By Joe Leatherdale

n the metal rube pro- ducing industry, solu- tions of stronc mineral -

acids are used to chemically mill or clean metal surfaces as intermediate or finishing operations. As the metal dissolves in the process solution, there is a corresponding decrease in unused (free) acid con- centration.

Although concentrated acid can be added ro the bath to make up for the loss in the unused acid level, the metal dis- solution rate generally decreases with increasing metal concentrations. When the rate of production reaches low enough levels, the process solution must be re- placed with a fresh bath.

This batch approach. to the processing solution may suffer other shortcomings, such as variability in production rates, variability in finish quality, additional rework due to the variability in the proc- ess bath, and hazardous handling of con- centrated solutions.

In addition to these process difficulties created, additional costs are incurred to neutralke and dispose of the spent acid solution, a solution which generally con- tains a great amount of unused acid.

Acid purification systems perform con- tinuous purification of the acid process solution for metal removal. Therefore. they can offer such benefits as:

I . Uniform product quality with re-

2. Reduced chemical consumption. 3. Reduced filter cake disposal to

landfill. 4. Improved safcty conditions for em-

ployees through reduced handling of hazardous materials.

5. Reduced costs of waste handling requirements.

6. Reduced operating costs.

Historical and Technical Review

A phenomenon referred to as “ x i d retardation” involves certain ion ex- change resins, similar to those used in water softeners, which have the ability to sorb strong mineral acids, such as hydrochloric (HCI), sulfuric (H2SO4), and nitric acid (HNOJ), from a solution containing unused acid and dissolved metals (metal salts).

The process is reversible, in that acid can be desorbed from the resin with water. Acid retardation thus presents an opportunity for separating acids from dissolved metals by nltcrnatcly conrncting rhe resin with metal-contaminated acid and thcn \~:iter.

Unfurtunatelv, only small voluines (It’

solution can be purified c;ich opcracinr: cyclc. The difficulrv thcn \vas r o find ;i

\v;i;. t o c tkienrlv prows.; tlw nc:J \vrrl ioiir

duced rework.

An ion exchange process known a “reciprocating flow ion exchange,” c Recoflor”, was developed to fully exploi the abilities of ion exchange to work wit. or recover Concentrated matcrials.

The process was commercialized i . 197 1. Its design characteristics includc the use ut‘:.

I . Fine rncsh resin henlis which ir

1 equivalent volumes of conventional res- ins. The result was to improve the reac- tion kinetics of the exchange processes.

4. Countercurrent flows of feed and regenerant solutions to maximize chemi- cal efficiency and reduce operating costs.

acid retardation was commercialized. The first industrial system was commissioned in 1977 for the purification of sulfuric acid anodizing solutions. 2. Short resin beds, which reduced The initial commercial applications

The system, called an acid purification of Recoflo ion exchange were in the unit (“apu”), has been applied to numer- recovery of metal plating solutions, such

3. Fixed, fully-packed resin beds that as hexavalent chromic acid and nickel OUs acids and acid The steps in the basic “apu” process are the upstroke resulted in plug flow movement of solu- salts. In 1975. reciprocating flow ion ex- and the downstroke. tions. This reduced intermixing and dilu- change was applied tu acid retardation,

tion ot fceJ and rcgcnennt phases. and became the vchicle [hrough which TPQfiVinret I992 49

) pressure drops and equipment size. .#‘

i,

a ,

Figure 3 shows an *apu'* for nitric/ hydrofluoric acid service. The resin bed is a cylindrical vessel 30 to 60 centimeters (12 to 24 inches) in height. Bed.diameter depends on the feed flow and typically varies between 15 and 180 centimeters (6 and 72 inches).

Applications Pickling, acid dipping, descaling,

chemical milling, and anodizing are all terms used in a variety of industries which include primary steel production (carbon and specialty metals), titanium forgings, copper, copper/beryllium, and aluminum processing.

