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 858xc2x0 C. to, say, about 635xc2x0 C. for the mixture. (It will be appreciated that in these circumstances the chloride ions will not be discharged.) Similarly, where it is desired to use sodium hydroxide, it may be preferable to mix it with a proportion of sodium carbonate to reduce the melting temperature of pure sodium hydroxide from 322xc2x0 C. to about 285xc2x0 C. for the mixture.
Where the device is operated at an elevated temperature, care may need to be taken during the addition of metal compound to replenish that used up in the process, because thermal shock could, for example, damage the solid electrolyte. The fresh compound may, for example, be added at a steady slow rate, or the solid electrolyte may be constructed to withstand thermal shock. This may, for example, be achieved by ensuring that the electrolyte has a radius of curvature, preferably a small radius of curvature, in all areas in at least 2 directions. For instance, in the case of tubular shaped electrolytes, the diameter would be reduced to the smallest practical value. Also, solid electrolytes such as beta alumina may be toughened by including about 12% zirconia in its structure. However, the preferred method in the invention is to use a separate compartment where the fresh metal compound is heated to a temperature close to that of the liquid surrounding the solid electrolyte. In one embodiment of the invention solid sodium hydroxide is melted in a separate container and the molten salt from this container is fed to the electrolysis section to keep the molten salt level there at a reasonably constant level. In a second embodiment, an aqueous solution of sodium hydroxide is dropped into a container of molten sodium hydroxide. Rapid drying and melting of the solution results. Again, the drying compartment is preferably sufficiently separated from the electrolysis compartment to prevent the solid electrolyte being damaged by thermal shock or chemical attack by water.
The power supply for the electrolysis process frequently constitutes a major part of the total cost, so attention is preferably given to minimizing its power and size. The voltage requirement may be minimized by using an easily decomposed salt, and by ensuring that all current carrying parts are as short as possible and have the highest cross-sectional area that is practical. The current requirement can be reduced by eliminating intermittent operation of the device. Since metal is often required to be introduced into the vessel in an intermittent mode, the invention preferably includes means for storing a small amount of metal within the device until it is needed. A means is then also included to feed the stored and produced metal when required. However, metallic sodium and other group 1A metals present a safety problem, therefore the apparatus preferably includes means to ensure that the minimum amount of metal is present at any given stage of the addition process. For this reason pressurized inert gas is the favoured method for pumping the molten metal from the electrolysis compartment into the vessel. Where a secondary pumping system is used to move metal from the apparatus to the vessel, it is desirable to include a sensor for the flow of metal so that the flow can be set at an optimum rate. Such a sensor may also aid in the detection of blockages in the metal feed pipe, for example. In the case where gas pressure is used, one or more gas pressure gauges are preferably used.
The solid-state electrolyte for sodium addition is preferably of sodium beta alumina. Sodium beta alumina has a sodium ion conductivity similar to that of molten salts with a negligible electronic conductivity over a wide temperature range but any other suitable sodium ion conducting electrolyte may be used. The solid-state electrolyte for lithium addition is preferably lithium beta alumina although, again, any other suitable lithium ion conducting electrolyte may be used.
Thus it is possible by means of the present invention to control the addition of metal to a melt by controlling the charge across the solid-state electrolyte. The amount of material that is pumped through the solid-state electrolyte is determined by Faraday""s law. For 26.8 ampere hours one mole of monovalent ionised metal is pumped through the solid-state electrolyte.
A sensor for the added metal, e.g. for sodium, can be inserted into the melt and the addition of the metal monitored and controlled up to a predetermined, desired level.
It can then be maintained at that level without need to add excess, thereby significantly reducing waste and fume and dross production and these advantages are achieved without any risk of failure of a container within the melt.
