Synthesis of calcium ferrite from waste gypsum board

Abstract

Herein, we investigate the chemical recycling of waste gypsum board obtaining synthetic calcium ferrite for making steel. Thermodynamic estimation and results of small-scale heating tests revealed that decomposition of gypsum was promoted in the partial pressure of O2 and lower partial pressure of SO2 by preventing the formation of CaS. The addition of Fe2O3 to gypsum helped reduce the decomposition temperature of gypsum, resulting in formation of calcium ferrite in a lower temperature. Decomposition of gypsum was incomplete after heating in air at 1 180°C, whereas calcium ferrite was successfully obtained by a two-step heat treatment of gypsum: forming CaS through carbon reduction and heating the fine mixture of Fe2O3 and CaS at 1 180°C. Calcium ferrite can also be obtained in a single heat treatment if we use the proper gas and material conditions. After trial tests using a large-scale rotary kiln, we confirmed that heating a sample containing gypsum, coke fine, and Fe2O3 at a temperature less than 1 200°C resulted in the formation of calcium ferrite accompanied with a high desulfurization degree of gypsum.