Mixing Chemicals with Soil JULlAN C . S M I T H CORNELL UNIVERSITY. ITHACA, N. Y .
Effective modification of soil properties with chemical additives requires thorough dispersion
of the additive into the soil. Field tests of chemical stabilizers have generally been less successful than predicted in the laboratory, principally because of inadequate mixing. Results of mixing tests oftraveling mixers and of stationary mixing plants are reported. T h e strength of stabilized soil i s proportional to the uniformity of t h e mix. Fpr thorough mixing of finegrained soils a stationary mixer preceded by a crushing or screening device i s required. Of the machines tested a reciprocating-conveyor continuous kneader is the most effective mixer for silts and clays.
C
HEMICAL treatment of soil requires two things-an effective additive and an effective way of getting the additive into the soil. Especially with trace additives, the chemical must be thoroughly and uniformly distributed through the soil. This mixing i s sometimes easy, sometimes not, but in any event the cost of incorporating the additive must be considered as part of the cost of the stabilization. Economic comparisons must be based on the total cost of the treated soil, including the cost of mixing and other processing, and not merely on the cost of the chemicals. In some cases mixing is so expensive with present techniques that stabilized soil could not compete with gravel even if the chemicals cost nothing. This paper presents 1. Evidence that good mixing is necessary in soil treatment 2. Good mixing criteria and methods of evaluation 3. Mixing machines now used for road-base stabilization and evaluation tests of laboratory and pilot plant mixers Need for Good Mixing
Extensive tests have shown that the performance of chemical stabilizers in the field usually fails to match performance in the laboratory. I n tests of various additives in heavy clays, for example, spot samples from test roads had strengths only 10 to 75% of the laboratory value. I n some cases rapid failure occurred in localized areas. The most probable reason was the lack of adequate dispersion. With portland cement, a widely used stabilizer for road bases, quantitative data on the distribution of additive are available for a satisfactory and a failed section of road. The variation of cement content with depth is shown in Figure 1 (2). The distribution was poor, especially in the failed section where the top layer contained very little cement. Even in the satisfactory section the variation in cement content was large, and much of the treated soil contained more cement than necessary. Good mixing would have prevented failure and permitted the use of emaller quantities of cement. Good Mixing Criteria
Although it is easy t o agree that good mixing is desirable, it is less easy to agree on a definition of good mixing. The only true measure of mixing is the uniformity of the product. Gases or miscible liquids can be mixed to give a single phase that is truly homogeneous; that is, a spot sample of any practical size can have exactly the same composition as any other sample. In a complex inulticomponent system of particulate solids, however, a mixture can never be truly uniform. Obviously in a pile of gravel containing Zinch stones of various kinds, one sample 10 cubic inches in size will never be identical with any other 10cubic-inch sample. Depending on the size of the particles in the mass there is some minimum size of sample which will indicate 2240
true uniformity in a perfectly mixed sample. Smaller samples will differ no matter how good the mixing may be. Mixing of particulate solids is a process of randomization, not one of arranging the components in an orderly regular pattern ( 4 ) . Thus analyses of small samples, even of thoroughly mixed material, will give results which are randomly distributed about the mean of the entire mass. The variance of the results depende on the sample size and the size of the particles in the mix (6). When the particle size is large, as in coarse gravel, a large total sample must be taken to make the variance due to sample size small in comparison with percentage of additive. With finegrained soils such as silts and clays, however, the variance caused by the effect of particle size is negligible even with very small samples. In the practical evaluation of soil mixing it is also necessary t o distinguish between highly localized variations in composition, and more gradual variations throughout the bulk of the soil. If mixing is poor, the properties of a given sample depend more on localized variations than on its total content of additive. With good mixing, the properties depend on more gradual variations in level of treatment. The choice of the distance over which a variation changes from bulk to local is an arbitrary one. For soil treatment the term “macroscopic variation” has been used t o refer to changes occurring over a distance of more than 1 inch; and i‘niicroscopic variation” to changes over distances of 1 inch or less (2). Figure 1 indicates macroscopic variations. In the products from a continuous stationary mixer, microscopic variations are a function of the effectiveness of the mixer; macroscopic variations-that is, changes in the percentage of additive in the product over appreciable periods of time-are caused by inadequacies of the feeders which proportion soil and additive. In evaluating mixer performance various empirical methods based on the performance of the treated soil have been used. A statistical method based on conditions a t zero mixing was proposed by Michaels and Puzinauskas (S). I n a later study of mixing coarse solids, Weidenbaum and Bonilla (6) used a statistical measure based on the standard deviation predicted for perfectly mixed samples of a fixed small size. The results reported here are expressed in terms of a mixirig coefficient, R, which is the reciprocal of the mixing index used by Michaels and Puzinauskas (S). The coefficient R is derived as follows. If n spot samples are taken and analyzed from a mix containing a mean fraction of additive pa, the standard deviation of the analytical results is u =
px;
d).‘
(1)
where xi = indicated content of any sample.