Almost any metal tube finishing proc- ess which uses strong mineral acids or mixtures thereof a n use acid purification to separate the dissolved metal from the unused acid content of the acid solution. Tk+pluLwunl\-wilb br p e n z e d tut&reh&:

1 S i o ~ s n i i i r P m P * tureehWEfl#oEiiM+

2- P i c k ) m [ t r i t e ~ f f * " E , ^ s ~ l - f u l

3 w & ~ W i f M i z i ~ i m & i ~ d

FIGURE 3 This is an ''upu"f0r nimc/hydrofruoric Md service. - soluble metal oxides) from most grades of stainless steel. Genenllv. these baths are

Ni tic/H ydrotluoric Pickling of Stainless Steels

Mixtures of nitric acid and hydrofluoric formulated with ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ( ~ ) acid (HF) are used to remove scale (in- wy+t1*5;Q~g,&(*10 to 15 percent y weight)

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FIGURE 4 This table gives typical results using an "apu" on nimc/hydiofiuoric acid service. - to 4 mean a consistent operating temperature Bath and the potential to reduce the temper- m6f*a. ature. This would result 'in reduced acid

fuming around the pickle tank as well.

Almost any metal tube finishing process which uses strong mineral acids or mixtures thereof can us acid purification to separa: the dissolved metal from the unused acid content

of the acid solution.

Generally, the pickfinpfl -

€+During the pickling process,*imn. Further, by maintaining a consistent ~ ~ ~ ~ ~ i ~ ~ ~ ~ ~ ~ n ~ & " r u m , and irickekare dissolved into bath composition a n d J . r i n l ~ t j q over the life of the bath. Incidents of u the acid. i ~ ~ ~ t a u i z a t i ~ t ~ ~ ~ ~ t ~ h ~ ~ ~ ~ ~ , , ~ c r - and over-pickling were sometim

Conccntrated nitric acid and hydro- fluoric acid must be added regularly to Thc dissolvcd k r s tevci woulJ r'

gcrs, d~~q t , , ge , soa te& thus, mqi n-zncountercd. c*l&,foF hcat,tmnsferl

maintain unused acid levels. However,

tv6Qdb(5 to 6 percent weight hy weight (tv/w)], metal salts begin to

and $imimrinp&wmintenancc associ- .c.emoval;,ancl cleaning of th; ngcrs.

r a k &nrencoF&asurution '

crystalliic out of solution, and the bath must he dumped. Frcqucntly, manual lnhor must be used to rcmove the iron crysrals from thc tank.

solved metals concentration can %be con- tinuously minninccl in the nngg of 2Qto 40 442 to 4 percent w/w). This enables the solution to be used indefinitely. Typ- ical results using the " a p " on this mixed acid service arc given in Figure 4.

Processing facilities may rcaliie krrdits e OF-am "apu': system,

including:

1;Wiminacing- the labor and hazard- ous disposal'cosrs; associated withx re- moving iron crystals from the bottom of theRicklr tub.

2. Eliminating the labor costs asso- cia& with regular dumping and refor&- larim of the pickle bath.

3~Increasdpicklingcapacity. due to the timesavings from elimination& these procedures.

Further intangibles come from im- proved safety conditions through reduced handling of hazardous materials; consis- tent, predictable pickling for consistent quality; and ease of production scheduling.

Additional savings can come from the heat exchangers and energy used in the pickle bath. Consistent pickle conditions

52 TPQIWinter 1992

With the %pv%ystem, themtaldis?

Case Study A case study performed in 1989 at

Awwise&Zuh~2L~d~uka, 3 hatch stainless stcel tube pickling opcracion in Markham, Ontario, Canada, illustrates the benefits derived from using the process.

Prior to installation of a unit, it was common practice to &mpa 20,000-liter (.%rZ8Q-gallorpb pickling tank and recharge

which had- formed ;id sett led t-thc bottom of the bath-pcrhc pickle solution could then be retumcd to the pickle tub.

Throughout these intcrvals, pickling times would increase as the dissolved metals level increased and the active concentration of hydrofluoric acid de- creased. Mitiomi*hydmfl u o k asi&was adile&u~easionsiiy whew the auwage @&ling. rime exceeded two.hour&%me nitric acid was also addcd to top off the level of the bath.

1 r%iWt%rP"i"a"&cP*~~kt=i~ t i m ~ t h & b a t h * t e m p e E a t u ~ ~ ~ ~ ~ ~ sume-. t i m e s l k d t c t 4 Q - t m 5 Q degrees.. Un- fortunately, the increased bath tempera- ture resulted in increased evaporative acid losses and objectional, unsafe working conditions around the pickling tank.

bbwas ldumpd. As discussed prev significant ferric fluoride sludgcs tend form in the pickling baths a t such hiti dissolved iron concentrations.

