US3552404A

US3552404A - Installation for the descaling of metals

Info

Publication number: US3552404A
Application number: US706108A
Authority: US
Inventors: Rudolf Kuhn
Original assignee: Degussa GmbH
Current assignee: Evonik Operations GmbH
Priority date: 1967-02-16
Filing date: 1968-02-16
Publication date: 1971-01-05
Anticipated expiration: 1988-01-05
Also published as: GB1205478A; AT293139B; FR1571367A

Abstract

Metals having metal oxide scales are descaled with molten sodium metal. The descaling can be preceded and followed by treatment of the metal with molten alkali metal hydroxide.

Description

United States Patent Rudolf Kuhn Koppern, Taunus, Germany Feb. 16,1968

Jan. 5, 1971 Deutsche Gold-und Silber-Scheideanstalt vormals Roessler Fruitful-tam Main, Germany [72] inventor [21] Appl. No. I 22] Filed [45] Patented [73 Assignee [32] Priority Feb. 16, 1967, July 13, 1967 [331 Germany [31 N05. D52,292 and 1153.583

[54] 1 INSTALLATION FOR THE DESCALING OF METALS 6 Claims; 2 Drawing Figs.

52 us. Cl. 134/64, 134/2, 134/15, 134/114 [51] Int. Cl B08b 3/08 [50] Field ofSearclI 134/2,3, 15, 64,114,122

[56] References Cited UNITED STATES PATENTS 876,182 1/1908 Herrick 134/64X 1,595,237 8/1926 Minton 134/15X 2,134,457 10/1938 Tainton m. 134/15X 2,390,007 1 1/ 1945 Sherman 134/64X 2,601,863 7/1952 Murphy 134/15 2,664,901 l/ 1954 Gehr et a1. 134/64X 3,067,758 12/ 1962 Hersh 134/64X 3,129,712 4/1964 Thomas 134/64X 3,293,159 12/1966 Mekjean et a1. 134/2X 3,358,980 12/1967 Taylor 134/122X 3,424,614 1/1969 Lichte 134/15 Primary Examiner-Morris O. Wolk Assistant Examiner-Joseph T. Zatarga Attorney-Stephens, Huettig and O'Connell ABSTRACT: Metals having metal oxide scales are descaled with molten sodium metal. The descaling can be preceded and followed by treatment of the metal with molten alkali metal hydroxide.

PATENTEU JAN 5 IBYI INVENTOR Rudblf Kuhn @ZM M azawzz ATTORNEYS INSTALLATION FOR THE DESCALING OF METALS BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to the removal of metal oxide scales from metals.

2. Description of the Prior Art It is known that hot or cold formed metal, such as rolled steel bands or drawn wire, must be'annealed, in order to obscales are removed by having the metalobject pickled in acid.

The treatment of steel objects in an acid pickling process for the removal of scales therefrom, 'however, gives rise to a number of disadvantages. Not-only does the waste water from the process create disposal and handling problems, as are common to all pickling installations, but, due to the fact that the through-put time for thest'eel in the acid pickling bath must be relatively long, when steel band or wire must be passed through the bath at a high rate of speed, then extraordinarily large pickling installations have to be provided in order to accommodate the needs of the process. Furthermore, and particularly in those cases where highly alloyed'steel is being processed,the pickling acid not only attacks the scales but the base material as well, and thus leads to expensive losses of metal. In addition, the acid pickling process also leads to the absorption of hydrogen by the metal being treated which causes undesirable embrittlement of the metal.

Because of such disadvantages in the use of aqueous pickling baths, salt bath processes have been developed which are significantly short and have more limited process times than is required with the'acid treatment processes. 'Such salt bath processes include those involving the use of molten alkaline salts, in which the scales are more or less given a preremoval treatment with the aid of direct electrical current in order to assure the quick removal of the scales in a subsequent heat treatment in the molten salt, as .well as processes in which a to be treated for the removal of scales therefrom.

All these prior art processes, however, including those which involve the use of reducing agents, have the disadvantage that they do not remove all-the metal oxide scales from the surfaces of the treated metal bands without the use of 'a subsequent acid pickling treatment, and where a reducing agent is used.an acid pickling pretreatment is also required.

- SUMMARY OF THE INVENTION An object of the present invention is to provide an installation in which to conduct the novel process of this invention.

A further object o f-the present invention is to provide an in- I 'stallation in which to conduct thenovel process of this invention.

