WO1998058092A1

WO1998058092A1 - Method and installation for recovering precious metals from solutions using laser-induced reduction and precipitation

Info

Publication number: WO1998058092A1
Application number: PCT/RU1997/000194
Authority: WO
Inventors: Evgeny Nikolaevich Bolshakov; Boris Borisovich Krynetsky; Jury Petrovich Kuznetsov; Andrei Grigorievich Perederei; Svetlana Sergeevna Fadeeva
Original assignee: Evgeny Nikolaevich Bolshakov; Boris Borisovich Krynetsky; Jury Petrovich Kuznetsov; Andrei Grigorievich Perederei; Svetlana Sergeevna Fadeeva
Priority date: 1996-04-02
Filing date: 1997-06-19
Publication date: 1998-12-23
Also published as: RU2081925C1; AU4475797A

Abstract

The invention relates to precious metal recovery from solutions at both high and low content of the metal being recovered in the solution. The essence of the method consists in that in technological process including mixing of the initial solution in the presence of a reducer followed by selective separation of the resulting metal-containing product from the solution, mixing of the initial solution in the presence of the reducer is carried out under exposure to laser radiation with a frequency corresponding the resonance frequency of excitation of ions of the metal being recovered. The process proceeds without any change in the solution temperature. The installation for precious metal recovery from the solution is made based on a centrifugal extractor and is provided with a laser system with frequency controlled radiation, whereby the extractor is configured with a window with a material transparent in infra-red, visual and ultra-violet wavelength ranges to transmit the laser beam in a mixing chamber. The method provides recovering metals from solutions down to the level less than 1 mg/l.

Description

METHOD AND INSTALLATION FOR RECOVERING PRECIOUS METALS FROM SOLUTIONS

The present invention relates to metallurgy, in particular to producing precious metals from liquids and may be used both at a high and at a low content of the metal to be recovered in a solution in the cases when conventional chemical methods of precious metal recovery are of little efficiency. It may find application as an independent invention and in a single technological chain together with known methods, including methods for cleaning industrial wastes, to recover precious and other strategically important metals.

In technology of precious metal recovery from solutions, precipitation methods are commonly known based on precious metal precipitation from the solution by adding to it chemical precipitants. As precipitants different reagents are utilised depending on the type of the precious metal to be recovered. Thus in a known method for platinum group metals recovery and purification alkali is added as a precipitant to the precious metal containing solution and then it is reacted with reducer (ethanol) and is subject to boiling (DE patent No.3340056, C 22 B 1 1/04, 1985). The resulting precipitate of precious metals is separated from iron and copper salts by a chemical method.

The above method as well as other relating thermal methods are characterised by a rather low metal separation selectivity, by low productivity due to extended period of precipitation process, slow rate of chemical processes in virtue of low activity of platinum group metals as well as by high labour consumption due to multiple operations and adverse factors of using biologically dangerous reagents in the technology. Such method is non-economical for treating solutions with low precious metal content. This method is also unacceptable for recovering metals from solutions contaminated with radioactive wastage in which, nevertheless, concentrations of such metals may reach 4 to 19 kg/t, i.e. may exceed by hundreds of times concentrations of natural deposits. The reason is that chemicals required to implement such technology (predominantly ferric or ferrous sulphates) must exceed in volume the initial volume of the treated solution by 4-5 times and this results not in decrease but in increase of total volumes of radioactive wastage which requires burying.

Methods are also known to recover precious metals from liquid solutions based on reduction of metal ions contained in the solution to be treated to the state of a solid neutral metal by adding a chemical reducer reagent to the solution.

