Pure halogen salts of alkaline and/or alkaline earth metals or a mixture thereof are prepared by (I) hydrolytically heating a suspension of 1 part by weight of a halogenic, organic waste material in a comminuted state in 1-10 parts by weight of an aqueous medium in the presence of a base to a temperature ranging from 200-300° C. at a pressure sufficient to maintain the water in a liquid state for a period of time sufficient to convert substantially all the organically bound halogen present to inorganic halides, and (II) separating the hydrolysate obtained in step (I) into a solid hydrolysate fraction and a liquid hydrolysate fraction, (III) neutralizing the liquid hydrolysate with hydrohalogenic acid, (IV) adding a flocculent to the neutralized hydrolysate; (V) separating the material from step (IV) into a solid containing fraction and an aqueous solution; and (VI) nanofiltering the aqueous solution to obtain a retentate and a permeate, wherein the permeate is further treated to obtain the pure salts.