INDUSTRIAL AND ENGINEERING CHEMISTRY
214
crop plants for the mineral nutrients present. It should also contribute to the formation of humus, since most of the humus found in soils results from a combination of lignin with bacterial proteins. ACKNOWLEDGMENT
Vol. 36, No. 3
LITERATURE CITED
(1) Aldeifer, R. B., and Merkle, F. G., Soil Sci., 51, 201 (1941). (2) Bollen, W. B., Pacific Pulp & Pupel. Ind., 16, 30 (1942). (3) Gribbina, M. F., Ph.D. thesis, Pa. State College, 1942. (4) Partansky, A. M., and Benson, H. K., Paper Trade J., 101, 29-35 (1936).
The work reported in this paper was made possible through a fello~shipestablished at The Pennsylvania State College by the Castanea Paper Company, of Johnsonburg, Pa.
( 5 ) Peele, T. C., J. Am. SOC.Agron., 32, 204 (1940). (6) Phillips, Met et al., J. Agl.. Research, 53,209 (1936).
JOURNAL Series Paper 1156, Pennsylvania Agricultural Experiment Station.
Basicitv Factors of Limestone and Lime J
EVALUATkON AS NEUTRALIZING AGENTS RICHARD D. HOAK, C. J. LEWIS, AND WILLARD W. HODGE’ Mellon Institute of Industrial Research, Pittsburgh, P a .
A procedure is presented for determining basicity factors as a menns for comparing the relative neutralizing values of limestones and limes. This method should be useful in water, sewage, and industrial waste treatment plants. Basicity factors are applied to the neutralization of waste liquor from the sulfuric acid pickling of steel. A procedure is given for the rapid determination of the acid value of such P liquor, in terms of sulfate ion, with high precision and an accuracy of 0.2%. A nomograph is presented from which the pounds of neutralizing agent per gallon of liquor can be read directly where the basicity
factor of the agent and the acid value of the liquor have been determined. Where waste pickle liquor is neutralized with limestone or lime the settling rates of the sludges produced are similar to those of water suspensions of the neutralizing agents alone. Increments in temperature and dilution increase sludge settling rates but not enough to justify their use economically. Such rates decrease with increases in pH, but are usually determined by the requirements of neutralization. A brief summary is given of the economic factors involved in deciding whether to use limestone, quicklime, or hydrated lime.
HE basicity factor of an alkaline agent is a measure of the available alkalinity of the agent that avoids dependence upon chemical analysis, which may not reflect the true value of the substance in the application intended. Although the method of determining basicity factors is not novel, the procedure has been developed as a means for determining the relative reactivity of related agents or of the same agent in different physical states. The user can then select from a number of possibilities that agent best suited to his purpose. The application of basicity factors to an actual problem is illustrated by the treatment of spent pickling liquor. This procedure should be adaptable to ascertaining the acid value of most waste acid liquors not containing metals whose hydroxides are soluble in excess sodium hydroxide.
T
limestone and lime. Sulfuric acid was employed to decompose the samples because the spent liquor from the sulfuric acid pickling of steel is a typical acid industrial waste to demonstrate the practical application of the method. The pulverized limestone was guaranteed 100% through 30 mesh. A portion of this material was ground until it all passed 100 mesh, to obtain greater homogeneity, and samples were analyzed by the standard chemical method. The average results on three samples follow:
BASICITY FACTOR O F LIMESTONE
Basicity factors were determined on a series of samples of different particle size prepared by grinding portions of the original 30-mesh material until all the portions passed through a given standard screen.
Basicity factors are of particular utility in evaluating the available alkalinity of limestone and lime because the chemical analysis of these substances is time consuming and the results give limited information concerning the reactivity of the material. (Unless otherwise indicated, “basicity factor” will be used in this paper to represent grams of calcium oxide per gram of alkaline agent.) The method evolved for determirting the basicity factor of limestone or lime reflects the value of the substance as a neutralizing agent under the conditions of use. I n the development of the procedure, samples of Bellefonte pulverized limestone and of shaft-kiln and rotary-kiln lime produced from the same source were selected as representative of good grades of high-calcium 1 Department of Chemical Engineering, West Virginis University, Morgantown. W. Va. (Advisory Fellow, Mellon Institute).
