US2936278A - Molten salt bath apparatus for electrolytic cleaning of metals - Google Patents

Description

y 1960 R. H. SHOEMAKER ETAL MOLTEN SALT BATH APPARATUS FOR ELECTROLYTIC Original Filed March 7, 1955 CLEANING OF METALS 3 Sheets-Sheet 1 f I a I I T\ 2 I I I I 2 I r INVENT0R5. W N. M, By Ma./m,

\ Mg- MGM y 1960 R. H. SHOEMAKER EIAL 2,936,278

MOLTEN SALT BATH APPARATUS FOR ELECTROLYTIC CLEANING OF METALS Original Filed March '7, 1955 3 Sheets-Sheet 2 O 5 f9. Q 2 =5 38 q SE15 &

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May 10, 1960 R. H. SHOEMAKER ET L MOLTEN SALT BATH APPARATUS FOR ELECTROLYTIC CLEANING 0F METALS Original Filed March 7, 1955 3 Sheets-Sheet 3- IN V EN TORS WA By a air/v N MOLTEN SALT BATH APPARATUS FOR ELEC- TROLYTIC CLEANING OF METALS Original application March 7, 1955, Serial No. 492,358. Divided and this application July 1, 1957, Serial No. 669,281

1 Claim. (Cl. 204-206) This application is a division of co-pending application Serial No. 492,358, filed March 7, 1955 and now abandoned, and relates to the cleaning of metals by electrolytic molten salt baths and aims to provide novel apparatus for this general purpose.

One object of the present invention is to provide a molten salt bath apparatus comprising essentially two separate baths, one of which is connected as an anode and the other as a cathode, with the moving metal strip being cleaned, immersed in both baths, serving as a means for electrically connecting the two baths and completing the circuit between the anode bath and the cathode bath.

A still further object is to provide a molten salt bath of 1 atent this general character wherein anodes and cathodes are 1 spaced longitudinally and alternately along the moving strip in the bath. These electrodes, alternate groups of which are connected to opposite sides of the circuit to function as anodes and cathodes respectively, are in the form of transversely arranged metal rods, and may be positioned in groups above and below the moving strip.

Because it is desired to have the moving strip as close as possible to these electrodes to improve efliciency, it is a further object of this invention to provide guide and guard rolls of insulation material with which the strip may come in contact and which guard the strip against contact with the electrodes and yet guide the strip very close to but slightly spaced from these electrodes.

Accordingly, it is an aim of this invention to provide guard means in the form of an insulation member which insulates the work piece from part of the apparatus with which the workpiece should not come in contact. In the embodiment described above, the insulation member guards the work piece from coming in contact with the electrodes.

Notwithstanding the arrangements outlined in the preceding paragraphs a further definite object exists whereby the operation could be successfully executed by the mere advantageous positioning of electrode and strip by the complete omission of insulation, if so desired.

A still further object is to provide an apparatus comprising two insulated baths, each having an immersed electrode of opposite polarity to the other, with the metal strip completing the circuit between the opposite polarity electrodes.

A still further object is to provide various novel arrangements for connecting the strip and the bath in electrolytic continuous strip cleaning.

Further aims and objects of the invention will be under stood upon reference to the appended drawings, which show various embodiments.

In these drawings:

Fig. 1 is a side elevation view of a metal cleaning apparatus employing two baths connected to opposite sides of the circuitto function as anode and cathode respectively, and with the metal strip being the only means connecting the two baths electrically.

Fig. 2 is a top plan view of Fig. 1.

Fig. 3 is a small scale view of a modification where two baths of Fig. 1 are incorporated in a single metal tank.

Fig. 4 is a side elevation view to small scale of an electrolytic molten salt cleaning bath employing electrodes disposed along the moving strip above and below it and connected alternately in groups to opposite sides of the circuit, with guard and guide rolls of insulation material included in the combination.

Fig. 5 shows an arrangement directly to the strip.

Referring now to the drawings, it will be observed that Figs. 1 and 2 show a molten salt cleaning apparatus of the character developed for continuously cleaning of strip metal such as steel by the electrolytic process described in the H. G. Webster Patent No. 2,468,006, dated April 19, 1949.

According to that patent, the metal cleaning process contemplates subjecting the work-piece, while immersed in a molten salt bath, alternately to opposite sides of a direct current circuit whereby the workpiece functions alternately as anode and cathode for electrolytic cleaning.

