John B. Farrow

The impact of polyacrylamide flocculant solution age on flocculation performance

Aqueous solutions of high molecular weight polyacrylamides used to flocculate mineral slurries undergo time-based changes in their properties. Previous studies of the impact of ageing on flocculation performance have focused on time-scales of weeks or months, which has little relevance to industrial practice. In this study, ageing times from 1 h to 6 days were examined. Flocculation was achieved continuously in a Couette mixing device (stationary outer cylinder, rotating inner cylinder). The extent of aggregation was assessed from batch settling tests and in situ size characterisation with a focused beam reflectance measurement (FBRM) probe. The polyacrylamide dosages required to achieve measurable flocculation decreased as the flocculant age was increased, with optimal performance attained at 72 h. Flocculation using a 1-h-old flocculant solution consumed 75% more polymer than with an optimally prepared stock solution. The relationship between hindered settling rate and FBRM chord length measurements was found to be independent of flocculant age, but was sensitive to shifts in aggregate density caused by variations in mixing intensity. It is proposed that the early stages of flocculant dissolution involved the release of discrete polymer chains from highly agglomerated species, the former dominating flocculation activity. Optimal ageing maximised the discrete polymer concentration available for flocculation, leading to a significant increase in the aggregate size distribution but did not appear to impact upon the aggregate packing structure (density).

Iron oxy-hydroxide crystallization in a hydrometallurgical residue

In some hydrometallurgical operations, the removal of iron from process liquors can be critical, with the method of choice commonly being crystallization (precipitation). It has been recently discovered that if the removal of iron occurs at pH 3, ferrihydrite, a complex and disordered compound, will generally be precipitated. A laboratory scale, computer controlled, continuous crystallizer was constructed to investigate the removal of iron from low pH solutions (precipitation). The significance of ferrihydrite precipitation and the effect of pH and silica concentration on the properties of the precipitate were investigated. Considerable insight has been gained concerning the crystallization and growth mechanisms under these conditions, including the unusual limit of ferrihydrite crystal growth. The use of in situ focussed beam reflectance measurement (FBRM) analysis has allowed the particle size differences experienced at low pH in ferrihydrite precipitation to be studied. The improved understanding of the crystallization of iron from hydrometallurgical process liquors has resulted in a methodology for more effective removal of iron.

Recent developments in techniques and methodologies for improving thickener performance

Abstract Research projects directed towards ‘Improving Thickener Technology’ have been conducted by the AJ Parker Cooperative Research Centre for Hydrometallurgy since 1988 through the Australian Mineral Industries Research Association with support from 25 Australian and international mineral processing companies. These projects have resulted in the development of a range of research tools and techniques to address flocculation and thickening issues. Laboratory and plant studies have focused on how key factors, such as mixing intensity, mixing duration and solids concentration, affect flocculation efficiency and, hence, thickener performance. A comprehensive computational fluid dynamics (CFD) model has been developed to predict likely full-scale performance under various process conditions, based upon a knowledge of thickener design, plant flows and laboratory assessment of the flocculation behaviour of the feed slurry. Application of this CFD model to problems within specific mineral processing operations has resulted in significant benefits, in one case leading to doubling of a thickener’s throughput. This paper presents an overview of the work conducted during these projects, focusing on the research tools, the nature of the information that has been gained and the implications for thickener performance. Brief details will be presented on research issues currently being investigated.