Introduction To Magnesium For Refractory Bricks

Where to get magnesium for refractory bricks

Application of magnesium refractory bricks

What factors affect the service life of magnesium bricks?

How to use and maintain magnesium refractory bricks

What are the special requirements for the use and maintenance of different types of magnesium refractory bricks?

Elemental magnesium for refractory bricks is mainly obtained from the following sources:

(1) Magnesite: This is one of the most important sources for obtaining magnesium. Magnesite is a carbonate mineral, the main component is magnesium carbonate ((MgCO₃)), the theoretical magnesium oxide content is as high as (47.82%). It is often processed by beneficiation and calcination, and after high temperature calcination, magnesium oxide can be obtained, which in turn is used for the manufacture of refractory bricks. China is one of the richest countries in the world in magnesite, the reserves account for a certain proportion of the world's total reserves, the main production area in Liaoning and Shandong.

(2) Dolomite ore: Dolomite is a complex salt of magnesium carbonate and calcium carbonate with the chemical formula (CaMg(CO₃)₂), of which the theoretical content of magnesium oxide is (21.8%). A mixture containing calcium oxide and magnesium oxide is first obtained by calcining dolomite, and then after further processing and separation, elemental magnesium can be obtained for refractory brick production. China also has the largest reserves of dolomite in the world, with abundant resources, and the main production areas are located in the provinces and regions of Liaoning, Henan, Shanxi, Ningxia, Hebei and Shaanxi.

 

(3) Seawater and salt lake brine: Seawater and salt lake brine contain a large number of magnesium ions. To extract magnesium from seawater, generally first add milk of lime (calcium hydroxide) to seawater, so that magnesium ions are converted into magnesium hydroxide precipitate, and then the magnesium hydroxide precipitate is filtered out, and then get magnesium oxide through calcination and other processes. Salt lake brines are treated in a similar way, for example, the Charkhan Salt Lake in Qinghai, China, which has huge reserves of magnesium chloride and is an important source of magnesium resources. The magnesium oxide products produced in this way are of high purity, with MgO content above (97%), and the chemical composition is easy to adjust, making it a high-quality raw material for refractory materials.

 

(4) The main component of magnesite is magnesium hydroxide ((Mg(OH)₂)), which can be decomposed to obtain magnesium oxide after high-temperature calcination, thus providing magnesium for refractory bricks. However, the reserves of magnesite are relatively small compared to magnesite and dolomite.

Magnesium is widely used in refractories, mainly because of its advantages such as high melting point, good chemical stability and thermal shock stability. The following are the main types of magnesium refractory bricks:

Magnesium bricks: mainly made of sintered magnesium sand, with high refractoriness and good resistance to alkaline slag erosion, commonly used in the iron and steel industry, flat furnaces, electric furnaces, furnace bottom and furnace walls and other parts of the furnace, to withstand high temperatures and slag erosion, to extend the service life of the furnace lining.

Magnesium-chromium bricks: made of magnesium sand and chromite and other raw materials, with good thermal shock resistance and erosion resistance, commonly used in non-ferrous metal smelting furnace, cement rotary kiln and other high-temperature equipment lining. In the cement rotary kiln, magnesium-chromium bricks can withstand the scouring of high-temperature materials and flames, and at the same time resist the erosion of alkaline gases and dust inside the kiln.

Magnesium-aluminum bricks: with magnesium sand and alumina as the main raw materials, they have high strength and thermal shock resistance, and are suitable for the lining of steel converters, electric furnaces, and heat storage chambers of glass kilns. In the steel converter, magnesium-aluminum bricks can withstand the drastic temperature changes and the scouring of molten steel, and improve the service life of the furnace lining.

The service life of magnesium bricks is affected by the quality of raw materials, production process, masonry quality, use of the environment and other factors, as follows:

Raw material quality

Purity of magnesium sand: magnesium bricks made of high purity magnesium sand, with less impurities and high content of magnesite, have higher refractoriness, strength and erosion resistance. If the magnesium sand CaO, SiO₂ and other impurities in the content is too high, will form a low melting point phase, reduce the high temperature performance of magnesium brick, shorten the service life.

Particle grading: Reasonable particle grading can make the structure of magnesium bricks denser. Coarse particles can provide skeleton support, fine particles fill the pores, reduce porosity, improve strength and resistance to slag erosion. If the grading is not appropriate, the porosity increases, easy to be eroded by the slag penetration, reducing the service life.

