29

Attrition and Breakage of aWestern Reference Oil Shale at

Process Temperatures

Ulrich Grimm and Glenn SwaneyU.S. Department of Energy

Morgantown Energy Technology Center

Morgantown,West Virginia

ABSTRACT

A high-temperature, rotating-drum attrition tester has

been used to study the friability of a Mahogany zone

(Parachute Creek member, Green River Formation) oil

shale. This DOE reference shale has a Fischer assay of

27.5 gal/ ton and was obtained from the Exxon Colonymine located nearParachute,Colorado. This feedmaterial

was screened to 9.5 to 12.5 mm, heated in the drum to the

temperature of interest, and tumbled a desired amount.

Testing conditions included nitrogen, air, and carbon di

oxide atmospheres; temperatures from ambient through

900C; and tumbling amounts of 0 to 2,400 revolutions.

Thebreakage functionof this shale is fairly similar to thatofawestern shale studied previouslyby theauthors, yield

ing a product size distribution that is typically trimodal or

quadrimodal. Attrition levels for air atmospheres were

somewhat higher than for nitrogen and produced more

multimodal distributionsunderotherwiseequivalent con

ditions. The primary loss in particle strength occurs in the

300C to 500C range, and is attributed to kerogen decom

position.Maxima in the particle-size distribution at 10 and

80mmwereevident.Thebreakagekinetics for the feed size

of this shale are nonlinear.

INTRODUCTION

Breakdown of kerogen during the retorting of oil shale

and combustion of the spent material generally weakens

the remainingmineralmatrix, leaving it substantiallymore

friable. Processing of thismaterial tends to generate fines,which can have detrimental environmental and economic

consequences due to contamination and separation prob

lems.Although in situ processeswould seem to circumvent

most of these attrition-related difficulties, they still must

deal with the fines that contaminate the product oil. The

aim of this work is to quantify the most significant factors

affecting this attrition process, including (but not limited

to) shale material properties, processing conditions, and

the attritingenvironment. The ultimate objective is to

develop a model capable of predicting the extent and

nature of attrition in a proposed process that requires little

or no experimental attrition data.

In this study, attrition is defined as an unwanted erosion

process inwhichdaughterparticlesare substantiallysmaller

than the parent, leading to a bimodal or multimodal par

ticle-size distribution. The subject of attrition frequently is

ignoredbecause it isa complex,poorlyunderstood process.

However, research in thisareahasbeenincreasingrecently,

particularly as more and more fluidized-bed applications

areconsidered and developed (Rayandothers, 1987).With

oil shale, thedifficultyof thegeneral problemofattrition is

considerably compounded by the complexmorphologyof

thematerial.Attrition isquantified inpracticalapplications

using arbitrarily defined indices, such as grindability and

hardness, but these permit only comparison of closely re

lated materials and conditions and provide no insight into

the attrition process itself.

Attrition tests are classifiable as either single-particle or

multiparticle, depending on the nature of the sample.

Multiparticle tests readily provide statistically significant

data but reveal little about the details of the attrition/-

breakage process. While single-particle tests can provide

suchdetails, theygenerallyrequireaprohibitivelyextensive

and labor-intensive experimental program. Becausemuch

of the interest in attrition of oil shale centers on fluidized-

bed processing, we have used a low impact-velocity,multiparticle tester, theRotatingAttritionTest facility (RAT),to study attrition and breakage of four shales to date

(Grimm and Swaney, 1989, 1990a, 1990b) (Figure 1). Based

on the ASTM D4058 drum test for measuring catalyst

friability, theRAT is a 25.4-cm-diameter Inconel 617drum

suspended and rotating in an electric furnace. Dependingon the particle size, the impact energy afforded by this

drum size is somewhat greater than or equivalent to that

found in a typical fluidized bed (Grimm and Swaney,1990). Hollow shafts allow the drum interior to be purged

with a selected process sweep gas, and a ceramic felt filter

on the exit shaft prevents loss of fines. A shelf mounted

inside thedrum preventsparticle segregationof thecharge

30 COLORADO SCHOOL OFMINES QUARTERLY

andensuresuniform tumbling.Alonizing thedrum interior

provides enhanced high-temperature corrosion and abra

sionresistance.Amoredetaileddescriptionof theapparatus

is provided in Grimm and Swaney (1989).

Furnace

^_^Rotary-Linear

\ ] / Bearings

Exhaust

/

Process

Atmosphere

Particulate

Material

Figure 1. Rolling Attrition Tester (RAT) schematic.

