Aluminum Hydroxide, Aluminium Trihydrate, 21645-51-2

ALUMINUM HYDROXIDE

Aluminum Hydroxide / Alüminyum Hidroksit / Al(OH)₃
CAS Number: 21645-51-2
EC Number: 244-492-7

1. PRODUCT IDENTIFICATION AND CHEMICAL IDENTITY

Aluminum hydroxide is an inorganic compound with the chemical formula Al(OH)₃. It is an amphoteric substance, meaning it can exhibit both basic and acidic properties. In nature, it occurs as the mineral gibbsite, which has monoclinic crystals. Aluminum hydroxide is a white, odorless, tasteless powder that is insoluble in water but soluble in acids and alkalis. It is a widely available, corrosion-resistant, lightweight material with a high melting point, good flame retardant properties, and the ability to release hydration water at high temperatures. The vast majority of commercially used aluminum hydroxide is produced by the Bayer process, which involves dissolving bauxite in sodium hydroxide at temperatures up to 270°C (518°F). The waste solid (bauxite residue) is removed, and aluminum hydroxide is precipitated from the remaining sodium aluminate solution. Aluminum hydroxide is widely used as an additive in electronic and polymeric applications, as a flame retardant filler, as an antacid in pharmaceutical preparations, and in various industrial applications.

2. PHYSICAL AND CHEMICAL PROPERTIES

Aluminum hydroxide is a white amorphous powder or crystalline solid that is odorless and tasteless. It has a density of 2.42 g/cm³ and decomposes at approximately 300°C to form aluminum oxide (Al₂O₃) and water. Water loss begins at 180-200°C, which is the basis for its flame retardant properties. It is practically insoluble in water (0.0001 g/100 mL) but soluble in both acids and alkalis due to its amphoteric nature. The pH of a 100 g/L suspension in water is 8-9 at 20°C. The purity of commercial grades is typically ≥ 99.9% Al(OH)₃. Loss on ignition is approximately 34-35% (as water). Specific surface area ranges from 2-50 m²/g depending on the grade, and particle size ranges from 1-100 µm. The refractive index is approximately 1.57. It has a high electrical resistivity, making it a good insulating material. The Mohs hardness is 2.5-3.5.

3. CHEMICAL PROPERTIES AND REACTIONS

Aluminum hydroxide is an amphoteric compound, which means it can react as both a base and an acid. This unique property makes it versatile in various chemical applications.

Reaction with Acids (Basic Behavior):

As a base, aluminum hydroxide neutralizes acids to form aluminum salts and water:

Al(OH)₃ + 3 HCl → AlCl₃ + 3 H₂O (aluminum chloride)

Al(OH)₃ + 3 HNO₃ → Al(NO₃)₃ + 3 H₂O (aluminum nitrate)

2 Al(OH)₃ + 3 H₂SO₄ → Al₂(SO₄)₃ + 6 H₂O (aluminum sulfate)

Reaction with Alkalis (Acidic Behavior):

As an acid, aluminum hydroxide reacts with strong bases to form aluminates:

Al(OH)₃ + NaOH → NaAlO₂ + 2 H₂O (sodium aluminate)

Al(OH)₃ + KOH → KAlO₂ + 2 H₂O (potassium aluminate)

Thermal Decomposition:

Heating aluminum hydroxide causes it to decompose to aluminum oxide and water:

2 Al(OH)₃ → Al₂O₃ + 3 H₂O (at ~300°C)

This endothermic reaction absorbs heat and releases water vapor, which contributes to its flame retardant properties.

Reaction with Carbonates:

Aluminum hydroxide can react with carbonates under certain conditions to form basic aluminum carbonates.

Amphoteric Dissolution:

Aluminum hydroxide dissolves in both strong acids and strong bases, demonstrating its amphoteric nature:

• In acids: Al(OH)₃ + 3H⁺ → Al³⁺ + 3H₂O

• In bases: Al(OH)₃ + OH⁻ → Al(OH)₄⁻ (aluminate ion)

Flame Retardant Mechanism:

When heated, aluminum hydroxide decomposes endothermically:

2 Al(OH)₃ → Al₂O₃ + 3 H₂O (absorbs heat, releases water vapor)

This reaction:

• Absorbs heat (cooling effect)

• Releases water vapor (dilutes flammable gases)

• Forms an aluminum oxide char layer (protective barrier)

4. PRODUCTION METHODS

The vast majority of commercially used aluminum hydroxide is produced by the Bayer process from bauxite ore.

