Management of thiosalts in mill effluents by chemical oxidation or buffering in the lime neutralization process

摘要

Laboratory studies were conducted to investigate the removal or management of thiosalts within the lime-neutralization process, to prevent or minimize the adverse effects of thiosalts that cause delayed acidity to downstream environment. The oxidizing reagent hydrogen peroxide (H_2O_2) and the pH stabilizing (buffering) reagents carbon dioxide (CO_2), sodium bicarbonate (NaHCO_3) and sodium carbonate (Na_2CO_3) were examined for removal and management of thiosalts, respectively. Chemical oxygen demand (COD) was determined to be a proxy for thiosalts and was employed for their rapid assessment. The Target Level of thiosalts harmless to aquatic life was found to be 30 mg/L or less. The optimized lime-neutralization process required a pH level of 9.5-10 and aeration. Over-liming to pH levels >11 did not provide excess alkalinity, hardness, or a decrease in thiosalt levels. Addition of H_2O_2 to either the acid or lime-neutralized water at a molar H_2O_2:S_2O_3 ratio of 1-1.5 removed thiosalts to safe levels. About 10-15 min. at room temperature was ample time low temperatures slowed down the process but the dosages were not affected. Removal of thiosalts from 170 to 30 mg/L caused a decrease in pH from 9.6 to 6.5. Among the buffering reagents studied, both NaHCO_3 and Na_2CO_3 provided adequate buffering and a stable pH of 7 to the lime-neutralized water; whereas CO_2 resulted in poor buffering and an unstable pH that remained below 6. In cold temperatures, NaHCO_3 and Na_2CO_3 also outperformed CO_2 with higher alkalinity and hardness. Na_2CO_3 addition to lime neutralized water at pH 9.5 was found to be the most cost-effective option. Other methods could have niche applications, depending on seasonal variations and temperature.