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
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SOIL STABILIZATION 20c
At the start of a mixing operation, before the mixer is turned on, soil and additive may be placed in the mixer without any intermingling. Under these conditions mixing is zero. If errors in analysis and the variance introduced because of particle size are both negligible, the standard deviation of a large number of spot samples a t zero mixing is ,70
= [Pdl
- 1.(0)10*6
100
80 60
(2)
40
The mixing coefficient, R , is defined as the ratio of the standard deviation a t zero mixing to the observed standard deviation
a
;20 z
w
I! This coefficient i s unity a t no mixing, and theoretically approaches infinity as the mixed product becomes more and more uniform. Commercial Methods of Soil Mixing
I n an ideal application the chemical additive would be sprayed or sprinkled over the area to be treated and would penetrate uniformly to the desired depth. Unfortunately this is rarely possible. Nearly always the soil must be excavated, blended mechanically with the additive, replaced, and compacted. These operations are done in two types of systems: 1. Traveling-plant or in-place mixers 2. Stationary mixing plants Traveling plants are driven over the area to be treated, excavating, mixing, replacing, and compacting the soil as they go. Stationary plants are usually mounted on trailers so they can be set up near the area being treated. Soil and additives are brought to the plant; mixed soil is trucked to the point of use, spread, and compacted. I n both m6thods the quantities of soil treated are very large-to 500 tons per hour per machine. In-Place Mixers. Typical in-place traveling mixers are rotary tillers and pugmills. A tiller contains sets of spring-mounted cutting blades on a horizontal rotating shaft. The additive is first spread over the surface of the soil to be treated. As the machine travels forward the blades cut out a layer of, soil and throw it backward against a hood carrying a strike-off screed a t its
0 1 -Road
Surface-
7
I
.I
E 3 W
0
5 5
0
IO
15
20
25
CEMENT CONTENT-per cent Figure 1.
