A method and an electrolysis cell for production of a metal from a molten chloride

The present invention relates to a method for production of liquid zinc and gaseous chlorine from a molten chloride electrolyte containing zinc chloride and an electrolysis cell for performing the method.

Several patents and other literature related to the electrolytic production of zinc from zinc chloride exist. These patents and reports essentially describe electrolytic cells with a single compartment containing all the electrodes. The present invention, on the other hand, describes an electrolytic cell with at least two compartments, of which at least one compartment contains the electrodes (electrode chamber) and at least one compartment lies next to the electrode chamber. The chambers are separated by a partition wall that allows for flow of electrolyte between the compartments. The electrodes are vertical, horizontal or tilted with some angle.

Bureau of Mines reports 8133 and 8524 (US Dep. of the Interior) both describe electrowinning of zinc from ZnCI ₂ fused salt cells. Report 8133 presents results from electrolysis using two monopolar electrodes, while report 8524 relates to electrolysis in both monopolar and bipolar cells. The electrodes in all cells are horizontal or slightly tilted from the horisontal position.

WO 2004/074552 A1 describes production of zinc and chlorine from molten ZnCI ₂ in bipolar electrolysis cells with tilted electrodes.

Further cell designs are known from electrolytic production of magnesium, and are used for both multi-monopolar (US 4,308,116) and multi-bipolar electrodes (GB 8800674). However, magnesium metal is lighter than the electrolyte and floats on top of it while zinc is heavier than the electrolyte and will collect on the bottom of the cell. The design of a cell with two or more compartments for electrolytic production of zinc will therefore differ considerably from an electrolytic cell for magnesium production.

EP 1364077 B1 describes an electrolytic cell for production of aluminium and oxygen from a molten fluoride/oxide electrolyte comprising non-consumable anodes. The described cell has separate compartments, one compartment for the electrodes, and one gas separation chamber. The purpose of the gas separation chamber is to ensure efficient removal of oxygen from the electrolyte. The produced aluminium sinks to the bottom of the cell where it

enters a third metal collection compartment to protect it from the oxygen dissolved in the electrolyte.

In accordance with the present invention as defined in the accompanying claims, there is now presented a novel method and electrolysis cell for production of Zinc that will ensure proper flow conditions in the electrolyte.

Due to the design of the cell and the corresponding method for operating same, the upward flow of the chlorine bubbles produced on the anode (3) creates a drag on the electrolyte which leads to an upward flow between the anodes and the cathodes. In a single compartment cell, this flow can lead to volumes with particularly turbulent electrolyte flow (upper part of the cell) and volumes with almost zero flow velocities (below the electrodes). Both situations are undesirable. High turbulence can lead to increased cell wear and recombination between chlorine and zinc, while low velocities can cause collection of sludge. In the following, the present invention shall be described by examples and figures where:

Fig. 1 shows the principal components of a cell with two compartments according to the present invention, shown in a cross sectional end view,

Fig. 2 shows the principal components of the cell shown in Fig. 1 , shown in a cross sectional top view,

Fig. 3 shows the principal components of the cell shown in Fig. 1 , shown in a cross sectional side view.

With reference to Fig. 1 there is in a cross sectional view shown an electrolysis cell with an electrolysis chamber 2 and one adjacent chamber 1. Figure 2 shows a top view of the same cell in the level of the cathodes with the same numerical references. It should be understood that several configurations of chambers are possible. One may for example have two separated electrolysis chambers sharing a central common adjacent chamber. In the Figures, reference numerals 3 and 4 are the anode and cathode, respectively. In the embodiment shown, the anode 3 is inserted through the top, while the cathode 4 is inserted from the side. It should be understood that the opposite configuration is equally possible, as are configurations with only top inserted electrodes, only side-inserted electrodes, or configurations with bottom-inserted electrodes. For bottom or side inserted electrodes,

proper cooling of the electrode head is important to avoid electrolyte leakage from the cell. Bipolar electrode configurations are also possible. In that case, only the end cathode(s) and anode(s) need to be inserted into the cell. The bipolar electrodes will be completely immersed into the electrolyte. Bipolar electrodes also allow for inclination of the electrodes. Inclination to nearly horizontal electrode configuration is possible. On inclined electrodes, chlorine is produced on the electrode surface facing downwards, and Zn on the surface facing upwards.

