fluorspar in China that was exported has declined substantially. As a result of the dramatic expansion of the Chinese refrigerant and fluoropolymer manufacturing industry in the 2000s, most acid-grade fluorspar mined in China is now consumed domestically. Only 8% of the acid-grade fluorspar mined in China in 2016 was exported.
Figure 3.3 Global mined quantities of acid-grade fluorspar during 1993 to 2016 [8].
In 2008, Mexico overtook China as the leading exporter of acid-grade fluorspar to the US and in 2009 became the largest fluorspar exporter globally. In 2016, Mexico supplied about 65% of the acid-grade fluorspar imported into the United States (see Figure 3.4) [8].
HF itself is also traded globally. it is, however, both toxic and corrosive making it difficult to transport great distances. Exported quantities are thus substantially lower than those of fluorspar, and HF export is mostly limited to nearby countries. For example, China primarily exports HF to Japan and South Korea, whereas nearly all Mexican HF exports go to the US. Likewise, most HF imported into the United States comes from Mexico (see Figure 3.5) [8].
Figure 3.4 Annual U.S. import quantities of acid-grade fluorspar (“acidspar”), 1993–2016. Mexico, China, and South Africa are the major countries of origin for acidspar. Other countries that have exported significant quantities of fluorspar to the United States include Vietnam, Spain, the United Kingdom, and Mongolia [8].
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