Abstract
In 2000 a large amount of waste gypsum (3.26x106 tons) consisting of sulphated absorbent (2.28x106 tons) and waste gypsum boards (0.98x106 tons) were discarded in Japan. Most of them have been landfilled. However, there is a shortage of dump yards. Furthermore, resources need to be recovered to reduce waste loads to the environment. The present work is concerned with the regeneration of CaO by reductive decomposition of spent gypsum (CaSO4). Reductive decomposition of CaSO4 was carried out by using a thermogravimetric analyzer at a temperature range of 1173K to 1273K in a reductive gas mixture of CO-CO2-H2-N2(CO and H2 : 2vol%-10vol%, CO2 : 0vol%-30vol%, N2 : balanced). In addition, the effect of iron oxide (Fe2O3) additive on the enhancement of CaSO4 decomposition was studied under CO: 2vol%, CO2: 30vol%, N2: balanced atmosphere at 1273K.
It was found that the regeneration of CaO from CaSO4 was most effective when CO2 was present in both CO-N2 and H2-N2 atmospheres, and depended on the CO/CO2 and H2/CO2 concentration ratios. The CO2 was considered to suppress the formation of an unfavorable reaction product CaS. CaSO4 conversion of about 90% was obtained in CO, H2: 2vol% and CO2: 30vol% at 1273K. It was also found that the rate of reductive decomposition of CaSO4 was accelerated by the addition of iron oxide.
It was recognized that CaO regenerated from CaSO4 had a higher SO2 sorption performance compared with CaO calcined from CaCO3 due to a larger pore volume and a larger pore diameter of the regenerated CaO.