Calcium oarbonate CaCOa Magnesium carboGate, MgCOx Metal oxides, Rz03 Acid-insoluble Moisture at 120’ C. Total Ignitionloss at l l O O o C .
92.80% 2.07 0.74 4.20
0.05 __
99.86 42.03
PROCEDURE. Weigh accurately a 2-gram sample of pulverized limestone, representative of its condition a t the time of use, into a 500-ml. Erlenme ’er flask containing 10 ml. of distilled water. Run in from a kuret the amount of 0.5 N sulfuric acid estimated to decompose the sample completely and add 30-35 ml. in excess. Boil the sample, adding water to make up evaporation loss, until decomposition appears to be complete. Cool to room temperature, wash down the inside of the flask, add two drops of phenolphthalein, and titrate to the usual end point with 0.5 N sodium hydroxide. Calculate the net acid required in terms of equivalent calcium oxidz. When several samples are run a t the same time, it is convenient to condense a part of the water vaporized during boiling by circulating cold water through narrow U-tubes inserted two
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1944
thirds of the way to the bottom of the flasks. Single tubes made from 6-mm. soft glass may be used, or a series can be made in one piece from a length of tubing. Each of the figures in Table I re resents the average of three determinations of basicity factor $r the several particle sizes. The average of six determinations of basicit factor on samples of this limestone, after calcining in a m d e a t 900" C. and slaking in boiling water by the procedure given later, was 0.5230 gram of calcium oxide per gram of sample. This value was taken to represent the maximumapossible basicity of the limestone under the conditions of c cination. Where a basicity factor is calculated from the chemical analysis the. value is 0,5334 or about 2% higher than the. actual basicity. The results in Table I were obtained by boiling the samples (except 30 mesh) until maximum basicity was realized. These findings illustrate the greater reactivity of the more finely pulverized limestones.
.
TABLE I. BASICITYFACTOR OF LIMESTONE AS GRAMS EQUIVALENT crto PER GRAMSAMPLE
Boiling Time, Hr.
30 0.4443 0.4571 0.4736 0.4855
'/2
1 l'/a 2 21/a 3
....
0,4958
Samples Ground to Pass Mesh No.: 65 80 100 0.4572 0.4798 0.4874 0.4879 0.5030 0.5153 0.5088 0.5170 0.5244 0.5211 0.5229 0.5249 .... ....
....
....
....
....
' 200
'
0.5231
.... .... .... ....
higher reactivity and total basicity than the shaft lime. This difference resulted because the rotary lime was burned at a lower and more uniform temperature, was ground to pass an SO-mesh instead of a 30-mesh screen, and was not subjected t o as much contamination during burning as the shaft lime. I t should not be concluded, however that lime from these two methods of manufacture will invar!lably exhibit this difference in reactivity. The relative effectiveness of sulfuric and hydrochloric acids in finding basicity factors were compared on a blast-furnace grade stone from western Pennsylvania. The average of three determinations of the chemical analysis of the stone and the lime made from it by calcining samples in a muffle for 4 hours at 1000" C. follows: Limestone Calcium carbonate Magnesium carbonate Metal oxides, Re08 Acid-insoluble Moisture at 120' C. Total
Lime (as CaO)
PROCEDUR~. Using a weighing bottle, weigh accurately about 1.3 grams of lime and transfer the sample to a small porcelain crucible. Place the crucible upright in a 250-ml. beaker containing 15 ml. water. Cover the beaker, heat the water to boiling, and tip the crucible to fill it with the hot water. Continue heating for several minutes to slake the lime completely. Scrub the crucible inside and outside with a rubber policeman, wash and remove it, run in from a buret the uantity of 0.5 N sulfuric acid required to re?ct with the lime, %en add 30-35 ml. in excess. Wash the mixture into a n Erlenmeyer flask, boil, make up evaporation loss with water, cool to room temperature, and titrate with 0.5 N sodium hydroxide to a phenolphthalein end point. The following data represent an average of three determinations in each case. The results are compared with those obtained when samples of the same materials were ignited in a muffle at 900' C. for 11/2 hours to eliminate differences caused by air slaking and unburned stone: Boiling Time, Hr. l/d 1/2
1 1 1/2
--Basicity Rotary 0.9340
.... .... ....
