US3537695A - Apparatus for centrifuging - Google Patents

Description

Nov. 3, 1970 a. c. ROBINSON. JR., ETAL 3,537,695 7 APPARATUS FORCEN'I'RIF'U'GING Filed Jan. 19, 1968 -5 Sheets-Sheet 1 V INVENTORS GROVER C. ROBINSON, JR. OGLE R. SINGLETON, JR.

JOHN L. JORSTAILJR.

ATTORNEYS "N v s.1970 G. c. ROBIN ON. JR; Em 3,531,695

v APPARATUS FOR CENTRIFUGING Filed Jan; 19, 1968 3 Sheets-Sheet 2 INVENTORS GROVER c. nosmsom, JR. oeuz R. SINGLETON JR. JOHN L. JORSTADJR.

ATTORNEYS 3 9 0 G- c. IROBINSON, JR., TAL v 3,537,695

APPARATUS FOR CENTRIFUGING Filed Jan, '19 "1968 x 3 Sheets-Sheet 3 INVENTORS GROVER c. ROBINSON, JR. OGLE R. SINGLETON, JR. JOHN L. JORSTAD,JR.

ATTORNEYS United States Patent 3,537,695 APPARATUS FOR CENTRIFUGING Grover C. Robinson, Jr., and Ogle R. Singleton, Jr., Richmond, and John L. Jorstad, .lr., Glen Allen, Va., assignors to Reynolds Metals Company, Richmond,

Va., a corporation of Delaware Filed Jan. 19, 1968, Ser. No. 699,183 Int. Cl. B04b 1/02, 1/12 US. Cl. 266-37 19 Claims ABSTRACT OF THE DISCLOSURE Apparatus for separation of liquid and solid phase metals from each other by centrifuging, comprising a sealing enclosure which encloses a rotatable bowl having a removable side wall.

This application is related to copending patent application Ser. No. 435,596, filed Feb. 26, 1965, now US. Pat. No. 3,374,089.

This invention relates to method and apparatus for the separation of a metallic liquid phase from a partially solidified melt. More particularly, the invention concerns a novel method for the segregation of hypereutectic alloys containing at least 50% aluminum and at least 20% silicon into solid and liquid phases at temperatures above the eutectic temperature.

It is well known that the properties of metallic alloys are influenced by the nature and proportions of the components. The components or their intermetallic compounds may be soluble mutually, wholly, or only partially up to certain limits of saturation, depending upon temperature. These solubility relations have great influence upon the progression of freezing of the alloy from the molten state and upon the structure of the resulting solids. In the case of single pure metals, or of pure eutectics or other intermetallic compounds which act essentially as a unit substance, freezing takes place at constant freezing temperature to an aggregate of homogeneous crystalline structure. With other relations of solubility, solidification proceeds selectively, being spread over a range of temperature.

The present invention is concerned particularly with the behavior and treatment of hypereutectic aluminum alloys, especially those containing, by weight, at least 50% aluminum and at least 20% silicon, the remainder comprising various amounts of iron or titanium or both. Thus, the amount of iron can range up to about 15%, and theamount of titanium up to about 10%; and the alloy also may contain about 0.1-4% carbon. Alloys of this type can be segregated into solid and liquid phases at temperatures above the eutectic temperature. The invention especially includes the separation, including purification, of thermally smelted alloys which usually contain (in addition to Al, Si, Fe and Ti) calcium, phosphorus, and contaminations such as carbides, oxides, and oxycarbides. The predominant carbide is SiC, which may run as high as 10% in any particular alloy.

Generally, on cooling a molten alloy of this type, soluble nonmetallics and TiSi start to crystallize out first. The crystallization continues in the general pattern set by the known Fe-Al-Si phase diagram. As an example, within the particularly useful range of at least 50% Al, at least about 20% Si, 0.5 to 10% Fe, 0.1 to 5% Ti and less than 2% carbon, the order of crystallization is as follows:

at 1675 F.: SiC-l-TiSi (little Si) at 1400 F.: TiSi -l-Si (little SiC and little FeAl Si at 1130 F.: Si+FcAl Si (little SiC and little TiSi If Ca is present, CaSi will be mainly in the crystals obtained at 1130 F. Such alloys have a eutectic which solidifies at about 578 C. and has a composition: of approximately 11.7% Si, 0.8% Fe, 0.08% Ti, 0.02% Ca, and the remainder being aluminum. On cooling from the molten state and separating at a practical temperature above the eutectic temperature, e.g. 590 to 650 C., hypereutectic liquid phases are obtained. The higher the temperature, the more the Si, Fe and Ti stay in solution. A typical composition separated as liquid at 610 C. is 13.5 %-14% Si, 1.2% Fe, and 0.1% Ti. Additionally, the amount of iron may be decreased, if desired, by adding copper (up to about 4%) or magnesium (up to about 1%), or both, prior to crystallization, to provide a lower melting eutectic composition.

