PROCESS FOR THE RECOVERY OF VALUABLE METALS

Abstract: Multistage process for the recovery of valuable metals from an acidic, aqueous leachate solution in which valuable metal ions are present in concentrations of from 10 to 40 g/l, which comprises (I) liquid/liquid extraction of the leachate solution, where the leachate solution is brought into contact at least once with an organic, water-insoluble extractant, (II) and subsequently bringing the extractant loaded with valuable metal ions obtained from (I) into contact at least once with an acidic aqueous phase, with the major part of the valuable metal ions being transferred into the acidic aqueous phase, (III) and subjecting the acidic aqueous phase obtained from (II) to an electrolysis or crystallization process for recovering the valuable metal, wherein the pH of the aqueous leachate solution is set to at least 1.2 and not more than 2.5 before and/or during the liquid/liquid extraction process (I).

Description: Process for the recovery of valuable metals The present application relates to a multistage process for the recovery of valuable metals from aqueous, acidic leachate solutions. Many valuable metals such as copper, silver, gold or palladium occur in nature not alone but in association with other ores. To recover the valuable metals from these ores, hydrometallurgical processes by means of which the valuable metal ions are separated from the other constituents of the ores by liquid/ liquid extraction using suitable complexing agents have become established. For this purpose, the ores or the ore concentrates obtained by flotation are first broken down chemically in a leaching process, forming aqueous solutions of the metal salts. The valuable metal ions are subsequently separated out by means of liquid/ liquid extraction. Metal recovery occurs in various ways depending on the type of metal, for example by electrowinning of copper. This is followed by a refining process dependent on the type of metal, for example the electrolytic refining of copper. The details of the leaching process are dependent on the type of valuable metal, the way in which it is bound in the ore and the type of materials accompanying the ore (gangue type) . The ore is frequently leached using aqueous acids, for example sulphuric, hydrochloric or nitric acid, or alkalis such as ammonia/ammonium carbonate or ammonium sulphate. However, apart from the known chemical processes, biological leaching by means of suitable strains of bacteria has become established in recent years. Thus, US 4,571,387 describes a process for leaching sulphidic copper ores, for example chalcopyrite (CuFeS 2 ) , in which the copper ores are brought into contact with sulphide-oxidizing strains of the bacterium Thiobacillus ferrooxidans in acidic aqueous solution and Cu 2+ ions as well as sulphur and sulphate or sulphuric acid are formed by oxidation of the ore. The Cu 2+ ions can then be processed further by liquid/liquid extraction. A similar process is described in US 4,729,788 which discloses the use of thermophilic bacteria of the Sulfolobus type for leaching sulphidic gold or silver ores. Although microbiological leaching has advantages over the classical methods (in particular, it requires less complicated and expensive plants and the amounts of inorganic acids or alkalis required for leaching can be reduced) , microbiological leaching does cause problems in the subsequent processing stage of solvent extraction (SX) . This is because the resulting leachate solutions contain not only the desired high proportions of valuable metals but also, as an intrinsic aspect of the leaching process, relatively large amounts of accompanying elements such as iron. In particular, the leachate solutions obtained have relatively high proportions of valuable metal ions. Furthermore, for example, microbiological leaching of sulphidic Cu ores (or ore concentrates) can give aqueous leachate solutions containing Cu ions in amounts of up to 50 g/1. Such high valuable metal concentrations are, on the one hand, desirable for efficient leaching, but on the other hand these high valuable metal concentrations cause problems in the SX process. This is because when such concentrated leachate solutions are treated with complexing agents, a significant drop in the pH of the aqueous phase occurs during extraction, since the complexing agents generally exchange the valuable metal ions for protons. However, as the pH drops the acid-dependent equilibrium between free complexing agents and loaded complexing agents is quickly shifted to the side of the free complexing agents, so that the proportion of valuable metal which can still be extracted from the