Study On The Preparation And Properties Of High-alumina Phosphate-bonded Plastic Refractory Materials

In recent years,phosphate binders and phosphate-bonded high-temperature composites have been attracting increasing attention due to their good properties.High-alumina phosphate-bonded plastic refractory materials,as one of the most important unshaped refractory,which possesses excellent mechanical properties at medium-high temperature(700-1400 ?),were widely used in the medium temperature boilers(700-1100 ?)as repair materials.However,due to the limitation of the calcination temperature,the strength of this refractory material can not be optimized at the working temperature.At present,there are a few literatures of enhanced the mechanical strength of high-alumina refractories at medium temperature.Therefore,it is important to design novel refractory materials and enhance the performance of refractory materials to match their operating temperatures.In this thesis,consititutes and performances of different phosphate binders and refractory materials were studied,the main results are summarized as follows:1.A series of aluminum phosphate binders with different molar ratios of Al/P were prepared by using aluminum hydroxide and phosphoric acid as raw materials.The properties of the binders were studied by the factors such as composition,viscosity,crystal transformation and water content of the binders.The results showed that the composition of the aluminum phosphate binder was Al(H2PO4)3 under molar ratio of Al/P = 1/3,and were mixtures of Al(H2PO4)3 and Al(H2PO4)(HPO4)under higher molar ratios of Al/P.Al(H2PO4)3 has the best bonding performance compared with other aluminum phosphate.Content increase of Al(H2PO4)(HPO4)would result in an decrease of the bonding performance of binders.In addition,as the temperature and water content increased,the viscosity of the aluminum phosphate binder would decrease,resulting in an increase of the wettability of the binder.2.High-alumina phosphate-bonded plastic refractory materials were prepared by using calcined alumina and silica fume as main fillers,calcium aluminate cement as accelerator,nano-copper oxide as sintering aid,brown fused alumina with a particle size of 0-1 mm and 1-3 mm as aggregates.The effects of the content of silica fume,addition of copper oxide sintering aids and addition of brown fused alumina on densification and strength properties of high-aluminum phosphate-bonded plastic refractories were studied by permanent linear change,apparent porosity,cold compressive strength,flexural strength,compositions and microstructures.The results are as follows:(1)The optimum content of silica fume in refractories is 8-12 wt.%;(2)With the addition of 5 wt.% nano-copper oxide,the copper aluminate spinel would be formed in high-alumina refractory by reaction of alumina and copper oxide at 900 ?,which resulting in the strength of compressive and flexural of the refractory has nearly doubled;(3)The addition of 40-50 wt.% brown fused alumina with a particle size of 0-1 mm can not only effectively reduce the dosage of binder,but also act as a skeleton in the refractory materials,resulting obviously improvements in the densification and flexural strength of refractories;(4)The strength of the refractory materials decreases with the addition of brown fused alumina with a particle size of 1-3 mm,caused by the decrease of the binding interface area.3.High-alumina phosphate-bonded plastic refractory materials were prepared by using boric acid as sintering aid.The effects of the addition of H3BO3 on densification and strength properties of high-aluminum phosphate-bonded plastic refractories were studied by phase transition behavior of boric acid and alumina,and the permanent linear change,apparent porosity,cold compressive strength,flexural strength and microstructure of refractory.The results indicate that slender 2Al2O3·B2O3 and stubby 9Al2O3·2B2O3 whiskers would be obtained at 700-950 ? and above 950 ? respectively by sintering the mixtures of Al2O3 and H3BO3,and the composition and microstructure of the generated aluminum borate whisker will not be affected by the molar ratios of Al/B which varied from 9/2 to ? at same temperature.Additionally,The densification and mechanical strength of the refractory materials can be improved significantly at the optimum adding amount of H3BO3.A plenty of bound water would be driven into the refractory materials and the expanded effect would be caused by the evaporated water at an excess amount of H3BO3,and at the same time,an excessive aluminum borate whisker would form,resulting in an internal stress in the interior of the refractory materials and a reduction of the strength.Besides,the optimum dosage of boric acid in the high-alumina refractory materials depends on the operating temperature.In conclusion,the optimum dosage of H3BO3 in the powder system of high-alumina phosphate-bonded plastic refractory materials are 5 wt.%,4 wt.% and 3 wt.%,sintered at 700 ?,900 ? and 1100 ?,respectively.