The instant invention relates to a process and plant for the transformation of dangerous wastes containing chromium six as contaminant into non dangerous wastes that can be stored without special care and will be degraded in the environment without time limit. The process basically consists of milling, extracting chromium six in liquid phase and under controlled conditions of stirring, time and temperature, proceeding then, through reduction, to transform the chromium six in chromium three and then precipitating as chromium trioxide, through gasification. The solid resulting from the transformation process can be used as raw material for the manufacturing of firebricks or eventually for the manufacturing of bricks used in the building industry through a process not included in the instant description.