Leaching kinetics of mechanically activated boron concentrate in a NaOH solution

Mechanical activation is an effective pretreatment method that can change the properties of boron concentrates, thus promoting B2O3 leaching from samples. Four boron concentrate samples were milled for 0, 10, 30, and 60 min and leached in a NaOH solution at various leaching temperatures and initial NaOH concentrations. We investigated the main control steps for leaching and obtained the kinetic parameters. We discuss the influences of the specific surface area and surface free energy on the apparent rate constants. During the early-stage leaching (0-20 min), the leaching process was controlled by a combination of the surface chemical reaction and diffusion through the product layer; however, the surface chemical reaction was the main controlling step with an apparent activation energy of 29.37-39.77 kJ/mol. During the late-stage leaching (>40 min), the leaching process of the milled samples was controlled by a combination of the surface chemical reaction and diffusion through the product layer; in contrast, that of the unmilled sample was controlled by the surface chemical reaction and showed a high apparent activation energy of 69.14 kJ/mol. With increasing milling time, the apparent activation energy and reaction order decreased, thus reducing the dependence on temperature and NaOH concentration. The leaching rate constants were related to the specific surface area, surface free energy, and other properties, such as crystal amorphization and chemical bonds stability. After milling for 60 min, the surface free energy increased from 26.88 to 33.99 mJ/m2, and the Lewis acid component increased from 0 to 3.31 mJ/m2, promoting the absorption of the solvent and OH- in the leaching solution, which also accelerated the leaching rate.