Alpha Natural Resources is planning a public offering of $500 million worth of senior unsecured notes, partly to refinance other debt.

The coal supplier said Thursday that it plans to use the offering’s net proceeds to partly fund a buyback of up to $350 million in convertible senior notes due in 2015. Alpha is starting a tender offer for the $350 million notes. The tender offer is set to expire on Oct. 25 unless extended or ended earlier.

Any remaining net proceeds from the notes offering will be used for general corporate purposes.

Moody’s Investors Service gave the $500 million notes offering a “B2,” or junk, rating. Those notes, which are due in 2018, will give Alpha some more time to pay back debt.

Last week Alpha Natural Resources Inc. announced that it was immediately closing eight mines — four in West Virginia, three in Virginia and one in Pennsylvania — and eliminating 1,200 jobs companywide by early 2013.

Shares of Alpha added 14 cents to $6.73 in midday trading.