US3211547A - Treatment of molten aluminum - Google Patents

Description

PERCENT OF HIGH PURITY METAL Oct. 12, 1965 N. JARRETT ET AL TREATMENT OF MOLTEN ALUMINUM Filed Jan. 31, 1963 TEMPERATURE 2 Sheets-Sheet 1 INVENTORS .z= .4 Noel Jrrelf, Ber/20rd If. Sid/Her;

AREA COVERED PER M|NUTE Sian/qy (Z-Tacobs, ZesierL/fnapp ATTORNEY Oct. 12, 1965 JARRETT ETAL TREATMENT OF MOLTEN ALUMINUM 2 Sheets-Sheet 2 Filed Jan. 31, 1963 -|NVENTORS A/oe/ hr/eff, Bernard/7570mm",

Sfan/ey C Ja tobs, Zesfe/"L Knapp.

ATTORNEY United States Patent 3,211,547 TREATMENT OF MOLTEN ALUMINUM Noel Jarrett, New Kensington, Pa., Bernard M. Starner,

Baltimore, Md., and Stanley C. Jacobs and Lester L.

Knapp, New Kensington, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania Filed Jan. 31, 1963, Ser. No. 268,168 11 Claims. (CI. 75-68) This application is a continuation-in-part of our application Serial No. 88,334, filed February 10, 1961, now abandoned, for Treatment of Molten Aluminum.

This invention relates to the use of the principles of preferential crystallization to produce, from molten bodies of impure aluminum, fractions of a higher purity than that of the original molten body. While not so limited in over-all utility, the methods of this invention find greatest usefulness in the commercial production of substantial fractions of aluminum of a purity of about 99.99 percent by weight, or better, from primary aluminum as usually produced by electrolytic smelting, which is usually of a purity of more than 99 percent by weight of aluminum.

The basic principles of preferential, or fractional, crystallization have long been known and proposed as a means of beneficiating impure aluminum. However, such principles have not, to our knowledge, been commercially applied in a successful manner to the obtainment from aluminum of lesser purity of fractions of relatively high purity.

The objects of this invention include methods by which impure aluminum may be economically beneficiated to obtain high purity fractions in such amount, in respect of other fractions, that the sum of the economic value of all fractions represents, over-all, an economic beneficiation of the impure starting material. More specifically the objects of the invention include the concentration, by fractional crystallization, of some impurities common to electrolytically-won primary aluminum into fractions of relatively low economic value, while simultaneously producing a substantial fraction of a purity of 99.99 percent by weight, or more, of aluminum and at least one intermediate substantial fraction the purity of which, in respect of aluminum, is not widely variant from that of the starting material. These and other objects of this invention will more fully appear from the following description thereof.

The present invention is chiefly concerned with the beneficiation of primary aluminum, and aluminum having an impurity content similar to primary aluminum. Primary aluminum is, and has long been, produced by electrolytic smelting of ore, which is usually oxide resulting from the beneficiation of ores of the bauxitic types. As produced, primary aluminum usually contains about 99.4 to 99.8 percent of aluminum, although amounts of lesser and of greater purity are also obtained. Continual improvements in the classical smelting processes, and in ore beneficiation, have made the production of primary aluminum-containing from about 0.4 to 0.1 percent of other metallics commonplace but such purities do not satisfy many of the demands which, by reason of intended use or desire for careful alloying control, are for metal of a purity of 99.99 and better. Heretofore the demand for aluminum of such purity, and even of lesser purity, have been largely met by subjecting the primary aluminum to electrolytic refining processes, used for several decades.

The appended drawings are here presented solely to facilitate description and explanation of the procedural concepts and steps of this invention and do not, in themselves, set forth the invention which is, as will appear, procedural in character. In those drawings:

FIG. 1 illustrates a portion of a stylized solidification diagram;

FIG. 2 illustrates, schematically for the most part in sectional elevation, a metal receptacle in which a fractional crystallization procedure is taking place;

FIG. 3 illustrates a modification of the structure shown in FIG. 2;

FIG. 4 is a curve generally illustrating a relationship between the time interval of a pressure applying step and the effect of said interval on product recovery;

FIG. 5 is a modification of the showing of FIG. 2 with added appurtenances;

FIG. 6 is an illustration in elevation of a modification of certain appurtenances shown in FIG. 5; and

FIG. 7 is a modification of the general showing of FIGS. 2 and 5 together withan added heating device.

Impurity, as the term is used herein and in the appended claims, designates a metallic component, other than aluminum, which is present in aluminum whether it be of primary or secondary origin. Such a component may, or may not, in some context and content, be a valuable alloying element in aluminum. There has been for many years a demand for aluminum metal of the highest possible purity. This demand arises partly because of the existence of uses where such purities are desirable and partly because such metal is desired as a base from which to compound alloys by the addition of other metals as carefully selected and adjusted components. Therefore, in the context of use What is a valuable metallic element in aluminum in one instance may be an impurity in another instance.

The methods of the present invention are directed to the beneficiation of aluminum in respect of what is here termed eutectic impurities by which is meant metallics which, when present in aluminum in sufi'icient amount, under ordinary conditions of solidification, form in the solidified metal a characteristic pattern or structure which contains aluminum and which has a lower melting point than pure aluminum. Such are the elements iron and silicon, for instance, which are present in small amount in primary aluminum. There commonly occurs in aluminum metallic impurities such as iron, silicon, gallium, titanium and, often, others. Of these iron, silicon and gallium are typical of what is here termed eutectic impurities. On the other hand, titanium is typical of impurities which are not here termed eutectic impurities because, on cooling, they solidify, usually as compounds or complexes with aluminum, at temperatures above the melting point of pure aluminum. Such impurities are here termed pertectic impurities. This term is not to be confused with the term peritectic which refers to an isothermal reversible reaction in binary alloy systems, in which a solid and a liquid phase react during cooling to form a second solid phase. It will be at once recognized that over a wide range of aluminum purity th terms eutectic impurities and pertectic impurities are not mutually exclusive since, for instance, iron may, depending upon the amount present, be one or the other. However, in any given body of aluminum an impurity will be, as here expressed, either an eutectic or a pertectic impurity and therefore either will, or will not, respond to the procedures of this invention. All of this is well known and the terms are here used to define a physical condition encountered rather than specific elements, although for practical purposes the iron, silicon and gallium impurities of primary aluminum are always eutectic impurities and titanium is always a pertectic impurity.

