Selective leaching of nickel from mixed nickel cobalt hydroxide precipitate
Introduction
Hydrometallurgical processing of nickel laterite ore is complicated by impurities such as chromium, iron, aluminium, manganese and cobalt. Cobalt, in particular, is a value metal which is always present in the unweathered precursor igneous rock (Freyssinet et al., 2005). Once leached, chromium, iron and aluminium can be selectively precipitated leaving nickel, cobalt and manganese in solution. Subsequent sulphide or hydroxide precipitation is typically used to achieve manganese separation from nickel and cobalt; the latter being less selective against manganese (Harvey et al., 2011). Nickel and cobalt however, tend to precipitate together as either hydroxides or sulphides, and also tend to co-deposit when reduced as their standard reduction potentials are similar, − 0.25 V and − 0.28 V vs. the standard hydrogen electrode, respectively (Bard et al., 1985). As a consequence, the separation of nickel from cobalt is a significant challenge for hydrometallurgists.
Nickel laterite ore is typically processed by variations of two flowsheets; ammonia leaching of roasted ore, or direct sulphuric acid leaching, with industry favouring the latter in recent times. Nickel and cobalt are recovered from acidic pregnant leach solutions (PLS) by one of three methods; (i) precipitation of a mixed nickel–cobalt sulphide; (ii) precipitation of a mixed nickel–cobalt hydroxide (MHP) or (iii) direct solvent extraction (DSX). Nickel and cobalt are then separated from each other and subsequently recovered from the resulting solutions by hydrogen reduction, electrowinning or precipitation. Nickel–cobalt separation technologies are summarised in Table 1.
Although mixed sulphide precipitation excludes more of the common impurities, mixed hydroxide precipitation is becoming increasingly popular as it “eliminates the costly and undesirable H2S precipitation step, and yields a product readily soluble in ammonia or dilute sulphuric acid with potential for the application of SX/EW” (Taylor, 1995). Mixed hydroxide precipitation has also proven to be a more robust technology than DSX. Bulong operation's DSX was noted to have high levels of impurities, in particular, calcium magnesium, and iron(II) (Donegan, 2006). Bulong also experienced a lengthy ramp-up due to considerable operational issues such as gypsum scale, crud build-up in the solvent extraction settlers and organic contamination of the nickel electrolyte (Donegan, 2006, Nofal et al., 2001, O'Callaghan, 2003). The high predicted operating costs and perceived higher technical risk of DSX (Mason and Hawker, 1998) has led to widespread consideration of the mixed hydroxide precipitation flowsheet and stimulates the current research to develop new methods of MHP refining with a simple and effective nickel and cobalt separation step.
One method of refining MHP, employed at the Queensland Nickel and Cawse refineries, is an ammonia ammonium–carbonate leach. In the leach, nickel and cobalt are recovered to solution along with copper, zinc and manganese (to a lesser extent); however, the majority of the iron, aluminium, magnesium, calcium, chromium and silicon remain in the solid phase (Dobson et al., 2001, MacKenzie et al., 2006). Ammonia and carbon dioxide are then recovered from the PLS by a steam strip to provide an appropriate ammonia concentration for subsequent solvent extraction. Prior to solvent extraction, cobalt in the PLS is oxidised with air to its trivalent form. During solvent extraction the Co(III) ammine complex remains kinetically inert in the aqueous phase, while nickel readily loads onto the LIX84I-containing organic phase. Nickel is eluted from the organic, in either acid (Cawse) or strong ammonia solution (Queensland Nickel), and is recovered as a high-purity cathode or nickel hydroxyl-carbonate. Cobalt is subsequently recovered from the raffinate by sulphide precipitation (Dobson et al., 2001, Fittock, 1997).
