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`
`(cid:61)(cid:76)(cid:81)(cid:70)(cid:3)(cid:40)(cid:79)(cid:72)(cid:70)(cid:87)(cid:85)(cid:82)(cid:90)(cid:76)(cid:81)(cid:81)(cid:76)(cid:81)(cid:74)(cid:3)(cid:58)(cid:76)(cid:87)(cid:75)(cid:3)(cid:42)(cid:68)(cid:86)(cid:3)(cid:39)(cid:76)(cid:73)(cid:73)(cid:88)(cid:86)(cid:76)(cid:82)(cid:81)(cid:3)(cid:36)(cid:81)(cid:82)(cid:71)(cid:72)(cid:86)(cid:29)(cid:3)(cid:54)(cid:87)(cid:68)(cid:87)(cid:72)(cid:3)(cid:82)(cid:73)(cid:3)(cid:87)(cid:75)(cid:72)(cid:3)(cid:36)(cid:85)(cid:87)(cid:3)(cid:68)(cid:81)(cid:71)(cid:3)(cid:41)(cid:88)(cid:87)(cid:88)(cid:85)(cid:72)
`(cid:39)(cid:72)(cid:89)(cid:72)(cid:79)(cid:82)(cid:83)(cid:80)(cid:72)(cid:81)(cid:87)(cid:86)
`
`(cid:38)(cid:87)(cid:89)(cid:78)(cid:72)(cid:81)(cid:74)(cid:100)(cid:100)(cid:78)(cid:83)(cid:100)(cid:100)(cid:40)(cid:70)(cid:83)(cid:70)(cid:73)(cid:78)(cid:70)(cid:83)(cid:5)(cid:50)(cid:74)(cid:89)(cid:70)(cid:81)(cid:81)(cid:90)(cid:87)(cid:76)(cid:78)(cid:72)(cid:70)(cid:81)(cid:5)(cid:54)(cid:90)(cid:70)(cid:87)(cid:89)(cid:74)(cid:87)(cid:81)(cid:94)(cid:5)(cid:123)(cid:5)(cid:52)(cid:72)(cid:89)(cid:84)(cid:71)(cid:74)(cid:87)(cid:5)(cid:23)(cid:21)(cid:21)(cid:22)
`
`(cid:41)(cid:52)(cid:46)(cid:31)(cid:5)(cid:22)(cid:21)(cid:19)(cid:22)(cid:22)(cid:28)(cid:30)(cid:20)(cid:72)(cid:82)(cid:86)(cid:19)(cid:23)(cid:21)(cid:21)(cid:22)(cid:19)(cid:25)(cid:21)(cid:19)(cid:25)(cid:19)(cid:25)(cid:26)(cid:30)
`
`(cid:40)(cid:46)(cid:57)(cid:38)(cid:57)(cid:46)(cid:52)(cid:51)(cid:56)
`(cid:28)
`
`(cid:27)(cid:5)(cid:70)(cid:90)(cid:89)(cid:77)(cid:84)(cid:87)(cid:88)(cid:17)(cid:5)(cid:78)(cid:83)(cid:72)(cid:81)(cid:90)(cid:73)(cid:78)(cid:83)(cid:76)(cid:31)
`
`(cid:50)(cid:70)(cid:88)(cid:88)(cid:78)(cid:82)(cid:78)(cid:81)(cid:78)(cid:70)(cid:83)(cid:84)(cid:5)(cid:39)(cid:74)(cid:88)(cid:89)(cid:74)(cid:89)(cid:89)(cid:78)
`(cid:53)(cid:84)(cid:81)(cid:78)(cid:89)(cid:74)(cid:72)(cid:83)(cid:78)(cid:72)(cid:84)(cid:5)(cid:73)(cid:78)(cid:5)(cid:50)(cid:78)(cid:81)(cid:70)(cid:83)(cid:84)
`
`(cid:22)(cid:23)(cid:29)(cid:5)(cid:53)(cid:58)(cid:39)(cid:49)(cid:46)(cid:40)(cid:38)(cid:57)(cid:46)(cid:52)(cid:51)(cid:56)(cid:100)(cid:100)(cid:100)(cid:22)(cid:17)(cid:22)(cid:26)(cid:23)(cid:5)(cid:40)(cid:46)(cid:57)(cid:38)(cid:57)(cid:46)(cid:52)(cid:51)(cid:56)(cid:100)(cid:100)(cid:100)
`
`(cid:56)(cid:42)(cid:42)(cid:5)(cid:53)(cid:55)(cid:52)(cid:43)(cid:46)(cid:49)(cid:42)
`
`(cid:55)(cid:42)(cid:38)(cid:41)(cid:56)
`(cid:22)(cid:28)(cid:30)
`
`(cid:44)(cid:74)(cid:84)(cid:75)(cid:75)(cid:5)(cid:45)(cid:19)(cid:5)(cid:48)(cid:74)(cid:81)(cid:88)(cid:70)(cid:81)(cid:81)
`(cid:46)(cid:82)(cid:85)(cid:74)(cid:87)(cid:78)(cid:70)(cid:81)(cid:5)(cid:40)(cid:84)(cid:81)(cid:81)(cid:74)(cid:76)(cid:74)(cid:5)(cid:49)(cid:84)(cid:83)(cid:73)(cid:84)(cid:83)
`
