Purification technology of flue gas from remelting process of aluminum alloy tailings

Vol.12 No. 3Trans. Nonferrous Met. Soc. ChinaJun.2002[ Article ID] 1003 - 6326( 2002 )3 - 0529 - 05Purification technology of flue gas from remelting process ofaluminum alloy tailingsLI Cai-ting(李彩亭), ZENG Guang-rming(曾光明), WEI Xian- xur(魏先勋),YUAN Xing- zhons(袁兴中), WANG Li-ping( 王丽平)( Department of Environmental Science and Engineering , Hunan University ,Changsha 410082 , China )[ Abstract ] Through a practical example of treatment of the flue gas from the remelting process of aluminum alloy tail-ings , the design and calculation method of exhaust hood , as well as the principles and the equipments of dust removal ,smoke abatement and harmful gas elimination were studied. Combination of centrifugal and wet dust removal can purifythe dust high efficiently. The carbon black and harmful gases in the flue gas can be removed by adding a small quantity ofactivator to the absorption solution. The application results are that the dedusting efficiency is 97. 43% ,C2 control effi-ciency is 88. 03% , the exhaust fume blackness is lower than Ringelman number I ,and the purification device resistance is1 126 Pa.[ Key words ] aluminum alloy tailings ; flue gas ; purification[ CLC number ]x51[ Document code ]A1 INTRODUCTIONciated with the specific situation , a purification planwas made. First , collect the flue gas , and then inputIn an aluminum alloy casting workshop of theit into the self-made purification device to make it in-automobile carburetor plant , there are ten electricteract with the absorption solution，thus the objectivefurnaces，among which six operate simultaneously inof dust removal , black smoke abatement and harmfulaverage for aluminum alloy melting. The tailings in-gas elimination can be realized. Theconcretecluding runners , feathers , remnants etc , will be gen-schematic diagram is shown in Fig. 1. The purifiederated during the process of casting and refinementflue gas after dehydration is discharged into the airafterwards. With the aim of economizing resources ,through the induced draft fan and chimney , and thethe tailings need to be remelted for reuse. However ,absorption solution will be recycled. The followingsome oily dirties , cuticle removers , emulsifiers andfour critical problems must be solved in order todusts will cover the tailings , large numbers of blacksmoke and dust will discharge during the remelting12 11process. Moreover , when refining , some C2 and HCIwill escape from the workshop1~3]. Without treat-ment , the air pollution is tremendously serious in thepA小\Aworkshop. The blackness level of the flue gas fromthe furnace opening reaches Ringelman number 5.The concentration of C2( standard state) in the air口口口口around the operating station is 3. 8 mg/ m2( the stan-dard of Clis 1 mg/m3 )4]. These pollutants are ex-tremely harmful to the health of the workers. Thplant lies in the dense community of the city ,so resi-dents are strongly against it.Fig. 1 Schematic diagram of purificationprocess of flue gas2 PURIFICATION MEASURE1- -Aluminum melting furnaces ;2- -Water pump ;3- -Activator concentrated pond ;4- -Precipitatingand cycling pond ;5- Automatic water compensationThere is no example on the purification of fluegas from the remelting process of aluminum alloy tail-中国煤化工device of flue gas ;mney ;9- -Muffler ;ings now. According to the properties of the flue gas ,YHC N M H G)iagonal rope ofthe project group carried out a small- scale test. Asso-exhaust hood ; 12- -Exhaust hood①[ Foundation item Prijcts ( 70171055 ; 50179011 ) supported by the National Natural Science Foundation of China ; Project supprted by theChinese Teaching agnd Reseatch Award Program for Outstanding Young Teachers in Higher Education Institutes in 2000[ Received 4n8261 - 12 - 26 ;[ Accepted date ]2002 - 01 -23530 .Trans. Nonferrous Met. Soc. ChinaJun.2002implement the above process: 1 ) collection of flueence ,Cgas ,2 ) dust removal ,3 ) black smoke abatement andThe total ventilating gas volume is calculated ac-harmful gas elimination , and 4 ) anticorrosion andcording to 60% operational rate of furnaces( six elec-abrasion resistance of the devicd 5- 81.tric furnaces operated simultaneously ). In addition，the air leak factor is considered in some extent as2.1 Collection of flue gasesLr= 6LC(4)2.1.1 Installation of exhaust hoodwhere C is coefficient , given as 1.1.Under certain process conditions ，whether theThe calculated ventilating gas quantity of everycollection of flue gases is effective is of great impor-hood is 0. 49 m?