THICKENING CALCIUM CARBONATE SLURRIES E. W. COMINGS University of Illinois, Urbana, Ill. calcium carbonate was selected for this purpose, and the slurries were prepared by suspending it in city water. Only one slurry concentration was used.
A continuous glass thickener, 8 inches (20 cm.) in diameter, was fed with a slurry of 45 grams per liter of precipitated calcium carbonate and allowed to reach a steady state. Concentration gradients through the thickening zone clearly show the effects of both the rate of flow of thickened sludge and the concentrating action of the rakes on the depth of the thickening zone and the concentration of solid in the thickened sludge. A thickener will produce a cloudy overflow either when its capacity is exceeded by too high a feed rate or when its underflow rate is reduced below the minimum required to discharge the solid fed to it. The concentration gradients show these operating limits to be distinctly different. Curves of height D S . time from batch settling tests are not sufficient for the design of continuous thickeners except when the slurries are noncompressible.
Batch Settling
A freshly prepared slurry containing 45 grams per liter of calcium carbonate was allowed to settle in a 2.4-inch (6.0-cm.) i. d. glass cylinder. A layer of clear water appeared a t the top, and a sharp line of demarcation separated this layer from the rest of the slurry, The progress of this line as it moved down the cylinder is shown in Figure 1 as a graph of height us. time. The carbonate soon began to pile up a t the bottom of the cylinder and formed a compression zone. The top of this pile was indicated by a poorly defined region an inch or more thick crossed by vertical lines through which water escaped from the upper part of the pile where, as shown below, the carbonate concentration increases rapidly with depth. The increasing depth of this pile, as the suspended solid falls on it from above, is shown as the lower line of Figure 1. The lower layers of this pile are continuously being compressed by the weight of the upper layers, but the new solid falling from above causes the pile to build up. When all ,of the solid has settled onto the pile, the two curves in Figure 1 combine, and the pile slowly compacts until it reaches an equilibrium height. The rate of settling, as determined by the upper line of demarcation, decreased suddenly when this line coincided vith the top of the pile, and from this point the rate was much less than in the initial period. After the above test the slurry remained in the batch settling cylinder for a few days and was then thoroughly mixed and the settling cupve again determined. Repetition of this procedure several times gave settling curves which varied to a considerable extent and indicated that the settling properties varied from one time to another. Portions of slurry of the same composition used above were then taken from the feed tank of the continuous thickener, described below, and settled under batch conditions in similar glass cylinders. These portions of slurry had been subjected to prolonged agitation by the paddles in the feed tank. The settling curves are shown in Figure 1 for portions taken a t intervals of 7, 9, 26, and 47 days, and indicate satisfactory reproduction of settling properties. Portions of the latter slurry, taken from the feed tank of the continuous thickener, were also settled in 2.25-inch (5.7cm.) i. d. glass cylinders. When the upper line of demarcation had settled to a predetermined level, samples of the slurry in the tube below this line were removed and analyzed for carbonate. Two 0.125-inch (0.317-cm.) 0. d. brass tubes were used to collect the samples. They were passed through short guide tubes of glass held in a rubber stopper a t the top of the settling cylinders. Samples were drawn alternately from one tube and then the other, working down from the top to avoid disturbing the lower layers. After a tube had been lowered into the sampling position, enough slurry was sucked through it to displace the liquid in the tube completely, and approximately 10 cc. were then collected in a separate bottle for analysis. After samples were taken, the slurry was discarded and a new portion was settled to another height and