Vertical distribution of cement i n stabilized soil-cement road base (2)
November 1955
IO
g *
0
(3
-
6
f 5 4 z
-
2
I
1
2
3
4
Finger-Prong Mixer, ' Picric Acid Tracer (3) Two-Arm Kneader, PX2Tracer
5
6
(3)
7
-I8
t
MIXING TIME-minutes Figure 2 Variation of mixing coefficent w i t h t i m e in mixing kaolinite in batch laboratory mixers (3, 5) Figures refer t o pounds water/pounds dry soil
lower edge. Several passes over the area are usually made to give thorough mixing. Traveling pugmills are single-pass machines. When the additive is a solid it is spread over the soil surface in front of the machine; with liquids the machines carry the additive and meter it into the soil. The Wood Roadmixer (Pettibone-Wood Mfg. Co., 6900 Tujunga Ave., North Hollywood, Calif.) is a horizontal pugmill carried endwise over the soil area, scooping up soil from a windrow and discharging it from the rear of the machine. The Madsen Road Pug (Madsen Iron Works, Huntington Park, Calif.) is similar, except that it contains a 2-shaft pugmill. The P and H Stabilizer (I-larnischfeger Corp., 4400 W. National Ave., Milwaukee 14, Wis.) both excavates and mixes. It contains four transverse horizontal shafts. The first carries cutting blades for cutting and pulverizing the soil. The second or blending rotor picks up the loose material and throws it through the metered sprays of liquid into the mixing unit, which is a 2-shaft pugmill with wide-faced paddles. The shafts turn in opposite directions rubbing and squeezing the mix. Because large stones occasionally occur in almost all soils, clearances in the mixers are not close, and the mixing blades are built to withstand heavy blows. These machines treat layers 7 to 11feet wide and 3 to 9 inches deep, a t speeds of 5 to 200 feet per minute (Table I). They are used commercially on fairly dry coarse-grained soils. They can process even very stony materials a t high rates and low cost. On wet clays they are not very effective, because the sticky soil does not discharge well and because i t is difficult to drive the machine over soft slippery areas. Stationary Mixers. Stationary mixing plants have several advantages over in-place mixers, chief of which is that they can handle wetter and stickier soil, and can operate under much less avorable weather conditions. Closer control of the proportions of soil and additive is possible in a stationary unit, and the
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ENGINEERING, DESIGN, AND EQUIPMENT
MIXING TIME-minutes Figure 3. Variation of mixing coefficient with t i m e i n mixing natural soils i n laboratory kneader
mixing action can be more positive, especially with fine-grained soils. On the other hand, a vigorous smearing action necessitates close clearances, so that the feed for the mixer must be free from large stones. In addition the large quantities of soil must be excavated and transported to and away from a stationary mixer a t considerable cost. Small quantities of soil for laboratory or small scale field tests are mixed in stationary batch mixers such as tumbling drums, mullers, finger-prong mixers, or 2-arm kneaders. For field mixing, however, the large quantities involved necessitate continuous machines such as continuous tumbling drums and pugmills, either single-shaft or twin-shaft. Other continuous mixers have been studied, although they are not used commercially. Evaluation Studies of Commercial Mixers
There have been few studies of the actual mixing effectiveness of traveling mixers. Figure 1 shows what may happen with a tiller mixer under adverse conditions, but the usual performance of tillers is much better than this. Probably with coarse-grained
Table 1. Name Type mixer Forward speed, ft./min. Width of road treated, f t .
soils and traveling mixers the macroscopic variations in the product are small and depend almost entirely on the method of proportioning the additive to the feed. Microscopic variations in a coarse soil may be very large, regardless of the effectiveness of the mixer. Studies have been made of the effectiveness of various stationary mixers, both batch and continuous, using the mixing coefficient, R, as a criterion. Results are reported for the following mixers: Batch Finger-prong Laboratory kneader Large Clearfield muller Continuous Centrifugal disk (Entoleter) Reducing-helix extruder (Rotofeed) Reciprocating conveyor (KO-Kneader). The Entoleter mixer (Safety Car Heating and Lighting Co., P.O. Box 904, New Haven, Conn.) blends by subjecting the solids to a high centrifugal force, smearing them a t high velocity over a horizontal 14-inch disk rotating a t 1750 to 3500 r.p.m. At the edge of each disk there are two rows of short vertical pins on the upper face. I n the Rotofeed mixer (Marco