US2877146A

US2877146A - Metallurgical pickling

Info

Publication number: US2877146A
Application number: US511872A
Authority: US
Inventors: Charles B Francis
Original assignee: Puriron and Chemicals Inc
Current assignee: Puriron and Chemicals Inc
Priority date: 1955-05-31
Filing date: 1955-05-31
Publication date: 1959-03-10
Anticipated expiration: 1976-03-10

Description

March 1959 c. B5 FRANCIS 2,877,146

' METALLURGICAL PICKLING Filed May 31, 1955 INVENTQR. Charles 5. Francis United States Patent METALLURGICAL PICKLING Charles B. Francis, Pittsburgh, Pa., assignor to Puriron Chemicals, Inc., a corporation of Pennsylvania Application May 31, 1955, Serial No. 511,872

6 Claims. (Cl. 134-13) This invention relates to the pickling of iron and steel articles in hot dilute solutions of sulfuric acid to remove from the surfaces of the articles iron oxides, such as rust and scale, and the invention consists in certain new and useful improvements in methods of metallurgical pickling.

The spent acid, or so-called pickle liquor, produced in pickling operations has, heretofore, been discharged into lakes, streams, rivers, and dumps, resulting in conditions of pollution to such an extent that the practice has been outlawed. In order to meet the anti-pollution laws industry in many cases neutralizes the pickle liquor, providing a liquid which may be discarded Without objectionable consequences. This practice is costly. In fact, in one steel plant of which I have knowledge the cost of pickle liquor disposal exceeds $50,000.00 per year.

I am aware that various processes have been proposed for treating pickle liquor, whereby useful acid and metal salts may be recovered, but so far as I am aware none of such prior processes has provided a complete solution of the problem. While such processes may be effective to neutralize the pickle liquor and recover the said products, the capital required for the essential apparatus and the cost of operating the apparatus so far exceed the value of the products that it is generally conceeded that a comercially practical solution of the problem has not been realized.

For example, it has been heretofore proposed to neutralize pickle liquor by the reaction of its contained acid with steel scrap, and to evaporate the liquid from the neutralized pickle liquor, whereby the ferrous sulfate may be recovered in crystalline form. The recovered ferrous sulfate is first dehydrated and then calcined, and in the latter step gaseous S0 and S0 are driven off and recovered for use in the production of commercial sulfuric acid. The resultant FE O may be then reduced to metallic iron in any one of several known methods.

As distinguished from such procedure, I evaporate only a part of the water from the pickle liquor and I preferably do so at such temperature that substantially none of the sulfuric acid in the liquor is lost. In the resulting concentrated pickle liquor, a major portion of the included ferrous sulfate is precipitated and recovered. The supernatant liquod in the concentrate is then withdrawn in the form of a sulfuric acid solution including residual quantities of dissolved ferrous sulfate, and this liquid is returned to the pickling apparatus, Where new acid and water are added to provide an acid solution of pickling concentration. With the qualifications that more than one-half of the water is evaporated from the pickle liquor and a major portion of the ferrous sulfate is precipitated, my treatment of pickle liquor is conducted in the liquid phase at temperatures and pressures inhibitive of a substantial loss of sulfuric acid.

The object of my invention is to provide a pickling operation in which the pickle liquor is processed for re-use.

'It is further my aim to achieve such object with a minimum capital expenditure for apparatus, and with low labor and maintenance costs.

It is further an object of my invention to process pickle liquor at a profit, and thus provide a solution of an existing industrial problem.

The pickling of iron and steel articles is carried out in either of two ways which are known as tub pickling and continuous pickling. In tub pickling one or more tanks are almost filled with hot dilute sulfuric acid (10 to 12%) and the articles to be cleaned are placed upon a rack and plunged up and down in the acid until clean. Then the cleansed articles are rinsed in hot water to remove adhering acid and salts. Each tank of acid is used until the acid solution is saturated with ferrous sulfate to the degree where it is no longer elfective as a pickling agent, whereupon the tank is emptied and filled with new pickling solution.

In exemplary way, I shall describe the invention as it is practiced in the continuous pickling of hot rolled strip metal, whereby those skilled in the art will readily perceive the manner in which apparatus and process may be modified to practice the invention in other pickling operations in industry.