/s and the total ventilating air quanti-tance for the air quality of workshop. In this work ,ty is 3.234 m'/s , namely 11 642 m'/h.an exhaust hood is set up at the top of electric furnacefor flue gas collecting. Because the ten electric fur-2.2 Dust removalnaces will not operate at the same time ,a valve wasThe flue gas containing dusts will enter the pu-installed on each exhaust hood in order to reduce en-rification device from the tangential direction by pass-ergy consumption and ensure the purification effect.ing through every exhaust hood ，branch pipe andIn consideration of the convenience of the crane load-main pipe. The absorption solution is sprayed into theing and unloading materials and the facilitation of op-purification device from the top. In the purificationerations of alloy refining ，slag removing and pouringdevice , the dusts with particle diameter over 10 μmand soon , the exhaust hood is designed to be rotaryare removed through the centrifugal force. Accordinglifting and low hanged style. The shortest distanceto the Stokes' law , the radial velocities of the dustbetween the bottom of the hood and the entrance ofparticles moving to the tunnel wall under the com-the furnace is 150 mm , and the moving distance ofbined actions of the centrifugal force and resistancethe hood is 650 mm. In order to reduce the distur-can be written asbance of cross flow and the loss of heat of the metalv,= pu}d2( 18μr )(5)liquid ,an inner hood is set in the exhaust hood. Thewhere r is particle rotation radius ,m; Vr is radialexhaust hood , which is the telescope-feed style , usesvelocity of the particle when the rotation radius is r ,the balancing ring to balance the mass of itself and them/s; p is particle density ,kg/m’;d is particle di-inner hood through pulleys. The rotary process of theameter ,m ; Vt is tangent velocity of the particle whenexhaust hood is realized by ball bearings and the ex-the rotation radiusis r ,m/s ipμ is kinetic viscosity ofhaust hood is hanged up with a diagonal rope thatgas,Pa scould reduce the twisting moment of the axial bearingEqn.( 5 ) indicates that the radial velocities ofand make the hood rotate skillfully and freely.particles with certain sizes vary directly as v哈and in-2.1.2 Calculation of ventilating gas volumeThe scale of the exhaust hood opening is calcu-versely as r. The designed purification device ,with ahigher Vt value , has a rmax value only about half oflated bythe tube radius of conventional cyclone separators ,D=d +0.8H(1)and its gases haven' t any radial centripetal flows.where D is diameter of the hood opening ,m ;d isThus , the radial moving velocities of the particles arehorizontal projection diameter of heat sources ,m ; Hcomparatively larger. Therefore , the particles withis vertical distance from the hood opening to the fur-diameter over 10 pum can be removed in the smallernace opening , m.Ventilating gas volume of single exhaust hood isdevice and in shorter time.The dusts with particle diameter less than 10 pumcalculated asL=Lo+ o'F'(2)have smaller Vr values , so they can not be removedwhere L is ventilating gas volume of every exhaustefficiently only by the centrifugal forces. However ,hood ,m'/s;o' is indraft velocity of gas in expandedthe wet dust remover , who removes the dust by colli-areas, m/s , given as 0.6 m/s; F' is the expandedsions and condensation of the dust particles and liquiddrops , can be used to reach a higher efficiency of dustarea of hood opening , which is the subtraction ofremoval. The collision probability of the dust particleshood opening area and sectional area of heat current，and liquid drops is represented by the dimensionlessm2 ; Lo is initial flow rate of heat flow above the heatcollisisource ,m2/s. Lo can be calculated as中国煤化工(6)Lo= 3. 81( AFhA3Ol43y3x 10-2(3)wherdMYHCNMHGy of dust paricle towhere A is coefficient ,given as 1.7 ; F is convec-droplet , m/s; ds is diameter of droplet , m; C istion and heat elimination area，m^ ; h is qualitative .Cunninghum slipping correction coefficient.dimension of heat sources ，namely the diameter ofFrom Eqn.