For an alloy of the character described, freezing from the melt results in segregation, with progressive enrichment of the molten composition toward the composition which has the lowest freezing point. The first portion or portions to freeze will have an opportunity to solidify in crystal form and with freedom for movement in the remaining liquid. The remaining liquid is finally forced to the composition of lowest and final freezing temperature, the so-called eutectic temperature and composition, constant for a particular alloy, whereupon it will solidify and occupy such space as may remain between the particles making up the solid phase frozen during the freezing range above the temperature of isothermal freezing. If the molten alloy is cooled to an intermediate temperature, the hypereutectic composition will remain liquid within the spaces in the solid phase.

Methods have been suggested in the prior art whereby a molten aluminum alloy is allowed to cool to form a crystal mass of noneutectic material, which is separated from a menstruum of liquid phase which may be eutectic or most commonly hypereutectic, preferably near to eutectic alloy, by centrifugation, e.g., in a basket centrifuge, but these methods have proven cumbersome and ineflicient. Other attempts have been reported using a centrifugal force, resulting in an acceleration at the periphery of 1,000 to 2,500 g., the aim being to move the lighter solids, e.g., silicon, towards the center for continuous removal. However, aside from the mechanical difficulties of operating hot mixtures with such high forces, the separation was incomplete and required a subsequent filtration.

In contrast, the present invention recognizes essential advantages in not removing solids at all, by producing a skeletal crystalline solid phase, having relatively high physical strength, and applying a centrifugal force below that which could destroy the structural integrity of the solid phase. In addition, the skeletal structure is used as a filter in which undesired solid contaminants are trapped. The invention accordingly provides means for obtaining such a skeletal structure having a compressive strength at temperatures around 1150 F., of 10-40 lbs./ sq. in., 'while the centrifugal force is limited to a force which will not change the structure and shape of the solid phase.

Therefore, in accordance with the present invention, there is provided a novel method for the separation of an alloy into solid and liquid phases involving the formation under controlled conditions of a cake of solid phase alloy, which is structurally rigid and capable of retaining its structural pores and channels when subjected to moderate centrifugal forces. The solid phase alloy obtained by the method of the invention comprises a rigid skeletal matrix having distributed throughout its inner portion, a multiplicity of interconnecting cells or pores, which communicate with each other, permitting the liquid phase to collect and to be separated by centrifugal or comparable means. Factors which influence the formation of strong skeletal crystalline phase and which are controlled in accordance with the invention include (a) the composition of the alloy, and the ratio of liquid to solid at a selected temperature and (b) the rate of cooling.

The novel method and apparatus of the invention are adapted for the separation of liquid and solid phases of metallic alloys in general, but will be illustrated with reference to the treatment of the aforementioned alloy of aluminum and silicon, for which the invention is especially adapted. The principles of centrifugal separation in accordance with the invention will be described as illustrative, but it is to be understood that the invention is not to be considered as limited thereto.

The method of the invention comprises the steps of introducing a raw molten alloy into the bowl of a centrifuge, allowing the substantially undisturbed (quiescent) mass of molten alloy to decrease in temperature by regulation of the temperature gradient of cooling to form in part a rigid skeletal solid phase containing: distributed throughout its inner portion, a multiplicity of interconnecting passages, and a liquid phase, said liquid phase being distributed in said passages, said spongy solid phase having strength sufiicient to withstand the application of centrifugal forces without injury thereto, and then centrifuging to separate said liquid phase from said solid phase. The solid phase is obtained in the form of a cake, comprising the selectively crystallized portion of the mixture.

The novel centrifuge bowl arrangement which is adapted to achieve the objective of controlled formation of a crystalline cake, in accordance with the invention, is of special design. Three embodiments of this apparatus are illustrated in the accompanying drawings, in which:

FIG. 1 is a schematic cross-sectional view of a centrifugal separator having a continuous outer shell and a removable inner cone;

FIG. 2a is a schematic cross-sectional view of a second form of a centrifugal bowl having a removable heatretaining wall;

FIG. 2b shows the apparatus of FIG. 2a, with the removable wall in a raised position;

FIG. 2c shows the apparatus of FIG. 2a, with the crystallized cake bound !by handing straps;

FIG. 2d shows the apparatus of FIG. 2a, with the crystallized cake bound by wire mesh; and

FIG. 3 is a preferred embodiment of the invention.

In the embodiment shown in FIG. 1, the centrifugal bowl 11 is mounted upon a drive shaft 12, driven by power means not shown. Molten alloy is supplied to the centrifuge bowl from a supply source shown generally at 13, which may be a melting furnace or a reduction furnace. A charge of molten alloy in the bowl 11 is allowed to cool to the desired temperature under controlled conditions, forming cake 16, which collects around a conical support 17 of refractory covered steel or similar material which is located in the center of the bowl and in alignment with shaft 12. The conical support rests upon a circular plate 18, kept in alignment with the shaft 12 by a central recess 19 into which the upper end of the shaft extends. The conical support 17 has attached at its upper end a shaft 20 by means of which the support and cake 16 can !be lifted out of the centrifuge bowl when the centrifuging operation is completed. Bowl 11 is provided at its upper edge with an annular flange 22, over which the liquid flows into annular collecting trough 21, which is provided with a splash baffie wall 23.

