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390” C. in less than 24 hours (Figure 16). Higher temperatures will give much more rapid reaction. 6. It is imperative that water in any form be excluded from the process.
The similarity of this process to two others abandoned as uneconomical makes desirable a discussion of its potential advantages over them. Comparing our method with the Deacon process for chlorine, the following advantages seem evident : 1. The temperatures required are somewhat lower. 2. The reactions involved are substantially irreversible under the conditions for rapid completion. 3. No catalyst seems necessary. 4. A potentially important intermediate product, sodiuni chlorosulfonate, is formed. 5. The process may involve at least one less intermediate step. 6. The materials handled are inherently less corrosive. 7. Chlorine is produced in the absence of nonoondensable gases. Comparing our method with the Hargreaves process for sodium sulfate, the following advantages seem apparent: 1. Control of the process is simpler and a higher purity of product may be expected. 2. The second product of the process, chlorine, is more valuable than hydrochloric acid. 3. The materials handled are less corrosive. 4. An intermediate product of potential value is obtained.
The possible advantages of our process over the present widely used electrolytic method for the manufacture of chlorine seem t o be as follows: 1. Proximity t o cheap electric power is unnecessary. 2. There is a rapidly increasing demand for both salt cake and chlorine, while the demand for chlorine has outrun the demand for camtic soda. 3 Since the over-all process is exothermic, it is theoretically possible to eliminate power and fuel costs almost entirely. 4. For customers who can use chlorine for bleaching, sodium sulfate for sulfate pulp, or sodium sulfite for sulfite pulp, shipment
Vol. 33, No. 12
of the intermediate sodium chlorosulfonate offers interesting possibilities as a means of economical transport of chlorine and sodium sulfate. 5. The intermediate product, sodium chlorosulfonate, is potentially valuable as a sulfonating agent. ACKNOWLEDGMENT The exploratory work of Ralph Miller brought to the authors’ attention the interesting possibilities of the process. His interested criticism during the progress of the study was a valuable asset. The financial support of the Chemical Foundation made the study possible. Patents covering the use of the process will be administered by the Chemical Foundation, Inc. LITERATURE CITED Bodenstein, M., and Fink, C. G., 2. physilo. Chem., 60, 1 (1907). Brown, E. H., J . Chem. Education, 10, 119 (1933). Clemm, Brit. Patent 15,152 (1899). Deacon, Ibid., 1908 (1871). Fales, I-I., and Kenny, F., “Inorganic Quantitative Analysis”, p. 278, New York, D. Appleton-CenturyCo., 1939. I. G. Farbenindustrie, German Patent 644,222 (1937). Iler, U. 9. Patent 2,219,103 (1940). Ishikawa, Masuda, and Hagisawa, Scieltce Repts. T6hoku Imp. Uniu., 23, 164 (1934). Kelley, X. K., U. S. Bur. Mines, Bull. 406 (1937). Laury, N. A. (to American Cyanamid Co.), U. S. Patent 2,254,014 (Aug. 26, 1941). Patosz, T., and Rabek, T. J., Przemysl Chem., 14, 529 (1930). Roberts, I., IND. ENQ.CHEM., ANAL.ED., 8, 365 (1936). Rose, H., Ann. Physik, 28, 120 (1833). Salley, D. J., J . Am. Chem. SOC.,61,834 (1939). Schmidt, Brit. Patent 249,474 (1926). Schulta-Sellack, C., Ber., 4 , 109 (1871). Stephens, H., J . Am. Chem. Soc., 52, 636 (1930). Traube, W., Ber., 46, 2513 (1913). U. S. Tariff Commission, Rept. 124, 4 (1937).
PRESENTED before t h e Division of Industrial a n d Engineering Chemistry a t t h e 102nd Meeting of t h e American Chemical Society, Atlantic C i t y , N. J. Based upon a dissertation presented by A. H. Tenney in partial fulfillment of t h e requirements for t h e degree of doctor of philosophy i n t h e Faculty of P u r e Science, Columbia University.
