Dewatered tailings practice – trends and observations

The traditional impoundment where a dam(s) retains pumped slurried tailings with solids contents typically in the range of 30–40% remains the most common method for storing the tailings from milling operations. However, there are a growing number of operations whereby the tailings are dewatered and placed in the tailings storage facility with less accompanying water. The degree of dewatering and the method of placement can vary significantly dependent upon design criteria and site specific constraints. This paper reviews the state of dewatered tailings practice in a global sense. The review provides a summary of trends in the practice as well as some valuable lessons learned from observations throughout the world. Like many things that are relatively new to an industry, both excessive optimism and pessimism about the value and outcomes from using dewatered tailings have developed. The paper provides insight to both ends of this spectrum and, through actual operating and design experience, offers pragmatic and supportable “real case” scenarios to many of the more contentious issues surrounding dewatered tailings practice. The paper addresses critical issues such as “dry landscape” concepts and beach slopes misconceptions for thickened/paste tailings. Finally, the paper provides some needed guidance in terms of nomenclature. 1 DEWATERED TAILINGS – WHAT ARE THEY? 1.1 Tailings water removal Essentially any tailings can be dewatered, by definition, if water is removed by some process between the point of resource extraction and their subsequent placement in the tailings management facility (TMF). The simplest form of explanation is in considering the traditional slurried milled tailings stream that is typically, for hard rock mining, in the 30 to 40% solids range. This slurry can have its effective solids content increased by dewatering through increasing mechanical intervention through the dewatering continuum as shown in Figure 1. However, classification of tailings in the dewatering “continuum” can be confusing. This is particularly true for thickened/paste tailings where the concept of segregating versus non-segregating becomes included in the definitions. There is clear understanding as to the meaning of segregating versus non-segregating tailings as separated by a segregation threshold. At solids contents higher than the threshold, and often as augmented by coagulants or flocculants, a tailings slurry after discharge does not separate into sand beaches and fines (also known as slimes or sludges) and