Basic Shaped Refractories
Basic Shaped Refractories
Basic shaped refractories are those refractories which have resistance to corrosive reactions with chemically basic slags, dusts and fumes at elevated temperatures. They are both MgO and CaO based refractories or in combination between them or in combination between MgO and Cr2O3. These refractories belong to MgO- CaO equilibrium system as shown in Fig 1.
Fig 1 MgO-CaO equilibrium system
Broadly basic refractories falls into one of the following five compositional categories (Fig 2).
- Products based on dead burned magnesite (DBM) or magnesia. These products are known as magnesite bricks.
- Products based on DBM or magnesia in combination with chrome containing materials such as chrome ore. Chrome containing magnesite bricks with about 5 % to 15 % Cr2O3 are known as magnesite chrome bricks while those with 15 % to 30 % chromium are called chrome magnesite bricks.
- DBM or magnesia in combination with spinel. In these basic bricks magnesia-rich spinel (MgO.Al2O3) replaces chrome ore. These bricks are called magnesite spinel bricks.
- DBM or magnesia in combination with carbon. These bricks are known as magnesite carbon bricks.
- Dolomitic products. These bricks are known as sintered dolomite bricks.
Fig 2 Categories of basic shaped refractories based on composition
Basic refractories are characterized by an extremely high refractoriness and good resistance to basic slags. Compared to fireclay bricks they do not have glassy phase. These refractories have low resistance both to thermal shocks and creep at temperatures close to 1500 deg C. The chemical-physical characteristics together with a very high thermal capacity and thermal conductivity make basic refractories ideal refractories for steel making processes. Hence basic refractories received increased importance with the introduction of basic oxygen steel making process.
One of the more important types of magnesite bricks are those which have low boron oxide contents and dicalcium silicates bonds. These chemical features give the bricks excellent refractoriness, hot strength and resistance to load at elevated temperatures. Magnesite bricks containing higher content of boron oxide has improved hydration resistance.
Basic refractories containing chrome has excellent slag resistance, superior spalling resistance and good hot strengths. With the use of high purity raw materials in combination with high firing temperatures, it is possible to produce direct bonded bricks where a ceramic bond between the chrome ore and periclase particles exists. These direct bonded bricks exhibit superior slag resistance and strengths at elevated temperatures.
Trivalent chromium (Cr+3) present in the magnesite chrome bricks can be converted into hexavalent state (Cr+6) by reactions with alkalis, alkaline earth constituents and other compounds that are present in some service environments. Hexavalent chrome is poisonous and water soluble , thus used refractories cannot be transported to the dumps after the furnace lining demolition.
Magnesite bricks in combination with spinel have increased importance these days because of a desire to replace chrome containing refractories because of environmental concerns. Bricks made with spinel and magnesite have better spalling resistance and lower coefficient of expansion than bricks made solely with DBM. These features minimize the chance of the brick cracking during service.
Carbon is added to magnesite refractories because it is not easily wetted by slag. One of the principal functions of carbon is to prevent liquid slag from entering the brick and causing disruption. Basic refractories containing carbon (C) include pitch impregnated magnesite bricks with carbon contents up to 2.5 %, pitch bonded magnesite bricks containing around 5 % carbon and magnesite carbon bricks which contains up to 30 % carbon. Magnesite carbon bricks are more corrosion resistant than pitch impregnated and pitch bonded magnesite bricks.
Dolomite bricks have a good balance between low cost and good refractoriness for certain uses. They also offer good metallurgical characteristics for certain clean steel applications.
Magnesia, dolomite, olivine and chrome- magnesia are commonly used basic refractory raw materials.
The term magnesite refer to the naturally occurring magnesium carbonate mineral. In refractory terms it is used to refer to the high temperature product magnesia (periclase). There are three general grades of magnesite produced from natural magnesite (MgCO3), magnesium hydroxide [Mg(OH)2] obtained from sea water or from brine deposits.
DBM is produced by sintering/firing MgCO3 at temperatures between 1450 deg C and 1760 deg C in shaft or rotary kilns. Based on the conditions used and the quality of the raw materials, this can produce materials with MgO contents between around 85 % to 98 %. For refractory applications, besides MgO, lime, silica and iron contents are also critical.
Seawater / brine magnesite is produced by adding limestone or calcined dolomite to sea water/brine and precipitating out magnesium hydroxide. The precipitate is formed into filter cake and can then either be fired at temperatures greater than 1800 deg C to form the magnesite or alternative the Mg(OH)2 is put through a low temperature decomposition at around 1000 deg C to form MgO. This is then cooled and pelletized before it is sintered in shaft/rotary kilns. The magnesia produced through this process can also have varying MgO contents between 90 % to 98 % MgO. However, if a high purity magnesia source is used in this process then the products can have MgO contents in excess of 99 %. As with DBM careful control of lime, silica and iron contents are crucial to ensure good refractory properties.
