- Time:Mar 13, 2023
There are many types of minerals in refractory clay, which can be divided into two types: clay minerals and impurity minerals.
1. Clay minerals Clay minerals are minerals such as hydrous aluminosilicates.
(1) Kaolinite family. Minerals of this group include kaolinite, dikaiite, pearl clay and silicon-rich kaolinite. The chemical formula of kaolinite is AI2O3 • 2SiO2 • 2H2O or Al4(Si4O10)(OH)8. It is formed by hydrothermal metamorphism or weathering of acidic and neutral rocks. Monoclinic crystal system, mostly dense fine-grained, loose scale-like and earthy collections. The crystalline fragments are often in the shape of earthworm curved columns. Under the electron microscope, it appears as pseudo-hexagonal scales. The appearance is white, off-white, light brown, yellow, light red, light green when containing impurities. The luster is dim, the hardness is close to 1, and the density is 2.58~2. 60g/cm3. Kneaded into powder by hand, it is hydrophilic; it is easy to disintegrate after mixing with water, and has strong reliability. Kaolin has high refractoriness, good formability, sinterability and chemical stability.
During the heating process, kaolinite loses crystal water at about 550~650°C and becomes metakaolinite. There are many opinions on the further decomposition of metakaolinite.
One is: it decomposes at about 980°C and transforms into a spinel phase, and begins to transform into mullite at about 1200°C.
The second is: metakaolin decomposes into amorphous Al2O3 and amorphous SiO2 when the temperature continues to rise (about 900~960°C); when the temperature rises to 1100~1200°C and higher, mullite and Cristobalite formed successively.
(2) Montmorillonite family. Smectite group minerals include beidellite and saponite. The chemical formula of montmorillonite is Al2O3 • 4SiO2•nH2O montmorillonite belongs to monoclinic crystal system, often in the form of earthy aphanitic block. Under the electron microscope, it is microcrystalline cotton floc, cloud or particulate. The appearance is white, rose red, sometimes light-colored and dull. The hardness is 1, and the density is about 2g/cm3. After adding water, the volume expands and becomes a jelly.
(3) Others. Water mica, gibbsite, etc.
2. Impurity minerals. The impurity minerals in refractory clay mainly include quartz and iron compounds; carbonates and sulfates of calcium and magnesium; rutile, mica residues and organic substances.
(1) Quartz is the main impurity in refractory clay. It is a barren material that exists in clay in the form of particles or large particles. When a large number of quartz particles exist, it will have an adverse effect on the plasticity and bonding of refractory clay, but it can reduce the drying and firing shrinkage of bricks. In addition, when it reacts with other impurities at high temperature, it will reduce the refractoriness of refractory clay.
(2) Iron compounds are one of the harmful substances ubiquitous in clay. No matter which mineral form it exists in, it will have adverse effects on refractory products. It is the root cause of defects such as dark spots, caves, and swelling of products. Because it acts as a flux at high temperature, thereby reducing the refractory performance of the product, the content of iron in refractory clay should be strictly controlled. The introduction of iron in the process should be minimized or measures should be taken to remove it.
(3) Calcium and magnesium carbonates and sulfates, mainly minerals such as calcite, dolomite, magnesite and gypsum. They mostly exist in fine dispersion or nodules, and their content is generally not much, but they are strong flux substances. When clay clinker or products contain free calcium oxide, when it absorbs water and pulverizes, it will cause cracks or voids in the product.
(4) Rutile introduces titanium minerals into refractory clay. In addition to rutile, there are anatase and brookite. Most of these minerals exist in a finely dispersed state. It is a strong flux. It has obvious adverse effects on the high temperature performance of the product - refractoriness and load softening temperature.
(5) The organic matter in refractory clay makes its appearance dark gray, gray black or black. It burns and escapes during the middle temperature stage of clay heating, which increases the porosity and shrinkage of firing. Refractory clay containing more organic substances has a dark appearance, but it may be off-white or white after calcination. The plasticity and bonding of this clay are generally good.
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