Refractory materials are basically inorganic non-metallic substances which are composed of thermally stable mineral aggregates, a binder phase, and additives. Refractory substances are supposed to be able to withstand the varying degrees of severity in terms of exposure to abrasive and corrosive solids, liquids, or gases at extremely high temperatures. The principal raw materials used in the production of refractories are: Oxides of silicon, aluminium, magnesium, calcium and zirconium, and some non-oxide compounds of carbides, nitrides, borides, silicates and graphite. There are a number of different types of refractory materials, in terms of physico-chemical composition and designs, which are manufactured depending upon the operating service conditions.
Refractories are mainly used in the metallurgical industries for the purposes of providing internal linings of furnaces, kilns, reactors, boilers, and other vessels for holding and transporting metal and slag. They are even used in non-metallurgical industries, where they are mainly used in fired heaters, hydrogen reformers, ammonia primary and secondary reformers, cracking furnaces, incinerators, utility boilers, catalytic cracking units, coke calciner, ducting, stacks, etc.
Modi Laboratary has one of the best laboratory facilities for testing of ‘Refractories and insulation materials’. Besides we have a team of experts who regularly visit the manufacturing sites for inspection of such materials. Some of the important refractory items tested at Modi Laboratary are:
Insulating materials are primarily used with the purpose of preventing heat loss or gain in the application areas, like manufacturing structures and heating chambers. They should possess low thermal conductivity and their heat capacities depend on their bulk densities and specific heats. They are generally porous, containing a large number of dormant air cells. Insulating materials can be classified into two broad categories: Organic and Inorganic materials. The organic foams include polystyrene, polyurethane, phenol foam, polyethylene foam etc. The inorganic ones include mineral wool, calcium silicate, cellular glass, micro porous silica, magnesia, ceramic fibre, vermiculite and pearlite.
This classification is based on the behaviour, i.e. chemical reaction of refractory materials towards slag. Three types are recognized:
These types of refractories are attacked by basic slags (alkalies), and are stable to acidic ones. Hence, these are used in those places where the atmosphere and slag composition are acidic. Examples include Silica (SiO2), Zirconia (ZrO2), etc.
These refractories are attacked by acidic slags, and are stable to alkaline slags, dusts and fumes at higher temperatures. Thus, they find use in places, like furnace linings, where the slags and atmospheric conditions are basic. In terms of chemical composition, they mainly include Magnesia (MgO), Dolomite (CaO∙MgO), and Chromite (main part of chrome ore).
These are chemically stable to both acidic and basic slags, hence are used in areas where the slag and atmospheric conditions are either acidic or basic. Some of the common neutral refractories are Carbon graphite (most inert), Chromites (Cr2O3), & Alumina (Al2O3). Graphite is extensively used in metallurgical furnaces for controlling the process of oxidation, since it is extremely stable and inert.
Based on their physical forms, refractories can be classified into ‘Shaped’ and ‘Unshaped’ refractories. The former is commonly referred to as refractory bricks and the latter as “monolithic” refractories
Depending on the method of manufacturing, the refractories can be classified into the following methods:-
The test methods adopted for analyses depend on the test item, its intended use and the client’s customized requirements, if any. Generally, standard test methods are followed like ASTM, ISO, IS, and some other accepted national and international methods. Creep Test in Compression (CIC) at high Temperatures
This test helps to determine the shrinkage of a refractory test item under a constant load, and exposed to a constant high temperature over a long (specified) period of time. Thus, this test measures the load-bearing capacity of the test item so as to maintain its dimensional stability. Creep testing of materials at high temperatures and constant load is a very important investigative tool used at many different sectors of refractory industries. Accurate high temperature creep data is essential for the proper design and construction of many structural components operating at inordinately high temperatures. The standard for refractory materials restricts compressive creep (deformation at a given time and temperature under stress) for normal working conditions to no more than 0.3 percent in the first 50 hours.
Upon heating, refractory substances can undergo either phase changes or mineral formations. These changes result in change in volume. Then upon cooling to room temperature, the material changes in dimensions, which is verified using reheat tests carried out at temperatures prescribed by ASTM for each class of refractory (with proper heating schedules). Finally, the permanent change in dimensions is calculated and expressed as permanent linear change.
