No abstract

Physico-chemical phenomena influenced by aging or reaction can result in rheological changes across several orders of magnitude, but the classical rheometry methods available for analysis of concentrated suspensions can face challenges in correctly measuring the yield stress of aging/reacting (evolving) materials and need some precautions to enable precise measurement of the evolution of the yield stress with time. Here, a creeping sphere method has been applied to measure time-resolved yield stress; the force required to pull a solid sphere at very low velocity is used to calculate yield stress using previous analytical solutions for local flow of a creeping sphere in yield stress materials. The measured yield stress values agree well with the data recorded using vane-in-cup geometry for timeindependent measurements using Carbopol gel. The creeping sphere is less affected by shear history because of the constantly changing shear region, and therefore measures yield stress changes in evolving materials such as cement for a long time period in a single run, without altering ongoing structural network bond formation.

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.