US 3840364 A
Description (OCR text may contain errors)
United States Patent [191 Flemings et a1.
-[451 Oct. 8, 1974 1 1 METHODS OF REFINING METAL ALLOYS  Assignee: Massachusetts Institute of Technology, Cambridge, Mass.
22 Filed: Jan. 28, 1972 21 Appl. No.: 221,796
 US. Cl. 75/63, 75/0.5 B, 75/64, 75/65, 75/68 R, 75/135, 264/12  Int. Cl ,C22b 7/00  Field of Search 75/63, 65, 68 R, 129, 135, 75/79, 93 R, .5 R, .5 B, 64; 264/12  References Cited UNITED STATES PATENTS 1,831,023 11/1931 Mathieu 75/68 R 1,957,819 5/1934 Cowan 75/63 2,180,139 11/1939 Deitz, Jr... 75/93 R 2,296,196 9/1942 Behr 75/63 2,362,147 11/1944 Mondolfo 75/68 R 2,471,899 5/1949 Regner 75/63 2,967,351 1/1961 Roberts et a1. 264/12 X 3,028,234 4/1962 Alexander et al. 75/134 3,102,805 9/1963 Messner 75/63 X 3,180,727 4/1965 Alexander et al. 75/134 3,189,444 6/1965 Olds et a1 75/148 3,194,656 7/1965 Vordahl 75/135 3,240,592 3/1966 Bray 75/135 3,300,296 1/1967 Hardy et a1 75/20 F 3,373,014 3/1968 Merles 75/63 X Schmidt 75/63 X 3,393,067 7/1968 Alexanderet al. 75/l30.5 3,468,658 9/1969 Herald et a1. 75/122 3,515,542 6/1970 Larsen 75/l22.5 3,574,609 4/1971 Finlay et a1. 75/153 3,600,163 8/1971 Badia ct a1 3,630,720 12/1971 Messner 3,667,742 7/1972 Toth 75/63 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-M. J. Andrews Attorney, Agent, or Firm-Arthur A. Smith, Jr.; Robert Shaw; Martin M. Santa 57] ABSTRACT A method of producing purified powders and of refining metal alloys from alloy scrap and the likeThe alloy is brought to a state which is partly liquid and partly solid and in mixture form, preferably by heating the alloy to create a melt and then cooling the melt to permit partial solidification thereof. The mixture is vigorously agitated to create a slurry, the temperature of the slurry being controlled to provide a predetermined percentage of liquid-solid therein. Then the liquid is separated from the solid either by selectively draining the liquid or by producing a stream of the slurry and injecting intothe stream a gas in the form of a high velocity jet, which atomizes and solidifies the liquid to form a particulate that is generally smaller than the solid portion of the mixture (i.e., the primary solid) and can beseparated therefrom by screening, for example.
Y 13 Claims, 4 Drawing Figures PATENTEDBCI 8 m4 SHEH 1 0f 2 FIG.
3,840,364 PATENTED 81974 sum 2 a a FIG.
METHODS OF REFINING METAL ALLOYS The invention herein described was made in the course of a contract with the Office of the Secretary of Defense, Advanced Research Project Agency.
The present invention relates to a method of purifying metal powders and of refining metal alloys from alloy scrap and the like and, particularly, to a method of purifying and refining by a partial-solidification and separation process.
The best known examples of use of partial solidification to refine metals are zone refining" and related techniques. In these techniques, a liquid-melt region is formed in the metal and is moved relative to the solid metal; the interface between the liquid and the remaining solid is maintained microscopically planar as solidification proceeds from one part of the solid to another. The solid forming in resolidification is of different composition than the liquid, alloy elements being either preferentially rejected or preferentially incorporated in the solid as it grows. These techniques are useful for obtaining ultra-high purity in already nearly pure material. They are not applicable for the purifying or refining of large quantities of alloys containing appreciable solute.
