Study On Simultaneous Desulfurization And Denitrification By Copper Slag Slurry

Due to the late start of the non-ferrous industry in controlling SO₂ and NO_x from smelting flue gas and acid-making tail gas,they are still exploring new integrated desulfurization and denitrification technologies based on the experience of other industries.Therefore,the development of green,efficient,and low-cost simultaneous desulfurization and denitrification technology is an urgent need for the non-ferrous industry.The industrial solid waste generated during non-ferrous smelting process can be used for wet flue gas desulfurization（WFGD）due to its high SO₂ reaction activity.Currently,copper smelting slag has been used as a new desulfurizing agent on a pilot scale,but the denitrification efficiency is unsatisfactory.This dissertation proposed the use of copper smelting solid waste as a new wet desulfurization and denitrification agent,combined with wet denitrification technology and advanced oxidation processes to carry out simultaneous desulfurization and denitrification research for the lack of suitable integrated SO₂ and NO_x removal technology in non-ferrous smelting flue gas and acid-making tail gas.To address the deactivation problem of denitrification in the raw copper slag system,the thermal modification pretreatment of copper slag was carried out,the relationship between phase transformation of copper slag and denitrification were studied.Secondly,the thermally-modified copper slag was utilized as a new H₂O₂-based Fenton heterogeneous catalyst to form an advanced oxidation system for simultaneous desulfurization and denitrification,and the generation pathways of free radicals and non-free radicals under this system were explored.Finally,the interaction of multiple parameters was investigated through response surface design in the expanded experimental study,and a quadratic polynomial regression model was established to effectively predict denitrification efficiency.The main research conclusions of this thesis are as follows:（1）The mechanism of the desulfurization and denitrification using raw copper slag/KMnO₄ composite slurry was investigated.The mineralogy study of the original copper converter slag showed that the copper slag is mainly composed of magnetite,fayalite,and quartz.The solid-liquid separation of copper converter slag slurry showed that the liquid phase part of slurry played a critical role in desulfurization and denitrification.During the reaction process,the leached metal ions such as Al³⁺,Cu²⁺,Fe²⁺,Fe³⁺,and Mg²⁺ions,have the effect of liquid-phase catalytic oxidation on sulfite oxidation,reducing the activation energy of the conversion from SO₂ to H₂SO₄ and enhancing the SO₂ absorption rate.For NO_x,the combination of Cu²⁺+Al³⁺ions group has a synergistic effect on NO_x removal,while the addition of Fe²⁺ions suppresses NO_xremoval efficiency.Through the detection of aqueous products and analysis of the surface properties of the copper slag before and after the reaction,it was found that the main reason for the gradual deactivation of the raw copper slag/KMnO₄ composite slurry for denitrification was the leaching of Fe²⁺ions.Fe²⁺preferentially reacted with MnO₄^-,resulting in the consumption of the oxidant and significantly inhibiting the denitrification efficiency.（2）Aiming at the problem of Fe²⁺ion leaching in the raw copper slag/KMnO₄composite slurry during the desulfurization and denitrification process,the thermal modification pretreatment method was proposed to improve the mineral structure of raw copper slag,and the mechanism of simultaneous desulfurization and denitrification of thermally-modified copper slag/KMnO₄ composite slurry was investigated.The effects of modifier type,modifier dosage,calcination temperature,and calcination time on the changes in copper slag phase structure and their effects on desulfurization and denitrification efficiency were investigated during the thermal modification process.Thus,Obtaining the optimized modification conditions were as follows:the CaO as modifier,800℃ of calcination temperature.The NO_x absorption capacity of CaO-thermally-modified copper slag/KMnO₄ composite slurry was approximately 29%higher than that of pure KMnO₄ solution.The study of copper slag phase transformation under high-temperature process using XPS and XRD characterization showed that the reducing substances such as Fe（Ⅱ）species（Fe₂SiO₄ and Fe₃O₄）and metal sulfides were reduced during calcination,accompanied by the generation of a new phase,CuO.The simulated phase experiments indicated thatα-Fe₂O₃ was the main active component that maintained high denitrification activity.The formation of CuO,Fe₂O₃,and Cu Fe₂O₄ led to a volcanic trend in denitrification efficiency with increasing calcination temperature.The gradual decrease in denitrification efficiency in the high-temperature range（900-1000℃）was mainly due to the consumption of Fe₂O₃ active substance by the Ca₃Fe₂（SiO₄）₃ phase formed in this range,which was the main reason for the decrease in NO_x removal efficiency.（3）Aiming at the problem of high cost of KMnO₄ oxidant and difficulty in treating MnO₂ precipitation,the research was conducted on the simultaneous removal of SO₂and NO_x using thermally-modified copper slag/H₂O₂ composite slurry.The results showed that the key to obtaining a high denitrification capacity for the thermally-modified copper slag/H₂O₂ composite slurry was to control the calcination temperature during thermal modification process,which effectively tuned the proportion of Fe,Cu,and Si elements on the surface of the copper slag,thereby increasing the Fe active sites on the surface of the copper slag and reducing the inhibition of Cu sites on the rapid self-decomposition of H₂O₂.Copper slag phase simulation results showed that the main active components of the thermally-modified copper slag（CS-CaO-900）wereα-Fe₂O₃andγ-Fe₂O₃,while CuO was the main reason for the rapid deactivation of the slurry.The chemical quenching experiments and EPR characterization results showed that the active oxygen species that played a major role in denitrification in the thermally-modified copper slag/H₂O₂ composite slurry were the surface-bound hydroxyl radicals(·OH_surface)and the superoxide radicals(O₂^·-)in the liquid phase,while singlet oxygen（¹O₂）had a relatively minor contribution to denitrification.NO was mainly oxidized by reactive oxygen species（ROS）to form the recyclable nitrogen-containing resource products NO₂^-and NO₃^-.The rate of nitrite generation is 4.6 times that of nitrate generation,indicating that NO can converted to nitrite through surface-bound hydroxyl radical oxidation,and converted to nitrite and nitrate by superoxide radical oxidation.Meanwhile,SO₂ was converted into all recyclable sulfur-containing products SO₄^2-.（4）The research on simultaneous desulfurization and denitrification of thermal modified copper slag/H₂O₂ composite slurry based on the expansion experiment was carried out.Single-factor experiments were performed to investigate the main reaction parameters in a bubbling reactor for small-scale tests and a spray tower for scale-up experiments.The results indicated that the main influencing factors were reaction temperature,initial slurry pH,and H₂O₂ concentration.Based on the spray tower wet expansion experiment（10 L/min）,at the inlet NO and SO₂ concentrations of 1000 ppm and 500 ppm respectively,the optimal reaction conditions were H₂O₂ concentration of4 mol/L,thermal modified copper slag slurry concentration of 5.0 g/L,reaction temperature of 50℃,52.8 of gas-liquid ratio,and 11.0 of initial slurry pH,and the optimized maximum NO_x removal efficiency reached 86%and SO₂ removal efficiency was 100%.Furthermore,the Box-Behnken response surface method was used to optimize the reaction parameters in a two-stage sequential reactor（bubbling reactor+spray tower）.The contribution of the three reaction parameters to the denitrification efficiency was in the order of initial slurry pH（X₂）>H₂O₂ concentration（X₃）>reaction temperature（X₁）.A quadratic polynomial regression model was established to describe the relationship between the maximum denitrification efficiency and the experimental response values.