A LABORATORY INVESTIGATION

OF COMPACTION OF SOLID WASTE

BY ROLLER CRUSHING

R. J. HARKER and M. A. JUDS University of Wisconsin

Madison, Wisconsin

ABSTRACT

This paper summarizes a pilot experimental program to investigate the feasibility of crushing solid waste by means of a cylindrical roller against

•

a plane carrier. Data are given indicating volumetric reductions, crushing forces, and required energy obtained in the laboratory for various metal, plastic, and glass containers. Significant compac­tions were achieved, and the process appears to hold considerable potential. No attempt was made to study food, or other organic materials.

INTRODUCTION

The large volumes of solid waste generated per capita in the developed countries has created a formidable problem with respect to the collection and eventual disposal of this material. Thus, compaction of this typically low density material becomes of paramount consideration in terms of collection vehicles and required landfill capacity.

Although solid waste is inherently heterogene­ous, it is usually composed of a large percentage of used containers which, as discarded, enclose voids. Most other components similarly contain voids of a more distributed nature. Still others, such as garbage, involve primarily soft, compres­sible materials, especially as intermixed with paper products.

A common approach to refuse compaction is the use of hydraulic press action to compress the material, as, for example, the conventional packer

blade collection vehicle. Because the compaction is usually applied simultaneously to a relatively large surface, the compressive stresses tend to be relatively low, In fact, except for any mechanical advantage present, the ratio of the unit pressure on the refuse to the hydraulic fluid pressure will be the ratio of the piston area to the contact area at the refuse.

An alternate method, which will be discussed here in detail, entails refuse compaction by means of a roller, providing an action similar to that of rolling steel. This results in high localized com­pressive stresses at the roll nip. The high stresses, in turn, provide the potential for a more nearly complete crushing action. For instance, cans may be flattened, plastic containers crushed, and bot­tles reduced to granular glass. Additionally, there is the possibility of substantial dewatering of moist refuse.

Practically, it is important that any proposed rolling system be capable of forcing the refuse into the crushing regime of the roll, and of delivering the material through the roll nip in a continuous or reciprocating action.

In the present pilot study, a reciprocating, flat, horizontal plate was used under a horizontal roller, with the roller bearings spring preloaded against the plate. Refuse material, typically cans and bottles, was forced under the roller by means of a cleat on the plate. The roller was not powered, but was rotated by contact.

Transducers recorded the horizontal and vertical forces developed and the rise of the roller

513

during a complete cycle for various specimens. These are apparently important design data for extrapolation to large capacity syst�ms.

The possibility of improved action by means of grooved, rather than simple planar surfaces, was also investigated; however, the simple, flat crush­ing surfaces generally proved preferable.

CONTACT STR ESSES

The crushing of a refuse object requires induc­ing stresses in the object which are in excess of the yield or ultimate, as stresses within the elastic range simply result in a springback. Certain plastics are more complex in this respect, demonstrating an elastic memory; that is, they tend to recover an origin�.shape after a significant time lapse follow­ing a severe deformation.

Even stresses in metal cans are all but impos­sible to determine because the crushing action is progressive, with the yielding pattern more in the nature of a gross buckling. In fact, this domain of complete and catastrophic failure is far beyond the usual engineering concerns for structural design, with assumptions of small de­flections and elastic behavior. Buckling modes are extreme and well past the usual stability

• regimes.

Nevertheless, it is instructive to consider the most elementary or classic mechanism, in which Hertz stresses are developed between a flat elastic specimen and a cylinder, Fig. 1.

Maximum compressive stress at the point of contact with the elastic range is

(1)

where

p - unit axial load = Q/L -

-

Q - total radial load -

L - length of cylinder -

D - diameter of cylinder -

E - modulus of elasticity -

v - Poisson's ratio -

Apparently, the developed stress varies as vp/D; that is, the stress increases with the radial load and with a decrease in diameter, but not linearly.

With a steel roller on a much weaker or softer specimen, El of the roller could be considered infinite and the first term minimal. Then

s = c (2)

Of particular importance is the concept that the stress is highly localized at the contact, and that the radial force Q acts at the roller bearings. As such, Q is a preload and not directly related to the actuating force on the specimen, which is tangen tial .

