Advantages and disadvantages of magnesia chrome bricks for copper converter

Magnesia chrome refractory is a refractory mainly composed of magnesia and supplemented by chromite; After magnesia is added to chrome ore and fired at high temperature, obvious physical and chemical reactions occur between magnesia and chrome ore. MgO in magnesia and Cr2O3 in chromite diffuse each other during sintering, forming point like secondary spinel in periclase and original spinel in chrome ore; According to the different processes and raw materials used, the magnesia chrome bricks form different degrees of secondary composite spinel and periclase secondary spinel composite during the cooling process, so as to form a structure where all phases are directly combined and the chromite phase is wrapped by magnesia.

In the development history of magnesia chrome refractories used in copper smelting industry, silica bricks were first used to build furnace linings. However, due to the FeO and SiO2 components in the slag and the SO2 acid gas in the smelting atmosphere, silica bricks have been severely eroded. From the 1950s to 1960s, alkaline bricks have been used in the copper smelting industry. Today, through continuous improvement of several generations, The production process and application of magnesia chrome refractories in copper smelting industry are more and more mature. The magnesia chrome refractories commonly used in copper smelting industry are as follows:

(1) Ordinary silicate bonded magnesia chrome brick

The common silicate bonded magnesia chrome brick is formed by burning the lower grade magnesia and lower grade chrome ore at 1550 ℃. Ordinary silicate bonded magnesia chrome bricks contain more silicate phase and less secondary spinel, forming a microstructure of silica phase wrapped periclase phase.

In ordinary silicate bonded magnesia chrome bricks, the low melting point silicate phase separates and wraps the high melting point periclase and chromite, the high melting point periclase and chromite are directly bonded with silicate without forming a direct bonding structure, and the low melting point silicate is interconnected to form a continuous phase; The silicate phase with low melting point is very easy to be softened and dissolved during use, which seriously affects the load softening temperature, thermal strength, volume stability and other properties of magnesia chrome bricks; In addition, cracks can be propagated through silicate glass phase, and molten slag is easy to penetrate and corrode from weak silicate glass phase, so the thermal shock resistance and slag corrosion resistance of silicate bonded magnesia chrome bricks are poor.

(2) Direct bonded magnesia chrome brick

Directly bonded magnesia chrome bricks are fired at 1700 ℃ with high-grade magnesia and chrome concentrates with low impurities such as silicate; In the direct bonded magnesia chrome brick, there is a small amount of silicate phase. The silicate phase exists in the matrix in the form of islands, and the direct bonding between periclase periclase and periclase composite spinel. Therefore, the direct bonded magnesia chrome brick has good high temperature resistance, corrosion resistance and thermal shock resistance.

In the 1960s, the magnesia chrome bricks prepared by direct bonding method had good performance, successfully replaced the magnesia chrome bricks prepared by silicate bonding, and were widely used in the burning belt of cement kiln.

However, there are also some problems in directly bonded magnesia chrome bricks:

(1) The magnesia chrome brick will react with the cement clinker on the firing zone of the cement kiln to produce low melting substances, such as C12A7 at 1335 ℃, C4AF at 1395 ℃, CMS at 1500 ℃, and C3MS2 at 1575 ℃. These low melting mineral phases will penetrate into the depth of the refractory brick, making it difficult to form a thick kiln skin on the surface of the refractory brick. As a result, the chemical corrosion resistance of refractory bricks becomes worse and the application time becomes shorter.

(2) When the cement rotary kiln is in oxidation atmosphere, trivalent Cr3+is oxidized to hexavalent Cr6+. Subsequently, the magnesia chrome bricks will be severely corroded by sulfate, thus transforming the Cr6+compounds into water-soluble substances. These substances containing hexavalent chromium ions will be discharged to the outside along with the used refractory bricks and waste gas emission. As we all know, hexavalent Cr6+compounds are highly toxic, which will seriously pollute the environment, cause serious harm to organisms, and even cause cancer.

(3) When the cement rotary kiln is in a reducing atmosphere, CO can react with the magnesia chromite in the magnesia chromite brick to produce magnesia cristobalite. It is also assumed that the reduction atmosphere inside the cement rotary kiln is converted into oxidation atmosphere, which will lead to the complete disintegration of the internal structure of the refractory brick, reducing the service time of the refractory brick.

(4) The cement kiln with magnesia chrome bricks is mostly made of dicalcium sulfate. When the kiln body is cooled, β- C2S becomes γ- C2S, which causes the kiln skin to be powdery and finally peeled off, shortening the application time of magnesia chrome bricks.

With the strengthening of environmental protection policies, people's awareness of environmental protection is also constantly improving. It is required to stop using all over the world. In view of this situation, a new idea of chrome free is proposed. Although chrome free refractory for cement kiln has been proposed for many years, chrome free refractory bricks that can replace magnesia chrome bricks are still not developed. Some researchers in the refractory industry have always changed the properties of magnesia chrome bricks in order to achieve the purposes of recycling materials, reducing the emission of harmful gases, and extending the service life