Catalytic oxidation of calcium sulfite in solution/aqueous slurry

Journal of Entionmenal Sciences Val. I6.No.6.pp .973- -977 ,2004CNI1-236291Y___Article D: 101.074242004)06-0973-050CLC number: X131.1 Document code: ACatalytic oxidation of calcium sulfite in solution/ aqueous slurryWU Xiao-qin, WU Zhong- biao" , w ANG Da-hui.hui( Deparment of Enironmental Engineering, Zhejiang University , Hangzhou 310027, China. E mail: wuzb@ mail . hz . zj. cn: zbwu@ zju. edu. cn)Abstract: Forced oxidation of calcium sulite aqueous slurry is a key step for the calcium-based flue gas desulfurization( FGD) residue.Experiments were conducted in a semi-batch system and a continuous flow system on lab scales. The main reactor in semi-batch systemisa 1000 ml volume flask. It has five necks for continuous feeding of gas and a batch of calcium sulite solution/aqueous slury. Incontinuous flow system, the main part is a jacketed Pyrex glass reactor in which gas and solution/aqueous slury are fed continuously.Calclum sulite oxidatin is a series of complex free radical reactions. According to experimental results and literature data. the reactionsare infuenced signiticantly by manganese as catalyst. At low concentration of manganese and calcium sulite， the reaction rate isdependent on 1.5 order of sulfite concentration, 0.5 order of manganese concentration, and zero order of oxygen concentration in whichthe oxidation is contolled by chemical kinetics. With concentrations of calcium sulfite and manganese increasing, the reactions areindependent gradually on the constituents in solution but are impacted by oxygen concentration. Manganese can accelerate the free-radicalreactions. and then enhances the mass transter of oxygen from gas to liquid. The critical concentration of calcium sulfite is 0.007 mol/L,manganese is 10 mol/L, and oxygen is of 0.2-0.4 atm.Keywords: calcium- based FGD; calcium sulite; manganese; catalyzed oxidationHSO; + 1/202→S0:- + H',(1)Introductionso; + 1/202→s0:(2)Caleium-based FGD process is the most common commercialIn fact， they are a sequence of very complex free radical reactionsmethod for removal of SO2 emission from combustion of fossil fuel.involving a number of highly reactive intermediate species:Scnubbing suries forned in the process consist of calcium sulfte andS0;- + Me"→. S0; + Me*=-'，(k)sulfate . The sluries presenly deposited in environment will bring serious.S0; + O2→.SO;， (k2)(4)waste problem. So practice use of the slurries becomes a topic of greal●SO;+So;-→so;-+.s0;,(k;)(5)interest in the study of caleium-based FCD process. Moreover, caleiumS0;+ s0;→250:-，(h)(6)sulfate is a neutral product, and is ully uilized in the building.SO; +.sO; →2S0; + O， (k,)(7)industry as Bypsum. Therefore， the sluries are preferred to calciumsulfate as an end product， and the simplest method for the surries2Me'-11 + 2H2O+ O2→2Me"+changed into calcium sulfate is forced oxidation by air.20H° + H,02. (K)(8)Catalyzed sulfite oxidation has been used to measure gas dispersersOf the radical reactions, it is assumed that the Reaction (4) isand other mas-transfer equipments for about one hundred years.rapid which would not constraint the total reaction rate. the reactions areAccording to the results achieved before. sulite concentration rangedterminated by radical species combination in Reaction (7). and Reactionfrom 0.2 to 0.8 mol/L and the kinetics of catalyzed sulite oxidation is(8) keeps thermodynamic equilibrium during oxidation. So it can beextremely complex which has many features in common of soluion phasedenived that the oxidaion rate expression by using the steady- siatefree radical reactions. From half of 1970s， sulfte oxidation was paidapproximation:more atention 10 for its use in waste product oxidation of FCD process(9)within the air pllution. In the case of caleiun- based serubbing thesulite ion concentrations will be low because of precipitation of calciumseo.(台) .sulfite. 。Therefore only investigations on very diluted sulfte solutions areMoreover, the concentration of sulfite in aqueous slury containingrelevant for the oxidation in calcium- based scrubbing slurries undercalcium is governed by the ollowing equilibrium only as long as thetypical FGD conditions. Some of the investigations will be discussedconcentration of calcium sulfate( final product) is below its precipitationbelow. Homogeneous syslem with sulfte concentration formn zero to 0.007level:mol/L was studied as well as heterogeneous system. Of the systems, onlygas-liquid mass transfer and sulite oxidation exist in homogencousWith the concentration of ealcium increasing due to the formation ofsystem，while the process of dissolve. oxidation and esysallizationmore soluble calcium sulfate. the concentration of So; diminishes tocoexists in heterogeneous system, which is the main difference betweenachieve the minimum value fixed by the additional equilibrium:homogeneous system and heterogencous system.CaSO..