WO1997001649A1

WO1997001649A1 - Method for selective separation of cadmium from an acidic aqueous solution

Info

Publication number: WO1997001649A1
Application number: PCT/GB1996/001350
Authority: WO
Inventors: Raymond Frederick Dalton
Original assignee: Zeneca Ltd
Current assignee: Syngenta Ltd
Priority date: 1995-06-24
Filing date: 1996-06-07
Publication date: 1997-01-16
Anticipated expiration: 1997-12-24
Also published as: ZA965343B; GB9512925D0; ID16515A; AU6009196A

Abstract

A method for the selective separation of cadmium from an acidic aqueous solution also comprising co-extractable metal ions is provided. The method comprises contacting an organothiophosphorus solvent extractant with the aqueous solution, the amount of solvent extractant being effective to reduce substantially the concentration of cadmium ions in the aqueous solution without significantly reducing the concentration of other heavy metal ions. Preferably the mole ratio of organothiophosphorus extractant to extractable metal ions is insufficient to extract substantially all of the extractable metal ions, and the organic phase is maintained in contact with the aqueous solution until at least a portion of the co-extractable metal ions are displaced by cadmium, thereby rendering the ratio of Cd:co-extractable metal ions in the organic phase greater than that in the aqueous solution.

Description

Method for selective separation of cadmium from an addle aqueous sol ution .

This invention relates to a chemical process and more particularly to a method for the selective removal of cadmium from aqueous solutions containing cadmium and other heavy metal salts.

There is considerable industrial interest in methods for the recovery of cadmium from aqueous solutions. Typical important applications are in the hydrometallurgical processing of spent nickel-cadmium cells, the removal of cadmium from wet process phosphoric acid to be used in fertiliser production, and the removal of cadmium from effluents and process streams for environmental reasons.

One of the largest areas for removal of cadmium, however, is in the production of certain metals from their ores - particularly lead and zinc, where cadmium commonly occurs with these metals and must be removed during the processing in order to obtain a pure metal product. There is currently considerable interest in the potential of solvent extraction in novel processes for the production of zinc from zinc bearing ores and secondary materials. In the case of an ore, this could be by the process of Leach-Solvent Extraction- Electrowinning, now widely practised for the mine site production of copper from oxidic and low grade sulphidic copper bearing material. Leaching of a zinc ore with dilute sulphuric acid produces a solution containing a mixture of metals, primarily zinc and iron, but also quantities of various other metals that are present in the ore and leachable with dilute sulphuric acid. Cadmium is in the same group of the periodic classification of the elements as zinc, and is nearly always present as a minor component in zinc ores, commonly at a level of between 0.5% and 2% of the level of zinc. Thus, a typical solution obtained by leaching a zinc ore and containing approximately 10g/l zinc, may also contain from 50-200 mg/l cadmium.

The recovery of zinc by electrowinning from a strongly acidic sulphate electrolyte is a delicate process, the efficiency of which is very sensitive to low levels of impurities. In addition, if present, less electropositive metals such as copper and cadmium are preferentially plated out, contaminating the final zinc product. Because of this, the common upper limit for cadmium in a zinc electrowinning process is as low as 0.5mg/l. It is therefore essential that cadmium be removed and isolated by some means during the process so that it does not contaminate the final zinc metal, and also is not returned to the environment in solution form. Various means are practised, or have been advocated, for the removal of cadmium from solutions, appropriate to the particular zinc extraction process requirements or operating conditions. For instance, most of the zinc produced from primary sources is by the Roast-Leach-Electrowin Process. In this process, a high grade zinc sulphide ore is roasted to the oxide and is then leached with spent, strongly acidic zinc sulphate electrolyte solution from the electrowinning process. The leaching is usually carried out in two steps referred to as the Acid Leach, and the Neutral Leach. The Acid Leach is an aggressive step which serves to leach the final traces of metal from the roasted ore. In the Neutral Leach step, there is a slight deficiency of acid over the metal oxides so the pH value rises to about 4-5, a level where some undesirable metals are precipitated, in the case of iron as Jarosite. Having precipitated the ferric iron it is then possible to remove numerous other impurity metals such as copper, cadmium and cobalt by the process of cementation using zinc dust - where the highly electropositive element zinc goes into solution as the sulphate and less electropositive elements, such as copper and cadmium, are precipitated in a finely divided metallic form.

The process of cementation, as practised in the Roast-Leach-Electrowin process for zinc, is not applicable to cadmium removal from a Leach-Solvent Extraction - Electrowin process. The reason for this is that the leach solution is both moderately acidic (pH 1.5-2) and also contains high concentrations of ferric iron. Quite unacceptable quantities of zinc dust would therefore be consumed in neutralising the excess acid, and by the side reaction of reduction of ferric to ferrous iron, before efficient cementation of cadmium occurred. Nor, because of the high acidity, could cementation be practised on the zinc electrolyte.

Precipitation processes based on the use of organic complexing agents have been advocated for the removal of cadmium from some acidic aqueous solutions, particularly for the removal of cadmium from phosphoric acid solutions to be used in fertiliser production. Such a process is described by Gradl et al in US Patent No. 4,452,768 where heavy metal ions, especially cadmium, copper, lead and mercury, are removed from wet process phosphoric acid by precipitation with a diorganodithiophosphoric acid ester in the presence of an adsorbent such as carbon black, porous resins or zeolites. The precipitation agent for the heavy metals is of general formula (RO)₂PSSH where R stands for a substituted alkyl, cycloalkyl or aryl group, typically 2-ethylhexyl or phenyl.

Other precipitation techniques have been described, sometimes combined with ion flotation in order to recover the precipitated metal complex. For example, US Patent No. 4,511,541 describes such a process in which heavy metals are removed from wet process phosphoric acid using as precipitant various thio-organophosphines and dialkyldithiophosphinates. It is clear, however, that there is a considerable lack of selectivity in this process and that if zinc is present, it is also precipitated along with for instance cadmium, copper and molybdenum. While simultaneous removal of zinc may be advantageous in purifying fertiliser grade phosphoric acid, it is clearly disadvantageous when attempting to remove traces of cadmium from a concentrated zinc solution in a metallurgical process.

A further feature of the various cadmium precipitation techniques based on use of organothiophosphorus compounds is that the efficiency and consumption of reagent is adversely affected by the presence of ferric iron. [Ref. E. Jdid, P. Blazy, J. Bessiere and R. Duranol - Removal of Cadmium Contained in Industrial Phosphoric Acid Using The Ionic Flotation Technique in "Trace Metals removal from Aqueous Solution", Ed. R. Thompson. Royal Society of Chemistry, London, 1986]. European Patent Application EP-A-0 033489 claims an improvement in the precipitation process based on using organothiophosphine reagents by incorporating a reduction step. Again, while this might possibly be applicable to Wet Process Phosphoric Acid solutions containing low concentrations of iron, it would be quite uneconomic to apply the process to a large scale zinc production where the leach solution would contain typically several g/l of ferric iron.