In the operation of the form shown in FIG. 1, the molten alloy is allowed to cool undisturbed until a rigid and strong skeletal structure has formed, the passages ,of which are permeated with molten liquid phase. The bowl is then subjected to rotation at a speed providing a force sufficient to expel the liquid phase from the interior of the cake and to the interior wall of the bowl, whereby the liquid phase rises along the inner surfaces of the bowl, and overflows the upper edge of the bowl into the circular trough 21 surrounding the bowl. When separation of solid and liquid phases is complete, the solid cake and supporting cone 17 are lifted out of the apparatus.

In the embodiment of the apparatus shown in FIG. 2a, the centrifuge bowl comprises a flat bed 30, mounted upon a driving shaft 31, which extends through the fioor of the bed to provide a projection of protuberance 32, which serves to anchor the solid cake when formed. The inwardly tapering wall 33 is lifted from the bed 30 after the solid cake has formed and immediately prior to centrifuging. The wall does not rotate and, when raised, acts in conjunction with splash baffle 37 and trough 34 to permit collection of the liquid phase ejected from the cake rotating with bed 30 (see FIG. 2b). The wall 33 and bed 30 are constructed of heat retardant and resistant materials such as steel lined with refractory ceramic. Molten starting alloy is supplied from a source 36, as shown. The apparatus of FIG. 2 may be fitted, if desired, with a central support such as 17 in FIG. 1, and, conversely, the apparatus of FIG. 1 may be used without such support. Although not shown, projection 32 may be extended above the charge 16 and, by means of spider arms, support a screen through which the liquid phase is extracted.

The physical strength of the crystal sponge permits it to suffer moderate, but suflicient centrifugal forces, without the support of an outer retaining wall. This in turn allows less centrifugal force, because the liquid does not require any vertical force component, as in the embodiment of FIG. 1.

In the embodiment shown in FIG. 20 the centrifuge bowl is comprised of a fiat bed 30 and an inwardly tapering wall as disclosed in FIG. 2b (not shown). The inwardly tapering wall shown in FIG. 212 has been lifted from the bed 30 after the cake 16 has been formed and prior to centrifuging. Immediately prior to centrifuging several or more (three are shown) bands or straps 38 are tightly secured around the circumference of the cake 16. The bands 38 are shown as being tightened and/or adjusted by means of screws 40, however, various other means may be used. Also, bands which have no adjusting means may be used. All other numerals of the elements in the FIG. 20 refer to the like elements of FIGS. 2a and 12b.

In the embodiment shown in FIG. 2d the elements with like numerals as used in FIGS. 2a, 2b and 2c are the same elements as used in those figures. The embodiment of FIG. 2d differs from that of FIG. 20 in that the straps or bands 38 of that figure are replaced with wire mesh 42 or the like, tightly drawn to completely encircle the circumference of the cake 16.

In each of the FIGS. 20 and 2d the banding 38 or wire mesh 42 has been described as being placed around the cake 16 after the inwardly tapering wall had been removed, however, the banding 38 or wire mesh 42 could be placed adjacent to the inwardly tapering wall prior to the pouring of the molten alloy into the centrifuge bowl. After the inwardly tapering wall is removed the banding 38 or wire mesh 42 would be located in place without the necessity of so doing after the molten alloy cooled to a quiescent state to form a coherent skeletal crystalline solid phase.

In each of the modifications of FIGS. 20 and 2d the banding 38 and wire mesh 42 are used as an aid in giving rigidity to the crystalline cake 16 during the centrifuging thereof. With this additional strengthening provided by the bands or wire mesh the cake 16 may be rotated at a higher rpm. in order to extract more of the liquid phase alloy from the cake without endangering a centrifugal disintegration of the cake due to hoop stress failure. Of course, there are other obvious strengthening means which may be used instead of handing or wire mesh. ,As an example, a combination of the wire mesh and handing could be used.

In the embodiment shown in FIG. 3, which is a preferred embodiment of the invention, the centrifugal bowl is comprised of a fiat bed or base 50 of a generally planar surface but not necessarily of that configuration, and inwardly tapering wall 52. The inwardly tapering wall 52 is removable from the flat bed 50 by loosening the hand screw cranks 54, which are secured tight by nuts 56. The inwardly tapering wall 52 may also be constructed to taper in an opposite direction from that shown such that the Wall 52 may be lowered below the base 50 during the centrifuging. The flat bed 50 and the inwardly tapering wall 52 are constructed of a ceramic material backed by steel plating 58 for added strength. The flat bed 50 is removably secured to a turntable 60 by dowel pins 62. The dowel pins 62 are axially slideable in the turntable 60 through holes 61 such that the flat bed 50 may be removed from the turntable 60 and replaced by another centrifuging bowl, by the mere insertion of lift means into lift eye means 64. Centered in the flat bed 50 is a dowel pin 66 which in turn centers a cake lifting spindle 68 provided with a lifting eye means 70. The inwardly tapering wall 52 is provided with lift eye means 72 to facilitate the removal thereof after the hand cranks 54 are loosened. The turntable 60 is supported and axially aligned by driving shaft 74 and bearings 76 and 77. The shaft 74 is driven by pulleys 78.

