RICHARD D . HOAK, C. J . LEW IS,
ANDW. W. HODGE1
M e llo n I n s t i t u t e , P itts b u r g h , Pa.Where p ick le liq u o r h a s b e e n trea ted w ith lim e a nd th e sludge is o xid ized, th e sto ic h io m e tr ic q u a n tity o f lim e is required; b u t w h ere th e slu d g e is d isch arged to a se ttlin g basin w ith a m in im u m o f o x id a tio n , ap p roxim ately 95%
o f th e th eo retica l a m o u n t o f lim e w ill afford c o m p lete trea tm en t. L im esto n e s vary w id ely in th e ir ra te o f reac
tio n w ith p ick le liq u o r , a n d t h e ra te d ep en d s u p o n p a rticle size, ch em ica l a n a ly sis , a n d a sp ecific rea ctiv ity p ecu lia r to a particu lar lim e s to n e . T h ere is a c ritica l p a rticle size, falling b etw een 200 a n d 325 m e s h , a n d differing for d ifferent lim e sto n es, for o p tim u m r ea c tio n b e tw e en th is m a teria l and pickle liq u o r. W here m a g n e siu m ca rb o n a te is pres
en t in lim e sto n e in excess o f a b o u t 2% , th e r a te o f rea ctio n
w ith p ick le liq u o r is rou g h ly in versely p rop ortion al to th e p ercen ta g e o f th is c o n s titu e n t. D o lo m itic lim e sto n e s are p ra ctica lly u sele ss for p ick le liq u o r tr e a tm e n t. T h e sp eci
fic rea ctiv ity o f lim e sto n e s is an im p o r ta n t fa cto r w h ich c a n n o t b e correlated and is d e te rm in a b le o n ly by tria l.
T h e ra te a t w h ich lim e sto n e rem oves iron from p ick le liq u o r is a fu n c tio n o f th e ra te a t w h ich ferrous iro n o xi
d izes. S u b sta n tia l eco n o m y in p ick le liq u o r tr e a tm e n t c a n b e realized by u s in g pu lverized h ig h -c a lc iu m lim e s to n e , to n e u tr a liz e free acid and p recip ita te part o f th e iro n , an d lim e to c o m p lete t h e tr e a tm e n t. T h e co m m e r c ia l o p era tio n o f a lim e s to n e -lim e s p lit tr e a tm e n t is d e
scribed.
T
HE removal of the oxide film from steel, preparatory to further processing, is usually accomplished by pickling the metal in a sulfuric acid bath. This treatment results in a waste liquor which is substantially an aqueous solution of ferrous sulfate and sulfuric acid. Many steel companies can dispose of this liquor only after treating it with lime to neutralize the free acid and precipitate the iron.
Lime treatment of waste pickle liquor is expensive; occasion
ally the cost of disposing of the spent liquor is as great as the cost of pickling. Colton (2) patented a process, described by Rent- schler (5), for manufacturing a building material from the sludge produced in lime treatment. In many instances, however, no useful by-products can be recovered economically. Under these circumstances it is essential that the treating agent be utilized as effectively as possible. An appreciation of the importance of the several factors involved in the treatment of waste pickle
1 W. W. Hodge is an Advisory Fellow a t Mellon In stitute. His address is West Virginia University, Morgantown, W. Va.
liquor contributes to the most efficient operation of the process under a particular set of conditions.
Although a number of acids other than sulfuric (hydrochloric, nitric, hydrofluoric, phosphoric) are used in pickling steel, the quantities are small in comparison with sulfuric acid. Part of the experimental work reported in this paper was conducted with a straight sulfuric acid liquor, and part with a liquor containing small amounts of nitric and hydrofluoric acids in addition to the sulfuric acid which comprised the bulk of the pickling agent.
The results of this investigation should be applicable to any of the waste liquors commonly resulting from pickling.
The authors (4) presented a scheme whereby determination of the basicity factor of an alkaline agent and the acid value of a waste liquor provides a rapid method for calculating the propor
tions of the two materials which should be combined to effect a desired result. This procedure has the advantage of quickly measuring the available basicity of an alkaline agent under the conditions of the treatment and eliminates speculation
concera-Hi
554 I N D U S T R I A L A N D E N G I N E E R I N G •JoL 37, No. 6
juaa, 1945 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 555 ing the applicability of basicities calculated from chemical analy
ses. T h e basicity factor-acid value relation was used as a basis for th e experim ental w ork to be presented because extensive in
vestigation has dem onstrated th e reliability of th e procedure.
LIM E TR E A T M E N T W IT H AGITATION from th e sludges were analyzed for soluble iron.
