US3254500A - Freeze-refining apparatus - Google Patents

Description

2 Sheets-Sheet 1 Filed Oct. 23, 1962 Heat Exchange INVENTOR Leonard E. Olds XTTORNEY June 7, 1966 L. E. oLDs FREEZE-REFINING APPARATUS 2 Sheets-Sheet 2 Filed Oct. 25, 1962 INVENTOR Leonard E. Olds @mflwm 6% ATTORNEYS United States Patent Filed Get. 23, 1962, Ser. No. 232,449 2 Claims. (Cl. 62124) This invention relates in general to freeze-refining and more particularly to the treatment of two-component liquid systems such as, for example, molten alloys and liquid organic mixtures, whereby two relatively pure fractions are produced through the controlled freezing of a portion of the mixture, the frozen portion comprising the higher-melting material and the non frozen' portion com-prising the lower-melting material. The invention is equally applicable to the separation of one or more components from a multi-component system.

Heretofore, several methods of separating higherand lower-melting fractions from systems containing two or more components have been employed. Perhaps the most well-known of these is the process known as zone refining, wherein a rod of relatively pure metal is subjected to the refining action of a molten zone traversing the length thereof. In this process, diffusion acts to sweep lower-melting impurities to one end of the rod, the impurities having a natural tendency to stay within the molten zone as it traverses the rod, migrating away from the advancing solid face. With multiple passes of the molten zone, it is possible to achieve extremely high purities when high-melting alloys are treated, and the process finds ready application, for example, in the purification of germanium and other metals intended for use in semiconductor devices. Of course, since diffusion is a time-dependent phenomenon, the effectiveness of purification will increase with a decrease in the speed of travel of the molten zone; the process is thus necessarily a slow one. Also, equipment and handling problems limit the diameter of the rod which can be treated to about a half-inch or less; the volume of material which can be treated is, therefore, quite small.

Other freeze-refining processes have been developed which are not dependent on time-consuming diffusion and which can treat a substantial volume of material. These may be called dynamic processes, in that they all eliminate the need for diffusion by vigorously agitating the molten material during the course of freezing and provide some mechanical means for separating the solid, highermelting fraction. Typical among the dynamic freezerefining processes are ones in which an internally-cooled impeller is rotated at a high rate of speed within a melt maintained at a certain temperature. Heat is extracted from the liquid -in contact with the impeller, and the higher-melting fraction is caused to freeze thereon. Bafiles of some sort are generally employed within the containing vessel to insure turbulent motion of the liquid and prevent formation of a vortex.

A variation of the foregoing process involves the use of a large, rotating drum into which the molten material is poured and theoutside of which is continuously cooled. Heat extracted from the exterior-causes freezing of the higher-melting fraction and centrifugal force causes solidified particles to collect on the interior periphery of the drum. A-fter'a period of time, the remaining liquid is poured off and the solidified fraction is removed either by mechanical means or by remelting.

While the foregoing dynamic processes are capable of treating relatively large volumes of material in reasonable periods of time and of achieving at least satisfactory separations, commercial acceptance thereof has been lacking, due at least in part to the fact that very large, expen- 3,254,500 Patented June 7, 1966 ice sive mechanical devices were required, construction of which had to meet the highest standards to avoid the everpresent hazards present when dealing with large volumes of molten materials moving at high speed. The hazards are particularly great, of course, when molten alloys are being handled. Additionally, the necessary high degree of temperature and heat-flow control required to successfullycarry out the process is difiicult to achieve when working with such cumbersome equipment.

It is accordingly an object of the present invention to provide a process wherein substantial volumes of material may be purified Within a reasonable time.

It is a further object of the invention to provide a process for freeze-refining materials which can be carried out on a routine, production basis with simple equipment.

It is yet another object of the invention to provide an improved freeze-refining process wherein heat-extraction may be readily controlled with a high degree of accuracy.

Various other objects and advantages of the invention will appear from the following description of several embodiments of the invention, and the novel features will be particularly pointed out hereinafter in connection with the appended claims.

