Fine-grained tailings (leach residue and gypsum) resulting from hydrometallurgical processes (acid leach followed by lime neutralisation) of nickel laterite ores are proving difficult to dewater using conventional dewatering methods (thickeners/vacuum filters). To generate a dense non-segregating slurry or paste that can be deposited on surface in the form of a stable stack, more innovative dewatering methods are required. The current work involves working on thickened nickel laterite tailings to produce a paste product for surface disposal. Achieving greater dewatering would offer advantages compared to conventional slurry deposition such as reducing the size of impoundments; reducing the size and costs of containment dams; and decreasing water sent to the impoundment. It also offers environmental advantages, including reduced leachate generation and decreased permeability of paste (homogeneous mass). Electrokinetic (EK) consolidation is a soil improvement technique that has been mostly used for soft fine-grained soils, such as silts and clays. The treatment involves applying a direct current across electrodes embedded into the soil. The current induces the movement of soilwater from the anode (positive pole) toward the cathode (negative pole). Dewatering and consolidation are achieved when the water is permitted to drain at the cathode and prohibited to enter the anode. The electrochemical reactions are associated with the EK process, which leads to a pH gradient in soil, generation of hydrogen and oxygen gases at electrodes and corrosion of consumable anodes, which must be addressed on a project-specific basis. The objective of this research is to study the viability of using EK to dewater tailings (leach residue and gypsum) from hydrometallurgical processes. The electroosmotic (EO) flow rate and coefficient of EO permeability (ke) are measured to assess the effectiveness of EK dewatering. The study includes comprehensive material characterisation and EK cell tests. The results of the study indicate: EO generated significant water flow in the tailings sample. The tailings have stable electrical conductivity, i.e. the applied current can be maintained relatively constant over time under a constant voltage. The ke values are in the range of 1.00 × 10-9 to 7.00 × 10-9 m2/V/sec, which is considered very favourable for EO dewatering. The electric conductivities of the saturated tailings are in the range of 2,0002,300 ?S/cm, and the tailings porewater has the electric conductivity of 8,000 ?S/cm, which is considered favourable in terms of power consumption for EK dewatering. The zeta potentials of the tailings before and after EO tests are stable in the pH range of 49, indicating EO dewatering would be effective in tailings without pre-treatment.
Water is employed as a cost-effective media of transporting tailings; a mixture of ground waste ore, water and chemicals used in metal extraction processes. Tailings are conveyed in pipelines from the plant to the tailings dam. From the dam, the water should be recycled back to the processing plant but herein lies the challenge. The hydraulic conductivity of fine-grained tailings is very low such that instead of draining, the water tends to accumulate in the dam. Not only does this led to pore water pressure build up which undermines the dams stability, but it also causes massive water losses by evaporation more so in arid regions. Dewatering of tailings prior to disposal has emerged as a solution that can be used to conserve water by producing paste or thickened tailings. Thickened tailings have higher shear strength, lower volume and they reduce dam closure costs. Filtered tailings can also be used as underground backfill for mine cavities or transported with conveyor belts and trucks to a designated point. The main limitation is that the benefits of dewatering are mainly technical and they do not yield a direct financial return. Processes that increase mineral production are vital in mine operations. Apart from the residue of the targeted metal, most tailings contain base metals like copper, nickel and zinc. Some mines have already established systems to extract valuable minerals from tailings; these include DRDGold, Lonmin and Sibanye mines in South Africa. Dewatering systems are designed to be in close contact with tailings and if they can perform the dual function of recovering water while extracting metals, they would fully meet the demands of the mining industry. One of the most efficient techniques which can harness both dewatering and metal extraction is electrokinetics. Electrokinetics involves the application of an electrical current to induce the flow of water from the anode to the cathode in a process called electroosmosis. Electrokinetics also induces the migration of ions in a phenomenon termed electro migration. The metal cations precipitate at the cathode where they are collected and dried. This paper evaluates the viability of using an electrokinetic system to dewater and extract metals from tailings. Introducing a mineral extracting function could well be the key to increase the usage of tailings dewatering techniques by mines.