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- Hedenstedt, Kristoffer, 1979
(author)
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Electrochemical Investigations of Water and Hypochlorite Reduction on a-and y-FeOOH
- 2015
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Licentiate thesis (other academic/artistic)abstract
- The cathodes in the electrochemical cells used for production of sodium chlorate process are made from mild steel. Due to the harsh environment in the chlorate cell, with oxidative species such as chlorate and hypochlorous acid, the cathodes corrodes. This corrosion might be beneficial, leading to surface enlargement and lower potential. It may however also have negative consequences, such as reduction of reaction intermediates and products, hypochlorite and chlorate, and the physical deterioration of the cathode. Since the electrochemical production of sodium chlorate is highly energy intensive, up to about 5.5 MWh is used per ton produced, it is of interest to reduce the energy consumption as much as possible. One way to lower the energy consumption is to understand how the cathode can be used as efficiently as possible in the process. Cathodes were taken from two different chlorate plants and were characterised with SEM, EDX and XRD. It was found that goethite was present on one of them while lepidocrocite was the corrosion product present on the second. It had also previously been established that the electrode showing lepidocrocite had longer starting period, i.e. before reaching sufficient current efficiency. Electrocatalytical properties were studied on pure phases of goethite and lepidocrocite electrodeposited onto titanium rotating discs and on carbon paste electrodes which were produced from pure powders. From Tafel plots it was seen that the activity towards hypochlorite reduction was lower for the iron oxyhydroxides than for a polished mild steel electrode. It was found that the activity for hypochlorite reduction followed the order of mild steel > goethite > lepidocrocite. It was determined, for all species, that the first electron transfer was the rate determining step and the transfer coefficient was 0.5. The study of hydrogen evolution on the species showed that the first linear sweeps had much lower activity compared to mild steel. However the activity changed during potentiostatic polarisation to almost the same as for the mild steel. Tafel slopes of 200-250 mV/decade showed that the first electron transfer was rate limiting but the transfer coefficient was small (0.3). It is presumed that the hydrogen evolution on all three different electrodes takes place on Fe(OH)2 which must first be formed on the surface. Since the transfer coefficient is low, the transition state is closer to the adsorbed hydrogen than to the water in solution. Hence the active site for water reduction is proposed to be the protonated surface group Fe(II)-OH2+ independent if the starting material is α-FeOOH, γ-FeOOH or mild steel.
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| 2. |
- Hedenstedt, Kristoffer, 1979
(author)
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Energy efficiency in the sodium chlorate process: From electrocatalysis to pilot plant investigations
- 2017
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Doctoral thesis (other academic/artistic)abstract
- Sodium chlorate is an important industrial chemical produced through an electrochemical manufacturing process. The global production rate is 3.6 million tons annually and consumes approximately 20 TWh of electrical power. The majority of the produced sodium chlorate is used as raw material to make chlorine dioxide for the bleaching of kraft pulp. This thesis aims to provide a deeper understanding on the mild steel cathode and the role sodium dichromate has in the electrolyte in the chlorate process. Such understanding would allow reduction of the energy consumption, in particular, as well as the overall manufacturing footprint. Two separate sodium chlorate plants have shown different performances in terms of current efficiency and corrosion of the mild steel cathodes. Surface characterisations and current efficiency measurements were performed on the two cathodes in order to evaluate the difference in performance between the samples. Two types of FeOOH were found on the individual cathodes: goethite (α-FeOOH) on the normally performing cathode and lepidocrocite (γ-FeOOH) on the poorly performing cathode. The two different FeOOH species were synthesised in pure form to elucidate if their electrocatalytic properties were the reason for their different performance. Both goethite and lepidocrocite showed lower activity for the reduction of water compared to polished mild steel but almost equally good towards hypochlorite reduction. The difference in performance of the pure phases can therefore not explain their differences in behaviour in large scale performance. However, in situ Raman spectroscopy revealed that the active species on the surface of the mild steel cathode was Fe(OH)2 and the kinetics for the reduction of the surface from Fe(III) to Fe(II) was also found to be different between the two types of corrosion products. These findings are the reason for the observed differences in current efficiency. Reduction of hypochlorite is the most important loss reaction in the chlorate process and Cr(VI) is added to the electrolyte to inhibit this reaction. A Cr(III) film formed at the cathode provide selectivity towards hydrogen evolution. The mechanism of hypochlorite reduction at Fe(III) and Cr(III) was studied by Density Functional Theory (DFT) calculations in order to understand the blocking effect of the Cr(III) film. The electro catalytic properties was shown to be very similar for Fe(III) and Cr(III) and cannot explain the blocking effect of Cr(III). However, the experimental results clearly demonstrated that the Cr(III) film was completely blocking of the hypochlorite reduction. It was concluded that it is the semiconductor properties of the materials that explain that the hypochlorite reduction at Cr(III) is inhibited while the reduction readily can proceed at iron (oxy)hydroxides. A pilot plant was used to investigate the long term effects from continuous operation. Three process parameters were tested in the pilot plant to investigate the formation of different corrosion products on the cathode surface and their effect on the energy efficiency. These three were concentration of dichromate, sulphate and the temperature of the electrolyte. The pilot plant studies revealed possibilities to optimise the current efficiencies and corrosion of the cathodes with respect to the operating and shutdown conditions. Finally recommendations are issued, as to how a sodium chlorate producer should relate to the results in order to minimize the losses in current efficiencies and cathodic corrosion.
