Roasting Activation: The Core Process for the Formation of Activated Alumina Properties

Roasting activation is the pivotal process for defining the properties of activated alumina spheres. Temperature and time directly influence the adsorption capacity, catalytic carrier compatibility, and service life of the final product by regulating its crystal phase structure, porous characteristics (including pore size, pore volume, and specific surface area), and mechanical strength. The effects of these two factors are not independent; rather, they require a coordinated "temperature-time" balance to achieve optimal performance. The following analysis explores the influence of temperature and time, combined with the underlying mechanisms of action:

1. The Influence of Calcination Temperature: The "Core Variable" for Crystal Phase and Pore Structure Formation

The calcination temperature plays a decisive role in determining the key properties of activated alumina spheres by affecting the dehydration degree, crystal phase transformation pathway, and pore-forming agent decomposition efficiency of alumina precursors (e.g., pseudo-boehmite). The effect of temperature varies significantly across different ranges:

1.1 Low Temperature Range (200-400 °C): Formation of Basic Pores with Insufficient Activity

Mechanism of Action:

During this stage, the process mainly involves "physical dehydration" and the "preliminary decomposition of pore-forming agents." Free water and crystalline water in the precursor (pseudo-boehmite) are gradually removed, resulting in the formation of a preliminary microporous structure (pore size < 2 nm).Pore-forming agents such as starch and ammonium bicarbonate begin to decompose, releasing gases like CO₂ and H₂O, leaving behind "initial pores" within the spheres. However, decomposition is incomplete, leading to partial carbonization of starch.

Impact on Performance:

Low specific surface area (typically < 150 m²/g), small pore volume (< 0.3 cm³/g), and weak adsorption activity (e.g., static water absorption rate < 15%), rendering it unsuitable for industrial applications such as drying and defluorination.

The crystal phase is predominantly "amorphous alumina," lacking active γ-Al₂O₃, and exhibiting poor mechanical strength (compressive strength < 0.5 kN/particle), making it prone to breakage during subsequent use.

Conclusion:This is only the "pre-processing stage." Further heating is required to activate the material's performance.

 1.2 Medium Temperature Range (400-800 °C): Formation of Active Crystal Phase and Peak Adsorption Performance

Mechanism of Action:

This range marks the "activation critical period," during which two key reactions occur:

Crystal Phase Transformation: Amorphous alumina fully transforms into γ-Al₂O₃ (active crystal phase), which has a loose "spinel structure" with large lattice gaps and abundant hydroxyl (-OH) active sites on its surface, making it ideal for adsorption and catalytic carrier functions.

Pore Optimization: Pore-forming agents are completely decomposed (e.g., carbonized starch oxidized to CO₂, ammonium bicarbonate fully decomposed), expanding micropores into mesopores (pore size 2-50 nm) and improving pore connectivity, which results in a rapid increase in specific surface area and pore volume.

Impact on Performance:

Optimal Adsorption Performance: Specific surface area reaches 250-400 m²/g (peak usually at 600-700 °C), pore volume increases to 0.5-0.8 cm³/g, and static water absorption rate ≥ 20%, achieving maximum adsorption capacity for water, fluoride ions, sulfides, etc.

Moderate Mechanical Strength: The initial growth of γ-Al₂O₃ grains (particle size < 10 nm) forms a stable skeletal structure, with compressive strength increasing to 0.8-1.5 kN/particle (5 mm particle size), meeting the strength requirements for most industrial applications such as gas drying and water purification.

Good Chemical Stability: The γ-Al₂O₃ crystal phase is stable with excellent acid and alkali resistance (except in strong concentrated alkali and hydrofluoric acid), and no impurities leach out.

Conclusion: In industrial production, the calcination temperature of adsorption-type activated alumina (used for drying, defluorination, etc.) is typically controlled within this range (core temperature 600-700 °C) to balance adsorption activity and strength.

 1.3 High Temperature Range (800-1200 °C): Decreased Activity and Increased Strength

Mechanism of Action:

 Above 800 °C, γ-Al₂O₃ undergoes "phase reconstruction" and "grain growth":

Crystal Phase Transformation: γ-Al₂O₃ gradually transforms into δ-Al₂O₃ and θ-Al₂O₃ (semi-inert crystal phases), and eventually into α-Al₂O₃ (inert crystal phase) above 1200 °C. α-Al₂O₃ has a dense structure, lacks active sites, and virtually loses its adsorption capacity.

