The development of a resin-in-pulp process for the recovery of nickel and cobalt from laterite leach slurries

The processing of laterite ores for the recovery of nickel and cobalt has increased as the reserves of exploitable sulphide ores have become depleted. The pressure acid leach process (PAL) has become the preferred option for the treatment of laterite ores. Difficulties associated with the poor settling characteristics of the pulp in the counter current decantation (CCD) circuit after pressure leaching has resulted in as much as 10% of the soluble nickel and cobalt reporting to the tailings. The objective of this project is the development of an alternative processing step for the recovery of soluble nickel and cobalt from the PAL tailings using resin-in-pulp (RIP) technology. Commercially available chelating resins with the iminodiacetate functional group have been studied for their suitability for the adsorption of nickel and other metal ions from PAL tailings pulp. The Amberlite IRC 748 resin was found to be superior despite its lower nominal loading capacity. The resin with the highest nominal capacity was observed to adsorb less nickel as a result of the adsorption of greater amounts of the impurity ions. The equilibrium loading for nickel on the preferred resin was found to be similar from the ammonium and protonated form of the resin although the kinetics of adsorption is greater when the resin is initially in the ammonium form. A comprehensive study has been made of the equilibrium adsorption of several metal ions on the resin as a function of the equilibrium pH of the solution. A relatively simple model of the equilibrium adsorption which includes the effect of pH has been developed and the results compared with the experimental data obtained in the M2+/Na+/H+ system. The model which requires two equilibrium constants has been found to fairly well describe the experimental results. A study of the kinetics of the loading of nickel and cobalt from both solutions and pulp has shown that the rate can be described in terms of a first-order approach to equilibrium. The kinetic and equilibrium parameters were used to simulate the performance of a multi-stage counter current resin-in-pulp operation A semiquantitative study of the elution of the adsorbed metal ions from the resin by dilute sulphuric acid solutions was also undertaken. The technical feasibility of the RIP process for the recovery of nickel and cobalt from the PAL tailings has been successfully demonstrated in both laboratory and pilot-plant studies using a five-stage adsorption process. Important parameters such as the operating pH and the residence times of pulp and resin in each stage were identified through the batch test work coupled to the simulation procedure. The optimum pH for adsorption was found to be in the range 4 to 5 as this pH is high enough to maximise the adsorption of nickel and cobalt while preventing precipitation of nickel and cobalt as hydroxides from the pulp. A method for minimizing the competition from more strongly loaded ions such as iron(II) and chromium(III) which are present in the pulp was also developed in the initial laboratory phase of the project and utilized during the pilot operation. Problems associated with the preparation of the pulp preparation, elution of the loaded resin and control of the adsorption train were resolved during several pilot plant runs on site at a local PAL plant. These and other minor improvements and adjustments to the operating procedure culminated in a successful continuous run for several days during which the target recovery of 90% for nickel and 60% for cobalt was exceeded throughout the run