Morphology characteristics and mode of CaO encapsulation during treatment of electrolytic manganese solid waste

Electrolytic manganese solid waste (EMSW) is composed of manganese, calcium, and other sulfates. Common practice in China is to treat EMSW with quicklime (CaO); however, the per unit mass treatment efficiency of CaO is low. Studies of the interface between the CaO and EMSW particle and their microstructural characteristics are limited; these interactions may explain the low treatment efficiency. We conducted leaching experiments and measurements of the secondary heat generated by hydration of CaO to assess the extent of excess CaO in EMSW. The microstructure of CaO was also analyzed. It was determined that excess CaO particles in the EMSW were encapsulated, which influenced CaO hydration and morphology. The outer layer of the encapsulated CaO contained high levels of calcium and sulfur, which postulated to be caused by CaSO₄ precipitates formed from the reaction of CaO hydration products with soluble sulfate. Three types of CaO encapsulation were identified: fully encapsulated CaO (55 % of the total CaO), partly encapsulated CaO (32 %), and self-encapsulated CaO (13 %). High concentrations of soluble sulfates in EMSW cause CaO encapsulation. These react to form CaSO₄, which could negatively influence mass transfer and result in low treatment efficiency of EMSW by CaO.