The procedural concepts of the present invention are premised on the well-known fact that when a body of molten aluminum containing eutectic impurities is subjected to cooling the crystals which are first to solidify are aluminum-rich, i.e. of higher aluminum content than the melt from which they come. Therefore, it is possible, by partial solidification of the melt, to produce a solid fraction which, if practically separable from the mother liquor, represents an aluminum of higher purity than the original melt. This known fact may be generally illustrated by reference to FIG. 1 where the two lines L and S, diverging downwardly from the melting point of pure aluminum, M, represent the liquidus (L) and the solidus (S) of a so-called equilibrium diagram of the solidification of a binary system of aluminum and of an eutectic impurity. In this diagram the ordinate represents increasing temperature and the abscissa represents increasing content of the eutectic impurity. Assuming an original impure aluminum melt of average composition represented by the broken line X, this melt, upon being cooled to the temperature at which line X intercepts the liquidus L will first, theoretically, produce solid aluminum-rich particles, often referred to as crystals, having the composition Z, represented by the intersection of the broken line Y with the solidus S. Continued solidification of the melt, which has now become enriched as to the eutectic impurity, results in incremental solidification of more aluminum-rich particles :at lower temperature points along liquidus L and having corresponding compositions indicated by the solidus- S. If the slope of solidus S .be steep, the successive increments thus solidified will not be widely variant from composition Z. Thus when melt of composition indicated by broken line U, which beg-ins solidification at the temperature at which line U, intersects liquidus L, is cooled it will, theoretically, produce particles or crystals of composition W, as indicated by the intersection of broken line V with solidus S. Since the slope of S is steep there is but little difference between the composition Z and W. Of course, in actual practice such equilibriums are but approximate. Also a simple binary relationship is rarely, if ever, present in impure aluminum. Nonetheless the generalities, as just stated. in respect of FIG. 1, are indicative of long known facts which point to fractional crystallization as a possible means of producing high melting point aluminum-rich crystals in particulate form While effecting thereby a concentration of the lower melting point eutectic impurities in the resulting mother liquor from which the solid aluminum-rich crystals tend to settle by reason of greater specific gravity or density. Despite such knowledge no commercially successful process of beneficiation of primary aluminum by tractional crystallization is known to exist and the art has heretofore relied on electrolytic refining for such purposes. The probable reasons for the previous lack of success of fractional crystallization for such purposes may be three-fold, viz; first, the occlusion or adhesion of mother liquor on the crystal surfaces makes difficult a clean-cut separation of fractionally crystallized solids from the residual melt; second, difficulty encountered in separating crystallized fractions of higher average purity from fractions of a lower average purity caused either because of their entrapment of more impure mother liquor or because of an unfavorable slope of the solidus; and third, the economic fact that the ultimate production cost of a desired high purity fraction must necessarily depend on the respective amounts, and the respective economic value, of all of the fractions which are eventually recovered. Whatever may have been the reasons why the known fractional crystallization principles have not previously been commercially applied to the beneficiation of primary aluminum, the procedural concepts of this invention provide methods by which fractional crystallization principles may be efficiently and commercially used to produce substantial amounts of aluminum of 99.99 percent by weight purity, or better,

of Example 1.

from primary aluminum such as that containing 0.1 to

cedural steps which may be broadly indicated :as an im-] proved crystallization or solidification step, a concurrently operated improved compacting or pressure applying step and a novel remelting and separation step. Any of these steps, and notably, the crystallization step and the remelting step are susceptible of use, separately and in combination, in the beneficiation of an preponderantly aluminum metal which contains eutectic impurities, but when these steps are used in an over-all treatment they are particularly useful to the aforesaid beneficiation of primary aluminumand aluminum of a similar purity. Since the methods of this invention do not effect the removal of the so-called pertectic impurities these, if to be removed or reduced, must be the subject of a separate treatment. For this purpose it is convenient to use a treatment in which the molten aluminum is cooled to a temperature just above the melting point of pure aluminum, or to a temperature just above the point at which solidification of substantially pure aluminum crystals or particles takes place. This will result in appearance in solid form of much of the pertectic impurity, usually as -a complex or compound, and such solids may be then removed by settling, filtering or by other means. If the aluminum to be beneficiated is free, or substantially free, of such pertectic impurities, or the pertectic impurities are not unwelcome in the finally purified metal, there is, of course, no need for such a treatment. Where the amount of the pertectic impurities is to be reduced we prefer to effect their removal before the impure molten aluminum metal is subjected to methods of this invention. We also prefer to accomplish this result by adding boron to the molten metal before any solidification of the impurities as a complex or a compound has occurred. The boron and the impurities form a dense precipitate, apparently as the consequence of some chemical reaction, and the precipitate settles relatively rapidly in the body of molten aluminum. An instance of a process of this type is described in the opening paragraph. Such processes of removing pertectic impurities form no part of this invention. If pertectic removal is desired any convenient method may be used. In the following description of the methods of this invention it is assumed that the metal under treatment has been pre-treated to remove pertectic impurities, if such is desired, or that the final fractions obtained by the practice of this invention will, to the extent desired, be treated to remove such impurities.

In the practice of the invention the impure aluminum to be beneficiated, is contained, in molten state, to form a molten body having an uncontained free boundary or surface maintained in heat emitting position with respect to air, or with some other gas, if such be preferred. For convenience of reference, air or other gases are herein generically termed air. At this uncontained or free boundary a direct molten metal-air interface exists unless, as is sometimes the case in respect of molten aluminum, it may be advisable to cover the surface of the molten aluminum with a fiux or other body which serves a known function, such as a protective function, but which does not essentially interfere with heat transmission between the molten surface and the air. The term molten metalair interface as used herein contemplates the presence, if desired, of such neutral bodies. The thermal conditions at the confined boundaries of the molten mass are so controlled, by insulation or by heating, that there is little or no heat flow outwardly at such boundaries, and particularly at those boundaries which bound the deposited mass of crystals or particles which are the result of the fractional crystallization about to be described. Es-

Sentially, therefore, the loss of heat from the thus contained molten body of impure aluminum will take place at the unconfined boundary or, in other terms, at the molten metal-air interface. It is a premise of this invention that the removal of heat of solidification which initiates and maintains fractional crystallization of the molten impure aluminum takes place in a zone which is at and immediately under the unconfined molten metal boundary, and parallel thereto, until a predetermined amount of the original molten charge has been crystallized. It is a further premise of this invention that the crystallization which takes place in this zone be essentially discrete or particulate in nature, as opposed to massive, and that the presence of agglomerates, groups or clusters of crystals be avoided to the greatest extent. The desirable discrete or particulate crystals may, and often do, increase in size as they descend by gravity from the zone of crystallization to the lower region of the melt, Where the crystals eventually accumulate. Massive freezing or formation of the melt is here understood to refer to freezing in which a solid substantially unitary body is formed. Massive freezing in the precipitation zone will produce a formation within which occurs permanently trapped metal of lower purity.

It is, as above mentioned, desirable that there be little, or no removal of heat of solidification at the confined boundaries of the molten impure aluminum, or mixture of crystals and molten liquor, as fractional crystallization proceeds. Expressed in another Way the tendency, if any, to energy flow in the molten metal should be directionally from the confining boundaries to the precipitation zone. The term, tendency, is used in recognition of the physical fact that the solidification phenomena taking place during fractional crystallization maintain the temperature of the mixture of mother liquor and crystals below the precipitation zone at a substantially constant temperature. Indeed, any pronounced heat input into the confined boundaries of the mass which produces more than local melting of the precipitated crystals at or near such boundaries would tend to defeat the fractional crystallization process. From a practical standpoint, where operations are on a commercial scale involving several thousand pounds of melt, it is somewhat difiicult to prevent some massive freezing at the side Walls Without taking special precautions to practically eliminate massive freezing. Where such special precautions are not economically justified we have found that as much as percent of the total melt may so freeze but such is not substantial and is generally acceptable subject to certain preferred conditions which are (a) that this massive side wall freezing is not materially in the zone of eventual deposit of the precipitated crystals (b) that it be so located as to not be carried into said zone by reason of the compacting operations, yet to be described, and (c) that it be removed before remelting operations or, conversely, that such remelting operations take place under circumstances which avoid substantial remelt of this massively frozen material. Any freezing of the confined molten metal at a confining boundary, caused by heat flow outwardly at that boundary, we have observed, tends to be massive in character. If such frozen metal is not transferred to the zone of deposit it represents a production loss. If it be transferred to the zone of deposit it carries to that zone firmly entrapped impurities which tend to downgrade the purity of the high purity fractions. What has just been said above as to conditions in respect to heat transfer at the confining boundaries is illustrated in FIG. 2 which indicates, schematically, a molten body of impure aluminum 10, confined by a receptacle 11 so as to present to the air a free boundary 12, which may also be considered to be the molten metal-air interface. As shown, fractional crystallization is in progress, in accordance with this invention, within the zone of crystallization indicated at 13 and the crystals as formed, being of greater density than the mother liquor, settle by gravity to the lower confines of the mass and there gather in a zone of deposit 14, i.e. the lower regions of the melt. In the practice of the invention crystallization is initiated essentially in the zone of crystallization l3 largely, as above mentioned, in the form of essentially discrete crystals or particles. However, it is difiicult in practice, without special precautions, to avoid some massive freezing at those confined boundaries adjacent the zone of crystallization and where special precautions are not economically desirable there often appears massive freezing of the type indicated at 15. Such massive freezing is acceptable if in insubstantial quantity as above indicated. Yet, if it be present in the shape and location indicated in FIG. 2 it may be bothersome, as indicated above, to further processing. Consequently, when such a type of massive confining boundary cooling occurs it may be differently located, such as by sloping the walls of the container, as shown in the partial view of FIG. 3 thereby to position the massively frozen portion 15 out of substantial register with the periphery of the zone of deposit 13. The improved crystallization step just described will, when practiced alone, produce in the zone of deposit a fraction of relatively high purity, i.e. with a minimum of occluded or entrapped mother liquor and, after crystallization has been carried to the point desired, may be separated from the mother liquor by decantation or by means later described, and then remelted and cast into desired form for further use.

