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

Abstract

The occurrence and separation relationships of Na and maceral groups in Zhundong coals were investigated in this study. The sequential extraction results indicate that the total Na content of all samples decreased with increasing sampling depth, and the level in inertinite-enriched samples (inertinite content 81.0%–84.0%) was significantly higher than that in corresponding raw coals and vitrinite-enriched samples (vitrinite content 90%). Additionally, H2O-Na (soluble salt species) and insoluble Na (acid-insoluble residues) were found to be concentrated in the inertinite-enriched samples. In combined SEM–EDS and microscope observations, local Na enrichments were detected in all raw coal and inertinite-enriched samples except for the vitrinite-enriched samples, but only inertinite-enriched samples were found to generally have over 10 wt% of Na enrichments, all of which occurred as NaCl. Moreover, Na is mostly filled or associated with cell-filling minerals in cells of fusinite. The maceral separation and Na removal of Zhundong coal were simultaneously achieved using triboelectrostatic separation. The vitrinite content in concentrates increased up to 60%, along with a reduced Na level, while the inertinite and sodium levels were both evidently raised in tailings. The obvious positive occurrence and separation correlation between sodium and inertinite offers new insight into, and a technical reference for, the sodium removal and maceral processing of Zhundong coal.

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

“…However, the traditional method has many drawbacks, such as using a neutral ammonium acetate solution to dissolve some carbonate minerals (CaCO 3 and MgCO 3 ) in advance, meaning that some mineral forms of calcium and magnesium are mistaken for ion-exchange forms [26,27]. In addition, some Na, when combined with organic matter, becomes insoluble in ammonium acetate solution and might be extracted using hydrochloric acid [28,29]. The above circumstances cause inaccurate classification results.…”

“…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.…”

“…This difference was present because Na-Water in coal mainly originates from the enrichment of high-alkaline water in coal-forming swamps and the infiltration of highly mineralized groundwater into coal seams after the sedimentary period [45]. Furthermore, the inertinite of coal has a more abundant pore structure, which makes it easier to enrich Na-Water in coal [28]. Meanwhile, the vitrinite-enriched samples (B3-V and B5-V) were less metamorphic and had more abundant oxygenated functional groups (organic acids) [46], which made it easier to form organic sodium with Na + .…”