Various X-ray and photographic film and film in the process are fine crystals of silver salt emulsion film body on the sheet coated with a thin layer of the base. About 1 mg of silver is contained per square centimeter of the film. This film mainly containing silver bromide, and silver chloride and containing a small amount of silver iodide. The silver salt on the film, when exposed to X-rays or light, produces a photoelectric effect that produces a physical and chemical reaction. The silver bromide crystals after exposed to light, due to the impact of electrons, ions from neutral atoms bromo lose electrons, positive ions and electrons obtained silver atoms reduced to silver metal. This process is called exposure. The reaction can be expressed by the following formula:
AgBr 
Ag+Br
The role of the developer is to further reduce the exposed silver salt to metallic silver, and to securely store it on the film for imaging. During the development process, the unexposed silver salt remains in the original silver salt state and stays on the film. Its role is roughly: exposed silver salt + developer Metallic silver + developer oxide + soluble bromide. If hydroquinone is used as a developer, its development is:
C6H4(OH) 2 H + +(C 6 H 4 O 2 ) 2 -
(C 6 H 4 O 2 ) 2 - +2Ag + =C 6 H 4 O 2 +2Ag
2H + +2Br - =2HBr
The role of the fixer is to dissolve the unexposed silver salt from the film. The main component of the fixer is sodium thiosulfate (Haibo), which does not affect the silver that has been reduced to metal, but reacts with the unexposed silver salt to form a soluble complex salt to dissolve it from the film. Come down. The response is:
Na 2 S 2 O 3 +AgBr NaAgS 2 O 3 +NaBr
NaAgS 2 O 3 +Na 2 S 2 O 3 Na 2 Ag(S 2 O 3 ) 2
2NaAgS 2 O 3 +Na 2 S 2 O 3 Na 4 Ag 2 (S 2 O 3 ) 3
3NaAgS 2 O 3 +Na 2 S 2 O 3 Na 5 Ag 3 (S 2 O 3 ) 4
For various films and films, the amount of silver remaining on the film per unit area and dissolved in the fixing solution is different due to the difference in exposure. Taking a standard exposure X-ray chestlet as an example, about one-fifth of the silver salt is reduced on the film, and four-fifths of the silver salt is exposed to dissolve in the fixer. Black and white movies and TV copies, the average reduction of their silver salts is roughly the same as that of X-ray films.
The image of the color film is imaged by the production of organic dyes in the multilayer structure of the film during the printing process. Therefore, all the silver enters the solution when the color film is printed, and the film does not actually contain silver. Although the color sheet for producing a multi-layer structure per square meter is more silver (about 1.05:1) than the black and white sheet, it can be concentrated in the fixing solution for recycling.
Since the silver and silver in the state of the film and the image are dissolved into the fixing solution during the printing process, the silver content is continuously accumulated by the repeatedly used fixing solution, and the sodium thiosulfate is continuously converted into the sodium thiosulfate silver salt. , and gradually reduce the fixing effect, so that the fixing time is constantly increasing. When the amount of silver reaches a certain saturation concentration, the fixing solution loses its effect and is scrapped. Specifically, the reasons for the scrap liquid scrap are:
(1) During the fixing process, as the film is placed and raised, the developing solution and the rinsing water are continuously introduced to dilute the fixing solution, and the concentration and acidity of the fixing solution are lowered.
(2) When the film is raised, a part of the fixing liquid is brought to the rinsing water, causing loss of the fixing solution.
(3) During the fixing process, the silver dissolved in the film is continuously combined with sodium thiosulfate to form silver thiosulfate, so that the content of free sodium thiosulfate in the fixing solution is continuously lowered.
Among the above reasons, the third item has the greatest impact. Therefore, the regeneration of the fixing solution is to recover the silver in the silver thiosulfate silver complex salt in the fixing solution, and re-convert it into sodium thiosulfate, and the fixing solution is regenerated. This method recovers silver from the fixing solution, and the fixing solution can be returned to use after silver extraction. However, some units process the fixer just to recover the silver.
