AU725314B3 - Electrolytic cell for production of magnesium - Google Patents

Description

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A PETTY PATENT

ORIGINAL

Name of Applicant: Actual Inventors: Address of Service: Invention Title: RUSSIAN NATIONAL ALUMINUM AND MAGNESIUM INSTITUTE (VAMI) Alexander N. Tatakin and Vera N. Boytseva.

BALDWIN SHELSTON WATERS MARGARET STREET SYDNEY NSW 2000 ELECTROLYTIC CELL FOR PRODUCTION OF MAGNESIUM The following statement is a full description of this invention, including the best method ofperforming it known to me/us:- ELECTROLYTIC CELL FOR PRODUCTION OF MAGNESIUM FIELD OF THE INVENTION This invention relates to the metallurgy of non-ferrous metals, and in particular, to an apparatus and method for electrolytic production of magnesium.

BACKGROUND OF THE INVENTION As conventional in the art to which the invention pertains, the magnesium metal can be produced upon passing direct electric current between anodes and cathodes positioned in an electrolyte or a molten electrolytic bath containing magnesium chloride. In this arrangement, the bath is maintained at a temperature above.the melting point of magnesium. Thus, the current heats the electrolyte or the bath and results in electrolysis of magnesium chloride contained therein. This causes molten magnesium metal to be released at the cathode surfaces. Since the metal is lighter than the bath it rises along the cathode surfaces.

Simultaneously, the chlorine gas rises through the bath from each anode surface to be collected outside the bath electrolytic chamber. In the process, it is essential to keep the released free magnesium metal isolated from the evolving chlorine gas and to prevent recombination of magnesium and chlorine. Such recombination decreases the efficiency of magnesium production since it returns the produced metal to the bath in the form of recombined magnesium chloride.

Typically the anodes are made of graphite, while the cathodes are fabricated as steel plates. The spacing between the respective cathodes and anodes is determined by the requirement that the released magnesium metal flowing upwardly along the cathode surfaces is kept away from contact with the chlorine gas liberated at the surface of the anodes.

A typical electrolytic cell for magnesium production includes an electrolytic or electrolytic chamber with anodes installed from a top area thereof. Cathodes are introduced through a side refractory wall of the cell and metal collecting compartment is disposed substantially perpendicular to electrodes (see German Patent 1,122,263 published August 23, 1962). In each cel, two operating cathodes each having only one working surface are positioned between successive anodes. The collecting chamber is separated from the electrolytic chamber by a self-supporting refractory curtain wall which is formed with an operational opening situated below the level of the bath or electrolyte in the cell. In this arrangement, the produced magnesium metal is moved to the collecting compartment in the following manner: in the course of electrolysis, the chlorine generated at the anodes forms an upwardly directed flow of electrolyte or bath in the spaces between the electrodes, i.e. in the spaces between anodes and cathodes.

this flow entrains the produced free magnesium and brings the metal to a channel situated above the cathodes. The magnesium metal moves along the cathode and through the operational opening in the curtain wall and enters a collecting chamber for accumulation therein. The electrolytic bath flow by-passes the cathodes and is returned back to inter-electrode spacing, while moving in the plane perpendicular to the surfaces of cathodes. With such circulation around the cathodes, the flow velocity is rather high. Thus, a part of the produced magnesium metal does not enter the collecting channel, but is entrained by the bath flow and is brought back to the inter-electrode spacing, wherein it undergoes highly undesirable recombination through the reaction with the evolving chlorine gas. This results in the losses of the produced magnesium metal and reduction of the electrical current efficiency of the process.

In the electrolytic cell disclosed by US Patent No.

4,055474 to Sivilotti the above discussed drawbacks are somewhat reduced by positioning of the electrodes at an angle to the working' surfaces. The anodes in the Sivilotti patent are formed as inverted semi-conical structures. The cathodes are located so as to increase the inter-electrode distance in the bottom to top direction and to increase the distance between bottom ends of the adjacent cathodes. Such design and the respective positioning of electrodes result in the limited reduction of the velocity of the electrolytic bath flow. It should be noted, however, that such reduction of the flow velocity is not sufficient and a certain amount of the produced magnesium metal is still captured by the flow and is directed back into the inter-electrode spacing.