The essence of the process of the present invention involves treating the metal to bedescaled with molten sodium metal, which treatment is preferably both preceded and followed by treating the metal with molten alkali metal hydroxide.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of an installation in which the process disclosed maybe conducted.

FIG. 2 is an end view of the installation of FIG. 1.

DESCRIPTION OF THE PREFERREDEMBODIMENTS It has now been unexpectedly shown, that by treatment thereof with molten sodium metal, and in particular at temperatures between about 230 and 700 C. and preferably at temperatures above 300 C., all types of steel, the unalloyed as well as the highly alloyed types, can be readily descaled. The reduction of the metal oxide scales is so complete with such treatment that subsequent acid pickling treatments are usually not necessary. The process has important advantages, therefore, in that material losses due to overpickling are avoided, even if for some reason the metal object being treated must remain stationary in the molten sodium, and in that hydrogen absorption by the metal, which is associated with acid pickling treatments, does not occur. In addition, due to the direct descaling of the metal by means of the sodium metal, the reducing time is confined to only a few seconds, so that through-put speeds can be achieved for the descaling process which have not been possible to date The molten sodium metal treatment is preferably conducted at 230 to 700 C in about 60 to 5 seconds.

The molten sodium treating process can be further improved if the metal to be treated is pretreated or dipped in molten alkali metal hydroxide for a short time before the molten metal treatment. The alkali metal hydroxide used is preferably sodium hydroxide. Molten mixtures of alkali metal hydroxides and alkali chlorides can also be used. The pretreatment with alkali metal hydroxide is preferably conducted at 230 to 700 C. for 5 to 60 seconds. The wetting and resulting descaling of the surface of the metal being treated is so free of objectionable features that the descaling process is completed within a very short time without any trouble.

The alkali metal hydroxide bath which-is used in conjunction with the actual pickling process, which latter process is conducted in the molten sodium metal, can be used as means for sealing the sodium metal processing chamber from the atmosphere. The alkali metal hydroxide melt which is used for pretreating the object to be descaled can also be used to pre heat such objects. An alkali metal hydroxide bath can also be used as a post-descaling treatment in order to act as a scavenger and remove from the metal object being processed not only pieces of sodium metal that may adhere to the metal object as it leaves the sodium metal bath; but also sodium oxide reaction products that may form during the descaling step. The pieces of sodium metal and sodium oxide reaction products that are taken up by the alkali metal hydroxide bath can be readily converted into sodium hydroxide upon contact with moisture in the atmosphere.

The molten sodium metal bath is preferably blanketed with an inert gas which will not react with the sodium metal. The purpose of this inert gas is primarily to seal off the molten sodium metal bath from gases or other components of the atmosphere which would either react with the sodium metal or with the metal being processed under the prevailing process conditions. Examples of the undesired gases or other compounds are oxygen, hydrogen, water. Nitrogen is the preferred inert gas to be used as the blanketing medium.

A metal processing installation in which the process can be carried out can be constructed in such a way that the molten sodium metal is allowed to float on top of the molten alkali metal hydroxide bath. The sodium metal bath can be isolated from the atmosphere by means of a tower type structure erected over the sodium metal bath. Elongated metal objects, such as, wire and bands, to be treated in the sodium metal bath can be fed continuously through the bath and the tower type structure surrounding the bath by means of feeder devices,

' such as guide rollers.

shaped container has a wide open top and a narrow open bottom whereby an interface between molten sodium metal and molten alkali metal hydroxide sets in inside the narrow open end of the funnel shaped container.

An example of such an installation is shown in the drawings. FIG. 1 shows a side view of the installation and FIG. 2 shows an end view of the same installation. The installation consists essentially of a furnace which has a wall 1 and a heating chamber la, an open topped trough or vat 2 for the molten alkali metal hydroxide 2a, an open topped container 3 for the molten sodium metal bath 3a, and airtight housing 4 and feeder or guide rollers 5.

Walls 4a of housing 4 extend on all sides down into bath 2a and housing 4 provides in combination with the molten alkali metal hydroxide 2a, an air tight area 7. Area 7 is thus bounded on its top and sides by housing 4 and at its base by the top of bath 2a. Area 7 completely isolates bath 30 from contact with the atmosphere, and during the operation of the installation area 7 is charged with an inert gas. The portions of the top of bath 2a which are outside walls 4a of housing 4 are in contact with the atmosphere.

