Magnesium-chromium bricks are basic refractories containing 55%? 80% MgO and Cr2O3%? 20% Cr2O3. They are composed of periclase, magnesia-zirconium composite spinel and a small amount of silicate phase. Composite spinel includes spinel solid solutions such as MgAl2O4, MgFe2O4, MgCr2O4 and FeAl2O4.
Magnesium-chrome bricks have developed rapidly since 1960s due to the improvement of raw material purity and firing temperature. At present, magnesia-chrome bricks can be divided into ordinary bricks, direct bonded bricks, co-sintered bricks, re-bonded bricks and Melt-cast bricks according to different production methods.
(1) Ordinary magnesia-chrome bricks
This is a traditional product, with chrome ore as coarse particles, magnesia as fine powder. Or the two materials are composed of graded particles, and the firing temperature is generally 1550? 1600 C. The micro-structure of the brick shows that there is little direct bonding between chromite particles and periclase, mostly silicate (CMS) cementation or crack isolation; there are few dissolving phases in periclase and few direct bonding in matrix. The mechanical properties of the brick are poor and slag corrosion resistance is poor.
(2) Direct bonding magnesia-chrome bricks
Direct-bonded magnesia-chrome brick is developed on the basis of ordinary magnesia-chrome brick. Its production features mainly include two points: the use of relatively pure raw materials, and the use of higher firing temperature. The so-called direct bonding refers to more direct contact between chromite particles and periclase in bricks, because there is less SiO 2 in raw materials (controlled below 1%? 25%) and less silicate formation, which is extruded into the corner of solid particles by means of high temperature firing. Thus, the direct bonding of solid phase can be improved.
Use of Magnesium Carbon Bricks
Direct bonded magnesia-chrome bricks have higher high temperature strength, slag resistance, corrosion resistance, erosion resistance, corrosion resistance, excellent thermal shock stability and volume stability at 1800 ~C due to their high degree of direct bonding.
(3) Co-sintered magnesia-chrome bricks
The production process of this kind of product is characterized by high temperature furnace burning the mixture of magnesia and chrome ore fine powder according to a certain proportion, realizing the solid state reaction for the purpose of producing secondary spinel and magnesia-chrome ore direct combination, and producing co-sintering material, using this material to produce sintered products or chemically bonded products.
The direct bonding and homogeneity of microstructures of co-sintered magnesia-chromium bricks are better than those of direct bonded bricks. The amount of periclase dissolving phase and intercrystalline secondary spinel is more. Co-sintered magnesia-chromium bricks have a series of better properties than direct bonded bricks, especially the high temperature strength, resistance to temperature sudden change and slag resistance.
Co-fired bricks can also be divided into two types: one is all-sintered bricks, the micro-structure of which is basically similar whether sintered or chemically bonded; the other is part of the co-fired bricks, with some ingredients, such as coarse-grained co-sintered materials, while fine-grained parts can be mixed with fine chrome ore and magnesia paper powder in a certain proportion into the bricks. In this way, the firing and chemical bonding products have different microstructures.
(4) Rebonded magnesia-chrome bricks
Magnesium-chromium mixed powders were melted by electrofusion and crystallized through melt to form materials with homogeneous microstructures consisting of magnesia-chromium spinel and periclase as main phases. The fused magnesia-chromium mixed powders were crushed to a certain particle size and mixed to form. The fused magnesia-chromium mixed powders were fired to prepare composite bricks or used directly as chemical bonding platform bricks.
The microstructural characteristics of the composite brick are high direct bonding and a large number of spinel dissolving phases: the base crystals containing a large number of dissolving phases essentially change the physical and chemical properties of periclase, such as reducing thermal expansion coefficient, improving thermal shock resistance and improving the resistance to acid-alkaline slag erosion. Re-bonded bricks have similar properties as Melt-cast bricks, but have better resistance to temperature sudden change and more uniform microstructures than Melt-cast bricks.
The magnesia-chrome brick is a fine matrix with uniform pore distribution and micro-cracks. It is more sensitive to temperature change than melt-casting. The high temperature performance of the product is between the Melt-cast brick and the directly bonded brick.
(5) Cast magnesia-chromium brick
The mixture of magnesia and chromite is melted completely in the electric arc furnace, and then the melt is injected into the refractory casting mould for casting. Stable periclase and spinel phases are formed during solidification, and fine crystalline structure is formed at the same time. Therefore, Melt-cast magnesia-chrome bricks have excellent high temperature strength and slag corrosion resistance.