US2428221A - Pickling process - Google Patents

Description

Sept. 30, 1947. w, HUDSON 2,428,221

PICKLING PROCESS Filed Feb. 6, 1942 2 Sheets-Sheet 1 AbfV TOR lmzm ATTORNEYS Sept. 30, 1947- J. w. HUDSON PICKLING PROCESS Filed Feb. 6, 1942.

2 Sheets-Shut 2 INVENT 3 w 9 r MiM ZM ATTORNEY Patented Sept. 30, 1947 PICKLING PROCESS James W. Hudson, Upper Darby, Pa., assignor to Walker Brothers, Conshohocken, Pa., a corporation of Pennsylvania Application February 6, 1942, Serial No. 429,739

3 Claims.

This invention relates to the pickling of iron and steel objects to remove oxides formed by atmospheric corrosion and hot processing. More particularly, the invention is concerned with a novel method for pickling which offers many advantages over the methods now commonly used, and with apparatus by which the new method can be carried on efiiciently.

In the manufacture of iron and steel articles, it is necessary in many instances to clean the articles thoroughly as a preliminary to subsequent operations, such as galvanizing, electroplating, etc., and such cleaning ordinarily involves operations of two different types. The first cleaning operation serves to remove the oils, greases, and other organic contaminants employed in machine operations, such as tapping and threading, and the second is employed for the removal of rust and mill scale.

Various methods are now available for removal of greases and oils and, for that purpose, the objects may be treated with alkalis or $01- vents, or subjected to electrolytic cleaning. While these various processes have objectionable features, they are in common use and the-selection of a particular process depends on the nature of the articles being treated and economic and other considerations.

Pickling, or the removal of iron oxides, is also practiced in various ways, but the most commonly perior thereto in that the operation can be carried on in a short time, is under close control, and does not result in the formation of products presenting a waste disposal problem. According to the new method, the articles are pickled by immersion in a hot solution of an acid which is capable of forming an azeotropic or constant boiling point solution with water. The temperature and strength of the bath are maintained by vaporizing that solution and directing the hot vapors into the bath where they condense and give up heat thereto. The bath is maintained at a selected level during the operation and overflow therefrom is collected andv from time to time heated to drive oil! the acid in the form of vapor and to produce a solid residue. The vapor thus evolved iscollected by appropriate means and recovered for further use in the operation while the solid residue is removed from the vaporizer and disposed or in any convenient manner.

The acid preferred for use in the new process is hydrochloric and a solution of that acid containing 20.24% hydrogen chloride has a boiling point which, at atmospheric pressure, is about 230 F, and is the maximum for hydrochloric acid solutions. Any solution of the acid containing less than 20.24% hydrogen chloride will boil at a temperature between 212 F, and about 230 F. and the vapor produced will be. relatively rich in water. When the acid reaches a concentration of used procedure involves immersion of the articles in a sulfuric acid bath. This acid is cheap and non-volatile! but the reaction is slow and theefliciency of a sulfuric acid bath is constantly changing in service. This makes agitation of the bath or of the objects desirable, particularly in the pickling of hollow objects, such as tubes. Also, control of the operation presents considerable dlfllculty, since the percentages of both the acid and of the reaction product, ferrous sulphate, present in the bath, as well as the temperature, are factors influencing the solution velocity of the bath. In the course of use, the accumulation of ferrous sulphate in the bath requires that the bath be renewed and it is a common practice to discard the entire bath when the acid content falls below a level which may be around 5%. The throwingaway of such a bath represents a considerable loss of acid and the disposal frequently presents a difilcult problem, since the bath would pollute any stream into which it might be discharged.

The present invention is directed to the pro- Vision of a pickling process which avoids the dis advantages of the practice outlined and is su- 20.24% hydrogen chloride, its boiling point is constant and advantage is taken of this property in pickling according to the new method.

For a better understanding of the invention, reference may be had to the accompanying drawings in which Fig. 1 is a diagrammatic view illustrating the steps in the complete cleaning operation of which the pickling process of the invention forms a P Fig. 2 is a View, partly in elevation and partly in vertical cross-section, through apparatus used in the pickling operation: and

Fig. 3 is a view at right angles to Fig. 2 showing a modified form of the apparatus on a reduced scale.

The apparatus shown in Fig. 1 is employed in the continuous cleaning of iron and steel objects which are conveyed through the various vessels in which the several operations take place. The objects to be pickled may be of various forms and the apparatus illustrated is appropriate for continuous cleaning of tubes or conduits i0 which are supplied to a conveyor generally designated II. The conveyor is arranged to carry the tubes into and out of the successive tanks and when the tubes require degreasing, they are first passed through tank l2 in which the degreasing operation is performed. It is to be understood, of course, that degreasing may be omitted, if there is no oil or grease on the objects.

