Refractories for Rotary Kilns in Five Industries

Home » Refractories for Rotary Kilns in Five Industries

Rotary kilns are cylindrical steel structures lined with refractory materials. They are employed in industries such as building materials, metallurgy, chemicals, and environmental protection for the calcination of materials through a rotating process. The cement industry utilizes rotary kilns the most, with thousands of them nationwide. Following closely are the black and non-ferrous metallurgical industries, where rotary kilns are used for the calcination of ores and intermediate materials such as iron, aluminum, copper, zinc, tin, nickel, tungsten, chromium, and more. Additionally, these kilns are employed for calcining materials like active lime used as a steelmaking flux, various applications of dolomite (such as in converter steelmaking slag-making agent, metallic magnesium raw material, refractory raw material, etc.), calcined magnesite, aluminum-magnesium spinel, alumina, hard clay, and other refractory materials. They are also used for the calcination of rare earths, ceramic particles, and the incineration of waste materials. The calcination temperature varies depending on the materials, ranging from below 1000°C to a maximum of 2000°C in some cases.

Despite their diverse applications, these kilns share a common working principle. Materials enter from the kiln’s tail and exit from the head. The kiln is divided into several zones based on temperature: the tail zone, preheating zone, calcining zone (or sintering zone), cooling zone, and the head zone (some industries further divide it into the front transition zone and the rear transition zone). Each zone utilizes different refractory materials, although the basic mechanism of refractory material damage is similar. The refractory lining inside the kiln rotates with the shell, subject to stresses generated by the kiln’s rotation, as well as the high-temperature material’s rolling impact, friction, erosion, and other effects. Due to the different types of materials being processed, the refractory lining materials also vary.

In order to improve the service life of the refractory lining, in addition to selecting the appropriate refractory materials, it is also necessary to improve the kiln construction technology, as well as the maintenance and repair technology of the kiln lining. In many industries rotary kiln, this paper selects representative cement kilns, dolomite and lime kilns, pellet kilns, zinc oxide kilns, waste incineration kilns, etc.

While the materials being calcined in rotary kilns vary across industries, the fundamental working principles remain the same. As a result, there are both similarities and differences in the refractory materials used for the linings. For instance, different zones of the kiln utilize refractory materials of varying qualities, and both the kiln head and tail often use high-alumina, low-cement, steel-fiber castables. The following sections will delve into the specific refractory materials chosen for different industries.

Although the calcined materials of rotary kilns in various industries are different, the working principle is the same, so the refractory materials lining also have the same and different, such as the lining is divided into different quality refractory materials, and the kiln head and kiln tail are made of high alumina and low cement and steel fiber castables. Different industries choose different refractory materials are described below.

insulation board application
insulation board application

Rotary Kiln in Cement Industry

In the early 1980s, the technology of refractory materials for cement rotary kilns was essentially established. Generally, for the inner drum of large SP and PC rotary kilns, direct-bonded magnesia-chrome bricks are used in the burning zone, high-alumina bricks in the decomposition zone, and alkali-resistant bricks or common clay bricks in the rear part of the drum. Silicon molybdenum bricks emerged in the late 20th century and early 21st century. Apart from the burning and the front and rear kiln mouths, different grades of silicon molybdenum bricks are used in other parts of the kiln. In today’s cement industry, the new dry-process kiln is the mainstream of technological development. The firing temperature is around 1450°C, and the temperature of the combustion gas in the kiln can reach over 1700°C, even approaching 2000°C.

To meet the requirements of the second generation of new dry-process cement technology, the selected refractory materials must possess various functions such as energy-saving, environmental friendliness, long life, high efficiency, safety, and stability. Therefore, magnesia-alumina spinel bricks are used instead of magnesia-chrome bricks in the burning zone (due to the production of toxic hexavalent chromium harmful to humans). Magnesia-alumina spinel bricks are also used in the transition zone, and a new composite brick with a three-layer structure (working layer, insulation layer, and insulation layer) is experimented with. The working layer is made of high-strength, wear-resistant, and corrosion-resistant mullite. The insulation layer contains zirconia-alumina fiberboard, and the insulation layer has high strength, low thermal conductivity, and acts as a framework connecting the working layer and the insulation layer. At 37-47m in the kiln, the shell surface temperature is reduced by 60°C compared to silicon molybdenum bricks. Additionally, high-performance refractory mortar, prefabricated components, and other innovations enable large cement rotary kilns to transition from annual major maintenance to major maintenance every three years.