Settling and Thickening of Aqueous Suspensions KARL KAMMERMEYER Drexel Institute of Technology. Philadelphia, Penna.
HE factors involved in the settling of suspensions can
T
roughly be divided into two groups. One group will comprise the factors actually coming into play during the settling of the particles from the original suspension and thus primarily determines the rate of sedimentation and therefore the area of the settling tank. The second group will include the factors which characterize the ultimate condition of a settled sludge-that is, the sedimentation volume-and therefore will largely determine the height of the tank. The object of this paper is t o discuss the effects of some of the factors of the second group upon the sedimentation volumes of the aqueous suspensions of calcium carbonate, barium sulfate, silica, and sludges from alum manufacture. More specifically, these factors are settling tube diameter, initial
weight concentration, slow agitation during settling (e. g., thickening), and, in connection with the latter, the effect of stirrer height. Some of the more recent investigations on the settling behavior of suspensions are those of Work and Kohler (7) and Comings (2), dealing with the settling of sludges with slow stirring, and of Egolf and McCabe (3) and Ward and Kammermeyer (6) on the quiescent settling of suspensions. It is to be expected that the ultimate settling height of a suspension will be different for the two types of settling, and a comparison of the behavior under these different settling conditions will be made in a subsequent section. It was felt, however, that there are two factors having a general bearing on the problem: (a) the effect of the tube
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P
r
-60
The behavior of aqueous Suspensions of calcium carbonate, barium sulfate, and silica was studied in settling, with and without stirring. The factors influencing the ultimate settling height, or the sedimentation volume, are primarily the initial weight concentration of the suspension and the type of settling-that is, quiescent or thickening. The relation between the ratio of HJH., (ultimate and initial settling height, respectively) and the initial weight concentration, C,,of suspensionscan be expressed by the equation H J H , = aC,",where a and n are related to particle size and compressibility, respectively. In the case of thickening, more than one equation may be needed to express the behavior; above a certain bulk concentration a decrease in packing may occur which is believed to be due to the incidence of thixotropy. Batch settling experiments with slow stirring can be considered as thickening with infinitely slow feed and underflow, and should serve as a performance standard of thickening equipment in regard to maximum attainable underflow concentrations. The degree of packing in settling, with and without stirring, can be measured by a thickening ratio. The effect of various experimental conditions was ascertained and recommendations based on the findings are made.
diameter on the settling of the suspension, in so far as wal effects are concerned, and (b) the effect of stirrer height in experiments where suspensions are being stirred while settling.
63 W
95 0
;
40
LL
W
30
Leo
2a IO 91 TUBEDIAMETER us. 8, FIGURE 1. VARYING (No STIRRING)
Tests carried out in a consistent manner with tubes having a diameter of less than 40 mm. may give comparable results. However, some experiments made with calcium carbonate suspensions (no stirring) in tubes of small diameters (i. e., 12 to 35 mm.) showed that the relation between weight concentration of the suspension and the ratio'
8. = Hu/Ho did not give a straight line on log-log paper, as was found to be the case in earlier work (6) with larger tube sizes. The slightly curved lines obtained in these tests indicated that such small size tubes should not be used. If the results obtained by different authors are to be compared] the tests should be carried out in tubes of sufficiently large diameter to eliminate wall effects. The investigations of Egolf and McCabe (a), Comings (a),Work and Kohler (7), and Ward and Kammermeyer (6) were all carried out in tubes with diamcters of 41 to 60 mm., and the reported data on calcium carbonate and silica should therefore lend themselves to direct comparisons. Such comparisons will be made in a later section. 1 Previous publications used t h e symbols So (7) and H u / H o (3,6). respectively. Suis suggested because it represents a measure of t h e ultimate sedimentation volume.
