5 Common Manufacturing Methods Of Refractory Materials

Refractories are manufactured by various methods. The common manufacturing methods include dry press process, fused cast, hand molded, formed and un-formed.

1) Dry press process
This process is ideally suitable to the formation of simple solid shapes. It is particularly suited to clays of very low plasticity. Clay is mixed with a minimal amount of water, then pressed into steel molds under pressures by hydraulic or compressed air rams. Because the dry press process is so simple and involves low capital equipment costs it is the most widely uased high –volume forming process for ceramics.

The manufacturing process has six general steps: 1) mining and storage of raw materials, 2) preparing raw materials, 3) forming the brick, 4) drying, 5) firing and cooling and 6) de-hacking and storing finished products.

2) Fused cast
Fused cast involves melting refractory material in a electric furnace followed by casting and annealing are treated with oxygen while in the molten state to place the constituents in the most highly oxidized state. This method minimizes the exudation of the glassy matrix of the refractory during service. The raw materials for the refractories may be oxidized before melting by heat treating to reduce the oxygen necessary for oxidizing the molten refractory. High density, small or large shapes are obtained. When appropriate, a finish is made by grinding with diamond tools.

3) Hand molded
The mold is made by hand. Hand molded refractories do not have the smooth surface of machine made brick. This method is especially well adapted to small

4) Formed
Firebrick is a common example of formed refractory. Formed refractories are manufactured by either fired or chemically bonded method.
Fired refractories is formed by heating the refractory material to high temperatures in a kiln to form a ceramic bond. This process gives the raw materials their refractory properties.
Chemically bonded refractory brick, also referred to as unfired brick, is formed with the aid of selected additives that set up at room temperature and provide structural integrity, eliminating the need for high-temperature sintering. It offers significant energy savings by eliminating the need for high-temperature processing. In addition, the many methods for modifying the chemical bond can develop new compositions to withstand a variety of severe environments encountered in many industrial processes.

5) Un-formed
Un-formed refractories, also called monolithic, do not comes in any specific form. Unformed refractories are made and marketed in granulated or plastic forms or as spray mixes. Thus, they can be used as patching materials for maintenance. Common unformed refactories include monolithic-plastic, ramming and gunning mass, castables, mortars, and dry vibrating cements. They are manufactured in various ways.

What Are The Causes Of Refractory Failures

Refractories are heat-resistant materials that constitute the linings for high-temperature furnaces and reactors and other processing units. In addition to being resistant to thermal stress and other physical phenomena induced by heat, refractories must also withstand physical wear and corrosion by chemical agents. Any failure of refractory could result in a great loss of production time.

The refractory material failure may caused by many different factors, such as chemical reaction and corrosion, spalling, material selection, plant operations, material storage, mixing, installation, curing, and drying. Only by understanding all aspects pertaining to the design and installation of the refractory material can one find the cause of the failure and help eliminate future failures.

The most common cause for failure of refractory is chemical reaction with the environment in which it is operating and chemical corrosion from molten slag and hot gas/molten salt. Chemical corrosion of a refractory is caused by slag attack at the refractory surface. The material selected must match the chemical environment that exists. For example, an acidic refractory should not be used in furnaces using basic fluxes, slag, etc. and vice-versa.

The porosity of refractory plays an important role in the chemical reaction. The more porous it is, the greater will be the depth to which the slag will penetrate and destroy the refractory. As the temperature increases, the rate of chemical reaction gradually increases. Sometimes, rise in temperature beyond the safe limit quickly brings about the destruction of the refractory. These chemical aspects are complementary to the engineering plant aspects and must be taken into consideration for a successful realization of the process.

Another important cause is spalling. It may be thermal, mechanical or structural. Thermal spalling may be due to unequal expansion or contraction caused by the difference in temperature at different parts. Mechanical spalling is mostly due to carelessness in loading the furnace or in the removal of materials from furnace, thereby damaging the refractory. Structural spalling takes place due to change in composition of the refractory because of reaction with slags, flux, etc. as a result its coefficient of expansion changes. Thus, different parts expand and contract to a different extent.

Improper material storage, mixing, installation, curing and drying will also cause refractory failure. Refractory material should always be stored in dry, well-ventilated conditions. Use fresh refractory materials and follow proper storage procedures to ensure that the refractory will not lose strength. Use potable water (suitable for drinking) for mixing. The use of the wrong type of water will hinder the ability of the refractory material to reach its proper strength. Using the right type of mixer, following proper mixing procedures, and staying within recommended pot life are other important installation factors. Using the wrong mixer or pneumatic gun could also affect the strength of the refractory material. Almost all refractory materials (except those that are phosphate bonded) must be cured prior to the drying process. Failure to properly cure a cement-bonded refractory material is the number one contributor to refractory failure and lack of longevity.

Problems with the quality of the refractory material itself is also an important reason for the failure of refracoties. A selection of the right refractories for a specific application is important.

What are Refractories?

Refractories are high temperature resistant materials. Refractory materials can retain their strength at high temperatures. They are used to make crucibles and are used in linings for high-temperature industrial furnaces, kilns and reactors and other processing units.

In addition to being resistant to thermal stress and other physical phenomena induced by heat, refractories must also resist abrasion wears and erosion by chemical agents. Refractory materials must be strong at high temperatures, resistant to thermal shock, chemically inert, and have low thermal conductivities and coefficients of expansion. Refractories are more heat resistant than metals and are required for heating applications above1000F(538C).

Refractories are produced from natural and synthetic materials, usually nonmetallic, or combinations of compounds and minerals such as alumina, fireclays, bauxite, chromite, dolomite, magnesite, silicon carbide, zirconia, and others. Aluminium oxide (Al2O3), Magnesium oxide (MgO) and Silicon oxide (SiO2) are the most important refractory materials, though fireclay is widely used as well. Zirconia is used when the material must withstand extremely high temperatures. Silicon carbide is another refractory material. It is very strong at high temperatures, but will burn in the presence of oxygen, if the protective silica coating comes off.

Refractories must be chosen according to the temperature inside the unit and the chemical nature of the material being processed. For example, carbon cannot be used when it will be in contact with oxygen, as it will burn. Acidic refractories cannot be used in a basic environment and basic refractories cannot be used in acidic environment because they will be eroded.

Zircon, fireclay and silica are acidic. They are generally not attacked or affected by acidic materials, but easily affected by basic materials. At high temperatures, acidic refractories may also react with limes and basic oxides. Dolomite and magnesite are basic. These are used on areas where slags and atmosphere are basic; they are stable to alkaline materials but could react with acids. Alumina, chromite, silicon carbide, carbon and mullite are neutral. These are used in areas where slags and atmosphere are either acidic or basic and are chemically stable to both acids and bases.

Refractories are made in varying combinations and shapes depending on their applications. Refractories are widely used in the blast furnaces, hot stoves, and open-hearth furnaces, cement kilns, glass furnaces, nonferrous metallurgical furnaces, ceramic kilns, steam boilers, and paper plants. Special types of refractories are also used in rockets, jets, and nuclear power plants.