Study On The Process Of Producing High-quality Gypsum And Hemihydrate Gypsum Whiskers By FGD

The flue gas desulfurization gypsum（FGD）is a byproduct of Coal-fired power station,which can be used for the production of plaster powder,αgypsum powder,gypsum products,gypsum mortar,cement additives and other building materials.Currently,the quality of FGD in market is very low,which is difficult to meet the basic requirements of the production of downstream products.Therefore,in this article,we design a process route to prepare high-purity gypsum in the H₂SO₄-Ca（OH）₂-H₂O system,and the production of calcium sulfate whiskers under both high pressure and atmospheric conditions,to achieve the utilization of gypsum resource.Based on the recrystallization characteristics of gypsum in acidic solution,atmospheric acid solution method is used to decoloring gypsum by controlling the preparation of crystallization and dissolution process,in this progress,SiO₂,Al₂O₃,flash and other meterials can be dissipated.And high purifed calcium sulfate dehydrate（CaSO₄.2H₂O）is parepared.Several factors containing sulfuric acid concentration,solid-to-liquid ratio,stabilizers,temperature,reaction time and aging time,which affect the quality of the product,are invistigated.The results indicates that:under normal pressure,H₂SO₄ amount of 10%,Ca（OH）₂ dosage of 0.5%,7.41%of the FGD,0.5%of the amount of polypropylene glycol,reaction in 120℃,with time 2.5 h,after aging 24 h,the crystal of calcium sulfate dihydrate looks good,with whiteness and purity reached 94.01%and99.24%,respectively.The particle size of CaSO₄.2H₂O is between 0.2 cm² and 1.0 cm².At the same time,this process route is also reserved in acidic system through dissolution,controlled crystallization process to prepare a high-quality semi-calcium sulfate whisker of exports,and under laboratory conditions to achieve the recycling of mother liquor.Respectively are equilibrium reactor and the collector thermostat heating magnetic stirrer experiment studied under different pressures.Several factors containing sulfuric acid concentration,solid-to-liquid ratio,stabilizers（PEG）,temperature（T）,reaction time（t_r）and aging time（t_s）,which affect the quality of the product,are invistigated.The results show that:under normal pressure,c H₂SO₄ was 1.00%,c _Ca（OH）2was 0.3%,the reaction temperature is 130℃,reaction time 55 min,after addition of0.5%polyethylene glycol hemi-calcium sulfate formed whisker（CaSO₄.1/2H₂O）had good morphology,the average length（L）is 89.72μm,and uniform particles.After addition of 2 mol/L hydrochloric acid（HCI）,produced gypsum whisker growth 1.2-1.4 times,and more slender.Under high pressure,c _H2SO42SO4 in an amount of 10%,c _Ca（OH）2a（OH）2 was 0.3%,the reaction temperature is 130℃,hemihydrate calcium sulfate whisker reaction time 35 min,after addition of 0.5%polyethylene glycol,is formed（CaSO₄.1/2H₂O）good morphology,the average length（L）is 98.72μm,not only had uniform particles,but also significantly greater than the length of the normal pressure,and the reaction time is shortened.In H₂SO₄-Ca（OH）₂-H₂O system,H₂SO₄ and trace amounts of Ca（OH）₂ reaction generated CaSO₄ crystal seed,then hydrothermal method is used to make CaSO₄.2H₂O in the FGD gypsum dissolved,a large amount of Ca²⁺and SO₄^2-in the solution gathered in CaSO₄ crystal surface,crystallization and grow up to give a good appearance gypsum products.The impurities of FGD gypsum are left in the mother liquor,the mother liquor after treatment can be recycled three times.