**5. Conclusion**

Leaching of steenstrupine concentrate was successfully achieved from both low grade (10% REO) and high grade (21% REO) concentrates. Leaching tests show increasing acid concentrations in solution improved the solid liquid separation and yielded high rare earth extractions. Incremental addition of leach feed to the reactor had little impact on metal dissolution but improved leach residue solid liquid separation significantly. The optimum acid dose was in the range of 0.39 to 0.53 t anhydrous HC/t of dry concentrate. Acid doses lower than 0.35 t anhydrous HCl/t dry concentrate result in poor filterability despite excellent leach extraction.

Filtration characteristics also improved when leaching was performed in a continuous reactor as opposed to a batch reactor. Increasing residence time in the leach also improved filtration characteristics at constant acid dose. Under the same acid regime the batch process gave a very viscous slurry while the continuous system gave a free flowing slurry.

High acid strength, longer residence time and continuous operation improve are the most important factors to produce a leach slurry which is suitable for solid liquid separation.

Further research could be performed to examine the impact of temperature on the solid liquid separation efficiency.
