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Influence of the electrolyte on the electrode reactions in the chlorate process
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The chlorate process is very energy intensive and a major part of the production costs are for electrical energy. Since the electricity prices are constantly increasing and may also vary periodically, the chlorate plants may be forced to adjust their production rate to the price at each moment in order to minimise their costs. Variation of current load requires increased knowledge regarding the electrode behaviour in a wide current range. In this thesis, the aim was to study the impact of the electrolyte on the electrode reactions in order to reduce the energy consumption. The work has mainly been experimental and additionally mathematical modelling has been carried out. A wide current range has been investigated in order to increase the understanding of the phenomena and to obtain results useful for low-load operation during the periods of high electricity cost.

To operate the anode as energy efficiently as possible, the anode potential should not exceed the critical potential (Ecr), where the slope of the anodic polarisation curve increases, most likely due to ruthenium(VIII)-formation, and where the side reaction of oxygen evolution increases. In this work, the influence of different electrolyte parameters on Ecr has been studied. It was shown that a higher chloride concentration and an increased temperature lowered Ecr, which was expected to increase the risk of exceeding Ecr. However, this was not observed due to a simultaneous favouring of the chloride oxidation. Hence it was concluded that the electrolyte parameters should be optimised so that the lowest possible anode potential is obtained, which would enable higher current densities without exceeding Ecr. A further conclusion is that the increased slope of the polarisation curve at Ecr was possibly related to the lower activity for chloride oxidation on ruthenium oxidised to ruthenium(VIII).

At full-load operation, the cathode potential was shown to be rather independent of the electrolyte composition despite a large variation of electrolyte parameters. The cathode composition appears to be more critical than the electrolyte composition when aiming at reducing the energy consumption. A strategy to increase the cathode activity could be to in situ apply a catalytic film onto the electrode surface. Therefore, Y(III) was added to a chloride electrolyte in order to form a yttrium hydroxide film on the alkaline cathode surface during hydrogen evolution. The yttrium-hydroxide film activated reduction of water (hydrogen evolution) and hindered hypochlorite reduction, proton reduction and nitrate reduction. The inhibiting properties are important for the prevention of side reactions, which currently are avoided by reducing Cr(VI) of the electrolyte on the cathode, producing an inhibiting chromium-hydroxide film. The studies on Y(III) increase the expectations for finding alternatives to the toxic Cr(VI).

The addition of chromate to the chlorate electrolyte gives a high cathodic current efficiency and chromate has buffering properties in the electrolyte. The role of the buffer has been investigated for the oxygen evolution from water (one possible anodic side reaction), as well as cathodic hydrogen evolution. Models have been developed for these systems to increase the understanding of the interaction between buffer, electrode reactions and mass transport; the results have been verified experimentally. The chromate buffer increased the limiting current significantly for the cathodic H+ reduction and the cathodic overpotential was reduced drastically at currents lower than the limited current. A too low overpotential could result in the cathodic protection being lost. The presence of chromate buffer increased the limiting current for the oxygen evolution from OH-. The modelling of these systems revealed that the homogeneous reactions connected to the electrode reactions were not in equilibrium at the electrode surface. Further, a good resolution of the interface at the electrode surface was crucial since the, for the electrode reactions, important buffering takes place in an nm-thick reaction layer.

Abstract [sv]

Framställning av klorat är mycket energiintensiv och kräver stora mängder elenergi. Stigande elpriser, som dessutom ofta varierar under dygnet eller säsongsvis, gör att man vill reducera onödiga förluster samt ibland försöka anpassa produktionen så att man när elpriset är högt minskar den, för att sedan öka produktionen igen då elpriset sjunker. Denna flexibla drift kräver ny kunskap om hur elektroderna beter sig i ett större strömintervall än vad som tidigare varit av intresse. Målet med detta arbete var att, med fokus på elektrolytens betydelse, identifiera möjliga förbättringar för kloratprocessen och därmed minska energiförbrukningen. Studierna har i huvudsak varit experimentella men även matematisk modellering har använts. Ett brett strömintervall har undersökts för att bättre förstå fenomenen och för att även kunna använda resultaten då höga elpriser gör att man vill köra processen vid lägre laster än normalt.

