Halogens and Halogen Compounds

Halogens and Fluorine Compounds

Description of Halogens and Halogen Compounds

Halogens are a group of highly reactive nonmetals located in Group 17 of the periodic table. They include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Halogen compounds are widely utilized in various applications due to their chemical properties, particularly in organic and inorganic chemistry.

Fluorine Sources

Major sources of fluorine include:

• Fluorspar (CaF2): A primary source known for its versatility and economic importance in the chemical industry.

• Fluorapatite: Naturally occurring mineral with a fluorine content ranging from 2-4% by weight, valuable for fertilizer production.

• Cryolite (Na3AlF6): A mineral historically mined in Greenland until 1963, used primarily in aluminum production.

• Estimated fluorine content in the Earth's crust is about 0.09%.

Fluorspar

In 1992, the extraction of fluorspar reached 4.054 million tonnes. Key Applications:

• Essential in the manufacture of raw steel, where fluorine aids in refining processes.

• Vital for aluminum production, acting as a flux in the electrolytic process.

• Integral to the fluorochemical industry, including refrigerants, pesticides, and pharmaceutical products.

Fluorspar Extraction

Extraction Method:

• Mining is employed to retrieve fluorspar from the earth.

• The mined fluorspar often contains impurities such as:

  • Heavy spar (BaSO4)

  • Galena (PbS)

  • Quartzite (SiO2)

  • Zinc blende (ZnS)

Concentration Process:

• Beneficiation: Involves mechanical grinding and flotation techniques to separate impurities, yielding 30-80% CaF2.

• The final concentration is typically 96-98% CaF2 (classified as acidspar). The remaining impurities usually consist of SiO2, CaCO3, and BaSO4.

Fluorine Manufacturing

Electrochemical Production:

• Produced from a salt melt formed by a mixture of potassium fluoride and hydrofluoric acid at a molar ratio of 1:2 to 1:2.2.

• The reaction temperature is maintained between 70-130°C.

• Potassium fluoride serves as a vital component, ensuring melt conductivity and undergoing continuous replenishment during electrolysis.

Electrolysis Cell

The electrolysis cell is chiefly constructed from materials such as:

• Cathodes and Cell Materials: Typically made from Monel alloys or steel to withstand the harsh environments.

• Anodes: Often constructed from degraphitized carbon.

• The system functions without a diaphragm, relying on steel skirts for separation.

Fluorine and Hydrogen Production

The fluorine and hydrogen produced during the process contains approximately 10% hydrogen fluoride (HF). After production:

• Hydrogen fluoride is subsequently removed through an alkaline scrubber.

• The fluorine gas undergoes treatment with sodium fluoride to minimize residual hydrogen fluoride content.

Uses of Fluorine:

• Primarily in the manufacturing of uranium hexafluoride, which is crucial for uranium enrichment processes.

• Production of sulfur hexafluoride, widely used in the electrical industry due to its insulating properties.

• It is liquefied for storage, typically in pressure cylinders mixed with nitrogen (10-20% fluorine).

Global Fluorine Production Capacity

The total global production capacity of fluorine stands at approximately 7,500 tonnes/year, with utilization rates broken down as follows:

• 75% for uranium hexafluoride

• 22.5% for sulfur hexafluoride

• 2.5% for tetrafluoromethane

Hydrogen Fluoride

Key Product in Fluorochemicals: In 1994, world production of hydrogen fluoride exceeded 1 million tonnes/year. Its industrial manufacturing process typically involves the chemical reaction of sulfuric acid with fluorspar (acid grade).

• A 5-10% excess of sulfuric acid is necessary to counteract impurities.

• Silicon dioxide generates silicon tetrafluoride, which can significantly reduce the yield of HF.

Applications of Hydrogen Fluoride:

• Crucial for manufacturing inorganic fluorides, such as aluminum fluoride.

• Plays a significant role in the production of various organofluorocompounds, which are increasingly in demand due to environmental considerations regarding ozone depletion.

• HF is pivotal in meeting the requirements for various fluorinated compounds, including historical refrigerants like R11 and R12.

Inorganic Fluorine Applications

Inorganics containing fluorine have several industrial applications, including:

• Glass Etching and Polishing: Utilized extensively to enhance surface qualities.

• Steel Pickling: Employed in cleaning metal surfaces by removing oxides and impurities.

• Semiconductor Production: Essential for etching patterns on silicon wafers.

• Commercially available as liquids or aqueous solutions containing 40-75% HF concentration.

  • Containers for storing >70% HF must be made of steel, while those for <70% are typically constructed from plastic lined with rubber.

