The main raw materials for magnesium-carbon bricks include: sintered magnesium sand or electrofused magnesium sand, scaled graphite, and organic binding agent.
The main raw material used in the production of magnesium carbon brick is magnesium sand. In the choice of magnesium sand to ensure the quality of magnesium carbon bricks, should be used in low impurity content, high purity, remelted by the electric furnace and the crystallization development of magnesium sand is more complete. In the use of magnesium carbon bricks, magnesium sand particles will be eroded its process is generally:
(1) At high temperature, magnesium sand particles react with graphite in solid phase:
MgO+C=Mg+CO(g)
Generation of magnesium vapor volatilized with CO.
(2) slag will be magnesium sand particles melting loss, the main material including slag and magnesium sand in the various types of oxide impurities.
(3) magnesium carbon bricks in use due to the surface oxidation decarburization, the brick body combined strength decline, due to slag erosion and scouring, so that the magnesium sand particles and brick body detachment wrapped into the slag.
In the higher inches, magnesium carbon brick choose to do the main raw material of magnesium sand should be a larger grain, less impurities and magnesium oxide containing although higher magnesium sand, such magnesium sand not only reduces the degree of silicate relative to the magnesite crystal division, so that the slag on the crystal boundary of the erosion rate is reduced, and improves the magnesium sand particles and graphite at high temperatures in the same time under the stability of the existence of. Moreover, because of this type of magnesium sand its density is larger, strong bonding force, in the magnesium sand processing process obtained particles with distinctive boundary prisms, and matrix mosaic bonding was strengthened, magnesium sand particles in the magnesium-carbon bricks in the stability of the magnesium was improved.
Magnesia brick is mainly used in the fused magnesia, this is because the fused magnesia than the sintered magnesia crystallization of coarse grains, the bulk density of the particles is greater, but the cost is higher than the sintered magnesia. In order to make the use of the effect and cost of organic combination, magnesium carbon bricks added to the fused magnesium sand is according to different levels of different ratios. In addition to the magnesium sand’s own performance of its particle ratio on the use of magnesium carbon brick performance also has a greater impact. As an aggregate of coarse particles in the ingredients accounted for a large proportion, and the determination of the critical size range has a more important impact on the performance of the brick.
The melting point of flake graphite is about 3700 ℃, it has a lamellar structure, high thermal conductivity and low coefficient of thermal expansion and modulus of elasticity, etc. It is an excellent carbon raw material for the preparation of magnesium carbon bricks. Graphite scale thickness with the purity of the increase and improve the antioxidant performance is also improved, high temperature loss of creep is small, Jasper is its price is also the higher. In the production of magnesium-carbon bricks need to be based on the actual situation to determine the amount and quality of graphite, but also to consider the production costs and the use of the effect. Many of the properties of magnesium carbon bricks and graphite have a close relationship with the graphite, with the graphite used although increased, magnesium carbon bricks thermal conductivity and thermal stability improved, but the bulk density, room temperature mechanical strength, coefficient of thermal expansion has decreased.
Combining agent is the key material for the production of magnesium carbon bricks. Nowadays, most of the production of magnesium-carbon bricks use synthetic phenolic resin as the binding agent.
The binding agent is the key material for the production of magnesium-carbon bricks. It should have the following conditions:
(1) The binding agent should have better affinity and wetting effect on magnesium sand particles and graphite at room temperature without age hardening.
(2)The polymerization reaction can occur through heat treatment, so that the magnesium carbon bricks have high strength; (3)The magnesium carbon bricks can be used at high temperatures without ageing hardening.
(3)There is a relatively high residual carbon rate when used at high temperature, and it can have polymerization reaction with carbon material, so that the brick body has good high-temperature performance; (4)The nature of magnesium-carbon bricks has a relatively high strength.
(4) Stable in nature, not easy to deteriorate.
Production process
Various raw materials are mixed in a high-speed mixer after batching, pressed and molded, and finally dried at a temperature of about 200 ℃ to make magnesium carbon bricks.
The dosing should be such that the probability of large and medium particles coming into contact with each other is as small as possible, and this is especially true for the contact between coarse particles. This is in order to obtain a continuous carbon binding network as the skeleton, magnesium sand particles filled in the skeleton network, magnesium sand particles do not have a common interface between the microstructure for the two different coefficients of expansion of magnesium sand and graphite, due to the former’s thermal expansion by the constraints of the carbon mesh as well as the presence of magnesium sand / carbon interface so that the matrix part of the toughness increased. The overall linear expansion is smaller, and the continuous carbon network skeleton is favorable for heat conduction, so the products have high thermal stability.
The mixing of magnesium-carbon brick mixture is an important process in the production of magnesium-carbon bricks, and the general mixing machine with high-speed stirrer and rotating disk is not effective in the preparation of the mixture with high graphite content. The reasons are as follows:
(1) The graphite, which has a light specific gravity, tends to float on the top of the mixture, so that it is not in complete contact with the other components in the formula; (2) The centrifugal force of the rotating disk tends to cause some degree of segregation of certain components.
(2) The centrifugal force of the rotating disk tends to produce a certain degree of segregation of certain components, and at the same time, due to the strong vortex and the individual particles in the mixture to produce loose effect, not conducive to the uniform distribution of the binding agent on the surface of magnesium sand particles, thus affecting the magnesium sand particles of the good bonding graphite.
(3) The crystal structure of each component in the mixture, especially the large flake graphite particles will be partially damaged by the use of a high-speed mixer, which prevents the magnesium-carbon bricks from forming the organization of a continuous graphite matrix;.
Based on the above reasons, a specially designed planetary mixer with distinctive features overcomes the shortcomings of the ordinary mixer, and achieves better results even when processing 30% of scaled graphite and magnesium-carbon bricks mixed with synthetic resin as binding agent.
In the molding process, friction presses are often used for multiple pressing and molding. In the pressing process should pay attention to the pressure, the number of times, as well as to prevent the generation of defects such as layer cracks.
Heat treatment process is an important process that affects the strength and carbon residue of the resin-bonded magnesium-carbon bricks, and thus need to closely determine the heat treatment temperature and heat treatment time and other thermal regimes compatible with the bonding agent, in order to make the magnesium-carbon bricks to obtain sufficient low-temperature strength.