Refractory is a material that can resist heat, pressure, or chemical corrosion and decomposition, and maintain its strength and shape at high temperatures. The main raw materials used to produce refractories are usually oxides of silicon, aluminum, magnesium, calcium, and zirconium. There are some non-oxide refractory materials, such as carbides, nitrides, borides, silicates, and graphite. Refractory materials must be able to withstand physical wear, high temperature, and chemical corrosion. Refractory materials are an important link between the thermal engineering system and the material to be heated. Performance affects both the quality of the product and the cost of heat treatment. Refractory materials are made from natural and artificial materials (usually non-metals) or a mixture of compounds and minerals, like as alumina, refractory clay, bauxite, chromite, dolomite, silicon carbide, and zirconia. Refractories come in various shapes and sizes. They can be pressed or molded for floors and walls, produced in interlocking shapes and wedges, or bent for installation in boilers and ladles. Some refractory parts are very small, with complex and delicate geometric shapes; others are in the form of prefabricated or molten cast blocks, and are heavy, weighing several tons. The production of refractory materials begins with the processing of raw materials. Raw material processing includes crushing and grinding, sorting by size, calcination, and drying of raw materials. The processed raw materials can then be dry blended with other minerals and chemicals for packaging and transportation as products. After the mixing process, the raw materials are formed into the desired shape. This process usually occurs under moist or humid conditions. Once the refractory material is formed, the material is fired. Firing includes heating the refractory material in a continuous or batch tunnel kiln to make a ceramic bond. This process makes the raw material fire-resistant. The final processing stage consists of milling, grinding, and sandblasting of the finished product. For some products, the final processing may also include tar and bitumen impregnation and product packaging.
Refractory materials can be manufactured by a variety of methods, including
dry pressing processes
fused cast process
hand moulding process
1. Dry Pressing Processes
This process is suitable for forming simple solid shapes. Especially suitable for clay with very low plasticity. The clay is mixed with a minimum amount of water and then pressed into the steel mould under the pressure of a hydraulic or compressed air cylinder. Because the dry pressing process is very simple and the equipment cost is low, it is the most widely used ceramic mass forming process.
The manufacturing process includes six general steps:
extraction and storage of raw materials
preparation of raw materials
firing and cooling
Remoulding and storage of finished products.
2.Fused Cast Products
Fused casting involves melting refractory materials in an electric furnace, then casting and annealing, and treating them with oxygen in the molten state to make the components in the most oxidised state. This method minimizes the exudation of the refractory glass matrix during use. The raw materials can be oxidized by heat treatment and then melted to reduce the oxygen required to oxidize the molten refractory. Obtain small or large high-density shapes. Where appropriate, finishing is done by grinding with diamond tools.
Some special-shaped refractories are usually hand-formed, and it is expected that their properties will be slightly different. The hand molding process produces low-strength, low-density refractory materials.
The formed refractory material is manufactured by firing or chemical bonding methods. The fired refractory material is formed by heating the refractory material to a high temperature in a kiln to form a ceramic bond. This process makes the raw material fire-resistant. Chemically bonded refractory bricks are formed with the help of selected additives that solidify at room temperature and provide structural integrity without the need for high-temperature sintering. By eliminating the need for high-temperature processing, significant energy savings can be achieved. In addition, many methods of changing chemical bonds can develop new compositions to withstand the harsh environments found in many industrial processes.
Unformed refractories, also called monolithic refractories, do not have any specific form. Unformed refractory materials are manufactured and sold in granular or plastic form or in the form of spray mixtures. Therefore, they can be used as maintenance patch materials. Common unformed modifications include monolithic plastic, ramming, and gun dough, molds, mortar, and dry vibrating cement. They are manufactured in various ways.
Machines used in the manufacturing process
Various machines use to produce refractory materials:
1. Mixing/kneading machine
There are two types of Mixing and kneading machine:
Fixed vessel and driven Vessel: The mixing homogenizes two or more bulk materials, and the kneader forms a uniform coating. The mixing and kneading machine is equipped with mixing blades or grinding wheels. Heating, cooling, or degassing equipment can also be applied to the container. Mixing and kneading machines are used to make shaped and unshaped refractory materials. However, the formed refractories are not processed further.
Refractory presses are divided into three categories:
impact and static presses
cold isostatic presses
The choice between the three sets of presses largely depends on the type of raw materials used.
Static and impact presses
Impact and static presses are usually equipped with vacuum degassing machines. The impact press has a higher maximum allowable pressure than the static press. However, static presses are increasingly used in the production of complex refractory materials, such as immersion nozzles and lids, and the production of industrial ceramics. The bricks formed by static pressure are flat, uniform, and dense.
The vibration press is divided into two types:
Air cylinder type and hydraulic cylinder type. An air cylinder-type vibrator is installed on the worktable, and the air cylinder compacts the material. The hydraulic vibration press has a built-in hydraulic pulse generator coupled with the pressure block, and the hydraulic cylinder compacts the material. Vibratory presses are usually used to compress refractory materials with complex shapes.
Cold Isostatic Press (CIP)
CIP is a molding device that provides uniform hydrostatic pressure on the entire surface of a powder-filled rubber mold. This method, also known as hydrostatic pressing or rubber pressing, is a material processing technique in which high fluid pressure is applied to a portion of the powder at room temperature to press it into a predetermined shape. The power part is integrated into a dense and compact form. Water or oil is often used as a pressing medium. CIP is based on the wet bag method (place the mold in a pressurized liquid) or the dry bag method (the mold does not touch the pressurized liquid). High-pressure forming provides uniform density, thereby reducing internal stress, eliminating cracks and delamination, enabling the manufacture of complex shapes and pressing multiple shapes at the same time.
The refractory material is fired to improve the fire resistance of the material. Unfired refractories are heat-treated to produce thermally stable refractories or crystals. Three types of kilns are used in industry:
In the tunnel kiln, refractory products continuously pass through the preheating, firing, and cooling zones. The combustion gas from the firing zone is usually used to preheat the refractory. Heat can be recovered from refractory materials heated by cooling and reused as combustion air.
About 80% of shaped refractories are fired in tunnel kilns.
Round Periodic Kilns
Round periodic kilns are usually used to firing silica bricks. These kilns can be used to fire large refractory products that cannot be fired in tunnel kilns and can easily adapt to changes in production.
The design of the shuttle kiln is similar to the firing zone of a tunnel kiln. Shuttle kilns effectively store heat and are used to burn clay and specialty bricks