Classification of Cement Kiln Refractory Materials

The primary function of cement kiln refractory materials is to serve as the lining of the cement kiln. They maintain the high temperatures required to burn cement clinker, protect the kiln equipment, and reduce heat loss. Refractory materials have a significant impact on cement production output, quality, energy consumption, and equipment life. From a refractory manufacturing perspective, cement kiln refractory materials have evolved from a few varieties, such as clay bricks, high-alumina bricks, and alumina cement blocks, to a family of nearly 100 materials. Today, cement kiln refractory materials include sintered alkaline refractories, sintered alumina-silica refractories, monolithic refractories and prefabricated components, and insulating refractory products.

Sintered alkaline materials include ordinary magnesia-chrome bricks, directly bonded magnesia-chrome bricks, low-chrome magnesia-chrome bricks, periclase-magnesia-aluminate spinel bricks (hereinafter referred to as magnesia-aluminate spinel bricks), magnesia-aluminate spinel zircon bricks, magnesia-aluminate spinel zirconium bricks, magnesia-ferro-spinel bricks, ferro-aluminate spinel bricks, chromium-containing magnesia-aluminate spinel bricks, magnesia-manganese spinel bricks, dolomite bricks, magnesia-dolomite bricks, dolomite-zirconium bricks, magnesia-dolomite-zirconium bricks, periclase calcium zirconate bricks, and composite alkaline bricks.

Shaped alumino-silica refractories include alkali-resistant clay bricks, high-strength alkali-resistant bricks, vault alkali-resistant bricks, alkali-resistant insulating bricks, phosphate-bonded high-alumina bricks, phosphate-bonded wear-resistant bricks, steel fiber-reinforced phosphate-bonded wear-resistant bricks, high-load soft bricks, kyanite bricks, anti-stripping high-alumina bricks, silica-molybdenum bricks, silica-molybdenum red bricks, and other specialty high-alumina bricks.

Shaped insulating refractory materials include conventional calcium silicate insulation products, hard calcium silicate insulation products, and refractory fiber products, as well as a wide variety of insulation products made from floating beads, ceramsite, diatomaceous earth, expanded perlite, hollow spheres, other lightweight materials, and combustible additives.

Monoshaped refractory materials include conventional refractory castables, low-cement refractory castables, ultra-low-cement refractory castables, cement-free refractory castables, steel fiber refractory castables, anti-explosion castables, anti-scaling castables, phosphate refractory castables, alkali-resistant castables, and insulating castables. In addition, there are various new types of monolithic refractory materials, such as self-flowing castables, pumpable castables, and injection castables.

In developed countries, monolithic refractory use already accounts for 50% of the total refractory material supply. To a large extent, the special properties of many monolithic refractory materials require controlled interfacial reactions. Taking low-cement castables as an example, cement is the most active substance, and α-Al2O3 powder and silica fume are potentially active substances. Cement hydration, the reaction between cement and particles, and the composition and structure of castables can be controlled by raw material composition, particle size, dosage, and admixtures. The conditions for achieving dispersion-agglomeration transition are: (1) the dissolution rate of the admixture is absolutely dominant; (2) the dosage of the curing agent is absolutely dominant; (3) in the early stage of hydration, the admixture controls the liquid phase properties, and its active functional groups can be adsorbed on the surface of solid particles, changing the zeta potential and producing a shielding effect, making the castable have good fluidity; (4) after the admixture is consumed, the curing agent takes control, causing the castable to quickly solidify and harden. Monolithic refractory materials originated from ordinary concrete, and their subsequent development also borrowed a lot from the achievements of modern cement concrete materials. Refractory Configurations for Various Parts of a Cement Kiln

Cement production technology development has primarily focused on the kiln process for calcining cement clinker. The configuration of refractory linings for various parts of a rotary kiln used in cement production is shown below:

1. Kiln Discharge Port and Conveyor

The linings for the kiln discharge port and conveyor are subject to severe mechanical wear and chemical attack, requiring high levels of abrasion resistance and resistance to temperature shock. The conveyor is typically constructed from high-alumina bricks with a 70-80% Al₂O₃ content, heat-shock-resistant high-alumina bricks, spinel bricks, and magnesia-chrome bricks. Heat-resistant concrete with corundum as an aggregate or silicon carbide bricks is used for the discharge port. Other refractory materials used for the front and rear kiln ports include corundum-based, alumina-based, low-cement refractory castables and steel fiber-reinforced refractory castables. The kiln head hood utilizes 16B steel fiber-reinforced high-alumina refractory castable.

