Occurrence Relationship between Sodium and Maceral Groups in Subbituminous Coal: A Case Study on Zhundong Coal and Shenfu Coal

Abstract

The occurrence characteristics of sodium and its potential relationship with macerals in Zhundong coal, as well as in Shenfu coal with average sodium levels, are investigated in this study. A new five-step sequential extraction method was first conducted for determination. The occurrence status of sodium and its related macerals in samples was determined by microscope and SEM–EDS (scanning electron microscope–energy dispersive X-ray spectroscopy). Soluble sodium salts (H2O-Na) make up the primary proportion of sodium in Zhundong coal, at approximately 50%, while various sodium species are distributed in Shenfu coal with nonobvious differences. Inertinite contains more sodium than vitrinite does in both coals, and the highest enrichment degree of sodium was discovered in inertinite from Zhundong coal, which presented primarily as NaCl crystals (H2O-Na) in fusinite cells, with a local weight percentage of over 15%. More specifically, H2O-Na and insoluble Na both tend to enrich in fusinite. Additionally, it is found that maceral-rich products can be gathered using triboelectrostatic separation, and a portion of sodium can thus be removed from the coal by removing inertinite. This study may provide new insights and references regarding sodium removal from Zhundong coal.

Occurrence Mode of Sodium in Zhundong Coal, China: Relationship to Maceral Groups

“…This was similar to the order of the Na-containing oxide level in the XRF test: YH4 > YH3 > YH1 > YH2 > YH5. A reasonable explanation was offered previously [44][45][46][47][48][49][50][51][52][53][54][55]: the ash-forming substances in Zhundong coal with low ash content are mainly from non-mineral inorganics (organically associated Na and soluble salts), which leads to the development of a close relationship between Na and ash yields. In addition, there are very few trace elements in the five raw coals.…”

“…According to the extraction steps, Na in coal can be classified into five types, i.e., "H 2 O-Na", "NH 4 Cl-Na", "HCl-Na", "NH 4 -EDTA-Na", and "insoluble-Na", which refer to inorganic sodium salts, Na associated with carboxyl, Na occurring in chelate complexes or attached to functional groups other than carboxylic groups, Na-containing acid-soluble minerals, and Na-containing acid-insoluble residues, respectively [47]. The complete procedure and relevant operational parameters are shown in Figure 2, and the approach was thoroughly described in our previous study [48].…”

Occurrence of Differences between Alkali and Alkaline Earth Metals (AAEMs), Including Sodium, Potassium, Calcium, and Magnesium, in the Maceral Groups of the Xiheishan Coal, Zhundong Coalfield, Xinjiang, China

“…Then, the sharp end of the tweezers was used to slightly crush the scraped fusain flakes (thickness of about 1 mm), and the tweezers were used to select the particles with typical charcoal appearance and no other impurities as the inertinite concentrates. The separation operation used was also described in detail in our team's previous study [21]. Liptinite was excluded from this investigation due to its severe scarcity (<3%) in Zhundong coal [22][23][24][25].…”

“…To solve these problems, we proposed a new five-step sequential extraction process, using deionized water, 0.1 mol/L of NH 4 Cl solution (buffered to pH = 8.5 with NH 3 •H 2 O), 0.1 mol/L of NH 4 -EDTA solution (buffered to pH = 9 with NH 3 •H 2 O), 1 mol/L of HCl, and digestive acids (HF, HNO 3 , and H 2 O 2 ) as the extraction solution, as it can obtain watersoluble AAEMs (AAEMs-Water), AAEMs bound to carboxyl groups (AAEMs-NH 4 Cl), AAEMs in the form of chelates or AAEMs combined with functional groups other than carboxyl groups (AAEMs-EDTA), acid-soluble carbonate-bound AAEMs (AAEMs-HCl), and insoluble AAEMs (AAEMs-I). This process was described in detail in our previous study, and some progress has since been made [21,28]. In this study, we compared the traditional four-step extraction process to the improved five-step extraction process.…”