Simultaneous Purification Of SO₂ And Heavy Metals In Zinc Smelting Flue Gas And Engineering Application

Nonferrous metals smelting flue gas is the main emission source of SO₂ and heavy metals in the atmosphere of china.With the increase of the atmospheric pollutant emission standard of non-ferrous smelting industry and emission standards of lead-zinc smelting industry including mercury?Hg?,lead?Pb?,cadmium?Cd?,arsenic?As?and other heavy metal pollutant becoming more strict,the deep purification and resource utilization of SO₂ and heavy metals have become an urgent problem in the nonferrous metals industry.Flue gas of zinc metallurgy is the main object for the paper,giving the characteristics of zinc smelting flue gas that high SO₂ concentration,high volume fluctuations,containing Hg,As,Cd,Pb and other heavy metals,a collaborative SO₂ and heavy metal smelting flue gas purification and resource utilization technology is proposed.In this study,in order to removal and recycle SO₂ and heavy metals from flue gas to produce ammonium sulfate product,SO₂ and heavy metals are absorbed by ammonium sulfide solution,then Hg⁰ is taken out by depth catalytic processing.Finally,the integration of technology is studied to complete the application of engineering research.The main contents of this paper are as follows:?1?The flow and equilibrium characteristics of S and heavy metals including Pb,Hg,As,Cd in the process of zinc roasting and smelting flue gas purification,and the removal effect of different purification devices on heavy metals are studied.The results show that after the smelting,the total sand?calcine+ dust?yield is 87%,the rest 13%go into the gas phase.Based on the initial contents of S,Pb,Cd,As and Hg in the ore,the proportion of S,Pb,As and Cd in the solid phase?calcine and dust?is 8.13%?9.65%,58.0%?67.5%,39.3%?47.3%and 75.3%?88.8%,respectively.The proportion of that in flue gas after dust removal is 90.35%?91.87%,32.5%?42.0%,52.7%?60.7%,11.2%?24.7%.99%of the mercury enters the gas phase,and only about 1%of the mercury remains in the solid phase.The removal efficiency of Pb,As and Cd in the hollow tower and packed tower was the highest in the purification craft of acid-making tailing gas.The cumulative purification efficiencies of Pb,As,Cd and Hg were 78.6%,99.9%,83.2%and 35%,respectively,after the flue gas was treated by empty tower,packed tower and electric demister.The Boliden mercury removal tower removes 55.1%of the total mercury.After Boliden mercury removal tower,the contents of Pb,Hg,As and Cd in the flue gas account for 3.3%,9.3%,2.9%and 1.9%of the total,respectively.As the electric mist eliminator and pipeline air leakage effects,the outlet SO₂ concentration of the Boliden was down to 4%?5.5%from 6%?7%of that of the dust collector.?2?Based on the analysis of thermodynamics,macro-kinetics,key influential factors and the mass transfer process of gas-liquid reaction in the SO₂-?NH₄?₂S-H₂O system,the main ions are HSO₃^-,S^2-,S₂O₃²,HS^-,H⁺,OH^-,SO₃^2-and NH₄⁺.According to the absorption reaction variation,the process of?NH₄?₂S absorbing SO₂ can be divided into 3 stages.Stage 1,solution pH>7,where the desulfurization efficiency is close to 100%,but at the same time will produce a large amount of H₂S gas,the main sulfur ions in the solution are S₂",S₂O₃^2- and SO₃^2-solution;pH=7?4 is the second stage,mainly SO₃^2-play a role in desulfurization solution,which has good desulfurization effect,no H₂S is found,the main sulfur ions in solution are HSO₃^-,S₂O₃^2- and SO₃²;pH<4 solution for the third stage,weaken the solution desulfurization capacity,the gas phase concentration of SO₂ began to increase sharply,the main remaining S₂O₃^2-solution in HSO₃^- and H₂SO₃.Therefore,the pH of 7-4 in absorption liquid is a suitable absorption region.On the basis of the experimental analysis,the macro-kinetics of desulfurization reaction using ammonium sulfide is divided into two processes.One is?NH₄?₂S solution absorption of SO₂,where thereaction rate expression is expressed as r1=7.34×10-3-e-5067/RT p?sO₂?0.936.The otheris?NH₄?₂SO₃ absorbs SO₂,where the reaction rate is r2 = 1.39 ×10-4e 1495.7/RT p?SO₂?1.026.?3?The influence of different reaction conditions on desulfurization and removal of heavy metals in ammonium sulfide solution and the method of recovery of sulfur resource are researched.The research on desulfurization and removal of arsenic by?NH₄?₂S solution prove that?NH₄?₂S can effectively remove SO₂ and As₂O₃ in flue gas,however,most of the arsenic enter into the solution.When the solution pH is in the range of 4?7,As₂S₃ is stable,and it is As₂O₃ which leads to arsenic concentration increasing.Under certain conditions,the increase of?NH₄?₂S solution concentration and the increase of temperature can promote the removal of mercury?Hg2+?,lead,cadmium and arsenic in flue gas.When the pH is about 6,the absorption solution has the weakest absorption effect on the four valuable heavy metals.SO₂ in the flue gas will inhibit the removal of heavy metals,the removal of Hg⁰ the removal of Hg⁰ was significantly inhibited.Mercury?Hg2+?,lead,cadmium and arsenic are removed by chemical action and physical action with?NH₄?₂S solution,while Hg⁰ is mainly partially removed by physical elution and gravitational sedimentation.The major precipitates produced by desulfurization and removal of heavy metals in ammonium sulfide solution are HgO,HgS,PbO,PbS,PbSO₃,CdO,CdS and As₂S₃.Ammonium sulfate is obtained by neutralization,precipitation,oxidation,evaporation and crystallization.?4?Hg⁰ in flue gas is well removed with the catalytic reaction by Fe/AC-0.15 catalyst made of activated carbon impregnated into FeC13 solution.Removal efficiency of Hg⁰ can be maintained at 100%in 770 minutes,and the corresponding adsorption breakthrough capacity is 22.8 mg/g under the condition of reaction temperature is 50 ?,the gas flow rate is 400 mL/min,the roasting temperature is 300? and the oxygen concentration is 5%.While the existence of SO₂ and H₂O inhibits Hg⁰ removal.XRD,XPS and TPR results show that Fe mainly exists in the catalyst surface in the form of Fe₂O₃,the chemical adsorption oxygen on the surface of the reactive oxygen species were believed to be involved in the oxidation reaction,physical adsorption and chemical adsorption processes effect the Hg⁰ removal simultaneously,the chemical adsorption is the main mode of action of Hg⁰ removal process.NH₃-TPD and CO₂-TPD show that both the alkaline sites and the acidic sites existed on the modified catalyst surface,and these two synergistically catalyze the removal of Hg⁰.The catalyst exhibits excellent mercury removal properties by the form of C-Cl and Fe-0 functional groups loaded on AC surface,which is testified by Infrared analysis.?5?The results of engineering application showed that ammonium sulfide solution has better removal effect on SO₂ and heavy metals,after secondary ammonium sulfide desulfurization tower,the stable concentration of SO₂ is less than 310 mg/m3,concentrations of heavy metal including Pb and Cd is stably less than 0.5 mg/m3,arsenic concentration is lower than 0.4 mg/m3,all heavy metals except mercury reached GB 25466-2010 emission limit,after depth purification of mercury,the content of mercury is less than 0.012 mg/m3,and ammonium sulfate products meet DL/T 808-2002 standards.