Electrolysis of carbon dioxide to solid carbon using a liquid metal cathode

Abstract

A process for producing solid carbon and gaseous oxygen from CO.sub.2 via electrolysis using an electrolysis apparatus is disclosed. The apparatus includes a chamber with an electrolyte inlet, an electrolyte outlet, a liquid electrolyte containing CO.sub.2 in the chamber, at least one cathode-anode pair, with the cathode including a liquid metal capable of catalysing reduction of CO.sub.2 to solid carbon at a selected operating temperature of the process. The process includes causing the electrolyte to flow from the inlet to the outlet in fluid communication with the cathode-anode pair, applying a voltage between the cathode-anode pair and causing solid carbon to form on the cathode from CO.sub.2 in the electrolyte and gaseous oxygen to be evolved at the anode from CO.sub.2 in the electrolyte.

Claims

1. A process for producing solid carbon and gaseous oxygen from CO.sub.2 via electrolysis using an electrolysis apparatus having a chamber with an electrolyte inlet, an electrolyte outlet, a pool of a liquid electrolyte containing CO.sub.2 in the chamber, at least one cathode-anode pair, with the cathode including a pool of a liquid metal capable of catalysing reduction of CO.sub.2 to solid carbon, and the liquid metal pool being in contact with the electrolyte pool, the process including supplying the electrolyte to the chamber via the inlet and discharging the electrolyte from the chamber via the outlet with the electrolyte flowing from the inlet to the outlet in fluid communication with the cathode-anode pair, applying a voltage between the cathode-anode pair and causing solid carbon to form on the cathode from CO.sub.2 in the electrolyte and gaseous oxygen to be evolved at the anode from CO.sub.2 in the electrolyte, and discharging solid carbon by transporting solid carbon from the cathode in the electrolyte to the electrolyte outlet and from the chamber via the outlet, and discharging gaseous oxygen from the chamber.

2. The process defined in claim 1 includes maintaining a pressure of 0-50 barg in the chamber.

3. The process defined in claim 1 includes supplying the electrolyte at a temperature up to 200 C. to the chamber.

4. The process defined in claim 1 includes applying a voltage in a range of 1 to 10 volts between the cathode-anode pair.

5. The process defined in claim 1 wherein the electrolyte includes dimethylformamide containing CO.sub.2 in solution.

6. The process defined in claim 1 includes separating solid carbon from the electrolyte discharged from the electrolyte outlet and returning the electrolyte to the chamber via the electrolyte inlet.

7. The process defined in claim 6 includes regenerating the electrolyte by adding CO.sub.2 to the electrolyte before returning the electrolyte to the chamber via the electrolyte inlet.

8. The process defined in claim 1 includes supplying the electrolyte to the chamber so that the electrolyte flowing through a gap between the cathode and the anode has a superficial liquid velocity in a range of 0.05-5 m/s.

9. A process for producing iron including: producing solid carbon and gaseous oxygen in accordance with the electrolysis process defined in claim 1, supplying iron ore, gaseous oxygen and a source of carbon to a direct smelter and direct smelting iron ore to molten iron and producing an off-gas containing CO.sub.2, with the carbon source for the direct smelter including solid carbon produced in the electrolysis process, and with CO.sub.2 in the off-gas from the direct smelter being used in the electrolysis process.

10. The process defined in claim 9 includes using gaseous oxygen from the electrolysis process as at least a part of the gaseous oxygen for direct smelting iron ore in the direct smelter.

11. A process for producing iron including: producing solid carbon and gaseous oxygen in accordance with the electrolysis process defined in claim 1 producing molten iron and an off-gas containing CO.sub.2 in a blast furnace, with CO.sub.2 in the off-gas from the blast furnace being used in the electrolysis process, and with solid carbon produced in the electrolysis process being used as a carbon source for the blast furnace.

12. The process defined in claim 11 includes mixing solid carbon from the electrolysis process and a binder and forming lumps of solid carbon, processing the lumps to coke, and supplying the coke to the blast furnace.

13. The process defined in claim 1 includes causing solid carbon to adhere only very weakly if at all to the liquid metal surface so electrolyte flow is sufficient to dislodge carbon and allow removal by simple convection.

