In the cementation extraction of gold from the cyanide solutions, zinc is normally used as the precipitating However, in certain cases aluminum may be a preferable precipitating agent. First, the universal availability of aluminum scrap makes it possible to use the inexpensive and readily available reagent. Second, in contrast with zinc, aluminum does not form complex compounds with the CN~- ions, therefore the cementation of the noble metals by aluminum is accompanied by regeneration of the cyanide:3Ag(CN)_2~- + Al + 4OH~- = 3Ag + AlO_2~- ++ 6CN~- + 2H_2O. (1)Third, the aluminum ions are easily removed from the solution by simple liming, which is important when using the recyclable solutions:2AlO_2~- + Ca~(2+) = CaAl_2O_4. (2)The use of aluminum in powder form to precipitate the noble metals has long been known (1). The primary condition when using aluminum to precipitate the noble metals from the cyanide solutions is the absence of calcium in the solution, i.e., the use of sodium hydroxide as the alkalizing agent. However, it is believed that aluminum is effective in the precipitation of silver, while for the solutions containing only gold the precipitation by aluminum does not yield satisfactory results (2).
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