Berong Nickel Laterite Complex: Nickel Provenance Using Sequential
Extraction Methods and SEM-EDX
Meryl Calibo, Carlo Arcilla, Ma. Luisa Tejada
Mineralogical and geochemical characteristics of the nickel ore deposit in Berong, Palawan, were investigated to better understand the occurrence, distribution, variation, and controls of nickel enrichment within weathered sections. Based on mineralogical and chemical compositions, four zones were delineated from surface to bottom of the weathering profile: (1) the goethite-dominated iron oxides and hydroxides zone of around 1-2% nickel concentration; (2) the transitional zone between the former and the mineralized serpentine zone; (3) the nickel-enriched serpentine zone, with nickel concentrations almost reaching 3%; (4) and the unmineralized serpentine zone equivalent to a weathered serpentinizeddunite bedrock. In the absence of garnierites, the higher nickel concentration in the saprolite layers may be explained by the downward migration of leached nickel ions during the lateritization of the top layers. Behavior of major and minor oxides as weathering progresses indicates that iron is another target metal that can be potentially mined along with nickel and cobalt. Despite being globally widespread and relatively easy to mine, nickel laterite ores prove to be difficult to process. Beneficiation of Ni and other economically extractable elements along with it greatly depends on the mixture of the feed material (silicate phases and oxide phases) which, in turn, is dependent on the mineralogy of the raw ore. The type(s), concentration and consumption of acid to be used in dissolution are controlled by the percentages of both gangue and host minerals trapping the Ni, whether by sorption or isomorphous substitution. A modified selective sequential extraction was designed to recover Ni from from its various host phases to optimize the beneficiation process using samples from different zones in a nickel laterite deposit in Berong, Palawan. This deposit is defined by, from top to bottom, an iron oxide-hydroxide zone, a transition zone made up of serpentine and iron oxides, a nickel-enriched serpentine zone, and a nickel-depleted serpentine zone, based on mineralogy and geochemistry. The occurrence of Ni as adsorbed and exchangeable cations, in carbonates, in amorphous iron oxides, within the structure of crystalline iron oxides and hydroxides, and in residual silicate layers within each of the zones was determined. Qualitative analysis of nickel in iron oxides, serpentine and talc using electron-probe microanalyzer supports the results of the extraction experiment. In limonite zones, dominated by goethite and other secondary iron oxides, > 90% of the total nickel reside in the crystal structure of Fe oxides. In the iron oxide – magnesium silicate transition horizon and in the nickel-enriched saprolite zone, respectively, > 80% and an average of 77% of the nickel are distributed in Fe oxides and within the octahedral layers of serpentine. The remaining Ni ions not taken up by these crystalline minerals are mostly associated with amorphous Fe oxides. An EPMA image, depicting relative abundance of Ni, Mg, and Fe in a section of weathered bedrock, implies that Ni is more closely associated with Fe than with Mg. An industrially significant output of this research is the finding that residual silicates, which take an enormous amount of acid to dissolve, need not be dissolved in order to optimize the beneficiation of nickel.