Sludge dewatering studies were carried out at the Brunswick Mining and Smelting Corporation’s No. 12 mill as part of a pilot plant program on the removal of heavy metals from base metal mining effluents. A mobile sludge dewatering trailer located on site was used to assess the feasibility of basket centrifugation, solid bowl centrifugation, vacuum filtration and pressure filtration as possible methods of dewatering. The effect of control variables on performance, and the applicability of existing bench-scale methods to design were studied for each process.
Cake solids of 25% and suspended solids recoveries of 95% were attainable on all four pilot-scale dewatering units. The use of a basket centrifuge may not be practical due to very short cycle times which result from the high sludge feed solids concentration (7%). The addition of cationic polyelectrolyte was necessary to achieve recoveries in excess of 75% with the centrifuges, but was not required with either the vacuum filter or the filter press. The filter press, which produced a filter cake of 38 to 45% and a filtrate quality of 0 to 10 mg/1 of suspended solids, produced the best cake solids and effluent quality. However, the vacuum filter would require the lowest operating cost of units tested.
Bench-scale work included penetration tests, Buchner funnel tests, capillary suction time, leaf filter tests and bench filter tests. Scale-up from bench to pilot plant was not possible for centrifugation with existing methodology. Filter yields and pressing time were predicted from bench tests for the vacuum filter and pressure filter respectively, and compared with the results obtained. Specific sludge characteristics such as zeta potential, particle size distribution and rheology were also determined and related to solids concentration and level of polymer addition.