In the accompanying drawings, I illustrate apparatus in which the method of this invention may be practiced, as follows:

Fig. 1 is a diagrammatic view in plan of two rows of pickling tanks for the continuous pickling of steel strip,

in combination with an apparatus in which the invention may be practiced; and

Fig. 2 is a diagrammatic view in vertical section of one of the settling tanks of the apparatus, the view being. taken on the plane II--II of Fig. 1 and shown on larger ously added to the wash and rinse tanks 5 and 6, whence it flows into tank 4 where it is mixed with sulfuric acid delivered from an acid storage tank in such proportions that a 20% solution of sulfuric acid is produced in tank 4. The acid in the tanks is in known way maintained at a temperature of approximately 200 F., and as the steel strip advances left-to-right through the tanks 1-6 in sequence the acid flows right-to-left from tank to tank, until, upon reaching the tank 1, the acid is spent, providing 'the pickle liquor with which this invention is primarily concerned. The sequence of movements of the strip and fluids in pickling line B is substantially the same as that described for line A. p

The pickle liquor in tanks 1 and 7 is an aqueous solution that contains in the order of 6% free acid, and is saturated with 19% ferrous sulfate and 0.095% manganese sulfate, the percentages being by weight, and the exact concentration of the metal salts depending on the particular temperature of the solution.

The pickle liquor is drawn from the tanks 1 and 7 through a pipe line 13 connected to a plurality of

evaporator units

14, 15, 16, and 17. The evaporator units are operated on the batch principle; that is, the pickle liquor is fed into a selected unit until it is fully charged, whereupon heat is applied to the liquor, a jet condenser (11, b,

being so determined that when the last unit is fully charged the first unit will have been emptied to receive a new batch of liquor. A pump 12 in line 13 may be provided to deliver the liquor from the pickling lines to the evaporator units. In this case, to provide for an emergency created by one of the evaporator units going down for repair or servicing, I provide an elevated storage tank S to hold the liquor drawn from the pickling lines until an evaporator unit becomes available.

While the invention is not immediately concerned with details of the construction of the evaporator units, I may mention that each unit comprises a tank, which may be formed of steel plate, having an acid-resisting lining of sheet lead. In this case each tank is cylindrical in form, having a conical bottom three feet deep, and a holding capacity of 4000 gallons. Each tank is provided with internal steam heating coils, or with external fuel burners (not shown), whereby the batch of liquor in the tank may be held to a temperature of approximately 200 F.

Under the effect of the applied heat and the jet condenser of the evaporator unit 50% to 60% of the water of the contained batch of pickle liquor is removed, as indicated by the double-headed arrows in Fig. 1, and it should be noted that, while an evaporator unit may be operated with the liquor heated to above 200 F., it should not be operated with the temperature of the liquor substantially in excess of 212 F. In any event the temperature of the liquor should not exceed 220 F., at which temperature the sulfuric acid tends to distill off with the water.

In the case of the four thousand gallon evaporator units herein illustrated, 60% of the water may be evaporated from the liquor in from five to six hours, according to the particular temperature at which the liquor is maintained. At the end of the evaporating phase there remains in the units 1702 gallons of pickle liquor concentrate, comprising 12,792 lbs. of water, 8101.6 lbs. of ferrous sulfate, and 2558.4 lbs. of sulfuric acid. This means that the concentrate comprises 34.54% ferrous sulfate and 10.91% sulfuric acid by weight.

A phenomenon to be noted in the case of aqueous sulfuric acid solutions is that the solubility of ferrous sulfate in the solution increases as the temperature of the solution is increased to 140 F., whereas a further increase in the temperature of the solution above 140 F. results in a decrease in the solubility of ferrous sulfate in the solution. The solubility of the salt at all temperatures decreases as the concentration of the sulfuric acid in the solution increases, up to an acid concentration of 60%, at which the salt is practically insoluble. My invention embraces an application of these phenomena.

In the typical application under consideration, where the temperature of the concentrated liquor is 200 F., and the solution contains 10.91% sulfuric acid, the liquor solution is saturated when the ferrous sulfate content is approximately 14% by weight. Thus, the difference between the 34.54% and 14%, or about twothirds of the ferrous sulfate, is precipitated in finely divided form, the particles being of micron size and being unsusceptible of separation from the liquid in a practical filtering operation. Due to such precipitation of the ferrous sulfate the acidity of the liquor concentrate increases to about 14%. If the temperature of the solution is allowed to fall below 200 F., more of the ferrous sulfate will be held in solution, and if the tempera ture is increased above 200 F. less of the sulfate will be held in solution. But for present purposes, the case is given where the temperature is held at 200 F., and the concentrated liquor in the evaporator unit is agitated by the escaping vapor, and/or by any suitable means known to the art, to keep the precipitated ferrous sulfate in suspension.

