A method of separation of sodium chloride and potassium chloride from a polymineral source containing sodium and potassium chlorides, comprising the following steps: (i) feeding a pre-ground polymineral source into the upper zone of a vertical three-zone reactor filled with a sodium chloride and potassium chloride saturated solution, in which zone the temperature is maintained between -2.5°C and -20°C, wherein a total concentration of bivalent cations in the liquid phase is kept below 2 wt.%, (ii) recrystallizing sodium chloride to form sodium chloride dihydrate and separating the formed sodium chloride dihydrate crystals from crystals of potassium chloride by flotation to produce a froth containing potassium chloride crystals, whereas the sodium chloride dihydrate crystals fall down to the middle zone of said vertical three-zone reactor, (iii) multistage sizing of the sodium chloride dihydrate crystals by an upward flow of liquid phase and simultaneous multistage countercurrent washing of the falling sodium chloride dihydrate crystals by the same upward flow of liquid phase, wherein the upward flow velocity is equal to the flotation velocity of sodium chloride dihydrate crystals whose hydrodynamic diameter is greater than the maximum hydrodynamic diameter of the polymineral source particles by not less than 20 microns; thus allowing the washed sodium chloride dihydrate crystals to enter the lower zone of the said vertical three-zone reactor, provided that the hydrodynamic diameter of the sodium chloride dihydrate crystals entering the lower zone of said vertical three-zone reactor exceeds the maximum hydrodynamic diameter of the polymineral source particles by at least 20 microns, (iv) recrystallizing the washed sodium chloride dihydrate crystals into anhydrous sodium chloride in the lower zone of the said vertical three-zone reactor filled with a sodium chloride saturated solution, in which zone the temperature is maintained between 0.5°C and 15°C, to produce a suspension of anhydrous sodium chloride. (i) feeding a pre-ground polymineral source into the upper zone of a vertical three-zone reactor filled with a sodium chloride and potassium chloride saturated solution, in which zone the temperature is maintained between -2.5°C and -20°C, wherein a total concentration of bivalent cations in the liquid phase is kept below 2 wt.%, (ii) recrystallizing sodium chloride to form sodium chloride dihydrate and separating the formed sodium chloride dihydrate crystals from crystals of potassium chloride by flotation to produce a froth containing potassium chloride crystals, whereas the sodium chloride dihydrate crystals fall down to the middle zone of said vertical three-zone reactor, (iii) multistage sizing of the sodium chloride dihydrate crystals by an upward flow of liquid phase and simultaneous multistage countercurrent washing of the falling sodium chloride dihydrate crystals by the same upward flow of liquid phase, wherein the upward flow velocity is equal to the flotation velocity of sodium chloride dihydrate crystals whose hydrodynamic diameter is greater than the maximum hydrodynamic diameter of the polymineral source particles by not less than 20 microns; thus allowing the washed sodium chloride dihydrate crystals to enter the lower zone of the said vertical three-zone reactor, provided that the hydrodynamic diameter of the sodium chloride dihydrate crystals entering the lower zone of said vertical three-zone reactor exceeds the maximum hydrodynamic diameter of the polymineral source particles by at least 20 microns, (iv) recrystallizing the washed sodium chloride dihydrate crystals into anhydrous sodium chloride in the lower zone of the said vertical three-zone reactor filled with a sodium chloride saturated solution, in which zone the temperature is maintained between 0.5°C and 15°C, to produce a suspension of anhydrous sodium chloride.