There has been recent interest in the possible application of solvent extraction technology to the removal of cadmium from aqueous solution. J. Preston et al (HYDROMETALLURGY, 36(1994), 61-78 and 143-160 describe a process for the selective solvent extraction of cadmium by mixtures of carboxylic acids and trialkylphosphine sulphides. The preferred mixture seems to be based on a 1 :1 molar ratio of triisobutylphosphine sulphide (TIBPS) and, as extractant, 3,5-diisopropylsalicylic acid in a kerosene type carrier such as Shellsol 2325 (ca. 20% aromatic content). The main advantage of such a mixture is claimed to be that the addition of TIBPS lowers the pH₀.₅ value (the pH at which 50% metal extraction occurs) for cadmium by up to 1.7 pH units whilst lowering it for zinc by approximately 0.5 pH units. Thus, cadmium may be recovered at a lower pH and with a greater separation from zinc. However, it is apparent that even with one of the most favourable compositions, the pH₀.₅ value for cadmium is still as high as 3.17. Thus, even in the presence of TIBPS, 3,5-diisopropylsalicylic acid is a relatively weak extractant, and is incapable of giving efficient cadmium removal from more acidic solutions such as those found in a Leach-Solvent Extraction-Electrowin

Process, without adjustment of the pH. Additionally, the extraction reaction itself liberates protons according to the equation

Cd²⁺ + 2RCOOH + 2 TIBPS > Cd(COOR)₂ (TIBPS)₂ + 2H ⁺ causing a further drop in pH as the extraction proceeds. It is quite evident from the work described by Preston et al that in order to obtain efficient extraction of cadmium it was necessary to add base in the form of sodium hydroxide or concentrated ammonia solution in order to maintain a pH in the range 3.7 - 3.8. It is also apparent from the case study given (HYDROMETALLURGY 36(1994) 143-160), that the process was applied to recovery of cadmium from a cadmium cement cake from an electrolytic zinc plant. Sulphuric acid leaching of this solution produced a feed containing 16 g/l cadmium, 3g/l zinc and only 26mg/l iron. Even so, iron was extracted strongly and built up in the organic phase. Preston et al report that the pH_{0 5} value for ferric iron with their synergistic extractant composition is as low as 1.23. It thus becomes readily apparent that this process is quite unsuited to the removal of cadmium from strongly acidic leach solutions (pH 1.5-2) in which cadmium is only a minor component and which also contain high concentrations of ferric iron.

In another recent publication, di(2,4,4-trimethylpentyl) thiophosphinic acid (R₂PS.OH) has been proposed for the removal of cadmium from crude wet-process phosphoric acid by solvent extraction (W.A. Rickelton, Journal of Metals, May 1992, 52- 54). Similarly A. Almel and M.P. Elizalde (Solvent Extraction of Cadmium II from acidic media by "CYANEX" 302. HYDROMETALLURGY 37 (1995) 47-57) describe the fundamentals of the extraction of cadmium by this dialkyl thiophosphinic acid, but give no information on selectivity or how it might be applied in a process. However, Preston et al (above) point out that above pH values of about 0.6, zinc is co-extracted with the cadmium, and that stripping of cadmium from the organic phase is also problematical, clearly teaching that thiophosphinic acids are unsuited to the separation of cadmium from zinc, particularly at higher pHs.

In European Patent Application EP-A-0573182, there is described a process for extracting metal values, especially zinc, from aqueous solutions of metal salts which comprises contacting the aqueous solution with an organic phase comprising a compound of the Formula (1):

wherein each of R¹, R², R³ and R⁴, independently, represents an optionally substituted hydrocarbyl or hydrocarbyloxy group or R¹ and R² together with the attached phosphorus atom and/or R³ and R⁴ together with the attached phosphorus atom form a 5- to 8- membered heterocyclic ring.

In International Patent Application No. PCT/GB94/02485, there is described a similar process wherein, in the compound of Formula (1), R¹ is an optionally substituted 2- alkylphenoxy group, each of R², R³ and R⁴ is a group selected from optionally substituted 2-alkylphenoxy, optionally substituted phenyl, optionally substituted alkyl and optionally substituted alkoxy and at least one optionally substituted 2-alkylphenoxy group has a tertiary alkyl substituent except the compound wherein each of R¹ and R² is 2-isopropyl-4- tert-nonylphenoxy and each of R³ and R⁴ is phenyl and the compound wherein R¹ is 2- methyl-4-tert-nonylphenoxy, R² is 2,4-dimethylphenoxy and each of R³ and R⁴ is phenyl. The extractant compounds of Formula (1) are taught to be very strong extractants for zinc, with pH₀.₅ values well below pH 2, and to exhibit exceedingly high selectivity for zinc over ferric iron. Such reagents therefore have considerable potential for application in a solvent extraction based process for the recovery of zinc from acidic sulphate leach solutions obtained by digesting zinc bearing ores. They also have considerable potential in the recovery of zinc from waste solutions and solutions obtained by the leaching of various zinc process by-products and secondary materials such as zinc bearing fumes, oxides and drosses.

However, extractants of Formula (1) of International Patent Application No. PCT/GB94/02485 are also taught to be strong extractants for cadmium. Therefore, in a conventional solvent extraction process, in which the extractant is employed in an amount sufficient to extract substantially all of the extractable metal, when an organic phase containing an extractant of Formula (1) is contacted with an aqueous feed solution containing both zinc and cadmium, both metals are extracted. Furthermore, during the course of the studies leading to the present invention, cadmium was found to be not readily given up at the strip stage where zinc loaded extractant is contacted with spent electrolyte which may typically contain 70g/l zinc and 180g/l sulphuric acid. Thus, in a conventional solvent extraction process, the cadmium will accumulate in the organic phase and will eventually reach concentrations at which small, but nonetheless significant (mg/l), quantities may be transferred to the electrolyte at the stripping stage. To avoid this could necessitate the taking of a sizeable bleed from the organic extractant, which would have to be stripped using forcing conditions such as strong hydrochloric acid, in order to maintain a cadmium level in the organic phase at an acceptably low level.

It has now been found that despite the strong co-extraction of cadmium and zinc, organothiophosphorus extractants usable for the solvent extraction of zinc may also be used for the almost complete removal of cadmium from a process stream in a pre- extraction step, despite the competing extraction of zinc.

