Application Selection of Refractories for Rotary Kilns in Five Different Industries
A rotary kiln, commonly known as a rotating kiln, is a steel cylinder lined with refractories. Operating through rotational motion, it is used for calcining materials in industries such as building materials, metallurgy, chemical engineering, and environmental protection. The cement industry employs the largest number of rotary kilns, with thousands across the country, followed by the ferrous and non-ferrous metallurgy sectors. Rotary kilns are utilized to calcine ores and intermediates for metals including iron, aluminum, copper, zinc, tin, nickel, tungsten, and chromium. They also process refractory raw materials such as activated lime (used as a flux in steelmaking), dolomite (for converter slagging, magnesium metal production, refractory feedstock, etc.), calcined magnesia, alumina-magnesia spinel, bauxite, and hard clay. Additionally, they are suitable for calcining rare earths, ceramsite, and waste incineration. Calcination temperatures vary by material: some processes reach a maximum of 2000°C, while others remain below 1000°C.
Despite differing operating temperatures, all rotary kilns share the same working principle: materials enter from the kiln tail and exit through the kiln head. Based on internal temperature gradients, the kiln is divided into sections such as the kiln tail, preheating zone, firing zone (also called the calcining zone), cooling zone, and kiln head (some industries further subdivide into transition zones). While refractory selections vary by section, the damage mechanisms are fundamentally similar: the refractory lining rotates with the steel shell, enduring mechanical stress from kiln rotation, as well as impact, abrasion, and corrosion from high-temperature materials. Refractory choices are tailored to the specific materials being processed.
To extend lining service life, key measures include selecting appropriate refractories, optimizing kiln construction techniques, and implementing effective lining maintenance and repair protocols. Among rotary kilns across industries, this article focuses on representative types: cement kilns, dolomite/lime kilns, pelletizing kilns, zinc oxide kilns, and waste incineration kilns. Although these kilns process different materials, their shared working principle results in both commonalities and differences in refractory selections. For example, all employ zone-specific refractories of varying grades, with high-alumina, low-cement castables reinforced with steel fibers typically used for the kiln head and tail. Industry-specific refractory applications are detailed below.
01 Cement Kilns
In the early 1980s, refractory technology for cement rotary kilns was essentially finalized. Generally speaking, in the cylinders of large-scale SP and PC rotary kilns, direct-bonded magnesia-chrome bricks were used in the firing zone, high-alumina bricks in the decomposition zone, and alkali-resistant bricks or ordinary fireclay bricks in the rear part of the cylinder. At the turn of the 21st century, silica-mullite bricks emerged. Except for the firing zone and the front and rear kiln mouths, different grades of silica-mullite bricks have been adopted for the other parts of the kiln cylinder.
Today, in the cement industry, new dry-process kilns have become the mainstream technology. The calcination temperature is around 1450°C, and the temperature of the combustion gas inside the kiln can exceed 1700°C, even approaching 2000°C. To meet the technical requirements of second-generation new dry-process cement production, the selected refractories must integrate multiple functions, such as energy conservation, emission reduction, environmental friendliness, long service life, high efficiency, safety, and stability.
As a result, magnesia-iron-alumina spinel bricks have replaced magnesia-chrome bricks (which tend to generate hexavalent chromium, a toxic substance harmful to human health) in the firing zone, and magnesia-alumina spinel bricks are used in the transition zone. Additionally, new composite bricks with a three-layer structure (working layer, thermal insulation layer, and heat barrier layer) have been trialed. The working layer is made of high-strength, wear-resistant, and corrosion-resistant mullite; the heat barrier layer is zirconium-containing alumina fiberboard; and the thermal insulation layer, which features high strength and low thermal conductivity, serves as a skeleton connecting the working layer and the heat barrier layer. This structure reduces the surface temperature of the kiln shell by 60°C at the 37-47m section, compared to silica-mullite bricks. Furthermore, the use of high-performance mortars and precast components has extended the maintenance cycle of large cement rotary kilns from one major overhaul per year to two major overhauls every three years.
02 Lime and Dolomite Kilns
Lime and dolomite share similar chemical compositions and properties, with roughly the same calcination temperature. Some enterprises alternate between lime and dolomite calcination in the same kiln. There are three main types of refractory linings used:
Type 1: Magnesia-alumina spinel bricks in the firing zone, high-alumina bricks in the transition zone, fireclay bricks in the preheating zone, and refractory fibers or lightweight bricks as the insulation layer. The kiln surface temperature ranges from 300°C to 350°C, with a lining service life of 1 to 2 years.
Type 2: Lightweight-heavyweight composite spinel bricks in the firing zone. The kiln shell surface temperature is below 250°C, and the lining service life extends to 2 to 3 years.
Type 3: A lining composed of precast blocks and castables. The kiln shell surface temperature is also below 250°C, with a service life of 2 to 3 years.
Baoshan Iron & Steel (Baosteel) replaced composite spinel bricks with andalusite bricks in the 16.1–19.55m section of the firing zone. This modification reduced the kiln shell surface temperature, lightened the kiln weight, and improved kiln coating adherence. After 2 years of operation, the andalusite bricks remained intact following kiln coating removal, while the composite spinel bricks suffered severe spalling. Due to andalusite's lower true density compared to spinel, using andalusite bricks throughout the firing zone can reduce the kiln weight by 35 tons and lower the shell temperature by 100°C.
