How to choose different magnesia-carbon bricks

Graphite is an indispensable component of magnesia-carbon bricks. The main material used in the production of magnesia-carbon bricks is flake graphite, a natural crystalline graphite belonging to the hexagonal crystal system. It has a layered structure, is insoluble in water, dilute acids, dilute alkalis, and organic solvents, and exhibits high temperature resistance, good electrical and thermal conductivity, and good lubricity. The specifications of graphite used in the production of magnesia-carbon bricks generally fall within the following ranges: fixed carbon 85%–98%, ash content 13%–2% (mainly SiO2, Al2O3, etc.), relative density 2.09–2.23, and melting point 3640K (volatile). However, graphite is very easily oxidized, which is its fatal flaw.

The reason why graphite is oxidized

During the field use of magnesia-carbon bricks, the graphite is oxidized mainly due to the following reasons:

(1) Free oxygen in the atmosphere;

(2) Various oxides in the slag;

(3) Impurities and oxides contained in the graphite itself in the magnesia-carbon bricks and oxides contained in the magnesia sand.

Classification of graphite grades

According to national standards, flake graphite is classified into four categories based on its fixed carbon content: high-purity graphite, high-carbon graphite, medium-carbon graphite, and low-carbon graphite. High-carbon and medium-carbon graphite are commonly used in refractory materials. However, because the national standard for graphite classification is complex and the names are not easy to remember, companies generally use their own inter-company standards.

Using a 100-mesh sieve as a standard, what passes through the sieve is considered negative, and what passes is considered positive. Simply put, positive-grade graphite flakes are larger than negative-grade graphite flakes. The fixed carbon content is added after the symbol to classify and name the graphite. This classification and naming method is intuitive and clear, making it easy for non-technical personnel to distinguish the graphite grade, facilitating identification of graphite grade during procurement and production.

Magnesia-carbon bricks commonly use graphite

The commonly used graphite types for producing magnesia-carbon bricks are as follows:

(1) -190, -192, -193, -194, -195, -196, -197, -198

(2) +194, +195, +196, +197, +198

The oxidation of graphite in magnesia-carbon bricks results in a loose structure, increased pores, and reduced strength. Impurities in the graphite accelerate its oxidation. Therefore, to obtain high-performance magnesia-carbon bricks, production tends to use graphite with high purity and large flakes. However, large-flake and high-purity graphite are both expensive. For industrial production, it's not feasible to blindly pursue high raw material specifications; price is also a crucial factor. After several years of exploration and investigation, it has been found that the demand for high-grade graphite is not large. The graphite with the highest demand is -194, -195, and -196. Of course, -190 and -192 graphite are also used in lower-grade magnesia-carbon bricks. The primary goal of a factory is to generate profit, and graphite costs account for a large proportion of the production cost of magnesia-carbon bricks. Therefore, the rational selection of graphite is of paramount importance.

Selection of Graphite for Different Types of Magnesia-Carbon Bricks

Below, we introduce the appropriate graphite grades for different types of magnesia-carbon bricks:

(1) Magnesia-carbon bricks for converters: During the smelting process, the magnesia-carbon bricks in the furnace cap area experience greater mechanical wear but are less susceptible to slag erosion. We use -194 graphite. The magnesia-carbon bricks in the furnace body are more susceptible to slag erosion, so we use -195 or -196 graphite. The magnesia-carbon bricks at the furnace bottom are mainly worn by the agitation of molten steel, with minimal slag erosion, so we use -192 graphite. These graphite grades generally meet the needs of various steel plants in China at present.

(2) Ladle slag line bricks: There are many types of ladle slag line bricks. Ordinary non-refined ladle slag line bricks have a fast turnover time, so -193 graphite is sufficient. Refined ladle slag line bricks generally use graphite of -195 or higher. The ladle walls and bottom are less susceptible to slag erosion, so -192, -193, and -194 graphite can be used. (3) Magnesia-carbon bricks for electric furnaces: During the smelting process of electric furnaces, the magnesia-carbon bricks in the molten pool area are less corroded by steel slag, and some of them are buried in the ramming material at the bottom of the furnace. Therefore, -192 graphite can be used for this part. In addition to being corroded by steel slag, the slag line area is also subject to the burning of electric arc. Here we use -195 or -196 graphite. The upper furnace wall area is not corroded much, so we use -190 graphite.