`(cid:22)(cid:26)(cid:25)(cid:5)(cid:53)(cid:58)(cid:39)(cid:49)(cid:46)(cid:40)(cid:38)(cid:57)(cid:46)(cid:52)(cid:51)(cid:56)(cid:100)(cid:100)(cid:100)(cid:24)(cid:17)(cid:25)(cid:22)(cid:23)(cid:5)(cid:40)(cid:46)(cid:57)(cid:38)(cid:57)(cid:46)(cid:52)(cid:51)(cid:56)(cid:100)(cid:100)(cid:100)
`
`(cid:56)(cid:42)(cid:42)(cid:5)(cid:53)(cid:55)(cid:52)(cid:43)(cid:46)(cid:49)(cid:42)
`
`(cid:56)(cid:84)(cid:82)(cid:74)(cid:5)(cid:84)(cid:75)(cid:5)(cid:89)(cid:77)(cid:74)(cid:5)(cid:70)(cid:90)(cid:89)(cid:77)(cid:84)(cid:87)(cid:88)(cid:5)(cid:84)(cid:75)(cid:5)(cid:89)(cid:77)(cid:78)(cid:88)(cid:5)(cid:85)(cid:90)(cid:71)(cid:81)(cid:78)(cid:72)(cid:70)(cid:89)(cid:78)(cid:84)(cid:83)(cid:5)(cid:70)(cid:87)(cid:74)(cid:5)(cid:70)(cid:81)(cid:88)(cid:84)(cid:5)(cid:92)(cid:84)(cid:87)(cid:80)(cid:78)(cid:83)(cid:76)(cid:5)(cid:84)(cid:83)(cid:5)(cid:89)(cid:77)(cid:74)(cid:88)(cid:74)(cid:5)(cid:87)(cid:74)(cid:81)(cid:70)(cid:89)(cid:74)(cid:73)(cid:5)(cid:85)(cid:87)(cid:84)(cid:79)(cid:74)(cid:72)(cid:89)(cid:88)(cid:31)
`
`(cid:42)(cid:83)(cid:77)(cid:70)(cid:83)(cid:72)(cid:78)(cid:83)(cid:76)(cid:5)(cid:89)(cid:77)(cid:74)(cid:5)(cid:70)(cid:73)(cid:77)(cid:74)(cid:88)(cid:78)(cid:84)(cid:83)(cid:5)(cid:84)(cid:75)(cid:5)(cid:73)(cid:78)(cid:70)(cid:82)(cid:84)(cid:83)(cid:73)(cid:18)(cid:81)(cid:78)(cid:80)(cid:74)(cid:5)(cid:72)(cid:70)(cid:87)(cid:71)(cid:84)(cid:83)(cid:5)(cid:75)(cid:78)(cid:81)(cid:82)(cid:88)(cid:5)(cid:89)(cid:84)(cid:5)(cid:88)(cid:89)(cid:74)(cid:74)(cid:81)(cid:5)(cid:88)(cid:90)(cid:71)(cid:88)(cid:89)(cid:87)(cid:70)(cid:89)(cid:74)(cid:88)(cid:5)(cid:90)(cid:88)(cid:78)(cid:83)(cid:76)(cid:5)(cid:88)(cid:78)(cid:81)(cid:78)(cid:72)(cid:84)(cid:83)(cid:18)(cid:72)(cid:84)(cid:83)(cid:89)(cid:70)(cid:78)(cid:83)(cid:78)(cid:83)(cid:76)(cid:5)(cid:78)(cid:83)(cid:89)(cid:74)(cid:87)(cid:81)(cid:70)(cid:94)(cid:74)(cid:87)(cid:88)(cid:5)(cid:59)(cid:78)(cid:74)(cid:92)(cid:5)(cid:85)(cid:87)(cid:84)(cid:79)(cid:74)(cid:72)(cid:89)
`
`(cid:50)(cid:84)(cid:81)(cid:89)(cid:74)(cid:83)(cid:5)(cid:57)(cid:78)(cid:83)(cid:5)(cid:38)(cid:83)(cid:84)(cid:73)(cid:74)(cid:88)(cid:5)(cid:75)(cid:84)(cid:87)(cid:5)(cid:40)(cid:70)(cid:87)(cid:71)(cid:84)(cid:83)(cid:18)(cid:38)(cid:78)(cid:87)(cid:5)(cid:56)(cid:52)(cid:43)(cid:40)(cid:88)(cid:5)(cid:59)(cid:78)(cid:74)(cid:92)(cid:5)(cid:85)(cid:87)(cid:84)(cid:79)(cid:74)(cid:72)(cid:89)
`
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`
`(cid:57)(cid:77)(cid:74)(cid:5)(cid:90)(cid:88)(cid:74)(cid:87)(cid:5)(cid:77)(cid:70)(cid:88)(cid:5)(cid:87)(cid:74)(cid:86)(cid:90)(cid:74)(cid:88)(cid:89)(cid:74)(cid:73)(cid:5)(cid:74)(cid:83)(cid:77)(cid:70)(cid:83)(cid:72)(cid:74)(cid:82)(cid:74)(cid:83)(cid:89)(cid:5)(cid:84)(cid:75)(cid:5)(cid:89)(cid:77)(cid:74)(cid:5)(cid:73)(cid:84)(cid:92)(cid:83)(cid:81)(cid:84)(cid:70)(cid:73)(cid:74)(cid:73)(cid:5)(cid:75)(cid:78)(cid:81)(cid:74)(cid:19)
`
`Tennant Company
`Exhibit 1022
`
`