Procedure. Seven 200-ml. portions of th e pickle liquor (acid value, 146.3 gram s sulfate per liter) were treated w ith quantities of high-calcium quicklime (basicity factor. 0.9292 gram calcium oxide per gram) ranging from 80 to 110% of th a t
natants were determ ined electrom etrically, 20-ml. sam ples were filtered, and th e filtrates were analyzed for iron. T he m ixtures
The results of these experim ents are given graphically in Figure 1. The curves show th a t, where a deficiency of lim e is used, the concentration of soluble iron first decreases; then as oxidation proceeds and th e p H falls, it increases to some equilibrium value.
This effect is more pronounced in th e sludges th a n in th e super
natants because of th e presence of lime particles which react slowly over several days in th e former. T he phenomenon is illus
trated clearly in Figure I E where th e percentage of iron pre
cipitated in the system s as a whole is p lo tted against time.
I t is significant th a t, where 95% of th e required am ount of lime was used, a phenolphthalein end p oint could be obtained 4 days after treatm ent. If an operator was using phenolphthalein as an indicator of complete treatm en t, he would obtain an end point when insufficient lime h ad been added. This reaction could be observed transitorially where greater deficiencies of lime were used and mixing was inadequate. W here th e basicity factor-acid value relation is employed, th e proportions of lime and pickle liquor can be calculated quickly, and no dependence need be placed on indicators w hich m ay yield m isleading results.
Attention is directed to th e 100% curves in Figures IB and D, ions are distributed relatively uniform ly throughout such a sludge, and if the m ixture is agitated and allowed to settle, those ions which were n o t in contact w ith th e entrapped lime particles appear in th e supernatant. A sufficiently long mixing period will result in complete precipitation of th e iron where the requi
site am ount of lime is used.
LIM E TREATM ENT W ITH Q U IESC E N T SETTLIN G In the experiments described in th e preceding section approxi
m ate equilibrium was attain ed m uch m ore rapidly th an would lagoon m ight n ot oxidize completely in several years.
T he use of th e basicity factor-acid value relation is based orf th e fact th a t treatm en t of pickle liquor w ith lime is equivalent to supplying enough calcium ion to combine w ith all of th e sulfate ion (er other acid ions) present. Oxidation of ferrous iron should result in a decrease in p H sufficient to perm it re-solution of iron
cipitate which forms is a basic salt ra th e r th an a simple hydroxide, and th e more rapidly the alkali is added, th e less basic is th e pre
cipitate. T he sulfate ion forms basic salts which are more stable th an those of other common anions. Ferric salts have an even more pronounced tendency th an ferrous salts to form basic compounds, and th e rate of oxidation of ferrous salts increases rapidly w ith th e increase in th e p H of the liquor when lime is added to it. This phenomenon has th e effect of removing from solution anions which would otherwise combine w ith calcium ions, leaving an excess of calcium hydroxide over the stoichiom et
ric requirem ent of th e liquor.
U nder these conditions it appeared possible to tre a t pickle liquor satisfactorily w ith somewhat less lime th an th e acid value of the liquor would indicate. The point was investigated by treatin g a sulfuric acid pickle liquor containing 55.75 gram s iron per liter, plus small quantities of nitric and hydrofluoric acids, in a m anner which would approxim ate th e practical operation of a lime treatm en t plant, w ith 90, 95, and 100% of the lime necessary to satisfy th e basicity factor-acid value relation.
Procedure. Three 25-gallon portions of a w aste pickle liquor (acid value, 202.6 grams sulfate per liter) were treated in 55- were added during th e mixing period in each case. T he m ixtures were allowed to stand undisturbed, except for th e removal of samples, u n til the runs were completed. Samples of sludges and supernatants were taken daily for 10 days, then every other day for about 10 days, and finally a t increasing intervals. R epre
sentative samples of th e sludges were taken by removing a cylin
der of sludge in a glass tube inserted to the bottom of each drum.
T he pH of each sample was determ ined electrom etrically, and filtrates from the samples were analyzed for iron.
T he results of these experiments are shown graphically in Figure 2. Duplicates of the 90 and 95% runs were m ade; th e results satisfactorily checked those shown for these two m ixtures in Figure 2. T he figure shows th at, when th e q u an tity of lime required by th e basicity factor-acid value relation is thoroughly mixed w ith pickle liquor, the iron is quickly and completely pre
cipitated. The single point showing iron in th e sludge after 5-day settling probably resulted from striking w ith th e sampling tube a small region in th e sludge where mixing had been incomplete.
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If th e sludge is agitated, however, to perm it oxidation to proceed, th e p H of th e sludge falls rapidly and iron appears in solution.
Figure 3 illustrates th e d a ta obtained when a portion of th e 95%
sludge, after having settled for 44 days, was ag itated w ith an elec
tric mixer.
