Magnesia chromium bricks'​ properties

The properties of a refractory product depend on its composition and structure, and the composition and structure are determined by the raw materials and the manufacturing process.
Today we will focus on the common types and characteristics of magnesium-chromium refractory products, the influence of raw materials and process conditions on the properties of magnesium-chromium products, the preparation, structure and properties of co-sintered and fused cast refractory products. As raw materials and processes for the production of magnesia chrome bricks are constantly improving. Magnesia-chromium refractory products are also increasingly rich in varieties, including silicate-bonded magnesia-chrome bricks, direct-bonding magnesia-chrome bricks, and high-quality magnesia-chrome bricks, such as fused cast magnesia chrome bricks, fused fusion magnesium chrome bricks, and semi-recombined magnesia chrome bricks. And fully synthetic magnesia chrome bricks.
The preparation, characteristics and micro-structure of the commonly used magnesia chrome bricks are summarized below.
1.1 Silicate Bonded Magnesia-chrome Brick
Silicate Bonded Magnesia-chrome Brick is also known as ordinary magnesia-chrome brick. The brick is made of chrome ore and magnesia with high content of impurities (mainly SiO2 and CaO), and the firing temperature is not high, at around 1550 °C.
Its micro-structural characteristics are: refractory grains are combined by a silicate having a low melting point or a softening point, so it is called a silicate-bonded magnesia-chrome brick.
The main crystalline phase of the matrix of silicate-bonded magnesia-chrome brick is granular periclase, and the gray-white film-like silicate (CMS) cemented phase is often between the magnesia and chrome ore particles or between the respective particles. Or isolated by pores, followed by a small amount of composite spinel (white) filled in the magnesia intercrystalline, but the degree of direct bonding is very low. The matrix is a dense network structure, the pores are mostly strips, and a small part is closed. Because the micro-structural characteristics of silicate bonding are not conducive to the high temperature mechanical properties and chemical corrosion resistance of ordinary magnesia refractory products, silicate bonded magnesia chrome bricks are often used in locations where performance is not critical.
1.2 Direct Bonded Magnesia-chrome Brick
Direct Bonded Magnesia-chrome Brick is based on silicate-bonded magnesia-chrome bricks to minimize the content of impurities (especially SiO2 and CaO) in the raw materials,that is, the content of impurities is low.
Magnesia-chrome brick prepared from chrome concentrate and relatively pure magnesia. Since the content of impurities in the brick is low, high temperature firing is usually used (the firing temperature is usually above 1700 ° C). Due to the use of higher purity raw materials, the amount of silicate bonded phase in the brick is reduced, the impurity content is small, and the refractory grains are mostly in direct contact, so it is called direct combination of magnesia chrome brick.
The main crystalline phase in the direct combination of magnesia chrome brick matrix is still granular periclase, and the crystal also contains a large amount of spinel desolvent phase (within the magnesia grain). The white precipitates, followed by a small amount of composite spinel (white) and an off-white film silicate phase filled between the periclolite grains. The main crystalline phase of periclase is mainly composed of periclase-periccite and periclase-chromium ore (composite spinel), that is, the direct combination of grains is dominant; the other part is through a small amount of silicate. The phase films are glued together. The direct combined microstructure characteristics significantly improve the high temperature performance, corrosion resistance and erosion resistance of the magnesia chrome brick, making it a popular magnesium chrome brick. The direct combination of magnesia-chrome bricks has been widely used in places with severe conditions such as non-ferrous smelting furnaces and cement kiln.
1.3 Electro-fusion combined with magnesia chrome brick
In some countries, magnesia chrome bricks made entirely of synthetic raw materials, co-sintered magnesia chrome or fused magnesia (or partially fused magnesia) are referred to as recombined magnesia chrome bricks. In China, fused chrome brick (Rebounded Magnesia-chrome Brick) is usually made of fused pre-synthesized magnesia chrome.
Because of the high purity of the raw materials, it needs to be fired at a high temperature or super high temperature above 1750 °C. 
In addition, because the process preliminarily obtains a magnesia-chromium composite with complete crystallized petrochemical, high direct bonding rate and uniform phase distribution by means of electro-fusion, a relatively uniform structure is obtained, and the calcite solution is also resistant to thermal shock resistance. The advantage of erosion resistance improves the performance of the product.
1.4 Semi-recombined Magnesia-chrome Brick
Semi-recombined Magnesia-chrome Brick is a magnesia-chrome brick between the electro-fusion recombined magnesia chrome brick and the direct combination magnesia chrome brick. The raw materials used in the production include both fused pre-magnesium chrome sand and chromium concentrate and magnesia. This kind of brick is also fired at a high temperature above 1700 °C, and the refractory grains in the brick are often directly combined. The advantage is that the thermal shock resistance is good, and the erosion resistance and erosion resistance are also good.
The characteristics of the semi-recombined magnesia chrome brick combining the characteristics of magnesia chrome brick, electrofusion and magnesium chrome brick. In the matrix part, the main crystalline phase is granular periclase, which contains a large amount of spinel desolvent phase, followed by composite spinel (white) and a small amount of grayish white film silicate phase (CMS). Magnesia intercrystalline. The matrix structure is in the form of a porous network. Most of the main crystalline phase magnesia is directly bonded by the periclase-composite spinel, and a few are cemented together by the silicate phase.
1.5 Fused Cast Magnesia Chrome brick
The main production process of Casting Magnesia-chrome Brick is: adding a certain amount of admixture with magnesia and chrome ore, mixing, compacting and burning, breaking into blocks, melting into electric arc furnace, re-injecting the inner mold is annealed to produce the mother brick; the mother brick can be cut and milled to form the required brick type. The structural characteristics of the fused cast magnesia chrome bricks are uniform distribution of components, direct contact between refractory grains, and silicates in the form of islands. The brick is particularly resistant to melt erosion, penetration and scouring. It is a product specially developed for non-ferrous smelting. It is most suitable for building in a continuous production equipment melting furnace such as the lower part of the flash furnace reaction tower and the sedimentation tank slag line area.
1.6 Chemical-combined Non-burned Magnesia Chrome Bricks
This type of brick is usually made of magnesia and chrome ore. It is made of sodium polyphosphate or sodium hexametaphosphate or water glass as a binder. It is baked at a temperature of about 200 °C because it does not need to be fired at a high temperature. Since the brick is not fired at a high temperature, the magnesia in the brick is hydrated, so the brick cannot be stored for a long time. In summary, there are many types of magnesium-chromium refractories. The commonly used magnesia-chrome refractories are directly combined with magnesia-chrome bricks (generally directly combined with magnesia and chrome ore), electro-fused with magnesia-chrome bricks (with fused magnesia chrome sand as raw material), and semi-recombined magnesia-chrome bricks ( Add part of fused magnesia chrome sand) and so on. Performance impact the difference in production process makes the structure of magnesium-chromium material significantly different, which in turn affects the performance of magnesia-chrome brick. For example, the direct combination of magnesia-chrome bricks has good thermal shock resistance, and the electro-fusion combined with magnesia-chrome bricks has strong anti-erosion performance, and the performance of semi-recombined magnesia-chrome bricks is somewhere in between. To this end, this section will discuss in detail the impact of the production process on the performance of magnesia chrome bricks.