According to a first aspect of the present invention, there is provided a method for the selective separation of cadmium from an acidic aqueous solution comprising cadmium ions and other heavy metal ions, said method comprising contacting the solution with an organic phase comprising an organothiophosphorus extractant compound in an amount effective to reduce substantially the concentration of cadmium ions in the aqueous solution without significantly reducing the concentration of other heavy metal ions, with the proviso that when the organothiophosphorus extractant compound is di(2,4,4- trimethylpentyl)thiophosphinic acid, the pH of the acidic aqueous solution is above 0.6. According to a second aspect of the present invention, there is provided a method for the selective separation of cadmium from an acidic aqueous solution comprising cadmium ions and other metal ions, by contacting an organic phase comprising an organothiophosphorus solvent extractant with the aqueous solution, at least one of the other metal ions being co-extractable by the solvent extractant together with, but displaceable therefrom by, cadmium, the mole ratio of organothiophosphorus extractant to extractable metal ions being insufficient to extract substantially all of the extractable metal ions, in which the organic phase is maintained in contact with the aqueous solution until at least a portion of the displaceable metal ions are displaced by cadmium, thereby rendering the ratio of Cd:displaceable metal ions in the organic phase greater than that in the aqueous solution.

Other metals which may be present in the acidic aqueous solution include metals which form complexes with, and are therefore extractable by, the organothiophosphorus extractant compound under the conditions at which contact between the aqueous solution and the organic phase occurs, but are displaceable therefrom by cadmium. It will be recognised that whether a metal forms a complex with the organothiophosphorus extractant under given conditions can readily be determined by methods conventional in the art. For example, an aqueous solution of the metal ion concerned can be contacted with an organothiophosphorus extractant comprising substantially no metal ions under those conditions, and then organothiophosphorus phase analysed to determine whether metal has been extracted or not. Whether or not a metal is displaceable from an organothiophosphorus extractant complex by cadmium can also be determined easily, for example, by preparing a substantially cadmium free organic solution containing an organothiophosphorus complex of the metal(s) whose cadmium displaceability is to be checked. This organic solution is then contacted with an aqueous solution comprising cadmium ions and the concentrations of the metal ions in both the organic and aqueous phases determined. Displaceable metal ions will be replaced by cadmium in the organic phase and substantial quantities found in the aqueous phase. Examples of such displaceable metals include cobalt, nickel, lead, manganese and zinc.

The total concentration of metal ions in the aqueous acidic solution is generally at least 1g/l, usually at least 4g/l, often at least 7g/l and commonly at least 10g/l. The total concentration of metal ions is usually no more than 100g/l, commonly no more than 75g/l and more commonly no more than 50g/l. The total metal concentration in the aqueous acidic solution may be made up entirely of metal ions extractable by the extractant under the conditions employed, or may comprise metal ions which are not extracted under the conditions employed. Most commonly, total extractable metal concentrations are in the range of from 2g/l to 40 g/l.

Cadmium usually comprises a minor proportion by weight of the extractable metals in the aqueous solution. The concentration of cadmium in the aqueous acidic solution may vary depending on the source of the solution, but may be up to 5g/l, and is often no more than 2 g/l and commonly no more than 1g/l. Most commonly, cadmium concentrations are in the range of from 0.001 g/l to 0.75g/l. Concentrations of cadmium above 5g/l may be encountered, for example when the aqueous acidic liquor is derived from recycling of Cd-Ni cells. The weight ratio of cadmium to other co-extractable, but cadmium displaceable metals in the aqueous acidic solution is often 1:6 or less, commonly 1:50 or less, and particularly in the range of from 1:100 to 1:15,000. When the cadmium- displaceable metal comprises zinc, the concentration of zinc in the aqueous acidic solution is usually from 1 to 70g/l, often from 2 to 50g/l and preferably from 3 to 35g/l. When the cadmium-displaceable metal comprises cobalt and/or nickel, the concentration of each of cobalt and nickel in the aqueous acidic solution is normally no more than 75g/l and preferably no more than 50g/l. The concentration of cobalt is usually from 0.1 to 20g/l, often from 0.2 to 2g/l and preferably from 0.3 to 1g/l. The concentration of nickel is usually from 0.5 to 30g/l, often from 1 to 20g/l and preferably from 3 to 10g/l.

Suitable organothiophosphorus extractant compounds for use in the method of the invention contain at least one P=S and/or P-SH group and particularly include dialkylthiophosphinic acids, particulariy those comprising alkyl groups having from 6 to 14, preferably from 7 to 10, carbons, particularly preferably branched alkyl groups, and most preferably compounds of Formula (1), particularly amidobis(thiophosphoryl) compounds disclosed in either European Patent Application EP-A-0573182 or International Patent Application No. PCT/GB94/02485, which are incorporated herein by reference.

Optionally substituted hydrocarbyl and optionally substituted hydrocarbyloxy groups which may be represented by R¹, R², R³ and R⁴ in Formula (1) comprise optionally substituted alkyl, alkoxy, aryl and aryloxy groups including any combination of these, for example optionally substituted aralkyl and alkaryl groups.

As examples of optionally substituted alkyl and alkoxy groups which may be represented by R¹, R², R³ and R⁴, there may be mentioned groups in which the alkyl or alkoxy moieties each contain from 1 to 20, for example from 1 to 10, carbon atoms. As examples of optionally substituted aryl and aryloxy groups, there may be mentioned optionally substituted phenyl and phenoxy groups.

As examples of heterocyclic rings which may be formed by R¹ and R² together with the attached phosphorus atom and/or by R³ and R⁴ together with the attached phosphorus atom, there may be mentioned rings wherein R¹ and R² together and/or R³ and R⁴ together having the following structures:

wherein each of X¹ and X², independently, represents O or S and in which one or more of the carbon atoms may optionally carry substituents. When any of R¹, R², R³ and R⁴ are substituted hydrocarbyl or hydrocarbyloxy groups or when the derived heterocyclic rings carry substituents, said substituents should be such as do not adversely effect the ability of the compounds of Formula (1) to complex with metals. Suitable substituents include halogen, nitro, cyano, hydrocarbyloxy, hydrocarbyloxycarbonyl, acyl and acyloxy and there may be more than one substituent in which case the substituents may be the same or different.

A preferred class of compounds of Formula (1) for use in the process of the invention includes compounds wherein each of R¹, R², R³ and R⁴ is an alkyl group, especially a secondary alkyl group. Good solubility in preferred solvents is provided when R¹, R², R³ and R⁴ taken together contain at least 16, and preferably at least 20, saturated aliphatic carbon atoms. For this purpose, a phenyl or phenoxy group may be regarded as equivalent to about two or three saturated aliphatic carbon atoms. In an especially preferred compound of Formula (1), each of R¹, R², R³ and R⁴ is 2-pentyl.

A second preferred class of compounds of Formula (1) is that in which at least one and especially at least two of R¹ to R⁴ are optionally substituted phenoxy groups, particularly alkyl substituted phenoxy groups wherein the alkyl groups contain from 1 to 20, for example from 1 to 10 carbon atoms.

