Process for the reclamation of acid from spent pickle liquor

A process for reclaiming pickling liquor acid from spent pickle liquor comprises injecting the spent liquor from a metal pickling process into a container having an upper section, a lower liquid holding section and a heating means. The spent pickle liquor in the container lower section is heated, and the water therein is vaporized and returned to the pickling process in a vapor state to be absorbed therein and provide heat thereto. A substantial portion of the unused acid is removed from the spent pickle liquor and is also returned to the pickling process without cooling.

BACKGROUND OF THE INVENTION 
This invention relates to the pickling of metals by passing the same 
through an acid bath, and in particular to the reclamation of unused acid 
from and concentration of spent pickle liquor produced by a metal pickling 
process. 
In the production of metals, and in particular ferrous metals such as 
steel, metal oxide scales often form on the surface of the metals. To 
remove such scale, the metal is typically passed through an acid bath 
referred to as pickle liquor. Frequently, the pickle liquor used is 
hydrochloric or sulfuric acid. As the metal is treated, metallic salts are 
produced by the corrosive action of the acid bath. The corrosive action 
also occurs on the unoxidized or descaled portions of the metal being 
pickled, which is an undesirable consequence of the pickling process as 
the metal so removed is wasted. Sometimes an inhibitor such as thiourea is 
included in the bath which somewhat inhibits the corrosion of the unscaled 
metal while having little effect on the descaling action. The formation of 
the metallic salts uses and thus reduces the concentration of the pickle 
liquor acid, and as will be discussed later, also often undesirably 
enhances the corrosive effect on the descaled metal. Hence, the metallic 
salts must be removed from the pickle liquor, and fresh acid added 
thereto. Spent pickle liquor which is relatively high in metallic salts 
and low in acid is thus typically bled off from the pickle liquor bath. 
The unused acid in the spent liquor is, therefore, lost unless recovered. 
Disposal of the spent liquor containing unused acid is difficult both due 
to total volume of the spent pickle liquor and low PH caused by the acid. 
Federal and state regulations on dumping of low PH materials are becoming 
increasingly strict, thus this low PH material generated from the pickling 
process normally requires neutralization before placing in a landfill, 
deep welling or other disposal site. 
A number of processes have been developed for converting the metallic salts 
of various ferrous metal pickling processes, wherein the pickling acid is 
hydrochloric acid, to iron oxides, often by combusting oxygen and fuel in 
the presence of the iron chlorides produces therein. However, these 
processes require substantially increased inputs of energy and often need 
careful monitoring of the operation. Hence these processes may require 
substantial additional energy and labor inputs as compared to the 
requirements of operating a pickling process without any recovery system. 
In addition, spent pickle liquor metallic salt concentration processes are 
sometimes used in conjunction with pickling processes using hydrochloric 
acid which include a single stage evaporator and the absorption or 
condensation of the acid-water gaseous vapors or overheads released from 
the evaporation before returning of the overheads to the pickling bath. 
This process in general uses separate energy supplies in the form of heat 
to vaporize the acid-water mixture and to heat the pickling baths to their 
proper operational temperature. Thus, most spent pickle liquor 
concentration processes require substantially greater energy input than 
does a pickling process which does not concentrate the spent pickle 
liquor. 
The marketability of some of the metallic salts is low and thus they are 
disposed of by standard methods of waste disposal, however, many of the 
salts such as iron chlorides have a relatively good commercial market. 
Normally salability improves as the salt concentration of the solution 
containing the metallic salts increases, and as the free or unused acid 
concentration therein decreases. Marketability is also significantly 
enhanced by a decrease in free or unused acid content of the concentrated 
metallic salts solution. Therefore, the unused acid removal and 
concentration of the remaining salt solution provided by the present 
invention, as will be discussed later, also improves the value and 
salability of the concentrated spent pickle liquor. In addition, the 
ferrous chlorides generated in the ferrous metal pickling processes using 
hydrochloric acid are not as marketable as ferric chlorides and it is thus 
desirable to convert the ferrous chloride to ferric chloride. 
SUMMARY OF THE INVENTION 
Therefore, the principal objects of the present invention are: to provide a 
process for reclamation of unused acid from spent pickle liquor; to 
provide such a process which absorbs volatile acid and water vapors in a 
gaseous state directly into the pickle liquor bath; to provide such a 
process which requires little additional energy and manpower; to provide 
such a process which concentrates spent pickle liquor; to provide such a 
process which uses multi-stage stripping to remove relatively volatile 
unused acid from the spent liquor; to provide such a process which will 
reduce corrosion of the metal being pickled; to provide such a process 
which converts ferrous chloride to ferric chloride; to provide such a 
process which separates relatively non-volatile acids from the spent 
pickle liquor by crystallization and removal of metallic salts contained 
therein, leaving the spent pickle liquor as essentially unused acid which 
can be returned to the pickle liquor bath; to provide such a process which 
is capable of an extended useful life, and is particularly well adapted 
for the proposed use. 
