Gold Refining5954149

Gold extracted by means of amalgamation or cyanidation carries a variety of many different impurities, including zinc, copper, silver, and iron. Two techniques are frequently utilized for purification:
The Miller process and also the Wohlwill process: The Miller process is based on the fact that practically all of the impurities present in gold are  combined with gaseous chlorine and are more readily than gold to reach temperatures equivalent to or greater than the melting point of gold. [http://metalrefinery.com gold refining] The impure gold is therefore melted and gaseous chlorine is blown into a resultant liquid. The impurities form chloride compounds that separate into a layer on the surface of the molten gold.

The Miller process is quick, simple, and easy (if you understand the chemicals and how to handle them) however it produces gold of only about 99.5 percent purity.

The Wohlwill process increases purity to about 99.99 percent by electrolysis. With this process, a casting of impure gold is lowered into an electrolyte solution of hydrochloric acid and gold chloride. Under the influence of an electrical current, the casting functions like a positively charged electrode, or anode. The anode dissolves, and then the impurities either pass into a solution or fall to the bottom of the electro refining tank as an insoluble slime. The gold migrates under the influence of the electrical field to a negatively charged electrode known as the cathode, where it is restored to a highly pure metallic state.

Though the Wohlwill process produces gold of high purity, it necessitates the producer has to maintain an on hand considerable inventory of gold (mainly for the electrolyte), and this is extremely expensive. Techniques based on direct chemical purification and recovery from solution as elemental gold can significantly speed gold processing and virtually [http://metalrefinery.com metal refiner] eradicate expensive in-process inventories.

The processing of gold scrap varies not just aided by the gold content but also with the amenability of the gold in the scrap to extraction. Therefore, the majority of the gold can be recovered by leaching techniques using cyanidation or aqueous chlorination, and the residue can then be treated by smelting to recover the balance.

In general, scrap which has a gold content of lower than 0.1 percent, if not quickly recoverable by leaching, is recycled back to a pyrometallurgical process. Metallic scrap gold from jewelry production is often melted on-site and reused.

Fire assay is taken into account as the most consistent method for accurately determining the content of gold, silver, and platinum-group metals (except osmium and ruthenium) in ores or concentrates.

This process entails melting a gold-bearing sample in a clay crucible with a mixture of fluxes (such as silica and borax), lead oxide (called litharge), along with a reducing agent (frequently flour). The fluxes lower the melting point of the oxide materials, enabling them to fuse, and the molten litharge is reduced by the flour to extremely fine drops of lead dispersed throughout the charge. The drops of lead will dissolve the gold, silver, and platinum-group metals, then coalesce and gradually descend through the sample to create a metallic layer at the bottom of the crucible. Subsequent to cooling, the lead "button" is separated from the slag layer and heated under oxidizing conditions to oxidize and eradicate the lead. The shiny metallic bead that is left consists of the precious metals. [http://metalrefinery.com metal refiner]  The bead is boiled in nitric acid to dissolve the silver (a process called parting), and the gold deposit is weighted.