Aluminum Fluoride

Aluminum fluoride is vital in the electrolytic manufacture of aluminum, where the electrolyte composition generally consists of:

• 80-85% Na3AlF6 (cryolite)

• 5-7% AlF3

• 5-7% CaF2 Modern processes have improved efficiency in fluorine recovery during aluminum production.

Other Applications Include:

• Functioning as a flux during welding processes.

• Melting enamels and facilitating glass production.

Hydrogen Fluoride Conversion to Aluminum Fluoride

Lurgi Process:

• Involves calcining aluminum hydroxide at temperatures ranging from 300-400°C.

• Reactions are conducted at 400-600°C in the presence of hydrogen fluoride, forming aluminum fluoride.

Cryolite

Applications:

• Predominantly utilized in aluminum manufacturing processes.

• Important in steel processing, serving as a flux in aluminum waste processing.

• Used in welding technology, glass production, and as an additive in abrasives and light metals.

Cryolite Manufacture

Manufactured starting from aqueous HF solutions derived from hexafluorosilicic acid. The production involves:

• Isolation of ammonium fluoride, which is then reacted with sodium aluminate to produce cryolite.

Alkali Fluorides

Key Fluorides:

• Sodium fluoride, potassium hydrogen fluoride, and ammonium hydrogen fluoride are essential chemicals in various industries.

• They can be manufactured by reacting HF or hexafluorosilicic acid with alkali hydroxides.

• Ammonium fluoride is specifically obtained via a reaction between ammonia and hydrogen fluoride.

Sodium Fluoride Applications

Sodium fluoride has extensive applications, including:

• Crucial in the production of organofluorocompounds.

• Used as a preservative and in dental products, notably sodium monofluorophosphate in toothpaste.

Hexafluorosilicates

Sodium and potassium hexafluorosilicate are produced by the reaction of alkali salts with hexafluorosilicic acid.

• Their applications are prominent in wood preservation, specifically magnesium hexafluorosilicate for treating wood against decay.

• Sodium hexafluorosilicate is predominantly used for water fluoridation efforts to enhance dental health in communities.

Uranium Hexafluoride

A critical chemical for uranium isotope separation (i.e., 235U and 238U), uranium hexafluoride is produced by reacting uranium(IV) oxide with HF to form uranium tetrafluoride, which is then converted to UF6.

Boron Trifluoride and Tetrafluoroboric Acid

• Boron Trifluoride: Synthesized by reacting borates with fluorspar or by combining HF with boric acid.

• Tetrafluoroboric Acid: Produced from the reaction of boric acid and HF, utilized in various applications including as a reagent in organic synthesis.

Sulfur Hexafluoride

Manufactured by the reaction of sulfur and fluorine, this exothermic reaction produces sulfur hexafluoride.

• Applications include high voltage fire extinguishers and protective gas for high voltage installations.

Organofluorocompounds via Electrochemical Fluorination

This industrial technique is crucial for ensuring carbon compounds retain their functional groups through an electrolytic process using nickel electrodes under specific conditions.

Chloralkali Electrolysis

The electrolysis of sodium chloride solution is critical for producing chlorine, sodium hydroxide, and hydrogen. Methods for extraction include:

• Natural deposits or seawater extraction.

• A purification process that involves removing impurities such as calcium and magnesium to yield high-purity products.

Manufacturing Processes for Chlorine

There are several processes used in chlorine production:

• Mercury Process (64% of usage): Involves electrolyzing NaCl solution on a mercury cathode.

• Diaphragm Process (24%): Employs titanium anodes separated by an asbestos diaphragm, yielding brine with lower purity.

• Membrane Process (11%): Utilizes ion-conducting membranes to achieve high purity at lower energy costs.

Evaluation of Processes

Mercury Process:

• Pros: Produces high purity sodium hydroxide.

• Cons: Associated with high energy consumption and environmental concerns related to mercury.

Diaphragm Process:

• Pros: Produces lower voltage with less purity but acceptable for certain applications.

• Cons: Results in dilute NaOH and chlorine gas that may contain oxygen, impacting its usability.

Membrane Process:

• Pros: Known for high purity outputs and reduced energy consumption.

• Cons: Limited membrane lifetime and significant initial investment required for setup.

Applications of Chlorine

In the year 2000, predicted usage for chlorine included:

• Various products such as PVC (polyvinyl chloride), pulp bleaching agents, propene oxide production, and critical water treatment processes.

Sodium Hydroxide Utilization Spectrum

Sodium hydroxide serves numerous applications across industries, including organic chemicals, inorganic chemicals, pulp and paper production, and textiles. Its versatility makes it an essential chemical in various manufacturing processes.