Settling

tanks

22, 23, and 24 and 25 are provided, in this case one tank for each of the four evaporator units, iand pumps 18 to 21 are arranged in lines 29 to deliver 4 the liquor concentrate from the evaporator units to the settling tanks severally.

The construction and operation of the settling tanks 22 to 25 form important aspects of my invention. Each tank has a cylindrical body with a conical bottom constructed of steel plate, lead lined. Conventional means (not shown) are provided for maintaining the contents of each settling tank at the specified temperature, such means as the steam coils, or the hot waste gases from fuel burners used to heat the evaporator units. The ca-. pacity of each settling tank is such that it will hold more than a single batch of concentrate produced in its associated evaporator unit. More particularly, I prefer that each settling tank shall have a capacity of at least three times the volume of one batch of liquor concentrate; that is, at least 5106 gallons in this case. Thus, three batches of concentrate may be introduced from an evaporator unit to its settling tank, in a period of from fifteen to eighteen hours.

Referring to Fig. 2, the diameter of the settling tank may be approximately eight feet and the height sixteen feet, while the distance between liquid levels L3 and L2 may equal five feet five inches. The distance between L2 and L1 may be the same. When the tank is filled to the level L3, the time required for the precipitated errous sulfate to settle out of the liquid standing between L2 and L3 is from eleven to twelve hours, leaving a clear solution containing about 14% sulfuric acid and from 12% to 15% ferrous sulfate. The hot supernatant or clear solution standing above level L2 is separated or drawn off from the concentrate by means of a pipe 32 and fed through pipe 34 into tank 3 of pickling line A and tank 0 of pickling line B.

The ferrous sulfate precipitated from the liquor above level L2 gradually settles through the liquid below l'evel L2 into the hopper shaped bottom of the tank. When the recovered acid has been withdrawn through pipe 32, a further batch of liquor concentrate is charged from the associate evaporator unit into the settling tank, and so it is with each batch of recovered acid removed from above level L2. There is a point of importance now to be noted.

Each additional batch of liquor concentrate added to the settling tank is introduced in such a manner as not to disturb the orderly settling of the precipitated ferrous sulfate. Means to this end comprise a pipe 30 of relativel'y large diameter with respect to the pipe 29 that carries the liquor concentrate from the evaporator unit to the settling tank, and the pipe 30 opens at its lower end through a flared mouth 33 into the conical chamber at the bottom of the tank. By virtue of this structural arrangement the addition of batch concentrate may be made to the settling tank at any time required, without interfering with the settling of the precipitated ferrous sulfate into the conical bottom 26 of the settling tank, it being noted that with the parts of the apparatus proportioned as shown in Fig. 2, the separation of the salt is greatly facilitated, since the downward rate of flow of the liquor concentrate through the tube 30 is about ten times the settling rate of the salt in the static suspension in tank 25, and the upward flow of the liquor concentrate emerging from the flared end 33 of the tube is about one-tenth of the said settling rate of the salt in the static suspension.

From time to time the accumulated precipitate in the bottom of the settling tank is, by opening a valve 35, released into a box 27 having a wall 28 of filter material. A floor 37 is pivoted at its left-hand end to the body of the box 27, and such floor slopes to a trough 36. Most of the liquid contained in the ferrous sulfate delivered into box 27 is filtered and drained into the trough 36. The trough 36 is arranged to receive the filtered liquid from the boxes 27 of all the settling tanks, and fromsuch trough the filtered liquid, comprising an effective acidsolution, is delivered through a pipe 41 (Fig. 1) into the lines leading to pickling tanks 3 and 9.