The above-described centrifuging apparatus is entirely encased in a heat resistant enclosure 80. The enclosure 80 is supported by vertical supports 82. The bearings 76 and 77 are supported by horizontal suports 83 and 84, respectively. Horizontal support 83 is lined on its upper surface with asbestos heat shielding means 85. A steelplated ceramic ring 86 is supported above supports 82 by a resilient ring 87. The ring 86 supoprts a rock wool filled outwardly tapering side wall casing 89. Shown on the lefthand side of casing 89 is a trough 90 which slopes downward and to the right such that the molten alloy to be deposited therein will flow toward and out of the discharge spout 91. Mounted above the casing 89 and in heat sealing relationship therewith is an inwardly tapering side wall casing 92. The multipart casing 80 is covered with a removable lid 93 which is in heat sealing relationship with the inwardly tapering side wall casing 92. The lid 93 is provided with a burner vent chimney 94. An adjustable damper 95 is mounted atop the vent chimney 94 for the purpose of damping of the casing enclosure 80. Lift eye means 96 provide means which facilitate the removal of the lid 93 by the insertion of lift means therein. Burner pipes 97 are conduits through which external heat is applied to the centrifugal bowl and to the ambient environment of the casing 80. Various types of heating means may be used, depending upon the desired temperatures to be attained inside the casing 80'.

In order to more nearly completely heat seal the casing 80 sealing means 100 are pressed upwardly against the bottom of the flat bed 50. The sealing means 100 are made in the form of an asbestos ring which is tightly" bound by several bands 101. These bands 101 insure that the asbestos ring will not be abraded when it is moved axially against ring 86. The asbestos ring 100 is backed by a ring backing channeled member 102 for added rigidity. The asbestos sealing ring 100 is supported and axially moved by solenoids 104 through link members 105. While the turntable 60 is in a stationary or nonrotating position the solenoids 104 are energized to press the asbestos ring 100 tightly against the bottom of the flat bed 50. The solenoids 104 are actuated and energized by electrical circuit means 106. In order to lower the asbestos ring 100 from engagement with the bottom of the flat bed 50 the electrical circuit means 107 are actuated. After the asbestos ring 100 is lowered several .001 inch to allow the turntable 60 to rotate, reflecting heat shielding means 108 provide a labyrinth path to the escapement of any heat from the casing 80 into the bearings 76 and 77 or to the drive pulleys 78. Such a design of the asbestos rings 100, in combination with the heat shielding means 85, adequately protects the bearings and the drive pulleys from excessive heat damage. Adjustable stop means 109 are provided to insure that the asbestos ring 100 does not drop lower than the desired prede- 6 termined tolerance when the solenoids 104 are deenergized,

In the operation of the embodiment shown in FIG. 3 the ambient environment of the casing 80, the flat bed 50 and the inwardly tapering wall 52 are preheated to a sufiicient temperature to insure that the molten alloys to be poured into the centrifuging bowl do not solidify upon their initial deposition therein. This preheating is accomplished by directing heat through burner pipes 97. During this preheating step the turntable 60 is stationary and the asbestos ring 100 is pressed upwardly and tightly against the flat bed 50. However, the asbestos ring 100 could be lowered a sufficient amount and the turntable rotated during preheating if it were found that a more even heat distribution were achieved in this manner.

After the ambient environment of casing and the centrifuging bowl are preheated to a sufficient temperature the molten alloys are supplied to the centrifuging bowl in the aforesaid manner. The lid 93 may be removed to supply the molten alloy to the bowl or the molten alloy may be introduced through hatch means (not shown) in the lid itself. The molten alloy is allowed to cool by controlled cooling, undisturbed until a rigid and strong skeletal structure has formed, the passages of which are permeated with molten liquid phase alloys. The amibent environment of casing 80, the flat bed 50, and the inwardly tapering wall 52 are preheated in order to allow the outside surface of cake 16 to crystallize at the same rate as the interior of the cake. Were the molten alloys poured into a cold bowl the outside surfaces of the cake might be chilled to form a surface skin which would be impervious to the flow of any liquid metal through the cake crystals during the centrifuging operation.

When the cake 16 has crystallized into a strong skeletal structure the lid 93 is removed from the casing 80-. The hand screw cranks 54 are loosened and by means of the lift eye means 72 the inwardly tapering wall 52 is removed from the interior of casing 80. The lid 93 is then immediately replaced. The aforesaid bands 38 described in FIG. 20 or wire mesh 42 described in FIG. 2d may be placed securely around the cake 16 for added rigidity. Of course, the banding or wire mesh may also be preplaced as previously described in the discussion of FIGS. 2c and 20. before the molten alloys are poured into the bowl. Further, banding means may be entirely dispensed with when centrifuging the cake as is done in the FIG. 2b modification. Also, the inwardly tapering wall 52 may be merely lifted out of position inside the enclosure 80 before the centrifuging of the cake thus eliminating the necessity of removing lid 93.

