The Research On Recovery Valuable Metal From Spent Pickling Liquor And Recycling Of Acid

Over one million cubic meters of spent steel stainless pickling liquid wouldbe produced in china in a year. Generally, the spent pickling liquor contains valuemetals such as Fe, Cr and Ni elt, as well as nitric and hydrofluoric acids. Currentlythe vast majority of spent pickling liquor was processed with neutralizationprecipitation method which formed pickling sludge stacked disposal. It not onlycaused negative effects to the ecological environment and human health, also wasteda huge of resources. So it is indispensable to recover of valuable metals and reducethe pollution of heavy metal ions in the environment.A method was presented to simultaneously recover the valuable metals fromstainless steel spent pickling liquor and recycle the spent acids in this paper. Thewhole technological process is as follows: the seeds were added to the spent picklingliquor induced crystallization precipitation of metal fluorides, which could recoverymetals. New acids were added to the filtered spent pickling liquor to make it getreused in the pickling process. This method not only recycled the spent picklingliquor but also got by-product metal fluorides.The crystal seeds were used to induce the Fe and Cr ions precipitate in order torecovery the metals and purify the acid. So the spent pickling liquor could be usedrepeatedly. In this paper, temperature, concentration of HF and amount of seedswere considered as the major factors to affluence the metals precipitation. Thecrystals growth kinetics was also analyzed in detail. The balance of materials forrecycling process was calculated, and the quantum of adding new acids wasobtained.The experimental results showed that: The crystallization product particle sizewas not uniform and showed difference on colors. The larger particles emerged asthe shape of the rules of six prisms and its Fe content was higher than that of smallones. The molar ratio of O and F was close to1:1. Crystallization precipitation experiments of spent pickling liquor under30℃,40℃and50℃were researched.The deposition process was divided into two stages: the first100minutes of theprocess and the rest of the300minutes. The deposition rate of FeF3increased withthe increase of temperature at first satge. While the precipitation rate of CrF3wasaffected little by temperature at first satge. The precipitation rates of FeF3and CrF3increased with the temperature decreased at later stage. Lower temperature couldincrease the amount of precipitation. With the increase of HF acid concentration, theamount of Fe(III) and Cr(III) in the spent pickling liquor got less, that mean moremetals were deposited. With the increase of HF acid concentration further, thecontents of Fe and Cr in the spent pickling liquor had a small change. The addingamount of seeds could increase the rate of crystalline precipitate.Activation energy for FeF3·3H2O and CrF3·3H2O deposition reaction werecalculated with Arrhenius equation and the activation energy were64.13kJ·mol-1and67.00kJ·mol-1respectively. Crystal growth velocity would change with thedegree of extra-saturation under different temperature. It could be concluded that thebest precipitation temperature of FeF3·3H2O was30℃and then followed by50℃.Crystal growth velocity of CrF3·3H2O was mainly affeceted by the degree ofsupersaturation.The spent picking liquor crystallization precipitation experiment was carriedout for many times cycles. And then the multiple of recycling spent acids in thespent pickling liquor was testified feasible. Recycle of1kg of spent pickling liquorfor the instance, the conservation of material was calculated in the recycle the spentacids process. The amounts of HNO3, HF and H2O that needed to add to the spentpickling liquor after crystallization precipitation in the part of adding new acids wereobtained to keep the material conservation.