SUPERIOR, Wis. — Husky Energy said it’s been given approval to rebuild its oil refinery in Superior, Wisconsin after an explosion and a series of fires last year injured 36 people and required much of the city to evacuate.

Husky, which does business as Superior Refining Company in Wisconsin, said it will start the $400 million plan immediately with the goal of resuming partial operations in 2021.

Residents and tribal leaders expressed frustration over Husky’s plans to continue using the highly toxic chemical hydrogen fluoride as part of its refining process. The city’s evacuation was largely because of the presence of hydrogen fluoride, which can be hazardous to human health. The tank containing the chemical wasn’t damaged by the explosion, which was caused by a hole in a valve.

The mayors of Superior and adjacent Duluth, Minnesota, also called on Husky Energy to remove hydrogen fluoride from its operations.

The Wisconsin Department of Natural Resources approved the permit.

Earlier this year, Company CEO Rob Symonds said company officials explored using alternatives to hydrogen fluoride, but there is no better option.

“The hydrogen fluoride alkylation unit, as it’s called, is fundamental to making gasoline,” Symonds said. “This is the gasoline that we all use in our cars today.”

The company has said it plans to make safety improvements to the existing alkylation unit, including a rapid acid transfer system to capture any hydrofluoric acid in case of a release.

Businesses and labor groups spoke in support of a rebuild permit at an August hearing, arguing the refinery has economic benefits. Husky has about 200 permanent jobs with a $27 million payroll. Another 350 jobs are expected to be created during the rebuild.

When it resumes operations, Husky expects to refine an average of 45,000 barrels of oil per day.