In the operation of the step of crystallization the solidification phenomena taking place hold the zone of crystallization at almost constant temperature While the heat of solidification is emitted at the molten metal-air interface. Consequently the rate at which crystallization takes place in said zone may be slowed or accelerated by adjusting the speed with which the emitted heat is removed from said interface during crystallization. This may be conveniently accomplished by manipulation of the temperature, and/or the movement, of the air at the interface. Too rapid a heat removal at the interface will, of course, lead to undesirable massive freezing across the zone of crystallization and thus prevent the formation of the essentially discrete crystals of desirable purity. Therefore the rate of cooling at the interface, and the extent of area of this molten metal-air interface, are factors essentially economic in character and the choice of the operator so long as formation of essentially discrete crystals, as distinguished from massive freezing, takes place in the zone of crystallization or precipitation. While the observation of the operator is usually essential to the maintenance of the rate of cooling at the interface so as to achieve in the zone of crystallization the desirable type of crystallization, it can be generally said that a heat transfer at the molten metal-air interface which is not greater than that which will induce precipitation of about pounds of crystals per hour per square foot of interface will accomplish the desired type of crystallization. In the beneficiation of primary aluminum, or of aluminum of similar purity, we have generally found that a cooling rate of at least 30 pounds of crystals per hour per square foot of interface is favorable to the formation of essentially discrete crystals in the zone of crystallization. Crystallization carried out according to the above principles will prove satisfactory unless economic factors prevail which indicate resort to the special procedures now to be described.

It has been found that there exists in the above described crystallization step a definite relationship between the rate of heat transfer at the molten metal-air interface, the amount of desired high purity fraction ultimately obtained and the amount of the otherwise acceptable massive freezing at the confined boundaries of the zone of crystallization and at the juncture of that zone with any compacting or tamping means which may be used as hereinafter described. As pointed out previously, it is difiicult to prevent some massive freezing but if such said junctures.

7 freezing is not in substantial amount and is suitably controlled as to location good results can still be obtained. On the other hand, it appears under such conditions that as the rate of heat transfer at the molten metal-air interface is increased there results a small but significant and constantly greater decrease in the amount of the desired high purity fraction which is ultimately recovered. Surprisingly it has been found that this decrease in amount of high purity fraction with increasing rate of heat transfer can be virtually eliminated if special precautions are taken to practically eliminate massive freezing. Absolute elimination of massive freezing is a theoretical but probably not a practical possibility where commercial sizes of melts are being processed, yet if massive freezing be practically eliminated, i.e. reduced to an amount of about 2 percent and, preferably, one percent or less by weight of the total melt the rate of heat transfer at the interface can be materially increased without affecting the amount of the high purity fraction ultimately obtained. The extent of the increase of heat transfer at the interface which can be obtained, under these conditions, without undesirable decrease in the amount of high purity fraction obtained, will depend on the exact conditions of the commercial operations but is readily observable by the operator. Usually the rate of heat transfer can be at least trebled, over that which would normally take place at the interface were the process allowed to proceed without manipulation. of the heat transfer rate, before any real decrease is observed in the amount of high purity fraction recovered. For instance where melts composed of several thousand pounds are processed in accordance with the principles of this invention under conditions where no manipulation of the rate of heat transfer at the molten metal-air interface is undertaken and the normal rate of heat transfer is accepted, While as previously noted the process is otherwise controlled to prevent substantial amounts of massive freezing, the time of processing is quite long because of a slow rate of crystallization. If under such condition one accelerates the heat transfer rate at the interface by, for instance, rapidly changing acceptable massive freezing this increase in heat transfer rate, and consequent saving of processing time, will take place without any real decrease in the amount of high purity fraction obtained. Whether or not this modification of the invention is useful in a given instance is a matter of economic balance between the benefits obtained by a lesser over-all crystallization time and the cost of accelerating heat transfer at the interface plus the cost of the special precautions necessarily involved in obtaining and maintaining temperature conditions at the confined boundaries of the zone of crystallization, and at the juncture of said zone with any compressing or tamping devices which may be present, so as to practically eliminate massive freezing at said boundaries and Where such acceleration is desirable it may be accomplished, for instance, by delivering compressed air or another suitable fiuid at the interface through a number of small jets and at such a rate as to change the air .at the interface several times a minute. The control of temperature conditions to practically prevent massive freezing is more diflicult but we have found it best accomplished by maintaining, at the confining wall which defines the contained boundaries of the zone of crystallization, a heat input which, while positive, is not any compacting or tamping elements which may be used has been accomplished by careful maintenance of heat input to said elements, as hereinafter generally described.

The crystallization step above described, and its various procedures, will not, when used alone, so benefit primary aluminum, or aluminum of similar impurity content, as to usually produce a commercially substantial fraction of an aluminum purity of 99.99 percent by weight. To this latter end, and to the general improvement of the efliciency of our improved crystallization step, this invention contemplates the use during the fractional crystallization step of a compacting or pressing step which will now be described.

In essence, the second step of our preferred process consists in compacting the precipitated crystals into a relatively firm mass in the lower region of the melt as precipitation proceeds. Compacting, as the term is here used, means generally the application of pressure to the crystals which settle from the zone of crystallization and approach, or come to rest in the zone of deposit or lower regions of the melt. Specifically, within the context of this invention, compacting means the application during the fractional crystallization process of a pressure, intermittently applied, to the crystal mass in the zone of deposit and, also, a pressure which is applied over only a portion of the upper surface of that mass at any one time. Whether the concentrate of crystal solids mixed with some mother liquor which lies in or near the zone of deposit, i.e., in the lower region of the confined mass, can be said to have an upper surface may be open to speculation but in any event the invention contemplates that the compacting pressure applied as crystallization proceeds shall be applied at any instant over not more than about one-half of the upper area presented by this crystal mass or concentrated slurry. When compacting pressure is thus applied in such fashion that substantially the entire upper area presented by the crystal mass is subjected to the compacting pressure at least once in about every ten minutes, then the amount of the higher purity fraction obtainable is doubled or tripled. If the upper area presented by the crystal mass is subjected to pressure compaction once every five to three minutes further improvement is obtained in respect of the amount of high purity fraction obtainable. Beyond about three minutes a further decrease in the time interval within which the over-all area is subjected to compacting brings little improvement. Compacting of the over-all area at a rate of less than about once in ten minutes brings some improvement but does not give the preferred result. Generally, the rate of increase of high purity fraction recovered in relation to the time interval in which the compaction takes place over the upper area of the crystall mass is indicated by the curve of FIG. 4 which expresses generally the relationship between the time interval, increasing along the abscissa and amount of final fraction of 99.99 percent of aluminum purity, increasing along the ordinate. This curve is but indicative of a time intervalfraction relationship which exists, although varying in absolute value with the amount of the high purity fraction desired, i.e. for example 15, 20 or 30 percent by weight of the original melt, and the purity of the fraction. This general relationship does not appear to alter essentially with the actual area compacted during any instance of application of pressure. Thus the same general relationship will be found whether the actual area compacted at a given instance is 1 percent or 50 percent of the over-all upper area of the crystal mass.

