Phosphate ores are one of the most important secondary sources of rare earth elements (REEs). The Esfordi phosphate concentrate (Yazd Province, Iran) contains 13.1% phosphate, 34.6% calcium, and 1.09% REEs. After selective leaching of the concentrate with 35% nitric acid at 60°C for 70 min, a pregnant leach (nitrophosphate) solution containing 143.6 g/L calcium, 53.1 g/L phosphorus, and other impurities was obtained. Sulfuric acid was used to remove calcium from the pregnant leach solution in the form of gypsum, and the effect of the stoichiometric ratio of sulfate to calcium was investigated. The results showed that at a sulfate-to-calcium molar ratio of 1, 99.5% of calcium, 95.2% of cerium, 92.8% of La, and 96.5% of Nd were precipitated from the solution. Phosphate co-precipitation with gypsum was < 10%. As the stoichiometric ratio of sulfate to calcium increased, the co-precipitation rate of phosphate decreased. The results of the present study showed that in the two-stage leaching method of REE production from phosphate ores, calcium precipitation in the form of gypsum is an effective method for purifying the solution from calcium.