Within this second preferred class of compounds of Formula (1), particular mention may be made of compounds wherein R¹ is an optionally substituted 2-alkylphenoxy group, each of R², R³ and R⁴ is a group selected from optionally substituted 2-alkylphenoxy, optionally substituted phenyl, optionally substituted alkyl and optionally substituted alkoxy and at least one optionally substituted 2-alkylphenoxy group has a tertiary alkyl substituent. A further preferred class of extractants of Formula (1 ) includes those compounds wherein R¹ and R² are each a 2-alkyl group, particulariy a 2-pentyl group, and R³ and R⁴ are each a 2-alkylphenoxy group, particularly 2-butylphenoxy, and particularly preferably a 2-t-buty I phenoxy group.

In the method according to the present invention, the organothiophosphorus extractants are present in an amount effective to reduce substantially the concentration of cadmium ions in the aqueous solution without significantly reducing the concentration of other heavy metal ions. In certain embodiments of the present invention, the amount of extractant can be the stoichiometric amount necessary, or an excess over the stoichiometric amount necessary, to extract all of the extractable metal ions in the aqueous acidic solution. Without wishing to be bound by any theory, it is believed that a stoichiometric amount, or an excess, of extractant can be successfully employed to reduce cadmium concentrations without significantly reducing the concentrations of other metal ions because the thermodynamic strength of the cadmium-extractant complex is greater than the complexes formed by the other co-extractable but displaceable heavy metal ions. Because of the isotherm nature of the extraction of metal ions, even though there is theoretically sufficient extractant to remove all of the metal ions, the strength of the cadmium-extractant complex is such that cadmium is extracted preferentially over the other extractable metal ions, thereby resulting in a concentration of other extractable metal ions in the organic solution significantly below that which would be expected. Usually, the amount of extractant does not exceed a 50% molar excess, preferably a 25% molar excess, and particularly preferably a 10% molar excess above the stoichiometric amount needed to extract all of the extractable metal ions. Often the mole ratio of organothiophosphorus extractant in the organic phase to extractable metal ions is no more than 5:1, more often no more than 3:1. In many preferred embodiments, the mole ratio of organothiophosphorus extractant

(including "free" extractant which is not complexed with a metal, and extractant which is in the form of a metal complex) in the organic phase to extractable metal ions is selected such that there is insufficient extractant to extract into the organic phase substantially all of the extractable metal ions in the aqueous solution with which the extractant is contacted. Often the mole ratio of organothiophosphorus extractant in the organic phase to extractable metal ions is no more than 2:1 , more often no more than 1.5:1 , commonly no more than 1 :1, and is advantageously up to 1 :30, more advantageously up to 1 :15, and preferably in the range of from 1 :2 to 1:8. The desired mole ratio of extractant to extractable metal ions can be achieved by recycling organic solution into contact with further extractable metal ion-containing aqueous solution.

The concentration of extractant in the organic solution is often from about 0.05 molar up to the solubility of the extractant in the particular organic phase, and is preferably from about 0.1M to about 0.5M, particularly from 0.2M to 0.3M. The volume ratio of organic solution to aqueous acidic solution is often chosen to be in the range of from 2:1 to 1 :30, with volume ratios in the range of from 1:1 to 1:10 being preferred. In many preferred embodiments of the present invention, the concentration of extractant in the organic solution, the flow rates of the aqueous and organic solutions and the volume ratio of organic solution to aqueous solution are chosen such that the amount of extractant in contact with the aqueous solution is sufficient to extract sufficient cadmium to reduce the cadmium concentration in the organic solution to the desired concentration, or below, and often to remove substantially all the cadmium from the organic solution, but not to extract unacceptable amounts of other co-extractable metals. If desired, the extractant compound can be used together with an agent which modifies the behaviour thereof in the extraction process, for example an alkylphenol, alcohol or ester which may be used in an amount of from 5% to 200%, especially from 10% to 50% by weight of extractant compound. Such compounds weaken the extractant but facilitate the subsequent stripping of metal therefrom. In this way, a very strong extractant may be adjusted in strength to the requirements of different feed solutions and different stripping solutions.

Alkylphenols which may be used as modifiers in conjunction with the extractant compounds of the invention include alkylphenols containing from 3 to 15 alkyl carbon atoms, for example 4-tert-butylphenol, 4-heptylphenol, 5-methyl-4-pentylphenol, 2-chloro- 4-nonylphenol, 2-cyano-4-nonylphenol, 4-dodecylphenol, 3-pentadecylphenol and 4- nonylphenol and mixtures thereof. The preferred phenols contain alkyl groups having from 4 to 12 carbon atoms, especially the mixed 4-nonylphenols obtained by condensation of phenol and propylene trimer.

Alcohols which may be used as modifiers in conjunction with the extractant compounds of the invention include saturated and unsaturated hydrocarbon alcohols and polyols containing 10 to 30, preferably 13 to 25 carbon atoms. The alcohols are preferably highly branched with the hydroxyl group located approximately midway along the hydrocarbon backbone. Optionally, the alcohols may contain an aromatic group or other functional group, particularly an ester group. Examples of particularly efficient alcohol modifiers include tridecanol, highly branched isohexadecyl alcohol and iso-octadecyl alcohol, the latter being 2-(1 ,3,3- trimethylbutyl)-5,7,7-trimethyloctanol.

Esters which may be used as modifiers in conjunction with the extractant compounds of the invention include saturated and unsaturated aliphatic and aromatic- aliphatic esters containing from 10 to 30 carbon atoms. The esters may be mono-esters or polyesters, especially di-esters. The esters are preferably highly branched. Optionally, the esters may contain other functional groups, particularly a hydroxyl group. Especially useful esters include 2,2,4-trimethyl-1 ,1 ,3-pentanediol isobutyrate and the benzoic acid ester thereof.

In the context of the present invention, 'highly branched' as applied to the alcohols and esters means that the ratio of the number of methyl carbon atoms to non-methyl carbon atoms is higher than 1:5. Preferably, this ratio is higher than 1:3.

If desired, mixtures of alkylphenols and/or alcohols and/or esters may be employed as modifiers.

The organic phase employed in the process of the invention typically contains a water-immiscible inert organic solvent. Organic solvents which may be used include any mobile organic solvent, or mixture of solvents, which is immiscible with water, is inert under the extraction conditions to the other materials present and is a good solvent for the extractant and the metal complexes thereof. Examples of suitable solvents include aliphatic, alicyclic and aromatic hydrocarbons and mixtures of any of these as well as chlorinated hydrocarbons such as trichloroethylene, perchloroethylene, trichloroethane and chloroform. Preferred solvents are hydrocarbon solvents including high flash point solvents with a high aromatic content such as SOLVESSO 150 commercially available from Exxon (SOLVESSO is a trade mark) and AROMASOL H which consists essentially of a mixture of trimethylbenzenes and is commercially available from Imperial Chemical Industries PLC (AROMASOL is a trade mark). Especially preferred, however, on grounds of low toxicity and wide availability are hydrocarbon solvents of relatively low aromatic content such as kerosene, for example ESCAID 100 which is petroleum distillate comprising 20% aromatics, 56.6% paraffins and 23.4% naphthenes commercially available form Exxon (ESCAID is a trade mark).

