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Performance and requirements of dolomite bricks used in VOD ladle molten pool area

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Performance and requirements of dolomite bricks used in VOD ladle molten pool area

VOD method ladle

In recent years, the use of dolomite as a raw material for high-quality refractory materials for the lining of external furnace vacuum treatment devices, especially VOD and VAD molten steel treatment equipment, has increased dramatically and can increase the lining life by more than 1 to 2 times. The main cause of corrosion of the magnesia-calcium lining used in VOD ladles is cracks produced in the refractory material, which is due to low thermal shock resistance and interaction with molten slag. At the same time, the CaO in the dolomite particles is completely converted into the liquid phase. When the optimal particle size of fused MgO and fused CaO sand is used in the ingredients, the above phenomenon can be eliminated and the life of the refractory lining is extended by about 50%.

In order to improve the service life of the lining in the ladle slag line area of the VOD method, Japanese Patent 59-170215 recommends the use of MgO-C bricks containing 10wt% C. Tar dolomite bricks are used to build the ladle wall, which can achieve the purpose of balancing corrosion and extending the life of the ladle.

The MgO-CaO-C bricks used in the steel-receiving parts of the VOD refining ladle have higher durability than magnesia-chrome bricks. This is because it contains graphite, which is inherently resistant to slag erosion and is difficult to cause structural peeling. It also exhibits the characteristics of this type of material because it contains CaO and is highly resistant to the erosion of low CaO/SiO2 ratio (low alkalinity) slag.

In addition, the Japanese steel pipe company Keihin Steel Works has tried magnesia-alumina spinel bricks made from special magnesium-alumina spinel sand in the low-corrosion area of a VOD/VAD refining ladle with a capacity of 50t that is used with an electric furnace. This product has good thermal shock resistance and good corrosion resistance against desulfurization slag containing higher CaO and Al2O3. Comparative tests with magnesia-chromium bricks on the furnace show that its durability is improved by 40% to 50%, see Table 1.

Brick type Brick A MgO-Al₂O₃ Brick B MgO-Al₂O₃ MgO-Cr₇O₂ brick
chemical composition,%
MgO 89 92 10
  Al₂O₃ 10 7 19
  Fe₂O₃ 8
Porosity, % 14.80 16.90 14.90
Bulk density, g/cm³ 3.06 2.98 3.17
Compressive strength, MPa 78 58 85
Flexural strength, MPa 16 11 15
Flexural strength, MPa1400℃ 15.1 10.1 11.0
erosion index 1.00 0.98 1.58

MgO-CaO-C brick is a high-quality carbon-containing alkaline composite refractory material developed in recent years. Because of the presence of CaO in this composite refractory material, it has high thermal stability, strong erosion resistance of molten slag (especially low-alkalinity slag), and it also has the advantages of purifying molten steel. Therefore, when the steelmaking temperature is high, the operation time is long, the alkalinity of the slag changes greatly, and the ratio of smelting stainless steel is high, it shows the advantage of longer service life than MgO-C bricks.

When graphite is the main source of carbon in MgO-CaO-C bricks, the purity, particle size and amount of graphite have an impact on the oxidation resistance and corrosion resistance of the product. This effect is shown in Figures 1-3. These three figures show that the corrosion resistance of MgO-CaO-C bricks increases with the increase of graphite purity (see Figure 1); the corrosion resistance is greatest when the graphite content is 15wt% (see Figure 3). Their weight loss rate in the oxidation test decreases with the decrease in graphite particle size and graphite dosage.

Performance and requirements of dolomite bricks used in VOD ladle molten pool area

Performance and requirements of dolomite bricks used in VOD ladle molten pool area

Performance and requirements of dolomite bricks used in VOD ladle molten pool area

There is no special difference between the production process of MgO-CaO-C bricks and that of MgO-C bricks, but the key to its success is to solve the hydration problem of CaO. When anhydrous phenolic resin is selected, the condensation water is represented by a hydroxyl value and should be controlled within 300~400mgK0H/g, which can also prevent the formation of pores and cracks in the formed bricks, and also prevent the occurrence of hydration of the bricks. Regarding the problem of preventing CaO hydration in magnesium-calcium-C bricks, the measures listed in the section of magnesium-calcium-based refractory materials for AOD furnaces can be used when producing MgO-CaO-C bricks.

At present, MgO-CaO-C bricks used in secondary smelting ladles should meet the following requirements:

(1) Select magnesia-calcium sand and magnesite with low impurity content as highly corrosion-resistant raw materials for making bricks; control the sintering method of these raw materials to reduce the porosity between grains to prevent slag from penetrating into the intergranules.

(2) Improve the comprehensive performance of this type of refractory materials by using no less than 5wt% high-purity graphite (15% graphite is optimal), while suppressing shrinkage due to sintering. This is to prevent molten steel or slag from intruding through the brick joints and causing premature damage to the brick joints.

(3) In order not to reduce the corrosion resistance, it is advisable not to add metal powder.

(4) Use anhydrous phenolic resin as the binder to prevent the hydration of CaO during brick making and reduce environmental pollution.

(5) Apply asphalt on the surface of the product to prevent structural deterioration caused by hydration of such products during storage and transportation.

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.

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