INDUCED CONTACT FORCES

The external force pattern of a specimen enter­ing the roll nip is a function of the roll geometry and the coefficient of friction. In Fig. 2, an ideal­ized specimen is shown entering driven tandem rollers in contact with each other. There are normal and frictional forces at the roll contact points and an external force, Q.

R OLLER

CONTACT STRESSES

...--SPECIMEN

PLATE

F IG. 1 BA S IC R E LAT IO N S H I P OF A S P EC IM E N TO A P LAT E-RO L L COM B I NAT IO N, I N W H IC H

MAX IMUM CO NTACT STR E S S E S ARE

PRODUC ED AT T H E N I P

5 14

Q

f

w

,

F IG. 2 A CY L I NDR ICA L' S P ECIM E N E NT ER I NG TWO EQUA L A ND TA NG E NT RO L L ER S I S SU BJ ECT ED TO NORMA L FORC E S D E V E LO P­ED BY EXT ER NA L FORC I NG A ND FR ICT IO N.

,. ....

.... .,

!2 40f-� It: 1&.1 U It: e 20

10

f,) - 2 (INTERNAL ROLL)

! I .!z) , 1--1., - 4

"TF' ) -<I)

FLAT PLATE)

-I

� � � ro � � 00

RATIO OF RADII ( rJ F IG. 3 CRUS H I NG TO CROWD I NG FORC E RATIOS

FOR VARIOU S G EOM ETRIC ARRA NG E­M E NTS, I ND ICAT I NG T H E IM PRO V ED FACTOR S O BTAI NA B L E U S I NG I NT ER NA L RO L L S

Consideration of this geometry leads to the relation of the radial crushing force developed to the driving force, Q, from which the ratio F n/Q can be calculated as shown in the lowest curve, Fig. 3, for an assumed coefficient of friction of 0. 10. Apparently, enlargement of the roll radius creates greater normal crushing forces. Also, how­ever, a driving force, Q, is required to maintain the contact and to thereby induce the crushing

5 1 5

effect: Gravity forces are usually small and are neglected.

If one of the rolls is inverted, becoming an­nular, Fig. 4, the force pattern is as shown. Again, the ratio (Fn/Q) can be calculated from this analysis for various roll radii, Fig. 3. Apparently, the internal geometry is more conducive to entrance and crushing than the external. In fact, the r2/r1 = 2 curve approaches infinity

/

Q I

-----

•

n

, F IG. 4 A N I NT ER NA L, OR PLA N ETARY RO L L ER,

D E V E LO P S CO NTACT FORC E S A T T H E TWO PO I N T S OF TA NG E NCY W I T H T H E S P EC I­M E N

asymptotically at (rl Iro) = 48. An infinite value for (Fn/Q) indicates an infinite crushing or· normal force. If the rolls have sufficient torque, the specimen will be crushed. Otherwise, the rolls will stall in this self-locking si tua tion.

As the internal cylinder becomes larger, this carrier surface approaches a plane, as r2 approaches infinity, Fig. 5. The locking asymptotic combina­tion occurs then at approximately rl Iro = 100. Physically, this means that a concave object on a plane having a diameter greater than one percent of the roll diameter will not enter the nip if the coefficient of friction is 0. 1 0 or less, even though both the roller and plate are driven. Apparently, a positive crowding force, Q, will be necessary in typical situations. It can be seen in Fig. 5 that the locked condition occurs if the line of action of the resultant contact forces becomes colinear on the line a-a. So, for a given geometry, there is a corresponding critical coefficient of friction, where f = tan-I Fr/ Fn.

Fig. 6 shows a freely turning roll with the specimen forced horizontally by a cleat on a driven tray. The roll can take no torque and cannot rise. There are no friction forces, and the three external forces are concurrent on the cylindrical specimen. From the geometry

5 16

( rl - 1)

sin 1:1 ro

(3) --

rl (To + 1)

I = tan 1:1 (4) -- ,

cos 1:1

These results, Fig. 7, indicate the crushing forces to be generally greater than the actuating force, Q, or the ratio to be greater than unity. The force multiplication obviously increases as the specimen becomes smaller with respect to a given roll diameter. Also, the roll contact force always exceeds the plate contact force.