Ammoniacal leaching of MHP is more selective towards nickel and cobalt when compared to acid leaching; however, it is sensitive to MHP oxidation and ageing (Jones and Welham, 2010). To increase the extraction of nickel and cobalt from MHP, the Queensland Nickel refinery employs a reductive leach (CoNiS as the reductant) to liberate nickel and cobalt from oxidised manganese and cobalt compounds (Anderson et al., 2009). It is also pertinent to note that MHP contains ammonia-consuming sulphate, resulting in significant losses of ammonia as ammonium sulphate that is not recovered by steam stripping.
Cobalt is the most problematic issue in the use of the LIX84I solvent extraction circuit. As the oxidation of Co(II) to Co(III) is never complete, any unoxidised cobalt loads with nickel onto the organic (MacKenzie et al., 2006, Skepper and Fittock, 1996). This loaded cobalt may contaminate the downstream nickel circuit, or more commonly oxidise on the organic and accumulate, reducing the subsequent nickel transfer capacity by 1.5 g-Ni/L per g-Co(III)/L (MacKenzie et al., 2006). Cobalt accumulation can be managed by reductively stripping cobalt from the organic in a separate stage. Copper also loads onto the organic at the same time as the nickel, requiring a separate copper removal process (MacKenzie et al., 2006). Without a dedicated copper removal process, the Cawse operation had difficulty maintaining copper in the catholyte below 10 mg-Cu/L (Dobson et al., 2001). The LIX84I functional group also degrades; however, regeneration can be carried out onsite (Ferguson et al., 1988).
Nickel recovery via ammoniacal leaching and SX presents two clear challenges; process sensitivity and impurity rejection. It is in these areas that selective acid leaching of MHP provides advantages. In this process as shown in Fig. 1, MHP is contacted with a strong oxidant and sulphuric acid solution. With the selection of a suitable oxidant and oxidant addition rate, cobalt and manganese are stabilised in the solid phase (Eqs. (1), (2)), leaving the majority of the nickel accessible for leaching at weakly acidic conditions (approximately pH 4.5), yielding a concentrated nickel sulphate solution (Eq. (3)). Nickel can be recovered from solution as a final product, for example, as metal or a compound such as nickel hydroxide or sulphate. The leach residue, a mixture of cobalt oxy-hydroxide and manganese oxy-hydroxide, may be treated by methods described by Chong et al. (in press).
Section snippets
MHP sample
One industrial sample of MHP was used in all the experiments, except for investigating the effect of MHP ageing on leach performance. The fresh MHP was approximately three months old and had been stored in a sealed container and plastic bag to minimise drying and/or oxidation. The MHP sample has an average moisture content of 43.5 wt. % and composition as given in Table 2. The size distribution of the sample, as shown in Fig. 2, was determined by a combination of wet and dry sieving, and
The effect of pH on metal deportment
The recovery of major (> 1 wt. % MHP dry basis) and minor (< 1 wt. % MHP dry basis) elements for pH 1.0 to 7.0 is shown in Fig. 3, Fig. 4, Fig. 5, respectively.
At pH 4.5 the maximum nickel extraction in this batch experiment, approximately 90%, was achieved whilst maintaining complete separation from cobalt and manganese as well as copper and iron. Approximately 90% of magnesium, 80% of calcium, 65% of zinc, 35% of silica, 25% of chromium and 11% of aluminium was also recovered to solution. The
Conclusions
The selective acid leaching of MHP provides a relatively simple means to recover and separate nickel from cobalt, manganese, copper and iron. These separations were achieved quickly (< 1 h) over a wide range of temperature (30–80 °C). Nickel leaching was only slightly affected by MHP ageing. Calcium and magnesium, the major solution impurities, can be partially removed from the MHP by prewashing at pH 7. These characteristics provide the basis for a robust and streamlined process for the refining
Acknowledgements
The authors gratefully acknowledge the financial assistance for this research provided by the Queensland Department of Employment, Economic Development and Innovation, as well as support from UniQuest. In addition, the authors recognise the contributions to this project by Sebastian Chong, Sheena Chen and Ana Rosa De Lima and input from various industry representatives.
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