Special refractory materials to withstand at high temperature

April 11, 2014 10:27 am

The refractory materials should be able to withstand at high temperature/sudden changes of temperature, load at service conditionsRefractory concretes are by definition materials which are resistant to high temperatures, mechanical stress, and thermal stress, chemical and abrasive attack. They are designed for specific applications predominately in the metal industry, but are also used extensively in the chemical, cement and glass industries. Refractory concretes rely on a complicated mix of aggregate and binder, the most common binder being high alumina cement (HAC). Aggregates used will vary depending on the intended application. Refractories come in two general types: preformed and monolithic. Preformed includes bricks and large-scale monoliths. Monolithic are generally described as gun mixes and castables used as linings and coatings normally fired in-situ.Role of refractoryRefractory materials have a crucial impact on the cost and quality of steel products. The diversification on steel products and their cleanliness requirement in recent years have increased the demand for high quality refractory. Steelmaking requires high temperatures of the order of 1,600  C. In addition steelmaking handles high temperature phases like molten steel, slag and hot gases. These phases are chemically reactive; refractory materials are required to produces steels. High quality refractory at a cheaper cost is the main requirement because cost of refractory adds into the cost of product.Why it required?To minimise heat losses from the reaction chamber to allow thermal energy dependent conversion of chemically reactive reactants into products because metallic vessels are not suitable.Refractory requirementsThe refractory materials should be able to withstand at high temperature/sudden changes of temperature, load at service conditions.Important properties of refractoriesMelting point: Pure substances melt sharply at a definite temperature. Most refractory materials consist of high melting particles bonded together. At high temperature, glass fuses and as the temperature rises, the resulting slag increases in quantity by partial solution of the refractory particles. The temperature at which this action results in failure of a test pyramid (cone) to support its own weight is called, for convenience, the melting point of the refractory. Table 5.4 shows the melting point of some pure compounds used as refractories.PorosityThe apparent porosity is a measure of the volume of the open pores into which a liquid can penetrate as a percentage of the total volume. This is an important property in cases where the refractory is in contact with molten charge and slags. A low apparent porosity is desirable as it would prevent easy penetration of the refractory size, and continuity of pores will have important influences on refractory behaviour. A large number of small pores is generally preferable to an equivalent number of large pores.Bulk density: ASTM C-134 using cubes 2 x 2 x 2 inchesBulk density (BD) can be defined as the ratio of the mass of the refractory specimen to its bulk volume (which includes the volume of the pore spaces). Bulk density gives an indication of the porosity of the refractory substance; low B.D. indicates high porosity and likewise low strength of the material and vice versa. A higher value of bulk density of a given refractory indicates enhanced volume stability, heat capacity and resistance to slag penetration. BD is expressed either in pounds per cubic foot (lb/ft3) or kilograms per cubic meter (kg/m3).Cold crushing strength ASTM C-133 using cubes 2 x 2 x 2 inchesThis test helps in determining the strength of a brick. It tells us how much load that refractory can bear in cold conditions. The concept of testing CCS of a refractory material has perhaps, come from metallurgy. This is because it is rather rare for any refractory brick that it would fail simply due to load on it in cold condition. Therefore, the determination of cold crushing strength does not appear to be important from that point of view. Abrasion test ASTM C-704 using plates 4.5 x 4.5 x 1Abrasion test or abrasion resistance test refers to the ability of refractory test specimens to resist the surface wear caused by the mechanical action of moving solids with high speed at elevated or room temperatures. This test determines the suitability of test items to be used in abrasive environments. The moving solid is the abrading material, which is either white fused alumina or black silicon carbide with specified grain size and chemical composition. This test helps in determining the relative abrasion resistance of refractory bricks at room temperature and can also be applied to castable refractories. The test result is calculated in terms of the loss in volume of the test items. This test method measures the volume of material in cubic centimetres abraded from flat surface at a right angle to a nozzle through which 1,000 g of size-graded silicon carbide grain is blasted by air at 65 psi in 450 seconds.