Significant characteristics of refractory materials used in hazardous waste rotary kilns and furnace drying process

Significant characteristics of refractory materials used in hazardous waste rotary kilns and furnace drying process

Hazardous waste rotary kiln

Hazardous waste rotary kiln incineration generally involves several stages, including drying, pyrolysis, combustion, and burnout. This process ensures the harmful components in the hazardous waste are fully decomposed and destroyed under high temperatures, forming high-temperature flue gas and slag. This high-temperature flue gas and slag can corrode and damage the refractory materials lining the rotary kiln.

Therefore, refractory materials used in rotary kilns for hazardous waste incineration should possess the following characteristics:

(1) High-temperature resistance. They should be able to operate for extended periods at temperatures above 800℃.

(2) High strength and good wear resistance. The refractory materials used for the rotary kiln lining need to have sufficient mechanical strength to withstand the expansion stress at high temperatures and the stress caused by the deformation of the kiln shell. Simultaneously, because hazardous waste and dust in the flue gas can cause wear to the kiln lining, strong wear resistance is required.

(3) Good chemical stability to resist the erosion of chemical substances in the flue gas. The substances that have the greatest impact on the quality of refractory materials are alkalis (potassium, sodium), halogens (chlorine, fluorine), and sulfur compounds. (4) Good thermal shock resistance, capable of withstanding alternating thermal stress during combustion. In situations of shutdown, startup, and unstable rotation, the temperature fluctuations within the kiln are significant. This necessitates that the refractory material not crack or peel under such drastic temperature changes.

(5) Good high-temperature volume stability. Although the coefficient of thermal expansion of the rotary kiln shell (generally carbon steel plate) is greater than that of the rotary kiln refractory material, the shell temperature is typically around 150–300℃, while the refractory material typically withstands temperatures above 800℃. This could lead to the refractory material having greater thermal expansion than the rotary kiln shell, making it more prone to peeling.

(6) Low porosity. High porosity allows flue gas to penetrate and corrode the refractory material.

Currently, in hazardous waste incineration plants both domestically and internationally, rotary kilns are typically constructed using refractory bricks, with refractory castables used only at the ends. Commonly used refractory bricks include mullite bricks and high-alumina bricks, while the castable is primarily high-alumina refractory castable. After the refractory brick lining of a rotary kiln used for hazardous waste treatment is laid, a kiln drying process is required. The purpose of this process is to remove moisture from the refractory material masonry, ensuring it can perform at high temperatures and maintain the strength of the masonry. The specific drying procedure is as follows:

Furnace Preparation:

1) Check if the refractory material masonry and pouring meet the specifications;

2) Prepare the materials needed for furnace drying, such as fuel, tools, water, compressed air, etc.;

3) Other preparations for the entire system, such as: fire protection system, electrical system, environmental conditions, etc.

Furnace Drying Curve

The main materials for rotary kiln masonry are high-alumina bricks, mullite bricks, and high-alumina castables. The furnace heating process mainly includes three key temperature points:

The first temperature point is 150℃. This temperature point is primarily for evaporating free water in the masonry. Because water turns into water vapor at 100℃, but evaporation is slow, it must be above 100℃ for effective drying. Based on experience, 150℃ is suitable; the dehydration at this stage is not significant and should not be too rapid. If the water is removed too quickly, the diffusion rate of residual internal water cannot keep up with the surface evaporation rate. The internal water will be heated and turn into steam, causing expansion, which leads to slurry shrinkage and cracking, reducing bond strength and weakening the masonry strength. This is especially true for castables. Therefore, during the process from room temperature to 150℃, the temperature rise should not be too rapid, ideally controlled at 10℃·h⁻¹. When reaching 150℃, it is necessary to hold the temperature for a period of time to ensure the masonry is fully dry.

The second temperature point is 300℃. At this temperature point, the main purpose is to eliminate the expansion and thermal stress caused by the evaporation of moisture in the masonry. Considering the large linear expansion coefficient of high-alumina bricks, the temperature increase from 150 to 300℃ should not be too rapid. High-alumina bricks are generally heated at a rate of 30–50℃·h⁻¹ to ensure uniform and stable expansion.

The third temperature point is 600℃. This temperature point is primarily for the evaporation of crystal water in the masonry. During the furnace drying process, this temperature point should be maintained for at least 48 hours to allow as much crystal water as possible to precipitate out of the masonry, ensuring thorough drying.

In the initial cooling phase, large amounts of cold air should be avoided from entering the furnace, as this could lead to excessively rapid cooling and affect the strength of the masonry. When the furnace temperature drops below 150℃, the masonry can be allowed to cool naturally, completing the furnace drying process.

Post-furnace drying inspection

The post-furnace drying inspection mainly involves checking the refractory material masonry. Observe the drying condition of the refractory materials, check the joints between the refractory bricks, and if any are loose, tighten them with locking steel plates; check for cracks in the refractory materials, and if cracks are larger than 3mm, fill them with refractory castable.