US1913929A - Process and furnace for remelting and fining crude metals - Google Patents

Description

June 13,1933. KERSCHBAUM Y 1,913,929

PROCESS AND FURNACE FOR REMELTING AND FINING CRUDE METALS Filed Sept. 18 1930 Patented June 13, 1933' PATENT OFFICE FRIEDRICH PAUL KERSCHBAUM, OF WINTER HAVEN, FLORIDA PROCESS AND FURNACE FOR REMELTING AND FINING CRUDE METALS Application filed September 18, 1930, Serial No. 482,830, and in Germany August 1, 1929.

The invention relates to a process and furnace for melting, or remelting and fining metals such as lead, zinc, aluminium, silver, 6 magnesium, and the like, and alloys, in

saline baths directly heated by electrical energy, in which, in addition to the remelting process, physical and/or chemical refining processes occur between the molten metal and saline melt.

The process offers great advantages, for example in remelting crude Zinc, which is in the form of electrolytic zinc cathodes, or of dust, such as the condensate from zinc shaft furnaces, and is to be worked up into commercial ingots.

In the existing processes, the superficial layers always found on the crude metals, the composition of which layers depends on the conditions. of their preparation-for instance, in the case of zinc dust, they consist substantially of ZnO, whilst, in electrolytic zinc, they may consist of zinc sulphate, chloride and other substancesrender the production of ingots from the crude metal, by melting, troublesome and wastfeul.

In present day practice, the remelting of such metals or alloys is performed in reverberatory furnaces, or in electric radiation 3 furnaces, with the assistance of suitable fining salts or mixtures of same, such as ZnCl MgCl or NaCl, which are capable of dissolving the oxide-bearing layers. Although usable ingots can be obtained in this manner, allowance must be made for a heavy loss of metal, which may amount, in the case of electrolytic zinc, to as much as 5% of the original Weight in the charge, and to still more in the case of zinc dust. In remelting volatile metals such as zinc, the loss arises partly through volatilization of the metal and metalliferous fining salts, in the form of fume, under the influence of the combustion gases of fuel-fired furnaces, and partly through the metal com ounds, and also the metal itself, of the c arge becoming dissolved in the fining salts. The resulting cont-aminated salts must then be removed, from time to time and replaced by fresh quantities, and

their content of metal (e. g. zinc) recovered for use by the aid of suitable means.

It has now been ascertained that a heatsaving, simple and continuous melting of solid or dust crude metals, practically free from loss, can be efiected by fusing the metal in a suitable fining saline melt which is heated to the requisite temperature by the direct introduction of electrical energy, especially in the form of alternating current, the saline melt acting as heating resistance and purifying compound at the same time. For remelting zinc for example, salts such as zinc chloride, magnesium chloride, ammonium chloride, and the like, or mixtures of same, may be used as fining electrolytes. The resistance of the saline bath can be regulated as desired by means of suitable additions to the bath, or the employment of suitable saline mixtures. A

In the salt-bath tageous to arrange that plates of electrolytic zinc, or zinc dust, introduced in a suitable manner, will form one or more sumps after having been melted by the heat of the bath.

A particularly suitable method of procedure consists in allowing the molten metal to pass from the first sump or sumps into a sump on a lower level, advantageously in the form of drops by way of a suitably designed outlet or overflow, in proportion as fresh metal is introduced. In this manner, the metal is subjected to the fining action-of the saline bath in a particularly effective furnace it is advanthe metal, such as way.

The above described method of operating greatly facilitates the solvent action of the saline melt on the superficial layers, and the additional effect is produced that gases contained in the metal--such as occluded hydrogen or merely mechanically adherent gases,

in the case of electrolytic zinc, or air confined in zinc dust-are wholly or partially liberated.

A further result consists in that the mechanical retention of particles of the superficial layer in the molten metal is prevented. Owing to its high degree of purity, the metal recovered in the lower sump is specially suitable for direct casting into ingots.

A particularly intensive contact between the molten metal and the saline melt can be obtained, for example by the provision, at a suitable point, of mechanical devices, such as stirrers, circulation pumps or the like, or by agitation with gases, orin a very simple mannerby pouring back the drawn-off molten metal.

A relative displacement of the metaland saline phase is particularly advisable when the elimination of impurities from the metal phase is desired, for example, foreign substances suspended or dissolved in the metal. An example of such a case occurs when it is desired to extract sodium from magnesium containing the same, by fusing it with MgCl During the continued employment of a saline melt for remelting crude metal, the impurities accumulate in the melt and diminish its fining action. In order to restore that property, according to the invention the saline melt may be regenerated, for example by continuously or intermittently adding thereto solid, liquid, gaseous or vaporous substances capable, by chemical reaction, of reversing the changes that have occurred in the melt, so as to restore useful constituents to the melt, in a suitable form, such as a salt, and to eliminate from the melt absorbed injurious substances, such as oxygen, by transformation, for example, into water.

