Refractory bricks for cement kilns, that's the way to go!

With the continuous increase in cement production, the speed of CRKs is accelerating, and the calcination temperature is constantly rising. More and more production lines are simultaneously processing waste materials, hazardous waste, and other substances. All these factors pose significant challenges to the service life of refractory bricks inside the cement kiln.

In addition to the impact of changes in kiln speed, elevated temperature process conditions, and co-disposal of cement kilns, the damage to refractory bricks is also influenced by chemical factors.

I. Chemical Damage Mechanisms

1. Basic SaltPermeation

Basic salt permeation refers to the infiltration and condensation of alkali salt compounds in the brick's voids, forming a horizontal permeation layer of alkali salts within the brick. The damage caused by alkali salt permeation can be observed as distinct salt deposition layers on the residual brick surface. The upper transition zone is mainly infiltrated and deposited with KCl, while the lower transition zone is primarily infiltrated with K₂SO₄. The inconsistent properties between the permeation layer and the original brick layer make it prone to spalling under thermal shock. Refractory brick damage due to alkali salt permeation is characterized by visible color changes and significant differences in chemical composition between the layers after salt permeation.

To mitigate the damage caused by alkali salt permeation, it is necessary to select refractory bricks with good resistance to alkali and minimize the basic salt content introduced into the kiln.

2. Alkali Erosion

Alkali elements such as Na and K in the cement kiln react with Al₂O₃ and SiO₂ in the refractory brick, leading to alkali spalling and subsequent brick damage.

Alkali salts erode the periclase and bonding phases in the high-temperature zone, causing a porous structure in this area. Extensive infiltration and erosion by cement clinker result in the formation of a dense altered layer. The deposition of alkali salts in the low-temperature zone increases the thickness of the dense altered layer. The dense and thick altered layer undergoes severe structural spalling during temperature fluctuations, particularly affecting magnesia-alumina spinel and magnesia-iron spinel.

To prevent damage caused by alkali erosion, it is essential to control the alkali-sulfur ratio (ASR) of refractory bricks at around. Choosing refractory materials with excellent alkali resistance and ensuring low porosity and impurity content in the refractory material can be beneficial.

3. Oxidation-Reduction Spalling

Oxidation-reduction spalling in refractory bricks is characterized by whitening on the working face or darkening on the non-working face (which can regain color after overburning). This type of damage occurs due to volume changes in Fe²⁺ and Fe³⁺ within the brick resulting from fluctuations in the kiln's oxidation and reduction atmospheres, leading to structural spalling. Refractory bricks containing iron elements, such as magnesia-iron bricks, are significantly affected by oxidation and reduction atmospheres.

Using low-iron, chrome-free bricks, especially in cement kilns with co-disposal configurations, requires proper fuel and air ratio management and improving coal powder fineness to effectively address refractory brick damage caused by oxidation-reduction.

4. Hydration Cracking

Hydration cracking is characterized by whitening on the surface of refractory bricks, with extensive hydration resulting in spider-web-like fracturing. This occurs due to the reaction between MgO and H₂O in the air, leading to volume expansion and cracking of the refractory bricks.

To prevent hydration cracking, it is crucial to store refractory bricks in a dry indoor environment. Expired magnesia bricks are not recommended for use, and careful inspection for hydration cracks should be carried out before use.

II. Selection of Refractory Bricks

To address the chemical damage issues that occur in cement kilns, a certain company has developed ZJMA magnesia-alumina spinel bricks and ZJMI magnesia-iron spinel bricks, which can effectively reduce the damage to refractory materials caused by chemical factors and prolong the service life of products.

1. Advantages and Application of ZJMA Magnesia-Alumina Spinel Bricks

By optimizing the selection of raw materials, ZJMA magnesia-alumina spinel bricks reduce the levels of SiO₂ and Fe₂O₃, which affect product stability, to a reasonable range, thereby enhancing the bricks' resistance to erosion. The introduction of alumina-based spinel during high-temperature sintering enables the in-situ production of spinel in the product and utilizes the micro-cracks generated during the process to improve the thermal shock stability of the product.

Magnesia-alumina spinel phase is stable at high temperatures and is not easily corroded by alkaline substances. Therefore, magnesia-alumina spinel bricks have lower ASR and are less prone to react with alkaline substances. The low solubility and chemical reactivity of magnesia-alumina spinel phase allow it to maintain structural stability in alkaline environments, making the bricks resistant to alkali erosion and prolonging their service life.

Since magnesia-alumina spinel bricks are widely used in the transition zone of cement kilns, which has higher temperatures and more complex environments, ZJMI magnesia-iron spinel bricks have been developed specifically for this area.

2. ZJMI magnesia-iron spinel bricks have the following advantages and application areas:

Composition: ZJMI magnesia-iron spinel bricks are primarily made from magnesia sand and iron-aluminum spinel. By introducing active spinel into the matrix composition, a network-like spinel structure is formed during the sintering process, providing resistance against sulfur, chlorine, salt, and alkali erosion and penetration.

Low ASR: The iron-aluminum spinel phase in magnesia-iron spinel bricks does not readily react with alkaline substances, resulting in lower ASR. This allows the magnesia-iron spinel bricks to maintain structural stability when exposed to alkaline environments.

Chemical Inertness: Magnesia-iron spinel exhibits high chemical inertness, low solubility, and reactivity towards alkaline substances. Therefore, magnesia-iron spinel bricks can effectively resist erosion and corrosion by alkaline materials in alkaline environments.

Environmental Friendliness: Magnesia-iron spinel bricks are chromium-free and low in iron content, making them an environmentally friendly alternative to magnesia-chrome bricks. They are widely used in the burning zone of cement kilns.

Both magnesia-alumina spinel bricks and magnesia-iron spinel bricks offer advantages such as low ASR, excellent alkali resistance, outstanding resistance to alkali salt penetration, and being chromium-free and low in iron content. These combined advantages enable these products to effectively address common chemical damage issues in cement kilns, ensuring efficient and safe operation while achieving energy savings, reducing costs, and meeting environmental requirements. They are ideal environmentally friendly alkaline refractory materials for cement kilns.