Method of concentrating an aqueous solution by multiple effect evaporating

In a method of concentrating an aqueous solution containing a first salt and a second salt, the solubility of the first salt increasing more with increasing temperature than the solubility of the second salt, by multiple effect evaporation wherein the multiple effect evaporators comprise at least three evaporator effects, wherein the multiple effect evaporators are backward fed, wherein the first evaporator effect is the hottest, wherein second salt is precipitated during evaporation, wherein the concentrated mother liquor from the multiple effect evaporators is forwarded to a zone operated at atmospheric pressure, and wherein mother liquor effluent from the evaporators is forwarded to a zone wherein the first salt is recovered, the improvement comprising:

• A. Operating the first evaporator effect at superatmospheric pressure;
• B. Operating at atmospheric pressure an evaporator effect which is preceeded in the direction of mother liquor effluent flow by at least one cooler evaporator effect;
• C. Feeding to the first evaporator effect a portion of the overflow from the evaporator effect cooler than the evaporator effect operated at atmospheric pressure, wherein the portion bypasses the evaporator effect operated at atmospheric pressure;
• and
• D. Feeding the mother liquor effluent from the first evaporator effect to the evaporator effect operated at atmospheric pressure.