Abstract: To improve recovery of nitrate-nitrite compounds from effluent of nitriding processes, a melt of the nitrate-nitrite salts is prepared and maintained at a temperature above about 400.degree. C; the effluent liquid is introduced into the melt, to evaporate the water and melt the salts contained therein to merge immediately into the melt, the melt being drawn off by an overflow. The evaporated water is filtered to recover any remaining salts, together with condensation water. Preferably, the apparatus includes a dispersion device close to the introduction duct of the effluent liquid into the melt, to prevent introduction of the effluent liquid in a solid stream; this dispersion may be an open pot with a perforated bottom located beneath the surface of the melt, a cartridge with steel wool, or the like.