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(11) | EP 1 920 087 B1 |
| (12) | EUROPEAN PATENT SPECIFICATION |
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| (54) |
ELECTROCHEMICAL REDUCTION OF TITANIUM OXIDE ELEKTROCHEMISCHE REDUKTION VON TITANOXID REDUCTION ELECTROCHIMIQUE D'OXYDE DE TITANE |
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| Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention). |
[0002] The present application relates to electrochemical reduction of metal oxides in the form of powders and/or pellets in an electrolytic cell containing a molten electrolyte to produce reduced material, namely metal having a low oxygen concentration, typically no more than 0.2% by weight.
[0003] The present invention is concerned with controlling electrochemical reduction of titanium oxide.
[0004] The present invention applies to situations in which the process is carried out on a batch basis, a continuous basis, and a semi-continuous basis.
[0005] The present invention was made during the course of a research project on electrochemical reduction of metal oxides carried out by the applicant.
[0007] During the course of the research project the applicant carried out a series of experiments, initially on a laboratory scale and more recently on a pilot plant scale, investigating the reduction of metal oxides in the form of titania in electrolytic cells comprising a pool of molten CaCl2-based electrolyte, an anode formed from graphite, and a range of cathodes.
[0008] The CaCl2-based electrolyte used in the experiments was a commercially available source of CaCl2, which decomposed on heating and produced a very small amount of CaO.
[0009] The applicant operated the electrolytic cells at a potential above the decomposition potential of CaO and below the decomposition potential of CaCl2.
[0010] The applicant found in the laboratory work that the cells electrochemically reduced titania to titanium with low concentrations of oxygen, i.e. concentrations less than 0.2 wt.%, at these potentials.
[0011] The applicant operated the laboratory electrolytic cells under a wide range of different operating parameters and conditions.
[0012] The applicant operated the laboratory electrolytic cells on a batch basis with titania in the form of pellets and larger solid blocks in the early part of the laboratory work and titania powder in the later part of the work.
[0013] The applicant also operated the laboratory electrolytic cells on a batch basis with other metal oxides.
[0014] Recent pilot plant work carried out by the applicant was carried out on a pilot plant electrolytic cell that was set up to operate initially on a continuous basis and subsequently on a batch basis.
[0015] During the course of the laboratory scale experimental work the applicant found that the concentration of CaO in the electrolyte had a significant impact on the process for electrochemically reducing metal oxides in the laboratory cells.
[0016] In particular, the applicant found that the concentration of CaO in the electrolyte should be controlled by maintaining the concentration above a minimum CaO concentration and below a maximum CaO concentration in order to optimise operation of the process.
[0017] More particularly, the applicant found that there were lower reduction rates and lower current efficiencies at higher CaO concentrations in the electrolyte.
[0018] In addition, the applicant found that there was a minimum CaO concentration below which there was practically no reduction of titania.
[0019] The applicant believes that, in any situation, the lower limit of CaO is a function of scavenger or other reactions that consume CaO.
[0020] For example, CaO and titania react spontaneously and form perovskite (CaTiO3). Thus, depending on the amount of CaO in the electrolyte and the amount of titania introduced into the electrolyte, the amount of CaO may decrease substantially as a consequence of the perovskite reaction.
[0021] In addition, calcium metal reacts with titania and forms calcium titanates, thus producing another sink for CaO.
[0022] In addition, the applicant found that the minimum and maximum CaO concentrations in any given situation are dependent to a certain extent on the physical characteristics of the titania.
[0023] An important physical characteristic is the surface area of titania in contact with electrolyte, particularly when this characteristic is considered in the context of a given mass of the electrolyte and a given mass of the titania.
[0024] In turn, the titania surface area for a given mass of titania is dependent on physical characteristics of the titania, such as powder/pellet shape and porosity.
[0025] The applicant also believes that the above-mentioned significance of the CaO concentration in a CaCl2-based electrolyte also applies to other alkali earth metal oxides, alkali metal oxides, and yttrium oxides. Specifically, the applicant believes that where one or more than one of the metal oxides is present in a CaCl2-based electrolyte, controlling the concentrations of the metal oxides within ranges is a means of optimising operation of an electrochemical reduction process for titanium oxide in a solid state in the electrolyte.
