Mined fluorspar is classified based on the quality of the ore which is measured by its calcium fluoride content. Fluorspar is typically found in three grades; acid grade or acidspar, ceramic grade and lastly metallurgical grade or metspar. Acidspar has a minimum calcium fluoride content of 97 % and is principally converted to hydrofluoric acid which is a reagent in fluorochemical syntheses. Ceramic grade spar has a calcium fluoride content of between 80 and 96 % with silicon dioxide content under 3 % and is used to produce enamels and glass. Metspar has a minimum calcium fluoride content of 80% and can be found with up to 15 % silicon dioxide, it is used in steel making as well as in the production of aluminium (Bide, 2011, Finger et al., 1960, Kirsch, 2004).
From the three grades of fluorspar typically found, acidspar is the most versatile by having a multitude of uses, some of which involve the preliminary conversion to hydrofluoric acid.
The production of hydrofluoric acid
Although hydrofluoric acid is not an organofluorine it is very important in the synthesis of various fluorocarbons such as chlorofluorocarbons, agrochemicals, refrigerants and pharmaceuticals (Bide, 2011).
Anhydrous hydrofluoric acid is produced by an endothermic reaction (Equation 2.1) involving the addition of acid grade fluorspar to concentrated sulphuric acid at temperatures of approximately 300 ○C in a rotary kiln reactor. The reaction produces gaseous hydrofluoric acid and calcium sulphate which is found as a solid. The reactor is usually operated at vacuum conditions as this has the dual advantage of reducing operating temperatures as well as aiding in the removal of the gaseous product.
The gaseous stream then undergoes various processing steps; thereafter anhydrous hydrofluoric acid with a purity of 99.98 % is produced. Aqueous hydrofluoric acid may be then obtained by the addition of water to the process stream. Figure 2.1 shows a simplified process flow diagram of the production process (Bide, 2011, Kirsch, 2004).
CaF2+ H2SO4 → 2 HF + CaSO4 (2.1)
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Dry acid grade fluorspar
Concentrated sulphuric acid
Rotary kiln reactor (± 300 ○C)
Solid calcium sulphate
Washing
column Condenser Distillation
column Hydrofluoric acid free
emmmisons
Anhydrous hydrofluoric acid
Aqueous hydrofluoric acid Gaseous
hydrofluoric acid
Water
Figure 2.1: Simplified process flow diagram of the production of anhydrous and aqueous hydrofluoric acid from the reaction of acid grade fluorspar and concentrated sulphuric acid (Bide, 2011).
The production of chlorofluorocarbons
One of the most infamous groups of organofluorine compounds are chlorofluorocarbons (CFCs) whose negative effects on the ozone layer were highlighted by the Montreal Protocol of 1987 which subsequently placed legal limits on the use of CFCs, carbon tetrachloride, hydrofluoroethers, halons and various other ozone depleting chemicals (Auffhammer et al., 2005).
In the mid-1900s, CFCs were essential to everyday life, gaining popularity through their use as refrigerants, as insulation, in air conditioning systems, and later as solvents and in electronics.
These chemicals were widespread because they were non-toxic, inert, could be produced in a range of boiling points (which made them desirable as refrigerants), were non-flammable, and non-carcinogenic (Haas, 1992).
In 1930 Midgley and Henne developed a continuous process for the production of dichlorodifluoromethane based on the work of F. Swarts. Dichlorodifluoromethane was produced by contacting carbon tetrachloride and anhydrous antimony bifluoride in the presence of antimony pentachloride according to Equation 2.2. The reaction proceeded with yields of up to 94% being readily achievable. Dichlorodifloromethane was then easily separated from the product by fractional distillation (Midgley Jr and Henne, 1930).
3 CCl4+ 2 SbF2 SbCl→ 3 CCl5 2F2+ 2 SbCl4 (2.2)
8 The production of hydrofluorocarbons
Hydrofluorocarbons are another important organofluorine which gained great importance when restrictions on the use of CFCs were implemented by the Montreal Protocol. Hydrofluorocarbons served as replacement refrigerants to CFCs and although they were not potential ozone depleting chemicals, they are greenhouse gases (Kirsch, 2004).
Hydrofluorocarbons can be synthesised by combining two types of reactions hydrogenolysis of bromine or chlorine and Lewis acid-catalysed halogen isomerisation. Figure 2.2 shows three typical synthesis routes of how 1,1,1,2-tetrafluoroethane is produced by the reaction of tetrachloroethene and hydrofluoric acid.
Figure 2.2: Typical synthesis routes to produce 1,1,1,2-tetrafluoroethane by the reaction of tetrachloroethene and hydrofluoric acid by the combination of two main reaction routes: hydrogenolysis of chlorine and Lewis-acid catalysed halogen isomerisation (Kirsch, 2004).
The production of fluoropolymers
A discussion of organofluorine synthesis would not be complete without the inclusion of fluoropolymers. Fluoropolymers were first discovered in the early 1930s by Ruff and Bretschnieder who synthesized gaseous tetrafluoroethylene, which later became a valuable monomer for perfluorinated fluoropolymer synthesis, such as polytetrafluoroethylene (PTFE).
Tetrafluoroethylene is synthesized by the pyrolysis of chlorodifluoromethane in the reaction steps shown in Figure 2.3.
PTFE was discovered accidentally by Dr. Roy Plunkett at DuPont (one of the world leaders in fluoropolymer industry) in 1938. This would later be seen as the most monumental discovery in the fluoropolymers industry (Teng, 2012). PTFE is produced in a batch process by the addition polymerisation of tetrafluoroethylene. The reaction temperature may be up to 100 ○C and between pressures of 0.7 and 3.5 MPa.
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Figure 2.3: Typical synthesis route to the commercial production of tetrafluoroethylene (Ebnesajjad, 2011).
Initially, research into fluoropolymers was not eagerly pursued due to its high melt viscosity and melt temperature as well as insolubility which made it difficult to process and dangerous to synthesize given the toxic nature of tetrafluoroethylene (Teng, 2012). This changed in the 1940s when fluoropolymers were seen as a viable material to be used in the purification of uranium hexafluoride during World War II, this meant that funding and equipment were readily available for this research.
The fluoropolymer industry continued to grow and today it is one of the largest global markets for fluorochemicals. The drivers of this market lie with the extremely desirable properties of fluoropolymers, such as low surface energy making it difficult for substances to adhere to them as well as being exceptionally chemically and thermally stable. These physical and chemical properties make them versatile and appropriate for use in electronics, chemical storage and transport, bio materials, coatings, textiles, construction and automotive industries (Ebnesajjad, 2011, Kirsch, 2004, Teng, 2012).
Fluoropolymers can be divided into two types; homopolymer and copolymers. Homopolyers are polymers which consist principally of a single repeating monomer whereas copolymers may consist of multiple monomers in each polymer segment. The polymers may then be further characterised by two different classes, namely; perfluoronated and partially fluorinated fluoropolymers. Perfluoronated fluoropolymers display the exceptional physical and chemical properties descried above, but this in turn decreases their processability. This inspired the innovation of partially fluorinated fluoropolymers which exhibited the necessary qualities of the perfluoronated fluoropolymers and was less difficult to process and mould.
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