Surface Modification Mechanism of Fine Coal by Electrochemical Methods

Jun.2006J. China Univ of Mining Tech (English Edition)Vol 16 No. 2Surface Modification Mechanism of FineCoal by Electrochemical MethodsZHU Hong, WANG Fang-hui, WANG Dian-zuo, OU Ze-shen'School of Science, Beijing Jiaotong University, Beijing 100044, chinaBeijing General Research for Non-Ferrous Metals, Beijing 100088, ChinaSchool of Chemical Engineering and Environment, China University of Mining& Technology, Xuzhou, Jiangsu 221008, ChinaAbstract: In order to reveal the surface modification mechanism of fine coal by electrochemical methods, the structuralchanges of the coal surface before and after electrochemical modification were investigated by Fourier Transform In-rared Spectra(FTIR)and Raman Spectra. The results show that under certain electrochemical conditions, the oxy.gen-containing functional group in the coal structure and the oxygen content of absorption could be reduced and thefloatability of coal improved. At the same time, the sulfur in the coal was reduced to the hydrophilic Which could betional grpe asily from coal. Thus electrochemical modification methods could be used to change the structure and func-Key words: fine coal; electrochemistry modification; modification mechanism; FTIR; Raman SpectraCLC number: TQ 028.9*4: X 701.31 IntroductionMine Xuzhou, were broken to 0.4-0.2 mm size andinjected with nitrogen a reductant; C6H6O3 a phThere is a vast amount of coal in China. but regulating agent; diluted HCl and diluted NaOhmost of it is difficult to clean due to its high ash and 2.2 Measuring apparatus and conditionsulfur content. More than half of the sulfur found inraw coal is in the form of pyrite, which is distributedThe following equipment was usedin coal in micro-fine sizes or combined with coal1)Fourier Transform Infrared Spectra(FTIR)Coal must be broken to pieces to increase the rate of Nicolet-510 type, resolving power of 4, backgroundrecovery of cleaned coal and to decrease sulfur and gain of 2, scanning number of 32, sample gain of 200,ash. Flotation is a very important and widely used scanning number of 120. Diameter of sample pool ofmethod for the removal of pyrite from high-sulfur I cm, sample size of 0.074 mm and sample weight of5 mgIt has been reported that after initial oxidation2)Raman Spectra. Renishaw Raman Systemthe pyrite surface containing oxidized products such Mki2000 type, laser wavelength of 514.5 nm, 1800as polysulphide and free sulfur is naturally hydro- back scattering installment, power of 9 mW, lensphobic. There are various kinds of oxygen-containing magnification of 20 times, continuous scanning of 10functional groups in coal. The more hydrophilic thecoal is, the more groups there are on the coal sur- 3 Results and DiscussionThe principle of electrochemical desulfurizationn coal flotation is to modify surface characteristics of3.1 FTIR spectra analyses of fine coal before andcoal and pyrite by controlling certain operationalafter surface modificationconditions. As a result, the purpose of desulfurizationUsing the reductant method and metal corrosionin flotation can be achieved by decreasing the flotati- interaction method to modify the coal, the FTIP spec-biliite surfaces and enhancing that of coal tra of fine coal before and after treatment are shownin Fig. I. We can learn from Fig. I that the changes ofcharachave some features in2 Experimental Materials and Conditions中国煤化工f→ HOH weakens at36002.1 MaterialsCNMHGon peak of >C=0weak17U-I0V cn 5)une absorption peakGas coal samples taken from Cacheng Coal of -s-S-,SH weakens slightly at 540-420Received 20 August 2005: accepted 15 October 2005Project 50174054 supported by the National Natural Science Foundation of ChinaCorrespsdisxutor. Tel: +86-10-51688469, +86-13161080285: E-mail address: zhuhol280263netZHU Hong et alSurface Modification Mechanism of Fine Coal by Electrochemical Metcm". These results show that both the reductant 4 Electrochemical Reaction Mechanismsmethod and metal corrosion interaction method canof Coal Surface Modificationdecrease the content of oxygen-containing functionalgroups on coal surfaces and enhance the hydrophoOne of main processes included in all electro-bicity of coalchemical reactions is the transfer of electrons, whichleads to changes in the electron cloud and the splitting of chemical bonds, followed by formation of newchemical bonds. The main fraction of coal has anganic aromatic-cycle structure, so an electro- chemi-cal reaction in coal belongs to the class of organicelectrochemical reactions, essentially a redox reac4.1 Mechanism of -OH group decrease by elec-3600280020001200400trochemical methodsWave number (cm")It can be seen in Fig. I and Fig. 2 that the -OHFig. 