In fact, extensive cleaning of the tail was usually needed a t dump time to rr m w e the accumulated sludge from rll tank bottom. These sludg3s might also .n to rcducc the hydrofluoric acid bail activity over long time intervals.

In November, 1988, a few months aft[ thc "apu" was installed, the optimui pickling bath operating Farametcrs we: dctermincd and adoptcd. Based a conccms for a healthier working environ mcnt and improved bath heater operatio

ncc rcquircments,,h?y, s SCE 3% approximamly, 2!

Typical results arc shown in Figure 55 Although analysis of chromium and nick el arc not shown, other tests have corn firmed that these metals are removed iir

proportion to the iron. These data show that the "apu" ii

unused. nitric acid, whiles removing &

d S 2 r a g e r gickling time! alled, is consistenii- sfand it is no Ionget

necessary to increase the bath temperaturu to compensate for reduced pickling rat= Acid losses due to fuming and the as sociated health and safety hazards haw been eliminated.

the metal contaxninatiop.

- Generation of sludge in the pickle tank

has been reduced, thereby eliminating periodic tank dumps with the associated production downtime and labor require- ments.

Pickling quality is consistently better. Recommended analytical methods were implemented. As a direct consequence of the reduced average pickling time and improved surface finish consistency, the

t b&/Lpe&g&during the fim fivemonths ’%. afropecation,,desoite. the facLthaLche

0 f+i&aLying tim& mu1 teci in 60 percent more production. In addition, this eval- -

uation was done during the initial period, during which familiarization wi th the system and the new analytical techniques was taking place.

Pollution Abatement Nitrate and fluoride ions are difficult to

treat and are considered pollutants. With environmental laws becoming increas- ingly strict, the reduction in nitrate and fluoride levels in the final effluent is also an important benefit.

Reduction in nitrate and fluoride levels are proportional to the reduction in pur- chases of the acids.

AdaR&+instalId om 2 srainless steel tube, pickling opentiom caw o f i ~ &e: fol- lowing henefits:

FIGURE 6 While h e Pickling rimes shown here for carbon steel in sulfuric acid may not be - representative of $1 operations, they are useful for comparative purposes on a percell- barb. 54 TPWinter I992

Sulfuric Acid Pickling of Carbon Steels

Sulfuric acid is most commonly usea batch pickling operations of tubc’G

-,dumFedr wherr the dissolved iroscontt m c h a 60 g/k to’ 100 @l(& todl&percenti wcight). At higher levels, the iron 11 -

crystallix out of the solution. Care must be taken not to allow eitr

the unused acid level to get tcm high,, the bath temperature to get too low,, this will depress the solubility of the rous sulphate. Ferrous sulphate crysa tgd ta interfere with efficient. operat: %[.bear exchangers- and, ta clog, tram

,pipesand pumps, resultingin longer ma tenance. schedules for. clean-ou recharging.

In addition, the spent bath can be ((

ficult to treat. Neutralization with lii generates large amounts of unwanted (L

cium sulphate (gypsum) along with iron hydroxide sludge.

While the pickle times shown in F ure 6 may not be representative of operations, they are useful for comparat purposes on a percentage basis.

A plant using a batch sulfuric a pickle typically formulates a fresh b

b

..I.

dW!RW iron (typically sulfuric acid - 110 g/l, iron - 80 g/l). These ba th compositions can be related to minimum pickling time (see Figure 6).

The heavy line shown in Figure 6 rep- resents the operating line of the pickle bath. I t shows t h e l q

pickle times to avoid either under- or overepickling.

(point B) aa&%This represents an average improvement of 16 to 18 s or approximately 20 percent. Thus, far -a sP&p workweeR.cbuld be slrorrcnecbmfiveda).

I6.;Eop-percemurrused.acib i s used, pisklink.rima cam bg red T ? t W ~ a 2 6 h o u ~ p e coaRfeliminmane. shift BezQy.

In addition to improving productivity, the system also stabilizes the pickling

FIGURE 7 Since the "+" recovers unsued acid in solution, higher sulfuric acid concenrr~rim - can be wed economically, as shown by these typical results.

operation. Pickling rates are cpnstant when the acid and iron levels are con- trolled, which makes it easier for operators to ensure that uniform, high-quality ma- terial is produced.

