What are the methods for producing ultrafine aluminum hydroxide micropowder?

What are the production methods for ultrafine aluminum hydroxide micropowder? The production of ultrafine aluminum hydroxide micropowder encompasses both mechanical and chemical methods.

What are the production methods for ultrafine aluminum hydroxide micropowder? The production of ultrafine aluminum hydroxide micropowder encompasses both mechanical and chemical methods. Chemical methods include sodium aluminate solution seed decomposition, metal alcoholate synthesis, microemulsion process, supergravity method, and hydrothermal coupling. However, apart from sodium aluminate solution seed decomposition—which offers low cost and suitability for industrial production—other chemical preparation processes are less practical for industrialization due to high equipment requirements and elevated production costs.

1. Mechanical Pulverization Method

The mechanical pulverization method involves washing and drying ordinary metallurgical-grade aluminum hydroxide, then processing it into aluminum hydroxide micropowder using an air jet mill or ball mill. Micropowder produced by mechanical methods exhibits coarse particle size, a broad particle size distribution, irregular particle shapes, and a maximum particle size reaching 15–20 μm. This product exhibits poor performance in wire and cable manufacturing, with suboptimal processability and low bending strength and elongation. Compared to chemically produced aluminum hydroxide, it has a lower oxygen index and inferior flame-retardant properties.

2. Microemulsion Method

W/O microemulsions are transparent or translucent thermodynamically stable systems composed of water, water-insoluble organic solvents, surfactants, and co-surfactants. The composition and properties of microemulsion systems using water, octylphenol polyoxyethylene ether, n-hexanol, and cyclohexane were investigated. When the weight ratio of n-hexanol to octylphenol polyoxyethylene ether was 23, the microemulsion system exhibited a broad and stable microemulsion phase region, making it an ideal system for preparing ultrafine particles. In this system, ultrafine aluminum hydroxide particles with an average diameter of 6 nm and excellent dispersion were prepared using the ammonia precipitation method.

3. Sol-Gel Method

The sol-gel method is one of the most extensively studied and applied techniques for preparing ultrafine powders. Using aluminum sulfate as the raw material, a stable sol was obtained by sol-gelation under conditions of pH 3.5, sol concentration 0.4 mol/L, addition of 0.3% water-soluble polymeric dispersant, and reaction temperature of 80°C. After 5 hours of heat treatment at 100–150°C, ultrafine aluminum hydroxide particles with an average particle size of 70 nm, good dispersion, and uniform particle size distribution were prepared.

4. Metal Alcoholate Method

Under suitable catalysis, hydrolysis occurs in an H₂O-NH₃ system. Filtration and drying of the precipitate yields ultrafine aluminum hydroxide particles with an average diameter of 90 nm. This method features simple operation and small sample requirements. Disadvantages include severe sample agglomeration and high organic raw material costs.

5. Aluminate Hydrolysis

Depending on seed type, sodium aluminate seed decomposition can be categorized into self-decomposition seed decomposition and mechanical crushing seed decomposition. The mechanical seed decomposition method yields products with finer average particle sizes, reaching 2 μm. However, since the seeds used for decomposition undergo mechanical grinding, large particles of 10 μm or even 15 μm inevitably remain in the final product. This constitutes a critical flaw in the production of flame-retardant materials, particularly in the manufacturing of wires, cables, and engineering plastics. Products prepared via the two-stage decomposition method using self-decomposition seeds exhibit fine particle sizes, with average diameters below 1 μm, narrow particle size distribution, high purity, excellent whiteness, uniform and regular particle shapes, and ease of dispersion. This represents one of several common production methods for ultrafine aluminum hydroxide micropowders.