Study On The Prepartion Conditions Of ?-hemihydrate Gypsum From Flue Gas Desulfurization Gypsum In Alcohol-salt System Under Normal Pressure

In this dissertation,short columnar and long rod-shaped?-hemihydrate gypsum??-HH?were prepared from flue gas desulfurization?FGD?gypsum via“alcohol-saltsolution”method in alcohol-salt system under normal pressureunder normal pressure.The main work and results are as follows:1.Using pre-treated flue gas desulfurization gypsum as raw material,the optimum concentration of?-HH in alcohol-salt system and the more suitable conditions of conversion reaction were investigated and determined through single-factor experiments:Glycerol-Ca-Mg-Na system:the concentration of CaCl₂,MgCl₂ and NaCl are 2.9,0.0315 and 0.2735 mol·L^-1 respectively,glycerol addition 6%,reaction time 4 h,the ratio of solid-liquid 1:4,reaction temperature 92°C,the as-prepared sample is a pure rod-like?-HH of hexagonal crystal type,with different diameter and aspect ratio of about 5:1 to 15:1.Ethanediol-Ca-Zn-Na system:the concentration of NaCl,ZnCl₂ and CaCl₂ are 0.2507,0.0587 and 2.8798 mol·L^-1 respectively,ethanediol addition 10%,reaction time4 h,the ratio of solid-liquid 1:4,reaction temperature 95°C,the as-prepared sample is a pure rod-shaped?-HH of hexagonal crystal type with aspect ratio of about 5:1¹0:1.2.Succinic acid and glucose were selected as the more suitable crystal shape controlling agents in the glycerol-Ca-Mg-Na system for the preparation of short columnar and long rod-shaped?-HH.When using succinic acid as the crystal shape controlling agent,the more suitable concentration is 0.0282 mol·L^-1.The as-prepared sample is short columnar?-HH with length to 20³5?m and aspect ratio of about 1:1²:1.When using glucose as the crystal shape controlling agent,the more suitable concentration is 0.0370 mol·L^-1.The as-prepared?-HH sample is long rod-shaped?-HH with length up to 100?m and aspect ratio of about 35:1.The mechanism of conversion from DH to?-HH is discussed preliminarily and it's considered that the transformation process is accordent to the“dissolution-recrystallization”mechanism,while the crystalization of the?-HH considered accordent to the“screw dislocation”mechanism by Frank.3.In the ethanediol-Ca-Zn-Na system,the more suitable concentration of succinic acid is 0.0423 mol·L^-1,using as the crystal shape controlling agent.The as-prepared sample is short columnar?-HH with length of 15³0?m and aspect ratio of about1:2¹:1.As to glucose,the more suitable concentration is 0.0555 mol·L^-1.The as-prepared sample is a long rod-shaped?-HH with length up to 80¹00?m and aspect ratio of about 20:1.4.The“2 h flexural strength”,“2 h compressive strength”and“drying compressive strength”of all the as-prepared samples were tested.The results showed that the short columnar?-HH samples obtained in the glycerol-Ca-Mg-Na-succinic acid system and ethanediol-Ca-Zn-Na-succinic acid system have relatively larger strength,which is consistent with the literature:short columnar?-HH has higher mechanical strength.The analysis of cross-sectional SEM images shows that the samples have the flaky,small and short rod-like structure and the relatively compact cross-sectional arrangement,which contributing to the relatively higher strength.