Acid Waste Management with Diffusion Dialysis

Large quantities of spent nitric and hydrofluoric acid mixtures are generated during the pickling of stainless steel. While neutralization of the spent acid is the most common method of handling this waste, membrane-based technologies such as electrodialysis and diffusion dialysis are emerging as alternatives. These technologies focus on removing the metal salt contaminants selectively while recovering the free acid.In this paper, we present results of a pilot investigation using diffusion dialysis to recover nitric acid and ammonium bifluoride from a spent pickling waste over a period of six weeks at an industrial facility processing stainless steel. An average free acid recovery of 86.1% was obtained. Iron, chromium and nickel salts in the reclaimed acid were lowered by 87.9 %, 88.7% and 79.5% respectively. The recovery of ammonium bifluoride in the reclaimed acid was very low necessitating its addition as a supplement to the reclaimed acid. Pickling performance with reclaimed acid supplemented with a mixture of fresh nitric acid and ammonium bifluoride was judged equivalent to that obtained by a fresh acid mixture. Implementation of the technology on a continuous basis would provide a means to control the metal salts level at predetermined levels thus improving pickling process efficiency and ensuring consistent performance. The acid strength and metals concentration of the pickling bath can be maintained with diffusion dialysis technology.A simple method to determine the size of the diffusion dialysis system is presented. A payback period of 4 years has been estimated for maintaining the 1500-gallon nitric acid and ammonium bifluoride-pickling bath with diffusion dialysis. Diffusion dialysis was determined to be both technically and economically viable for adoption by the facility. However, longer term testing is recommended to determine the chemical stability of the membranes.