The present invention, briefly stated, improves upon previous dynamic freezing processes by employing stationary mechanical elements only, and letting the liquid provide its own agitation by the action of gravity. Thus,

in its simplest embodiment, the invention comprises a containing vessel holding a quantity of material which is to be refined, the vessel being located above the refining apparatus and providing a static head of pressure for the operation thereof. A conduit consisting of either tube, pipe or covered launder connects with the bottom of the containing vessel. This conduit passes, through a zone of controlled temperature or, when desired, temperature gradient, and as molten material passes therethrough in gravity-flow, the higher melting fraction freezes to the walls thereof, and the lower melting fraction passes through as a liquid. The length, diameter and slope of conduit employed, which may conveniently be wound in the shape of a spiral or helical coil to conserve space, is calculated to be sufficient to accommodate the capacity of the containing vessel without becoming clogged .or

otherwise impeding the flow of molten material. After a charge of material has passed one or more times therethrough and the lower-melting liquid removed, the temperature in the controlled zone around the tube is increased and the higher-melting fraction allowed to flow out.

As is well known, the flow of a liquid through a tube can be either laminar or turbulent. While deposition of the higher-melting fraction from a stream flowing in a turbulent manner is found to be more satisfactory than from a laminar stream, as might be expected, it has been found to be entirely possible to achieve satisfactory deposition from a laminar stream under most conditions. The conditions of freezing are found to be comparable to those in a rotating drum, i.e., the fiow of molten material is substantially parallel to the deposition surface, so it is not surprising that deposition takes place under both sets of conditions.

While not wishing to be bound by any particular theory of operation, it is believed that the theories controlling the operation of zone refining are equally applicable to the through a solid rod, the molten zone is effectively within the rod, the walls of which gradually close in about it. It can thus be theorized that the same general boundary conditions which apply to zone refining may also be applied to the invention. In this regard, it is important to recognize that three separate boundary conditionss exist; a momentum boundary layer, a diffusion boundary layer, and a thermal (energy) boundary layer. While the complete set of boundary layer equations which describe the combined momentum, mass, and energy transport have been worked out for a two-dimensional, steady laminar boundary layer by Eckert and Drake (Heat and Mass- Transfer, McGraw-Hill Book Co., New York, p. 456 (1959)), they are most diffi-cult to solve. By making certain assumptions, these equations can be substantially simplified, as shown by Johnston and Tiller (Fluid Flow Control During solidification: Magnetic Stirring in the Plane of the Solid-Liquid Interface, AIME Transactions, 1961, vol. 221,'p. 331), and the operability of the invention can be confirmed on a theoretical level as well as the practical. The latter is attested by the examples appended hereinbelow.

It is believed that a better understanding of the invention will be gained by referring to the following detailed description thereof, taken in conjunction with FIGURE 1, which is a simplified schematic-view of an embodiment of the apparatus utilized in carrying out the invention with high-melting materials, such as alloys, and FIGURE 2, which illustrates an embodiment of the invention wherein a single source of fluid supplies a plurality of freezing conduits, thus rendering the apparatus capable of substantially continuous operation.

With reference to FIGURE 1, it will be seen that a conventional refractory crucible 1 is employed to contain the molten alloy 2. The crucible 1 is fitted with a bottom-pouring device 3 of the type well known in the art. Of course, any convenient means may be employed to contain the raw material; when low-melting materials are being treated, such as, for example, organic waxes, no special precautions need be taken to keep the material molten. With alloys and other high-melting materials, however, it is often convenient to employ heating means such as the heating coil 4 to maintain the melt at a uniform temperature.

Hot metal 2 from the crucible 1 is poured into the holding tank 5, where it provides a suitable head of pressure for the apparatus. From holding tank 5, the metal flows by gravity into and, in part, through the tubing 7,

terial being treated, the heat transfer characteristics of the tubing 7 and the temperature and other properties of the raw material. Thus, for example, an agitated icewater mixture is satisfactory for the freeze-refining of water-peroxide systems. With many organics, a tank of water at a suitable temperature is operable. For certain low-melting alloys a heated oil bath is preferred, and for other alloys, where substantially elevated temperatures are required to keep the lower-melting fraction molten throughout, various furnace type structures and a gaseous medium are required.

