A digital imaging based approach to characterise arsenic minerals in colemanite ores

The presence of considerable amount of arsenic in borate concentrate produces penalty in marketing and makes its removal necessary. In this perspective, an automated procedure based on structural and textural characterization of a colemanite ore, in order to evaluate the degree of liberation of the arsenic minerals, adopting image analysis techniques, is presented. Samples of a colemanite (Ca2B6O11·5H2O) concentrate coming from Turkey have been collected. It has been revealed the occurrence of colemanite in different sizes and shapes, most commonly in nodules (up to 50cm), sometimes with thin discontinuous layers of clay. The samples are affected by relevant concentration of two arsenic minerals, realgar (AsS) and orpiment (As2S3). Polished sections have been submitted to scanning electron microscopy (SEM) and optical microscopy in order to acquire suitable digital images. The arsenic minerals have been found either as disseminated crystals or in veinlets along colemanite fractures with a very thin size (up to 100μm). In order to characterise and quantify the complex and variable distribution of the As minerals, fully imaging based strategies have been set up and applied. The obtained results enabled to define suitable and reliable comminution strategies in order to liberate the arsenic minerals. The developed procedure can be successfully applied to other ores, constituted by mineral association characterised by complex textures.