Factor of QuicklimeIgnited Shaft 0.9363 0.8674 0.8809 0.8842 0.8880
.... .... ....
Ignited 0.8891
.... ....
....
For comparison with basicity factors, available calcium oxide was determined by the usual sugar test. Three determinations on samples of the above rotary kjln lime averaged 0.8533. This result indicates some recarbonation of the lime during storage. The basicity factor includes .not only the available calcium oxide but all substances in the lime which react with acids. Although the above rotary- and shaft-kiln limes were made from limestone from the same source, the rotary lime had a
-
Acid Limestone (as CsCOa)
Upon calcination of limestone some of the calcium and magnesium oxides may combine with silica, alumina, and iron oxide to form compositions which me insoluble even in moderately concentrated acids; the available neutralizing value of the substance is thereby reduced. On the other hand, certain complex calcium and magnesium silicates in limestone may decompose when the stone is calcined. The temperature of calcination and the particle size of the product are important factors governing the availability of its neutralizing value. Basicity factors were determined on samples of rotary-kiln and shaft-kiln lime, produced from the same source of limestone, to define the difference in reactivity which may exist between these two types. The rotary kilns were fired with pulverized coal; the shaft kilns were externally fired with run-of-mine coal.
% 92.19 1.47 1.67 4.57 0.10 100.00
Lime Calcium oxide Magnesium oxide Metal oxides Acid-insoluble
% 88 11 1.28 2 84 77
__
100 00
Basicity factors were determined with sulfuric and hydrochloric acids, following the rocedure given above. The limestone was ground to 30 mest and a sample heated with excess acid for one hour just below the boiling point; the calcined limestone was heated with excess acid for 5 minutes before titrating. Results follow:
....
BASICITY FACTOR OF LIME
275
&SO4
HC1
HC1
Time 1 hr. 1 hr. 5 min. 5 min,
Basiaity 0.5726 0.9185 0.6168 0.8781
These results illustrate the greater ra idity with which basicity factors may be obtained with hydrochroric acid. Any common acid can be used in the method, provided it is not excessively volatile, but the choice of acid should be governed by the application intended. Basicity factors determined for a given boiling period will obviously vary with the same strength of different acids and with different strengths of the same acid unless the ultimate basicity is obtained. I n the procedures given, boiling was continued until the ultimate basicity was realized, but it is evident that the method provides a means for establishing the relative reactivity of lime or limestone under any given set of conditions. The basicity factors determined by this procedure would provide a rapid means for comparing the reactivity of limes employed in lime-soda softening of water ( 4 ) and in the coagulation of sewage (6),as well as in the treatment of spent pickling liquoys discussed specifically here. ACID VALUE OF WASTE LIQUOR
The acid value method provides an extremely rapid means for determining the total sulfate content of a waste pickling liquor. The accurate chemical analysis of a sample of such liquor requires considerable time. Where the liquor is to be completely treated with lime, the advantages of a method for the rapid determination of the neutralizing value of the liquor are manifest. Neutralization is the combination of hydrogen and hydroxyl ions t o form water. Waste pickle liquor is a solution comprising sulfuric acid, ferrous sulfate, and ferric sulfate. Where an alkaline agent--e.g., lime slurry-is added t o such a liquor, the hydroxyl ions from the calcium hydroxide react successively with the hydrogen ions set free from ( a ) the dissociation of sulfuric acid, (b) the hydrolysis of ferric sulfate, and (e) the hydrolysis of ferrous sulfate as the hydrogen ion concentration of the solution decreases. For each equivalent of hydrogen ion uniting with hyklroxyl ion to form water, an equivalent of sulfate ion combines with an equivalent of calcium ion to form calcium sulfate. Neutralization of a sulfuric acid liquor with lime or limestone, including complete precipitation of iron, may therefore be considered a union of calcium and sulfate ions. To compute the amount of alkaline agent required to neutralize a given quantity
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
276
of acid liquor, it is then necessary to know only the total sulfate content of the liquor. A method has been developed whereby total sulfate can be determined with a n accuracy of 0.2’%,
