Formation mechanism of calcium hexaluminate

To investigate the formation mechanism of calcium hexaluminate（CaAl_（12）O_（19）, CA_6）, the analytically pure alumina and calcia used as raw materials were mixed in CaO/Al_2O_3 ratio of 12.57：137.43 by mass. The raw materials were ball-milled and shaped into green specimens, and fired at 1300-1600°C. Then, the phase composition and microstructure evolution of the fired specimen were studied, and a first principle calculation was performed. The results show that in the reaction system of CaO and Al_2O_3, a small amount of CA_6 forms at 1300°C, and greater amounts are formed at 1400°C and higher temperatures. The reaction is as follows： CaO ·2Al_2O_3（CA_2） ＋ 4Al_2O_3 → CA_6. The diffusions of Ca^（2＋） in CA_2 towards Al_2O_3 and Al^（3＋） in Al_2O_3 towards CA_2 change the structures in different degrees of difficulty. Compared with the difficulty of structural change and the corresponding lattice energy change, it is deduced that the main formation mechanism is the diffusion of Ca^（2＋） in CA_2 towards Al_2O_3.

Effect of Alumina Micropowders on Properties of Calcium Hexaluminate

Calcium hexaluminate （ CA6 ） was synthesized by mixing light-weight calcium carbonate and Al2O3 mi- cropowder （calcined α-Al2O3, activated α-Al2O3 or α-Al2O3） according to the stoichimetric ratio of CA6, and reaction sintering at 1 200, 1 300, 1 400 and 1 500 ℃fin 3 h, respectiely. Efcts of the three alu- mina micropowders on the phase composition, micro- structure and properties of CA were investigated. The re- sults show that ： （ 1 ） for the three Al2O3 micropowders, the reaction to generate CA6 in specimens basically com- plete. at about 1 500 ℃ ; CA6 generated in all specimens is planar, bat the array modes are slightly different; （2） the specimel＇s prepared from, calcined α-Al2O3or ρ-Al2O3 shrink almost, while the specimens prepared from activated α-Al2O3 expand ; （ 3 ） the cold crushing strength of the specimen prepared from activated α-Al2O3 is the highest, reaching 42.5 MPa when only CA6 exists （ after firing at 1 500 ℃ ） ; （ 4 ） the specimen prepared from ρ-Al2O3 has the lowest generation temperature of CA6 and the highest apparent porosity, reaching 70. 1% when only CA exists （after firing at 1 500 ℃）; （5） the specimen prepared from calcined α-Al2O3 has the biggest bulk density.

Influence Researches of Fused Calcium Hexaluminate Amount on Metal Corrosion Resistance of Alumina Refractories

Calcium hexaluminate is one from such perspective materials. It is characterized by high refractoriness （ above 1 830 ~C ） , as well as properties stability in reducing atmosphere, high alkali- and slag-resistance, low thermal conductivity and wettability by molten metals. In this work influence researches of fused calcium hexaluminate material additive in amount of 5%, 15% amt 25% on the main properties of alumina refractories were carried out. It is established that, the refractories containing the additive of 25% calcium hexaluminate material are characterized by the highest properties, slightly impregnated with metal melt and the highest metal corrosion resistance. The alumina purging plugs fi）r blowing of metal with calcium hexaluminate material additive are produced and intended to one metallurgical plant of Ukraine for carrying out tests.

Insulating Permanent Lining of Calcium Hexaluminate Based Castable for 300t Ladle in Baosteel

Minimizing of heat loss of liquid steel through the ladle lining led to the integration of increasing insulation function of the permanent lining and high-quality insulating layer between the permanent lining and the ladle steel shell.In this paper,a study was made on physical and thermomechanical properties and corrosion resistance of calcium hexaluminate based castables.The chemical and physical properties of CA6 based castables fabricated in this work can fulfill general requirements of the permanent lining in ladle.The permanent linings of CA6 based castables offer superb thermal insulation and corrosion resistance,compared to conventional refractory systems of 300t ladle in Baosteel.Both functions of insulation and safety work effectively in whole process of ladle operation.

Synthesis of Calcium Hexaluminate from Waste Slag of Aluminum and Oyster Shell

Waste aluminum slag and oyster shell were used as raw materials to synthesize calcium hexaluminate（CA6）. The effects of different source materials of CaO and sintering temperature on the structures and properties of CA6 were investigated,respectively. The results show that compared to calcium oxide,oyster shell can lower the formation temperature of CA6,hence CA6 can be detected at 1300 ℃ by using oyster shell as the starting raw materials. Increasing the sintering temperature can promote the crystal growth. CA6 crystals show typical platelet shape,and its optimum sintering temperature falls in the 1450～1550 ℃ region. The bulk density is 1.54～1.83 g/cm^3,the apparent porosity is 44.1～55.2% and the flexural strength is 10.8～25.3 MPa.

