Mode of occurrence of calcium in various coals

Calcium in coal can be classified into two parts: mineral matter and cationic metals associated with organic matrix. Chemical leaching with aqueous ammonium acetate (NH4OAc) solution has been widely used for the purpose of removing only ion-exchanged metals. However, it still remains uncertain whether the leaching can remove only these metals, because the leaching experiments have been mainly carried out using low rank coals. In this study, the leaching of three Argonne premium coals was carried out under different conditions to examine the effect of operating variables on leaching, and furthermore the species leached out were identified. Calcium leaching of Pocahontas #3 (POC) coal by aqueous NH4OAc solution was examined in detail. Under our standard condition, the leaching yield was 80%, almost the same as for aqueous HCl leaching. Furthermore, we analyzed the mineral matter in the three coals before and after the NH4OAc leaching by X-ray diffraction analysis (XRD) and computer controlled scanning electron microscopy (CCSEM). According to the results from XRD and CCSEM, it was found that a substantial portion of calcite was removed by the NH4OAc leaching. In conclusion, not only ion-exchanged calcium but also calcite was leached out with NH4OAc solution.