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The calcium carbonate is separated by filtration, and the solvent is returned to the extractor, as the ammonium salt is regenerated in the carbonator (Figure 1). In this process, calcium is selectively extracted from steelmaking slag using ammonium salt solutions in the extraction step: This concept has the potential to reduce CO 2 emissions and simultaneously utilize steelmaking slag. To minimize transportation costs, the limestone is calcined before transportation to the PCC production facility, which is normally located at the paper mill site.Īn alternative production concept for calcium carbonate production is being developed that omits the need for fresh limestone and its calcination ( Eloneva et al., 2009). PCC production requires also relatively pure limestone. The production of PCC binds CO 2 as carbonate, but CO 2 is also emitted when limestone is calcined for providing lime for the process. PCC is normally also brighter than GCC, since organic impurities and some metal oxides can be separated during the PCC production process ( Naydowski et al., 2001). By adjusting the precipitation process parameters, the shape and size of the crystals produced can be controlled to optimize their optical properties for use in paper making. In conventional production of synthetic (i.e., precipitated) calcium carbonate, flue gas containing CO 2 is bubbled though a hydrated lime slurry (calcium hydroxide), from which calcium carbonate precipitates. Precipitated calcium carbonate (PCC) is also used in papermaking as a coating and filler material. Therefore, GCC has a broad distribution of shapes and sizes, which reduces their optical performance ( Clark, 1992). Ground calcium carbonate (GCC) is manufactured by grinding high quality limestone to very small sizes and is mostly used as a pigment, included as an externally applied coating in coated papers. However, both filtration technologies seem suitable for filtering and washing calcium carbonate prepared with the studied method as a residual chloride content as low as 10 ppm of the filtered solids can be achieved with quite a small amount of washing water and the filtration rate is fast.Ĭalcium carbonate is the most commonly used filler material in paper making ( Naydowski et al., 2001). As the vacuum filtration tests used a filter cloth with higher permeability than that of the pressure filtration tests, a slightly higher filtration rate was expected. Vacuum filtration gave slightly higher filtration rates (500–610 kg/m 2h at the lowest residual chloride contents of the cakes), but the cake residual moisture also stayed higher (25–26 wt-%). Pressure filtration gave a high capacity (400–460 kg/m 2h) and a low cake residual moisture content (12–14 wt-%). The filtered cakes were analyzed by their residual moisture content, chloride content, and conductivity, and the filtrates by their residual solids content, chloride content, and conductivity.
#Scale up of a rotary vacuum filter series
The tests included pressure filtration and vacuum filtration and the test series contained altogether 21 different filtration cycles with varying combinations of filtering, washing, and drying steps. The order of maximum filtration rates achievable of the calcium carbonate slurries was determined by experimental work. In this work, different separation processes, including washing, filtering, and drying, were tested on two calcium carbonate slurries produced from steel converter slag and CO 2 by a laboratory-scale pilot facility, with the aim of obtaining a solid product with a low chloride content using a minimum amount of washing water. As calcium carbonate is precipitated in a solution containing ammonium chloride, calcium chloride, and ammonia, the product needs to be washed and hence filtered. The concept uses steelmaking slag as raw material and has potential to fix CO 2 emissions and utilize steelmaking slag, simultaneously. In this work, experiments were performed to determine the filterability of calcium carbonate produced with an alternative calcium carbonate production concept.