The implications of sulphides in GCC feed and the potential for their removal during alkaline amine flotation
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Ground calcium carbonate (GCC) slurries are commonly used in paper to improve properties such as whiteness, printability, opacity and gloss. The brightness R457 (i.e. the reflectance at a wavelength of 457 nm) and colour specifications constitute the main quality parameters for GCC slurries, and must be within certain tolerances set by the paper industry. Hence, the removal of any dark or colouring minerals from the raw material is essential to obtain high quality. Norsk Mineral is one of the world’s biggest suppliers of raw material for GCC production. The raw material is a coarse grained calcite marble extracted from the Akselberg deposit situated in Brønnøysund, 250 km north of Trondheim, Norway. Here, some 2 million tonnes of GCC raw material are produced annually by the subsidiary Brønnøy Kalk. The beneficiation of this raw material involves purification through reverse flotation followed by micronisation of the concentrates. The resulting fine grained slurries (d80 ≈ 1 μm) comprise the GCC product manufactured for the paper industry. Graphite and silicates represent the main contaminant phases in the Akselberg (Watne 2001). These contaminants are easily separated from the calcite marble by froth flotation, thus resulting in a white, high-quality product. However, substantial volumes of the deposit consist of a sulphide bearing marble, where the main contaminants are pyrite and pyrrhotite, the latter being the most abundant. The first part of the PhD study included an attempt to identify and quantify the different crystallographic phases of pyrrhotite since they display different magnetic, oxidative and flotation properties. The magnetic colloid method, SEM (scanning electron microscope) and EMP (electron micro probe) confirmed the presence of both pyrrhotite phases. Although the accurate quantification was not achieved, the experience gained is still documented in the thesis. Mineralogical and chemical investigations of the sulphide bearing marble revealed that it consists of >97% calcite with ≤1.3% pyrrhotite and pyrite grains (1 μm – 3 mm) evenly distributed in the calcite. A mineral liberation analysis (MLA) showed that approximately 95% of the sulphides were liberated in the <250 μm flotation feed. The next step of the project was to test the colouring effect that different contaminants, and especially pyrrhotite and pyrite, display on actual GCC products. Of the 14 contaminants tested, pyrrhotite, graphite and chalcopyrite displayed the most detrimental reduction in brightness, where a contamination level as low as 0.01 wt% compromised product quality in the finest slurry (90% <2 μm). By evaluating the colour information in the CIEL*a*b* parameters, pyrrhotite, graphite and chalcopyrite were shown to introduce a more bluish colour than the other contaminants tested. The contaminant particle size produced a significant effect on brightness and properties such as inherent optical differences, grindability and smearing effects of soft mineral were suggested as possible explanations for the variation in colouring properties of the different contaminant phases. Furthermore, the brightness as a function of contaminant concentration displayed a non-linear behaviour and it was shown that the system could be linearized by using the Kubelka-Munk model. The applicability of the Kubelka- Munk model in order to predict the GCC brightness was tested and the model was found to be usable. Since it was shown that sulphides significantly colour GCC products, the potential for removing pyrrhotite and pyrite during typical amine GCC flotation in saturated CaCO3 solution was tested. Microflotation results showed that both sulphides float to a certain degree, but that recoveries are highly dependent of pH. The overall highest recoveries were obtained at a pH close to 8. In addition, pyrrhotite recoveries were shown to be highly dependent on the extent of surface oxidation. The overall highest recovery of pyrrhotite was found at pH 8 for a sample that, prior to flotation, had been exposed to air for no more than 5 min. The flotation results showed strong correlation with zeta potential measurements, EDTA extraction results and redox (Eh) measurements. Finally, the flotation solution chemistry was shown to significantly influence the sulphide recoveries. The overall highest microflotation recoveries were produced in saturated CaCO3 solution. Significantly lower recoveries were obtained when using CaCl2 solution or deionised water. Bench scale flotation experiments showed that by using an amine collector concentration ≥300 g/t, recoveries became independent of pH and oxidation level. With lower collector concentrations, pH must be kept at 8.3-8.4 (the natural CaCO3 solution pH) and surface oxidation must be limited. The main findings in this PhD study have shown that sulphides undoubtedly reduce the brightness of GCC slurries. However, sulphides can to a certain degree be floated in a typical amine GCC flotation by controlling the operating parameters pH, surface oxidation or collector dosage. Bench scale flotation experiments with mixtures of the main raw material (i.e. the graphite speckled marble) and the sulphide bearing marble SBM in the flotation feed showed that substantial amounts of sulphide bearing marble can be blended into the raw material without compromising the brightness cut-off value. Brønnøy Kalk would benefit from improved subdivision of production blocks based on geological parameters as opposed to today’s regular pattern. Several recommendations as to how the sulphide bearing marble can be detected in the mine are given. In addition, it is highly recommended that the company performs a more detailed mineral liberation analysis of the sulphide bearing marble in order to evaluate the potential for increased removal of sulphides within the existing beneficiation process or in a separate processing line.