Other objects and advantages of this invention will become apparent from 
the following description taken in connection with the accompanying 
drawings wherein are set forth, by way of illustration and example, 
certain embodiments of this invention. 
The drawings constitute a part of this specification and include exemplary 
embodiments of the present invention and illustrate various objects and 
features thereof.

As required, detailed embodiments of the present invention are disclosed 
herein, however, it is to be understood that the disclosed embodiments are 
merely exemplary of the invention which may be embodied in various forms. 
Therefore, specific structural and functional details disclosed herein are 
not to be interpreted as limiting, but merely as a basis for the claims 
and as a representative basis for teaching one skilled in the art to 
variously employ the present invention in virtually any appropriately 
detailed structure. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
In general FIG. 1 shows schematically an embodiment of a process comprising 
a metal pickling process in combination with a novel process of reclaiming 
unused acid from spent pickle liquor. 
The metal pickling process contemplated by the present invention can be 
used to pickle a wide variety of metals and may use a wide variety of 
pickling acids. In the embodiment illustrated in FIG. 1 the metal to be 
pickled is a ferrous metal, and the pickling acid is hydrochloric acid. 
However, the description of this specific pickling process is not intended 
to exclude pickling processes comprising other metals or pickling acids, 
as demonstrated by the modified embodiments discussed hereinafter. 
The spent pickle liquor acid reclamation process, as used in combination 
with the above mentioned metal pickling process, receives spent pickle 
liquor from the metal pickling process, separates a major portion of the 
unused acid from the metallic salts contained in the spent pickle liquor, 
returns the unused acid to the metal pickling process, and concentrates 
the remaining spent pickle liquor solution which contains the metallic 
salts. In the embodiment illustrated in FIG. 1, the spent pickle liquor 
acid reclamation process operates such that spent pickle liquor is 
withdrawn from a ferrous metal pickling process, water is vaporized from 
the spent pickle liquor in a container, a major portion of the unused 
hydrochloric acid is stripped from the spent pickle liquor in a 
multi-stage stripper by allowing the vaporized water to rise therethrough 
and act as a stripping gas, the acid-water mixture is returned in a 
gaseous state to the metal pickling process, and the concentrated pickle 
liquor containing the metallic salts is removed to storage or other 
disposition. However, the description of this specific acid reclamation 
process is not intended to exclude other acid reclamation processes as 
will be described and claimed herein. 
In the embodiment illustrated in FIG. 1 the metal pickling process 
comprises an acid bath, that is, a vat or tub containing a somewhat dilute 
acidic solution (pickling liquor, including a pickling acid (the process 
as described herein uses hydrochloric acid) along with beneficial and 
nonbeneficial reaction products of the pickled metal and pickling acid. 
The acid bath is heated to a selected temperature for optimum scale 
removal from the metal, yet with minimum corrosion of the descaled metal. 
The optimum temperature is normally a function of acid concentration in 
the bath, and certain other factors as will be discussed later. The 
ferrous metal is selectively passed through the pickle liquor. The metal 
may be a long pliable strip or pieces which must be conveyed by some 
means. As pickling acid is used up or consumed in the pickling process, 
mostly by reaction with the scale and base metal resulting in the 
consequent production of iron chlorides, fresh acid must be added and 
spent pickle liquor must be withdrawn from the vat. 
In FIG. 1, the bath is composed of four tubes 10, 11, 12 and 13. The 
ferrous metal comprises a long pliable strip 15 which is guided by rollers 
(not shown) through tubes 13, 12, 11 and 10 in that order. During normal 
operation, fresh make-up acid of selected concentrations is delivered to 
tub 10 by pipe 12. During continuous operation fresh acid flow may be 
regulated by a valve 21. The pickle liquor 22, having partially reacted 
with the metal in tub 10, overflows tub 10 through line 24 into tub 11. 
The liquid in tub 11 is thereby of weaker concentration in acid and higher 
in iron chloride concentration than that in tub 10. Likewise each tub 
overflows into the next consecutive tub in the opposite order with which 
the strip 15 passes therethrough, such that lower acid concentrations and 
higher iron chloride levels exist in each consecutive tub. Thus, the strip 
15 passes through the tub 13 with the weakest acid concentration first, 
and the tub 10 with the strongest acid concentration last. 
The invention does not require the particular alignment of the tubs as 
discussed above. The process according to the invention would also be 
functional with a pickling process having greater or fewer tubs than four, 
including one, and would be functional if the ferrous metal strip were 
interjected into the pickling liquor in a manner other than as described, 
such as by dipping in sections. 