Use of the "apu" on sulfuric acid pick- ling solutions can overcome these disad- vantages. Continuous bsbpwification ensurcsttrar dissolved.irorr-concennations do rrotexceed%ol"ubilitp. levels.

Since the "apu" recovers unused acid in solution, higher sulfuric acid concen- rations can be used economically. Typical results are given in Figure 7.

Aluminum Anodizing in Sulfuric Acid

aluminum tubes is the anodizing process. The tube is placed into a &b.sui&ric acid, typically at a concentration aLh.50 g/P"WB@g/./) ( 1 4 percent w/w to 22 percent wlw). The tube io made anodic, and electrical current is passed through the solution.

The result is the formation of a hard, p ~ q ~ a l u m i n m s x i d e f & n on the sur- face of the tube which acts as the pro- tective layer for the base metal.

An important part in the finishing of.

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~ ., I

As the anodizing process proceeds, 5. Processlpcorgo-gling aluminum dissotves

&, thereby reducin corrosion. A nominal i n the electrolyte re

i n dissolved aluminum

A typical operating scenario for an anodizer is to process product until the aluminum concentration reaches l#$Lto

this p&@b tk saluaom is de- canted, and made u p with fresh dilute sulfuric acid, resulting in. at concentration 06 Z g/l ta 10 glt aluminum. The disadvan- tages of this approach are:

1, The loss ofwable acid.

2. Additionar caustic consumption. to uualize excess usable acid.

3. Imrmptiomofpoduction.

9r Employees are required to handle concentrated hazardous materials.

The application of an acid purification unit to this solution provides the follow- ing benefits:

ulMlgedK srr&& acid. w h k h was last t l ” g b ~ m m W S i i r l r d . b E u m - inumunrcenHms.

2. Amequivalenn reducrion in the a u s t i c sodat fNaoH)” consumed- in, the plant%neutraIization facility. Caustic soda is considerably morr costlyt than sulfuric aci& and; a5 a: resulr; represents the major chemical savings form an “apu” system.

3. Easier control 0.6: pH in t h e plant neutralizatiom facility through the- elim- ination of 6atch“‘dumps of acid.

4. Improved product consistency by, providing consistent production conditions.

Many extruded tube facilities use a caustic soda solution to clean aluminum from the surface of the production dies. As a result, the excess acid from anodizing bath dumps can be used to neutralize the spent die cleaning solution.

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l k w + m i m d a pur-

a cleaning solutions can be regenerated us- ing a crystallization process that recovers all of the.caustic soda and produces a- saleable aluminum nihydrate byproduct.

For facilities which use both anodizing and die cleaning, an “apu” system in concert with a die cleaning regeneration system can result in significant reductions in operating costs while producing the benefits of controlled operations.

Acid Purification Unit-Limitations

The “apu” process is limited to its ability to separate dissolved metal salts from free or unused acid. X z i c L w h i c h combined wi th the?.med twform,,che metal salt is Ipt. byp product. oc uraste stream from the “apu” muststill be neutralized irr ar treatment. facility. qnd, where applicable laws, require, the metals precipitated and separated from the plant effluent. While the “apu” makes this process easier, it does not eliminate it.

Work is currently under way to recover the byproduct streams from the Uapu” system. A demonstration planc has been set up for the recovery of nitric and hy- drofluoric acid from the metal salt by- product, and work is proceeding on the recovery of a usable byproducr from an “apu” system for anodizing service.

Summary The acid purification process can help

reduce operating costs associated with acid processing solutions used in tube produc- ing operations.

The process can help provide chemi- cal savings, labor savings, reduced waste treatment costs, increased productivity, consistent finishes, reduced fuming, and employee safety.

’ &sl3 r,

/

Joe Leatherdale, P. Eng., is technical sales representative with Eco-Tec, Inc., PickennR, Onmio, C a d . O n g i d y presented at “Tube Producing and Fabricating in the Job Shop Environment” conference, March 2-4, 1992. Ft. Luuderdafe, Florida, sponsored by Tube and Pipe Fabricators AssociationlFabricators B Manufacturers Association, International (TPFAIFMA).

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