This causes undesirable recombination of magnesium and chlorine and reduction of the electrical current efficiency.

A further important drawback of the electrolytic cell of the Sivilotti patent is similar to the drawbacks of other prior art references, that cathodes having only one operational surface are utilized. Such structure results in the increased distance between the anodes causing the reduction of the electrolytic cell production capacity.

Summary of the Invention An important object of the invention is to increase the production capacity of electrolytic cell by means of improvement of the electrical current efficiency and lowering the consumption of electrical power.

One aspect of the invention provides an electrolytic cell for production of magnesium having an improved circulation of the electrolyte or electrolytic bath and facilitating delivery of magnesium metal to and collection of the metal at the collection chamber. The electrolytic cell of the invention also prevents recombination of magnesium and chlorine and includes a refractory wall housing defining at least one electrolytic chamber containing the electrolytic bath and adapted for conducting of electrolysis aud at least one metal collection chamber for collecting of a magnesium metal at an upper area thereof. The electrolytic and collecting chambers are separated by a refractory curtain wall extending substantially upwardly within the refractory structure from a floor to a top part thereof. Mutually spaced cathode and anode means are arranged in an alternate substantially parallel array in the electrolytic chamber and extending transversely relative to the partitioning wall.

The refractory curtain wall is formed with first and second operational openings, wherein the first operational opening is situated above the cathode means and below the surface of the bath and the level of the magnesium metal in the collecting chamber. The second operational opening is situated at the floor of the refractory structure. The electrolytic bath is circulated from the electrolytic chamber along surfaces of at least the cathode means through the first operational opening into the collecting chamber.

There, the flow of the fused bath is directed downwardly and through the second operational opening back to the electrolytic chamber substantially without entraining the collected magnesium metal.

As to another aspect of the invention, the refractory housing is formed with two electrolytic chambers, separated by the collecting chamber. The anode means consists of a plurality of upright anode elements and the cathode means includes a multiplicity of cathode elements interspread with the anode elements in space relation thereto. Each cathode element has two operating surfaces situated opposite each other.

Still a further aspect of the invention provides the electrolytic cell for production of magnesium in which each cathode element is a substantially flat steel plate extending transversely to and spaced from the curtain wall.

Each cathode element having two working surfaces positioned opposite each other and facing two successive anode elements. A first baffle member is provided interconnecting ends of the cathode elements situated at the curtain wall. A second baffle member interconnects the cathode elements at a wall of the electrolytic chamber remote from the curtain wall, so that the cathode elements and the baffle members form an envelope surrounding the anode elements.

Brief Description of the Drawings.

Fig. 1 is a schematical top plan section view of the electrolytic cell of the invention according to section line 1-1 of Fig. 2; and Fig. 2 is a side elevational section view of the cell according to section line 2-2 of Fig. 1, with arrows showing the direction of the molten bath or electrolyte movement.

Description of the Preferred Embodiment Referring now to Figures 1 and 2 wherein the preferred embodiment of the electrolytic cell of the invention for production of magnesium is illustrated. The electrolytic cell housing 10 is a refractory wall structure formed with two electrolytic chambers 2 which are separated by a collecting chamber 3.

A refractory curtain or partitioning wall 4 is positioned between the collecting and the electrolytic chambers. These curtain walls extend substantially upwardly within the refractory housing of the electrolytic cell from an area of the bottom floor 12 to a top part thereof. As conventional in the present art, the walls and the floor of the electrolytic cell can be made of heavy refractory construction utilizing refractory blocks. Each electrolytic chamber 2 includes an outlet duct at its top portion for removal of chlorine gas. The electrolytic chambers 2 are enclosed at the top, so as to provide gas-tight seal.

7 Anode and cathode means form a part 'of the electrolytic chambers 2. The anode means is typically in the form of multiple plate-like graphite elements 6, each having two working surfaces 14 and 15 opposing each other.

The anode members are projected downwardly into the electrolytic chamber 2, so that the lower edges of the anode members are situated near the bottom floor area 12.