More realistically, provision will be made to permit the crushing roll to rise as the specimen resists compaction, Fig. 8. Downward force on the roll will be its weight plus a spring effect, the lat­ter assumed not to be preloaded. The relationship is then for the angle

(y) - ( ro - 1) rl rl

sin 1:1 = (5) ro

( r I + 1)

a

Q

FIG.5

Q

a

I

- I

FORC E PATT ER N D E V E LO P ED BY A RO L L ER O N A P LA N E, W IT H BOTH DRI V E N

\. •

F IG. 6 A COMPR ES S I B L E S P EC IM E N I N CO NTACT W IT H A F IX ED P I VOT RO L L ER A ND A

DR I V E N P LAT E CAU S E S O NLY NORMA L FORC E S O N T H E S P EC IM E N

5 17

o �

5

� 3 w o :5 2 ...

o � w � � � w ro � w � RATIO OF RADII Cl/rO)

FIG.7 THE SPECIMEN OF FIG. 6 IS SUBJECTED TO INITIAL NORMAL FORCES WHICH VARY WITH THE RATIO OF RADII, (r,/ro)

Fn2 = (W + Ky)

e Q

Q

y

FIG.8 SPRING-LOADED, PIVOTED ROLLER RISES AS CRUSHING FORCES DEVELOP ON A CYLINDRICAL SPECIMEN

518

2.0

15

1.0

�=I W

0 �--�. 2----- .�4-----.6�--�.B----�--��1� 2----�14 X/,

rl

FIG.9 BEHAVIOR OF THE MECHANISM OF FIG. 8 DURING PROGRESSIYE ENGAGEMENT OF AN INCOMPRESSIBLE SPECIMEN

The dimensionless displacement equation is

.---______ __ • (6) J ro ro y y

- 4 (-) -2(1- - ) (-) _ (_ )2 rl r I rl rl

And the forces ratios are

(kW

rl) [(E.g -I) + (!.Q. + I) sin OJ + I

r I r I

tan 0

(�) tan 0 W

Fnl Q -

W =(

W)

II cos U

Forces are related to a k nown constant roller weight as reference.

(7)

(8)

(9)

The nature of these relations is shown in Fig. 9, in which the spring factor k r I /W = I, and the ratio ro/rl = 1/3. Driving force, Q, is maximum at initial contact, dropping to zero when the specimen is located directly below the roller. Resulting contact loads usually exceed the driving force, becoming equal and opposite at the dead center position.

The roller rise from zero to the specimen diameter

is also shown, relative to the horizontal displace­ment.

This analysis assumes an incompressible speci­

men, and Fig. 9 shows this behavior. If the speci­men diameter reduces during the cycle, the pattern will be altered. This effect is difficult to model be­cause the specimen is usually not behaving elastical­ly. Nevertheless, the equations are valid, and cal­culations can be made as indicated for any as­sumed (ro/rl) and displacement ratio within the cycle.

VOLUMETRIC REDUCTION

A given volume of original refuse is composed of solids and voids, the latter dispersed between various specimens as well as internal to most. In crushing, only the voids can be minimized.

In this study the following parameters are used

to specify volumetric results:

519

Vo solid volume

V 1 total initial volume

VI initial void volume

V 1 total initial volume

,

•

•

• j

J

F IG. 1 0 O V ERALL V I E W OF EX P ER IM E NTAL ROLL· I NG F IXTUR E, S HOW I NG T H E DRUM, CA NTIL E V ER S PR I NGS, A ND R ECORD I NG I N STRUME NTAT IO N

V2 _ final void volume

V I total initial volume

For a given quantity of material the density ratio is the ratio of the final to the initial volume, or may be termed the compaction factor.

PI (W/Vd - c ( 10) --

Efficiency is related to the elimination of as much of the void volume as pOSSible,

E VI -V2 I _ (2) - -

- -

VI VI I - (p I )

( 11)

P2 I - C - = .. -

I _ (VO ) I _ (VO ) VI VI and becomes unity if crushing completely elimin· ates the original voids, V2 = O.

