Preparation And Anti-hygroscopicity Study Of Modified Calcined Dolomite

The problem of moisture absorption for calcined dolomite during the processof production and storage affect the development of primary magnesium industryseriously.Calcined dolomite, which the chemical composition is (CaO·MgO),is theproduct of dolomite(MgCO3·CaCO3),which burn at high temperatures during theprocess of Pidgeon’s magnesium reduction. The moisture absorption of calcineddolomite may causes a range of problems,such as declination of calcineddolomite’s activity, decrease of primary magnesium reduction ratio and increaseof production cost of each company. The total magnesium output in China is about769700tons in2013, accounting for88.5%of the total world’s magnesium output.The reduction rate of magnesium is about80%.for producing1ton primarymagnesium, it will need to consume about11tons of dolomite,1.06tons offerrosilicon,7tons of coal. It also discharge about7tons of magnesium reductionslags,35tons of carbon dioxide gas and a lot of fine dust. Thus the study ondecrease of the calcined dolomite moisture absorption may increase the outputrate of primary magnesium effectively and decrease the consumption of non-renewable raw materials such as dolomite, coal, ferrosilicon, etc. It also canreduce the discharge of magnesium reduction slags and greenhouse gas emissions.So it will have some significance for our country primary magnesium industry to a healthy direction such as energy conservation, emission reduction, green,environmental protection, sustainable development and so on.The research object of this paper was calcined dolomite and the modifier ofthis paper was sodium stearate. The experiment took physicochemical treatmentsto modify the powder of calcined dolomite for special calcined dolomite, whichhad excellent performance of moisture-proof. The experiment used infraredspectrum analyzer, contact angle tester,scanning electron microscope and physicaladsorption instrument to test the calcined dolomite before and after modification.The mechanism of moisture-proofed calcined dolomite had been studied and gotthe results as follows:(1) It showed that the fastest grown of moisture rate occured during0~100minutes, the tendency of grown liked as a line; the grown of moisture rate becameslow down during100~400minutes; the grown of moisture rate became very slowafter400min. the grown of moisture rate became slow down famouslyduring720~2880minutes.(2) The moisture of calcined dolomite was caused by its more large specificsurface area and micropore. The contact angle on the surface of calcined dolomiteclosed to0°, it showed that the calcined dolomite belonged to typical hydrophilicmaterials.(3) The molecules of H2O and CO2from the atmosphere were absorbed onthe surface of calcined dolomite under the effect of physical absorption,and thenthe reaction of hydration and carbonation appeared.(4) The analyze of orthogonal design indicated that the influence factors ofpreparation of anti—hygroscopic calcined dolomite were:the mass fraction ofmodifier, modification temperature and modification time.(5) An integral and uniform film of stearate was covered on the surface ofthe modified calcined dolomite at a condition of same temperature,time and5%of modification.The contant angle of modified calcined dolomite was about128°. The hygroscopic rate and surface contant angle of the calcined dolomite tendedto be stabilized after modification.The content of modifier, modifying temperatureand time of optimum condition were5%,55℃and25minutes.(6) The hygroscopic rate of the calcined dolomite before modification stillincreased after720minutes in atmospheres, but the hygroscopic rate of thecalcined dolomite nearly invariable after modification. The hygroscopic ratebefore and after modification changed from1.89%to0.31%. The hygroscopicrate reduced by84%.(7) Sodium stearate had chemical reactions, formed chemical bonds andadded long chain alcohol on the surface of the calcined dolomite. Long chainalcohol and parts of sodium stearate were adsorbed on the surface of the calcineddolomite for its adsorption and became a film of stearate, which had properties ofthe moisture resistant capability.