Effect of sodium compounds on the sintering propensity of coal-associated minerals

•The influence of NaCl and Na2CO3 on sintering of a mineral mixture was investigated.•Both NaCl and Na2CO3 increased the sintering by 20–80% at 500–800 °C in air.•Increased sintering was attributed to enhanced sulphation of limestone to anhydrite.•NaCl decreased sintering in the mixture treated at 700–1000 °C in N2.•Na2CO3 increased sintering in the mixture treated at 500–800 °C in N2.

This study focused on the chemistry involved in sintering of coal-associated minerals during combustion and inert conditions. Changes in mechanical strength were recorded with compressive strength measurements. This method was used to investigate the influence of sodium addition (as NaCl and Na2CO3) on the sintering strength of a prepared mineral mixture at temperatures ranging from 500 °C to 1000 °C. Supporting data were collected with thermogravimetry (TG), differential thermal analysis (DTA), scanning electron microscopy (SEM) and X-ray diffraction (XRD). It was found that the addition of both NaCl and Na2CO3 increased the sintering of the mixture by between 20% and 80% at temperatures ranging from 500 °C to 800 °C in air. The increase in sintering was most likely due to the enhanced sulphation of limestone to anhydrite, resulting in a more extensive sintered network. Anhydrite decomposes at temperatures higher than 900 °C. At 900 °C and 1000 °C the sintering observed for the mixture with additives was comparable to the sintering observed for the baseline mixture. The decomposition of anhydrite therefore weakened the sintered network to the same level as the baseline mixture. In an inert atmosphere (N2) the same strength development as in the mixture heat-treated in air was not observed in mixtures with sodium additives. Anhydrite was not formed in N2 due to the lack of O2 and SOx. A decrease in the sintering propensity upon heat-treating the samples from 700 °C to 1000 °C in N2 for the experiments containing NaCl as additive was noted. This was most likely due to the volatilization of NaCl that weakened the structure. The addition of Na2CO3 increased the sintering propensity of the mixture heat-treated in N2 from 500 °C to 800 °C. The mechanism for this increase is currently unknown and should be further investigated.