In other partial-solidification processes, the solid forms, not with a plane front, but dendritically, so that over large portions of a solidifying casting or ingot, solid and liquid are intimately mixed. In this case, as in zone refining, the solid forming is of different composition than the liquid remaining. Refining can be effected during solidification, provided the intimately admixed I liquid can be separated from the solid. Such separation can be accomplished by filtering the liquid from'the mixture by the use, forexample, of a porous refractory or a granular filter bed. Oxides and other non-metallics are removed from aluminum in this way as is the Fe-Sn intermetallic from molten tin. Another separation technique is centrifugal separation of the interdendritic liquid from the solid. The US. Bureau of Mines pioneered refining of alloys in this way, notably tinand Zinc alloys. In France, Pechiney centrifuges have been used to separate zinc extracted aluminum from aluminum-silicon alloys. More recently, a removable wall centrifuge for aluminum-silicon alloys has been discussed. In this interesting concept, the alloy is partially solidified within a stationary centrifuge. The circumferential wall of the centrifuge is then removed and the centrifuge rotated. Conditions are adjusted such that the skeleton of the partially solidified alloy does not rupture and centrifugal force causes the interdendritic liquid to flow radially outward. Irrespective of the particular separation procedure employed, however, there is a great deal of trapping of the liquid within interstices of the solid, which makes removal difficult.
A series of processes had been developed or proposed in which a local portion of a melt is caused to solidify at least partly, the bulk remaining fully liquid. The liquid is then vigorously agitated with the effect that some of the interdendritic enriched liquid from the partially solidified region is washed out into the bulk fluid. In a modification of this technique, it has been proposed to add grains of pure metalas seeds for crystallization.
In an Alcoa process for producing high purity aluminum, heat of fusion is removed from the top surface of a bath of molten aluminum. The crystals that form tend to settle downwards and are aided in this by tamping" with a graphite bar. After approximately percent of the melt is solidified, the remaining (impure) liquid is decanted. The solid which has formed is next directionally melted from the top down, with the liquid flowing through the bed of settled crystals. This liquid is drawn off continuously as melting proceeds. The purity of the liquiddrawn off increases as melting progresses. For example, in one experiment, initially the composition was 99.8% Al. The last thirty percent of liquid to be drained in the remelting operation averaged 99.9% Al and the last 10% averaged 99.94% Al. A modification of the Alcoa process has been proposed, in which initial crystallization is caused to occur not on the top surface of the melt but on a cooled finger within the melt itself.
Most, but by no means all, impurity elements in liquid metals lower the melting point of the purer metal. Examples are iron and silicon in aluminum alloys, when present in amounts less than the eutectic composition. In these cases, the liquid that is extracted from the partially solidified bulk alloy is less pure than the solid which has formed; thus, the solid remaining is the desired, purified material. Examples of alloy elements which raise the melting point of the purer material are iron and silicon in aluminum alloys when present in amounts greater than eutectic. For suchalloys, the liquid extracted is the refined portion of the metal. Another class of elements which raises the melting point of aluminum consists of elements which form a peritectie that is rich in aluminum, examples being chromium and zirconium. Efficiency of many of the separation processes described above can be altered by adding elements which modify the phase diagram and, so, the efficiency of separation. As an example, addition of silicon and/or magnesium to aluminum moves the composition of the iron rich eutectic closer to pure aluminum and so lowers the limit to which iron can be reduced in alloys where the iron is originally present in large (hypereutectic) quantities. Addition of boron to aluminum causes formation of borides with elements such as titanium and vanadium. These are virtually insoluble in molten aluminum, are heavy and settle, after'which the purer aluminum is decanted.
Accordingly, an object of the present invention is to teach a new process for refining, by partial solidification, alloy scrap and the like.
A further object is to provide a process which can be employed to purify or refine pure material, in small quantities, to ultra-high purity, but one that can be used, as well, to refine large quantities of alloys containing appreciable solute.
, A still further object is to provide a partial solidification process which is faster, more economical and more efficient than previously discussed processes intended to accomplish the herein-discussed purposes.