Important methods for recovering silver from the fixing solution and the washing water include a metal replacement precipitation method, a sulfurization precipitation method, and an insoluble anode electrolysis method.
1. Metal replacement precipitation method
The metal displacement precipitation method is one of the convenient methods for recovering silver from the fixing solution. The method can use metals such as iron , copper , zinc , aluminum and magnesium, and the most commonly used metal iron. Before the replacement, it is preferred to add 0.5% by volume of concentrated sulfuric acid to the fixing solution. The main disadvantage of the metal replacement method is that the replacement metal dissolves into the solution, so that the fixing solution cannot be returned to use. For example, the replacement of iron is to add iron or iron filings or iron powder to the acidic fixing solution, and the silver is replaced by a reduction precipitate:
3NaAgS 2 O 3 +Fe=3Ag↓+Na 2 Fe(S 2 O 3 ) 3
The replacement operation is to add about 5 mL of concentrated sulfuric acid per liter of the fixing solution under stirring until the solution turns yellow-green. Do not add excess sulfuric acid. Since the excess sulfuric acid decomposes NaAgS 2 O 3 , the solution is milky white turbid, and the sulfur content in the displaced silver is increased. However, the sulfuric acid is added too little, and the silver deposited on the iron is not easily washed. When the fixing solution is left for too long and acidified to yellow-green due to absorption of carbon dioxide in the air, sulfuric acid may be added with or without addition.
For replacement under static conditions, thin iron sheets or iron filings are generally used. Before use, the oil and oxide on the surface of the iron are removed by immersion with hot water and dilute hydrochloric acid, and washed with water and then added to the fixing solution. At the beginning of the replacement, the solution is blackened by the dissolution of iron and the formation of sulfides, and finally the solution is colorless and transparent. The replacement process takes about 48 hours.
After the replacement is completed, the supernatant is decanted and water is added to wash the silver on the iron piece. The washed product contains fine silver powder, carbon, iron oxide, silver sulfide, etc., and is black. After standing to precipitate, the supernatant was decanted, filtered and washed with water 1 or 2 times. Then, the mixture is transferred into a beaker, and an iron piece of the same weight and an appropriate amount of concentrated hydrochloric acid are added for boiling for 15-20 minutes to reduce the silver sulfide and remove the hydrochloric acid soluble matter. It was further washed with water and decanted twice, filtered, and washed with distilled water until there was no Cl - . The crude silver powder obtained after drying contains more than 98% of silver.
It is reported that steel wool (or zinc wire, aluminum scrap or brass scrap) is placed on a perforated plate (false bottom) provided in the lower portion of the cylindrical plastic replacement tank, and the fixing liquid is supplied to the porous pipe from the liquid supply pipe at the center of the displacement tank. Below the false bottom plate, the solution rises countercurrently through the porous plate and reacts with the replacement metal, and is discharged from the overflow drain pipe on the upper side of the groove, and the displaced silver powder falls into the bottom of the groove. In theory, 3.9 kg of silver can be replaced per kilogram of iron. When used to treat a fixing solution containing 2.5 g of lanthanum silver, 4 kg of steel wool actually recovered 3.42 kg of silver. When a vinyl-based hydrocarbon or a tertiary amine-based corrosion inhibitor is added to the fixing solution, the corrosion of the solution against iron can be prevented, and the silver replaced per kilogram of steel wool is increased from 1.48 kg to 1.56 kg.
The fixing solution was passed through two displacement columns equipped with cast iron powder having a particle size of 100 to 2000 μm, and 90% or more of silver was recovered from the solution after about 30 hours.
Add 1 to 30 g of citrate per liter of fixer to form a complex with silver, and then replace it with aluminum or aluminum wire (or scrap brass heat sink on the car) for about 1 minute. Replace the silver inside.