The method according to the present invention is often carried out at a temperature of up to 75°C, and particularly a temperature in the range of from 15 to 50°C. The contact time between the organic phase and the aqueous acidic solution can vary depending on factors which affect the kinetics of the metal ion extraction and particularly the kinetics of the displacement of metal ions by cadmium. Examples of such factors include the temperature, a higher temperature generally tending to reduce the contact time; the nature of the extractant; and the amount of agitation, with increasing agitation generally reducing contact time. In many preferred embodiments of the present invention, the organic phase is contacted with the aqueous phase until the mole ratio of organothiophosphorus extractant : cadmium in the organic phase is 10:1 or less, and preferably in the range of from 6:1 , particularly preferably from 4:1 , to 2:1. The organic phase is contacted with the aqueous phase until the mole ratio of organothiophosphorus extractantxadmium in the organic phase is often at least 50%, commonly at least 75%, preferably at least 90%, and particularly preferably at least 95% of the equilibrium cadmium content of the extractant under the extraction conditions. However, in some embodiments, particularly when it is desired to reduce the cadmium concentrations to extremely low levels, it can be desirable for the organic phase to be contacted with the aqueous phase until a much lower cadmium loading is achieved, for example until the mole ratio of organothiophosphorus extractantxadmium in the organic phase is 10 to 30% of the equilibrium cadmium content of the extractant under the extraction conditions. Typical contact times range from a few minutes, such as from 2 to 5 minutes to 30 minutes, for instance in a continuous process, up to several hours, such as from 1 to 2 hours up to 9 or 10 hours. The contact between the organic solution and the aqueous acidic solution can be achieved in batch, semi-continuous or continuous manner. When semi-continuous or continuous operation is employed, the flow-rates of solutions and/or the number of extraction stages are engineered to give the necessary contact or residence times to achieve the desired loadings of cadmium in the organic phase, and the desired separation from other co-extracted and displaceable metals.

The weight ratio of cadmiumxo-extractable, but displaceable metal, and particularly zinc, in the organic phase produced by the method of the present invention is usually at least 1:3, and preferably at least 1:1. In certain embodiments, it may be desirable to employ more than one cadmium separation stage according to the present invention. These can be operated under substantially similar conditions, but it may be advantageous to employ different conditions in each stage. For example, a first stage could be operated under conditions in which the organic phase achieves a high cadmium loading, such as a weight ratio of cadmiumxo-extractable, but displaceable metal of at least 1:1 , and a second stage could be operated could be operated under conditions in which the organic phase achieves a low cadmium loading, such as a weight ratio of cadmiumxo-extractable, but displaceable metal of from 1:10 to 1:5. In such a process, the first stage could be employed to remove the majority of the cadmium from the aqueous organic solution, with the second stage being employed as a "polish" to remove any cadmium not removed in the first stage. Because of the low cadmium loading which may be employed in the second stage, it may be desirable for substantially less extractant to be employed in the second stage than in the first stage to avoid the stripping of relatively large amounts of other heavy metal ions.

In the method of the present invention, the concentration of cadmium ions in the aqueous acidic solution is reduced substantially, but without a significant reduction in the concentration of other heavy metal ions. The reduction in concentration of other heavy metal ions that can be tolerated will depend on many factors, including particularly the nature and value of the other metal ions. Often, and particulariy in the case of zinc, the reduction in concentration is no more than 15%, commonly no more than 10%, preferably no more than 5% and particularly preferably no more than 2% by weight of the concentration of the metal ion in the aqueous solution prior to contact with the organic phase.

The pH of aqueous acidic solutions in the method of the present invention is commonly 6.5 or less, particularly 6.0 or less, and is often at least 0.6, and particularly at least 1.0. For the selective separation of cadmium from a solution containing cadmium ions and zinc ions, the pH of the solution is often in the range of from 1.5 to 3.5, and particularly from 2.0 to 3.0. For the selective separation of cadmium from a solution containing cadmium ions and cobalt and/or nickel ions, the pH of the solution is often at least 2.5, and particularly in the range from 3.0 to 6.0. The method of the invention is of particular value for the selective separation of cadmium from an acid aqueous solution containing cadmium ions and zinc ions, the cadmium being present in a minor proportion by weight relative to the zinc. Without wishing to be bound by any theory, it is believed that although on initial contact with the zinc bearing feed solution the extractant phase rapidly attains a high loading of zinc, this zinc is unexpectedly displaced very rapidly by the cadmium.

The acidic aqueous solution containing cadmium and zinc ions will typically be a solution obtained by leaching a zinc ore with acid, especially sulphuric acid. The concentration of cadmium ions in the solution will generally be less than 20%, especially less than 10%, more especially less than 5% and most especially less than 1% of the concentration of zinc ions, on a weight basis. Other metal ions, for example Fe(lll) ions may also be present in the solution.

In one aspect, the method of the invention comprises the steps of : (i) contacting an aqueous phase containing a major proportion by weight of zinc ions and a minor proportion by weight of cadmium ions on a weight basis in an extraction zone with an organic phase containing an organothiophosphorus extractant in an amount effective to reduce substantially the concentration of cadmium ions in the aqueous solution without significantly reducing the concentration of zinc, and preferably the mole ratio of organothiophosphorus extractant : cadmium plus zinc being insufficient to extract substantially all of the cadmium and zinc, whereby to extract a major proportion of the cadmium but not of the zinc into the organic phase in the form of a complex of the cadmium with the extractant; (ii) separating the organic phase containing cadmium complex from the extracted aqueous phase; (iii) contacting the organic phase containing cadmium complex with an aqueous strip solution whereby to strip cadmium from the organic phase into the strip solution; (iv) separating the stripped organic phase from the aqueous strip solution, and, preferably, (v) recycling the stripped organic phase to the extraction zone. If desired, the above sequence may be varied after stage 2 by contacting only a minor proportion of the organic phase containing cadmium complex with the aqueous strip solution, the major proportion of the organic phase being recycled direct to the extraction zone to ensure a high cadmium loading. Under these circumstances, the proportion of organic phase containing extracted cadmium complex that is contacted with the strip solution is selected to be such that the cadmium loading of the organic phase reaches the desired level. The proportion contacted with the strip solution is often selected such that the amount of cadmium removed in the strip per unit time is about equal to the amount of cadmium entering the extraction zone per unit time, and will typically be up to 10% w/w of the total organic phase, and preferably from 1 to 5% of the total organic phase. The stripped minor proportion of organic phase is then separated from the aqueous strip solution and recycled to the extraction zone as before.