An elongated metal object which is to be treated using the process of the present invention is fed into and through the installation, in the direction of the arrow, by means of guide rollers 5. As shown by the drawings, the metal object passes first through the NaOH bath, then through the Na bath and again through the NaOH bath before it leaves the installation. The furnace maintains both the NaOH bath and the Na bath in a molten condition. The NaOH bath is heated directly through its container wall 2 by means of heat generated in heating chamber In. The sodium is in turn heated by the molten NaOH.

The sodium bath 3a is housed in funnel shaped container 3. Container 3 may be suspended from above in the NaOl-l bath or it may be supported by a base which is itself resting on the bottom of trough 2. The walls of the lower half of container 3 slope inwardly to provide a narrow, neck 6 which is open at the bottom thereof. Neck 6 extends into the NaOH bath 2a in such a way as to leave a distance, on all sides of it, between it and the walls and base of trough 2. As a result of this arrangement there is no horizontal wall separating the molten sodium in the bottom of neck 6 from the molten NaOH in trough 2 so that the molten sodium (y 0.95 kg/dm) floats on the molten sodium hydroxide ('y 1.8 kg/dm). Container 3 is thrust so far into the molten NaOh that the interface between the molten sodium and the molten NaOl-l is inside neck 6. Thus, since neck 6 has a definite length, care must be taken during the operation of the installation to provide that fluctuations in the amounts of sodium and sodium hydroxide in their respective containers do not cause the interface between the sodium and the sodium hydroxide to be located outside of neck 6.

By reason of the arrangement of the installation as shown in the drawings various advantages are obtained in conducting the process of the present invention in such an installation. When using the disclosed installation products such as the M1 0 reduced metals, and impurities which are produced or formed during the descaling step in the molten sodium metal eventually end up in the molten sodium hydroxide. This arises due to the fact that such materials are all heavier than the molten sodium. They sink or slide, therefore, to the bottom of container 3 and through the open bottom of neck 6 into the molten sodium hydroxide. Such materials either then further fall to the bottom of trough 2 or they are taken up by the molten sodium hydroxide. The sodium oxide materials will dissolve in the molten sodium hydroxide. The dissolved N320 will not be an impurity in the molten sodium hydroxide since the Na O will react with moisture in the air that is absorbed into the molten sodium hydroxide and will form sodium hydroxide. i.e.. Na O-j H O- 2 NaOH. The moisture in the air can enter the sodium hydroxide where the molten sodium hydroxide is exposed to the atmosphere. i.e.. that portion of the molten sodium hydroxide bath which is outside walls 411. The usual impurities which are present or form during the descaling step. notably reduced metal;

two materials and thereby prevents a large scale dissolution of the sodium in the alkali metal hydroxidel' Since the lower portion of container 3 is completely immersed in the molten sodium hydroxide, the molten sodium hydroxide can be used as an air tight sealing means, in combination with airtight housing 4, in order to keep moisture and other materials in the air from reaching and reacting with the molten sodium. Housing 4 is so constructed that continuous metal wire or bands can be processed therein under an inert atmosphere from the time the wire or band enters the molten sodium hydroxide for the first time until it leaves the molten sodium hydroxide at the end of the process. in this way slits are not needed in walls 4a in order to provide ingress and egress means for the wire and bands. Particles of sodium metal that may adhere to the band after the band leaves the molten sodium bath are readily taken up in the molten sodium hydroxide the second time the band passes through bath 2a, and thus before the band passes out into the outside atmosphere.

As can be seen from FIG. 2, housing 4 is constructed wide enough so that there is sufficient open space on the surface of bath 20 between walls 4a of housing 4 and walls 3b of container 3 as to allow for the removal of impurities in the form of sludge from under container 3 in bath 2a through such open surface space.

Instead of using a single common bath housed in a single trough for treating the continuous metal objects in two separate molten alkali metal hydroxide bath treatments, two separate alkali metal hydroxide baths placed in separate troughs can also be used. In such instance the sodium melt could also be housed in a separate trough shaped container of its own and the three troughs could still all be housed under a single air tight housing.

Those elements of construction which are to be used in the building of the installation and which are to come in contact with molten sodium during the operation of the installation, such as the inside of container 3 or the guide rollers which are partly immersed in bath 3a, must be made of a material which will not be attacked by the molten sodium. Such elements of construction should, therefore, be constructed of a sodium resistant material such as nickel, or be plated with such sodium resistant materials.