1 FTIR spectra of gas coal by differentmodification methodsgroups decrease in coal structures. That is, the ab-eak of -oh at 3400 cm weakens after1# is the coal sample without treatmenis the coal sample treated with the reductant method;treatment by the reductant method and by the metal3# is the coal sample treated with metal corrosion interaction reductioncorrosion interaction method. The peak is characteristic of -oh3.2 Raman spectra analyses of fine coal before boxyl group or from hydrogen bond association reac-and after surface modificationtions. There are two reasons to explain this phnomenonRaman spectra before and after treatment by1)Hydration reaction. Coal mined and exposedelectrochemical reduction are shown in Fig. 2. Line I to air absorbs moisture very easily, and as a result, ais the Raman spectra of raw coal. Results show that hydration reaction takes place on the surface(shownthere are some changes in peak-intensity and in Fig 3)to increase -OH groups, which can affectpeak-width after the reaction, in particular the weak- the double-electric-layer and decrease the floatabilityening of the absorption peak of-OH at 34 cm and of coal.-Oh groups that are produced afterward,of >c=0 at 1000 cm. Because black suspension isnot innately, have an infirm connection with the hystrawy capable of absorbing laser beams with a drogen bond in coal, so they can be easily removedwavelength of 514.5 nm, Raman spectra provide us from coalwith representative characteristics of the sample surface. From the Raman spectra we see that electro-2)Reduction. Innate -Oh groups in coal arechemical reduction can decrease the oxygencontaiing reduced by a reductant or by attaining electrons afterfunctional groups in coal structure, modify its surface, treatment by electrochemical methods. There may beand increase its surface hydrophobicity. The FTIRthe following reactionstra above also verify the same changes. The re-Coal-CH2-OH +RedH-+Coal-CH3+Oxof Raman and FTIR spectra confirm each otherCoal-CHyOH+H+e -+Coal-CH3 + H2OResearch results indicate that electrochemical reduc- where Red h is the reductant and Ox is an oxidatedtion is an effective surface modification methodstate substance called a reductant after oxidationC-OH≥600OH>C=0+H2O+- C-OHO+H,0+=C4000=C-OH=C-OH中国煤化工)0action on coal surfaces30004000CNMHGRaman gain(cm))4.2 Surface modification mechanism of coalFig 2 Raman spectra of gas-coal before andtreated by the reductant methodafter electrolysis reductionIt can be seen from analyses of FTIR spectthat long frame coal, gas coal and fat coal have a betJ. China Univ of Mining& Tech(English Edition)Vol 16 No. 2ter reduction effect than coke coal and lean coal. The formed through wire and electrodes In the circuit, thehigher the coal rank, the fewer the innate oxygen- electric current outside the electrolysis cell has ancontaining functional groups; the more firm the con- effect on the electric current of the solution inside thenection force, the more difficult the reduction per- cell. The electrolyte in the solution becomes ionsformed. Take low rank gas coal as an example, which which flow to form an ion electric current. Thus, thecontains many functional groups such as -OH, -Oelectron electric current and ion electric current form>C=O. -COOH and-s-, and -SH in its a complete electric circuit. Substrate substances oncoal structures. Using C6H6O3 as a reductant, effec- the anode are oxidized by loss of electrons to formtive collision will take place between the surface of oxidized products and substrate substances oncoal particles and the reductant and the -OH bond in cathode are reduced by attaining electrons to formthe coal structure will be split and combined with H reduced products. All of the above-mentioned oxidafrom the reductant to form the resultant h,o. there tion and reduction reactions are conducted on the ba-may be the following reactions:sis of Faraday's law. In fact, electrolysis reactions areCoal-COOH C6H3(OH)3made up of a series of processes such as electro-Coal-CHO + C6H4O2+H2O,chemistry(electrons flowing), chemistry and physicsCoal-CHO +C6H3(OH)3These complicated electrolysis reactions can be→Coal-CH2OH+C6HO2,plained as shown in Fig 4Coal-CHr-OH + C6H3(OH)Coal-CH3+C6H4O2 +H2OThe absorption peak of-OH at 3400 cmwa二,·P4,Pweakens, that of >c=o at 1700 cm also weakensand that of -CH, and -CH3 at 2999 cm and 2872cm enhance, respectively. Results show that >C=OFig 4 Reaction process of electrodeand-OH groups are reduced to-CH2 and -CH3I)Substrate substance S in the electrolysistion reaches the electrode surface by diffusing anc4.3 Surface modification mechanism of coal swimming: 2)S of electrode surface becomes antreated by metal corrosion interactiontive intermediate( due to chemical reaction; 3)of the electrode surface becomes an absorptioIt is well known that active metal can produce a termediate I,ad because of physical or chemical ab-reducing atmosphere. Micro-batteries will be formed sorption; 4)I, d of electrode surface forms a new ab-when active metal particles contact with coal particles sorption intermediate I"ad through electron transfer orbecause active metal has a lower balance potential electrochemical process; 5)I"ad of the electrode surthan