The pollution of cement industry refractory materials is mainly caused by chromium-containing refractory materials. The main ways are: First, during the calcination of cement clinker, when chromium-containing refractory materials are in the presence of oxidizing atmosphere and alkaline oxides, at a certain temperature, part of Cr3+ will be converted into Cr6+, which is harmful to humans, and mixed into cement clinker with the peeling refractory materials, resulting in excessive chromium content in cement; second, the chromium-containing waste bricks are not handled properly, and are washed into the ground by rainwater and pollute the groundwater source.
Since the 1990s, my country's cement industry has developed rapidly, and chromium-containing refractory materials have been widely used in cement kiln firing zones, and have achieved good results. However, at the same time, the chromium pollution caused by chromium-containing refractory materials has become more and more widespread. In order to solve the environmental pollution problem caused by magnesia-chrome bricks used in cement kilns, chromium-free has become a top priority. After the unremitting efforts of the majority of refractory workers, chromium-free alkaline refractory materials have made great progress in the past 20 years. The chromium-free refractory materials developed for cement kiln firing zone mainly include periclase-spinel bricks, magnesia-iron spinel bricks, magnesia-iron-aluminum composite spinel bricks, etc.
Magnesia-iron-aluminum composite spinel bricks are another new energy-saving and environmentally friendly refractory material developed on the basis of the original magnesia-iron spinel bricks.
PART 1 Production of magnesia-iron-aluminum composite spinel bricks
1.1 Raw material standards
The raw materials entering the factory are first analyzed for chemical composition in the laboratory, and their main chemical composition must meet the requirements.
1.2 Raw material preparation
The qualified raw materials are coarsely crushed by the jaw crusher, and part of them enter the cone crusher for medium crushing. After screening, the particles ≤5mm are screened into different particle sizes according to the process requirements and enter different silos. The part of the particles >5mm returns to the cone crusher for crushing, and the other part enters the drum mill or Raymond mill to be processed into fine powder ≤0.088mm for standby.
1.3 Mixing
Using sulfite pulp waste liquid as a binder, first add the granular material to the wet mill and mix for 3 to 5 minutes, add 4/5 of the binder and mix for 3 to 5 minutes, then add the fine powder and mix for 5 to 10 minutes (during this period, add the binder appropriately according to the dryness and wetness of the mud material). After the mud material is evenly mixed, it can be discharged for blank forming.
1.4 Forming
Use a 630-ton program-controlled press to form. The semi-finished brick blanks are required to have no pockmarks, no cracks, regular size and shape, meet the semi-finished product standards, and the semi-finished product volume density ≥3.12g/cm3.
1.5 Firing
The semi-finished brick blanks that pass the inspection enter the tunnel dryer, are dried at 110℃×24h, and are loaded into the high-temperature tunnel kiln after passing the inspection and fired in a weak reducing atmosphere. After leaving the kiln, they are inspected and stored.
PART 2 Performance of MgFeAl composite spinel bricks
01 Good corrosion resistance
First of all, MgFeAl composite spinel bricks have high purity and less low-melting products. The contact area between the main crystal phases of periclase, MgAl spinel, MgFe spinel, and FeAl spinel is large, the degree of direct bonding is high, and the low-melting products are distributed in the triangular area formed between the crystals, showing good corrosion resistance. Secondly, under the high temperature of firing and use, MgO and Al2O3, Fe2O3 or FeO of FeAl spinel undergo a series of reactions to generate secondary spinel. These reactions are accompanied by a certain volume expansion, which improves the density of the material and can effectively prevent the penetration of cement low-melting products. At the same time, the components in the brick react with the infiltrated cement components to generate a high-melting point phase, which increases the viscosity of the low-melting products and delays their penetration process. Therefore, MgFeAl composite spinel bricks have good corrosion resistance.
02Good kiln skin performance
Cement clinker is made of limestone, clay and iron raw materials as the main raw materials, and is prepared into raw materials in appropriate proportions. At high temperature, the cement raw materials produce melts and react with the surface of refractory bricks, penetrate into the bricks through the pores of the bricks, and the infiltrate enters the bricks and solidifies at a position below 1200℃, forming an initial kiln skin with a "mechanical anchoring" effect. The initial kiln skin is then bonded to the clinker particles, making the kiln skin gradually thicker. The main components of the kiln skin are silicates, aluminates and ferroaluminates. Magnesium-iron-aluminum composite spinel bricks contain certain Al2O3, Fe2O3 or FeO components. The appropriate A/F value is conducive to the formation of a stable kiln skin and plays a self-protection role.
03Good thermal shock resistance
Fe-aluminum spinel has the characteristics of small thermal expansion coefficient and high thermal conductivity, which can significantly reduce the thermal stress of magnesium-iron-aluminum composite spinel bricks. Magnesium-iron-aluminum composite spinel bricks contain various phases such as periclase, iron-aluminum spinel, magnesium-iron spinel, and magnesium-aluminum spinel. Due to the different thermal expansion coefficients of various phases, there are a large number of microcracks inside the material, which increases the toughness of the material. Therefore, magnesium-iron-aluminum spinel bricks have good flexibility and thermal shock resistance.
04 Lower thermal conductivity
The kiln lining bricks of cement kilns are all single-layer structures. Long-term high-temperature operation of the kiln body is prone to cause deformation of the cylinder, affecting the operation of the kiln and reducing cement production. Therefore, it is required to reduce the thermal conductivity of the kiln lining as much as possible. Compared with directly bonded magnesia-chrome bricks, magnesium-iron-aluminum composite spinel bricks are multi-phase composite materials with lower thermal conductivity, which can reduce the surface temperature of the cylinder, reduce heat loss, and correspondingly reduce fuel consumption, creating good conditions for improving the operation rate and service life of cement kilns.
Although magnesia-alumina spinel bricks have good thermal shock resistance, their kiln skin performance is poor, and the thermal conductivity is too high; although magnesia-iron spinel bricks have good performance, they contain chromium components and are prohibited products; magnesia-iron-aluminum composite spinel bricks have good comprehensive high-temperature performance and are currently the first choice for refractory materials for cement kiln firing zones.
PART 3 Application of magnesia-iron-aluminum composite spinel bricks
Magnesia-iron-aluminum composite spinel bricks have been widely used in nearly 100 cement kilns at home and abroad. From the application of various cement companies over the past three years, magnesia-iron-aluminum spinel bricks have excellent high-temperature performance and strong adaptability, which exceeds the level of the original direct combination of magnesia-chrome bricks, and can completely realize the chromium-free cement kiln firing zone materials.
Magnesia-iron-aluminum composite spinel bricks have good comprehensive high-temperature performance such as anti-erosion, thermal shock resistance, and wear resistance, and low thermal conductivity, and are easy to form a stable kiln skin. They are currently the ideal choice for chromium-free refractory materials for cement kiln firing zones. Judging from the application of many cement companies in recent years: magnesia-iron-aluminum composite spinel bricks run smoothly and have strong adaptability, exceeding the level of the original directly combined magnesia-chrome bricks, and completely realizing the chromium-free refractory materials in the cement kiln firing zone.
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