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Metal Casting Technologies : September 2006
78 TECHNICAL FEATURE ■ The water in mono-phosphate evaporates at 150°C with max 5 bar steam pressure. For di- phosphate the water is released at 213°C and the steam pressure of 20 bars may exceed the strength of the material. ■ Two component products use a molar excess of H3PO4 compared to MxO. The vapor pressure of phosphoric acid and monoaluminium-phosphate solutions are lower than water. Thus, they are less explosive than other (water containing) castable refractories during heat up (15) It should be noted that the whole dehydration process of phosphate bonded castable refractories hasn't been properly evaluated yet. The thermo-mechanical properties of phosphate bonded materials are different than those with a cement bond. After setting, additional curing time is not needed. Poly- and meta -phosphate compounds formed during condensation possess a ring and chain structure with a stability up to 800°C (1500ºF). (16) This flexible structure leads to a low modulus of elasticity and contributes to the thermal shock and abrasion resistance. Above 900°C (1650ºF) first melting of the MxO- phosphate compounds of the binder system occurs. Depending on the raw materials, heat setting phosphate bonded materials behave similar because of a reaction of phosphates with MxO impurities. Thus, phosphate bonded castables with alkali additions can show lower hot modulus of rupture and refractoriness under load numbers compared to cement formulations. Depending on the quantity and kind of alkali additive, this melt phase is of a temporary nature and further reacts with alumina to melts of higher viscosity. In many burner tile applications a certain quantity of melt phase at service temperature reduces the stiffness of the material and improves the thermal shock properties, which is desired. Between 900°C (1650ºF) and 1600°C (2900ºF) amorphous phosphates and phosphate melts are being formed. Depending on the kind of MxO- phosphate and quantity thereof, occurring melts can attack the aggregates of the raw materials at process temperatures above 1350°C (2460ºF). Resulting volume changes are due to the formation of additional melt (shrinkage) or formation of new compounds like Mg-spinel and mullite (expansion). Above 1600°C (2900°F)aluminium-phosphate starts to decompose to aluminium-oxide and P205 which is volatile at this stage. Material testing of phosphate bonded materials As aforementioned cold setting phosphate bonded materials with MxO- phosphates as a binder component show partial melting at temperatures between 900°C (1650°F) and 1400°C (2550°F) Table 4 shows some alkali- and alkali earth metal oxides used as a setting agent. There is some evidence that occurring phosphate melts are temporary in nature and change at increased temperatures to different compounds. Some test methods, however, do not reflect this materials characteristic. Critical tests are: ■ Thermal expansion under load > 1100°C (2000°F). ■ Hot modulus of rupture > 1100°C (2000°F) ■ Refractoriness under load The results obtained from these tests can be significantly influenced by the occurring melt phase at test temperature. For instance samples pre fired at 1100°C (2000°F) with a holding time of 24 hours showed 10% HotMOR increase and with a holding time of 168 hours an increase of 65% at 1100°C (2000°F). Typically, published data references test results after only a five hour holding time. Another example is thermal expansion under load. Due to the load, plastic flow and the coefficient of thermal expansion are mixed and resulting high permanent linear change is then overstated. The next example shows the conversion from calcium phosphate to Anorthit. This reaction is evident from XRD analysis. Calcium- monophosphate as part of the binder component is being formed at ambient temperatures by the reaction of phosphoric acid and calcium oxide. During heat up this monophosphate converts into calcium - metaphosphate as shown in Table 3. Calcium metaphosphate has a melting point of 975°C (1800°F). After firing at 1600°C (2900°F) with 10hours holding time the XRD diagram (Figure 2) shows that all phosphate has been converted to AlPO4. Calcium oxide on the other hand has been reacted with mullite and converted into Anorthite with a melting point of 1550°C (2800°F) www.metals.rala.com.au Figure 2. XRD analysis Formula 6B -- 1600 C/10h (2900°F) Table 4. Melting points of alkali phosphates (17) Chemical Name Melting Point formula 0C (0F) Na4 P2O7 Na pyrophosphate 985 (1805) K4 P2O7 K pyrophosphate 1109 (2030) Ca (PO3)2 Ca metaphosphate 975 (1790) Ca2 P2O7 Ca pyrophosphate 1350 (2460) Mg(PO3)2 Mg metaphosphate 1165 (2130) Mg2 P2O7 Mg pyrophosphate 1383 (2520) Al (PO3)2 Al metaphosphate 1060 (1940)