Recovering silver from other silver-containing waste materials

Fire smelting-electrolysis, as well as methods for recovering silver from photosensitive materials, are also suitable for processing other silver-containing materials. Here are some ways to recover silver from other silver-containing materials.

First, the fire method smelting

U.S. Patent describes a pyrometallurgical method is to use copper or nickel as the trapping agent and the noble metal scrap smelting alloy material together and then subjected to pickling treatment, or electrolytic recovery of a noble metal alloy feed.

A Austrian Patent Recommended: recovery of precious metals, silver and other precious metals from a slag containing the residue or concentrate feed or a silver-containing alloy as trapping agent, wherein the noble metal can be quantitatively recovered in the smelting fire, compared with the effect of lead or Copper is good as a collector.

Relevant literature reports: Oxygen refining can be used to treat silver scrap without other precious metals. If the raw material containing 96% to 98% of silver is melted, the surface of the molten metal is subjected to oxygen refining for 20 to 30 minutes while maintaining the furnace temperature at 1050 to 1150 ° C to produce silver having a purity of 99.9%.

Second, the electrolysis method

Electrolysis is suitable for extracting silver from metal scraps (such as currency, solder, silk sheets, jewelry, decorations, metal scraps and alloys), as well as from silver-containing solutions (such as fixer, plating solution). Silver is extracted from washing water and various silver-containing waste liquids. The key is how to choose the appropriate electrolysis operation system and electrolyte formula according to the nature and composition of the raw materials or feed liquid. The following is a brief description of some processing examples.

For example, if a silver-copper alloy containing less than 3% of silver is used, copper is electrolyzed by using a sulfuric acid or copper sulfate electrolyte to recover silver from the anode slime. An alloy containing 5% to 25% of silver is electrolyzed from a sulfuric acid-containing electrolyte to produce a cathode copper having a purity of 99.9%. The silver remains in the form of a skeleton on the anode, and the purity can reach 92% to 96%. In order to improve the quality of the cathode copper, a small amount of chloride should be added during electrolysis to precipitate a trace amount of silver into the solution. Alloys containing more than 25% silver should be blended with an appropriate amount of copper. The anode is controlled to contain silver in the range of 10% to 25%, so that the anode is not passivated during the electrolysis process and a large amount of silver enters the solution.

A stainless steel solder alloy containing 5% to 30% silver and 2% to 15% copper can be electrolyzed in a nitric acid or silver nitrate solution. Under the conditions of a cell voltage of 0.1 to 0.5 V and an area current of 30 A/m ² , silver having a purity of 95% to 99% can be produced.

The silver- palladium plating on the copper substrate can be electrolyzed under the conditions of a sulfuric acid concentration of more than 100 g/L, a liquid temperature of 40 to 55 ° C, and a current density of 5 A per kg of material.

The silver is recovered from the silver- cadmium oxide-brass composite contact material, and the metal titanium titanium basket anode and the stainless steel cathode are used in an electrolyte of potassium fluoride 70-100 g ∕L and silver fluoride 20-40 g ∕L. Electrolysis was carried out at 40 ° C, 10 A ∕ dm ² and a cell voltage of 0.76 to 1.28 V. The purity of the cathode silver can reach 99.3%, and the silver recovery rate is 98.2%.

A gold-based alloy containing 8% to 10% of silver and 5% of copper, first electrolyzing gold in a hydrochloric acid medium, and recovering silver from the anode slime.

Using a electrolytic cell equipped with a rotating titanium anode, 290 L of waste electrolyte containing (g/L) silver 21.3, potassium 30.8, cadmium 1.98, copper 0.24, zinc 0.05 was treated, and electrolysis was carried out for 8 h at a current density of 10 A ∕dm ^{2 to} recover silver 1.783. Kg.

A batch of three-stage tandem electrolytic cells were used to treat a batch of silver nitrate electrolytic waste liquid containing 40-90 g of silver and 60-130 g of copper. Electrolysis was carried out under conditions of a total cell voltage of 8 to 9 V, an area current of 400 A ∕ m ² , a liquid temperature of 40 to 45 ° C, and an electrolyte circulation rate of 0.25 L ∕ h. The first trough produces two-thirds of the total silver, with a purity of 99.9%; the second trough produces one-third of the total silver, with a purity of 98.6% to 99.7%; Copper with a copper content of 99% or more has a purity of 98% or more. The total current efficiency is 95% to 96%.

Third, the dissolution method

Dissolution method, which can recover silver from various silver scraps, silver alloys and silver-plated products, such as:

The silver is recovered from the silver-zinc battery by breaking the battery, picking up the metal block, crushing and dissolving it in nitric acid, and then adding copper to replace the crude silver for further refining. Or silver nitrate was added to a solution of sodium chloride, the silver precipitate was generated silver cyanide, together with scrap iron feed displacement reduction refining.

Recycling gold and silver of various product coatings, using 80%-85% cyanide, 0.05%-1% lithium , 12%-20% nitrobenzoate, 0.05%~0.25% additives, will They are dissolved in water to make an aqueous solution of 25-35 g ∕L, heated to 25-35 ° C, and the product is immersed in the solution for 30 s to completely dissolve the gold and silver plating on the product.

