Characterization and kinetic analysis of acid leaching of low carbon fraction in coal gasification slag

Abstract

Coal gasification slag (CGS), a byproduct of coal chemical processes, can be repurposed as functional materials through acid treatment to remove impurities such as Al, Ca, and Fe.This study investigates the acid-leaching behavior of these impurities by examining the slag’s mineralogical structure and elemental distribution. The process involved initial carbon-ash separation via physical sieving (0.85 mm), yielding a low-carbon slag (L-CS) with < 3% carbon content and a 48.33% yield, characterized by dense, lamellar aluminosilicate glass with uniformly dispersed and encapsulated Ca and Fe. The L-CS was subjected to heat treatment at varying temperatures, followed by hydrochloric acid leaching. Results revealed that heat treatment broke and rearranged the original Si-O-Si(Al) bonds, partially converting [AlO4] structural units into more stable [AlO6] and leading to the sequential formation of magnetite and esseneite phases. This structural reorganization significantly reduced the leaching rates of Al, Ca, and Fe. Kinetic analysis demonstrated that the leaching of Fe and Ca was governed by a combination of internal diffusion and interfacial chemical reactions, whereas Al leaching was controlled solely by interfacial reactions. The Ca and Fe components were leached more readily than Al. Under optimized conditions (70 °C, 180 min, 8% HCl, liquid-to-solid ratio 8:1), the leaching rates reached 74.17% for Al, 90.01% for Ca, and 83.34% for Fe. The acid-leached residue primarily retained amorphous aluminosilicate phases with minor quartz, exhibiting a more ordered structure than the precursor. Morphologically, the original dense structure was disrupted, forming fractured surfaces composed of 40–60 nm nanospheres. The residue possessed a specific surface area of 101.44 m²/g and an average pore diameter of 5.734 nm. These findings indicate that the acid-leached CGS residue has significant potential for direct application as a mesoporous adsorbent material or for uses requiring enhanced reactivity.