Effect of Tube Diameter Suspensions of various solids were settled without stirring in tubes having different diameters in the range 14 to 59 mm. inside diameter, and the results obtained with suspensions of calcium carbonate, barium sulfate, and silica are shown in Figure 1. The concentrations of the suspensions used in these tests were: calcium carbonate] 14.0,17.0; barium sulfate, 26.2, 36.0; and silica, 16.6 per cent of solids by weight. As was to be expected, the smaller tube diameters gave a lower degree of settling than the larger ones, and the ultimate height, H,, reached a constant value at tube diameters greater than about 40 mm. with suspensions of calcium carbonate, silica, and the higher barium sulfate c o n c e n t r a t i o n s , while the barium sulfate suspensions of lower concentration continued to show a change in packing beyond this tube diameter.
F r w DORRTHICKENERS ARRANQED FOR CONTINUOUS COUNTERCURRENT DECANTATION AT THE. GOLDCYANIDE. MILLOF NEPTUNE. MINING COMPANY I N NICARAGUA
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Effect of Stirrer Height I n the discussions of the paper by Work and Kohler (7), the question was raised as to whether the stirrer should extend all the way up through the settled sludge, or if a shorter stirrer would have been satisfactory. Definite information on this point was not available at that time; therefore i t seemed desirable to carry out some tests with varying stirrer heights.
OF STIRRER HEIGHT UPON S, FIGURE 2. EFFECT
Figure 2 shows the results obtained with an aqueous suspension containing 19 per cent calcium carbonate by weight. I n accordance with expectations, S, became constant when the ratio of stirrer height to ultimate height of suspension became equal to 1. The construction of the stirrer was of the type described by Work and Kohler, and the stirrer speed was maintained a t 2 r. p. m. In order to obtain settling with a level surface, it was necessary to have the stirring member extend as close as possible to the wall of the settling tube; otherwise the suspension settled with an uneven surface, concave upward.
Effect of Initial Height on Suspension Data presented by Egolf and McCabe ( 3 ) on lead chromate suspensions showed that S, varied with the initial height of the suspension in such a manner that closer packing-that is, a lower S, value-was obtained with a greater H, value. These tests were made without agitation. Similar tests carried out in our laboratory to check this behavior showed that suspensions of calcium carbonate, barium sulfate, and silica also gave a variation of S, with different initial settling heights. I n contrast to these findings, Work and Kohler (7) found that calcium carbonate and alumina slurries, when settled from different initial heights, reached the same value of 8, after 5-day settling with stirring a t 4 r. p. m. I n view of this discrepancy settling experiments were carried out with suspensions of calcium carbonate, barium
CONCENTRATION- WEIGHT PERCENT SOLID
FIQURE 4. SETTLING OF AQUEOUS SUSPENSIONS
80 70 60 50
40
30
20 10
0
I
1
I
I
I
I
I
I
I
I
I
l
l
1
I
l
l
I
l
FIGURE3. EFFECTOF VARYINGINITIAL SETTLING HEIGHTH a UPON S,
l
sulfate, and silica containing varying amounts of solids by weight, while stirring a t 2 r. p. m. I n each case the tests were carried out with a constant concentration and the same tube diameter, and extended over a period of 6 days. In most cases S, varied noticeably with the initial height in the same manner as in the experiments without stirring. Figure 3 shows curves obtained for the test suspensions with and without stirring; for comparison, the data of Egolf and McCabe (8) for a 1.8 per cent lead chromate suspension are included. Owing to the fact that the publication of Work and Kohler does not give sufficient data to calculate the weight per cent compositions of the suspensions, it is not possible to show any of their results for comparison. There is a possibility that a t high solid concentrations of the suspension or a t high initial
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Courtesy, The Dorr Company, Ine.
A PICKET FENCE: THICKENER
settling heights, S, becomes independent of the initial height. This possibility is indicated by the curve of a 27 per cent barium sulfate suspension, and the curves for calcium carbonate which have a rather steep slope, particularly in the case of stirring, may reach a constant value of S, a t greater initial heights. Most of the data of Work and Kohler were obtained by settling from heights of 55 to 152 cm.; this is probably the reason why their calcium carbonate slurries did not show any change in S, with different initial settling heights.
