The sodium nitrate is put through a vacuum crystallization process to transform potassium chloride to produce potassium nitrate in the following ways: a low-temperature mother liquid is prepared according to a phase diagram; sodium nitrate and potassium chloride are added and the temperature conversion is held at 75-90 C; the slurry is precipitated at 75-90 C until the solids amount to 30-45 percent, and the crystallized high temperature salt is separated through centrifugal action and the clear liquid enters a vacuum crystallizer after filtration; the high temperature salt is washed and the washing liquid is sent directly to the vacuum crystallizer; the temperature of the vacuum crystallizer is kept at 25-40 C, and the potassium nitrate crystal is precipitated; the slurry is pumped into an elevated slot after vacuum crystallization, and the slurry is introduced into a centrifuge for centrifugal separation to obtain the primary potassium nitrate product; the primary product is washed, re-dried and packaged. This invention results in convenient amounts, mild isolation and transformation conditions, and a short production process, which can save more than 18 percent of the water and 20 percent of the energy. The operation is simple and the process easy for continuous production and automation. It also has low production costs and good economic benefits.
