Metal cementation method is one simple method of recovering silver from the fixer. 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.
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