→Ca”+ s0;(11)1 Mechanism中国煤化工iation. and srlzationIn aqueous slurry, calcium sulfte fored oxidation mainly appearsappear t:MHC NMH G'eous slumry .in liquid phase. The oxidation reactions are generally described as2 Experimental apparatus and procedurefollows( Srivastava.1968; Mishra, 1976; Linek, 1981):The experiments were carried out in homogeneous system and inFoundation item: The National Hi-Tech R&D Program( 863) of China(No. 2001AA642030 - 1); * Corresponding author974WU Xiao-qin et al.Vol.16heterogeneous system. The former was done to determine the rate3expression for calcium sulfte in solution. The lab scale experimentalapparalus were thermostatic stiming tanks for semi-batch process ancontinuous Alow process(Fig.1 and Fig.2). The seni-batch process也consists of a mechanically agitated thermostatic reactor, which is a 1000ml volume fask. The flask has five necks for a continuous feedinge of gas飞10and a batch of solution/aqueous slurry. In the continuous flow process.the main reactor is mnade of Pyrex glass, which is a jacketed, and 0.1312m ID eylinder with a hemispherical botomn. In both proceses. an axialstirer was used to provide thorough mix in liquid/ aqueous slury. Thetrrer blade is 0.03 m width and 0.06 m impeller diameter. Thetemperature was set al 45C in all experiments. Air was bubbled into thebottom of the reactor and then up through solution/ aqueous slurry at aflow rate of 3 Umin measured by a rotameter. In continuous flowFig.2 Continuous flow experimental apparatussystem, the liquid flow rate was kept constanly at 50 ml/min by a1.2. gas cylinders(Oz，N2); 3, 4, 5. rolamneters: 6. reactor; 7,8. slumytank; 9.catalyst solution tanks; 10. pristltile pump; 11.tail gas abober; .peisalie pump. and the gas flow gave a liquid holdup volume of 3.9x12. termostat; 13. wet flowmneter10* m' . The residence time of solution/ aqueous slumy in continuouslow system is 470s. The stirer speed n was kepl constanly in theBaSO。 precipitalion method. The precipitales formed in experiments wereexperiments as much as 400- -500 revolutions per minute .observed under microscope in polanized light.3 Experimental resultsn83.1 Catalyzed oxidation of calcium sulfte in semi- batch systemResults of lypical calcium sulfite oxidation shown at discrete times17-8are presented in Table 1 and Fig. 3 and 4. The relations betweenoxidation rale and total calcium sulfte( solid and liquid in heterogeneous6system) and catalyst concentration are shown in Fig. 5 and 6respectively .Fig.1 Smi-bach experimental apparatus1. tail gas abyorber; 2. termostat: 3. fask with five necks; 4. termometer:TableI Fasbex 105 with diterent Csn and Coun, in semi-batch system5. ectromagnetism constant speed stirer; 6. gas distributing unit; 7.Cvmol/(L. min)、mol/Lconpound lectrode; 8. rolamneter: 9. air pump: 10. pH meter0.00 1.00E-5 1.00E4 1.00E-3 1.00E-2 1.00E.1The liquid phases were clear solution or aqueous slurry of calciumC u .mol/L~sulfte by disolving analytical calcium hydroxide into deionized water.0.00236.04 4.82 18 18 18 18and then they contacted with pure SO2 with the contolled pH value in0.00467.54 21.8 30.8 30.8 30.8 30.2the range of3.0-6.0. The econcentration of Ca2' and tolal So;- both0.007010.35 23.230.2ranged from 10~’ to 0.024 moVL. In the experiments. manganous0.012218.84 33.0 45.8 42.4 49.4 50.2sulfate has been selected鸱the catalyst. Manganese is one of the minor0.023513.33 29.5 32.5 40.0 44.3 53.60.07745.18 38.548.149.061.0coal elements which can be transferred from coal to ash, condensedphase and fue gas during the coal combustion. In FCD system,In homogeneous system，experiments showed that the oxidation ratemanganese ion is the secondaryrich metalis the highest itially , and then descends sharply with sulfte consumingAccording to the measurement of FGD system of some Hangzhou Powerwithout catalyst( Fig.3 and Fig.5). The oxidation rate is around 6.04 xStation and lieraures data ( Weisnicht, 1980; Yan， 1999)， the10-”mol( L. min) at sulfte concentration of 0.0023 mol/L. while 5.57concentration of manganese in FCD system can reach to the magnitudesx 10“mol/(L. min) at half of the initiad concentation( Fig.5). Thereof 10- mol/L. After calcium sllfte oxidation, it would enter into wasteare also a series of experiments performed by using dfferent manganesewater system for futher treatment which is nol the topic included in thisconcentrations between 10。to 0.1 mol/L without pH control, andtudy. The concentration of manganese was varied in the range of 0-experimental results indicate that manganese has a significant efet on0.1 mol/L.the oxidation at low concentrations(C <7x 10- mol/L, and Cx