Treatment And Resourceful Utilization Of Spent Pickling Liquor Containing Heavy Metal

According to the China's environmental requirements for the steel metallurgy industry increasing,it's a serious problem that products containing amounts of harmful heavy metal impurities exceeding the standard by current disposal methods to steel spent pickling liquors?SPL?.In this paper,the SPL from a steel plant was selected as the experiments sample.The pure iron salt or iron mineral products were obtained by stepwise crystallization,oxidation precipitation and fractional crystallization combined with oxidation precipitation,respectively.Separation conditions and determination of the maximum experimental parameters,to investigate the main factors affecting the separation of iron and zinc and the quality of the final product,to achieve the separation of iron and zinc in the waste acid solution and to obtain pure iron products.1.Fe and Zn was effectively separated by step-wise crystallization of steel spent pickling liquors containing ferrous and zinc under the conditions of ordinary pressure and negative pressure,relatively pure crystal products were obtained,and the optimum process conditions for realizing the separation of Fe and Zn were investigated.The ratio of moles of Fe to Zn in the initial SPL was 4.5:1.The optimum parameter conditions were determined by atmospheric and negative pressure step-wise crystallization experiments.The optimum parameters are as follows:130°C of distillation temperature for 90 min and 4°C of crystallization temperature for 12 h under atmospheric step-wise crystallization;0.06 Mpa of vacuum,105°C of distillation temperature for 60 min and4°C of crystallization temperature for 12 h under negative pressure step-wise crystallization.Under conditions of the optimal parameters,the obtained molar ratios of Fe and Zn were 26.56:1 and 41.95:1.The mass fraction of zinc in the crystal was between 1.5%and 2.0%.The mass recovery rates of iron were 78.37%and 71.11%,respectively.2.The optimum conditions for oxidative precipitation of the steel spent pickling liquor were explored.The results shown that:1 L of steel spent pickling liquors was oxidized with 35 g sodium chlorate,2 mol/L NaOH was used to adjust the pH to 4.0and immediately solid-liquid separation.And then using the acid washing liquor with pH 4.0 to wash the precipitate repeatedly.The best iron-zinc separation effect could be achieved by washing three times or more.The iron mineral with the lowest zinc content was obtained,where in the mass fraction of zinc was only 0.1%.X-ray diffraction analysis?XRD?revealed that the dried iron mineral was goethite.The main factors affecting the zinc content of the products are as follows:Firstly,excessive sodium hydroxide concentration or other high concentration of alkaline reagent will cause the local pH rising rapidly in the system,resulting in local formation of iron-zinc composite salts such as zinc ferrite.Therefore,the purity of the product was affected.Secondly,the pH of the SPL was adjusted to 4.0 to form the iron hydroxide colloid,and the zinc ion was adsorpted in the colloid,which affect the separation of zinc from the SPL.Thridly,the zinc-containing solution in the final solid precipitation gap cannot be completely removed.3.The feasibility of step-wise crystallization combined with oxidation precipitation was examined.Since the ratio of the mass of iron to zinc in the remaining mother liquor from the step-wise crystallization was significantly lower than that of the initial SPL,the zinc content in the obtained precipitate by oxidation precipitation was higher.Finally,the condition parameters of oxidative precipitation were simulated used of Visual Minteq what is an ion balance analysis in water software.And compared with the experimental conditions and phenomena.The precipitation pH range of each of Fe²⁺,Fe³⁺and Zn²⁺was basically consistent with the experimental results.Meanwhile,it was verified that the positively charged Fe₃?OH?₄⁵⁺form precipitates,and some forms of negatively charged zinc,such as Zn?OH?₃^-and Zn₂?OH?₄^2-,were adsorbed,which affected the separation of iron and zinc.4.An iron-based pigment was prepared using the purified iron compound products obtained from oxidation precipitation of SPL.The characterization of the pigment was performed by Visual chroma identification,XRD,scanning electron microscopy?SEM?and transmission electron microscopy?TEM?.The results shown that the products achieved from high temperature calcination of the iron pigment precursor was iron pigment red iron oxide??-Fe₂O₃,a=5.035,b=5.035,c=13.750,c/a=2.731?from the chromaticity,the crystal phase and lattice constant of the particles,and the morphology of the particles.Meanwhile,when the calcination temperature was below 800°C,as the calcination temperature increaseing,the particle size of the obtained iron oxide red particles was smaller,the dispersibility was better,the size was more uniform,and the pigment quality was better.After the iron-based pigment precursor was calcined at800°C for 2 h,the SEM and TEM shown that the particle size of the iron oxide red particles was mainly 600?300 nm and 400?200 nm.The particle size was basically uniform and the morphology was clearly distinguishable.The necessity of investment construction,technical feasibility,economic feasibility and risk factors and countermeasures were discussed through the feasibility analysis.In the economic feasibility analysis,it was found that the gross profit of the process is 68.58%,far exceeding the overall gross profit rate of most environmental listed companies.