The use of cyanide as a gold lixiviant necessitates a high pH to ensure the speciation of cyanide that favors CN⁻ rather than HCN. This speciation is crucial for increasing the safety and efficiency of cyanide reagent use. A pH of 11 is optimal, although some plants opt for pH levels of 10.5. The gangue minerals in gold ores contain a variety and large quantity of Si-containing minerals. Si dissolution from these minerals increases with rising hydroxide (OH⁻) concentration, especially at pH levels of 10 and above. There are two main negative impacts resulting from the use of NaOH in gold processing circuits: silica-gel formation and sodium and associated sulfate accumulation.
NaOH, a strong alkali, dissolves Si from Si-containing gangue minerals and causes deprotonation of silanol groups (Si-OH), resulting in the formation of Si-O-Si bonds and the polymerization of silica into a gel-like substance. The formation of Si-gel is problematic in gold leaching circuits due to its viscosity, negative impact on heap leaching hydraulic conductivity, clogging and blockages in pipes and transfer pumps, and coating of activated carbon.
In contrast, the use of Ca(OH)2 does not result in Si-gel formation in gold processing circuits. NaOH, being a stronger alkali with higher solubility than Ca(OH)2, causes a greater extent of Si dissolution from Si-containing gangue minerals and the formation of gel-like polymerized silica. When Ca(OH)2 is used, the extent of Si dissolution is significantly less. Even if some Si is dissolved, it reacts with Ca2+ from Ca(OH)2 to form stable calcium silicate precipitates on the surfaces of gangue mineral particles, without gel-like properties. The net effect is reduced dissolved Si and the virtual absence of polymerized Si-gel in any part of the gold processing circuit.
A secondary problem resulting from the use of NaOH in gold processing circuits is that Na⁺ remains soluble (typically with sulfate as the counter anion) and accumulates within the processing solution circuit. This is because there are no precipitation reactions occurring within gold processing circuits that can remove sodium from solution. The build-up of sodium and associated sulfate is detrimental to gold processing for several reasons, including the negative impact sulfates have on the lime slaking (hydration) reaction and subsequent utilization efficiency of the slaked lime reagent. The only exits for sodium and associated sulfate from the processing circuit are via uncontrolled leakage into the surrounding environment or expensive reverse osmosis methods.
In contrast, the use of Ca(OH)2 does not result in the accumulation of soluble Ca2+ and associated sulfate, because of the precipitation effect of gypsum (CaSO4·2H2O), which removes calcium and sulfate from solution.