Читать книгу Concise Handbook of Fluorocarbon Gases - Sina Ebnesajjad - Страница 36

Fluorine is the 13^th most common element on the Earth but it occurs virtually only as inorganic compounds. There are three industrially significant fluorine minerals: fluorite or fluorspar, fluorapatite and cryolite. Fluorspar consists of CaF₂, seen in Figure 3.1 and comes in two grades. The more pure variety is called acid grade and is used for HF production. The other fluorspar grade (lower) is used in metallurgy in iron and steel casting, primary aluminum production, glass manufacture, enamels; welding rod coatings, cement production, as a flux in steelmaking and in other applications [6].

Fluorapatite or fluoroapatite is a phosphate with the chemical formula of Ca₅(PO₄)₃F [Figure 3.1(b)]. It is the most abundant fluorine ore but it contains little fluorine. The third fluorine mineral Cryolite [Figure 3.1(c)] has a chemical formula of Na₃AlF₆, used in aluminum production. Cryolite has the highest amount of fluorine but it is a relatively scarce ore.

The dynamics of supply and consumption of fluorspar are quite complex. Fluorspar price and annual consumption/import for the United States is presented in the rest of this section. Figure 3.2 is helpful in clarification of some of the issues relevant to fluorspar. Some of the factors impacting the imported quantities and the prices are listed below:

1 The general economy and business cycle such as recessions and booms.

2 Demand for fluorinated and other materials that requiring HF.

3 United State stopped production of fluorspar after 1996.

4 USA has been importing acid grade fluorspar mostly from China, Mexico and South Africa. These three countries are the major three producers of CaF2 in the world.

5 During the early years post stoppage of domestic fluorspar China was the primary supplier to the US. In the recent years Mexico is the top supplier.

6 A supply of fluorspar is maintained by the Defense National Stockpile Center. The Center sells the fluorspar as required.

7 The Montreal Protocol (1987) banned most chlorofluorocarbon (CFC) gases. A two-phase replacement plan was developed: 1. To transition from CFCs to Hydrochlorofluorocarbons (HCFCs) as an intermediate solution; 2. To replace HCFCs with Hydrofluorocarbons (HFCs) and 3. To replace all previous fluorocarbons with low Ozone Depleting and low Global Warming replacements such as hydrofluoroolefins.

8 Most CFCs were phased out by 1996; the rest were removed by 2010; HCFCs replaced CFCs. The replacement required a one-time large block of production to replace all fluids in refrigeration and air conditioning units in addition to other applications of CFCs.

9 European Union countries replaced all HCFCs with HFCs by the end of 2010.

10 In 2010 the Chinese Ministry of land and Resources placed a quota on the annual production rate of fluorspar limiting at 11 million tons.

11 In the recent years many programs have been implemented to capture and recycle fluorocarbons in addition to reducing consumption of these materials.

12 Some of the fluoropolymer manufacturing, specifically polytetrafluoroethylene, has moved away from the US, to China and India.

13 A reason fluorinated chemicals are more expensive than hydrocarbons is the cost of fluorspar.

Figure 3.1 Photographs of fluorine minerals: (a) fluorspar, (b) fluorapatite, and (c) cryolite Sources: (a) www.themineralgallery.com/elmwoodroom.htm, Courtesy: www.mindat.org and the Hudson Institute of Mineralogy (b) www.mindat.org/photo-16871.html, Courtesy: David Soler, (c) www.mindat.org/min-1161.html, Courtesy: JGW, May 2016.

Figure 3.2 Historical US imports and prices of acid grade fluorspar [7].

As shown in Figure 3.3, China mines more acid-grade fluorspar than any other country in the world, at 2,500 ktons annually. Mexico is the second-largest producer of acid-grade fluorspar at around 600 ktons annually, followed by South Africa (150 ktons). Contributions from the rest of the world have declined since the mid-2000s, with the remaining balance of acid-grade fluorspar production at about 300 ktons in 2016 [7].

From the mid-1990s to early 2000s, most of the acid-grade fluorspar mined in China was exported to the major refrigerant producing regions (US, EU, Japan). Starting from around 78% in 1999, the fraction of all mined acid-grade 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].