30 U.S.C. § 213

Any lease to develop and extract phosphates, phosphate rock, and associated or related minerals under the provisions of sections 211 to 214 of this title shall provide that the lessee may use so much of any deposit of silica or limestone or other rock situated on any public lands embraced in the lease as may be utilized in the processing or refining of the phosphates, phosphate rock, and associated or related minerals mined from the leased lands or from other lands upon payments of such royalty as may be determined by the Secretary of the Interior, which royalty may be stated in the lease or, as to the leases already issued, may be provided for in an attachment to the lease to be duly executed by the lessor and the lessee.