( 6 ),it is clear that N is directly pro-electric furnace. crucible ; Ap is horizontal projectionportional to V and inversely proportional to ds.area of hearbMHes, m2 ; Ot is temperature differ-Thus , increasing the relative velocity of dust particlesVol.12 No .3Purification technology of flue gas531 .and droplets and reducing the diameters of the .3.1 m/s, the resistance of equipment of less thandroplets are the major ways to improve the dust re-1 200 Pa can be obtained , which shows that the e-moval efficiency. In the purification device ，the com-quipment is able to dispose higher flow of gas in aponents can enhance the relative velocity of gas andsmaller device , and to minimize the space.liquid , reduce the value of ds which can atomize theabsorption solution , and increase the valueof N. As a2.3 Black smoke abatement and harmful gas re-result ,a considerably high efficiency of dust removalmovalcan be acquired. Whereas , the droplet cannot be tooWhen the aluminum alloy tailing is remelted ,small ,or it will be flown along with the air currents ,the oil dirt and emulsifier on its surface will producewhich will greatly reduce the relative velocity of gashigh carbon substances , including carbon black ( ben-and liquid. The optimum diameter range of liquidzene ring matter ) and soot ( cyclic hydrocarbon mat-drop is 500~ 1 000 μn[5].ter ) through the processes of cracking , dehydrogena-The test of dust removal efficiency is taken in ation , combination ,and cyclizatiorf I The dischargessimulated test device.' I he experimental dust is stan-of the soot and carbon black ( hereafter called carbondard pulverized coal powder with meso position diam-black ) will generate black smoke. The carbon black iseter dso= 16.02 pμm. The inlet dust concentration istoo light ( relative density is about 0. 05 ) to be re-5 g/m( standard state ). The tests show that the ratiomoved by the cyclone separators. There is an inter-of liquid and gas is of slight effect on the resistance ofmolecular force between the surface of carbon blackdust remover. The relationship between the liquid/and water molecular , which is not powerful enough togas ratio and the dust removal efficiency is shown inovercome the cohesive force among water molecules.Table 1. It shows that the dust removal efficiency in-Therefore, for the poor hydrophilic force betweencreases proportionally to the rise of the ratio of solu-water and carbon black , the conventional wet dust re-tion and gas under certain gas volumetric flow rate.movers could hardly gather the carbon black effective-When the ratio is 0.2 L/m3 the dust removal efficien-1[9~11].Through theoretical researchest121 ，Wallkercy will over 98. 5% , namely the gas and liquid arefully mixed in the dust remover and the effect of dustmethod experiment and small-scale tests ，it wasremoval is excellent. The relationship between gasfound that the combination phases between the carbonvelocity in tower section and resistance of the equip-black and the absorption solution can be improved byment is shown in Fig.2，which indicates that the re-adding extremely small quantity of activators to thesistance increases with the increase of gas velocity.absorption solutions of wet dust removers. The abilityWhen the gas velocity of tower section is less thanof the absorption solution infiltrating the carbon blackdepends on the relative values of hydrophilic and hy-Table 1 Test results of dust removal efficiency( % )drophobic energy of activator. If the hydrophobic en-ergy is too strong , no enough affinity will exist be-Gas volumetric flowRatio gas to liquid(L: m~3)tween the activator and the water phase , and if therate/( m' h- 1 )0.20.4 .0.60.8hydrophilic energy is too strong , the surface activity40098. 6099. 1099.23 99. 30of the activator will decrease greatly which makes ithard to be combined with the carbon black. Only if60098.7598.80 98. 9099.25the ratio range of hydrophilic and hydrophobic ener-gies of the activator lies between 4. 5 ~ 6. 5，the2.0[bridge grafting effect between the carbon black andwater can be sufficiently generated. According to ex-1.6periment and optimized selection ，the main compo-nent of the activator is a kind of macromolecule soapmade of industrial products. The thick activator solu-s 1.25tion is put into the chemical medicine box above therecycling precipitation pond for periodically activator80.8adding. The water pump put the absorption solutionfrom the pond into the purification device. Under the0.45effec中国煤化工vice，the gas , liquidandroughly and contactedwithYHc N M H Gdge drafting efet ofactivator , the absorption solution is able to effectivelyGas velocity in tower section/(m's~ ')collect the carbon black. Thus the object of smoke a-batement can be realized. The collected carbon blackFig2, Relationship between resistancecondenses and floats up the pond for periodically) ad落捞velocity in tower sectionscraping out , and the absorption solution is recycled.Trans. Nonferrous Met. Soc. ChinaJun.2002In a similar way ,this kind of activator also has an ex-ing. Table 2 shown that the base metal of coating Ccellent purification effect for oil mist.had some stain pots , and shown a poor performanceDuring the process of aluminium alloy refinementof anticorrosion. Coating