When the ferrous sulfate precipitate delivered into a box 27 is relieved of its excess liquid, the floor 37 is dropped, thereby delivering the deliquefied ferrous sulfate to an endless conveyor 38, which transports the ferrous sulfate to a rotary drying kiln 39, in which the residual moisture may be removed to the degree necessary to provide apowdered ferrous sulfate of desired specifications. The acid in this residual moisture may be neutralized by mixing about 0.2% of ferrous hydroxide with the ferrous sulfate as it enters the dryer 39. The ferrous hydroxide is prepared by dissolving a portion of the ferrous sulfate in water, adding thereto a basic material, such as NaOH, or NH Ol-I, and separating the precipitated ferrous hydroxide, Fe(OH) Returning to a consideration of the recovered pickling acid, it may be mentioned that, in order to insure that the steel being pickled is thoroughly cleaned, I prefer that the last acid tank through which strip passes shall contain fresh or new acid solution, and it is for this reason that the recovered acid is fed (via line 34) into the next-tothe-last acid tank 3 of the four-tank pickling line A, or into the center of the last acid tank of the three-tankline B. New acid is introduced from fresh acid storage tank 40 into the tanks 4 and 9, together with wash water from tanks 5 and 10, respectively, and such additional water as may be required to give a solution of desired concentra tion of free acid, say a solution in the present case. The volume of Water added may be equal to that evaporated, which for the 4,000 gallon evaporator units in this case equals 1066x4000 gals. 75% 60%, or 19188.00 lbs., while the weight of the 100% acid added equals After mixing the recovered acid with the new acid the composition of the pickling solution is found, as follows:

--19,188 or 2132 lbs.

Pounds Water in recovered acid 12, 792 Water added 1 19, 18 Acid in recovered solution 2, 558 New acid added 2, 132 Residual of ferrous sulfate in recovered solution 2, 709

Total 39, 379

Percentage of acid in the mixture 4, 690

=11. 9% 39, 379 Percentage of ferrous sulfate in the mixture 2, 709 F 1 /o 0 7 or 68,571 gallons instead of the 48,000 gallons that has in accordance with prior practices had to be neutralized and discarded. Dealing with these quantities of acid recovered, the savings equal about $276.00 per day. The value of the ferrous sulfate precipitated and recovered, calculated at $30.00 per ton on a dry basis, is $972.00 per day. In other words the total value of the recovered products is $1248.00 per day, and, allowing $348.00 per day for labor cost and capital charges for the apparatus, the savings enjoyed in the practice of my invention is $900.00 per day, or over a third of a million dollars per year in the single plant taken as an example in this case.

In modification of the method described, the pickle liquor concentrate produced in the evaporator units may, after the removal or separation of the precipitated ferrous sulfate, be cooled below 140 F., say to a temperature of 70 F., or lower. At temperatures 'below F., the solubility of ferrous sulfate in the pickle liquor concentrate decreases as the temperature decreases, and a substantial quantity of the ferrous sulfate remaining in solution, after that which was precipitated during the evaporating phase, is released in the concentrate in crystalline form and rapidly settles to the bottom of the settling tank, thereby leaving the acidity of remaining liquid at a value of from 25% to 30%, with the residual ferrous sulfate in the solution equal to from 4% to 5%.

The concentrate may be cooled in the settling tank (which tank in the modified process is not heated) 'by evaporating additional water from the concentrate. The additional evaporation may be effected by' bubbling cool air through the concentrate in the tank, or by flowing the concentrate over a slat-type cooling apparatus. In the modified method, as inthe case of the method first described, the supernatant liquid in the form of a sulfuric acid solution including a residual quantity of dissolved ferrous sulfate is returned to the pickling baths for re-use, and of this modified method it may be noted that the quantity of residual ferrous sulfate in the recovered solution is less and the acid concentration greater than in the recovered solution of the method first described.

In further modification, it is contemplated that in some cases the concentrated pickle liquor produced in an evaporator unit may be allowed to remain in such unit. The concentrate may be cooled in the evaporator unit, whereby such unit serves also as the settling tank, operated on the charge and draw batch principle.

From the foregoing specification it will be understood that the process of my invention consists in treating pickle liquor principally in the liquid phase, as distinguished from the gaseous phase in which S0 and S0 are re covered from the constituents of pickle liquor and reconverted into sulfuric acid. Within practical limits, I cleanse pickle liquor of the major portionsof the soluble ingredients or salts which accumulate in pickling acid in service, and thereby I recover at minimum cost and with relatively inexpensive capital equipment a pickling acid which contains in solution a residual but unobjectionable quantity of ferrous sulfate, in consequence of which the problem of pickle liquor disposal is solved, but it is done so in such manner that a substantial profit is realized.

Still further modifications may be practiced within the knowledge of those skilled in the art, without departing from the spirit of the invention defined in the appended claims.