The acid concentration of the pickle liquor 22 varies as discussed above 
throughout the pickling process. In a conventional process using 
hydrochloric acid, the concentration of the fresh make-up acid may be in 
the nature of about 30% by weight acid (the concentration of acid can be 
varied by water dilution as required for desired process results). The 
average concentration in the first tub may be in the nature of 15% acid by 
weight. This varies, however, and is somewhat a function of acid bath 
temperatures and residence time of the ferrous metal strip. The 
concentration of the acid in the final tub 13 is in the nature of about 8% 
by weight, whereas in conventional pickling processes which do not reclaim 
the unused hydrochloric acid, the acid concentration in the final tub is 
typically 2% by weight. This low acid concentration in conventional 
pickling processes is normally dictated by the economics of disposing of 
as little unused acid as possible and yet still pickling the metal strip 
15 within a reasonable residence time. As will be discussed in greater 
detail, such low acid concentrations in the final tub also require high 
acid temperatures and necessarily result in coexistant high iron chloride 
concentrations; both high temperatures and high iron chloride 
concentrations in turn result in increased corrosion of the metal strip 
over what is necessary for proper scale removal and metal protection in 
the pickling process. 
The acid used in the pickle liquor processes in FIG. 1 is hydrochloric acid 
which is an aqueous solution of hydrogen chloride (HC1). However, it is 
not intended that the use of the invention should be limited to processes 
using hydrochloric acid. The invention, as embodied in FIG. 1, would be 
functional with any acid which has a relatively low boiling point. As used 
herein, a volatile acid is understood to mean one which vaporizes upon 
heating before or with water mixed therewith, or which vaporizes within a 
close temperature proximity to the temperature at which water mixed 
therewith vaporizes (in particular, not more than 50.degree. Centrigrade 
(C) above the water) and which in all cases must necessarily volatilize 
before the metallic salts mixed therewith. Thus, while hydrochloric acid 
is referred to frequently in this embodiment, other volatile acids, for 
example hydrochloric acid and nitric acid, would also be functional 
according to the metal pickling process contemplated herein. 
While many pickling processes are used to pickle ferrous metals, as is 
described herein, this is not meant to exclude a process in which metal to 
be pickled is other than ferrous, for instance aluminum. 
Spent pickle liquor is pickling liquor of relatively low acid concentration 
and high metallic salt concentration wherein the liquor is no longer 
functional in pickling the metal strip 15 in view of the economics of 
discarding unused acid and actual ability to further pickle the metal in 
each individual situation. In the illustrated pickling process, spent 
pickle liquor overflows tub 13 into collection pot 28. The spent pickle 
liquor is transferred by conduit 30 to a pump 31. An automatic level 
controller 29 is connected with the spent pickle liquor collection pot 28, 
and opens and shuts a control valve 32 as necessary to maintain a constant 
level in the spent liquor collection pot 28. 
In conventional pickling processes without unused acid recovery, the spent 
pickle liquor is frequently delivered by conduit 30 to storage, a waste 
removal system, or to other appropriate disposition. The spent liquor in 
such a system contains unused acid which must normally be neutralized 
before final disposition. Due to the large bulk of spent pickle liquor 
generated from such systems, substantial cartage expense and/or burial 
expense in landfills or deep wells is required. 
As illustrated in FIG. 1, the unused acid reclamation process basically 
requires three elements: a tower, a liquid level holding pot and heating 
means. There is no restriction that these elements be separate vessels or 
apparatus or that they be all part of the same structure. It is also 
understood that each element could be composed of a multiplicity of 
separate structures, that is for instance that the tower could be 
comprised of several separate vessels, each including several separation 
stages which will be discussed in greater detail hereinafter. 
In the illustrated embodiment, the major elements are contained in a 
container or column 33 having an upper stripping section 34 (the tower) 
and a lower liquid level holding pot 35 and a thermo-syphon reboiler 36 
(the heating means). 
The column upper stripping section 34 comprises a multi-stage separation 
tower. (By multi-stage separation is meant the common chemical engineering 
use of the terminology, for instance see King, Separation Processes, 
published by McGraw-Hill 1971). The upper section 34 may be comprised of 
plate or tray separation stages, a packed column using such trade or 
generically named packing devices as Raschig rings, Lessing rings, 
tellerettes, Pall rings, Intralox saddles, Berl saddles, or other 
multi-stage separation processes, especially those suitable for gas 
stripping from liquid. The illustrated column upper section 34 is a vessel 
containing Raschig ring packing. 
The column lower section 35 comprises a liquid holding pot. The lower 
section 35 may be a separate vessel from the upper section 34, however, in 
the illustrated embodiment both sections 34 and 35 are one continuous 
vessel. 
The heating means may be any suitable device for adding heat to the liquid 
in the liquid holding pot 35. This may be a heating device located 
internally of the pot 35, such as an electric heating coil, or externally, 
such a mechanical recirculation system including a pump and a heat 
exchanger, wherein the liquid is circulated by the pump from the holding 
pot 35 to the heat exchanger for heat input therein and then returned to 
the holding tank 35. In the illustrated embodiment, the liquid in the 
holding pot 35 is conveyed by conduit 36 to a thermo-syphon (that is, the 
heated liquid in the tank rises, drawing new liquid into the bottom) 
reboiler tank 37 and then returned to the holding tank 35 by conduit 38 
after heating. A closed steam coil supplies heat to the reboiler tank and 
conveys away condensate after conventional trapping (not shown). 
