Synthesis, Rietveld refinement and DSC analysis of Al-goethites to support mineralogical quantification of gibbsitic bauxites

This study describes a procedure to synthesize and characterize a series of Al-goethites (varieties with Al-for-Fe substitution ranging from 9 to 28%) to serve as heat-of-reaction (dehydroxylation) standards aiming the quantification of Al-goethite in gibbsitic bauxites using differential scanning calorimetry (DSC). The goal of this study is to establish the proposed procedure as a supporting method for the comparison and validation of old and new (different methods of) mineralogical quantification methods of gibbsitic bauxites, since the methods commonly used are rather questionable in terms of precision and accuracy. Although the application of DSC in an aluminum industry setting may be unlikely due to practicality and time-of-analysis constraints, the technique may prove to be useful for validating methods currently being developed. The increasing order of Al-for-Fe substitution in planned syntheses was confirmed by XRD, XRF, DSC and SEM results, which showed that a small database of goethite dehydroxylation standard enthalpies could be established in this study. The production of standards of multiple goethite varieties is so important than one goethite standard alone, because such mixtures are thermodynamically common in nature, and therefore very common in bauxites. The elevated dilution of the Al-goethite phase in the test bauxite did not allow DSC electronic signal, which implied the need for physical separation/concentration, in this case sieving and cyclone to concentrate the goethite in the fractions below 0.15 mm.