phosphoric acid manufacturing facilities respectively as understood by those skilled in the art these and other salt solutions such as recycled leachate together with substantially pure water may be used separately or mixed in various proportions to form leaching media of compositions found economically and technically desirable as also understood by those skilled in the art the leaching of the calcine may be applied as a multi stage process with the number sequence and nature of the leaching stages such as to be most economically and technically advantageous in the preferred embodiment of the instant invention north carolina phosphate rock is heated to sufficiently high temperatures to volatilize essentially all of the congeneric organic matter and to minimize undesirable sulfide formation laboratory tests show that if lower temperatures than those specified herein are used as for example at between about 700 u00b0 c to about 770 u00b0 c a carbonaceous char resistant to gasification forms this char can be removed only by heating at higher temperatures in this temperature range of 700 u00b0 770 u00b0 c formation of the undesirable acid evolved sulfide is at a maximum and complete elimination of this sulfide during the calcination step is only possible if calcination is carried out at temperatures of greater than 800 u00b0 c if the calcination temperature is too high as with for example temperatures of 850 u00b0 c or greater complete elimination of the organic matter may not occur since the sudden collapse of the francolite structure during calcination with resultant melting in the rock particles may effectively encapsulate and immobilize a portion of the organic matter therein as discussed previously heating at these high temperatures also causes a marked and.