Aluminum and magnesium castables commonly used five types of binding agent
In aluminum-magnesium castables, there are five main types of commonly used binding agents:
01 Water Glass Binding System
Water glass consists of alkali metal silicates with good bonding strength. Depending on the type of alkali metal oxide, it is divided into sodium water glass (Na2O・nSiO2), potassium water glass (K2O・nSiO2) and potassium-sodium water glass (K・NaO・nSiO2). It is mainly dried naturally or dehydrated by heating to form a gel, which produces bonding strength.
02 Phosphoric acid and aluminum phosphate combined system
Industrial phosphoric acid has the molecular formula H3PO4・0.5H2O, prismatic crystals, soluble in water in any proportion. There are three types of phosphoric acid, the most stable of which is orthophosphoric acid, referred to as phosphoric acid (H3PO4). The binding mechanism of phosphoric acid is to combine with oxides in the material to form compounds to produce binding strength.
Aluminum phosphate can also be used as a refractory binding agent, which is usually produced by reacting phosphoric acid with aluminum hydroxide, and is divided into aluminum phosphate monohydrogen and aluminum phosphate dihydrogen.
Using high alumina bauxite clinker and electrofused spinel fines as the main raw materials and phosphoric acid as the binding agent, the spinel high alumina castables prepared have good thermal shock resistance and slag resistance. Phosphoric acid reacts with free MgO in spinel to produce magnesium dihydrogen phosphate, which then automatically polymerizes into magnesium phosphate, resulting in strength. This combination also increases the construction time and improves the hydration resistance of the castables.
MgO+2H3PO4=Mg(H3PO4)2+H2O (1)
nMg(H2PO4)→nMg·2nPO3+2nH2O (2)
03 MgO-SiO2-H2O binding system
This bonding system is a fine powder cohesive bond, which is commonly found in bauxite-based castables. The introduction of CaO into this system should be avoided, because the introduction of CaO will cause the system to generate a low melting point phase at high temperatures, which is detrimental to the high temperature performance of the material and has a significant effect. Finer SiO2 micropowders can react with water as follows:
SiO2+H2O=Si-OH++OH-(3)
The advantage of this system is that the material has high strength after medium temperature treatment: M-S-H contains less water of crystallization, which is conducive to fast baking and drying; SiO2 reacts with MgO to generate magnesium olivine at high temperature, which improves the high temperature performance of the material; and SiO2 improves the fluidity of the castables; the disadvantage is that the resistance to slag erosion is weak.
04 Hydrated Aluminum Oxide Binding
Among the many crystalline forms of Al2O3, only ρ-Al2O3 can be spontaneously hydrated at room temperature, and its bonding mechanism as a castable material is the hydration to form trihydroxyalumina and boehmite sols with the following reaction:
ρ-Al2O3+2H2O=AI(OH)3+AlOOH (4)
ρ-Al2O3 is an amorphous substance, and its internal Al-O disordered arrangement and shortage of valence make it more active with fast hydration reaction. At room temperature, the autocatalytic reaction determines that the hydration rate of ρ-Al2O3 becomes larger with the increase of temperature. However, the ρ-Al2O3 hydration reaction is more violent and difficult to control, resulting in a less stable material.
The study compares the performance differences between hydrated alumina-bonded and cement-bonded aluminum-magnesium castables, and the results show that the aluminum-magnesium castables with the addition of 3 wt% of hydrated alumina-bonded aluminum-magnesium are better in terms of slag resistance, permeability and thermal shock resistance compared to the cement-bonded castables.
The characteristics of different bonding systems for combining aluminum and magnesium castables were investigated, and it was shown that the ρ-Al2O3 formed around the ρ-Al2O3 at high temperatures produces a small shrinkage due to the dehydration of hydrated alumina, and due to this shrinkage the annular region formed has the effect of preventing crack extension and relieving the stress, which improves the thermal shock resistance of the material.
05 Aluminate Cement Binding System
At present, the most industrial applications of aluminum and magnesium ladle permeable brick and other castables are mostly combined with calcium aluminate cement. Aluminate cement as the binding agent of the castables at room temperature to form 2CaO-Al2O3・8H2O and Al (OH) 3 colloid, with a greater strength of the release; medium temperature treatment after the lower strength: 1400 ℃ above the temperature of the treatment to generate CA6, the material strength increased significantly; over the use of high temperatures are prone to produce structural spalling and limit the use.
Low cement refractory castables (LCC) and ultra-low cement refractory castables (ULCC) were studied to reduce the amount of cement. In traditional refractory castables, the cement addition is 10%-15%, while in ULCC the cement addition is only 2wt%-3wt%.
Aluminate cement refractory castables strength change mechanism shirts, its relative compressive strength (to 110 ℃ drying compressive strength for 100%) and the relationship between the heating temperature. As shown in Figure 1, it can be seen from the figure, the aluminate cement castables molding initial condensation, standard maintenance can be obtained high room temperature strength; drying strength is reduced, which is due to the hydration product 2CaOAl2O38H2O and AI (OH) 3 dehydration leads to.
The high-temperature compressive strength of aluminate cement-bonded refractory castables is characterized by the following: when heat-treated below 1000°C, its high-temperature compressive strength is not much different from the cold compressive strength; as the temperature rises, the liquid phase appears, which reduces the high-temperature strength of the material; and when the temperature continues to rise to 1350°C, its high-temperature compressive strength is only 2MPa.
After the material is heated at a temperature of about 300 ℃, the crystalline transformation is fast, and more free water is excluded, therefore, the relative strength is reduced by a larger amount, generally 18~25%. Between 300 ~ 900 ℃, the material in the free water and the vast majority of the combined water burning loss, significant increase in apparent porosity. At temperatures between 900 and 1200°C, a chemical reaction occurs, generating CA and CA2, forming a new mineral structure and volume contraction. At the same time, due to the lower temperature, lower sintering, the internal structure of the material is loose, and the strength is significantly reduced, about half of the strength of the material after drying. 1200 ℃ treated specimens, observed under the microscope, the organizational structure is composed of blocks separated from each other, the size difference is not large, so the strength is the lowest. After heating at 1300~1400°C, the strength rebounded and increased substantially, which was attributed to the formation of the stabilization product CA6 and the realization of ceramic bonding .
The results of the study show that: at 1300 ℃, corundum and diatomic calcium aluminate are the main crystalline phase of the synthesized material and the CA2 + m2O3 → CA6 reaction begins to take place; at 1400 ℃, CA2 is reduced in large quantities, and a large number of CA6 is generated; when the temperature continues to rise to 1500 ℃, the reaction is over, and the main crystalline phase of the material is corundum and CA6.
Due to the anisotropy of CA6 grain growth, its crystalline shape is mostly flaky or needle-like structure, similar to the thermal expansion coefficient of alumina, and is highly compatible with alumina, adding it to alumina-based ceramics or coatings and other materials, there is a significant increase in the mechanical properties of the material.CA6 is a product of chemical reaction in the matrix portion of the material at high temperatures, and its flaky crystalline shape can be intersected with the spinel grains to form a similar network structure, which can effectively improve the strength of the material.
Zinfon Refractory Technology Co.,Ltd
We are a refractory material supplier integrating R&D,production,construction,warehousing and commerce.
We are offering various magnesia and alumina refractories including both shaped and unshaped products, raw materials and related chemical products.
We are certified to ISO9001, ISO14001, ISO45001 and other national and local certifications as follows: