Figure 3.5 U.S. imports of hydrofluoric acid, 1993–2016 [8].
3.3 Fluorocarbon Compounds
There are a number of routes to introduce fluorine into organic compounds [9–11]. Those methods include direct fluorination of carbon; halogen exchange between chlorocarbons and hydrofluoric acid; fluorination of hydrocarbons using electrochemical and catalytic methods; and fluorination techniques using metal fluorides [12]. Chapter 6 describes some of the commercial techniques for fluorocarbon preparation in detail.
3.4 Hydrofluoric Acid
The commercial manufacture of fluorocarbons requires converting fluorine’s inorganic ores to a suitable intermediate. That would in turn be used to introduce fluorine into organic compounds. A suitable compound would react with hydrocarbons (though not too reactive), inexpensive and is safe would be ideal. The most frequently used agent, commercially speaking, has proven to be hydrofluoric acid (HF), far from a perfect choice (Table 3.3).
HF when combined with water forms a highly corrosive acid that can even etch glass. Skin contact, inhalation, ingestion and contact with eyes must be avoided because of the extreme danger hydrofluoric poses. Safety data sheet (SDS) of HF must be consulted prior to its handling.
3.4.1 Manufacturing Hydrofluoric Acid
Hydrofluoric acid is manufactured by the reaction of acid-grade fluorspar (≥97% CaF2) with sulfuric acid (H2SO4). The basic reaction is shown in Eq. (3.1).
Figure 3.6 displays a process diagram for commercial production of HF [14]. In the first step fluorspar is dried for 30-60 minutes in a horizontal rotary kiln that is heated to 200-250°C. Dry fluorspar and a small excess of sulfuric acid are fed continuously to the front end of a stationary pre-reactor or directly to the kiln by a screw conveyor. The pre-reactor mixes the components prior to charging to the rotary kiln. Calcium sulfate (CaSO4) is removed through an air lock at the opposite end of the kiln. The gaseous reaction mixture - HF and the excess H2SO4 from the primary reaction are removed at the front end of the kiln along with entrained particulates. Silicon tetrafluoride (SiF4), sulfur dioxide (SO2), carbon dioxide (CO2), and water produced in secondary reactions are also removed along with the HF.
Table 3.3 Typical physical properties of hydrogen fluoride and hydrofluoric acid [13].
| Concentration | 49% HF | 70% HF | 100% HF (AHF) |
| Freezing Point | -33°F (-36°C) | -95°F (-71°C) | -118°F (-84°C) |
| Boiling Point | 223°F (106°C) | 146°F (63°C) | 67.1°F (19.5°C) |
| Density (68°F) | 9.6 lbs/gal | 10.1 lbs/gal | 8.3 lbs/gal |
| pH | <3.4 | <3.0 | <1.0 (10% solution) |
| Flash Point | Not Flammable | ||
| Vapor Pressure (68°F) | 23 mm Hg | 132 mm Hg | 771 mm Hg |
| Vapor Density | 2.4 (air = 1.0) | ||
Figure 3.6 Hydrofluoric acid manufacturing process flow diagram [14].
The particulates are separated from the gas stream using a dust separator and returned to the kiln. Sulfuric acid and water are removed using a pre-condenser. HF vapors are next condensed in refrigerated condensers forming crude HF (impure) sent to storage tanks. The remaining gas stream passes through a sulfuric acid absorption tower or acid scrubber, to take out most of the residual HF and H2SO4. The gases exiting the scrubber are passed through water scrubbers to recover SiF4 and HF as fluosilicic acid (H2SiF6). The water scrubber gases are passed through a caustic scrubber before release to the atmosphere. Stored HF and H2SO4 are distilled to obtain HF at 99.98% purity. Lower concentration HF are prepared by water dilution [14].
3.5 Aliphatic Fluorinated Organic Compounds
Commercial fluorocarbons are classified as aliphatic compounds which means they have saturated or unsaturated linear chemical structures. Cyclic fluorocarbons are considered part of the aliphatic group but they are not used in any significant quantity in the applications of the rest of the aliphatic fluorocarbons. Consequently, cyclic compounds are not included in this chapter.
Carbon forms its strongest bond with fluorine. The credit for demonstrating the stability of the C–F bond goes to the French chemists Dumas and Peligot, who heated dimethyl sulfate with potassium fluoride and obtained methyl fluoride [see Eq. (3.2)] [15].
The first nucleophilic replacement of another halogen by fluorine was attributed to a genius, the Russian musician and gifted chemist Alexander Borodin [16]. He synthesized benzoyl fluoride by replacement of chlorine in benzoyl chloride using Fremy’s Salt (KF+HF) [17]. The reaction, called halex (abbreviation for halogen exchange) has continued to be the most significant way to produce C–F bonds on a commercial scale [12].
The pioneering work of Belgian chemist Frederic Swarts breathed a new life into the lagging chemistry of aliphatic fluorine compounds. Swartz conducted halogen exchange on polychlorides and polybromides through the use of combined antimony trifluoride and bromine (SbF3 + Br2). He elucidated dehalogenation reaction using Zn and dehydrohalogenation using K2CO3 could selectively remove halogens other than fluorine leading to the formation of fluorinated olefins. Swarts has been credited with the first synthesis of CCl2F2 by Midgley and Henne of the Frigidaire Co. (part of General Motors), who pioneered the use of fluorinated hydrocarbons in the refrigeration industry [18, 19].
In the 1950s and 1960s, study of the fluorocarbons began leading to developments for biological activity. Fluorocarbons, for instance, such as Fluroxene® (CF3CH2OH=CH2) started a massive change in the types of inhalation
