The product is made from high-quality sintered magnesium and calcium sand as raw materials. Depending on specific requirements, the MgO/CaO ratio is selected, followed by high-temperature sintering and impregnation with asphalt or paraffin under vacuum pressure. The product exhibits excellent high-temperature performance and slag resistance, making it suitable for AOD furnaces, VOD furnaces, and other refining furnaces.
Introduction
High-purity dense magnesium sand and sintered magnesium dolomite sand or dolomite sand are used as raw materials. Depending on the service environment, the appropriate MgO/CaO ratio is selected, and an anhydrous bonding agent is used. The mixture is molded at the proper temperature and then fired at high temperature.
The products have strong resistance to low-iron, low-alkalinity refining slags outside the furnace and are conducive to desulfurization and dephosphorization, helping to remove inclusions from steel and purify it. When used in cement kilns, dolomite bricks have a strong affinity with cement clinker, easily adhere to the kiln surface, and form a uniform thickness.
Magnesium-calcium products are suitable for stainless steel refining furnaces, such as AOD furnaces, VOD furnaces, and LF-VD ladles of secondary refining devices. When used as a substitute for magnesium-chromium bricks on the firing belt of cement kilns, their application range is becoming increasingly extensive.
Physical And Chemical Specifications
| Grades | Unit |
ZMGa 15A |
ZMGa 15B |
ZMGa 20A |
ZMGa 20B |
ZMGa 25A |
ZMGa 25B |
ZMGa 30A |
ZMGa 30B |
ZMGa 40A |
ZMGa 40B |
|
CaO |
% |
13-17 |
13-17 |
18-22 |
18-22 |
23-27 |
23-27 |
28-32 |
28-32 |
38-42 |
38-42 |
|
∑(A+F+S) |
% |
3 |
4 |
3 |
4 |
3 |
4 |
3 |
4 |
3 |
4 |
| Apparent Porosity |
% |
8 |
8 |
8 |
8 |
8 |
8 |
8 |
8 |
8 |
8 |
| Cold Compressive Strength |
MPa |
55 |
50 |
55 |
50 |
55 |
50 |
55 |
50 |
55 |
50 |
| Refractoriness Under Load |
℃ |
1700 |
1680 |
1700 |
1680 |
1700 |
1680 |
1700 |
1680 |
1700 |
1680 |
Advantage
1. Excellent Erosion Resistance
The brick has strong resistance to alkaline slag and is not easily infiltrated or eroded by the slag, which can significantly extend the service life of high-temperature equipment.
At high temperatures, it does not easily react with alkaline oxides in molten steel, preventing spalling or deterioration of the brick. Its stability is superior to that of neutral or acidic refractory materials, such as clay bricks and high-alumina bricks.
2. Unique Steel Purification Function
The CaO component in the brick can react with sulfur, phosphorus, and non-metallic inclusions in molten steel to form low-melting-point compounds that can be removed with the slag.
This effectively reduces the content of harmful elements in molten steel and improves its purity. It is especially suitable for smelting ultra-low-carbon steel and high-quality alloy steel, which require high cleanliness of molten steel.
3. Good High-Temperature Performance
The brick maintains high structural strength at temperatures above 1600 ℃, retaining good volumetric stability and resisting softening or deformation.
It also exhibits excellent thermal shock resistance, withstanding drastic temperature changes during the start-up and shutdown of high-temperature equipment, thereby reducing the risk of cracking.
Process
1. Pre-treatment of raw materials
This step is the foundation for ensuring the quality of the bricks. The main raw materials and auxiliary raw materials need to be processed separately, with a focus on controlling purity and particle size.
Processing of main raw materials: Natural dolomite is calcined above 1600℃ to decompose into MgO and CaO. When using artificial raw materials, magnesium sand and lime are crushed to different particle sizes and mixed according to the required proportion.
Removal of impurities and anti-hydration treatment: Impurities such as SiO₂ and Fe₂O₃ in the raw materials are removed through magnetic separation or screening. Anti-hydration agents such as Al₂O₃ and B₂O₃ are added to form stable compounds with CaO, preventing moisture absorption and subsequent chalking.
2. Dosing and mixing
The proportion of each component is precisely controlled according to product performance requirements. Raw materials are evenly dispersed and combined with a binding agent through mixing.
Dosing: According to the target MgO content, weigh the heavy burnt dolomite, magnesium sand, lime, water-resistant agent, and mineraliser according to the formula. The error should be controlled within ±0.5%.
Mixing: Mix the dry materials for 10–15 minutes, then add the binding agent and continue mixing for 20–30 minutes to form a homogeneous slurry. The mixing process should be sealed to prevent CaO from coming into contact with air and moisture.
3. Forming
After mixing, the slurry is pressed into bricks of the desired shape and size. The goal is to ensure uniform density and avoid internal defects.
Moulding: The slurry is evenly filled into steel molds to ensure complete filling without air bubbles.
Pressurisation: Hydraulic presses are used to form bricks. The pressure is usually 15–30 MPa, with high-performance bricks requiring pressures above 40 MPa. During pressing, the method of "slow press and quick release" is used to avoid delamination or cracking. The density of the formed bricks should be ≥2.8 g/cm³.
