About Bauxite

- Dec 03, 2018 -

Bauxite

Introduction

Chemical formula Al2O3.H2O, Al2O3.3H2O and a small amount of Fe2O3.SiO2, an alumina ore. It is often called "iron-bauxite" because it contains yellow iron to red iron. It is the main raw material for aluminum smelting.
Alumina is a colloidal mixture of three aluminum hydroxides in different ratios. According to its use, it is divided into metallurgical grade, chemical grade, refractory grade, grinding grade, cement grade and so on.
Used to make refractory materials, this bauxite is called refractory grade bauxite.
And the alumina clinker with a proper ratio of AL2O3/Fe2O3 and AL2O3/SiO2 is used for melting alumina.
Bauxite has a strong ability to resist acid and alkaline slag, and has high-temperature strength. It is widely used in steel, non-ferrous and other industries, but its resistance to rapid cooling and heat, high temperature and volume stability is not good, resulting in deformation and spalling of materials. Reduce the life of the material. At present, the method of adding synthetic mullite is basically adopted in China to overcome its shortcomings. However, the price of synthetic mullite is high, which limits its use. Bauxite has a wide range of uses in the refractory and aluminum industries. The most basic use of bauxite is in the metallurgical sector of the production of aluminum, which accounts for 85% of the total consumption of bauxite. The remaining bauxite consumption can be divided into two categories: special alumina and "other". The special alumina in the alumina is used to produce nitrogen-alkali aluminum, calcined alumina and activated alumina, which accounts for 10% of the total consumption of bauxite. The “other” category refers to alumina used directly for mining such as refractory materials, cement and grinding industries. In terms of market size and value, refractory bauxite plays an important role in this non-metallurgical market end use.

High alumina bauxite

The main minerals of high alumina bauxite in China are diaspore (monohydrate), boehmite (monohydrate), kaolin and pyrophyllite. It often contains impurities such as kaolinite, iron minerals and titanium minerals. It can be divided into diaspore-kaolinite type (DK), boehmite-kaolinite type (BK) and diaspore-pariec type (DP), etc., mostly in DK type.
Sintered DK type bauxite clinker is generally classified into special grade, I, IIA, IIB, etc. according to its A12O3 content. The sinterability of DK type high alumina bauxite is closely related to its A12O3 content; the composition of the bauxite after burning is closer to the composition of mullite, the more difficult it is to sinter. The two main factors affecting the sintering of such bauxite are secondary mullite and liquid phase action. The main chemical components of bauxite are Al2O3, SiO2, Fe2O3, TiO2, which accounts for about 95% of the total composition. The minor components are CaO, MgO, K2O, Na2O, MnO2, and organic and trace components Ga, Ge and so on. Tri-alumina is the main chemical component of bauxite. It exists mainly in the form of aluminum hydroxide, followed by aluminosilicate, a clay mineral. Silica is generally in the form of clay minerals and is rarely free. quartz. The minerals of bauxite raw materials in China are mainly diaspore minerals, a small amount of boehmite, and gibbsite minerals in some areas. Generally, the content of aluminum oxide in alumina is generally between 45% and 80%. The content of aluminum and silicon in bauxite is inversely related. The content of ferric oxide is mostly between 1% and 1.5%. The content of calcium and magnesium is lower. The content of potassium and sodium is generally less than 1%. The content of TiO2 is generally 2%~4%, the amount of buckling reduction is about 14%.