Settling with Varying Concentration An earlier paper (6) showed that suspensions settling without stirring gave a straight-line relation on log-log paper be-
tween S, and the percentage of solid in the suspensions. To see if this relation could be extended to settling with stirring, a number of experiments were carried out by settling suspensions of calcium carbonate, barium sulfate, and silica when stirring at 2 r. p. m. Typical curves for suspensions of these materials obtained by settling with and without stirring are shown in Figure 4. These charts clearly indicate that the straight-line relation also holds in the case of settling with stirring up to a certain concentration, which for the suspensions of calcium carbonate and barium sulfate are about 14 and 25 per cent solids, respectively, while the suspensions of silica do not show any break over the range covered. The curves representing settling without slow stirring remain straight over most of the concentration range investigated. An extension of these curves to the intersection with S, = 1 is not possible, however, as a break may occur at the higher concentrations as shown in Figure 4A for calcium carbonate suspensions. The intersection of this upper part of the curve with S, = 1 should give the concentration of that suspension which is stable-that is, which will not show any settling without stirring. The slope of the lines as expressed by the exponent n in the equation 8" = arc* (1) is a measure of the compressibility of the settled sludge. The actual amount of compression due to the pressure of the super-
natant sludge layers is not great when dealing with column heights of about 50 cm., but i t is sufficient to show up in the experimentally determined exponents, n; which for settling without stirring were found to be 0.946, 0.966, and 1.144 for barium sulfate, calcium carbonate, and silica, respectively. This indicates that the barium sulfate and calcium carbonate sludges were slightly compressible (n < 1) while the silica sludge was not. The values of coefficient a were found to be 0.020,0.0385,and 0.0064for barium sulfate, calcium carbonate, and silica, respectively, and as stated in a previous paper (6') are believed to be related to the average particle size of the materials under consideration. The curves representing settling of calcium carbonate and barium sulfate suspensions with stirring below the break are shifted in the direction of closer packing, as would be expected. Furthermore, the slope of these latter curves is less than that of the curves representing settling without stirring, and this change in slope is such that the effect of the concentration of the suspension upon S, is less than when settling without stirring. The values of exponent n and coefficient CY in Equation 1 for these curves were found to be 0.822, 0.014, and 0.902, 0.0243 for barium sulfate and calcium carbonate, respectively. A comparison of these n values with those obtained without stirring shows that the sludges not only settled to a lower volume but also became more compressible through the mild agitation of the stirrer. This effect can be expressed numerically by the ratios of the exponents without and with stirring-e. g. n/n8-which are 1.152 and 1.071 for barium sulfate and calcium carbonate, respectively; this indicates that the barium sulfate suspensions which were more compressible than the calcium carbonate suspensions at first showed a greater increase in compressibility than calcium carbonate when settling with stirring. Above the break in the curves the slopes changed in such a manner that the sludges packed to a lesser extent and also became less compressible. The extension of this part of the curves beyond the highest concentrations covered by the present work is not permissible until established by further experimental work.
