Research On Mechanism Of Anti-moisture Calcined Dolomite Prepared By In-situ Dispersion Polymerization

The problem of moisture absorption for calcined dolomite during the process of production and storage affect the development of primary magnesium industry seriously.Calcined dolomite(CaO·MgO) is the product of dolomite which is calcinedat high temperatures during the process of Pidgeon’s magnesium reduction.Thesurface of calcined dolomite dry up immediately and generate a layer of scaleafter absorbing a small amount of water due to the strong moistureabsorption.When the temperature reach to600~800℃in reducing pot,thereducing rate of magnesium reduce because of the reducing capacity of Sireduce after the reaction between moisture and Si,the reduced magnesium isoxidized to MgO again by H2O and CO2.The product of moisture absorption ofcalcined dolomite dissociate in vacuum and relatively low temperature whichmake the residual pressure of reduction reaction zone increase and sublimation speed of magnesium slow down.Moisture absorption of calcined dolomite is oneof the unresolved problem for magnesium industry,it has attracted theenterprise’s high attention.So,how to prevent calcined dolomite absorbingmoisture to keep the activity of calcined white for a long time has the vitalsignificance for improving reducing rate of magnesium and reduce the enterpriseproduction cost.The research object of this paper was calcined dolomite,Anti-moisturecalcined dolomite is made when Poly (Perfluoroalkylethyl acrylate)(PFM) waseffectively grafted onto calcined dolomite through the initiation of anchoredACPA during in-situ dispersion polymerization. The experiment usedXPS,IR,Thermal Analyzer,Elemental Analyzer,Contact Angle Tester,SEM andSurface areas and porosity analyzer to test the calcined dolomite before and aftermodification and polymerization. The mechanism and preparation method ofmoisture-proofed calcined dolomite had been studied and got the results asfollows:(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 ratebecame slow down during100-400minutes; the grown of moisture rate becamevery slow after400min. the grown of moisture rate became slow down famouslyduring720-2880minutes. So the hygroscopicity of the calcined dolomiteincreased with the increase of moisture adsorption time,but its growth rateslowed down gradually. The molecules of H2O and CO2from the atmosphere were absorbed on the surface of calcined dolomite under the effect of physicalabsorption,and then the reaction of hydration and carbonation appeared.(2)The calcined dolomite belonged to typical strong hydrophilic substancesbecause its contact angle of the calcined dolomite closed to0°.The large specificsurface area and microporous surface structure of calcined dolomite made itselfto adsorb water and CO2of environment easily.(3)The-OH of surface of calcined dolomit took alcohol reaction withSi-OCH3of Silane coupling agents (KH560) and grafted on the surface of thecalcined dolomite and introduced epoxide group to the surface of the calcineddolomit. ACPA anchored on the surface of calcined dolomite by means of thering-opening condensation reactions between-COOH in the ACPA and epoxygroup of the surface of calcined dolomite,The anchoring efficiency is close to50%.(4)The initiator ACPA anchored on the surface of calcined dolomitedecomposed and generated free radical at given temperature. When themass fraction of FM was7%,the grafting ratio and grafting efficiency ofPFM/calcined dolomite was about0.5%and26.6%.(5)The grafting ratio of PFM on the surface of calcined dolomiteincreased with increasing of the polymerization conversion while the graftingefficiency decreased.The grafting ratio and grafting efficiency had a linearrelation with the polymerization conversion during polymerization ofcalcined dolomite with different content ofACPA. (6)The hygroscopicity of PFM/calcined dolomite decreased with increasingof the grafting ratio of PFM.When the grafting ratio of PFM was0.5%,thehygroscopicity decreased98%than the untreated calcined dolomite.Thehygroscopicity and contact angle had the same changing trend.And the contactangle is132.8°.The hydrophobicity of PFM/calcined dolomite is well.Theresults show that decreasing the surface energy of calcined dolomite is aeffective way to improve its hygroscopicity.(7)Both specific surface area and pore volume decreased. SEM analysisshows that there are some uniform layers of polymer membrane in themicroporous structure and on the surface of calcined dolomite,and part of thelayers sealing the pore.When the grafting ratio of PFM was0.5%,the hickness is about8nm which is showed in XPS information.