Purification of slag-derived leachate and selective carbonation for high-quality precipitated calcium carbonate synthesis

Much research has been performed to synthesize precipitated calcium carbonate (PCC) from calcium extracted from blast furnace (BF) slag. A significant challenge has been obtaining a chemically pure sample with a homogeneous crystal structure and narrow particle size distribution, which can have profitable marketability (e.g. in the papermaking industry). The aim of this study was to investigate the influence of alternative and combinatory processing strategies on the chemical, morphological and mineralogical properties of BF slag-derived PCC. In a first phase, the physicochemical removal of impurities from the leachate, prior to the carbonation, was investigated, by means of temperature reduction (20 → 1 °C) and pH elevation (4.4 → 8.4); this led to selective precipitation of Al, Mg and Si. In a second phase, the operational conditions of the carbonation step were optimized, by lessening the severity of the operational conditions (90 → 30 °C and 6 → 2 bar, CO2) and by reducing the amount of acid-neutralizing base (1.0 → 0.85 M,Na+/M,CH3COO−) added during the carbonation process; this led to more selective calcium carbonation. Using these strategies, it was possible to synthesize chemically pure PCC (>98% Ca) with uniform morphology (scalenohedral), homogeneous mineralogy (>88% calcite) and narrow (1.09 uniformity), small (D50 = 1.1 μm) particle size distribution.