The explosion happened around 8:45 a.m. at G&S Metals in Manchester, a little more than 60 miles (97 kilometers) south of Atlanta. About 20 workers were inside at the time, Manchester Fire Chief Greg Lehtimaki said.
The injured men were all plant employees, Lehtimaki said. Their names were not immediately released.
Three of the men were taken to Grady Memorial Hospital in Atlanta, where spokeswoman Denise Simpson said one patient was in critical condition and two were in serious condition. Four with less serious injuries went to other hospitals, Lehtimaki said.
The cause of the explosion was not yet known. The blast broke natural gas lines, causing a fire. The blaze was extinguished within 15 minutes, Lehtimaki said.
A city official said that a half-mile area around the plant was being evacuated. However, by midmorning, spectators lined a roadway behind the plant, and it was unclear if an evacuation order was ever issued.
The company buys, sells and processes and converts aluminum scrap, and provides metallurgical services, said Carolyn McKinley, executive director of the Meriwether County Chamber of Commerce. The material is used for auto wheels, brake parts and structural components and other applications in the casting industry.
Ken Rauch, human resources manager at the plant's parent company, said Thursday morning that he and other executives were about to get on a plane to get to the plant.