In this study a two stage hydrometallurgical treatment of eudialyte concentrate is presented, aiming on recovering rare earth elements and suppressing silica gel formation, The proposed treatment incorporates a preprocessing step called "Fuming", which is the addition of an acidic solution to eudialyte concentrate heated at boiling temperatures, followed by the leaching step, where the treated concentrate is leached at ambient or low temperature. Fuming pretreatment with sulfuric or hydrochloric acid, and subsequently water leaching of the treated concentrate, resulted to >90% recovery of rare earths avoiding silica gel formation which is a major challenge to overcome during eudialyte dissolution. Scanning electron microscopy analysis, indicate that "Fuming", transforms eudialyte into a mixture of metal salts and a siliceous secondary precipitate. Leaching of the treated concentrate dissolves soluble metal salts into solution whereas the secondary siliceous precipitate remains in the residue with the rest gangue minerals such as aegirine and feldspar. Advantages of the proposed treatment are low acid consumption, high rare earth recovery yields, avoidance of additives such as fluoride ions and avoidance of silica gel formation. (C) 2016 Elsevier Ltd. All rights reserved.