HICKENERS are used industrially to reduce the amount of liquid in slurries containing such materials as calcium carbonate, gypsum, metallurgical tailings, and sewage. Coe and Clevenger ( I ) and later Deane ( 2 ) described four zones which appear in the batch settling process. B , C, and D ( 3 ) . Considering a These zones are designated -4, vessel in which a slurry is being settled, zone A is a layer of clear liquor a t the top of the vessel. Zone B is a layer in which the concentration of solid is relatively constant with depth. For slurries which are not too dilute, a distinct line marks the boundary between zones A and B. The height of this line as a function of time is termed the “batch settling curve”. Below zone B is transition zone C, followed by compression zone D a t the bottom of the vessel. It is in this lower zone that thickening takes place. From data on thickening metallurgical pulps, Coe and Clevenger concluded that the only requirement for thickening these pulps was sufficient time for the liquid to flow from the interstices between the solid particles, and that the depth of solid in the thickening zone was not important. For this special case they derived a formula for determining the volume of a continuous thickener from batch settling data. Few published data are available on the operation of continuous thickeners, and these only give compositions of the feed and underflow. This paper reports data for batch settling and for continuous thickening and discusses the relationship between the two. The method of preparation and the previous treatment of slurries influence their settling and thickening properties (4,6). To minimize variations in these properties, the solid in suspension should be stable and not susceptible to changes with time. A commercial grade (Snowtop) of precipitated
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I FIGURE1. BATCH SETTLING CURVES The lower curves compare the settling behavior of a freshly prepared slurry with one which was agitated in the feed tank for various lengths of time. Diameter of settling cylinder, 2.4 inchea (6.0 om.). The upper curve is for a slurry containing 65 grams calcium carbonate per liter found in the up er part of the continuous thickener in run 16 a n 3 allowed to settle in lace at the conclusion of the run. Diameter of)aettling cylinder, 7.62 inches (19.3 om.).
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sampled. The analysis was carried out on 3-cc. aliquots by neutralizing with hydrochloric acid, boiling, and back-titrating with sodium hydroxide, using methyl orange as an indicator. Figure 2 gives the curves of concentration us. height obtained in this manner when the initial height of the slurry was 44 inches (112 cm.), and it had been allowed to settle to heights varying from 35 to 6.25 inches (89 to 15.9 cm.) before the samples were taken. Ten t o fifteen minutes were required for taking samples, and during this time the upper line dropped only 1 to 1.5 inches (2.5 to 3.8 cm.). As a check on the accuracy of the data, the same amount of slurry was used in each test, and the weight of solid is therefore always the same. This weight in grams was calculated from the data by graphical integration. Table I gives the calculated weight for each test. It averaged about 10 per cent below the value for 44 inches before any settling took place.
element of solid in the zone. While the pile is undergoing compression, the force of gravity on this element of solid is balanced partially by support from the semirigid solid below it and partially by the fluid friction from the liquid being forced out of the lower layers of solid and flowing upward around it. When settling is complete, the weight of this element is supported entirely by the solid below it. The lower layers are thus subjected to greater pressure, and if the pulp is compressible, these lower layers will be more firmly compacted. The weight of the water above is not effective in compressing the element since this is hydrostatic pressure and is exerted within the element as well as at its boundaries.
WEIGHT OF SOLID FOR EACH TEST TABLEI. CALCULATED Height of batch settling line Inches Centimeters Cslcd. weight of solid in cylinder, gram
44 112
35
30
25
89 76 64
20 51
15 38
10
25
6.25 15.88
120 107 113 111 108 103 105 107
Comparing the curves in Figure 2 with Coe and Clevenger’s description, the clear layer of liquid above the settling line is zone A . From this line down through the region of uniform concentration is zone B. The lowest region, in which the concentration is increasing rapidly with depth, is zone D, the compression zone. Zone C is not well defined in these tests. There is little change in composition with height in zone B and only a slight increase with time. The solid particles or groups of particles fall through the liquid in this zone in a loosely knit mass. Some classification evidently takes place with the larger or heavier particles falling through this mass and accounting for the decreasing rate of settling with time. Without this classification, settling in this zone would take place a t a constant rate equal to the terminal velocity for the loosely knit masses of solid. I n the compression zone the concentration increases from the top down, and the deeper the pile, the higher is the concentration in the bottom layers. A generalized picture of the forces acting in this zone is obtained by considering an
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FIGURE 2. CONCENTRATION GRADIENTS IN BATCH SETTLING Initial height of slurry 44 inches (112 o m . ) ; diameter of. settling oylinder, 2.25 inches (5.7 om.); ultimate height of settling h e , 6.25 inohes (15.9 om.).