MODELING

Two size-reduction theories have been extant formanyyears. In the first, Rittinger's surface theory, energy input

is proportional to the surface area formed. The second,

Kick's law, states that energy isproportional to thevolume

or weight of the comminuted product. It is generally con

sidered that Kick's law applies to impact pulverizing and

Rittinger's law to finegrinding (BritishMaterialsHandlingBoard, 1987). Raw shale consists of variousmineral grains

bound by kerogen; retorting the shale and subjecting it toan attriting environment removes most of this binder and

tends to resolve the material into its component, char

acteristic mineral grains, which are mostly <10 mm (L.J.

Shadle and D.C. Galloway, pers. comm.). It is an interest

ing observation that for attrition such as this,which forms

a characteristic fines distribution, the fines account for

nearly all the surface area formed,which isproportional to

theirvolume ormass.Thusboth Rittinger's law andKick's

law essentially are equivalent in this case (Ray and others,1987). These both imply that an attriting process that con

sumesenergyat a constant ratewill form fines at a constant

rate. In experimental studieson the attrition coal in a fluid

ized bed, Knowlton (1986) observed that the rate of fines

generation initially is rapid but then levels to a steady rate

as Kick's or Rittinger's law would indicate. He attributed

this nonlinear induction period to the breakage of sharpedges on the surfacesof the particles. Studies ofother size-

reductionprocesseshave shown that feedparticlespossess

a"memory"

of their previous treatment,whichmust fade

away before materials with different histories can be sub

jected toa faircomparison. Inotherwords, simplyknowingthe composition and particle-size distribution of a sample

is not enough to predict how it will attrite; details such as

the presence of microcracks and surface roughness can

play a crucial role. As samples are subjected to a common

processing environment, these differences tend to vanish.

Mechanisms of particle breakage/attrition can be

grouped into three categories according to decreasingimpact energy (1) shatter or fracture,where high energyproduces a wide distribution of fragment sizes; (2) chip

ping or cleavage, where the energy is just enough to load

a fewregionsof theparticle to the fracturepoint,generatingseveral large fragments; and (3) abrasion,where low ener

gy causes localized stressing, shattering a small region on

the periphery of the particle.

Over the years a large volume of literature on

comminution and grinding has evolved,much of which is

aimed at thedesign ofballmill circuits.Recently, however,

fully and semi-autogenous grinding have become im

portant comminutionmethods (Menacho, 1986). If a large

amount of material is charged, the RAT can behave

effectively as a tumbling mill, but the expense of sample

preparationdictates thatweusea small charge in thedrum

that is completely lifted by the shelfon each pass, so it actssomewhat like an impact crusher. In either case, the grind

ing (attriting/breaking) action is basically autogenous,

which is intrinsically difficult to analyze in that all three

mechanismsofparticlebreakageareapparentandgenerallysignificant.

One of the simplest kinetic treatments of breakage

assumes that the process is analogous to a first-order de

composition reaction:

dmi/dt = -kimi (1)

Here m; is themass fraction in size i, t is time, and k{ is the

breakage rate constant for size i (Kelley and Spottiswood,1982).

Herbst and Fuerstenau (1980) have used a population

balance and the assumption of linear kinetics to derive the

following result for batch grinding:

where

y(t) = TJ(t) T-l X

0 i<)1 l=]

Tv =

ft kk. lii

it) =exp(-fc,f) =

0t*)_

(2)

(3)

(4)

Here xand yare the feed and product sizedistribution vec

tors, respectively.Thebasic parametersare /c,-, thebreakage

rate function, and fe,y, the breakage distribution function.

ATTRITION AND BREAKAGE OFWESTERN REFERENCE OIL SHALE 31

Thebreakage rate function has in the past been referred to

as the selection function.

EXPERIMENTAL

The oil shale used was a Mahogany zone (Parachute

Creek member, Green River Formation) shale obtained

originally from the ExxonColonymine located near Parachute,Colorado. This is a reference shale (here designated

WOS-86) that has been extensively characterized (Miknis

and Robertson, 1987). Its Fischer assay is 27.5 gal/ton; itcontains 4.2%mineral carbon and 66.9% ash; and it is com

posed predominantly of quartz, ankerite, and calcite. This

material was crushed and sieved into five size fractions

9.5 to 12.5 mm, 6.3 to 9.5 mm, 2.36 to 6.3 mm, 1.18 to

2.36mm, and 0.60 to 1.18 mm. The largest feed size was

dictated by the available material; the remaining sizes

were selected to cover the experimentally practical range.

