Whenever revenue bonds have been issued under this Act, the entire revenue received thereafter from the operation of the plant and system shall be deposited in a separate fund, designated the “Coal Processing Fund of the County of ….”, which shall be used for paying the cost of maintenance and operation of such plant and system and bonds issued hereunder. Each such county may provide by resolution for the expenditure of a reasonable portion of the revenue derived from the operation of the plant and system for research to improve the quality of the prepared fuel and the by products of its preparation, to improve the efficiency of the facilities used in such preparation and for the promotion of the use thereof.
     Rates charged for coal processing shall be sufficient to pay the cost of maintenance and operation and to pay the principal of and interest upon all bonds issued under this Act, for the construction of such plant and system, and such rates shall not be reduced while any of said bonds are unpaid.
     Revenue bonds issued under this Act shall be redeemable at any interest-paying date: Provided, that a written notice is mailed to the holder of each such bond thirty days prior to such redeemable date, notifying such holder that said bond will be redeemed on the next interest-paying date; or, if the holder of such bond is unknown, then a notice published thirty days prior to such redeemable date in a newspaper of general circulation in such county describing the bond to be redeemed, and the date of such redemption, shall be sufficient notice to such holder, and upon such notice having been mailed to the holder of the bond, or upon such notice being published in the newspaper in case the holder of the bond is unknown, all such bonds shall cease bearing interest from and after the date the county has elected to redeem them.