Semi-continuous process for preparing phosphorus trichloride

There is disclosed an improved process for the semi-continuous production of phosphorus trichloride by the reaction of elemental phosphorus and chlorine in a liquid phosphorus trichloride layer. In accordance with the present invention periodic termination of the process by discontinuing phosphorus feed thereby creating a heel containing residual phosphorus is rendered more safe and less waste is produced by the improvement which comprises chlorinating the residual phosphorus at a substantially constant flow rate and continuing the chlorine feed rate until after observing at least one of either a step-wise decrease in the pressure within the reactor or a step-wise decrease in the flow rate of phosphorus trichloride removed from the reactor and then terminating the chlorine feed when said pressure and flow rate achieve a substantially steady state at a lower value.