After the inwardly tapering wall 52 is removed, and the lid 93 is replaced and the asbestos ring is lowered by means of solenoids 104 to a position such that its bottom surface is in contact with the stop means 109. The top surface of the asbestos ring 100 is now clear of the flat bed 50. The flat bed 50, which now supports the cake 16, bound by banding or wire mesh or unbound, is rotated to a sufficient r.p.m. to provide a centrifugal force sufiicient to expel the liquid phase means from the interior of the cake 16.

The liquid phase metal by the means of centrifugal force is thrown into trough 90. Since the trough is inclined, the liquid phase metal will flow by gravitational force toward and out of spout 91. In order to recover a greater quantity of the liquid phase metal from the cake, the interior ambient environment 80 of the apparatus is maintained at an optimum temperature during the centrifuging thereof by means of the burner pipes 97. While the interior ambient environment 80 has been described as that of air the heated environment may be inert or a reducing agent may be added thereto, to reduce the formation of oxide particles in the liquid metal.

Still a greater quantity of aluminum may be recovered from the cake by the reheating and the recentrifuging 7 thereof. There is no necessity in replacing the inwardly tapering wall 52 in place if excessive heat is not applied to the cake 16 before the recentrifuging thereof.

After the cake 16 has been centrifuged the lid 93 is again removed to allow for the removal of the cake 16. Lift means are then inserted into lift eye means 70. The cake 16 is removed from the fiat bed 50 by the lifting of the lifting spindle 68 with lift means inserted into lift eye means 70. The cake is then removed from the spindle 68 or another spindle is placed in the dowel hole centered by dowel pin 66. The inwardly tapering wall 52 is replaced and secured to the fiat bed by the tightening of the hand cranks 54. The asbestos ring 100 is reraised to a position to press tightly against the bottom of the flat bed 50 in heat sealing relationship therewith. The lid 93 is replaced and the apparatus is again ready to repeat the centrifuging operation with another charge of molten alloys.

The dowel pins 62 provide means whereby the fiat bed 50 may be replaced by another flat bed, in case of damage thereto or if cake sticking problems are incurred, by the mere insertion of and lifting of the flat bed by lift means in the lift eye means 64. Because of this expedient any damaged bowls may be quickly and expediently replaced on the turntable without the loss of valuable centrifuging time.

The temperature of centrifuging must be selected to insure that the particular alloy exhibits a selectively crystallized solid phase, which in volume corresponds to a solidzliquid ratio of at least 1:10. If less solid is present, the resulting structure does not sufficiently cake together. A preferred range of operation is 6590% liquid and 35-1 solids, although the operation can be conducted at equal parts liquid and solid and even down to about 20% liquid by volume. The recoverable fraction of the available liquid decreases, however, as the absolute amount of liquid decreases. This relationship is not linear, but decreases sharply as the initial proportion of liquid falls below the range from about 65% to about 40%. The rigidity of the cake and the efliciency of separation depend also on heat withdrawal within the critical range of temperature below 1800 F., where most of the crystallization takes place. An alloy tapped from the reduction furnace at about 3000 F. can be quickly cooled down to approximately 2000 F., e.g., by adding an appropriate amount of solid metal, which may, for example, have been obtained by casting a previous tapping into pig moulds. However, below 1800 F., the crystallization must be controlled and must be substantially undisturbed. The withdrawal of heat should be done at a rate resulting in an average temperature drop of not more than approximately 50 F. per minute, preferably less. Uniformity of temperature distribution throughout the alloy should be controlled, allowing a gradient of temperature of the order of F. It is advisable to contain the alloy in preheated and well insulated refractory material and to apply suitable heat, e.g., by hot gases, on the exposed surface. Best results are obtained when, for example, a mass of 1200 lbs. alloy is cooled from 1800" F. to 1130" F. over a period of 5-6 hours, e.g., at an average rate of 2 F. per minute. In such case, the alloy is contained in a well insulated bowl and the bowl is covered with a lid, minimizing radiation losses. However, for commercial speed of operations, artificial cooling may be applied, up to an average rate of about 20 F. per minute. It is not advisable to cool much faster, because the network of the skeletal cake becomes finer in size and enmeshes the liquid in smaller enclaves, which either necessitates greater centrifugal force for separation or results in less efiiciency.

The choice of rate of solidification also depends on the specific alloy. Larger contents of SiC, Fe, or Ti, e.g., more than 5% of any one, tend to produce a denser network at the same cooling rate compared to alloys having more primary silicon and less Fe or Ti.

Yield and rate of extraction of liquid from the coherent cake structure are influenced unexpectedly by certain variables in the processing steps. For example, in the operation of several size centrifuges up to 1000-lb. capacity it was found that the control characteristic was neither peripheral velocity nor centrifugal g force but was unexpectedly related directly to the rotational speed squared times the outer radius of the skeletal cake times the sum of the internal and external radii of the cake. This may be compared to the theoretical relationship developed by the US. Bureau of Mines in RI:5007, November 1953, that the control characteristic was directly proportional to the rotational speed squared and inversely proportional to the radius of rotation, for centrifuging molten metal by a method in which no solid skeletal structure was provided. This difference is indicative of the fundamental dissimilarities of the present method, which has the benefit of being better adapted for use in commercial sizes and amounts.

