Migration behavior of impurities during the purification of waste graphite powders.

Metal-laden solid wastes (e.g., waste graphite powders) have attracted great attention owing to their hazardous effects on the surrounding soil and water. Additionally, the metal-bearing impurities also hinder the reutilization of waste graphite powders. Thus, it is necessary to remove these inorganic impurities and figure out the removal mechanism of impurities in the purification process. In this study, an alkaline roasting-water washing-acid leaching (AWA) method was used to upgrade the waste graphite powders, and the migration behavior of diverse impurities has been qualitatively and quantitatively investigated. A graphite product with high impurity removal efficiencies is attained under optimal conditions. The removal of impurities mainly follows three routes: (1) V-, P-, and S-bearing impurities were complete removed (some formed soluble salts during alkaline roasting, and the remainder was dissolved in acid); (2) most Al-, K-, and Si-bearing impurities were removed by alkaline roasting, with the remainder was dissolved in the acid-leaching process; and (3) Fe-, Mg-, Ti-, Ca-, and Zn-bearing impurities were decomposed at high temperature and reacted with alkali to form hydroxides or oxides, which was subsequently dissolved in acid. In addition, the treatment of the generated wastewater is also discussed. The uncovered migration mechanisms of diverse impurities would guide the purification and reutilization process of other metal-bearing solid wastes efficiently.