The tank I2 contains an organic solvent, such as trichlorethylene, and is of such length that, as the conveyor passes the tubes through the solution, the tubes are satisfactorily degreased. From tank i 2, the conveyor carries the tubes over the inclined floor l3 from which the solvent draining from the tubes may run back into the tank. The solvent employed in degreasing is kept at the appropriate temperature by suitable means and vapors given oil are collected in hood l2a containing condensing means. The condensed liquid is returned to the tank and from time to time, the solvent bath .is renewed as may be required.

The degreased articles pass from the floor I3 into the pickling tank l4, shown in detail in Figs. 2 and 3. .The tank contains a, solution I5 of hydrochloric acid, which is maintained at a selected level by an overflow line I3 leading to a vessel II.

The vessel is also connected to the bottom of the tank by a valved connection I8 and has a valved inlet l3. From the bottom of the vessel, 9. line 20 containing a valve 2| leads into a vaporizer 22 heated by external means illus trated conventionally at 23. Lines 24 and 23 connect the line 20 to sources of compressed air and steam, respectively, and flow through lines 24 and 25 is controlled by valves 26 and 21 in the respective lines.

The vaporizer is connected by a line 28 and a branch 29 containing a valve 30 to a fractionating column 3|, from the top of which a line 32 leads to a condenser 33. The condenser has an air inlet 34 at the top and a bottom outlet 35 connected by a branch 36 containing a valve 31 to vessel 11. Another valved branch 38 from outlet 35 leads to any suitable discharge point. The line 28 from the vaporizer has another branch 39 containing a valve 40, the branch 39 extending into tank l4 and terminating near the bottom thereof within a heat exchanger section comprising horizontal partitions 4|, 42. These partitions are of such arrangement that vapor discharged from the branch line 39 enters liquid in thespace beneath partition 4| and, as the liquid volume by condensation of the vapor, the lengthwise of the: tank to the opposite end and returns between partitions 4| and 42 to enter the main body of the bath.

The objects carried through the pickling tank M on the conveyor pass over a, floor 43 from which the excess acid may drain off and flow back into tank i4. Beyond floor 43, the conveyor carries the objects through a tank 44 containing wash water and thence over a floor 45 from which the water draining from the objects may return to tank 44. Beyond the floor 45, the conveyor carries the objects into a tank 46 containing an alkaline solution and thence over floor 4'! from which solution'draining from the objects returns to tank 46. At the end of floor 41, the objects are discharged from the conveyor and transported to the appropriate delivery point.

A hood 48, exhausted by a fan 49, overlies a portion of floor l3, the tank I4, and the floor 43. The fumes carried away through the hood enter a condenser 50 wherein the acid is condensed by cooling means, the condensate being led away The exhaust gases from the through a pipe 5|.

acid may be considered as condenser are piped into stack 52, which is preferably outside the building in which the pickling contain less than 20.24% hydrogen chloride is introduced into vessel l1 through valve inlet [3. The liquid in the vessel is then allowed to run into the vaporizer 22 through pipe 20 by opening of valve 2!, and after the desired amount of acid has thus been fed into the vaporizer, valve 2! is shut on and the burner 23 is started.

At this time, valve 30 is open and valve 43 is closed. If the solution introduced into the vaporizer contains less than 20.24% hydrogen chloride, heating of the solution results in evolution of vapor which is relatively rich in water. The vapor is then passed through outlet line 28 and branch 29 into the fractionating column 3|; As the water is the lower boiling point member of the binary solution, of which 20.24% hydrochloric the other constituent. water vapor is the diluent from the column and passes through pipe 32 into condenser 33. The distilled water collecting in the condenser may then be drawn off through outlet 35 and passed through line 36 into vessel i! for use as make-up water for the acid solution employed in pickling or may be discharged through outlet 35 and line 38 and disposed of in the usual way.

A thermometer 53 is mounted in the column 3| near the upper end thereof and when the temperature of the vapor, as indicated by the thermometer, exceeds 212 F., valve 40 is opened and a short time thereafter valve 30 is closed and valve 24 is opened successively. These operations result in the fractionating column being emptied and cause the vapors from the vaporizer to enter the heat exchange section of tank i4 and air from line 24 to bubble through the liquid in the vaporizer. The vapors condense in the heat exchange section of tank l4 and give up heat to the liquid in the tank. When the temperature of the liquid'in the tank reaches 230 F., the supply of fuel to the burner 23 is throttled.