It has also been found, in accordance with the invention, that extraction for these aluminous alloys is optimum under solid to liquid volume ratios of 1:10 to 1:2 and is good at a solid to liquid ratio of 1:1, below which the extractability falls off unexpectedly and sharply to effectively no recovery at 4:1 solid).

There have also been found, in accordance with the invention, effects of particle size upon extractability. Primary Si and intermetallic particle size is increased in any aluminous alloy by controlling and slowing down the quiescent cooling rate, by pretreatment to remove the normal carbon containing contaminants to less than about 2% as carbon, as well as by temperature-of-separation selection to reduce the solid content of separation to as close as the 1:10 ratio as possible. An increase in particle size improves the ease of extraction not directly, but unexpectedly proportionately to the effective diameter of the solid particles to the second power.

Final centrifuge speeds employed may range from about to about 2500 rpm, in relation to the size of equipment as indicated by the foregoing control characteristic, and in accordance with other considerations set forth herein.

The composition of the liquid phase will be in the range: Si 12l8%, Fe 0.81.8%, Ti 0.1-0.3%, balance Al. The liquid phase thus obtained has a composition which is similar to those of aluminum-silicon die casting alloys, and can be used for this purpose. The liquid phase also may be used in other types of castings such as those used for making wrought products, especially if the liquid phase is modified by incorporating aluminum having a lower iron content; or it may be used as a beneficiated raw material for production of substantially pure aluminum by known refining processes such as, for example, the aluminum monohalide process.

The described method also offers a means to purify thermally smelted raw alloys from undesired contents of Ca or P. If Ca remains in a casting alloy with more than 0.05%, its effect is harmful for sharp reproduction of patterns. In particular, sharp edges may be rounded off.

If P remains in a casting alloy above 0.005%, it interferes with the well known sodium modification of the Si eutectic. The result is a coarser crystal shape of the silicon within the eutectic and consequently inferior strength characteristics.

It has been found, surprisingly and unexpectedly that the centrifugal treatment of the invention, whereby a skeletal structure of silicon and compounds containing silicon is obtained from an aluminum-silicon alloy, as previously described, also achieves an excellent removal of calcium and phosphorous from the liquid phase alloy, these elements being scavenged and remaining in the skeletal solid phase. Thus the liquid phase is suitable for die casting operations as obtained, being low in calcium (about 0.03%) and phosphorous (about 0.001%).

The solid cake is found to function as a cleaning agent 9 for the liquid phase alloy trapping calcium, e.g., in the form of CaSi between the crystals of silicon as they are formed. It is believed that phosphorous present is also trapped in the silicon crystals in the form of nuclei of aluminum phosphide around which the silicon crystals form.

.The performance of the method of the invention is illustrated by the following examples, which are not to be considered limiting.

The following examples concern an alloy containing approximately 30% Si, 4% Fe, 2% Ti, balance essentially aluminum, treated in the removable wall centrifuge of FIG. 2a, to provide a 9.3 cm. exterior radius of the cake at the bottom (and 9 cm. at 4 cm. above the bottom), with a 2.7 cm. radius of the post.

EXAMPLE I The alloy was treated to be essentially free of carbon impurities. The alloy at 1900 F. was poured into the centrifuge bowl. Cooling time from 1500 F. to 1300 F. was 13 minutes. At 1175 F., the Wall was removed. At 1130 F., the alloy was accelerated to about 2500 rpm. in about 30 to 50 seconds and held for about 70 seconds of rotating time prior to deceleration.

63% by weight of the charge was recovered as filtrate product of composition: 14.4% Si, 1.3% Fe, 0.1% Ti, balance essentially aluminum. The original charge was estimated to contain 69.5% liquid at 1130 F. and of that amount of liquid 90.5% was recovered. The residue had an analysis of 59.5% Si, 6.0 Fe, 6.4% Ti, and 25% Al.

EXAMPLE II The alloy was treated to be essentially free of carbon impurities. The alloy at 1900" F. was poured into the centrifuge bowl. Cooling time from 1700 F. to 1500 F. was four minutes. At 1500 F., the wall was removed. At 1475 F., the alloy was accelerated to about 1000 rpm. in about 30 seconds and held at that speed for 1 /2 minutes prior to deceleration.

87% by weight of the charge was recovered as filtrate of composition: 25% Si, 3.5% Fe, 0.39% Ti, balance essentially aluminum. The original charge was estimated to contain 90.5% liquid at 1475 F. and of that amount 95.7% was recovered.

The residue had analysis of 56.7% Si, 1.6% Fe, 14.0% Ti, and 24.8% Al. The central portion of the residue contained 60% Si, 1.4% Fe, 16.7% Ti, 19% Al, and was more representative of results obtainable either in a larger unit, or if hot gases at 1400 F. were supplied as a blanket over the spinning alloy. Over 80% of the input Ti was trapped in this residue.

