Technology And Mechanism Of Arsenic Selective Leaching From Copper Smelter Fly Ash Based On Process Intensification

As the biggest copper producing and consuming country in the world,China has been suffering from the eco-environmental pollution caused by mining and smelting,which produced huge amounts of hazardous wastes.Compared the one from mining activities,fly ashes from the smelting process were characterized as smaller particle size,larger surface area and more easily inhaled by persons,thus,they could migrate for a longer distance and caused a more severe risk on the local environment.Furthermore,smelting of nonferrous metal has been recognized as the most important pollution source of arsenic,due to the amount of arsenic released from these activities accounted 40%of its total amount in the atmosphere,among which 80%was in charge of copper smelting.Thus,it is meaningful for arsenic pollution control to remove it from copper smelter fly ashes all over the world.In addition,it could be reused,attributed to the high amount of Cu,Pb,Zn and valuable elements such as In,Ge,Ga.However,as a crucial impurity,arsenic not only increased the burden of the reusing process,but also decreased the grade of products.With the decreasing of copper concentrates,the solid waste from the smelting process?such as slags and fly ashes?would be regarded as one of the most important secondary copper.How to reuse effectively the above wastes has become one of the most important research issues.Nowadays,most research still focus on improving the technology and the mechanism for the leaching of valuable elements,however,As showed unstable and was more easily leached out by acid and base compared with other elements.The copper fly ashes were previously re-melted together with copper concentrates,however,higher arsenic enriching in the smelter decreased the purity of cathode copper,and the decreasing of ore grade could cause more arsenic into the subsequent smelting procedure.Thus,it is meaningful to develop effectively technology for the As selective removing from fly ash before returned it into smelting processes.The leaching agent of NaOH and Na₂S could remove As selectively from fly ash in theory,nevertheless,the above leaching procedure still facing the dilemma,including of more dosing,longer time and lower leaching rate.The situation has become worse with the increasing of arsenic content the fly ashes.Considering that occurrence of As significantly controlled its leaching process,the phase composition of As and other metals were crucial for the improving the leaching technology.Furthermore,it remains unclear of the microcosmical mechanism of NaOH and Na₂S on As leaching.Therefore,this paper chose copper fly ashes as the main study object,and analyzed their chemical and phase composition,surface topography,ANC\BNC.Combined with the investigation of leaching pattern of the valuable elements?Cu?Zn?Bi?Sn?Mo?Sb?Ba?Al?and hazardous elements?Pb?Ni?As?Cd?Co?,the relationship between the leaching and phase compositions of metals and the effects of solubility-controlled mechanism,adsorption-controlled mechanism and mass transfer-controlled mechanism on the metal leaching were determined.Moreover,to develop effective As removing technology which also should be conformed to cleaner production standard,this paper also determined the selective leaching efficiency of As among the microwave,ultrasound,ball milling method,and revealed the mechanism of NaOH and Na₂S on its leaching from the microcosmic scale.The main contents and conclusions are as below:?1?E stablished the relationship betw een the leaching pattern and their phase com positions of m etals in the studied fly ash based on therm odynam ics analysis,and determ ined the effects of solubility-controlled m echanism,adsorption-controlled m echanism and m ass transfer-controlled m echanism on the m etal leaching.The elements of Zn?Cd?As?Al?Fe?Na?K?Cl?Zn could easily washed out,indicating their readily-soluble compounds were existing in the fly ash.Their release is controlled by the matrix distributions in the ash particles.At the acid conditions,little Pb was leached out and mainly controlled by the solubility of PbSO₄,and Cd and Mo could be adsorbed onto the ferric or aluminum oxides.Arsenic leaching was highly related with its phase composition and the adsorption mechanism.As^? in the ashes was mainly existed as As₂O₃,which could be easily released and migrated in the environment.The As^V bonded with Zn and Cu and formed Cu-Zn complex precipitates.At pH 3,arsenic was mainly adsorbed onto the ferric oxides and formed bidentate binuclear?As-Fe=3.25??and bidentate mononuclear?As-Fe=2.85??complexes.Furthermore,the absorbed arsenic was existed as AsO₄^3-,due to disappearing of As-Fe at 2.85?.At pH 5,arsenic was mainly adsorbed onto the newly formed aluminum oxides,with the As-Al at 3.19?.?2?D eterm ined the m ain factors and controlled processes of A s selective leaching from the copper fly ash by the agents of N aO H and N a₂S assisted w ith m icrow ave,and interpreted the m echanism for A s selective leaching from therm odynam ic angle.Compared with the traditional As leaching technology warmed in the water bath,microwave highly increased the final As leached rate?increased by 14%?and shortened the leaching time to 1/9 of the one in the traditional process.In a whole,this novel process assisted with microwave could achieve the selective leaching of As from Cu,Zn,Pb,Cd effectively.According to the orthogonal test,the above metals showed various leached rates in the process assisted with microwave.The leached rate of As was the highest one among them and effected by the content of extract.A little part of Cu,Cd,Zn also leached out and their leached rates effected by different factors,of which Cd,Pb leaching not only related with the content of agents,but also with the temperature and the ratio of liquid and solid;meanwhile,Zn leaching highly depended on the ratio of liquid and solid.The usage of NaOH was the main factor for As leaching in this leaching process and its leaching rate extremely increased with the adding of NaOH and reached 100%at M_NaOH:Ash=0.3:1,however,further dosing caused adverse effect on the As selective leaching from Zn.The