EXTRACTION OF METAL VALUES FROM AQUEOUS LIQUIDS

Abstract: Metal values (generally in ionic form) are extracted from aqueous solutions or suspensions using an extraction medium comprising non-viable cell walls (for example, waste material from yeast extract production), which complex with the metal values. The complexed cell walls are then separated from the aqueous solution or suspension. The extraction medium may be in the form of a slurry, or a flocculent mass comprising the cell walls treated with a chelating agent. The method can be used for extraction of noble metals, or toxic or radioactive metals.

Description: Extraction of Metal Values from Aqueous Liquids The present invention is concerned with a method of extracting metal values from aqueous liquids. Many industrial processes produce as by-products aqueous liquids containing metal values in solution or suspension; it is often desirable, and sometimes essential, for the metal values to be extracted from such aqueous liquids. For example, it is desirable, and sometimes maπdatory j to extract toxic or radioactive metals (such as cadmium, lead, arsenic, uranium, plutonium or other radiαnuclides) from aqueous effluents before the latter can be discharged into, for example, a river or a stream. There is, indeed , often a statutory prohibition on the discharge of effluent containing more than a minute con¬ centration of radioactive metal. There are, of course, cases where the metal values present in an aqueous liquid are commercially valu¬ able (for example, when the metal values are of noble metals such as gold, silver or platinum); it is desirable to extract and recover such metals from the aqueous liquid. Various methods are known for extracting metal values from aqueous liquids; for example, by treatment with an ion-exchange resin or by the use of live microorganisms. We have now discovered that non-viable cell walls are effective for the extraction of metal values from aqueous liquids. According to the invention, therefore, there is provided a method of processing an aqueous liquid containing metal values, which comprises contacting the - aqueous liquid with an extraction medium comprising non- viable cell walls so as to cause said metal values to complex with said cell walls, and separating said complexed cell walls from the aqueous liquid. The metal values are preferably separated from the complexed cell walls, for example, by desorption under acid conditions, by electrolysis, or by dest¬ ruction of the extraction medium ( for ' example, by , pyrolysis or burning). The cell walls used in the method accord- ing to the invention are either dead or, if still alive, non-viable; there may be present with the contents of- the cells or, preferably, substantially free of the former contents of the cells. The cell walls are pref¬ erably broken up and dead; nevertheless, it may be possible to carry out the method according to the invention in the presence of a small amount of living cells in addition to the dead material. The cell walls are preferably those of micro¬ organisms or plants; suitable microorganisms are Saccharomyces yeasts (for example, Saccharomyces c erevisiae or uvarum) . Other suitable microorganisms are those grown commercially for the production of pharm¬ aceutical materials or precursors therefor; examples of such materials are filamentary fungi such as Penicillium fungi. Many suitable materials comprising non-viable cell walls are available as waste products; for example, waste material available in the commercial product¬ ion o f yeast extract is eminently suitable for use in the method according to the invention. The method according to the invention is preferably carried out in a - reaction vessel and it may be carried out as a continuous process, a semi-continu¬ ous process or a batch process. In the case of a continu¬ ous or semi-continuous process the vessel is preferably a column or fluidized bed of elongated shape, with an inlet at or near one end of an outlet at " or near the other end thereof. The vessel is preferably disposed with its greatest length upright or inclined to the 10 horizontal and with its inlet at or near the bottom and its outlet at or near the top. The extraction medium may comprise a flocculent mass comprising the cell walls treated with polyvalent ions, for example, non-deleterious cations such as calcium, T5 or chelating anions such as borate, phosphate, ethylene diamine tetraacetate (EDTA) or nitrilotriacetate (NTA). In the case where the extraction medium is in the form of a flocculent mass, it can be immobilised and used in a similar manner to a filter. 20 In an alternative embodiment of the present invention, the extraction medium may be in the form of an aqueous slurry, which preferably contains the non- viable cell walls in an amount of 1 to 5?. by weight, based on the weight of the slurry. The proportion may be higher 25 than 5?ά (for example, when the slurry is concentrated) or lower than 1 % (for example, as low as 0.1 %) . The concen¬ tration selected depends on the concentration of the metal value to be extracted and on the type of cell walls. When a slurry is used, it may contain a stab¬ 30 ilizer to prevent or slow down deterioration of decom¬ position of the cell walls. The stabilizer is preferably a salt of a non-deleterious cation (such as an alkali metal) and an anion such as azide or sorbate. Examples of suitable such stabilizers are sodium or potassium 35 azide or sorbate. Another possible class of stabilizers are organic acids, such as sorbic acid. The slurry may be housed in a reaction vessel which is preferably of elongate shape, and is preferably disposed with its greatest length upright or inclined 4Q to the horizontal; there is preferably an inlet at or near the bottom and an outlet at or near the top thereof. ciruugh the inlet, intermittently or continuously, and liquid is withdrawn from the outlet. The flow rate is sufficiently slow for the removal of cell wall material with the -liquid leaving through the outlet at least partially restrained by gravity. Nevertheless, if desired^ the contents of the vessel may be stirred. In one method, t.he vessel is charged with slurry and is used for a pe iod without any additional extraction medium being introduced. In a modified method, however, more extraction medium is introduced during the process. For example, the medium may be added to the aqueous solut¬ ion or suspension shortly before it is introduced into the vessel. The solution or suspension may remain in contact with the extraction medium for any suitable period of time, but the period may be at leastiDne hour and. may be several hours in duration. A typical timescale is 1 to 5 hours (depending on the particular extraction process). The liquid leaving the vessel is likely to contain quantities of solid material, much or all of which may comprise the extraction medium. The solid material may be removed in any suitable manner. In the case of highly dispersed material, such as the walls of dead yeast cells, it may be necessary to separate the solids from the liquids by a centrifug- ing process, but in the case of coarser material it may be possible to separate the solids by filtration; for example, use may be made of vibrating screens. In order to simplify the separation of the extraction medium from the liquid the cell walls may be treated in " such a manner that they tend to aggregate or floccu¬ late. This treatment may be carried out before extraction of metal values in the method according to the invention , for example, by treatment with polyvalent, ions as described above. Flocculation may-alsα be carried during or after extraction of metal values; for example, by treatment with polyvalent ions or a particulate ion exchange resin. In an alter¬ native method of inducing aggregation or flocculation, Ibis αead cell walls may be treated wit 'an amide. The invention may be of use in removing cadmium ions from effluents from processes carried out in the steel and nickel industries, and in removing gold, platinum and silver from industrial effluents from a wide variety of processes such as certain processes carried out in the ceramic or plating industries. The invention may also be used in removing radioactive metals such as uranium and pluton- iu or other radionuclides from effluents containing such metals. For example, there may be a vessel, contain¬ ing the extraction medium, permanently connected in an effluent line so that in the event of an error occurr¬ ing such that radioactive material is inadvertently discharged through the line it automatically under¬ goes treatment in the vessel. The metal values extracted in the method according to the invention are preferably initially present in the form of simple hydrated ions, although they may be present in other ionic species. The aqueous liquid being treated by the method according to the invention is generally a solution or suspension in water.

