Method for the recovery of silver value from aqueous solution

Abstract: An efficient method for the highly selective recovery of silver value in an aqueous solution containing silver ions in a low concentration, such as depleted photographic fixing solutions, comprises extracting the silver ions from the aqueous phase with an organic solution containing 0,0-bis(2-ethylhexyl) hydrogen thiophosphate as an extractant solution, the silver ions contained in the extract solution being back-extracted with a hydrochloric acid solution containing thiourea. Alternatively, the silver-containing starting aqueous solution may be contacted with beads of a porous resin bearing the thiophosphate compound so that the silver ions are selectively adsorbed on the resin beads and then the silver ions are leached out of the resin beads with an organic solvent.

Description: The present invention relates to a method for the recovery of silver value from aqueous solutions or, more particularly, to an efficient method for the separation and recovery of silver value, in particular, even from aqueous solutions containing silver in the form of silver ions in a very low concentration. As is known, silver is a kind of noble metal and widely used in, for example, photographic materials, electronic parts, and ornamental jewelry. It is a very important social issue from the standpoint of resource preservation to re-use the silver value contained in waste materials by separating and recovering the silver value from aqueous waste solutions containing silver ions in very low concentration such as wastes of photographic film, depleted photographic fixing solutions and waste etching solutions and washings in manufacturing processes for electronic parts. Known methods for the recovery of silver value from an aqueous solution containing silver ions include the electrolytic method, precipitation method, ion exchange method and metal collector method. It is, however, generally a very difficult problem to recover efficiently silver value from an aqueous solution containing silver ions in extremely low concentration or to separate selectively and recover silver value alone from an aqueous solution containing not only silver ions but also ions of several other kinds of metals. Among the above mentioned methods for the recovery of silver, the ion exchange method using an ion exchange resin has an advantage because the method is suitable for continuously collecting silver from a large volume of aqueous solution but the method is not applicable to an aqueous solution containing several kinds of metal ion. The metal collector method is not practicable due to the very high cost because special expensive reagents must be used in a large amount. In the Journal "Tohoku Kogyo Gijutsu Shikensho Hokoku", (1992) Volume 25, pages 47-51 (Chemical Abstracts, Vol. 117, No. 12, 21 September 1992, Abstract No. 115533) the authors, Wakui, Matsunaga and Suzuki describe the use of bis(2-ethyl)hydrogen monothiophosphate as a solvent for selective extraction of ions from an acidic solution of HNO 3 or HCl, the extracted silver being back extracted from the solvent by acidic solutions. Journal of Membrane Science, Vol. 40, No. 1, 20 May 1992, pages 31-39 describes the recovery of silver ions from an aqueous solution which is brought into contact with a porous membrane of polypropylene film which is impregnated with mixtures of DPTA/Kerosene. The extracted silver is re-extracted by contacting the membrane with an aqueous solution of sulfuric acid and thiourea. The present invention accordingly has an object to provide an efficient and inexpensive method for the recovery of silver value even from an aqueous solution containing silver ions in extremely low concentration or containing silver ions together with ions of other kinds of metals in combination. The present invention has been made as a result of the extensive investigations undertaken by the inventors arriving at a discovery that silver ions can be efficiently and selectively separated and recovered from an aqueous solution containing silver ions in extremely low concentration or containing silver ions together with ions of other kinds of metals by the use of a specific thiophosphate compound. Specifically, the method of the present invention for the recovery of silver value from an aqueous solution containing silver ions comprises the steps of: (a) bringing the aqueous solution containing silver ions through a column filled with a porous organic resin bearing 0,0-bis (2-ethyl-hexyl) hydrogen thiophosphate to effect adsorption of silver ions from the aqueous solution onto the porous organic resin; and (b) isolating silver from the porous resin by leaching with an organic solvent to obtain a silver-containing organic solution from which the silver ions are back extracted with an aqueous hydrochloric acid solution containing thiourea. In the drawings: Figure 1 is a graph obtained in Reference Example 1 showing the residual concentrations of ions of silver, copper, cadmium and iron after extraction as a function of the concentration of hydrochloric acid in the aqueous phase. In the method of the present invention, a specific compound 0,0-bis(2-ethylhexyl) hydrogen thiophosphate is used as a silver collector, which is a known compound expressed by the structural formula C 4 H 9 -CH(C 2 H 5 ) -CH 2 -O-PO(SH)-O-CH 2 -CH(C 2 H 5 )-C 4 H 9 . This compound is used supported on a porous organic resin to be brought into contact with a starting aqueous solution containing silver value in the form of silver ions. The porous organic resin to carry the thiophosphate compound is selected from those which are insoluble in water and in the organic solvent used in the leaching step. Examples of suitable porous organic resins include foamed polyethylenes, foamed polypropylenes, foamed polystyrenes, foamed polyvinyl chlorides, foamed polyurethanes and foamed