Study On Preparation Of Copper-based Sulphides And Capture Of Gaseous Mercury From Smelting Flue Gas

Non-ferrous metal smelting is the main source of anthropogenic gaseous mercury emissions in China.Due to the inability to remove the gaseous zero-valence mercury（Hg⁰）in flue gas from conventional flue gas purification units,mercury mainly exists in the form of gaseous zero-valence mercury（Hg⁰）in exhaust gas emissions.In recent years,transition metal sulfides have been widely used in flue gas mercury removal due to its strong anti-interference and multiple active sites.Among many metal sulfides,copper sulfide has attracted much attention in the field of mercury removal from non-ferrous smelting flue gas because of its strong mercury affinity,high sulfur resistance and low price.Although copper sulfide adsorbent itself can be suitable for mercury removal under high sulfur flue gas,it is difficult to be used in actual non-ferrous metal smelting production due to the limitation of temperature and adsorption capacity.Based on the above problems,two strategies of preparing sulfur-rich copper sulfide（CuS_x）adsorbent to increase the adsorption capacity and cationic doping modified copper sulfide(Cu_xM_（1-x）S_y)to broaden the reaction temperature range were proposed in this study.The effects of adsorbent preparation and adsorption parameters on the removal performance of Hg⁰ were investigated in detail,and the mechanism of Hg⁰ removal was further studied in order to achieve the efficient removal of Hg⁰from non-ferrous smelting flue gas.（1）CuS_x adsorbents with different sulfur-copper ratios were prepared by various methods,and the process parameters involved in the preparation process were optimized.The experimental conclusions are as follows:the optimal adsorbent is CuS_1.34 prepared by one-step hydrothermal,which can be applied to the low and medium temperature range from 75°C to 150°C.Meanwhile,the atmospheres of O₂,SO₂ and HCl in the flue gas have less influence on the CuS_1.34 adsorbent,which has better anti-oxygen and anti-sulfur properties and can be applied to the removal of Hg⁰ in a chlorine-free atmosphere.Long-time adsorption capacity test results showed that the adsorption capacity of CuS_1.34 adsorbent could reach 5.237 mg/g at the breakthrough threshold of 25%,which is significantly higher than the conventional metal sulfide adsorbents reported so far.The kinetic fitting part and the study of adsorption mechanism showed that the process of Hg⁰ adsorption by CuS_1.34 adsorbent was in accordance with the pseudo primary kinetic model,and the fitted saturation adsorption capacity was 21.25 mg/g;short chain sulfur(S₂^2-)was the main active site of Hg⁰ adsorption by CuS_x adsorbent,and the number of S₂^2-active sites showed a positive feedback trend with the increase of sulfur-copper ratio.The specific reaction mechanism is that the Hg⁰ in the flue gas reacts chemically with the active short-chain sulfur(S₂^2-)on the adsorbent surface,and Hg⁰ is converted into stable Hg S,which remains on the adsorbent surface to achieve the removal of Hg⁰ from the flue gas.（2）Three typical transition metal elements,Co,W and Mo,were selected to modify CuS_y,and Co was identified as the best modified cation,and the performance of Cu_xM_（1-x）S_y adsorbent for Hg removal was investigated,and the following experimental conclusions were drawn:the optimal adsorbent is Cu_0.95Co_0.05S_y adsorbent,which can be applied to the middle and high temperature range of 100°C～200°C.When the temperature reaches 200°C,the mercury removal performance can still reach about 88%.Compared with the CuS_1.34 adsorbent in Chapter 3,the Cu_0.95Co_0.05S_y adsorbent still has excellent oxygen and sulfur resistance under med ium and high temperature conditions,and can be applied to the removal of Hg⁰ under high temperature oxygen-rich and sulfur-rich flue gas conditions.The adsorption capacity test results showed that the adsorption capacity of Cu_0.95Co_0.05S_y adsorbent was 5.327 mg/g when the breakthrough threshold was 25%,which was basically the same as that of CuS_1.34 adsorbent.To further enhance the adsorption capacity of the adsorbent on Hg⁰,the adsorption capacity of Cu_0.95Co_0.05S_y adsorbent was increased by soaking with polysulfide compound solution,and the adsorption capacity of the soaked Cu_0.95Co_0.05S_y adsorbent was 8.525 mg/g.The regeneration cycle experiment was conducted on the adsorbent,and when the cycle number was 4 times,the mercury removal rate could still reach 93.4%,which demonstrated excellent regeneration cycle performance.The regeneration cycle performance was excellent.The kinetic fitting results showed that the adsorption process was in accordance with the pseudo primary kinetic model before the immersion of（NH₄）₂S₈ solution,and the theoretical saturation capacity was 27.26 mg/g.The main active site in the adsorbent was identified as S₂^2-by Raman,XPS and Hg-TPD,and the Co doping could promote the formation of more S₂^2-active sites on the adsorbent surface and improve its high temperature stability.Co doping can promote the formation of more S₂^2-active sites on the adsorbent surface and improve its high-temperature stability,which in turn broadens the reaction temperature and improves the performance of Hg⁰ removal.