Energy conserving limestone calcining system

Energy conserving limestone calcining process of first and second separate successive steps which comprises:

• in the first step, supplying sensible heat to limestone to maintain the limestone at a temperature sufficient to dissociate the calcium carbonate present to calcium oxide and carbon dioxide and thereby form a hot mass and liberated carbon dioxide, in accordance with the equilibrium dissociation reaction of calcium carbonate to calcium oxide and carbon dioxide which is dependent on the partial pressure of carbon dioxide at said temperature, and for a duration sufficient to achieve in the first step at least about 60% partial conversion of the limestone by the sensible heat in the hot mass, and thereby provide in the first step a retained sensible heat containing partially calcined hot mass in which the retained heat in the mass in the first step is sufficient for subsequently independently substantially completing the conversion of the remaining unconverted limestone in the hot mass in the second step substantially in the absence of supplying further sensible heat thereto in the second step, while removing in the first step the liberated carbon dioxide from the vicinity of the hot mass at a rate sufficient to maintain the carbon dioxide partial pressure in the first step below the corresponding equilibrium pressure at said temperature, and thereafter
• in the second step, continuing the dissociation reaction for independently substantially completing the calcining of the limestone, substantially in the absence of supplying further sensible heat to the partially calcined hot mass from the first step, by subjecting the sensible heat containing partially calcined hot mass from the first step to a vacuum in the second step sufficiently to remove substantially completely the further liberated carbon dioxide and for a duration sufficient to achieve in the second step substantially complete conversion of the limestone independently by the retained sensible heat contained in the partially calcined hot mass and substantially in the absence of supplying further sensible heat thereto and thereby provide a substantially completely calcined hot mass.