aqueous leachate solution also falls. For example, the extraction of copper can virtually no longer be carried out economically at pH values in the region of 1.0. It is therefore an object of the present invention to provide an improved extraction process by means of which even leachate solutions containing high proportions of valuable metal ions can be subjected to liquid/liquid extraction. A further problem is posed by the accompanying metals in the leachate solution. Thus, most commercially exploited copper ores also contain iron which is likewise brought into solution by the leaching process. In the extraction process, this iron can lead to selectivity problems which reduce the yield of valuable metal. It is therefore a further object of the present invention to provide a process in which valuable metal ions can be extracted selectively and in economically viable yields from leachate solutions in which they are present in association with other metal ions. It has been found that the above objects are achieved by combining the liquid/liquid extraction process known per se with particular pH control . The present application accordingly provides a multistage process for the recovery of valuable metals from an acidic, aqueous leachate solution in which valuable metal ions are present in concentrations of from 10 to 40 g/1, which process comprises (I) liquid/liquid extraction of the leachate solution, where the leachate solution is brought into contact at least once with an organic, water- insoluble extractant, (II) and subsequently bringing the extractant loaded with valuable metal ions obtained from (I) into contact at least once with an acidic aqueous phase, with the major part of the valuable metal ions being transferred into the acidic aqueous phase, (III) and subjecting the acidic aqueous phase obtained from (II) to an electrolysis or crystallization process for recovering the valuable metal, wherein the pH of the aqueous leachate solution is set to at least 1.2 and not more than 2.5 before and/or during the liquid/liquid extraction process (I) . Valuable metals in the context of the present invention are metals from the group consisting of copper, silver, gold and palladium which can be brought in ionic form into the aqueous phase by leaching of their ores or ore concentrates obtained by flotation. It is immaterial whether the valuable metal is present in the ore in elemental or ionic form. The ores typically contain the valuable metals in the form of oxides, sulphides, selenides, tellurides, carbonates, nitrates, silicates or halides. In the process of the invention, particular preference is given to using leachate solutions containing copper ions as valuable metal . The leachate solutions are therefore preferably obtained by leaching sulphidic ores, for example chalcopyrite or bornite. Apart from the valuable metal ions, further metal ions can be present in the leachate solutions . These are preferably iron ions which are present in amounts of from 0.001 to 25 g/1. The iron ions are preferably in the form of Fe 3+ ions with lesser amounts of Fe 2+ ions. From 60 to 99 mol% of Fe 3+ and from 40 to 1 mol% of Fe 2+ ions are preferably present. In the process of the invention, particular preference is given to iron-containing leachate solutions containing virtually exclusively Fe 3+ ions . It is also preferred that the aqueous leachate solutions are obtained by biological leaching of ores containing valuable metals or their flotation concentrates using thermophilic microbes of the Sulfolobus type. Here, the ores or concentrates are leached with the aid of the thermophilic bacteria in the temperature range from 40°C to 90°C. The process as set forth in steps (I) to (III) is known per se and can be carried out in the plants or apparatuses described in the prior art. It is an essential aspect of the invention that the aqueous leachate solution is subjected to pH control at particular points within the process . In the interests of providing an overview, Figure 1 shows an embodiment of the invention on which the following description is based but which is purely by way of example and has no limiting character. In the figure, the solid lines relate to the transport of organic phases while the broken lines show the transport of aqueous phases. The aqueous, acidic leachate solution (feed) is introduced into a mixer/settler El and brought into contact with a solution of an organic complexing agent . The organic phase is separated off and the pH of the remaining aqueous phase is adjusted by bringing the aqueous phase into contact with a suitable inorganic or organic base which may be present in solid form or as a solution or