Today I will share the magnesium oxide in the electric heating tube with new and old customers. There are a lot of dry goods in this sharing, and I hope to better assist my growing friends. I also hope that everyone will like and choose our products better.
Magnesium oxide powder refers to the fused crystalline magnesium oxide block that is crushed and matched with different particle sizes or numbers in a certain proportion, directly or modified, used in tubular electric heating elements as an insulating medium that conducts heat at high temperatures. In the electric heating tube, it mainly plays the role of fixing the center position of the resistance wire, ensuring that the metal shell is not charged (insulating), and conducting the heat emitted by the resistance wire.
According to the production method and main purpose, it can be divided into the following four categories,
1. Common type Classification code: P.
2. Low temperature and moisture-proof type Classification code: D.
3. Medium temperature moisture-proof type Classification code: Z.
4. High temperature type Classification code: G.
Performance requirements of magnesium oxide powder:
1. When MgO powder is at working temperature, it must have high thermal conductivity, so as to quickly transfer heat to the surface of the tube, and make the resistance wire closer to the tube wall temperature;
2. When the working temperature is within 1100℃, MgO powder should have good insulation properties;
3. MgO powder must have a certain degree of granularity. The shape is generally round rather than flakes currently produced in my country. Because the round powder has high density, good fluidity, and a certain percentage of particles, it is not easy to add powder Damage the heating wire to ensure that the powder can be filled without difficulty;
4. MgO powder should not corrode heating wire materials and pipes at room temperature or high temperature.
5. The water absorption rate of magnesium oxide powder shall not be higher than the following regulations:
(1), common type ≤1.5%
(2), low temperature and moisture-proof type ≤0.05%
(3), medium temperature and moisture-proof type ≤0.10%
(4), high temperature type ≤0.10%
The resistivity of magnesium oxide powder shall not be lower than the value specified in the following table:
Temperature ℃ 600 700 800 875 900 975 1000 1100
Resistivity Ω. cm 5*109 1.5*109 3*108 8*107 5*107 1.4*107 1.0*107 3*106
Note:
1. The selected test temperature is between the above temperature points, and the resistivity is determined by linear interpolation,
2. Select two points T1 and T2 in the test temperature range from T minus 50℃ to T plus 100℃, and T2 minus T1 is equal to 100℃, high temperature magnesium oxide T2 is equal to 975℃, and T1 is equal to 875℃. Particle size analysis of electrical grade magnesium oxide powder;
After the magnesia ore is crushed, the size of the particle size is different, if a certain number of ratios are used, it has the following advantages.
1. It can increase the powder density and reduce the working temperature of the resistance wire, thereby increasing the life of the electric heating element;
2. It can overcome the "sieving" effect; improve the utilization rate of MgO powder;
Based on actual use experience, I have summarized the following data, which are now provided for your reference:
Diameter 6.6~8.0(mm) Diameter 8.5~12(mm) Diameter 12~18(mm)
Particle size (mesh) Content (% by weight) Particle size (mesh) Content (% by weight) Particle size (mesh) Content (% by weight)
60~140 73.8 50~140 76.30 40~140 86
140~200 15.4 140~200 11 140~200 7.4
200~325 7.15 200~325 10 200~325 4.5
~325 3 ~325 2.4 ~325-
Analysis of the state of magnesium oxide powder under heating:
The porosity of the MgO powder compressed in the tubular heating element is 15% under normal conditions, that is, the density of the MgO powder at this time is: the true density of the MgO powder minus the porosity, that is, 3.85*(1-15%)=3.05 G/cm3. If the temperature of the heating element is sufficiently high when the heating element is working, the oxygen in the pores will interact with the resistance wire and the pipe. Due to this reaction, part of the oxygen pressure will be reduced. The final state of the part of the oxygen pressure determines the oxygen and the metal parts of the element. The affinity between. According to data, some oxygen pressure may drop to 10-13~10-19ata. Under such low oxygen pressure, the nature of the fine particles of fused MgO powder changes, that is, the common MgO powder turns black. Under oxidizing conditions, MgO powder mainly evaporates in molecular form and does not decompose at high temperatures. MgO powder may be partially reduced. MgO is decomposed as follows:
MgO solid≒Mg+1/2 O
According to data: the decomposition pressure of magnesium oxide powder at different temperatures can be calculated by the following formula:
10logP=-(A*104)/T+BlogT+C*10-3+D*10-5 T-2+E
Where P: decomposition pressure (value); T: temperature, 0 K (range 9320K~13930K);
A=2.6061; B=0.2680; C=-0.62578; D=0.0932; E=7.3377.
After substituting the data, it is calculated:
When T1=9230K (650℃), then P1=4.68016*10-21
When T2=10730K(800℃), then P2=3.92101*10-17
When T3=11730K(900℃), then P3=4.43868*10-15
When T4=12230K(950℃), then P4=3.52367*10-14
When T5=12730K(1000℃), then P5=2.37276*10-13
It can be seen that when the pressure in the heating element drops by 10-19~10-13 ata at 800~1000℃, MgO has the conditions for reductive decomposition. Only by adding a "reducing atmosphere", the necessary conditions for the reduction and decomposition of MgO are sufficient, so that the MgO powder starts to turn black. If the cross-sectional area of the heating element is affected, the insulation of the heating element will decrease and deteriorate. As a result, the leakage current increases. Some sources also believe that this reaction is reversible. If annealed in the air at 900°C, the original properties of the powder will be restored.
What problems should be paid attention to in the use of electrical grade magnesium oxide powder?
1. For stainless steel components with a relatively long tube (such as 4-6 meters), they must be annealed at a high temperature of 900°C before bending. In this case, the resistance wire should be made of nickel-chromium wire instead of iron-chromium wire as much as possible. Because aluminum (Al) is particularly easy to deprive the oxygen in the MgO powder at 900℃ (aluminum and oxygen have a strong affinity), the result is partial reduction of magnesium oxide, that is, MgO changes the surface of magnesium, which will make the MgO powder black. Of course, nickel is used. Chromium wire is much better, because the affinity of chromium and oxygen is small, and the affinity of nickel and oxygen is also very small, so the MgO powder must be reduced, that is, the reduction temperature is much higher.
2. Increasing the compression density of MgO powder can reduce the working temperature of the resistance wire, but it is meaningless for annealing (referring to stainless steel 900 ℃) for the use of electricity, because the former has a temperature gradient, and the latter has temperature at various points equal.
3. Reducing the thickness of the insulating layer of the MgO powder can also reduce the working temperature of the MgO powder and increase the life of the heating tube.
4. Prevent the generation of organic matter and rust spots on the inner wall of the pipe. Therefore, the pipe, especially the iron pipe, must be pickled and dried to wipe the rust. After the process is completed, the pipe is quickly filled with powder and must not be left for too long. Especially in the high temperature season to avoid rust.
5. Since the MgO powder itself has organic matter that will cause the magnesium powder to turn black, it is recommended to roast the used MgO powder before filling the powder.
We remind more customers: MgO powder or block should not be exposed to sun and night dew, and should not be exposed to rain in the open air to prevent acid rain and harmful gas from being immersed in the block and powder. The quality of magnesium powder determines one of the excellent factors of electric heating tubes