Visual representation of such technology is provided in the enclosed flow-chart disclosed in Livingston S. "Chemistry of Ruthenium, Rhodium, Palladium, Osmium, Iridium, Platinum", M.: Mir, 1978, p.25, Fig.3. To a technological solution prepared by dissolving platinum group metal concentrates in nitrohydrochloric acid ferric or ferrous sulphate is added (II) upon which the resulting precipitate is separated from the liquid phase. Contaminated gold recovered as a precipitate is re-melted into anodes which are utilised in electrolytic gold refining. In turn, the filtrate is subjected to further treatment to recover other precious metals contained in the solution (platinum, palladium, etc.) In order to recover from the basic solution all platinum group metals, the known technology employs multiple methods considerably differing in operation sequence, in reagents used (reducers and precipitants) as well as in methods of precious metal refining. The above-discussed recovery method considered as a prototype essentially has the same inherent drawbacks as with the earlier discussed analogue:

- low productivity due to low activity of platinum-like metals and extended periods of chemical processes (for example, in order to recover rhodium - to produce "rhodium mirror", - trichloride rhodium solution is boiled in chlorhydric acid in the presence of formic acid for 24 hours);

- high labour intensiveness and material consumption on the volume of equipment used (in order to implement the method and technology for all platinum-like metal group, multiple electrolytic cells, tempering and melting furnaces, chemical reactors, mixers and water cleaning equipment as well as ventilation exhausting equipment providing security in work with cyanides in gold extraction are used);

- limited use for solutions containing few precious metals;

- low efficiency which is compensated only by high price of extracted product;

- ecological dangers of environmental pollution due to use of cyanides;

- non-acceptability for extraction precious and other strategically important metals from solutions contaminated with radioactive wastage;

- low recovery selectivity for an individual metal from a complex solution containing a mixture of different metals including non-precious, such as iron, copper, nickel and other metal ions used as a reducer.

None of the methods discussed allows to extract from a solution all of platinum group metals contained therein by its sequence of similar operations and techniques in result of which in practice technologies are used that represent a combination of multiple various methods constituting a multi-step process with a huge number of various operations and methods. The main reason for such multitude of operations, as seen from the flow-chart in Livingston S. "Chemistry of Ruthenium...", is the fact that at the step of metal ions reduction to the neutral state, selectivity of each metal recovery is not ensured and only a contaminated product (conglomerate) is achieved which incorporates both other precious metals and extraneous admixtures which requires further product treatment (predominantly, refining by various techniques).

From equipment used to extract precious and other metals, an installation is known for separating a metal-containing product from solutions comprising a centrifugal extractor and means for dosing the treated solution (USSR author's certificate No.528100, B 01 D 11/04, 1976). Such installations are employed in nuclear power engineering extraction technology since they are easily operated and reliable. Their combination of intensive mixing of liquid and solid components with effective phase separation under the effect of centrifugal forces provides high productivity even with comparatively small sizes of these arrangements. However, they are not adapted for performing laser extraction processes.

The method is based on the task to implement such interaction between precious metal ions and reducer which would result in selective metal recovery ensured already at the stage of metal reduction.

Resolution of this objective of a method for precious metal recovery from liquid solutions, including precious metal reduction and subsequent separation of the metal-containing product from the solution, is attained by the fact that reduction is carried out by superposing laser radiation having a frequency corresponding to the resonance frequency of ion excitation for ions of the metal to be extracted with selective metal-containing product separation.

Selective metal extraction at the reduction stage is provided by the fact that under the impact of laser radiation (which radiation frequency is selected so as to correspond the frequency of metal ions free oscillations for the metal to be extracted) this particular metal ions are excited and, upon transition in a more active state, interact with the reducer faster than ions of other metals present in the solution which have not acquired such "activity input". In such a process low activity of platinum group metals promotes selective recovery of the desired metal from the complex solution.

Solution of the present objective is also ensured by the fact that

- solution laser radiation treatment is performed in the process of its mixing;

- separation of the metal-containing product and its removal from the radiation zone are carried out continuously as the product forms;

-formic acid, or acetic acid, or oxalic acid, or malonic acid, or formiate salts, or acetate salts, or oxalate salts, or malonic acid salts, or acid hydrazides. or methanol, or ethanol, or uranyi salts are used as a reducer.