4. Drying (pre-firing)
Moisture and volatile components of the binding agent are removed to prevent cracking during subsequent high-temperature sintering.
Low-temperature drying: Bricks are dried in a kiln at 60–80 ℃ for 4–6 hours to remove free water slowly.
Medium-temperature pre-sintering: The temperature is raised to 120–200 ℃ and held for 8–12 hours to allow the binding agent to volatilize slowly and the internal structure of the brick to cure. After drying, the moisture content of the brick should be ≤0.5%.
5. High-temperature sintering
This is a critical step that determines the final performance of dolomite bricks. High temperature promotes sintering of the raw material particles and the formation of a stable microstructure.
Heating stage: Place the dried bricks in a tunnel kiln or inverted flame kiln. Heat from room temperature to 1400 ℃ at a rate of 5–10 ℃/h to avoid cracking caused by large temperature differences.
Holding sintering: Maintain a temperature of 1600–1700 ℃ for 10–16 hours so that MgO and CaO particles fully diffuse to form solid solutions. Impurities such as SiO₂ react with CaO to form low-melting-point compounds, which are either discharged with kiln gas or form a small amount of liquid phase to promote sintering.
Cooling stage: Cool slowly to room temperature at a rate ≤8 ℃/h to prevent micro-cracks caused by thermal stress.
6. Post-processing and inspection
After sintering, the bricks undergo processing and quality inspection to ensure they meet standards.
Appearance processing: Remove sintering residues from the brick surface. Cut and trim bricks that deviate from the required size. The length, width, and height should be within ±1 mm.
Quality inspection: Test bulk density, apparent porosity, and compressive strength at room temperature. High-temperature flexural strength and hydration resistance are tested on samples. Only qualified bricks are packaged and shipped.
Application
With strong alkali resistance, steel purification ability, and good high-temperature stability, the core application of dolomite bricks focuses on the metallurgical industry. They are also used in specific scenarios in the building materials, chemical, and other high-temperature industries. Bricks with different MgO contents correspond to different usage requirements.
1. Core application field: Metallurgical industry
Metallurgy is the most important application of dolomite bricks, especially for steel refining and converter smelting, where they meet the dual requirements of erosion resistance and steel quality improvement.
Converter furnace lining: Dolomite bricks with low to medium MgO content resist the erosion of highly alkaline slag in converters and prolong furnace lining life. Compared with traditional dolomite bricks, they have higher high-temperature structural strength, reducing the risk of furnace leakage.
Ladle refining furnaces (LF furnace, RH furnace): Dolomite bricks with high MgO content are used for furnace lining or slag lines. The CaO in the bricks reacts with sulfur, phosphorus, and Al₂O₃ inclusions in steel to reduce the content of harmful elements, making them suitable for refining high-quality steels, such as ultra-low-carbon steels and bearing steels.
Non-ferrous metal smelting furnaces: In ferronickel smelting furnaces, copper converters, and similar equipment, dolomite bricks resist erosion from molten metal and alkaline slag while avoiding contamination of metal products by Cr and other harmful elements, meeting environmental protection requirements.
2. Important application areas: Building materials industry
Dolomite bricks are mainly used in high-temperature equipment in the building materials industry to resist erosion from alkaline substances, typically in cement rotary kilns.
Cement rotary kiln firing/transition zones: Dolomite bricks with 50–70% MgO content resist alkaline slag formed by cement raw materials at high temperatures. They are more cost-effective than magnesia-aluminium spinel bricks and do not compromise cement quality due to Al₂O₃ content.
Lime kiln lining: During high-temperature limestone calcination, lime kilns produce high concentrations of CaO vapor. The composition of dolomite bricks is compatible with this environment, minimizing chemical reactions that cause spalling, and their service life is superior to that of clay bricks.
3. Other applications: Chemical and environmental protection industries
In some high-temperature chemical reaction equipment and environmental protection incinerators, dolomite bricks can adapt to specific alkaline working conditions.
Chemical reaction furnaces: For example, in soda ash production calcining furnaces, dolomite bricks resist erosion from molten soda ash, preventing impurities from corroded bricks from contaminating the product.
Waste incinerators: Flue gases produced during waste incineration contain corrosive components such as Cl⁻ and SO₄²⁻, and the slag is alkaline. Dolomite bricks can resist both acid and alkali erosion, making them suitable for lining grates or hearths and improving equipment corrosion resistance.
Application Notes
Moisture-proof storage: CaO in dolomite bricks easily absorbs moisture and reacts to form Ca(OH)₂, which can lead to pulverization. Therefore, sealed packaging is required for storage, and open stacking or contact with a humid environment should be avoided.
Masonry requirements: Special alkaline mortar should be used to ensure that the brick joints have the same properties as the bricks and to prevent slag from penetrating the joints.
Working condition matching: Low MgO bricks (<50%) are suitable for low- to medium-temperature, low-erosion working conditions (such as ordinary converters), while high MgO bricks (>60%) are suitable for high-temperature, high-erosion working conditions (such as refining furnaces). The type of brick should be selected according to the actual working conditions.
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