Another consideration of importance in the idealized laboratory tests is the matter of mUltiple

520

specimens versus single. In the former instance both the internal voids and the external are con· sidered. In the latter only internal voids are con· sidered. Obviously the volumetric considerations are quite different with respect to tangent cylinders or random cylinders than a single cylinder.

TEST FIXTURE

A laboratory mechanism was designed and built to study the feasibility of rolling compaction of refuse, as well as to quantify to some extent the possible reduction in volume and related crushing and driving forces. This model is shown in Fig. 10 and schematically in Fig. I I .

The unit employed a steel drum 406.4 mm. ( 16 in.) in diameter and 298.4 mm. ( 1 1.75 in.) long mounted in antifriction bearings, in turn bolted to pivoted horizontal arms, allowing the roller axis to rise. The arms were constrained against rising by cantilever beam springs, such that the effective total spring rate at the roll center was 928,000 N/m (5300 Ib/in.). Dead weight of the roller was 667N ( 150 Ibs.)

A specimen carrier plate was located below the roller, and driven horizontally by means of a pair

Ry Q=R.

FIG.11 SCHEMATIC OF LABORATORY MECHANISM, SHOWING PIVOTED ARMS AN D RESILIENT CONSTRAINT AT THE ROLLER BEARINGS. Q IS PRODUCED BY THE CRANK-DRIVEN PINION AND RACK

of rack and pinions with a hand-operated crank

at the pinions. The plate was also supported from below by a second roller directly under the main roll. An angle cleat on the plate provided a pos­itive horizontal crowding force on the specimen,

Fig. 12.

MEASUREMENTS

Forces were measured by providing load cells at the pivots, 03, Fig. 13. These consisted of

cylindrical cantilever beams with strain gages, which, after calibration, indicated horizontal and vertical force components at this point.

Then from static equilibrium the horizontal and vertical components of the resultant force between the roll and the specimen could be calculated.

In order to measure displacement, a potentio­meter was coupled to the rear of the roller, Fig. 14.

Thus as the drum rotated a corresponding voltage signal was provided. Then by using a two-channel

XY plotter the values of Rx and Ry were recorded during a test cycle against the angular displacement,

cp. The latter only represents horizontal displace­ment of the plate and specimen if there is no slippage. When slippage occurred, this could be determined during a reversal cycle.

V ertical displacement of the roll was measured with dial gages mounted between the pivoted arms and the stationary bed of the fixture.

521

Although sometimes difficult, the various cans, bottles, and cartons tested were measured and calculated to determine volumetric changes during crushing. Data for each run were processed by means of a computer program.

In all, cycle data were obtained on nearly two hundred tests. These involved various types, sizes, and combinations of cans, containers, and bottles, including aerosol, and plastics. Compaction statistics will be detailed later. Most tests used simple cylinder

on plane contact; however, variations at the plate included longitudinal ridges and rubber belting as a cushion.

EXPERIMENTAL RESULTS-FORCES

As an example of data obtained during compac­tion, curves are shown in Fig. 15 for a cycle in

which three food cans with tops removed were entered in T formation. The drum revolved 90° ,

with maximum normal force of about 8000 N (1800 lb.) at a roller rise of about 8 mm. (0.31

in.). This induced load, FnJ, results from an ap­horizontal force, Q, of 1200 N (270 lb.) for a

multiplication factor of nearly seven; however, a larger driving force of about 2350 N (530 lb.) is required earlier during the entering phase.

It is also of interest to note that Q becomes negative and (1 exceeds 90° near the end of the cycle. Physically this means that specimen spring­back tends to drive the plate forward. The plate

FI G 12 TWO C,NS ,N sEe' E5 EN' Ee' NG ,HE eoLL

. NIP. DRIVE.N BY T\-IE. CLE.AT oN T\-IE.

CARRIE.R PLATE.

FIG.13 LOAD C ELLS W ER E I NSTALL ED AT T H E ARM P I VOTS TO M EA SUR E HOR IZO NTAL

A ND V ERTICAL COMPO N E NT S OF T HIS R EACTIO N

FIG.14 DRUM ROTATIO N WA S D ET ERMI N ED W IT H A POT E NTIOM ET ER DIR ECTLY COUPL ED TO T H E DRUM S HAFT

523

I

Pi" 1\::.

