The injured worker was standing on a platform at a lower level of the tank when it exploded, the company said. The worker's condition was not immediately released.
The cause of the explosion was not known and is under investigation, the company said.
Company spokesman Ron Zimmerman said up to 10 people were working in the area when the explosion happened in what he described as "a storage tank for pulp." No fire resulted from the explosion, he said.
Identities of the victims were not being released Tuesday night, Zimmerman said.
Capt. Mike Drury of the nearby Merrill Fire Department said his rescue crew was called to help free the employees about 1:40 p.m.
but was called off before it arrived. Emergency workers on the scene used a crane to rescue them, he said.
"There was an explosion and a couple of employees were trapped for a short period of time," Drury said.
The Lincoln County Sheriff's Department did not immediately return a telephone message.
The main part of the mill remained in operation Tuesday afternoon. Workers walked out with somber faces.
One man was met by a woman in a car, and they embraced and hugged for nearly a minute. The man talked with her for several minutes and then went back into the administrative office, telling a reporter he couldn't comment.
A worker leaving the plant said employees had been told not to discuss details of the explosion with reporters, referring questions to Zimmerman, who didn't immediately respond to a message seeking further information.
Packaging Corp. of America, which makes containerboard and corrugated packaging products, operates four paper mills and 67 corrugated product plants in 26 states. According to the Lake Forest, Ill.-based company's Web site, it employs 8,350 people nationwide and posted sales of $2.3 billion last year.
The Tomahawk mill has three semi-chemical corrugating machines and produces more than 572 million tons annually, the company said.
--- AP writer Carrie Antlfinger in Milwaukee contributed to this report.
(Copyright 2008 by The Associated Press. All Rights Reserved.) AP-NY-07-29-08 2010EDT
