Compressional dewatering of flocculated mineral suspensions

Abstract

The recovery of water for reuse in minerals processing is an important issue in the reduction of water use in mining, as is the dewatering of tailings to reduce the propensity of "wet" tailings dams. It is common in these dewatering operations to use thickeners whereby polymeric flocculants are combined with the dilute feed to the thickener to aid sedimentation and the subsequent consolidation of particulate materials. Thickeners are the work horses of water recovery and tailings dewatering, and although the basics are well understood, there is still no ability to reliably predict the throughput and underflow density from a thickener based on traditional sedimentation tests and flux analysis. The reasons for the discrepancy between full scale operations and predictions based on laboratory tests are numerous, including difficulties in mimicking the flocculation conditions experienced in the field and the shear conditions in the thickener. The latter effects are little understood and hard to quantify and include effects due to flocculated aggregate densification, complimentary shear and compressional effects and aggregate break-up. A set of test protocols to quantify these effects and their contributions to compressional dewatering in a thickening environment has been developed. The data and analysis show that aggregate densification is a dominant contributor to the difference between observed and predicted behaviour based on laboratory tests. Also of interest is the shear rate, concentration, and time dependence of the aggregate densification process. A basic model of the process has been developed along with network yielding tests conducted using rheometry.