Fused magnesite is formed by melting sintered magnesia in an electric arc furnace. The resulting product is cooled and ground into aggregate and powders.
The three types of magnesite described above are used in a variety of refractory applications. DBM is mostly used in the manufacture of basic monolithics such as gunning repair products, tundish working linings and precast shapes (tundish dams/weirs). Magnesia spinel bricks are produces with the addition of small amounts of alumina to improve thermal shock resistance. Fused magnesite tends to have superior properties to sintered magnesite and as such is incorporated into refractory products used in high wear areas such as slag lines etc.
Dolomite is a calcium magnesium carbonate mineral (CaCO3. MgCO3). The natural raw material cannot be used in refractories and must be dead burnt at high temperatures. This is done with the addition of some iron or silica to assist in stabilizing/semi stabilizing the dolomite by removing CaO as calcium ferrite or calcium silicate. Synthetic dolomite clinker can also be produced by the high temperature reaction of calcium hydroxide and magnesium hydroxide with a small amount of an iron compound. The chemistry of this synthetic material can be adjusted and the material offers improved corrosion resistance compared to the dead burnt natural dolomite due to the lower levels of impurities present in the material. Dolomite bricks have one major disadvantage over mag-carbon bricks which is its tendency to disintegrate when stored for a short period of time due to reaction of free lime with moisture in the air.
In recent years, the trend has shifted to developing highly engineered basic refractories. Development of technology for engineered basic refractories is for addressing specific wear mechanisms by employing special additives in the refractory compositions. These additives generally constitute less than 6 % of the total mix, although levels at 3 % and below are probably the most common. Example of these special additives include zirconia (used for improving spalling resistance of burned basic refractories), carbon, metallic additives such as powdered aluminum, magnesium, or silicon ( used for improving hot strength and oxidation resistance), boron carbide, B4C (for improving oxidation resistance in magnesia carbon bricks).
Dolomite bricks are made both in dead burnt and carbon bonded compositions. The carbon bonded varieties include both pitch and resin bonded versions. Some of the carbon bonded dolomite bricks contain flake graphite and are analogous to magnesite carbon bricks.
Magnesite bricks are made from DBM and are characterized by good resistance to basic slags as well as low vulnerability to attack by iron oxide and alkalis. They are employed as sub hearth bricks for electric arc furnaces and sometimes as back up lining for basic oxygen converters. They are also used as slide gate refractories or in pouring nozzles. They are often impregnated with pitch.
The main component of magnesite bricks is the mineral periclase (MgO). Its properties determine the behaviour of the brick material to a large extent. The most important properties of periclase are high melting point (around 2800 deg C), high thermal conductivity (4.5 W/K.m at 1000 deg C), high thermal expansion (2 % at 1400 deg C), and no phase modification.
The refractoriness of a magnesite brick is often determined by the amount and the type of imprity within the grain. The refractoriness of the DBM is improved by lowering the amount of impurites or adjusting the chemistry of the impurities or both.
Magnesite refractories are either burned or chemically bonded. They are divided into two categories on the basis of chemistry. The first category of the bricks are made with low boron magnesite with boron oxide usually less than 0.02 %. These bricks have lime to silica ratio of 2:1 or more. This ratio is intentionally adjusted to get high hot strength. Bricks with lime to silica ratio greater than 2:1 are often of higher purity than the dicalcium silicate bonded bricks. The scond category of magnesite brick usually has lime to silica ratios between zero and one. These bricks may have high boron oxide content (more than 0.1 % B2O3). These bricks have good hydration resistance. These bricks are made with lower purity natural DBM with MgO content of 95 % or less. These bricks have lower refractoriness as compared to bricks made with very pure magnesites (MgO content more than 98 %).
Magnesite chrome and chrome magnesite bricks
The reaction between chrome ore and magnesite outline the fundamental chemistry of the magnesite chrome bricks. Magnesite chrome bricks can be either silicate bonded or direct bonded. Silicate bonded bricks have a thin film of silicate minerals that surrounds and bonds together the magnesite and chrome ore particles. Direct bonded bricks have the direct attachment of the magnesia to the chrome ore without intervening films of silicate. Direct bonding is obtained by combining high purity chrome ores and magnesites and firing them at extremely high temperatures.
Direct bonded bricks have high strength at elevated temperatures, better slag resistance and better resistance to peel spalling than silicate bonded bricks. The balance of properties of the bricks is a function of the magnesite to chrome ratio. For example, a direct bonded brick containing 60 % magnesia have better spalling resistance than one containing 80 % magnesia, although the latter might be considered a better choice in a high alkali environment. The changing balance of properties as a function of ratio of magnesite to chrome ore makes it possible to choose products best suited for an individual application.