dispersion. The aqueous phase which has thus been set to a pH in the range from 1.2 to 2.5 is again brought into contact with the organic complexing agents in further mixer/settlers E2 and E3 . The aqueous phase resulting from the last extraction stage ( "raffinate" , solution depleted in copper) is discharged from the system. The organic phase loaded with metal ions is then conveyed to step (II) and stripped in SI and S2 using a strongly acidic aqueous solution (H 2 S0 4 ) . This is preferably carried out in mixer/settler systems, preferably using two mixer/ settlers connected in series. As a result of contact with the strongly acidic aqueous solution, the metal ions go back into the aqueous phase and can now be passed to a further recovery step (III) , preferably electrolysis (electrowinning) . The organic complexing agents used in the liquid/liquid extraction (I) in the process of the invention are essentially hydrophobic compounds which undergo specific or nonspecific interactions and/or coordination with the valuable metal ions so as to balance and/or shield the charge of the valuable metal ion so that the polarity is reduced sufficiently for the solubility of the ions in the aqueous phase to be decreased while the solubility of the complex in the organic phase is increased. Such complexing agents include particular phosphorus compounds, organic carboxylic acids, phenols, ketoximes, aldoximes and further chelating agents known m this context to those skilled in the art . Examples of specific classes of substances are organic phosphoric, phosphonic or phosphimc acids, m particular dι(2- ethylhexyl) phosphoric acid or trioctylphosphme oxide (TOPO) , tributyl phosphate (TBP) or dibutyl carbmol (DBBP) , also acetylacetone, oxalic acid, citric acid, 2 , 2 ' -bipyridyl , salicylaldehyde or ethylenediammetetraacetic acid. According to the invention, particular preference is given to the use of aldoximes and/or ketoximes as complexing agents. Such compounds are commercially available under the trade name LIX from Henkel Corp . In a preferred embodiment, step (I) is carried out using organic extractants containing from 1 to 35% by weight, preferably from 2 to 25% by weight and in particular from 2 to 20% by weight, of organic complexing agents. The complexing agents are dissolved in suitable organic solvents, for example aliphatic or cycloaliphatic or else aromatic hydrocarbons or mixtures thereof, chlorinated hydrocarbons, ketones or ethers having high flash points (from at least 70°C to about 110°C) and also mixtures of these compounds. The flow rates of the organic (O) and aqueous (A) phases can be or have to be matched as a function of the Cu concentration in the leachate solution ("advanced flow rates" of O/A) . At a Cu content of from about 12 to 15 g/1 in the leachate solution, it has been found to be advantageous to set an O/A ratio of from 1.6 to 2.0. This ratio can also be increased in principle, but the extraction performance then deteriorates . The stripping of the loaded organic phase is preferably carried out using an aqueous sulphuric acid solution. The aqueous solutions for stripping typically contain from 100 to 220 g/1 and m particular from 160 to 180 g/1 of H S0 . The latter concentration is particularly preferred for subsequent Cu electrowinning, with the stripping solution (B.E.) containing about 30-35 g/1 of Cu. During the stripping stage (II), the acid-labile formation equilibrium of the valuable metal ion complexes is shifted back toward the starting compounds as a result of the high proton concentration. As a result, the valuable metal ions are set free and go into the aqueous phase. This acidic aqueous phase loaded with valuable metal ions is then subjected in (III) to known work-up procedures such as crystallization or, preferably, electrolysis in order to recover the valuable metal . The way in which process step (I) , namely the liquid/liquid extraction, is carried out is of particular importance, since the pH control according to the invention is carried out before and/or during this process step. In the simplest case, the pH control step can be carried out prior to introduction of the aqueous leachate solution into the extraction stage (I) . This pH control prior to the extraction is preferably achieved by diluting the untreated leachate solution with water. However, other methods of pH control described further below can, in principle, also be employed in this process step. The dilution method is particularly advantageous when the leachate solution contains large amounts of iron ions which would precipitate in the form of solid iron hydroxides on addition of bases. However, this method does produce