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The exponent n and coefficient a for the settling of silica suspensions with stirring were 1.270 and 0.00295, respectively, which indicates closer packing of the still incompressible sludge. The silica suspensions gave considerable trouble in the experiments with stirring, as the sludge frequently packed to such an extent that the stirrers froze. This is the primary reason why data on high concentrations are not yet available for this material. The question arises as to why the curves representing settling without stirring do not show a similar break-that is, a change to less dense packing a t some intermediate concentration, The answer apparently is that the highest bulk concentrations attained, which correspond to the intersection of the curve with S, = 1, are not so high as the bulk concentrations reached a t the break points. The bulk concentrations under discussion are the concentrations in weight per cent in the settled portion of the sludge and can be calculated from the initial weight concentration C, (at height H J , the value of S,, and the densities of the solid and the suspending liquid. The curves without stirring (Figure 4,A and B ) show that a t S, = 1 the corresponding concentrations which represent an approximation to that of a stable suspension are about 30 and 62 per cent by weight of calcium carbonate and barium sulfate, respectively. The calculated bulk concentrations for the break points, howevcr, were 43 and 71 per cent of solids by woight, definitely higher than the former values. A wholly satisfactory explanation of the occurrence of the break in the curves representing settling with stirring is not readily available. It might be expected that the more compressible barium sulfate suspcnsions would shorn the break a t lower initial concentrations, but this is not the case (Figure 4, A and B ) . It is possible, however, that the change in degree of packing may be due to the incidence of thixotropy. Thixotropy, defined as an isothermal reversible sol-gel transformation, results in the setting up of a gel structure with the retention of a certain amount of water which, although not very rigid, probably shows some increased resistance to compression. According to Freundlich (4) and Akamatu ( I ) ,
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substances which exhibit thixotropy give large sedimentation volumes, and the decrease in packing above the break means the retention of more water and therefore larger sedimentation volumes. It is known that suspensions of many finely powdered substances exhibit thixotropy a t higher solid concentrations, and in recent work on the determination of viscosities we have found that thixotropy will occur a t concentrations of about 35 and 60 per cent by weight of calcium carbonate and barium sulfate, respectively. As mentioned previously, the calculated average weight per cent concentration a t the break of the calcium carbonate and barium sulfate curves were 43 and 71 per cent, respectively; and although these values are somewhat higher than would be indicated by the behavior in the viscosity determinations, they present a t least a qualitative confirmation. Increase in concentration results in increasing viscosities, and the more viscous medium would offer increased resistance to the flow of water from the thicker sludge; but this behavior would not explain the relatively sharp break indicated by the experimental data, and a more gradual decrease of degree of packing would be expected. The relative sharpness of the breaks can perhaps be explained as follows: The material which will be most effective in imparting thixotropic behavior has a very small particle size-e. g., one micron or less ( 5 ) . As the particle size distributions were such that only fractions of a per cent of the particles of calcium carbonate and barium sulfate had diameters of less than one micron, over-all concentrations had to be reached such as existed a t the break points, so that a sufficiently large quantity of the fine particles was present to enable them to exert their thixotropic behavior.
Thickening Ratio The effect of stirring while settling as shown in Figure 4 is primarily an increase in the degree of packing of the sludge, while a change in compressibility may or may not occur. This change in packing can be expressed by the thickening ratio, which may be taken as the ratio of S, without stirring to 8, with- stirring. If change in slope would not occur between the two types of settling but only a parallel shift of curves, this ratio would be a constant, independent of concen t r a t i o n . However, owing to the change in slope, the thickening ratio becomes a function of weight concentration. As shown by Figure 5, it can be represented by straight lines on semilog paper for weight concentrations greater than about 5 per cent of calcium carbonate and barium sulfate, and a t values greater than 10 per cent for silica. It would be expected that this ratio will also be affected by particle size distribution and temperature of settling.
Thickener Rating
A P A I R O F 2 0 0 - F o O T DORRTHICKENERS I N SQU.4RE TANKS PRECEDED B Y DORRCO FLOCCUPRECIPITATE FROM 100,000,000 GALLONS PER DAYOF SOFTENED ~ A T E R FOR LATORS REMOVE Los ANGELESA N D CITIESOF SOUTHCOASTAL PLAIN
In his publication on thP thickening of calcium carbonate slurries, Comings (2) s h o w e d t h e e f f e c t of a number of factors on thickener
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performance and pointed out that batch settling experiments do not give sufficient information for the design of continuous thickeners. The batch settling tests in question were carried out without stirring’ and i t appears that Comings’ conclusions are well founded on this basis. However, if the batch settling tests are carried out with slow stirring, the results obtained might be compared to a thickener performing at infinitely slow feed and underflow and therefore at what could be considered 100 per cent efficiency.
