Maintaining a high pH in the leachate solution is an important requirement for cyanide efficiency and gold recovery. This case study describes one large gold mine customer, who was targeting 130,000 ounces of gold per year via heap leaching. Due to ore properties, the pH of the percolating leachate was decreasing down the heap, negatively affecting cyanide speciation and gold recovery efficiency.
Historically, the heap irrigation solution was adjusted to a pH of 10.3 using a 50% caustic soda (Na(OH)) solution. The customer experienced scaling and clogging of drip emitters when higher pH levels were targeted. Caustic solutions are known to dissolve silica from ore resulting in silicate fouling.
The customer was interested in evaluating the performance of Lhoist’s high-quality engineered calcium hydroxide (Ca(OH)₂) reagent that can be dosed as readily as Na(OH) without clogging drip line emitters. In addition to the operational benefits, the customer was interested in the non-hazardous properties of calcium hydroxide compared to a caustic reagent.
Lhoist recommended the use of SLS45 as a safer and more cost-effective solution to increase pH. SLS45 is an engineered calcium hydroxide product, offering quick release of hydroxyl ions to increase pH without the risk of overdosing or silica dissolution from ore and the associated fouling effects.
Field trial equipment, including a tank and pumps, were deployed for a three-month field trial. SLS45 was dosed to the barren solution, before recirculation back to the heap. The pH was slowly increased over the trial period and the effects on cyanide consumption and gold recovery recorded.
The trial of SLS45 was a success: SLS45 could be substituted for caustic soda and achieved the same process performance when operating at the same target irrigation pH of 10.3. Additionally, a significant benefit was that the irrigation pH could be increased to 10.8 with SLS45 without causing fouling issues observed when using caustic soda. The higher pH increased the hydroxide flux into the heap, resulting in reduced cyanide consumption and increased gold recovery from the solution, as illustrated below.
The trial period with SLS45 was considered successful and yielded the following benefits:
SLS45 provides a cost-effective, non-hazardous alternative to caustic soda, resulting in reduced cyanide consumption and increased gold recovery.