The furnace can be heated to over 1600℃ and still stand steadily, not because of the “stone”, but because of the metallic magnesium hidden in the magnesium bricks.
1.Why does MgO have to be present in magnesia bricks?
The skeleton of magnesia brick is sintered magnesia, which has only one chemical formula, MgO.
First, its melting point is 2852°C. This is significantly higher than molten iron, so it’s not the first part of the furnace wall to soften.
Secondly, MgO is alkaline. In this way, once it encounters the acidic slag running around in blast furnaces and electric furnaces, it will immediately “neutralize the acid and alkali” and slow down the corrosion rate.
Furthermore, MgO grains grow but do not crack at high temperatures. In other words, the brick body will always maintain its “skeleton” intact, so that the furnace can have a long life.
2.From “magnesium” to magnesium bricks: how functions are activated
After natural magnesia enters the factory, it is first re-burned at 1900℃. After the loose Mg(OH)₂ is completely burned into dense MgO, the density increases from 3.0 g/cm³ to 3.5 g/cm³. The pores in the bricks shrink instantly, and molten iron and alkaline vapor cannot penetrate.
Then add 3% to 5% high-purity graphite. The carbon “sews” the MgO particles together. When thermal shock occurs, the microcracks are buffered by the graphite and the bricks no longer fall off.
Finally, it is formed under high pressure and fired in a tunnel kiln at 1600℃. The MgO grain boundaries interlock with each other, pushing the strength to over 60 MPa. At this point, the refractory properties of magnesium are fully activated, eliminating the need for furnace overheating or slag buildup.
3.How to choose furnace lining according to magnesium content
Although they are all called “magnesia bricks”, the effects of different MgO contents are very different. If you make the wrong choice, the life of your furnace will be shortened by half a year.
It can be said that whether it is used in the firing zone of steelmaking converters, refining furnaces or cement rotary kilns, magnesia bricks can effectively withstand high temperatures, provide basic safety guarantees for the furnace structure, and prevent melting or softening caused by high temperatures.
In the slag line area of the molten iron ladle, the FeO content in the slag is high, so high-purity magnesia bricks are used. MgO first reacts with FeO to form high-melting-point magnesium fulvite, which blocks the corrosion on the surface.
In the firing zone of cement kiln, alkali-sulfur circulation is heavy, so magnesia-chrome bricks can be selected. The chromite phase “pins” the thermal shock cracks, which is both sufficient and economical.
The upper layer of the glass kiln heat storage chamber has high temperature but little slag, so magnesia-zirconium bricks are used instead. Zircon absorbs thermal expansion and the lattice does not block the pores.
Understand the function of magnesium and then match the indicators to the corresponding ones, and the furnace will naturally live longer.
In addition, when choosing refractory bricks, try to find a high-quality refractory brick supplier who can provide a full set of test reports. If the supplier is well selected, there will be fewer emergency stops when repairing the furnace later.