A fixing liquid 320L containing 6 to 7 g of silver and pH 4.5 was taken, and 1.95 kg of silver was recovered by passing through a displacement tank containing 500 g of aluminum-magnesium alloy scrap at a flow rate of 135 mL of ∕min.
Second, the sulfide precipitation method
The sulfurization precipitation method is based on the maximum affinity of sulfur and silver. It is known that the solubility product of Ag 2 S is 6 × 10 -50 , which is one of the most difficult to dissolve in water. The sulfurization precipitation method has the addition of sodium sulfide and hydrogen sulfide. The main reactions of these two methods are:
Na 2 S+2NaAgS 2 O 3 Ag 2 S↓+2Na 2 S 2 O 3
H 2 S+2NaAgS 2 O 3 Ag 2 S↓+Na 2 S 2 O 3 +H 2 S 2 O 3
In the sodium sulfide method, a sodium sulfide solution is added to the fixing solution at room temperature under stirring. The amount of sodium sulfide added is 1 to 1.5 kg of sodium sulfide per kg of silver. Hydrogen sulfide method, hydrogen sulfide gas is introduced into the fixing solution at room temperature.
The end point of the operation of the sulfide precipitation method is to take 2 to 3 drops of the clear liquid in the late stage of the vulcanization operation on the filter paper, and then drop a drop of the sodium sulfide solution to the wet edge of the droplet. If the paper shows a black or dark brown precipitate, indicating that the silver is not yet at the end, the sodium sulfide (or hydrogen sulfide) should be added with continued stirring. The operation is terminated until the edge of the droplet is light yellowish brown. Then, it is allowed to stand for 1 to 2 days, the supernatant is extracted, and then heated to boiling to make Ag 2 S agglomerate. After a little cold, it is filtered, washed and dried.
Simple methods for recovering silver from silver sulfide are:
(1) Nitric acid oxidation
After the silver sulfide is wetted with water, it is dissolved by adding dilute nitric acid (acid: water = 1: 2 to 3). At this time, silver sulfide is decomposed by nitric acid and precipitates monomer sulfur. Then, it was filtered, and then brine was added to the AgNO 3 filtrate to form a silver chloride precipitate. After standing, the supernatant was removed, and then heated to boiling to cause the silver chloride to aggregate, and then filtered and washed. The washed silver chloride is added to the alkaline solution to reduce hydration:
AgCl+2NH 3 Ag(NH 3 ) 2 +Cl -
4Ag(NH 3 ) 2 + +2N 2 H 2 4Ag↓+2N 2 +8NH 3 +4H 2 O
After the produced silver powder is filtered, washed with water and dried, it contains 99.5% of silver.
(2) Iron sheet replacement method
To each 100g of silver sulfide, 250mL of concentrated hydrochloric acid and about 75g of thin iron piece are added, heated to boiling in a fume hood (with hydrogen sulfide release), and then moved to the asbestos mesh mat to continue heating, which is completely reduced to powder after about 1 hour of silver. The heating was decanted, the supernatant was washed, water was added and the residual iron pieces were picked up, filtered, washed with distilled water until dry without Cl.
(3) Aluminum chip replacement method
The silver sulfide is placed in a solution containing NaOH, and the aluminum filings of -4 mesh are added, and the silver is reduced to metallic silver:
2Al+8NaOH+3Ag 2 S+2H 2 O 3Na 2 S+6Ag↓+Na 2 Al 2 O 4 +6H 2 O
The reduced metallic silver powder is filtered off, and after drying, a flux and an oxidizing agent are added for pyrometallurgical refining. This method has been applied to the industrial production of the Nipissing Mining Company in Ontario, Canada.
