Basic refractories were so named because they exhibit resistance to corrosive reactions with chemically basic slags, dusts and fumes at elevated temperatures. Some classes of basic refractories have been developed that exhibit excellent resistance to rather acidic slags.
Broadly speaking, basic refractories generally fall into one of five compositional areas:
1. Products based on deadburned magnesite or magnesia.
2. Products based on deadburned magnesite or magnesia in combination with chrome-containing materials such as chrome ore.
3. Deadburned magnesite or magnesia in combination with spinel.
4. Deadburned magnesite or magnesia in combination with carbon.
5. Dolomitic products.
The principal raw materials used in the production of basic refractories are dead-burned and fused magnesites, dead-burned dolomite, chrome ore, spinel and carbon. In recent years, the trend has shifted to developing highly engineered basic refractories. One result of this effort has been the development of technology to address specific wear mechanisms by employing special additives to the refractory composition. These additives generally constitute less than 6% of the total mix, although levels at 3% and below are probably the most common.
The term high-alumina brick refers to refractory brick having an alumina content of 47.5% or higher. High-alumina brick are classified by their alumina content according to the following ASTM convention. The 50%, 60%, 70% and 80% alumina classes contain their respective alumina contents with an allowable range of plus or minus 2.5%.
Mullite brick - predominantly contains the mineral phase mullite.
Chemically-bonded brick - usually phosphate-bonded brick in the 75% to 85% alumina range. An aluminum orthophosphate bond can be formed at relatively low temperatures.
Alumina-chrome brick - typically formed from very high purity, high-alumina materials and chromic oxide. At high temperatures, alumina and chromia form a solid solution which is highly refractory.
Alumina-carbon brick - high-alumina brick (usually bonded by a resin) containing a carbonaceous ingredient such as graphite.
Refractory fireclay consists essentially of hydrated aluminum silicates with minor proportions of other minerals. There are 5 standard classes of fireclay brick: superduty, high-duty, medium-duty, low-duty, and semi-silica. These classes cover the range from approximately 18% to 44% alumina, and from about 50% to 80% silica.
Superduty - these bricks have good strength and stability of volume at high temperatures and an alumina content of 40% to 44%. Some superduty brick have superior resistance to cracking or spalling when subjected to rapid changes of temperature.
High-duty - these bricks are used in large quantities and for a wide range of applications. Because of their greater resistance to thermal shock, high-duty fireclay brick can often be used with better economy than medium-duty brick for the linings of furnaces operated at moderate temperatures over long periods of time but subject to frequent shutdowns.
Medium-duty - these bricks are appropriate in applications where they are exposed to conditions of moderate severity. Medium-duty brick, within their serviceable temperature ranges, can withstand abrasion better than many brick of the high-duty class.
Low-duty - these brick find applications as backing for brick with higher refractoriness, and for other service where relatively moderate temperatures prevail.
Silica refractories are well adapted to high-temperature service because of their high refractoriness, high mechanical strength and rigidity at temperatures almost up to their melting points, as well as their ability to resist the action of dusts, fumes, and acid slags. Silica brick are divided into Type A and Type B based on the brick's flux factor.
Monolithic or monolith-forming refractories are special mixes or blends of dry granular or cohesive plastic materials used to form virtually joint-free linings.
Types of Monolithic Refractories
Important properties of refractories which can be determined most readily are chemical composition, bulk density, apparent porosity, apparent specific gravity and strength at atmospheric temperatures. These properties are often among those which are used as controls in the manufacturing and quality control process. The chemical composition serves as a basic for classification of refractories. The density, porosity and strengths of fired products are influenced by many factors. Among these are type and quality of the raw materials, the size and "fit" of the particles, moisture content at the time of pressing, pressure of pressing, temperature and duration of firing, kiln atmosphere and the rate of cooling.