Alumina and Alumina Refractories


Alumina and Alumina Refractories

Alumina (Al2O3) refractories are the part of alumina- silica (SiO2) group of refractories and belongs to the SiO2 -Al2O3 phase equilibrium system as shown in diagram at Fig 1. They differs from fire clay refractories in term of Al2O3 content and normally have Al2O3 content of more than 45 %. The raw material base for these refractories are different than the fire clay bricks.

Silica Alumina phase diagram

Fig 1 SiO2 – Al2O3 phase diagram

As seen in the diagram, refractoriness increases with the increase in the Al2O3 content. The eutectic at 1595 deg C has a composition of 94.5 % SiO2 and 5.5 % Al2O3. As the Al2O3 content is increased, the melting point of the refractory increases to a maximum of 2054 deg C which is the melting point of pure corundum. The only stable compound in the system is mullite, which has a defective space lattice and decomposes into corundum and liquid phase at around 1840 deg C.

The classification of Al2O3-SiO2 refractories as per the Al2O3–SiO2 phase equilibrium diagram is given in Tab 1.

Tab 1 Classification of Al2O3-SiO2 refractories as per the Al2O3–SiO2 phase equilibrium diagram
Range of Al2O3  Phases as per common terminology General performance of refractories in conditions of the absence of slag corrosion or alkali attack
Al2O3 less than 50 % Fireclay (Chamotte); Phases on phase diagram are mullite and glass; can contain free SiO2 Normally made from 100 % fireclay, Highest quality grades (super duty bricks) usable to about 1600 deg C,  Usually contain 38 % to 42 % Al2O3 and are based on fireclay minerals
Al2O3 50 % or 60 % Sillimanite, andalusite, or kyanite; Phases on phase diagram are mullite as major phase and glass as minor phase; can contain free SiO2 These refractories cannot be made from 100 % clay since Al203 content of clays are lower, Made from minerals containing 60 % Al2O3 and contain some fireclay; Can be made with bauxite and clay; Can be used for temperatures greater than 1700 deg C
Al2O3 70 %; Mullite Phases on phase diagram are  mullite as a major phase; Refractories made with bauxite contain corundum, mullite, and glass Made either from ‘bauxitic clay’ or calcined bauxite and clay; Can be used for temperatures greater than 1750 deg C
Al2O3 80 % and 85 % Bauxite; Phases on phase diagram are corundum as the major phase along with minor quantity of mullite, and glass Made from calcined bauxite
Al2O3 90 % Phases on phase diagram are corundum as the major phase  with a substantially minor amount of mullite and glass. Fused Al203 is used for bricks having contact with liquid steel; Normally contain fused Al2O3 for enhanced abrasion resistance Made from tabular and/or fused synthetic alumina aggregates, Can be used for temperatures greater than 1800 deg C

 

History of alumina refractories

During the first half of 1940s major development of Al2O3 refractories took place when the use of Al2O3 – SiO2 bricks based on bauxite started as an alternative for basic refractories. The development of the Bayer process for manufacturing aluminum brought about the manufacture of calcined and tabular

alumina—generating a new line of high Al2O3 refractories. Processing technology in grinding, separation, mixing, pressing, and firing improved dramatically in the latter half of 1940s  giving a technology edge in the bricks manufacture. Developments with refractory castable eventually caused Al2O3 – SiO2 refractories consumption to peak which began a decline starting in the late 1970s and the early 1980s. Technical improvements in all types of refractory products caused a reduction in refractory consumption rates. The end result has been a decline in production of Al2O3 -SiO2 refractories.

Production of alumina refractories

 For the production of high Al2O3 refractories, both natural raw materials (such as kyanite, sillimanite, andalusite, and bauxite etc.) as well as synthetic materials (sintered mullite, fused mullite, calcined alumina, sintered corundum, and fused corundum etc,) are used.

The minerals kyanite, sillimanite and andalusite have the same chemical composition according to the general formula Al2O3.SiO2. Their crystal structure differs widely according to their formation and origin, and so do the properties dependent upon it e.g. density. During firing these minerals change into mullite and a liquid phase, although this transformation occurs at different temperatures. The transformation of kyanite and andalusite already starts at about 1330 deg C, whereas sillimanite begins to form mullite in appreciable quantities only above 1550 deg C. Depending on the starting material, this mullite formation leads to varying firing expansion. Due to its high density of 3.5 to 3.6 gm/cc, kyanite increases in volume by about 15 %  on firing and must be calcined before use. The expansion of sillimanite and andalusite is only about 5-8 volume percent.

Bauxite is a rock which consists largely of alumina hydrates and usually contain impurities such as iron and titanium oxides. Only bauxite with a high Al2O3 content and low iron oxide content is suitable for refractory purposes. Since bauxites give away their water during heating accompanied by a marked reduction in volume, they are generally sintered. During this sintering, corundum and mullite are formed, together with a small amount of liquid phase and low melting titanates. Despite the presence of low melting phase, bauxite can only be densified to a compact sinter at high temperatures.

Fused corundum is preferably melted from bauxite in electric arc furnaces. Very pure grades are manufactured from calcined Al2O3. Fused Al2O3 is slow to react and for this reason the more reactive sintered corundum is now used more often. This is produced from finely ground Al2O3 by sintering below its melting point. As a result of this sintering process  the physical properties of the refractory material are excellent and in particular, a uniform crystal bond is obtained in the brick.

Mullite is also  produced by sintering or fusing; the initial materials are clay or kaolin which are enriched to the mullite composition by adding calcined Al2O3. In order to obtain sintered mullite with little glassy phase, raw materials low in fluxing agents and, above all, low in alkalis are used.