The silver plating on the bronze product was immersed in an aqueous solution of 10 g/L sodium ethylenediaminetetraacetate in an aqueous solution of hydrogen peroxide to dissolve the silver plating on the product. This solution does not corrode the bronze matrix and is non-toxic.

Silver recovery lens thermos bile and other glass products, the silver decorations, using 2% to 10% sulfuric acid and 1% to 5% anhydrous chromic acid solution, immersion in liquid temperature 10 ~ 12min 16 ℃, i.e., the conversion of silver It is silver sulfate. A salt solution is added to the solution to cause silver chloride to be purified. It can also be dissolved with dilute nitric acid solution (acid: water = 1:6 to 8) under stirring, and then precipitated into silver chloride, and then iron chloride is reduced to crude silver and then purified.

Fourth, precipitation method

The sodium sulfite reduction precipitation method can precipitate silver from various silver-containing waste liquids. The recovery rate of silver is as high as 99%, the purity is large, and the cost is low.

To each liter of mirror wash water (containing silver 2.0×10 ^-4 , pH 9 ), 150 mL of 12% NaOH solution was added, and after 7.5 h, 98.8% of silver was precipitated.

Hydrochloric acid (or salt strontium) is added to the silver-containing wastewater to precipitate silver chloride and then purified. This method is a conventional method for recovering silver in a silver-containing waste liquid, and the recovery rate of silver is high and the interference of a large amount of impurity ions existing in the solution can be excluded.

Five, the treatment of silver chloride

The treatment of silver chloride is mostly carried out by a fire reduction smelting method, especially in the case of processing large quantities. The method comprises mixing dry (or dried) silver chloride with sodium carbonate and iron filings, and performing reduction smelting at 1050 to 1150 ° C to produce metallic silver.

The dry silver chloride is passed through a hydrogen gas in a tubular furnace at 360-400 ° C (small amount should be supplied when hydrogen is started to prevent the oxygen present in the furnace from reacting strongly with hydrogen to cause explosion), as long as the silver chloride is pure, It can produce silver of high purity.

Silver chloride is placed in an acidic solution of dilute sulfuric acid or hydrochloric acid, and then iron scraps are substituted for metallic silver, but the replacement process is slow. It can also be reduced with sodium borohydride or zinc powder in an acidic solution at 60 to 80 °C.

The hydrazine hydrate reduction silver chloride method is to first wash the silver chloride with hot water, and then add 0.3 to 0.4 kg of hydrazine hydrate and 1.2 to 1.6 kg of ammonia water per kilogram of silver for reduction.

100 parts of dried silver chloride was added to 350 parts of hot sulfuric acid, heated for 10 minutes, and then cooled to 50 to 60 ° C, and poured into cold water. The obtained silver sulfate crystal has a purity of more than 99%, and the recovery rate of silver is 90%.

The silver chloride Filter cake was leached at 25 ° C with a solution of calcium chloride saturated dimethyl sulfoxide, and the insoluble impurities were removed by filtration to make the Ca(AgCl ₂ ) ₂ in the filtrate larger than 196 g ∕L. By adding water equivalent to 30% to 40% by volume to the filtrate, pure silver chloride can be precipitated. After the filtrate is distilled off, it can be returned to use. In order to control the content of impurities, the leachate should be discarded periodically after being recycled for a period of time.

The Japanese patent recommends that the high-purity silver chloride containing 74% silver be obtained from the treated copper anode mud by wet method, and then converted into AgOH by treatment with 114g of NaOH solution, and then reduced by glucose to obtain a purity of 99.999%. The recovery rate of silver powder and silver was 97.2%.

The dry silver chloride was dissolved in 28-29% ammonia solution and reduced with 0.94 mol of ascorbic acid, and the recovery rate of silver was 100%. Ascorbic acid reduces silver in view of the fact that the reduction potentials of the former at pH 4 and pH 7 are 0.127 V and 0.34 V, respectively, so that the standard potential + 0.8 V silver can be reduced at pH 4-7. Test about 128g of silver chloride per liter of suspension, add 4mL of concentrated hydrochloric acid and 100mL of concentrated nitric acid per liter of liquid, heat to 100 °C and slowly stir to slag until the slag is completely white (the supernatant contains impurities usually light Green) so far. A saturated solution of potassium permanganate was added dropwise to the boiling solution until the solution was a brown and held 1min. Heating was continued until the brown color disappeared. After cooling, the supernatant was decanted and filtered on a dense filter paper. The filtered silver chloride is immediately transferred to the dissolving dish (to prevent the formation of thunder silver-Ag ₃ N or AgH ₂ N to dry to no moisture or to explode for a long time), add concentrated ammonia water to completely dissolve under stirring, and then add 0.94 mol. Ascorbic acid solution (containing 176.1 g of ascorbic acid per liter of distilled water) until no more silver precipitates. After standing, the ammonia solution was decanted for reuse. The precipitated silver was filtered, washed twice with deionized water, dried and then ingot. This method is also suitable for the treatment of silver bromide and silver cyanide-containing solution. However, when the silver cyanide solution is treated, the silver chloride may be first precipitated with sodium hypochlorite and then treated.

Six other methods

There are many methods for recovering silver from waste materials, such as pressure leaching, pressurized reduction, ion exchange, activated carbon adsorption, solvent extraction, mechanical brushing, flotation, cyanide precipitation, and heating. Decomposition method and ray irradiation method, etc., will not be introduced one by one here.

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