Uses, Preparation, and Market Prospects of Aluminum Fluoride

Aluminum fluoride (molecular formula: AlF3) is commonly used in industry in the form of the crystalline hydrate AlF3·3H2O. Aluminum fluoride trihydrate appears as a colorless, white powder with a triclinic crystal structure, while anhydrous aluminum fluoride exists as large, hexagonal-system crystals (an ionic crystal). It has a density of 2.88 g/cm³, a melting point of 1040°C, and a boiling point of 1291°C. It is insoluble in cold water, soluble in hot water, sparingly soluble in acidic and alkaline solutions, and insoluble in most organic solvents. It does not react when heated with liquid ammonia or concentrated sulfuric acid, nor when fused with potassium hydroxide. Its chemical properties are highly stable; it does not decompose upon heating but instead undergoes sublimation.

• It is primarily used in aluminum production to lower the melting point and increase the electrical conductivity of the electrolyte.
• It serves as a fermentation inhibitor in alcohol production, acts as a flux and glaze component for ceramics and enamels, and functions as a flux in non-ferrous metal smelting.

Currently, over 95% of industrial aluminum fluoride is consumed by the aluminum industry. It serves as an additive to the cryolite-alumina electrolyte solution, compensating for alumina loss, lowering the electrolyte's molecular ratio and primary crystallization temperature, and helping to control the thermal balance during the aluminum electrolysis process; it is an essential raw material for the electrolytic aluminum industry.

1. (1) Wet Process

   Using acid-grade fluorspar powder, concentrated sulfuric acid, and aluminum hydroxide as raw materials, the hydrogen fluoride gas generated from the reaction of fluorspar powder and concentrated sulfuric acid is first absorbed in water to produce a 30% hydrofluoric acid solution. This solution then undergoes a liquid-solid mixed-phase reaction with aluminum hydroxide to form a supersaturated aluminum fluoride solution. Finally, the product is obtained through crystallization, filtration, and high-temperature calcination for dehydration.

2. (2) Dry Process

   Like the wet process, this method utilizes acid-grade fluorspar powder, concentrated sulfuric acid, and aluminum hydroxide as raw materials. The key difference lies in the process flow: the hydrogen fluoride gas generated in the initial step passes through a gas-phase purification system to remove dust and sulfuric acid. It is then mixed with dried aluminum hydroxide (with a moisture content of 12%) and fed into a double-layer fluidized bed. Here, the calcination-dehydration and fluorination reactions take place, ultimately yielding the aluminum fluoride product.

3. (3) Aluminum Salt Process

   Common aluminum salts used to produce aluminum fluoride include aluminum chloride, aluminum sulfate, aluminum silicate, and aluminum fluorosilicate.

   The aluminum chloride method involves heating an aqueous solution of aluminum chloride with hydrofluoric acid to 400°C to drive off hydrogen chloride gas, thereby producing aluminum fluoride. While the process route is simple, the use of expensive and highly corrosive hydrofluoric acid results in high production costs.

   The aluminum sulfate method involves heating low-grade fluorspar ore with aluminum sulfate to generate calcium sulfate (CaSO4) precipitate and an aluminum fluoride solution; the final product is obtained through filtration, crystallization, drying, and dehydration. Although this process route is relatively complex and relies on fluorspar—a resource in tight supply—it entails lower equipment costs compared to other methods.

   Aluminum silicate is most directly sourced from natural aluminum ores, which are reacted directly with hydrogen fluoride to produce aluminum fluoride. While this method offers a simple process route, the use of expensive and highly corrosive hydrogen fluoride gas significantly drives up both production and equipment costs.

The new global industrial revolution, characterized by the low-carbon economy, will inevitably drive the aluminum fluoride industry toward higher quality and greater efficiency. Anhydrous aluminum fluoride, serving as an alternative to the standard dry-process product, will represent the industry's state-of-the-art technology in the near future; indeed, the development of aluminum fluoride products with superior performance is an inevitable trend. Furthermore, utilizing fluorine resources derived from phosphate fertilizer by-products to establish new supply channels—thereby producing anhydrous aluminum fluoride or high-performance variants—represents a necessary strategic choice for the future of the industry.