Recovery of metal sulphates and hydrochloric acid from spent pickling liquors

Spent pickling liquors are classified like hazardous materials due to their high concentration of metals and presence of different acids and several best available technologies are developed to treat this kind of wastes. In this paper a new process combining essentially the evaporation and crystallization section is developed to produce respectively the aqueous solution of hydrochloric acid and iron-zinc sulphate-eptahydrates, the first used in the same process of pickling while the second to produce fertilizers. The innovative aspect is that two heat pumps are used in replacement of industrial boilers: in literature there are not works about the use of heat pumps for regeneration of spent pickling liquors. Several advantages are obtained in this way: the reduction on ton of equivalent petrol, equal to 39%, and air pollutants, in particular carbon dioxide equal to 59%, according to the European Union climate package. A simulation and an eco-friendly and energy efficient design of the process is carried out. Then an anova analysis to find the influence of several factors on energy efficiency and production of chemical composts is developed. The results show that the optimal operating parameters are following; for evaporation section: pressure 0.2 bar and temperature 357 K; for de-oiling section: outlet temperature 333 K; for the crystallizer section: dissolver temperature 308 K, evaporator and crystallizer temperature 303 K, evaporator pressure 0.03 bar, recycle ratio of mother liquors to evaporator 0.1, evaporator recirculation flow-rate 9000 kg/h. This eco-innovation will provide a simple, economic and environmental solution to a problem about management of special hazardous waste. The future construction of the plant will verify the obtained results.