Another object is to teach a way of producing purified metal powders.
Still another object is to provide a process which allows refining of a greater range of alloy types than hitherto has been possible employing prior-art techniques.
These and still further objects are discussed hereinafter and are particularly delineated in the appended claims.
By way of summary, the objects of the invention are attained in a process for separating a metal alloy into the constituent metals of which the alloy consists by first heating the metal alloy to produce a mixture, part liquid and part solid. The mixture can be arrived at by first melting the alloy and then cooling the liquid melt until freezing occurs; or it can be formed by heating the alloy until enough of the alloy is in the liquid state to permit stirring or agitating. The liquid-solid mixture thus formed is vigorously agitated to produce a slurry, the temperature of the slurry being adjusted to a predetermined level which, in combination, with the agitating, establishes the percent liquid and the percent solid of the slurry. The agitation in the disclosed embodiment is effected by a rotating blade disposed within the mixture and operable to create shear-type movement in the mixture during the mixing process. The liquid portion of the slurry is then drained or otherwise separated from the solid portion thereof.
The invention will now be discussed with reference to the accompanying drawing in which:
FIG. 1 is an elevation view, schematic in form and partially cut away, of apparatus adapted to perform the herein-described process;
FIG. 2 is a reproduction of a photomicrograph showing a beginning metal alloy which is purified into ultrahigh purity powders or is refined to a higher purity than the original alloy;
FIG. 3 is a reproduction of a photomicrograph of the alloy material, after solidification, that remains in a crucible after refining by employing the drainage or filtration techniques herein discussed; and
FIG. 4 is a reproduction of a photomicrograph of the now-solidified liquid drained in the process mentioned in connection with FIGS. 2 and 3.
In t p esiq qibe ug e i i etailberemie producing purified powders and for refining alloy scrap, the initial material is heated to form a melt. Thereafter energy is withdrawn from the melt, until solidification begins, to form a mixture which is part liquid and part primary solid. The liquid-solid mixture is agitated throughout the stages of solidification to break up formation of a continuous dendritic network and thereby produce a slurry. Further heat is withdrawn until a desired percent liquid-solid is attained, at which point the liquid is separated from the solid. Two distinct methods of separating the solid phase, i.e., primary solid, of the slurry from the liquid phase have been developed. Both are discussed in some detail later; but, briefly, one separation method involves draining or filtering the liquid in the slurry from the solid and the other method involves forming the slurry into a stream which is subjected to a high pressure gas jet (e.g., nitrogen) which atomizes and solidifies the liquid phase. The thus-solidified particulate can then be distinguished on the basis of size from the primary solid.
The preparation of the slurry is, however, similar irrespective of the separation technique employed. Thus, the melt is partially solidified while it is vigorously agitated by one or more rotating propeller blades 1, in FIG. 1. As distinct from some of the prior-art processes previously discussed, it is not simply the bulk liquid that is agitated, but the entire liquid-solid mixture which is designated 2 and in which the liquid is designated 3 and the primary solid 4 (the particles 4 shown are, of course, of exaggerated size for purposes of the present explanation). The vigorous agitation causes the liquidsolid mixture to behave as a fluid slurry to fractions solid as high as about 0.5. A combination of the vigorous agitation and relatively slow rate of heat removal from the solidifying slurry causes it to beessentially isothermal and to have uniform distribution of fraction solid throughout.
The structure of the solid grains that form during this vigorous agitation is often very different from the usual dendritic structure that forms during usual solidification of castings and ingots (see, for example, FIG. 1 in an application for Letters Patent S.N. 153,819, filed onJune I6, 1971, by two of the present inventors. The solid particles in some cases approach small spheroids, and this is an important aspect of the processes described herein. Because the fine dendritic structure is absent, the segregated liquid is much easier to separate from the solid than in prior-art methods-there is, ideally, no liquid in small interdendritic pockets. In other alloys, such as that shown in FIGS. 2 and 3, the primary solid forms as a faceted phase, without obvious dendrites, even when no agitation is employed. Agitation however, causes alloys even of this lattermentioned type also to behave as'a slurry suitable for the refining treatment discussed herein.