Burned chrome magnesite bricks may be of either direct bonded or silicate bonded variety. The direct bonded bricks are used under more severe service conditions.
Chemical bonded magnesite chrome and chrome magnesite bricks do not have the high temperature strength, load resistance or slag resistance of burned compositions. They are used usually a lower cost compositions to balance out wear profiles in various applications. These bricks are sometimes used with steel casing. In service steel casing oxidizes and forms a tight bond between the bricks.
Magnesite spinel bricks
A family of magnesite spinel refractories has been developed by combining the constituent raw materials in various ways. Some magnesite spinel bricks are made by adding fine alumina to compositions composed mainly of magnesia. On firing, the fine alumina reacts with the fine magnesia in the matrix of the brick to form an in situ spinel bond. An alternative is to add spinel grain to a composition containing magnesia.
One of the principal benefits of combining spinel and magnesia is that the resulting compositions has better spalling resistance than bricks made solely with DBM. Spinel additions also lower the thermal expansion coefficient of magnesite compositions.
A desire to use chrome free basic brick for environmental reasons has increased the importance of the magnesite spinel bricks.
Carbon containing basic bricks
Carbon containing basic bricks are categorized in three types namely (i) pitch impregnated burned magnesite bricks containing about 2.5 % carbon, (ii) pitch bonded magnesite brick containing around 5 % carbon , and (iii) magnesite carbon bricks containing 8 % to 30 % carbon. Bricks with carbon content 10 % to 20 % are more common.
While all bricks in these categories have both magnesite and carbon, the term magnesite carbon brick is used for bricks having carbon content more than 8 %. Pitch impregnated and pitch bonded bricks have just enough carbon to fill their pore structure. In magnesite carbon bricks, the carbon addition is too large to be considered a pore filler. These bricks are considered composite refractories in which the carbon phase has a major influence on the brick properties.
Burned pitch impregnated magnesite brick is made with a dicalcium silicate (melting point around 2130 deg C) bond. Use of this bond in combination of tight chemical control of other oxides gives these bricks excellent hot strength. The carbon derived from the impregnating pitch when the brick is heated in service prevents slag constituents from chemically altering the dicalcium silicate bond, preserving the hot strength and high refractoriness. Carbon also prevents the phenomenon of peel spalling, where the hot phase of a brick cracks and falls away due to slag penetration in combination with temperature cycling.
Pitch bonded magnesia bricks are having excellent thermal shock resistance, high temperature strength and good slag resistance.
In magnesite carbon bricks, the high carbon content is achieved by adding flake graphite. The high oxidation resistance of flake graphite contributes to the reduced erosion rates of these bricks. In addition , the flake graphite results in very high thermal conductivities compared to most refractories. These high thermal conductivities are a factor in the excellent spalling resistance of the magnesite carbon bricks. By reducing the temperature gradient through a brick, the high thermal conductivities reduces the thermal stresses within the brick. High thermal conductivity also results in faster cooling of the brick between heats and thus reduces potential for oxidation.
These days magnesite carbon bricks are made with good slag resistance and hot stability. A high degree of slag resistance and good high temperature stability have been found to be advantageous in the hotter and more corrosive service environments. High temperature stability of magnesite carbon bricks is achieved by utilization of high purity graphites and magnesites. Since flake graphite is a natural mined material, there are impurities associated with it. These may be minerals such as quartz, muscovites, pyrite, iron oxides and feldspars. Although most of the impurities of flake carbon are removed by floatation processes, most graphites contain only a limited amount of impurities known as graphite ash. Some of the ash components such as silica and iron oxide, are easily reduced by carbon and thus will result in a loss of carbon from the brick and a reduction of hot strength at elevated temperatures. Magnesia can also be reduced by carbon at high temperatures. Hence for high temperature stability , high purity magnesites are used. Magnesites with very low boron oxide contents are especially desirable.
For improving the performance of carbon containing magnesite bricks, special additives are used. These additives include powdered metals such as aluminum, magnesium, and silicon. One reason for adding these metals is to improve oxidation resistance. The metals consume oxygen that would otherwise oxidize carbon. The aluminum and silicon also cause the pore structure to become finer after the brick is heated. This happens due to the formation of aluminum carbide(Al4C) and silicon carbide (SiC) by reaction between metal and carbon in the brick. The finer pores results in decreased permeability of the brick and inhibit oxidation by making it more difficult for oxygen to enter the brick structure. Another reason for adding metals is to improve the hot strength of the magnesite carbon brick. Improvement in the hot strength takes place due to the formation of carbide bridges within the matrix of the magnesite carbon brick.