by adjusting the pH in the second stage, thereby increasing the leaching performance with respect to the remaining metals. The results indicate that the pulp density could possibly be further increased, and that the duration of the stage could be reduced while still achieving acceptable yields. In summary, the bioleaching process of the invention provides the following advantages during a one-stage bioleaching process with moderate mesophilic and thermophilic cultures: • A higher "total" bio-oxidation rate relative to the arsenic-containing material. • Higher metal performance by virtue of achieving more complete oxidation at higher temperatures. For example, because a certain amount of refractory gold (not readily recoverable) is present in iron pyrite. • Material containing arsenic that requires high temperatures can be treated, for example chalcopyrite concentrate containing arsenic. * An interesting point from an environmental aspect is that arsenic tends to be separated by precipitation in a more stable way at high temperatures. * Lower requirements for indirect cooling by water (lower water consumption) and a lower need for heat exchange surfaces. ß The higher cooling water temperatures that come out, that is, 60-65 ° C, compared to 40-45 ° C, allow the heat generated to be used more effectively to heat rooms and other spaces, etc. * the cyanide consumed to extract gold from the bioleaching residue will probably decrease the dependence on a more complete oxidation of sulfur to sulfate.

Claims (8)

1. CLAIMS 1. A method of leaching sulfidic material containing metals and arsenic under oxidizing conditions and with the help of microorganisms, characterized in that the material is mixed with an acidic aqueous solution to obtain a pulp, to carry out the leaching process in a first stage of leaching at a pH below 2, and at a temperature in the range from room temperature to a temperature of about 55 ° C in the presence of an active amount of microorganisms of the mesophilic and / or moderate thermophilic type, with which most of the arsenic contained in the material, and possibly also a part of its metal content is / are extracted by leaching, and the arsenic leached from the material is successively oxidized to a pentavalent state; and extracting by leaching the remaining leachable metal content of the material in a subsequent leaching step, in the presence of an active amount of an extremely thermophilic microorganism, at a temperature within the effective range of the microorganism, and greater than the temperature of the microorganism. The first stage of leaching.
2. 2. A method according to claim 1, wherein raising the pH in the second leaching step to a level at which the pentavalent arsenic present in solution is caused to transform to a solid state and separated by precipitation as arsenate containing metal, which is separated from the leaching solution together with unleaded material in the pulp subsequent to the completion of the second leaching stage.
3. 3. A method according to claim 1 or 2, wherein the first leaching step is carried out at a temperature of 45-55 ° C and at a pH of 1.0-1.34.
4. A method according to any of claims 1-3, wherein the second stage of leaching is carried out at a temperature of 65-70 ° C and at a pH of 1.5-2.0.
5. 5. A method according to any of claims 1-4, wherein Sulfol obus me tal l i cus is selected as the extremely thermophilic microorganism.
6. 6. A method according to any of claims 1-5, wherein the first stage of leaching is continued for a period of time such that the arsenic concentration of the pulp will not be toxic to the extremely thermophilic microorganism.
7. 7. A method according to any of claims 1-6, wherein the volume of leach solution is adapted to provide a pulp density from 10 to 25%.
8. 8. A method according to any of claims 1-6 for biolixing. pyrite-arsenic-iron minerals that contain refractory gold or concentrates of such minerals, where there is an additional stage in which the leaching residue is treated with cyanide to extract its gold content and also its content of other precious metals. SUMMARY OF THE INVENTION The present invention describes a method of leaching sulfidic material containing metals and arsenic under oxidizing conditions and with the help of microorganisms, characterized in that the material is mixed with an aqueous acidic solution to form a pulp, and because the material is leached in a first stage of leaching at a pH below 2, and at a temperature in the range from room temperature to a temperature of about 55 ° C in the presence of an active amount of microorganisms of the mesophilic and / or moderate thermophilic type, with which most of the arsenic contained in the material, and possibly also a part of its metal content is / are extracted by leaching, and the arsenic leached from the material is successively oxidized to a pentavalent state. The remaining leachable metal content of the material is extracted by leaching in a subsequent leaching step, in the presence of an active amount of an extremely thermophilic microorganism., at a temperature within the effective range of the microorganism, the temperature is higher than the temperature of the first stage. The pH is suitably elevated in the second stage to a level at which pentavalent arsenic present in solution is caused to transform to a solid state, and separated by precipitation as a metal-containing arsenate, which is separated from the solution of leaching together with the unleached material in the pulp after the completion of the leaching process.