[0026] The metal oxides may be present as an impurity or impurites in the feed material used to produce the CaCl2-based electrolyte and/or by way of specific addition to the electrolyte.
[0027] According to the present invention there is provided a process as defined in the appended independent claim, to which reference should now be made. The invention may therefore advantageously provide a process for electrochemically reducing a titanium oxide in a solid state in an electrolytic cell that includes an anode, a cathode, a molten CaCl2-based electrolyte containing CaO, and titanium oxide feed material in contact with the molten electrolyte, which electrochemical process includes applying an electrical potential across the anode and the cathode and electrochemically reducing titanium oxide feed material in contact with the molten electrolyte and producing reduced material, and which process is characterised by controlling the process by controlling the concentration of CaO in the electrolyte.
[0028] Preferably the process includes controlling the concentration of CaO in the electrolyte by maintaining the concentration of CaO in the electrolyte between lower and upper concentration limits.
[0036] The process includes controlling the concentration of CaO in the electrolyte by reference to the surface area of contact of the titanium oxide with the electrolyte.
[0037] The process includes controlling the concentration of CaO in the electrolyte by reference to the surface area of contact of the titanium oxide with the electrolyte when considered in the context of the mass of the electrolyte and the mass of the titanium oxide.
[0038] The practical option for controlling the CaO concentration in the electrolyte is to control the mass ratio of the electrolyte and the titanium oxide in the cell.
[0039] In the context of controlling the CaO concentration in the electrolyte above the minimum concentration limit, the process includes controlling the mass ratio of the electrolyte and titanium oxide in the cell to be at least 10:1 in situations where the initial CaO concentration is below 0.3, more preferably below 0.2 wt.% in the electrolyte and the titanium oxide is in a form of pellets and/or powders having a specific surface area of 0.1 to 100 m2/g of titanium oxide.
[0040] The process may include controlling the CaO concentration in the electrolyte by adding CaO to the electrolyte during the course of the process.
[0041] The process may include controlling the CaO concentration in the electrolyte by selecting an initial CaO concentration that is sufficiently high.
[0042] The electrochemical process may be carried out on any one of a batch basis, a semi-continuous basis, or a continuous basis.
[0045] Preferably the electrochemical reduction process includes applying a potential across the anode and the cathode that is above the decomposition potential of CaO and below the decomposition of CaCl2.
[0046] The electrochemical reduction process may be carried out as a single stage or a multi-stage process.
[0047] In more general terms, according to the present application there is provided a process for electrochemically reducing a titanium oxide in a solid state in an electrolytic cell that includes (a) an anode, (b) a cathode, (c) a molten CaCl2-based electrolyte containing any one or more than one metal oxide selected from the group including alkali earth metal oxides, alkali metal oxides, and yttrium oxides, and (d) a titanium oxide feed material in contact with the molten electrolyte, which electrochemical process includes applying an electrical potential across the anode and the cathode and electrochemically reducing titanium oxide feed material in contact with the molten electrolyte and producing reduced material, and which process is characterised by controlling the process by controlling the concentration of the selected metal oxide or metal oxides in the electrolyte.
[0049] The above-mentioned laboratory scale experimental work carried out by the applicant was carried out in a high temperature laboratory electrolytic cell.
[0050] The cell comprised a reaction vessel, a furnace, a crucible assembly, an electrode assembly, and a power supply.
[0051] The reaction vessel was manufactured from a high temperature stainless steel and had an internal diameter of 110mm and height of 430mm, and a water-cooled flange.
[0052] The vessel was contained within a resistance-heated furnace capable of reaching 1400°K. A positioning pedestal within the vessel allowed for the crucible containing the molten salt to be properly fixed within the vessel.
[0053] A water-cooled lid was a critical feature of the reaction vessel and had provision for a viewing port which facilitated accurate positioning of the electrodes and the thermocouple within the molten salt bath. The viewing port also allowed the surface of the molten salt to be monitored during the reaction.