In either case, the extracted and cadmium-depleted aqueous phase obtained in stage 2 above may be subjected to a treatment comprising the steps of : (a) contacting said extracted aqueous phase containing zinc ions in a second extraction zone with a second organic phase containing an extractant for zinc, e.g. an organothiophosphorus extractant, whereby to extract a major proportion of the zinc into said second organic phase in the form of a complex of the zinc with the extractant;

(b) separating the second organic phase containing zinc complex from the further extracted aqueous phase;

(c) contacting the second organic phase containing zinc complex with a second aqueous strip solution whereby to strip zinc from said second organic phase in the second strip solution;

(d) separating the stripped second organic phase from the second aqueous strip solution, and

(e) recycling the stripped organic phase to the second extraction of zinc.

Zinc may then be recovered from the second aqueous strip solution obtained in Stage D by a conventional electro-winning procedure. The invention thus also provides a method for recovering zinc in high purity, substantially free from cadmium, from an aqueous solution containing, on a weight basis, a major proportion of zinc and a minor proportion of cadmium.

Certain embodiments ofthe invention will now be described, without limitation, with reference to Figure 1. Figure 1 is a diagrammatic flow chart illustrating an extraction system utilising an improved process described herein. This is merely one example of a suitable extraction system involving 3 extraction and 2 stripping stages. However depending on the feed composition, the extraction and stripping conditions, and various other factors such as throughput and yield, the number of extraction and stripping stages may be varied in a manner familiar to those skilled in the art of solvent extraction. The system illustrated in Figure 1. includes a cadmium removal unit comprising a mixer-settler stage 1 employed for cadmium extraction and a further mixer-settler stage 2 for cadmium stripping and a zinc recovery unit comprising three mixer-settler stages 3, 4 and 5 employed for zinc extraction, two mixer-settler stages 6 and 7 for zinc stripping and an electro-winning bath 8 for electrodeposition of metallic zinc.

In operation, an acidic aqueous leach solution containing, on a weight basis, a major proportion of zinc and a minor proportion of cadmium is fed to the mixer tank of mixer-settler 1 where it is mixed with an organic phase comprising a solution of an organothiophosphorus extractant in a water-immiscible organic solvent, for example kerosene. The outlet of the mixer tank feeds to a settling tank where the organic phase, now containing the cadmium-extractant complex in solution is separated from the aqueous phase. The bulk of the organic phase is recycled to the mixer tank to achieve a high cadmium loading but a small bleed stream is passed to mixer-settler 2 where it is contacted with hydrochloric acid to strip out the cadmium and regenerate the extractant before being recycled to mixer-settler 1.

The aqueous phase from 1 containing zinc but substantially free from cadmium is fed to the mixer-settler stages 3, 4 and 5 where it is contacted countercurrently with a second organic phase comprising a solution of an organothiophosphorus extractant in a water-immiscible solvent. The separated organic phase leaving 3 and now containing the zinc-extractant complex in solution is fed to mixer-settler stages 6 and 7 where it is contacted countercurrently with an aqueous strip solution comprising stripped or spent electrolyte from the electro-winning step, the stripped organic phase being recycled to mixer-settler stage 5 of the zinc extraction unit and the aqueous phase being fed to the electro-winning bath 8. The depleted aqueous feedstock (raffinate) leaving 5 is either discharged or recirculated for further leaching.

In order to strip the cadmium from the extractant, strong hydrochloric acid, for example 5M or more, may be used but in certain preferred embodiments of the present invention, it is preferred to use a mixture of a dilute solution of hydrochloric acid with an alkali metal or alkaline earth metal chloride or ammonium chloride. Typically, the concentration of dilute hydrochloric acid is up to 2M, and preferably 1M or less. The total concentration of chloride is usually at least 4M, preferably at least 5M, and often up to about 10M. Stripping is highly efficient and it is possible after stripping to obtain an aqueous cadmium solution containing relatively little free acid. Because of the low free acid and the higher relative amounts of cadmium to other metals, particularly zinc, cadmium may be recovered from this solution by neutralisation and cementation using scrap iron or zinc dust.

This invention is further illustrated, but not limited, by the following Examples. Example 1

An extractant solution was prepared containing 200 g/l of a compound of Formula 1 wherein R¹=R³=2-tert-butylphenoxy, R²=2-sec-butylphenoxy and R⁴=phenyl and 30g/l tridecanol as modifier in ESCAID 100.

An aqueous feed solution was prepared containing 22.9g/l zinc, 3.9g/l ferric iron and 0.5g/l cadmium as their respective sulphates, in dilute sulphuric acid solution at pH 2.0.

Various volume ratios of organic extractant solution and the prepared aqueous feed solution were equilibrated by being stirred together vigorously in glass vessels at 40°C for a period of 6 hours.

The phases were then allowed to disengage and separated. After cooling to 25°C, each solution was filtered and analysed for cadmium, the organic samples by atomic absorption spectrophotometry and the aqueous samples by inductively coupled plasma spectrometry (ICP). The results, showing the distribution of cadmium between the aqueous phase and the extractant phase for various phase ratios of contact are as follows:

Aqueous : Organic Phase Ratio 1:1 2:1 5:1 10:1 20:1

Cd in aqueous phase (mg/l) <0.1 <0.1 0.1 1.4 8.2

Cd in organic phase (g/l) 0.535 1.11 2.51 5.23 10.05

It will be noted that even at high aqueous to organic phase ratios and in the presence of a large excess of zinc, the cadmium is extracted preferentially with levels remaining in the aqueous phase reduced to a few mg/l or less.

Example 2

This Example demonstrates the speed and effectiveness with which cadmium will displace zinc from the extractant, thus allowing the extractant to be used to recover cadmium in the presence of a large excess of zinc.

As organic extractant solution, there was taken the same composition as described in Example 1. This was pre loaded with zinc by stirring 100ml vigorously at room temperature (25°) for 2 hours with 350ml of an aqueous solution containing 23g/l zinc as the sulphate, with dilute sodium hydroxide solution being added to maintain a pH in the range 3.5 to 4.

An aqueous solution was prepared so as the contain approximately 10g/l cadmium at pH 2.0. It was subsequently analysed and shown to contain 10.28 g/l cadmium and 5mg/l zinc. Equal volume portions of the zinc loaded extractant solution and the aqueous cadmium solution were stirred at 1000 rpm in a glass vessel at 40°C. At various time intervals after the initiation of the stirring, a sample of the dispersion was withdrawn without stopping the stirrer. Each sample was allowed to separate into two phases which were then filtered and set aside for analysis. The results for the analysis of cadmium and zinc in each phase was as follows:

Time (min) 0 1 5 15 30 60

Cadmium in organic (g/l) 4 X 10^"4 11.94 12.06 12.10 12.24 12.08 Cadmium in aqueous (mg/l) 10.28 0.7 1.0 0.9 1.0 0.9 Zinc in aqueous (g/l) 5 x 10^"3 0 5.36 5.32 5.48

(zinc level in aqueous at 1 and 5 minutes not determined due to insufficient sample)

This clearly demonstrates the rapidity with which zinc is exchanged for cadmium, with near equilibrium loading of cadmium being attained in only one minute.

