Depolarised gas anodes for aluminium electrowinning

藏巴science DirectTransactions ofNonferrous MetalsSociety of ChinaELSEVIER PressTrans. Nonferrous Met. Soc. China 20(2010)2152-2154www.tnmsc.cnDepolarised gas anodes for aluminium electrowinningGMHAARBERG.E KVALHEIM. A.P. RATVIKSJ. XIAO T. MOKKELBOST1. Department of Materials Science and Engineering,Norwegian University of Science and Technology(NTND), No-7491 Trondheim, Norway;2. SINTEF Materials and Chemistry, NO- 7465 Trondheim, NorwayReceived 23 October 2009; accepted 30 March 2010Abstract: Consumable carbon anodes are used in the electrowinning of aluminium by the Hall-Heroult process. Emissions of cO2may be eliminated by introducing an inert oxygen evolving anode, which however will require a higher anode potential. Analternative approach is to use a natural gas or hydrogen gas anode to reduce the CO2 emissions and lower the anode potentialPreliminary laboratory experiments were camied out in an altermative molten salt electrolyte consisting of CaCIz-Cao-NaCl at 680CPorous anodes of platinum and tin oxide were tested during electrolysis at constant current. The behaviour of inert anode candidatematerials such as tin oxide and nickel ferrite were also studiedKey words: anodes; natural gas; aluminium electrowinningcurrent efficiency, energy consumption and cell size1 Introduction(amperage) have been greatly improved. However, theprocess still suffers from the use of consumable carbonAluminium is produced solely by the Hall-Heroult anodes, which leads to emissions of CO2 of about 1.5 kgprocess, which was patented independently by Hall and per kg Al in addition to the extra cost and labour forHeroult in 1886. The overall primary cell reaction isproducing and replacing anodes. Considerable researchefforts have not led to the development of an inert2 Al203(diss)+=(s)Al(+cO2(g)(1) oxygen evolving anode, although some candidatematerials have been identified. Among them SnOz andThe standard Gibbs energy is 689.309 kJ/mol at 960C. nickel ferrites are considered promising. By using anPure anhydrous solid alumina is dissolved in a molten inert anode the cell reaction becomesfluoride electrolyte based on cryolite(Na3AlF6)containingsome AIF, as well as CaF2lll. Modern cells are equipped IAl2O3(diss -Al()+=O2(g)with so called prebaked carbon anodesrating at955-965oC. The current efficiency with respect to with a standard Gibbs energy of 1 283. 316 kJ/mol at 960aluminium can be as high as 96% and the corresponding C. Hence, the decomposition voltage will increase byenergy consumption may be -14 kW.h/kg Al in cells about 1 V when using an inert anode. This increase mayrunning at-300 ka or higher. The annual production of be compensated by being able to redesign the cellsprimary aluminium was about 38 million tons in 2007, reduce the ohmic voltage drop in the electrolytemaking it the most important electrowinning process[2An alternative way of reducing the anodic coAlthough the basic principles of the process remain production is to use an oxidizable gas to depolarise theunchanged, great technological developments have taken anode process. By introducing methane gas to the anode,place. The main improvements are related to the reaction can change toenvironmental issues. Scrubbing of the exit gas hasminimized the emissions of harmful and toxic gaseous Iand particulate constituents to the working atmosphere22H:4=AO21H2Oand to the air. also some key performance data such asCNMHGCorresponding author: G M. HAARBERG; Tel: +47-95071825: E-mail: geir. m haarberg @materialntnu.noDol:l0.1016S10036326(09)604349G.M. HAARBERG, et al/Trans. Nonferrous Met. Soc. China 20(2010)2152-21542153having a stibbs energy of 683.080 kJ/mol at 960 that the gas was forced through the anode. The desired°C. Usingarising gas such as methane will also anode process should take place at the three phasepotential and hence lower the energy boundary between gas, anode and electrolyte. The anodepotential was recorded by using a Ag/AgCl referenceIf hydrogen gas is supplied to the anode, the electrode consisting of a mullite tube containing thereaction will becomemolten salt electrolyte with AgCl and a Ag wire1A2o3(is)+3x12)=A0+3Og)(4)Ag/AgCl reference electrodeSIn such a case, hydrogen will be produced from methaneIn a separate process.The use of renewable electrical energy is of greatimportance, since electrolysis relielectrical energy. The total COz emissions associatedt or snCathodewith electrowinning are more dependent on the source ofelectricity. Today, the average global COz emissionscoming from the generation of electricity is about 550 gMoltenCO,/(kW.h), and this figure must be reducedCaCI.