Can High-Alumina Bricks Be Used for a Barbecue Grill?

High-alumina brick formulation

1. Binder: Soft or semi-soft clay is usually used as the binder, with a small amount of pulp waste liquor added to improve formability and green strength. Expansion of the green body caused by secondary mullitization is the primary consideration in determining the amount of binder clay. When making Grade I and Grade II high-alumina bricks, the corundum content or mineral composition of the bauxite clinker is highly non-uniform, so to limit secondary mullite formation, not too much binder clay should be added. When using Grade III bauxite clinker, the amount of binder clay can be set according to forming and firing conditions without considering secondary mullitization. In practice, clay usage ranges from 5% to 10%.

2. Blending clinkers of different grades: High-alumina bricks can be produced from blends of different raw material grades to adjust the Al2O3 content and improve the matrix composition. Adjacent grades should preferably be blended, with the higher-Al2O3 clinker added as fine powder so it reacts fully with the clay, allowing secondary mullitization to occur evenly in the matrix and mullitizing the matrix.

When the Al2O3 content of the mix needs adjusting, it is generally better to blend clinkers of different Al2O3 contents rather than change the amount of clay.

High-alumina brick production process

1) Raw material quality and binder

The grade of bauxite clinker is chosen according to the grade of brick being produced. The clinker grade is determined by chemical analysis, mineralogical identification and physical property testing, and raw materials are then strictly selected.

Generally, clay is used as the binder to give the mix cohesion, aiding forming and giving the green brick sufficient handling strength. If the mix's cohesion is affected, an organic binder such as sulfite pulp waste liquor is usually added, since too much clay would lower the Al2O3 content of the brick matrix.

The typical particle size distribution for high-alumina brick is: coarse particles (3 mm or 5 mm critical size) 50%–60%; medium particles 10%–15%; fine powder 35%–40%. The finer the powder, the more pronounced the sintering effect.

High-alumina bricks generally contain 5%–20% clay; large, complex-shaped bricks may contain 30%.

2. Forming and drying

High-alumina bricks can be made by various forming methods, though semi-dry pressing is most common, with a mix moisture of 3%–6%. Bulk density or porosity of the green brick must be strictly controlled during forming to ensure quality.

Formed bricks must be dried; residual moisture when entering the kiln should be below 2%.

3. Firing

The main physicochemical changes during firing are as follows:

Below 200°C, residual moisture in the green brick is driven off.

From 200°C to 1250°C, kaolinite in the binder clay dehydrates and decomposes, forming mullite and free SiO2.

Above 1250°C, α-Al2O3 in the clinker combines with free SiO2 to form secondary mullite, accompanied by volume expansion. Secondary mullite is generally undesirable but unavoidable. The more clay added, the more pronounced the secondary mullitization; even without clay, it still occurs, because the mineral distribution in bauxite clinker lumps is non-uniform and the components redistribute after crushing and mixing.

For Grade I and Grade II high-alumina bricks, setting height generally does not exceed 750 mm; firing temperature is 1440–1460°C with a 40–48 h soak in a down-draft kiln, or 1540–1580°C in a tunnel kiln. For Grade III bricks the setting height is 10000 mm [as stated in the original], fired at 1380–1420°C with a 24–32 h soak in a down-draft kiln, or 1430–1460°C in a tunnel kiln.