Claims: 1. A method of extracting metal values from an aqueous liquid, which comprises contacting said liquid with an extraction medium comprising non-viable cell walls so as to cause at least some of said metal values to complex with said cell walls, and separating said complexed cell walls from the aqueous liquid. 2. A method according to claim , in which said cell walls are substantially free of the former contents of the cells. 3. A method according to claim 1 or 2, in which said cell walls comprise fungal material and/or waste material from yeast extract production. 4. A method according to any of claims 1 to 3, in which said extraction medium is a flocculent mass comprising said cell walls treated with polyvalent ions. 5. A method according to claim 4, in which said polyvalent ions comprise chelating anions. 6. A method according to claim 4 or 5, in which the extract¬ ion medium is employed in a column or a fluidised bed. 7. A method according to any of claims 1 to 3, in which the extraction medium is in the form of an aqueous slurry containing said non-viable cell walls in an amount of 1 to 5?ή by weight. 8. A method according to claim 7, in which the slurry contains a stabilizer, such as an alkali metal azide or sorbate. 9. A method according to any of claims 1 to 8, in which the complexed metal values are separated from said cell walls . 10. A method according to any of claims 1 to 9, in which the metal values are of toxic, radioactive or noble metals.

Inventors: GREENSHIELDS RODERICK NORMAN, GREENSHIELDS, RODERICK, NORMAN
Applicants: HARMON FRANCES LTD, FRANCES HARMON LTD
Application year: 1985
Main classification: C22B3
IPC codes: C22B3