The protective role of refractory bricks in the firing zone in rotary kilns

The main role of refractory bricks in rotary kilns is to protect the kiln barrel from damage by high temperature gases and high temperature materials and to ensure normal production. In industrial production, the use cycle of refractory bricks in firing zone is very short, which often leads to unplanned kiln stoppage for maintenance and is a key factor affecting good, high production, low consumption and annual operating rate of cement kilns.

A. Erosion mechanism of refractory bricks

Whether it is a wet kiln, or a new dry rotary kiln, in the clinker calcination process, due to the kiln gas temperature is much higher than the material temperature, kiln each rotation, the surface of the kiln lining is subject to periodic thermal shock, the temperature change range of 150 ~ 250 ℃, in the kiln lining 10 ~ 20mm surface range of thermal stress. The kiln lining is also subjected to the radial and axial mechanical stresses of alternating brick masonry due to the rotation of the kiln, as well as the scouring and abrasion of the calcined materials. Due to the simultaneous generation of silicate melt, it is easy to interact with the surface of the kiln lining refractory bricks to form an initial layer in a high temperature environment, and simultaneously penetrate along the pores of the refractory bricks to the interior of the refractory bricks and adhere to them, causing changes in the chemical composition and phase composition within 10-20 mm of the surface layer of the refractory bricks and reducing their technical properties. When the sintering range of the material is narrow or the formation of short flame rapid firing to produce local high temperature, it will make the minimum temperature of the kiln skin surface is higher than the solidification temperature of the liquid phase of the material, the surface layer of the kiln skin that is changed from solid to liquid and fall off, and from the surface and deep into the initial kiln skin after the formation of a new kiln skin initial layer. When this situation occurs repeatedly, firing belt kiln lining gradually from thick to thin, or even completely off, resulting in local exposure of the kiln cylinder body and red kiln. In fact, the firing zone kiln lining damage is exactly the case, the thickness of the residual brick in the high-temperature region is generally arc distribution with a large radius of curvature, and sometimes the bottom of the arc falls on the inner surface of the kiln.

Second, the protection of refractory bricks

  1. The impact of the physical properties of refractory bricks

Slag resistance refers to the ability of refractory materials to resist chemical erosion, in the formation of the initial layer of kiln skin and when the material viscosity is large or produce local high temperature prompted by the kiln skin off the case, slag resistance is very important.

Porosity and thermal conductivity play an important role in the formation of the initial layer of kiln skin, and in the case of partial shedding of the kiln skin, refractories with greater porosity and thermal conductivity help the timely replacement of the kiln skin. But at the same time, it may show a great destructive effect, so that the thin layer of refractory bricks peeling off.

Corey refractory bricks in its production process, its physicochemical changes generally have not reached the equilibrium state at firing temperature. There are also insufficiently fired refractory bricks, and thus in the role of rotary kiln and then subjected to high temperature, most of the refractory bricks due to the generation of its own liquid phase and the filling of pores, the occurrence of irreversible refired shrinkage. Therefore, high temperature volume stability, must be taken into account when selecting refractory bricks for the firing zone.

Thermal surface lamination is the main form of damage to the kiln lining of the rotary kiln firing zone after thermal shock; if local kiln skin shedding occurs at the same time, it will make the refractory bricks use cycle much shorter.  

  1. Combustion and fuel nozzle on the impact of refractory bricks

When using coal as fuel, coal volatile matter and ash play a decisive role, directly affecting the flame shape. Higher volatile matter and ash content of the lower coal powder, can make the black flame head shortened, the formation of low-temperature long flame calcination. For the protection of kiln lining is generally beneficial, but the volatile fraction is too high, the fire is too fast, so that the kiln clinker temperature up to 260 ℃ or more, the secondary wind temperature of more than 900 ℃, it is very easy to burn the nozzle, so that its deformation or burned gap, resulting in disorderly flame shape, before it is replaced to damage the kiln lining. The volatile matter of coal is too low (less than 0%) and the ash content is too high (more than 28%), and the incomplete combustion of a large amount of pulverized coal will settle in the material to burn and release a lot of heat will also damage the kiln skin.

Fuel nozzle structure in the production is often not enough attention, nozzle shape and outlet size mainly affects the degree of mixing of pulverized coal with the primary wind and spray speed, sometimes to strengthen the mixing of wind and coal, you can also add wind fins in the nozzle, but pay attention to the cyclone wind rotation amplitude is too large to sweep the kiln skin.

  1. Raw material composition fluctuations on the impact of refractory bricks

When the aluminum rate is too high, the liquid phase viscosity is large, the kiln skin a large number of collapse, operation is not easy to control, the protection of the kiln lining is not conducive to the production practice of aluminum rate is generally controlled at 1.3 ~ 1.6; when taking high saturation ratio, high silicon rate, low liquid phase ingredients easy to produce viscous material washout, abrasive kiln skin so that the kiln thin serious damage to the kiln lining, the production practice of silicon 2.5, saturation ratio should not exceed 0.92, when the silicon rate of 2.8. Saturation ratio should not exceed 0.90.

Raw material feeding fluctuations are more harmful to the kiln lining. When there is too much raw material in the kiln, it is necessary to turn off the exhaust air at the end of the kiln and increase the amount of pulverized coal to force the fire to burn, so that the heat load of the firing zone increases rapidly and the kiln lining is seriously damaged. When the kiln is too little material, coal powder flame obviously downward, the area of the kiln skin in the high temperature will fall off, thinning, puff to the thinner layer of material, if not timely adjustment of wind and coal volume, it is easy to burn the kiln skin and refractory bricks. In addition, the fluctuation of raw material feeding amount will lead to unstable thermal system in the kiln temperature is too large kiln skin off or damaged.

Through the above analysis, when the kiln clinker temperature is up to 1260℃ or more and the secondary air temperature exceeds 900℃, it is easy to burn the nozzle, make it deformed or burnt and gap, produce disorderly flame shape, and make the kiln lining easily damaged. Clinker three rate value is generally controlled at KH0.91±0.01, silicon rate 2.6±0.1, aluminum rate control between 1.3 to 1.6, to protect the refractory brick life cycle and improve the strength of clinker is extremely beneficial.