Electrolysis or Miller's procedure for obtaining gold?

Electrolytic refining is one of the most popular procedures for obtaining gold.

Purest gold is obtained by electrolysis 'Wohlwillovim' procedure. Simple electrolytic cell as glass or porcelain bowl. Anodes are contaminated at least 950 fineness of gold, weighing 12 kg and measures 280x230x12 mm and the cathode of a rolled gold plate 999.9 fineness and measures 300x75x0, 25mm.

The cell is filled with electrolyte solution, which contains per liter 50 to 100 g of gold in the form of ions (AuCl4), and an equal weight of hydrochloric acid. Electrolyte temperature should be maintained at 60-70C. Gold dissolves from the anode as a translator in the ion (AlCl4), and the cathode is deposited as a metal:

Anode, Au +4 Cl-(AuCl4) - +3 e (3)
Cathode (AuCl4) - +4 +3 e Au-Cl (4)

Hydrochloric acid plays in dissolving anode. When the electrolyte solution would not have been acidified, the anode should be pulled chlorine.

Electric potential between the electrodes is up to 2V, and the current density up to 1900 A/m2. Such a large current density prevents a large amount of gold clusters in solution electrolytes during refining. At such a high current density cathode is tough, but very compact. Anodes are cast thinner at the lower end and at the edges, to be able to use and to take as long as possible, but it is interrupted when the electrolysis is about 90% of the anode material is melted. To prevent silver chloride anode polarization occurring at the anode dissolution and reaction of silver with hydrochloric acid (occurs only if the proportion of silver in the metallic anode 5% and higher), the electrodes are sometimes connected to a pulsating direct current.

As the gold is deposited on the anode from the cathode, while the silver and other metals electrolytically just melt away, eventually the concentration of gold in solution is greatly reduced. In order to compensate for this loss, it added highly concentrated electrolyte solution of gold (III) chloride or completely replace the electrolyte. Replacing electrolytes is applied when the gold contains platinum metals, electrolysis is performed as long as the concentration of platinum and palladium in the electrolyte has increased to 60 g/dm3.

Along with some gold iridium, osmium and ruthenium can be found in the mud of silver chloride is deposited near the anode. These metals and gold due to sludge mechanical ripping the anode. After treatment with zinc chloride of silver plus a small amount of sulfuric acid, which produces zinc chloride and metallic silver, minted silver will contain gold, and what can be gained from the anode mud of the silver refinery.

Cathodes made of pure gold is removed from the cell for refining every 48 hours, wash up, smelted in an induction furnace and cast into ingots. Of so refined gold 10% is used for designing new cathode and back again in the process.

Electrolytically refined gold has a very high purity (average share of gold from 99.95 to 99.98%). With this the proceedings, although they get less pure gold, are preferred over electrolytic because it takes less time. Miller refining process was completed in one day, and the days of electrolysis gold reserves at the refinery, and he also slows circulation and sales. Although electrolysis obtained gold fineness of 999.9 may be used for monetary purposes, it is mainly used in industry and science. To obtain greater fineness of gold is not enough to simply electrolytic or chemical refining. Consecutive electrolysis can produce gold fineness 999.99, 999.999 and finesse only viable extraction solvents and zone melting.

Most refineries in the world produces gold fineness of 996, while Russian fineness 999th The degree of purity of gold is sufficient for casting in gold bullion held as monetary backing. Small gold bars of a commercially available gold are heavy 12.5 kg and shapes are small bricks. Surface gold bullion offered for sale must be flawless, no cracks, breaks or scratches. Molten gold, temperature, slightly more than the melting point, the left into molds which are coated on the inside with a thin layer of soot. When we pass by cooling the molten surface of the reducing flame to prevent oxidation of minor amounts of silver. During casting bullion taken probes for accurate determination of fineness. When the lever correspond with the requirements, mark stamped with a number, fineness and called refinery.

Read more about Gold refineries processes.