comparatively large volume flows which require the entire plant to be made appropriately large. In this method, the leachate solution is preferably diluted with two or three times its volume of water. For this reason, the pH control of the aqueous leachate solution is preferably carried out only after the first extraction step. For the purposes of the present application, the term "aqueous leachate solution" is also used to refer to such a solution which has already been subjected to one or more extraction steps. Here, pH control can take the form, for example, of a titration using a suitable base. Before carrying out the process of the invention, the leachate solutions have pH values of less than 1.2, with typical values being in the range from 0.8 to 1.1. The copper contents are from 10 to 40 g/1. The iron contents can be in the range from 0.0001 to 25 g/1. In addition, depending on the type of ore which has been leached, other metal ions such as aluminium, lead, cadmium, calcium, chromium, cobalt, manganese, magnesium, sodium, nickel and zinc ions may be present in amounts of from 0.001 to 1.0 g/1. As bases for setting a pH in the range from 1.2 to 2.5 as specified according to the invention, it is possible to use either inorganic or organic bases. Suitable inorganic bases are the oxides or hydroxides of alkali metals and/or alkaline earth metals, for example sodium hydroxide or calcium hydroxide (for example as "slaked lime" or "quicklime"). Preference is given to using calcium hydroxide or sodium hydroxide in the form of aqueous solutions or slurries. These solutions or slurries typically contain the inorganic base in amounts of from 10 to 30% by weight, preferably from 20 to 25% by weight. The neutralization can also be carried out using solid bases. During the pH control of the aqueous leachate solution, for example, the bases are added until the desired pH range has been set. The aqueous leachate solution which has been adjusted in this way is then subjected to the further process steps. Typical amounts of base solution, as 25% strength by weight NaOH, are in the range from 5 to 30 g of base solution/litre of leachate solution. In principle, organic bases such as tertiary amines can also be used for controlling the pH. These are marketed by the Applicant under the trade name ALAMINE ® . Quaternary ammonium compounds, for example of the ALIQUAT ® type (Henkel) , are also suitable. However, in the case of these organic bases, the pH control step has to be carried out in the form of a separate SX stage, i.e. a separate extraction circuit. In such a procedure, the amines in the form of 30 - 40% strength by weight solutions in suitable solvents, e.g. kerosene, are brought into contact with the aqueous leachate solution. It is also preferred that the pH control of the aqueous leachate solution be carried out during a three-stage liquid/liquid extraction as shown in Figure 2, where the leachate solution is first brought into contact with the organic extractant in two mixer/ settlers, pH control is subsequently carried out and this is followed once more by a mixer/settler system for extraction using the organic extractant . In all cases, the process of the invention can be carried out continuously or batchwise. However, it is generally carried out continuously. Examples The process of the invention was carried out in a plant as shown in Figure 1. Here, El to E3 are each mixer/settlers in which the aqueous leachate solution is sequentially brought into contact with the organic extractant. SI and S2 are stripping stages in which the loaded organic phase is brought into contact with the aqueous acidic phase. B.E. (barren electrolyte) is the Cu-containing sulphuric acid stripping solution which is used for stripping the Cu-laden organic phase. P.E. (pregnant electrolyte) is the Cu-enriched sulphuric acid solution obtained after stripping; this is passed to Cu electrowinning (Cu-EW) . Buffer vessels serve for intermediate storage of the stripped organic phase and the Cu- laden organic phase and make it possible for the process to be carried out continuously. A leachate solution (feed) having a Cu concentration of 13.9 g/1 and a pH of 3.8 was used. The iron content was 8 mg/1. In addition, according to the invention, after the first mixer/settler El in the liquid/liquid extraction there was installed a further mixer/settler for adjusting the pH ( "pH adjustment") in which the organic phase loaded with valuable metal was brought into contact with an aqueous 25% strength by weight NaOH solution. The extraction was carried out using a 30% v/v solution of LIX 973N ® in Shellsol ® D70 as organic phase in each case. Four trials with and