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Metal Casting Technologies : September 2006
Alumina reacts with phosphoric acid above 127°C (260°F) to aluminium phosphate and shows considerable strengths above 350°C (660°F). The most often applied phosphate binder in the refractory industry is mono- aluminiumphosphate (MAP) in liquid or spray-dried form. If setting properties at ambient temperature are desired an additional setting agent is needed. This can be an alkali- or alkali earth metal oxide component (MxO) like sodium, magnesia, calcium or compounds thereof. The disadvantage of a spray dried MAP additive in a single component material is the poor shelf life due to the strong hygroscopic behavior. Alternatively, phosphoric acid can be used as a liquid in a two component product. Depending on the reactivity both MAP and phosphoric acid need to be diluted to slow down the exothermic reactions. Phosphoric acid is a tri-basic acid and reacts with different metal oxides to form salts. Some of these salts function as a refractory binder. In an aqueous solution the phosphoric acid reacts in three stages according the molar ratio MxO / P2O5 of 1:1, 2:1 or 3:1. Table 2 shows as an example the reactions between magnesia and phosphoric acid and the resulting three different stages. Magnesia and magnesium hydroxide are widely used as setting additives for phosphate containing formulations. The reaction type 1 and type 2 are exothermic. However, reaction 1 is much stronger than 2 whereas reaction 3 doesn't show cold setting properties. The phosphates being formed at ambient show dehydration of chemically bonded water at elevated temperatures. This condensation process is described in Table 3. Phosphate bonded products with a binder system of a two component mono- and a single component di- phosphate are commercially available. The advantage of a two component mono-phosphate bond is the faster setting and heat up time. Flame impingement experiments with both kinds of materials have shown that mono-phosphates are less susceptible to explosive spalling than di- phosphate bonded materials. Possible explanations for this are: ■ The mono-phosphate formation is highly exothermic, which cause dehydration during setting. ■ The quantity of chemically combined water in the mono- phosphate is higher than in di- phosphates. This can have an influence on the steam pressure in the capillary system of the material during bake out. 1. Formation of mono- phosphate: MgO + 2 H3PO4 = Mg (H2PO4)2 + H2O 2. Formation of di- phosphate: Mg(H2PO4)2 + MgO = 2 MgHPO4 + H2O 3. Formation of tri- phosphate: 2MgHPO4 + MgO = Mg3(PO4)2 + H2O Table 2. Table 3. 1a. Mg(H2PO4)2 = Mg(PO3)2 Mg- metaphsophate + H2O 2a. 2 MgHPO4 = Mg2P2O7 Mg- phyrophosphate + H2O 3a. Mg3(PO4)2 = Mg3(PO4)2 Mg- orthophosphate 150ºC (302ºF) 213ºC (415ºF) 700ºC (1292ºF) 77 WHO'S WHO OF METALS -- ANNUAL 2006/7