The cathode means consists of a plurality of cathode elements 7 typically fabricated in the form of steel plates.

The cathode elements 7 are arranged at the localities between successive anodes, so that the electrodes alternate in mutually substantially parallel array along the electrolytic chambers. The -cathode elements 7 extend substantially from an outer wall of the housing to the curtain wall of the respective electrolytic chamber. As illustrated in Fig. i, each cathode element 7 has two planar working surfaces 16 and 17 disposed in facing spaced relation to the working surfaces of the corresponding anode elements.

The cathode elements 7 extend longitudinally within the respective electrolytic chamber for a distance slightly longer than the extent of the respective anodes. In the vertical direction, each cathode element 7 extends substantially upwardly from the level which is slightly above the lower end of the corresponding anode element 6 to a higher level which is below the level of the molten bath in the respective electrolytic chamber 2. In the invention, the active area in vertical direction of each anode working surface may be considered to extend between about the lower end of the anode and the upper level corresponding to the upper level of the respective cathode. The active vertical area of each cathode working surface includes the entire cathode surface.

8 Each curtain wall 4 contains a first operational opening 5 and a second operational opening 9 separated by a solid portion of the wall. The first operational openings are provided at a level above the upper region of cathodes 7 and below the level of electrolytic bath whereas the second operational openings 9 are situated in the respective curtain walls at the floor area 12.

In the electrolytic chamber, the cathodes elements 7 can be connected by a baffle member 8. As best illustrated in Figure i, one baffle member interconnects ends of the cathodes elements situated at the curtain wall 4, whereas another baffle member interconnects the cathode elements at a wall of the electrolytic chamber which is opposite to the curtain wall. The baffle members 8 can be in the form of a single element interconnecting all cathodes alternatively, or can be in the form of a plurality of shorter members connecting two successive cathodes. Thus, in the electrolytic chamber 2 each par of successive cathode elements 7 and interconnecting portions of the baffle members 8 form an envelope surrounding the anode elements interposed between this pair of elements. As illustrated in Fig. 1, in the preferred embodiment of the invention, the great majority of the anodes is surrounded by these envelopes.

In an alternative embodiment, the cathode means can be in the form of a self-supportive rigid frame situated within the electrolytic chamber. Similar to the above discussed embodiment, the frame is formed by the baffle members interconnecting both ends of the cathode elements. However, in view of the self-supportive status, the frame does not require anchoring of the cathode elements within the outer walls of the housing. Thus, this arrangement prevents destruction of the outer refractory walls in the area of potential engagement with cathode elements. The frame defined by the cathode elements and the baffle members also form envelopes completely surrounding the respective anode elements. The frame construction increases the active surfaces of the electrodes and intensifies circulation of the electrolytic bath within the cell.

In operation of the apparatus of the invention, the electrolytic chamber 2 is filled to a predetermined level with the electrolyte or electrolytic bath containing magnesium chloride. By means of a suitable source of energy, a direct electric current is passed through the bath between the facing each other working surfaces 14,15 of the anodes and 16,17 of the cathodes. .Continuous passage of the electrical current results in electrolysis of the molten chemicals, including magnesium chloride contained in the bath. Free magnesium metal is deposited in the molten state on the surfaces of the cathodes. Since the magnesium metal is lighter than the bath, it flows upwardly along the working surfaces of the cathodes to be ultimately received and accumulated in the collecting chamber. Simultaneously, the chlorine gas is continuously evolved at each anode and rises from the anodes to be collected in a gas space above the bath level.

In the course of electrolysis, the chlorine gas generated at the anode elements 6 forms.-an upwardly directed flow of the electrolytic bath in the spaces between the electrodes. This flow entrains the produced metal magnesium. Through the first operational openings 5 in the curtain wall 4, the bath flow containing magnesium metal enters the collecting chamber 3. The magnesium metal is accumulated in the top region of the collecting chamber 3.

While in the collecting chamber, the flow of electrolytic bath is directed downwardly and through the second operational opening 9 at the bottom floor area 12 is circulated back to the electrolytic chamber 2. In the apparatus of the invention, the flow of the bath is smoothly returned to the electrolytic chamber substantially without entraining of the accumulated magnesium metal.