PROCEDURE. Pipet 5 ml. of the sulfate liquor into 50 ml. of distilled water in a 250-ml. beaker. Run in from a buret 25-30 ml. of 0.5 N sodium hydroxide in excess of that required for complete reaction. Heat to boiling for 2-3 minutes, stirring t o avoid loss by bumping. Prepara a filter by placing a KO.42 Khatman (or equivalent) filter paper in a Buchner funnel, moisten, and apply vacuum. Pour about 10 ml. of a thin slurry of acidwashed asbestos on the filter, and rotate the funnel rapidly to throw the fiber to the edges of the paper and form a good seal. Turn off vacuum and pour the boiling mixture on t,he filter. Immediately begin applying vacuum gradually, taking 15-20 seconds t o turn it on full, to prevent the finely divided precipitate from passing the filter. Wash the residue with four 10-ml. portions of water. In more than fifty determinations by this method, such a washing procedure ha3 been found adequate to remove all free alkali; however, the final wash may be tested for alkali with phenolphthalein paper. Add about 1 ml. bromothymol blue for each 200 ml. of filtrate, and titrate with 0.5 N sulfuric wid to pH 7.0. Determine the end point by adding a little less than 0.5 ml. of bromothymol blue to 10 ml. of the titration mixture in a comparison tube and matching the color with the st:i:idard for pH 7.0. Calculate the net sodium hydroxide required to grams of sulfate ion per liter.
I /I
I
TITRATION O F SPENT P I C K L E LIOUOR WITH SODIUM HYDROXIDE
l i i l i l l I
2
3
4
5
6
7
8
DH
Where the acidity value is desired for application in lime or limestone neutralization, the method may be simplified by titrating the excess sodium hydroxide to a phenolphthalein end point. Use of this indicator yields results which are slightly high; but where basicity factors are determined with phenolphthalein, the results are slightly low, and in neutralizations, these effects practically cancel and cause an inapureciable error -_ in calculations. The method has been developed to provide the rapidity and accuracy required for practical work. It is imuortant that the liquor Eontah a negligible concentration of metals whose hydroxides are soluble in excess alkali. For example, a liquor which contained 5% of zinc sulfate yielded an arid value which
Vol. 36, No. 3
was about 2% high. Kumerous determinations have slionn that the method outlined is accurate to 2 parts er 1000. Owing t o the fact that in neutralizations it ifi the availagility of hydrogen ions which is important, the procedure should be applicable to most waste acid liquors. NEUTRALIZATION OF SPENT PICKLE LIQUOR
Where spent pickle liquor is treated with an alkaline agent to neutralize the free acid and precipitate all the iron, a complex of reactions occurs whose precise mechanism can be established only with difficulty. Considerable data illustrating the desirability of a simple method for determining the quantity of alkaline agent required for a given volume of pickle liquor have been collected in the authors’ laboratory. The equilibria which occur upon the addition of small increments of an alkaline agent to pickle liquor are approached the more rapidly as the concentration of iron in the liquor is decreased by dilution. As a study of equilibrium p H in an undiluted liquor mould have required too long a time, changes in pH during neutralization were followed in relatively dilute solutions. Preliminary work was carried out with sodium hydroxide to avoid complications arising from the limited solubility of lime in water. A 10-cc. portion of piclde liquor of known acid value was diluted t o about 500 cc. and stirred with an electric mixer. Small increments of 0.5 N sodium hydroxide were added; and the pH was determined electrometrically when, after each addition, the pH had reached practical constancy. The curve in Figure 1is representative of the kind of equilibrium data ,collected in a series of these neutralizations. At point A most of the free acid and all the ferric iron may be assumed to have reacted. The portion of the curve from A to B represents the reaction of most of the ferrous iron, and from B to C the rapid increase in pH as the last traces of ferrous iron are precipitated. During the titration, when point B is reached, small additions of hydroxide result in an increase of one or more pH units; but upon continued stirring the p H gradually falls to an approximate equilibrium value at which change occurs very slowly. This vertical portion of the curve represents an assumed equilibrium governing the reaction between the alkali and the ferrous iron; if all the free alkali has reacted and the ferrous hydrate formed begins to oxidize, the pH decreases at a muck lower rate. A t point C small additions generally result in a gradual increase of pH to a maximum value, although when the reaction is nearly complete, the pH may slonrly decline from its initial value. If the stoichiometric quantity of sodium hydroxide has been added, further small additions of hydroxide result in a gradual decrease of the pH from its initial value. Since a week or more is required to establish an equilibrium curve, the mixtures frequently stood overnight, and each morning the pH was found to have fallen to a point considerably below the equilibrium value obtained the night before. These decreases in p H on standing obviously resulted from the oxidation of ferrous iron at a low rate. The observed fluctuations in pH may be