The waste fixer is regenerated after being silvered by sodium sulfide and can be returned to use. When the hydrogen sulfide method is employed, since hydrogen sulfide has a weak acid property and a reducing action, when a large amount of hydrogen sulfide is introduced into the fixing solution, sulfur is precipitated due to oxidation of hydrogen sulfide itself and destruction of sodium thiosulfate. The fixing solution becomes cloudy and cannot be returned to use. And hydrogen sulfide is highly toxic, so it should be avoided. For the same reason, some researchers believe that when using sodium sulfide, a certain amount of sodium hydroxide should be added to the fixing solution before adding sodium sulfide to make the fixing solution neutral or weakly alkaline (with stone). The paper is measured to increase the OH - ion concentration and to avoid the generation of a large amount of hydrogen sulfide by the sodium sulfide under weakly acidic conditions.
During the fixing process, the concentration of sodium thiosulfate decreases due to dilution and consumption. During the silver extraction process, the potassium strontium was destroyed and the acetic acid was neutralized. Therefore, after the fixing solution is silvered, it is necessary to add the medicine before returning to use. According to some testers, each 45L (10 gallons) should be supplemented with 2 to 4 kg of sodium thiosulfate and 300 to 400 g of potassium. And add an appropriate amount (usually around 400mL) glacial acetic acid, so that the fixing solution is weakly acidic.
Third, insoluble anode electrolysis
The extraction of silver from the fixing solution by electrolysis has received extensive attention from all over the world. In the past three decades, there have been dozens of methods and equipment for electrolytic silver extraction that have been tested and recommended by various researchers. The advantage of the insoluble anodic electrolysis method is that metal silver having a purity of more than 90% can be directly obtained without adding impurity ions to the fixing solution, thereby not affecting the return use of the solution.
If the amount of the fixing solution is large and the amount of recoverable silver is large, electrolytic cell electrolysis using a thyristor or a silicon rectifier can be used.
Since September 1974, a film studio in China has used the equipment shown in Figure 1 to extract silver from the fixing slag. The 38 batches of industrial production tests are summarized as follows:
Figure 1 Schematic diagram of electrolytic silver extraction equipment system
1-silicon rectifier; 2-silver machine; 3-plastic pump; 4-reservoir;
5-film processor overflow manifold
The silvering machine used in the test was a cylindrical plastic tank with a graphite anode at the center, a diameter of 750 mm, a height of 500 mm and an area of ​​0.395 m 2 . Outside the anode is a stainless steel cathode with a length of 1000 mm and a width of 500 mm. The anode to cathode area ratio was 1.26:1. The pole spacing is 35 to 40 mm.
The electrolyte for the test contained 2.5 to 9.3 g of silver, 240 to 260 g of sodium thiosulfate, 20 g of sodium sulfite, and 20 mg of glacial acetic acid. The fixing solution was treated with 510 L per batch under the conditions of a cell voltage of 2 to 2.2 V, an area current of 175 to 195 A ∕ m 2 , a liquid temperature of 20 to 25 ° C, and an electrolyte circulation linear velocity of 4.82 m/s. After electrowinning for 3~4h (fixing solution containing silver 3~4g∕L) or 5~6h (fixing solution containing silver 5~6g∕L), the solution contains silver to 0.5~0.7g∕L, and returns to the printing workshop. .
Test results: silver recovery rate of 95.76%, electrolytic silver purity of 90% to 93%, current efficiency of 72.51%, electricity consumption per ton of 4100 ~ 4700kW · h, direct production costs of 2000 ~ 2500 yuan / t silver.
Under the above test conditions, the rate of silver precipitation at the anode is very fast within 2 to 4 hours from the start of energization, and the precipitation rate decreases as the silver content in the solution decreases. When the solution contains silver to 0.5 ~ 0.8g ∕ L, it can be returned to the printing workshop. If the silver content of the solution is required to fall below 0.5 g/L, the current density should be greatly reduced during the extended electrolysis time. Because the dissociation of sulfur at a large current density causes the solution to turbid, and the small energy can be returned to use.
