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Refractories are a class of ceramic materials that are designed to protect the vessels that contain high temperature processes. To do that, they must withstand specific wear mechanisms at high temperatures and provide known insulating characteristics.
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Although there are variations in how wear mechanisms are described, generally five categories are sufficient:
Abrasion – the direct or sliding impact of dry material against the refractory.
Erosion – the washing of a liquid against the refractory.
Corrosion – a chemical reaction that changes the properties of the refractory.
Thermal Cycling – repeated changes in temperature that cause an accumulation of micro-cracks in the refractory. If the thermal cycling happens suddenly enough to break the refractory immediately, it is called thermal spalling.
Load – weight bearing in excess of the refractory compressive strength. Like most other physical properties, the compressive strength varies with temperature.
These wear mechanisms can occur in combinations, or can be progressive, one leading to another. In almost all cases, higher temperatures increase the susceptibility of the refractory material to any particular wear mechanism.
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A variety of minerals, chemicals and synthetic materials can be blended to make refractory products. These blends of raw materials are carefully selected to provide a combination of physical properties that address the known wear mechanisms and the typical operating temperatures inherent to the process. Finished refractory products can be in the form of brick, castables (refractory concrete), tempered plastics (like modeling clay), wet or dry mortars and dry granular blends called ramming mixes. Ceramic fiber products are also refractories.
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Finished refractory products like castables, plastics and ramming mixes that have no pre-defined shape can be used to form large structures with no inherent joints, essentially monoliths. The word monolith comes from the Greek word monotlithos which roughly translates to "consisting of a single stone”.
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Gunite refers to an application method for refractory monoliths. The method involves a specialized piece of machinery called a gunning machine or a gunite rig. The machine uses a high volume of air to convey dry or damp refractory material through a hose to a spray nozzle. In appearance this hose and nozzle will remind you of a fireman’s water hose, but both items are specially designed for use with refractory materials. Enough water is introduced at the nozzle to thoroughly wet the refractory and make it sticky enough to adhere to the surface of the vessel it will protect. Gunning operations can be run to repair or build refractory linings that are cold, but the greatest value of a gunite mix is for repair of refractory linings that are still hot.
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Shotcrete refers to an application method for refractory monoliths. You may have seen concrete being pumped through heavy duty hoses into bridge piers or road surfaces that might be in hard to reach areas. Heavy duty pumps can be used to transport refractory concretes too, providing a speed and accessibility advantage over traditional placement methods. Pumpable refractories are generally placed behind forms much the same as castable refractories are. The shotcrete method is an extension of pumping wherein the pumpable refractories are gelled and stiffened with chemical activators during placement. These activating chemicals are aspirated and injected into the pumped material by use of a specially designed nozzle just before the pumped refractory leaves the hose. In this way the material can be used to build a refractory lining without the use of forms. The pumping and shotcrete methods are generally the fastest way to install refractories with a controlled water addition.
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If you are using cement bonded monoliths cooler temperatures will slow the setting time and increase the possibility that weaker, less permeable cement bond phases can be created. This will result in less than optimal physical properties, and increase the chance of trapping steam during the dry out process, resulting in cracking of the refractory. Higher temperatures speed the setting reaction and reduce working time. The target temperatures for storage and installation of cement bonded products is usually considered to be between 70 and 80 degrees F for the best combination of working time, setting time and bond phase development. These temperature ranges should be monitored for the dry material being mixed, and for the ambient temperature during the curing process.
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Curing refers to the time and temperature needed to develop the intended bond phase. For air setting mortars, curing is simply the air drying process. For heat setting mortars, curing is the application of external heat to form the bond. For cement bonded products, curing is the reaction of the calcium aluminate cement with the added water. Curing is normally different than the dry out process.
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One way to organize or classify refractory products is by their chemical acid/base ratios. Basic refractories typically have a high proportion of basic content like lime (CaO) and magnesia (MgO). Conversely, “acid” refractories typically have high proportions of acidic content like silica (SiO2). Refractory products containing high proportions of minerals like alumina (Al2O3) are considered neutral. |
The first thing that must be understood is what wear mechanism, or combination of wear mechanisms must be endured by the refractory lining, and at what temperature. With this information at hand, contact your refractory manufacturer to discuss your process in detail. In addition to the expected operating conditions they will want to take a number of things into consideration, including your operating practices, the installation method that is most practical and the time frame in which you want to place the new refractory lining into service.
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You might be surprised. In general, any process operating at a temperature that exceeds the useful service temperature of the vessel construction must have a refractory lining. Refractory linings are used by many industries. Ferrous metals like iron and steel are processed in refractory lined vessels from the time the metal is released from its ore to the heat treating of the finished product. Primary and recycled aluminum producers use a variety of refractory materials. The power industry burns coal, oil or natural gas in boilers protected by refractory linings. The paper industry makes wood pulp in refractory lined process vessels that recycle chemicals and generate process heat. Glass manufacturers, plants that produce cement and limestone, thermal oxidizers that reduce pollution emissions, and even industrial bakery ovens use refractory products.
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Refractory concretes need a binder. You could think of the binder as the glue that holds the rocks together. In most cases the binder in castables, gunning mixes and shotcrete mixes is a calcium aluminate (CA) cement, a binder similar to calcium silicate cement (Portland cement) but designed to handle higher temperatures without breaking down. The CA cement is kind of like the Portland cement in that water is not only needed to start the binder reaction, and actually becomes part of the bond matrix. “No Cement” castables do not utilize a cement binder, rather the bond matrix is made up of a set of chemicals that form a bond matrix without the use of a cement. In most cases, the chemical reactions that create “no cement” binders are activated by water, but do not incorporate water as part of the bond. In some applications, no cement castables offer advantages over cement bonded products, especially in acidic environments or processes that operate at exceptionally high temperatures.
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