Experimental study on the leaching effect of different chelating agents on the constant metal ions of lignite

Chelating agents can increase the porosity of coal by leaching metal ions from it. Therefore, selecting the most suitable chelating agent based on the characteristics of the coal type is crucial when applying chelating agents. In this study, lignite from southwest China was chosen as the sample. Four chelating agents, namely tetrasodium iminodisuccinate (IDS), diethylenetriaminepentaacetic acid (DTPA), tetrasodium aspartate diacetate (ASDA), and tetrasodium glutamate diacetate (GLDA), were evaluated for their impact on the leaching of constant metal ions (CMIs). The leaching effect of CMIs was characterized and analyzed using inductively coupled plasma, scanning electron microscopy, and Brunauer–Emmett–Teller measurements. The results indicated that the ASDA chelating agent was the most effective in leaching Ca²⁺, Mg²⁺, Fe^2+/3+, and Al³⁺. Under the optimal concentration condition of 2500 mg L⁻¹, the leaching effect of CMIs from different chelating agents could be ranked as: ASDA > DTPA > IDS > GLDA. The contact angle of the ASDA chelating agent with the coal sample decreased from 48.2° at 1 s to 26.5° at 20 s. The metal minerals on the surface of the coal dissolved under the action of the ASDA chelating agent, and the micropores on the coal surface transitioned to mesopores and macropores. The pore volume of coal samples increased from 0.0254 cm³ g⁻¹ to 0.0276 cm³ g⁻¹, and the pore size increased from 3.26 nm to 4.06 nm. As the pore size of the coal increased, the permeability also significantly increased.