Detailed Production Process (Bayer Process - Most Common):

1. Digestion: Bauxite ore (crushed and ground) is mixed with concentrated sodium hydroxide solution and heated to 150-270°C under pressure (3-5 atm). The aluminum oxide in the bauxite dissolves as sodium aluminate:

Al₂O₃·xH₂O + 2 NaOH → 2 NaAlO₂ + (x+1) H₂O

2. Clarification: The insoluble impurities (bauxite residue/red mud, containing iron oxides, silica, and titanium oxides) are removed by settling and filtration.

3. Precipitation: The clarified sodium aluminate solution is cooled and seeded with fine aluminum hydroxide crystals. This causes aluminum hydroxide to precipitate:

NaAlO₂ + 2 H₂O → Al(OH)₃↓ + NaOH

4. Washing: The precipitated aluminum hydroxide is washed to remove residual sodium hydroxide and impurities.

5. Filtration: The washed aluminum hydroxide is filtered to remove excess water.

6. Drying: The filter cake is dried to achieve the desired moisture content.

7. Milling and Classification: The dried product is milled and classified to achieve the desired particle size distribution for specific applications.

8. Packaging: The final product is packaged in 25 kg bags, big bags, or bulk containers.

5. APPLICATIONS AND INDUSTRIAL USES

Aluminum hydroxide is a versatile compound with a wide range of applications across various industries due to its flame retardant properties, amphoteric nature, availability, low cost, and non-toxicity.

5.1. Flame Retardant (Most Important Application)

Aluminum hydroxide is one of the most widely used flame retardant additives in the world. Its flame retardant mechanism is based on the endothermic decomposition that releases water vapor, which dilutes combustible gases and cools the material. The aluminum oxide residue also forms a protective char layer on the material surface, insulating it from further heat. It is preferred because it is colorless (like most polymers), inexpensive, non-toxic, and provides good flame retardant properties. Typical loading levels in polymers range from 30-60% by weight.

5.2. Plastics and Polymer Additives

Aluminum hydroxide is widely used as a filler in plastics and polymer composites. It improves mechanical properties, reduces cost, and modifies surface properties. It is particularly important in PVC processing, epoxy resins, and polyolefin composites.

5.3. Pharmaceutical and Medical Applications

Aluminum hydroxide is widely used as an antacid in over-the-counter and prescription medications. It neutralizes stomach acid without raising the gastric pH above 7, thus not triggering excessive acid secretion (like sodium bicarbonate can). It is also used as a phosphate binder in patients with chronic kidney disease. As an aluminum-based adjuvant, it is used in some vaccines to enhance the immune response.

5.4. Cosmetics and Personal Care Products

Aluminum hydroxide is used in toothpaste as a mild abrasive and filler. It is also used in various cosmetic and personal care products for its soothing and protective properties.

5.5. Industrial and Chemical Applications

Aluminum hydroxide has numerous industrial and chemical applications. It is used as a glass additive, paper coating pigment, ceramic raw material, catalyst support, and in water treatment. It is also used in paints, coatings, rubber compounds, textiles, and construction materials.

6. PHARMACEUTICAL APPLICATIONS (ANTACID USE)

Aluminum hydroxide is a widely used antacid in the management and treatment of acid indigestion, heartburn, gastric ulcers, and GERD. As an antacid, it works by neutralizing gastric acid through a simple acid-base reaction:

Al(OH)₃ + 3 HCl → AlCl₃ + 3 H₂O

The onset of action is rapid (within 1-2 minutes), and the duration of action is 30-60 minutes (longer when taken with food). Aluminum hydroxide is preferred over sodium bicarbonate because it does not raise gastric pH above 7, thus not triggering excessive acid secretion (acid rebound). It is available in various dosage forms, including tablets, capsules, chewable tablets, liquids, and powders. It is often combined with magnesium hydroxide (to offset the constipating effect of aluminum) and simethicone (for anti-gas effect). Common brand names include Maalox, Mylanta, and Gaviscon (often in combination with other ingredients).