The objects are then passed through the bath in tank l4 and the rust and scale thereon are attacked by the acid and removed. The pickling operation employing hydrochloric acid with the bath maintained at a temperature of about 230 F, is extremely rapid andpickling operations which may take hours when carried on by sulfuric acid methods can be successfully completed in minutes by the new method.

As the pickling proceeds, the condensation of vapors in tank i4 results in overflow of liquid into vessel i1 and this liquid contains ferrous chloride, which is produced by the attack of the acid on the objects and by interaction of the reaction products. The reactions which take place may be represented by the following equations:

centrated to 20.24% acid, as above described,

and used in the ordinarymanner in the process. As a result of the vaporization of the lower boiling point solution formed in tank I4, as described, the pickling bath tends to maintain its acid concentration at 20.24% hydrogen chloride and the solution overflowing from tank I4 into vessel I! thus contains approximately 20.24% hydrogen chloride. The overflow is collected in the vessel until a suitable liquid level therein is reached and the solution is then allowed to flow into vaporizer 22. Valve 40 is then shut off and valve 30 opened and heat is applied to the vaporizer as before to drive off vapor which enters the fractionating column. Any excess water present is thus removed and valve -40 is then opened and thereafter valve 30 is closed, so that the vapor produced from further heating enters the heat exchanger section of the tank as above described.

The liquid in the vaporizer now contains ferrous chloride and probably a small percentage '6 Any ferrous chloride that may have escaped oxidation in the initial reaction is then converted into ferric chloride as follows:

The ferric chloride thus produced reacts with steam with formation of the oxide and evolution of acid as indicated in the preceding equation. In both reactions above described, the iron oxide resulting from the reaction is indicated as FezOs (ferric oxide), but it is to be understood that the type of iron oxide obtained will vary with variation in the pressures and temperatures and the degree of hydration.

In all operations in which a solid residue is to be obtained, it is desirable to introduce compressed air into the vaporizer just before the latter is opened to recover the residue. This serves to' expel all vapor from the vaporizer and also prevents liquid from the condenser from sucking back into the vaporizer.

of ferric chloride which has not been reduced.

By continuing the heating of the vaporizer, a solid residue, mainly of ferrous chloride crystals, may be obtained and this solid material is then removed from the vaporizer through a suitable discharge opening. Thereafter liquid from vessel I1 is introduced into the vaporizer and the sequence of operations, involving elimination of any excess water that may be present and the delivery of the vapors of the constant boiling point solution of acid into the tank I4, is repeated.

In some instances, it may be desirable to recover the iron from the pickling bath in the form of ferric chloride rather than ferrous chloride, and this is accomplished by introducin air v through lines 24 and 20 into the liquid in vaporizer 2|, while the solution therein is being heated to dryness. The air aids in the agitation of the solution and the reaction is as follows:

After completion of the reaction, the solution may be heated to dryness and the residue, which will be mainly ferric chloride with minor amounts of ferrous chloride remaining unoxidized in the reaction, may be removed. It is to be understood that there may also be basic chlorides present in the residue obtained either when ferrous chloride or ferric chloride is the main constituent of the residue.

When the solid residue taken from the vaporizer 22 is either ferrous chloride or ferric chloride, the formation of the residue represents a loss of acid and it may be desirable for economic or other reasons to recover the acid and to produce a residue in the form of iron oxide. For this purpose, the ferrous chloride in the solution in the vaporizer may first be converted to ferric chloride, after which both steam and air are introduced into the vaporizer through lines 2, 25, and 20. The ferric chloride then reacts with the steam as follows:

The heating of the solution may be carried on to dryness in the vaporizer, or, if desired, the solution may be concentrated in the vaporizer almost to dryness and then removed in liquid formto be given its'final treatment at another time or at another place. One reason for this is that if the vaporizer were to be used only for vaporization, it could be constructed somewhat like a steam boiler to effect vaporization rapidly. Such a vaporizer, however, might not be able to stand the higher temperature required for decomposition of the iron salts. Also, such a vaporizer could probably not be conveniently provided with means for removal of solid material. In the practice of the process, therefore, it is immaterial whether the concentration of the solution is carried to dryness in the vaporizer or elsewhere.

