Deep analysis of the content of magnesia chrome bricks

① Cast magnesium inscription brick: Cast magnesium inscription brick is a refractory product made from magnesia and chrome ore by electric melting and pouring. The production process of cast magnesium inscription brick is shown in Figure 1. It is characterized by large and isolated pores, dense products, high strength, corrosion resistance and sensitivity to temperature changes. The chemical property of magnesia brick is alkaline. Compared with magnesia brick and chrome brick, it has good thermal vibration resistance, stable volume at high temperature and high load softening temperature.

② Direct bonded magnesia bricks: The direct bonded magnesia bricks are made by mixing sintered magnesia and chromite. It is required that the content of SiO2 in raw materials is low, and the raw materials are fired at a high temperature above l700 ℃ to form a direct combination between periclase and chromite particles. The typical physical and chemical properties of directly bonded magnesia fused brick are: MgO 82.61%, Cr203 8.72%, SiO2 2.02%, apparent porosity 15%, and bulk density 3.08g/cm3. The compressive strength is 59.8MPa, the load softening temperature is 1765 ℃, the thermal shock resistance is 11OO ℃, water cooling for 14 times, and the bending strength is 8.33MPa.

③ Silicate bonded magnesia chrome bricks: Silicate bonded magnesia chrome bricks are sufficient to sinter magnesia and chrome ore as raw materials, and are prepared by mixing them in proper proportion and firing at high temperature. The mineral composition of the product is periclase, spinel and a small amount of silicate. Silicate bonded magnesia bricks are produced with magnesia for brick making and ordinary refractory chrome ore as raw materials. SiO2 in magnesia is less than 4%, Mg0 is more than 90%, and Cr203 in magnesia is 32% - 45%. After being mixed and formed with sulfite as the binder, they are fired at about 1600 ℃. In order to prevent abnormal expansion of products during firing, a weak oxidation atmosphere must be maintained in the kiln. Chemical composition of the product: SiO2 2.98% - 4.50%, MgO 61.75% - 72.69%, Cr203 10.04% - 14.90%. Physical properties: apparent porosity 18% - 21%, normal temperature compressive strength 36.1-50 OMPa, load softening temperature 16001640 ℃.

④ Re bonded magnesia chrome bricks and semi re bonded magnesia chrome bricks: Re bonded magnesia chrome bricks are made from fused magnesia chrome sand by re sintering. The sintered property of fused magnesia chrome sand is poor. The products are fine grain matrix with uniform porosity distribution, and have small cracks. The sensitivity to rapid temperature change is better than that of fused cast bricks. The high temperature performance of the product is between the fused cast brick and the direct bonded brick. The typical physical and chemical properties of the combined magnesia brick are: MgO 68%, Cr203 15%, SiO2 3%, and apparent porosity 14% The bulk density is 3.20g/cm3, the compressive strength is 52.8MPa, the load softening temperature is 1740 ℃, and the flexural strength is 7.86MPa.

Semi rebonded magnesia chrome bricks are made from fused magnesia chrome sand, magnesia, chromite or pre reactive magnesia chrome sand. The products have some characteristics of re bonded magnesia chrome bricks and directly bonded magnesia chrome bricks or pre reactive magnesia chrome bricks. The typical physical and chemical properties of semi re bonded magnesia bricks are: MgO 71.58%, Cr2O3 16.45%, SiO2 2.75%, and apparent porosity 13% The compressive strength is 46.7MPa, the load softening temperature is l760 ℃, and the bending strength is 9.09MPa.

⑤ Pre reaction magnesia chrome brick: the pre reaction magnesia chrome brick is made of all or part of the pre reaction magnesia chrome sand. The production cost is lower than that of recombined magnesia chrome bricks. Part of the reaction between magnesia chromite is completed during clinker calcination, so the apparent porosity of the product is lower than that of the direct bonded brick with equivalent composition High temperature strength. The typical composition of pre reaction magnesia chrome brick is: MgO 62.8%, Cr203 15.3%, SiO2 3.25%, apparent porosity 17%, compressive strength 51.3MPa, and load softening temperature 1650 ℃.

⑥ Unburned magnesia chrome bricks: Unburned magnesia chrome bricks are made by using sintered magnesia and chromite as raw materials, adding a small amount of chemical binders, and heat treating at a lower temperature to harden the products. Some products can be hardened at room temperature, while others need to be heated to an appropriate temperature to make products have a certain strength. When products are used at high temperatures, ceramic bonding or high temperature resistant phases are formed. The typical composition of unburned magnesia chrome brick is: MgO 52.73%, Cr203 18.08%, SiO2 5.0%, apparent porosity 10.9%, compressive strength 58MPa, and load softening temperature 1520-1530 ℃.

The so-called secondary spinel is the spinel formed by high temperature treatment or chemical reaction, which is different from the original chrome ore. There are three main types:

(1) Intercrystalline spinel - liquid-phase crystallization.

Silicate phase in magnesia chrome brick mainly comes from gangue in chrome ore.

The distribution of gangue in chrome ore is uneven, so the liquid phase formed after melting is difficult to be evenly distributed, and will be locally enriched. After the chromite surface dissolves in the liquid phase, most of the re precipitates are idiomorphic secondary spinels.

(2) The transition constitutes spinel.

Transition composition spinel refers to the reaction zone formed between chromite or spinel and periclase, which is the transition zone of gradual composition. Such reaction zone contributes the most to direct bonding. Some parts realize interphase "fusion", and some show microcracks.

(3) Desolved spinel.

Desolvation phase in periclase crystal is the third form of spinel, which has no effect on direct combination between phases, but plays an important role in changing the properties of periclase solid solution, and can improve the corrosion resistance of acid medium; Improved thermal shock resistance and toughness.