Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2001-06-12
2003-06-17
Valentine, Donald R. (Department: 1742)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C204S244000, C204S245000, C204S246000
Reexamination Certificate
active
06579438
ABSTRACT:
TECHINICAL FIELD
This invention relates to electrolytic reduction cells for the production of molten metals from molten salts, where the molten metal density is less than that of the electrolyte, and to methods of operating such cells. More particularly, the invention relates to electrolytic reduction cells of this type having reservoirs for the collection of the molten metal produced by the cells.
BACKGROUND ART
Magnesium and, to a lesser extent, lithium metals are normally produced on a commercial scale by the electrolysis of their chloride salts contained in a heated molten electrolyte in an electrolytic reduction cell. As electrolysis proceeds, metal is produced in molten form (since its melting point is lower than the temperature of the molten electrolyte) and, being less dense than the electrolyte, the molten metal floats to the surface of the electrolyte, where it collects and is periodically removed.
Most such reduction cells contain a metal recovery section separate from an electrolysis section. The metal recovery section takes the form of a relatively quiescent section of the cell in which metal separation may proceed effectively. In most cases, a barrier or partition is provided between the electrolysis section and metal recovery section so that the separated metal in the metal recovery section, which floats on the surface of the electrolyte, is maintained out of contact with chlorine gas, the other product of electrolysis. Electrolyte is recirculated from the metal recovery section back to the electrolysis section so that there is always sufficient electrolyte for the electrolysis process. Any barrier or partition provided for this purpose generally has channels or openings below the level of the metal layer to permit such recirculation. To assist in the electrolyte circulation, some electrolytic reduction cells of this type use level control devices to control the liquid level (metal plus electrolyte) in the metal recovery section. For example, an open bottomed bell or “submarine” immersed in the electrolyte which is connected to an inert gas supply may be used, where gas pressure is used to adjust the amount of liquid stored in the bell, which thereby alters the liquid level in the cell.
Modern electrolysis cells, particularly those of the multipolar type, have high productivity, but at the same time generate excess heat which must be removed to maintain the electrolyte temperature at a constant target level. This is often accomplished using an air to liquid heat exchanger, immersed, for example, in the metal recovery section.
The productivity of such multipolar cells has been increased to the point that either the capacity of the metal recovery section must be increased to allow for the storage of more metal between periodic metal removal operations (metal tapping), or alternatively, the frequency of metal removal must be increased. Neither of these solutions is particularly satisfactory. The provision of larger metal recovery sections would mean that cell size would be increased, thus increasing the size of metal production facilities. More frequent metal tapping results in reduced efficiency of cell operation. The very desirable gains in efficiency of metal production are therefore producing their own problems regarding plant design and operation.
Furthermore, modern electrolytic cells for the production of magnesium operate at temperatures very close to the melting point of the electrolyte in order to maximize current efficiency. This means also that the cell operating temperature lies close to the freezing point of the magnesium product. When the magnesium is collected on the electrolyte surface as in conventional cells, it can become semi-solid, or at least very viscous and difficult to tap. The conventional solution to this problem is, by some means, to heat the entire metal pad in the metal recovery section prior to tapping. This, of course, raises the electrolyte temperature and reduces current efficiency for a part of the cell operation. Heat exchangers as described above can be used to maintain the temperature at a relatively constant level, even when extra heat input is used during tapping, but in large capacity cells, the heat exchanger sizes necessary to accomplish this during and after a tapping operation become prohibitively large and expensive and require large cell sizes to accommodate them.
PCT patent publication WO 97/28295, published on Aug. 7, 1997 in the name of Olivo Sivilotti, discloses a process and apparatus for electrolysing metal chloride salts. In this patent document, metal from a metal collection section is circulated to a reservoir provided within the cell submerged beneath the molten electrolyte, and is then periodically tapped from the reservoir. The reservoir is positioned approximately centrally of the cell to ensure proper electrolyte circulation, which is associated with the particular intended method of operation of this particular cell. The central submerged reservoir is provided in order to maintain the molten metal out of contact with the refractory cell walls as much as possible, to prevent reaction with the refractory material and consequent contamination of the metal. The disadvantage of this design is that it is very specialized, complex and consequently expensive. Existing cells cannot easily be modified to accommodate this design. The central location of the reservoir tends to maximize heat equalization between the reservoir and the cell, which can result in reductions of current efficiency.
There therefore is a need for a less complex and more practical solution to the problem of increasing metal storage in metal production cells.
DISCLOSURE OF THE INVENTION
An object of the present invention is to improve the efficiency and ease of production of metal in electrolytic reduction cells where the density of the molten metal produced is less than that of the electrolyte.
Another object of the present invention is to provide a process and electrolytic apparatus for producing molten metals less dense than the electrolyte in which increased volumes of molten metal can be accommodated within electrolysis cells, particularly those with large production capacity, without having to resort to cells of much larger size, to tapping operations at much greater than normal frequency, or to the use of excessively large and expensive heat exchangers
Yet another object of the present invention is to enable molten metal in electrolytic reduction cells to be kept at least temporarily at temperatures above those of the molten electrolyte without reducing cell efficiencies.
According to one aspect of the invention there is provided an electrolysis cell for producing a molten metal having a density less than a density of a molten electrolyte used for producing said metal in said cell, comprising: at least one electrolysis section for the electrolysis of a salt of said metal contained in a molten electrolyte to form droplets of said metal in molten form contained in said electrolyte; electrodes within said at least one electrolysis section for effecting said electrolysis; a metal recovery section for separation of said metal from said electrolyte to form a molten metal layer, having an upper surface, floating on an upper surface of said molten electrolyte; a liquid-filled reservoir communicating with an upper part of the metal recovery section for the collection of molten metal from said molten metal layer by overflow of said layer into said reservoir; liquid transfer apparatus communicating with said reservoir for enabling molten metal from said layer to accumulate in said reservoir by displacement of liquid already present in said reservoir, without removing said liquid permanently from said cell: and a tapping device for periodically removing molten metal from the cell. By “overflow” of the molten metal from the recovery section to the reservoir, we do not necessarily mean that the upper surfaces of the molten metal layers in these parts of the cell have different vertical levels. Indeed, these surfaces may be continuous (i.e. at the s
Christensen Jorgen
Creber David K.
Ficara Pasquale
Holywell George C.
Vandermeulen Meine
Alcan International Limited
Valentine Donald R.
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