FIG. 1 is a schematic illustration of the basic heating and mixing apparatus used in the experiments later mentioned. In the figure, the mixture 2, consisting of the primary. solid pieces 4 suspended in the liquid 3, forms a slurry upon being agitated by the rotating propeller blade 1. The rotating blade 1 creates shear-type strains within the slurry. Heat is supplied by an electric resistance furnace 7; and the propeller 1 is driven by a shaft 8 powered by a variable-speed electric motor assembly 9. In one separation process, after theagitated slurry has reached a desired temperature (and so desired fraction solid), a tap hole'5 in the crucible shown at 10 is opened. Vigorous agitation is maintained especially in the vicinity of the tap hole 5 and the slurry 2 then flows out this hole. On leaving-the hole, it is struck by a number of converging gas jets provided by nozzles 6 to atomize it as is done in standard commercial practice with fully liquid melts. In this case, the gas jets atomize the fully liquid portion of the melt to very fine particles which subsequently solidify. These fine particles, having formed from fully liquid material, possess the average composition ofthe liquid (usually enriched in impurity). The gas jet cannot, of course, atomize the solid particles of metal entrained in the stream. These particles, larger than those of the atomized liquid, are low in impurity. Refining is now accomplished simply by separating (as by Screening) the larger size fractions from the smaller.
In the second separation process, the metalis mixed and cooled, as described above, to obtain a partiallysolid slurry. A tap hole is again opened in the crucible, but this time, once the slurry is formed, the rotation speed of the blades 1 is lowered so the metal in the immediate vicinity of the tap hole 5 is not so vigorously agitated. Now, a metal stream again flows through the tap hole 5, but this time only the liquid portion goes through the hole; the bulk of the solid phase stays behind. It is believed that the reason for this behavior is that the solid particles give the slurry a thixotropic nature. A semi-rigid skeleton of the solid grains forms in regions of the not so vigorously agitated melt. This rigid skeleton then effectively acts as a fine filter, holding back the particles from the more vigorously agitated portion of the melt. While the second process of separation by filtration may be performed by using the solid grains as the sole filtering medium, as just indicated, a ceramic filter may additionally be used to aid in performing the filtering step of applicants method. When a ceramic filter is used, the filtering action may be accelerated if pressure is also applied to the slurry.
Although two primary methods of separation have been disclosed above, resort may also be had to still a third method of separation, if those skilled in the art find it more desirable. Such third method involves the use of centrifugal force to effect the separation.
The process described herein for purifying metal alloys has several potential advantages over existing processes. One of these is that the form of the solid in the liquid-solid mixture is, potentially, fully non-dendritic, and so it is possible more completely to separate the liquid from the solid than in existing processes. In priorart processes, such as centrifuging, relatively high pressures are required to effect separation. 1n the present process, the nature of the slurry is such that either the liquid-solid mixture, or selectively the liquid, can be made to flow under a relatively low pressure head (in the experiments reported gravity alone sufficed).
Three different experiments are discussed in the immediately following paragraphs: the first two were on a Sn-% Pb alloy while the third experiment was on an Al-30% Si-6% Fe-1% Ti alloy. The first experiment was on a Sn-15% Pb alloy that was vigorously agitated during solidification down to 40% solid (by weight), then atomized.
Experiment I: Atomization of Liquid-Solid Lead-Tin With mixing initiated at a temperature above liquidus, the furnace was turned off and mixing continued as the temperature dropped. Mixing equipment consisted of a variable-speed motor and three-bladed propeller. The propeller was rotated in a manner so as to push material downward toward the center bottom of the crucible and upward at the sides.