[0054] Carbon monoxide and carbon dioxide are the main by-products of the electrochemical reduction process. In order to monitor the progress of reduction and to ensure that the only source of oxygen was the titanium dioxide, special care was taken in controlling the gas atmosphere. A measured flow of high purity argon gas was passed through a furnace containing copper turnings at 873°K, then through a furnace containing magnesium flakes at 673°K before entering the vessel. After passing through the vessel, the gas stream was stripped of any chlorine-containing species and moisture and continuously analysed for CO and CO2.
[0055] A solid electrolyte based oxygen analyser monitored the partial pressure of O2 in the off-gas immediately after the furnace.
[0057] Furnace pressure, applied voltage, reference voltage, cell current, electrolyte temperature, oxygen, carbon monoxide and carbon dioxide content of the off-gas were monitored and logged for subsequent analysis as a function of time.
[0058] As is indicated above, the chemistry of the molten electrolyte was found to have a significant impact on the process operation. Consequently, the applicant ensured that the composition of the electrolyte at the start of a run was well controlled.
[0059] The electrolyte was prepared from analytical grade dihydrate, CaCl2.H2O obtained from APS Chemicals. Typically, 680g of molten salt were used in the experiments. Prior to melting, a dehydration step was carried out as follows.
[0060] The CaCl2.H2O was slowly heated under vacuum to 525K and kept at this temperature for at least 12 hours during which time the weight loss due to water removal was monitored. Once there was no more weight loss, the anhydrous salt was transferred from the vacuum oven into a platinum crucible and placed in a melting furnace. The salt was melted at 1275°K and kept at this temperature for 30 minutes to allow further removal of any residual water. The molten salt was then cast into a preheated steel mould, removed once solid, and transferred while hot to a drying oven held at 400K.
[0061] By using this standardised procedure for preparing the CaCl2 electrolyte, it was possible to obtain reproducible quality material.
[0062] In the majority of experiments the titania pellets were prepared from 99.5% minimum purity rutile (Alfa Aesar) although occasionally other titania sources were used. The titania pellets were prepared so that control of the total porosity and pore size was exercised by sintering at a predetermined temperature.
[0063] In one series of experiments, the concentration of CaO in the electrolyte was systematically increased up to 2.5 wt.%.
[0064] Specifically, experiments were carried out with concentrations of 0.14, 0.20, 0.42, 1.4, and 2.4 wt.% CaO in the electrolyte and with the electrolyte in molten form at 1275K. The experiments were carried out for a total of 4 hours with an applied voltage of 3V. The IR losses of the cell were calculated to be less than 0.2V.
[0066] Figure 1 is a plot of cell current (in Amps) versus time (in seconds) for each of the above-mentioned experiments. The Figure shows that there was a decrease in cell current over time with each of the CaO concentrations in the electrolyte. The Figure also shows that the cell passed more current with increased concentrations of CaO in the electrolyte. This finding, of itself does not indicate that there was increased redusction of titania at increased CaO concentrations.
[0067] Figure 2 is a plot of the concentration of oxygen in titanium in the reduced pellets versus the CaO concentration in the electrolyte at the end of the experimental runs of 4 hours. The Figure shows that reduction of titania to titanium was inhibited as the concentration of CaO in the electrolyte increased. Specifically, the Figure shows that there were higher concentrations of oxygen in the titanium produced in the cell as the CaO concentrations in the electrolyte increased. In other words, the Figure shows that the rate of reduction decreased as the concentration of CaO in the electrolyte increased.
[0068] In addition, when read together, Figures 1 and 2 indicate that significantly higher current consumption was required to achieve lower levels of reduction of titania at increased CaO concentrations. Significantly, on the basis of Figures 1 and 2, operating at CaO concentrations of 0.5 wt.% or less produced high levels of reduction - to less than about 0.5 wt.% oxygen - with reasonable current consumption
[0069] Figure 3 is a plot of the amount of carbon removed from the anode in grams and the overall current efficiency versus the CaO concentration in the electrolyte at the end of the experimental runs of 4 hours. The Figure shows that the amounts of electricity and carbon consumed in the process were proportionally higher as the concentration of CaO in the electrolyte increased. In particular, the Figure shows that the current efficiency decreased as the concentration of CaO in the electrolyte increased.