För att driften av anoden ska vara så energieffektiv som möjligt bör anodpotentialen inte överskrida den kritiska potentialen (Ecr), där den anodiska polarisationskurvan får en högre lutning (troligtvis pga Ru(VIII)-bildning) och bireaktionen syrgasutveckling ökar. I detta arbete har påverkan av olika elektrolytparametrar på Ecr undersökts. Det visade sig att en ökad kloridkoncentration och ökad temperatur sänkte Ecr. Trots att detta borde göra att Ecr lättare överskrids, blev inte detta fallet eftersom kloridoxidationen samtidigt gynnades. Slutsatsen blir därför att elektrolytparametrarna bör optimeras så att lägsta möjliga anodpotential uppnås, vilket då även gör att strömtätheten kan ökas utan att Ecr överskrids. Slutsatsen är vidare att polarisationskurvans högre lutning vid Ecr kan ha att göra med att rutenium oxiderat till rutenium(VIII) har lägre aktivitet för kloridoxidation.

Vid full last visade sig katodens potential vara relativt oberoende av elektrolytsammansättningen trots att denna varierades kraftigt. Katodens sammansättning verkar vara viktigare att ta hänsyn till än elektrolytens för kunna åstadkomma en större energibesparing. Ett alternativ till att öka katodens aktivitet skulle vara att in-situ belägga elektrodytan med en katalytisk film. Försök gjordes att sätta till Y(III) till kloridelektrolyt för att under vätgasutveckling fälla ut en yttriumhydroxidfilm på den alkaliska katodytan. Yttriumhydroxidfilmen aktiverade vattenreduktion (vätgasutveckling) och inhiberade hypokloritreduktion, protonreduktion och nitratreduktion. De inhiberande egenskaperna är viktiga för att förhindra bireaktioner, vilka idag hindras av att Cr(VI) i elektrolyten reduceras på katoden och bildar en hindrande kromhydroxidfilm. Försöken med Y(III) visar att det finns goda möjligheter att hitta alternativ till det miljöfarliga Cr(VI).

Kromattillsatsen i kloratelektrolyt ger förutom ett högt katodiskt strömutbyte även en buffrande effekt till elektrolyten. Effekten av buffert har undersökts för en av de anodiska bireaktionerna, syrgasutveckling ur vatten, samt för vätgasutvecklingen på katoden. Dessa system har modellerats för att bättre förstå samspelet mellan buffert, elektrodreaktioner och materietransport och resultaten har verifierats experimentellt. Kromatbufferten ökade gränsströmmen för katodisk H+-reduktion betydligt och katodöverpotentialen sjönk kraftigt vid lägre strömmar än gränsströmmen. Detta kan vara ett problem om överpotentialen sjunker så lågt att elektroden inte är katodiskt skyddad. För syrgasutvecklingen ökade närvaron av kromatbuffert gränsströmmen för syrgasutveckling ur OH-. Modellering av dessa system visar att de homogena reaktioner som var kopplade till elektrodreaktionerna inte var i jämvikt vid elektrodytan. Vidare visade det sig vara mycket viktigt med en bra upplösning av gränsskiktet vid elektrodytan, då den buffring som är viktig för elektrodreaktionerna sker i ett mycket tunt reaktionsskikt (nanometertjockt).