Example 3

The procedure of Example 1 was repeated using as extractant phase a solution containing 108.3 g/l of the commercial product CYANEX 302 (Cytec) containing as active agent 84.8% of bis(2,4,4-trimethylpentyl)monothiophosphinic acid. This is a 0.3 molar concentration of the extractant molecule R₂PS.OH, where R = 2,4,4-trimethylpentyl, and the reagent was diluted to the required concentration in ESCAID 100. The aqueous feed solution contained approximately 23g/l zinc, 4g/l ferric iron, and was shown by analysis to contain initially 0.459 g/l cadmium.

Various volume ratios of organic extractant solution and aqueous feed containing zinc and cadmium were equilibrated by vigorous stirring in glass vessels for 6 hours at 40°C. The phases were then separated, filtered and analysed as before:

Aqueous : Organic Phase ratio 1 :1 2:1 5:1 10:1 20:1

Cd in aqueous phase (mg/l) <0.2 <0.2 1 12 168

Cd in organic phase (g/l) 0.49 0.95 2.44 4.54 6.13

Example 4 A laboratory scale solvent extraction test circuit was assembled as shown in schematic form in Figure 2. The test rig comprised a single extraction mixer settler, 1 , and one strip mixer settler, 2, arranged as shown. The mixer units had a volume of 270 cm³ and the settler units a volume of 440 cm .

An aqueous feed solution was prepared containing approximately 23 g/l zinc and 100 mg/l cadmium by dissolving the appropriate quantities of the respective sulphates in water, and adjusting to volume with dilute sulphuric acid to a final pH of 2.0. The organic extractant solution was of the same composition as that described in Example 1 above. The one stage extraction circuit was operated at an organic to aqueous (O/A) phase ratio of 1:10, initially with the organic phase being recycled from the reservoir 3 back to extraction without stripping so as to build up the concentration of cadmium in the organic phase. After 85 litres of aqueous feed solution had been run through the circuit, the organic phase was analysed and found to contain 9.77 g/l cadmium, 4.79 g/l zinc. The aqueous raffinate discharged at this point contained 15 mg/l cadmium and 23.08 g/l zinc.

The conditions were then changed to introduce the strip circuit shown in Fig 2. The extraction O/A ratio was changed to 1 :2 and the strip circuit was run at an O/A ratio of 1:1 with an organic bleed representing 4% of the organic flow being taken from the reservoir and stripped with 5 molar hydrochloric acid.

The object was to strip the cadmium from the loaded organic, which was then recycled back to the cadmium extraction circuit so as to produce a near cadmium free raffinate which could be fed to the zinc recovery circuit.

After passing a total of 185 litres of feed through the circuit, analysis of the various streams yielded the following :

Raffinate 3mg/l Cadmium, 22.69 g/l Zinc

Loaded Organic (El) 6.76 g/l Cadmium, 5.62 g/l Zinc

Stripped Organic (SI) 94 mg/l Cadmium, 8 mg/l Zinc

Acid extract 3.75 g/l Cadmium, 4.74 g/l Zinc

This Example shows that the cadmium level of a feed containing 23 g/l zinc, 100 mg/l cadmium pH 2.0 can be reduced to 3 mg/l cadmium in a single extraction stage, leaving a zinc level in the raffinate of 22.69 g/l, which can then proceed to a main zinc extraction circuit.

Example 5

In this Example, the organic extractant phase comprised an ESCAID 100 solution containing 209g/l (0.3 mole/litre) of an extractant of Formula 1 wherein R¹=R²=2-tert- butylphenoxy; and R³=R =2-ethylhexyloxy. No modifier was incorporated into the organic phase.

Using this extractant composition, the procedure described in Example 2 was followed, the organic phase first being loaded with zinc by stirring with an aqueous zinc solution containing 23 g/l zinc.

As before, equal volumes of the zinc loaded extractant solution and an aqueous solution containing 10g/l cadmium at pH 2 were stirred at lOOOrpm and at 40°C. Samples were taken at suitable time intervals, the phases separated and analysed with the following result:

Example 6

This example illustrates the improvement in stripping of cadmium from the loaded extractant solution by use of a solution of hydrochloric acid containing additional chloride ions.

An organic extractant solution, of composition described in Example 1 , was pre¬ loaded with cadmium and zinc as follows. A 200 ml portion of the extractant solution was stirred vigorously for 2 hours at 25°C with 200 ml of an aqueous solution containing approximately 10 g/l cadmium and 2 g/l zinc. The phases were then separated, the organic solution filtered, and shown by analysis to contain 10.21 g/l cadmium and 1.16 g/l zinc.

Stripping experiments were carried out employing two different aqueous strip solutions. The first (A) comprised simply 1.0 M hydrochloric acid. The composition of the second strip solution (B) was 0.865 M hydrochloric acid and 2.0 M calcium chloride, giving a total chloride ion concentration of 4.87 M.

Portions of the pre-loaded organic extractant solution (O) were equilibrated with portions of the aqueous strip solutions (A) at various ratios by stirring vigorously at 40°C for 5 hours. The phases were then separated, filtered and analysed for cadmium and zinc. The results were as follows :

Using Strip Solution A (1.0 M HCI)

O/A Ratio Cd in organic Cd in aqueous Zn in organic Zn in aqueous

(g/i) (g/i) (g/i) (g/i)

1/1 9.47 1.32 0.035 0.98 2/1 9.91 1.51 0.067 1.91 3/1 10.41 1.53 0.085 2.77

This example clearly demonstrates the advantage of adding extra chloride ion to the hydrochloric acid strip solution, so that effective stripping may be obtained using a relatively dilute concentration of hydrochloric acid. Compared to solution A, the stripping with solution B can reduce cadmium in the organic phase to an acceptably low level and produce high concentrations of cadmium in the aqueous strip solution. Consequently, there is also a much high ratio of cadmium to zinc in the resulting aqueous extract. All these factors, combined with the relatively low acidity of the resulting strip solution, are high favourable to the subsequent recovery of the cadmium, e.g. by neutralisation and cementation with zinc.

Example 7

The procedure of Example 2 was repeated using, as organic extractant, a solution as described in Example 3, that was 0.3 molar in bis (2,4,4- trimethylpentyl)monothiophosphinic acid, obtained by appropriate dilution of the commercial product CYANEX 302 with ESCAID 100. This solution was pre-loaded with zinc, as described in Example 2, and found by analysis to contain ca. 5g/l zinc. Equal portions of this zinc loaded organic solution and an aqueous solution containing 10g/l cadmium at pH2 were stirred together vigorously at 40°C. Samples of the dispersion were removed at time intervals, allowed to separate, and analysed as before. The results of the analysis of cadmium and zinc in each phase as a function of time were as follows:

Aqueous Organic

Time (min) Cd g/I Zn g/I Cd g/I Zn (g/l)

0 10.25 0 0 4.32

30 sec 3.6 4.2 7.2 0.66

1 min 3.6 4.2 7.4 0.55

2 min 3.5 4.3 7.2 0.54

5 min 3.6 4.4 7.4 0.53

1 hour 3.4 4.4 7.5 0.55

This example demonstrates that the exchange of zinc for cadmium is, again, very rapid (equilibrium reached by 30 sec), although the cadmium levels in the aqueous phase were not reduced to the very low levels achieved in Example 2.