Selective metal recovery at the metal reduction stage provided by the above operations and conditions due to the fact that in combining the operations of mixing and solution irradiation with laser beam, not only processes of chemical interaction of the treated solution with the reducer are accelerated, but there is reduction of time required to contact the interacting components and to extract the desired metal without simultaneous involvement into the process of other metals and admixtures contained in the solution. Solving the present task of providing the possibility to conduct laser reduction in an installation for recovering precious metals from liquid solutions comprising a centrifugal extractor and means for dosing chemical reagents is achieved by its being provided with a laser system, whereby the extractor is configured to have a window made of a material transparent in UV visible and IR wavelength range for laser radiation to pass through. Due to metal recovery selectivity, thus already at the stage of its reduction significant decrease in the amount of required equipment is ensured, the process labour intensiveness is reduced and productivity, efficiency and security are enhanced. Decrease in the amount of equipment, the same type of operations for all metals to be extracted allow for this process to be readily automated which is specially important for security provision in working with radioactive solutions (wastage). The method is advantageous for its ecological safety both due to the lack of need to use cyanides and to the fact that after intensive photochemical reactions of metal reduction and recovery no additional adverse wastes are form, all added reagents decompose during photochemical effects into water, nitrogen, methane, CO₂ while during treatment of radioactive wastage a high degree of recovery is secured with retention of original volumes of radioactive materials being treated.

Fig.1 demonstrates the flow-diagram for implementing the method for precious metal extraction from solutions;

Fig.2 shows the outline diagram of the installation for extraction of precious metals and other strategically important materials from solutions.

In order to implement the proposed method, the metal ion solution to be treated is mixed with the reducer, exposed to laser radiation having a frequency corresponding to the resonance frequency of ions of alternately extracted metals, and the resulting metal-containing product is separated (Fig.1 ). Hereby several options of said operations sequence are possible:

- mixing, laser radiation treatment and separation of the metal-containing product are performed in separate technological arrangements interconnected with flow-over channels, i.e. alternately, with the steps being separated in time;

- solution mixing and laser radiation treatment are performed jointly in time while the metal-containing product is separated individually;

- solution mixing, laser radiation treatment and metal-containing product separation are combined and conducted in one technological apparatus having functions of the three arrangements, mixer, laser reactor and separator.

The best sequence to implement the steps of the proposed method is time- combined performance of the three above-identified operations since it contributes to enhanced productivity of precious metal recovery and, additionally, to providing selectivity of recovery of metals present in the solution: rapid removal from the solution of the first metal to be extracted which has made transition to the solid phase recovers solution properties corrupted by the presence of metal particles and creates the necessary prerequisites for extracting the second, third, etc., metals.

The installation diagram shown in fig.2 illustrates this particular option of conducting the process. The installation operates as follows.

The components to be mixed leaving compression feed tanks 1 (for the solution being treated) and 2 (for reducer) at predetermined rates with the assistance of a proportion doser 3 which enables to strictly maintain stream proportions reach through ducts 4 a centrifugal extractor 5, enter its mixing chamber 6 and are mixed with mixer 7. The mixing chamber 6 of extractor 5 is formed with a window 8 configured from a material transparent in IR, visual and UV wavelength ranges through which monochromatic re-adjustable radiation in IR, visual and UV wavelength ranges passes from a laser system 9 to the mixing chamber 6 of the extractor 5. In order to extract each particular metal, the resonance frequency of ions of the metal being recovered is known in advance. It is determined spectrophotometrically using reference mono-solutions of particular precious metal according to the appropriate absorption peak of the frequency-readjustable radiation. To increase the treated zone of the mixing chamber 6, the irradiating laser has a widened beam. The product in the form of solid finely dispersed suspension of the metal-containing product or in the form of emulsion produced as a result of mixing and laser beam treatment at metal ion extraction with an organic solvent is fed with a transporting means 10 to a rotor 11 where under action of centrifugal forces it is separated into phases. Pure phases from the rotor reach annular receiving collectors 12 and 13 for a heavy and a light phases, respectively, and are removed from the extractor. The centrifugal extractor is rotated by a DC electric motor 14. The total consumption of the starting components varied from 1 to 30 l/h which corresponds to change in duration of solution laser radiation treatment from 20 to 0.7 s.

