JP2019157230A - Recovery method of silver from silver halide-containing scrap raw material - Google Patents

Abstract

To provide a method for efficiently recovering high-purity silver from a scrap raw material containing silver halide at a state of coexistence with many kinds of impurities.SOLUTION: There is provided a method for separating and recovering silver from a scrap raw material containing impurities such as alumina, silica, and iron in addition to silver halide, having a process for adjusting pH of a slurry manufactured by adding water to the scrap raw material with alkali hydroxide and reduction treating the silver halide under a condition with pH of 12 or more, a process for adding nitric acid to a solid component obtained by solid solution separation of the slurry after the reduction treatment to leach silver, a process for solid solution separating a nitric acid solution containing silver obtained by the exudation from the leach residue and then adding sodium chloride or hydrochloric acid to the nitric acid solution to generate silver chloride, and a process for adjusting pH of the slurry manufactured by adding water to the silver chloride with alkali hydroxide and reduction treating again under a condition of pH of 12 or more.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for recovering silver from a silver-based scrap raw material containing a sparingly soluble silver halide coexisting with impurities.

  Wet processing and dry processing are known as methods for separating and recovering noble metal from scrap raw materials containing noble metal, and acid dissolution or cyan dissolution is generally used in the former wet processing. For example, when a silver-based scrap raw material containing silver is wet-treated, first, leaching treatment with nitric acid is performed to dissolve silver. In this case, when dissolving with aqua regia instead of dissolving with nitric acid, silver settles as silver chloride and remains in the residue together with other undissolved substances. Since the remaining silver chloride has low solubility in not only water but also acid or alkaline solution, recovery by wet processing is difficult, and generally it is often processed by dry method. However, in the dry treatment, since exhaust gas contains chlorine and hydrogen chloride, problems such as corrosion are likely to occur in the exhaust gas treatment facility, and a large capital investment is required for this anti-corrosion measure.

  Therefore, Patent Document 1 discloses a method of leaching Ag from a residue containing silver chloride as a main component using a complexing agent to recover metallic silver. It is described that silver can be recovered by transferring Ag to the back-extracted solution side and reducing it. Patent Document 2 discloses a technique of extracting silver using a trialkylphosphine sulfide as an extractant for a hardly soluble silver compound raw material. It is described that a trialkylphosphine sulfide is dissolved in an organic solvent in order to eliminate the contained diphosphine sulfide compound, and the diphosphine sulfide compound is reacted with a predetermined metal to be changed into an organophosphorus compound that does not affect silver extraction. ing.

  Further, Patent Document 3 discloses a wet processing technique for recovering crude silver with very few impurities by leaching a raw material containing a hardly soluble silver compound with an ammonia solution and then reducing the leaching solution with a reducing agent. . Patent Document 4 discloses, as a method for separating and purifying high-purity silver chloride from a refined intermediate containing a sparingly soluble silver compound and an impurity element, a leaching step of leaching the refined intermediate in a sulfite aqueous solution, A method comprising a silver chloride production step of acidifying the leaching product solution containing silver obtained in the leaching step to precipitate silver chloride, and a silver chloride purification step of purifying the silver chloride by oxidizing it with an oxidizing agent. It is disclosed. This method does not require a pretreatment of a refining intermediate, and it is described that it is not necessary to carry out dry refining of metal silver or repurification by electrolysis when producing metal silver using it as a raw material.

JP 2003-105456 A JP 2002-003958 A JP 2000-297332 A International Publication No. 2005/023716

  However, the techniques described in Patent Documents 1 and 2 need to provide a dedicated extraction device and a back extraction device for performing processing by a solvent extraction method separately from the dissolution reaction tank. Further, the process becomes special, and the extractant itself may become a part of impurities. The technique of Patent Document 3 requires the addition of excess ammonia, and when the ammonia concentration is increased, there is concern about the formation of explosive silver compounds such as thunder silver, and caution is required in terms of safety. In the technique of Patent Document 4 described above, for example, when silver chloride is reacted with sodium sulfite in a sulfite aqueous solution as a hardly soluble silver compound to form a silver sulfide complex salt, reaction conditions such as sulfite ion concentration, pH, and temperature are set. Since it is necessary to maintain each within a suitable range, it is difficult to adjust, and problems such as a decrease in actual yield occur when these reaction conditions deviate from appropriate values. As described above, the conventional technique for recovering silver from silver halide by wet processing has a problem in that the process is special and complicated, and in some cases, a special chemical is required, so that the cost is high. It was.

