Research On Key Technology Of Preparation Of Gypsum And Building Gypsum From Waste Sulfuric Acid

Chemical enterprises usually generate a lot of waste acid in the process of production,if companies just simple discharge of waste acid,which is not only a waste of resources,but caused a great harm to the environment.In recent years,the state has been strict with industrial waste emissions,although many companies come up with calcium carbonate or lime neutralization of waste sulfuric acid,the gypsum can’t be used,as a result behind the seemingly simple question really has a high technical threshold.The process conditions of preparing gypsum are discussed in this paper,mainly studied crystal modifier,process parameters such as stirring speed,sulfuric acid adding rate,initial suspension concentration,temperature,seed,and material factors such as the concentration of sulfuric acid and calcium carbonate size on the influence of gypsum particle size,morphology and the free water content.The gypsum is tested by laser particle size analyzer,scanning electron microscopy(SEM),X-ray photoelectron spectroscopy(XPS)and X-ray diffraction(XRD).The process conditions of preparing building gypsum are also discussed in this paper,mainly studied the calcinations temperature and the pretreatment process on the influence of building gypsum performance including the mechanical properties,particle size and morphology,then comparison with the building gypsum prepared by routine process.The main research results are as follows:The effect of crystal modifier is studied on organic acid and phosphonic acid,in which the citric acid has better effect of crystal.The building gypsum which they prepared morphologies is like diamond-shaped,grain size is about 25μm,and the minimum free water content can reach 25.5%,far below the free water content prepared by conventional method.Thus it can instead of natural gypsum in the building material industry applications.The different effect of crystal is related to the adsorption between crystal modifier and gypsum through XPS analysis.The study conduct an experiment of many factors on the performance of gypsum when neutralization.The results show that the stirring speed is 180 r/min to 250r/min,sulfuric acid adding rate is 3 ml/min to 7ml/min,the initial suspension solids content is 10%to 30%,calcium carbonate was added by 2 to 4 times,the temperature is 10℃to 40℃,the concentration of sulfuric acid is 50 g/L to 200g/L,the calcium carbonate size is 5.8μm to 16.44μm,then the free water content of gypsum far below the content prepared by conventional method.Thus it can replace natural gypsum in the building material industry applications.The optimum process parameters for preparing gypsum have been determined by orthogonal experiment,the results of gypsum performance are listed in the order:limestone surface area>crystal concentration>stirring speed>initial suspension solid content.The optimum process parameters are citric acid at a concentration of O.Olmol/L,stirring speed of 250r/min,the initial suspension of 30%solids content,limestone surface area of 450m2.For calcium hydroxide,due to the strong reaction with sulfuric acid in the neutralization,the supersaturation of gypsum is difficult to control,thus it is easy to generate fine particle and high water content.Therefore calcium hydroxide shouldn’t be selected.The building gypsum accord with the building gypsum standard after pickling with pH=1 and pH=2.Comparison research on mechanical properties and microstructure analysis of the building gypsum prepared by the optimum method and the building gypsum prepared by conventional method show that 2h compressive strength can reach 5.0MPa,2h flexural strength reach 2.5MPa,however,the latter 2h compressive strength only reach 1.7MPa and 2h flexural strength only reach 0.6MPa.It can be seen through their morphology analysis that the former fracture surface morphology is a long rod-like crystal,while the latter is like flake,and the structure is loose.