PHILADELPHIA (Reuters) – Workers at the fire-damaged Philadelphia Energy Solutions refinery took the first steps this week toward ridding the plant of roughly 30,000 barrels of modified hydrofluoric acid, a dangerous undertaking that has rarely, if ever, been performed under similar circumstances.

In the minutes after the blaze and a series of explosions at the PES refinery on June 21, operations workers activated an emergency system that swiftly dumped the acid into a protective vessel before it could spread a harmful fog over the city.

Hydrofluoric acid is a highly toxic chemical used to make high-octane gasoline in more than one-third of U.S. refineries. Exposure can cause people’s eyes to burn and lead to severe health problems, including death. It has been used in refinery operations for 70 years, and labor unions and environmentalists have warned about its presence in densely populated areas.

PES entered Chapter 11 bankruptcy last week for the second time in less than two years, citing damage and business interruption from the fire.

The bulk of neutralizing is expected to begin next week, said Kathy Matheson, spokesperson for the Philadelphia Fire Department, one of several city, state and federal agencies that will help monitor the process.

The U.S. Environmental Protection Agency is installing multiple additional monitors around the refinery to detect any possible HF release, EPA spokeswoman Terri White said.

Refinery workers have begun neutralizing remnants of the modified hydrofluoric acid, or HF, in the drained unit before addressing the larger supply held in the emergency container, sources familiar with the plan said.

The method for removing the bigger amount of acid is still not clear, but whatever approach is taken will be complicated, experts said.

“The site itself would have to be absolutely secured so that there was no danger from residual HF or from damaged equipment for people who go into the area where the HF has to be removed. That’s the first priority,” said Mike Wright, director of health safety and environment at the United Steelworkers union.

Wright said he was not aware of another situation where such a large amount of HF was neutralized or removed from a U.S. refinery, particularly after a catastrophic fire.

“This is a unique situation,” he said.

Most of the PES refinery’s roughly 650 union workers are scheduled to be laid off on Aug. 25 as the 335,000 barrel-per-day refinery, the largest and oldest on the East Coast, closes.

The company is considering keeping on some workers to manage the site in the months ahead, including to oversee possible demolition of the damaged HF alkylation unit, sources familiar with the plan said.

HF is popular for its efficiency and ability to be reused, making it cheaper than alternatives. But it can burn through skin and bone and form a potentially deadly mist at room temperature that can travel for several miles. Modified HF, which PES uses, is intended to better contain a toxic release.

Cleaning up and transporting HF is dangerous, and the damage caused by the fire will complicate the process, said Sally Hayati, an engineer and activist who advocates for refineries to stop using HF.

“That could be a very hazardous operation,” Hayati said. “It has to be done very carefully.”

Neutralizing the HF on-site could be tricky because of the volume of the reserves, Hayati said. The emergency container was likely fitted to hold just the amount of acid that was dumped into it on the morning of the fire, she said, leaving little room for the amount of chemicals needed to neutralize.

The HF will likely have to be transferred to trucks for removal from the site or first moved to other containers to be neutralized.

“That transfer is a hazardous operation, but obviously it’s going to have to be done one way or another,” Hayati said.