Removal of mercury from glycine leaching solutions by precipitation

This study investigates the removal of mercury (Hg) from amalgam waste generated by artisanal and small-scale gold mining (ASGM) through precipitation following glycine leaching. Glycine has emerged as a promising alternative in gold extraction due to its low toxicity, biodegradability, and effective leaching performance. Within this system, the removal of Hg from the pregnant leach solution is a critical step for ensuring both process efficiency and environmental safety. To this end, a comparative assessment was carried out using sodium sulfide (Na2S) and potassium ethyl xanthate (KEX) as precipitating agents for Hg removal from glycine-based leachates. Key operational parameters—including temperature, reagent dosage, and reaction time—were optimized using the Central Composite Design (CCD) methodology. Na2S achieved highly efficient Hg removal (>99%) in the form of HgS, with no detectable loss of gold (Au); however, significant co-precipitation of silver (Ag) was observed. In contrast, KEX attained a maximum Hg removal efficiency of approximately 95%, while preserving valuable metals with minimal losses. Notably, at elevated dosages, KEX exhibited a tendency to form micelles above its critical micelle concentration (CMC), highlighting the necessity of precise dosage control. Both reagents present distinct trade-offs in glycine-based Hg removal, Na2S offers high efficiency but limited selectivity, whereas KEX provides better selectivity at the expense of slightly lower Hg removal. Consequently, KEX may be preferred in applications where minimizing losses of valuable metals is a priority. However, it was found that KEX adversely affects Au adsorption onto activated carbon—likely due to organic residues blocking active sites—whereas Na2S showed no such detrimental effect. When ion-exchange resin was used instead of activated carbon, neither precipitant exhibited a negative impact on Au recovery.