Magnesia-Calcium Refractory Materials for Rotary Kilns

The main reasons why some Western countries use dolomite bricks in cement rotary kilns are as follows: First, the kiln has a high operation rate (>80%). Second, the price of dolomite bricks is relatively low. Third, there are regulations to prevent chromium pollution, and waste magnesia chrome bricks cannot be discarded casually.

The advantages of dolomite bricks are high temperature resistance, kiln-hanging ability, and resistance to clinker chemical erosion. The disadvantage is that it has poor thermal shock resistance, hydration resistance, and gas erosion resistance such as CO2 and SO2. Experiments have shown that dolomite bricks used in the stable section of the kiln skin in the firing zone can achieve a lifespan equivalent to that of magnesia-chrome bricks (about 1 year). However, even if measures such as enriching magnesium, increasing zirconium, and reducing permeability are taken, magnesium dolomite or dolomite bricks are not suitable for use in sections where the kiln skin is unstable. If used in these sections, magnesium dolomite bricks, especially dolomite bricks, only have a lifespan of 3 months.

Magnesia-Calcium Refractory Materials for Rotary Kilns

If a large amount of ZrO2 is added to produce MgO-CaO·ZrO2 series materials, refractory products with excellent performance can be produced. However, the cost of raw materials is high, making the product difficult to sell.

The main problem with magnesia-based or dolomite refractories is hydration during manufacture, storage, and use.

① Dense and dead-burned magnesia-calcium sand or dolomite sand must be produced. If the dolomite sand is not dense, the produced dolomite bricks will be easily hydrated. If the sand is not burned, the size and performance of the dolomite bricks will change during firing and use, which will affect the life of the dolomite bricks.

② The produced dolomite sand must be sealed and stored in a water-tight place, and the storage period should not exceed 1 month. If the weather is humid or the temperature increases, the storage period should be shortened. In addition to protecting against rain and moisture, special attention should be paid to preventing fog and dew!

③ When crushing magnesia-calcium sand, pay attention to dust prevention. High CaO sand dust is highly damaging to human skin and mucous membranes, and can also contaminate other refractory materials.

④ After crushing dolomite sand, use it as soon as possible. It is best to grind the dolomite sand into bricks within 24 hours after crushing and put them into the kiln for calcining.

⑤ If the fine powder part of bricks is made of magnesia, it can significantly reduce the difficulty of brick making and help improve the life of the material. However, the magnesia raw materials used must be dry and cannot contain free water or excessive crystal water (loss on ignition <0.5%). If there is more magnesium hydroxide in the brick, dehydration at about 350°C will cause the CaO in the magnesia to hydrate rapidly.

⑥ When molding, it is necessary to use an anhydrous binder with a moisture content of <0.5%. Both the brick making material and the anhydrous binder must be preheated and dehydrated. Paraffin is a commonly used anhydrous binding agent. Paraffin wax is non-toxic and cheap. In particular, the physical and chemical properties of paraffin wax, such as melting point, flash point and volatilization characteristics, are ideal. However, in order to facilitate molding and reduce damage, paraffin with a lower melting point should be used in winter, and paraffin with a higher melting point should be used as a binding agent in summer. When using, be careful not to overheat the paraffin to prevent a fire. Do not use paraffin when it is wet to prevent splashing. In addition, complete fire prevention measures need to be taken in advance. If managed properly, the use of paraffin binders can ensure reliable and safe production.

⑦ When forming, it is best to use imported automatic hydraulic presses. Magnesia-calcium or dolomite bricks cannot use conventional fire mud to adjust the brick joints. If the brick shape error is large, the service life of dolomite refractory materials will be greater than that of magnesia refractory materials.

⑧ When firing, especially for dolomite bricks with high CaO content, a flash heating process must be used to quickly raise the brick surface to above 500°C. Otherwise, the refractory bricks produced may have serious cracks.

⑨ After the firing is completed, perform wax sticking treatment. Dip a slightly hot brick into liquid paraffin and take it out quickly so that the liquid paraffin can penetrate into the pores of the brick, seal the surface of the brick, and block the passage of water vapor intrusion after solidification. Some manufacturers use asphalt coating, but asphalt is harmful to the human body and is not welcomed by workers in the cement industry. In this way, the storage time of bricks can be extended to 13 months. When unpacking and building the kiln, the bricks can be effectively protected in a short period of time.

⑩ Place the bricks in aluminum foil, vacuum them, and pack them airtight. If there are no conditions, you can also use waterproof plastic or thermoplastic airtight packaging, and add lime protection inside the packaging box. From this, a shelf life of about 1 year can be obtained.

When converting from magnesia refractory materials, including magnesium chromium, magnesium aluminum, magnesium iron, and other materials, to magnesia-calcium refractory materials, the production line needs to be carefully cleaned. If spinel raw material particles are mixed into magnesia-calcium brick materials, melting holes will appear in the burned magnesia-calcium products.

When switching from magnesia-calcium refractory materials to magnesia refractory materials, the production line needs to be more thoroughly cleaned. If magnesia-calcium raw material particles are mixed into magnesia brick materials, cracks will appear in the dried or burned magnesia products. If the f-CaO content of the mixed magnesium-calcium raw materials is low and the quantity is small, the harm may be greater. Because f-CaO in these magnesium-calcium raw materials is not only small in quantity but also wrapped in MgO, which cannot be fully hydrated in a short time and cannot produce sufficient destructive power. Therefore, some refractory bricks do not crack when drying or firing, and no problems are found during random inspections. It was not until unpacking and inspection before building the kiln that a large number of briquettes were found to have hydration cracks for no apparent reason, which may cause irreparable losses.

Because free CaO has the characteristics of rapid hydration and strong pollution. Dolomite refractory materials need to be produced in a specialized and large-scale manner. Manufacturers of magnesium-calcium refractory materials need the support of the market for stainless steel, clean steel, and other refractory materials used in the production of special steel.

Dolomite bricks use aluminum foil packaging, vacuum evacuation, and waterproof coating to prevent hydration. Under normal circumstances, dolomite bricks using the above measures can be stored for 1 year. Things to note when using dolomite bricks:

• ①Do not damage the waterproof packaging during transportation.

• ②The warehouse must not be exposed to rain or moisture.

• ③Do not get the bricks wet during the kiln-building process.

• ④Do not use fire mud to build dolomite bricks or refractory materials close to dolomite bricks.