China's Refractories

With satisfactory high temperature properties, thermal shock resistance and mechanical properties, refractories in the MgO-Al₂O₃ system have become promising alternative to MgO-Cr₂O₃ counterpart used as RH snorkel lining. High purity dead burned magnesia, fused magnesia and sintered spinel were used as raw materials and sulfite pulp waste as the binder, specimens of the refractories in the MgO-Al₂O₃ system were prepared for the purpose of investigating the influences in terms of the addition of three kinds of spinels, with Al₂O₃ 50%, 66% and 78%, by mass, respectively, and the size of MA50 spinel on properties of the specimens. Relatively superior comprehensive properties can be achieved when 16% of MA50 spinel with sizes under 1 mm is adopted.

Reactive alumina is a well-established group of raw materials for refractory castables to improve their rheological behavior. In this article the influence of bimodal reactive alumina and standard (<325#) calcined alumina on workability, packing density and strength of alumina based castables is examined. It will be shown that there is a significant effect on water demand and apparent density of the castable, which is depending on the percentage of reactive alumina on the one hand and on the type of reactive alumina on the other hand. The effect of the alumina on setting time and cold crushing strength has shown to be insignificant in this set of experiments.

To study the effect of the particle size of industrial alumina powder on the properties of active alumina micropowder (α-Al₂O₃ micropowder), the fine powders with the median diameter of 50, 30, 10, and 5 μm, respectively, were obtained by grinding industrial alumina powder (the median diameter of 80 μm), and the active alumina micropowders were prepared by firing the industrial alumina powders before and after grinding at 1 320 ℃ for 5 h. The effect of the particle size of the industrial alumina powder on the microstructure and properties of the active alumina micropowder was researched. The results indicate that the initial particle size of the industrial alumina powder has a great influence on the grain morphology of the active alumina micropowder; when the median diameter is larger than 30 μm, the α-Al₂O₃ grains show worm-like aggregates state after calcination and when it is around or less than 10 μm, the α-Al₂O₃ are round, spherical and well dispersed grains with the particle size of 0.3-1.0 μm. With the initial particle size of the industrial alumina powder decreasing, the α-Al₂O₃ phase transition rate increases and the true density gradually increases; moreover, the α-Al₂O₃ powder shows better dispersity and flowability.

This scientific paper gives consideration to the information on the structure of the triple component system, in particular MgO-Al₂O₃-TiO₂ that serves as a basis for the production of thermal resistance materials. The structure of such binary systems as MgO-Al₂O₃, Al₂O₃-TiO₂, MgO-TiO₂ was described and the data available for the MgO-Al₂O₃-TiO₂ system were given. Thermodynamic data on all the system compounds were also presented and used for the computation of a change in the free Gibbs energy in the temperature range form 800 K to 1 900 K for the basic exchange reactions. It was established that the triangulation of the MgO-Al₂O₃-TiO₂ system was changed in the three temperature intervals: in the temperature range lower than 1 537 K TiO₂ existed as the polymorphous modification, i.e. anatase; in the temperature range from 1 537 K to 2 076 K TiO₂ existed as a polymorphous modification in form of rutile and tialite was stable; and at the temperatures above 2 076 K the availability of stochiometric compound of Al₄TiO₈ was possible. In the temperature range lower than 1 537 K the two-phase equilibra of Al₂O₃-MgTi₂O₅, MgTi₂O₅-MgAl₂O₄, MgTiO₃-MgAl₂O₄, and Mg₂TiO₄-MgAl₂O₄ were stable; in the temperature range from 1 537 K to 2 076 K the two-phase equilibria of MgTi₂O₅-Al₂TiO₅, MgTiO₃-Al₂TiO₅, MgTiO₃-Al₂O₃, MgTiO₃-MgAl₂O₄, and Mg₂TiO₄-MgAl₂O₄ were stable, and above 2 076 K the MgTi₂O₅-Al₂TiO₅, MgTi₂O₅-Al₄TiO₈, Al₄TiO₈-MgTiO₃, Al₄TiO₈-Mg₂TiO₄, Al₄TiO₈-MgO, and Al₄TiO₈-MgAl₂O₄ systems were stable.