The embodiment illustrated in FIG. 1 includes two heat exchanges 42 and 43 
for cooling concentrated spent pickle liquor as will be discussed 
hereafter, and a constant liquid level overflow tank 44 which maintains 
the liquid level in the holding pot 35, also as will be discussed further 
hereinafter. 
To use the acid reclamation process, the spent pickle liquor is first 
transferred by conduit 30 from the spent pickle liquor collection pot 28 
through a valve 46 and a distribution nozzle 47 into the top of the column 
33. If desired, a recycle stream of spent pickle liquor can also be 
directed back to the high acid concentration tubs 10 and 11 through a line 
51 and a valve 52 to dilute the acid concentration therein. The recycled 
acid mixes with the fresh make-up conveyed in the line 20, and the mixture 
is delivered to the tubs 10 and 11 by lines 53 and 54 through valves 55 
and 56 respectively. Dilution of the fresh make-up acid by recirculation 
of spent pickle liquor is not always required, but may be appropriate 
where the use of the process disclosed herein creates an overconcentration 
of acid in the high concentration tubs 10 and 11 and especially in the tub 
10. 
The spent pickle liquor is distributed by the nozzle 47 into the top column 
upper stripping section 34, whereupon it cascades down through the packing 
contained therein to the holding pot 35. The liquid collects in the 
holding pot 35 and the reboiler tank 37. Closed steam in the nature of 150 
pounds per square inch gauge (PSI) is applied through the steam coil 40 to 
the reboiler tank 37 whereupon a portion of the water and the HC1 are 
vaporized from the spent pickle liquor container therein. As was mentioned 
before, other heating means are functional and would include steam of 
lesser or greater pressure, for instance, 200 PSI steam. The steam flow to 
the reboiler tank may be controlled by conventional automatic temperature 
control means (not shown). The water vaporized in the reboiler tank 37 is 
returned to the column 33 through conduit 38 and therein rises through the 
column upper stripping section 34. In the upper section 34, the water 
vapors strip the HCl from the downflowing, cascading spent pickle liquor 
in a stage equilibrium process, wherein the stripped HCl enters into the 
gaseous phase with the water vapors. The acid concentration of the spent 
pickle liquor reaching the holding pot 35 is basically dependent on the 
original acid concentration of spent pickle liquor, and the number of 
stages present which may be calculated by standard chemical engineering 
techniques. 
The water and stripped HCl vapor normally exit the column 33 under 
pressure. The pressure of the vapors varies with operation conditions, 
heating means used, and type of pickling acid used. In the embodiment 
illustrated in FIG. 1, the pressure of the vapors is in the nature of 3 
PSI, however, this could vary to much higher pressures (over 100 PSI) or 
may at times be a vacuum. The water and HCl vapors, sometimes referred to 
as column overheads, are conveyed from the column 33 in a gaseous state by 
line 50 directly to the tubs 10, 11, 12 and 13 through lines 61, 62, 63 
and 64 and associated valves 65, 66, 67 and 68 respectively. The line 50 
is insulated or otherwise treated to avoid condensation of the vapors 
therein. The return of the column overhead vapors directly to the pickling 
bath without condensation is important, as it allows the heat entrapped in 
the vapors to be used to raise the temperature of the tubs 10, 11, 12 and 
13. In many conventional process open or closed steam is used directly on 
the pickle liquor 22 in the tubs 10, 11, 12 and 13 to heat the same to 
pickling process temperatures. The potential driving force of this steam 
for separating the acid from the spent pickle liquor and concentration 
thereof is thereby not used in the conventional processes. In the present 
invention the steam heat input is exchanged in the reboiler tank 37 with 
spent pickle liquor with the resultant creation of water and HCl vapors at 
a pressure in the nature of 3 PSI, which vapors are thereby returned to 
the tubs 10, 11, 12 and 13 for recovery of unused acid and for the 
resultant heating thereof. While there are of course some heat losses 
between the reboiler tank 37 and the tubs 10, 11, 12 and 13 due to normal 
inefficiencies, in this embodiment as illustrated in FIG. 1 and described 
above, there is a substantially equivalent amount of energy in the form of 
heat applied to the tubs 10, 11, 12 and 13 through the overhead vapors and 
the fresh make-up acid (having been heated in the heat exchanger 42), as 
is applied to the reboiler tank 37. 
The column overheads are normally distributed directly into the pickle 
liquor 22 in the tubs 10, 11, 12 and 13, whereby the vapors are absorbed 
into the solution therein. This may be accomplished for instance by 
sparging the vapors from distribution manifold tubes or other devices such 
as porous nozzles or plates directly into the pickling liquor 22. Or an 
aspiration jet could be used to distribute the overheads wherein a pump 
(not shown) circulated pickle liquor from tub 10 through an aspirator jet 
(not shown) to which the overheads are fed whereby the overheads are 
absorbed and heat the recirculated pickle liquor which is then returned to 
the tub 10. In the illustrated embodiment the overheads return to the tubs 
10, 11, 12 and 13 through sparging nozzle 70. Normally the vapors are 
returned to the tub 10, however, as shown above they may be directed to 
the other tubs 11, 12 and 13, if it is desirous to increase the acid 
concentration therein. 
