A nearby resident says he heard a "tremendous explosion and saw a huge plume of white vapor."
"The bricks in the top of the roof, you know, started coming out and then I'm run over here to see what's going on and I saw a lot of smoke over there by the train track and everybody start running," said Reynaldo Guzman, who works near the explosion site.
Two people are being treated at the hospital: The employee who was reportedly mixing the chemicals and a delivery man. Both are expected to be OK.
Columbus Foods Company handles 20,000 and 40,000 pounds of biodiesel food a day. Authorities are looking into whether sulfuric acid was mixed with glycerin.
"Somehow two got mixed in the same tank and created enough heat to cause an explosion," said Deputy District Chief Bruce Gebien, Chicago Fire Department.
The building was heavily damaged; it's roof and walls collapsed.
"It's obviously scary when anything happens and people could be hurt. Fortunately, we were able to avoid anymore serious injuries," said Rick Cummisford, Columbus Vegetable Oil.
A second alarm was called moments later at 2931 West Grand when more than a dozen Metra and Union Pacific workers at a Metra yard two blocks away from Columbus Food.
"They were taken to six different hospitals as a -- as a precautionary measure just to be sure that they were checked out and determine if anything was really toxic that they were exposed to," said Judy Pardonnet, Metra.
Pieces of the building were reportedly found in a nearby Metra station parking lot, and some streets in the area are closed off for cleanup.
Officials said the explosion did not pose any danger to residents in the area, but did damage some other businesses.
"My men were pretty well shook up because it felt like the building just rocked up and down," said Jim Antoni, Mastercraft Furniture.
Antoni said his business had about $1,000 worth of damage to the building.
Nine employees were at Columbus Foods Company when the HAZMAT occurred. They were decontaminated on the scene.