2. Firing Zone

The firing zone operates at a relatively high temperature, approximately 1200-1500°C. Magnesia-chrome bricks, directly bonded magnesia-chrome bricks, sodium polyphosphate-bonded magnesia bricks, alkali-resistant bricks, spinel bricks, magnesia-zirconium bricks, and silica-molybdenum bricks are commonly used. These refractories typically exhibit high strength both in cold and hot states, as well as excellent thermal shock resistance, and are becoming increasingly popular.

Magnesia-chrome bricks: Directly bonded magnesia-chrome bricks offer high resistance to high temperatures, SiO2 corrosion, and redox reactions. They also possess high high-temperature strength, mechanical stress resistance, and excellent kiln lining properties, making them widely used in the firing zone.

When used in cement kilns, magnesia-chrome bricks, under the influence of alkali (or sulfur), convert stable trivalent chromium into highly oxidizing hexavalent chromium. Chromium compounds exceeding 10 mg/m³ in the gaseous atmosphere and 0.5 mg/l in aqueous solutions can pose serious health risks. Since the mid-1980s, industrialized countries have enacted a series of environmental and health regulations, comprehensively monitoring cement kiln exhaust dust, magnesia-chrome brick waste, and cement plant wastewater. Consequently, the use of magnesia-chrome bricks has been subject to certain restrictions.

Spinel Bricks: Appearing in the 1990s, spinel bricks not only offer strong kiln lining resistance but also offer a range of advantages in terms of resistance to alkali, molten sulfur, and liquid clinker corrosion, thermal shock, and mechanical stress from kiln deformation, as well as thermal load resistance. These bricks outperform magnesia-chrome bricks and have become the mainstream of alkaline brick technology worldwide.

Magnesia-zirconia Bricks: The most significant characteristic of zirconia is the microcracks formed around the particles, which absorb external stress and provide high fracture strength in both hot and cold conditions. In a series of comparative tests with spinel bricks, magnesia-zirconium bricks demonstrated significant advantages in terms of resistance to harmful substances such as SO₃, CO₂, and alkaline vapor; resistance to liquid clinker corrosion; resistance to the effects of redox atmospheres; and compressive strength. However, magnesia-zirconium bricks require the addition of large amounts of scarce zirconium oxide, resulting in a high price and uncertain raw material supply.

3. Transition Zone

The transition zone, adjacent to the firing zone, is characterized by high and fluctuating kiln temperatures, frequent kiln lining lining bridging and severe chemical attack. Commonly used refractory materials include high-alumina bricks made from corundum and bauxite (50-80% Al₂O₃), directly bonded magnesia-chrome bricks, standard magnesia-chrome bricks, and spinel bricks. In recent years, silica-molybdenum bricks have been widely used in cement kiln transition zones. These bricks feature a high softening point, high strength, high adhesion, low thermal conductivity, and excellent spalling resistance. They also offer superior resistance to penetration by kiln charge, coal melt, and volatile components primarily composed of sulfate and alkali oxides, and exhibit superior corrosion resistance compared to alkaline bricks. This brick also offers better thermal shock resistance than alkaline bricks and boasts far greater structural strength. It is highly resistant to combined damage from mechanical stress, thermal stress, chemical reactions, overheating, thermal fatigue, and other factors.

4. Cooling Zone

The cooling zone temperature remains relatively high (approximately 1100-1300°C), but chemical corrosion is less severe than in the previous zone. High-alumina bricks, magnesia-chrome bricks, phosphate-bonded high-alumina bricks, magnesia-alumina spinel bricks, and corundum-based high-strength, low-cement refractory castables are generally used.

5. Decomposition Zone

In the area connecting the decomposition zone and the preheating zone, thermal and chemical stresses are minimal, so clay bricks, high-alumina bricks, and ordinary magnesia-chrome bricks can be used. In the area connecting the decomposition zone and the wave-passing zone, higher wear resistance and high-temperature resistance are required. High-alumina bricks with an Al₂O₃ content of 50-60%, ordinary magnesia-chrome bricks, spinel bricks, special high-alumina bricks, and anti-scaling high-alumina bricks can be used. In addition, calcium silicate boards, a series of high-strength insulating bricks, high-alumina, high-strength, low-cement refractory castables, and 50S anti-scaling castables can be used as thermal insulation materials.