14. A process for producing solid carbon and gaseous oxygen from CO.sub.2 via electrolysis using an electrolysis apparatus having a chamber with an electrolyte inlet, an electrolyte outlet, a pool of a liquid electrolyte containing CO.sub.2 in the chamber, at least one cathode-anode pair in the electrolyte pool, with the cathode including a pool of a liquid metal capable of catalysing reduction of CO.sub.2 to solid carbon with the liquid metal pool having a depth of 1-50 mm, the process including maintaining a pressure of 0-50 barg in the chamber, supplying the electrolyte at a temperature up to 200 C. to the chamber via the inlet and discharging the electrolyte from the chamber via the outlet, with the electrolyte flowing from the inlet to the outlet in fluid communication with the cathode-anode pair, applying a voltage in a range of 1 to 10 volts between the cathode-anode pair and causing solid carbon to form on the cathode from CO.sub.2 in the electrolyte and gaseous oxygen to be evolved at the anode from CO.sub.2 in the electrolyte, and discharging solid carbon by transporting solid carbon from the cathode in the electrolyte to the electrolyte outlet and from the chamber via the outlet, and discharging gaseous oxygen from the chamber via a gas outlet.

15. The process defined in claim 14 includes supplying the electrolyte to the chamber via the inlet at a temperature between ambient and 90 C.

16. The process defined in claim 14 includes maintaining the pressure between 0-15 barg.

17. The process defined in claim 14 includes applying the voltage in a range of 1.5-3 volts between the cathode-anode pair.

18. The process defined in claim 14 includes causing solid carbon to adhere only very weakly if at all to the liquid metal surface so electrolyte flow is sufficient to dislodge carbon and allow removal by simple convection.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is described further by way of example only with reference to the accompanying drawings, of which:

(2) FIG. 1 is a schematic diagram of an embodiment of the electrolysis apparatus of the invention that includes an electrolysis cell with single electrode pair and a solid anode (Embodiment A);

(3) FIG. 2 is a schematic diagram of another embodiment of the electrolysis apparatus of the invention that includes an electrolysis cell with single electrode pair and an anode featuring apertures for oxygen bubble escape (Embodiment B);

(4) FIG. 3 is a schematic diagram of another embodiment of the electrolysis apparatus of the invention that includes an electrolysis cell with multiple electrode pairs, each with a solid anode (Embodiment C);

(5) FIG. 4 is a schematic diagram of another embodiment of the electrolysis apparatus of the invention that includes an electrolysis cell with multiple electrode pairs and anodes featuring apertures for oxygen bubble escape (Embodiment D);

(6) FIG. 5 is a schematic diagram of another, although not the only other possible, embodiment of the electrolysis apparatus of the invention that includes an electrolysis cell with a vertically oriented electrode pair and a cathode having a porous medium;

(7) FIG. 6 is a schematic diagram of an embodiment of a process and apparatus for producing iron using an HiSarna direct smelting unit in accordance with the invention;

(8) FIG. 7 is a schematic diagram of an embodiment of a process and apparatus for producing iron using a blast furnace in accordance with the invention;

(9) FIG. 8 is an image of the setup of the electrolysis apparatus during laboratory work carried out by the applicant;

(10) FIG. 9 is a current density curve for the electrolysis using a solution of 66 wt % MEA+34 wt % water and 0.1M NH.sub.4BF.sub.4 during laboratory work carried out by the applicant;

(11) FIG. 10 is an image of the electrolysis apparatus after the electrolysis reaction using a solution of 66 wt % MEA+34 wt % water and 0.1M NH.sub.4BF.sub.4 during laboratory work carried out by the applicant;

(12) FIG. 11 is a SEM image and EDS spectrum of the solid carbon produced by the electrolysis reaction using a solution of 66 wt % MEA+34 wt % water and 0.1M NH.sub.4BF.sub.4 during laboratory work carried out by the applicant;

(13) FIG. 12 shows the current density curves for the electrolysis using a solution of 10 wt % PEI+90 wt % water and 0.1M NH.sub.4BF.sub.4 during laboratory work carried out by the applicant;

(14) FIG. 13 is an image of the electrolysis apparatus after the electrolysis reaction using a solution of 10 wt % PEI+90 wt % water and 0.1M NH.sub.4BF.sub.4 during laboratory work carried out by the applicant;

(15) FIG. 14 is a current density curve for the electrolysis using a solution of pure MEA and 0.1M NH.sub.4BF.sub.4+2.5M H.sub.2O during laboratory work carried out by the applicant;

(16) FIG. 15 is an image of the electrolysis apparatus after the electrolysis reaction using a solution of pure MEA and 0.1M NH4BF4+2.5M H.sub.2O during laboratory work carried out by the applicant;