optimized factors of this leaching process described as below:M_NaOH:Ash=0.3:1,L/S=20:1,M_NaOH:Ash=0.4:1,T=80?,leaching time=10min.According the SEM patterns,thermal stress successfully dissociated ash particles into smaller particles,and thus increased the reaction activity.Furthermore,microwave could oxidize partly As^? into As^V.The phase of As^V adsorbed onto Fe and Al oxides with the Zn was the crucial one controlled its leaching.The two agents effected synergistically on As leaching.Enough dosage of NaOH facilitated to the desorption of As from the inner-sphere bidentate binuclear and bidentate mononuclear complexation of aluminum-oxides.Meanwhile,Na₂S prevented arsenic adsorbing onto ferric oxides by forming iron sulfides.?3?D eterm ined the prim ary influence factors and the control steps of arsenic leaching rates in copper sm elting dust by N a₂S-N aO H leaching process w ith assistance of ultrasound m ethod and revealed arsenic selective leaching m echanism in a m icrostructural perspective.Comparing with the traditional water bath,ultrasound not only improves the As leaching rate in copper smelting dust,but also reduces 4/5 reaction times.The leaching process with Na₂S-NaOH assistant with ultrasound reduces As and increases Cu content in soot to 0.58%and 2.34%,and the leaching rate of As decreases 0.3%,Cu increases 0.2%,compared with bath alkaline leaching,respectively.It is conductive to further reclamation of those smelting ash with high arsenic and low copper concentration.Orthogonal and single factor experiments show that the leaching rates of different heavy metal elements have distinction in Na₂S-NaOH leaching process with assistance of ultrasound.Meanwhile,As has the highest leaching rate and NaOH dosage affects it,but excessive NaOH increases the leaching rate of Zn and Pb,which against As separate leaching.Na₂S is the main influence factor of As and Pb separation leaching.Liquid-solid ratio and reaction temperature impact leaching velocity.Leaching equilibrium time is lengthening with the increase of liquid-solid ratio,and it is reduced with temperature rising.According to the result of separation leaching effects with As and other heavy metals,the optimum conditions of Na₂S-NaOH leaching process with assistance of ultrasound are below:M_NaOH:Ash=0.3:1,M_Na2S:Ash=0.4:1,liquid-solid ratio is 20 mL/g,80 W discharge power,stirring speed is 400 r/min,15 min discharge time and temperature is 60?.Kinetics indicates that alkaline assisted with ultrasound leaching of As in copper smelting dust is controlled by internal diffusion and interfacial mass transfer at lower temperature,but Interface chemical reaction rate haven't distinct function,compared with water bath leaching system.SEM-EDS showed that ultrasound has dispersion effect to soot,prompting the ash particle changes from solid scope to loose,with the particle size decreased and specific surface area enlarged.The surface cavitation of ash particles in liquid-solid leaching system is further promoted.XRD results showed that ultrasound could not change the nature of the chemical reactions occurring at the leachates of NaOH and Na₂S.Whether for the leaching process with water bath or for the one assisted by ultrasound,the major phases PbSO₄ and ZnSO₄ in the fly ashes were both formed into PbS,ZnS and ZnO.XPS analysis of As indicated that ultrasound made for transforming part percentage of As^? into As^V,which has an important thermodynamic meaning for arsenic leaching.?4?D eterm ined the m ain factor of the leaching efficiency and the rate of A s in the N aO H-N a₂S leaching system during ball m illing process.It is noteworthy that ball milling can significantly increase the leaching efficiency of As.Compared with traditional water bathing methods,after ball milling,the leaching efficiency of As is increased by 11%,and the content of As in the leaching residue is reduced too?only0.5%?,the heavy metal content reduced by 5.04%compared with the original soot.Therefore,the selective of As leaching in such soot is attainable,and its efficiencies is mainly effected by rotate speed and the radio of soot and ball.The leaching efficiency of As increased rapidly with increasing rotate speed,when increased rotate speed from100r/min to 350r/min,the efficiency of arsenic increased by 49%and then decreased along with the higher rotate speed.Increasing the liquid-solid ratio during the ball milling process prevents Zn leaching,while the efficiency of Pb shows opposite trend.When the liquid to solid radio is 20:1,the leaching efficiency of As reaches the maximum while the efficiency of Zn and Pb are the lowest.The As leaching efficiency shows a tendency of increase at the beginning and then decreased.The results shows that As leaching efficiency increase from 77%to 85%with an actual increase of the radio of material and ball from 0.2 to 0.6,With further increase,the As leaching efficiency is significantly reduced and then decreased to 46%when the radio of material and ball is 1.2.Therefore,the ratio of material/ball and the volume of grinding ball should be reasonably distributed in practical applications.The As leaching efficiency gradually increased with the leaching time prolonged,after 60 min,the leaching efficiency remained relatively stable.The optimum conditions for arsenic leaching at NaOH-Na₂S leaching system with ball milling process are determined as:rotate speed of 350r/min,liquid to solid of 20:1,material-to-ball radio of 0.6,leaching time of 30min.The decomposition of PbSO₄ and complex compounds can be promoted along with the generation of the micro-contact points by milling process.In addition,it can also distort the crystal structure,increase the surface active sites of particles,and then oxidize the part of As^? to As^V,thereby increasing As leaching rate.The innovations of this paper were summarized as below:determined the relationship between the leaching pattern of the valuable elements?Cu?Zn?Bi?Sn?Mo?Sb?Ba?Al?,hazardous elements?Pb?Ni?As?Cd?Co?and their phase;with contrastive analysis of atom distances between Fe,Cu,Zn,Al,O and As in various residues and leachates,the adsorption-controlled and synergistic mechanism of NaOH and Na₂S on As leaching were both determined from the microcosmic scale.