Improved sequential extraction method for determination of alkali and alkaline earth metals in Zhundong coals

• Effects of extraction condition on AAEM determination in were experimentally studied.
• An improved sequential extraction method was proposed with clearly defined conditions.
• The new method has higher accuracy and is less time consuming.
• AAEM occurrence modes and contents in three Zhundong coals were determined by the new method.
• Mineral patterns in raw coal and extraction residues were analyzed by XRD.

Zhundong coal is one of the most important coals in China due to its huge reserve, but it causes severe fouling and slagging problems when it is burnt in boilers for its high AAEMs (alkali and alkaline earth metal species) contents. Accurately determination of the occurrence modes and contents of AAEMs in Zhundong coals is a necessary step to overcome the ash-related problems. Nowadays, different researchers used different methods or procedures to determine AAEMs in Zhundong coals, and the data were scattering. Based on a series of experiments on the effects of extraction conditions, an improved extraction method with clearly defined conditions for each extraction step was proposed. 0.1 mol/L NH4Cl (pH = 8.5) instead of 1 mol/L NH4OAc (pH = 7) was used to extract the exchangeable AAEMs in Zhundong coals to avoid the high solubility of carbonates in the extraction solution. The improved method was more accurate and took much less time than the conventional one. With the proposed extraction method, the occurrence modes and contents of AAEMs in three Zhundong coals were measured. The results revealed that Na is mainly water soluble (Naw/Natotal = 50–85%) which is halite or in the form of surface bound Na+, while Ca and Mg are mainly acid soluble (Caac/Catotal = 60–90%, Mgac/Mgtotal = 45–90%) which are carbonates together with a little amount of sulfates. Exchangeable AAEMs organically bonded to the coal matrix are not the main occurrence mode of AAEMs. Acid insoluble Ca-species could be rankinite. The amounts of AAEMs in the same occurrence mode could be remarkably different among coals from different Zhundong district. The improved sequential extraction method can be extended to determine AAEMs in high carbonate-containing coals.

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