Nickel recovery from spent electrolyte by nanofiltration

Abstract

South Africa is the world's leading producer of Platinum Group Metals (PGMs) and holds more than 80% of the world's reserves. Although PGMs are the most important products in the ores, base metals are valuable by-products, particularly nickel. Metallic nickel is produced by electro-winning from the acidic nickel sulphate solution, and the spent electrolyte still contains high levels of nickel. In the current process, the spent electrolyte is neutralised by the addition of caustic that results in precipitation of nickel hydroxide in a sodium sulphate solution. The main disadvantages of this process are that low-value sodium sulphate is produced by adding high-value caustic and that three major process units are required. In this paper the possibility of using a DOW NF membrane to separate the nickel from an acidic solution is studied under simulated industrial conditions. The experiments were carried out in a lab-scale, cross flow, flat-sheet membrane contactor. The experimental conditions include a nickel concentration at 30 to 50 g/L, a pH range of I to 2 and a temperature of 25 to 40°C. Pressure differences of 20 to 55 bar were chosen to examine the effect of pressure on the selectivity of the system and to achieve meaningful flux values. The nickel rejection was > 97.5% for all the chosen combinations of operating conditions and as high as 99.2% at pH 2 and nickel concentration of 40 g/L. Overall the rejection of nickel was higher at pH 2 compared to pH 1. With respect to the hydronium ions, negative rejection was observed. The flux depended on the nickel concentration of the feed, pressure difference over the membrane and temperature of the solution, with no significant influence by pH. Although temperature had a large affect on the flux, no influence on nickel rejection was observed. With the introduction of sodium the flux reduced immensely but only a 10% reduction in rejection of nickel was found. Fouling caused by scaling occurred and a notable reduction of flux was found during the long run. From this experimental work, it can be concluded that the results are very promising towards the introduction of nanofiltration technology into the challenging world of hydrometallurgical separations in the minerals process industry. The nickel can be separated from the sulphuric acid with large rejections (> 97.5%), and reasonable fluxes (20 - 50 kg.m-2.h-1). Sodium, which is present as sodium sulphate, significantly decreases ,the flux, and further studies should be carried out on the replacement of sodium sulphate in the electro-winning process.