90

-8 •

.40 <.> "'00 1.00 80 ... 0 2000 F" --- .. z :lI -0800 - . 800 70 ... '" - _.30 - -' - '" '" ... ...

... 1500 -' ... '" z � 600 - :r 60 '" 0 .60 0: 0 Z 0: Z IL - .20 ... 0

1000 -0: -' .... LlJ 400 -' .40 5 500 -' 0 0:

200 500 a

"' 20 W 40 � 60 ro ROLL ANGLE •• (DEG)

.20

... 0: 0:

0 40

FIG. 1 5 TYP ICAL R E SULTS DUR I NG A CRUSH I NG CYCL E I N VOL V I NG THR E E CA N S, WITH

R E SULTS CALCULAT ED E V ERY 10 0 FROM CO NT I NUOUS X-Y R ECORDI NGS

then requires no forcing beyond the 74° point. [n fact, since e = 90° at tills condition, the contact force on the roll is exactly vertical.

J;:nergy required fonhis particular crushing operation is determined by considering the product of the horizontal force and horizontal displace­men t.

Numerically this integral is 382J (3,380 lb. in.) for the three units, and this external energy is

converted to strain energy as the metal is dis­torted.

Plastic containers behaved somewhat similarly to metal in terms of requiring sustained and ap­preciable forces during a large portion of the cycle. Glass bottles, however, shattered almost completely after initial failure, therefore requiring a relatively small energy input. The breaking forces varied depending upon the attitude, and the size and tillckness of the specimen. Based on numerous tests the breaking force usually ranged from 2670 to 5340 N (600 to 1200 lb.). Corresponding horizontal driving forces similarily ranged from about 2200 to 4450 N (500 to 1000 lb.).

EXPERIMENTAL RESULTS-COMPACTION

The glass con tainers generally suffer fairly complete fracture as they are compressed between the roll, the cleat, and the plate, and are reduced to granular form, but with some residual pieces, Fig. 16. Fig. 17 shows the nature of the broken glass resulting from different containers. An even greater reduction seems possible by increasing the roll pressure.

As seen in Fig. 18, the soft drink can is a well­behaved and predictable element, converted from cylindrical to nearly a flat geometry. The aerosol can, Fig. 19, similarly is completely fla ttened on

FIG. 16 GLA SS DEBRIS TY P ICALLY R E SULT I NG FROM CRU SH ED BOTTL E S

524

-

F IG. 17 TY P E S OF R E S IDUAL MATER IAL

PRODUC ED BY-CRU S HI NG VAR IOUS GLA SS CO NTAI N ER S, ILLUSTRAT I NG A HIG H

DEGR E E OF COM PACT IO N

FIG. 18 TY P ICAL A P P EARA NC E OF B E V ERAG E CA N S B EFOR E A ND AFT ER CRU S H I NG

I N T H E MOD EL

525

•

\

FIG. 19 T H E NATUR E OF A N A EROSOL CO NTA I N ER A S CRU S H ED W IT H A FLAT PLA T E: L EFT, A ND O N A R I DG E, C E NT ER

(/) SINGLE MULTIPLE w • t!) UNIT UNITS

0 SPECIMEN

1 z C E C E

I 12 OZ. SODA CANS x .21 .80 .23 .77

2 12 OZ. c;ODA CANS .32 .69 .31 .70

3 OPEN TOP CANS x .22 .79 .24 .76

4 OPEN TOP CANS .39 .62 .38 .63

5 12 OZ. BEER BOTTLES .31 .87 .1 6 .94

6 28 OZ. SODA BOTTLES .33 .85 .18 .93

7 16 OZ. SODA BOTTLES .46 .77 .24 .90

8 FOOD JARS :21 .97 .10 .99

9 PLASTIC CONTAINERS .59 .42 .48 .53

10 LIQUOR BOTTLES (THIN) x .36 .82 .15 .94

I I LIQUOR BOTTLES (THIN) .31 .89 .13 .96

12 LIQUOR BOTTLES (THICK) .64 .54 .29 .84

FIG.20 SOM E TYP ICAL R E SULTS OF VOLUM ETR IC R EDUCT IO N BY CRUS H I NG, COM PAR I NG MULT I P L E WIT H SI NGL E U NITS, BA S ED O N COM PACTIO NS A ND EFFICI E NC I E S

O BTA I N ED

a simple plate. The effect of longitudinal edges on the plate is clearly seen in the cen ter.