Even though the vapor pressure in the overhead is usually somewhat less 
than the steam supplying heat to the reboiler 37 through line 40, the 
heating values as measured by enthalpy are quite similar. Thus, in the 
present process little heating effect is lost by injecting the heat from 
the steam at the reboiler tank 37, rather than, directly inputing same 
into the tubs 10, 11, 12 and 13; yet steam has indirectly served two 
useful purposes by acting as a driving force in separating the unused acid 
from the spent pickle liquor and concentrating the spent pickle liquor by 
vaporizing water threfrom. This has been accomplished with very little 
additional input of energy over what would normally be used in the metal 
pickling process with consequent little additional energy expense. A major 
savings in expense is realized because of the concentration of the spent 
pickle liquor due to evaporation of water therefrom, resulting in reduced 
storage expense and substantially reduced transportation and disposal 
expense or if the liquor is salable, as is liquor containing iron 
chlorides, the quality and value of the liquor is greatly increased by 
concentration thereof. Because the concentrated acid is substantially 
reduced in unused acid concentrations relative to the untreated spent 
pickle liquor (any reduction is beneficial), the cost of neutralizing the 
spent pickle liquor before disposal is also similarly reduced. 
Returning attention to FIG. 1, the concentrated spent pickle liquor exits 
the bottom of the column liquid holding pot 35 through line 73 and enters 
heat exchanger 42, wherein fresh cold make-up acid in line 20 is heated by 
the residual heat from the concentrated spent pickle liquor. This results 
in additional energy savings in the form of reduced heat consumption. The 
concentrated spent pickle liquor exits heat exchanger 42, and is conveyed 
by line 75 to the second heat exchanger 43, wherein the concentrated spent 
pickle liquor may be further cooled by water if necessary. 
The cooled spent pickle liquor is carried by line 77 from heat exchanger 43 
to an overflow tank 44, which serves to maintain the proper operating 
liquid level in the column liquid holding pot 35. Level maintenance is 
accomplished by an overflow conduit 79 at the liquid level desired, which 
consequently maintains about the same operating liquid level in the 
overflow tank 44, the liquid holding pot 35, and the reboiler tank 37, as 
is shown by phantom line 80, with some slight variances due to pressure 
surges. A normal operating level 82 for liquid 83 in the column lower 
section 35 can be seen in FIG. 1. The concentrated spent pickle liquor 
having substantially reduced acid content which overflows tank 44 is 
transferred to storage (not shown) for later disposition which may include 
beneficial recovery and use of the metallic salt or disposal as waste. The 
liquid level as shown by line 80 should be at least high enough to 
substantially cover the steam coil 40, or other heating means in the 
reboiler tank 37 with spent pickle liquor to be concentrated. The overflow 
tank 44 has a gas relief valve 81 on top thereof to release gaseous 
buildups therefrom. The relief valve 81 relieves to a caustic scrubber 
system (not shown) which vents and scrubs the pickling bath. 
The amount of the acid in the spent pickle liquor to storage according to 
the embodiment of FIG. 1, is reduced from the amount in the spend pickling 
liquor as withdrawn from the pickling process. The concentration of the 
acid in the liquor to storage may vary according to the number of stages 
in the upper stripping section 34, as was previously discussed, especially 
as dictated by the economics of adding additional stages versus the 
reduction in concentration. Concentrations would be expected to normally 
be in the range of a trace to 2.0 by weight acid. (Note, the concentration 
of acid is increased by the reduction in the volume of the spent pickle 
liquor in the column 33, thus even though the concentration of acid in and 
out of the column 33 may be similiar, the amount of acid will be 
substantially reduced by passage through the column 33.) 
The examples which follow serve to illustrate the use of the invention in 
the reclamation of acid from spent pickle liquor generafted by a ferrous 
metal pickling process as described above and shown in FIG. 1. These 
examples are presented only to offer an exemplary comparison between a 
metal pickling process with no acid reclamation, and a typical embodiment 
of the present invention, wherein the metal pickling process is used in 
combination with the acid reclamation process previously described. The 
following examples are set forth only to provide a vehicle to describe the 
comparative benefit and effects of the present processes and are not 
intended to be limiting. Each example is based on a given steel strip 
production, having a pickling residence time for the metal of 2.6 minutes, 
and an assumed 200,000 square feet of metal to be exposed to the acid 
during a period of one hour (that is, approximately 4.8.times.10.sup.6 
surface square feet of metal are treated each full operative day). All 
percentages in the examples are by weight. 
EXAMPLE 1 
A conventional ferrous metal pickling process is used with no system of 
unused acid recovery. The fresh make-up acid is about 20 Baume' HCl (20 
Be' is about 31% HCl with the remaining portion being water.) The average 
concentration in the first pickling tub is 15% HCl, the average 
concentration in all of the pickling tubs is 8.5%. The appropriate 
pickling bath temperature for the above residence time and acid 
concentration is about 176.degree. Fahrenheit (F.) according to A. N. 