`

`459
`
`Canadian Metallurgical Quarterly, Vol 40, No 4 pp 459-469, 2001
`© Canadian Institute of Mining, Metallurgy and Petroleum
`Published by Canadian Institute of Mining, Metallurgy and Petroleum
`Printed in Canada. All rights reserved
`
`ZINC ELECTROWINNING WITH GAS DIFFUSION ANODES:
`STATE OF THE ART AND FUTURE DEVELOPMENTS
`
`M. BESTETTI1, U. DUCATI1, G. KELSALL2, G. LI3, E. GUERRA4 and R. ALLEN5
`
`1Department of Applied Physical Chemistry, Polytechnic of Milan 20131, Milan, Italy
`2T.H. Huxley School, Imperial College London SW7 2BP, UK
`3Cominco Research, Cominco Ltd. Trail, British Columbia, Canada, V1R 4S4
`4Department of Metals and Materials Engineering, University of British Columbia
`Vancouver, British Columbia, Canada, V6T 1Z4
`5E-TEK Division, De Nora North America Inc., Ashland, MA, 01721, USA
`
`(Selected from: Electrometallurgy 2001 Symposium, 31st Annual Hydrometallurgical Meeting, Editor A. Gonzalez)
`
`Abstract — Hydrogen gas diffusion anodes (HGDA) have been considered previously as alternatives to
`oxygen-evolving lead alloy anodes in metal electrowinning. Despite their higher capital costs, the sub-
`stantial decrease in electrowinning production costs potentially offered by HGDAs is causing renewed
`interest in their industrial use. This paper reviews the more recent applications of hydrogen gas diffusion
`anodes in industrial electrowinning. Using data from the literature, a simplified economical assessment
`is presented for a zinc electrowinning tank house implementing hydrogen gas diffusion anodes; optimum
`operating current densities of about 3 kA m-2 are predicted.
`
`Résumé — Des anodes de diffusion de gaz hydrogène (ADGH) ont été considérées antérieurement
`comme remplacement des anodes d’alliage au plomb produisant de l’oxygène pour l’extraction
`électrolytique du métal. En dépit de leur coût plus élevé en capital, la diminution substantielle des coûts
`de production par extraction électrolytique offerte potentiellement par les ADGHs suscite un intérêt
`renouvelé pour leur utilisation industrielle. Cet article révise les applications les plus récentes d’anodes
`à diffusion de gaz hydrogène dans l’extraction électrolytique industrielle. En utilisant les données de la
`littérature, on présente une évaluation économique simplifiée d’un ensemble de cellules d’extraction
`électrolytique de zinc ayant mis en application les anodes à diffusion de gaz hydrogène; on prédit des
`densités de courant d’opération optimum d’environ 3 kA m-2.
`
`INTRODUCTION
`
`Conventional electrowinning of zinc from aqueous elec-
`trolytes involves water decomposition at oxidized lead alloy
`anodes. This results in the evolution of oxygen, an intrinsi-
`cally slow and therefore energy intensive process.
`Increasing environmental and economic pressures have and
`will continue to heighten pressures to decrease energy con-
`sumptions in such processes. In aqueous electrowinning pri-
`marily, this involves decreasing the anode thermodynamic
`and overpotential contributions to the cell voltage, hence
`decreasing specific electrical energy consumptions [1].
`
`One approach to this objective in conventional zinc
`electrowinning is to replace the oxygen evolving lead
`alloy anodes with hydrogen oxidation to hydrogen ions in
`gas diffusion anodes1. These multiphase structures have
`been developed primarily for use in fuel cells and the
`
`engineering is even more challenging in electrowinning
`processes.
`
`At present, hydrogen is produced from a variety of
`feedstocks (natural gas, coal, biomass and water) by using
`several technologies (reforming, gasification and electroly-
`sis). However, steam reforming of methane is the most
`common and least expensive method for hydrogen produc-
`tion accounting for about 48% of world hydrogen produc-
`tion with a unit cost of $.60 – 1.14 (U.S.)kg-1 [2]. Hydrogen
`is also the main byproduct of the chlor-alkali industry. In
`1999, the world chlorine use was estimated as 42.8 million
`tonnes [3], resulting in co-production of 1.2 million tonnes
`of hydrogen [4]. In principle, this corresponds to ca. 35
`
`1 The standard Gibbs free energy change values, at 25 °C, for the reactions
`ZnSO4 + H2O = Zn + 1/2O2 + H2SO4 and ZnSO4 + H2 = Zn + H2SO4 are,
`respectively, DG∞ = 384.159 kJ/x (E∞ = 1.99 V) and DG∞ = 147.018 kJ/x
`(E∞ = 0.76 V).
`
`CANADIAN METALLURGICAL QUARTERLY
`
`