partly explained by the rate of oxidation of ferrous to ferric iron, which increases with the pH. But since the precipitation of certain metals, including iron, with alkalies has been shown ( 1 ) to result in the formation of compounds, even in dilute solutions which are less basic than the corresponding hydroxide, the equilibrium curves would have to be supplemented by considerable analytical data to provide an adequate explanation of the mechanism of the reaction. Such data could be obtained only by very tedious methods because of the constantly changing ratio of ferrous to ferric iron, and were deemed to be outside the scope of this investigation. Where a suspension of hydrated lime in water was titrated with pickle liquor, a curve similar to Figure 1 was obtained, and the p H data exhibited the same type of variation as those found where sodium hydroxide was used.
March, 1944
I N D U S T R I A L A N D ENa I N E E R I N O C H E M I S T R Y
An appreciation of the complexity of the reactions involved, and of the slight dependence which may be placed upon a random pH value, in the reaction between pickle liquor and an alkaline agent leads to a recognition of the fallacy of expecting to obtain efficient treatment by adding a lime slurry until, for example, a pink color is produced by phenolphthalein. Depending upon the reactivity of the lime, its rate of addition, and the thoroughness of the agitation, an operator might finish his treatment with too much or too little lime for satisfactory results. Numerous trials in the authors’ laboratory have demonstrated, where pickle liquor is treated with a quantity of lime based upon the basicity factor-acid value relation, that the acid is neutralized and the iron completely precipitated if the mixture is allowed to stand for a considerable period. Where a hard-burned lime is used, slaking may be incomplete and the heavy sludge will carry the coarser particles into a zone of relative unreactivity; in such a case it is desirable to add an excess of 1 to 2% of lime to ensure the complete prebipitation of iron. The excess lime will be most efficiently utilized if it is added after the theoretical amount has been allowed to react for 15 to 30 minutes. It is important that lime develop a maximum temperature during slaking in order that the larger lumps slake completely and the most reactive slurry result. Slaking will occur most satisfactorily if just enough water is added to the lime to produce a thick slurry during the reaction period; if the mass tends to become dry, a little more water should be added. Slaking with too little water will cause “burning”, with too much, “drowning”. Both conditions reduce the reactivity of the slurry. The slaked lime should be diluted with 4-5 parts of water, by weight, based upon the original weight of lime. A slurry of this density has been found to be the highest practicable concentration for use in pickle liquor treatment. Where the slurry is added to the liquor, the reaction time will be reduced by efficient agitation. Mixing may be accomplished mechanically or by the admission of air or live steam through perforated pipes. DOLOMITIC LIME AS AN ALKALINE AGENT
The preceding discussion of pickle liquor treatment applies particularly to high-calcium limes. Where dolomitic limes are used, the much lower solubility of magnesium hydrate, relative to calcium hydrate, is an important factor. Pickle liquor can be successfully treated with dolomitic lime if basicity factor-acid value data are used to control the process. Properly employed in acid waste treatment, dolomitic lime has the advantage of producing a smaller sludge volume than high-calcium lime. However, care must be exercised that no more than a small excess is used if the advantage is to be realized. A series of experiments was performed in which pure calcium and magnesium oxides, whose basicity factors had been determined, were combined in various proportions to represent six gradations from a high-calcium to a high-magnesium lime. Where these synthetic limes were slaked and added to pickle liquor in amounts calculated from the basicity factor-acid value relation, substantially all the iron was precipitated and the sludge volume decreased proportionately as the percentage of magnesium oxide in the limes increased. A calculated quantity of a slurry of high-calcium lime was added to two of the mixtures in which the reaction had gone to completion. The first few drops of this slurry increased the p H to about 10.5, where it remained practically constant until most of the slurry had been added. This treatment resulted in an increase in sludge volume and the almost complete removal of magnesium from the supernatant. The precipitation of magnesium hydroxide by calcium hydroxide from a solution of magnesium sulfate was checked by titrating a solution of magnesium sulfate with lime water. The titration curve showed a rapid increase in p H to about 10.5, where it remained practically constant until most of the magnesium had been precipitated.
