The use of Caustic Magnesia as the precipitant of uranium from ion exchange column eluates, having been decided upon for the flow sheet of the Port Pirie Chemical Treatment Plant, it became necessary to ascertain the conditions under which optimum reactivity consistent with economic operation would be expected. The investigational work which is reported hereunder was carried out with this object in view, and laboratory operations were intended to simulate plant conditions as closely as possible. Preliminary work which had determined the usefulness of the process, had suggested that excess of magnesia could very easily be used raising the pH of the eluate to such a point that resolution of Uranium took place. By determining more closely the reactive magnesia as part of the magnesia, better control of the process would be obtained. Increasing the reactivity to its peak would give this efficiency, and it was felt that ball milling of the Causemag slurry to improve the percentage of Mg as Mg (OH)2 would be the most effective way to achieve the desired result.