is of doubtful value, except for suspensions which settle rapidly; otherwise the time involved for the fiuccessive settling operations, as well as transfers of settled sludges, would be excessive. The curves, however, are presented to illustrate the graphical method, since this type of settling does not involve the considerable degree of concentration obtained in one operation under thickening conditions. For operation under thickening conditions, Figure 6 shows that with feed concentrations as low as 1 per cent, the underflow used as feed to the second unit would be such as to bring the underflow from that unit on the part of the curve showing decreasing concentrations (i. e., about 63 per cent). The significance of this phenomenon is that for optimum degree of thickening the feed to any one thickener unit should be kept below the “break c~ncentration” which is dependent on the suspended material and thought to be largely determined by the amount of very fine particles in the suspension. INITIAL CONCENTRATION- WEIGHT PERCENT SOLIDS I n practice this could readily be taken care of by adjustFIGURE 5. THICKENINQ RATIOus. INITIAL WEIGHTCONCENTRATION ing the underflow rate from any one of the thickeners, as an increase in underflow rate will decrease the underflow concentration (8). These considerations apply On this basis, batch settling tests with stirring represent a only if the suspension under investigation shows a break in standard by which the performance of thickeners can be the S, (or C,) us. C, curve and if the sludge is compressible. measured in terms of maximum attainable underflow concentration. Using this method of rating, a rough cdculation gives a value of 80 per cent underflow efficiency for the lowest underflow rate shown in Figure 4 of Comings’ publication (8) converted to a thickener height of about 50 cm. It should be pointed out that this value is only approximate, as it was necessary to compare data which had been obtained on somewhat different batches of calcium carbonate and by operation at different heights-that is, about 130 om. in Comings’ apparatus and 50 cm. in the batch settling tests. If the values of S, were independent of the initial settling height, H,,, then the bulk concentration in the sedimentation volume could be compared directly with the underflow concentration of a thickener of any height, and on this basis the performance of Comings’ thickener at the lowest underflow rate would have resulted in about 90 per cent of the maximum attainable underflow concentration. In Figure 4A, point C represents the behavior of Comings’ calcium carbonate. suspensions, and although it is somewhat below the curve established for the material used in this investigation, a comparison seems permissible.
Thickener Operation The interpretation of the S, VS. C, curves in Figure 4, A and B , obtained while settling with stirring (that is, under thickening conditions) indicates that most efficient thickening is obtained when operating a t concentrations below the break. The solid curve I in Figure 6 presents a plot of C, against C,, the bulk concentration calculated for H,; and a sharp reversion in settled bulk concentration is seen to take place at the concentration corresponding to the break in the log-log plot. Figure 6 also shows curve I1 for C,’ vs, C,,’; it is the same curve as C, vs. C,, except that the coordinate axes are reversed. This method of plotting permits the graphical stepping-off of the number of theoretical thickening units required to perform a certain degree of concentrating. The application of this procedure is seen more clearly from curves I11 and IV, which are plots of C, VS. C, and C,’ IS. C,’ for settling without stirring, These curves intersect at the theoretical concentration of the stable suspension and form a closed field within which graphical steps can be performed giving the number of theoretical settling units required t o perform a certain degree of thickening by successive settling steps. The practical application of settling without stirring
FIGURE 6. INITIAL us. FINAL BULKCONCENTRATION OF BARIUM SULFATE SUSPENSIONS
Figure 2 shows that the stirrer height had a pronounced effect on the degree of thickening’ and therefore thickener equipment should be designed in such a manner as to provide for stirring members above the rake and extending throughout the so-called compression zone of the sludge. If the bottom rake only is used, its effect will extend a certain distance up through the sludge, as shown in the figures presented by Comings (2); and the conclusions appear justified that the concentration break shown in these figures would occur at a greater distance from the bottom of the thickener and result in somewhat increased underflow concentration. This principle is employed in the picket thickener type of equipment.