`

`460
`
`M. BESTETTI, U. DUCATI, G. KELSALL, G. LI, E. GUERRA and R. ALLEN
`
`million tonnes per annum of zinc that could be produced at
`a current efficiency of 0.9 with hydrogen diffusion anodes;
`zinc production in 1999 was 7.7 million tonnes [3,5]. For
`example, a zinc plant producing 105 tonnes per annum
`would require 4¥107 Nm3 hydrogen (see below). Hydrogen
`can be transported by pipelines or by truck, rail or ship in
`liquefied or in gaseous form. The primary methods for
`hydrogen storage are compressed gas, liquefied hydrogen,
`metal hydride and carbon based systems. Transportation
`and storage costs must be accounted for in the economical
`assessment of electrowinning processes using gas diffusion
`anodes. A survey of the economics of hydrogen technolo-
`gies can be found in Reference 2.
`
`STRUCTURE OF GAS DIFFUSION ANODES
`
`The structure of a gas diffusion electrode is shown
`schematically in Figure 1 [6]. Gaseous hydrogen permeates
`through the porous structure of the electrode, dissolves into
`the electrolyte and diffuses to the electrocatalyst, when it is
`oxidised to protons. As the performance of hydrogen diffu-
`sion anodes depends mainly on the dissolution of the
`hydrogen gas in the electrolyte and its diffusion towards
`reaction sites, higher hydrogen solubilities and higher
`transport rates improve the performance of such anodes.
`
`In order to attain high current densities, porous elec-
`trodes must satisfy some basic requirements:
`
`1) Catalysts must have high activities.
`
`2) Pores must neither fill with electrolyte because of cap-
`illary action (“flooding”, “percolation”) nor must the
`gas pressure become so large that electrolyte is com-
`pletely expelled from the pores.
`
`3) The shorter the distance from gas dissolution sites to
`catalyst sites, the higher the gas feed rate and conse-
`quently, the higher the sustainable current densities.
`
`The main parts of a gas diffusion electrode are the gas
`supplying layer and the reaction layer. The current collec-
`tor is imbedded in the structure so as to be in electrical con-
`tact with the electrocatalyst region. To prevent electrolyte
`percolation, the side of the electrode exposed to the elec-
`trolyte is made hydrophobic by a layer that constitutes a
`barrier for the electrolyte; electrode performance also
`depends on the characteristics of that hydrophobic layer.
`
`Furuya [27] performed zinc electrowinning experi-
`ments using gas diffusion electrodes prepared by hot press-
`ing together the reaction and the gas supplying layers at
`600 kg cm-2 and 380 °C. The reaction layer was made from
`hydrophobic carbon black (45%), hydrophilic carbon black
`(35%), PTFE (20%) and a platinum catalyst (0.56 mg cm-2).
`The gas supply layer was made from hydrophobic carbon
`black (70%) and PTFE (30%). The thickness of the reaction
`and gas supplying layers was 0.1 mm and 0.5 mm, respec-
`tively. A similar structure was fabricated by Nikolova et al.
`[7,8] for their experiments on zinc and nickel electrowin-
`ning using a gas supplying layer coupled with an active
`layer containing PTFE coated carbon black and tungsten
`
`Gas phase
`
`Zone of high current density
`(Three-phase zone)
`
`Solution
`
`ring-like
`reaction zone
`
`Electrode wall
`
`Gas phase
`
`Electrolyte
`space
`
`long
`
`short
`
`transport paths
`
`Fig. 1. Single pore of a gas diffusion electrode (left) and three-phase zone in a gas diffusion electrode (right). Reproduced by permission of Wiley-VCH.
`
`CANADIAN METALLURGICAL QUARTERLY
`
`