Thus, the produced free magnesium metal accumulated in the collecting chamber is isolated from the evolving chlorine gas and the undesirable recombination of magnesium and chlorine is prevented.

As illustrated by the arrows A in Fig. 2, in the electrolytic cell of the invention, the molten electrolytic bath or electrolyte circulates in planes substantially parallel to the working surfaces of electrodes. Actually, it circulates in the planes which is substantially parallel to the working surfaces of the cathodes as well as the anodes.

This enables the invention to exclude the use of additional beyond cathode spacing, to reduce the spaces between the electrodes, ultimately resulting in a more compact arrangement of electrodes in the electrolytic compartment.

Furthermore, in the-apparatus of the invention the working surfaces of the electrodes and current intensity are increased accompanied by the simultaneous decrease of current density and voltage. Thus, the invention provides the controlled movement of the electrolytic bath for delivery of magnesium into the metal collecting chamber and provides a compact positioning of the electrodes in the electrolytic chamber. All these factors substantially contribute to the increased productivity of the electrolytic cell, increased current efficiency and substantial reduction of the electrical power consumption.

Furthermore, in the invention, the anode elements 6 can be introduced into the electrolytic chamber 2 from the bottom area of the cell. In this arrangement, the heat 11 losses from the surface of the electrolytic compartment are lower, compared to the cells in which the anodes elements are introduced from the top region thereof. Thus, the apparatus of the invention is capable of carrying out the process of electrolysis at the low rate of the electrical current density and reduced voltage. Still further, the maintenance conditions of such electrolytic cell are improved.

Although the electrolytic cell for the production of magnesium has been discussed hereinabove with reference to specific embodiments, it should be understood however, that further modifications of the invention, such as for example, variations in the number and location of electrolytic and collecting chambers are within the scope of the invention.

For example, the electrolytic cell of the invention can be arranged with one metal collecting and one electrolytic compartment, etc.

Claims (3)

1. An electrolytic cell for production of magnesium, comprising: a housing, a refractory lining within the housing defining at least one electrolytic chamber containing an electrolytic bath and at least one metal collecting chamber for collecting of a magnesium metal at an upper area thereof, said electrolytic and collecting chambers being separated by a curtain wall situated within said housing and configured to permit circulation of the electrolytic bath between said chambers; a plurality of upright anode elements interspread with a plurality of cathode elements in spaced relation thereto within said at least one electrolytic chamber, said cathode elements extending transversely to and spaced from said curtain wall; wherein a first baffle member interconnects said cathode elements at said curtain wall, a second baffle member interconnects said cathode elements at a wall of said electrolytic chamber remote from said curtain wall, so that a pair of said successive cathode elements and respective portions of said baffle members situated therebetween form an envelope surrounding the respective anode element interposed between said pair of cathode elements.

2. The electrolytic cell according to claim 1, wherein said cathode elements contain two working surfaces positioned opposite each other and facing working surfaces of successive anode elements, and said first and second baffle members and said cathode elements are interconnected so as to form a rigid frame within said at least one electrolytic chamber surrounding the anodes elements.

3. The electrolytic cell according to claim 1 or 2, wherein said rigid frame is supported by at least said refractory lining; said curtain wall is formed with first and second operational openings separated by a solid portion of the curtain wall, said first operational opening is situated above the cathode elements and below the intended level of the electrolytic bath and the second operational opening positioned at the bottom of the housing; so as to allow said electrolytic bath to circulate from said at least one electrolytic chamber in a plane substantially parallel to working planes of at least said cathode elements through said first operational opening into said at least one collecting chamber, I d in said collecting chamber, to flow off downwardly and through said second -13- operational opening back to the electrolytic chamber without substantial entrainment of the magnesium metal. DATED this 11th Day of August, 2000 RUSSIAN NATIONAL ALUMINUM AND MAGNESIUM INSTITUTE (VAMI) Attorney: PAUL G. HARRISON Fellow Institute of Patent and Trade Mark Attorneys of Australia of BALDWIN SHELSTON WATERS