The compacting pressure is preferably applied to any increment of area in such manner as to not unduly displace adjacent previously compacted material. The absolute pressure used, of course, is a function of the area and configuration of the pressure applying surface. Good ,results have been generally obtained at pressures in the neighborhood of 15 pounds per square inch using a flat pressure face which, during use, becomes somewhat rounded or conical by reason of local freezing of metal of packing on that face. Such a specific figure is, however, only indicative because, as compacting proceeds during the fractional crystallization step, the mass develops a characteristic firmness which resists the compacting and tends to shorten the stroke of the compacting means as further precipitation builds up the depth of the crystal mass. In other words, the original physical state of a relatively dense slurry of crystals and mother liquor is changed by the compaction as the mother liquor is forced from between the crystals and returns to the upper regions of the total melt, thereby decreasing the mobility of the crystals of the mass in the zone of deposit and, consequently, increasing the firmness of that mass. In the practice of our compacting step, the pressure applying device used should preferably be heated before and, if necessary, during, use to reduce massive freezing on the pres sure device to a minimum. Also so much of the pressure device as comes in contact with the melt should be made of a non-contaminating material, such as graphite, to avoid undue addition to the melt of unwanted impurities.

The compacting step just described may be further illustrated by reference to FIG. where, again, the schematic showing of FIG. 2 is repeated with the addition of two pressure applying devices consisting of graphite blocks 16 mounted in rods 17. Since these pressure devices may be actuated manually or automatically with substantially equal result, no actuating means are shown. In commercial practice the blocks or tampers 16 may be mounted on conventional air cylinders to effect the desired vertical reciprocation and the assembly may be so indexed as to automatically, in some sequence, so move the blocks or tampers laterally between strokes as to ensure that the entire upper area of the crystal mass will be covered in the time interval desired. In the form shown the blocks or tampers 16 are of such length, in respect of the depth of the melt that only a portion thereof is immersed in the melt at any stage of their vertically reciprocating movement. If it is desired to always maintain the tamper in the melt it may be differently shaped, as shown in FIG. 6 so that the depth of the tamper body is so reduced as to allow it to complete its entire reciprocating movement within the melt. The depth of the melt will vary since, as later herein pointed out, the methods of this invention allow solidification of as much as 80 percent by weight or more of the total melt in some instances.

It has been our experience that in the practice of our invention actual agitation of the melt and mixing of previously deposited crystals is best kept at a minimum. For that reason we prefer a vertical, or substantially vertical, movement of the compacting device or tamper in respect of the zone of solidification. Where avoidance of an insubstantial amount of massive freezing is not of importance to the operation, or where an accelerated heat transfer rate at the interface without diminution in the amount of high purity fraction recoverable is not an economic desirability it may be found desirable to completely remove the tamper elements from the melt after each pressure applying stroke so that stirring of the melt is avoided during the movement, or indexing, of the tamper element to its next pressure applying position. Where, however, it is desirable to limit massive freezing on the tamper element to the greatest possible extent it is advantageous to avoid lifting the tamper element and to merely raise it vertically in the melt to the extent necessary to be clear of the compacted crystal mass before the element is moved, or indexed, to its next pressure applying position. In such event it is desirable to so govern lateral displacement of the tamping element in the melt that any resulting agitation of the melt be gentle and that vertical or swirling movement be kept at a minimum in the melt. Where, in any case, it is desirable to reduce massive freezing on the tamper element to the least amount possible the result may be best obtained by heating, externally or internally, the elements in such 10 manner that the temperature at their juncture with the molten metal-air interface be at or slightly above the temperature of the molten metal.

The third and final step of our preferred method of fractional crystallization may be termed a separating and remelting operation. These separating and melting steps are useful in the removal and recovery of any crystallized fraction which is deposited in the lower regions of a molten impure aluminum melt, whether it be compacted or not, and whether or not it be initially crystallized in accordance with the methods of this invention. However, the principles of recovery and remelting about to be described are particularly beneficial when used in connection with the crystallization step and compacting step of this invention in effecting a substantial yield of high value fractions from primary aluminum or aluminum of similar impurity content.

Referring now to the schematic showing of FIGS. 2 and 5 it will be noticed that in each instance the receptacle 11 is provided with a pouring spout 18 the receiving entrance of which is located at, or slightly below, the level of the floor, or lowest point, of the receptacle 11. A conventional type of plug 24 can be used to close the spout. Associated with this spout is a burner, or other heating device, 19 by which the spout may be kept, during use, at a temperature above that obtaining within the receptacle. This is a desirable precaution because the slurry of crystals and mother liquor within the receptacle are at a temperature relatively close to the freezing point of much of the mother liquor and consequently the mother liquor is sluggish and tends to move slowly and freeze in the spout unless extra heat is applied at this point.

When the crystallization has proceeded to the point desired the spout 18 is promptly opened and as much of the mother liquor as will flow therethrough is removed. Since this mother liquor now contains much of the eutectic impurities present in the original melt, it is desirable that such of the liquor as lies above the zone of deposit flow directly to the spout without undue additional contact with the crystals most of which lie in the lower regions of the receptacle, i.e. the zone of deposit. This may be accomplished to some extent by heating the spout prior to opening it and by using some tool, such as a graphite rod to form a channel or path of least resistance downwardly through the deposited mass of crystals and close to the inner wall of the receptacle adjacent the spout. Good results will also be obtained, however, if the receptacle is provided with an additional spout 20 (shown in FIG. 5) located at the level or approximate level of the final upper surface of the deposited mass of crystals. If such a spout 20 is used it likewise is provided with a spout heater 21 and plug 24 for the purposes previously described in respect of spout 18. The provision of this second or higher level spout 20 allows the mother liquor lying above the deposited crystal mass to be drawn off before the lower spout 18 is opened, therefore preventing undue contact between the overlying impure mother liquor and the deposited crystals.