Operational details of the RAT are given inGrimm and

Swaney (1989). The experimental program employed

nitrogen, air, and carbon dioxide atmospheres; pyrolysis

temperaturesof300C, 350C, 400C, 425C, 450C, 475C,

500C, 525C, 550C, 575C, 600C, 675C, 750C, and

900C; and tumbling amounts of 0, 200, 400, 600, 1,200,

1,800,and 2,400 revolutions.Because the65 runsperformed

did not permit complete analysis of all possibilities, a base

case of nitrogen, 500C, 9.5 to 12.5 mm feed, and 400

revolutions was chosen as most typical of actual retort

conditions. The effect of all parameters (temperature, feed

size, atmosphere, and tumbling amount) were studied

individually from this base case. In addition, the effects of

temperature, tumblingamount, and feed sizewere studied

for all atmospheres.

Foranalysis,productmaterial was sieved into 16fractions

(upper size shown) 12.5 mm, 11.2 mm, 9.5mm, 8.0 mm,

6.3 mm, 4.75 mm, 3.35 mm, 2.36 mm, 1.70 mm, 1.18 mm,

850mm, 600mm, 425mm, 300mm, 212mm, and 150mm.

The -150 mm material then was analyzed byCoulter-

counter using 256 channels in the range of 9 to 200 mm.

Filtering these data by combining channels to eliminate

noisewasdone on an individualbasis,but typically 20 size

fractions were retained.

RESULTS

Figure 2 gives theweight loss in theRAT for this shale as

a function of pyrolysis temperature for the three at

mospheres, togetherwith thematerial-balanceFischerassay

reported byMiknis and Robertson (1987). Data points for

each temperaturewereobtained by averaging results from

all feed sizes used at that temperature and atmosphere.

Two stages of weight loss are apparent, especially for the

nitrogendata, corresponding to breakdown of thekerogen

from 300C to 500C and carbonate decomposition from

40

30

20

10

WEIGHT LOSS (%)

Material: WOS86

Feed Size: Averaged

Time: 400 revolutions (10 min)

Analysis:

I I Water content

EZZ Oil & gas

I I Residual carbon and carbonates

Atmosphere:

o N2CO2

Air

j I

300 400 500 600 700

TEMPERATURE (C)

800 900

Figure 2.Weight loss vs temperature forMahogany oil shale in

nitrogen, carbon dioxide, and air atmospheres.

600C to 900C.The runsperformed underair showhigher

weight loss at all but the highest temperatures because

oxygenaccelerates thedecompositionofkerogen. It should

be noted that itwas decided to pyrolyze the samples for 2

to3hours, similar toactualprocessingconditions.However,

thedataofMiknisandothers (1988) indicate thatwithin the

retorting temperature range, complete yield of oil is not

attained formanyhours.Therefore,because the samples in

general are not pyrolyzed to absolute completion, and be

cause destruction of kerogen is faster under air, samples

heated in air are pyrolyzedmore thoroughly as a result of

a rate effect aswell as a chemical effect. The carbon dioxide

atmosphere suppresses carbonate decomposition in the

range of 600C to 800C, at which point carbon dioxide

vapor pressure over CaCOs approaches 1 atmosphere.

A typical complete product size distribution is shown in

Figure 3 for500C, air, and 600 revolutions. Ascanbe seen,the product ismultimodal, consisting primarily of eroded

feed and fines in the range of 5 to 150 mm. The fines are

divided into two size-distribution peaks at about 10 mm

and 80mm, sizes thatprobablyarecharacteristic of specific

minerals within the shale. The appearance of this distri

bution is quite similar to that of the Anvil Points western

shale studied previously by the authors.

Figures 4, 5, and 6 illustrate the cumulative produce size

distributions versus temperature fornitrogen, air, and car

bon dioxide atmospheres, respectively. The bottom two

areas in each graph correspond to the fines (<150mm) and

the two peaks mentioned above; the upper five divisions

correspond to the feed sizes used.

Breakage and attrition increase rapidly between 350C

and450C,wherekerogenanddawsonitearedecomposed .

Between525C and 675C under carbon dioxide, andmoreso under nitrogen, susceptibility to attrition decreases.

32 COLORADO SCHOOL OFMINES QUARTERLY

WEIGHT FREQUENCY

1.6'

1.4

1.2

1.0

0.8

0.6

0.4

0.2

W0S863 Conditions

500C

Air

600 Revolutions

Feed 9.5-12.5 mm

1 1i 1 1 ii i i i i 1 1 1 ii i i i 1 1

9.5 - 12.5 mm Feed 400 Rev

0.0001 0.001 0.01 0.1

PARTICLE SIZE (cm)

1.0

Figure 3.WOS-86 RAT product distribution.

