| Summary: | 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. === Thesis (M. Ing. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2007.
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