(17) FIG. 16 shows the current density curves for the electrolysis using electrolytes having different concentrations of DMF+MEA+0.05M NH.sub.4BF.sub.4+1M H.sub.2O during laboratory work carried out by the applicant;

(18) FIG. 17 is a CO.sub.2 absorption curve for different concentrations of MEA in the solution in the electrolysis using electrolytes having different concentrations of DMF+MEA+0.05M NH.sub.4BF.sub.4+1M H.sub.2O during laboratory work carried out by the applicant;

(19) FIG. 18 is an image of the electrolysis apparatus after the electrolysis reaction using a solution of DMF+MEA+0.05M NH.sub.4BF.sub.4+1M H.sub.2O during laboratory work carried out by the applicant.

DESCRIPTION OF EMBODIMENTS AND EXPERIMENTAL WORK

(20) The present invention comprises an electrolysis process and apparatus for reducing CO.sub.2 via electrolysis at an industrial scale and producing solid carbon and gaseous oxygen.

(21) The present invention is described below in relation to a number of, although not the only, embodiments of the invention and experimental work in relation to aspects of the invention.

Overview of the Embodiments of FIGS. 1-5

(22) In some embodiments, for example as described in relation to FIGS. 1-4, the electrolysis apparatus has one or more pairs of generally horizontally oriented cathode and anode plates, with the angle relative to the horizontal being less than 10 degrees. Each upward-facing cathode plate shown in the Figures has a generally non-conducting base and an upstanding perimeter wall that forms a tray which contains a static pool of liquid metal, as described herein, with a depth in the range 1-50 mm. The metal pool is electrically connected to the cathode and effectively becomes part of the cathode. The anode is positioned above the cathode as a parallel plate, with distance between the top of the liquid metal and the bottom of anode plate in the range 10-100 mm. The cathode and the anode are appropriately insulated from earth and connected to a direct current power source. The cathode potential is negative 1 to 4 V with reference to an Ag/Ag+(10 mM AgNO3 in acetonitrile) electrode. Under these conditions, in use, the electrolyte is supplied to the chamber via the inlet and is discharged from the chamber via the outlet. The electrolyte flows from the inlet to the outlet in fluid communication with the cathode-anode pair. The voltage applied between the cathode-anode pair causes solid carbon to form on the cathode from CO.sub.2 in the electrolyte and gaseous oxygen to be evolved at the anode from CO.sub.2 in the electrolyte. Solid carbon is discharged by being transported from the cathode in the electrolyte to the electrolyte outlet and from the chamber via the outlet. Gaseous oxygen is discharged from the chamber via a gas outlet.

(23) In other embodiments, for example as described in relation to FIG. 5, the cathode includes a vertically arranged flow-restricting element in the form of a porous medium across or through which the liquid metal, as described herein, percolates slowly under gravity. The porous medium is selected such that a more or less continuous liquid metal surface is presented to the electrolyte, whilst at the same time slowing gravity-induced downward flow such that the integrity of the liquid metal interface with the electrolyte is preserved. Using this type of porous medium allows the orientation of the cell to depart dramatically from horizontal, up to vertical. Under these conditions, in use, liquid metal passes through the porous medium, flowing under gravity from the top to the bottom. In order to maintain such a system, liquid metal that exits the porous element at the bottom is collected and returned to the top in such a way that, in use, the top region maintains a more or less continuous liquid metal-electrolyte interface at all times.

(24) The described embodiments of FIGS. 1-5 operate at a temperature in the range 0 to 200 C., and pressure in the range ambient to 50 bar g. The liquid metal may be any suitable metal-containing substance that is substantially liquid at the operating temperature of the cell and is capable of catalysing reduction of CO.sub.2 to solid carbon under the operating conditions. The liquid metal may include an active agent for catalysing carbon dioxide reduction, such as Ce or any other suitable substance.

(25) In the described embodiments of FIGS. 1-5, the electrolyte is a liquid at cell operating temperature with a relatively high capacity for holding dissolved carbon dioxide in solution. Water will be present in this electrolyte since this is an essential part of the reaction sequence for the embodiments.

(26) The selection of an electrolyte will depend on a number of factors, including pressureif the cell is to operate at the lower end of the range, then it could be advantageous to use a solvent such as dimethylformamide because this will significantly increase the mole fraction of dissolved carbon dioxide. At the higher end of the pressure range it may become advantageous to use water alone, since carbon dioxide solubility increases under these conditions.

