Rare earth elements (REE5) have an ever-increasing variety of applications in modern technology. In the present work, the possibility of extraction of rare earth elements cerium (Ce), lanthanum (La), neodymium (Nd) and thorium (Th) from a monazite concentrate containing 25% rare earth elements' oxide was investigated. In this regard, the acidic digestion and leaching step, and their effective parameters were studied in detail using design of experiment (DOE) which hadn't been investigated before. The optimum conditions for the performed experiments in acidic digestion step were: temperature of 225 degrees C, time of 3.5 h and sulfuric acid to concentrate weight ratio (L/S) of 2.5. Using these conditions, the recovery of 92.34%, 91.44%, 92.1% and 88.94% were obtained for cerium, lanthanum, neodymium and thorium, respectively. In leaching step, the optimum conditions were: temperature of 75 degrees C, time of 15 h and water to digested mass weight ratio of 7.5. Using these conditions, recovery of 88.47%, 89.45%, 87.35% and 82.19% were obtained for rare earth elements cerium, lanthanum, neodymium and thorium, respectively. In precipitation step, rare earths' oxalates were precipitated from the leach liquor using oxalic acid. Then, the precipitate was calcined in a furnace at 800 degrees C for 1 h and finally, a mixture of rare earth elements' oxides with a purity of about 84% was obtained. (C) 2016 Elsevier B.V. All rights reserved.