I claim:

1. The method which comprises pickling a ferrous article by passing it sequentially through a series of pickling baths of hot dilute sulfuric acid solution wherein there is a movement of such solution from bath to bath in a direction counter to the movement of said article and an accumulation of dissolved ferrous sulfate to form pickle liquor of progressively greater ferrous sulfate concentration along the series of baths from the work entering end to the leaving end of the series, withdrawing heated pickle liquor from a point adjacent to the work-entering end of the series and forming the with drawn liquor into a plurality of batches, evaporating a substantial portion of the contained water from said batches severally While the heated liquor is below the temperature at which sulfuric acid is evolved and effecting a precipitation of suspended ferrous sulfate in the resulting concentrate, recovering by gravity separation the precipitated ferrous sulfate from the concentrate, recovering supernatant liquid from the concentrate in the form of a sulfuric acid solution including a residual quantity of dissolved ferrous sulfate, returning the recovered sulfuric acid solution to the series of baths at a point removed from the work-entering end thereof, adding new sulfuric acid to the series of baths at a point more removed from the entering end of the series than the point at which the recovered acid is added, and adding water to the series of baths for the formation of said recovered acid and new acid into an acid solution of pickling concentration.

2. The method which comprises pickling a ferrous article by passing it sequentially through a series of pickling baths of hot dilute sulfuric acid solution where there is a movement of such solution from bath to bath in a direction counter to the movement of said article and an accumulation of dissolved ferrous sulfate to form pickle liquor of progressively greater ferrous sulfate concentration along the series of baths from the worleentering end to the leaving end of the series, withdrawing heated pickle liquor from a point removed from the leaving end of the series of baths and forming the withdrawn liquor into a plurality of batches, evaporating a substantial portion of the contained Water from the pickle liquor in said batches severally at a temperature between 140 F. and 220 F. and efiectinga precipitation of suspended ferrous sulfate, cooling the resultant concentrate to a point substantially below 140 F. and effecting the crystallization of ferrous sulfate remaining in the concentrate, recovering supernatant liquid from the concentrate in the form of a sulfuric acid solution including a residual quantity of dissolved ferrous sulfate, and returning the recovered solution to the series of baths at a point removed from the work-entering end of the series.

3. The method which comprises pickling a ferrous article by passing it sequentially through a series of pickling baths of hot dilute sulfuric acid solution wherein there is a movement of such solution from bath to bath in a direction counter to the movement of said article and an accumulation of dissolved ferrous sulfate to form pickle liquor of progressively greater ferrous sulfate concentration along the series of baths from the work-entering end to the leaving end of the series, withdrawing heated pickle liquor from a point removed from the leaving end of the series of baths and forming the withdrawn liquor into a plurality of batches, evaporating a substantial portion of the contained water from the pickle liquor in said batches severally at a temperature between 140 F. and 220 F. and effecting a precipitation of suspended ferrous sulfate, cooling the resultant concentrate to a point substantially below 140 F. and effecting the crystallization of ferrous sulfate remaining in the concentrate, recovering supernatant liquid from the concentrate in the form of a sulfuric acid solution including a residual quantity of dissolved ferrous sulfate, returning the recovered solution to the series of baths at a point removed from the work-entering end of the series, adding new sulfuric acid to the baths at a point more removed than the last-named point from said entering end of the series, and adding water to the baths for the formation of said recovered acid and said new acid into an acid solution of pickling concentration.

4. In pickling ferrous articles in a bath of hot dilute sulfuric acid solution accompanied by an accumulation of dissolved ferrous sulfate and the formation of plCklC liquor, the method which comprises withdrawing heated pickle liquor from said bath and forming such liquor into a plurality of batches, evaporating water from said batches severally, while the heated liquor is below the temperature at which sulfuric acid is evolved, and obtaining a solution concentrate saturated with ferrous sulfate, cooling the concentrate to below 140 F. and effecting the crystallization of ferrous sulfate heptahydrate, recovering the crystallized ferrous sulfate from the com ccntrate, recovering from the concentrate supernatant liquid comprising a sulfuric acid solution, returning said recovered solution to said pickling bath and adding thereto an acid solution containing water substantially equal in volume to the water evaporated from the withdrawn pickle liquor and containing acid in quantity to maintain the bath at pickling concentration.

