Magnesium Aluminum Refractory Materials

Magnesium aluminum refractory materials are used in the metallurgical industry for lining steel nickel furnaces and aluminum smelting furnaces, electric furnace covers, steel ladle linings, refining vacuum treatment device linings outside steelmaking furnaces, and steelmaking open hearth heat storage chambers in non-ferrous metal smelting furnaces; Used in the building materials industry for lining the high-temperature zone of cement calcination kilns, transition zones of cement kilns, and high-temperature zones of lime kilns; Used in the glass industry for lattice bricks or lattice bodies in glass furnace heat storage chambers; Used in the refractory material industry for parts with large temperature changes in refractory kilns.
6. Magnesium aluminum refractory products
The ladle is an indispensable and important equipment in the steelmaking industry. As a refractory material for the ladle lining, it has undergone several major changes. Previously, traditional sintered clay bricks and third grade high alumina bricks were used as refractory materials, which showed poor resistance to peeling and erosion during use. The service life was low, averaging only 10 times, and after treatment, it could only reach about 20 times. With the development of steelmaking technology and the increasing demand for refractory materials for ladle lining in steel refining, the use of integral casting magnesium aluminum ladle lining material was promoted in the early 20th century. This type of lining has good overall integrity and significantly improves its service life, but it has the problem of long baking time for steel ladles and serious slagging. For most small and medium-sized steel mills without dedicated equipment, unpacking is difficult. In order to compensate for this deficiency, aluminum magnesium unburned bricks were successfully applied. The lining bricks are easy to build, have a short baking time, and are easy to unpack, but there are still problems with easy slag removal and poor resistance to peeling. On this basis, in order to overcome the problem of easy slag formation in the lining of aluminum magnesium non fired brick ladle, aluminum magnesium carbon non fired brick was developed in the late 1990s to replace aluminum magnesium non fired brick. This material combines the properties of aluminum magnesium materials and carbon containing materials. The main advantages of aluminum magnesium carbon unburned bricks are: no brick seam melting loss, good slag resistance and thermal vibration resistance, overcoming the structural peeling caused by the infiltration of molten steel and slag, and significantly improving the service life. This product has attracted people's attention due to its non burning or low-temperature firing, which saves energy, reduces costs, and has significant economic benefits.
The excellent properties of magnesium based products combined with spinel (magnesium aluminum spinel), such as slag resistance, peel resistance, and creep resistance, have long been known. The former Soviet Union began research in 1942 and product development in 1964, but Europe did not show significant interest in this product until the late 1970s. Japan began using magnesia alumina spinel bricks in the cement industry in 1976. In recent years, there has been an increasing amount of research on magnesium aluminum spinel and its products internationally.
Magnesium aluminum spinel (MgO · Al2O3) has a high melting point, low thermal expansion, low thermal stress, and good thermal vibration stability. At the same time, it has stable chemical properties and strong resistance to alkaline slag. It is the core point for the use of aluminum magnesium unburned bricks and one of the key substances to improve their service life. Due to the gradual maturity of artificial synthesis technology for magnesium aluminum spinel in recent years, it has become possible to directly produce ladle bricks using synthesized spinel materials, which can significantly improve their characteristics.
The quality of magnesia alumina spinel is one of the key issues related to whether magnesia alumina spinel bricks can achieve suitable results. Through laboratory experiments and the introduction of relevant data, it has been found that magnesium rich spinel exhibits good corrosion resistance, structural peeling resistance, and thermal vibration resistance within the range of 20% to 30%. However, there are various conditions and factors in the production process of steel ladles, and it is reasonable to require the amount of magnesium aluminum spinel added to be between 30% and 40%. Therefore, in the process of brick making, in addition to adding the produced artificial spinel, it is also necessary to add fused magnesium powder and corundum powder to generate secondary spinel during use In order to improve the performance of the brick body's quality, special attention should be paid to the selection of raw materials to control the content of harmful impurities, especially low melting point substances Na20 and K2O.
