Preparation of magnesium hydroxide from leachate of dolomitic phosphate ore with dilute waste acid from titanium dioxide production

• A method is proposed for removal of Mg from phosphate ores with dilute waste acid.
• Mechanism for leaching of dolomite from dolomitic phosphate ores is analyzed.
• A multi-step process for purification and recovery of Mg from the dilute leachate.
• Column-shaped Mg(OH)2 nanoparticles have been prepared with NH3 as the precipitant.
• SDS modified Mg(OH)2 crystals exhibit a growth habit with preferential orientation.

A novel method for the removal of magnesium from dolomitic phosphate ores by using dilute waste acid (DWA) from titanium dioxide pigment production was experimentally studied in this work. The pH of liquid phase and particle size of granulized phosphate ore have been found to be crucial to the efficient removal of magnesium from phosphate ore. Operating under the optimum leaching conditions, a 98.31% removal of Mg with 0.02% P2O5 loss was obtained. It was demonstrated that dolomite can be selectively dissolved by H+ in the DWA and the ability of H+ to attack the apatite is inhibited by the strong interionic strength of DWA. By using the solubility differences of corresponding elements, a two-step precipitation process was presented to remove Fe selectively from Mg. A process for further purifying the synthetic leachate by a precipitation/leaching method and oxidation was also investigated. Subsequently, magnesium hydroxide (Mg(OH)2) was prepared by precipitation from the refined solution with ammonia as the precipitant and sodium dodecyl sulfate (SDS) as the modifier. The phase structure and morphology of Mg(OH)2 were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. The prepared nanoparticles of Mg(OH)2 were found to exhibit a column-shaped structure, demonstrating that the anions of DS− adsorbed onto the surface of Mg(OH)2 nucleus contribute to the stacking of Mg(OH)2 regular-hexagon platelets along the c-axis perpendicular to the [001] plane. The adsorption of SDS on the surface of Mg(OH)2 was supported by Fourier transform infrared spectroscopy (FTIR).