phenolic resins which may be either crosslinked or uncrosslinked. In particular, foamed resins of a crosslinked polymer of an acrylic acid ester are preferred. These porous resins should desirably have no substituent groups having ion exchangeability such as amino groups and sulfonyl groups because these groups influence the absorption behavior of the porous resin to the metal ions. The porous resin is used preferably in the form of a powder having a particle size distribution of 20 to 200 mesh fineness and the resin should have a specific surface area of 50 to 800 m 2 /g and an average pore diameter of 5 to 40 nm. The procedure for obtaining the porous organic resin bearing the thiophosphate compound is as follows. Specifically, the thiophosphate compound is dissolved in an organic solvent having a relatively low boiling point such as acetone or hexane and the porous resin particles after thorough washing and drying are added to the solution and agitated for several hours followed by evaporation of the solvent to dryness. The amount of the thiophosphate compound supported by the porous resin particles is usually in the range of from 1 to 200 parts by weight or, preferably, in the range from 10 to 70 parts by weight per 100 parts by weight of the porous resin powder. The silver value contained in the starting aqueous solution should be in the form of a monovalent silver ion Ag + or in the form of a complex thereof having solubility in an aqueous phase. The aqueous solution can contain cations of other metals including alkali metals, alkaline earth metals, aluminum, iron, manganese, nickel, zinc, cadmium and copper and anions including nitrate ions, chloride ions, thiosulfate ions and sulfate ions, although sulfate ions and chloride ions are undesirable because these anions form a water-insoluble precipitate with the silver ions. Accordingly, for example, the concentration of chloride ions should be kept sufficiently high, because precipitates of silver chloride can be solubilized in the presence of chloride ions of high concentration by forming chloro complexes; to be at least 1 mole/liter when the concentration of silver ions in the aqueous solution is 10 -4 mole/liter. The concentration of thiosulfate ions in the aqueous solution, on the other hand, should be reduced to 0.1 mole/liter or lower, if necessary by dilution with water because thiosulfate ions have an inhibiting effect on the recovery of the silver value from the aqueous solution. The pH value of the starting aqueous solution containing silver ions should be adjusted so as not to exceed 4, if necessary, by the addition of an acid because, when the pH value of the aqueous solution is too high, elimination of the thiophosphate compound from the porous organic resin may take place when the aqueous solution is contacted with the porous resin particles bearing the thiophosphate compound. The acid used for the adjustment of the pH of the aqueous solution is preferably nitric acid because nitric acid has little adverse influence on the formation of precipitates and on the porous resin. In the method of the present invention, a porous resin bearing the thiophosphate compound can be used also for a starting aqueous solution in which the concentration of silver ions is relatively high while a solution of the thiophosphate compound in an organic solvent is most advantageously applied to a starting aqueous solution of a relatively low silver concentration so that it is advantageous that an aqueous solution of a relatively high silver concentration is subjected beforehand to another method for the recovery of silver such as the solvent extraction method or the electrolytic method before the aqueous solution is treated in step (a) of the inventive method. When the starting aqueous solution contains, besides silver ions, ions of other metals such as aluminum, manganese, nickel, zinc, iron, cadmium and copper, sufficiently selective separation of the silver ions therefrom can be accomplished by adjusting adequately the pH value of the aqueous solution so as to modify the separation factor between the silver ions and the ions of respective other metals. In step (b) of the inventive method, the silver value contained in the organic solution of the thiophosphate compound is subjected to a back extraction treatment into an aqueous phase which is performed with an aqueous hydrochloric acid solution containing a compound capable of forming a complex with the silver ions, namely thiourea. For example, the back extraction treatment can be performed satisfactorily with an aqueous solution containing at least 1 mole/liter of hydrogen chloride and at least 1 mole/liter of thiourea. A convenient and efficient method for practicing the method of the present invention is that the starting aqueous solution containing silver ions is passed through a column filled with particles or beads of the porous resin bearing the thiophosphate compound to cause adsorption of the silver ions onto the porous resin followed by leaching of the silver ions from the porous resin by passing an organic solvent through the column. The organic solvent used in this leaching treatment is not particularly limited provided that the solvent can dissolve the thiophosphate compound. Examples of suitable organic solvents include hydrocarbons such as hexane, heptane, benzene, toluene, xylene and kerosene, chlorinated hydrocarbons such as chloroform, dichloromethane, trichloroethane and chlorobenzene and ketones such as acetone, methyl isobutyl ketone and methyl ethyl ketone. Sufficient leaching of the silver ions can be accomplished by passing the organic solvent in a volume of 2 to 3 times the empty volume of the column though dependent on the flow rate therethrough. When a water-immiscible organic solvent is used for leaching, the organic solution obtained is subjected to a back extraction treatment with an aqueous hydrogen