Preparation of Whitening Dihydrate Gypsum and Short Columnar α‐Hemihydrate Gypsum with FGD Gypsum

Abstract

Using FGD gypsum as the raw material, whitening and large prismatic dihydrate gypsum can be obtained by dissolution-crystallization and decoloring process with α-hemihydrate gypsum whiskers prepared in MgSO 4 -CaCl 2 -H 2 O solution, which can transform into whitening short columnar α-hemihydrate gypsum. The effects of aging temperature, pH, dosage of sodium dodecyl sulfonate (SDS), time on composition and morphology of products were investigated. The research shows that: whitening and large prismatic dihydrate gypsum with average length of 350μm and the whiteness of 82.57% can be obtained with 6g SDS (mass ratio of SDS/FGD gypsum was 1.2) addition and pH of 8 after 3.5h under 60°C. The effects of different morphologies of dihydrate gypsum on α-hemihydrate gypsum were discussed, which indicates that granularity and morphology of α-hemihydrate gypsum are directly related to dihydrate gypsum. And short columnar α-hemihydrate gypsum with the length of 100μm and the whiteness of 85.68% can be prepared by dihydrate gypsum obtained under optimum conditions mentioned above.the impurities sticking to the surface of FGD gypsum, but can't remove internal impurities, as a result, the whiteness of products is still low. So physical purification methods can't improve the whiteness of products dramatically.In acid or saline solution, dissolutionrecrystallization of FGD gypsum can release the impurities of internal crystals and obviously improve whiteness. Heiska P [5] invented a method to purify FGD gypsum in acid solution using magnetic separator. The whitening dihydrate gypsum purified can be used for coating or filler pigment for paper or board. Mao [6] found that CTAB can be used to achieve the control of crystal morphology in HCl solution by experimental and MD simulation. And needle-like crystals with high whiteness and good aspect ratios were obtained. Zhang [7] and Lu [8] studied the preparation of calcium sulfate whiskers with high whiteness in HCl-CaCl 2 -H 2 O and H 2 SO 4 -Ca-(OH) 2 -H 2 O respectively by physical method and chemical method. Although the whiteness of FGD gypsum can be improved obviously, acidation method is more suitable for α-calcium sulfate hemihydrate whiskers with large length-diameter ratios and short columnar crystals can't be obtained in acid solution due to fast growth rate along C-axis.Fu [9] and Shi [10] studied decoloring and whitening process of anhydrite or phosphogypsum by acid-leaching method. The material was firstly calcined under high temperature (>600°C) and then soaked in acid solution at a definite temperature, which can make the whiteness of anhydrous gypsum reach 90% or more. However, the process has many disadvantages, such as high energy consumption, serious equipment corrosion under strong acid and harsh reaction conditions. Zhang [11] researched the addition of anhydrous gypsum, high-strength gypsum and bleaching packing on the performance of compound gypsum and the whiteness of the products can be improved to 80.1%. Although the whiteness can be improved, different perfo...