Hydrofluoric Acid

Basic Information

CAS:7664-39-3 (30–50% concentration)

Melting Point: -35 °C (35.5% HF aqueous solution)

Boiling Point: 120 °C (35.5% HF aqueous solution)

Density: 1.18 kg/m³ (40% HF solution)

Appearance: Colorless solution

Molecular Weight: 20.01

Odor: Pungent odor

Molecular Formula: HF

Application

1. Chemical Industry

In aluminum production, hydrofluoric acid is used to manufacture aluminum fluoride, synthetic cryolite, sodium fluoride, and magnesium fluoride. Within the aerospace sector, it primarily serves as a catalyst for producing liquid propellants for jet aircraft and missile jet fuel propellants.

2. Refining Industry

In refineries, hydrofluoric acid-catalyzed alkylation units convert components like butane and butene into high-octane gasoline through alkylation reactions.

3. Etching Applications

Hydrofluoric acid etches glass to engrave patterns, mark graduations, and inscribe text. The Canadian Diamond Drilling Association conducted experiments with various hydrofluoric acid mixtures. They found that regardless of the mixture, the etching action ceases within 4 hours. That is, within 4 hours, the acid is consumed and the chemical reaction is complete. They also discovered that a 4% hydrofluoric acid solution produces the clearest etching marks on glass test tubes.

4. Graphite Purification

The hydrofluoric acid purification method involves preheating graphite and adding it to a reactor equipped with a stirrer along with a specific proportion of hydrofluoric acid or a dilute acid-fluoride and ammonium fluoride salt system capable of producing hydrofluoric acid. After thorough wetting, stirring is timed. The reactor temperature is controlled by a thermostat. Upon reaching the specified time, excess acid solution is promptly removed, and the filtrate is recycled. The filter cake is washed with hot water until neutral, then dehydrated and dried to obtain the product. Additionally, due to its ability to dissolve oxides, hydrofluoric acid plays a crucial role in the purification of aluminum and uranium.

5. Geological Sample Decomposition

Although hydrofluoric acid is a relatively weak acid, its fluoride ions possess strong coordinating ability. Reacting with silicon, these ions form volatile silicon tetrafluoride, granting hydrofluoric acid unique decomposition capabilities for silicate rocks and minerals. This property makes it widely applicable in geological sample digestion. In geological sample digestion, hydrofluoric acid is rarely used alone; it is typically combined with high-boiling-point acids. For instance, it can be mixed with nitric acid, perchloric acid, sulfuric acid, or phosphoric acid to decompose samples such as silicates, phosphate ores, silver ores, quartz, niobium ores, and aluminum-rich ores. The combination of hydrofluoric acid and perchloric acid can also decompose minerals like feldspar, mica, quartz, magnetite, beryl, and pyrite.

6. Refrigerants

Hydrofluoric acid is also used in synthesizing various fluorinated organic compounds, including polytetrafluoroethylene and Freon refrigerants.

7. Semiconductors

The semiconductor industry employs it to remove oxides from silicon surfaces. In transistor fabrication, photoresist is applied over a SiO₂ mask on silicon. Mercury light passes through the mask's apertures onto the coating, causing the exposed areas to develop holes in the developer solution. After cleaning, the holes are etched out using hydrofluoric acid. Impurities then diffuse through these holes into the silicon material, forming n-type and p-type regions. This completes the full transistor fabrication process involving oxide masking, printing, photolithography, and diffusion.

8. Other Applications

Hydrofluoric acid can be used to remove glass coating layers from microfilament surfaces. At room temperature, it takes approximately 150 seconds to remove a 10 μm thick glass coating layer. Additionally, hydrofluoric acid is employed in the “acid pickling” process to remove oxygen-containing impurities from stainless steel surfaces.