Arch Coal Inc. says it's started $14.60 a share tender offer for rival International Coal Group.
Arch says the offer that started Monday expires June 14.
The St. Louis-based coal industry giant agreed to buy ICG for $3.4 billion May 2.
The deal is designed to exploit growing demand for high-priced coal used to manufacture steel. Arch says the combined companies would be the nation's second largest supplier of metallurgical coal.
Met coal commands premium prices often two or three times those charged for coal used to produce electricity. Government figures show exports grew to 55 million tons last year, from 42.5 million in 2008.