Example 8

The procedure of Example 7 was repeated but using an aqueous solution comprising ca 5g/l cadmium at pH2. The results were as follows: Aqueous phase analysis Organic phase analysis t Cd (g/l) Zn (g/l) Cd (g/l) Zn (g/l)

0 5.13 0 0 4.32

30 sec <1 mg/l 3.25 5.4 1.31 1 min <1 mg/l 3.25 5.53 1.45 5 min <1 mg/l 3.5 5.45 1.3 15 min <1 mg/l 3.3 5.35 1.27 1 hour <1 mg/l 3.35 5.48 1.34

The experiment demonstrates again, the rapidity of the exchange of zinc for cadmium in the organic phase, equilibrium being attained again in ca 30 seconds. In this case however, cadmium has been reduced to very low levels (<1 mg/l).

Example 9

This example demonstrates the speed and efficiency of the displacement of cobalt by cadmium from a solution of the compound of Formula 1. An extractant solution was prepared containing 200g/l of the compound of Formula

1 used in Example 1 , together with 30g/l tridecanol, in ESCAID 100. This solution was loaded with cobalt by stirring vigorously with an aqueous solution containing 20g/l cobalt and adding base as the extraction proceeded to maintain a pH of 5.0 (cobalt is not extracted at all at low pH values). Analysis showed the level of cobalt in the organic phase to be 7.8 g/l.

Equal volume portions of the extractant loaded with cobalt, and an aqueous solution containing ~10g/l cadmium at pH 2.0 were stirred vigorously at 40°C and sampled at time intervals, as described in Example 2.

After 1 minute there was 6g/l cobalt in the aqueous phase and no cadmium detectable. The organic phase contained 2g/l cobalt, 10.5g/l cadmium. These concentrations did not change materially with further agitation.

Example 10

This example demonstrates the speed and efficiency of the displacement of nickel by cadmium from a solution of the compound of Formula 1.

The procedure of Example 9 was repeated with the exception that the organic phase was pre-loaded with nickel by vigorous stirring with a solution containing 20g/l nickel, and base was added as the extraction proceeded to maintain a pH of 6.5. Analysis showed the organic phase to contain 7.9g/l nickel. Equal portions of the nickel loaded organic phase and an aqueous solution containing 10g/l cadmium at pH2 were contacted as described in Example 2. The analysis of the aqueous phase with time was as follows:

Aqueous Organic t Ni Cd Ni Cd

0 0 Non detected (<1 ppm) 7.9 0

1 min 6g/l Non detected (<1 ppm) 2.29 10.06

2 min 6.13g/l Non detected (<1 ppm) 2.20 10.25 30 min 6.2g/l Non detected (<1 ppm) 2.17 10.35 60 min 6.1 g/l Non detected (<1 ppm) 2.18 10.37

This again shows that equilibrium is essentially established in under 1 minute, with quantitative removal of cadmium from the aqueous phase.

Claims

1. A method for the selective separation of cadmium from an acidic aqueous solution comprising cadmium ions and other heavy metal ions, said method comprising contacting the solution with an organic phase comprising an organothiophosphorus extractant compound in an amount effective to reduce substantially the concentration of cadmium ions in the aqueous solution without significantly reducing the concentration of other heavy metal ions, with the proviso that when the organothiophosphorus extractant compound is di(2,4,4-trimethylpentyl)thiophosphinic acid, the pH of the acidic aqueous solution is above 0.6.

2. A method for the selective separation of cadmium from an acidic aqueous solution comprising cadmium ions and other metal ions, by contacting an organic phase comprising an organothiophosphorus solvent extractant with the aqueous solution, at least one of the other metal ions being co-extractable by the solvent extractant together with, but displaceable therefrom by, cadmium, the mole ratio of organothiophosphorus extractant to extractable metal ions being insufficient to extract substantially all of the extractable metal ions, in which the organic phase is maintained in contact with the aqueous solution until at least a portion of the displaceable metal ions are displaced by cadmium, thereby rendering the ratio of Cd : displaceable metal ions in the organic phase greater than that in the aqueous solution.

3. A method according to either of claims 1 or 2, wherein the organothiophosphorus extractant compound has the formula :

wherein each of R¹, R², R³ and R⁴, independently, represents an optionally substituted hydrocarbyl or hydrocarbyloxy group, or R¹ and R² together with the attached phosphorus atom and/or R³ and R⁴ together with the attached phosphorus atom form a 5- to 8- membered heterocyclic ring.

4. A method according to any preceding claim, wherein co-extractable metal comprises cobalt, nickel and/or zinc.

5. A method according to any preceding claim, wherein the mole ratio of organothiophosphorus solvent extractant to extractable metal ions is no more than 5 : 1 , and preferably in the range of from 1 : 2 to 1 : 8. 6. A method according to any preceding claim, wherein the pH of the aqueous solution is from 0.

6 to 6.0.

7. A method according to any preceding claim, wherein the total concentration of 5 extractable metal ions in the aqueous acidic solution is from 2g/l to 40g/l, the concentration of cadmium being from 0.001 g/l to 0.75g/l.

8. A method for the separation of cadmium from an aqueous acidic solution also comprising zinc ions, comprising the stages of: o (i) contacting an aqueous phase containing a major proportion by weight of zinc ions and a minor proportion by weight of cadmium ions on a weight basis in an extraction zone with an organic phase containing an organothiophosphorus extractant in an amount effective to reduce substantially the concentration of cadmium ions in the aqueous solution without significantly reducing the s concentration of zinc, and preferably the mole ratio of organothiophosphorus extractant : cadmium plus zinc being insufficient to extract substantially all of the cadmium and zinc, whereby to extract a major proportion of the cadmium but not of the zinc into the organic phase in the form of a complex of the cadmium with the extractant; o (ii) separating the organic phase containing cadmium complex from the extracted aqueous phase; (iii) contacting the organic phase containing cadmium complex with an aqueous strip solution whereby to strip cadmium from the organic phase into the strip solution; and 5 (iv) separating the stripped organic phase from the aqueous strip solution,

9. A method according to claim 8, wherein the contact in stage (i) is maintained until the cadmium concentration in the organic phase is at least 50% of the equilibrium cadmium content of the extractant under the extraction conditions. 0

10. A method according to claim 8 or 9, wherein the aqueous strip solution in stage (iii) comprises up to 2 molar hydrochloric acid solution and an alkali metal, alkaline earth metal or ammonium chloride, such that the total concentration of chloride ions is at least 4 molar. 5