Pyrometric Cone Equivalent gives an indication about the ‘Softening Temperature’ (or the range of temperatures), which is the temperature at which the refractory substance will deform under its own weight. This also means that this test gives an idea of the ‘Refractoriness’ of the test substance, but it is not an accurate estimate of the softening temperature because of the complex chemical nature of refractories. Basically, in this test the softening of the test refractory material is indirectly determined through comparisons with standard cones. Although this test does not provide a clear picture as to how a refractory would perform in actual service conditions, it provides a useful idea about the relative refractoriness of fireclay and some classes of high-alumina refractory materials. However, this test is not relevant for basic and certain other refractories.
Refractories under Load (RUL) This test estimates the deformation or fragile behaviour of refractory products (especially ceramics) subjected to a constant load and increasing temperature. For example, in case of bricks this test measures the temperature at which the bricks will collapse, in actual service conditions under the application of a similar load.
Abrasion 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 item.
Air Permeability Test This test is used to measure the rate of flow of gases like air or nitrogen through refractory bricks or monoliths. The permeability is determined by forcing a known volume of the gas through a cube of definite dimensions for a specified of time, and maintaining the desired pressure difference. There is a relationship between permeability of the refractory and the number of its closed pores. Knowledge of this parameter is extremely useful in the application of refractory in furnaces involving molten metal, slag etc., and hence determines its slag resistance. Uniform permeability is an indication of the absence of cracks in the refractory.
This test helps in determining the microscopic or morphological structure of materials using thin sections or polished surfaces of the refractory specimen. The microscopic views or images help in elucidation of the internal structure of refractory materials, which enables their characterization.
This test gives a measure of the acid resistance capacity of the refractory test material. The results from this test can be utilized for manufacturing of efficient acid resistant bricks and other refractory substances which are used in furnace linings. The results are also used for quality control purposes.
Thermal conductivity can be defined as the amount of heat flowing from the hot face to the cold face of a refractory lining. Its units are BTU/h, W/ (m0K), etc. Thermal conductivity value depends upon the chemical and mineralogical compositions and also on the application temperature. It usually changes with rise in temperature. The amount of heat flowing through a refractory wall depends on a number of factors: (1) Conductivity value of the refractory, (2) Temperature drop across the surface from the hot face to the cold face, (3) Area of the wall, (4) Time of heat flow, (5) Thickness of the wall. Since, different test methods for measuring thermal conductivity produce different results, it is important to specify test method used while reporting such results. The applications of refractories vary with the intended use, like in cases where heat transfer is required though the brick work, for example in recuperators, regenerators, muffles, etc. the refractory should have high conductivity. Low thermal conductivity is desirable for conservation of heat by providing adequate insulation.
This test helps in determination the proportion of particles of known diameter within a sample (of known mass). The test specimen is either passed through a set of standard sieves in its natural state, or if a significant amount of binding material, such as clay, is present, then the sample needs to be washed over a small aperture sieve to remove the binding substance.
This test measures the amount of water that a refractory can absorb under controlled experimental conditions. This is an important property of burned refractory brick and shapes. The results of water absorption tests are used for a number of reasons: Quality assurance, Research & Development, Design of various structures, deciding criteria for selection for use in industrial applications.
‘Apparent Porosity’ gives a measure of the volume of open pores (or voids) present on the surfaces of the refractory sample. It is calculated in terms of the fraction of total volume of open pores to that of the total volume of the sample. This is an important parameter concerning slag resistance and permeability of the refractory. The desirability of the number of pores depends on the intended use of the refractory substance. Porosity has a significant effect on the heat flow pattern in refractories.
This 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 for any refractory brick it is rather; rare that it would fail simply due to load on it in cold condition and therefore, the determination of cold crushing strength does not appear to be important from that point of view.
Bulk density (B.D.) 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. B.D. is expressed either in pounds per cubic foot (lb/ft3) or kilograms per cubic meter (kg/m3).
The modulus of rupture (MOR) refers to the flexural breaking strength of the refractory specimen. Technically speaking, it can be defined as the maximum stress/load a rectangular test piece of specified dimensions can withstand in a 3-point bend test until it breaks. It is expressed as N/mm2 , MPa, or other units. When this test is conducted at elevated temperatures the parameter is known as Hot Modulus of Rupture (HMOR). HMOR gives a fair estimate of the structural integrity and abrasion characteristics of the test refractory specimen.
Since refractory materials are mainly meant to be used under extreme conditions of temperature and also exposed to severe corrosive and abrasive gases, liquids, and molten metals, maintenance of the dimensional stability of such materials is a key area of concern. This property is important in the designing of proper structures so as to ensure perfect fitting in the place of use for the refractories.