Pore Collapse: Rapid grain growth (from 10 nm to > 50 nm for γ-Al₂O₃ grains) fills the gaps between particles, reducing mesopores and shrinking or even closing pores, leading to a significant decrease in specific surface area and pore volume.

Skeleton Densification: The grains become tightly packed, and the volumetric density of the spheres increases (from 2.5 g/cm³ to over 3.5 g/cm³), resulting in a significant improvement in mechanical strength.

Impact on Performance:

Sharp Decline in Adsorption Activity: Specific surface area decreases from 400 m²/g to below 100 m²/g (and may fall below 50 m²/g at 1000 °C), with pore volume dropping below 0.3 cm³/g. Adsorption capacity is only 1/3 to 1/5 of that of medium-temperature products, making it unsuitable for adsorption applications.

Extremely High Mechanical Strength: Compressive strength increases to 2.0-3.0 kN/particle (5 mm particle size), making the material wear-resistant and highly heat-resistant (can withstand temperatures above 1200 °C). It is ideal as a "high-strength catalytic carrier" for high-temperature applications (e.g., fluidized bed reactors that endure severe airflow erosion).

Conclusion: For "high-strength, low-activity" products intended for specialized applications (e.g., high-temperature catalytic carriers), calcination temperatures should be strictly controlled to avoid exceeding 800 °C.

 2. The Influence of Calcination Time: Regulating Reaction Completeness and Performance Stability

Within a specific temperature range, the calcination time primarily affects the degree of crystal phase transformation, the decomposition efficiency of pore-forming agents, and the uniformity of the pore structure. If the time is too short or too long, performance defects may arise, necessitating careful control in conjunction with temperature.

 2.1 Short Time (Not Meeting Insulation Requirements): Performance Does Not Meet Standards

Problem Manifestation:

Incomplete Crystal Phase Transformation: If the material is kept at a medium temperature (e.g., 600 °C) for only 1 hour (typically 3-5 hours), the conversion rate of γ-Al₂O₃ may be only 60-70%, leaving a large amount of amorphous alumina, which results in insufficient adsorption active sites and a 20-30% lower specific surface area than standard values.

Residual Pore-Forming Agents: Starch, ammonium bicarbonate, etc., may not be fully decomposed, leaving behind residual carbon and ammonium salts that can block pores, increasing the proportion of closed pores. Additionally, residual ammonium salts may release odors or contaminate the treated medium during subsequent use (e.g., affecting water quality during defluorination).

Uneven Porosity: Reaction progress differs between the inner and outer layers, leading to "saturation of the outer layer and underutilization of the inner layer" during adsorption, causing a decrease in adsorption capacity. The sphere may also crack due to internal and external stress differences.

Typical Case: Activated alumina balls held at 600 °C for 1 hour exhibit a static water absorption rate of only 16% (standard ≥ 20%) and a fluoride adsorption capacity of 0.8 mg/g (standard ≥ 1.0 mg/g), which cannot meet the requirements for fluoride removal in drinking water.

 2.2 Suitable Time (Insulation Duration Matched with Temperature): Stable and Uniform Performance

Core Role:

Ensure Complete Reaction: At the target temperature (e.g., 600 °C for 3-5 hours), the conversion rate of γ-Al₂O₃ reaches ≥ 95%, the decomposition rate of pore-forming agents is 100%, pores are uniform (mesopore proportion ≥ 80%), and performance deviation within the same batch is < 5%.

Eliminate Internal Stress: The slower reaction process allows temperature and composition differences inside and outside the spheres to balance, preventing internal stress from forming due to rapid local crystal phase transformation, thus reducing breakage during subsequent use (annual wear ≤ 0.1%).

Industrial Experience: Lower temperatures require longer insulation times (e.g., 5-6 hours at 400 °C, 2-3 hours at 700 °C). Essentially, "the reaction rate is slower at lower temperatures, requiring more time for complete conversion."

2.3 Long Duration (Exceeding Necessary Insulation Time): Increased Cost and Slight Performance Degradation

Problem Manifestation:      Increased Energy