(27) In the described embodiments of FIGS. 1-5, the electrolyte flows in a closed loop. It will be loaded with carbon dioxide in a saturator vessel to bring it close to saturation. This loaded electrolyte will then be pumped into the main electrolyser cell and will pass between the cathode and anode plates. The superficial velocity of the electrolyte in the liquid metal-to-anode gap will be in the range 0.05-5 m/s. Solid carbon flakes will be generated at the interface between the liquid metal and the electrolyte, while oxygen bubbles will be generated at the anode plate.

(28) In the described embodiments of FIGS. 1-5, the reaction products (solid carbon and gaseous oxygen) are managed in such a way that they do not interfere with ongoing electrolysis.

(29) Typically, the solid carbon will be in the form of flakes. Carbon flakes adhere only very weakly (if at all) to the liquid metal surface, so electrolyte flow (depending on velocity) may be sufficient to dislodge carbon and allow removal by simple convection. If adherence becomes more of an issue, techniques such as ultrasonic agitation or physical wave generation on the surface of the liquid metal (by mechanical or other means) may be used.

(30) Oxygen bubbles collecting at the underside of the anode could compromise electrical conductivity and slow the reaction if their volume fraction becomes too high. Simple convection of the electrolyte may be sufficient to manage this but, if not, appropriate apertures in the anode (holes or slots) may be provided to allow upward escape of oxygen bubbles. In the flat metal pool embodiments of FIGS. 1-4 the anode may also be angled to a modest degree (relative to the cathode) to further promote oxygen bubble removal, either in discrete stages (with individual gas outlets) or as a whole. In the porous cathode medium embodiment of FIG. 5, any angle may be used, with oxygen bubbles (most likely) rising counter-current to the flow of electrolyte. It should also be noted that operating pressure will have a significant impact on the volume of oxygen bubbles in the system, with higher pressures leading to reduced bubble volumes.

(31) In the described embodiments of FIGS. 1-5, as electrolyte approaches the electrolyte outlets, it is carbon dioxide-depleted and contains both carbon flakes and (at least some) oxygen bubbles. A gas-liquid separation stage allows oxygens to leave the system without carrying significant electrolyte with it. At the liquid outlet, a carbon filtering system removes product carbon from the electrolyte. This carbon filtration system may be any suitable system for removing substantially all the solid carbon from the electrolyte whilst maintaining the electrolyte in the liquid phase. From here depleted electrolyte will be sent to the saturator to complete the cycle.

(32) The described embodiments of FIGS. 3 and 4 include a stack of several anode/cathode pairs of electrodes within a common electrolyte bath for cost and efficiency reasons. In the case of a solid anode, this is accomplished by providing a layer of insulating material on the top surface of the anode and placing a second cathode-anode assembly on top (and so forth). If the anode is not solid (viz contains apertures for progressive upward escape of oxygen bubbles) then each cathode-anode assembly needs its own oxygen collection chamber at the top of the anode. Multi-stack assemblies are still possible with insulating layers between each cathode-anode pair, but in this case each oxygen offtake chamber will need a two-phase flow control device at the outlet in order to maintain reasonable fluid mechanics. This controller may be any suitable device, including a vertical lift column with re-injection of product oxygen gas (at a controlled rate) in order to manage suction pressure drop.

(33) It is not necessary to circulate liquid metal at all in the described embodiments of FIGS. 1-4.

(34) When the liquid metal includes catalysts such as Ce, the catalyst can regenerate itself locally, its working redox cycle can take place within a small local zone close to the metal-electrolyte interface. However, this does not mean that periodic (partial) liquid metal change-out is undesirable. It may be advantageous to replace a portion of the liquid metal inventory on a regular cycle (perhaps once a day) in order to clean and re-activate it by replacing or supplementing catalytically active ingredients before it is returned to service. In addition, the described embodiment of FIG. 5 includes liquid metal circulation.

(35) In the described embodiments of FIGS. 1-5, the electrolyte containing dissolved CO.sub.2 is pumped into the chamber via the electrolyte inlet. Typically, the electrolyte will contain water and may or may not include another solute such as dimethylformamide (depending on pressure). As the electrolyte passes through the cell it becomes depleted in CO.sub.2 and will eventually be pumped out via the electrolyte outlet, carry solid carbon. The electrolyte is then re-loaded with CO.sub.2 before being pumped back to the electrolyte inlet.