Co., Saginaw, Mich.) a helical agitator rotates in a closed conical chamber, with feed entering a t the large end and discharging from the small end. The agitator helix is modified to give a recirculating and folding action to the mix. I n the unit tested the chamber was 14 inches in maximum diameter and about 4 inches in diameter at the discharge. The agitator was driven a t 228 r.p.m. by a 5-hp. motor. The KO-Kneader (Baker-Perkins, Inc., Saginaw, Mich.) contains a single shaft with inclined paddles rotating in an open trough or a closed cylinder. The walls of the mixing chamber carry stationary blades meshing with those on the revolving shaft. The shaft reciprocates with each revolution so t h a t the moving paddles pass between the stationary blades, smearing the mix repeatedly and moving i t from the feed opening to the discharge. I n the machine studied, the cylinder was 4 inches in diameter and about 4 feet long. The motor size was 10 hp.; the shaft speed was 39 to 69 r.p.m. Coefficients of Mixing. Mixing coefficients were evaluated for these machines by soil-mixing studies by mixing a small amount of a tracer with the soil, taking several 1-cc. samples a t random from the mix, and analyzing for the tracer. Tracers that have been used include picric acid on dextrose, analyzed colorimetrically (S),and radioactive tracers PSS( 5 ) and Corn (1). The results with laboratory batch mixers on various soils are shown in Figures 2 and 3. The sources and Atterberg limits of the soils are given in Table 11. Figure 2 shows some typical results with kaolinite clay in the finger-prong mixer and in the 2-arm kneader (5, 6). As mixing time increases the uniformity index rises very sharply a t first, then levels out after 3 or 4 minutes of mixing. The shape and position of the graph depend on the
Characteristics of Self-Propelled In-Place Road Mixers Seaman Pulvi-Mixer Wood Roadmixer Madsen Road Pug P and H Stabilizer
Maximum depth of treatment, inches
6
Approximate throughput tons/ hr Machine wt., lb. Motor size, hp.
1000-2oooa
Single-shaft Pugmill 7-45 Soil previously excavated & windrowed Soil previously excavated & windrowed 200
... ...
24,000 125
.
Tiller 60-200 7
2-shaft Pugmill 5-40 Soil previously excavated & windrowed Soil previoualy excavated & windrowed 200-550
2-shaft Pugmill 6-33 9-1 1
44,000 85 propulsion 170 mixing 255 total
-
7-9
200-300 55,000 280
Several pasyes usually required.
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SOIL STABILIZATION moisture content of the soil. With a soil-water ratio of 0.30 pound of water per pound of dry soil mixing is more rapid, and the product after prolonged mixing is more uniform, than when the ratio is 0.50. The complete variation of index with moisture content is complex, however; mixing is most rapid with soilwater ratios of 0 or 0.30, less rapid with ratios of 0.20 or 0.40, and least rapid with a ratio of 0.50 ( 3 ) .
Table II. Soil Sand
Properties of Soils Used i n Mixing Tests
Soil Series or Type 91% below 5 mesh 12% below 200 mesh
Limit Plastic Liquid
Source Ithaca, N. Y.
Plasticity Index
..
..
Dunkirk Mars hall Kaolinite
18 21 19 31
21 36 52 60
3 15. 33 19
Kaolinite
32
46
14
0
The lowest curve in Figure 2 shows the variation in a 2-arm kneader with a soil-water ratio of 0.51 ( 5 ) . The curve is similar to that for the finger-prong mixer but mixing is a little less rapid. This may be, because the kneader has a larger volume (500 cc. versus 180 cc.) and the kneader blades do not intermesh as do the prongs of the finger mixer. Similar graphs for four natural soils in a 2-arm kneader are illustrated in Figure 3. As the soil becomes more plastic mixing becomes slower. After about 4 minutes of mixing the increase in mixing coefficient becomes very slow. Ultimately, however, all the graphs probably reach a common asymptote. Typical mixing coefficients found with several soils in various batch and continuous mixers are given in Table 111. These indexes for the continuous mixers were computed from 36 spot samples taken from all the soil discharged in a 5-minute period. For comparison the index was measured of crudely hand-mixed soil with the additive still plainly visible; it ranged from 7 to 12. The data in Table I11 indicate a muller is very effective with sands but much less effective with fine-grained soils. A centrifugal disk mixer is not effective with soils. The product from the Rotofeed mixer was not highly uniform; that from the KoKneader was very thoroughly blended. The product uniformity from both the Rotofeed mixer and the KO-Kneader increased as the soil became more plastic. The indexes reflect macroscopic variations in the feed rates of soil and additive during the runs as well as any lack of homogeneity in small volumes of the product.
