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What are the production methods of magnesium aluminate spinel?
The methods for synthesizing magnesium aluminum spinel are mainly sintering method and electrofusion method. The sintering method refers to a raw material containing magnesium hydroxide, sintered alumina and the like, and a magnesium-containing raw material such as magnesium carbonate or magnesium hydroxide, which is compounded according to requirements, and is finely ground together, pressed (ball), and rotated in a rotary kiln or above 1750 ° C. The high-temperature calcination in the kiln can obtain the magnesium-aluminum spinel synthesized by the sintering method, and specifically, it can be divided into a one-step method, a one-step method and a two-step method.
One-step sintering of magnesite + bauxite raw material → dry co-grinding → forming → firing → spinel clinker
One-step semi-sinter sintering of light burnt magnesium powder + bauxite raw material → dry co-grinding → forming → firing → spinel clinker
Two-step sintering synthetic magnesite + bauxite raw material → dry co-grinding → forming → light burning (about 1300 ° C) → crushing → forming → firing → spinel clinker
In addition, the activated spinel raw material ball is calcined at a low temperature of 1200 to 1300 ° C to obtain an active spinel. Unlike the sintered spinel, the active spinel contains unreacted w (Al 2 O 3 ) 10 ~15%, w(MgO) 5%~10%.
In the spinel synthesized by the sintering method, since the raw material contains impurities such as SiO2, CaO, and Fe2O3, the synthetic sand often contains impurity minerals such as MgSi2O4 and CaMgSiO4 and excess Al2O3 and MgO in addition to MgAl2O4.
The electrofusion method is used to synthesize magnesium-aluminum spinel sand, and various magnesium-containing aluminum-containing raw materials can be selected. In the raw material for synthesizing spinel, the w (MgO) content has been selected in the range of 35% to 50%. The highest or too low MgO content is detrimental to the melting of the synthetic sand. The high viscosity makes the melt difficult to cast, and the addition of chromium ore is beneficial to the melting and pouring of the melt.
The formulated mixture can be melted in a tilting electric furnace or a vortex melting furnace. The vortex melting furnace can melt various formulations of fused spinel by heating a selected proportion of the mixture in the furnace to a temperature above 150~250 ° C (the limit temperature of the molten pool is 2300 ° C) Therefore, it is possible to melt a material having a melting point of not higher than 2150 ° C. The positions of the electric frit are different, and the structure is different. Generally, there are many honeycomb vents in the upper part and the periphery, and the frit having the theoretical composition of the spinel has the largest porosity, but contains an excessive amount of MgO or a Cr2O3 frit. The porosity is low. Therefore, the key to the production of fused spinel sand is how to obtain a product with a uniform structure, while properly venting pores to reduce the porosity of the product. In addition, the addition of Cr2O3 can also improve the high temperature resistance of the molten material.
The spinel frit has a spinel mass fraction above 80%, with the remainder being silicate and glassy. In the spinel frit, the impurity-free spinel crystallized above the lowest eutectic temperature is a primary spinel, and the spinel solid solution precipitated below the lowest eutectic temperature is called a secondary spinel. Typically, the secondary spinel crystallizes in the upper portion of the frit, while the primary spinel crystallizes in the lower portion of the frit.
The lattice parameters of Cr2O3-free fused spinel are similar to those of normal spinel. When Cr2O3 is added, the lattice is obviously distorted, indicating that Cr2O3 is dissolved in the spinel lattice according to the displacement solid solution.
By controlling the cooling rate of the furnace body, an fused spinel having a different degree of crystallization can be obtained. When a magnesium spinel product is produced using a spinel having a high structural defect, it is ensured that the desired sintering activity is obtained at the time of firing.
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