Anhydrite was synthesized from Flue-gas gypsum (FGD gypsum), a waste product from the Mae Moh Power Plant. FGD gypsum was precleaned by a hydrocyclone classifier, acid leaching and sedimentation, respectively. After that, the precleaned FGD gypsum was transformed to anhydrite by Semidry Acid-Anhydrite method. It was found that the optimal condition for synthesis was to calcine at 100 ํC for 5 hours using the ratio of dried FGD gypsum : distilled water : 3 M sulphuric acid equal to 200 g : 100 ml : 100 ml, respectively. Before neutralization, the obtained anhydrite was washed with distilled water to get rid of iron compounds, which would affect its color. Consequently this also reduced the content of CaCO3 neutralizer to about 8 times. After neutralization, anhydrite consisted of small prismatic crystals having an average size of 2 micro m high purity (all 97%), and high brightness (80%). In the paper processing, addition of anhydrite (50% of oven dry pulp weight) increased opacity of paper. At the optimal condition of refining pulp, 6,000 rev, mechanical properties such as tensile strength, tear strength and burst strength were maximum. Furthermore the effect of adding anhydrite to polyethylene plastics, LLDPE and HDPE was also investigated.
