Gravity and magnetic separation of sulphides from the Akselberg calcite marble
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Calcite marble from Brønnøy Kalk AS’ Akselberg deposit is raw material for production of Ground Calcium Carbonate (GCC) used as filler and coating pigment in paper industry. One of the most important parameters for GCC production is brightness. 10 % of the deposit is of sulphide speckled marble quality, with the brightness detrimental sulphides pyrrhotite and pyrite. Existing flotation methods at the production plant is not capable of removing the sulphides, and it is therefore of interest to test whether gravity and magnetic methods can be useful for removing pyrrhotite and pyrite, as an iron content of 0.02 % is enough to reduce a high quality calcite marble brightness to below the required product brightness of 93 %. Gravity separation was performed with the Mozley Laboratory Mineral Separator (LMS), a Knelson KC-MD 3 centrifugal concentrator and the Mozley Multi-Gravity Separator (MGS). Magnetic separation was tested with the Jones P40 Wet High Intensity Magnetic Separator (WHIMS). The material was then analyzed with the Tappi Brightness T525 method, X-Ray Fluorescence (XRF) major element analysis and the acid insoluble residue method. The study shows that there is a potential for removal of sulphides and therefore a better resource utilization of the Akselberg calcite marble by magnetic methods, but that further investigations must be carried out on mineralogy and liberation of the products, as well as optimizing and combining of methods for further reduction of sulphide content in order to obtain the necessary brightness of the calcite concentrate. Gravity methods did remove some sulphides, resulting in a higher brightness, but the calcite loss during the process was too high in order for these methods to be interesting.