The filtrate was collected and reseparated by centrifugal action at 1130 F., yielding a product of 14.4% Si, 1.3% Fe, 0.1% Ti, balance essentially Al, when centrifuged again at 1000 rpm. This provided a recovery of 63% of the filtrate charge and 55% of the original charge. Under comparable conditions, one separation at 1130 F. would yield 53% recovery. The resdiue composition was 48% Si, 8.2% Fe, 1% Ti, and 36.1% A1.

Spinning as above described was accomplished in a room temperature air environment. Yield is increased and control facilitated, however, when the top surface and sides of the alloy mass are provided with an environment of gases above the melting point of the liquid phase (about 1070 F.) and not substantially hotter than the desired separation temperature, thus permitting the metal in the centrifuge to approach thermal equilibrium at the temperature for separation. Without this provision, thermal gradients up to 40 F. occur in a 15- lb. load (and up to 75 F. in a 1000-lb. load), using average cooling rates below 1800 F. of 25 F. per minute and F. per minute, respectively. Nonuniform temperature is undesirable because it tends to cause a nonuniform liquid product. The provision of a temperature-maintaining capability has the added advantage of permitting a greater time leeway for initiating operation of the centrifuge, which is a practical convenience. Under conditions comparable to Example I, a copious supply of hot gases (1125 F.) was maintained around the cake after the wall was removed. The cake also was insulated from the rotating bed by six layers of Fiberfrax material. The yield increased to 68% of the charge.

The alloys discussed herein are conveniently prepared by a thermal smelting operation in which metallic ore such as bauxite, alumina or low-silica clay is reduced by carbon, preferably in an electric arc furnace. Some of the carbon can be replaced by substances of a metallic nature, as known in the art. The aluminous metal produced may contain dissolved or entrained non-metallics, especially carbon and carbides, also oxides and nitrides. If the carbon containing impurities were present at a level of 6 to 8%, then one spinning at 1000 r.p.m. maximum would reduce yield to 46% to 36% from a typical yield of 53% due to increased solids ratio and nucleant eifects reducing particle size.

The following additional example illustrates the effect of copper or copper and magnesium additions in depressing iron content of the separated liquid phase.

EXAMPLE III (a) A raw alloy having the following composition by weight was treated in the manner of Example I:

Si, 40.2; Fe, 3.6; Cu, 0.7; Mn, 0.3; Mg, 0.2; Ni, 0.1;

Ti, 1.8; A1, Bal.

The recovery was 45.3%, with the beneficial alloy having the following analysis:

Si, 12.7; Fe, 0.65; Cu, 1.6; Mn, 0.12; Mg, 0.37;

Ni, 0.22; Ti, 0.1; A1, Bal.

(b) In contrast, a raw alloy containing substantially no copper and magnesium, having the following composition, was treated in like manner:

Si, 40.3; Fe, 3.3; Cu, 0.1; Mn, (01; Mg, .05;

Ni, .OS; Ti, 0.10; A1, Bal.

to yield 54.4% recovery of an alloy analyzing as follows:

Si, 14.0; Fe, 1.1; Cu, .09; Mn, .02; Mg, 0.1;

Ni, .05; Ti, 0.10; Al, Bal.

It is apparent from the results of these comparative tests that the final iron content was considerably lower where copper and magnesium were present prior to centrifuging. The most useful range of additions is between about 1 and 3% copper, and from 0.1 to about 1% magnesium, in order to reduce the iron content of the liquid product below 0.8%. Spin separation then is performed below 1130 F and preferably below 1100 F. Over of the added copper typically is recovered in the liquid phase alloy.

Suitable methods for the removal of non-metallic insolubles prior to centrifuging include (a) high temperature settling, i.e., above 1800 F., and decantation of the upperpurified-portion; (b) high temperature centrifuging, i.e., above 1600 F. to remove purified liquid from a high concentration of non-metallics and some metallic compounds such as TiSi and (c) filtration at temperatures above 1800 F. through alumina, carbon, SiC, anhydrous quartz or other inert beds. On these beds, it has been found that particle size should be controlled such that the minimum size is not much less than effective diameter and can range upwards from this value. About 8" of bed of A? to 1 /2" metallurgical coke was found to reduce the total carbon content of a melt of raw alloy from about 2% to about 0.2% in a one pass straight-through filter at about 2600 F.

Although the preceding description has emphasized recovery of the liquid phase, certain useful characteristics also are exhibited by the residual solid phase. Due to removal of the more ductile, lower melting point metal, the cake is easier to crush or grind, and is susceptible to chemical leaching for the purpose of liberating higher melting point brittle phases such as, for example, TiSi Si, or an Al-Fe-Si compound; and the porous cake itself may be used for catalyst support or as a packing for chemical reaction towers.