Any known system providing generation of readjustable laser radiation in the resonance frequencies range of ion excitation of the recovered metals may be utilised as the laser system 9 in the proposed installation. In conducting experiments, a laser system was used which comprised a dye laser 15 pumped by an excimer laser 16 was used. In the process of solution mixing and laser radiation at the frequency corresponding to the resonance frequency (free oscillation frequency) of ions of the metal to be recovered, the latter, as a result of such interaction, make transition into an excited state and, therefore, acquire high chemical activity which significantly accelerate their reaction with the reducer. Effectiveness of such metal ion excitation may be judged, for example, by the fact that irradiation of trichloride rhodium solution in hydrochloric acid in the presence of formic acid as a reducer results in forming metallic rhodium already in 30 minutes whereas conventional boiling of hydrochloric acid solution, under otherwise equal conditions, takes 24 hours to accomplish it. And selective rhodium recovery according to the proposed method occurs at the metal reduction stage rather then after refining from metal conglomerate as is the case with known solution heating technologies.

In the process of separation of metal-containing product combined with mixing, the metal being reduced is extensively removed from the solution which enables to start the next cycle, that of recovering the next metal. The next cycle may also be the cycle of repeated solution treatment with laser radiation with the same or different reducer or with reducer being a mixture of several reducer types

Apart from formic acid in the proposed method the following may be used as reducer: acetic acid, oxalic acid, malonic acid, acetates, formiates, oxalates, malonates, hydrazme, methanol, ethanol and other reagents which when exposed to laser radiation display reducer properties, as well as a mixture of these reagents It seems impossible to indicate in advance the specific type of reducer needed to recover the specific metal since the choice of reducer depends on a variety of factors characterising the solution being treated. In selecting the reducer for specific metal recovery, of importance is component composition of the solution, valence of metal ions, solution acidity, etc. Thus, if the particular solution contains even two metals which are recovered for the same time from the start of irradiation with the same reducer (e.g. oxalic acid), then in order to provide recovery selectivity, it may be expedient to use another reducer means which would allow these metals recovery in one cycle of laser irradiation to be set apart in time. Necessity of such choice will become apparent when resonance frequencies of these ions excitation are quite close. Possibility of such setting apart in time of metal recovery is readily available. For instance, the process of metallic palladium laser generation in the presence of oxalic acid begins, virtually, instantaneously from the start of the solution laser irradiation at the resonance frequency. In the presence of formic acid, palladium reduction begins in 20-25 minutes of irradiation at this frequency while in the presence of formic acid with addition of alcohol it starts in 12-15 minutes.

Depending on solution characteristics, even the resonance frequency of ion excitation for the same metal may be somewhat different since ion ligand surrounding may be different in different media.

The method is illustrated with examples set out below of platinum group metal extraction from liquid solutions, examples being sided with in a table. Experiments included in the group of examples were conducted on the installation of Fig.2. An excimer laser λ = 308 nm with pulse power 70-80 mJ and pulse frequency of 10 Hz, a frequency-controlled dye laser or an argon laser, λ = 458 with power output of 260 mW served in the experiments as radiation sources. The resonance frequency was adjusted by the corresponding absorption peak.

Metallic phase produced in a number of cases upon separation was a finely dispersed metal powder useful for utilisation already as such (e.g. as a catalyst) or for re-melting. In one case, in reducing iridium (IV) to iridium (111), the resulting product was separated by means of liquid extraction on the same installation using a number of extractants (e.g. tributyl phosphate in kerosene).