This most likely is caused by peripheral coke formation,which increasesparticle strength,making itmore resistant

toattrition (AdamsandMahajan, 1987).With the exception

of data taken under air, attrition increases from 675C to

800C,presumablyattributable tocarbonatedecomposition.

From800C to900Cundercarbondioxideand air, attrition

decreases probably because ofmineral sintering and slag

ging, which may be related to the formation of silicates.

Because this is not apparent for the data under nitrogen, it

seems that oxygen plays an important role in thismineral

fusion process. It should also be noted that the lowest level

of attrition for spentmaterial was obtained under carbon

dioxide at 900C

Figure 7 illustrates the dependence of the product size

distribution on amount of tumblingunder carbon dioxide

at 500C for the 9.5- to 12.5-mm feed size; results for the

othergaseswere similar.Similar to the findingsofKnowlton

(1986), after a brief period of rapid breakdown, attrition

tends toward a steady-state rate as the particle surfacesbe

come rounded, losing thememoryof theirprevioushistory.

The effectof feed sizewas studied at500C and 750C for

all atmospheres.Figures8 and 9 showproductdistribution

as a function of feed size for tumbling 400 revolutions

undernitrogenat500Cand750C, respectively. Inessence,

thesedata represent thebreakagedistribution function. As

a size fraction approaches that of the feed, the percentage

undersizemust become 100%; hence, the lines descendingfrom the topof thegraphareexpected. It ismore interestingthat the production of fines seems to level off or exhibit a

maximum for the 6.3- to 9.5-mm feed size. The decrease in

fines for smaller feed sizes results from reduced impact

energy. For the largest size, impact energy exceeds the

strengthof theasperities,and the rateofattrition is roughlyproportional to the specific surface area,which is less than

for the smaller particles.

N 60-

- 40 fjn

40 - 150 tan

160 - 840 fjm

0.84 - 1.18 mm

1.18 - 3.36 mm

2.36 - 6.3 mm

6.3 - 9.5 mm

9.5 - 12.6 mm

400 500 600

Temperature (deg C)

Figure 4. Attrition vs temperature under nitrogen.

n 60

WOS-86

Air

9.5 - 12.5 mm Feed

400 Revolutions

B888888|- 40 ym

40 - 150 ijm

150 - 840 jUll

0.84 - 1.18 fjm

1.18 - 3.36 mm

2.36 - 6.3 mm

6.3 - 9.5 mm

9.5 - 12.5 mm

400 500 600

Temperature (deg C)

Figure 5. Attrition vs temperature under air.

WOS-86

CO,9.5 - 12.5 mm Feed

400 Revolutions

400 500 600

Temperature (deg C)

Figure 6. Attrition vs temperature under carbon dioxide.

The estimation of the process matrix, or breakage rate

constants, are depicted in Figure 10 for the largest feed

sizes under all atmospheres at 500C. Because the

ATTRITION AND BREAKAGE OF WESTERN REFERENCE OIL SHALE 33

WOS-86 COj 9.5 - 12.5 mm Feed 600t:

160 - 840 /urn

2.36 - 6.3

1000 1500

RAT Revolutions

Figure 7.Attrition vs time under carbon dioxide at 500C.

WOS-86 Nfc 500V: 400 Rev

40

0.15 0.2 0.3 0.5

Feed Size (cm)

Figure 8. Attrition vs feed size under nitrogen at 500C.

disappearance of the feed is given by exp(-fr,f), /c, is given

by the slope of the line on this semilogplot. Clearly the

breakage is not first order, since the plots are strongly

nonlinear. Austin and others (1973) attributed this type of

behavior in laboratoryballmills to the fact that theparticles

have a distribution of strengths and are subjected to a

distribution of forces.Thismeans thatweaker particles are

destroyedmuch faster;as theyaredepleted, the rateof feed

disappearance reduces.

FRACTION OF FEED REMAINING

0.1

0.01 -

0.001

fj

WOS86

500C

:

o

6

D

O

Atmosphere:

o N2? CO2

A Air

-

AO

:

a

OD

-

A

A

i i i i i i i

A

i i i i

500 1000 1500

RAT REVOLUTIONS

2000 2500

Figure 10. Feed decomposition at 500C.

Figure 11 is a plot of the estimated initial breakage rate

constants (k{, s) as a function of feed size for all atmospheres

at 500C and 750C, according to the power-law re

lationship,

Vfcl =[(*iW*i*2)a5]a

(5)

WOS-86 Mj 75ffC 400 Rev 1.0RATE CONSTANT RATIO, kj/k.