6. Preheating Zone

The lining of the preheating zone must have sufficient alkali resistance and thermal insulation properties. Alkali-resistant, insulating clay bricks are primarily used in industrial applications. Using lightweight bricks can reduce the kiln shell temperature by 60-100°C compared to clay bricks of the same thickness. For dry-process kilns, this can reduce unit heat consumption by 21-38 kJ/kg of clinker.

7. Preheater System

The preheater system requires lining materials with excellent alkali resistance and thermal insulation properties, such as a series of alkali-resistant bricks and alkali-resistant castables. Alkali-resistant clay bricks are primarily used within the preheater and precalciner's cylindrical and conical sections, as well as within the connecting pipes. Fireclay masonry is used. The roof can be constructed with firebricks, backed by mineral wool or concrete. Castables are often used at elbows. Dense semi-silica clay bricks are used in areas such as the kiln tail riser to prevent alkali corrosion.

Alkali-resistant castables: Under alkali corrosion, a glaze layer forms on the surface of the alkali-resistant castable after use. This glaze layer forms a substance called kAs2, surrounded by a glassy matrix. At lower temperatures, a liquid phase forms on the castable surface. This viscous liquid phase seals surface cracks and prevents alkali from penetrating the interior of the refractory material.

8. Cooler System

The material temperature inside the cooler fluctuates most, and refractory erosion is uneven. This is particularly true at the cooler neck and between the tertiary air duct inlet and the cooling neck. Furthermore, dust accumulation and the expansion of the masonry can damage the side walls. Refractory materials used in the cooler system include mullite high-strength, wear-resistant castables, refractory bricks, lightweight castables, insulating bricks, and insulating panels. Ordinary magnesia-chrome bricks and high-alumina bricks can be used in the throat area and high-temperature zones; clay bricks can be used in medium- and low-temperature zones.

For some large rotary kilns, the lower portion of the kiln hood experiences a relatively high heat load. If the curing temperature of conventional high-alumina refractory castables is not properly controlled, cracking and blockage can occur. The top of the kiln hood, near the tertiary air duct, is subject to relatively severe erosion from dust-laden airflow. Furthermore, the top castable is difficult to construct, requiring high material flow and early strength.

9. Other Areas

In addition to the areas mentioned above, all other critical equipment in cement kilns requires refractory lining. Tertiary air duct elbows and dampers are subject to significant thermal fluctuations and are eroded by high-temperature clinker particles. This makes the castable prone to loosening and flaking, making them the most susceptible to wear during cement plant operation. Wear-resistant castables are typically used. C4 and C5 cones, their discharge pipes, the calciner cone, and the smoke chamber are particularly susceptible to scaling and are difficult to remove. Manual cleaning with iron tools inevitably causes mechanical damage to the refractory castable. Severe scaling requires kiln shutdown for treatment. High-strength, anti-scaling silicon carbide castables are recommended. They have a maximum operating temperature of 1400°C, a bulk density of 2.50 g/cm³ or higher after drying at 110°C, a silicon carbide content of 55% or higher, a flexural strength of 11 MPa or higher at 110°C for 24 hours, and a compressive strength of 80 MPa or higher at 110°C for 24 hours. In addition, C1-C3 should use high-strength alkali-resistant castables, and C4 and C5 (except the cone) should use high-temperature, high-strength alkali-resistant castables. The maximum operating temperature is 1400℃, the bulk density after baking at 110℃ is ≥2.20g/cm3, the alumina content is ≥45%, the flexural strength at 110℃×24h is ≥8MPa, and the compressive strength at 110℃×24h is ≥80MPa. The decomposition furnace uses high-strength wear-resistant castables with a maximum operating temperature of 1600℃, the bulk density after baking at 110℃ is ≥2.70g/cm3, the alumina content is ≥80%, the flexural strength at 110℃×24h is ≥10MPa, and the compressive strength at 110℃×24h is ≥100MPa.