(36) This re-loading step involves dissolving CO.sub.2 gas into the electrolyte. One option for doing this is disclosed in WO 94/01210 in the name of Technological Resources Pty Ltd (12). WO 94/01210 describes a method for efficiently creating small gas bubbles in a high pressure liquid body by use of venturi aspirators. Although there are several ways to promote gas dissolution into liquids, this option is considered particularly well suited and is a strong candidate for electrolyte re-loading with carbon dioxide. The disclosure in WO 94/01210 is incorporated herein by cross-reference.

FIG. 1 Embodiment

(37) FIG. 1 is a schematic diagram of Embodiment A of a process and an apparatus for electrolysis of carbon dioxide into solid carbon and gaseous oxygen according to the invention.

(38) With reference to FIG. 1, the apparatus includes an electrolysis chamber 101 and a CO.sub.2 saturator 102. Carbon dioxide 103 is dissolved in the electrolyte in saturator 102 via a venturi aspirator which forms part of saturator 102. Loaded electrolyte 105 containing substantial dissolved carbon dioxide is fed into electrolysis chamber 101 via electrolyte pump 104.

(39) Inside chamber 101 a pool of electrolyte 106 is maintained, with an oxygen-rich gas space 107 above. Cathode 108 comprises a large horizontal solid plate with a fully surrounding weir constructed from non-conducting material 109 on its upper face. A liquid metal pool 110 with a depth of 5-10 mm is maintained on the upper face of the cathode, in direct electrical contact with cathode 108.

(40) Anode 111 comprises a parallel flat plate set 30-80 mm above the surface of liquid metal 110. Power supply 112 is connected to the cathode-anode pair to maintain a voltage in the range 1 to 10 volts, typically 2-6 volts, more typically 2-4 volts.

(41) In use, loaded electrolyte 105 is pumped from left to right as shown, at a superficial liquid velocity in a gap between liquid metal 110 and the bottom of anode 111 in a range 0.1-1 m/s.

(42) Both oxygen bubbles 113 and carbon flakes 114 are transported to the right by electrolyte convection. As they leave the cathode-anode gap (at right hand extreme), oxygen bubbles rise into gas space 107 and from there pass through demister 115 where any residual electrolyte is removed and returned to cell 101. Final oxygen product 116 is removed for compression and re-use or else is vented.

(43) Electrolyte containing carbon flakes 117 enter filter 118 where solid carbon is removed. Carbon product 119 is collected and removed for storage or re-use.

FIG. 2 Embodiment

(44) FIG. 2 is a schematic diagram of Embodiment B of a process and an apparatus for electrolysis of carbon dioxide into solid carbon and gaseous oxygen according to the current invention.

(45) With reference to FIG. 2, all numerals have been incremented by 100 and otherwise have the same meaning as those in FIG. 1, apart from anode 211 and oxygen bubbles 213.

(46) In FIG. 2, anode 211 contains apertures that allow progressive release of oxygen bubbles 213 upwards and away from the reaction zone. If conditions in the electrolyser with Embodiment A (FIG. 4) are such that there is a large gas fraction trapped under the anode and this compromises anode efficiency, then the Embodiment B (FIG. 1) becomes a more preferred embodiment.

FIG. 3 Embodiment

(47) FIG. 3 is a schematic diagram of Embodiment C of a process and an apparatus for electrolysis of carbon dioxide into solid carbon and gaseous oxygen according to the current invention.

(48) With reference to FIG. 3, the general layout and operation of components is the same as that in Embodiment A (FIG. 1) apart from electrode arrangements.

(49) FIG. 3 shows 6 electrode pairs stacked on top of one another in a common electrolyte pool. Insulation layer 320 separates each pair of electrodes, allowing each pair to operate without electrical interference from the pair above or the one below.

(50) If current densities for the arrangement in Embodiment A (FIG. 1) are too low for acceptable economics, then a stacked arrangement such as Embodiment C (FIG. 3) is an option for improving cost efficiency.

FIG. 4 Embodiment

(51) FIG. 4 is a schematic diagram of Embodiment D of a process and an apparatus for electrolysis of carbon dioxide into solid carbon and gaseous oxygen according to the current invention.

(52) With reference to FIG. 4, the general layout and operation of components of this embodiment are the same as that in Embodiment C (FIG. 3) apart from anode 411, oxygen bubbles 413 and flow control devices 421.