does pulp. While the oxidation reaction is taking face or interface becomes the absorption resultant Paplace with the anode metal, electrons transfer to the by chemical reaction; 6)Pad forms deabsorption reathode coal surface and oxygen-containing func- sultant P by deabsorption: 7)P diffuses or swims intional groups in the coal are reduced. With the help of electrolysis solution, and then the entire electrolysFTIR spectra, it is shown that the >C=0 and -OH reaction is completed. Hence, the processes 1)and 7)groups in coal structures decrease and so do the mentioned above are physical ones, processes 2),4)-CH2 and -CH3 groups. The reaction mechanism and 5) are chemical ones and processes 3)and 6)aremay be the following reactionsabsorptive ones. The electron transfer process 4)isAnode (meta):M→Mthe most important action of electrochemistry in theseCathode: Coal -C0 +ne +Helectrolysis reactionsCoal-CH3+ OHThere may be three models to explain the elecCoal -CO+ne- Coal-H+COtrolysis reduction mechanism on coal surfacesCoal -cooh+ne- Coal-H+co1)Electron transfer mechanismwhere M is the metalCoal particles on cathode surface directly ac-This shows that oxygen-containing functionalquire electrons and the entire reaction process is rep-groups will be removed from the coal surface after resented in Fig. 5ontact with active metal to form micro-batteriesThus, this increases hydrophobicity of coalChemicalProductCoal中国煤化工∞→p4.4 Surface modification mechanism of coaltreated by electrochemical reductionCNMHGFig. 5 The entire reaction process of coal particlesElectrochemical reduction is carried out in anlectrolysis cell in which oxidation and reduction takeplace on the anode and cathode separately. An elec-Coal particles diffuse from the solution to thetron electie current outside the electrolysis cell is cathode surface, accompanied by conducting electronZHU Hong et alSurface Modification Mechanism of Fine Coal by Electrochemical Methodstransfer and chemical reactions. Then reaction prodnNa+ne→nNaucts rediffuse from the cathode surface to the solutionn Na+[coal]-n Na+Icoall"The following reactions may be deducedO Reduction of ether-bondIcoal+H'→ coal] hRCH2-O?R5 Conclusionshrch3 +r CH3 +H,O2 Reduction of hydroxyl groupThe modification mechanisms of coal surfacesROH +2H++→RH+H2O.can be explained by three electrochemical reduction3 Reduction of carbonyl groupmethods1) Deoxygen reactions+2H+2.k)CHOH+10Reduction of ether -bonda Reduction of carboxyl groupRCH2-0-CHR+ 4H]RCooH +h+e→HRCH3+RCH3+H20RH+CO2↑,RCooH +h+eRCHO+H2OReduction of carboxyl group:RORCOOH+[H]→RH+CO2RCHo+H+eCH2OHRCH2OH+H"+e—→RCH3+H2OReduction of carbonyl group:2)Active H actionR—CO_R+2(H→R—CHOH-R'+H2OFirst, H2O conducts an electrolysis reaction onReduction of phenol hydroxyl groupthe cathode surface and releases active free H- whichROH+2[H]→RH+H2Ohas high energy2)Desulfurization reactionsH"+e→HR-SH+NaOH→R_SNa+H2OThen active H and oxygen-containing functional→RH+NaHS+H2Ogroups such as-OH, >C=O and -COoH on the3)Under certain electrochemical conditions, thecoal surface conduct chemical reactionsmodification of the coal surface can decrease theAr-OH+H→[Ar]+H2O,oxygen-containing functional groups in the coalAr-CO+H→[Ar]+H2Ostructure and improve the floatability of coalAr-COOH+H.→[Ar+H2O,[Ar·]+HArH,Acknowledgement[Ar]+[Ar]→Ar-A3)Supporting electrolyte actionFinancial support provided by National NaturalScience Foundation of China(Project 50174054)forWhen NaCl plays a role as a supporting electro- this work is gratefully acknowledgedlyte, the reaction mechanism may be the followingreactionsReferences[] Lee k T, Bhatia S, Mohamed A R. Removal of sulfur dioxide using absorbent synthesized from coal fly ash: Role of oxygenand nitrogen oxide in the desulfurization reaction. Chemical Engineering Science, 2005, 60(12): 3419-3423.[2] Zhu H, Ou ZS. Study on the Mechanism of Pyrite Surface Oxidation and Coal Desulfurization in Flotation. Taiyuan: ShanxiScience and Technology Press, 1999: 1275-12781 QiJ w,Zhu S Q. Xie ww. Desulphurization and deashing of ultrafine coal using selective bilateral flocculation. Journal ofChina University of Mining& Technology, 2005, 34(2): 156-159.(In Chinesej Zhang L, Sato A, Ninomiya Y, et al. In situ desulfurization during combustion of high-sulfur coals added with sulfur capturesorbents.Fuel,2003,82(3):255-266[5] Tao Y J, Lu M X, Wang Z Y, et al. An entrapment model of water flow on fine coal flotation. Journal of China University ofMining Technology 2004, 14(1):1-56] LiDT, Li w, Li B Q In situ diffuse reflectance FTIR study on water in lignite. Chemical Journal of Chinese Universities2002,23(12):2325-2328.( n chinese)[7] Zhu H, Yang Y F, Zhao W. Study of enhancing desulphurization mechsulphur coal in flotation by electrolyticreduction. Journal of China University of Mining Technology, 2003中国煤化工[8] Sui J C, Xu M H, Qiu J H, et al. Factors influencing conversion of pradiation. Journal ofChina University of Mining Technology, 2005, 15(1): 16-20CNMHG