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Analyzing the composition of magnesium chromium bricks

2023-08-23 11:26:02
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(1) Composition of magnesium chromium bricks

Magnesium chromium bricks are alkaline refractory products containing 55% - 80% MgO and 8% - 20% Cr₂O₃. They are mainly composed of periclase and composite spinel XO · Y₂O, with XO being mainly MgO and FeO; Y₂O is mainly composed of Cr₂O, Al₂O, and Fe₂O. The molar numbers of XO and Y₂O are equal, and the excess Y₂O is solidly dissolved in the composite spinel. There are also small amounts of silicate phases (magnesium olivine and calcium magnesium olivine). 


(2) Raw materials and production processes used

Magnesium chromium bricks are mainly made of high-quality sintered magnesia and chromite (Cr₂O 30% - 15%, CaO 1% - 5%) as raw materials. The production process of magnesia brick is generally similar to that of magnesia brick. Use inorganic magnesium salt solution as a binder for unburned magnesium engraved bricks. The loosening effect caused by the expansion of spinel generated by the reaction between MgO and CrO, Al₂O or iron oxides during the firing process can be achieved by using pre synthesized magnesia sand bricks, which must be fired in an oxidizing atmosphere above 1600 ℃. If there is a change in the atmospheric properties, the Fe₂O in chromite is affected by the oxidation-reduction reaction, forming various valence oxides of iron. At the same time, Cr₂O is also reduced to produce compounds of different valences. Under repeated reactions, bricks may be damaged, so products with high MgO and low Cr₂O should be used as much as possible.

According to the raw materials and process characteristics used in the product, it can be divided into fused cast magnesium engraved bricks, directly bonded magnesium engraved bricks, silicate bonded magnesium engraved bricks, rebonded magnesium engraved bricks, semi rebonded magnesium engraved bricks, pre reacted magnesium engraved bricks, and unburned magnesium engraved bricks.

① Melt cast magnesium brick: Melt cast magnesium brick is a refractory product made from magnesia and chromium ore by electric melting and pouring. Its characteristics are large and isolated pores, dense products, high strength, corrosion resistance, and sensitivity to temperature changes. The chemical properties of magnesia brick are alkaline, and compared with magnesia brick and magnesia brick, it has good thermal vibration resistance, stable volume at high temperatures, and high load softening temperature.

② Directly bonded magnesia brick: Directly bonded magnesia brick is made by combining sintered magnesia and chromite. The raw material is required to have a low SiO₂ content and be fired at high temperatures above 1700 ℃ to form a direct bond between periclase and chromite particles. The typical physical and chemical properties of directly bonded magnesium fused bricks are: MgO 82.61%, Cr₂O 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 1100 ℃, and the water cooling is 14 times. The flexural 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 appropriate proportions and firing them at high temperatures. The mineral composition of the product is periclase, spinel, and a small amount of silicate. The production of silicate bonded magnesia brick is based on brick making magnesia and general refractory grade chromite as raw materials. The SiO₂ content in magnesia is less than 4%, MgO is more than 90%, and Cr₂O in magnesite is 32% - 45%. After being mixed and formed with sulfite as the binder, it is fired at around 1600 ℃. To prevent abnormal expansion of products during firing, a weak oxidation atmosphere must be maintained in the kiln. The chemical composition of the product: SiO₂ 2.98% - 4.50%, MgO 61.75% - 72.69%, Cr₂O 10.04% - 14.90%. Physical properties: Apparent porosity 18% - 21%, room temperature compressive strength 36.1- 50.OMPa, load softening temperature 1600 ℃ - 1640 ℃.


④ Rebound magnesium chromium bricks and semi rebound magnesium chromium bricks: Rebound magnesium chromium bricks are made from fused magnesium chromium sand as raw material through re sintering. The sintering performance of fused magnesium chromium sand is poor, and the product is a fine-grained matrix with uniform pore distribution and small cracks. It is more sensitive to sudden temperature changes than fused cast bricks. The high-temperature performance of the product is between fused cast bricks and directly bonded bricks. The typical physical and chemical properties of magnesia brick combined with magnesia are: MgO 68%, Cr₂O 15%, SiO₂ 3%, and apparent porosity of 14% Volume density 3.20g/cm³, compressive strength 52.8MPa, load softening temperature 1740 ℃, flexural strength 7.86MPa.


Semi rebonded magnesia chrome bricks are made from fused magnesia chrome sand, magnesia sand, chromite ore, or pre reacted magnesia chrome sand. The product has some characteristics of re bonded magnesium chromium bricks and directly bonded magnesium chromium bricks or pre reacted magnesium engraved bricks. The typical physical and chemical properties of semi rebonded magnesium engraved bricks are: MgO 71.58%, Cr₂O 16.45%, SiO₂ 2.75%, and apparent porosity of 13% The compressive strength is 46.7MPa, the load softening temperature is 1760 ℃, and the flexural strength is 9.09MPa.


⑤ Pre reactive magnesium chromium bricks: Pre reactive magnesium chromium bricks are made from all or part of pre reactive magnesium chromium sand. The production cost is lower than that of combined magnesium chromium bricks. The partial reaction between magnesia and chromite is completed during clinker calcination, so the apparent porosity of the product is lower than that of directly bonded bricks with equivalent composition High temperature strength. The typical composition of pre reacted magnesium chromium bricks is: MgO 62.8%, Cr₂O 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 from sintered magnesia and chromite as raw materials, with a small amount of chemical binder added, and heat treated at lower temperatures to harden the products. Some can harden the product at room temperature, while others require heating to an appropriate temperature to give the product a certain strength. When used at high temperatures, the product forms a ceramic bond or high-temperature resistant phase. The typical composition of unburned magnesium chromium bricks is: MgO 52.73%, Cr₂O 18.08%, SiO₂ 5.0%, apparent porosity 10.9%, compressive strength 58MPa, and load softening temperature 1520 ℃ - 1530 ℃.



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