Winterbottom and J. P. Reed, Journal of Iron and Steel Inst., 159-204, 
1932. The metal corrosion rate at this temperature is about 2.7 pounds of 
metal per each square foot of metal exposed to the acid for a period of 
one day. Therefore, a total of 11.7 tons of metal are lost to acid 
corrosion each day and this metal-acid reaction forms a corresponding 
stoichiometric amount of iron chlorides (mostly ferrous chloride). 
On the assumed basis of a medium size steel mill operating about 500 shifts 
of 8 hours each shift per year, spent pickle liquor production would be in 
the nature of 5 million gallons per year and comprise about 23% ferrous 
chloride, 2% HCl, and a remainder portion mostly of water. Energy to heat 
in the form of open steam is sparged into the first tub or into the other 
tubs as necessary to retain an operating temperature of about 176.degree. 
F. 
EXAMPLE II 
A ferrous metal pickling process is used in combination with a novel pickle 
liquor unused acid reclamation process as disclosed herein by the 
embodiment illustrated in FIG. 1. The fresh make-up acid is as in Example 
I about 31% HCl. The average concentration in the first tub 10 is about 
15% HCl. The average concentration throughout all of the tubs is about 
11.5% HCl. The appropriate pickling bath temperature for a 2.6 minute 
metal residence time and the above acid concentration is about 162.degree. 
F. The metal corrosion rate (which is highly temperature dependent) at 
this temperature is about 1.7 pounds of metal for each square foot of 
metal exposed to the acid for a period of one day. Therefore, a total of 
about 7.7 tons are lost to acid corrosion each day and this metal-acid 
reaction forms a corresponding stoichiometric amount of iron chlorides. 
Spent pickle liquor production would be in the nature of 5 million gallons 
per year comprising about 13.6% ferrous chloride, about 8% HCl, and the 
remainder portion mostly of water. This spent pickle liquor is transferred 
to the spent pickle liquor unused acid recovery process. The steel 
production treated by the pickling process is the same as in Example I. 
The concentrated spent pickle liquor which is delivered from the unused 
acid reclamation process to storage is roughly one-third of the volume of 
spent pickle liquor in Example I or about 1.75 million gallons per year. 
The concentrated spent pickle liquor is about 36% ferrous chloride, about 
0.2% HCl and the remaining portion is composed mostly of water. All of the 
overheads from the column 33 are returned in a substantially vapor state 
to the first pickling tub 10 wherein they mix with the fresh make-up acid. 
The overheads having a composition of about 13% HCl and a remaining 
portion comprising mostly water. Energy in the form of heat is added to 
the reboiler tank 37 as closed steam and used to vaporize water therein 
which strips the HCl from the spent pickle liquor in the column 33; most 
of this energy is then transferred to the tub 10 for heating the pickle 
liquor 22 by means of the heat transferred by overhead vapors from the 
column 33. Additional energy in the form of heat may be added at the tubs 
10, 11 12 and 13 if necessary, (for instance, especially on startup). The 
process preferably is continuously operating in a somewhat steady-state 
condition, that is there are no major variations in flow rates, pressures 
or temperature. The equipment of the acid reclamation process as used 
herein is basically self operating (where necessary automatic controllers 
operate level, flow and temperature control valves). Hence, very little 
additional operational labor is required for the unused acid reclamation 
process as composed to a process having no acid recovery such as Example 
I. 
The benefits of a metal pickling process used with the unused acid 
reclamation process according to the embodiment in FIG. 1 and as 
previously described can be compared with a metal pickling process not 
using an acid reclamation pickling process according to results of 
Examples I and II respectively. The process according to Example II has 
the following advantages over Example I. 
(a) The pickling bath can be run at a higher concentration in the final tub 
13 (also the intermediate tubs 11 and 12 are at a higher average acid 
concentration), since unused acid is reclaimed and not wasted. 
(b) The higher acid concentration allows faster pickling rates or, as 
shown, allows a reduction in the pickling bath temperature of about 
14.degree. F. whereby corrosion of the metal is reduced by about 4.0 tons 
per day given the assumed pickling rates. This results in savings due to 
reduced metal loss and reduced acid usage. 
(c) The reduction of the pickling bath temperature also results in less 
heat consumption by the bath in obtaining processing temperature, because 
metal and pickle liquor are at a lower pickling temperature, and in the 
reduction of heat loss to vaporization of the pickling liquor while in the 
pickling bath. Thus there is less heat required for the pickling bath with 
consequent energy savings. 
(d) The amount of spent pickle liquor is reduced by about 3.25 million 
gallons per year (again under the rates assumed for the examples). This 
results in reduced storage, transportation, and waste disposal expense. 
The spent pickle liquor is also concentrated, whereby recovery or 
beneficial use of the iron chloride may be made without additional expense 
of concentration. 
