US3423189A

US3423189A - Zone melting

Info

Publication number: US3423189A
Application number: US520451A
Authority: US
Inventors: William G Pfann
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1966-01-13
Filing date: 1966-01-13
Publication date: 1969-01-21
Anticipated expiration: 1986-01-21
Also published as: GB1175084A; CH486903A; DE1644038B1; FR1509326A

Description

Jan. 21', 1969 w. e. PFANN ZONE MELTING Fi led Jan. 15, 1966 2052p muqmtbm m M20 050: #243000 W450 QZEQMI lNl/ENTOR W G. PFA NN ATTORNEY United States Patent 7 Claims ABSTRACT OF THE DISCLOSURE The specification describes a zone refining apparatus in which the zones are moved longitudinally through a horizontally disposed continuous cylindrical charge. Zone dimensions are maintained uniform by rotating the cylinder and are minimized by forced cooling.

This invention relates to zone melting.

A typical zone melting process is a multipass operation and it is often advantageous to provide as many molten zones as possible within a given length of material to minimize the size of the apparatus and the processing time. This goal establishes a small zone length as a general objective in zone refining methods.

Zone melting processes which are carried out in an essentially closed tube would ordinarily operate with the tube disposed vertically. The unfortunate consequence of this arrangement is that heat convection contributes directly to a lengthening of the molten zones. It may be theorized that the adverse effects of heat convection may be minimized by disposing the tube horizontally and passing the zones along its horizontal axis. However, when the tube is placed horizontally, the effect of heat convection is to distort the zone boundaries such that the upper region of the zone is considerably longer than the lower region. Planar and parallel zone boundaries are desirable for various known reasons but especially for providing a large number of zones in a small length.

According to one aspect of the present invention the zone length is dramatically reduced by the combined expedients of disposing the tube horizontally and passing the zones along the horizontal axis, coupled with a slow rotation of the tube about its own axis. Slow rotation is intended as meaning from about one-half to twenty-five r.p.m.

As suggested in the foregoing discussion, the horizontal position of the tube largely restricts the direction of natural convection currents to the plane of the zone. If the tube is stationary, the convection currents, confined in the upper regions of the tube, begin to spread horizontally which results in a widening of the liquid zone. The convection flow pattern is of course due to variations in the density of the liquid. The major driving force for the convection flow is gravity. When the tube is rotated slowly the gravitational influence is altered and the characteristic convection flow pattern which produces the horizontal spread at the top of the tube is destroyed. The convection flow pattern remaining with the tube rotating is likely to be angular. Intuitively, it will be appreciated that an anguler flow pattern in the plane of the zone contributes to uniform zone boundaries.

It will often be found advantageous to forcibly cool the solid regions between the zones. This results in a further decrease in the zone length and in many cases will be found essential to a practical refining operation. This is especially true of low melting solids, particularly those with only moderate heat conductivities. Accordingly, a preferred form of this invention involves the use of alternate heating and cooling means disposed around a rotating horizontal tube. As is well known, the zones may be "ice passed by moving either the heating means or the tube. Usually the latter is more convenient in the multizone operation of this invention. Any known cooling means may be employed. However, as another aspect of this invention a cooling means is employed which is especially eflective for the desired purpose. One outstanding feature of this cooling means is that it flows the cooling fluid in direct contact with the tube itself and confines it to a precise flow path; yet it is not physically attached to the tube and the tube can move freely through the cooling as well as the heating zones while the cooling and heating apparatus is held stationary.

These and other aspects of the invention will become more apparent upon consideration of the following detailed description. In the drawing:

FIG. 1 is a front view, in perspective, of a multipass zone melting apparatus constructed according to the principles of the invention; and

FIG. 2 is a perspective view partly in section illustrating the operation of the novel cooling apparatus.

In the apparatus of FIG. 1 the tube 10, containing the material to be refined, is carried by a support jig (not shown) for supporting the tube in a horizontal position and slowly moving the tube back and forth in the direction indicated by the arrows. The support jig is also designed to rotate the tube slowly about its axis. The heating wires used to form the molten zones are indicated at 11 and consist here of several similar Nichrome wire rings disposed around the tube 10 and spaced therefrom a distance which permits both adequate heating and free movement of the tube. The distance is not critical and the rotation of the tube, according to an essential feature of the invention, allows for irregularities in the spacing of the wire from the tube and in the circumferential uniformity of the heating sources which might otherwise create difliculties. Various alternative heating means may be employed such as U-shaped heaters although ring heaters have been found to be particularly effective.

The cooling means are a series of rings 12 connected to hollow inlet tubes 1'3 at the top and outlet tubes 14 at the bottom. The rings may be of any appropriate material but preferably consist of a metal with good heat conductivity, such as copper, silver, gold, molybdenum, or an alloy such as brass. It is important that the ring have a significant axial dimension for reasons which will become apparent.

The operation of the cooling means is illustrated in FIG. 2 and forms a distinct part of the invention.

