Metallurgical Treatment
Crude platinum can be refined either by dry or by wet methods, the following being brief outlines of the two processes:
By the wet method, the crude platinum is dissolved in aqua regia, with excess of hydrochloric acid. Evaporation is continued until the whole of the nitric acid is expelled. By addition of a solution of ammonium chloride, the platinum is then precipitated as ammonium platini-chloride. This precipitate is heated to redness, when chlorine and ammonium chloride are given off, and spongy platinum remains. The last is next granulated, after fusion by the oxy-hydrogen blowpipe in a small lime furnace. Platinum obtained by this method may contain small amounts of iridium, rhodium and palladium.
In the dry method, introduced by Delville and Debray, the crude platinum is smelted with galena in a small reverberatory furnace. A portion of the lead is reduced to the metallic state by the iron in the charge, and forms a fusible alloy with the platinum. The osmiridium present settles to the bottom of the furnace, and may thus be removed. Litharge is then thrown in to form more alloy, and some glass to act as a flux. The alloy is cupelled, and the residual platinum is then melted in a lime furnace with the oxy-hydrogen flame. The platinum thus obtained often contains iridium and rhodium.
A combination of these two methods is also sometimes employed[[3]] p. 403.
The electrolytic process of gold-refining was introduced to treat platiniferous gold. In the gold chloride, or Wohlwill method, iridium and osmiridium are insoluble, and remain with the anode slime. Platinum and palladium, if nearly pure, are also insoluble, but when present alloyed with gold and silver, pass into the solution and remain there. In a hot bath it is stated that the platinum present should not exceed 50 gm. per litre. According to T. K. Rose, a cold bath containing only 20 gm. per litre causes a certain quantity of platinum to be deposited with the gold[[6]].
H. F. Keller, in “Platinum, the Most Precious of Metals” (Journal of the Franklin Institute, November 1912) deals fully with the extraction and refining of platinum.