Example 1 illustrates possibilities of the described method when working with radioactive solutions. The experiment used a complex solution containing palladium p²⁺ ions and uranyl UO ions. This solution is a model technological solution produced in treatment of nuclear power installation wastage. When

2+ acted upon by light, uranyl UO₂ cc ions in acidic solutions convert, as a result

2+ of oxidation-reduction reactions, to the U₂ state which is characterised by strong reducing capability. Under the action of this reducer, Pd ions convert to the neutral state which becomes the inception of the solid (metallic) phase. As a result of such interactions of the solution being treated with laser radiation at the palladium ion excitation frequency, palladium phase transition from the solution to the metallic state, appearance of palladium black, was observed.

Palladium reduction process may run at laser resonance excitation of its ions in the presence of other reducers: methanol, ethanol, oxalic acid, formic acid, etc., as well as from solutions of acid of different natures (nitric acid, sulphuric acid, hydrochloric acid).

Reducer may be initially present in industrial technological solutions (e.g. uranyl, ethanol). -Example 2 illustrates this version. Palladium reduction in the presence of ethanol is generally carried out by heating solution to 60- 70°C. Ethanol is a two-electron reducer and oxidises to aldehydes during the process.

Under laser radiation, palladium (II) reduction process runs at the room temperature at a rate substantially higher than that observed for thermal processes. Examination of processes of metallic palladium formation in the presence of ethanol under laser radiation has shown that the reaction rate depends on ethanol concentration in solution.

Palladium reduction rate has also been shown to depend on solution acidity. The upper acidity limit at which no black precipitate formation of freshly precipitated palladium was observed was 4 M/l with conventional chemical method of palladium reduction in which solution warming is used, and palladium black does not form already at acid concentrations over 0.07 M/l. Thus the laser exposure method allows to work with high acidity solutions.

Examples 3-5 demonstrate the possibility to extract palladium from solutions comprising other organic and inorganic reducers: oxalic acid H₂C₂O₄, hydrochloric hydrazine N₂H₄HCI, formic acid HCOOH, under excimer laser irradiation at the wavelength of 308 nm.

The process of palladium laser generation -in the presence of oxalic acid, H₂C₂O₄, was found to commence virtually instantaneously from the start of irradiation. However, the solution acidity threshold was in the region of 0-1.5 M/l of the acid (EΞxample 5). If solution is irradiated at the wavelength of 430 nm (which correlates more with the absorption band of palladium solution in the presence of oxalic acid), then the solution acidity threshold at which metallic palladium formation is observed, significantly rises up to 4-5 M/l (.Example 6). Hydrochloric hydrazine N₂H₄HCI also substantially impacts palladium reduction process (-Example 3). Formation of the black precipitate of metallic palladium is observed already in several minutes (5-8 min) after the start of irradiation.

In the presence of formic acid (.Example 4), reaction runs significantly slower, formation of the black precipitate begins in 20-25 minutes of irradiation.

At the same time, adding alcohol initiates metallic palladium formation in 12-15 minutes.

This activity sequence of reducers H₂C O₄< N₂N₄.HCI< C₂H₅OH< HCOOH in the process of palladium ion reduction coincides with the sequence of palladium absorption band short-wave shift in the presence of these ligands in electron spectra.

An aquated palladium ion has an intensive absorption band at 380 nm. The presence of other ligands such as chloride-ion, oxalate- and hydroxalate-ion, ethyl alcohol, formic acid, hydrochloric hydrazine, shifts the palladium absorption band. Thus, in a hydrochloric acid solution, absorption peak for Pd²⁺ is observed at 465 nm, while addition of alcohol shifts the palladium absorption band in the short-wave region up to 435 nm. In the presence of oxalic acid, the palladium absorption peak is observed already at 404 nm while addition of hydrazine shifts the band even more into the short-wave region up to 394 nm.

-Examples 7, 8 show the research results for processes of platinum laser excitation.