(e) The spent pickle liquor to storage is substantially reduced in unused 
acid content (here from 2% by weight in Example I to 0.2% by weight in 
Example II in a reduced volume as shown in (d) above. This reduces 
expenses due to neutralization of the acid before disposal and reduces the 
amount of unused acid lost. Also concentrated metal salt solutions having 
lower free or unused acid concentrations are more marketable than 
corresponding solutions having higher free acid concentrations. 
(f) The average iron chloride concentration in the pickling bath is 
reduced. The iron chlorides in the pickle liquor have a corrosive effect 
on metal and processing equipment which is directly related to 
concentration, thus creating savings in reduced loss of pickled metal and 
reduced repair expense of equipment. 
In addition to the above benefits, the process of Example II requires 
little additional operational labor or energy input over the process of 
Example I. 
Also, by way of comparison, typical conventional spent pickle liquor 
recovery process, such as described by Kolek (J. F. Kolek, "Hydrochloric 
Acid Recovery Process", Chemical Engineering Process, Vol. 69, p. 47 
(1973.)), do provide some improvement over a pickling process having no 
recovery in the nature of reclaiming a portion of the spent pickle liquor 
unused acid, reducing the amount of the unused or free acid in the spent 
pickle liquor, and concentrating the spent pickle liquor. However, such a 
conventional recovery process does not save energy by returning the 
evaporator overhead vapors directly to the pickling bath before 
condensation nor yield a concentrated spent pickle liquor solution as low 
in free acid concentration as can be produced by the inventive process 
disclosed herein. In addition such conventional processes are typically 
more expensive than the process disclosed herein both in process 
equipment, requiring absorbers and attendant apparatus, and in energy 
cost, requiring separate energy supplies both to heat the evaporator and 
the pickling bath. 
A modified embodiment of the invention is illustrated in FIG. 2, wherein a 
modification is made to a line 30a which feeds spent pickle liquor to a 
column 33a. Since the metal pickling process and the acid reclamation 
process shown in FIG. 2 are otherwise substantially the same as those 
shown in FIG. 1, similar parts appearing in FIG. 1 and FIG. 2 are 
respectively represented by the same, corresponding reference numeral 
except for the addition of the suffix "a" to the numerals in the modified 
embodiment. The line 30a transfers the withdrawn spent pickle liquor to a 
chlorinator 85. A line 88 transfers the chlorinated spent pickle liquor to 
the column 33a wherein the liquor is dispersed by nozzle 47a. Chlorine 
from a storage area (not shown) is transferred to the chlorinator 85 by a 
line 86 and distributed therein by a sparging nozzle 87. Excess chlorine 
is transferred from the top of the chlorinator 85 by a line 90 through a 
gas pressure control valve 91 to a caustic scrubber (not shown). 
As was previously discussed, many processes pickle ferrous metal using 
hydrochloric acid with the consequent production of ferrous chloride. Thus 
in the modified process shown in FIG. 2, the spent pickle liquor has a 
major portion of ferrous chloride therein. This ferrous chloride is not as 
marketable as ferric chloride, therefore it is desired to convert the 
former to the latter. The pickle liquor is therefore injected into the 
chlorinator 85 while chlorine is sparged therethrough. The chlorine mixes 
with the spent pickle liquor and reacts with a substantial portion of the 
ferrous chloride therein to produce ferric chloride. The pickle liquor now 
containing a reduced portion of or negligible ferrous chloride, is 
injected into column 33a through line 88. The remainder of the process is 
essentially similar to that previously discussed in relation to the 
process of FIG. 1 except for the following benefits. The chlorine-ferrous 
chloride reaction is exothermic and thus adds energy in the form of heat 
to the spent pickle liquor, thereby requiring less heat input into the 
reboiler tank 37. Also, ferric chloride is normally a more marketable 
commodity than is ferrous chloride, and ferric chloride does not salt out 
(come out of solution) as easily as ferrous chloride thereby making 
processing easier and reducing salt build up on the coil 40 and other 
processing equipment. 
Another embodiment of the invention is shown in FIG. 3, and includes a 
modification to the unused acid reclamation process to allow recovery of 
unused acids which are not relatively volatile but where the reaction 
product metallic salts formed by the reaction of the acid and metal in the 
metal pickling process will crystallize upon concentration of the pickle 
liquor withdrawn from the metal pickling process. Since the metal pickling 
process, including the various lines related thereto, are substantially 
the same as that shown in FIG. 1, as are many parts of the reclamation 
process, similar parts appearing in FIG. 1 and FIG. 3 are respectively 
represented by the same, corresponding reference numeral except for the 
addition of the suffix "b" to the numerals in this modified embodiment. 
Many metal pickling processes use acids which are not readily 
volatilizable as was previously discussed, or use non-volatile in 
conjunction with volatile acids. Thus, while water can be vaporized from a 
pickle liquor containing these salts, the non-volatile unused acid cannot 
be. However, many of the metallic salts formed in metal pickling process 
crystallize when the spent pickle liquor is concentrated by the 
evaporation of water. Thus the embodiment of the reclamation process as 
illustrated in FIG. 3, provides for the return of the unused relatively 
nonvolatile acids when the metallic salts will crystallize. 
