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Sillimanite andsilicate bonding clays bentonite

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Unformatted text preview: licate Bonding clays (bentonite) - composition - stability - Binder systems may be organic or inorganic, humidity tural Hardener: (as in Table 1). - or synthetic Special sand moulding processes have been CO2, dicalcium silicate, Fe-Si, cement veloped to enable moulding with less effort and ill, saving time and expenses, produce better quality oulds and cores and effectively help in improving oductivity [1]. Table 1 . Sand binders. Source Natural Synthetic Clays Sodium silicate Bentonites Inorganic _ Cement Plaster Phosphate binders Sulphite lye Organic Oil binders Core oils Sugar Aminoplast resins(UF) Phenoplast resins(PF) Furan resins Epoxy resins Polyester resins Polyurethane resins – Ferro-silicon process – Carbon dioxide process Differences between these various processes lie in the quality of hardener used, the type of catalysts or other additives, and in the nature of the chemical Sodium silicate binder aqueous solution of sodium silicate glass SiO2 + Na2CO3 -> xNa2O.ySiO2 + CO2↑ at 700-1200oC - SiO2 - network former - Na2O (Li2O, K2O) - network modifier - Na2O and SiO2 - form a series of compounds, which dissolve in water orthosilicate (2Na2O.SiO2) metasilicate (Na2O.SiO2) disilicate (Na2O.2SiO2) water glass (Na2O.3.3SiO2) Mass ratio= Na2O:SiO2 = 1 : 2.1 to-1 : 2.5 Processes based on sodium silicate binder - cement molding - dicalcium silicate process - ferro-silicon process - carbon dioxide process Na2O SiO2 silicate, which is always highly alkaline, This reaction involves depolymerization of2 O)Si(OH) 5 ]2H2 0 ions may be 6] -2 + H2O (6) OH [(H silica -+ [Si(OH) through hydration and dissolution to Si(OH) 4, polysilicateto a pH belowin equilibrium w neutralized by acid and disilicate anions, silica 10 .5, the and [On H 2Oother -hand, when 2a solution ] of a(5) 4, metasilicate 6]-2 Si(OH)(H 0)Si(OH) 5 Si(OH) 4 + hydration and dissolution to + OH soluble through by addition of hydroxide ion to form silicate 2[S 2H2 0 i(OH) followed the which can be followed by addition alwayssilicate ionalkaline, the olymerization processacid, a expressthe silicate silicate,conversionis of hydroxide ion to form ions decompose to silicic weight aggregatesby ion . The which of silica to high...
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