Experimental study on metal removal characteristics of Zhundong coal to produce ultra-clean coal using acid-alkali method

The large-scale application of coal-based deash carbon materials has put forward growing requirements for the removal technology of ash within coal. Extensive research has demonstrated that the contents of ash and metal elements have a large influence on the performance of the coal-based electrodes made from ultra-clean coal. However, there is a lack of research on the ash and mental elemental removal characteristics of Zhundong coal. In the present study, one kind of Zhundong coal characterized by a comparatively low ash content, was utilized to examine the characteristics of ash and metals removal during the acid-alkali ash removal process. An inductively coupled plasma optical emission spectrometer (ICP-OES) was utilized to analyze the metal element concentrations in the solution during ash removal and the metal element contents within the coal samples. The removal characteristics of metal elements were further analyzed based on the detection results. The experimental results indicate significant improvements in the removal of ash and metals through acid-alkali multi-stage leaching. Under the specific experimental condition here, a two-stage acid-alkali ash removal process achieves coal with the lowest ash content of 1.6% and the removal of 98.0% of Fe, 95.2% of K, and 94.6% of Na, with the removal of Al reaching up to 40%. In addition, the Fe removed in the acid leaching step accounted for 95% of the total amount removed, and adding an acid pretreatment step based on acid-alkali leaching can remove 90.2% of Fe within coal. Both the acid leaching and alkali leaching steps exhibit a certain removal proportion for Al, K, and Na elements, which are closely related to the existing form of these elements within the coal. Iron-containing minerals are mostly acid soluble and distributed on the surface of coal particles. Potassium-containing minerals are mainly alkaline soluble, resulting in a removal rate of over 60% in the alkali leaching step of acid-alkali leaching combined with acid pretreatment. The low removal rate of Al (∼40%) is due to the wide variety of Al-containing minerals, such as kaolin, illite, and aluminates, which are less removed during the reaction with alkali due to temperature limitations. The present study can offer improved knowledge on mineral removal to obtain ultra-clean coal for utilization as function materials.