[0070] The above results and other results not summarised here and consideration by the applicant of the results indicate to the applicant that CaO is an important parameter for controlling an electrochemical reduction process to produce optimum results in terms of reduction rate and current efficiency, and that the process could be controlled for example by maintaining the CaO concentration within lower and upper concentration limits. Specifically, above results and other results not summarised here indicate to the applicant that the CaO concentration in the electrolyte should be controlled to be in the range of 0.05-0.5 wt.%.
1. A process for electrochemically reducing a titanium oxide in a solid state in an electrolytic
cell that includes an anode, a cathode, a molten CaCl2-based electrolyte containing CaO, and titanium oxide feed material in contact with
the molten electrolyte, which electrochemical process includes applying an electrical
potential across the anode and the cathode and electrochemically reducing titanium
oxide feed material in contact with the molten electrolyte and producing reduced material,
and which process is characterised by controlling the process by controlling the concentration of CaO in the electrolyte
by controlling the mass ratio of the electrolyte and titanium oxide in the cell to
be at least 10:1, the initial CaO concentration to be below 0.3 wt.% in the electrolyte
and the titanium oxide to be in a form of pellets and/or powders having a specific
surface area of 0.1 to 100 m2/g of titanium oxide.
2. The process defined in claim 1 includes controlling the concentration of CaO in the
electrolyte by maintaining the concentration of CaO in the electrolyte between lower
and upper concentration limits.
3. The process defined in claim 2 includes maintaining the concentration of CaO in the
electrolyte at or below a maximum CaO concentration of 0.5 wt.%.
4. The process defined in claim 2 includes maintaining the concentration of CaO in the
electrolyte at or below a maximum CaO concentration of 0.3 wt.%.
5. The process defined in any one of claims 2 to 4 includes maintaining the concentration
of CaO in the electrolyte at or above a minimum CaO concentration of 0.005 wt.%.
6. The process defined in any one of claims 2 to 4 includes maintaining the concentration
of CaO in the electrolyte at or above a minimum CaO concentration of 0.05 wt.%.
7. The process defined in any of claims 2 to 4 includes maintaining the concentration
of CaO in the electrolyte at or above a minimum CaO concentration of 0.1 wt.%.
8. The process defined in claim 2 includes maintaining the concentration of CaO in the
electrolyte in the range of 0.1-0.3 wt.%.
9. The process defined in claim 2 includes maintaining the concentration of CaO in the
electrolyte in the range of 0.15-0.25 wt.%.
10. The process defined in any one of the preceding claims includes controlling the concentration
of CaO in the electrolyte by reference to the surface area of contact of the titanium
oxide with the electrolyte.
11. The process defined in any one of the preceding claims includes controlling the concentration
of CaO in the electrolyte by reference to the surface area of contact of the titanium
oxide with the electrolyte when considered in the context of the mass of the electrolyte
and the mass of the titanium oxide.
12. The process defined in any one of the preceding claims includes controlling the CaO
concentration in the electrolyte by controlling the mass ratio of the electrolyte
and the titanium oxide in the cell.
13. The process defined in any one of the preceding claims in which the initial CaO concentration
is below 0.2 wt. % in the electrolyte.
14. The process defined in any one of the preceding claims includes controlling the CaO
concentration in the electrolyte by adding CaO to the electrolyte during the course
of the process.
15. The process defined in any one of the preceding claims includes controlling the CaO
concentration in the electrolyte by selecting an initial CaO concentration that is
sufficiently high.
