Gu Huazhi

Preparation of Microporous Corundum Aggregates and their Applications in Lightweight Alumina-Magnesia Castables

AbstractThe industrialization process all over the world has brought with it problems of energy being consumed too fast and a serious energy crisis has developed. The use of heat insulation refractories is a main factor used to reduce industrial furnace energy. Heat insulation material that is nearer the work layer provides better insulation. Two areas work are discussed in this paper. Firstly, in order to prepare the kind of high-performance microporous aggregate used for the alumina-magnesia castable in ladle linings, with α-Al2O3 micropowder as the main raw material, a microporous corundum aggregate was prepared by the method of wet grinding. The distribution of pore size, the thermal conductivity and the microstructure of the microporous corundum aggregate were tested by means of mercury porosimetry, a laser flash thermal analyzer and scanning electron microscopy, etc. The results show that:nThere are large numbers of pores in the microporous corundum aggregate, which are uniformly distributed on the inside, the internal corundum grains grow well and there are short columnar crystalsCompared to the ordinary tabular corundum aggregate, the closed porosity of the microporous corundum aggregate is higher, the apparent porosity and the bulk density are lower, moreover, the pores inside it are smallerWhen the thermal conductivity of the aggregates is compared, that in the microporous corundum aggregate is significantly lower than that of ordinary tabular corundum (its bulk density is 3.63 g/cm−3, this is 42% less).Secondly, a lightweight alumina-magnesia castable was obtained using this microporous corundum aggregate, fused magnesia powder and corundum powder as the main raw materials. In terms of the alumina-magnesia castable, the sintering properties, the strength after baking and the heat treatment at different temperatures, the thermal conductivity coefficient and the resistance to slag were tested in the experiment. The results show that:nCompared with the ordinary alumina magnesia castable which is made of tabular alumina, the lightweight castable has a lower linear change rate, greater apparent porosity and smaller bulk density. Its strength after baking and medium temperature heat treatment can reach the requirements of industrial production; however, the its strength improves obviously after heat treatment at 1500°CWhen the thermal conductivity of castables are compared, the thermal conductivity of two kinds of castables is basically the same at 350°C. When the test temperature rises to 600 and 800°C, the thermal conductivity of the lightweight castable made of the microporous corundum aggregate is significantly lowerWhen the resistance to slag is compared, the two kinds of castables have basically the same corrosion index; the penetration index of the lightweight alumina-magnesia castable is 14.3% larger.

Water Modelling of the Blowing Effect in Ladles with Purging Plugs of Different Slit Structure and Distribution

The purging plug is an indispensable refining component; its slit structure and distribution are important factors that influence refining efficiency. We have fabricated a purging plug with adjustable slit structure and we have studied the mixing time of molten steel and the removal rate of inclusions in the ladle with a purging plug of different slit structure and blowing parameters by water modelling. The effects of the slit width, slit distribution and blowing parameters on the application of the purging plug in a ladle are discussed herein. The results show that if the gas flow rate is less than 0.89 m3/h, the mixing time difference is obviously larger; however, when the gas flow rate reaches or exceeds 0.89 m3/h, the mixing time difference becomes very small. When the slit width is more than 0.05 mm, the mixing time and inclusion removal rate are improved significantly. A smaller slit width is better for the formation of small bubbles and the removal of small inclusions, while bigger slits are conducive to stirring and a removal rate of inclusions of more than 50 µm. Bottom blowing using a d-type purging plug with collocation of slits with different widths and interval gap distribution not only ensures the removal rate of inclusions of more than 50 µm, but also highly improves the removal rate of inclusions of less than 50 µm. Thus, the total inclusion removal rate increases significantly and is as high as 67.21%. In order to guarantee good refinement, the distance of the location of the purging plug from the centre of the circle should be greater than 0.4 R, which is a critical location for better blowing metallurgy and avoidance of more serious wear of local lining refractories.

Effect of SiO2 Micropowder on Properties of Calcium Hexaluminate Castables for Coal Catalytic Gasifiers

Furnace lining materials for coal catalytic gasifiers should have good high-temperature strength and alkali slag corrosion resistance. Calcium hexaluminate castable is a promising application direction. In this paper, the effect of the addition of SiO2 micropowder on the performance of a calcium hexaluminate castable was evaluated through sintered properties, high-temperature strength, anti-alkali slag performance, and microstructural analysis. The results show a significant increase of grain and matrix bonding strength and a decrease of porosity with SiO2 micropowder addition. The castables with 3 mass-% SiO2 micropowder possessed 14.3 MPa CMOR (Cold Modulus of Rupture) and 107.6 MPa CCS (Cold Crush Strength) after heat treatment at 800 °C for 3 h. The hot flexural strength is 11.8 MPa and the retained residual flexural strength of the castables reached 37.13 % after five thermal shock cycles (water cooling) at 1100 °C. The potassium element in the slag has a very small amount of penetration and aggregation at the interface between the sample and the slag.