Place, publisher, year, edition, pages
Stockholm: KTH , 2008. , 55 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2008:22
Keyword [en]
Chlorate, chloride oxidation, critical anode potential, chromate, DSA, hydrogen evolution, iron, mass transport, oxygen evolution, REM, RDE, steel
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-4681ISBN: 978-91-7178-918-1 (print)OAI: oai:DiVA.org:kth-4681DiVA: diva2:13397
Public defence
2008-04-18, V2, Teknikringen 76, Stockholm, 10:00
Opponent
Supervisors
Note

QC 20100901

Available from: 2008-03-27 Created: 2008-03-27 Last updated: 2012-12-18Bibliographically approved
List of papers
1. Investigation of the oxygen evolving electrode in pH-neutral electrolytes: Modelling and experiments of the RDE-cell
Open this publication in new window or tab >>Investigation of the oxygen evolving electrode in pH-neutral electrolytes: Modelling and experiments of the RDE-cell
2007 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 0019-4686, Vol. 52, no 13, 4513-4524 p.Article in journal (Refereed) Published
Abstract [en]

A model has been developed to illustrate the complex interplay between the acidifying electrode reactions for oxygen evolution, mass transport and homogeneous reactions in pH-neutral electrolytes. Modelled polarisation curves of the oxygen evolution reaction were verified by polarisation curves experimentally measured in 5 M NaClO4 on a RDE of DSA material. The conditions in the simulations and in the experiments were similar to those in the chlorate process (high ionic strength, 70 degrees C, chromate-containing electrolyte, DSA electrode), in which the oxygen evolution reaction is one of the possible side reactions. The model predicted the concentration gradients of H+, OH-, CrO42- and HCrO4- during oxygen evolution on the RDE. It was found that an important part of the chromate buffering effect at high current densities occurs in a thin (in the order of nanometers) reaction layer at the anode. From comparisons between the model and experiments, a buffering reaction has been proposed. The most likely reaction for the chromate buffering in the investigated system is CrO42- reacting with water to HCrO4- and OH-. In the chlorate process, where chromate is a buffer and oxygen evolution is a side reaction, it is likely that chromate promotes oxygen evolution from OH-.

Keyword
oxygen evolution, pH buffer, rotating disk electrode, mathematical modelling, chromate
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-5722 (URN)10.1016/j.electacta.2006.12.048 (DOI)000245476100028 ()2-s2.0-33847359224 (Scopus ID)
Note
QC 20100831. Uppdaterad från submitted till published (20100831).Available from: 2006-05-11 Created: 2006-05-11 Last updated: 2010-08-31Bibliographically approved
2. Critical Anode Potential in the Chlorate Process
Open this publication in new window or tab >>Critical Anode Potential in the Chlorate Process
2006 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, Vol. 153, no 1, D14-D20 p.Article in journal (Refereed) Published
Abstract [en]

Anodic polarization curves on dimensionally stable anodes (DSAs) of RuO2/TiO2 in chlorate electrolyte bend to a higher Tafel slope at the critical potential (E-cr) of approximately 1.2 V vs Ag/AgCl. Operating the chlorate process above E-cr leads to increased oxygen evolution and higher potential losses. In this study the impact of different electrolyte parameters and electrolyte impurities on the risk of reaching/exceeding E-cr was investigated. A dependency of Cl- concentration on E-cr of about -90 mV/dec C-Cl(-) was found at pH 2. An addition of Na2Cr2O7 to chlorate electrolyte is necessary in order to keep a high current efficiency on the cathode but was found to increase the anode potential and thereby increase the risk of exceeding E-cr at galvanostatic operation. Additions of impurities as 30 g/L Na2SO4 or 100 ppm Si (added as SiO2) resulted in increased anode potentials, but adding 1.4 g/L KH2PO4 or 1 g/L HF did not have significant short-term impact on the potential. The anode potential as well as E-cr decreased with increased temperature. A high temperature is beneficial in terms of the decreased anode potential, which outweighs the negative effect of a decrease in Ecr.

