Purification of Slag-Derived Leachate and Selective Carbonation for the Production of High-Quality Precipitated Calcium Carbonate

Much research has been performed to synthesize precipitated calcium carbonate (PCC) from calcium extracted from blast furnace slag (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). To date, methodologies for achieving these properties have not been clearly elucidated in scientific literature. 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.

The studied process commences with the traditional approach of leaching calcium from the BF slag (e.g. using an organic acid, most commonly acetic acid). In this study, high calcium extraction extent was sought to produce a solid residue suitable for subsequent zeolite synthesis, as reported in our prior study (Chiang et al., 2014, Chem. Eng. J. 249, 260-269). After separation from the post-extraction BF slag residue, the produced leachate is carbonated at controlled temperature, pressure and duration. However, without processing improvements, large amounts of impurities (particularly magnesium, aluminum and silicon), which are co-leached from the BF slag, end up in the post-carbonation precipitates. Furthermore, the PCC attains markedly heterogeneous mineralogy and morphology.

To overcome these issues, the present research undertook a two-fold approach. In a first phase, the physicochemical removal of impurities from the leachate prior to the carbonation was investigated. By reducing the temperature of the leachate, the solubility of silicon was lowered, aiding in its removal by physical means. Furthermore, the removal of magnesium, aluminum and suspended silicon was accomplished by elevating the pH of the leachate prior to physical separation; this action induced the precipitation of Mg and Al hydroxides, while the Si co-precipitated with the formed voluminous flocs.

In a second research phase, the operational conditions of the carbonation step were optimized in order to produce a post-carbonation precipitate with homogeneous crystal structure and uniform morphology. In addition, the undesired co-precipitation of magnesium carbonate was prevented, thus improving the chemical and mineralogical purity of the product. This was achieved by lessening the severity of the operational conditions and by reducing the amount of acid neutralizing base (e.g. sodium hydroxide/bicarbonate) added during the carbonation process, leading to more selective calcium carbonation.

The implementation of all aforementioned strategies to the traditional BF slag-derived PCC producing process resulted in the synthesis of 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. This product is potentially suitable for application as a papermaking filler or paper coating pigment.