Abstract
Ultramafic nickel ores are difficult to process because they contain serpentine, an anisotropic mineral with a nonspherical morphology and multiple pH-dependent surface charges. Dehydroxylation of serpentine in ultramafic nickel ores by microwave treatment is proposed to improve the processability of these ores. Upon heating, serpentine is converted to olivine, an isotropic mineral that is benign in mineral processing circuits. The microwave heating of two ultramafic nickel ores is explored in this paper, as well as effects on mineralogy and grindability. The first ore was sourced from the Okanogan nickel deposit in Washington State, USA, while the second ore was obtained from the Vale-owned Pipe deposit located in the Thomson Nickel Belt in Manitoba, Canada. The ultramafic nickel ores were found to heat well upon exposure to microwave radiation and the heating behaviors were a function of the imaginary permittivities. The temperatures achieved during microwave treatment were sufficient to dehydroxylate serpentine, and the serpentine content in ultramafic nickel ores was reduced by 63–84%. The grindability of ore with consistent texture (OK ore) improved dramatically with microwave treatment, whereas the grindability of ore with inconsistent texture (Pipe ore) was found to decrease. Pentlandite liberation and specific surface area improved for both ores with microwave treatment. Ultimately, microwave pretreatment did not decrease the energy required for grinding under the conditions studied. However, energy savings may be realized when overall process improvements are considered (e.g., grinding, rheology, flotation, material handling, dewatering and tailings treatment).
Highlights
Ultramafic nickel ores are difficult to process, and the difficulty is largely due to the presence of serpentine (Mg3 Si2 O5 (OH)4 )
The objective of this paper is to explore the microwave heating of ultramafic nickel ores and effects on the mineralogy, ore grindability, specific surface area and pentlandite liberation
Despite similarities in mass loss with microwave heating time, the OK and Pipe ores demonstrated disparate heating behavior, which diverged at longer microwave exposure times
Summary
Ultramafic nickel ores are difficult to process, and the difficulty is largely due to the presence of serpentine (Mg3 Si2 O5 (OH) ). Serpentine is an anisotropic mineral with multiple pH-dependent surface charges and a nonspherical morphology. The processing of ultramafic nickel ores involves upgrading by froth flotation, on which serpentine minerals have a negative effect. The surface charge of the serpentine brucite-like basal plane is opposite that of the valuable nickel-bearing mineral, pentlandite ((Ni,Fe) S8 ), at the pH of flotation (pH 9–10) [1]. Serpentine greatly increases the viscosity of ore slurries due to its morphology and anisotropic nature [1,4,5], which inhibits the separation of minerals and necessitates that grinding and flotation be conducted at low solids content [3,6]. It is hypothesized that the Minerals 2018, 8, 524; doi:10.3390/min8110524 www.mdpi.com/journal/minerals
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