chloride solution containing thiourea. When an organic solvent miscible with water is used for leaching, the solvent is removed by evaporation from the solution and the residue is re-dissolved in a hydrochloric acid solution for further purification. The inventive method described above can be advantageously applied even to a silver-containing aqueous solution which contains silver ions in an extremely low concentration or contains chloride ions in a high concentration when conventional methods can hardly be applied to such an aqueous solution. Moreover, the inventive method provides a means for selectively recovering the silver value alone from an aqueous solution containing ions of other kinds of metals. In the following, the method of the present invention is described and illustrated in more detail by way of examples. Reference Example 1. A 10 ml portion of each of five aqueous solutions containing silver ions in a concentration of 10 -5 mole/liter and hydrogen chloride in a concentration 1 to 5 moles/liter, which also contain manganese ions Mn 2+ , iron ions, Fe 3+ , nickel ions Ni 2+ , zinc ions Zn 2+ , copper ions Cu 2+ and cadmium ions Cd 2+ each in a concentration of 10 -4 mole/liter, was thoroughly shaken for 1 hour with admixture of 0.1 g, as dried, of thiophosphate-bearing resin beads, which were prepared from 100 parts by weight of porous resin beads of an acrylic acid ester-divinylbenzene copolymer having a specific surface area of 450 m 2 /g, average pore diameter of 9.0 nm and particle size distribution of 20 to 60 mesh fineness and 100 parts by weight of 0,0-bis (2-ethylhexyl) hydrogen thiophosphate and the aqueous solution freed from the resin beads was subjected to determination of the residual concentrations of the ions of the respective metals to calculate the values of % immobilization of the metal ions of silver, iron, cadmium and copper are shown in Figure 1 which gives the % unimmobilization, i.e. the ratio of the residual concentration of the metal ions after the resin treatment to the concentration before the treatment, in the ordinate as a function of the hydrogen chloride concentration in the aqueous phase. Absolutely no immobilization was noted for the ions of manganese, nickel and zinc. Reference Example 2. A cylindrical column having an inner diameter of 8 mm and a length of 50 mm was filled with 1.00 g of the same thiophosphate-bearing resin beads as used in Reference Example 1 to form a resin bed through which 20 ml of an aqueous solution containing 10 -6 mole/liter of silver ions, 1 mole/liter of sodium chloride and 0.1 mole/liter of hydrogen chloride were passed at a flow rate of about 48 ml/hour and the outflow solution was taken in fractions which were subjected to the determination of the concentration of silver ions therein to find that silver ions could not be detected in each of the fractions. Example A 25 ml portion of a depleted photographic fixing solution containing about 350 ppm by weight of silver was acidified by the addition of 1 ml of concentrated hydrochloric acid and diluted with 250 ml of water. After standing as such overnight, the thus diluted aqueous solution was filtered through filter paper to remove insoluble matter. A 10 ml portion of the filtrate was shaken with addition of 0.1 g of the same thiophosphate-bearing porous resin beads as used in Reference Example 1 and small portions of the aqueous phase were periodically taken and analyzed for the residual concentration of the silver ions to find that about 97% and about 99% of the silver ions in the starting aqueous solution were immobilized onto the resin beads after aqueous solution were immobilized onto the resin beads after 1 hour and after 6 hours, respectively, of shaking.

Claims: A method for the recovery of silver value from an aqueous solution containing silver ions which comprises the step of: (a) passing the aqueous solution containing silver ions through a column filled with a porous organic resin carrying 0,0-bis(2-ethylhexyl) hydrogen thiophosphate to effect adsorption of silver ions from the aqueous solution onto the porous resin; (b) leaching out the silver ions from the porous organic resin with an organic solvent; and (c) subjecting the organic solvent containing the silver ions to back-extraction with an aqueous solution of hydrogen chloride containing thiourea. The method for the recovery of silver value from an aqueous solution containing silver ions as claimed in claim 1 in which the porous organic resin is in the form of a powder having a specific surface area in the range from 50 to 800 m 2 /g, an average pore diameter in the range from 5 to 40 nm and a particle size distribution in the range from 20 to 200 mesh fineness. The method for the recovery of silver value from an aqueous solution containing silver ions as claimed in claim 1 or claim 2 in which the amount of the 0,0-bis(2-ethylhexyl) hydrogen thiophosphate is in the range from 1 to 200 parts by weight per 100 parts by weight of the porous organic resin. The method for the recovery of silver value from an aqueous solution containing silver ions as claimed in any one of claims 1 to 3 in which the porous organic resin is a foamed resin which is a crosslinked copolymer of an acrylic acid ester. The method for the recovery of silver value from an aqueous solution containing silver ions as claimed in any one of claims 1 to 4 in which the organic solvent used in step (b) is selected from hydrocarbons, chlorinated hydrocarbons and ketones.

Inventors: TABASHI OSAMU, WAKUI YOSHITO, ITABASHI, OSAMU, WAKUI, YOSHITO, Tabashi, Osamu, c/o Tohuko National Industrial, Wakui, Yoshito, c/o Tohuko National Industrial
Applicants: AGENCY IND SCIENCE TECHN, JAPAN AS REPRESENTED BY DIRECTOR GENERAL OF AGENCY OF INDUSTRIAL SCIENCE AND TECHNOLOGY
Application year: 1995
Main classification: C22B3
IPC codes: C22B3, C02F1, G03C5, C22B7, B01D11, C22B11, C01G5