without pH adjustment were carried out. The results are shown in Tables la to Id: Table la: Process without pH adjustment Table lb: Process with pH adjustment between El and E2 (pH 1.8) : Table lc: Process with pH adjustment between El and E2 (pH = 2.0) : Table Id: Process with pH adjustment between El and E2 (pH 2.2) : Two further trials in which the pH control step was located downstream of the second mixer/settler E2 m the liquid/liquid extraction as shown in Figure 2 were carried out . The results are shown in Table 2a and 2b: Table 2a: Process with pH adjustment between E2 and E3 (pH 1.4) : Table 2b: Process with pH adjustment between E2 and E3 (pH 1.5) : Comparison of the concentration m the raffmate (the aqueous phase m E3) shows that the copper concentration can in each case be reduced to a value of about 10 mg/1 of Cu or less by means of the process of the invention, while values of greater than 0.3 g/1 of Cu are obtained without pH control. The process of the invention thus leads to considerably improved efficiency of the extraction step compared with conventional processes. Key to Figures for Finisher (numbers refer to copies marked up in red) : 1. Figure 1 2. Raffinate 3. pH adjustment 4. Buffer vessels P.E. and B.E. and Feed remain unchanged (also in Figure 2) . 5. Figure 2 6. Raffinate 7. pH adjustment 8. Buffer vessels

Claims: Claims 1. Multistage process for the recovery of valuable metals from an acidic, aqueous leachate solution in which valuable metal ions are present in concentrations of from 10 to 40 g/1, which comprises (I) liquid/liquid extraction of the leachate solution, where the leachate solution is brought into contact at least once with an organic, water-insoluble extractant, (II) and subsequently bringing the extractant loaded with valuable metal ions obtained from (I) into contact at least once with an acidic aqueous phase, with the major part of the valuable metal ions being transferred into the acidic aqueous phase, III) and subjecting the acidic aqueous phase obtained from (II) to an electrolysis or crystallization process for recovering the valuable metal, characterized in that the pH of the aqueous leachate solution is set to at least 1.2 and not more than 2.5 before and/or during the liquid/liquid extraction process (I) . 2. Process according to Claim 1, characterized in that the leachate solution contains copper ions as valuable metal . 3. Process according to Claim 1 or 2 , characterized in that the leachate solution contains iron ions in concentrations of from 0.001 to 25 g/1. 4. Process according to Claim 3, characterized in that the iron ions are in the form of from 60 to 99 mol% of Fe 3+ ions and from 40 to 1 mol% of Fe 2+ ions. 5. Process according to any of Claims 1 to 4, characterized in that the leachate solution is obtained by biological leaching of ores containing valuable metals or their flotation concentrates using thermophilic microbes of the Sulfolobus type. 6. Process according to any of Claims 1 to 5, characterized in that the leachate solution runs through at least two separate extraction stages in (I) . 7. Process according to any of Claims 1 to 6, characterized in that the organic extractants used in (I) contain one or more complexing agents selected from the group consisting of aldoximes and/or ketoximes dissolved in an organic solvent . 8. Process according to any of Claims 1 to 7, characterized in that the organic extractants used in (I) contain complexing agents in amounts of from 1 to 35% by weight, preferably in amounts of from 2 to 25% by weight and in particular in amounts of from 2 to 20% by weight. 9. Process according to any of Claims 1 to 8, characterized in that the pH of the aqueous leachate solution is adjusted by addition of water before and/or during the liquid/liquid extraction (I) . 10. Process according to any of Claims 1 to 9, characterized in that the pH of the aqueous leachate solution is adjusted during the liquid/liquid extraction (I) by bringing the leachate solution into contact with aqueous solutions or dispersions of organic or inorganic bases. 11. Process according to any of Claims 1 to 10, characterized in that the pH of the aqueous leachate solution is adjusted during the liquid/liquid extraction (I) by bringing the leachate solution into contact with an aqueous solution or dispersion of alkali metal and/or alkaline earth metal hydroxides or oxides .

Inventors: KEHL RALF, KOEPPL DIETGER, SCHWAB WERNER, KEHL, RALF, KOEPPL, DIETGER, SCHWAB, WERNER
Applicants: COGNIS DEUTSCHLAND GMBH, KEHL RALF, KOEPPL DIETGER, SCHWAB WERNER, KEHL, RALF, KOEPPL, DIETGER, SCHWAB, WERNER
Application year: 2000
Main classification: C22B
IPC codes: C22B3, C22B15