alkalinity but this procedure inherently provides further advantage and that is a decrease of the processing costs if back extraction as described herein is not carried out all water must be removed usually evaporated from the aqueous lactam solution which prior to removal generally has a lactam content of 30 35 by weight in view of the high evaporation heat of water 540 kcal kg the evaporation step by itself requires a considerable amount of energy on the other hand and according to the procedures described herein if instead of water one uses a suitable organic solvent having a much lower evaporation heat like chloroform evaporation heat 59 kcal kg must be evaporated as a result of the back extraction a substantial saving on the evaporation cost is obtained as simply less thermal energy is required to accomplish the same result for instance in the evaporating of the water and chloroform from an aqueous lactam solution containing 30 by weight of lactam and 2 by weight of chloroform 1260 keal are necessary per kg of lactam however by back extraction with chloroform a solution of 35 by weight of lactam and 3 8 by weight of water in chloroform is formed overall the evaporation of the chloroform and water requires 167 keal per kg of lactam thus a substantial savings is obtained as the thermal energy required is only one tenth that of the aqueous based solution fig1 is a schematic illustration of one.