Figure 2 shows a small electrolytic device recommended in Japan in 1980. Its cathode is made of stainless steel and has a bell-shaped shape that can vibrate vertically. The cathode has a height of 220 mm, an outer diameter of 102 mm, and an inner diameter of 97 mm. There is a graphite anode inside and outside the cathode. The outer anode is 200mm high, the inner diameter is 142mm, the inner anode is 200mm high, and the outer diameter is 60mm. During the electrolysis process, the fixing solution flows between the outer anode and the anode, passes between the cathode and the inner anode, and finally exits from the inner center of the inner anode. The cell volume is 1.3L. At room temperature and circulating liquid volume of 6.2 L ∕h, current density of 2 ~ 8A ∕ dm 2 under the conditions of electrowinning, silver recovery rate of 98.5% or more, the production capacity of each device is 52 ~ 181g / h.
Figure 2 bell-type electrolyzer
1-cathode; 2-outer anode; 3-inner anode
Some hospitals use electric bell transformers, and the secondary coils are revolved into 1.8V, 1A, and full-wave rectified by a selenium stack or copper oxide to reduce voltage to 0.7V, 1A. The distance between the anode of the stainless steel plate and the anode of the stone plate is 20 mm. The solution is not circulated, and an area current of 100 to 200 mA ∕dm 2 is used. The supply current is regulated by a variable resistor. In general, 0.5 to 1.0 g of silver can be recovered from the fixing solution for each 360 mm x 430 mm chest sheet.
Fourth, other methods of recovering silver from the fixing solution
Other methods for recovering silver from the fixer are:
(a) sodium borohydride reduction method
The trade name of sodium borohydride is SBH. It can be used to reduce gold and silver from acidic or alkaline fixing solutions, cyanide liquids, chlorinated liquids and precipitated silver chloride, and can also reduce platinum , palladium and the like therefrom. Its reduction reaction is:
8M n + +NaBH 4 +2H 2 O 8M+NaBO 2 +4H 2 ↑
For example, there are two kinds of sodium borohydride produced by Morton Vulcanized Rubber Company of the United States, one is a powder or tablet containing 98% of NaBH 4 , and the other is a liquid containing 12% of NaBH 4 and 40% of NaOH. The latter is low in price and is more convenient for reduction under alkaline conditions. There is also a DMAB which is a derivative of NaBH 4 , dimethane borane [(CH 3 ) 2 HN·BH 3 ], which also has a similar reduction. When these products are used, their reduction depends on the BH 4 - content therein. According to the above reaction formula, the theoretical consumption (g) of NaBH 4 required to reduce each gram of precious metal can be calculated from the relative atomic mass and chemical price of the metal:
Reduction metal amount ∕g Au + Ag + Pt 4 + Pd 2 + Ir 4 + Rh 3 +
NaBH 4 theoretical quantity / g 0.0245 0.0447 0.099 0.0908 0.100 0.141
In practical applications, the amount used in the alkaline solution is about 1.1 times the theoretical amount, and in the acidic solution is about 2.0 times the theoretical amount. If the solution contains an oxidizing agent, such as a color plate fixing solution containing Fe 3 + , it will consume NaBH 4 and be reduced to Fe 2 + , so the silver is reduced from the color plate fixing solution, and the NaBH 4 consumption is more than that in the black and white plate fixing solution.
The reduction operation is carried out according to the silver content in the fixing solution, and slowly added (10 to 20 minutes after completion) with stirring, the amount of NaBH 4 diluted with water is required, and gray black particles of silver are generated in the solution and hydrogen is evolved. After the reaction is completed, it is allowed to stand for a while to completely precipitate the silver particles, and then the supernatant is decanted, followed by filtration and drying. If the silver in the test solution has been completely reduced, a few drops of NaBH 4 solution can be added to the supernatant. If no gray-black precipitate appears, the reaction end point is reached.