Figs. 1 and 2 show the work piece in the form of a moving strip ill of steel or the like being cleaned while continuously moving through molten salt baths 11 and 12 arranged longitudinally in the line of travel of the strip. These baths are electrically separated and the metal tanks forming these baths are connected as shown diagrammatically in Fig. 2 to opposite sides of a direct current circuit so that the tanks 11 and 12 each function respectively as a cathode and an anode, and consequently, the metal strip 10 passing through each bath is first subjected to cathodic conditions'and then anodic, and also serves to interconnect the baths electrically.

Transversely disposed and mounted in the tank 12 are rollers 14 and 14a for supporting and guiding the strip immersed in its movement through the baths. These rollers are normally formed of metal and consequently would form electrodes for establishing electrical contact through the molten salt bath between the moving strip 10 and the tanks 11 and 12. In such a case electrical current to tank 12 would, to a large extent, be shunted across the rollers and thereby bypass the salt bath. Therefore, it is desirable to insulate the rollers from the tank to obtain maximum current flow through the salt bath. As seen in the drawing the rollers are mounted on bushings 15 and 15a which are of non-conducting material.

In tank 11, a support and guide roller 16, mounted in bushnig 17, is provided. Since the cleansing action of the process takes place in tank 12, it is not important whether or not this roller 16 is electrically insulated from the tank 11. The operation will not be materially afiected in either case.

The circuit between bath 11 and bath through the metal strip 10 being cleaned. The arrangement described in the foregoing provides a simple yet efiicient apparatus for electrolytic cleaning of continuously moving metal strip while immersed in a molten salt bath.

Fig. 3 shows an apparatus comprising two insulation tanks 21 and 22 which provide two baths through which the strip moves. These tanks 21 and 22 are disposed within a single metal tank 23.

The rollers indicated generically at 2 3 for supporting the strip and guiding the strip while immersed in the baths are not connected electrically to the metal tank 23 and hence have no useful function electrically. Instead, independent electrodes such as are shown at 25 and 25 are employed for the two baths 21 and 22. Each electrode comprises a group of metal rods supported and immersed within the bath adjacent to but out of contact with the strip and the electrodes 25 and 26 are electrically connected to oppowherein current is applied 12 is completed \2 site sides of the electric circuit. The strip completes the circuit.

This apparatus, described above may be further simplified by omitting the electrodes in one bath, such as electrodes 25 in tank 23 and simply connecting the electrical circuit directly to tank 23 without insulation in the same manner as the connection to tank 11 in Fig. 1 is made.

Fig. 4 shows an arrangement including a metal tank 31 with supporting and guiding rollers 32 along which the immersed strip 33 may pass through the bath while immersed in the molten salt bath.

Longitudinally spaced along the strip, above and below it, are groups of transverse metal rods forming electrodes 34 and 35 similar in construction and function to the electrodes 25 and 26 described in reference to Fig. 3. However, alternate groups of electrodes 34 and 35 of Fig. 4 are connected to opposite sides of the circuit. In the embodiment shown, four groups of rods are used to provide four electrodes of which the first and third are connected as cathodes and the second and fourth as anodes.

Because it is desired to maintain the distance between the electrodes and the strip as small as possible, the rods on opposite sides of the strip are spaced as close to each other as possible without however coming in contact with the strips. Because the strip is flexible and may move and twist in its passage through the bath accidental contact may sometimes occur with consequent short circuiting, arcing, and burningof parts. To avoid this there are included guide and guard rollers 38 on opposite sides of the strip which guard the strip in its motion through the electrodes and the bath but which prevent the strip from coming into contact with the electrode. These guard rollers may be in the form of transversely arranged metal rods supported at their ends on the tank with each rod being surrounded for its full length or at intervals by perforated disks or rings or the like of insulation material which form in effect insulation rollers or guides along which the strip may pass.

If desired, shields, either metallic or insulating, might also be employed.

Fig. shows an arrangement wherein tank 41 contains electrodes 42 on opposite sides of the strip 43 passing beneath rollers 44 through the bath 45 with electrodes 42 insulated from the tank 41, from the strip 43, and from rollers 44, and connected to one side of a D.C. source. The other side of the DC. source is connected to rollers 47 outside'the tank over which passes the strip '43 whereby the latter is connected to such latter side of the DC. source, with the contact between strip 43 and rollers 47 being of suflicient area and quality to insure against arcing between such strip 43 and rollers 47 and consequent burning.

As further described in the Webster Patent 2,468,006, work pieces, deposited in a perforated metal basket, may be immersed in the bath. An electric current is then applied to that metal basket of such a nature that the work piece, or the basket, forms a cathode, with the bath receptacle or any other element forming the anode.