In any event, and whether or not differently located spouts are used, the mother liquor is drained off, to the extent that its removal can be so achieved, immediately after the fractional crystallization has been completed. Surface occluded or trapped mother liquor is not mobile, is not readily drained and is, consequently, not usually decantable or drainable. During this drainage process the temperature within the receptacle is maintained as closely as possible to that at which fractional crystallization took place, thereby to prevent substantial freezing of the mother liquor, or conversely, substantial melting of the crystals. When practical drainage is complete heating is applied to the crystal mass to remelt and recover the crystallized material which, upon remelting, flows from spout 18 into any suitable mold or other cooling device or holding receptacle.

it may be cast in suitable shape for re-use. cient of the upper portion of the crystal mass has been If the crystallized material is of such'aver'age purity afiter drainage of the mother [liquor that it is acceptable per seas the end product the rernelting of the crystal mass may be accomplished in any manner. However, when the impure aluminum which is being beneficiated, i.e, the starting material is primary aluminum and it is desired to obtain a substantial firaction of a purity of 99.99 percent, or more, of aluminum it will be found that such substantial fraction is usually obtained only if the remel-ting is so accomplished that the upper portion of the crystal mass in the zone of deposit is first melted as a separate fraction or fractions and thereafter the lower portion of the crystal mass is remelted as .a final fraction. it will be this final fraction which is of the desired highest purity. Such a sequential remelting operation may be performed in several ways but best results are obtained when the remelting heat is applied to the upper surface of the now drained crystal mass, and over that surface to obtain a progressively downward sequential melting of the crystals in substantially horizontal increments. As previously mentioned when the sdlidus S is steep (see FIG. 1) the difference in composition of successively crystallized crystals is probably not great and indeed not as great as may be the differences in the composition of the mother liquor which surrounds or is adherent to a settled crystal into the lower regions of the receptacle. Therefore if it be assumed that some of the mother liquor through which the crystal initially passes is occluded, or otherwise held on the crystals surface then it would follow that those crystals which first are formed and settle to the zone of deposit will have on their surface mother liquor of lesser impurity content than those crystals which later are formed. If this assumption is correct then the result follows since as crystallization proceeds the concentration of impurities in the mother liquor increases. In any event it will be found that if the crystals lying in horizontal increments in the upper portion of the drained mass of crystals are first rcmelt-ed and the molten remclt drained off a separation can thus be efl ected of remelted fractions of increasing purity, the last progressively downward melted horizontal increments producing the highest purity fraction.

In the prefierred practice of this invention in the beneficiationof aluminum of a purity of '99 percent or better to obtain a substantial fraction of a purity of 99.99 percent or better we proceed in the manner generally indicated in FIG, 7 wherein is shown the receptacle '11 of FIG. and its associated (appurtenances, at a point where the drainable mother liquor has been drained from the compacted crystal mass which now lies in the zone or" deposit 13. At this point the pressure applying devices shown in FIG. 5 have been removed and there has been moved over the receptacle the cover plate 23 in which are located a plurality of heating devices 22 which can be of any practical type but are here schematically indicated as gas burners, the gas connection-s not being shown. This cover plate may be positioned at the opening of the receptacle, as shown. However, if there exists a massive freezing on the inner walls of the receptacle 11 such as that indicated at 14 in FIGS. 2 and 3 and it is desired that none of this be remel-ted then the cover plate 23 may be so as to be insertable within the receptacle ll in a plane above r e surface of the deposited crystals but below the massively frozen material on the side walls. In either event once the cover plate 23 is positioned the heaters are turned on and remelting of the upper portions ;of the crystal mass lying in the zone of deposit is begun. As the crystals remelt the remelt finds its way to the bottom of the receptacle and thence out of spout [1*8 where When s-ufliremelted to torm the amount of traction desired, the metal issuing from spout 18 is diverted to other in olds or receptacle-s which receive the next fraction and so on until all of the crystals have been remelted. The remelting temperature is not high and is maintained at but a few degrees above the temperature at which the crystals rernelt. Such a precaution is advisable to prevent a buildup in the remelted metal of temperatures such as would cause undue melting of other crystals which the remelted metal may contact on its'passage downward and, eventually to the pouring spout or spouts 18.

The exact amount and purity of each fraction remelted in accordance with the principles of this invention will depend on economic considerations which can be illustrated by the ollowi-ng example. If a starting impure primary aluminum has a purity of about 99.6 percent by weight of aluminum it is usually possible in the preferred practice of this invention, as its various steps are above described, to obtain a final remelted fraction of a purity of 99.99 percent by weight aluminum, or better, which 'frac't-ion represents about 20 to 30 percent by weight of starting weight of the metal of 9916 percent purity. If fractional crystallization were carried to a point where the drained weight of the mother liquor is about 15 percent by weight of the starting weight of impure aluminum, this fraction value of the 98 percent purity fraction less, of course,

the cost of crystallization process and recovery just above described. Considerations such as these will therefore govern the decision of the operator as to how many fractions are taken during the downward remelt-ing of the deposited crystals and the extent of any such traction. Thus, for example, if a fraction representing 25 percent of the starting material at 99.98 percent purity plus a fraction representing 10 percent of the starting material at 99.99 percent purity plus the remainder at 99.6 :per-

cent purity would be of a higher value than a larger 99.99 percent purity fraction plus the remainder fraction at 99.6 percent purity then the operator can accordingly manipulate the diversion of the remelt metal resulting from the downward melting of the crystal mass. The figures just given are illustrative only of economic balances which will determine the number and size of the fractions into which the remelt is sequentially divided.

In any event in the practice of our invention on a commercial scale in the beneficiation of primary aluminum of a starting weight of 200 to 2000 lbs. we have achieved fractions of 99.99 percent aluminum purity, or better, representing 20 to 30 percent by weight of the starting material Without downgrading more than approximately 15 percent by weight of the starting material and such downgrading has resulted in aluminum metal of a purity acceptable to many uses. In most cases the actual metal loss incurred has been no more than that known to occur, by reason of oxidation and scum formation and manipulation loss such as spillage and the like, when like. amounts of aluminum are melted and handled for any purpose.

Some examples of the practice of'th-e methods of this invention are as follows:

Example 1 percent by weight. The starting weight of this material was melted in a holding furnace and to this melt there was added about 0.07 pounds of an aluminumboron alloy Containing about 3% by weight of boron. After the aluminum-boron alloy addition had been melted and the mix completely stirred, the resultant melt was held in a quiescent condition for about 30 minutes, at a temperature of 720 C. to permit precipitation and settling out of the pertectic impurities. Thereafter the melt was tapped off the settled pertectic impurities and used as feed material, this melt now had a total pertectic impurity content of only 0.005 percent by :weight.

The thus treated melt was then placed in an insulated holding receptacle which had previously been preheated so that the inner surface had a temperature of about 665 C. The depth of the molten metal body thus contained was approximately 11 inches and the unconfined upper surface, which was exposed to air, presented a molten metal-air interface of about 0.4 square feet. The unconfined metal surface was allowed to cool to 660 centigrade thereby to establish an intiation of crystallization. During the fractional crystallization process the temperature at the molten metal-air interface was controlled to cause a crystallization rate at the interface of about 90 pounds per hour per square foot of said interface. This fractional crystallization was initiated in a zone immediately at, under and parallel to the unconfined metal surface. The depth of the zone was about 3 inches. To prevent any surface crust formation from blocking contact of molten mother liquor with the air at the interface this zone of crystallization was, from time to time, gently agitated with a graphite rod. Fractional crystallization was allowed to proceed for about 60 minutes at the end of which time about 80 percent by weight of the original molten metal was in solid crystallized form. The crystals formed in the crystallizations zone were discrete. No massive cooling was observed within the crystallization zone. As crystals formed they settled into the lower region of the container and formed a dense layer of crystals, with some mother liquor, at the bottom of the container. Throughout the process the insulated boundary walls of the container were maintained at a temperature of about 600 C. on the side and 670 C. on the bottom. This temperature differential insured that no massive freezing took place in the receptacle in the zone of deposit i.e. the lower region of the receptacle into which the precipitated crystals had come to rest. Some side wall freezing did, however, occur immediately adjacent the crystallization zone. The total amount of metal thus massively solidified was about 4 pounds. It formed in a band of about 1 inch at and immediately under the zone of solidification. The maximum depth of this band, which was irregular in depth, was about 2 inches. When, as aforesaid, about 80 percent of the original weight charged was solidified, a side drain hole located at the level of the bottom of the receptacle was opened, thus draining from the receptacle the drainable mother liquor. Previous to opening the drain hole the drain hole walls had been preheated. Drainage of the mo mother liquor took place over a period of 1 minute which represented 3 percent by Weight of the starting charge and containing about 5 /2 percent of the eutectic impurities present in the starting weight of the primary aluminum. The draining of the mother liquor being completed, the upper surface of the drained mass of crystals in the receptacle was subjected to a more or less evenly distributed heat to produce a remelted fraction which drained from the receptacle within a temperature range of about 660 to 670 C. This heating was continued progressively downward until the crystal mass was downwardly remelted. A first melting portion of the remelt resulting from this downward horizontal remelting of the crystal mass, which portion represented about 75 percent of the mass, was segregated and had an aluminum purity of 99.8 percent, the impurities remaining being as follows:

0.12 percent by weight of iron, 0.084 percent by weight of silicon, 0.018 percent by weight of gallium and all other impurities 0.008 percent by weight. The remaining or last melted portion of the downwardly remelted crystal mass was segregated into a final fraction having an aluminum purity of 99.97 percent and a final weight of about 12 pounds. The impurity content by weight of this final fraction was 0.008 percent iron, 0.017 percent silicon, 0.005 percent gallium and 0.005 percent of all other impurities. No compression of the crystal mass in the zone of crystallization took place during this process. About 1 percent of the total charge was lost by way of spillage or by oxidation. Assigning no recovery value to oxidized or spillage losses, the value of the fractions finally obtained exceeded the value of the starting material by about 17 percent.

Example 2 The material processed was 1000 pounds of impure aluminum having a purity of about 99.90 percent aluminum. Pertectic impurities including titanium, chromium, vanadium and zirconium were present in an amount of about 0.001 percent by weight. Eutectic impurities totalled 0.094 percent by weight of which by weight iron was 0.036 percent, silicon 0.043 percent and gallium 0.008 percent. This metal was first subjected to a preliminary pertectic treatment, as described above in Example 1. Thereafter the thus treated metal was transferred to a holding receptacle which had been previously preheated so that the inner surface had a temperature of about 665 to 670 C. The thus contained molten body had a depth of 15 inches and presented to the air a free uncontained upper surface of about 5.6 square feet. This molten metal-air interface was adjusted to a temperature of 660 C. thereby initiating crystallization. The temperature of the metal was maintained to establish a zone of crystallization initiation at, under and parallel to the molten metal-air interface of a depth of about 14 inches and within this zone to precipitate crystals at a rate of about 60 pounds per hour per square foot of area of said interface. Throughout the following crystallization, which took place over 2.1 hours the temperature of the confining walls of the chamber was maintained just above the temperature of the metal with which they came in contact thereby minimizing massive freezing at the confining boundaries of the mixture of mother liquor and crystals. No massive freezing was observed within the zone of crystallization but some side wall freezing did occur immediately adjacent the metalair interface. As soon as crystallization began the compression of the crystals which by gravity settled into the lower region of the molten body, was begun. Compression was accomplished by the approximately vertical strokes of graphite bars having a pressure surface of about 30 square inches each. Four of these bars were used and at the top of each vertical stroke each bar Was displaced laterally to another position thereby to make pressure contact on its next stroke with a different portion of the crystal mass. The crystal mass was spread over an area roughly equivalent, considered in square feet of vertically projected area, to the area of the molten metalair interface. The vertical strokes of the graphite bars were so timed, and so displaced after each stroke, that this entire area was contacted by the pressure face of the bar once every one half minute. The pressure bars, or tampers, were of such length that a portion thereof ex tended above the molten metal-air interface at all times. During each vertical stroke the bar rose free of said interface, and was therefore exposed to gas burners located to heat the bars as they rose on each stroke, thereby to minimize local cooling and massive freezing on the pressure bars. As crystallization proceeded the stroke of the bars was shortened as the bed of compacted crystals increased in thickness and compactness and resisted the pressure of the bars. After the fractional crystallization had placed in crystal form about 70 percent of the original metal, by which time the compacted bed of crystals in the zone of deposit was about 14 inches in thickness, the pressure bars were withdrawn, a preheated drain hole located at the bottom of the periphery of the compacted mass was opened, and the drainable mother liquor was drained therethrough, thus terminating fractional crystallization. This drainable mother liquor constituted 15 percent by weight of the original charge and contained 35 percent of the total eutectic impurities of the original charge. To the upper surface of the now drained and compacted mass was applied an even heat adjusted to remelt the crystals and to produce a remelt having a temperature of 660 to 670 C. which, upon being formed, ran by gravity to and through the aforesaid drain'hol'e. As a first remelt fraction there was collected the remelt of approximately, the top 70 percent of the crystal mass. This first fraction weighed600 pounds, was 60 percent of the original charge, had an aluminum purity of 99.90 percent by weight and, by weight, a pertectic impurity content of 0.000 percent and a eutectic impurity content of 0.10 percent, of which latter the iron was 0.38 percent of the fraction, the silicon was 0.045 percent and the gallium was 0.008 percent. As a second 'and final remelt fraction there was collected the remelt of the remainder of the crystals. This second fraction weighed 250 pounds, had an aluminum purity of 99.99 percent. This final fraction contained only 3 percent of the original eutectic impurities, calculated by weight of the total fraction. In this entire process the metal loss to spillage and oxidation was about 1 percent by weight of the starting material. Assigning no recovery value to this spillage and oxide, the total value of the drained mother liquor fraction, the first remelt fraction and the recovered remelt fraction was 31 percent greater than the original value of the starting material, all values, as in the first example, being calculated on the basis of then prevailing market price.

Example 3 In a similar run to that of Example 2 the drainage of drainable mother liquor took place in two steps as follows. First drainage was through a preheated drain hole located just above the level of the compacted crystal mass. Thereafter a preheated drain hole located on the periphery of the compacted crystal mass, and on a level of the bottom of said mass was opened and the balance of the drainable mother liquor removed. This two step draining procedure increased the aluminum yield of the upgraded fractions to some extent, about 80 percent absolute.