According to the acidity and alkalinity of the fixing solution, add NaOH or acetic acid to adjust the pH=5.5, slowly add 5-10 times of NaBH 4 solution with water under strong stirring, continue stirring for 30-40 minutes after 30 min addition, and wait for the silver precipitate after standing. The supernatant is withdrawn, and the silver powder is filtered, washed with water, dried, and then fluxed to melt the ingot. The recovery rate of silver is more than 99%, the residual liquid contains silver to 10 mg/L, the precipitated silver grade is 96% to 98%, and the grade of the smelted ingot can be increased to 99%.
When sodium hydroxide is added to the fixing solution to adjust the pH to 6-8, the addition of 22 kg of silver per kg of NaBH 4 can be calculated. After the silver reduction precipitation, the residual NaBH 4 in the solution does not affect the return of the fixing solution.
(2) Ion adsorption method
The silver is adsorbed from the waste fixing solution by using an alkaline solution of an intermediate product viscose waste-fiber xanthogen which produces chemical fiber, and the solution contains silver 2.84 g ∕L, pH 7, and the viscosity: fixing solution = 1:10 At room temperature, static adsorption for 24h, silver adsorption recovery rate of up to 100%. Tests have shown that the degree of esterification of sodium cellulose xanthate has no effect on the adsorption of silver. However, the pH of the fixer has a great influence on the adsorption rate of silver. For example, when the pH is 6, the adsorption rate drops to 82%; when the pH is 9, a gelatinous precipitate that is difficult to filter is formed.
From the cellulose-containing fiber, 93% to 99% of silver can be recovered from the alkaline fixing solution. Under neutral and acidic conditions, the adsorption recovery of silver was 25% and 88%, respectively. The silver-adsorbed fiber was incinerated at 300 to 500 ° C to obtain ash containing 21% of silver.
If cellulose fibers containing particulate zinc sulfide (or water-insoluble copper sulfide) are used, silver can be selectively recovered from solutions of different chemical components. Even if the solution contains very poor silver, a high recovery rate can be obtained. When used to recover silver from a fixer, the cost is only 10% to 12% of the value of the recovered silver.
Using a strong basic anion exchange resin, 91.7% of the silver in the fixing solution can be recovered.
The silver in the fixing solution can be precipitated using an organic compound containing an S or N functional group. For example, take 5L of fixer solution containing 3.2g ∕L X-ray film automatic processor, add 5% NaOH solution to adjust pH to 7, add 75mL of ALM-648 (Nippon Soda KK) solution to the solution, stir for 3min and filter. A precipitate of 50 g was obtained. The silver content of the filtrate can be reduced to 0.72×10 -6 , and the recovery of silver in the precipitate is greater than 99.99%.
5. Recovering silver from waste film and printing paper
The main methods for recovering silver from waste film and printing paper are incineration and dissolution.
(1) Incineration
This method is widely used to process large quantities of film and paper. Known as the 10-GSX incinerator developed by the US Air Force Weapons Laboratory with a daily processing film of 6.8kg (Fig. 3), the US Air Force Strategic Reconnaissance Center invented in 1970, using propane as fuel, daily processing film and paper 362kg. The 1150 incinerator (Fig. 4) and the large incinerator designed by Kodak, the largest photosensitive material manufacturer in the United States, to process 2041 kg of film per hour. Although the incineration method destroys the high-priced base, it is also easy to cause the loss of silver in the smoke, but this method has the advantages of large volume, low cost, and easy operation.
To reduce the loss of silver in soot, Kodak's large incinerator consists of two combustion chambers equipped with a cooling and dust collection system. This type of furnace is to slowly burn a film and paper like a small amount of air in the first combustion chamber. And in the second combustion chamber, the excess air is supplied to completely combust the combustible gas, the temperature can reach 750 ° C, and the silver in the flue gas is recovered by the electrostatic precipitator after the water spray is cooled to 316 ° C, and the produced ash is incinerated. The ash content of the film contains 46% to 52% of silver, and the ash content of the paper contains 0.6% to 0.7% of silver. The collected soot can be smelted in an electric arc furnace. It can also be dissolved in dilute nitric acid, precipitated with hydrochloric acid, and then smelted with sodium carbonate, or dissolved in dilute nitric acid and sent to electrolytic silver extraction.