Effect of Calcium Chloride Addition on Properties of Corundum Spinel Castable

Corundum spinel castable was prepared using tabular corundum as aggregates,white fused corundum powder,spinel powder and alumina powder as the matrix,pure calcium aluminate cement as a binder,and extra adding calcium chloride(0,1%,2%,and 3%,by mass).The effects of the CaCl2 addition on the cold physical properties,the hot strength,the thermal shock resistance and the microstructure of the castable were studied.The results show that,for the corundum spinel castable fired at 1550℃,with the increase of the CaCl2 addition from 0 to 3%,the cold strengthes first increase,then decrease,the apparent porosity increases,the volume density decreases,and the linear change rate first decreases and then increases,while the overall change is not significant;however,the hot modulus of rupture and the thermal shock resistance are obviously improved.This is mainly due to that,CaCl2 is evenly distributed in the castable in the form of solution,and reacts with Al2O3 to form small flake CA6 crystals,which evenly distributed in the sample matrix strengthening and toughening the material.

Influence of Pore Forming Agent on Properties of Micro-pored CA_6-MA Material

Micro-pored CA6 -MA lightweight material with CAM： MA mass ratio of 7：3 was prepared using Al（OH） 3, MgCO3 and CaCO3 as starting materials, and anthracite. sweet potato starch and anthracite ＋ sweet potato starch as pore forming agent （PFA） with an addition of 10 mass%, 20 mass% and 30 mass%, respectively. The starting materials were dry mixed, wet co-milled in a ball mill for 1 h. slip cast into cylindrical specimens with a diameter of 60 ram. and then calcined at 1 450 ℃ for 3 h. With the increase of PFA addition,, apparent porosity increases, and bulk density decreases. The influence of different PFAs on properties of the micro-pored LW CAM -MA aggregate was investigated. The achieved CAM - MA, by adding 30% sweet potato starch, has a porosity of 76. 8%, bulk density of 0. 78 g · cm^ - 3 and median pore size of 1.90 μm.

Recycling of High Ferrous Bauxite Reducing Slag for Synthesis of CaAl_2Si_2O_8-Al_2O_3-CaAl_(12)O_(19) Composite

CaAl2 Si2 O8- Al2 O3- CaAl12 O19） （CAS2- Al2 O3 -CA6 ） composite was synthesized through reaction sintering alumina and bauxite reducing slag. The CAS2-Al2 O3-CA5 composite was mainly composed of α-Al2O3 , CAS2 , and CA6. Gehlenite （Ca2 Al2 SiO7 , C2 AS） phase was effectively transformed to CASe and CA6 through high-temperature reaction sintering under weak oxidizing atmosphere at 1400 ℃for 4 h. SEM （scanning electron microscopy） and EDS （energy dispersive spectroscopy） analysis indicated that black and needle-shaped Al2O3, rhombic or irregular polygonal shaped FeAl2O4 , and glassy phase Ca2 Al2 SiO7 disappeared after the reaction sintering. The light gray and flaky hexagon crystals of CaAl12 O19 （10 μm） and the grainy particles of Al2O3 （2-7 μm） were observed in the CASe-Al2 O3-CA6 composite. The gray crystals of CASe act as the binding phase and are distributed around CA6 and Al2O3. CAS2-Al2O3-CA6 composite exhibits high refractoriness and service temperature, which are 1 650 ℃ and 1 450 ℃, respectively. Reaction sintering of alumina and bauxite reducing slag is a feasible method for the synthesis of CAS2-Al2 O3-CA6 composite.

Enhancing mechanical properties and air permeability of corundum porous materials by in situ formation of LaAl11O18 in bonding phase

Corundum porous materials of particle-packing type with different contents of in situ formed LaAl11O18were prepared using tabular corundum,reactive alumina and La2O3 powder as raw materials.The effects of the introduction of LaAl11O18 on the microstructure.phase composition and properties of the porous materials were investigated.The specimens were characterized by scanning electron microscopy.X-ray diffraction and mercury porosimetry.Results show that platelet-like LaAl11O18 is formed in situ by the reactions of La2O3 and Al2O3.With a certain amount of La2O3 added.the cold crushing strength,cold modulus of rupture and hot modulus of rupture(1400 x 0.5 h) of the specimens are increased,and the air permeability is improved simultaneously.However.upon further increasing the amount of La2O3 added,the mechanical properties and air permeability of the porous materials then decrease gradually owing to the increased numbers of pores and cracks in the bonding phase.The enhanced mechanical properties of the specimens with La2O3 added are attributed to the strengthening effects of plate-like LaAl11O18 in the bonding phase and to the activated sintering of both Al2O3 powder and corundum coarse aggregate for the diffusion of La3+in Al2O3 lattice.In addition,the improved air permeability of the specimens should be related to the decreased content of pores in the bonding phase and the reduced number of interfacial cracks.