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at the periphery and was siphoned into the overflow trough and thence flowed by gravit to a storage tank. The rake consisted of short, brass, angL strips fastened to a rotating crossbar, The speed of the rake was constant in all the tests a t 1.1 r. The equipment was operated continuously and was shut only for repair. Several days were sometimes required before a steady state was reached in the thickener; during this time the amount of solid in the unit changed and resulted in a concentration in the storage tank that differed from the feed concentration. The underflow and overflow streams were mixed in the storage tank, the after their composition was adjusted to 45 grams per liter, the mixture was returned to the feed tank. The establishment of steady-state conditions was indicated by a calcium carbonate balance on the thickener consistent to within 5 per cent and by the thickening line at the top of the compression zone r e maining stationary. After a steady state had been reached, samples were collected in the same manner as for the batch tests except that a sin le sampling tube was used. The tube was fitted with a collar ancfthumb screw, which was set at the distance the sam lin tube was to pass through the guide tube, the top of the g u i g tute acting as a datum level.
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Effect of Underflow Rate FIGURE 3. FLOW DIAGRAM O F CONTINUOUS THICKENING APPARATUS
The curve in Figure 2, taken when the settling had progressed t o within 6.25 inches (15.88 cm.) of the bottom, shows the concentration gradient after the cylinder had been allowed to stand for several days and had reached equilibrium. The concentration definitely increases with depth, indicating that this pulp is compressible and that when thickening t o a minimum liquid content the depth of the compression zone is important as well as the time required for the liquid t o be squeezed out of the zone. On the other hand, for pulps which settle to a relatively rigid structure which will withstand considerable pressure without deforming, the depth of the compression zone will have relatively little effect on the concentration, but i t will be necessary to allow sufficient time for the water t o flow out of the interstices.
Continuous Thickening The apparatus employed in making the continuous runs is shown in Figure 3.
A series of runs was made with the feed rate held constant at approximately 60 grams per minute t o determine the effect of variations in the underflow rate. The results of this series are shown graphically in Figure 4, in which the calcium carbonate concentration is plotted against the distance below the datum level for each underflow rate. A practically clear overflow was obtained from all runs shown in Figure 4. The level of the overflow was 4.31 inches (11.0 cm.) below the datum level. From this point the concentration increased slightly down to a clearly defined line where it increased rapidly at first and then more slowly. Radial traverses several inches below this line gave no indication of variation in concentration at different points at the same level. As the rate of underflow was decreased, the height of this thickening line increased, and a more concentrated underflow was obtained. Continued decrease in the underflow rate caused the thickening line to rise t o the overflow level and a cloudy overflow resulted. This condition then represented the maximum underflow concentration which could be produced in the present thickener at this feed rate. In each run the concentration below the thickening line increased with depth along a smooth curve to a point about 49 inches below the datum level or 3 inches (7.6 cm.) above the top of the rake. This is the zone of simple compression cor-
Prepared slurry containing 45 grams of calcium carbonate per liter was introduced into the 50-gallon (190-liter) copper feed tank which was equipped with a three-paddle agitator and stationary baffles to maintain a uniform suspension. This slurry was pumped to the top of the thickener by a specially constructed This pum as well as a similar one g:?%ng the under&w of thickened sludge from 2 the bottom of the thickener, was driven by a Hi-lo pulley, gear reduction unit, and four-step V-belt pulley arrangement, which allowed continuous speed control over a wide range. The pumps were made by forming a rectangular trough on the inside of a semicircular cut in a brass block. A rubber tube lay in this trough and brass rollers passed along the tube, squeezing its contents from one end to the other. At least two rollers were in contact with the tube a t all times, eliminating the need for check valves. The flow rates controlled by these pumps were relatively constant and gave variations of not more than 3 per cent in 24 hours. Therubber tubes required changing after 3 t o 14 days of continuous service. The thickener was made of four one-foot (30.5-cm.) lass cylinders, 7.62 inches (19.3 cm.) i. d., provifed with gaskets and held together by tie bolts. The resulting column was placed in an 0 100 200 300 4 00 angle-iron frame and equipped with a feed ring, CACO CONCENTRATION, GRAMS/LITER overflow trough, and rake. The slurry from the feed tank entered the feed ring through two copper FIGURE 4. CONCENTRATION GRADIENTS IN CONTINUOUS THICKENING tubes and discharged upward into the center of The curves show the effect of variation in the underflow rate. The break in the curvea the thickener. Clear liquid overflowedintoa ring near the bottom of the thickener WV&Scaused by the rake. Overflow was clear
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responding to that found in batch settling. At the lower underflow rates another sharp increase in concentration occurred, beginning about 3 inches above the rake and extending to the bottom of the thickener. Below 225 grams per liter the experimental points fall close to a smooth curve;
FIGURE5. ZONES IN A COMMERCIAL THICKENER Curves 1,2,and 3 above the zone of rake action were obtained by moving curve 4 down keeping i t parallel to itself. The zones in an actual thickener are sho& in their positions relative to curve 3.
