Crystallization of Jarosite with Variable Al3+ Content: The Transition to Alunite

Structural Effects of Aluminum and Iron Occupancy in Minerals of the Jarosite-Alunite Solid Solution

The alunite supergroup of minerals contains several hydroxysulfate mineral phases that commonly occur in acidic natural and engineered environments. The main division of the mineral supergroup defines two minerals, jarosite and alunite, based on the relative structural occupancy by Al or Fe, respectively. However, intermediate members of the jarosite-alunite solid solution have not been extensively characterized, especially in the environment. Here, we link the mineral unit cell sizes measured by X-ray diffraction, peak shifts in Raman spectra, fitting parameters in Mössbauer spectroscopy, and elemental quantification by EDX spectroscopy to known amounts of Al substitution in two synthetic series of Al-substituted jarosite (up to Al-for-Fe substitution of 9.5%) and unknown Al substitution in a natural jarosite isolated from an acid sulfate soil. Strong correlations were observed between the Al substitution of the jarosite samples and unit cell size, position of several vibrational peaks in Raman spectroscopy, and the temperature of magnetic ordering. In addition, elemental mapping provided a robust way to characterize the Al content of jarosite. As the techniques were effective in quantifying the Al or Fe content of jarosite-alunite supergroup mineral samples, without the need for sample dissolution, the findings support the application of these spectroscopy techniques to characterize natural jarosite-alunite samples. Using these techniques, we demonstrate at least 5% Al-for-Fe substitution in a jarosite sample from an acid sulfate soil. Application to environmental samples is especially useful in cases where it is otherwise difficult to directly measure the Al content of a mineral sample or when Al-for-Fe substitution influences the spectral responses to substitution at other sites in the crystal structure.

The High Temperature Co-Processing of Nickel Sulfide and Nickel Laterite Sources

The pressure oxidation of low-grade nickel sulfide concentrate with high iron sulfides content generates significant amounts of sulfuric acid that must be neutralized. This acid can be utilized to leach metal values from ores such as nickel laterites. The present study demonstrates the use of a low-grade nickel concentrate generated from Poseidon Nickel Mt Windarra ore to enable additional nickel and cobalt extraction from a Bulong Nickel Operation nickel laterite blend. The co-processing of these materials at 250 °C, with oxygen overpressure, using total pulp densities of 30% or 40% w/w, and a range of nickel concentrate to nickel laterite mass ratios between 0.30–0.53, yielded base metal extractions of 95% or greater. The final free acid range was between 21.5–58.5 g/L, which indicates that enough in situ sulfuric acid was generated during co-processing. The acid was shown from mineralogical analysis to be efficiently utilized to dissolve the laterite ore, which indicates that the primary iron hydrolysis product was hematite, while the aluminum-rich sodium alunite/jarosite phase that formed hosts approximately 5% of the hydrolyzed iron.