A group boosted its Kaiser Aluminum holding.

Headed by Tulsa, Okla., businessman J. A. Frates, the group said it acquired 1.7 million more Kaiser shares last week and increased its stake to 14.4% from 10.48%. The Frates group said it paid an average of $16.875 for the additional shares. Oakland-based Kaiser recently rejected the group’s offer of $20 a share for the company.