A column 33b in FIG. 3 may be essentially the same as the column 33 in FIG. 
1, especially where the unused acid is a composition of both volatile and 
nonvolatile acids, such as nitric acid and sulfuric acid. However, the 
packing in the upper section 34b would not be essential if only 
non-volatile acids were present. The column 33b could be any vessel which 
would contain the spent pickle liquor while heating thereof. The heating 
means 37b used to vaporize the water in FIG. 3 is likewise the same as in 
FIG. 1, however, any suitable liquid heating source such as an electric 
heater could also be used. Thus, the acid reclamation process of the 
present embodiment may be essentially the same as the embodiment as shown 
in FIG. 1 up to the point where the concentrated spent pickle liquor exits 
the column 33b through line 73b with the exception that non-volatile acids 
do not go through the overheads vapor line 50b, however, this is not meant 
to exclude substitutes of other suitable equipment as discussed above. The 
metal pickling process associated with the embodiment of FIG. 3 is also 
essentially the same as the process shown in FIG. 1 with the possible 
exception of using other or additional acids and/or metals to be pickled 
than are used according to the embodiment shown in FIG. 1. 
As shown in FIG. 3, the concentrated pickling liquor exits the column 33b 
through line 73b and is delivered by a pump 95 through line 96 to a filter 
98. The solids removed by the filter 98 are delivered to storage from the 
filter 98. The filtered liquor exits the filter 98 and is returned to the 
pickling bath through line 99. 
While certain equipment is illustrated schematically in FIG. 3, other 
interchangeable equipment would work suitably as well. Thus, the pump 95 
could be replaced by other means of delivering the pickle liquor from the 
column 33 to the filter 98, such as gravity. Also the filter 98 could be 
replaced by other means for separating liquids and solids, such as a 
centrifuge. In addition, the control of the liquid level in the column 35b 
is regulated by an automatic controller (not shown) which operates the 
pump 95. This level control could be maintained by other devices such as 
an overflow tank as was previously discussed. 
An example of a metal pickling process, wherewith the embodiment 
illustrated in FIG. 3 could be used, would be a ferrous metal pickling 
process wherein the pickling acid is sulfuric acid. In this example spent 
pickle liquor containing major portions of water, ferrous sulfate and 
sulfuric acid would be delivered to the column 33b through line 30b and 
collect in the liquid holding pot 35b wherein the level is automatically 
controlled. The pickle liquor in the holding pot 35b is heated by 
circulation through a thermo-syphon reboiler tank 37b, whereby a 
substantial amount of water is vaporized and returned to the metal 
pickling process in a vaporized state through the overheads line 50b 
passing through a line 38b and the column 33b. The spent pickle liquor in 
the holding pot 35b is concentrated by the vaporization of the water such 
that a major portion of the metallic salt crystallizes, herein ferrous 
sulfate crystallizes as ferrous sulfate monohydrate. The concentrated 
spent pickle liquor from the holding pot 35b is delivered to the filter 98 
wherein the crystals are removed from the liquor. The liquor then exits 
the filter 98 and is returned to the pickling bath. The metallic salt 
crystals are removed from the filter 98 by the conventional processes and 
delivered to storage or other suitable disposition. 
The modified embodiment shown in FIG. 3 provides for economization of 
energy in the form of heat input, as did the embodiment shown in FIG. 1. 
In the present modified embodiment much of the heat in the steam used to 
vaporize the water in reboiler tank 37b is transferred through the 
overhead return line 50b back to the pickling bath also in the form of 
heat contained in the vaporized water which is not condensed prior to 
return to the pickling bath and in the form of heat contained in the 
preheated acid by return of the unused acid separated from the spent 
pickle liquor without cooling. Thus, according to the process of the 
invention, the heat which is normally applied directly to the pickling 
bath can be applied at the reboiler tank 37b with substantially the same 
heating effect in the pickling bath and still derive the benefits of 
recovery of unused pickle acid and concentration of the solution 
containing the metallic salts of the pickling process (here the metallic 
salts are removed in solid state). 
While sulfuric acid is a common pickling acid and was thus discussed in 
detail above, other nonvolatile acids are not intended to be excluded. 
Also other metals than ferrous metals are not intended to be excluded. 
Thus, for instance, the modified embodiment as shown in FIG. 3 would 
contemplate an aluminum pickling process using phosphoric acid as the 
pickling acid. 
In addition to those metal mentioned in pickling processes hereinbefore 
described, other metals such as cobalt, copper, lead, magnesium, nickle, 
silver, titanum, zinc and alloys thereof are also contemplated by the 
invention. Likewise other acids or mixtures of acids than those mentioned 
above are also contemplated by the invention, such as organic acids, 
fluoboric acid, and a mixture comprising nitric acid, hydrochloric acid, 
or hydrofloric acid. 
It is to be understood that while I have illustrated and described certain 
embodiments of my invention, it is not to be limited to the specific forms 
or arrangements of equipment, flows, and materials herein described and 
shown.