lmm@magnesiacarbonbricklmm.com

86 411-84174804

Technical support and construction service team

A brief discussion on the application of dolomite refractory bricks in cement kilns

Contact Us

Special product design, please send specific data and drawings to our mailbox or form.

Table of Contents

Abstract: Environmental protection issues have become a top priority for countries around the world. As the main refractory material in the cement kiln firing zone, magnesia-chromium bricks are increasingly harmful to the environment. The performance advantages of dolomite bricks, which have been widely used abroad, in the firing zone are analyzed in detail. On this basis, the doubts about the use of this brick and its countermeasures are analyzed and introduced.

Keywords: cement kiln; firing zone; refractory bricks; dolomite bricks; magnesia-chrome bricks; chromium pollution

Preface

Magnesia-chrome bricks are the earliest and most commonly used main refractory materials in the firing zone of cement kilns in my country. However, their environmental hazards are increasingly exposed, so there is an urgent need for environmentally friendly, healthy, and low-cost alternative products. Magnesia-iron (aluminum) spinel brick is a refractory product developed and applied in my country’s cement kiln firing zone in recent years. It is mainly used to replace magnesia-chromium bricks. Dolomite bricks have been used in foreign countries for more than 50 years. They are the most commonly used refractory products in cement kiln firing zones abroad and have also been successfully used in many large domestic cement kilns. Dolomite bricks have excellent performance in thermal shock stability, compressive strength, alkali resistance and clinker chemical corrosion resistance, and there is no environmental pollution problem from chromium. And under normal process conditions, the average life of its products exceeds 12 months. Therefore, the cost performance of dolomite bricks is higher than that of other chromium-free products (such as magnesia-aluminum and magnesia-iron spinel bricks), and it is the development direction of chromium-free development in my country.

Analysis of the application advantages of dolomite bricks in the firing zone

Long service life

At present, the commonly used refractory bricks in the firing zone mainly include zirconium-containing dolomite bricks, magnesia-chromium bricks, and magnesia-aluminum (iron) spinel bricks. Their physical and chemical performance indicators are shown in Table 1.

Table 1 Physical and chemical performance indicators of several commonly used refractory bricks in the firing zone

ItemsZirconium-containing dolomite brick Z50-CZirconium-containing dolomite brick Z60-CMagnesia-chrome brickMagnesia-alumina spinel brick Magnesia-iron spinel brick
w(Al₂O₃)/%≤1.0≤1.039.5~11.54
w(Fe₂O₃)/%≤1.0≤1.09≤0.54
w(SiO₂)/%≤1.2≤1.21≤0.41
w(MgO)/%≥45≥5775~8086~9085~90
w(CaO)/%≤50≤45   
w(ZrO₂)/%≥1.2≥1.2   
w(CrO)/  8  
Volume density/(kg·m³)2.85~2.952.90~3.00 2.90~3.02.95
Apparent porosity/%12.0~16.012.0~16.0 13.0~17.015
Normal temperature compressive strength/MPa4040404040
Flexural strength/MPa88555
Load softening temperature (T0.6)/℃≥1600≥16501650≥1600≥1600
Thermal conductivity (1000℃)/(W·m²k-‘)3.23.22.533
Thermal shock stability (air temperature 950℃)/time)≥100≥100≥100≥100≥100
Linear expansion rate (1000 ℃)/%1.31.31.21.11.2

As can be seen from Table 1:

(1) The content of Al₂O₃ and Fe₂O₃ in dolomite bricks is relatively low, totaling about 2%, while the total content of Al₂O₃ and Fe₂O₃ in magnesia-chromium bricks and magnesia-aluminum (iron) spinel bricks is about 10%. During the use of refractory bricks, the two low-melting-point mineral phase components, Al₂O₃ and Fe₂O₃, easily react with clinker or other components and cause damage to the refractory bricks. Because the content of these two components in dolomite bricks is very small, the service life of refractory bricks is increased.