In a solution, platinum compositions may exist in solutions in two valences Pt(IV) and Pt(ll).

Unlike palladium, there is no pure aquated platinum ion in any valence, ^• therefore it is possible to consider electron spectra of only complex platinum ions. For example, in the PtCU spectrum, two distinct regions are discernible: two intensive absorption bands at 330 and 390 nm which are referred to transitions of charge shift from ligand to metal (L -> M) and weak bands at 220 nm. For complexes, however, that contain stronger ligands ligand field bands are shifted to a more short-wave region. Electron spectra Pt(IV) consist of ligand field bands and charge transfer bands which partly overlap and are at 450 nm.

2-

Platinum PtBrβ solutions irradiation in the presence of alcohol C₂H₅OH, oxalic acid H₂C₂0₄, formic acid HCOOH results only in Pt(IV) reduction to Pt(ll) which is observed by change of solution colouring from orange to pale-yellow and by absorption spectra. Further changes in the solutions were not observed (Example 7).

However, when Pt(IV) reduction to Pt(ll) under laser radiation in the presence of formic acid is first conducted and then ethanol is added to the solution and the solution is irradiated once again, formation of black suspension of metallic platinum can be observed (Example 8).

Examples 9, 10 show experiments on separation of palladium and platinum mixture. Depending on the wavelength used, reducer type and acidity of the medium, it is possible both joint precipitation of palladium and platinum and selective separation thereof.

Similar experiments were conducted with rhodium solutions. Like palladium, rhodium is a simpler system for examination since it is present in a solution only in the Rh(lll) state. Electron absorption spectrum of RhC^.^O in hydrochloric acid solution exhibits a single strong absorption band at 460 nm. Adding oxalic acid or its salts shifts the absorption band to 566 nm, while in the presence of formic acid the rhodium (III) absorption peak is at 425 nm.

-Example 1 1 demonstrates that producing rhodium solutions in the presence of alcohol during 1 hour did not result in any change in the solutions. No change of absorption spectra before and after irradiation was observed.

Rhodium solutions irradiation for 30 minutes in the presence of formic acid (-Example 12) changes the solution colour from purple-red to yellow and, correspondingly, the rhodium absorption spectrum changes. In 10 minutes of incubation, from the irradiated solution a grey residue of metallic rhodium begins to precipitate. The process is made substantially more rapid by adding oxalic acid (.Example 13) instead of formic acid. Resulting rhodium oxalates, however, are photochemically active only at 560 nm. Precisely because of this, laser activation is used here in the 560 nm band yielding grey powder of metallic rhodium.

Apparently, in irradiating Rr.CI₃.nH₂O solutions in the presence of reducers, formation of univalent Rh(l) complexes occurs:

RhCI₃ . nH₂O + HCOOH (H₂C-₂O₄) -> Rh (I) which in keeping in the air decompose to rhodium (0).

-Example 14 illustrates the possibility of selective palladium and rhodium separation. It presents no problems to selectively separate rhodium from palladium and platinum due to their principal differences in chemical properties.

Like compositions of platinum, iridium compositions exist in a solution in two valences Ir(IV) and Ir(lll). However, only Ir(IV) compositions are active

2- photochemically. In IrClβ electron spectra great amount of peaks is observed at

320, 410, 425, 490 nm. Therefore, iridium (IV) solution irradiation can be carried out at various wavelengths. IrClβ solution irradiation in the presence of reducers (oxalic acid H₂C₂O₄, alcohol C2H₅OH, formic acid HCOOH) has shown that in most cases reduction of Ir(IV) to Ir(lll) takes place. However, the process rate strongly depended on the nature of reducer: Ir(IV) reduces practically instantaneously in the presence of oxalic acid (-Example 15) but in the presence of ethanol no changes were observed under irradiation for 30 min (-Example 16). Radiation wavelength also affected the rate of reduction process of Ir(IV) to Ir(lll): under solution irradiation at 308 nm reduction took 2-3 min, however at 458 nm this process ran slower during 30-40 minutes.