7. FLAME RETARDANT MECHANISM

The flame retardant mechanism of aluminum hydroxide is based on three synergistic effects:

1. Cooling Effect: The decomposition of aluminum hydroxide is endothermic (absorbs ~1.0-1.3 kJ/g). This absorbs heat from the combustion zone, reducing the temperature and slowing the rate of pyrolysis and combustion.

2. Dilution Effect: The decomposition releases approximately 34-35% by weight of water vapor. This water vapor dilutes the combustible gases in the gas phase, reducing their concentration and slowing the combustion reaction.

3. Barrier Effect: The remaining aluminum oxide (Al₂O₃) forms a protective char layer on the material surface. This layer acts as a thermal barrier, insulating the material from further heat and reducing the transfer of flammable gases.

These mechanisms make aluminum hydroxide one of the most effective and widely used flame retardant additives in the world.

8. SAFETY AND HEALTH INFORMATION

Aluminum hydroxide is generally considered to be of low toxicity and is widely used in food, cosmetics, and pharmaceutical applications. However, dust inhalation and prolonged high-dose ingestion should be avoided.

8.1. Toxicological Data

8.2. Health Effects

Special Precautions:

• Phosphate Depletion: Prolonged use as an antacid may lead to phosphate depletion (hypophosphatemia) due to the formation of insoluble aluminum phosphate in the gastrointestinal tract.

• Aluminum Accumulation: In patients with renal impairment, aluminum can accumulate, leading to aluminum toxicity (osteomalacia, encephalopathy, and anemia).

• Dust Inhalation: Chronic inhalation of aluminum hydroxide dust may cause pulmonary effects; use appropriate respiratory protection during handling.

9. FIRST AID MEASURES

10. SAFETY INFORMATION (GHS CLASSIFICATION)

Aluminum hydroxide is not classified as a hazardous substance under GHS. However, fine powder may cause respiratory irritation.

Signal Word: WARNING

Hazard Statements (H-Codes):

Precautionary Statements (P-Codes):

11. FIREFIGHTING MEASURES

12. ACCIDENTAL RELEASE MEASURES

13. STORAGE AND HANDLING

Aluminum hydroxide should be stored in cool, dry, well-ventilated areas in tightly closed containers. It is slightly hygroscopic and may absorb moisture from the air, which can cause caking. It should be protected from moisture. Contact with strong acids and strong bases should be avoided. Proper grounding should be used if static electricity accumulation is a concern.

Handling Notes:

• Avoid dust generation (use enclosed systems, local exhaust ventilation)

• Avoid contact with eyes (use protective goggles)

• Avoid breathing dust (use dust mask or respirator)

• Keep away from moisture (may cause caking)

• Use in well-ventilated areas

14. TRANSPORT INFORMATION

Aluminum hydroxide is not classified as a dangerous good for transport. It is shipped in 25 kg bags, FIBCs, or bulk containers. During transport, protect from moisture, impact, and excessive heat.

15. REGULATORY INFORMATION

Regulatory Status:

16. OTHER NAMES AND DATABASE IDENTIFIERS

Database Identifiers:

17. ENVIRONMENTAL INFORMATION

Aluminum hydroxide has low toxicity to aquatic organisms. It is an inorganic compound and does not biodegrade. It has low bioaccumulation potential and low mobility in soil (insoluble in water). In large quantities, it may affect soil pH. Waste disposal should be carried out according to local, state, and federal regulations. Landfill or recovery is recommended.

18. APPLICATION SUMMARY TABLE

19. PHYSICAL FORMS

20. SUMMARY AND CRITICAL WARNINGS

SUMMARY:

Aluminum hydroxide (Al(OH)₃, CAS 21645-51-2) is an inorganic, amphoteric compound (can act as both a base and an acid). It is a white, odorless, tasteless powder that is insoluble in water but soluble in both acids and alkalis. The molecular weight is 78.00 g/mol, density is 2.42 g/cm³, and it decomposes at approximately 300°C to form aluminum oxide and water. It is produced primarily by the Bayer process from bauxite. Its most important applications include: flame retardant filler in plastics and polymers (the largest application), antacid in pharmaceutical preparations, filler in cosmetics and toothpaste, and various industrial applications in glass, paper, ceramics, and water treatment. As a flame retardant, it decomposes endothermically, releasing water vapor and absorbing heat, thus inhibiting combustion. As an antacid, it neutralizes stomach acid without causing acid rebound. It has low toxicity and is not classified as a hazardous substance under GHS. It is stored in cool, dry, well-ventilated areas, protected from moisture. Packaging: 25 kg bags, FIBCs, or bulk containers.

