**Abstract**

The Ilimaussaq complex in Greenland contains a rare earth bearing mineral called Steenstrupine. This mineral is a complex sodium rare earth phospho-silicate which also contains significant uranium and thorium. The mineral can be beneficiated via froth flotation to produce a mineral concentrate ranging between 15 and 23% rare earth oxide. Leaching of the mineral concentrate is required to dissolve the contained values and recover them using hydrometallurgy. Steenstrupine contains high amounts of acid soluble silica which can result in the formation of silica gel during leaching. Laboratory scale testwork was performed to determine which leaching conditions offer the control of silica and high extraction of values such as rare earth elements and uranium. A range of leach parameters where investigated to determine which are most significant to leach performance. Optimised parameters consisting of acid strength, residence time and nature of the operation were determined as significant. In conclusion the operating range identified produces high rare earth extractions while yielding a leach residue with suitable solid liquid separation performance. This process may be applied to other rare earth bearing minerals which contain high proportions of soluble silica.

**Keywords:** rare earth elements, leaching, silica gel, hydrometallurgy, steenstrupine, hydrochloric acid leaching