above this concentration the scattering is more pronounced. The solid undoubtedly begins to form a semirigid structure above this concentration, and definite evidence of channeling was noticed at times while samples were being drawn in this concentration range. The mild agitation of the rakes is sufficient to break up this structure, and the weight of solid above produces a more compact rearrangement of the individual particles. At the higher underflow rate the concentration does not become great enough to set up this structure, and the rake has little more concentrating effect than simple compression. The “picket type” thickener is designed to extend the influence of the rakes farther up into the compression zone. It is equipped with vertical arms extending upward from the rake and, for pulps of the type used here, improves the thickening action. Although the thickening which takes place in the lower zone of a batch settling process is similar to that in a continuous thickening process, the results in Figure 4 show that there are also distinct differences. First, the upper zone in the continuous process is not composed of clear liquid only but is a region in which the solids are settling under such a low concentration that the mechanism is one of free settling. Secondly, zone B of uniform concentration noted in Figure 2 is not found in Figure 4,and there is no region in which the feed concentration persists. The feed is diluted a t once by mixing with the upper layers in the thickener. Thirdly, the conditions within the simple compression zone itself are subject to wider variation in the continuous process since both the rate a t which solid is fed to this zone and the depth of the zone can be controlled by varying the feed and underflow rates. And fourthly, the action of the rakes produces a higher concentration in the underflow than can be secured a t the bottom of a batch settler. The method of Coe and Clevenger for predicting continuous thickener operation from batch settling tests was a p plied to run 19 shown in Figure 4. For this purpose slurry from the feed agitator tank was allowed to settle, and sufficient clear liquid was removed t o give a concentration of 95 grams per liter in the remaining portion. The latter was
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thoroughly mixed and allowed t o settle in a 1000-cc. graduated cylinder, and the height us. time relation determined through the compression period. The maximum average concentration in the compression zone attained a t the ultimate height was 288 grams per liter. The maximum concentration attained in the experimental thickener by simple compression alone exceeded this by 20 per cent, while the actual underflow concentration was 47 per cent greater. The above method predicted that a concentration of 288 grams per liter could be obtained with a thickener 2 inches (5 cm.) deep when using the feed rate employed in run 19. Actually ten times this depth was required. The curves of concentration vs. height in Figure 4 are similar to one another and the lower ones can be reproduced by moving the highest one down, keeping it parallel to itself, and leaving off the portion below the 49-inch level. The concentration is roughly proportional to the fourth root of the distance below the top of this zone. A series of curves obtained from this relation is shown in Figure 5 in their relative positions in a commercial thickener. The curves indicate that depths greater than 4 feet result in only small increases in the underflow concentration. Curve 4 is for the greatest depth of compression zone and corresponds with the lowest underflow rate. Curves 3, 2, and 1 for higher underflow rates were obtained by moving curve 4 down parallel to its original position. The curves in the zone of rake action were approximated from the experimental data. The zones normally present in the thickener are shown for curve 3.
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FIGURE 6. EFFECT OF FEEDRATE Run 16 shows the concentration gradients when the feed rate is increased enough to cause a cloudy overflow.
The uppermost of these is the clarification zone. A depth of from 1 to 3 feet is usually allowed to give a clear overflow. The feed is introduced below the surface under this zone and mixes with the dilute liquid in the upper part of the thickener. I n the resulting low concentration the solid settles without interference between particles to the top of the thickening zone. The thickening zone and the zone of rake action are similar to those in the experimental thickener.