5. in pickling ferrous articles in a bath of hot dilute sulfuric acid solution accompanied by an accumulation of dissolved ferrous sulfate and the formation of pickle liquor, the method which comprises withdrawing heated pickle liquor from said bath and forming such liquor into a plurality of batches, evaporating water from said batches severally, while the heated liquor is at a temperature between 140 F. and 220 F., and obtaining a solution concentrate saturated with ferrous sulfate, cooling and agitating the concentrate at about atmospheric temperature and effecting the crystallization of ferrous sulfate heptahydrate, recovering the crystallized ferrous sulfate from the concentrate, recovering supernatant liquid from the concentrate in the form of a sulfuric acid solution, returning the recovered sulfuric acid solution to said pickling bath, and adding acid and water thereto for the formation of an acid solution of pickling concentration.

6. in pickling ferrous articles in a bath of hot dilute sulfuric acid solution accompanied by an accumulation of dissolved ferrous sulfate and the formation of pickle liquor, the method which comprises withdrawing heated pickle liquor from said bath and forming such liquor into a plurality of batches, evaporating Water from said batches severally while the heated liquor is below the temperature at which sulfuric acid is evolved, precipitating a substantial portion of the ferrous sulfate in finely divided and suspended form in the resulting concentrate of each batch, settling such precipitate in the concentrate and separating the resulting sediment from the concentrate, evaporating water from such concentrate and cooling the concentrate to below 140 F. to effect a crystaL lization of ferrous sulfate heptahydrate, recovering the crystalline ferrous sulfate from the concentrate, recovering supernatant liquid from the concentrate in the form of a sulfuric acid solution, and returning the recovered sulfuric acid solution to said pickling bath and adding water thereto for the formation of an acid solution of pickling concentration.

References Cited in the file of this patent UNITED STATES PATENTS 1,392,781 Marsh Oct. 4, 1921 2,155,854 Barnes Apr. 25, 1939 2,591,067 Herrmann Apr. 1, 1952 2,602,023 Simms July 1, 1952 2,668,130 Martin Feb. 2, 1954 2,721,562 Irvine Oct. 25, 1955

Claims

1. THE METHOD WHICH COMPRISES PICKLING A FERROUS ARTICLE BY PASSING IT SEQUENTIALLY THROUGH A SERIES OF PICKLING BATHS OF HOT DILUTE SULFURIC ACID SOLUTION WHEREIN THEREIS A MOVEMENT OF SUCH SOLLUTION FROM BATH TO BATH IN A DIRECTION COUTER TO THE MOVEMENT OF SAID ARTICLE AND AN ACCUMULATION OF DISSOLVED FERROUS SULFATE TO FORM PICKLE LIQUOR OF PROGRESSIVELY GREATER FERROUS SULFATE CONCENTRATION ALONG THE SERIES OF BATHS FROM THE WORKENTERING END OT HTE LEAVING END OF THE SERIES, WITHDRAWING HEATED PICKLE LIQUOR FROM A POINT ADJACENT TO THE WORK-ENTERING END OT THE LEAVING END OF THE SERIES, WITHDRAWDRAWN LIQUOR INTO A PLURALITY OF BATCHES, EVAPORATING A SUBSTANTIAL PORTION OF THE CONTAINED WATER FROM SAID BATCHES SEVERALY WHILE THE HEATED LIQUOR IS BELOW THE TEMPERATURE AT WHICH SULFURIC AACID IS EVOLVED AND EFFECTING A PRECIPITATION OF SUSPENDED FERROUS SULFATE IN THE RESULTING CONCENTRATE, RECOVERING BY GRAVITY SEPARATION THE PRECIPITATED FERROUS SULFATE FROM THE CONCENTRATE, RECOVERING SUPERNATANT LIQUID FROM THE CONCENTRATE IN THE FORM OF A SULFURIC ACID SOLUTION INCLUDING A RESIDUAL QUANTITY OF DISSOLVED FERROUS SULFATE, RETURNING THE RECOVERED SULFURIC ACID SOLUTION TO THE SERIES OF BATHS AT A POINT REMOVED FROM THE WORK-ENTERING END THEREOF, ADDING NEW SULFURIC ACID TO THE SERIES OF BATHS AT A POINT MORE REMOVED FROM THE ENTERING END OF THE SERIES THAN THE POINT AT WHICH THE RECOVERED ACID IS ADDED, AND ADDING WATER TO THE SERIES OF BATHS FOR THE FORMATION OF SAID RECOVERED ACID AND NEW ACID INTO AN ACID SOLUTION OF PICKLING CONCENTRATION.