`

`ZINC ELECTROWINNING WITH GAS DIFFUSION ANODES: STATE OF THE ART AND FUTURE DEVELOPMENTS
`
`461
`
`carbide as a catalyst. In both cases, the gas was supplied to
`the diffusion electrode from a gas chamber structure in an
`arrangement similar to that shown schematically in Figure 2.
`
`in preference to tungsten carbide because of their higher
`catalytic activity. Hydrogen was supplied to the carbon
`cloth layer through grooves in the substrate (Figure 4).
`Such anodes are no longer produced by E-TEK.
`
`1
`
`2 H2O
`
`LABORATORY, PILOT
`AND INDUSTRIAL TESTS
`
`H2SO4
`ZnSO4
`
`5
`
`Prototech (U.S.A.) investigated the influence of zinc sul-
`fate and sulfuric acid concentrations, and of current densi-
`ty on cell voltage and energy consumption for zinc elec-
`
`1. Tungsten carbide gas
`diffusion anode
`2. Zinc cathode
`3. Gas chamber
`4. Electrolyte chamber
`5. Water jacket for
`temperature control
`
`H2
`
`H2
`
`1'
`
`7
`
`3 1
`
`1'
`
`5
`
`5
`
`T0
`
`0
`
`H2
`Outlet
`
`0
`
`I
`
`H2
`Inlet
`
`I
`
`Groove
`
`45
`
`41
`
`40
`
`44
`
`G'
`
`G
`
`42
`
`3
`
`4
`
`H2O
`
`Fig. 2. Schematic arrangement of the cell with a gas diffusion electrode
`[7,8].
`
`Instead of using a separate gas plenum structure, it is
`actually more convenient to flow hydrogen over a large
`electrode surface area through a carbon cloth electrode
`substrate. Figure 3 represents the laminated structure of an
`anode used in metal electrowinning by Allen et al. [9,11].
`A thick catalysed carbon cloth coated with an anti-percola-
`tion layer is epoxy bonded to a solid substrate material.
`Lead was generally used as metal substrate in sulfuric acid
`electrolytes and platinum group metals catalysts were used
`
`A B C D E F G
`
`L
`
`I
`
`Inlet
`
`Inlet
`
`Inlet
`
`A. Substrate metal
`B. Priming epoxy
`C. Bonding epoxy
`D. Carbon cloth
`E. Catalyst layer
`F. Polysulfone layer
`G. Microporous
`membrane
`
`Fig. 3. The sequence of laminations applied to the anode substrate (E-
`TEK).
`
`Fig. 4. Examples of gas distribution in plenum-free gas diffusion elec-
`trodes (Allen, Metallgesellschaft AG).
`
`CANADIAN METALLURGICAL QUARTERLY
`
`43
`
`

`

`462
`
`M. BESTETTI, U. DUCATI, G. KELSALL, G. LI, E. GUERRA and R. ALLEN
`
`trowinning using hydrogen diffusion anodes which resulted
`in a patented process [12]. For 100 g dm-3 H2SO4 at 55 °C
`and a current density of 388 A m-2, increasing the Zn(II)
`concentration in the range 0 - 100 g dm-3 increased current
`efficiencies from 0 to 96%, whereas above 100 -
`120 g Zn(II) dm-3, current efficiencies were 95 - 96%
`(Figure 5). Cell voltages increased with increasing Zn(II)
`concentration due to sulfate ions forming hydrogen sulfate
`ions, decreasing proton activities and so increasing elec-
`trolyte resistances. Under such experimental conditions,
`specific electrical energy consumptions were insensitive to
`Zn(II) concentration in the range of 50 - 120 g dm-3.
`
`This value must be compared with the cost of hydrogen
`produced, for example, by steam reforming of methane. If
`the electrolyte contains 300 g H2SO4 dm-3, according to
`Figure 6, the current efficiency of the zinc deposition
`process at 388 A m-2 is 60 % and the cell voltage is ca.
`1.1 V when the energy requirement for Zn electrowinning
`is still low (Figure 7). By comparison, hydrogen production
`in conventional water electrolysis cells requires cell volt-
`ages of 1.8 - 2 V [14,15].
`
`Table I lists contributions to the cell voltage (U) for zinc
`electrowinning, conventionally and using a hydrogen gas
`diffusion anode at 450 A m-2 and pH = 1 [16]. Assuming a
`current efficiency (F) of 0.90 for both processes, then
`Equation 2 gives
`
`(
`SEEC kWh tonne Zn
`
`) =–1
`
`2
`3.6
`
`F U
`MZn
`
`F
`
`(2)
`
`Fig. 6. Effect of H2SO4 concentration on reactor performance at
`50 g Zn(II) dm-3 and 388 A m-2 (Prototech).
`
`Fig. 7. Effect of current efficiency and cell voltage on specific electrical
`energy consumption.
`
`Fig. 5. Effect of Zn(II) concentration on reactor performance at
`100 g H2SO4 dm-3 and 388 A m-2 (Prototech).
`
`Figure 6 shows that increasing H2SO4 concentrations
`decreased current efficiencies because of the enhancement
`of the hydrogen evolution reaction rate. However, their
`effect in decreasing the electrolyte resistance, decreasing
`ohmic potential drops and cell voltages led to a corre-
`sponding decrease in specific electrical energy consump-
`tion which exhibited a minimum at ca. 100 g H2SO4 dm-3.
`Increasing current densities to 776 A m-2 then 967 A m-2
`produced minima at ca. 125 g H2SO4 dm-3 and
`150 g H2SO4 dm-3, respectively. The hydrogen diffusion
`anodes performed well in even more concentrated sulfuric
`acid solutions with the optimum concentration being about
`4 molar [13]. Moreover, the decrease in current efficiencies
`with increasing acid concentration is less of a drawback
`than in conventional zinc electrowinning, as hydrogen
`evolved at the cathode is reusable at the anode. The value
`of the cathodically by-product hydrogen, eH2,by-prod.,
`(US$ kg-1) is given by:
`
`.
`26 8
`Ue
`
`(1)
`
`Ue
`3600
`
`=
`
`ˆ ¯˜
`
`2
`M
`
`F
`
`H
`
`2
`
`Ê ËÁ
`
`=
`
`e
`
`2 ,
`H by – prod
`
`.
`
`CANADIAN METALLURGICAL QUARTERLY
`
`