The chemical formula of magnesium aluminum spinel (also known as spinel) is MgO-Al2O3, containing 28.3% Mg0 and 71.7% Al2O3. Spinel is only an intermediate compound in the Mg0-Al2O3 binary system phase diagram, with a melting point of 2135 C. Bartha pointed out that compared with magnesia chrome spinel, magnesium aluminum spinel has the main advantages of strong corrosion resistance to reducing atmospheres such as free CO2, free SO2/SO3, and free K2O/Na2O, as well as good thermal stability and wear resistance. At present, the Na2O content of most spinel refractory materials used in industrial production is between 8% and 15%.
The production process of magnesia alumina spinel bricks is as follows.
Magnesia sand (or fused alumina spinel sand) - crushing - sieving (fine grinding - fine powder) - raw materials - mixing - forming - drying - high-temperature firing - products
The production process of magnesium aluminum spinel is as follows.
Magnesia sand and fused magnesia alumina spinel sand are crushed according to the process to achieve the required particle composition and fine powder fineness of the aggregate Strictly weigh and mix the prepared aggregates and fine powders according to the process requirements, and add a certain amount of binder; Mix in the mixer for 15-20 minutes, form on the friction press, and dry the semi-finished product for 8-16 hours; Put the dried bricks into a tunnel kiln and burn them, with a maximum firing temperature of 1660-1680 ° C
The raw materials for magnesia alumina spinel unburned bricks are selected from mid-range sintered magnesia sand, pre synthesized magnesia alumina spinel, silicon micro powder, and alumina micro powder.
Due to the high temperature required for firing non fired bricks, they can be directly used for masonry after baking. Therefore, the strength requirements for non fired bricks at room temperature are relatively strict, and the bonding agent used must also meet the following requirements: good bonding effect on non fired bricks mainly composed of magnesia sand, sufficient mold strength for pressed brick blanks, and a room temperature strength of over 40 MPa after baking to meet transportation and masonry requirements; The binder is easy to use Do not complicate the production process: The bonding performance of the binder should be strong, and the amount added should be small to avoid significantly reducing the high-temperature performance of unburned bricks.
At present, two types of binders are usually selected: one is a water glass solution and uses sodium fluorosilicate as a hardening agent; The other type is polyphosphate as a binder, with a dosage of 0.8% to 1.0%. After treatment with 200T x 24 hours, the room temperature compressive strength has reached 80MPa; When using a mixture of water glass and sodium fluorosilicate as a binder, the amount added is relatively large. In theory, the high temperature performance of the product will be affected by introducing too much Na2O.
For magnesium refractory materials, when polyphosphate is used as a binder, the melting points of Mg (H2PO4) 2, Mg (PO3) 2, MgHPO4, and Mg2HP2O4 generated from magnesium sand are all very high. During the heating process, polymerization can also occur, which helps to improve the strength of the material, without phase transition and causing the structure of the binder to become loose, resulting in high strength at room temperature to medium temperature. From the perspective of production technology, polyphosphate can be added in solid form or dissolved in water in liquid form, making it more convenient and simple to use. It can meet the needs of large-scale industrial production, but the production cost is slightly higher.
Determination of spinel addition amount: Rich magnesium spinel is introduced into unburned bricks in the form of powder, with the purpose of pre embedding a portion of spinel crystal nuclei in the unburned brick matrix, so that magnesia sand and alumina micro powder can undergo spinel petrification reaction based on this crystal nucleus during use, which can easily generate MA-MA bonding phase, thus enabling unburned bricks to sinter as soon as possible and produce a crystal phase structure similar to that of magnesia alumina spinel fired products, in order to achieve the purpose of alkali corrosion resistance and peel resistance. In theory, the bonding strength of this MA-1/1 phase is higher than that of the MA-M phase. And it has little sensitivity to the atmosphere inside the kiln and possible local reduction conditions. In terms of its addition amount, the introduction of magnesium rich spinel as a powder should not be too much, otherwise it will affect the sintering of unburned bricks during use. It is more appropriate to control the introduction amount of magnesium rich spinel between I% and 1.5%.
7. Application of magnesia alumina spinel bricks in cement rotary kilns:
With the maturity of domestic rotary kiln technology, magnesia alumina spinel bricks are widely used in the unstable kiln skin area with tail in cement rotary kilns. Its performance is mainly reflected in: good hanging kiln skin performance, excellent thermal shock resistance and anti peeling performance, and strong physical and chemical stability. Among various refractory bricks used in cement rotary kilns, magnesia alumina spinel bricks are a relatively comprehensive type of refractory brick.