FIG. 2 is a perspective view with the tube 10 and its contents 15 shown in section. The coolant flows in the tube 13 at the top, conveniently with a gravity feed. The fluid flows around the tube 10 through the annular space 16 between the tube and the cooling ring 12. The charge 15 is maintained essentially solid in the vicinity of the cooling ring. The fluid exits through outlet tube 14. The outlet tube is not essential but does contribute to the effectiveness of the apparatus by producing a hydrodynamic head on the fluid in the annular space 16 and actually increases the fluid velocity. It also aids in confining the coolant in the annular space 16 due to the siphoning effect and provides a convenient means for collecting the coolant at the several ring coolers.

The annular space 16 is not closed as is evident from FIG. 2 since providing a seal to the tube 10 which permits the tube to rotate is a difficult problem. However, with the arrangement shown, surface tension confines the coolant to the annular space 16 and no liquid seal is necessary. Part of the heat removed from the charge is carried off by the coolant and part is removed by conduction to the metal ring and attached tubes. It is important that the inner surface be wetted by the coolant which usually reqiures the ring to be thoroughly clean.

The effect of using alternate heating and cooling means according to this particular embodiment is shown in FIG. 1. The liquid zones are shown at 17 separated by solid regions 18.

The following specific example is given to illustrate the effectiveness of the invention.

The tube 10 was a 2.54 cm. O.D. Pyrex tube approximately 50 cm. long. The heating rings were one turn of 0.5 mm. diameter Nichrome wire. The heaters were connected in series and powered by 60 c.p.s. current. A current of 4 or 5 amperes is approximately adequate to produce a desirable molten zone in the apparatus illustrated. Minor adjustments can be made for each heater by providing a variable shunt resistor across each heater. Once the heaters are set the apparatus can run for days at a time without further control of the heaters. The heating rings were spaced 2.5 cm. apart and a total of heaters were used. Eleven coolers are provided as shown in FIG. 1. The cooling elements 12 (FIG. 2) are copper rings 2.60 cm. I.D. having an axial width of 0.6 cm. inlet (13) and outlet 14 tubes are connected to the ring as shown in FIG. 2. The inlet and outlet tubes are copper having a 0.5 cm. diameter. The coolant used was water with the flow rate adjusted to about 70 cmfi/ min.

The charge material was naphthalene which orginally was a yellowish white showing obvious contamination. The mean zone length was 0.6 cm., roughly one-quarter of the tube diameter. The tube was moved axially at a rate of approximately 0.8 cm./hr. and rotated at 1.3 r.p.m. In this apparatus the heaters and coolers are stationary. Obviously the reverse arrangement can be used. After the passage of several zones, the material at the front end of the tube becomes whiter in color and obviously purer than the original charge.

In using this rotating tube technique voids may appear in the zone. For example, if the initial solid charge is sufficiently porous, and a molten zone is formed in it, the liquid may not occupy the entire cross section of the tube, even allowing for the increase in volume on melting. A void, or bubble, appears at the top of the zone. As the zone advances, the void moves with it, and a nonporous solid is formed behind the zone. If, however, the void occupies more than half the cross section, a hollow pipe will appear behind the zone along the axis of the charge.

Another way that a void can be formed is contraction of an established molten zone that occupies the entire cross section of the tube. Contraction produces solid, which occupies less volume than the liquid frozen to produce it, and hence a void appears.

In either case, the void is carried to the end -of the charge, and a nonporous solid is produced behind the zone. Movement of the void to the end of the charge corresponds, of course, to transport of matter toward the beginning of the charge.

The formation of nonporous solid behind the zone in the convective heating technique exacerbates the problem of tube breakage (for materials that increase in volume on melting). If the front end of the container is unyielding and is completely filled with nonporous solid, the volume increase when a zone enters the charge is likely to crack the container. Such tube breakage can be avoided in several ways.

The simplest way, when feasible, is to place a slidable silicone rubber plug (or other inert material) at each end of the charge in an otherwise open tube. Repeated zone passes move the plug at the beginning of the charge backwardthat is, opposite to the direction of zone travel-- and produce a larger and larger void at the end of the charge. The void can be removed by pushing in the end plug when the end of the charge is liquid.

This backward migration of the charge can be appreciable. For example, passing 27 zones averaging 0.9 cm. long through a charge of naphthalene moved the front plug about 4.5 cm. (which corresponds well with the density change on melting). The migration gradually moves the beginning of the charge out of the array of reciprocating heaters. This might be desirable from a purification point of view, but it is undesirable in that starting a zone some distance from the front end of the charge may crack the tube, for reasons mentioned. We have usually chosen to move the heater array with the charge by changing the positions of the limit switches that control the reciprocating motion.