Thereupon the current is reversed so that the work pieces or the basket containing them is made the anode, and the receptacle or some other element is made the cathode.

Thereafter the current is reversed again so that the work pieces or the basket containing them is a cathode. This is the final step in the preparation of the work piece surface for the further processing.

The bath is of a molten salt type and contains a composition comprising a mixture formed by fusing or melting together a predominant or major proportion of a caustic alkali such as sodium hydroxide or potassium hydroxide in the proportion of 75% to 90% and minor proportions of 7% to 13% sodium chloride, sodium fluoride used in proportions 2% to 6% and sodium aluminate used in proportions of 0.5% to 5%, all proportions by weight.

The preferred bath is a molten salt bath consisting of about 85% of sodium hydroxide, about 10% sodium chloride, about 4% sodium fluoride, and about 1% sodium aluminate, by weight.

The sodium fluoride is used for purposes of removing sand inclusions from the metal surface. Where the metal has no sand as impurity the sodium'fiuoride may be omitted. For sand-free metal the bath may have a preferred composition 89% caustic soda, 10% sodium chloride and 1% sodium aluminate.

As variations in the process, it is possible to repeat the steps of the process, as much as needed, without taking the work piece out of the bath, and merely by continued applications of current, reversing the current from time to time. This may be prolonged to any extent needed in order to continue the treatment of the metal and to attain a higher and higher degree of purity of the ferrite surface until the desired maximum is attained.

For other types of work pieces, other variations are possible. If the work piece is covered with scale, partly or wholly, the current is first applied with the work piece as a cathode; however, if there is no scale, this step may be omitted and the first application of current'may be with the work piece as the anode.

If the casting has only sand on its surface, the same may be removed by using the work piece as the cathode.

If the work piece contains nickel or other ingredients, which are insoluble in the bath, even with the electric current applied, then we first use the process hereof and thereafter immerse the work piece in a suitable acid to remove the nickel or similar ingredients and then we repeat the process hereof to the extent necessary to get the desired results.

The temperature of the bath may be between 600' and 1000 degrees Fahrenheit, but optimum results have been obtained at 850-900 degrees Fahrenheit.

The electrical current is slow alternating or successively reversed direct, of 4 to 6 volts, with a preferred intensity of 50 amperes per square foot. We have found that five minutes has proven satisfactory when the work piece is an anode and ten to thirty minutes when the work piece is a cathode. 7

Now having described the apparatus herein disclosed for the cleaning of'metal electrolytically in molten salt baths, reference should be had to the claim which follows.

We claim: I

A molten salt bath apparatus for cleaning of metal strip by passing the same continuously through molten salt contained in the apparatusas an electrolysis bath under electrolyzing conditions for removal of oxides and scale thereon, comprising an elongated metal tank of substantial depth adapted to contain molten salt therein as an clectrolyzing bath, metallic guide rollers near the inlet and outlet ends of said tank supported from the walls thereof, but electrically insulated therefrom, said rol ers being adapted to guide metal strip into and out of said tank, constraining said strip to a fixed path of movement through the molten salt therein, several electrodes, each of opposite polarity, in a longitudinally dis posed series separated one from the next, each electrode being supported at an intermediate height of said tank for immersion in the molten salt out of direct electrical contact with the walls thereof, each of the series of electrodes being supported closely adjacent to the path or" travel but out of direct contact with the metal strip passing through the bath continuously in a fixed path,

each electrode comprising a group of several cylindrical,

rods mounted in a pair of parallel planes transverse to the path of travel of said metal strip, each rod being mounted in a plane closely adjacent to another rod of the group and electrically connected to each other red at its end whereby to form a unitary electrode of two iancs of rods between which the metal strip may pass and he subjected on both surfaces to large current densities from the planes of rods, a series of guide rollers dis posed in tangential pairs, each pair of rollers being mounted between and adjacent to groups of electrodes of opposite polarity and there positioned to support and guide said metal strip passing through the nips of each pair of said tangential rollers, said tangential rollers comprising, at least on their arcuate surfaces, electrically non-conductive material chemically resistant to attack by said molten salt at the temperature of said bath, whereby said tangential rollers guide and rigidly support said metal strip in close passage to said electrodes for close electrical influence at high current densities, but out of actual electro-conductive contact therewith.

References Cited in the file of this patent UNITED STATES PATENTS Kirschner Dec. 2, Kelvie Apr. 14, Fosburg Sept. 8, Yerger et a1. July 11, Hall Feb. 3, Forsberg June 19, Webster Apr. 19,

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