1. Prozess zum elektrochemischen Reduzieren eines Titanoxids in einem festen Zustand
in einer elektrolytischen Zelle, die eine Anode, eine Kathode, einen geschmolzenen
Elektrolyt auf CaCl2-Basis, der CaO enthält, und ein Titanoxid-Ausgangsmaterial, das mit dem geschmolzenen
Elektrolyt in Kontakt ist, beinhaltet, wobei der elektrochemische Prozess das Anlegen
eines elektrischen Potentials an die Anode und die Kathode und das elektrochemische
Reduzieren von mit dem geschmolzenen Elektrolyt in Kontakt befindlichem Titanoxid-Ausgangsmaterial
und Erzeugen von reduziertem Material beinhaltet und wobei der Prozess dadurch gekennzeichnet ist, dass der Prozess durch Regeln der Konzentration von CaO in dem Elektrolyt durch Regeln
des Massenverhältnisses des Elektrolyts und Titanoxids in der Zelle, so dass es wenigstens
10:1 beträgt, die anfängliche CaO-Konzentration unter 0,3 Gew.-% in dem Elektrolyt
ist und das Titanoxid in einer Form von Pellets und/oder Pulvern mit einer spezifischen
Oberfläche von 0,1 bis 100 m2/g Titanoxid ist, gesteuert wird.
2. Prozess nach Anspruch 1, der das Regeln der Konzentration von CaO in dem Elektrolyt
durch Halten der Konzentration von CaO in dem Elektrolyt zwischen einer unteren und
einer oberen Konzentrationsgrenze beinhaltet.
3. Prozess nach Anspruch 2, der das Halten der Konzentration von CaO in dem Elektrolyt
auf oder unter einer maximalen CaO-Konzentration von 0,5 Gew.-% beinhaltet.
4. Prozess nach Anspruch 2, der das Halten der Konzentration von CaO in dem Elektrolyt
auf oder unter einer maximalen CaO-Konzentration von 0,3 Gew.-% beinhaltet.
5. Prozess nach Anspruch 2, der das Halten der Konzentration von CaO in dem Elektrolyt
auf oder über einer Mindest-CaO-Konzentration von 0,005 Gew.-% beinhaltet.
6. Prozess nach einem der Ansprüche 2 bis 4, der das Halten der Konzentration von CaO
in dem Elektrolyt auf oder über einer Mindest-CaO-Konzentration von 0,05 Gew.-% beinhaltet.
7. Prozess nach einem der Ansprüche 2 bis 4, der das Halten der Konzentration von CaO
in dem Elektrolyt auf oder über einer Mindest-CaO-Konzentration von 0,1 Gew.-% beinhaltet.
8. Prozess nach Anspruch 2, der das Halten der Konzentration von CaO in dem Elektrolyt
in dem Bereich von 0,1 bis 0,3 Gew.-% beinhaltet.
9. Prozess nach Anspruch 2, der das Halten der Konzentration von CaO in dem Elektrolyt
in dem Bereich von 0,15 bis 0,25 Gew.-% beinhaltet.
10. Prozess nach einem der vorhergehenden Ansprüche, der das Regeln der Konzentration
von CaO in dem Elektrolyt durch Bezug auf den Flächeninhalt der Kontaktfläche des
Titanoxids mit dem Elektrolyt beinhaltet.
11. Prozess nach einem der vorhergehenden Ansprüche, der das Regeln der Konzentration
von CaO in dem Elektrolyt durch Bezug auf den Flächeninhalt der Kontaktfläche des
Titanoxids mit dem Elektrolyt beinhaltet, wenn im Zusammenhang mit der Masse des Elektrolyts
und der Masse des Titanoxids betrachtet.
12. Prozess nach einem der vorhergehenden Ansprüche, der das Regeln der CaO-Konzentration
in dem Elektrolyt durch Regeln des Massenverhältnisses des Elektrolyts und des Titanoxids
in der Zelle beinhaltet.
13. Prozess nach einem der vorhergehenden Ansprüche, bei dem die anfängliche CaO-Konzentration
in dem Elektrolyt unter 0,2 Gew.-% ist.
14. Prozess nach einem der vorhergehenden Ansprüche, der das Regeln der CaO-Konzentration
in dem Elektrolyt durch Zugeben von CaO zu dem Elektrolyt während des Prozessverlaufs
beinhaltet.
