As early as 1960s, magnesia-alumina spinel brick was developed, but it was not put on the market. In order to solve the problems in the production and use of chromium, Japan introduced magnesia brick containing spinel in 1970s. That is, sintering spinel is added to magnesia, and magnesia spinel brick is fired in tunnel kiln at high temperature (1 900C), and spinel is used to replace chromium oxide and iron oxide.
At that time, various magnesia-alumina spinel bricks emerged one after another, but their service life was not as good as that of chrome-magnesia bricks. After finding out the cause of damage, the performance of this brick has been fundamentally improved, which can be comparable to that of chrome-magnesium brick.
The excellent properties of spinel (magnesia-alumina spinel) combined with magnesia products such as slag resistance, spalling resistance and creep resistance have long been known. The former Soviet Union started research from 1942 and developed products from 1964. However, Europe did not show great interest in this product until the end of 1970s, and Japan began to use magnesia-alumina spinel brick in cement industry from 1976. In recent years, the international research on magnesia-alumina spinel and its products is increasing.
In the 20th century, China began to study the synthesis of magnesia-alumina spinel from bauxite and magnesite (or light burned MgO). In recent years, a series of studies have been made on the preparation technology of magnesia-alumina spinel bonded magnesia products, and many achievements have been made. However, high-purity products with excellent properties have always been limited by high sintering degree. Some people in China use active spinel powder to reduce the foreign ultra-high temperature sintering process (1 850c) to 1 660C. However, it is still difficult for ordinary refractory kilns with coal as fuel. Therefore, domestic fire experts and scholars have studied the properties of high purity magnesia brick and spinel bonded magnesia brick with self-made activated magnesia-alumina adhesive as binder. Using the activity of magnesium-aluminum glue, these two products with excellent properties were fired at 65438 0550℃, which made it possible to fire these advanced products in kilns and lakes of ordinary refractory factories.
Refractory researchers and experts in some large domestic iron and steel enterprises have recently developed and applied magnesia-alumina spinel unburned bricks. Based on the production practice of active lime brick kiln, magnesia-alumina spinel unfired brick was developed. The application shows that the service life of magnesia-alumina spinel unburned brick fired by active lime rotary kiln exceeds 1 year. For a long time, direct bonded magnesia-chrome brick has excellent slag resistance and corrosion resistance, and is widely used in large-scale alkaline rotary kiln. However, with the increasingly serious problem of environmental protection, hexavalent chromium produced after the use of magnesium-chromium products has become a recognized problem in the world. Therefore, magnesia-alumina refractories with good spalling resistance, low thermal expansion rate, less tissue degradation and corrosion resistance were developed and applied to alkaline rotary kiln as the best substitute for magnesia-chrome refractories. Since the appearance of unshaped refractories in the early 20th century, they have been widely used in metallurgical industry. Today, the output of unshaped refractories in some industrialized countries accounts for almost half of the total refractories. At present, the most widely used refractory in iron and steel industry is magnesium-aluminum unshaped refractory, which accounts for about 85% of the total output of unshaped refractory, and is widely used in converter, ladle, heating furnace and blast furnace, as well as almost all metallurgical thermal equipment.
Magnesia-alumina spinel (MgO·al2o 3) has high melting point, small 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. This is the core of using aluminum-magnesium baking-free brick and one of the key substances to improve its service life. Due to the gradual maturity of synthetic magnesia-alumina spinel technology in recent years, it is possible to directly produce ladle bricks from synthetic spinel materials, which can significantly improve their properties.
The quality of magnesia-alumina spinel is one of the key problems related to whether magnesia-alumina spinel bricks can achieve suitable results. Through laboratory test screening and related data introduction, it is found that magnesium-rich spinel has good corrosion resistance, structural spalling resistance and thermal shock resistance when it is 20%-30%. However, there are many operating factors in ladle production, so it is reasonable to require the addition of magnesia-alumina spinel to be 30%-40%. Therefore, in the process of brick making, in addition to the artificial spinel produced, fused magnesium banknote powder and corundum powder must be added to generate secondary spinel in order to improve the performance of brick tomb. In order to control the content of harmful impurities in raw materials, especially Na20 and K2O with low melting points, special attention should be paid to the selection of raw materials.
The chemical formula of magnesia-alumina spinel (also known as spinel) is MgO-Al2O3, containing 28.3% MgO and 7 1.7% Al2O3. Spinel is only an intermediate compound in the phase diagram of MgO-al2o 3 binary system, and its melting point is 2135 C. Bartha pointed out that magnesia-alumina spinel has better corrosion resistance, thermal stability and wear resistance than magnesia-chrome spinel. The Na2O content of most spinel refractories produced and used in industry at present is about 8%- 15%.