(53) In this case, electrode pairs are again stacked on top of one another as in Embodiment C. The key difference is that each anode 411 has apertures for progressive oxygen bubble release, together with an individual oxygen collection chamber 413 and a flow controller 421. As with Embodiment B, this variant will be preferred if the gas fraction immediately below each anode becomes too high to support efficient operation.

(54) Each oxygen collection chamber 413 will have both electrolyte and gas flowing through inside it. This involves reasonably complex two-phase flow which will need to be controlled carefully. Pressure drop across each individual oxygen chamber will need to be roughly the same (as those of other chambers) despite the difference in liquid head from the chamber outlet to the head-space of electrolysis chamber 401. It will therefore be necessary to provide each oxygen chamber 413 with its own flow restrictor or control device for this purpose.

FIG. 5 Embodiment

(55) FIG. 5 is a schematic diagram of an electrolysis cell with a vertically oriented electrode pair utilising a cathode involving flow-restricting element in the form of a porous medium 509.

(56) The primary difference relative to the embodiment D of FIG. 4 is that, instead of a more or less static liquid pool of metal, this embodiment has the flow restricting element in the form of a porous medium 509 connected to the cathode.

(57) The porous medium (and other suitable options for flow restricting element) allows liquid metal to percolate slowly downwards in such a way that a more or less continuous metal surface is presented to the electrolyte. Metal that has percolated through the porous medium 509 is collected in a chamber 510 and returned to the top of the porous element in order to retain cathode interface integrity.

(58) An advantage of this arrangement is that oxygen bubbles will rise under gravity and be easier to remove. It is also possible to arrange such a system at an angle other than vertical to assist further with oxygen bubble removal.

(59) Overview of the Embodiments of FIGS. 6 and 7

(60) FIGS. 6 and 7 show embodiments of an iron making process and apparatus in accordance with the invention that include the electrolysis process/apparatus of the invention, for example the embodiments shown in FIGS. 1-5.

FIG. 6 Embodiment

(61) A conventional HIsarna direct smelting process uses technical-grade oxygen, fine coal and BF-quality iron ore fines. It produces hot metal plus an off-gas with >90% CO.sub.2; from here it is relatively easy to get to pure CO.sub.2. The electrolysis process/apparatus of the invention, for example the embodiments shown in FIGS. 1-5, closes the loop by reacting the CO.sub.2 back into carbon and technical-grade oxygen.

(62) FIG. 6 shows a green steel application of the electrolysis process/apparatus of the invention, for example the embodiments shown in FIGS. 1-5, and a HIsarna direct smelting unit for producing molten iron.

(63) HIsarna smelter 601 converts iron ore 602 and recovers carbon fines from storage 607 directly into hot metal 603 and CO.sub.2-rich off-gas. This off-gas is then compressed and cooled (with removal of non-condensable gas species) prior to being stored as a liquid in tanks 708. When intermittent green power 606 becomes available, electrolysis apparatus 605 (for example, one of the embodiments of FIGS. 1-5) starts up and converts CO.sub.2 into oxygen 608 and solid carbon 609.

(64) The amount of carbon needed in the HIsarna plant, exceeds the amount recovered from CO.sub.2 when hot metal 603 contains around 4% carbon. This carbon deficit can be made up in a number of ways, including by feeding another source of non-fossil carbon (e.g. dried biomass) into HIsarna plant 601.

FIG. 7 Embodiment

(65) The embodiment of a process and an apparatus for producing molten iron show in FIG. 7 is based on the use of a blast furnace.

(66) A key difference relative to direct smelting process/apparatus shown in FIG. 6 is that solid coke lumps are needed in the blast furnace in order to maintain shaft porosity. This requires conversion of at least a portion of recovered carbon for the electrolysis process/apparatus into lumps with suitable strength, using a binding agent such as bio-oil, tar or other suitable material.

(67) Blast furnace 701 converts iron ore 702 and synthetic coke from coke ovens 712 into hot metal 703. Top gas from blast furnace 701 is captured in CO.sub.2 scrubber 701a, and from there is sent to CO.sub.2 tank storage 704. When intermittent green power 706 becomes available, electrolysis apparatus 705 starts up and converts CO.sub.2 into oxygen 708 and solid carbon 709.

(68) Some fine carbon from storage 709 may be used as a substitute for pulverised coal injection (PCI) in blast furnace 701, but at least a portion needs to be formed into lumps in briquette plant 710 using a binding agent such as bio-oil, tar or other suitable medium 711. Green briquettes can then be converted into synthetic coke in coke plant 712 before being returned to blast furnace 701.