`

`ZINC ELECTROWINNING WITH GAS DIFFUSION ANODES: STATE OF THE ART AND FUTURE DEVELOPMENTS
`
`463
`
`where MZn is the molar mass of zinc, the specific electrical
`energy consumptions (SEEC) are 3.19 and 1.41 kWh kg-1
`of zinc, respectively. The use of hydrogen gas diffusion
`anodes also affects the heat balance such that significantly
`less electrolyte cooling would be required compared with
`that for conventional electrowinning.
`
`Table I – Components of Zinc Electrowinning Cell
`Voltage (Prototech)
`
`Voltage
`Component
`
`Conventional
`(V)
`
`H2 Depolarised
`(V)
`
`EA + EC
`h
`C
`h
`A
`IR
`Ucell
`
`2.04
`0.06
`0.86
`0.54
`3.50
`
`0.81
`0.06
`0.14
`0.54
`1.55
`
`Ruhr-Zink GmbH, together with Lurgi GmbH and
`Prototech, started the development of hydrogen diffusion
`anodes in the 1980s. They built a demonstration pilot scale
`installation comprising 41 gas diffusion anodes in Datteln,
`Germany with the aim of becoming commercially viable in
`1990 [17,18]. The advantages claimed by Ruhr-Zink
`GmbH for this technology were the decreased energy con-
`sumption and decreased concentrations of acid mist that
`pollute the atmosphere of tank houses and require expen-
`sive ventilation. In addition, improved zinc purity
`(99.999%) was claimed. The deposits were fine, smooth
`and thick and consequently, attractive for the galvanizing
`industry. However, the plant is no longer in operation
`because of the sharp decrease in zinc prices.
`
`Two-sided, free standing hydrogen anodes for
`immersed tank electrolysis were demonstrated in active
`areas of up to 0.6 m wide and 1.0 m deep. These electrodes
`were developed in cooperation with Lurgi AG with a view
`to replace lead anodes in an older tankhouse at Ruhr-Zink
`GmbH in Datteln, Germany. More modern facilities using
`“jumbo” anodes would be more difficult to retrofit.
`However, these anodes were tested at 1500 A m-2; higher
`current densities would have to be supported by higher gas
`flows which could potentially cause excessive internal
`pressure at the point of entry. Hence, the distance of the
`extremities of the electrodes from the entry point of the gas
`feeds is the key factor in their design limiting their immer-
`sion depth. Hydrogen utilization in these anodes is around
`90 - 95%; the balance is lost to the electrolyte without
`being oxidised.
`
`tinuously for a year at 500 A m2 in a typical zinc electrowin-
`ning solution (150 g H2SO4 dm-3 60 - 70 g Zn(II) dm-3, 40
`°C). This was attributed to catalyst poisoning by the carbon
`monoxide impurity in the hydrogen gas, although inorganic
`or organic impurities in the electrolyte may have con-
`tributed; HGDAs would be more prone to these effects than
`conventional lead alloy anodes. The zinc deposits obtained
`were uniform with no dendrite or nodule formation.
`
`Fig. 8. Time dependence of a (E-TEK) hydrogen gas diffusion anode
`operated at 500 A m2 in 150 g H2SO4 dm-3 60 - 70 g Zn(II) dm-3 at 40 °C.
`
`As hydrogen distribution and transport is more facile in
`plate and frame cell designs, higher current densities are
`possible compared with immersed tank construction.
`Electrodes with microporous anti-percolation coatings
`manufactured by E-TEK in a plate and frame configuration
`adopted apparent anode potentials of 300 to 400 mV (SHE)
`(uncompensated for IR) at current densities of 5 - 6 kA m-2
`in 150 g H2SO4 dm-3 at 40 °C. Electrodes for plate and
`frame electrolyzers are far less expensive to manufacture
`than immersed tank electrodes and are considered the pre-
`ferred route for new processes [19]. However, for elec-
`trowinning processes, the reactor design has to be capable
`of facilitating intermittent or continuous removal of the
`metal product.
`
`Wiesener et al. (20) reported cell voltage reductions of
`about 1.5 - 1.7 V at 200 - 1000 A m-2 for zinc electrowin-
`ning using gas diffusion anodes with tungsten carbide as
`the catalyst layer, in electrolyte containing 50-130 g Zn dm-3
`and 100 g H2SO4 dm-3 at 40 °C, relative to the convention-
`al process.
`
`The performances of hydrogen anodes have been
`assessed in short term experiments which showed no degra-
`dation in their behaviour over 200 hours. However, Figure 8
`shows an increase of ca. 150 mV in the potential of a
`100 cm2 E-TEK hydrogen gas diffusion anode operated con-
`
`In electrowinning copper using a hydrogen diffusion
`anode [21], depending on the anode potential, copper can
`be deposited on both the cathode and anode, thereby deac-
`tivating the anode catalyst since there is ca. 0.3 V driving
`force for the reaction:
`
`CANADIAN METALLURGICAL QUARTERLY
`
`