Further with reference to Fig. 1 , reference numeral 5 is indicating the Zn pool. As Zn is produced, it will collect on the bottom of the cell, and regular metal tapping is required. At the upper part of the cell, there is arranged a chlorine outlet 6. Metal can be removed through opening 9 and ZnCI ₂ addition can be performed through one opening 10. Depending on the height between the cell bottom and the cell lid, the metal can be sucked off or pumped out of the cell. Due to the density of Zn, suction is only efficient for heights below approx. 1.5 m. At larger heights, pumping is required. Addition of ZnCI ₂ is preferably made into the electrolysis chamber since ZnCI ₂ usually has a higher density than the electrolyte. The mixing in of ZnCI ₂ is more effective in the electrolysis chamber than in the adjacent chamber since convection is stronger in the electrolysis chamber. ZnCI ₂ addition in to the adjacent chamber is, however, also possible. ZnCI ₂ can be fed as either a liquid or a solid. Reference numerals 7 and 8 are indicating the partition walls (in cross sectional view) separating the electrolysis chamber from the adjacent chamber.

Figure 3 shows a side view section through the partition wall with the same numerical references as Figs. 1 and 2. By sufficient immersion of partition wall 8, separation of the atmospheres in the two chambers is achieved. The electrolysis chamber then contains mainly chlorine, while the adjacent chamber contains mainly air or a suitable inert gas. Partition wall 7 will assist the generation of a circular electrolyte flow indicated by the arrows in Fig. 1. The velocity of the electrolyte can be controlled by adjustment of the gap between wall 7 and 8, and/or the gap between wall 7 and the bottom of the cell. With reference to Fig. 3, reference numerals 11 and 12 indicate support pillars for the upper and lower partition walls.

The purpose of a cell design with two or more compartments for the production of zinc is to set up a controllable flow of the electrolyte in the cell. The upward flow of the chlorine bubbles produced on the anode (3) creates a drag on the electrolyte, which leads to an upward flow between the anodes and the cathodes. In a cell with two or more compartments, the upward flow of electrolyte can be directed from the electrode compartment 2 to the

adjacent compartment(s) 1 , and from the adjacent compartment(s) the electrolyte will flow back into the electrode compartment below the electrodes, thereby creating a circular flow. The downward flow in the adjacent chamber is preferably slower than the upward flow in the electrode chamber, which can be achieved by a large flow cross-section in the adjacent chambers.

This circular flow has several advantages: The electrolyte flow can be of a rather substantial laminar nature; the flow through the adjacent chamber(s) allows for efficient separation of small chlorine bubbles and electrolyte; small metal droplets that settle slowly will settle in the adjacent chamber(s) rather than recombine with chlorine in the turbulent flow above the electrodes; the residence time of low-density solid oxide particles in the electrolyte will increase, thereby reducing sludge formation by allowing for more efficient chlorination (M _x O + Cl ₂ = MCI ₂ + O ₂ ). Chambers separate from the electrode chamber also have the advantage that metal removal and chlorine extraction can be separated. Otherwise special means to avoid Cl ₂ leakage from the cell during metal removal must be implemented.

In the cell, several materials choices can be made. The anode is preferably a carbon material. Graphite is preferred due to its relatively low electrical resistance. The cathode can also be a carbon material, but electronically conductive ceramics such as TiB ₂ , can also be used. Inert or near inert metals such as Mo, W and Nb can be applied. The advantage of conductive ceramics and metals over carbon is that carbon does not wet liquid Zn, and therefore the Zn is produced as very fine droplets. Larger Zn droplets are advantageous from both a current efficiency and metal collection point of view.

The cell itself can be made from a steel shell lined with suitable brickwork, e.g. alumina based, silica based, carbon materials, silicon nitride based, silicon carbide based, aluminium nitride based, or combinations of these.

The electrolyte must contain ZnCI ₂ . The ZnCI ₂ should preferably be free from moisture, oxides and hydroxides, but some contaminations can be accepted. In addition, it is preferable to use one or more other chlorides to increase electrical conductivity, reduce the viscosity, hygroscopicity, and the vapour pressure of ZnCI ₂ . Typical chlorides to add are LiCI, NaCI and KCI, but also alkali earth chlorides and other alkali chlorides can be used. The ZnCI ₂ concentration can range from a few weight percent up to 80 w%. The temperature of the electrolysis can range from the melting point of Zn (420 ⁰ C) and upwards.