Every person engaged in the business of mining iron ore or operating an iron ore mine in the State of Alabama shall pay to the State of Alabama a license or privilege tax by the twentieth of each month for the privilege of operating said iron ore mine during the current month in which such payment is due an amount equal to $.03 per ton, of 2,240 pounds, on all iron ore mined during the last preceding month in which said mine was operated according to the run of the mine, whether such mine is an open mine or an underground mine, but no such tax shall be paid to any county in this state. Railroad weights shall govern where said iron ore is loaded on railroad cars in determining the amount of iron ore mined. In order that the industrial development of the state may be best preserved and promoted and in order that any deleterious effect of the tax levied in this section may be minimized, the Department of Revenue is authorized and empowered to lower, with the approval of the Governor, as in its knowledge of prevailing conditions may, from time to time prove expedient and advisable for the best welfare of the state, but not to raise, the rate on which the tax is computed. Any action by the state for the recovery of the tax levied under this section shall be commenced, or the assessment therefor made, within 12 months from the shipment by any means of such iron ore from the mine. Unless commenced within such period, the same shall be forever barred.