In the refractory industry, the terms "calcined alumina" and "activated alumina" are buzzwords, as they have become important synthetic raw materials for refractories, particularly in some high-performance monolithic refractories. However, there is ambiguity regarding the distinction between the two and whether the terminology is appropriate.
First, it's important to clarify what is calcined alumina, used in refractory materials. It's the product obtained by calcining aluminum hydroxide or industrial alumina at a certain temperature, partially or almost completely converting it into α-Al2O3. Calcination refers to heating the material at a certain temperature to cause a decomposition reaction, resulting in the loss of water of crystallization or volatile components. Broadly speaking, any aluminum oxide obtained by heating aluminum hydroxide to remove water of crystallization, even if it's a transitional state before the most thermodynamically stable α-Al2O3, such as γ-Al2O3 or θ-Al2O3, is considered calcined alumina, commonly known as industrial alumina or commercial alumina.
The term "activated alumina" has two meanings. Alumina used in non-refractory applications, such as desiccants, adsorbents, and catalysts, is often specifically referred to as "activated alumina." This porous, highly dispersed solid material has a large specific surface area. Its microporous surface possesses the properties required for catalysis, such as adsorption, surface activity, and excellent thermal stability. Therefore, it is widely used as a catalyst and catalyst support in chemical reactions. This type of activated alumina is typically produced from two types of raw materials: γ-Al₂O₃, obtained by heating and dehydrating gibbsite or byerite. The other is pseudo-boehmite, produced from aluminates, aluminum salts, or both. Obviously, this type of activated alumina is not suitable for use in refractories. Activated alumina for refractories must meet the following requirements:
1) be calcined alumina with α-Al₂O₃ as the primary crystalline phase;
2) have a primary crystal size down to the submicron level;
3) possess a grain morphology and size distribution conducive to close packing.
This term, originally coined by Almatis, stands for reactive alumina. Its hallmarks are its ability to control particle size distribution within the submicron range and its improved sinterability. Its activity is measured by its sinterability: ultrafine alumina powder is pressed into pellets and sintered at a specified temperature, such as 1540°C, for a period of time. The closer the bulk density of the resulting alumina sintered body approaches the theoretical density of α-Al₂O₃, the higher its activity. In Japan, activated alumina is simply referred to as "easy-sintering alumina." So-called activated alumina is simply a specific type of calcined alumina that has smaller crystals than commercially available raw materials, making it easier to sinter.
Obviously, the smaller the primary crystal size of calcined alumina used in refractory materials, the higher its specific surface area, and the higher its sintering activity. After thorough grinding to break up agglomerates, the median diameter can be used to characterize the primary crystal size. For example, the median diameter (D50) of Almatis's CT3000SG, RG4000, and CL370 activated aluminas is typically 0.5μm, 0.6μm, and 2.5μm, respectively. In contrast, the typical D50 value of conventional calcined alumina is often above 3μm.
The term "activity" is a relative term. Currently, there's no universally accepted upper limit for particle size that qualifies as "active," and most assessments are based on qualitative or semi-quantitative criteria. Sinterability depends on multiple factors, including specific surface area, median diameter, particle shape, and particle size distribution, and should not be judged solely by particle size. When discussing α-Al₂O₃ micropowders, the industry seems accustomed to interpreting calcined alumina as inactive, implying that it's calcined at higher temperatures and has larger crystal sizes. However, using the collective term "calcined alumina" to refer to a specific part of the alumina, while referring to another part as "activated alumina," is neither rigorous nor scientific. Technical parameters should be used to characterize the activity and other properties of alumina micropowders.
