Weyerhaeuser Company has sold its containerboard packaging and recycling business to International Paper (IP) for $6 billion in cash, subject to post-closing adjustments.Weyerhaeuser said it expects to use a substantial portion of the after-tax proceeds from the sale to pay down debt. The transaction includes nine containerboard mills, 72 packaging locations, 10 specialty-packaging plants, four kraft bag and sack locations and 19 recycling facilities.