Cement Manufacturers Association (CMA)

38 temperatures approaching their actual fusion point. This behavior contrasts with that of many other refractories, for example, alumino-silicate materials, which begin to fuse and creep at temperatures considerably lower than their fusion points. Magnesite: Magnesite refractories are chemically basic materials, containing at least 85% magnesium oxide. These are made from naturally occurring magnesite (MgCO 3 ) and Silica (SiO 2 ). Good quality magnesite bricks usually results from a CaO-SiO 2 ratio of less than 2 with a minimum ferrite concentration, particularly if the furnaces lined with the refractory operate in oxidizing and reducing conditions. Dolomite: The natural double carbonate dolomite (CaCO 3 – MgCO 3 ) can be converted to refractory dolomite (CaO- MgO) by high temperature firing. High purity dolomite is greater than 97% CaO + MgO. Dolomite refractories is found to be the most compatible material with the cement kiln clinker as it provides excellent coating stability, has very good resistance to thermal shock & alkali attack in varying kiln operating conditions. These refractories with Zirconia enrichment are used for crack arresting. Chromite Refractories: Here, a distinction must be made between chrome-magnesite refractories and magnesitechrome-refractories. Chrome- magnesite material usually contains 15-35% Cr2O3 and 42-50% MgO whereas magnesite-chromite refractories contain at least 60% MgO and 8-18% Cr 2 O 3 . Zirconia Refractories: Zirconium dioxide (ZrO 2 ) is a polymorphic material. There are certain difficulties in its usage and fabrication as a refractory material. It is essential to stabilize it before application as a refractory. This is achieved by incorporating small quantities of calcium, magnesium and cerium oxide, etc. Its properties depend mainly on degree of stabilization and quantity of stabilizer as well as the quality of original raw material. Zirconia refractories have a very high strength at room temperature which is maintained up to temperatures as high as 2700°F. They are, therefore, useful as high temperature construction materials for furnaces and kilns. The thermal conductivity of zirconium dioxide is found to be much lower than that of most other refractories and the material is therefore used as a high temperature insulating refractory. Since Zirconia exhibits very low thermal losses and does not react readily with liquid metals, it is particularly useful for making refractory crucibles and other vessels for metallurgical purposes. Zirconia is useful refractory material for glass furnaces primarily since it is not easily wetted by molten glass and because of its low reaction with them. Monolithic Refractories: Monolithic refractory, the name generally given to all unshaped refractory products, are materials installed as some form of suspension that ultimately harden to form a solid mass. Most monolithic formulations consist of large refractory particulates (an aggregate), fine filler materials (which fill the interparticle voids) and a binder phase (that gels the particulates together in the green state). Types of Monolithic Refractories 1. Castable Refractories 2. Insulating Castables 3. Plastic Refractories: 4. Ramming Mixes • Patching Refractories • Coating Refractories 5. Mortars: • Gunning Mixes: G • Fettling Mixes Insulating Refractories: These are high porosity refractories with low thermal conductivity used in order to reduce the rate of heat flow (heat losses) and thus to maximize heat conservation within the furnace. These have lower densities in range of 4 to 70 lb/ft3 . The three basic types of insulating materials for industrial use are: 1) thin (less than 20 micrometers), low-density (less than 12 lb/ft3 ) fibers made from organic or inorganic materials; 2) cellular material in closed or open cell form made of organic or inorganic material; and 3) flaked or granular inorganic materials bonded in the desired form. In most cases, glass (silica), mineral wool, high alumina, mulite, or zirconia are the base materials and can be used to temperatures as high as 2900°F. This class of materials has a lower density that varies from 4 lb/ft3 to 12 lb/ft3 and offers higher thermal resistance compared to firebricks. In all cases, thermal conductivity of the insulation increases significantly as temperature increases