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A Statistical Model for Slurry Thickening

Shahid Azam
Environmental Systems Engineering, University of Regina, Regina, SK, Canada
Syed A. Imran
Centre for Sustainable Infrastructure Research, National Research Council, Regina, SK, Canada

Keywords: statistical model, slurry thickening, large-strain consolidation, synthetic polymers

ABSTRACT: Thickening is the first step in the design of sustainable (cost effective, environmentally friendly, and socially viable) tailings management solutions for surface deposition, mine backfilling, and sub-aqueous discharge. The high water content slurries are converted to materials with superior dewatering properties by adding long-chain synthetic polymers. Given the solid and liquid composition of a slurry, a high settling rate alongside a high solids content can be achieved by optimizing the various polymers parameters: ionic type (T), charge density (C), molecular weight (M), and dosage (D). This paper developed a statistical model to predict field performance of a selected metal mine slurry using laboratory test data. Results of sedimentationconsolidation tests were fitted using the method of least squares. A newly devised polymer characteristic coefficient (Cp) that combined the various polymer parameters correlated well with the observed dewatering behavior as the R2 equalled 0.95 for void ratio and 0.84 for hydraulic conductivity. The various combinations of polymer parameters resulted in variable slurry performance during sedimentation and were found to converge during consolidation. Further, the void ratio-effective stress and the hydraulic conductivity-void ratio relationships were found to be e = a σ′ b and k = 10 (c + e d), respectively.

Introduction
Economic prosperity is influenced by the exploration of new sources of energy and materials along with the development of existing reserves. The Canadian mining industry contributes $40 billion to the economy and accounts for about 4% of the Gross Domestic Product. This important activity is associated with large volumes of ever increasing mine wastes of variable nature and extent. The mine tailings (slurries generated during ore processing) are particularly challenging because of their slow settling rates and high standing toxic waters in the waste containment facilities (Eckert et al., 1996). Numerous tailings dam failures in different parts of the globe have been reported to result in massive contaminant releases causing acute public distress over the conventional practice of tailings disposal (Vick, 1983). In order to minimize its environmental footprint, the Canadian mining
industry has been quite proactive in developing novel waste management methods such as thickening of the
slurry tailings. This paper is part of the ongoing research at the University of Regina that focuses on developing
case-specific engineering solutions for base-metal mining operations by investigating the innovative method of
slurry thickening.

The conventional practice of mine waste disposal usually generates large volumes of soil slurries that must be contained in tailings dams over a number of decades. Thickening is the initial step in the design of cost effective, environmentally friendly, and socially viable tailings management solutions for surface deposition, mine backfilling, and sub-aqueous discharge. Through a properly designed thickening process, the slurries can be converted into paste-like materials and deposited on the ground requiring minimal continuous monitoring (Robinsky, 1999). The production of thickened slurries by adding long-chain polymers as flocculating agents is
not well understood in geotechnical engineering. Complex colloid-water-polymer interactions govern the hindered sedimentation and large-strain consolidation behaviour of such materials (Azam 2004). To develop a process that achieves a high dewatering rate together with a high amount of water (that is, high solids content of the settled slurry) in the gravity thickener requires screening a number of synthetic polymers in the bench-scale laboratory tests. This allows the selection of appropriate polymers for a given slurry composition: solid particles (size distribution, mineralogy, and surface charges) and liquid medium (pH, electrical conductivity, and electrolyte concentration).

The main objective of this paper was to develop a statistical model to understand and improve the dewatering
behaviour of a selected metal mine slurry during thickening. The test data was obtained from a comprehensive
laboratory investigation program completed earlier (Azam 2003). The synthetic polymers used in the sedimentation-consolidation testing were characterized by their ionic type (T, -1 for anionic polymer and +1 for cationic polymer), charge density (C), molecular weight (M, x 106 g/mol), and dosage (D, ppm). The test data  were fitted using the method of least squares in conjunction with the newly devised polymer characteristic coefficient (Cp) that combined the various polymer parameters.

Literature Review
Mine tailings offer unique challenges related to the design, construction, operation, and reclamation of the containment facilities (Edil and Fox, 2000). The geological origin and the mining operation govern the geotechnical properties of the placed materials (Morgenstern and Scott, 1995). The tailings usually segregate during hydraulic transport and subsequent deposition. The coarse-grained fraction is used for dam construction thereby forming a basin to store the chemical-rich water and the fine-grained materials in suspension (Vick, 1983). A slow settling rate of the fine tailings along with high standing toxic waters in the ponds require that the containment facilities be carefully managed for a long time (Eckert et al., 1996). Several tailings dam failures around the world resulted in huge contamination causing acute public distress over conventional tailings disposal. 

The industry has been proactive in developing novel waste management methods to minimize its environmental
footprint (Concha and Burger, 2003). A sustainable innovation is slurry thickening that can produce a nonsegregating tailings stream with bulk of the process water removed and recycled back for ore beneficiation
(Robinsky, 1999). Theoretically, the low water content engineered materials occupy a much smaller disposal area that can be reclaimed as a useful landscape in a relatively short time. The gravity thickener (a cylindrical vessel with inverted conical base with or without a slow rotating rake mechanism) is used to develop a tailings stream with superior settling characteristics during sedimentation in the thickener and consolidation in the tailings pond (Jewell et al., 2002). Developing such a slurry is the most challenging issue in the design and implementation of an efficient tailings management program since sedimentation is governed by physicochemical interactions whereas consolidation is primarily a load-deformation process (Chalaturnyk et al., 2002).

The dewatering slurry in a continuous thickener exhibits a clear liquid at the top, an intermediate settling zone, and a bottom compression zone. Upward drainage allows collection of the liquid overflow at the top and of the solid underflow at the bottom. For a given set of operating conditions, the thickener retention time largely depends on the rate and amount of slurry dewatering (Bustos et al., 1999). The behavior of a slurry, which is governed by ore geology and the metal extraction process, can be improved by agglomerating individual particles and/or particle groups to develop multi-particle flocs that can settle rapidly (Torfs et al., 1996). Long-chain polymers are generally used because of their effectiveness in flocculation and negligible environmental impact. Based on cost-benefit analysis, an optimum flocculant is selected and added to a nonsegregating feed slurry at a low solids content (Xu and Cymerman, 1999). These opposing prerequisites of the feed slurry help maximize physicochemical phenomena at phase boundaries thereby resulting in a distinct microstructure (Hogg, 2000). The initial flocculated morphology (derived from solid mineralogy, liquid chemistry, and polymer properties including ionic type, charge density, molecular weight, and dosage) can result in an increased initial hydraulic conductivity (Azam and Sadiq, 2006). However, a high settling rate may or may not be associated with an increased dewatering amount during sedimentation and consolidation.

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