(69) Many modifications may be made to the embodiments described in relation to FIGS. 1-7 without departing form the spirit and scope of the invention.

(70) By way of example, whilst the cathode of the embodiment shown in FIG. 5 includes a flow-restricting element in the form of a porous medium, the invention is not so limited and extends to any suitable flow-restricting element that is formed and positioned in the chamber so that the liquid metal can percolate, typically slowly, under gravity through the flow-restricting element from an upper inlet to a lower outlet. Other examples of the flow-restricting element include mesh-based elements or solid elements having a series of surface features that cause a flow restriction.

(71) By way of example, whilst the embodiments of the electrolysis process and apparatus shown in FIGS. 1-5 are described with reference to applications in ironmaking and steelmaking in the embodiments shown in FIGS. 6 and 7, the invention is not so limited, and the electrolysis process and apparatus of the invention can be used in other applications, such as: (i) Centralised (for example, fossil fuel-derived) CO.sub.2 collection with buffer storage, in conjunction with the electrolyser and the electrolytic process of the invention, for example, using intermittent renewable power, and carbon product disposal in disused mines or other applications. (ii) Remote CO.sub.2 removal from air (for example, based on renewable energy), coupled with the electrolyser and the electrolytic process of the invention and carbon product disposal in disused mines or other applications. (iii) Carbon dioxide recirculation systems for applications requiring such systems, such as nuclear submarines.
Summary of Experimental Work

(72) Experimental work in relation to the present invention was carried out by the applicant, including the Department of Chemical Engineering at the University of Melbourne, Melbourne, Victoria.

(73) The purpose of the experimental work was to demonstrate that the invention can produce solid carbon and O.sub.2 gas from CO.sub.2 via electrolysis with a liquid electrolyte containing CO.sub.2 in solution and a cathode-anode pair, with the cathode being in the form of a liquid metal as defined herein capable of catalysing reduction of CO.sub.2 to solid carbon, without the cathode fouling over time.

(74) The experimental work included but was not limited to the experimental work summarised below: The experimental work was conducted on a small-scale batch basis. Electrolysis of CO.sub.2 with a liquid metal cathode (cerium-containing Galinstan) and different electrolyte solutions in an electrolysis apparatus. The electrolysis apparatus was set up as a beaker with the liquid metal cathode and a metal wire anode connected to a power supply, as shown in FIG. 8. The electrolysis apparatus included a Ag/AgCl reference electrode (RE), 3M KCl as a reference electrolyte, and an applied voltage of 0.120V vs. Normal Hydrogen Electrode (NHE). The liquid metal cathode formed a layer at the bottom of the beaker. In some embodiments of the experiment, smaller amounts of liquid metal forming a flattened droplet or marble of liquid metal were used in place of the liquid metal layer (in order to preserve reagent materials). The electrolyte formed a top layer above the layer (or marble) of liquid metal. A number of different electrolytes were tested. The electrolytes included solutions of organic solvents (such as MEA or polyethylenimine (PEI) or DMF), electrolyte salts (such as NH.sub.4BF.sub.4), and other CO.sub.2 absorbing agents.
Experimental ResultsSummary Solid carbon and O.sub.2 were produced in each experiment described below. The reaction rate for solid carbon generation using electrolytes containing amines (such as MEA or PEI) was several orders of magnitude greater than published experimental work carried out without a CO.sub.2 absorbing agent (7). The current density results indicate that the liquid metal cathode did not foul during the duration of the experiments. The current densities in the experiments were low but explicable and not a concern.

(75) The current densities were calculated based on the surface areas of the liquid metal cathodes, which is significantly larger than the surface areas of the anode wires.

Experiment 1

(76) A solution of 66 wt. % MEA+34 wt. % water and 0.1M NH.sub.4BF.sub.4 was used as the electrolyte in the electrolysis of CO.sub.2 to produce solid carbon. Galinstan was used as the liquid metal cathode. CO.sub.2 was injected in the solution before electrolysis. The applied voltage was 1.5V vs. RE. The current density (j) curve is shown in FIG. 9. The Figure shows that the current density decreased quickly from the start of the experiment to a value of 0.5 mA/cm.sup.2 and remained substantially constant for the remainder of the duration of the experiment. This substantially constant current density is an indication that the liquid metal cathode did not foul during the duration of the experiment. The image of the apparatus after the electrolysis reaction is shown in FIG. 10. It is evident from the Figure that solid carbon was produced. This is evident from the solution colour with small carbon particles dispersed in the solution and small carbon particles on the surface. The surfaces of solids in the beaker were viewed in a SEM and elemental analysis of samples were performed using EDS, with the results are shown in FIG. 11. The 2 SEM images in the Figure show the solid carbon. The EDS results in the Figure shows that the solids contained 63.3 wt. % carbon.