A substantial amount of gas may be given off during the method, so that the arrangement of the first electrode should preferably be such as to minimise the effect of the gas on the electrolytic process. For example, gas produced by the electrolysis may have difficulty escaping between the anode and the electrolyte. The distance between the anode and the electrolyte may need to be a compromise between being sufficiently small to provide efficient electrolysis and sufficiently large to enable gas produced at the anode to escape. In one embodiment, use is made of the fact that gas produced at the anode will decrease the overall density of the source material (i.e. molten metal compound or metal compound solution) into which it discharges. This density difference is used to create a flow of source material between the anode and the source material in a direction which aids the removal of gas from this region. Additionally or alternatively, a pump can be used to circulate the source material and thus aid the removal of the gas. Advantageously, the anode may be gas permeable, for example porous. The first electrode may, for example, comprise a gas permeable electrically conductive layer on the solid-state electrolyte.
The arrangement of the second electrode relative to the container can be such as to minimise the inventory of molten metal. Alternatively, the molten metal can be produced electrolytically on a continuous basis and maintained in a reservoir between the container and the vessel and pumped through as and when required. The rate of electrolysis can thereby be boosted.
The first electrode may, for example, be generally in the form of a cylinder, preferably a hollow cylinder. Advantageously, the first electrode and the solid-state electrolyte may be shaped such that they are separated by an approximately constant minimum distance over substantially their entire opposing surfaces. This may substantially prevent the formation of a concentration of current at a particular point in the solid-state electrolyte, which could cause its premature failure. This is particularly important when the electrolyte is formed from beta alumina.
The apparatus of the invention preferably includes a control means, for example a timer and/or a monitoring means, which causes the molten metal compound or metal compound solution to be replaced periodically; the method of the invention preferably includes a step of replacing the molten metal compound or metal compound solution periodically. This periodic replacement (or xe2x80x9cflushing-outxe2x80x9d) of the molten metal compound or metal compound solution preferably substantially prevents the build-up of precipitates which may, for example, be formed from impurities or from reaction of the metal compound with air. For example, if sodium hydroxide is used as the source material for the metal (in this case, sodium), it may react with carbon dioxide in the air to form carbonate which will normally electrolytically decompose more slowly than the sodium hydroxide and may therefore build up with time and form a precipitate which could form a blockage. Alternatively, the production of carbonate may increase the melting point of the source material above the operating temperature, causing solidification which may prevent the source material contacting the first electrode.
As the container in the apparatus of the invention is positioned outside the vessel containing the melt, a wider range of operating temperatures of the container can be employed enabling a wider range of metal compounds to be used. In particular, the operating temperature of the apparatus may be minimized (compared to that of the melt vessel) thereby normally enabling the use of more economical materials and a simpler construction. Sealing of the system, if required, is also generally more easily implemented.
Moreover, the design of the apparatus of the invention avoids the thermal shock problems associated with the prior art designs where the container has to be immersed in the melt in the vessel and, particularly for aluminium melts, overcomes the problem that solid-state electrolytes are unstable in molten aluminium.
The apparatus preferably includes a conduit, for example a feeding tube, to transport the molten metal to the melt. The conduit may be fully enclosed so that the metal is isolated from the external environment, for example it may be submerged in the melt. This is particularly important for the addition of sodium, for example. The conduit may be a simple tube or the like, but it is preferably a rotor, for example as illustrated schematically in FIG. 5. The conduit may be formed from a refractory material, e.g. a ceramic material (alumina is one possibility), or it may be formed from a metal which has a higher melting point than the temperature of the melt, e.g. it may be formed from steel.
Alternatively, the apparatus may include means, preferably a pump, which conveys the melt material out of the vessel for addition of the metal to the melt material in a location exterior to the vessel. Preferably, the melt material is conveyed into, or adjacent to, the apparatus for addition of the metal to the melt material in, or adjacent to, the apparatus.
The apparatus will normally include an outer housing enclosing the other components, for example for thermal insulation (to protect the operators) and also to aid its positioning and mounting with respect to the melt vessel.