I t is clear from Figure 2 th a t, when as little as 90% of th e re
quired am ount of lime is used, th e iron cannot be com pletely pre
cipitated, either in sludge or su p ern atan t, w ith quiescent settling.
PU LVER IZED LIM E STO N E TR E A T M E N T
Lime trea tm e n t of pickle liquor has been common for m any years, b u t th e developm ent of processes for using lim estone for th is purpose has received scant atten tio n . Lim estone would appear to be a logical alkaline m aterial for pickle liquor trea tm e n t because of its wide geographical distribution and relatively low price. I t has been used occasionally, usually by p erm itting th e liquor to percolate through beds of lum p lim estone, b u t th e results
Ho. 6
have usually been unsatisfactory. W here a s furic acid pickle liquor reacts w ith lum p limestone, th e deposit of a relatively im pervious lay®r 0 appears to be approxim ately inversely propor
tio n al to th e q u a n tity of m agnesium carb o n ate it th e radii of th eir cations. Voznesenskil, Evlonova, an d S uvorova (6) found th a t dolom ite reacted
lune. 1945
w
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 557 ard screen were analyzed chemically, u ltim ate basicity factors were determ ined w ith hydrochloric and w ith sulfuric acid, and basicity factors were calculated from th e chemical analyses.The d a ta ap p ear in T ab le I I . T he - 3 2 5 mesh fractions of th e samples were analyzed to ensure th a t u ltim ate basicities could be obtained w ith sulfuric acid in a 30-m inute boiling period so th a t these basicities could be com pared w ith those calculated from the chemical analysis of these samples.
There is a critical particle size which varies w ith different types of lim estone; am ong th e lim estones studied, this size is finer th an 200 mesh. I t is im p o rtan t to recognize, however, th a t fineness and basicity are interdependent factors. For practical purposes the ra te of reaction of lim estones increases little when particle sizes finer th a n th e critical are used. T his is im p o rtan t because a m aterial m uch finer th a n 200 mesh contributes little to th e effi
ciency of th e process, an d grinding costs increase rapidly as finer and finer screen sizes are specified. A pulverized lim estone of which 80 to 85% passes 200 m esh would be satisfactoiy, other things being equal, and in general ordinary pulverized limestones will m eet such a specification. In fact, a large proportion of the
—200 mesh m aterial will also be —325 mesh, and it is unnecessary to pay the higher price for a superfine m aterial.
Table I I illustrates th e close agreem ent betw een ultim ate basicity factors an d those calculated from th e chemical analysis.
However, only u ltim ate basicity factors agree w ith those calcu
lated from analysis. W here th e conditions of trea tm e n t require th a t a basicity factor lower th a n th e u ltim ate be used, th e error might be su b stan tial if th e factor was calculated from an analysis.
Table I I I presents sulfuric acid basicity factors determ ined on
The d ata reveal considerable physical and chemical differences between the lim estone sam ples. Portions were stirred w ith a pickle liquor to stu d y differences in th eir rates of reaction.
Procedure, One liter of pickle liquor (acid value 209.3 grams sulfate, 63.78 gram s iron) was placed in a 4-liter beaker equipped with an electric mixer. T he liquor was diluted w ith an equal volume of w ater to avoid an inconveniently high viscosity and to provide an am ple volum e for rem oval of samples. A quantity of pulverized lim estone (as received) equivalent to the acid value, based upon a one-hour sulfuric acid basicity factor, was added. Samples of th e slurries were w ithdraw n a t intervals, pH values determ ined electrom etrically, and filtrates analyzed for iron. T he m ixtures were stirred constantly from beginning to end of a run, except for th e least reactive lim estones where it was necessary for th e m ixtures to sta n d overnight. A negligible change occurred where such m ixtures stood w ithout agitation.
The d a ta from these experim ents are presented graphically in
activity of a limestone and its magnesium carbonate content.
TIME.H0URS reactivity of magnesian limestones m akes them relatively useless for pickle liquor treatm ent. T he curve for magnesium carbonate indicates th a t th e physical com bination of th e magnesium car
bonate in limestone affects th e reactivity of th e stone. T he magnesium carbonate used in this experim ent was U .S.P., and its specification perm its the sale of h ydrated basic carbonate as well as h ydrated norm al carbonate under this quality. A nhydrous carbonates are present in th e approxim ate m olecular proportions of dolomite. These experim ental curves illu strate th e effect of the specific reactivity of a given limestone. For example, com
paring limestones C and G, on the basis of fineness and basicity factor th e former would appear to be th e more desirable agent whereas in practice the la tte r is considerably more effective.