Keyword
Anodes, Anodic polarization, Electrolytes, Impurities, Oxygen, Thermal effects, Anode potential, Chlorate process, Electrolyte impurities
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-8148 (URN)10.1149/1.2135216 (DOI)000234142400050 ()2-s2.0-33645515270 (Scopus ID)
Note
QC 20100831Available from: 2008-03-27 Created: 2008-03-27 Last updated: 2010-11-22Bibliographically approved
3. Effects of electrolyte parameters on the iron/steel cathode potential in the chlorate process
Open this publication in new window or tab >>Effects of electrolyte parameters on the iron/steel cathode potential in the chlorate process
2009 (English)In: Journal of Applied Electrochemistry, ISSN 0021-891X, E-ISSN 1572-8838, Vol. 39, no 1, 71-81 p.Article in journal (Refereed) Published
Abstract [en]

This study focuses on how different electrolyte parameters of the chlorate process affect the cathode potential for hydrogen evolution on iron in a wide current-density range. The varied parameters were pH, temperature, mass transport conditions and the ionic concentrations of chloride, chlorate, chromate and hypochlorite. At lower current densities, where cathodic protection of the electrode material is important, the pH buffering capacity of the electrolyte influenced the potential to a large extent. It could be concluded that none of the electrolyte parameters had any major effects (< 50 mV) on the chlorate-cathode potential at industrially relevant current densities (around 3 kA m(-2)). Certainly, there is more voltage to gain from changing the cathode material than from modifying the electrolyte composition. This is exemplified by experiments on steel corroded from operation in a chlorate plant, which exhibits significantly higher activity for hydrogen evolution than polished steel or iron.

Keyword
Chlorate cathode, Chlorate process, Hydrogen evolution, Iron, Steel
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-8149 (URN)10.1007/s10800-008-9642-z (DOI)000268729700010 ()2-s2.0-58149503301 (Scopus ID)
Note
QC 20100901. Uppdaterad från submitted till published (20100901). Tidigare titel: Effects of electrolyte parameters on the chlorate cathode potentialAvailable from: 2008-03-27 Created: 2008-03-27 Last updated: 2010-09-01Bibliographically approved
4. Cathodic reactions on an iron RDE in the presence of Y(III)
Open this publication in new window or tab >>Cathodic reactions on an iron RDE in the presence of Y(III)
2008 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, Vol. 155, no 10, E136-E142 p.Article in journal (Refereed) Published
Abstract [en]

During electrolysis of a solution containing Y(III) ions, a hydrous Y(OH)(3) film forms in the alkaline layer close to a hydrogen-evolving cathode. The film hinders the reduction of dissolved oxygen and activates the reduction of water, hydrogen evolution. The ability to hinder certain reactions while catalyzing hydrogen evolution may be useful in electrolysis applications. In this work the electrochemical properties of an in situ formed yttrium-hydroxide film were studied on an iron rotating disk electrode (RDE) in 0.5 M NaCl with addition of YCl3, NaClO, and of NaNO3. It was found that the film also hinders the reduction of protons, hypochlorite ions, and nitrate ions. At low concentration of Y(III) or at high current density, when the hydrogen evolution was vigorous, no activation of hydrogen evolution was observed. Under these conditions the film still hindered the reduction of ions. The reactant in the catalyzed hydrogen evolution reaction is most likely water molecules within the hydrous film. Nitrate ions were easily reduced on an iron cathode when no Y(III) ions were present in the solution. When studying effects of yttrium addition to a chloride solution the use of YCl3, rather than Y(NO3)(3), as Y(III) source is recommended.

Keyword
Concentration (process), Current density, Disks (machine components), Dissolution, Dissolved oxygen, Electrochemical properties, Electrochemistry, Electrolysis, Hydrogen, Ions, Nitrates, Nonmetals, Oxygen, Reduction, Rotating disks, Sodium chloride, Yttrium, Applications., Cathodic reactions, Chloride solutions, High current densities, Hydrogen evolution, Hydrogen evolution reaction, In-situ, Low concentrations, Nitrate ions, NO activation, Reduction of protons, Rotating disk electrode, Water molecules, Yttrium addition
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-8150 (URN)10.1149/1.2958299 (DOI)000258976500042 ()2-s2.0-51849102885 (Scopus ID)
Note
20100901. Uppdaterad från submitted till published (20100901).Available from: 2008-03-27 Created: 2008-03-27 Last updated: 2012-05-30Bibliographically approved

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