Silicon carbide castables of different SiC contents (86% and 71%, by mass) were prepared using white fused corundum, silicon carbide particles and fines, activated alumina powder, silica fume and pure calcium aluminate cement as main starting materials, heat treating at 1 000 ℃ for 3 h, and oxidizing in steam atmosphere at 1 000 ℃ for different durations (100, 200, 300, 400 and 500 h). The mass and volume before and after oxidation, the bulk density, the apparent porosity and the cold compressive strength were tested. The phase composition and the microstructure before and after oxidation were analyzed by XRD and SEM. The results indicate that: (1) within 300 h of oxidation duration, silicon carbide shows an increasing oxidation rate; however, the oxidation rate is low during 300-500 h of oxidation duration; 2) the oxidation rate of the specimen with 71% SiC is slightly higher than the one with 86% SiC; 3) with the increasing oxidation degree of silicon carbide, the apparent porosity of the specimens tends to increase, followed by the declining bulk density and cold compressive strength.

The high chrome refractories were crushed, ground into powder and sampled. A mixture solution of sodium carbonate and sodium hydroxide was used to extract the powder samples. The extracted solution from the samples was mixed and reacted with diphenylcarbazide to form colored substance. The hexavalent chromium content in the samples was determined by spectrophotometry. The effects of the extracting conditions, the color reagent reaction conditions, the sample mass, the working curve and the trivalent chromium coexistence on the results were studied. The experiments about determination, recovery and precision were carried out. The relative standard deviation was less than 5% and the recovery rate was 98%-104%.

In order to study the effect of the curing temperature on the intermediate temperature properties of calcium aluminate cement bonded corundum castables, the prepared castables were cured at 5, 10, 25, 40 and 50 ℃, respectively, dried at 110 ℃ for 24 h and heat treated at 800 and 1 100 ℃, respectively. Then the apparent porosity, the cold modulus of rupture and the cold crushing strength were measured. The phase composition of castable matrix specimens treated under the same conditions and the influence of the curing temperature on the intermediate temperature strength of the castables were also analyzed. The results show that with the increase of the curing temperature, the hydration degree of calcium aluminate cement increases, which promotes the uniform distribution of hydration products with Al₂O₃ after decomposition, thus enhancing the intermediate temperature strength of castables.

In order to develop new basic light-weight refractory raw materials, natural forsterite (<0.045 mm) and magnesite (<0.045 mm) were batched according to the chemical composition of forsterite (2MgO · SiO₂), wet milled, semi-dry molded and calcined at different temperatures. Then cylinder samples with diameter of 36 mm were prepared. The effects of the wet milling jar rotation speed, the calcination temperature and the anthracite addition on the properties of the samples were researched. The results show that: when the calcination temperature exceeds 1 300 ℃, all the mineral phases have converted to the desired phases; with the increase of the rotation speed and the calcination temperature, the bulk density of the samples increases, the apparent porosity decreases and the compressive strength improves. By comprehensive consideration, 400 r · min^-1 and 1 450 ℃ are taken as the optimal scheme. High addition of anthracite makes the samples light, so series of light-weight raw materials with uniformly distributed micro-pores can be gained. The light-weight raw materials achieved were used for insulation refractory castables, obtaining good application.

In order to reduce the hydrolysis rate of Si powder and inhibit the swelling and cracking of cast samples, Si powder was treated at different temperatures for 5 h, and then slurries and SiC-Si₃N₄ materials were prepared. The effect of the preheating temperature on the composition of the Si powder, the hydrolysis rate as well as the nitridation of the SiC-Si₃N₄ materials was studied. The results show that with the increase of the pretreatment temperature of Si powder, the oxidation degree of Si powders increases, the viscosity of the slurry with PAAS dispersant decreases gradually, and the hydrolysis reaction of Si powder decreases gradually; the Si powder pretreated at 700 ℃ or lower has little effect on the nitriding process, while the Si powder pretreated at 800 ℃ has a great effect on the nitriding process, which is not conducive to the nitriding sintering of the samples. Therefore, the Si powder pretreated at 700 ℃ for 5 h has the best properties, which not only has lower hydrolysis rate and good slurry fluidity, but also has mild effect on the nitriding reaction.