-w

: W0S86? y

0 7- 400 revolutions

Power Law:

05 kj/k, = ((XjXi+1/x,x2)-S)a

Atmosphere, temperature:

o N2 500C

0.3 D N2 750C

A Air 500C

n? O Air 750C* yCO 2 500C

0.15 CO2750C^^

0.1 D .

0.07

0.05o

Figure 9.Attrition vs feed size under nitrogen at 750C.

0.07 0.1 0.15 0.2 0.3 0.5

RELATIVE FEED SIZE

Figure 11. Rate constants vs feed size.

0.7

34 COLORADO SCHOOL OFMINES QUARTERLY

commonly observed in batch grinding systems. Here x,and x,+i are the upper and lower sieve sizes, respectively,for size fraction i, and a is a filled constant. Because thismaterial is so easilybroken and attrited, it is not possible to

accuratelymeasure these rate constants. However, all the

data show a similar trend, leveling off toward the smallerfeed sizes. This is consistentwith some results on rod mill

breakage of ore minerals, including quartz (Kelly and

Spottiswood, 1982).

CONCLUSIONS

Much more work is needed to develop a predictive

model, including the incorporation of a fines-distributionexpression into the breakagematrix, and finding relevantparameters to characterize the effect of temperature and

shale composition. In spite of this, several conclusionsmaybe drawn at this point:

1 . WOS-86 is very friable after kerogen pyrolysis. This

would make it difficult to use in processes with high face

velocities of combustion/gasification gases for residual

carbon burnout.

2. Temperatures above 900C and a carbon dioxide at

mosphere may be advantageous in promoting mineral

fusion to reduce the formation of fines.

3. Agglomerating, sintering, or other strengtheningmineral reactions in the range 600C to 900C are favored

by oxidizing atmospheres.

4. Product distributions are multimodal, with

characteristic grain sizes of 10 and 80mm evident.

5. Breakage kinetics for the feed size of this shale are

nonlinear,due to thewidevariability in individual particle

strengths.

REFERENCES

Adams, D.C, and Mahajan, O.P., 1987, Morphology of retorted

oil shale particles: Energy & Fuels, v. 1, p. 23-28.

Austin, L.G., Shoji, K., and Everett,M.D., 1973, Anexplanation of

abnormal breakage of large particle sizes in laboratory mills:

PowderTechnology, v. 7, no. 1, p. 3-7.British Materials Handling Board, 1987, Particle attrition:

Clausthal-Zellerfeld, BRD, Trans Tech Publications.

Grimm, U., and Swaney, G.,1989, Eastern oil shale attrition and

breakage: testing and modeling, in Proceedings of the 1989

EasternOilShaleSymposium:UniversityofKentucky, Institute

forMining andMinerals Research.

Grimm, U., and Swaney, G., 1990a, Attrition and breakage of a

New Albany shale at process temperatures, in Lazar, D.J., ed.,Proceedings of the 1990 Eastern Oil Shale Symposium:

University of Kentucky, Institute for Mining and Minerals

Research.

Grimm, U., and Swaney, G., 1990b, Attrition and breakage of

western oil shale, in Gary, J.H., ed., Twenty-Third Oil Shale

Symposium Proceedings: Colorado School of Mines Press,p. 162-168.

Herbst, J.A., and Fuerstenau, D.W., 1980, Scale-up procedure forcontinuous grinding mill design using population balance

models: International Journal of Mineral Processing, v. 7,

p. 1-31.

Kelly, E.G., and Spottiswood, D.J., 1982, Introduction to mineral

processing: NewYork, Wiley-Interscience, 491 p.

Knowlton, T.M., 1986, Attrition and entrainment studies related

to fluidized-bed gasifiers: Institute ofGas Technology, Project

61077 Quarterly Rept. (April 15-July 14).

Menacho, J.M., 1986, Some solutions for the kinetics of combined

fracture and abrasion breakage: Powder Technology, v. 49,

no. 1, p. 87-95.

Miknis, F.P., and Robertson, R.E., 1987, Characterization ofDOE

referenceoil shales:Mahoganyzone,ParachuteCreekmember,Green River formation oil shale, and Clegg Creek member,

New Albany shale: U.S. Dept. of Energy Rept. DE-FC21-

86MC11076.

Miknis, F.P., Turner, T.F., Ennen, L.W., Chong, S.-L., and Glaser,

R., 1988, Thermal decomposition of Colorado and Kentuckyreference oil shales: U.S. Dept. of Energy Rept. DOE/MC/

11076-2625.

Ray, Y.-C, Jiang, T.-S., and Wen, C.Y., 1987, Particle attrition

phenomena in a fluidized bed: Powder Technology, v. 49,no. 3, p. 193-206.