Key Properties:

Major Application Areas:

Key Safety Points:

• LOW TOXICITY: Widely used in food, pharmaceutical, and cosmetic applications

• SLIGHTLY HYGROSCOPIC: Protect from moisture; may cake if exposed to humidity

• MILD IRRITANT: May cause mild eye irritation; dust may cause respiratory irritation

• STORAGE: Store in cool, dry, well-ventilated area, protected from moisture

• PPE RECOMMENDED: Dust mask, safety goggles, protective gloves (for handling powder)

• PHOSPHATE DEPLETION: Prolonged antacid use may cause phosphate depletion

• RENAL IMPAIRMENT: Use with caution in patients with renal impairment

CRITICAL WARNINGS:

Flame Retardant Properties: Aluminum hydroxide is one of the most widely used flame retardant additives in the world. Its flame retardant mechanism is based on an endothermic decomposition reaction that releases water vapor (diluting combustible gases) and leaves an aluminum oxide char layer (providing a thermal barrier). It is preferred because it is colorless (like most polymers), inexpensive, non-toxic, and provides good flame retardant properties. Typical loading levels in polymers range from 30-60% by weight.

Amphoteric Nature: Aluminum hydroxide is amphoteric, meaning it can react with both acids and bases. This property is the basis for its use as an antacid (reacts with gastric acid) and its solubility in both acids and alkalis. In acid: Al(OH)₃ + 3H⁺ → Al³⁺ + 3H₂O. In base: Al(OH)₃ + OH⁻ → Al(OH)₄⁻.

Antacid Use: Aluminum hydroxide is widely used as an antacid in the treatment of heartburn, acid indigestion, and gastric ulcers. It works by neutralizing stomach acid without raising the gastric pH above 7, thus avoiding acid rebound (a problem with sodium bicarbonate). However, prolonged use can lead to phosphate depletion (due to formation of insoluble aluminum phosphate) and, in patients with renal impairment, aluminum accumulation and toxicity (osteomalacia, encephalopathy, and anemia).

Phosphate Binder: Aluminum hydroxide is used as a phosphate binder in patients with chronic kidney disease to reduce phosphate absorption. It forms insoluble aluminum phosphate in the gastrointestinal tract, reducing phosphate uptake and lowering serum phosphate levels.

Vaccine Adjuvant: Aluminum hydroxide is used as an immunologic adjuvant in some vaccines to enhance the immune response to the vaccine antigen. It is one of the most common adjuvants in use today.

Dust Precautions: Although aluminum hydroxide has low toxicity, dust inhalation should be minimized. Chronic inhalation of aluminum hydroxide dust may cause pulmonary effects. Use appropriate respiratory protection during handling. Avoid dust generation; use local exhaust ventilation.

Phosphate Depletion Warning: Prolonged use of aluminum hydroxide as an antacid can lead to phosphate depletion (hypophosphatemia) due to the formation of insoluble aluminum phosphate in the gastrointestinal tract. Symptoms of phosphate depletion include weakness, bone pain, and osteomalacia. Patients on long-term therapy should have their phosphate levels monitored.

Renal Impairment Warning: In patients with renal impairment, aluminum hydroxide can lead to aluminum accumulation and toxicity (osteomalacia, encephalopathy, and anemia). Use with caution in patients with renal impairment, and monitor aluminum levels.

Important Disclaimer: This Technical Data Sheet (TDS) is for informational purposes only. The information provided is based on available data and is believed to be accurate. However, the user is solely responsible for determining the suitability of the product for their specific application and for complying with all applicable regulations and safety requirements. For complete safety, handling, storage, and regulatory compliance information, always refer to the official Safety Data Sheet (SDS/MSDS) provided by the manufacturer/supplier. Users are responsible for testing the product in their own processes and applications. All local, national, and international regulations must be followed when working with aluminum hydroxide.