Al2O3-SiO2 refractories are manufactured from a blend of sized raw material aggregates and clays by mixing, forming, drying, and firing. These are traditional processes in manufacturing any ceramic product. Since refractories are used at elevated temperatures, they are particularly sensitive to contamination and particle size segregation. By minimizing contamination and particle size segregation refractories  with a smaller variance in physical properties can be produced.

Properties of refractories containing with 50 % and 60 % of Al2O3

Refractories containing 50 % and 60 % Al2O3 exhibit improved refractoriness over fireclay products. There are two fundamental mineral mixtures used in the production of these refractories, and the physical properties of the refractories depend, in part, on which mineral mixture was used in the manufacture.

The most straightforward way to produce these refractories is to use with 50 % or 60% Al2O3 aggregates (i.e., bauxitic kaolin or andalusite). Another way to produce these refractories is to use a mixture of bauxite and fireclay. This latter method is called the bauxite dilution method since the bauxite (at 88 % Al2O3) is diluted with calcined fireclay and raw clay (contains around 40 % Al2O3) to produce the required Al2O3 content. Properties of the refractories produced by bauxite dilution method are generally inferior.

It is important to note that refractories containing bauxite or andalusite typically exhibit high reheat expansion while clay refractories do not. In refractories containing andalusite and also containing clay, this tendency for high reheat expansion may not be observed. Thus, there is a fundamental difference in the refractories within the same class with respect to permanent expansion characteristics. In linings requiring the extreme tightness (as in rotary kiln applications) the reheat expansion is extremely important in good lining life.

By contrast, high reheat expansion may be associated with high spalling tendency, i.e., low spalling resistance. In this regard, refractories produced from bauxitic kaolin, i.e., ‘clay base’, may have superior spalling resistance. This is because of their finer texture, namely smaller average pore size, and due to the absence of permanent expansion reactions on heating.

The TiO2 content of refractories may indicate the fact that they contain calcined bauxite aggregate (if around 2.5 %). Bauxite can also be recognized on a broken or saw-cut surface as a gray-appearing aggregate to the naked eye.

Properties of refractories containing with 70 % of Al2O3

Refractories containing 70 % of Al2O3 became a workhorse in industrial furnaces since second half of 1940s because of their high use, high duty ratings, and durability in many processes where slag corrosion or other reactions take place. These refractories can be produced either from bauxitic clays having 70 % Al2O3 or by using appropriate mixtures of calcined bauxite (88 % Al2O3) and fireclay (around 40 % Al2O3). As in the case of 60 % Al2O3 refractories, the mineral base of the firebrick makes a significant difference in physical properties and thermal response of 70 % Al2O3 refractories.

The refractories based on calcined bauxitic clays show much higher reheat expansion (PLC) and higher spalling loss than the refractories based on bauxitic clay. On the other hand, the refractories based on calcined bauxite may have superior performance where erosion resistance is required.

The microstructure examination s of the two types of 70 % Al2O3 refractories show that in the refractories made from calcined bauxitic clays, the coarse aggregate particles are surrounded by a finely textured matrix. The distribution of medium size particles around coarse aggregates is excellent. This microstructure suggests excellent spalling resistance. By contrast, the refractories made from calcined bauxite has a completely different appearance. The dark bauxite particles predominate the microstructure. A glassy matrix containing mullite surrounds all particles. A comparison of the microstructures  provides graphic evidence that the refractories of same Al2O3 percentage can have different reheat (PLC) properties.

 Properties of refractories containing with 80 % and 85 % of Al2O3

Refractories with 80 % and 85 % Al2O3 were originally developed for use in aluminum smelting and holding furnaces. It is rare that they find application in other types of furnaces. These refractories are based on calcined bauxite, as it is the closest mineral in Al2O3 content to their overall composition. The resistance to aluminum attack is, in part, due to the resistance of the bauxite to solution in molten aluminum and to salt fluxes that may cover the metal bath.

These refractories have not been successful in ferrous industry applications. The reason may be the relatively poor refractoriness of the bond phase (glass and mullite) holding together highly refractory calcined bauxite aggregate particles. In an aggressive slagging situation, the bauxite aggregate is eroded out of the refractory brick, and wear rates are usually unacceptably high.

 Properties of refractories containing with 90 % and 99 % of Al2O3

Refractories with 90 % and 99 % Al2O3 are amongst the highest strength and erosion resistant refractories. They are made from synthetic Al2O3 aggregates, and some types may contain fused Al2O3 for special erosion resistance. There are several distinct types of 90 % Al2O3 refractories. The fused Al2O3 provides very high erosion resistance to flowing liquid steel. The microstructure of this type of refractory shows fused alumina aggregate particles (white with rounded black pores) are surrounded by a gray matrix containing a lighter mullite phase. The type called as ‘tabular alumina—corundum matrix’ is made from coarse super calcined Al2O3 aggregates and reactive calcined Al2O3 fines to produce a ‘direct bonded’ microstructure where Al2O3 to Al2O3 bonding (corundum to corundum) predominates. This provides an obvious increase in hot modulus of rupture (HMOR). The microstructure of this type of refractories show tabular Al2O3 aggregate particle which resides, and connected to the matrix through bonds with smaller corundum crystals. There is a practical limit on Al2O3 content of about 96 % Al2O3 ( contains about 3.7 % SiO2) in refractory brick for the highest-temperature applications. At compositions of higher Al2O3 content, the products cannot be sintered in conventional gas fired kilns at sufficient temperatures to have good density and PLC (reheat) properties. While 99 % Al2O3 refractory bricks exist, they are primarily used for low temperature applications such as in chemical processes.