Sintering of alumina

The high-temperature mechanical properties of sintered alumina materials depend on the glass effect and the crystallization effect. The closer the A12O3 content is to 70%, the better the high-temperature strength and the smaller the creep. The introduction of appropriate amounts of MgO and zircon is beneficial to high-temperature mechanical properties. However, K2O, CaO, TiO2 and Fe2O3 are harmful impurities and should be minimized. The heating change of bauxite is divided into three stages: decomposition stage, secondary mullite stage and recrystallization stage.
Decomposition stage: At this stage, the boehmite and kaolinite in the bauxite began to dehydrate at 400 degrees Celsius, and the reaction was intense at 450 to 6 Baidu, and completed at 700 to 800 °C. After the dehydrated boehmite forms the corundum illusion, the shape of the original diaspore remains, but the edge is blurred and gradually transforms into corundum at high temperature. The high alumina stone is dehydrated to form metakaolinite, and above 950 °C, the metakaolin is converted into mullite and amorphous SiO2. The latter is converted to cristobalite at high temperatures.
Secondary mullite stage: Above 1200 ° C, the free silica decomposed from the corundum and kaolinite formed by the dehydration of the boehmite continues to react to form mullite. In the second mullite petrochemical, about 10% of the volume expansion occurs. At the same time, below 1300~1400 °C, impurities such as iron and titanium in bauxite and other impurities form a liquid phase with aluminum silicon, and iron and titanium enter the mullite lattice. Solid solution. The formation of the liquid phase contributes to the progress of the secondary mullite and also prepares conditions for recrystallization sintering.
Recrystallization stage: In the secondary mullite stage, some degree of sintering has begun due to the formation of the liquid phase, but the process is very slow, and the recrystallization sintering begins to proceed rapidly only after the secondary mullite is completed. Above 1400 ° C ~ 1500 ° C, corundum and mullite crystals grow up due to the action of the liquid phase, about 100 ~ 300 microns at 1500 ° C, and 60 and 90 microns to 1700 ° C, respectively. The high-temperature mechanical properties of sintered alumina materials depend on the glass effect and the crystallization effect. Since natural raw materials generally contain more impurities, especially the increase of alkali metal oxides not only increases the glass phase content, but also causes decomposition of mullite at high temperatures. Therefore, in the synthesis of mullite, in order to reduce the adverse effects of impurities, so that the synthetic material obtains a high mullite content, high-purity raw materials should be used as much as possible to reduce the amount of impurities. According to the characteristics of high-alumina bauxite resources in China, more than 70% are medium-low grade ore, and there are high impurity content, uneven mineral distribution and difficult sintering, which leads to high utilization. The closer the A12O3 content is to 70%, the better the high-temperature strength and the smaller the creep. The introduction of appropriate amounts of MgO and zircon is beneficial to high-temperature mechanical properties. However, K2O, CaO, TiO2 and Fe2O3 are harmful impurities and should be minimized.


Impurity

Glassy:

The vitreous is a precipitate of mullite formed by the kaolin in the bauxite during the calcination process, and its melting point is 730 ° C, when there are K 2 O, Na 2 O, Fe 2 O 3 . When present, it is easy to form a ternary or quaternary low-melting eutectic, which causes the refractoriness to decrease at a high speed, so it needs to be strictly controlled during high-temperature sintering, but a suitable glass phase can improve the thermal shock resistance of the material and the material. The degree of sintering.
Titanium mineral
There are various forms in the sintered high-alumina bauxite, such as a solid solution of TiO2, Ti2O3, TiN, TiC, TiO and corundum. Titanium oxide is very active in bauxite. It has not been found in the study to have any bad influence on the finished product. On the contrary, it has certain effect on the thermal expansion of corundum and the use of drilling salt as an inoculant to refine the grain size of the brick surface. Good effect, so there is no limit to the TiO2 content.

Iron mineral

The iron minerals present in the sintered bauxite are mainly Fe2O3, Fe3O4, iron spinel, etc. The melting point depends on the impurities contained in it, and can be converted into α-Fe2O3 as a matrix mineral under high-temperature oxidation atmosphere. At around 1560 ° C, iron minerals are still harmful impurities of low melting point and need to be controlled. Some studies have suggested that the Fe2O3 content control in the range of 1.2% to 1.5% has little effect on the high-temperature performance of the surface. Strong magnetic separation and pickling processes are used in the production to remove iron.

Other minerals

Alkali and alkaline earth metal oxides such as Na2O, K2O, CaO, MgO in bauxite can be combined with SiO2 and A12O3. Combined with the formation of multiple low-melting materials such as stone, nepheline, etc., this can be strictly controlled according to the use requirements. Especially in the ore dressing process, it is required to strictly remove the "red slag" and "yellow slag" in the sintered bauxite.

High alumina smelting bauxite

Strengthening smelting and blast furnace longevity projects have become the main work of modern blast furnace production. In terms of longevity of blast furnaces, the daily use of gun mud for blast furnaces plays a very good role in the maintenance of blast furnace hearths, and the intensified smelting and longevity of blast furnaces have placed increasingly stringent requirements on the blast furnace mud.
In the requirements, the protection of the hearth is of great significance for the longevity of the blast furnace. For this reason, the mud must be able to maintain a stable and sufficient depth of the iron mouth while smoothing the iron to reduce the carbon brick around the iron outlet. Damaged to achieve the purpose of protecting the hearth. At present, the gunner supplier can improve the performance of the mud to meet the requirements of blast furnace tapping by improving the raw material grade and the introduction of high-efficiency admixture. However, the application of these technical measures has increased the technical cost of high-performance gun mud and reduced its cost performance. The chemical composition and mineral phase of the homogenized bauxite products are relatively stable. In view of this, the high-priced brown corundum in the high-performance anhydrous gunpowder is replaced by homogenized bauxite to improve its cost performance.