The production process of magnesia-alumina spinel brick is as follows.
Magnesium oxide (or fused alumina spinel sand)-crushing-screening (fine grinding-fine powder)-raw materials-mixing-molding-drying-high temperature sintering-products.
The production process of magnesia-alumina spinel is as follows.
Magnesite and fused magnesia-alumina spinel sand are crushed according to the process, and the aggregate particle composition and fine powder fineness meet the process requirements. The prepared aggregate and fine powder are strictly weighed and mixed according to the process requirements, and a certain amount of binder is added. Mixing in a mixer for 15-20 minutes, molding on a friction brick press, and drying the semi-finished product for 8- 16 hours; The dried bricks are fired in tunnel kiln, and the highest firing temperature is 1660- 1680℃.
The raw materials of magnesia-alumina spinel baking-free brick are medium-grade sintered magnesia, pre-synthesized magnesia-alumina spinel, silica micropowder and alumina micropowder.
Because baking-free bricks do not need high-temperature firing, they can be directly used for masonry after baking, so the requirements for room-temperature strength of baking-free bricks are strict, and the binder used should also meet the following requirements: it has a good bonding effect on baking-free bricks mainly made of magnesium oxide, and the pressed brick blank should have sufficient mold strength, and the room-temperature strength after baking can reach more than 40 MPa, meeting the requirements of transportation and masonry; The binder is easy to use and does not complicate the production process: the binder should have strong bonding performance and small addition amount, so as not to greatly reduce the high temperature performance of unfired bricks.
At present, two kinds of binders are usually selected: one is sodium silicate solution with sodium fluorosilicate as curing agent; The other is polyphosphate as an adhesive. When the dosage is 0.8%- 1.0%, the compressive strength at room temperature after 200t×24h treatment has reached 80MPa. However, when the mixed solution of sodium silicate and sodium fluorosilicate is used as an adhesive, the addition amount is large. Theoretically, it will attract too much Na2O, which will affect the high temperature performance of the product.
For magnesium refractories, when polyphosphate is used as binder, the melting points of Mg(H2PO4)2, Mg(PO3)2, MgHPO4 and Mg2HP2O4 generated by polyphosphate and magnesium oxide are all very high, and the polymerization reaction which is helpful to improve the strength of the material can occur during the heating process, and the structure of the binder will not be loose, so the strength of the binder at room temperature to medium temperature is high. From the production process, polyphosphate can be added in solid state or dissolved in water in liquid state, which is more convenient and simple to use and can meet the needs of large-scale industrial production, but the production cost is slightly higher.
Determination of spinel addition: Magnesium-rich spinel is introduced into unburned brick in the form of powder, in order to pre-store a part of spinel crystal nucleus in the unburned brick matrix, so that magnesium oxide and alumina micropowder can react with spinel based on this crystal nucleus, and MA-MA combined phase is easily generated in the use process, so that unburned brick can be sintered as soon as possible, resulting in a crystal phase structure similar to that of magnesia-alumina spinel sintered products, thus achieving the purposes of alkali corrosion resistance and spalling resistance. Theoretically, the bonding strength of Ma-11phase is higher than that of MA-M phase. But also insensitive to the atmosphere in the kiln and possible local reduction conditions. As far as its addition amount is concerned, the introduction amount of magnesium-rich spinel as powder should not be too much, otherwise it will affect the sintering of unfired bricks in use, and it is more appropriate to control the introduction amount of magnesium-rich spinel at I%- 1.5%.
The function of micropowder and its addition amount are determined as follows.
A part of sio2 micropowder and Al203 micropowder are introduced into magnesia-alumina spinel brick. The function of Sio2 _ 2 micropowder is mainly to increase the strength at room temperature and promote sintering; The main function of Al-203 micropowder is to react with the fine magnesia powder in the tomb to generate magnesia-alumina spinel, so as to improve the thermal shock stability and erosion resistance of unfired bricks. According to the systematic analysis of MgO-Al2O3-SiO _ 2 phase diagram, it is harmful for SiO _ 2 micropowder to be adsorbed on unburned bricks mainly composed of magnesia. However, if the content of SiO _ 2 is less, SiO _ 2 will preferentially form forsterite with magnesia in the use process, thus forming a part of ceramic bonding phase in the baking-free brick, which is conducive to promoting the sintering of the baking-free brick as soon as possible. With the increase of Al2O3 powder content, the thermal shock stability of unburned brick shows an upward trend, because with the increase of Al2O3, the amount of magnesia-alumina spinel in unburned brick will also increase, and its thermal shock stability will certainly increase, but the addition of Al2O3 powder should not be too much.