A one-hour basicity factor was arbitrarily chosen for these comparisons because it represents a reasonable mixing period in practice. W here pickle liquor is treated w ith quicklime, th e
ro 6 I N D U S T R I A L A N D E N G I N E E R I N G C H Æ M J.SX R .Y ' VoL
F ig u re 4. R e p r e sen ta tiv e P h o t o m i c r o g r a p h s o f t h e E i g h t L i m e s t o n e s
gicity facto r-acid value relation affords a convenient and accurate control; w here pulverized lim estones are used, basicity factors serve as approxinfeitions for high-calcium limestones b u t are relatively useless for magnesium stones.
L IM E ST O N E -L IM E S P L IT TREATM EN T
T he findings presented in th e preceding sections suggested th a t economies in pickle liquor trea tm e n t m ight be realized by using pulverized lim estone to neutralize th e free acid and precipitate p a rt of th e iron, and quicklime to complete th e treatm en t. Labo
ra to ry studies showed th a t such a process was feasible and, if operated under th e m ost favorable conditions, th a t th e savings from its use w ould be substantial.
A steel com pany cooperated in th e investigation b y p erm itting th e au th o rs to m ake plant-scale runs of this process in its pickle liquor tre a tm e n t equipm ent. A series of experim ental runs was m ade in w hich 800-gallon batches of pickle liquor were trea ted
in th e sam e p la n t w hich w as c u rren tly using quicklim e to provide com plete trea tm e n t. T h e tria l ru n s successfully d em o n strated th e p racticab ility of th e process an d th e com pany, a fte r a 30-day te s t period w ith its own operators, ad o p ted th e s p lit tre a tm e n t because of its econom y over th e use of quicklim e alone.
I n th e operation of th e process pulverized lim estone is added while th e liquor is a g itated rapidly, u n til th e sludge tu rn s light yellow. As all pickle liquors con tain m ore o r less ferric iron, th e addition of lim estone u n til triv a le n t iro n p re cip itates is a n in d i
cation1 th a t all of th e free acid has been n eu tralized . A g itatio n of th e slurry is continued w ith o u t fu rth e r additions u n til exam in atio n of a sam ple dipped o u t w ith a p ad d le indicates t h a t evolu
tio n of gas has p ractically ceased. I t is im p o rta n t t h a t th e b u lk 1 This is a sharp end point which is easily recognized if sufficient ferrio iron is present in th e liquor. W here a tria l shows th a t the liquor contains too little ferric iron, th e addition of oxidized sludge to th e reaction vessel will provide enough. Because tre a tm e n t ta n k s are n o t pum ped d ry , each b a tc h after th e first should contain ample ferric iron.
June, 1945 ^ I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
of the entrained carbon dioxide be expelled to prevent its subsequent reaction w ith the lime.
Only 5 -8 m inutes are required to ensure the elim ination of m ost of th e gas. M ilk of lime is then ru n in from th e lim e-slaking ta n k until a pink color is obtained w ith phenol- phthalein on a spot plate, and th e trea tm e n t is complete. A lthough phenolphthalein first turns pink a t a p H of about 8.3 and it is desired to finish th e trea tm e n t a t p H 9 or higher, usually enough tim e will elapse be
tween taking th e test and tu rn in g off th e lim e feed for th e desired result to be attain ed .
Evolution of carbon dioxide in th e lim estone treatm ent causes no difficulty. In th e p lan t operation, lim estone is added as rapidly as possible (fourteen 80-pound bags in 3 m inutes) to a reaction ta n k w ith a freeboard of less than 4 feet, an d it does n o t froth over. In stallation of an exhaust fan is desirable for winter operation to keep th e concentration of carbon dioxide in th e room a t a low level;
such a fan also removes lim estone d u st and acid m ist and th u s perm its th e ferric end point to be seen clearly. T he tim e required to trea t a b atch by th is process is no greater than when lime alone is used; in stallation of a bin for feeding bulk lim estone would reduce the time som ewhat.
CONCLUSIONS
Although a quicklim e slurry is commonly used for the purpose, th e cost of pickle liquor treatm ent dan be su b stan tially reduced in many areas under favorable conditions by em
ploying a com bination of lim estone and lime.
The economy which can be realized by sub
stituting the lim estone-lim e split treatm en t for treatm ent w ith lim e alone depends largely on the cost of pulverized lim estone. In some areas the prices of crude bulk quicklime and pulverized lim estone will be equivalent in terms of calcium oxide, and there will be no great advantage in using limestone. Fre
quently, however, pulverized lim estone can be
obtained locally a t a considerable saving; it can sometimes be purchased for 1 to 2 dollars per ton. W here limestone can be bought a t a lower cost th a n th e equivalent am ount of lime,
obtained locally a t a considerable saving; it can sometimes be purchased for 1 to 2 dollars per ton. W here limestone can be bought a t a lower cost th a n th e equivalent am ount of lime,