The bonding agent of magnesia carbon brick is mainly liquid thermosetting phenolic resin, and the drying process of magnesia carbon brick is actually the curing process of thermosetting phenolic resin, so it is necessary to dry the magnesia carbon brick after forming.

With the increase of temperature, the thermosetting phenolic resin will undergo a very complex curing reaction, which not only depends on the reaction temperature, the structure of the raw material, the activity of the phenol hydroxyl group, but also depends on the catalyst used in the synthesis of the resin. The thermosetting phenolic resin is generally divided into two stages when it is heated and cured.

The first stage:

When the temperature is lower than 170 ° C, the main reaction is that the molecular chain is lengthened, hydroxymethyl groups react with active hydrogen on other molecules, remove a molecule of water, and form methylene bonds. The hydroxyl group reacts with the hydroxyl group on other molecules to remove one molecule of water and form a dibenzyl ether. The above two processes are the main reactions of the first stage curing reaction, in addition to a number of other reactions.

The second stage:

When the temperature exceeds 170℃, the reaction of the second stage begins to be intense, until about 200℃, the reaction of phenolic resin at high temperature is very complex, mainly including the further reaction of dibenzyl ether, dibenzyl ether bond is unstable, easy to decompose into methylene bonds, and escape formaldehyde; We still have the phenol alcohols that we didn't finish in the first stage. The second stage of the reaction is characterized by the curing material into reddish-brown, the deeper color may be dark brown, then the main generated is methylene benzoquinone and its polymer, the reaction generated more and more solid, the viscosity is getting larger and larger, their main form of connection is methylene bond, its curing temperature is 170 ~ 250℃.

According to the curing principle of thermosetting phenolic resin, the heat treatment temperature of magnesium-carbon brick is generally set at 200℃. The holding time is generally more than 10 hours, the reason why the heat treatment temperature is set at 200 ° C, because this temperature can ensure that the resin is completely cured, and will not consume too much heat energy, and it is easy to reach this temperature with ordinary fuels and kilns, which is a more economical temperature.

In order to ensure the heat treatment temperature of 200 ° C, various enterprises have designed many different forms of kilns according to their own characteristics. According to the heating form of the kiln can be divided into coal kiln, resistance wire kiln and microwave heat treatment kiln, these three types of kiln has its own characteristics, the following will be introduced one by one.

Magnesia carbon brick commonly used heat treatment kiln type

1. Coal kiln

Coal kiln is the first heat treatment kiln adopted by the early magnesia-carbon brick manufacturers. The flue is distributed in the kiln when the coal kiln is designed. As shown in Figure 1, the newly repaired part is the damaged flue. When the coal is burned, the high-temperature flue gas passes through the flue, thus increasing the temperature in the kiln. When the coal kiln is designed, there is basically no electrical auxiliary equipment, only a thermometer at the head and the end of the kiln to observe the temperature in the kiln.

Coal kiln as a heat treatment method is used by people, it must have its certain characteristics. When magnesium carbon brick first appeared, people did not pay much attention to its appearance, the use of the effect is better to attract people's attention, so this very primitive kiln entered the line of sight, as long as the effect of magnesium carbon brick heat treatment can be achieved, so many of his shortcomings were ignored.

2, resistance wire kiln

With the development of magnesia carbon brick, people's requirements for heat treatment kiln are getting higher and higher, at this time, many shortcomings of coal kiln begin to reveal, such as the instability of kiln temperature, the appearance of the product is not beautiful, and the smoke burning traces are serious. Therefore, people began to design resistance wire kilns to replace coal kilns. Figure 2 shows the resistance wires arranged inside the resistance wire kilns for heating. Compared with coal kilns, resistance wire kilns have many advantages:

(1) Fast heating speed, uniform temperature, accurate temperature control;

(2) Good production environment, no dust, no smoke marks;

(3) Full automatic control, no human interference, high safety;

(4) Because the resistance wire kiln is uniformly heated throughout the kiln, and the temperature is uniform and stable, the whole kiln body has a long life, and basically it can be done without maintenance, and the only thing that needs to be replaced is the resistance wire.

From the above points, it can be seen that the use of resistance wire kiln is the trend of modern production, and the complete replacement of coal kiln is just around the corner, and in fact, this is also proved. Of course, compared with the resistance wire kiln, the coal kiln is not useless, it has a non-negligible advantage, that is, the cost of heat treatment is low, according to the actual production comparison, the cost of heat treatment of a ton of magnesia-carbon bricks in the coal kiln is lower than the cost of heat treatment of a ton of magnesia-carbon bricks in the resistance wire kiln is 30~50 yuan. At this stage, in addition to a few relatively backward manufacturers and the pursuit of low-cost manufacturers also retain a part of the coal kiln, the rest of the manufacturers are basically resistance wire kilns.

3, microwave heat treatment kiln

Microwave heat treatment kiln is a new type of heat treatment equipment, although its theoretical basis has been established long ago, and has long been applied to microwave ovens, but its real investment in industrial production time is not too long, many of its technology and experience is not as rich as the resistance wire kiln, especially the heat treatment process of magnesia carbon brick, not only the process of moisture volatilization, It's the curing process of the phenolic resin.

For drying moisture only, microwave heat treatment kilns have the following characteristics:

(1) Fast drying speed, saving time. Conventional methods such as: steam drying method, electric heating drying method, hot air drying method, etc., the water content of about 30% of the body dehydration to the water content of less than 1%, at least more than 20 hours, and the use of new microwave heat treatment equipment only takes more than 20 minutes to complete; It takes more than ten hours for the billet with water content of about 10% to dehydrate to water content of less than 1% by conventional methods, and it only takes more than ten minutes to use the new microwave heat treatment equipment. The water content of about 5% of the body dehydration to water content of less than 1% of the conventional method takes six to seven hours, the use of microwave heat treatment equipment can be completed in a few minutes.

(2) High energy efficiency, easy to operate. Because the microwave is directly acting on the material, there is no additional heat consumption, and the conventional heating drying equipment is affected by the environment, and the heat loss is very large, especially in winter. And the microwave heat treatment equipment heating speed is fast, with the use of open, do not need to heat up and cool down like the conventional heating drying equipment, saving heat. At the same time, the power is adjustable and the operation is convenient.

(3) Good effect, can maintain the primary color of the material, no cracking. Microwave heating does not require a medium for heat conduction, microwave directly acts on the material, making it heat itself, heating speed, uniform temperature, will not cause material fission phenomenon.

(4) Assembly line operation, good working environment. Compared with ordinary heat treatment methods, microwave heat treatment equipment is simple and clear, covers a small area, less ancillary equipment, as long as there are the most basic conditions of water and electricity can be put into production. In contrast, it can generally save 30% to 50% of electricity, optimize the working conditions, and save the working area. And the operation of the equipment is simple, the operation of the whole equipment only needs about three people, and the noise is small during operation, which greatly improves the on-site working environment.