(2) The high-temperature flexural strength of zirconium-containing dolomite bricks is higher than that of spinel bricks and magnesia-chrome bricks, so the thermal properties of dolomite bricks are more superior. Because flexural strength is one of the most important indicators of refractory materials used in cement kilns, its size depends entirely on the sintering degree and structural strength of the product. The flexural strength indicates the degree of direct bonding of the product. If the direct bonding rate is high, the flexural strength must be high; and if the flexural strength is high, the ability to resist shear stress damage must be high.

The kiln skin is easy to hang and the kiln skin is strong

Zirconium-containing dolomite refractory bricks contain higher calcium oxide. During use, the calcium oxide in the bricks can react with C₂S in the clinker to form tricalcium silicate on the surface of the refractory bricks, as shown in Figure 1. This protective layer has good refractoriness and good compatibility with clinker, which helps to form a thick and stable kiln skin. This layer of kiln skin protects the lining from chemical and thermal attack and direct wear, while also improving the thermal efficiency of the cement kiln.

Figure 1 The kiln skin formation mechanism of dolomite bricks

It can be seen from Figure 1: (1) The two types of calcium silicate (C₂S, C₃S) in the kiln skin have high melting points (1900℃ and 2130℃ respectively), so they have good high temperature resistance. (2) A strong, stable and high-temperature resistant kiln skin transition layer is formed on the dolomite bricks. This transition layer is mostly composed of stable tricalcium silicate minerals and is closely combined with the bricks.

The petrographic structure of dolomite bricks is shown in Figures 2 and 3. As can be seen from Figures 2 and 3, dolomite bricks have two different crystal phases, namely the continuous lime crystal phase and the discontinuous magnesium oxide crystal phase. CaO is distributed throughout the brick, so the contact area between the brick and the kiln skin almost covers the entire brick surface. In this way, the comprehensive contact between the bricks and the kiln skin can effectively inhibit the kiln materials and gaseous corrosive substances in the kiln (C₂S and C₂F in the clinker, sulfur and alkali, etc.) from penetrating through the kiln skin into the hot surface layer of the bricks. . In addition, the purity of dolomite brick itself is extremely high, its w (CaO+MgO) can reach 97%, the content of impurities (SiO₂, AL₂O₃, Fe₂O₃) is very small, and the formation of low-melting compounds is very small. Therefore, under the closely contacting and dense kiln skin, there is rarely a metamorphic alteration layer in the high-purity dolomite bricks. This is obviously extremely beneficial to extending the service life.

Figure 2 Microstructure of dolomite brick and kiln skin section

Figure 3 Dolomite microstructure of two different crystal phases, magnesium oxide and calcium oxide

The kiln skin of magnesia chrome bricks is not as easy to hang as dolomite bricks, and the kiln skin and bricks cannot be highly tightly bonded. The clinker melt, sulfur, alkali, etc. can easily penetrate into the hot surface layer of the bricks through the less dense interface layer. . In this way, the hot surface layer under the kiln skin will inevitably become a metamorphic layer. Whenever the kiln is stopped for cooling, the kiln skin turns black, and the β-CS in the kiln skin is converted into y-C,s. At the same time, volume expansion occurs, causing the kiln skin to pulverize. Within a few working shifts after the kiln was ignited and opened, the kiln skin peeled off frequently over a large area, and the deteriorated layer on the hot surface of the bricks was also “teared off”. At this time, the new hot surface of the magnesia-chrome brick is quickly exposed to the flame, and is easily deteriorated due to excessive thermal expansion and erosion, creating conditions for it to be “teared off” with the new kiln skin again. This damage phenomenon is more serious in kilns that are turned off and re-ignited, and frequently started and stopped. Therefore, the service life of magnesia-chrome bricks will be significantly lower than that of dolomite bricks.

