Autoclave Ammonia Leaching of Silver from Low-Grade Copper Concentrates

Abstract

Autoclave ammonia leaching of silver from low-grade copper concentrates is studied with use of Zhezkazgan concentrates of the following composition, %: 9.55 Cu, Fe 4.84, S 5.6, 27.03 Si, 109.6 g/ton Ag. According to X-ray phase analysis the main minerals present in the concentrate are, %: 29 quartz (SiO2 ), 33 plagioclase ((Ca, Na)(Al, Si)AlSi2O6 ), 16 mica, 10 chalcocite (Cu2S), 2 chalcopyrite (CuFeS2 ), and 7 pyrite (FeS2 ). It is shown that during ore processing in which gold and silver are in close association with sulfide minerals, precious metals are not extracted by cyaniding even after ultrafine grinding. The proportion of such persistent raw materials is more than 30% of the total ore reserves containing precious metals in the world. Use of the most common technologies of autoclave sulphuric acid and neutral leaching for quantitative oxidation of gold and silver containing sulphides in this case is limited due to the formation of argentojarosite, inert with respect to the cyanide ion. In order to avoid its formation and to increase the degree of leaching silver directly at the autoclave stage, its extraction in ammonia is investigated. With direct autoclave leaching copper extraction into solution is 93–97%, while silver extraction is not observed in some experiments, which may be due to the sorption activity of aluminum and silicon compounds present in the concentrate. Preliminary mechanical activation and surface resynthesis have a favorable effect on recovery rates of copper and silver in solution. An additional operation of concentrate silica removal provides extraction of 97% copper and 83% silver into solution with [NH4OH] = 7 mole/dm3, L:S = 10:1, τ = 3 h, pO2 = 0.6 MPa, t = 140°С.