  The present invention has been made in view of the problems of the above-described conventional wet processing technology for silver-based scrap raw materials. From the scrap raw materials in which silver halide is coexisting with many kinds of impurities, the present invention is efficient. Another object of the present invention is to provide a method for recovering highly pure silver.

  In order to achieve the above object, a silver recovery method according to the present invention separates silver from a scrap material containing at least one component of alumina, silica, iron, nickel, and copper as impurities in addition to silver halide. And reducing the silver halide under a condition of pH 12 or more by adjusting the pH of the slurry prepared by adding water to the scrap raw material, and the reduction treatment Solid-liquid separation of the later slurry, the step of leaching silver by adding nitric acid to the solid content containing the impurities, and after solid-liquid separation of the nitric acid solution containing silver obtained by the leaching from the leaching residue A step of producing silver chloride by adding sodium chloride or hydrochloric acid to the nitric acid solution, and adjusting the pH of the slurry prepared by adding water to the silver chloride with an alkali hydroxide to a pH of 12 or more. It is characterized by a step of reduction treatment again under.

  ADVANTAGE OF THE INVENTION According to this invention, highly purified silver can be efficiently collect | recovered by the wet method from the scrap raw material contained in the state in which the hardly soluble silver halide coexisted with the impurity.

It is a process flow figure showing a recovery method of silver of an embodiment of the present invention.

  Hereinafter, a silver recovery method according to an embodiment of the present invention will be described in detail with reference to FIG. The silver recovery method according to the embodiment of the present invention includes, in addition to poorly soluble silver halides such as silver chloride and silver bromide, poorly soluble oxides such as alumina and silica, iron, nickel and copper as impurities. For the slurry in which industrial water is added to silver-based scrap raw material coexisting with at least one component, first, in the first impurity removal step S1, the pH is adjusted to 12 or more using an alkali hydroxide such as caustic soda, and the liquid temperature is preferably Halogenated at a redox potential of −600 to −800 mV (reference electrode: silver / silver chloride electrode) by adding a reducing agent such as hydrazine in a state heated to about 50 to 90 ° C., more preferably about 70 ° C. Reduce silver. This reduction treatment is considered to cause the reactions of the following formulas 1 to 3.

[Formula 1]
AgX + NaOH → AgOH + NaX (wherein X is a halogen element)
[Formula 2]
2AgOH → Ag ₂ O + H ₂ O
[Formula 3]
2Ag ₂ O + N ₂ H ₄ · H ₂ O → ₄ Ag + 3H ₂ O + N ₂ ↑

By the reaction of these formulas 1 to 3, silver is dissolved in nitric acid. As shown in the above formula 2, since AgOH is an unstable substance, it is easily decomposed into Ag ₂ O and H ₂ O. By performing solid-liquid separation of the slurry after the reduction treatment by solid-liquid separation means such as filtration, silver, iron, nickel, and copper are recovered as a solid content side together with a part of alumina and silica, and the solid content is the remainder. It is separated from the liquid phase side containing alumina and silica and chlorine ions, bromine ions and sodium ions.

  Next, nitric acid is added to the solid content in the second impurity removal step S2. Thereby, silver is leached into nitric acid to obtain a nitric acid solution as a silver leaching solution. In addition to silver, iron, copper, and nickel can be dissolved in nitric acid, but alumina and silica cannot be dissolved. Therefore, the slurry after the nitric acid leaching treatment is subjected to solid-liquid separation by solid-liquid separation means such as filtration. And impurities made of oxides such as silica are removed from the silver leachate (nitric acid solution). Next, in the third impurity removal step S3, silver chloride having low impurity quality can be produced by adding a stoichiometrically necessary amount of hydrochloric acid or sodium chloride to the nitric acid solution containing silver. By solid-liquid separation of the solution containing silver chloride by solid-liquid separation means such as filtration, solid silver chloride can be separated from the liquid phase side containing iron, copper, and nickel as impurities.

  Next to this silver chloride, an aqueous solution of alkali hydroxide such as caustic soda is added to adjust the pH to 12 or more in the silver powder recovery step S4 as in the first impurity removal step S1, and the liquid temperature is preferably 50 to 50. Silver chloride is reduced at a redox potential of −600 to −800 mV (reference electrode: silver / silver chloride electrode) by adding a reducing agent such as hydrazine while heated to about 90 ° C., more preferably about 70 ° C. Process to produce metallic silver powder. High purity silver powder can be recovered by solid-liquid separation of the slurry containing silver powder by solid-liquid separation means such as filtration.

