H20sioh5 the metasilicate ion 2 sioh6 2to monomer

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Unformatted text preview: roposed to be 6. [(H20)Si(OH)5]-+ the metasilicate ion, -2 + [Si(OH)6]-2to monomer, [ Si2(OH)to] corresponding --sp.[Si(OH)6 -2 toH 2 O 2 - losses water .6)atmosphere depending on SiO2/Na2O ratio; be 6. silicon 13 silica-water system, the ''Ys[Si2(OH)5 ]_ +H2 0 pH is proposed to coordination above OH- (pH = 13 studies [6], show that number of(7) .6 20W Further So + .6 and 10 .9, the disilica stable; and between)to]-2 13[Si(OH)6]-2 --sp.[ Si2(OH pH higher molecular weight ambientproposed to be 6. relative humidity show that silicon is temperature & theform at 13 .6)6-8 thermo-chemical 2reaction Further studies [6], according to theprincipal anio formed dimmer isabove pH 13 .6 rea OHmetasilicate ion, corresponding to monomer, is (pH = pH 0 corresponding to the following - gels - Further studies [6], ''Ys[Si2(OH)5 ]_ +H2theion, (7) to So correspondingOH monomer, is [(H 2 0)Si(OH) 5 ] - ~ Si(OH)4 +above+the disilicate 0H OH stable; andpolymerization of below pHpH 13 .6 between pH show that 10 .9, 10 metasilicate ion, higher molecular weight polymer ca 13 .6 and OH - progressive the metasilicate ion, corresponding to monomer, is; and between pH 13 .6 and following reaction: formed according to the 10l.9, the disilicate O stable OH, ion,2 0)Si(OH) 5 ] - ~ Si(OH)4 + OH - + [(H2 corresponding to dimmer is the principal anion 0H OH OH, [Si(OH)5.[Si(OH)4 ],,.Si(OH)5] -2 + [Si( OH SiO t;etrahedron-> colloid and 10 .9, the disilicate stable(pH— 10.9) and between pH 131.6 --> 2Gel H2O ion, corresponding to dimmer is the principal anion H 2O /x(S iO )x + (8) [ OH, Iranian Polymer Journal / Volume 10 Number 4OH, ;Si, (200!) ion, corresponding to dimmer is the principal anion .Si(OH) ] l + Sodium silicate binder "ii" "ii" "ii" "ii" - - - - CO2 process core making or core molding Na2O.mSiO2 + nH2O + CO2 = Na2CO3 + mSiO2.nH2O + Q (heat) silica hydrogel film sand CO2 sand excess CO2 moisture Over gassing: Na2CO3 + H2O + CO2 = 2Na2HCO3 loss of strength Benefit - reaction alters pH --> gelation - it does not require baking - no breakage, sagging of unhardened core - no need of expensive drier - higher SiO2:Na2O(2:1 or 3:1) faster gassing less CO2 usage - collapsibility is poor, can be increased by adding wood flour, spl. additives Refractory coatings graphite, talc, silica, zircon, silicates, olivine (Mg, Fe)2SiO4 Benefit: - reduce or eliminate metal penetration - reduce or prevent burn on - prevent erosion of sand by molten metal - provide smoother surface of the castings - improve overall casting quality - reduce scrap - reduces clearing costs...
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