PHOSPHATE-ASSISTED REMOVAL OF NICKEL AND TRACE METALS FROM GOLD-PROCESSING TAILINGS WATER USING A THERMALLY MODIFIED SUGAR-REFINERY LIME WASTE–BENTONITE COMPOSITE

This study evaluates the combined use of a thermally modified sugar-refinery lime waste (defecate)–bentonite composite and monoammonium phosphate (MAP) for the removal of nickel and associated trace elements from gold-processing tailings water. Defecate was calcined at 600 °C for 30 min and blended with local bentonite at 80:20 by mass. Tailings-pond water containing 66.0 mg/L Ni, 21.0 mg/L Cu, 21.0 mg/L Co, 4.80 mg/L Sr, 2.40 mg/L U, 1.30 mg/L Mn, 1.00 mg/L Se and 0.88 mg/L As was treated with 5 g/L of composite alone and with 5 g/L of composite plus 15 mL/L of 10 % MAP solution. The composite alone removed 99.87 % of Mn, 99.98 % of U and 98.86 % of As, but only 80.30 % of Ni, 66.67 % of Co and 47.62 % of Cu. Co-dosing MAP raised nickel removal to 97.53 %, reducing the residual concentration from 13.0 to 1.63 mg/L, and gave 93–96 % removal of Cr, Al and Zn, while potassium remained unchanged at 0.21 % removal. A phosphate budget constructed from the measured concentration changes shows that 91.9 % of the dosed phosphorus is consumed by calcium and only 8.1 % by the target metals, which reconciles the 15-fold difference between the laboratory dose (1.50 g/L) and the design dose adopted in the process flowsheet (0.10 g/L): the latter corresponds almost exactly to the stoichiometric requirement of the target metals alone (0.107 g/L). The laboratory dose also introduces an estimated 189 mg/L of ammonium into the treated water, which the stoichiometric dose reduces to 13.5 mg/L. Sequencing calcium removal ahead of phosphate addition is therefore proposed as the governing design principle for this reagent pair.