(Example)
First, as a raw material for silver powder, aqua regia-dissolved residue scraps containing 5% by mass of silver remaining after recovering gold, platinum, and palladium using aqua regia were prepared. When this scrap material was subjected to fluorescent X-ray analysis, it contained 5.7% by mass of chlorine and 0.3% by mass of bromine. In addition, it contained alumina and silica, which are hardly soluble oxides, as impurities. After adding water to 10 g of the scrap raw material to make a slurry with a slurry concentration of 100 g / L, sodium hydroxide was added to the slurry to adjust to pH 12 and the liquid temperature was heated to 70 ° C. to promote the reaction. . Under this condition, hydrazine hydrate as a reducing agent was added to the reference electrode until the oxidation / reduction potential using a silver / silver chloride electrode became −700 mV, and reduction treatment was performed. When X-ray fluorescence analysis was performed on the solid content obtained by solid-liquid separation of the slurry after the reduction treatment by filtration, neither chlorine nor bromine was detected.

  Next, after the solid content after the above reduction treatment is dissolved in 100 mL of nitric acid aqueous solution (nitric acid 1: water 1 having a concentration of 67.5% by mass on the basis of volume), the residue is removed by filtration, When silver was analyzed, the leaching rate of silver into nitric acid was 99.4%. Next, 1 mL of hydrochloric acid having a concentration of 35 mass% was added to the aqueous nitric acid solution in which the silver had been leached at least twice the theoretical equivalent to produce silver chloride. The silver chloride is recovered by filtration, and water is added thereto to make a slurry, and then sodium hydroxide is added to adjust to pH 12, and the hydrazine hydrate is used as a reference electrode under the condition that the liquid temperature is heated to 70 ° C. Then, a reduction treatment was performed by adding to an oxidation-reduction potential of −700 mV using a silver / silver chloride electrode. As a result, a silver powder having a low impurity quality of 99.9% could be obtained.

(Comparative Example 1)
When the aqua regia dissolution residue scrap material prepared in the same manner as in the example was dissolved in 100 mL of nitric acid in the same manner as in the example except that the reduction treatment with hydrated hydrazine at pH 12 was not carried out, almost no silver was obtained. Most of the residue remained on the residue side without leaching.

(Comparative Example 2)
Except that the pH was adjusted to 10 instead of 12, the first reduction treatment was carried out in the same manner as in the example. As a result, chlorine was contained in the solid content obtained by solid-liquid separation after the first reduction treatment. 0.6% by mass and 0.1% by mass of bromine were contained. Next, the solid content after the reduction treatment was dissolved in 100 mL of nitric acid in the same manner as in the example, and silver in the aqueous nitric acid solution was analyzed. As a result, the leaching rate of silver into nitric acid was 86.4%. It was found that more silver remained in the residue than in the example. That is, in the reduction treatment at a pH of about 10, the reduction reaction of silver halide becomes incomplete, and as a result, the silver halide having low solubility in nitric acid remains in the residue, so the silver recovery rate is low. I understand that

S1 First impurity removal step S2 Second impurity removal step S3 Third impurity removal step S4 Silver powder recovery step

Claims (3)

A method of separating and recovering silver from a scrap material containing at least one component of alumina, silica, iron, nickel, and copper as impurities in addition to silver halide,
Adjusting the pH of the slurry prepared by adding water to the scrap raw material with an alkali hydroxide to reduce the silver halide under a pH of 12 or more, and solid-liquid separation of the slurry after the reduction treatment, A step of leaching silver by adding nitric acid to the solid content containing impurities, and a solid solution separation of the nitric acid solution containing silver obtained by the leaching from the leaching residue, and then adding sodium chloride or hydrochloric acid to the nitric acid solution And adding silver to produce silver chloride, and adjusting the pH of the slurry prepared by adding water to the silver chloride with alkali hydroxide and reducing again under conditions of pH 12 or higher. And silver recovery method.   The silver recovery method according to claim 1, wherein the silver halide is silver chloride or silver bromide.   The silver recovery method according to claim 1 or 2, wherein the reduction treatment of the silver halide is performed with hydrazine.

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