Production process

Molding pressure: appropriate molding pressure can make the brick billet density uniform, dense structure. If the pressure is too low, the density of the billet is low and the strength is poor; if the pressure is too high, it may lead to cracks and other defects in the billet, all of which will affect the service life of magnesium bricks.

Firing system: the firing temperature and time are crucial to the performance of magnesium bricks. Insufficient firing temperature, square magnesite crystallization is incomplete, magnesium brick strength and erosion resistance is poor; too high a temperature or too long a time, will make the brick grain is too large, the resistance to thermal shock is reduced.

Masonry quality

Brick seam size: the brick seam should be as small and uniform as possible, too large a brick seam will make the high temperature furnace gas and slag easy to penetrate, erosion of the brick, but also reduces the overall strength of the masonry, in the thermal stress, the brick seam is easy to crack, spalling, affecting the service life of magnesium bricks.

Masonry flatness: uneven masonry surface, will make the magnesium brick in the use of the process of uneven force, local stress concentration, resulting in premature damage to the brick body. And the uneven surface is also easy to cause slag adhesion and erosion, accelerating the destruction of magnesium bricks.

Operating environment

Temperature fluctuation: Frequent temperature fluctuation will make the magnesium brick produce thermal stress. When the thermal stress exceeds the capacity of magnesium bricks, cracks will occur inside the bricks, and with the expansion of the cracks, the bricks will spall off and be damaged, which will seriously shorten the service life.

Slag erosion: different composition of the slag on the magnesium brick erosion degree is different. If the slag contains a large number of acidic oxides, it will have a chemical reaction with magnesium oxide in magnesium bricks, forming low melting point substances, accelerating the erosion of bricks and reducing the service life of magnesium bricks.

Mechanical wear: In the process of use, the material scouring, mechanical collision, etc. will cause wear and tear of magnesium bricks. Such as in the blast furnace iron outlet and other parts of the iron and slag high-speed scouring, will make the surface of magnesium bricks continue to wear, the thickness of thinning, until the loss of performance.

Before use

Check the quality: inspect the magnesium refractory bricks block by block to see whether there are cracks, missing corners, falling edges and other defects. For bricks with serious defects, they should be picked out in time and avoided to be used for masonry to prevent them from becoming weak points in the process of use and affecting the stability and service life of the overall structure.

Pre-laying: For complex furnace types or critical parts, pre-laying is recommended. Through pre-laying, the matching problems between bricks and bricks, such as size deviation, uneven brick joints, etc., can be found in advance, and timely adjustments and improvements can be made to ensure the quality and precision of formal masonry.

Drying treatment: If the magnesium refractory bricks are stored in a humid environment, they need to be dried before use. Because the moisture will vaporize quickly at high temperature and generate pressure, which may lead to cracking or spalling of the brick body. The bricks can be placed in a dry and ventilated place to dry naturally, or low temperature drying can be used to remove the moisture.

In use

Temperature control: avoid rapid temperature changes, strictly in accordance with the baking curve for baking operation, slowly rising temperature, so that the magnesium refractory bricks gradually adapt to the high temperature environment, to prevent cracking of the brick body due to excessive thermal stress. In the normal use of the process, it is also necessary to maintain a stable temperature in the furnace to minimize temperature fluctuations. At the same time, it is necessary to prevent localized overheating and ensure that the temperature in the furnace is evenly distributed.

Prevent chemical erosion: according to the working environment of the furnace, select the appropriate magnesium refractory bricks. For example, in the alkaline environment, to prevent acidic substances from entering the furnace and magnesium refractory bricks chemical reaction; in the environment with fluorine and other special media, to choose the magnesium refractory bricks with corresponding anti-erosion ability. In addition, should also avoid sudden changes in the composition of the slag, reduce the erosion of the brick body.

Avoid mechanical damage: in the loading, discharging and furnace operation process, we should pay attention to avoid the direct impact of the material on the magnesium refractory bricks and friction, to prevent the surface of the bricks due to mechanical abrasion caused by spalling or damage. For the parts which are easy to be abraded, such as furnace door, iron outlet, etc., additional protective measures can be taken, such as installing protective plates.

