Inorganic matter in Victorian brown coals

• Quantification of the minerals that occur in very small amounts in Victorian brown coals was enabled by sequential leaching.
• The organically associated inorganic elements were largely removed by water, ammonium acetate and NH4-EDTA.
• NH4-EDTA effectively removed significant amounts of inorganic elements that would be extracted only by strong acids.

A common method to quantify the mineral matter in coals is to remove the carbonaceous matter by low-temperature oxygen plasma ashing (LTA) followed by X-Ray Diffraction (XRD) analysis. However, some low-rank coals, such as Victorian brown coals (lignites), have low levels of mineral matter and contain significant amounts of non-mineral inorganics (organically associated elements and salts dissolved in the coal pore's water), which form artifact phases during low-temperature ashing. The presence of these phases also lowers the concentration of the low abundant mineral species in the ash below the detection limit of the XRD. Some of the non-mineral-matter components can be removed by leaching of the coal prior to ashing, but the leaching conditions have to be chosen so that the mineral matter is not removed. In this study, sequential leaching was performed on three Victorian brown coals (Hazelwood, Yallourn and Loy Yang) in order to increase the concentration of the mineral species in the low-temperature ashes by removing the non-mineral inorganics. We introduced diammonium ethylenediaminetetraacetate (NH4-EDTA) solution into the leaching process, because of its potential to extract ion-exchangeable cations without dissolving acid-sensitive mineral matter or contaminating the extract solution with introduced metal cations. The leaching protocol was stepwise extraction by water, ammonium acetate, NH4-EDTA, hydrochloric acid and nitric acid. Sequential leaching enabled quantification of the minerals that occur in very small amounts in the coals. K-feldspar, pyrite, augite and kaolinite were the additional minerals quantified in the leached coals beside quartz, plagioclase (calcian albite), marcasite and calcite, which were found in the original coals. Most non-mineral inorganics, which produce large amounts of artifact phases in the low-temperature ashes, were removed by water, ammonium acetate and NH4-EDTA. NH4-EDTA effectively removed significant amounts of inorganic elements, which most likely occur in coordination complexes and would be extracted in conventional leaching studies only by strong acids. This procedure makes it possible to discriminate between inorganic elements that occur organically associated and mineral matter that is dissolved by strong acids.