In order to obtain proper freeze-refining it is necessary, of course, to maintain both temperature and rate of heat exchange within the controlled area 6 as constant as possible. While in some instances it is posible to maintain the required conditions over a sufiicient period of time by merely providing a large tank of cooling medium, as, for example, the aforementioned ice-water mixture, it is generally necessary to provide heat transfer means 9 and a suitable pump 10 for circulation of the cooling medium. By providing a heat exchange 9 with a capacity sufficient to extract heat from the cooling. medium at substantially the same rate that the heat of solidification of the freezing liquid is evolved from the tube 7, it is possible to maintain conditions substantially constant within the controlled area 6.

Where liquid cooling mediums are employed, it is necessary that vigorous agitation be employed, so that temperatures throughout the controlled area are substantially constant; where gaseous mediums are used the natural circulation due to action of the pump 10, coupled with proper design of the enclosure, is generally sufficient.

The materials of construction are dependent, again, primarily on the particular temperatures required to separate the mixture being treated. Those skilled in the art will fully realize what materials are best suited for a given system; use of several materials is illustrated in the appended examples.

While the optimum conditions of operation of the invention for any particular raw material may be calculated from empirical data, the calculation is sufficiently complex to be impractical for most operations. Moreover, such calculations will only give a reasonable approximation of operating conditions, as in many cases the empirical.data available are inexact, and in most cases the applicable equations have been worked out for ideal systems in a laboratory rather than on a produc tion scale. Also, many assumptions have been made in their determination. Thus, one skilled in the art, with a phase diagram of the system to be treated and knowledge of the heat capacities of the components of his equipment and heat of solidification of the higher-melting fraction, will be able to estimate the conditions required for a proper separation of components, and with the aid of a few trial-and-error runs will be able to determine the optimum conditions for the particular system involved.

It is worthy of emphasis to note that, in the course of any given run of the apparatus, the optimum conditions will be subject to some change. While in most instances this will not be of sufficient magnitude to require compensation of the heat exchange, there are occasions when this must be done. Thus, as freezing along the walls of the tube proceeds, the heat capacity of the wall builds up, and the volume of molten material passing through the tube (per unit time at constant pressure) will decrease.

Thus, in certain instances it is necessary to reduce the rate of heat exchange or build up fluid pressure in the tube. This problem is easily remedied, however, by use of interlocking controls between temperature-sensing means located outside the tube 7 and the heat exchanger pump.

It is important to remember that any alloy freezerefining operation is limited, in terms of refining efficiency, to the phase relationships existing between the components of the system being treated. Thus, for example, where a eutectic with or without peritectics exists in the phase diagram and the raw material contains a greater amount of the solute element than is required for the eutectic alloy, the lower-melting fraction will approach the eutectic composition but further purification can not be achieved. Of course, the higher-melting fraction may be treated in successive stages until a suitable purity is attained. In all cases the lever rule will hold true and determine the composition of the 'lowerand higher-melting fractions, and a program of multiple stage refining will be readily worked out by one skilled in the 'art. A convenient method is to employ several conduits or tubes running from one feeding vessel. The feed liquid can then be fed to each tube for a certain period of time in seriatim (i.e., one after the other), and the higher-melting fraction removed from the tube by re-heating immediately after it 3 tions recovered can be conveyed to other units which willachieve further purification or recycled to the first unit for individual treatment.

The foregoing is illustrated in FIGURE 2, wherein like numerals as employed in FIGURE 1 are used to illustrate equivalent parts. In this figure, numerals 1-5 indicate a heated crucible with bottom pouring and a holding tank, as in FIGURE 1. Tank 5, however, is connected .to a header 12, which supplies a plurality of freezing coils 7a-7d (four shown) through valves 14a- 14d. Each coil is provided with heat and temperature regulating means 6a-6d, heat exchange means 9a-9d, pump means a-10d and receiving vessels 8a-8d. By means of valves 14, the flow of liquid 2 from tank 1 can be passed from coil to coil, and product will be dis charged from the unit substantially continuously.