-Example 17 illustrates the possibility of gold recovery from a solution. Electron spectrum of HAuCU solution has a single clearly expressed and very intensive absorption peak at 310 nm. Because excimer laser emits precisely in the 308 nm region, gold recovery in the laser radiation field proceeds practically instantaneously at acid concentrations up to 5 M/l.

This is also the case with silver. The absorption peak for silver (I) is at 300 nm. Therefore its reduction does on as easily as that of gold (Example 18).

The possibility to separate silver and gold is provided by the rate of solution flow and, correspondingly, by the length of solution treatment with laser radiation (-Example 19). When extracting gold' from a solution containing both gold and silver ions, a high flow rate is maintained which is determined also by initial gold content in the solution, subsequently,, upon separation of gold, the solution containing already exclusively silver, is treated with laser radiation repeatedly to recover metallic silver.

Because actual solutions of radioactive and industrial wastes contain mixtures of different metals, -Example 20 illustrates the possibility of consecutive recovery of precious metals from solutions containing both noble metal ions: platinum, rhodium, iridium, gold, silver, and metal ions of copper, nickel, iron, etc. As a result of laser irradiation of the above complex solutions, selective recovery of gold, the silver, metallic palladium, platinum and rhodium was established. Iridium is extracted as iridium (III) using appropriate extractants.

This invention may be utilised with high efficiency also in cleaning industrial wastage with concurrent recovery of precious and strategically important metals.

Claims

1. A method of recovering precious metals from solutions including precious metal reduction followed by separating the resulting metal-containing product from the solution, distinguished in that reduction is carried out mixing the initial solution in the presence of a reducer under exposure to laser radiation at a frequency corresponding to the resonance frequency of excitation of ions of the metal being recovered and with selective separation of the metal-containing product.

2. A method of claim 1 , distinguished in that selective separation of the resulting metal-containing product in the presence of reducer is carried out by adjusting the frequency of laser excitation of ions of the metal being recovered to the absorption peak.

3. A method of claims 1 and 2, distinguished in that formic acid is used as the reducer.

4. A method of clams 1 and 2, distinguished in that acetic acid is used as the reducer.

5. A method of claims 1 and 2, distinguished in that oxalic acid is used as the reducer.

6. A method of claims 1 and 2, distinguished in that sodium formyate, or ammonium formyate, or potassium formyate is used as the reducer.

7. A method of claims 1 and 2, distinguished in that sodium acetate, or ammonium acetate, or potassium acetate is used as the reducer.

8. A method of claims 1 and 2, distinguished in that sodium oxalate, or ammonium oxalate, or potassium oxalate is used as the reducer.

9. A method of claims 1 and 2, distinguished in that hydrazine is used as the reducer.

10. A method of claims 1 and 2, distinguished in that methanol is used as the reducer.

11. A method of claims 1 and 2, distinguished in that ethanol is used as the reducer.

12. A method of claims 1 and 2, distinguished in that a mixture of several differing from each other reducers is used as the reducer.

13. A method of claim 12, distinguished in that introduction in the solution is performed consecutively: following solution mixing in the presence of one reducer, under exposure to laser irradiation, a second reducer is added to the solution and repeated solution treatment is carried out with laser irradiation mixing it with the reducer with subsequent separation of the metal-containing product.

14. A method of claim 13, distinguished in that in introducing the second reducer, laser radiation frequency is adjusted to the absorption peak of the metal being recovered.

15. An installation for precious metal recovery from solutions comprising a centrifugal extractor and a means for dosing reagents, distinguished in that it is additionally provided with a laser system, the centrifugal extractor being configured with a window made of a material transparent in ultra-violet, visual and infra-red wavelength ranges to transmit laser radiation in a mixing chamber.

16. An installation of claim 15, distinguished in that the laser system is radiation frequency controlled.