Sludges from Alum Manufacture Feed and underflow samples were obtained from a threeunit Dorr thickener system employed in the manufacture of alum. Since this system is not always operated continuously,
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Required Experimental Conditions The discussions of the various factors which affect the ultimate settling height indicate that the experimental determinations should be carried out to meet a number of conditions which will eliminate or reduce variations inherent in the procedure. These conditions are as follows: 1. The size of the settling tubes should be such as to eliminate or reduce wall effects-that is, 40-mm. inside diameter or greater. It is believed that t h e 40-mm. i. d. tubing, which is available commercially at reasonable cost, can be used t o give satisfactory results with most sludges. 2. When experiments with stirring are carried out, the stirrer should extend the whole length of the settling tube, and the stirrer speed should be correspondingly lower for larger tubes to assure a thickening Courtesu, The Dorr C o m p a n y , Inc. action and avoid turbulence at the A FEWO F THE THICKENERS AT THE ALL-ANERICAN CANAL DESILT~NC PLANT, W H I C H perimeter. REMOVE 70,000 TONS O F SILT DAILYFROM 8,000,000 GALLONS O F TURBID COLORADO 3. To eliminate or reduce the RIVERWATER effect of initial settling height upon the value of S,, the settling tubes should be fairly high- that is, about 75 cm. (30 inches) or more. With many sludges, however, a height of 50 cm. will give satisfactory the concentration distribution of the sludges over the differresults. ent units is not always uniform, and the data obtained from 4. The control of temperature, which has not yet been menthe analyses of the sludges should be applied only to the pertioned, is desirable but experience indicates that fluctuations of * s o F. (2.8" C.) will not seriously impair the validity of the H , formance of the individual thickening units rather than to the determinations. three-stage system as a whole. The following table presents the data obtained on the samples:
Conclusions
% Solids Wk&t
Feed t o u n i t 1
Underflow from u n i t 1 UndrrHow Irtsrn un!t 2 UndarEow from unit 3 4
2.30
7.b8
4.54 16.30
"BB. of Liquor a t 84O F. (29' C.)
1.330 1.330 1.100 1.024
Su 0.0865
0.816 0.220 0.498
Performance, % of Attainable Underflow Conoentration
38.55 90.2 24.2 54.3
Based on actual ooncentration of underEow from unit.
The rating of 33.5 per cent of attainable underflow concentration obtained with the feed to the system is based on the actual underflow concentration from the first thickener. However, this procedure is not strictly correct as the sludge coming from the first thickener did not correspond directly to that coming in with the feed. This fact is brought out by the settling behavior of the underflow sludge which gave a rating of 90.2 per cent of attainable underflow concentration. The reason for the difference in behavior of the two sludges is that a coagulating agent is added to the feed a t a point between the sampling place and the first thickening unit, and the sludge withdrawn from the first unit contains a considerable portion of the coagulating agent. The effect of the coagulating agent is therefore twofold-to increase the rate of settling and to give a rather voluminous sludge having relatively poor packing properties. The ratings of 24.2 and 54.3 per cent of attainable underflow concentration for the second and third unit, respectively, indicate that most of the coagulating agent was washed out of the sludge in the subsequent thickening units. These ratings, aa well as the rating on the feed to the system, are rather low, and this may be due to the fact that the system is being operated close to the limit of its design capacity.
Experimental determinations of the ultimate settling height of aqueous suspensions of calcium carbonate, barium sulfate, and silica, without and with slow stirring (thickening), have shown that the relation between the initial weight per cent concentration, C,, and the ratio, S., can be expressed by equations of the type, S, = aC0" I n the case of thickening compressible sludges, more than one equation is necessary to express the results, and a break occurs a t bulk concentrations which are greater than the greatest concentration of the suspension which will be stable (that is, not show any settling) without agitation. The occurrence of the break above which less packing occurs is believed to be due to the incidence of thixotropy. The effect of stirring is to give a closer packing of the solids, and with compressible sludges an increase in compressibility is also found. Incompressible sludges of materials such as silica remain incompressible whether thickened or not. The increase in packing obtained by thickening can be expressed by a "thickening ratio" which is taken as the relation of S, without to S, with stirring for the same initial weight concentration of the suspension. This ratio is apparently a function of the initial weight concentration of the suspension. The sedimentation volume, as represented by S,, obtained in batch settling experiments with slow stirring is suggested as a standard by which the performance of thickening equipment can be expressed in regard to maximum attainable weight concentration of solids in the underflow. This concept is developed by considering the batch settling tube as a thickener with infinitely small feed and underflow. Actual operating data on sludges from alum manufacture are presented.