The elements of construction which are to come in contact with the molten alkali metal hydroxide should be such as are capable of withstanding attack from such hydroxides.

The sealing of the guide roller shafts 8 which are to pass through housing 4 can be done with stuffing' Boxes in order to maintain the air tight integrity of housing 4. The heat generated in the heating area 10 of the furnace can be generated electrically, as by using resistance elements, or by burning gas or oil. Heat generating electrodes can also be in serted directly into the baths.

The following examples are merely illustrations of th process and are not intended as a limitation upon the scope thereof.

Example 1 A test piece of ST-high grade steel was heat rolled and annealed in the atmosphere and oxide scales appeared on the" surface of the test piece. The surface of the testp'iece was then treated for 5 to 10 seconds at 400 C. in molten sodium hydroxide (=l00 percent pure) and then for 5 to l0 seconds at 450 C. in molten sodium metal percent-pure). The stirface of the thus treated'm'etal piece wasclear of oxide'sca'les:-'

When test pieces of the same type of heat rolled and annealed ST steel were descaled in the Customary pickling bath processes using a HCL or H 50. pickling-bath heated to 40 to 80 C. the treatments required a pickling bath time of at least two minutes in order to achieve the samedescaling results as were obtained with the process of the present invention. 0.4 percent C, 1 percent Mn. t

Example 2 Test pieces of a highly alloy chromesteel identified as workpiece numbers 4006 to 4021 were heat rolled and annealed in the atmosphere whereupon oxide scales appeared on the surface of the test pieces. The surface of some of the test pieces were then treated for 5 to 10 sec. at 500 C. in molten sodium hydroxide 100 percent pure) and then for 5 to l seconds at 400 to 500 C(inmolten sodiummetal 100 percent pure). The surface of the thus treated metal pieces were clear of oxide scales.

When some other of the test pieces were descaled in a pickling treatment using a mixture of HNO; and HF as the pickling bath at 40 to 60 C. the treatment required a pickling bath time of more than minutes in order to provide the same descaling effect as was provided by the process of the present invention. 18 percent Cr.

A test piece of highly alloyed chrome-nickel steel identified as work piece number 4300. was hot rolled and annealed in the atmosphere whereupon oxide scales appeared on the surface of the test piece. The surface of the test piece was then treated for 5 to l0 sec. at 400 C. in molten sodium hydroxide (100 percent pure) and then for 5 to seconds at 500 C. in molten sodium metal.(l00 percent pure). The layer of oxide scales was thus removed from the surface of the test piece.

When a test piece of the same type of'steel as workpiece 4300 which had been hot rolled and annealed as the piece used above was .descaled in a pickling treatment using a mixture of HNO and HF as the pickling bath at 40 to 60 C. the treatmentrequired a pickling bath time of 5 minutes in order to provide the same descaling effect as was provided by the process of the present invention. l8 percent Cr 8 percent Ni The molten sodium bath should contain sodium which is at least 100 percent pure. The alkali metal hydroxide bath should contain alkali metal hydroxide which is at least 100 percent pure.

The metals which may be descaled in accordance with the present invention include alloyed and nonalloyed iron and steel as well as copper, alloys of zirkonium and titanium, iron free alloys basing on Ni, Co and Cr. g

The metals being processed in the process of the present invention may have any shape or configuration. The process is particularly suitable to the treatment of objects which may be fed into themolten sodium metal bath in a continuous fashion, such as wire, cable, strips, bands, sheets, and similar elongated objects.

l. A metal object treating apparatus comprising, in combination, a first container, a bath of molten alkali metal hydroxide in said container suitable for pretreatment of the surface of said object, a second container open at the. top, a bath of molten sodium metal in said second container suitable for the treatment of the surface of said object after said baths in a molten state during said pretreatment and treatment, feeder means for feeding a metal object in and out of said baths, airtight housing means. and an inert atmosphere in said housing means over at least said bath of molten sodium metal and in contact therewith at said open top.

2. An apparatus according to claim 1 wherein the cross-sectional area of said housing means is less than the cross-sectional area of the upper surface of said first container and the bottom of said housing means extends into said first container above the bottom thereof whereby when said first container contains a molten alkali metal hydroxide bath, said bath seals said housing means, and the open top of said second container extends above the top of said first container, said housing means providing a space for an inert atmosphere above the area of the open top of said second container and at least the immediately adjacent area above said first container.