As previously discussed, there are two possible indices of performance with respect to a compac­tion process. One is a simple ratio C, of the final to the original volume of a sample. The other, perhaps more meaningful, takes into account the potential reduction on the basis of void content, and is here termed the efficiency, E.

Summary results are tabulated in Fig. 20 with representative samples tested indicated. Maximum compactions and efficiencies are found, with the glass containers, particularly the food. Minimum results occur with plastic containers which is related to the tendency of these materials to recover shape, even when severely deformed.

In three cases longitudinal steel ridges were attached to the carrier plate to learn the effect

526

of localizing the compressive stresses. No significant trend is apparent, and it appears that the plane crushing surface is adequate. In fact, with glass the ridges act as spacers, protecting the material be­tween the ridges from complete compaction.

REVERSE CYCLE

With a reciprocating carrier, as used in these tests, the plate must be returned for reloading. Obviously there is a probability of partially crushed material being forced back, precluding entry of new material.

This difficulty was resolved by providing a. backstop on the roller, actually a cam type link bearing against the center of the roll, Fig. 10. Thus the drum could be driven by the plate and the material in the forward direction. In reverse the drum remained stationary, acting as a dam . against return of a specimen. Apparently, as previously indicated, material will not pass through the nip unless acted upon by a positive displacing force.

Material remaining ahead of the nip will then be forced in a forward sense by new material loaded between it and the cleat.

CONCLUSIONS

This pilot project has shown that rolling is a feasible method of crushing refuse, and the results may be briefly summarized as follows:

I) Maximum effectiveness is achieved with glass containers, with efficiencies in the order of 90 per­cent.

2) For metal cans the efficiency is about 70 percent.

3) For plastic containers the efficiency is approximately 50 percent.

4) With a cylinder-plane combination material must be positively displaced into the nip.

5) Forcing and compaction loads are relatively high if substantial crushing is to be obtained.

Obviously actual refuse will be of a much more random and diverse nature than the basic contain­ers studied here; however, the quantitative data

, obtained should be of considerable benefit in estimating potential results. The forces measured should also be useful in the design of actual equip­ment for this purpose.

527

ACKNOWLEDGEMENT

This project was supported by the Wisconsin Alumni Research Foundation of the University of Wisconsin-Madison.

REFERENCES

[1] Cross, Frank l., "Compactor Sizing and Selection", Polution Engineering, Vol. 5, No. 7,

July 1973, pp. 34-37. [2] Ham, R. K. and Reinhardt, J. J., "Final Report

on a Demonstration Project at Madison, Wisconsin, to I nvestigate Milling of Solid Wastes Between 1966 and

1972" [3] Howard, D., Savage, G., and Trezek, G. J.,

"Mechanical Properties of Some Refuse Components",

Compost Science, Vol. 13, No.6, Nov.!Dec. 1972, pp. 10-15.

[4] Johnson, W. J., "Refuse Production Plant, Montreal-Quebec", Engineering Journal, Vol. 52,

No. 6, June, 1969, pp. 15-21. [5] Lawrence, D. E. "A Report on Investigations

Made into Refuse Disposal Methods Suitable for an

Inland Town and its Surrounding Rural Area", The

Institute of Municipal Engineers Journal, Vol. 96,

No. 6,June, 1969,pp. 169-179. [6] Shilesky, D. M., "A Compendium of Philosophy,

Proposals, and Action for the Recovery of Energy from

Municipal Solid Waste", Waste Age, Vol. 5, No. 2, March/April, 1974, pp. 72.

[7] Smith, M. l., "Refuse Shredding-A Major Change in Waste Management", Public Works, Vol. 104, April,

1973, pp. 74-76.