Figure 3 10-GSX type incinerator
1-second combustion chamber; 2-assisted burner; 3-secondary combustion chamber;
4-exhaust gas; 5-ash tray; 6-spray nozzle
Figure 4 1150 type incinerator
If the film is incinerated at (500 ± 5) °C, the obtained film ash is firstly washed with 4% NaOH solution, and the leaching residue is washed with hot water, and then treated with a 2 mol/L sulfuric acid solution containing 10% H 2 O 2 . Leaching the slag for 2 h allowed 91.75% of the silver to enter the solution.
(2) Dissolution method
1. Nitric acid dissolution method. The film was added to a 5% HNO 3 solution, heated to 40-60 ° C for 10 min, and the silver was completely dissolved from the film.
2. Acetic acid method. This method is to cut the waste film, add acetic acid to dissolve at 32 ~ 38 ° C, silver into the solution. The silver in the solution is extracted by electrowinning.
3, heavy catalytic potassium chromium. In the method, the chopped film is placed in hydrochloric acid or bromic acid solution, and potassium dichromate is added as a catalyst, and the reduced silver on the film is converted into silver halide. It is dissolved in sodium thiosulfate and sent to the electricity to extract silver. However, this method produces sewage due to the addition of dichromate, and the conversion process of dichromate can be completed in only a few seconds, which causes difficulty in operation and thus easily affects the recovery rate of silver.
4. Alkali dip method. This method involves digesting the comminuted film in 10% NaOH solution to transfer the silver into the alkaline solution. Further, sulfuric acid is added to neutralize to pH 6 to 7, and silver forms a silver sulfide precipitate.
5. Protease decomposition method. The method is to dissolve the film with a protease-containing pH 6.8-7.2 aqueous solution at 30-70 ° C, so that the protein in the substrate can be decomposed into a peptide and a chloro acid, and the silver layer on the substrate is detached to obtain silver slag. . Under the same conditions, an aqueous solution using chymase, amylase or lipase is also effective. When this method is used to process the photosensitive and washed film, if the silver halide residue is to be obtained, it should be treated with a solution of dichromate and hydrochloric acid to convert the reduced silver on the film into silver chloride. Treatment with protease or the like.
Further, by dissolving the pulverized film at 165 to 225 ° C for 15 minutes using an organic solvent aryl ether, phenone ether, dicarboxyacetate or the like, the polyester matrix can be dissolved to obtain metal silver and silver halide precipitate.
The silver halide precipitate obtained by the above various methods can be stirred and leached with silver in an aqueous solution containing 200 to 300 g of sodium thiosulfate, 30 g of sodium sulfite, and 30 mg of glacial acetic acid. The leachate is sent to the electricity to extract silver.
6. Recovering silver from photosensitive waste emulsion
There are many ways to recover silver from waste emulsions used in the manufacture of photosensitive materials plants. More important methods are: waste latex containing silver (2.75 ~ 8.0) × 10 -4 , first add about 1.5% of the weight of waste latex NaOH, heated to 85 ° C to destroy gelatin. Further, Na 2 S 2 O 3 equivalent to 0.15% by weight of the waste latex was added, and silver halide was decomposed at 85 °C. Finally, NaBH 4 , which is equivalent to 0.09% by weight of the waste latex, is added, and the silver is reduced to metal. The precipitated silver is easily separated, and the solution containing silver is less than 2 × 10 -4 % after sinking silver.
Japan-patent recommended alkali metal salt melting method is to add 0.3 to 5 mol (molecular) of solid alkali metal hydroxide, carbonate or bicarbonate per mole of (molecular) silver to the waste latex (at least A), after the heat and melt decomposition, the metallic silver is separated from the inorganic compound. This method recovers 99% of silver from waste such as waste emulsion, film or printing paper. The purity of silver can reach 99.3%.
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