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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
The experimental conditions are set forth which should be observed in order to obtain the greatest possible degree of reproducibility and to permit comparisons of experimental data obtained by different investigators.
Acknowledgment Considerable credit is due to J. M. Hiscott and Carl Pacifico, students in chemical engineering, who carried out most of the experimental work. Nomenclature initial concentration, % ’ solids by weight corresponding C,, to H , C,, = concentration, o/o solids by weight corresponding to H , Ho = initial settling height Hu = final or ultimate settling height =
Su
-
n
= =
01
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exponent which is a function of compressibility constant which is a function of particle size
Literature Cited (1) Akamatu, H., J. SOC.Chem. I d . Japan, 13,456 (1938). (2) Comings, E.W., IND. ENQ.CEEX.,32,663 (1940). (3) Egolf, C. B.,and McCabe, W. L., Trans. Am. Znst. Chem. Engrs., 33,020(1937). (4) Freundlich, H., “Thixotropy”, p. 18, Paris, Hermann & Cie., 1935. ( 5 ) Stern, A. G., U. 5. Bur. Mines, Rept. Investigations 3556 (1941). ( 6 ) Ward, H. T., and Kammermeyer, Karl, IND.ENQ.C H E x . , 32, 622 (1940). (7) Work, L. T.,and Kohler. A. S., Trans. Am. Inst. Chem. Engrs., 36,701 (1940). PRasENrED
before t h e Division of Indusrrial and Engineering Chemistry
at tho 102nd l l c e t i n g of tho Arnericnn Chemical Society, Atlnntic C i t y ,
N. J.
Hu/Ho
Manganese in Deep Reservoirs EDWARD S . HOPKINS
AND
GEORGE B. MCCALL
Bureau of Water Supply, Department of Public Works, Baltimore, Md.
S
OLUBLE manganese is probably more often present in Data published in 1931 indicated that the seasonal presimpounded raw water supplies than is generally recogence of soluble manganese in waters from deep reservoirs nized. This condition may also be a characteristic of is due to the solution of this element as a bicarbonate. This material is dissolved by the relatively large concenfiltered water when stored in unlined reservoirs. Previous studies by Baylis ( 2 ) advanced the hypothesis tration of carbon dioxide present in these waters. A previous article stated that the manganese seemed to be that the dissolved manganese was leached from the underlying rock and unstripped soils. Weston (IS) stated that the leached from the rock strata underlying the reservoir, and manganese was dissolved by the action of organic and mineral that the continuous deposition of silt on the bottom of acids. An investigation by Hopkins and McCall (7) in 1931 such a reservoir would ultimately overcome this condition. Investigation during the past ten years indicates that indicated that the solution of manganese was caused by anaerobic fermentation of vegetation on the bottom of the unsilting will not overcome this phenomenon. On the contrary, newly deposited muck on the bottom of reservoirs stripped Loch Raven Reservoir, with the liberation of large will continue the trouble for many years in the future, quantities of carbon dioxide as a by-product which dissolved since the sediment contains a large proportion of decomthe manganese from the muck and underlying soil. The posed manganese-bearing vegetation. This is proved by biological activity was accelerated a t temperatures above new data obtained from studies of two reservoirs covering 20’ C. During the autumn “turnover” the water dropped different geological formations. During this ten-year period the manganese on the bottom of a very deep reservoir was practically constant each year, while the shallower reservoir was affected by seasonal “turnover”. The reservoir having seasonal turnover was completely depleted of manganese a t this period, with a subsequent resolution each succeeding year. Since removal of manganese is a coagulation problem in conjunction with water purification plant operation, the isoelectric point of manganese hydroxide has been deterINTERIOR OF MONTEBELLO FILTER PLANT AT BALTIMOR~ mined.