B shown good results inand forming slag , some Cl2 and HCl are dischargedboth the surface and the base metal in the test , andand then are eliminated in the purification device a-was the best one in anticorrosive property.long with those harmful flue gases and dust. The re-Thus , the device has the features of low manu-actions between Cl2 and water in the purification de-facturing cost , small size，less mass and excellent an-vice are shown asticorrosive and abrasion resistant properties that canCl2+ H2O= HCl+ HOCl(7)make the device have a service life for over 8 years.8HOCl= 6HCl + 2HClO3 +O2(8)2HOCl= 2HCl+O2(9)The alkali components MeOH ( Me representsA一▲certain metal element ) in the activator will have aneutralized reaction with the acidic matters in the flueg2gas and the acidic products from reaction( 7)~(9)by which some harmless water and salts are generat-ed. In this way , the harmful flue gases can be re-●一Coatingmoved as- -Coating BMeOH+H+ = H2O+ Me+( 10)▲-Coating C2.4 Anticorrosion and abrasion resistance of deviceThe original flue gas contains both dust and cor-rosive gases , so whether the materials are anticorro-sive and abrasion resistant is of great importance forthe device durability. By extensive investigation and-200 300400500600analysis , it shows that the singular metal material isSoaking time/hnot enough for anticorrosion and abrasion resistance ,such that the 5 mm thickness steel panel will be per-Fig.3 Curves of coating mass changesforated due to the erosive and grinding process forvs soaking timeabout 20 d in practical application. At the same time，Table 2 Anticorrosion experimental results forthe strength of singular nonmetal material cannot bedifferent coatings( in0.1 mol/L HSO4 ,60 C )guaranteed. Therefore ，according to the situation andthe environment of the device and its components ,Changes inCorrosion ofItemcoating massdifferent combination or complex of materials is used(g m~2)coating surface base metalat different places in order to exert the individual ad-SurfaceNovantages of different materials. The metal materialsCoating A-15.1luster lostcorrosioncan guarantee the strength and the complex materialscan guarantee the anticorrosive and abrasion resistantAlmost theCoating B14.0properties.sameThe inner tube of the equipment is made fromCoating C25.4.ColorA littlepottery , while the inner wall of the device body isslightrusting potscovered with inorganic nonmetal material whoseRockwell hardness reach HRC 68 , and gives it a goodanticorrosion property. The outer surface of the inlet3 PURIFICATION EFFECTwater tube is covered with a special treated anticorro-sion complex material. The complex material B is se-After put into use ,the system works safe , regu-lected for realistic application after testing. The testlar，convenient for controlling and management andmethod is that the samples are firstly hanged in 1efficient for purification , which was highly praised bymol/L H2SO4 solution at 60 C , and then the massthe workers and around residents. Through the testchanges of the samples were determined , at the sameof en中国煤化工. partment，the concen-time the changes of the coating surface and corrosiontratio, Cl2 ) in the workshopof the base metal were observed. The experimentalis lowMYHC N M H Gurified indexes are farresults are shown in Fig.3 and Table 2. The coatingsuperior to the requirement of Class I of the NationalA lost some mass , which was mainly caused by theEmission Standard of Pollutants. The effect of purifi-acid corrosion and the coating material part dissolved ,cationof flue gas is shown in Table 3. It shows thatmeanwhile the, coating B and coating C got mass be-the dust purified efficiency is 97.43% , the Cl2 puri-cause of the n怒摘slightly permeating into the coat-fied efficiency is 88. 03% , the exhaust fume black-Vol.12 No.3.Purification technology of flue gas533 .Table3_ Effect of purification of flue gasItemInletOutlet[ REFERENCES ]Operation gas volumetric flow rate[1] Lopez F A ,Pena M C ,Lopez-Delgodo A. Hydrolysis and11 136 10 961/(m3 h-1)heat treatment of aluminum dust[J ] Journal of the Air& Waste Management Association ,2001 ,51(6 ):903 -Gas volumetric flow ratd Standard91309560912.state)X(m3 h-' )[2] Trovant M, Argyropoulos S. Finding boundary condi-tions :a coupling strategy for the modeling of metal cast-Dust concentratior( Standard state )143735.3ing processes( I )- -Experimental study and correlation/(mg m-3)development[ J ] Metallurgical and Materials Transac-tions , 2000 ,31B2):75 - 86.Mass flow rate of dust/(kg h- 1)13.12 0. 337[3] Trovant M, Argyropculos S. 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