This regeneration is preferably effected by passing into the saline melt a current of a suitable gas or gaseous mixture, such as dry hydrogen chloride, or, if desired, a gas in association with dry air. By passing hydrogen chloride for example through a bath of molten zinc chloride which has become contaminated with oxygen-bearing compounds of zinc, the oxides or oxychlorides present in the melt are transformed into chlorides, whilst the water which is also formed is carried off from the hot melt by the current of gas. Chloride melts may also be regenerated .by the employment of ammonium chloride.

The regeneration of the saline bath can be effected in a particularly simple and complete manner by means of direct electric current, the saline melt being employed as electrolyte. In remelting zinc, for example, under zinc chloride, the chlorine deposited on the anode during such electrolytic regeneration, is capable, at the relatively high temperature of the melt (above the melting point of Zn, about 4=50500 (3.), of acting upon the oxide dissolved, or in process of dissolving, at the anode, with formation of ZnCl and liberation of oxygen, and thus effecting a continuous or intermittent regeneration of the saline melt, according to the method chosen. At the same time, the zinc which has passed into solution is redeposited from the saline melt on to the cathode by the purifying current.

In such case, the metal sumps formed by the molten metal are preferably employed as electrodes for the direct current. Alternatively, only one of said sumps may be employed as an electrode, a suitable auxiliary electrode, such as graphite, being employed as the other. In particular, the lower sump of refined metal may be employed as an electrode-preferably as cathode. In this method of operating it may be of advantage to reverse the direction of the current at certain intervals, for example by means of an automatic reversing switch. The heating current itself, or a direct current superimposed on the alternating heating current, may be employed as the refining current is desired.

Alternating and direct current in alternation may also be employed as heating current, through the heating-current electrodes, in which case the periods allotted to the direct current may be shorter than those allotted to the alternating current, being regulated according to the degree of purification it is desired to obtain by the passage of the direct current.

A typical embodiment of a furnace suitable for carrying out the hereindescribed process is illustrated in the accompanying drawing in which 1 is a receptacleconsisting, for example, of suitable ceramic material-for containing the saline bath 2, and the crude metal (introduced, for example, in the form of plates 3 or metallic dust 4). 5 and 6 are lattice bars, or the like, for retaining the metal to be melted, and disposed in a suitable manner, horizontally, vertically or obliquely, for example as louvres (6). 7 is an upper sump of molten metal, for the accommodation of which a collecting channel 12 is provided. 8 is the lower sump. 9/9 are the electrodes for the admission of the heating current. 10/10 are the direct-current electrodes (which may be interchangeable) for the electrochemical regeneration of the saline melt. Substances for the regeneration of the saline melt may be introduced through 12.

Instead of a continuous upper sump, a plurality of separate sumps may, of course, be employed.

It has been found advisable, in this form of remelting furnace, to cause the thermal energy to be generated from the heat of the current mainly in the deepest part of the saline bath, close above the lower metal sump. In order to enable this to be done, the cross section of the saline bath is preferably reduced above the lower sump, for example by disposing displacement members 11, of nonconductive or badly conductive material, just above the lower sump 8, as by introducing a granular or arenaceous mass which is specifically heavier than the saline melt and is insoluble in the latter. Such mass may consist, for example, of fused alumina, Zinc blende and the like.

The heating may be effected by supplying electrical energy in any convenient form, preferably alternating (e. g. three-phase) current, or, if desired, direct current, by means of the electrodes 9/9, or also by suitably arranging the melting chamber by the use of. induction currents, high-frequency currents, and the like.

The process of the invention ofl'ers, inter alia, the following advantages A metal of very high purity is obtained which is practically free from solid and liquid impurities especially of oxide character and also from extraneous gaseous substances.

The utilization of energy is extremely complete, owing to the direct contact of the metal with .the heat-supplying saline melt, in which the heat is internally generated.

The impurities such as oxides introduced with the crude metals and passing into the saline melt are eliminated from the latter in a simple manner and without loss of material, and either continuously or intermittently, by regenerating the melt by means of suitably introduced substances, or by electrolysis the metal taken up by the purifying bath being recovered, as such, at the cathode, for example in the lower sump serving as cathode.

In contrast to the processes with indirect heating, the system does not require to be heated, at any point, to a higher temperature thanis needed for remelting. In this manner, the material of the receptacle is protected in a high degree.

Losses of heat to the outside can be largely eliminated by constructing the walls of the furnace of material of low heat conductivity, and also by providing special heat insulation.

I claim:

1. A process for melting and refining crude zinc which comprises contacting the crude zinc with a molten bath containing zinc chloride and maintaining the temperature of the bath above the melting point of zinc by means of an alternating electric current passed therethrough.

2. A process as defined in claim 1 in which the bath is regenerated by passing hydrogen chloride therethrough.

3. A process as defined in claim 1 in which the bath is regenerated by passing a direct current therethrough.

salt of the group consis magnesium chloride, ammonium chloride, temperature of the b y means 0 erethrough. ereof, I aflix my signature.-

CHBAUM.

point of zinc b current passed th In testimony wh FRIEDRICH P. KERS

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