According to a large amount of practice, under the same process and equipment conditions, the service life of dolomite bricks is usually longer than that of magnesia-chromium bricks of the same grade.

Excellent thermal shock stability

Zirconium oxide is added to the zirconium-containing dolomite brick products. When the dolomite bricks are calcined, the zirconium oxide reacts with lime to form zirconate [zirconate is a very good refractory material (melting point is 2430°C)]. And form micro-cracks in the brick. The existence of microcracks can effectively limit the expansion of stress cracks throughout the brick, thereby improving the thermal shock stability of the brick and reducing thermal spalling during kiln shutdown and the fracture of refractory bricks caused by thermal spalling.

In addition, dolomite bricks also have the unique feature of being able to deform without cracking. Under pressure, in a high-temperature load test (1550℃, 0.17N/mm²), dolomite bricks deformed by more than 10% without cracks, while magnesia refractory materials cannot deform by more than 1% before cracks occur. In actual use, when refractory bricks are directly exposed to high temperatures, especially in the lower transition zone, the hot surface expands greatly and exerts great force on the brick lining. In this case, magnesium oxide or magnesium spinel bricks will fracture, while dolomite bricks will simply creep. The performance comparison of bricks used in the cement kiln firing zone is shown in Table 2. As can be seen from Table 2, the advantages of using dolomite bricks in the firing zone are obvious, including alkali resistance, chemical corrosion resistance, long service life, and the remaining bricks are non-toxic.

Application concerns and countermeasures of dolomite bricks

As we all know, dolomite bricks have shortcomings such as being difficult to transport and store, and the bricks are prone to hydration with moisture in the environment, which may lead to the inability to continue to be used. The following is an analysis and countermeasures for this.

(1) Corresponding measures for transportation and storage. Dolomite bricks can be vacuum packed with aluminum film and pasted with ordinary tape, which can eliminate and reduce the contact between the bricks and the air and reduce the risk of the bricks reacting with moisture in the air. Even if the aluminum film is damaged during transportation and hoisting, the hazard is not great. When aluminum film vacuum packaging is used, the shelf life of dolomite bricks can be more than one year.

(2) Industry self-discipline, power rationing and price guarantees are common in the cement industry, and kiln shutdowns for about half a month often occur. After domestic and foreign use experience, there is no need to worry about hydration problems when using dolomite bricks in the firing zone. Because there is a stable, strong and thick kiln skin on the surface of dolomite bricks, it can effectively block the contact between the bricks and the air, and also prevent the risk of hydration. Dolomite bricks are used in areas without kiln skin. If the kiln is shut down within a month, the bricks can still be left without any treatment, and the quality of the bricks will not be greatly affected. If the kiln shutdown time is uncertain or much longer than one month, simple treatment on the surface of the bricks can prevent hydration of the bricks.

Conclusion

Based on the above, the use of zirconium dolomite bricks in cement rotary kilns is promising, and its popularity will be another milestone in the use of refractory materials in kilns in my country’s cement industry. In foreign countries, most of the firing zones of rotary kilns have been switched to dolomite bricks. Of course, our country should also fully pay attention to and strengthen research and promotion in this area, so that we can withdraw from the ranks of chromium pollution sources as soon as possible.

LMM YOTAI established in 2007. Our production technology comes from Japanese Yotai. As an experienced and international player in the refractories industry. We have succeeded in expanding both the breadth of its product range and the depth of its services. From raw material selection, refractory portofio & optimization, installation & services & recycle of used refractories on site to further reduce client’s Opex & Capex in refractory consumption per ton steel output, meanwhile improve product quality of client.

Our Product have been supplied to world’s top steel manufacturer Arcelormittal, TATA Steel, EZZ steel etc. We do OEM for Concast and Danieli for a long time

Facebook
Twitter
LinkedIn
Pinterest
Reddit

Comprehensive solution of Refractory for steelmaking

Fill out the form below, and we will be in touch shortly.
Contact Information