WDIO reported workers were being evacuated from the building.
Kesler said the tank contains hydrofluroic acid, which is a strong inorganic acid used mainly for industrial purposes.
Brad Boos, president for Hunt Electric North, which had 37 workers at the refinery at the time of the explosion, said one of their employees was hurt by debris/shrapnel from the first explosion but is expected to be OK.
There are no other details of the extent of the injuries or how many, or how many casualties there are at this time.
Police blocked roads around the refinery in Superior, which is home to about 27,000 people and borders Minnesota to the north and the tip of Lake Superior.
Husky Energy of Calgary, Alberta, bought the refinery past year for $492 million.
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At least 20 people were reportedly injured at an oil refinery in Wisconsin on Thursday after a sudden explosion that witnesses said rattled the air like a "sonic boom", FOX News reported. Smoke was seen coming from the refinery just after 10:00 a.m.
"It felt like a bomb", said Katey Geistfeld, who works at the Challenge Center at the nearby Mariner Mall. Superior police later tweeted that the fire had reignited and urged residents living within a one-mile radius of the refinery to leave. U.S. Department of Labor spokesman Scott Allen said the Occupational Safety and Health Administration fined Calumet Superior LLC $21,000 over emergency response and flammable liquids violations in 2015.
"We'll have more information as it becomes available, " he told the News Tribune.
The refinery's most recent Risk Management Plan was submitted in 2012 and states: "In the unlikely event of a catastrophic release, the refinery, working in conjunction with local emergency management staff, is well prepared to respond and mitigate adverse consequences to the community or the environment".
Husky purchased the refinery from Calumet Specialty Products Partners in a $435 million deal past year, BNO News reports. Husky said there were no changes planned for the facility but was planning to continue a $30 million upgrade started by Calumet. It's Wisconsin's only refinery, and it produces gasoline, asphalt and other products. The facility can process up to 38,000 barrels of oil a day.
Check back for updates.