In the foregoing, I have described the introduction of steam and air simultaneously into the tive in decomposition of the iron salts in the form of a dry or semi-dry residue. For example, if the solution in the vaporizer contains iron mainly in the form of ferrous chloride with a minor proportion of ferric chloride and this solution is heated, the iron salts form lumps deposited at the bottom of the vaporizer. If the shell of any such lump is mainly of ferrous chloride and steam and air are admitted into the vaporizer, the iron salt reacts with the oxygen of the air to form ferric chloride which then reacts with the steam to form iron oxide. Any ferric chloride in the lump then exposed to steam and air reacts with the steam and, as the reactions proceed, more of the iron salts within the lump are exposed. The reactions continue until the entire lump has reacted with the steam and air and the salts have been converted to iron oxide and hydrogen chloride has been produced. Neither steam norair alone would produce these results, since the air reacts only with the ferrous chloride and the steam only with theless-of which salt is exposed and in what form.

r The apparatus so far described includes only a single vaporizer and the pickling operation must be discontinued from time to time for removal of the iron compounds from the bath. If it is desired to carry on the pickling operation without interruption for indefinite periods, the modified form of apparatus illustrated in Fig. 3 may be used. That apparatus includes two vaporlzers 22a and 22b and the discharge lines 28a, 28b therefrom have respective branches 29a, 29b, connected together and leading into the bottom of the column 3|. The branches 29a, 29b are provided, respectively, with valves 30a, 30b. Branches 29a, 2% from the respective lines 28a, 281) are also connected together and lead into the tank M, the branches being provided, respectively, with valve 40a, 40b. The Vaporizers 22a, 22b are individually connected to the vessel I! by suitable lines (not shown), each of which has a valve similar to valve 2| and is connected to sources of compressed air and steam through valved lines similar to those designated 24 and 25.

In the practice of the new method as above described, the acid is supplied to the pickling tank during the pickling operation in the form of vapor of the constant boiling point solution of the acid and this procedure is preferred, since all the advantages of the invention are thereby obtained. However, under some conditions of operation, it may be undesirable or inconvenient to supply the acid in that form, as, for example, because the pickling is to be carried on at one location and the reclamation of the pickling bath at another or because it is desired to employ a pickling bath of a concentration less than that of the constant boiling point solution. In that situation, suitable modifications may be made in the pr'acticeof the method to meet local requlrements and modifications that will be readily apparent include recovery of the acid in the form of a constant boiling point solution from the spent bath and use of this acid, either diluted vor not, in a bath heated extraneously. When that practice is followed, advantage is taken only 01' the reclamation steps of the new method, but the convenience of carrying on that operation at a part of the plant remote from that where the articles are pickled or the desirability of using a bath of less concentration than the constant boiling point solution may, in'some instances, oiiset the loss of advantagesavailable when all the steps of the new method are carried out.

I claim:

1. A method of pickling metallic objects which comprises immersing the objects in a hot bath of a constant boiling point solution of hydrochloric acid, withdrawing part of the bath as the pickling operation proceeds, fractionally distilling the part of the bath withdrawn to drive 011 water vapor therefrom and produce a constant boiling point solution of hydrochloric acid, discarding the water vapor, heating the said solution to vaporize it and leave a solid residue, and introducing the vapors produced in the heating operation into the bath to be condensed therein.

2. A method of pickling metallic objects which comprises immersing the objects in a hot bath of a constant boiling point solution of hydrochloric acid, withdrawing part of the bath as the pickling operation proceeds, fractionally distilling the part of the bath withdrawn to drive off water vapor therefrom and produce a constant boiling point solution of hydrochloric acid, discarding the water vapor, heating the constant boiling point solution to vaporize it and to leave a solid residue, introducing air into the solution during the heating thereof, and introducing the vapors of the constant boiling point solution into the bath to be condensed therein to maintain the temperature and eifectiveness of said bath.

3. A method of pickling metallic objects which comprises immersing the. objects in a hot bath of a constant boiling point solution of hydrochloric acid, withdrawing part of the bath as the pickling proceeds, fractionally distillin the part of the bath withdrawn to drive off water vapor therefrom and produce a constant boiling point solution of hydrochloric acid, discarding the water vapor, heating the solution to drive 011' vapors therefrom and to concentrate it, introducing the vapors of said solution into the bath to be condensed therein to heat the bath and maintain the eifectiveness thereof, discontinuing the heating of the concentrated solution, and, in a subsequent operation, heating the concen trated solution to dryness.

JAMES W. HUDSON.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 389,058 Cole Sept. 4, 1888 1,272,917 Cooke July 16, 1918 1,146,071 Hoifman July 13, 1915 1,853,330 Barstow Apr. 12, 1932 2,017,773 Smith Oct. 15, 1935 2,123,434 Paulson July 12, 1938 2,193,051 Wood Mar. 12, 1940 2,228,135 Franwick Jan. 7, 1941 2,235,658 Waterman Mar. 18, 1941 FOREIGN PATENTS Number Country Date 273,307 Great Britain Apr. 5, 1928

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