Table 111.
Mixing Coefficients,
Batch Mixers, 3-Min. Mix 2-Arm Soil kneader Muller Sand 100 125Q Silt 67 29.4 Dunkirkclay 28.5 26.3 Marshall clay 20.8 16.7 a 1.5-min. mix.
R, with Natural Soils
Continuous Mixers Centrifugal Reducing Reciprocating disk helix conveyor 15.9 38.6 22.2 20.4 45.5 18.2 22.7 50 27.8 83
...
...
Power Requirements. A good mixer must produce a uniform product with a minimum power requirement. Michaels and Puzinauskas (3) showed that efficiency of energy utilization in mixing kaolinite depends strongly on the soil-water ratio. The efficiency is highest with either bone-dry soil or very wet, almost liquid soil. With clay a t its plastic limit mixing is rapid, but larger amounts of energy are needed than a t other moisture contents. I n many practical problems, however, it is not possible to select November 1955
the moisture colitent for maximum efficiency, and the mixer must handle the material in its most tenacious condition. Measurements were made of the power requirement of the KO-Kneader with various soils a t throughput rates between 200 and 750 pounds per hour and various conditions of plasticity. The power load on the empty machine was 2 hp.; the increase when soil was being processed was always less than 0.1 hp., or not more than 1 hp.-hour per ton of soil mixed. Michaels and Puzinauskas report a maximum value for good mixing ( R = 100 or more) or kaolinite in its most plastic condition of 1.7 hp.-hour per ton. Some soils are more tenacious than kaolinite, but with nearly all soils the power load on a large commercial mixer would not be excessive.
0 Figure 4.
5 IO 15 M I X I N G TIME- minutes
20
Strength of soil-cement versus mixing t i m e (2)
Traveling in-place road mixers use much less power for mixing. The mixer of the Madsen Road Pug requires about 0.5 hp.-hour per ton. The Wood Roadmixer supplies 0.75 hp.-hour per ton, both for mixing and for propelling the vehicle. The P and H Stabilizer utilizes 1 hp.-hour per ton for propulsion, excavation of the soil, and mixing. Thorough mixing of dry free-flowing soil needs less power than that of wet plastic soil. For example, good mixing of bone-dry kaolinite may be achieved with an input of only 0.04 hp.-hour per ton (3). Performance of Product. The practical measure of the effectiveness of a mixer is the performance of the product. An evaluation method used by Clare ( 2 ) is based on the comparison of the strength of mixed samples with the strength of “well mixed” samples from a laboratory mixer. The strength of soilcement specimens made in a batch laboratory kneader increases with time to a maximum, as shown in Figure 4 ( 2 ) . Samples taken from a field mixer are divided into two parts: From one part test specimens are molded directly; the other part is given an additional 10 minutes of mixing in the kneader before molding. This method is useful but provides no real correlation between strength and uniformity. Since the strength and mixing coefficient vary in much the same way with time in a batch mixer, a correlation between them
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ENGINEERING, DESIGN, AND EQUIPMENT soils, a high degree of uniformity on a microscopic scale is required for good performance. Here the expense of crushing and thorough blending would be justified. Conclusions
Mixing is a major problem in the chemical treatment of soils, especially with plastic soils and trace additives. For use in the field a mixing method must be adaptable to very large quantities of soil of widely varying consistencies and moisture contents, and must allow for the presence of occasional large stones in the soil. A high degree of uniformity] both macroscopic and microscopic, usually means good performance of the treated soil; however, i t is not always economical or necessary to have high uniformity. Excessive amounts M I X I N G COEFFICIENT- R of power are not required to achieve high Figure 5. Strength of soil-cement versus mixing coefficient (I) uniformity, even in plastic soils, in the appropriate mixers. For treating heavy soils, especially with would be expected. The relationship between the strength of trace additives, a stationary mixer is preferable to a traveling soil-cement and the mixing co-efficient was studied by Baker ( I ) , mixer. Precrushing of the soil fed to a mixer with close clearances using radiactive Cow as a tracer additive. His results are given is needed, and good control of the feed rates is important. With in Figure 5. There is a good correlation between strength and heavy soils, mixers that repeatedly smear the mix are more efmixing coefficient. The highest strengths and largest coefficients fective than those that mix by tumbling or tossing. Reciprocatwere obtained with thorough hand mixing. ing-conveyor continuous kneaders are the most effective mixers Mixer Accessories. The macroscopic uniformity of the product available for highly plastic soils. from a continuous mixer depends on the uniformiBy of the feed Development of effective mixing equipment and ‘accessories rates. With in-place mixing of solid additives such as cement, should improve the performance of such additives now in common common practice is to spread the additive over the area with use as cement, and probably is essential for the succeseful use of rakes or with a distributing machine. Both methods lead to some many newer chemical additives for soil treatment. nonuniformity ( 8 ) . Liquid additives are fed t o traveling mixers a t controlled rates. With stationary mixers the feeds of both Acknowledgment soil and additive can be controlled, although feeding sticky clay a t a constant rate is not easy. In clay working plants a feeder The mixing studies with natural soils were conducted a t consisting of a hopper with two parallel helical conveyors atCornell University under the sponsorship of the Research and tached to the bottom is standard equipment, and would be adaptDevelopment Laboratories, Corps. of Engineers, United State8 able to the chemical treatment of soil. Army. Thanks are due to V. H. Rodes and J. H. Reynolds of Stationary mixers with close clearances must be protected E.R.D.L. for their technical assistance, and to Baker-Perkins, from stones larger than l/* or */, inch. With coarse dry soils it Inc., Saginaw, Mich.; Marco Co., Saginaw, Mich.; Safety Car is possible to screen larger stones, but with plastic soils the only Heating and Lighting Co., New Haven, Conn., manufacturers feasible method is to crush the soil before it enters the mixer. of mixing equipment. The Marshall clay was obtained through This may be done, even with wet sticky clays, in a 2-roll clay the courtesy of D. T. Davison and John B. Sheeler of Iowa State disintegrator, also standard equipment in clay working industries. College, Ames, Iowa. Economic Limit of Mixing. Despite the influence of uniformity on strength, a high degree of uniformity may sometimes Literature Cited not be economical or even necessary. Clare ( 8 ) has shown that (1) Baker, C. IT.,Jr., “Strength of Soil Cement as a Function of a certain amount of untreated clay in soil cement has almost no Degree of Mixing,” presented at 33rd Annual Meeting, Highadverse effect on performance, provided the untreated lumps are way Research Board, Washington, D. C., January 1954. not too large or too numerous. Where the function of the additive (2) Clare, K. E., “Some Problems in Mixing Granular Materials is to upgrade the soil-that is, to increase its apparent particle Used in Road Construction,” presented at Public Works and size from that of clay to that of sand or gravel-thorough and Municipal Services Congress, London, November 1954. complete dispersion of the additive is obviously unnecessary. (3) Michaels, A. S., and Pueinauskas, V., Chem. Eng. Progr., 50, The added expense of thorough blending, especially if precrushing 604 (1954). is required, would show little or no return in the form of improved (4) Quillen, C. S., Chem. Eng., 61, KO. 6, 178 (1954). (5) Van Soye, C. C., and Williams, P. M., Senior Research Thesis, performance. Cornell University, Ithaca, N. Y., June 1954. On the other hand (Figure 1) some mixers overtreat parts of (6) Weidenbaum, S. S., and Bonilla, C. F.;Chem. Eng. Progr., 51, the mix, so that more total additive than necessary is used. 27-5 (1955). Greater macroscopic uniformity would save considerable additive. ACCEPTED September 2, 1955. RECEIVED for review February 11, 1955. With many chemicals, especially trace additives in fine-grained
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