Example 4 The benefit grained from accelerated heat dissipation from the metal-air interface without the occurrence of massive freezing and without decreasing the yield of high purity metal is illustrated in the following example where a 1550 pound charge of aluminum of 99.89% purity was processed. The aluminum contained a total of 0.007% by weight of pertectic impurities, including titanium, chromium, vanadium and zirconium, 0.045% by weight of iron, 0.037% by weight of silicon and 0.013 by weight of gallium. The charge was melted in a holding furnace and 2 pounds of an aluminum-boron alloy added which contained about 3% by weight of boron. After the alloy was thoroughly stirred it was held in an undisturbed condition over night at a temperature of 720 C. to permit precipitation and settling out of the pertectic impurities. A much shorter time could have been used had the work schedule permitted it. The melt was then tapped oil to a circular refractory lined receptacle having vertical side walls which had been preheated so that the surfaces to come in contact with the molten metal were at a temperature of about 700 C. The receptacle was filled to a depth of 15 inches and the free uncontained upper surface of the metal pool had an area of 8.7 square feet. The metalair interface of the melt was cooled to 660 C. and controlled jets air blown over the surface of the melt to cause 'as to reduce and control formation of aluminum-rich crystals while'preventing massive freezing in the crystallization zone and substantially eliminating it on the walls of the receptacle and graphite compacting bars. A heat balance was maintained at the interface which produced crystallization without massive freezing and yet heat was transferred through the interface at an accelerated rate. In this manner crystallization was promoted at a rate of 124 pounds per hour per square foot of surface at the metal-air interfaceand the operation was continued for a period of 60 minutes. Under these conditions crystals were rapidly formed and settling progressed but no massive freezing occurred. The settled crystals were compacted by the vertical strokes of a heated graphite bar having a pressure applying area of square inches, the lateral position of the bar being changed after each stroke so that substantially the entire top surface of the crystal bed was subjected to the compression strokes within a period of one minute. The strokes, of course, became shorter as the depth of the bed increased. When about 70% of the charge had been crystallized, the compression operation was terminated and a tap hole opened which was located in the side wall just above the bottom of the receptacle. The mother liquor drained from the bed amounted to 13% by Weight of the original charge. Heat was then applied to the surface of the crystal bed and downward melting of the bed was started, the tap hole being open and permitting continuous discharge of the liquid metal. The first fraction collected, 465 pounds, which represented 30% of the Wight of the initial charge, had a purity of 99.81%, the iron content being 0.089%, the silicon content 0.070% and the gallium content 0.023%. A second fraction, representing 40% of the original charge, was collected as the downward melting proceeded amounting to 620 pounds which had a purity of 99.90%, the iron content being 0.045 the silicon content 0.037% and the gallium content 0.013%. A third and final fraction was collected by melting the remainder of the crystal bed which amounted to 465 pounds, or 30% of the weight of the initial charge, that had a purity of 99.99%. Assigning Ito recovery value to spillage and oxide, the total value of the fractions obtained was 18% greater than that of the value of the starting material, all values being calculated on the basis of the then prevailing market prices. It is apparent that a high yield of high purity metal was obtained by maintaining a proper heat balance at the metal-air interface even though the rate of heat transfer was accelerated. The output of high purity metal was not sacrificed as a result of the accelerated cooling of the melt.

In the commercial practice of the preferred method of this invention, using starting materials of a weight of ton, it has been possible to regularly produce a final remelt fraction of 99.99 percent by weight of aluminum, or better, in an amount of 16 percent of the total weight of the starting material, with other fractions of such value that, at then prevailing market values, the value of the starting material is upgraded at least 19 percent.

The results of the practices of the principles above described may, in useful degree, he benefited by control of other variables. For instance it has been observedthat it is beneficial to so shape the walls confining the melt as to eliminate sharp, or reentrant, angles and, also, to so slope the walls as to facilitate gravity flow to the drainage spout or orifice from which the mother liquor, or the melt resulting from the remelt of the deposited crystals, is drained. Also, for example, it is beneficial to so proceed to a minimum oxide or dross at the molten metal-air interface. Such control may take place according to well-known principles by way of mechanical removal, such as skimming, or by way of preventation of oxide formation by the use of inert atmospheres. Whether, and to what extent, such control is beneficial is entirely economic, depending upon whether the resultant increased heat transfer at the interface is worth the cost of the control.

It will be immediately apparent to those skilled in th art that the procedural principles of this invention may be useful in the beneficiation of impure aluminum of various impurity contents, although such principles have found their greatest usefulness in connection with the specific object of this invention which is the beneficiation of primary aluminum, or like aluminum, by concentration of eutectic impurities in a downgraded fraction with the concurrent production of substantial fractions of 99.99 percent aluminum purity, or better. The preferred practice of our invention has by example and otherwise been explained in detail but we do not desired to be limited to such specific description except as expressed in the appended claims.

We claim:

1. In a process wherein molten impure aluminum containing eutectic impurities and more than about 99 percent by weight of aluminum is fractionally crystallized to initially place in solid form at least a part of the aluminum as a higher purity product thereby to effect a concentration of at least a part of the eutectic impurities in the resulting mother liquor from which said solid aluminum of higher purity tends to settle in the form of crystals by reason of their greater density, the improvement of obtaining from said impure aluminum one substantial remelt fraction of an aluminum purity greater than that of said impure aluminum, at least one other remelt fraction and a mother liquor fraction, by containing said impure aluminum to form a molten body thereof having an unconfined molten metal-air interface, removing heat of solidification at and through said interface at a rate to maintain formation of aluminum-rich crystals in a zone of crystallization which is at, under and substantially parallel to said interface while preventing substantial loss of heat at the contained boundaries of said body, and, during said crystallization, compacting the crystals thus formed as they settle to the lower regions of said body, by intermittently applying pressure to the upper area of the settled crystal mass, at any given application thereof, over an area of not more than about one-half of said upper area, continuing said crystallization and said concurrent compaction until a predetermined portion of the original molten body has been crystallized, thereafter separating a major portion of the mother liquor from the confined body and applying heat over the upper surface of the thus produced mass of crystals to cause a progressively downward remelting of said mass and, :as said remelting proceeds, separating the remelted metal into at least two successive fractions.

2. The improved process of claim 1 in which the rate of heat transfer at said interface is accelerated while the transfer of heat at the contained boundaries of the said body is controlled to practically prevent massive freezing of said melt at said boundaries.

3. In a process wherein molten impure aluminum containing eutectic impurities and more than about 99 percent by weight of aluminum is fractionally crystallized to initially place in solid form at least a part of the aluminum as a higher purity product thereby to effect a concentration of at least a part of the eutectic impurities in the resulting mother liquor from which said solid aluminum of higher purity tends to settle in the form of crystals by reason of their greater density the improvement of obtaining from said impure aluminum a substantial separated final fraction of aluminum purity of at least about 99.99 percent by weight by containing said impure aluminum to form a molten body thereof having an unconfined molten metal-air interface, removing heat of solidification at and through said interface at a rate to maintain formation of aluminum-rich crystals in a zone of crystallization which is at, under and substantially parallel to said interface while preventing substantial loss of heat at the contained boundaries of said body, and, during said crystallization, compacting the crystals thus formed, as they settle to the lower regions of said body, by intermittently applying pressure to the upper area of the settled crystal mass, at any given application thereof, over an area of not more than about one-half of said upper area, the rate of pressure application being such that the pressure is applied over substantially the entire upper area of said mass at least once in about every ten minutes, continuing said crystallization and said concurrent compaction until a predetermined portion of the original molten body has been crystallized, thereafter separating a major portion of the mother liquor from the confined body, applying heat over the upper. surface of the thus produced mass of crystals to cause a progressively downward remelting of said mass, and, as said remelting proceeds, separating the remelted metal into at least an initial fraction and, finally, remelting the lower portion of said crystal mass to obtain a final fraction.

4. The improved process of claim 3 in which the rate of heat transfer at said interface is accelerated while the transfer of heat at the contained boundaries of said body and at the juncture of said interface with pressure applying elements is controlled to practically prevent massive freezing of said melt at said boundaries and said junctures.

5. In a process wherein molten impure aluminum containing eutectic impurities is fractionally crystallized to initially place in solid form at least a part of the aluminum as a higher purity product thereby to effect a concentration of at least a part of the eutectic impurities in the resulting mother liquor from which said solid aluminum of higher purity tends to settle in the form of crystals by reason of their greater density, the improvement of containing said impure aluminum to form a molten body thereof having an unconfined molten metal-air interface, removing heat of solidification at and through said interface at a rate to maintain formation of aluminum-rich crystals in a zone of crystallization which is at and under and substantially parallel to said interface while preventing substantial loss of heat at the contained boundaries of said body, continuing said crystallization until a predetermined portion of the original molten body has been crystallized, thereafter separating a portion of the mobile mother liquor from the confined body and remelting the thus produced body of crystals to obtain at least one fraction of higher aluminum purity than the original impure aluminum.