`

`464
`
`M. BESTETTI, U. DUCATI, G. KELSALL, G. LI, E. GUERRA and R. ALLEN
`
`2
`
`+ + Æ
`–
`2
`e
`
`Cu
`
`0
`;
`Cu ECu
`
`+
`
`2
`
`Cu
`
`=
`
`
`
`0 334.
`
`(
`V SHE
`
`)
`
`(3)
`
`to be driven spontaneously by
`
`Æ
`
`+
`
`+
`
`2
`
`H
`
`H
`
`2
`
`2
`
`– ;
`0
`e EH H
`
`+
`
`2
`
`=
`
`(
`0 0. V SHE
`
`
`
`)
`
`(4)
`
`As sites for hydrogen oxidation are blocked on the
`anode, the anode potential rises, eventually causing it to
`behave as a copper anode as the potential surpasses the
`reversible copper potential. Hence, there is a degree of self-
`regulation in the anode behaviour. However, to achieve the
`maximum benefit in minimizing anode potentials, metals
`electrowon should have reversible potentials lower than
`that of the hydrogen electrode.
`
`Janssen [22] reported the use of a hydrogen gas diffu-
`sion anode developed for phosphoric acid fuel cells for
`electrotinning of strip steel; chromium, zinc, iron-zinc and
`zinc-nickel alloy coatings on steel may also be applied by
`this method. The difference in anode potential in running
`the tinning process with a hydrogen diffusion anode or with
`an oxygen evolving anode was approximately 1.6 V at
`1000 A m-2 resulting in lower energy consumption com-
`pared to the conventional process. Part of the saving in cell
`voltage was in the ohmic potential drop. The absence of the
`oxygen bubbles of the conventional process resulted in an
`additional saving of ca. 0.1 V. The gas bubbles generated in
`the conventional process create voidage (e
`g) in the elec-
`trolyte decreasing its (effective) conductivity (s
`eff) relative
`to the bulk phase conductivity (s) according to the
`Bruggeman equation [23]
`
`Additional advantages included decreased emissions of
`acid mist, stability of the electrolyte with respect to addi-
`tives as anode potentials preclude oxidation of the organics
`used, improvement in coating quality and no sludge forma-
`tion at the anode because of Sn(II) oxidation to the less sol-
`uble Sn(IV).
`
`Exposito et al. [24] reported hydrogen diffusion anodes
`operated ca. 1.3 V lower than oxygen evolving DSAs at
`1000 A m-2 in lead electrowinning from fluoborate elec-
`trolytes in which a hydrogen anode enables the use of an
`undivided cell with consequently lower ohmic potential
`drops.
`
`Rambla et al. [25] studied the electrowinning of nickel
`in a batch reactor using a hydrogen diffusion anode in sul-
`phate electrolytes containing variable amounts of Ni2+
`(10 g dm-3 up to saturation) and 35 g dm-3 of H3BO3 at pH
`3.5. Results showed that the energy cost increased linearly
`from 1 to 3 kWh kg-1 for current densities of 100 - 500 A m-2
`at a Ni2+ concentration of 74 g dm-3, room temperature and
`an interelectrode gap of 2 cm.
`
`Zinc Electrowinning at High Current Densities
`Since the industrial zinc electrowinning process was devel-
`oped during 1914-1918, a few reports [26] have appeared
`demonstrating its feasibility at higher current densities than
`those typical of industrial practice (400 - 600 A m-2). In the
`last two decades, such process intensification has been
`extended by incorporation of hydrogen gas diffusion
`anodes [27,28,29,30]; an example using a rotating alumini-
`um disc cathode is shown in Figure 9.
`
`(5)
`
`Table II shows the performance figures for convention-
`al zinc electrowinning and that proposed by Furuya [27].
`
`100 cm
`
`1
`
`7
`
`0.4 cm
`
`4 5 6 3
`
`4
`
`2
`
`3
`
`]
`)
`
`
`
`1 5.
`
`s
`eff
`
`)
`
`(
`x, y, z
`s
`
`[
`
`1
`
`=
`
`(
`
`e
`g
`
`–
`
`x, y, z
`
`1. Rotating aluminium
`disc cathode: Ø=1 m
`2. Hydrogen anode,
`active area: 4¥100 cm2
`2. Electrolyte inlet
`3. Electrolyte outlet
`4. Hydrogen inlet
`5. Hydrogen outlet
`7. Electric motor
`
`Fig. 9. Zinc electrowinning cell incorporating hydrogen gas diffusion cathodes [27].
`
`CANADIAN METALLURGICAL QUARTERLY
`
`