After use

Timely repair: After stopping the furnace, the magnesium refractory bricks should be inspected in time, and the small area damage, cracks and other problems should be repaired in time. When repairing, choose the refractory material with similar performance to the original brick, and use the appropriate repair process, such as applying repair material, spray patching, etc., in order to restore the integrity and performance of the brick body.

Overall maintenance: according to the use of magnesium refractory bricks and the condition of the furnace body, regular overall maintenance. Such as every certain period of time for a comprehensive inspection of the furnace body, assess the remaining life of the refractory bricks, for serious damage, affecting the safe operation of the parts, timely replacement. At the same time, the insulation layer, seals and other related parts of the furnace body are also inspected and maintained to ensure that the whole furnace body has good performance.

Proper storage: If there is any remaining magnesium refractory bricks, store them properly. A dry, ventilated warehouse should be selected to avoid moisture and rain. Storage should be categorized and placed, to avoid different specifications, different properties of the bricks mixed, and at the same time to prevent the bricks from being squeezed and collision, so as not to cause damage!

Magnesium refractory bricks are mainly ordinary magnesium bricks, magnesium chromium bricks, magnesium-aluminum bricks, magnesium-calcium bricks and other types of bricks, which have their own special requirements for use and maintenance due to different compositions and performance characteristics, as follows:

Ordinary Magnesia Refractory Brick

Use: ordinary magnesium bricks have high refractoriness, but relatively poor thermal shock resistance, suitable for use in alkaline furnace parts with relatively stable temperature and no strong thermal shock, such as permanent lining of steelmaking converter. The use of temperature fluctuations should be strictly controlled to avoid rapid cooling and heating, otherwise it is easy to lead to brick cracking and spalling.

Maintenance: need to focus on the sealing of the brick seams, due to its relatively high porosity, slag is easy to penetrate through the brick seams erosion of the brick, found that the brick seams have expanded or poor sealing, should be promptly hooked with magnesium fire clay treatment.

Magnesia Chrome Brick

Use: Magnesium-chromium bricks have good high-temperature strength and erosion resistance, especially for chromium-containing slag has a better resistance, commonly used in glass kiln heat storage chamber, cement rotary kiln transition zone and other parts. However, it should be noted that it may be converted to hexavalent chromium at high temperature and produce harmful gases, so it is necessary to ensure that the environment is well ventilated, and at the same time, avoid long-term contact with strong alkaline substances in order to prevent chromate from being destroyed and affecting its performance.

Maintenance: Pay attention to the treatment of waste magnesium-chromium bricks after use, they can not be discarded at will, and need to be recycled specifically in accordance with environmental requirements to prevent chromium pollution. In the maintenance inspection, we should pay special attention to the surface of the brick body whether there are color changes due to chromium ion migration and other abnormalities, if so, we need to further check the internal structure and performance of the brick body.

Magnesia Alumina Spinel Brick

Use: Magnesium aluminum brick contains alumina, thermal shock resistance is better than ordinary magnesium brick, can be used in some parts of the temperature fluctuation, such as electric furnace wall. In the process of use, avoid direct contact with materials with high silicon content, because silicon may react with the components in magnesium-aluminum bricks at high temperatures, generating low melting point substances and reducing the refractory performance of the bricks.

Maintenance: daily maintenance, to pay attention to the surface erosion of magnesium-aluminum bricks, due to its main components of magnesium oxide and alumina at high temperatures may be reacted with some of the alkaline or acidic components of the slag, so once found on the surface of the signs of erosion, we should analyze the composition of the slag in a timely manner to adjust the operating process, and if necessary, the erosion of the parts for repair.

calcium magnesium brick

Use: Magnesium-calcium bricks have good resistance to alkaline slag erosion and high high-temperature strength, and are commonly used in steelmaking converters, refining furnaces and so on. However, it has strong hygroscopicity, and must be strictly dried before use, otherwise the rapid evaporation of water at high temperatures will lead to brick body explosion. At the same time, it should be avoided in the environment with a large amount of water vapor to prevent the generation of calcium hydroxide, affecting the performance of the brick.

Maintenance: Choose a dry environment when storing, if you find magnesium-calcium bricks have moisture phenomenon, you need to re-dry treatment before use. At the end of the furnace service, the magnesium and calcium bricks in the furnace should be cleaned as soon as possible to avoid its long exposure to the air to absorb moisture and pulverization.

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