Distribution of impurity elements was studied during crystallization of nickel ammonium sulfate hexahydrate (double salt). It was shown that the formation of the double salt results in deep recovery of nickel from solution without preliminary isolation of excessive sulfuric acid. It was found that synthesis of the double nickel salt from tail mother liquors of copper electrolysis plants is accompanied by the formation of amorphous precipitate of hydroxides of impurity elements. The principal possibility of separation of the nickel double salt and the amorphous part employing rising current with alternating hydrodynamic regime was demonstrated. The coefficients of purification from impurities in the course of the synthesis of the nickel double salt were determined and high effectiveness of purification from non-isomorphic impurity elements was demonstrated. It was found that the synthesis of the double nickel-ammonium salt from the tail mother liquors of copper electrolysis plants is accompanied by increased capture of zinc by the crystals. Washing the crystals of the double salt did not give appreciable purification from zinc impurity. The main characteristics of the purification process of the double nickel ammonium salt from isomorphic impurities using isohydric crystallization were determined. The experimental coefficient of co-crystallization of isomorphic impurity of zinc with mother crystals of the double ammoniumnickel salt in the course of isohydric crystallization was determined. It was found that the value of the co-crystallization coefficient determined experimentally is close to the calculated according to the Doerner – Hoskins equation. It was found that the differential Doerner – Hoskins law is realized during isohydric crystallization process. The results of determination of the effectiveness of purification can be used in developing technological processes allowing production of the double salt with required zinc and other isomorphic impurities content.