In addition, the method of the present invention is applicable to the beneficiation of silicon containing impurities which are readily soluble in aluminum. This involves adding aluminum to produce a melt having at least about 25% silicon, and then operating near the lower limits of solid content and cooling rate to preferentially yield edge-connected crystals of silicon as the solid phase. Such crystals may be separated mechanically.

Another advantage of the invention is that a high melting point purified metal phase can be solidified out at temperatures well below the melting point. For example, an alloy of 40% Al60% Si (which is completely molten at 2200 F.) can be treated to yield half of its weight as a substantially pure silicon cake, when centrifuged at about 1300 E, although the melting point of silicon is about 2600 F. The coherent silicon matrix can be reinfiltrated by a liquid phase, iron or tin, for example, to produce an artificial composite having useful properties.

While present preferred embodiments of the invention have been illustrated and described, it will be apparent that the invention may be otherwise variously embodied and practiced within the scope of the following claims.

What is claimed is:

1. Apparatus for the treatment of molten metal to produce a coherent skeletal solid phase and a liquid phase therewith, and for the centrifugal separation of said solid and liquid phases, comprising a rotatable support means and an associated frusto-conically shaped nonporous side wall means which has an interior surface which tapers in a direction inwardly away from said rotatable support means, wherein said rotatable support means and said wall means form a nonporous bowl adapted to contain a charge of molten metal, and said wall means being removably attached to said support means to provide an opening therebetween for the removal of said liquid phase metal during the rotation of said support means.

2. Apparatus according to claim 1, wherein centering means are provided with solid phase removal means.

3. Apparatus according to claim 2, wherein said bowl comprises solid phase centering means removably atr direction from that of the centering means which tapers outwardly toward the rotatable support means.

5. Apparatus for the centrifugal separation of liquid phase metal from a coherent skeletal solid phase metal in which said liquid phase is entrained, comprising rotatable support means for said skeletal solid phase, an associated frusto-conically shaped nonporous removable sidewall means in cooperation with said support means thereby forming a bowl to contain a charge of molten metal, said wall means and said rotatable support means confining the resulting skeletal solid phase produced upon cooling of the charge of molten metal to effect selective crystallization thereof, means for centering the skeletal solid phase during the rotation thereof, means for separating the associated nonporous wall means from said support means prior to rotating the skeletal solid phase, and means for rotating the rotatable support means and said skeletal solid phase while maintaining said wall stationary to extract the liquid phase entrained therein.

6. Apparatus according to claim 5, wherein said rotatable support means and said wall means form a bowl which is rotatably supported within a sealing enclosure to provide for selective cooling the charge of molten metal contained in the bowl.

7. Apparatus according to claim 6, wherein said sealing enclosure is provided with liquid phase metal withdrawal means.

8. Apparatus according to claim 7, wherein said wall means has an interior surface which tapers in a direction inwardly away from the rotatable support means, said centering means tapers outwardly toward said rotatable support means and is provided with a solid phase removal means.

9. Apparatus according to claim 8, including heating means for preheating the bowl before it receives a charge of molten metal and for preheating the ambient environment of said enclosure to prevent the formation of an impervious surface skin on the skeletal solid phase.

10. Apparatus according to claim 9, including sealing means comprising a heat resistant seal adapted to maintain a seal between said enclosure and said rotatable support means, and shielding means coacting with the seal to reduce heat losses when the seal is moved out of its sealing relationship with the enclosure and the rotatable support.

11. Apparatus according to claim 10, wherein said sealing means is a controlled heat sealing means.

12. Apparatus according to claim 8, wherein said enclosure is provided with removable cover means to facilitate the removal of said wall means from the rotatable support means to allow for the centrifuging of said skeletalsolid phase.

13. Apparatus according to claim 12, wherein said means for centering the said skeletal solid phase during the rotation thereof is removably affixed to said rotatable support means.

14. Apparatus according to claim 8, wherein said means for centering said skeletal solid phase is provided with lift means to facilitate the removal of the skeletal solid phase from said rotatable support member.

15. Apparatus according to claim 8, wherein said bowl is removably afiixed to a support turntable to facilitate the removal of the bowl from enclosure.

16. Apparatus according to claim 5, including banding means adapted to constrain said skeletal solid phase during the centrifuging thereof.

17. Apparatus according to claim 7, including banding means adapted to constrain said solid skeletal solid phase during the centrifuging thereof.

18. Apparatus according to claim 5, wherein said wall means has an interior surface which tapers outwardly toward the rotatable support means, said centering means tapers outwardly toward said rotatable support means and is provided with a solid phase removal means.

19. Apparatus according to claim 5, wherein said Wall means has an interior surface which tapers in a direction inwardly away from the rotatable support means, said centering means tapers outwardly toward said rotatable support means and is provided with solid phase lift means.

References Cited UNITED STATES PATENTS 1,827,678 10/1931 Stramaglia 210-380 X 2,046,369 7/1936 Dake 164302 X 2,789,757 4/1957 Melton 233ll X J. SPENCER OVERHOLSER, Primary Examiner I. E. ROETHEL, Assistant Examiner US. Cl. X.R.

2lO-69, 380; 233l6

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