15. Prozess nach einem der vorhergehenden Ansprüche, der das Regeln der CaO-Konzentration
in dem Elektrolyt durch Auswählen einer anfänglichen CaO-Konzentration, die hoch genug
ist, beinhaltet.
1. Procédé de réduction électrochimique d'un oxyde de titane sous forme solide en une
cellule électrolytique qui inclut une anode, une cathode, un électrolyte fondu basé
sur CaCl2 contenant du CaO, et une matière de charge d'oxyde de titane en contact avec l'électrolyte
fondu, le procédé électrochimique incluant l'application d'un potentiel électrique
sur l'anode et la cathode et la réduction électrochimique de la matière de charge
de l'oxyde de titane en contact avec l'électrolyte fondu et la production d'un matériau
réduit, le procédé étant caractérisé par le contrôle du Procédé par le contrôle de la concentration de CaO dans l'électrolyte
par le contrôle du rapport de masse de l'électrolyte et de l'oxyde de titane dans
la cellule, devant être d'au moins 10:1, la concentration initiale en CaO étant inférieure
à 0,3 % en poids dans l'électrolyte et l'oxyde de titane étant sous la forme de pastilles
et/ou de poudres possédant une superficie spécifique de 0,1 à 100 m2/g d'oxyde de titane.
2. Procédé selon la revendication 1, incluant le contrôle de la concentration de CaO
dans l'électrolyte en maintenant la concentration de CaO dans l'électrolyte entre
les limites de concentration inférieure et supérieure.
3. Procédé selon la revendication 2, incluant le maintien de la concentration de CaO
dans l'électrolyte à ou en dessous d'une concentration maximale en CaO de 0,5 % en
poids.
4. Procédé selon la revendication 2, incluant le maintien de la concentration de CaO
dans l'électrolyte à ou en dessous d'une concentration maximale en CaO de 0,3 % en
poids.
5. Procédé selon l'une quelconque des revendications 2 à 4, incluant le maintien de la
concentration de CaO dans l'électrolyte à ou au-dessus d'une concentration minimale
en CaO de 0,005 % en poids.
6. Procédé selon l'une quelconque des revendications 2 à 4, incluant le maintien de de
la concentration de CaO dans l'électrolyte à ou au-dessus d'une concentration minimale
en CaO de 0,05 % en poids.
7. Procédé selon l'une quelconque des revendications 2 à 4, incluant le maintien de de
la concentration de CaO dans l'électrolyte à ou au-dessus d'une concentration minimale
en CaO de 0,01 % en poids.
8. Procédé selon la revendication 2, incluant le maintien de la concentration de CaO
dans l'électrolyte dans la plage de 0,1 à 0,3 % en poids.
9. Procédé selon la revendication 2, incluant le maintien de la concentration de CaO
dans l'électrolyte dans la plage de 0,15 à 0,25 % en poids.
10. Procédé selon l'une quelconque des revendications précédentes, incluant le contrôle
de la concentration de CaO dans l'électrolyte par référence à la superficie de contact
de l'oxyde de titane avec l'électrolyte.
11. Procédé selon l'une quelconque des revendications précédentes, incluant le contrôle
de la concentration de CaO dans l'électrolyte par référence à la superficie de contact
de l'oxyde de titane avec l'électrolyte lorsque considéré dans le contexte de la masse
de l'électrolyte et de la masse de l'oxyde de titane.
12. Procédé selon l'une quelconque des revendications précédentes, incluant le contrôle
de la concentration de CaO dans l'électrolyte par le contrôle du rapport de masse
de l'électrolyte et de l'oxyde de titane dans la cellule.
13. Procédé selon l'une quelconque des revendications précédentes, dans lequel la concentration
initiale en CaO est inférieure à 0,2 % en poids dans l'électrolyte.
14. Procédé selon l'une quelconque des revendications précédentes, incluant le contrôle
de la concentration de CaO dans l'électrolyte par l'ajout de CaO à l'électrolyte au
cours du procédé.
15. Procédé selon l'une quelconque des revendications précédentes, incluant le contrôle
de la concentration de CaO dans l'électrolyte par la sélection d'une concentration
initiale en CaO qui est suffisamment élevée.