In Aurora, North Carolina, the fertilizer company Nutrien operates one of the world’s largest fertilizer plants. Although the facility’s focus is on phosphate-containing crop nutrients, it also generates hydrofluorosilicic acid (HFS) as a by-product. Nutrien markets some of the acid for water fluoridation, but the company must dispose of much of it by other means it declines to comment on.
Meanwhile, 1,300 km away in Calvert City, Kentucky, the French chemical maker Arkema has a problem of its own: rising prices and unstable supplies for the raw materials it needs to produce fluoropolymers and fluorinated refrigerant gases.
In June, the firms announced an agreement intended to address both problems. Nutrien will build a plant in Aurora, the first of its kind in the US, that will convert by-product HFS into anhydrous hydrogen fluoride (HF). Arkema will pay $150 million to secure a long-term contract for the plant’s output when it starts up in 2022.
Although novel to the US, the Aurora plant will be one of several worldwide that are starting to challenge the current method of making hydrogen fluoride from the mineral fluorspar. The new approach could solve an environmental problem for fertilizer companies while improving both economics and raw material supply security for fluorochemical makers.
We think of farms as idyllic, literally pastoral, places where our basic foodstuffs are grown. But producing the fertilizers needed to make a modern farm productive can be a messy business, particularly when it comes to phosphate fertilizers.
In Aurora and other phosphate fertilizer production centers, companies use giant dragline buckets to scrape topsoil off acres of land and get at deposits containing clay, sand, and phosphate-rich rock. After the companies separate the clay and sand, they react the rock with sulfuric acid, yielding phosphoric acid. Next, they react phosphoric acid with ammonia to form ammonium phosphates, bedrock fertilizers for farmers because they provide nitrogen and phosphorus, which along with potassium are the three essential plant nutrients.
But the sulfuric acid also reacts with other elements in the rock to generate by-products, most notably silicon tetrafluoride gas. In the fertilizer industry’s early years, companies vented the gas into the environment, damaging nearby vegetation and livestock. Today it is absorbed in water, forming HFS.
The silicon content makes HFS a less versatile raw material than HF, with applications limited to water fluoridation and making certain fluoride salts. And the fertilizer industry generates much more HFS than is needed for these uses, explains Olivier Ruffiner, business manager for fluorine at the Swiss engineering firm Buss ChemTech. “The majority is neutralized and gets pumped into the sea,” Ruffiner says. “That’s a huge problem.”
Companies like Buss have recognized this problem since the 1980s, but early attempts to scale up laboratory methods of converting HFS to HF didn’t go well. “All of them failed,” Ruffiner says. In the mid-1990s, Buss acquired rights to technology, developed by the Polish firm Lubon, that uses sulfuric acid as a catalyst to turn HFS into hydrogen fluoride and silicon dioxide. After some 4 years of effort, Buss managed to develop a commercially viable process.
Buss’s initiative coincided with a push by authorities in China’s phosphate-producing regions for recovery and use of HFS. In 2008, the Chinese fertilizer firm Wengfu Group became the first company to adopt the Buss technology. Today, Ruffiner says, Wengfu operates five such plants across China, and it is considering two more facilities. A report by the British minerals research firm Roskill says Wengfu is planning an HFS facility in Morocco, home of OCP Group, the world’s largest phosphate fertilizer producer.
It seems likely that the North Carolina project will employ the Buss technology as well, but Buss, Arkema, and Nutrien all declined to discuss the matter.
Regardless of the technology provider, the intended recipient of the resulting HF is Arkema’s Calvert City plant. The facility started making fluorochemicals in 1956, reacting fluorspar, which is mostly calcium fluoride, with sulfuric acid to yield hydrogen fluoride and calcium sulfate.
When the plant opened, more than three-quarters of the fluorspar mined in the US came from an area straddling the nearby Kentucky-Illinois border. Today, chemical-grade fluorspar is no longer mined in the US. Instead, it is imported mainly from China and Mexico. China was the source of more than half of the fluorspar mined around the world last year, according to the US Geological Survey; Mexico accounted for 17%.
Arkema closed the Calvert City site’s hydrogen fluoride plant in 1996 and switched to bringing HF in by rail from Mexico, where Orbia’s Koura unit is the dominant producer. Arkema converts about half the imported HF into fluorinated polymers such as its Kynar-brand polyvinylidene fluoride. The other half goes to make refrigerant gases like R-32 (difluoromethane), of which Arkema is the sole US producer.
Supplies of fluorspar have become precarious in recent years as demand grows and Chinese officials crack down on environmentally questionable mining operations. Industrial Mineral Forums & Research, a market research firm, reports that the number of fluorspar mines in China fell from more than 1,200 in 2013 to 251 by the end of 2018. Compounding the supply shrinkage are growing trade frictions between the US and China.
Overall, according to Arkema, US supplies of fluorspar and hydrogen fluoride are becoming increasingly tight and volatile. The company began working with Nutrien on the new approach in 2017, opting for a facility with a capacity of up to 40,000 metric tons (t) per year. Environmental benefits, Arkema says, include use of an unwanted by-product instead of mined fluorspar and a decrease in both energy consumption and greenhouse gas emissions compared to the traditional process.
Ruffiner says interest in Buss’s technology is mostly environmental as authorities crack down on waste-disposal practices once deemed acceptable. “The regulations are getting tighter and tighter, and they can’t do it as they have in the past,” he says.
But economics play a role as well. The price of fluorspar is more than 3 times what it was in 2000, according to Roskill. Meanwhile, as a mostly unwanted by-product, HFS is essentially free. Roskill production cost calculations suggest that Wengfu’s plants have “significant” economic advantages over their fluorspar-based competitors, says Kerry Satterthwaite, an analyst with the research firm.
And there’s plenty more HFS to be captured. Phosphate rock contains about 3% fluorine, Satterthwaite says. Based on Roskill’s estimate of world phosphoric acid output, she puts HFS production at roughly 1.4 million t per year, of which less than a third is recovered for water fluoridation and fluoride salt production. Satterthwaite sees considerable benefit to directing more of this by-product to HF.
Still, Wengfu and—maybe—Arkema aside, customers are not beating down Buss’s door, Ruffiner acknowledges. Buss has been in contact with Mosaic, the largest US phosphate fertilizer producer, as well as other large firms in China, India, and the Middle East. Further contracts have yet to emerge. “All are theoretically interested,” Ruffiner says, “but most are state-owned, so the move forward is not that easy.”