When the temperature had fallen to the point where the alloy was 40% solid, the rotational speed of the propeller was increased to 200 RPM and maintained. at that value; a A inch hole in the crucible bottom was opened to allow the slurry to emit as a stream; and atomization was begun. Nitrogen at 200 psi was applied and this caused the liquid in the stream to be blasted away from the primary solid. The liquid then solidified into fine powder. The powder was then screened. The larger particles (10 to 30 mesh) in this process should contain 2-3% lead and the smaller particles (100 mesh and up) should contain 23% lead. The first effort, however, resulted in about 12% for the small particles and about 2223% for the small, sti1l a substantial improvement.
Experiment 11: Draining of Liquid-Solid Lead Tin The same initialprocedure as in Experiment 1 was used, the temperature of the slurry was lowered until the alloy was 50% solid, at which point the rotation rate was slowed from about 200 RPM to about 50 RPM and the inch hole in the crucible bottom was opened.
Substantial amounts of liquid material gravity drained from the slurry with very few solid particles being pulled down. The drained liquid contained about 22% lead and the residue in the crucible about 12% lead.
Experiment 111: Draining of Liquid Solid Aluminum Silicon Iron Titanium Alloy Table 1 (A) 750C=l'cmperature When Tapped Silicon Iron Titanium lnitial 29.8% 6.10% 1.08% Draincd 22.6% 4.64% .43%
(B) 600C= Temperature When Tapped Silicon Iron Titanium Initial 30.0% 6.19% 0.94% Drained 13.1% 1.77% 0.15%
In a separation method like that employed in Experiment 1, both the liquid and the solid phases are forced through a hole located in the base of the crucible and the resulting, mixture is atomized by a gas jet in the manner employed in usual liquid atomization processes, as above mentioned. The high flow velocity gas jet breaks up the liquid into small spherical powders thereby separating the liquid from the initiallysolidified, primary solid. By varying gas pressure, i.e., velocity, and/or nozzle geometry, the size of the atomized liquid powders can be varied over a wide size range. It will be appreciated that the angular velocity of the mixer, temperature of the melt, size of the drain hole 5, will vary with the alloy being purified in either separation process.
In the separation method employed in Experiments 11 and 111, the rotational speed of the propeller-like blade 1 and/or location or direction of rotation thereof can be varied so that only the liquid phase is drained through the hole in the crucible. In this situation, the solid-particles have a tendency to bond together, thereby producing a natural filter for drainage of the liquid. This type of materials separation can "be applied to purificatioon of any multi-component alloy system because the different alloy elements normally segregate during solidification.
The picture of the initial alloy material for an experiment like that described in Experiment III is shown, greatly enlarged, in FIG. 2 and in the vigorouslyagitated state. The material left in the crucible is pictured in FIG. 3 and the material, drained as a liquid and solidified, is shown in FIG. 4.
In the experimental work reported, as mentioned, apparatus like the apparatus shown schematically in FIG. 1 was used, and this apparatus has one rotatable propeller and one drain hole in the bottom of the crucible 10. It will be appreciated that more than one propeller and more than one hole can be employed in other more sophisticated apparatus and that mixers like those disclosed in said application may be employed in some circumstances.
Modifications of the invention herein described will occur to persons skilled in the art and all such modifications are deemed to be within the spirit and scope of the invention as defined in the appended claims.
What is claimed is: v
1. A process for refining a metal alloy, that comprises heating the alloy to form a mixture, part liquid and part solid, vigorously agitating the entire liquid-solid mixture thus formed in such a manner as to create shear type strains therein to produce a slurry containing liquid and primary solid, the solid being uniformly distributed throughout the liquid controlling the heat transfer to and from the mixture, the rate of heat transfer and the agitating acting in combination to establish the percent liquid and the percent primary solid of the slurry, and separating the liquid portion of the slurry from the primary solid portion thereof, said rate of heat transfer and said agitating being maintained during said separating to maintain the mixture in the slurry form and to maintain the desired percent liquid and percent primary solid in the slurry.
