Fused Magnesia Chrome Brick

With magnesia and chrome ore as basic refractory materials, it is produced by melting in an electric arc furnace, casting and forming, heat preservation, and slow cooling, cutting, and other processes. The process conditions for the production of fused magnesia-chrome bricks are as follows:

Ingredients: lightly burned magnesia: chrome ore = 50: 50, add a small number of additives, mix and grind, form a billet, and dry.

Melting and pouring: the billet is melted in a three-phase electric arc furnace, and the melting temperature is 2450-2550 °C; the melt is poured into a model composed of graphite plates, and the pouring temperature is 2300-2380 °C.

Annealing: The casting body is placed in an incubator filled with vermiculite for slow cooling and annealing, and the total annealing time is about 25d.

Cutting and grinding: After the casting body is demolded, it is cut and ground into the desired brick shape.

3.1 Masonry of electric furnace lining

The HDG-3 electric furnace is used, the transformer power is 2200kV-A, the bottom of the furnace is made of zirconium ramming material and graphite carbon bricks, and the lining is made of clay light bricks and mid-grade magnesia-chrome bricks.

3.2 Pouring template

The template is made of graphite. In order to prolong the service life, the graphite plate is wrapped with a quartz sand plate to ease the temperature difference. The upper part of the model is covered to leave gates and caps, as shown in Figures 3-25.

Mould design: 230mm x 357mm x 800mm;

Pouring quality: 250kg.

Mould design: 260mm x 400mm x 800mm;

Pouring quality: 280kg.

Template: inner mold graphite, thickness 40mm;

Outer mold: quartz sand, thickness 40mm.

3.3 Raw materials

According to the technical conditions of fused magnesia-chrome bricks, water-washed chrome ore, high chrome ore, light-burned magnesia, brick-making magnesia, industrial alumina, titanium dioxide, and fluorite powder are selected as raw materials.

The raw materials are batched, mixed, and milled into a blank, and dried at 600°C.

3.4 Electrofusion process

The main process parameters of the electrofusion process are shown in the table.

Fused magnesia chrome brick

The melting temperature measured with an optical pyrometer was 2450-2550°C, and the pouring temperature was 2300-2350°C. After the fused magnesia-chrome brick is cast and formed, it is annealed in an incubator filled with vermiculite, and the total annealing time is about 25d. But for the microstructure of the brick, the distribution of thermal stress, and the mitigation of volume effects, the most critical time is the first few hours after casting.

The solid line is the measured curve, and the dashed line is the estimated annealing curve of the surface of the cast brick.

3.5 Properties of fused magnesia-chrome bricks

The physical and chemical properties of the demolded ingots were tested.

Through the tests of these different batches and numbers of fused magnesia-chrome bricks, the complete batching, melting and pouring process parameters have been determined, which can ensure the production of high-quality fused magnesia-chrome refractories.

4. Microstructure analysis of fused magnesia-chromium refractories

The chemical composition and microstructure of the trial-produced fused magnesia-chromium refractory were analyzed. Firstly, the chemical composition of fused magnesia-chrome bricks was analyzed. The C-5 sampling point is in the center of the product, and the sampling points of other numbers are successively transitioned to the outer surface of the product in two directions. From the chemical compositions listed in Table 3-13, it can be seen that the chemical compositions of different sampling points, with C-5 as the symmetrical point, show a very regular trend of gradual change to both sides, such as SiO2 content, point C-5 The content is the lowest and increases in the direction of C-1 and C-7 in turn. The same rule is also reflected in the content of A12O3, Fe2O3, and Cr2O3. On the contrary, the content of MgO is the highest at C-5, which decreases in turn in both directions. These laws indicate that in infused magnesia-chrome bricks, periclase solid solution is the first solid phase to crystallize, silicate is the melt with the lowest melting point, and finally solidifies.

It can be seen from the chemical analysis results that the composition of fused magnesia-chrome bricks is quite complex, which determines that the mineral composition and microstructure of fused magnesia-chrome bricks are also quite complex.

In the composition of fused magnesia-chrome bricks, the periclase phase is the first to condense and crystallize. These periclase phases are rounded and surrounded by other minerals. The perfectly rounded areas with white spots in the figure are the periclase phases, which are surrounded or separated from each other by white stripes (spinel), and grey infiltrating areas with a lower melting point (silicate phase).

You can also see a few small white areas with neat edges and corners, this is the spinel phase, the crystalline state is complete, and it grows from the silicate phase with a lower melting point under ideal conditions. Surrounded by a large area of silicate phase (grey infiltrating area).

Using an electron microscope energy spectrometer to analyze the composition of the main crystal phase in the fused magnesia-chrome brick, it can be found that it is a complex composition, the content of magnesia (mass fraction) is only 51.63%, while the content of Cr2O3, A12O3, Fe2O3 is higher.

Comparing the chemical composition of spinel with the solid solubility of A12O3 and Cr2O3 in an MgO melt towel, it can be seen that the relative proportions of A12O3 and Cr2O3 are consistent and very close to the theoretical value. It shows that since the solid solubility of A12O3 in the MgO melt is relatively low, as long as the content of A12O3 is increased in the composition of the fused magnesia-chrome brick, there will be more intergranular spinels in the phase structure of the fused magnesia-chrome brick. exist.

There is also a phase with a lower melting point in the fused magnesia-chrome brick, which is the silicate phase. The silicate phase includes forsterite, calcium forsterite, and even glass phases with a lower softening point. Their existence depends mainly on SiO2 content infused magnesia-chrome bricks. The silicate phase is the last condensed phase, so it often exists in the gap between the periclase phase and the intergranular spinel phase. Excessive silicate is of course detrimental to the refractory performance of fused magnesia-chrome bricks. However, if the control quantity is reasonable and the structure is reasonable, it is also beneficial to alleviate the thermal stress and volume effect of the fused magnesia-chrome brick.

The crystalline structure of fused magnesia-chrome bricks is not only related to the chemical composition but also closely related to the cooling conditions of the coagulation process.