How Arsenic Affects Cyanidation Leaching of Gold

How Arsenic Affects Cyanidation Leaching of Gold

The Influence of Arsenic in the Cyanidation of Gold Ores is explained by W. B. Blyth has he is of the opinion that the presence of arsenic in an ore does not necessarily cause that ore to be refractory to cyanide treatment. He thinks the idea arose from the fact that arsenic is so often accompanied by antimony, pyrrhotite or graphite, stating that “the two former minerals cause premature precipitation of the gold almost as fast as it goes into solution, if they are present in sufficient quantity.”

The trouble in cyaniding semi-oxidized ores of arsenic he attributes rather to the presence of acid ferrous salts, and if he is right a preliminary water wash or possibly a weak acid wash ought to meet the case. (It should be noted here that the paper referred to does not deal with the cyanidation of silver ores containing arsenic.)

In detailing a number of experiments made on a certain antimonial gold ore Mr. Blyth states that by milling in water and running the ore into dams where it was settled, dried, and allowed to oxidize by cultivation of the surface, the pulp changed in color from a dark grey to a light yellow, and that whereas by immediate cyanidation the ore had only yielded 50% of its gold, after the oxidation treatment 93% extraction could be obtained. A similar result was later accomplished as far as the sand was concerned by milling the pulp without lime and then separating the sand from the slime. He says,

“Owing to the solubility of antimony in alkaline cyanide it is not possible to add lime to the mills and achieve good results. Consequently the abraded iron which mainly goes with the slime is reduced to the ferrous state and independently of the antimony effectually upsets the extraction from this product when treated direct without lime. In connection with the sand, however, it is found that the residue rises proportionately to the lime used and the best results are obtained when no lime at all is used. About one-half lb. of lead nitrate is used per ton of ore and the sand is slimed in an acid solution. The consumption of cyanide is naturally heavy, but the good extraction counterbalances losses in this direction.”

In regard to the use of cyanide solutions without protective alkali the writer found with one Mexican gold ore that while in a solution with a normal protective alkalinity only about 68% of the gold could be extracted, in a solution showing no protective alkali the extraction was raised to 88%. The ore contained several per cent, of arsenic but no antimony was reported.