The quality identification of high-alumina bricks can be carried out from the following aspects:
I. Visual Inspection
Color: The color of high-alumina bricks is generally related to their alumina content. Generally speaking, the higher the alumina content, the whiter the color. Grade III high-alumina bricks appear yellowish, Grade II ones are yellowish-white, and Grade I ones are relatively white. If the surface color is uneven or no crystalline phase can be observed, it may indicate insufficient sintering temperature, resulting in poor strength during service.
Surface Quality: High-quality high-alumina bricks feature smooth surfaces with few black spots. Black spots are mainly formed by ferric oxide during calcination. Large-area black spots indicate excessive ferric oxide content in the bricks, which may stem from flawed raw material proportioning and shorten the service life of the bricks. In addition, there should be no large fused deposits on the surface.
Flatness: The surface of high-alumina bricks shall be flat without obvious bending, warping or unevenness. Bricks with uneven surfaces will increase construction difficulty during masonry and impair the appearance quality and service life of furnace linings.
Dimension: High-alumina bricks shall have regular dimensions with minor edge length deviations, generally within permitted tolerances. Excessive dimensional deviation may compromise masonry quality, resulting in uneven brick joints and reducing the overall strength and tightness of furnace linings.
II. Sound Inspection
Strike two bricks against each other. High-quality high-alumina bricks produce a clear, crisp sound rather than a dull thud. This is because premium high-alumina bricks feature a dense internal structure that enables good sound conduction when tapped.
III. Weight and Bulk Density
Weight: The weight of high-alumina bricks is related to their grade and alumina content. Generally speaking, Grade I high-alumina bricks weigh approximately 4.4 kg, Grade II ones around 4.0 kg, Grade III ones about 3.7 kg, and Special Grade high-alumina bricks roughly 4.8 kg. However, the weight is also affected by production processes and other factors, so a fluctuation of 0.1 to 0.2 kg above or below the standard weight falls within the normal tolerance range.
Bulk Density: Bulk density is one of the key indicators for evaluating the quality of high-alumina bricks. It can be calculated by weighing the brick and measuring its dimensions. In general, the higher the bulk density, the better the strength and refractory performance of the brick. Bricks with a bulk density below 2.3 are usually Grade III high-alumina bricks; those with a bulk density ranging from 2.3 to 2.45 are generally Grade II high-alumina bricks; bricks with a bulk density of 2.45 and above are typically Grade I high-alumina bricks.
IV. Chemical Analysis Test
Alumina Content: The main component of high-alumina bricks is alumina. The higher the alumina content, the better the refractory performance of the bricks. However, the alumina content can only be determined through chemical analysis. Generally speaking, bricks with an alumina content of 55% are Grade III high-alumina bricks, those with 65% are Grade II, and those with 75% are Grade I.
Other physical and chemical indicators: In addition to alumina content, physical and chemical indicators such as strength, bulk density, reheating linear change and refractoriness under load of high-alumina bricks can also be tested. These indicators can more comprehensively reflect the quality and service performance of high-alumina bricks.
In summary, the quality of high-alumina bricks shall be comprehensively judged from multiple aspects including appearance, tapping sound, weight and bulk density, as well as chemical analysis. When purchasing and applying high-alumina bricks, products with appropriate grades and quality shall be selected according to actual service requirements.