Another way to prevent cracking, which obviates use of a sliding plug, is to seal a glass partition in the tube at the location of the beginning of the charge. A small hole is left in the partition near its periphery. When a zone forms at the beginning of the charge, expansion forces liquid out of the zone through the hole. Once the pressure is relieved, no more liquid flows out, because of the difliculty of nucleating a void in the zone. This method also produces a larger and larger void at the end of the charge, which can either be left there, or replaced with fresh charge material as dsecribed below.

A third way to prevent cracking is to leave an unmelted plug of solid at either end of the charge, with no other restraint. If the solid is deformable, or if it does not stick to the tube, it can behave just as the silicone plug described above. Again a void will grow at the end of the charge. If the plug of solid at the end of the charge is long, one can melt some of this solid and, by tilting the apparatus briefly, let the void bubble out, and at the same time let fresh charge material mix with the impure material in the last zone length.

If the substance of the charge contracts on melting the cracking problem should be less serious. The expected behavior, for a charge tube initially filled wit-h nonporous solid, is formation of a void when the zone enters the charge, travel of the void with the zone to the end of the charge, and disappearance of the void as the zone leaves the charge.

The translation rate of the zone must be tailored to the effectiveness of the heat sink and the natural convection in the zone. If the rate is increased beyond the proper value, the zone lags behind the heater, the melting interface becomes convex toward the liquid, and the freezing interface becomes concave toward the liquid. For 2.5 cm. tubes, and for a number of organic compounds, about 2 cm./hr. has been found to be a suitable maximum rate with respect to control of zone shape and zone size.

The rotation rate should be great enough to prevent the tapering efiect discussed earlier for a stationary horizontal tube yet not so great as to seriously alter the flow pattern due to natural convection. Rates of from 0.5 r.p.m. to about 25 r.p.m. have been found suitable for tubes of a few centimeters in diameter. If the rotation rate substarrtially exceeds these values in many cases control over the zone shape and dimensions is lost.

These parameters are exemplary and particularly adapted to materials having relatively low thermal conductivity. The invention in a preferred form is applied to materials having a thermal conductivity of less than 0.01 cal/cm. sec. C. The specific values of the various parameters for a different apparatus and charge material may best be determined empirically. In refining higher melting materials a forced cooling arrangement may be unnecessary. This will also be the case for refining lower melting materials if a wider zone spacing can be tolerated.

Whereas this discussion has been presented largely in terms of zone refining the invention is obviously applicable to other related processes such as zone leveling, normal freezing and the growth of large-diameter single crystals. The term zone melting is used herein in its generic sense.

Various additional modifications and extensions of this invention will become apparent to those skilled in the art. All such variations and deviations which basically rely on the teachings through which this invention has advanced the art are properly considered within the spirit and scope of this invention.

What is claimed is:

1. A zone melting apparatus comprising a hollow cylindrical elongated linear tube having an axis along its length, for containing the material to be refined, means for mounting the tube in a horizontal position, at least one heating element, said element surrounding a substantial portion of the circumference of the tube and spaced therefrom to permit relative axial movement between the tube and said heating element, means for causing relative axial movement between the tube and said heating element and means for rotating the tube around the axis.

2. A zone melting method which comprises circumferentially heating an elongated linear horizontally disposed tube containing a material to be melted to develop a molten zone in said material while continuously rotating the tube about its axis and passing the molten zone along the axis of the tube.

3. The method of claim 2 wherein the tube is rotated at a speed of from 0.5 to 25 revolutions per minute.

4. The method of claim 2 wherein the material being refined has a thermal conductivity of less than 0.01 cal./ cm. sec. C.

5. A zone melting apparatus comprising a horizontally disposed elongated tube having an axis along its length for containing the material to be melted, means for passing a molten zone axially along the tube, cooling means arranged to cool at least one of the zones adjacent the molten zone, said cooling means comprising a cylindrical ring encircling but spaced from the tube and having an axial dimension which is substantially less than the tube diameter, and means for introducing a liquid coolant into the upper region of the space between the tube and the cooling ring so that the cooling liquid will flow by gravity around the periphery of the tube, the space between the tube and the ring being such that the cooling liquid is confined to the space between the tube and the ring by surface tension.

6. The apparatus of claim 1 wherein a cooling means is provided adjacent to the heating means.

7. The apparatus of claim 6 wherein the cooling means is a liquid coolant.

References Cited UNITED STATES PATENTS 2,823,102 2/ 1958 Selken 23-273 2,967,095 1/ 1961 Herrick 23-273 3,124,633 3/1964 Van Run 23-301 3,139,653 7/1964 Orem 23-273 3,189,419 6/1965 Wilcox 23-301 3,258,314 6/ 1966 Redmond 23-301 2,998,335 8/1961 De Hmelt 23-301 FOREIGN PATENTS 191,165 9/1964 Sweden.

OTHER REFERENCES Pfann Zone Melting (pp. 62-71), 1958. Wiley and Sons.

NORMAN YUDKOFF, Primary Examiner.

G. P. HINES, Assistant Examiner.

US. Cl. X.R. 23-273