The removal of the collected solid from the tube is a relatively simple matter, merely requiring that the temn (1-g) where S is the amount of impurity or solute (i.e., mass), remaining in the liquid after a fraction g has been frozen, and S is the amount of impurity or solute contained in the original melt. The factor K can be defined .as a measure of the weight fraction of the solute element (S/S concentrating in the liquid when a given volume fraction (g) has solidified. It is to be noted that this expression differs from the K factor which is obtained by dividing the percent impurity in the liquid into the percent impurity of the entire solid deposit. Of course, freeze-refining is capable of concentrating impurities in either the solid or liquid phase, depending upon whether the impurities increase or decrease the melting temperatures. In the former case, impurities will freeze out, and the higher the K factor, the better the separation. In the latter case, impurities will concentrate in the liquid and the lower the K factor, the better.

Example I From an aluminum alloy smelter 412 pounds of a 70% Al-30% W alloy were produced. This alloy is freeze refined into a 98.6% Al fraction by passing it through a series of alumina launders arranged in the shape of a helix so that one launder discharges directly into the next. The alloy is held in a heated bottom pour vessel where it remains above 1250 C. by virtue of the superheat from the smelting furnace and the heat in the vessel. The bottom pour plunger is adjusted so that the 412 pounds of alloy is metered into the launder over the halfhour period.

The launders are fired alumina troughs, 1 /2 inches inside diameter and 3 /2 feet long, with inch walls. Each launder is covered with a A" alumina plate and the launder is supported in the middle by refractory brick. The helix formed by the launders has a 5-foot diameter with four launders to a loop. There are ten loops, each with a 10 slope. A pump at the discharge of the helix recycles the aluminum alloy to the top launder but not to the holding vessel.

The alumina helix is placed in a refractory brick enclosure and held by means of a closely controlled combustion process to 705 C.il C. The aluminum-tungsten alloy is then recycled through the helix launder for 30 minutes, after which time 203 pounds of liquid remains in the helix structure. This liquid is taken from the bottom of the helix and analyzes 98.6% Al. The gases 6 in the brick enclosure are then heated to bring the helix above 1300 C.; this causes the alloy solidified on the walls of the launder to melt and discharge at the bottom of the helix. Two hundred eight pounds of the high melting alloy is collected. This alloy analyzes 42% Al and is recycled to the smelting furnace.

The temperature of the brick enclosure is then adjusted to 666 Oil CL, andv the 98.6% Al liquid is then circulated for further purification. After one hour the liquid has reached a composition of 99.2% AI and no further change occurs at that temperature. Two hundred pounds of the purified liquid are obtained.

The heating chamber is then heated to 750 C. and the deposited solid melts and is discharged. This material amounted to 3 pounds and analyzed 64% Al.

Example II For this test, a laboratory unit consisting of a holding vessel maintained at a temperature of 5 C. is employed. The fluid level in the vessel is maintained constant at a 6-inch level by pumping in one gallon per minute of a 10% NaCl aqueous solution. The holding vessel is connected with any one of eight identical cooling coils. The cooling coils consist of 30 feet of %-inch I.D. copper tubing wound into a helical spiral. The diameter of the spiral is 16 inches and each turn is spaced inch apart. During the cooling cycle the coil is maintained at 19 C.- 'l C., and during the melting cycle it is allowed to come to room temperature.

The solution is allowed'to flow under its own head through the first coil until pressure in the line increases, as shown by an increase in the height of the solution in the holding vessel. A concentrated brine solution containing 24.8% NaCl is discharged from the end of the helix. The pressure build-up in the line is the result of freezing of purified water, and usually occurs after 15 to 20 minutes of operation. At this time flow is diverted to the next cooling coil and the refrigerant removed from around the first coil. This coil is allowed to come to room temperature and after one hour a quantity of pure water is discharged.

