A refinery in Superior, Wis., that exploded and burned last year, injuring 36 people and forcing the evacuation of thousands of residents, will be rebuilt and reopen in 2021, company officials announced Monday.

Calgary, Alberta-based Husky Energy said it has "received the required permit approvals to begin reconstruction activities at the site" and expects to begin work immediately on the $400 million project, reopening the refinery in 2021 at its full capacity of 45,000 barrels of oil a day.

A worn valve led to an explosion at the refinery in April 2018, sending giant clouds of black smoke into the sky that were visible from 30 miles away. It took firefighters more than 12 hours to put out the flames.

Superior authorities issued a mandatory evacuation for much of the city of 27,000 residents while three schools and a hospital closed, sending students and patients to take shelter in neighboring Duluth.

The evacuations were ordered because of concerns that a release of hydrogen fluoride (HF) from the refinery could produce a deadly gas cloud. The chemical is used as a catalyst to boost octane in gasoline produced at about half the nation's oil refineries.

Under the worst-case scenario, up to 180,000 people in the region surrounding the Lake Superior port community could have been hurt or killed if the blast had compromised a tank of hydrogen fluoride that sat within 200 feet of the Husky fire, according to a report by the U.S. Environmental Protection Agency (EPA).

Husky said earlier this year that it would continue to use hydrogen fluoride in the refining process at the Superior plant.

"After a thorough technology review, engineering analyses, risk assessments and discussions with subject matter experts, the Refinery determined alternative technologies were not proven or involved significant risks," Husky said in an explanation posted on its website. "The community's concerns about the use of HF are understood. Refinery workers — many who live in the community — are aware of the need to maintain the highest safety standards."

The company said the rebuilt refinery would have several new measures in place to guard against HF release. They include an enhanced leak detection system with laser detectors, along with a system allowing the rapid transfer of HF to an independent, secure holding tank in the event of an accident.

The company also will add several layers of water mitigation. Water can dilute HF, making it less dangerous.

Earlier this year, the U.S. Chemical Safety Board, an independent agency that investigates chemical accidents, encouraged the EPA to review whether the risk-management plans of refineries are sufficient to prevent "catastrophic releases" of hydrogen fluoride.

Duluth Mayor Emily Larson has called for the EPA to take action to prevent future disasters linked to HF use in refineries. Earlier this year, the Duluth City Council also asked the EPA to reconsider whether refineries should be allowed to use HF. Larson was not available for comment Monday.

Across the bay in Superior, officials have supported the continued operation of the refinery, which employs about 200 people.

"The mayor is supporting the rebuild," said Rani Gill, chief of staff for Superior Mayor Jim Paine. "As far as the hydrogen fluoride, we'll work with them. They have to do what they have to do."

The Superior City Council passed a resolution earlier this year expressing support for rebuilding the refinery.