CHICAGO—An explosion at a northern Illinois chemical plant Monday morning sparked massive fires that sent flames and huge plumes of thick black smoke high into the air and debris raining onto the ground, prompting evacuations. After 7 a.m., emergency crews rushed to the scene of the fire near Rockton, northwest of Chicago, at Chemtool Inc., a company that manufactures lubricants, grease products and other fluids, and is, according to the company, the largest manufacturer of grease in the Americas. Rockton Fire Department Chief Kirk Wilson said about 70 employees who were at the plant when firefighters arrived were evacuated safely, and that one firefighter suffered a minor injury. Chemtool’s parent company, Lubrizol Corp., later said there were closer to 50 employees present when the plant was evacuated. Following reports that the plumes of smoke were so big they were being picked up on weather radar, the Rockton Police Department posted …