10 Defects in Refractory Products Fired In Tunnel Kilns

When firing refractory products in a tunnel kiln, there are many factors that may result in defects in products. The main defects include under-burning, over-burning, cracks, mute brick, black core and embossing, white bricks, black bricks, bloating, limestone burst and efflorescence. 

(1) Under-burning

When firing bricks in tunnel kilns, since the firing temperature is low or the high temperature firing time is short or the temperature holding time is not enough, the reaction between different components is not sufficient and the molten liquid in the brick is too few, resulting in yellow and mute products with poor strength. The way to eliminate the defect is to control the firing temperature strictly and properly extend the holding time. 

(2) Over-burning

When firing bricks in tunnel kilns, since the firing temperature is too high or the holding time is too long, there is too much molten liquid generated in the bricks. This makes the bricks have large shrinkage and the size smaller than the required size. Although the structure is dense and the strength is high, they are still unqualified products. The way to eliminate the defect is to control the firing temperature strictly and properly increase the cooling rate if the holding time is too long. 

(3) Cracks

There are three types of cracks and the causes are different. 
A. hairline crack
If the bricks are cooled too fast, the surface of the bricks are cooled and shrink while the internal of the bricks is still hot and expand. Internal stress will be generated between in the surface and inside the bricks. Hairline crack is caused by the internal stress. The way to eliminate the hairline crack is to keep the firing temperature stable and the cooling not too fast, strictly control the moving rate of the cars and stop using air supply by the kiln end fans. 

B. Burst

Since the green bodies are wet, the moisture content is too high and the preheating is fast, the moisture is vaporized dramatically in a short period of time and then accumulates in the brick. When the vapor pressure exceeds the strength of the bricks, they burst. The way to eliminate the burst is to control the moisture content, extend the preheating time and rise the temperature slowly. 

(4) Mute bricks 

There are many reasons for mute bricks. Some are in the brick itself. The mute bricks can be distributed throughout the kiln. Some are caused by operational errors. Mute bricks are distributed in some part of the kiln. Therefore, for mute bricks, check and analyze its distribution and causes and then take appropriate action. 
The way to eliminate this defect is:
A. To store the dried green bodies in a dry environment and timely discharge low temperature wet flue gas in the kiln to avoid resurgence;
B. To fire the high moisture green bodies in time and not to place them in an alternating hot and cold environment;
C. To fully consider the moisture content of green bodies and adopt a reasonable heating rate;
D. To take reasonable cooling measures and to control the cooling rate, not too large or too small. 

(5) Black core and embossing

Black core and embossing are common in internal combustion bricks. Internal combustion bricks are green or blue inside the bricks since Fe3+ is reduced into FeO or Fe3O4 due to the lack of oxygen. Those low valence iron oxides have a strong fluxing action. Therefore, these parts have more liquid generated and more shrinkage, resulting in uneven phenomena. 
The ways to eliminate the black core and embossing include:
A. Changing the stacking way to reduce the overlapping area;
B. Properly extending the holding time and burn the internal fuels completely;
C. Adopting long-time low temperature firing and controlling the heating rate to avoid too early vitrification;
D. Supplying enough air to ensure complete combustion of fuel;
E. Increasing the kiln ventilation rate and burn the bricks thoroughly.

(6) White brick

There are always sulfides in the raw materials and added combustion materials, especially in the low quality coal. During the firing process, sulfides generate sulfur dioxide by the combustion reaction. Sulfur dioxide is oxidized to sulfur trioxide in an oxidizing atmosphere. This sulfur-containing gas with damp flue gas is absorbed by the low temperature bricks when passing through the preheating zone and generates sulfate on the surface after firing. The sulfate makes the surface white. The white bricks can be reduced or eliminated by reducing the moisture content in the green bodies, reducing the density of the brick stack, shortening the preheating zone and increasing the ventilation amount. 

(7) Black brick

During the firing process, green bodies are buried in the unburned coal char and ash and separated from air. High valence iron is reduced into low valence iron and red bricks turn to black or blue. This kind of brick is called black bricks, whole black or part black. As long as the stacking of the bricks meets the requirements, the assignment of the ventilation is reasonable, the particle size is appropriate and the coal is added a few once but frequently, black bricks can be reduced or eliminated. 

(8) Bloating

When the bricks are over burned, the molten liquid is formed too early in the firing process, resulting in the vitrification of the bricks in the surface, but the reaction of gas generation inside the bricks does not end. The gas cannot escape from the high viscosity melt in the blank and accumulates in the bricks, resulting in bloating. Reduce the internal combustion substances in the bricks, control the firing temperature and heating rate, adopt long-time burning at low temperature, slow the firing process and properly control the amount of excess air. All these can eliminate the bloating. 

(9) Limestone Burst

Limestone minerals in the raw materials may generate quick lime during the firing process. After the bricks are out of the kiln, quick lime is hydrated and digested and becomes slaked lime under the action of vapor in the atmosphere. The volume of the lime expands 1.5-3.5 times. The volume expansion stress is great and causes the burst of the bricks and results in flaking and crushing. The way to eliminate the burst of limestone is to enhance the crushing of raw materials and keep the particle size under 1mm. Extend firing time and appropriately increase the firing temperature by 30-50 degree to promote the reaction of limestone with silica compounds to form silicates. It not only can eliminate limestone burst, but also can improve the mechanical strength of the brick. 

(10) Efflorescence 

Efflorescence is caused by the recrystallization of solutions of soluble salts in the brick on the surface. In serious cases, it will cause scaly peeling in the bricks and affect the durability of the brick. If you extend the firing time, increase raw material crushing fineness, and fire the bricks totally when producing multi-hole bricks and hollow bricks, this can reduce or eliminate the efflorescence problems. 
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