The chemical composition of high alumina bauxite is mainly Al2O3, SiO2, Fe2O3, TiO2, CaO, MgO, K2O, Na2O, etc. After high temperature sintering, most of the Al2O3, Fe2O3 (w(Al2O3)>95wt%, w( Fe2O3)>80wt%) into the crystalline phase; most of SiO2, TiO2 into the crystalline phase, but with the change of Al2O3 content, the proportion of SiO2, TiO2 into the crystalline phase fluctuates greatly, w(SiO2) in 44~92wt % fluctuates, w(TiO2) fluctuates between 62 and 94wt%; most of CaO and MgO enter the glass phase, and almost all K2O and Na2O enter the glass phase, see Figure 1 for details. The influence of impurities on bauxite clinker is as follows:
1) TiO2
1) TiO2 is an impurity with a relatively high content in high alumina bauxite, usually with a content of 2~4wt%. In grade II high alumina bauxite, most of the TiO2 is dissolved in the mullite lattice, and a small part of TiO2 enters the glass. TiO2 is not good for the sintering of grade II high alumina bauxite, but it is beneficial to the high temperature mechanical properties of the product. ; In the special grade, I grade and III grade bauxite, TiO2 enters the glass more, which makes the liquid phase increase and the liquid phase viscosity decrease, which promotes the sintering process, but it is not good for the high temperature performance of the product.
The fate of TiO2 in the high bauxite sintering process: - Part of TiO2 enters the corundum and mullite lattice to form a solid solution; a part of TiO2 reacts with Al2O3 to form Al2O3·TiO2; the rest of TiO2 enters the glass phase and reduces the viscosity. The last two directions are detrimental to the high-temperature mechanical properties of the product. The formation of solid solution and the reduction of liquid viscosity make TiO2 significantly reduce the sintering temperature of high alumina bauxite clinker. About every 0.4wt% TiO2 increase, the sintering temperature can be reduced by 20℃; TiO2 is controlled at 2.3~2.5wt% Within the range, the total amount of crystalline phase in the porcelain can be the highest, and the solid solubility of Fe2O3 in the crystalline phase can be maximized, so that more SiO2 can enter the glass phase.
2) Fe2O3
Fe2O3 is a harmful impurity in high alumina bauxite. The content of corundum phase and mullite phase is not greatly affected by the content of Fe2O3, but the increase of Fe2O3 content will increase the content of glass phase, and the effect is relatively obvious. For every 1wt% increase in Fe2O3 content, it will increase by about 0.7wt% % glassy phase. During the sintering process of high alumina bauxite, most of Fe2O3 enters the main crystal phase (corundum and mullite) to form a solid solution, a small part enters the glass phase, and a part forms a solid solution with Al2O3 and TiO2 (Al2O3·TiO2, Fe2O3·TiO2) , the presence of Fe2O3 is beneficial to promote the sintering process.
Compared with TiO2, Fe2O3 has a lower ability to reduce the sintering temperature during the sintering process. This is because the amount of liquid phase increases very little with the increase of Fe2O3 content. The effect of Fe2O3 to reduce the sintering temperature depends on the sintering atmosphere. Fe2O3 in the atmosphere has little effect on reducing the sintering temperature, and obviously narrows the sintering range; in the reducing atmosphere, Fe2O3 can significantly reduce the sintering temperature, because the appearance of FeO will produce more liquid phase.
3) K2O and Na2O
K2O is a harmful impurity oxide of high alumina bauxite, and the content of main crystal phase and glass phase changes greatly with the increase of K2O content. For every increase of 1wt% K2O, the glass phase increases by about 7wt%, the content of mullite phase decreases by 15~20wt%, and the content of corundum phase increases by 8~13wt%. Figure 2 is a schematic diagram of the effect of K2O on the phase composition of sintered high alumina bauxite.
The increase of K2O content will destroy the original network structure and increase the harmful impurities in the glass phase, which is detrimental to the high-temperature mechanical properties of high alumina bauxite. The role of K2O in the sintering process of high alumina bauxite is first to hinder the process of secondary mulliteization, and secondly to decompose the formed mullite (mainly primary mullite).
The influence of K2O content on the chemical composition of materials mainly includes the following aspects:
(a) Crystalline phase. With the increase of K2O content in high-alumina bauxite, SiO2 in the high-alumina bauxite crystal phase decreases rapidly after firing, the content of Al2O3 increases, and TiO2 decreases slightly, that is, it has the greatest impact on SiO2. For high bauxite such as I, there is a turning point when K2O is 1.75wt%: when K2O increases from 0.01wt% to 1.75wt%, the proportion of SiO2 entering the crystalline phase decreases from 87.5wt% to 16wt%, that is, SiO2 enters the crystalline phase The rate of decrease decreases rapidly; as K2O continues to increase, the rate of decrease becomes slower. For II bauxite, when K2O increases from 0.01wt% to 3.51wt%, the proportion of SiO2 entering the crystalline phase decreases from 84wt% to 11wt%, which is basically a linear relationship. That is to say, regardless of high alumina bauxite such as I or II, when the K2O content is less than 1.75wt%, all K2O enters the glass phase after sintering; and when the K2O content exceeds 1.75wt%, some K2O enters the crystalline phase. Such as high bauxite such as II, when the K2O content is 3.51wt%, about 25wt% of K2O enters the crystalline phase.
(b) Glass phase. With the increase of K2O content in high-alumina bauxite, SiO2 and K2O in high-alumina bauxite glass phase increase after firing, and the remaining oxides decrease.
Na2O is similar to K2O and is also a very harmful impurity.
4) CaO
The CaO content in bauxite in China is generally 0.2wt%, and in some areas it reaches more than 2wt%. With the increase of CaO content, mullite showed a sharp decrease, and anorthite was generated and increased. At this stage, as the amount of CaO increases, the amount of mullite produced decreases rapidly, and anorthite appears and gradually increases. Generally, for every 1wt% increase, mullite decreases by 9wt%, anorthite increases by 4~5wt%, corundum increases by 3~4wt%, and glass phase content increases by 0.7~0.8wt%. Until the CaO content exceeds a turning point (about 6 wt% CaO for I grade high bauxite, and about 10 wt% for II grade high bauxite CaO content), all mullite disappears, its network structure is destroyed, and the high melting point crystallization The direct bonding rate of the phase decreases significantly, resulting in a significant decrease in the performance of the material.
(5) MgO
The effect of MgO content on the phase composition of sintered high alumina bauxite is closely related to the Al2O3 content in the high alumina bauxite. For sintered high alumina bauxite such as I, for each increase of 1wt% MgO, corundum decreases by 2.5wt%, and mullite increases by 3.3wt%. This stage has little effect on material properties.
