Magnesia-chrome bricks are alkaline refractory products containing MgO 55%-80%, Cr₂O₃ 8%-20%, with periclase and composite spinel XO·Y₂O₃ as the main crystal phases, among which XO is mainly MgO and FeO; Y₂O₃ is mainly Cr₂O₃, Al₂O₃, Fe₂O₃. The molar numbers of XO and Y₂O₃ are equal, and the excess Y₂O₃ is dissolved in the composite spinel. There is also a small amount of silicate phase (forsterite and calcium forsterite).
Raw materials and production process used:
Magnesia-chrome bricks use high-quality sintered magnesia sand and chromite (Cr₂O₃ 30%-15%, CaO 1%-5%) as the main raw materials. The production process of magnesia-chrome bricks is generally similar to that of magnesia bricks. Unfired magnesia-chrome bricks use inorganic magnesium salt solution as a binder. During the firing process, due to the loosening effect caused by the expansion of MgO and CrO₃, Al₂O₃ or iron oxides when they react to form spinel, pre-synthesized magnesia-chrome sand can be used to make bricks, which must be fired in an oxidizing atmosphere above 1600℃. If the nature of the atmosphere changes, Fe₂O₃ in chromite is affected by the redox reaction to form various valence oxides of iron. At the same time, Cr₂O₃ is also reduced to produce compounds of different valences. Under repeated reactions, the bricks are damaged, so products with high MgO and low Cr₂O₃ are used as much as possible.
According to the raw materials and process characteristics of the products, they can be divided into fused-cast magnesia-chrome bricks, directly bonded magnesia-chrome bricks, silicate-bonded magnesia-chrome bricks, rebonded magnesia-chrome bricks, semi-rebonded magnesia-chrome bricks, pre-reacted magnesia-chrome bricks and unfired magnesia-chrome bricks.
1. Fused-cast magnesia-chrome bricks
Fused-cast magnesia-chrome bricks are refractory products made of magnesia sand and chromium ore by electric melting and casting. They are characterized by large pores and isolated existence, dense products, high strength, corrosion resistance, and sensitivity to temperature changes. The chemical properties of magnesia-chrome bricks are alkaline. Compared with magnesia bricks and chrome bricks, they have good thermal vibration resistance, stable volume at high temperature, and high load softening temperature.
2. Directly bonded magnesia-chrome bricks
Directly bonded magnesia-chrome bricks are made by mixing sintered magnesia sand and chromite. The raw material is required to have a low SiO₂ content and be sintered at a high temperature above 1700℃ to form a direct bond between periclase and chromite particles. The typical physical and chemical properties of directly bonded magnesia-chrome bricks are: MgO 82.61%, Cr₂O₃ 8.72%, SiO₂ 2.02%, apparent porosity 15%, and bulk density 3.08g/cm³. Compressive strength 59.8MPa, load softening temperature 1765℃, thermal vibration resistance 1100℃ water cooling) 14 times, flexural strength 8.33MPa.
3. Silicate-bonded magnesia-chrome bricks
Silicate-bonded magnesia-chrome bricks are made of magnesia sand and chrome ore as raw materials, mixed in appropriate proportions and fired at high temperature. The mineral composition of the product is periclase, spinel and a small amount of silicate. The production of silicate-bonded magnesia-chrome bricks uses brick-making magnesia sand and general refractory grade chrome ore as raw materials. SiO₂<4%, MgO>90% in magnesia sand, Cr₂O₃ 32%-45% in chrome ore, sulfite is used as a binder, mixed and formed, and fired at about 1600℃. In order to prevent abnormal expansion of the product during firing, a weak oxidizing atmosphere must be maintained in the kiln. The chemical composition of the product: SiO₂ 2.98%-4.50%, MgO 61.75%-72.69%, Cr₂O₃ 10.04%-14.90%. Physical properties: apparent porosity 18%-21%, compressive strength at room temperature 36.1-50.OMPa, refractoriness under load 1600-1640℃.
4. Rebonded magnesia-chrome bricks and semi-rebonded magnesia-chrome bricks
Rebonded magnesia-chrome bricks are made by re-sintering fused magnesia-chrome sand as raw materials. Fused magnesia-chrome sand has poor sintering properties, and the product is a fine-grained matrix with uniform pore distribution and tiny cracks. It is more sensitive to 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 rebonded magnesia-chrome bricks are: MgO 68%, Cr₂O₃ 15%, SiO₂ 3%, apparent porosity 14%. Bulk density 3.20g/cm³, compressive strength 52.8MPa, refractoriness under load 1740℃, flexural strength 7.86MPa.
Semi-recombined magnesia-chrome bricks are made of fused magnesia-chrome sand and magnesia, chromite or pre-reacted magnesia-chrome sand. The products have some characteristics of recombined magnesia-chrome bricks and direct-bonded magnesia-chrome bricks or pre-reacted magnesia-chrome bricks. The typical physical and chemical properties of semi-recombined magnesia-chrome bricks are: MgO 71.58%, Cr₂O₃ 16.45%, SiO₂ 2.75%, apparent porosity 13%. Compressive strength 46.7MPa, load softening temperature 1760℃, flexural strength 9.09MPa.
5. Pre-reacted magnesia-chrome bricks
Pre-reacted magnesia-chrome bricks are made of all or part of pre-reacted magnesia-chrome sand. The production cost is lower than that of recombined magnesia-chrome bricks. The partial reaction between magnesia and chromite is completed during the calcination of clinker, so the apparent porosity of the product is lower than that of direct-bonded bricks with equivalent composition, and the high-temperature strength is high. The typical composition of pre-reacted magnesia-chrome bricks is: MgO 62.8%, Cr₂O₃ 15.3%, SiO₂ 3.25%, apparent porosity 17%, compressive strength 51.3MPa, load softening temperature 1650℃.
6. Unfired magnesia-chrome bricks
Unfired magnesia-chrome bricks are made of sintered magnesia sand and chromite as raw materials, adding a small amount of chemical binder, and heat treatment at a lower temperature to harden the product. Some products can be hardened at room temperature, while others need to be heated to an appropriate temperature to give the product a certain strength. When the product is used at high temperature, it forms a ceramic bond or a high-temperature resistant phase. The typical composition of unfired magnesia-chrome bricks is: MgO 52.73%, Cr₂O₃ 18.08%, SiO₂ 5.0%, apparent porosity 10.9%, compressive strength 58MPa, load softening temperature 1520-1530℃.