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| 3. |
- Hedenstedt, Kristoffer, 1979, et al.
(author)
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In-situ Raman spectroscopy of α- and γ-FeOOH during cathodic load
- 2017
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In: Journal of the Electrochemical Society. - : The Electrochemical Society. - 0013-4651 .- 1945-7111. ; 164:9
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Journal article (peer-reviewed)abstract
- Water reduction on corroded iron surfaces is technologically and fundamentally important. Here, the technological interest originates from the chlorate process where water reduction is the main cathodic process. Fundamentally, water reduction on oxide surfaces raises questions on the stability of the oxide and the nature of electrocatalytic surface sites. Two iron oxyhydroxides, alpha- and gamma-FeOOH, were electrodeposited on titanium substrate and their reduction processes were followed in detail with in-situ Raman spectroscopy, using low incident laser power to avoid sample damaging. Polarization to negative potentials show two reduction peaks for gamma-FeOOH and one peak for alpha-FeOOH prior to hydrogen evolution. The characteristic Raman peaks gradually disappear as the potential is made more negative but no new peaks can be observed. delta-FeOOH was detected as an intermediate phase upon oxidation of the reduced surface layer. This indicates that Fe(OH)(2) is formed during cathodic polarization and initially re-oxidized to the isostructural delta-FeOOH. Characteristic Raman signals of the original phases appear upon further oxidation in air. (C) The Author(s) 2017. Published by ECS. All rights reserved.
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| 5. |
- Hedenstedt, Kristoffer, 1979, et al.
(author)
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Study of Hypochlorite Reduction Related to the Sodium Chlorate Process
- 2016
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In: Electrocatalysis. - : Springer Science and Business Media LLC. - 1868-2529 .- 1868-5994. ; 7:4, s. 326-335
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Journal article (peer-reviewed)abstract
- Reduction of hypochlorite is the most important side reaction in the sodium chlorate reactor leading to high energy losses. Today chromate is added to the reactor solution to minimize the hypochlorite reduction but a replacement is necessary due to health and environmental risks with chromate. In order to understand the effect of different substrates on the hypochlorite reduction, α-FeOOH, γ-FeOOH, Cr2O3 and CrOH3 were electrodeposited on titanium and subjected to electrochemical investigations. These substances are commonly found on cathodes in the chlorate process and can serve as model substances for the experimental investigation. The mechanism of hypochlorite reduction was also studied using DFT calculations in which the reaction at Fe(III) and Cr(III) surface sites were considered in order to single out the electrocatalytic properties. The experimental results clearly demonstrated that the chromium films completely block the reduction of hypochlorite, while for the iron oxyhydroxides the process can readily occur. Since the electrocatalytic properties per se were shown by the DFT calculations to be very similar for Fe(III) and Cr(III) sites in the oxide matrix, other explanations for the blocking ability of chromium films are addressed and discussed in the context of surface charging, reduction of anions and conduction in the deposited films. The main conclusion is that the combined effect of electronic properties and reduction of negatively charged ions can explain the reduction kinetics of hypochlorite and the effect of chromate in the chlorate process.
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