1. The board of supervisors of any such mine drainage district may issue the bonds of such district as provided in section 242.480, except that in lieu of the basis of the issuance thereof as provided in section 242.450, the total amount of bonds so issued shall not exceed seventy-five percent of the income of such district from all sources for the number of years for which such bonds are to be outstanding, such income to be estimated by the board of supervisors upon the basis of the report of the chief engineer of the district provided for in section 242.720, and it shall be the duty of said board, in making the levy of the pumping tax or royalty and in prescribing the tax or charge for crushing and cleaning ores or minerals by such concentrating plant or plants, to take into account the maturing principal and accruing interest on all bonds of the district at any time outstanding, and to make ample provision for the payment thereof, and, to that end, it shall be the duty of the board of supervisors at any time, and from time to time, when it becomes apparent that such taxes or charges theretofore fixed will not produce the sums required for the payment of such bonds and the interest thereon and the operating and maintenance expenses of the district, to increase such taxes or charges so as to produce fully the sums required, as aforesaid; provided, however, that such pumping tax shall not exceed the limitation herein provided.

2. The board of supervisors, in order to secure the payment of such bonds and the interest thereon, shall have power and authority to pledge, mortgage or convey the property of such district, both real and personal, the proceeds to be derived from all such taxes or charges, after deducting therefrom the sum or sums required to pay the operating expenses of the district and to provide an adequate reserve fund for maintenance, depreciation and contingencies.

3. Before such bonds shall be negotiated, they shall be presented to the state auditor of Missouri for registration as provided in section 108.240 and the board of supervisors may sell such bonds as provided in section 242.480, or may sell the same to any corporation, commission or agency created or authorized by the Congress of the United States to purchase such bonds.