2. A method of refining metal alloys from alloy scrap that comprises, heating the alloy scrap to the molten state, withdrawing heat from the molten material thereby reducing the temperature of the molten material below the liquidus temperature of the alloy at which temperature solidification begins to form a mixture which is part liquid and part primary solid, continuing the withdrawal of heat to continue the solidification process to provide a substantial fraction solid whilesimultaneously vigorously agitating the bulk liquid-solid mixture so the mixture behaves as a slurry comprising liquid and particulate solid uniformly distributed throughout the liquid, forming a stream of the slurry and introducing a fluid in the form of a jet under high pressure into the stream, thereby separating the liquid in the slurry from the primary solid therein.
3. A method as claimed in claim 2 that includesvarying the velocity of the fluid in the jet thereby varying the size of the particles formed by it.
4. A method of refining metal alloys from alloy scrap that comprises, heating the alloy scrap to the molten state, withdrawing heat from the molten material thereby reducing its temperature below the liquidus temperature of the alloy at which temperature solidification begins toform a mixture which is part of liquid and part primary solid, continuing the withdrawal of heat to continue the solidification process to provide a substantial fraction solid while simultaneously vigorously agitating the bulk liquid-solid mixture so the mixture behaves as a slurry comprising the liquid and particulate solid uniformly distributed throughout the liquid, filtering the liquid from the liquid-solid mixture, thereby to separate the liquid from the primary solid, the latter constituting the sole filtering medium through which the liquid passes during separation.
5. A method as claimed in claim 4 in which the agitating in the filter region is reduced to prevent disturbing the primary solid in that region.
6. A method of purifying a metal alloy that comprises heating the metal alloy to produce a mixture, part liquid and part solid, vigorously agitating the bulk liquidsolid mixture in such a manner as to create shear type strains therein to form a slurry, comprising the liquid and particulate solid uniformly distributed throughout the liquid, while maintaining the temperature thereof at some predetermined level which, in combination with the agitating, establishes the percent liquid and the percent solid of the slurry, thereby minimizing the amount of continuous dendritic network present in the liquidsolid mixture, and causing the primary solid material to assume a shape approaching that of spheroids, and separating the liquid portion of the slurry fromthe solid portion thereof while maintaining agitation of the bulk of the liquid-solid mixture.
7. A method as claimed in claim 6 in which the metal alloy is heated only to said temperature, agitating being commenced only after enough of the alloy is in the liquid state to suspend the solid.
8. A method as claimed in claim 6 that includes heating until all is in the liquid form and thereafter withdrawing energy from the liquid to effect partial solidification thereof, thereby to produce the liquid-solid mixture, the mixture being agitated to form said slurry.
9. A method of refining metal alloys from alloy scrap that comprises, heating the alloy scrap to the molten state, withdrawing heat from the molten material thereby reducing the temperature of the molten material below the li quidus temperature of the alloy at which temperature solidification begins to form a mixture which is part liquid and part primary solid, continuing the withdrawal of heat to continue the solidification process to provide a substantial fraction solid while simultaneously vigorously agitating the bulk liquid-solid mixture in such a manner as to create shear type strains therein so the mixture behaves as a slurry comprising liquid and particulate solid uniformly dis tributed throughout the liquid, thereby minimizing the amount of continuous dendritic network presentin the liquid-solid mixture and causing the primary solid material to assume a shape approaching that of small spheroids, and separating the particulate suspension of primary solid particles thus produced in the slurry from the liquid while maintaining agitation of the bulk of the liquid-solid mixture.
10. A method as claimed in claim 9 in which said separating is effected by selectively drainingthe liquid from the liquid-solid mixture.
11. A method as claimed in claim 9 in which centrifugal force is employed to effect said separating.
12. A method as claimed in claim 9 in which the liquid in the slurry is drained from the solid through a ceramic filter to effect said separating.
13. A method as claimed in claim 12 in which pressure is introduced to the slurry to accelerate the separating process.