Caterpillar Inc., Peoria, Ill., is buying mining-equipment manufacturer Bucyrus International Inc., South Milwaukee, Wis., for $8.6 billion. Both companies’ boards of directors already have approved the deal, which is expected to be completed by the middle of next year, subject to regulatory approvals and to endorsement by Bucyrus stockholders. Caterpillar is buying the mining-equipment manufacturer to expand Cat’s leadership in the mining equipment industry. Caterpillar spokesman Jim Dugan emphasized the complementary nature of the two companies’ products. In December 2009, Terex sold its mining-equipment business to Bucyrus for a reported $1.3 billion. Caterpillar now inherits that acquired unit in its Bucyrus purchase.