The operation was continued until the 8th coil had been used, at which time the stream was again diverted back to the first coil. Thus, it was possible to operate the unit continuously. The average production from the unit was 36 gallons of purified water and 24 gallons of concentrated brine per hour.

Example III The same experimental equipment and conditions employed in Example II were used to separate a dilute solution of hydrogen peroxide and water. In a 1-hour run, a very satisfactory K factor of 0.13 was obtained. It is to be noted that the apparatus and process of the invention Will provide even better separations with concentrated HOOH solutions (i.e., where conventional distillation procedures involve substantial hazards.

It will be understood that various changes in the details, materials, steps and arrangements of parts, which have been described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

Having thus described my invention, what it is desired to secure by Letters Patent is:

1. Apparatus for freeze-refining liquids having at least two components with different melting points that comprises:

feeding means comprising a vessel equipped with temperature regulating means and capable of maintaining said liquid at a preselected temperature and a preselected static pressure; stationary conduit means in the shape of a helical coil attached to said feeding means for receiving said liquid therefrom and providing a gravity-flow passage for said liquid therethrough; temperature-regulating means surrounding said conduit means and capable of maintaining said conduit means at a preselected temperature and extracting heat of solidification of said liquid therefrom through the walls of said conduit, and also capable of raising the temperature of said conduit to a temperature above the melting point of solid frozen therein; and recovery means beneath said conduit means capable of recovering liquid passing through said conduit means and remelted solid frozen therein as separate-fractions. 2. The apparatus as claimed in claim 1, and additionally comprising:

header means operably connected to said feeding means, 1 a plurality of said conduit means connected to said header means for receiving liquid therefrom; I means capable of feeding liquid from said header means into each of said conduit means in seriatim;

and independently controllable temperature regulating means surrounding each of said conduit means.

References Cited by the Examiner UNITED STATES PATENTS Johnson 62124 Howard.

Burdick 62123 Lindenmuth 22200.5 Zumbo 62124 Carpenter 6258 Taylor 62--123 X Larsen 22200.5 Smith 62180 Trevel et al.

Brandin et al. 62352 X Wenzelberger 62124 Hohn -63 Schomer 266-37 Beattie et al. 6258 Johnson 7563 Council et al 62352 X ROBERT A. OLEARY, Primary Examiner. D. L. RECK, Examiner.

H. W. CUMMINGS, W. E. WAYNER,

Assistant Examiners.

Claims (1)

1. APPARATUS FOR FREEZE-REFINING LIQUIDS AT LEAST TWO COMPONENTS WITH DIFFERENT MELTING POINTS THAT COMPRISES: FEEDING MEANS COMPRISING A VESSEL EQUIPPED WITH TEMPERATURE REGULATING MEANS AND CAPABLE OF MAINTAINING SAID LIQUID AT A PRESELECTED TEMPERATURE AND A PRESELECTED STATIC PRESSURE; STATIONARY CONDUIT MEANS IN THE SHAPE OF A HELICAL COIL ATTACHED TO SAID FEEDING MEANS FOR RECEIVING SAID LIQUID THEREFROM AND PROVIDING A GRAVITY-FLOW PASSAGE FOR SAID LIQUID THERETHROUGH; TEMPERATURE-REGULATING MEANS SURROUNDING SAID CONDUIT MEANS AND CAPABLE OF MAINTAINING SAID CONDUIT MEANS AT A PRESELECTED TEMPERATURE AND EXTRACTING HEAT OF SOLIDIFICATION OF SAID LIQUID THEREFROM THROUGH THE WALLS OF SAID CONDUIT, AND ALSO CAPABLE OF RAISING THE TEMPERATURE OF SAID CONDUIT TO A TEMPERATURE ABOVE THE MELTING POINT OF SOLID FROZEN THEREIN; AND RECOVERY MEANS BENEATH SAID CONDUIT MEANS CAPABLE OF RECOVERING LIQUID PASSING THROUGH SAID CONDUIT MEANS AND REMELTED SOLID FROZEN THEREIN AS SEPARATE FRACTIONS.

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