`

`ZINC ELECTROWINNING WITH GAS DIFFUSION ANODES: STATE OF THE ART AND FUTURE DEVELOPMENTS
`
`465
`
`9
`
`15
`
`H2
`
`2
`
`4
`
`13
`
`1 3
`
`5
`
`11
`
`10
`
`16
`
`12
`
`14
`
`H2
`
`Fig. 11. Metal electrowinning on an aluminium strip with a gas diffusion
`electrode.
`
`(6)
`
`ˆ¯
`
`jg
`s
`
`+
`
`h – E – h
`A
`C
`c
`
`+
`
`A
`
`ÊË
`
`=
`U – E
`
`a (mean) current density j passing a distance g between anode
`(A) and cathode (C) in an electrolyte of (mean) conductivity
`s. Figure 12 shows the effect of current density on cell volt-
`age and current efficiency for an electrowinning reactor
`incorporating a hydrogen diffusion anode in an electrolyte
`containing 160 g H2SO4 dm-3 and 60 g Zn(II) dm-3 at 21 °C.
`
`Under simplified assumptions (for a more detailed
`description see References 31 and 32), the total cost per
`unit mass of zinc produced is given by
`
`(7)
`
`Ue
`F
`
`ˆ ¯˜
`
`2
`F
`M
`
`Zn
`
`Ê ËÁ
`
`+
`
`1
`3600
`
`*
`aI
`F
`j
`
`ˆ ¯˜
`
`2
`F
`M
`
`Zn
`
`Ê ËÁ
`
`=
`
`TC
`
`1
`3153600
`
`+
`
`MC
`
`˘˚˙
`
`F
`–
`F
`
`1
`
`–
`
`r
`
`ÈÎÍ
`
`1
`
`e
`
`H
`
`2
`
`ˆ ¯˜
`
`H Z
`M M
`
`2
`
`n
`
`Ê ËÁ
`
`+
`
`CANADIAN METALLURGICAL QUARTERLY
`
`Table II – Operating Characteristics and Energy
`Requirements in Zinc Electrowinning [27]
`
`Using
`Using
`Pb Anode H2 Anode
`
`e
`
`Current density, j / A m-2
`Current efficiency, F
`Cell voltage, U / V
`Specific electrical energy
`consumption / kWh (tonne Zn)-1
`Hydrogen requirement/
`Nm3 H2 (tonne Zn)-1
`
`580
`90%
`3.6
`
`3300
`
`-
`
`5000
`90%
`1.8
`
`1640
`
`380
`
`The advantages of using hydrogen diffusion anodes lies in
`their use at very high current densities at the same current
`efficiency as in conventional zinc electrowinning; indeed,
`their captial cost necessitates high current density opera-
`tion. At the current densities specified in Table II, the cell
`voltage and specific electrical energy consumption are
`decreased by >50% and the purity (99.999%) of zinc pro-
`duced with hydrogen diffusion anodes is higher than that
`obtained with conventional lead alloy anodes. However,
`such comparisons neglect the capital and hydrogen costs of
`gas diffusion anodes which also require very pure elec-
`trolytes to avoid poisoning of the electrocatalyst by inor-
`ganic and organic impurities. Possibly, this might necessi-
`tate the purification of the electrolyte by solvent extraction.
`
`Furuya [31,32] has also proposed other reactor designs
`for high current density electrowinning, involving a rotat-
`ing drum cathode or an aluminium foil as shown in Figure
`10 and Figure 11, respectively.
`
`Fig. 10. Metal electrowinning on a rotating drum with a gas diffusion
`electrode.
`
`OPTIMIZATION OF OPERATING
`PARAMETERS
`The thermodynamic (E), overpotential (h) and ohmic con-
`tributions to the cell voltage (U) are related by
`
`

`

`466
`
`M. BESTETTI, U. DUCATI, G. KELSALL, G. LI, E. GUERRA and R. ALLEN
`
`Fig. 12. Effect of current density on cell voltage and current efficiency.
`Experimental conditions: 60 g Zn(II) + 160 g H2SO4 dm-3 and
`T = 21 ± 4 °C; g = 4 mm [27].
`
`Fig. 13. Fixed and variable costs as a function of current density for
`r = 0 and r = 1. Curves are drawn for eH2 = 1 US$ kg-1, a = 2500 US$ m-2,
`I* = 15 % y-1 and e = 0.03 US$ kWh-1.
`
`being fixed. This may be an important issue whenever the
`hydrogen evolved at the cathode can be recovered from very
`acidic solutions and reused at the anode. Hence, the curves
`in Figure 15 were calculated using the experimental data
`provided by Juda [12] (P0 and P1 are respectively drawn for
`r = 0 and r = 1) and by Furuya [27] (F0 and F1).
`
`The operating conditions for P1 and P0 are the same for
`Figure 6 and those for curves F1 and F0 are related to
`Figure 14. The curves are markedly different because of the
`diversity in operating conditions employed especially in
`current density. From Figure 15 it can be concluded that at
`high current densities the costs decrease with increasing
`sulfuric acid concentration. As an example, for curves F1
`
`Fig. 14. Effect of H2SO4 concentration on reactor performance at
`50 g Zn(II) dm-3 and 5000 A m-2.
`
`where TC is the total cost and MC is the maintenance cost.
`The variable costs (VC) include the electrical energy cost,
`the hydrogen cost and the savings attributable to the recov-
`ery of the hydrogen byproduct at the cathode. The fraction
`of cathodically evolved hydrogen that is recycled at the gas
`diffusion anode is symbolised with r. The optimized condi-
`tions can be obtained by minimization of the total cost with
`respect to temperature, current density, composition and
`anode-to-cathode gap (g). Such problems can be solved
`analytically by knowing the two functions
`)
`(
`]
`] [
`[
`F F=
`T, j, Zn , H SO g
`2
`4
`)
`(
`] [
`]
`[
`=
`U U T, j, Zn , H SO g
`2
`4
`
`and
`
`(8)
`
`, ,
`
`The economics of a process incorporating hydrogen