The recovery of rare earth elements from spent nickel-metal hydride batteries by hydrometallurgical processing has become increasingly important in recent years. The present work investigated the effect of temperature, systems of adding the reactant, the molar ratio of sodium and lanthanum, and the initial concentration of six sulfate impurities (Ni, Co, Al, Mn, Fe, and Zn) on the crystallization of the monohydrate of sodium lanthanum sulfate double salt (NaLa(SO₄)₂·H₂O) crystals from synthetic leachate solutions. The sodium sulfate reactant was added as an acidic solution by pumping or batchwise as a solid anhydrate salt to a pregnant lanthanum sulfate solution. Compared to precipitation with acidic sodium sulfate solution, precipitation with solid sodium sulfate yielded smaller single crystals, a greater tendency to form aggregates, and lower crystal purity. The lowest overall impurity and highest lanthanum quantity in crystals were obtained by semi-batch reactant adding performance of Na₂SO₄ solution at 70°C with Na/La molar ratio of 3. Real-time monitoring of the count rates of different chord length fractions clearly showed the influence of crystallization temperature on the precipitation kinetics.