Quality control of a recrystallized Proterozoic limestone used for the industrial production of quicklime in a Twin Shaft Regenerative (TSR kiln: a South African case study

The dark grey bedded limestone from the Ouplaas Mine in Daniëlskuil, South Africa, is currently used to feed both the new 560 tpd Twin Shaft Regenerative (TSR) kilns, recently installed at the local manufacturing lime plant by Cimprogetti Spa. In order to support the client for setting up the quality control of the lime, compositional and microstructural analyses of the limestone, coupled with the technological characterization of limes burnt in laboratory at different temperatures, have been performed (Vola & Sarandrea, 2014). Especially, samples from two different quarry benches (A & B) have been studied to evaluate their burning attitude, and quicklime reactivity. This limestone belongs to the Kogelbeen Fm. of the Campbellrand-Malmani Carbonate Platform, Transvaal Supergroup, which is approximately 2500 Myrs old. It presents characteristic microbial stromatolite laminations, composed of prevalent planar, small scale domal, multi-layered sheets of problematic microorganisms, mainly microbes, originally deposited in a marine shallow subtidal environment. The most of the microfacies are strongly recrystallized. Micro and speudosparites, with ghosts of peloids and mud intraclasts, are dominant with also abundant late diagenetic poikilotopic calcite cement, generally associated with burial diagenesis, mechanical compaction, and pressure solution that produced swarms of thick black stylolites rich of pyrite and graphite (kerogen). The quantitative phase analysis (XRD-QPA) permitted to identify the impurity content, which is mainly ascribed to clay minerals, i.e. illite and chlorite, microcrystalline quartz (chert), and, locally, fabric dolomite. This limestone generally shows a sensitive burning attitude, and an overburning tendency at high temperature, so that the average slaking reactivity (EN 459-2) on limes burnt at 1050°C is very high (t60= 1.3 min. and Tmax= 72°C), but rapidly decreases at 1150°C (t60= 7.5 min. and Tmax= 68°C). Just two different layers of the quarry present significant exceptions, and one of them maintains an high reactivity also at 1150°C (t60= 2.3 min. Tmax= 67°C). The colour of the lime is mainly brown, due to the high content of manganese within the parental limestone. Light brown colour corresponds to soft-burnt limes, dark brown colour to medium-burnt limes, and dark-grey or black colours to hard-burnt limes. This information has been useful to control and setting up important kiln parameters, such as the heat distribution and the burning temperature of the plant. Considering the stratigraphic quarry log, and the reactivity of each layer, it was possible to calculate the average reactivity of each bench, and, subsequently, the expected reactivity of different mixtures feeding the kiln. The following scenarios have been reported: 1) A (100%): t60= 6.5 min.; 2) A (50%) + B(50%): t60= 7.3 min.; 3) B (100%): t60= 8.3 min. This study confirms that specific characteristics of the limestone, such as texture, degree of cementation, presence of dolomite, opaques, and clayey impurities, can significantly impact on quicklime reactivity, as well as, thermodynamic conditions of the process. Moreover, stratigraphic data, associated with laboratory tests, can be successfully adopted to select the best raw mix, reducing the impact of materials of a lesser quality.