Challenges in processing nickel laterite ores by flotation
Introduction
Nickel is an important metal with the total global consumption of about 2 million tons per year which has grown rapidly since the 1940s (Fig. 1) (Mackey, 2011). Nickel is sourced from two different types of ores, sulphide and laterite. The majority of the world's nickel resources occur as laterite ores which are complex, low grade, and expensive to treat using conventional smelting and high-temperature and/or high-pressure autoclave methods (Xu et al., 2013). While about 70% of the nickel resources are contained in laterites, only 40% of the world's nickel production comes from these ores. The main areas with the large nickel laterite resources in the world are New Caledonia, Australia, Indonesia, South America (Colombia and Brazil), The Philippines, India, and Russia (Einhorn, 2015). It should be noted that these days nickel laterites are more attractive for production of nickel as the amount of high-grade nickel sulphide ores has been diminished (Janwong, 2012). Therefore more economic processes to recover nickel from these resources should be developed.
Researchers have tried to improve nickel laterite flotation recovery by using a number of feed preparation techniques. A summary of the nickel grade upgrade using flotation reported in the literature is presented in Table 1. It can be seen that only minor nickel upgrades have been reported. Thus, the challenge of processing nickel laterite ores still exists. In fact, no physical separation technique (including flotation) has been able to dramatically upgrade nickel in laterite ores (Quast et al., 2015a). The only isolated case with an acceptable nickel upgrading is the result by Denysschen and Wagner (2009). They used dense medium separation ahead of flotation of a low-grade nickel ore at Tati Nickel Mine in Botswana. However, this work is not included in Table 1 as the ore they used was indeed a nickel sulphide, but the authors had classified it as lateritic probably because of its low grade.
It should be added that segregation of laterite ores prior the flotation process has shown some improvement in upgrading nickel. The segregation process relies on the addition of calcium chloride or sodium chloride and carbon allowing the formation of nickel and iron chlorides at temperatures between 900 and 1150 °C. Iwasaki et al., (1961) and Nagano et al., (1970) have reported nickel concentrate with 22–37% and 5–7% nickel using saprolitic and goethite ores, respectively. Harris et al. (2013) have also applied segregation to nickel lateritic ores before flotation and obtained an average nickel grade of 4–5% (Table 1).
Reverse flotation has been also used for an Indonesian iron-rich laterite ore to float siliceous minerals. Calcine laterite product was obtained from reduction at 900 °C by transforming limonite–goethite to magnetite. The reverse flotations was used to separate iron from nickel mineral using amine thioacetate, as collector (Purwanto et al., 2011). One stage rougher reverse flotation resulted in 0.5% nickel grades (at 33% recovery) which is not commercially acceptable.
Quast et al. (2015a) have reviewed the application of various techniques of pre-concentration of nickel laterite ores prior to hydrometallurgical or pyrometallurgical process. They concluded that the complex mineralogy of nickel laterite ores makes it difficult to achieve any significant nickel upgrading by physical techniques (including flotation). They have also stated that since nickel-bearing minerals are often finely disseminated through the laterite ores, liberation of such minerals may not be easily achievable. They have also published their flotation experimental results in a different paper (Quast et al., 2015c). The flotation of even a selected size fraction (38–75 μm) resulted in only a minor increase in the nickel content (from 1.0% to 1.4% at a Ni recovery of 43%). Therefore, further investigations in flotation of laterite ores are undoubtedly warranted. In particular, a more comprehensive review on topics influencing nickel laterite flotation needs to be conducted. The new review should cover both old and most recent publication in fine grinding, flotation of fine particle, aggregation of such particles, and the interactions between such fine particles in the flotation process. This will be heart of the current review. This paper aims to comprehensively review previously published work on processing nickel laterite ores. In particular, it focuses on the mineralogy of laterite ores, liberation of nickel bearing particles, and challenges in separation of these particles by flotation. Other factors which influence the flotation process including selective aggregation of fine particles, and particle–particle interactions are also discussed.
Section snippets
Mineralogy of nickel laterite ores
The main nickel-bearing minerals are Mn and Fe oxides, serpentine, garnierite, and chlorite. Table 2 shows an example of Ni-bearing minerals reported in literature (Massoura et al., 2006). Table 3 also shows some important minerals which are often present in nickel laterites (Rao, 2000).
In general, nickel laterite deposits are usually divided into 3 different zones of limonite (oxide), nontronite (clay), and saprolite (silicate) zones (Janwong, 2012). Saprolite is the lowest layer that reflects
Liberation of nickel-containing particles
It is known that selective grinding using stirred milling technology can preferentially liberate valuable minerals contained in soft minerals (Yue and Klein, 2004). It has been reported that stirred milling can selectively improve the degree of liberation compared to the traditional ball milling (Xiao et al., 2012). This may even increase the performance of mineral separation methods (Sen et al., 1987). Although, on the other hand, fine grinding may cause the pulp viscosity to be increased. The
Separation by flotation
It is well known that the recovery of fine particles (e.g. those produced by stirred media milling) is generally low in conventional flotation cells. An example of flotation recovery curve of different size fractions of chalcopyrite is presented in Fig. 5. While the low recovery of coarse particles may be due to the presence of complex particles or sedimentation of heavy coarse particles, the low recovery of fine particles is often attributed to the low particle–bubble collision (Dai et al.,
Selective aggregation of fine particles
It is evident that the flotation rate of fine particles can increase with increasing the particle size (Miettinen et al., 2010). In principle, flocculation or dispersion of all minerals should be possible by controlling the pulp ionic composition, or by using proper polymers or dispersants (Somasundaran, 1980). In general, dispersion or aggregation of mineral particles can be obtained via two basic mechanisms, charge or electrostatic, and steric interactions (Farrokhpay, 2009). Several studies
Interactions between fine particles in flotation
Interaction between fine particles can increase the pulp viscosity as well as the amount of gangue slime particles on the surface of minerals. This may reduce particle breakage rate during the grinding process (Klein and Hallbom, 2002, Yue and Klein, 2004) and also selective collector adsorption in flotation (Bremmell and Addai-Mensah, 2005). To probe these particle interactions, the surface potential and rheological properties of the ore suspension need to be determined at different conditions
Conclusions
Until now, flotation process has not been successful to recover nickel from laterite ores. This has been associated with the complex mineralogy of the laterites. Therefore, the mineralogical analysis is essential to identify the type and amount of nickel-bearing minerals, as well as the degree of their liberation. Nickel in laterite ores is often finely disseminated in various minerals in very fine size. Grinding to such a fine size is difficult using the traditional grinding tools. Therefore,
Acknowledgment
The financial supports from Labex “Ressources21” (Strategic Metals in the 21st Century) is gratefully acknowledged (Investissements d'Avenir-grant agreement no. ANR–11–LABX–0030).
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