refractory bricks for cement rotary kiln1

Lime and Dolomite Kilns

The chemical composition and properties of lime and dolomite are similar. They have approximately the same calcination temperature, leading some enterprises to alternate between burning lime and dolomite in the same kiln. The refractory lining for these kilns generally falls into three types:

Magnesia-Alumina Spinel Brick for Calcination Zone

  • Burning Zone: Magnesia-alumina spinel brick
  • Transition Zone: High-alumina brick
  • Preheating Zone: Clay brick
  • Insulation Layer: Refractory fiber or lightweight brick
  • Surface Temperature: 300–350°C
  • Lining Lifespan: 1–2 years

Spinel Composite Brick for Calcination Zone

  • Burning Zone: Spinel composite brick (light and heavy)
  • Surface Temperature: Below 250°C
  • Lining Lifespan: 2–3 years

Prefabricated Blocks and Castables for Lining

  • Composition: Prefabricated blocks and castables
  • Surface Temperature: Below 250°C
  • Lining Lifespan: 2–3 years

These variations in refractory lining design cater to different temperature requirements and offer varying lifespans. The choice between these types depends on factors such as the specific needs of the production process and the desired longevity of the refractory lining.

Pellet Kiln (Grate Cooler – Rotary Kiln)

In recent years, the pellet kiln has experienced rapid development, with a total production capacity of pellets exceeding 60 million tons annually. The kiln head temperature of the rotary kiln ranges from 1150 to 1180°C, while the kiln tail temperature is between 1050 and 1080°C. The temperature in the material receiving area of the grate cooler reaches 1100–1300°C. The selection of refractory materials depends on the usage conditions of different parts. The wear-resistant high-alumina castable is mainly used for the chain grating machine, and lightweight mullite castable is also employed. Some castables may incorporate anchor bricks. For the rotary kiln, high-alumina mullite prefabricated bricks with anchor devices and castables of the same material are typically interspersed and mixed for construction. In the hanging brick section of the first cooling zone of the grate cooler, phosphate bricks are replaced with fired high-alumina bricks, and the furnace roof is changed to low-cement castable, extending the lifespan from a few months to two years.

Zinc Oxide Kiln

Rotary kilns are the primary equipment for the pyrometallurgical production of zinc oxide. In the charge, coke powder and other reducing agents are added, and by leveraging the volatile nature of zinc metal, zinc vapor enters the flue gas. Under high-temperature conditions and in the presence of oxygen, zinc oxide is produced. The extensive production of dust during steelmaking, containing zinc, has led to the development of zinc oxide rotary kilns alongside the growth of the steel industry. In a certain factory, the drying and preheating zone of the rotary kiln uses clay bricks, while the reaction zone, which affects the lifespan, employs a variety of refractory materials, extending from the reaction zone to the cooling zone.

In a specific factory in Shandong, a mixture of calcined bauxite, fused magnesia, synthetic magnesia-alumina spinel, SiO2, Al2O3 micro-powder, combined with pure calcium aluminate cement, water-reducing agents, explosion-proof fibers, castables, and prefabricated bricks, anchored with fixtures, has proven effective in the high-temperature zone of the zinc oxide rotary kiln. This has created favorable conditions for the production of composite products.

Alumina Oxide Rotary Kiln and Other Applications

Alumina Oxide Kiln

Burning Zone Material Temperature: 1050–1300℃.

Traditional high-alumina bricks and magnesia bricks are not ideal.

Improved Approach: Using anti-stripping high-alumina bricks to line the kiln shell, which has proven to be more effective.

Petroleum Coke Kiln

Historical Approach: Used to involve casting the lining, with long construction periods, significant shrinkage, and short lifespan.

Improved Approach: Adopting prefabricated composite bricks with anchor pins and castable for comprehensive construction, resulting in better performance.

Waste Incineration Kiln

Operating Temperature: Approximately 800–1200℃.

Challenges: Due to the complex composition of waste, it causes severe erosion on refractory materials.

Options: Some choose to build the high-temperature zone with chrome corundum bricks. Others opt for a double-layer composite of clay bricks and zirconium-chrome corundum bricks.

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