Effect of Feed Rate In Figure 4 there is no zone B of constant concentration. However, if the feed rate is increased until the overflow be-
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comes cloudy, such a zone appears in the continuous thickener although its concentration is not that of the feed. To illustrate this point, the feed rate was increased from 60 to 70 grams per minute with the underflow rate approximately that of run 15. A slight increase in the height of the thickening line resulted, with the overflow remaining clear. The feed rate was then increased t o 84 grams per minute, and a definite line indicating a sharp increase in concentration rose gradually from the top of the compression zone. Free settling continued t o take place from the overflow level down to this line. After 3 days this line reached the overflow level, and the overflow became cloudy and analyzed 4.6 grams per liter. Figure 6 shows the concentrations down through the thickener under these conditions as run 16 compared with those of run 15. The concentration above the compression zone was remarkably constant t o within a few inches of the overflow, and this concentration was approximately 50 per cent greater than that of the feed. The run was continued 27 hours longer, and four additional samples were taken. They are shown in Figure 6 as triangles and indicate that the concentration had not changed in that time. This region of uniform concentration was also observed in several preliminary runs under conditions not directly comparable to run 16. At the conclusion of run 16 the feed and underflow were shut off, and the batch settling curve was determined on the material in the thickener a t that time. This is shown in Figure 1. The rate of settling was constant, probably because classification had taken place in the thickener and left a slurry of uniform particle size. This rate was somewhat less than any rate observed in the batch settling of the feed slurry prior to the compression period. A comparison of the observed batch settling rates before compression for the feed slurry with those existing above the compression zone of the continuous thickener indicates that when the feed rate tvas below the maximum for a clear overflow, free settling took place and the settling rate in the thickener was greater than for the feed slurry. When the feed rate exceeded this maximum, hindered settling occurred and the settling rate decreased immediately to less than that for the feed. The feed rate which will just initiate this hindered settling determines the limiting capacity of the thickener since i t is the maximum rate a t which the solid can reach the compression zone.
Conclusions In general, the underflow concentration from a continuous thickener is a function of both the depth of the thickening zone and the time that the solids are held in that zone. In this general case, thickener design must a t present be based on data from small experimental units or on past experience in thickening similar slurries. In the special case of noncompressible slurries the depth of the thickening zone is not so important as the time of retention in that zone. For such slurries the method proposed by Coe and Clevenger may be used to design thickeners from batch settling data. Slurries of the latter type will show little change in concentration with depth through the compression zone after the ultimate height is reached in batch settling tests. The specification of thickener height will depend on an economic balance for the installation. The principal costs will include the interest and amortization charges on the thickener itself, interest charges on the material held in the thickener, and small labor and power costs. If the thickener is one of a series in a countercurrent washing operation, the underflow concentration influences the number of thickeners required to provide a given washing efficiency. Where the liquid content of the underflow from a thickener is to be further reduced by filtration or drying, the amount of liquid to be removed by these subsequent, more costly, operations will be influenced by the concentration of solids in the underflow.
Acknowledgment The assistance of the following undergraduates is gratefully acknowledged: J. B. McCord and J. W. Latchum for construction and preliminary operation of the thickener, P. F. Flamm and L. Bennett for the concentrations gradients during batch settling, J. S. Griffith for miscellaneous assistance, and C. De Bord for analysis of samples in connection with a grant from the Sational Youth Administfation.
Literature Cited (1) Coe, H . S., and Clevenger, G. H., Trans. Am. Inst. Mining Met. Engrs., 55,356 (1916). (2) Deane, W. H., Trans. Am. Electrochem. Soc., 37, 71 (1920). (3) Perry, J. H., Chemical Engineers Handbook, p. 1349, New York. McGraw-Hill Book Co., 1934. (4) Samuel, J. O., Trans. Inst. Chen. Engrs. (London), 16, 47 (1938). (5) Stewart, R. F., and Roberts, E. J., Ibid., 11, 124 (1933).