The feed inlet, kiln head, and kiln tail are prone to damage and spalling due to impact and abrasion from bulk materials, resulting in the shortest service life among all sections. Wuhan Iron & Steel (Wugang) adopted wear-resistant and alkali-resistant castables, which operated for over 36 months without repairs. This castable uses homogenized bauxite clinker as aggregate, 1–0mm silicon carbide and its fine powder, SiO₂ and Al₂O₃ micro-powders, and ultra-low cement bonding. Its performance and application data are shown in Table 1.
For the firing zone, phosphate-bonded unburned composite bricks are adopted. The dense phase is mainly composed of homogenized bauxite clinker as aggregate, with 4% andalusite added as an expansion agent to enhance thermal shock resistance. The insulation layer is formulated with alumina hollow spheres and expanded perlite at a 2:1 ratio, achieving integral molding. The physical properties of the product are presented in Table 2.
Two types of lining structures are adopted: composite bricks combined with homogeneous bricks, and heat-insulating bricks. The service life exceeds 3 years, and the problem of kiln shell overheating has also been resolved. The performance requirements for phosphate-bonded unburned bricks are specified in Table 3.
In 1992, Anshan Iron and Steel (Ansteel) replaced magnesia-chrome bricks with magnesia-alumina spinel bricks in the firing zone of its rotary kiln and later switched to composite phosphate bricks. The lining structure was upgraded from a double-layer design to a composite structure, with the masonry method changing from dry laying to wet laying, and from sequential arrangement to staggered laying. Subsequently, a composite lining system of precast blocks and castables was adopted-both the precast blocks and castables were embedded with metal anchors for direct consolidation onto the steel shell, with a layer of thermal insulation material integrated between them. This modification reduced the kiln shell temperature from 320°C to 222°C. A further improvement involved adding a layer of NJS nano-scale microporous insulation felt inside the cylinder, lowering the shell temperature from 220°C to 176°C.
03 Pelletizing Kilns (Grate-Kiln Process)
In recent years, the pelletizing industry has developed rapidly, with total annual pellet production capacity exceeding 60 million tons. For the rotary kiln, the temperature at the kiln head ranges from 1150°C to 1180°C, and at the kiln tail from 1050°C to 1080°C. The temperature in the receiving area of the circular cooler reaches between 1100°C and 1300°C.
Refractory selection should be based on the operating conditions of the different sections:
For the grate: Wear-resistant high-alumina castables and lightweight mullite castables are mainly used, with anchor bricks incorporated into some castable formulations.
For the rotary kiln: High-alumina mullite precast bricks with metal anchors are typically laid alternately with castables of the same material, using a mixed masonry pattern.
For the circular cooler: In the first cooling zone, the hanging bricks were upgraded from unburned phosphate bricks to fired high-alumina bricks, and the roof was replaced with low-cement castables-extending the service life from several months to 2 years.
04 Zinc Oxide Kilns
Rotary kilns are the primary equipment used for zinc oxide production via the pyrometallurgical process. Reducing agents, such as coke breeze, are blended into the furnace charge. By taking advantage of the high volatility of zinc, zinc vapor is released into the flue gas and reacts with oxygen at high temperatures to form zinc oxide.
The iron and steel industry generates substantial amounts of dust containing zinc. As the iron and steel sector has developed, zinc oxide rotary kilns have also advanced. In one factory's rotary kiln, fireclay bricks are used in the drying and preheating zones. The specifications of refractory products used, from the reaction zone (a critical factor affecting service life) to the cooling zone, are presented in Table 4.
In this factory, a 5mm-thick asbestos sheet is installed between the kiln shell and the refractory bricks, but the thermal insulation effect is unsatisfactory. It is anticipated that thermal insulation composite products will be adopted instead.
The magnesia-alumina-chrome bricks produced in Luoyang have demonstrated excellent performance in service, and their physical and chemical properties are presented in Table 5.
A factory in Shandong uses castables and precast bricks formulated with bauxite clinker, fused magnesia, synthetic magnesia-alumina spinel, SiO₂ and Al₂O₃ micro-powders, bonded with pure calcium aluminate cement, and supplemented with water reducers and explosion-proof fibers. Fixed in place by metal anchors, these refractories have demonstrated excellent performance in the high-temperature zone of zinc oxide rotary kilns, creating favorable conditions for the production of composite refractory products.
05 Rotary Kilns for Alumina and Other Applications
For alumina kilns, the material temperature in the firing zone ranges from 1050°C to 1300°C. Previous trials with high-alumina and magnesia bricks yielded unsatisfactory results, while spallation-resistant high-alumina bricks have demonstrated excellent performance in kiln coating formation and service life.
In petroleum coke calcining kilns, castable linings were traditionally used, but these suffered from long construction cycles, high shrinkage, and short service lives. The adoption of integrated masonry with precast composite bricks (equipped with anchor pins) and castables has significantly improved performance.
For waste incineration kilns, although the operating temperature ranges from 800°C to 1200°C, the complex composition of waste causes severe corrosion of refractories. Some operators use chrome corundum bricks for the high-temperature zone, while others opt for a double-layer composite structure of fireclay bricks and zirconium-chrome corundum bricks.
In laterite nickel ore kilns (used for the direct reduction production of ferronickel), the lining requires high-performance refractories. Microporous magnesia-alumina-zirconia bricks have shown exceptional durability, with only about 10% wear after one year of service.