Study Of Separation Of Heavy Metals In Electrolytic Zinc Slag By Magnetic Adsorbent Material

Zinc is an important basic material and a strategic reserve resource of national economy and plays an important role in electroplating,aerospace and military industries.In recent years,the capacity and output of zinc mineral resources of China is the first in the world.With the development of electrolytic zinc enterprise,the environmental problem from it is more serious environmental pollution and ecological damage.In particular,the leaching residue of electrolytic zinc?LREZ?is the largest and most dangerous source of pollution in the industry.There were mainly five problems in the disposal process of LREZ:?1?The concentration of zinc,cadmium and lead is high.In LREZ leachate,zinc and cadmium are of high concentration which was hundred times the nation standard;?2?Zinc and cadmium are water soluble and easy to transfer;?3?The stock of LREZ was of great amount.In terms of the existing problems,this paper put forward the methods and thoughts that relied mainly on separation heavy metal ions in LREZ with hazard-free treatment in the end of pipe.In this study,the heavy metal ions of LREZ were adsorbed in a magnetic adsorbent-water-waste ternary system.Magnetic adsorbents with L-arginine,silica gel and g-C₃N₄ as adsorptive functionalities were prepared.The magnetic adsorbents with strong anti-interference ability were selected as magnetic adsorbents in the ternary system.The main conclusions as follows:?1?In the experiment of washing waste residue,the concentration of heavy metal and pH of the leachate have reached the national standard when the water-slag ratio was 10,the washing time was 5 and the mixing time was 90 min.The results of LREZ long-term stability show that the concentration of Zn???,Cd???and Pb???would rises slightly within 30 days,while the concentration of Zn???,Cd???and Pb???remain stable within 30-300 days.?2?Two kinds of magnetic adsorbents were prepared by using L-arginine as adsorptive functionalities.Fe₃O₄-CS-L was prepared by chitosan grafting and MNPs-L was prepared by one-step hydrothermal synthesis.Fe₃O₄-CS-L is a three-layer core-shell structure magnetic adsorbent,which the outermost layer of L-arginine and the middle layer of chitosan were grafted on the magnetic Fe₃O₄ layer in the glutaraldehyde bridge.The maximum adsorption of Zn???,Cd???and Pb???by Fe₃O₄-CS-L was 256.41 mg/g,156.99 mg/g and 128.63 mg/g,respectively.The mainly mechanism of adsorption is chemical adsorption.The adsorption was less affected by ionic strength.In one-step hydrothermal synthesis method,L-arginine or decomposing functional groups were modified on the surface of Fe₃O₄ under high temperature and pressure.The maximum adsorption of Zn???and Cd???by Fe₃O₄-CS-L was 150 mg/g and 120 mg/g,respectively.The mainly mechanism of adsorption are chemical adsorption and electrostatic attraction.The adsorption was more affected by ionic strength.?3?The core-shell structure of magnetic adsorbent?Fe₃O₄-Si-COOH?was prepared by uses of in-situ introduction method and the maximum adsorption of Fe₃O₄-Si-COOH for Zn???,Cd???and Pb???was 155,110 and 93 mg/g,respectively.The adsorption order of Zn???,Cd???and Pb???by Fe₃O₄-Si-COOH is Pb????Zn????Cd???.The main mechanism of Fe₃O₄-Si-COOH adsorption of heavy metal ions is the ion exchange reaction between hydrogen ions and heavy metal ions.The desorption rate of Fe₃O₄-Si-COOH can reach to 93%by desorption with 0.8mo/L HCl for 150 minutes.The adsorption capacity of reused Fe₃O₄-Si-COOH could maintain 80.4%after five cycles.?4?The structures of magnetic adsorbent with g-C₃N₄ as adsorptive functionalities have a greater impact on the adsorption capacity of metal ions.Fe₃O₄was only embedded on the edge of g-C₃N₄ and the g-C₃N₄ sheet did not open in Fe₃O₄&g-C₃N₄ which was prepared by one-step hydrothermal method.However,Fe₃O₄@g-C₃N₄ which was prepared by the layered stacking technique can fully open the g-C₃N₄ sheet and fully utilize the adsorption capacity of the g-C₃N₄ sheet.The main adsorption mechanism of Fe₃O₄&g-C₃N₄ and Fe₃O₄@g-C₃N₄ is the coordination between conjugated?-electron pairs of heptazine?C₆N₇?units or triazine ring?C₃N₃?units and sp² C-N with metal ions.The adsorption capacity of Fe₃O₄&g-C₃N₄ for Zn???was lower than that of Pb???and Cd???.The maximum adsorption capacity of Fe₃O₄@g-C₃N₄ for Zn???,Pb???and Cd???were 425.53,433.03 and 189.39 mg/g,respectively.?5?Taking the magnetic response and the adsorption capacity in the actual wastewater as screening requirements,the magnetic adsorbents of Fe₃O₄-Si-COOH and Fe₃O₄@g-C₃N₄ were selected as magnetic adsorbent in ternary system.Magnetic separation efficiency of the highest separation is the first magnetic adsorbent,after the water,the final residue.The separation sequence which corresponding to highest separation efficiency of magnetic adsorbent are the first magnetic adsorbent,after the water,the final residue.It is found that Fe₃O₄@g-C₃N₄ had a better effect than Fe₃O₄-Si-COOH by comparing the removal efficiency of the metal ions in the ternary system.The treated waste has reached the national standard at the slag:water:Fe₃O₄@g-C₃N₄ mass ratio of 1:10:1.4.In the ternary system,there is precipitation dissolution equilibrium,zinc carbonate,cadmium carbonate and lead sulfate will dissolve.In the adsorption equilibrium of PAM-magnetic adsorbents,Fe₃O₄@g-C₃N₄is more competitive with metal ions than PAM.The adsorption capacity of Fe₃O₄@g-C₃N₄ was mainly affect by compressing the thickness of the electric double layer,producing ion exchange competition and changing the total number of sites of the adsorbent.