277
These experiments demonstrate clearly that, where an excess of dolomitic lime has been used to treat pickle liquor, the calcium oxide component of the lime reacts with the magnesium sulfate ALKALINE m T Pounds per gallon
BASICITY Grams equivalent COO per qrn. somple
ACID VALUE Grams SO6 per l i t e r
L
0.3
50
0.4
so
0.5 70
0.6
0.7
80
0.8 0.9
90
1.0
100
L
I40
160 180
A Connect Scales A a n d
E
C B with a straightedge a n d read t h e result a n Scale C
Figure 2. NOMOGRAPH FOR ACID WASTE TREATMENT
which has been formed until all the excess calcium hydroxide is converted to calcium sulfate. The use of a large excess of dolomitic lime results in both a waste of lime and an increase in the sludge volume. BASIC OPEN-HEARTH SLAG AS AN ALKALINE AGENT
Basic open-hearth slag has frequently been suggested as a neutralizing agent for pickle liquor. The basicity factor was determined with sulfuric acid‘ on a typical slag sample (28.4% CaO, 7.10% MgO) ground to pass a 200-mesh screen. This value is compared with basicity factors for limestone and lime: Rotary-kiln quicklime Shaft-kiln quioklime Hydrated lime Limestone Basic open-hearth slag
0.934
0.881
0.718 0.623 0.072
This comparison shows that open-hearth slag is not an effective neutraIizing agent, even if ground to 200 mesh. I n addition, where spent pickle liquor is treated with this material, the sludge produced tends to form a hard cementlike cake. BASICITY FACTOR-ACID VALUE NOMOGRAPH
Figure 2 is a nomograph based on basicity factors and acid values, constructed as an aid in estimating the weight of neutralising agent required per gallon of pickle liquor. Where both the basicity factor of a lime and the acid value of a pickle liquor have been determined to a phenolphthalein end point and the quantity of lime required for treatment, determined from the nomograph, is added to the liquor, it will be found that the iron has been removed completely. Where the lime employed slakes incompletely, it may be necessary to add a small excess, as pointed out elsewhere in this paper.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vo!. 36, No. 3
Such an effluent could probably be discharged into a municipal sewer system without damage unless the sewage were treated in an activated sludge plant. I n order to produce a satisfactory effluent from a limestone treatment plant, it would be necessary to apply a finishing treatment with lime whereby the final p H could be increased to 8.2-8.5. SUMMARY OF ECONOMIC FACTORS
EFFECT OF N E U T R A L I Z I N G AGENT O N SLUDGE S E T T L I N G RATE
201
I
io
I 20
I
I
30 40 S E T T L I N G TIME, MIN.