-Caoconsiderably in the future if lowering of the COz fromthe electrowinning process itself will be significant.Fig 1 Schematic diagram of experimental electrochemical cellRefs. [3-5]. CHEN et al[] have proposed a new concept 3 Results and discussionfor electrowinning of metals and alloys from molten saltsMolten calcium chloride can dissolve large amounts ofSeveral experiments were carried out by introducingcalcium oxide, while the solubility of other oxides may methane gas to the anode during electrolysis. However,be low. By attaching a solid metal oxide to the cathode, no change in the anode potential was observed, and itoxide ions dissolve in the electrolyte whereas the metal is was concluded that the kinetics of the methane oxidationreduced without going into solution By using an inert reaction must be very slow. Therefore, methane wasoxygen evolving anode,cept is potentiallyreplaced by hydrogen as depolarising gasinteresting for industrial use. The production of sevElectrochemical studies confirmed that O2(g) wasmetals has been demonstrated in laboratory and pilotplant experiments. Other researchers have also published evolved on both Pt and SnO2 anodes. Fig. 2 shows theexperimental results from similar studies[7-8](a)2 Experimental区三04Laboratory experiments were carried out underNoArH2controlled conditions at 680C Model electrolytes basedon molten CaCIz containing Cao were used in theseexperiments instead of the corrosive molten fluorideelectrolyte used during aluminium electrowinning. AlsoAgCI was added in order to control the cathode reaction,which would be silver deposition rather than calciumformation. Experiments were carried out204atmosphere of dry argon to avoid the introduction andcontamination by moisture and oxygen02Supply of pure methane gas and hydrogen gas was madeduring electrolysis to study the effect of depolarising theanode process using oxygen evolving anodes of platinum中国煤化工nd Sno. The behaviour of inert anodes of tin oxide and Fig.2CNMHtential(a) and cellnickel ferrite was studied in separate experimentspotentat 0. 1 A(25 mA/cm)The experimental set-up is shown in Fig. 1. The with Pt anode in CaClr-Naci(70%0-30%) -Cao(15.4%)anode was porous with a gas tight seal to the inlet tube so AgCl(3, 6%)melts at 953 K and gas flow rate of 10 cm/minG M. HAARBERG, et al/Trans. Nonferrous Met. Soc. China 20(2010)2152-2154anode potential and cell voltage during galvanostatic ferrites have been proposed for aluminiumelectrolysis in molten CaCl -NaCI-CaO-AgCl using a Pt electrowinning NiFe2O4 anodes with addition of Nio, Nanode. The anode potential was found to be stable and and Cu were prepared and tested during electrolysis. Theconstant in argon atmosphere and during argon supplymetal phase of the anodes corroded badly, but Niosignificant depolarisation was observed after hydrogen seemed to give a better performance of the anodessupply, with the potential change being about 0.3-0.5 V. Pre-electrolysis using the anodes led to better behaviourA certain delay time for the action of hydrogen of about and less corrosion. The results from these studies have10 min was observed. This is due to the rather slow gas been published [9]flow rate through the anodeFig 3 shows the anode potential and cell voltage 4 Conclusionsduring galvanostatic electrolysis in the molten CaCl2-Cao based electrolyte using a SnO, anode. The anode1) Laboratory studies showed that by introducingpotential was found to be fairly constant during argonsupply through the anode. After a delay of about 15 min, electrolysis in molten CaCl2-Cao based electrolytes, thethe anode potential was found to decrease by about o. I V. anode potential was significantly lowered(0.3-0.5 v)The anode potential was unaffected by doubling the flow using a platinum anode. Using an inert tin oxide anoderate of hydrogen. It is not clear why the response of the the potential was lowered by about 0.1vSnOz anode was less pronounced than the Pt anode.2) The results are promising for the prospects ofHowever, changes in the active electrode area due to reducing the CO2 emissions and the energy consumptiondifferent wetting properties of the two anode materialsng of aluminium by delmay be an explanation.anode process using hydrogen or methane gasOther candidate inert anode materials were alsotested in molten Cackz-Cao based electrolytes. Nickel References[ THONSTAD J, FELLNER P, HAARBERG G M, HIVES JKVANDE H. STERTEN AFundamentals of the Hall-heroult process M]. Dusseldorf:⊙04Aluminium-Verlag, 2001: 353.[2]U.s.GeologicalSurvey.2010-09-28.http://www.usgs.gov/810 cn10 'min 20 cmp[3] FERRAND M L. Bulletin de la societe francaise des electricians [].letin of the French Society of Electricity, 1957, 79: 412-4[4] STENDER VV, TROFIMENKO V V. One solution to the anodeoblem in electrolytic production of aluminum [] Khim TekhnoLt/ks1969,12:41-45[] KRONENBERG M L, Gas depolarization graphite anodesaluminium electrowinning []. Joumal of the Electrochemical Society,9,116:160110.6[6] CHEN G Z, FRAY D J, FARTHING T W. Direct electrochemicalreduction of titanium dioxide to titanium in molten calcium chloride04H210 cm/min10cm/min 20 cm/min[7 SUZUKI R O, TERANUMA K, ONO K Calciothermic reduction oftitanium oxide and in-situ electrolysis in molten CaCh[8] OKABE T H, WASEDA Y. Producing titanium through anelectronically mediated reaction [] Journal of Metals, 1997, 49:Fig3 Time dependencies of anode potential(a) and cellpotential difference (b)during electrolysis at 0.06 A (15 9) KVALHEIM E, HAARBERG GM,MARTINEZAM, JAHRENHMinert anodes foren evolution in molten salts [). ECSTransactions, 2009. 16: 367-374(70%309%)CaO(16%}Agc(4.5% melts at953K(Edited by YANG Bing)中国煤化工CNMHG