Experiment 2

(77) A solution of 10 wt. % PEI+90 wt. % water and 0.1M NH.sub.44BF.sub.4 was used as the electrolyte in the electrolysis of CO.sub.2 to produce solid carbon. Galinstan was used as the liquid metal cathode. CO.sub.2 was injected in the solution before electrolysis. The applied voltage was 1.2V vs. RE. The current density curves are shown in FIG. 12. There are two curves. The left-hand curve shows that the current density decreased linearly until it reached a value of 0 mA/cm.sup.2 as the voltage was decreased from 3V vs RE to 1.2V vs. RE. The current density remained constant as the voltage was decreased from 1.2V vs. RE to 0.6V vs RE. The right-hand curve shows that the current density decreased quickly to a value of 0.4 mA/cm.sup.2 and slowly increased for the remainder of the duration of the experiment. The image of the apparatus after the electrolysis reaction is shown in FIG. 13. It is evident from the Figure that solid carbon was produced. This is evident from the solution and colour and large carbon flakes in a lower section of the beaker.

Experiment 3

(78) A solution of pure MEA and 0.1M NH.sub.4BF.sub.4+2.5M H.sub.2O was used as the electrolyte for the electrolysis of CO.sub.2 to produce solid carbon. Galinstan was used as the liquid metal cathode. The solution was left in air and no CO.sub.2 was injected in the solution before electrolysis. The applied voltage was 1.4V vs. RE. The current density curve is shown in FIG. 14. The Figure shows that the current density increased quickly for an initial time period, then decreased very quickly and then remained substantially constant for the remainder of the duration of the experiment. This substantially constant current density is an indication that the liquid metal cathode did not foul during the duration of the experiment. The image of the apparatus after the electrolysis reaction is shown in FIG. 15. It is evident from the Figure that solid carbon was produced. The solid carbon is in a lower section of the beaker.

Experiment 4

(79) Solutions of 5 different concentrations of DMF+MEA and 0.05M NH.sub.4BF.sub.4+1M H.sub.2O were used as electrolytes for the electrolysis of CO.sub.2 to produce solid carbon.

(80) Galinstan was used as the liquid metal cathode.

(81) Co.sub.2 was injected in the solution before electrolysis.

(82) The applied voltage was 2.0V vs. RE.

(83) The current density curves are shown in FIG. 16. The Figure shows that increasing proportions of MEA in the combinations of DMF+MEA in the electrolytes had an impact on the current density. For all the different concentrations of MEA in the electrolyte solution, the current densities decreased linearly until a value of 0 mA/cm.sup.2 as voltage was decreased from 3.0V vs RE to 1.0V vs RE.

(84) FIG. 17 shows the relationship between the CO.sub.2 absorption for different concentrations of MEA in the electrolyte solution is shown in FIG. 17. In particular, the Figure shows that the CO.sub.2 absorption increased with increasing MEA as a percentage of DMF+MEA.

(85) The image of the apparatus after the electrolysis reaction for one of the experiments is shown in FIG. 18. It is evident from the Figure that solid carbon was produced. The solid carbon is in a lower section of the beaker.

(86) Whilst the experimental work was conducted in a beaker at a small-scale and not carried out in a full-scale equipment, the applicant believes that the same or similar results would be obtained if carried out on a larger scale.

(87) Modeling carried out by the applicant indicates that the experimental data for the electrolytes tested can be extrapolated to other electrolytes.

(88) Moreover, whilst the experimental work was carried out on a small-scale batch basis, the applicant expects that the results are equally applicable to the embodiments of the electrolysis apparatus of the invention shown in FIGS. 1-5, noting that the purpose of the experimental work was to demonstrate that the invention can produce solid carbon and O.sub.2 gas from CO.sub.2 via electrolysis with a liquid electrolyte containing CO.sub.2 in solution and a cathode-anode pair, with the cathode being in the form of a liquid metal as defined herein capable of catalysing reduction of CO to solid carbon, without the cathode fouling over time.

REFERENCES

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