6. The process of claim 5 characterized by the fact that during the crystallization the aluminum-rich crystals are compacted in the lower region of said body by applying to said crystals as they settle intermittent pressure which during any given application is applied to not more than half of the total upper area of the mass formed by said crystals.

7. In a process wherein molten impure aluminum containing eutectic impurities is fractionally crystallized to initially place in solid form at least a part of the aluminum as a higher purity product thereby to effect a concentration of at least a part of the eutectic impurities in the resulting mother liquor from which said solid aluminum of higher purity tends to settle in the form of crystals by reason of their greater density, the improvement of containing said impure aluminum to form a molten body thereof having an unconfined molten metal-air interface, removing heat of solidification at and through said interface at a rate to maintain formation of aluminum-rich crystals in a zone of crystallization which is at, under and substantially parallel to said interface while preventing substantial loss of heat at the contained boundaries of said body, continuing said crystallization until a predetermined portion of the original molten body has been crystallized, thereafter separating still liquid mother liquor from the confined body and remelting the resultant body of crystals.

8. In a process wherein molten impure aluminum containing eutectic impurities is fractionally crystallized to initially place in solid form at least a part of the aluminum as a higher purity product thereby to effect a 19 concentration of at least a part of the eutectic impurities in the resultant mother liquor from which said solid aluminum of higher purity tends to settle in the form of crystals by reason of their greater density, the improvement of obtaining at least two fractions of higher purity product by containing said impure aluminum to form a molten body thereof having an unconfined molten metal-air interface, removing heat at and through said interface at a rate to maintain formation of aluminumrich crystals in a zone of crystallization which is at, un-

der and parallel to said interface while preventing substantial loss of heat at the contained boundaries of said body, continuing said crystallization until a predetermined portion of the original molten body has been crystallized, thereafter separating the drainable portion of the mother liquor from the confined body, applying heat over the upper surface of the thus produced crystal mass to cause a progressively downward remelting of said mass and, as said remelting proceeds, separating the remelted metal into at least two successive fractions.

9. In a process wherein molten impure aluminum containing eutectic impurities is fractionally crystallized to initially place in solid form at least a part of the aluminum as a higher purity product thereby to effect a concentration of at least a part of the eutectic impurities in the resulting mother liquor from which said solid aluminum of higher purity tends to settle in the form of crystals by reason of their greater density, and wherein the crystals formed during said crystallization are by application of pressure compressed into a compact crystal mass from which a substantial part of the mobile portion of the mother liquor is removed, the improvement of obtaining and separating at least two remelt fractions from said crystal mass by applying, after removal of the mobile mother liquor, heat over the upper surface of the remaining crystal mass to cause a progressively downward remelting of said mass and, as remelting proceeds, separating the remelt into at least two successive fractions.

10. In a process wherein molten impure aluminum containing eutecticimpurities is fractionally crystallized to iniitally place in solid form at least a part of the aluminumas a higher purity product thereby. to effect a concentration of at least a part of the eutectic impurities in the resulting mother liquor from which said solid aluminum of higher purity tends to settle in the form of crystals by reason of their greater density, said process including the steps of containing said impure aluminum to form a molten body having an unconfined molten metal-air interface, of removing heat of solidification at and through said interface t-o maintain formation of aluminum-rich crystals in a zone of crystallization which is at, under and substantially parallel to said interface, of continuing said crystallization to produce in crystal form a predetermined fraction of the original molten body and of separating and remelting the said fraction, the improvement consisting of manipulating temperature conditions at the said interface to accelerate the rate of heat transfer at said interface while controlling the transfer of heat at the contained boundaries of said body to practically prevent massive freezing of the melt at said boundaries.

11. The improved process of claim 10 characterized by the fact that acceleration of heat transfer at said interface is effected by contacting the molten metal at said inerface with a moving body of cooling air.

References Cited by the Examiner UNITED STATES PATENTS 2,087,347 7/37 Larsen -65 2,198,673 4/40 Loevenstein 75-63 2,382,723 8/45 Kirsebom 75-93 2,471,899 5/49 Regner 75-63 2,676,882 4/54 Hatch 75-63 2,739,110 3/56 Meister 75-65 2,912,321 11/59 Brennan 75-63 2,974,032 3/61 Grunert 75-68 3,069,240 12/62 Armond 23-296 3,102,805 9/63 Messner 75-81 DAVID L. RECK, Primary Examiner. BENJAMIN HENKIN, Examiner,

Claims (1)

1. IN A PROCESS WHEREIN MOLTEN IMPURE ALUMINUM CONTAINING EUTECTIC IMPURITIES AND MORE THAN ABOUT 99 PERCENT BY WEIGHT OF ALUMINUM IS FRACTIONALLY CRYSTALLIZED TO INITIALLY PLACE IN SOLID FORM AT LEAST A PART OF THE ALUMINUM AS A HIGHER PURITY PRODUCT THEREBY TO EFFECT A CONCENTRATION OF AT LEAST A PART OF THE EUTECTIC IMPURITIES IN THE RESULTING MOTHER LIQUOR FROM WHICH SAID SOLID ALUMINUM OF HIGHER PURITY TENDS TO SETTLE IN THE FORM OF CRYSTALS BY REASON OF THEIR GREATER DENSITY, THE IMPROVEMENT OF OBTAINING FROM SAID IMPURE ALUMINUM ONE SUBSTANTIAL REMELT FRACTION OF AN ALUMINUM PURITY GREATER THAN THAT OF SAID IMPURE ALUMINUM, AT LEAST ONE OTHER REMELT FRACTION AND A MOTHER LIQUOR FRACTION, BY CONTAINING SAID IMPURE ALUMINUM TO FORM A MOLTEN BODY THEREOF HAVING AN UNCONFINED MOLTEN METAL-AIR INTERFACE REMOVING HEAT OF SOLIDIFICATION AT AND THROUGH SAID INTERFACE AT A RATE TO MAINTAIN FORMATION OF ALUMINUM-RICH CRYSTALS IN A ZONE OF CRYSTALLIZATION WHICH IS AT, UNDER AND SUBSTANTIALLY PARALLEL TO SAID INTERFACE WHILE PREVENTING SUBSTANTIAL LOSS OF HEAT AT THE CONTAINED BOUNDARIES OF SAID BODY, AND DURING SAID CRYSTALLIZATION, COMPACTING THE CRYSTALS THUS FORMED AS THEY SETTLE TO THE LOWER REGIONS OF SAID BODY, BY INTERMITTENTLY APPLYING PRESSURE TO THE UPPER AREA OF THE SETTLED CRYSTAL MASS, AT ANY GIVEN APPLICATION THEREOF, OVER AN AREA OF NOT MORE THAN ABOUT ONE-HALF OF SAID UPPER AREA, CONTINUING SAID CRYSTALLIZATION AND SAID CONCURRENT COMPACTION UNTIL A PREDETERMINED PORTION OF THE ORIGINAL MOLTEN BODY HAS BEEN CRYSTALLIZED, THEREAFTER SEPARATING A MAJOR PORTION OF THE MOTHER LIQUOR FROM THE CONFINED BODY AND APPLYING HEAT OVER THE UPPER SURFACE OF THE THUS PRODUCED MASS OF CRYSTALS TO CAUSE A PROGRESSIVELY DOWNWARD REMELTING OF SAID MASS AND, AS SAID REMELTING PROCEEDS, SEPARATING THE REMELTED METAL INTO AT LEAST TWO SUCCESSIVE FRACTIONS.

Images