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Refining silver is a multi-step process, in which many different paths can be taken to reach the same end result. Here, the most common of which will be covered.
Most chemical methods starts by dissolving silver scrap in a 50/50 mix of nitric acid and distilled water. Roughly 1.2 ml of nitric is required per gram of silver. The silver nitrate is then filtered from any residual solids and the silver is reduced back to metal by one of several methods.
Cementing on copper
This is the easiest way to get back silver but it will also recover palladium and platinum in solution. The purity of the copper used will also affect the final purity. It must be clean and free of oxidation. It must also be fully submerged into the solution, but not sitting on the bottom of the vessel. Copper strand wire is a poor choice for this task, as the copper goes into solution at different rates along its surface, this can lead to little flakes and strands of copper contaminating the relatively pure silver cement. The best choice for this task is clean copper bus bar, heavy gauge copper wire, or copper pipe/tubing that has been beat flat with a hammer (and cleaned up again there after).
After the silver has cemented out of the nitrate solution, the barren liquid is decanted off. The silver cement must be cleaned thoroughly of all residual copper nitrate. A test to see if the copper nitrate has been fully removed, is to take a small sample of the rinse water and add ammonia. If there is copper still in the rinse water, it will turn blue. If the test is positive, more rinsing is required. After the silver cement is clean, it must be dried before melting.
(Main article: Silver chloride)
Silver Chloride Method The silver nitrate may have traces of palladium or platinum depending on the source of the silver. If that is the case, and the platinum group metals are in recoverable amounts, silver chloride conversion may be preferred. That is done by adding either hydrochloric acid or sodium chloride to the silver nitrate solution. After the addition, an immediate cloud of white curdy silver chloride will appear (this can also be a testing method to see if all silver has cemented out of solution, if you choose to cement with copper).
The silver chloride, must also be rinsed clean of the nitrate solution. After it is cleaned, do not allow it to dry. Drying the silver chloride will greatly complicate the conversion and recovery of silver.
There are several ways to convert the silver chloride back into metal:
These are just a few, numerous methods exist, but these are the simplest. Silver chloride mixed with lye is a good choice, as the Silver oxide product can be melted without further reduction to elemental silver. The key to any of the methods mentioned, is agitation. Movement of the solution is required so that the reaction can take place. It is a contact reaction, so if there is no contact, there is no reaction. And the melting of unreacted silver chloride will yield less than desirable results. After complete conversion, rinsing and drying is the next step.
Sodium formate reduction
(Main article: Silver nitrate reduction using sodium formate)
Silver nitrate can be reduced back into metallic silver using sodium formate. The product is very pure even with base metal nitrates in the solution.
(Main article: Silver cell)
In an electrolytic cell silver is dissolved at the anode and pure silver crystals are deposited at the cathode. In a silver cell the anode consists of raw silver and the anode is usually made from stainless steel. The electrolyte is made up of silver nitrate and minor additions of nitric acid and copper nitrate. The amount of dissolved copper affects the way silver crystals are formed and how hard they stick to the cathode.
Crystallization of silver in a cell will yield exceptional purity, so long as standard procedures are followed. Such as, bagging the anode with proper material and scraping crystals down to prevent a short circuit. Also, contamination levels in the electrolyte should be monitored closely to ensure codeposition of copper, palladium, and other soluble metals doesn't transpire.
Silver cells are usually used by hobby refiners to increase the purity of silver from cementing with copper.
(Main article: Moebius cell)
A Moebius cell is easy to construct and can run longer times without supervision. Drawbacks are larger floor space, more power consumption and more silver locked up in the electrolyte.
Most hobby based silver cells are of the Moebius type, especially cells made with a stainless steel bowl.
(Main article: Thum cell)
The Thum cell is popular in bigger refineries. It takes up less space and uses less power as the distance between the electrodes are low. Mechanical raking of the cathodes keeps the cells from shorting out. A drawback is that the anodes can't be run completely to finish but needs to be remelted into new anodes.
(Main article: Melting silver)
The product silver from the methods mentioned above can range from 97% purity for cementing on copper up to 99.999% for a carefully run silver cell.
Both fine powdered silver from cementing or silver chloride reduction or crystals from a silver cell should be properly dried before melting. Once melted, it can be poured into anodes for a cell, cast into bars or made into casting grains.
- GRF : Lou on some ways to treat silver feed stock
- GRF : 4metals formate reduction of silver nitrate
- AMS online : REFINING ELECTROLYSIS OF THE SECONDARY SILVER ALLOYS In Slovak but with easily understandable diagrams. 10 pages pdf.
- Gold-N-Scrap : Silver recovery and refining
- Hoke : ELECTROLYTIC SILVER REFINING, page 255
- C. W. Ammen : Recovery And Refining Of Precious Metals, 2:nd edition 1997, Page , 204-211