Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2001-02-06
2003-07-29
Valentine, Donald R. (Department: 1742)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S367000, C205S687000, C204S241000, C204S244000, C204S245000, C204S246000, C204S247000, C204S292000, C204S294000, C204S295000
Reexamination Certificate
active
06599413
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
This invention relates to the addition of trace amounts of metal to a melt.
It is particularly concerned with the addition of a metal from Group 1A of the Periodic Table to a melt of another metal, e.g. aluminium or zinc. Thus the Group 1A metal may be, for example sodium or lithium.
The invention is most preferably concerned with the addition of sodium to molten aluminium or an aluminium alloy and, although it will be appreciated that it is not intended to be limited thereto, it will be described for convenience below with specific reference to those metals.
The addition of trace amounts of sodium, e.g. amounts less than about 200 ppm, to an aluminium melt is well known. It can result in improved quality of castings and the castings can be more easily removable from the mould and subject to a reduction in shrinkage.
Conventionally, sodium has been added to the aluminium melt in metallic form as sticks or in aluminium cans or in the form of tablets of a sodium compound and while these methods have the advantage of simplicity they are very inefficient. Owing to the violence of the reaction that occurs much of the added sodium is lost by oxidation and considerable smoke generation is caused. Frequent additions are, therefore, necessary and the method is very wasteful, environmentally unfriendly and cannot provide a controlled amount of effective addition.
A method of overcoming these disadvantages is disclosed in EP-A-0688881. This teaches a method of adding sodium to a melt of aluminium or aluminium alloy in which an electrode comprising molten sodium or a molten sodium compound is immersed in the aluminium melt and is separated from the melt by a solid-state electrolyte which conducts sodium ions. A direct voltage is provided between that electrode and the melt by the provision of a second electrode in the melt. While providing a number of advantages in principle, this technique can lead to problems in the melt, e.g. if there is any failure of the solid-state electrolyte container.
It is an object of the present invention to provide a further improved means of metal addition.
Accordingly, the invention provides a method of adding a metal to a melt of a material in a vessel, in which a molten compound of the metal or a solution of a compound of the metal is provided in a container, the container being positioned outside the vessel, the compound is electrolytically decomposed and ions of the metal are caused to pass through a wall of a solid-state electrolyte which is a conductor therefor, from a first side of the wall to an opposite second side thereof, and to combine with electrons at the second side of the wall and then to flow as molten metal from the container into the melt.
In another aspect the invention provides an apparatus for adding a metal to a melt of a material in a vessel, the apparatus comprising a container for a molten compound of the metal or a solution of the compound of the metal, the container being positioned outside the vessel, means to electrolytically decompose the molten or dissolved compound, a wall positioned inside the container and formed of a solid-state electrolyte which is a conductor for ions of the metal, whereby the metal ions formed can pass through the wall from a first side to an opposite second side thereof, a source of electrons at the second side of the wall to combine with the metal ions, and means to pass the molten metal so formed from the second side of the wall into the melt.
For embodiments of the invention in which the container is for a molten compound of the metal, the apparatus preferably includes means to heat the compound of the metal to molten form.
For embodiments of the invention in which a solution of a compound of the metal is used, the solvent is preferably an organic solvent, for example acetamide or glycerol. When a solvent is used, the invention preferably includes means for preventing substantial loss of the solvent through evaporation or boiling.
As indicated above, the melt in the vessel will normally be a metal melt, e.g. of zinc or, preferably, aluminium but it will be appreciated that the invention is applicable in principle to non-metallic melts.
Also as indicated above, the metal to be added to the melt will normally be a metal of Group 1A of the Periodic Table and the invention is particularly useful for the addition of sodium.
The metal compound is preferably an ionic compound but the invention is equally applicable to the use of non-conducting metal compounds. A mixture of a plurality of metal compounds (ionic or non-ionic) may be used.
Where the or each metal compound is ionic, current may be passed between a first electrode positioned in the molten compound and a second electrode positioned beyond or at the second side of the wall of the solid-state electrolyte, whereas if one or more non-conducting metal compounds is/are used, the first electrode should be porous and be positioned to lie on the first side of the wall.
Thus electrolytic decomposition of the metal compound is effected, molten metal being discharged at the second electrode and anionic species being discharged at the first electrode. The metal compound is preferably a metal salt, for example a metal hydroxide, carbonate or oxalate salt. The anionic species preferably discharge to form one or more gases, e.g. where sodium hydroxide is used as the metal compound, water vapour and oxygen are produced, and where sodium carbonate is used as the metal compound, carbon dioxide and oxygen are produced. (It will be appreciated that where water vapour is produced, it should normally be ducted away to prevent any possible contact with the melt in the vessel.)
At the start up of the process, priming may be needed at the second side of the wall of the solid-state electrolyte. This may be achieved by contact between the second side and the second electrode or by the provision of an amount of the molten metal.
The wall of solid-state electrolyte may conveniently form a container. In one embodiment this container also provides the container in which the metal compound is held. Thus the first electrode for the required passage of current extends into the metal compound in the container or lies on the interior (first side) of the wall. The metal ions, therefore, pass through the container wall to the outside, are discharged and liquid metal then passes from the outside of the wall via a passage to the melt in the vessel. In a second embodiment the container formed of solid-state electrolyte is positioned inside another container. This outer container may conveniently act as one of the electrodes for the required passage of current.
In this second embodiment the metal compound may either be contained in the inner solid-state electrolyte container or outside that container but inside the outer container. The metal ions then either flow through the wall of the inner container from the inside to the outside or vice versa and the electrical circuitry is arranged accordingly as desired. Liquid metal is, therefore, provided with a passageway from inside or outside the inner container, as appropriate, to the melt in the vessel.
The electrodes may be formed of any suitable electrically conducting materials. Thus the first electrode may be formed, for example, of nickel, stainless steel or graphite and the second electrode may be formed, for example, of nickel, iron or steel depending on the metal compound used.
Where the metal to be added to the melt is sodium, the sodium compound to provide the source of sodium ions may be, for example, as indicated above, sodium hydroxide or sodium carbonate. Whatever compound is used, it should preferably be compatible with the solid-state electrolyte, should preferably be non-toxic and should preferably produce harmless by-products.
Where it is desired to use sodium carbonate, it may be preferable to mix it with a proportion of sodium chloride to reduce the melting temperature of pure sodium carbonate from 858° C. to, say, about 635° C. for the mixture. (It will be appreciated that in the
Copcutt Robert Charles
Dekeyser Jacky
Doughty Gregory
Fray Derek John
Foseco International Limited
Nixon & Vanderhye P.C.
Valentine Donald R.
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