- tionallarea of said first container, the molten alkali metal hydroxide bath being in contact with the molten sodium metal bath and sealing the bottom of saidsecond container, said molten sodium metal bath floating on said molten alkali metal hydroxide bath, heating means for maintaining both baths in a molten state, feeder means for feeding a metal object sequentially through saidvfirst bath, then through said second bath and again through said first bath, airtighthousing means, an inert atmosphere within said housing means, said opening of said wide top'of said second container disposed above the level of the molten alkali metal hydroxide in said first container'and extending into said inert atmosphere within the cross-sectional area of said airtight housing means whereby an inert atmosphere is maintained above said sodium metal.

4. An apparatus as in claim 3 inwhich all surfaces of said second container which contact sodium metal are made of nickel or are nickel plated.

5. An apparatus according to claim -3 in which said alkali metal hydroxide is sodium hydroxide.

6. An apparatus suitable for treating a metal object in a plurality of baths, one of said baths. beinga molten alkali metal tainer being positioned within, but out of contact with said pretreatment said second container having a narrow, open ,7

bottom, said bottom being'positionedwithin, but out of contact with said first container, the opening of said bottom being disposed within the'intemal cross-sectional area of said first container, .the molten alkali metal hydroxide-bath being in second container, the opening of said narrow bottom being disposed within the internal crosssectional area of said first container, heating means suitable for maintaining both of said baths in molten state, feeder means for'feeding a metal object sequentially through said first container, then through saidsecond container and then again. through said first container, airtight housing means, comprising a top and sidewall, said wide top of said second container being disposed within the cross-sectional area of said housing means and below the top of said housing means.

Claims

2. An apparatus according to claim 1 wherein the cross-sectional area of said housing means is less than the cross-sectional area of the upper surface of said first container and the bottom of said housing means extends into said first container above the bottom thereof whereby when said first container contains a molten alkali metal hydroxide bath, said bath seals said housing means, and the open top of said second container extends above the top of said first container, said housing means providing a space for an inert atmosphere above the area of the open top of said second container and at least the immediately adjacent area above said first container.
3. A metal object treating apparatus comprising in combination a first container open at the top thereof, a molten alkali metal hydroxide bath in said container suitable for the pretreatment of the surface of said metal object, a second, funnel shaped, container having a wide, open top and a narrow, open bottom positioned within, but out of contact with said first container, a molten sodium metal bath in said second container suitable for the treatment therein of the surface of said metal object after said pretreatment, the opening of said narrow bottom being disposed within the internal cross-sectional area of said first container, the molten alkali metal hydroxide bath being in contact with the molten sodium metal bath and sealing the bottom of said second container, said molten sodium metal bath floating on said molten alkali metal hydroxide bath, heating means for maintaining both baths in a molten state, feeder means for feeding a metal object sequentially through said first bath, then through said second bath and again through said first bath, airtight housing means, an inert atmosphere within said housing means, said opening of said wide top of said second container disposed above the level of the molten alkali metal hydroxide in said first container and extending into said inert atmosphere within the cross-sectional area of said airtight housing means whereby an inert atmosphere is maintained above said sodium metal.
4. An apparatus as in claim 3 in which all surfaces of said second container which contact sodium metal are made of nickel or are nickel plated.
5. An apparatus according to claim 3 in which said alkali metal hydroxide is sodium hydroxide.
6. An apparatus suitable for treating a metal object in a plurality of baths, one of said baths being a molten alkali metal hydroxide bath and a second of said baths being a molten sodium metal bath disposed on top of said alkali metal hydroxide bath and covered with an inert atmosphere, said apparatus comprising, in combination, a first lower container open at the top thereof suitable for holding said molten alkali metal hydroxide, a second, funnel shaped container suitable for holding said molten alkali metal, said second container having a wide, open top and a narrow, open bottom, said second container being positioned within, but out of contact with said second container, the opening of said narrow bottom being disposed within the internal cross-sectional area of said first container, heating means suitable for maintaining both of said baths in molten state, feeder means for feeding a metal object sequentially through said first container, then through said second container and then again through said first container, airtight housing means, comprising a top and sidewall, said wide top of said second container being disposed within the cross-sectional area of said housing means and below the top of said housing means.