I 50
I
60
SLUDGE SETTLING RATES
The objectives in the treatment of spent pickle liquor are a compact sludge and an effluent which can be discharged to a water course or to a municipal sewer without deleterious effects. Various common alkaline agents produce sludges which settle at widely different rates-an important consideration in choosing an agent for a particular purpose. The sludges formed where pickle liquor is treated with softburned lime, hard-burned lime, hydrated lime, or pulverized limestone have settling rates generally similar to the settling rates of these agents in water alone. Sludge settling rates were measured by treating equal volumes of pickle liquor with these agents and allowing the sludges formed to settle in 100-ml. graduated cylinders. Figure 3 gives curves based upon averages of several determinations for each agent. A number of measurements were made which indicate that, other things being equal, sludge settling rates decrease as the p H of the slurry increases. This finding represents another reason for avoiding an excess of alkaline agent. Where the treatment of pickle liquor is carried out efficiently, the effect of p H on sludge settling rates is not important. Increases in temperature result in increases in sludge settling rates. However, the cost of heating the slurry to attain the increased rate (10-1573 would not ordinarily be economical unless waste heat were available. Dilution of the liquor before, or of the slurry after, treatment has an adverse effect on settling rate. Although the rate is higher in the diluted suspensions, it is not sufficiently so to overcome the disadvantage of the greater depth of liquor through which the settling must occur. The sludge formed where pickle liquor is treated with pulverized limestone settles more rapidly and to a smaller final volume than that produced by lime. However, the iron cannot be completely precipitated by limestone even where a considerable excess is employed and the mixture is boiled and aerated; the effluent from such a treatment would be unsatisfactory for discharge into a stream without further treatment.
The evaluation of limes and limestones for neutralization processes involves a variety of physical, chemical, and economic factors which will be discussed briefly. Pulverized limestone is the cheapest of the materials under consideration, and it enjoys the lowest freight rate. It is readily obtainable and can be stored indefinitely in a dry place without special precautions. It presents no industrial hazards with ordinary precautions. As a neutralizing agent it produces a sludge which settles rapidly. Limestone, however, represents the lowest available basicity, and‘in order to gain full value of this factor, it must be more finely pulverized than the commercial 3O-mesh product. The price of ZOO-mesh limestone will ordinarily be double that of the 30-mesh material. The liberation of large quantities of carbon dioxide in neutralizing acid wastes may cause objectionable foaming. Also, in liquors containing ferrous iron, the carbon dioxide results in the formation of soluble ferrous bicarbonate which appears in the effluent and can be precipitated only by raising the p H to about 8.5 by finishing the treatment with lime or some other alkaline compound. Quicklime provides the highest available basicity a t the lowest delivered price. It must be slaked before use or much of its basicity will not be realized. It produces sludges with the slowest settling rates. It cannot be stored satisfactorily if exposed to air, and it is an active skin irritant. Hydrated lime occupies a position intermediate between the two materials just discussed. Its fineness (325 mesh) causes it to react quickly. It can be applied through a chemical dry feeder; but this practice is not recommended unless i t is thoroughly wetted with water before addition to a waste containing acids which form insoluble calcium salts, in order that its full basicity may be utilized. The sludge from this compound settles somewhat more rapidly than that produced by quicklime. Commercial hydrated limes exhibit a moderate range of available calcium oxide, owing to variations in the degree of hydration. The material stores reasonably well in paper bags. It is a skin irritant and may be an industrial hazard unless adequate precautions are taken. It is generally the most expensive of the three agents in terms of delivered cost per unit of basicity. ACKNOWLEDGMENT
This work was undertaken as a part of the research program of the Special Stream Pollution Committee of the American Iron and Steel Institute; R. D. Hoak is Senior Fellow of the Multiple Industrial Fellowship maintained by this institute a t Mellon Institute, Other processes for waste pickle liquor treatment have been reported by Hodge (S), and the stream pollution problem has been dealt with in general by Hoak (2). LITERATURE CITED
(1) Blitton, H. T. S., “Hydrogen Ions”, Vol. 2 , pp. 36-4d, London,
Chapman & Hall, 1942. (2) Hoak, R. D., Chem. Industries, 49, 170 (1941). (3) Hodge, W. W., IND.ENQ.CHEM.,31, 1364 (1939). (4) Hoover, C. P., “Water Supply and Treatment”, pp. 78 et seq., Washington, Natl. Lime Assoc., 1941. (5) Rudolfs, Willem, “Principles of Sewage Treatment”, pp. 57 et seq., Washington, Natl. Lime Assoc., 1941.
PRESENTED before the Division of Water, Sewage, and Sanitation ChemCHEMICAL SOCIETY, Pittaburph, istry at the 106th Meeting of the AXERICAN Pa.
