Cyanide is used in leaching phase of hydrometallurgical recovery of gold. The toxicity and environmental risks have created a need for safer alternatives. The goal of this thesis is to study leaching of gold with cyanide replacing alternatives that are utilized in normal air pressure and below temperatures of 100⁰C.In the theoretical part of the thesis primary and secondary sources of gold were studied. Hydrometallurgical recovery of gold among bioleaching and biosorption as process phases in the production were included in the study. The potential cyanide leaching alternatives examined were ammonium thiosulphate, halides, thiourea and thiocyanate. Solutions were studied bothby electrochemical and reactor leaching tests. The behaviour of gold was examined with polarisation runs to determine the adequate redox potential levels for dissolution of gold. Potential measurements were utilized to find out the suitable oxidant concentrations for each solution inorder to raise the redox potential to the level where gold dissolution can take place. Weight loss measurements were used to observe the dissolution rate of gold in the solutions containing allthe needed components for leaching. Complete leaching solutions were utilized in the reactor tests for leaching of gold from autoclave treated ore concentrate and flotation treated groundprinted circuit board (PCB) material that originated from mobile phones. In order to follow an assumption for higher the gold recovery rate, a batch of ore concentrate was bioleached and a batch of flotation concentrate of ground PCB material was acid treated as pretreatment.Based on preliminary polarisation runs and potential measurements, ammonium thiosulphate was evaluated to dissolve gold slower than on the rate predetermined for cyanide solution, 2,5mg/cm2h. A halide alternative, chloride-hypochlorite, exhibited a dissolution rate of 8,6 mg/cm2h in redox potential of 900 mV vs. SHE. Gold dissolved in thiocyanate in a rate of 3,4 mg/cm2h in redox potential of 620 mV vs. SHE. Thiourea had a dissolution rate of 1,65 mg/cm2h in redox potential of 450 mV vs. SHE. In the reactor tests the recovery rates of gold from ore concentrate were 24 % for thiourea, 23 % for chloride and 10 % for thiocyanate. Bioleached material showed recoveries of 10 %, 23 % and 9 % respectively. Gold recoveries from the acid treated PCB material were 16 % for thiourea, 24 % for chloride and 11 % for thiocyanate. Recoveries for PCB material without pretreatment with acid were 32 % for chloride and 21 % with thiocyanate. Thiourea recovery result was found to be faulty due to apparent error in metal analysis.Although the electrochemical tests showed adequate dissolution rates for gold, the recoveries were found to be generally poor in the successive reactor tests. Low recovery rates may be due to gold dissolution disturbing elements in the original materials, premature oxidation of complexants or reprecipitation of gold because of low level of redox potential. Copper and nickel were observed to leach effectively from PCB material. Bioleaching lowered gold recovery rate due to possible sulphur compounds precipitated on the gold surface.
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