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Magnesium Separation from Dolomitic Phosphate by Acid Leaching



Of all the impurities in phosphate rock magnesium can be one of the most troublesome. In the manufacture of phosphoric acid high magnesium increases sulfuric acid consumption and results in lower production rates and yields. When high magnesium acid is used to make Diammonium Phosphate (DAP) solid fertilizer, supplemental nitrogen in the form of higher cost ammonium nitrate or urea must be added.

The magnesium problem will grow increasingly critical as the Bone Valley reserves are depleted and mining activities move into the southern extension with its higher magnesium phosphate rock. High magnesium will mean that some phosphate rock will not be mined and just left in the ground and some of the rock that is mined will have to be discarded. While a number of techniques for magnesium removal have been have been proposed, no truly satisfactory solution to the problem has been adopted.

FIPR and others have devoted a great deal of time and money to solving this magnesium problem. Processes for magnesium removal from both phosphate rock and phosphoric acid have been successfully demonstrated in the laboratory but have failed to receive wide spread acceptance in the industry. The technique reported on in this paper treats the phosphate rock with a weak acid to solubilize the dolomite in the rock so that it can be removed as a solution. The magnesium can be recovered as a readily salable product that contributes to the economics of the process.

Processes of this type could reduce the number of acres that have to be mined each year, reduce the cost of producing DAP, and provide a valuable byproduct that would contribute to the industry’s financial stability.


A chemical process was tested to leach unwanted magnesium from dolomitic phosphate ore with sulfuric acid under automated pH control. Product carbon dioxide is removed by slurry spray in air or by boiling at reduced pressure. Magnesium is recovered for sale in the form of hydroxide or light oxide of good quality. By this method, up to 85% of MgO can be removed from rock with negligible loss of phosphate. Rock so purified is less costly to process into phosphoric acid by the wet process. Such technology may extend the life of Florida phosphate reserves, as low-magnesium rock is depleted.

Three phases of batch laboratory testing have been completed using a computer to control pH and log data. Neutralization of the leachate with lime was also tested, and the rate and endpoint were measured. Correlations developed from the results provide the basis for an economic systems model. The model predicts nearly 35% internal rate of return after accepting a 40% capital cost penalty for innovative technology. The model predicts optimum economic return at a leaching temperature of 76°C (169°F), 40 grams/100 ml starry density, pH 3.37, 214 minutes leaching time, 40 minutes neutralization time, using a single tank each for leaching and naturalization. No particle size reduction is indicated beyond that normally used with phosacid feeds.