CE 527 Solid Waste Management
Materials Recovery
(Chapter 9 and 12 of textbook)
Dr. S.K. Ong
1. Materials Recovery Facilities (MRF)
- are defined as a centralized operation where commingled and/or source separated recyclables are
processed mechanically or manually.
- prepare the recyclables to meet market specifications for sale
 Relatively new in the solid waste management field
- first MRF was established in Groton, Connecticut in the late 1970s (USEPA, 1991), still
operating but does not accept paper or plastic stream
- in 1990, there are about 104 MRFs identified (US EPA, 1991)
 Separation of waste materials is a necessary operation of materials recovery. There are generally two
types of MRF:
- MRFs for source separated wastes, e.g., curbside programs
- MRFs for commingled MSW- waste separated manually or mechanically on site into various
groups
 Advantages of MRF
- allows recyclable materials from a municipality or region to be pooled and processed uniformly
to meet buyers' specification, provide the needed volume and constant source of materials and to
maximize recycling and recovery of materials
- collection needs are usually simplified as separation are done at the MRF
- instead of several compartment as for curb side recycling, a single or two compartments
will be needed
- collection time and costs reduced
- increased participation by generators
- reduce MSW going to landfills
 Disadvantages
- may discourage generators from reducing waste
- high start up cost
2. Engineering Consideration
Design of MRF requires the estimation of quantities of materials that can be recovered and the
appropriate design loading rates. Loading rates (tons/day) are used to select and size equipment
properly.
3. Typical Process Flow Diagram
- consists of unit operations and facilities to achieve a specified waste generation goal or goals
- typical flow diagrams for source separation see Figure 2 – 5 and 6-34 (notes), Fig. 9 – 28 (book)
 Will discuss several unit operations within the processing facility. They are:
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4. Size Reduction
- reduce size of materials so that the materials can be handled efficiently by conventional
processing and materials handling equipment
- breaks and expose materials so that they can be separated and recovered
- shredded materials/waste burns more readily increasing its value as a fuel
- size reduction equipment developed originally for mining industry where the materials is for
homogeneous feed
- terms typically used are shredding, milling and grinding
 Type of Equipment
- _________________________(horizontal shaft or vertical shaft)
- central rotor with pinned radial hammers that are free to swing
- horizontal hammer mill comes with a discharged grate below the rotor which
determines the size of the product
- vertical hammer mills moves materials by gravity down the sides of the housing with
larger clearances between the housing at the top of the mill and progressively narrows
towards the bottom (sometimes they are called grinders)
- widely used for refuse shredding
- high maintenance for rebuilding and replacing hammers
_______________________
- have two counter-rotating blades or knives that cut and shear the waste
- low speed devices
- excellent for plastics, tin cans and ductile materials
______________________
- solid waste and water added
- cutting blade and rotor converts materials to pulp
- solids content of slurry is 2.5 to 3.5% range.
- hydropulper common for materials separation in paper recycling and as a front end for
biogasification of materials
4.1 Selection of Hammer mills
- most hammer mills selection is conducted through trial runs
- models have been developed to predict final particles size distribution for a given input.
But because of the heterogeneous nature of SW, most of the models are very specific and
cannot be universally applied (see Savage et al., 1986. Unit Operations Models for Solid
Waste Processing, Noyes Publication)
The most widely used particle size descriptor is the Rosin-Rammler model, first proposed
in 1933. The equation is a generalized expression for sigmoidal curves.
Y = 1 – exp(-x/xo)n
Where n = constant, xo is the characteristic particle size defined as the size at
which 63.2% (1 – 1/e = 0.632) of the particles (by weight) are smaller.
The linear form is written as
log [ ln (1/1-Y)] = n [log x – log xo]
- two important parameters are used, mass loading (ton/hr) and power requirements
- Hammermills are highly ineffieicnet with only 0.1 % to 2.0% of the energy supplied
appearing as increased surface energy of the product solids.
- Power requirements for shredding can be modeled using empirical relationships or form
manufacturers' literature
- common equations include:
2
dE
 CL  n
dL
where E is the energy required per ton/hour of materials processed (hp. hr/ton)
L is the particle size
C and n are constants
The equation assumes that the energy needed to achieve a small size change of dL in a
unit mass of materials is inversely proportional to the size of the particle L
If n = 1, then
integrating
dE
 CL 1
dL
E = C log(L1/L2)
where (L1/L2) is the size reduction ratio, E is the work done to reduce the size from L 1 to
L2.
This is also known as the _______________________
If n = 2, we have E = C (1/L2 - 1/L1)
_________________________
Rittinger's law agrees better with rough grinding operations
Kick's Law - better approximation for fine grinding
For n = 1.5, the general equation becomes the Bond Law where it is assumed that the
work done in crushing and grinding is directly proportional to the total length of new
cracks formed in the materials being reduced in size.
E  Ei
LF  Lp
LF
100
Lp


1 
 1
E  10 E i 


 Lp
LF 


where E = specific work (kWh/ton) required to reduce a unit weight of material with 80%
finer than some diameter LF in micrometers, to a product with 80% finer than some
diameter Lp in micrometers. Ei is the Bond Work Index, a factor that is a function of the
materials processes.
or
Work Indices for common industrial materials are tabulated below
Materials
Coal
Glass
Granite
Slag
Refuse
Paper
Aluminum cans
Work Index (kWh/ton)
The rotor of shredders are usually expressed as WR2 where W is the mass of the rotor
assembly and R the radius of the hammermill tips. A wide range of WR2 is offered by
manufacturers from about 50,000 to 150,000 lb-ft2.
Typically a shredder for MSW has a width/diameter ratio greater than 1.0, a hammer
weight of 150 lb, arotor inertia of 35,000 lb-ft2, a hammer tip speed of 14,000 ft/min. As
a rule of thumb a MSW shredder must be designed for at least 15 kWh/ton/
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4.2 Environmental Issues
- Noise: A 3 ton/hr hammer mill produces 95 - 100 dBA (normal grinding sound) plus high
impact noises. OSHA requirements limit noise levels to 90 dBA over am 8-hour working day
- Dust: possible vector for transmission of pathogens and microorganisms. OSHA standards - 15
mg/m3 of total dust over an 8-hour period. Dust level can be 7 to 13 times higher than OSHA
standards. Plate counts near hammer mills may have 20 times more bacterial count than ambient
which contains about 880 organisms/m3 of air.
- Explosion: from breakage of aerosol cans
5. Size Separation
- hand sorting
- air separation
- screening
- magnetic separation
- inertial separation
- flotation
- optical sorting (based on color)
- electrostatic separation
 Separation of a mixture of materials into two or more portions by means of one or more
screening surfaces or separation devices
 usually with a go or no go screen
5.1 Performance Characteristics
Performance of size separation equipment can be evaluated in terms of
- recovery
- purity
- efficiency
For illustration purposes, if the waste stream has two components, X and Y. Assume input mass a
Xo and Yo.
After separation, there are two streams: X stream one with more X - X1 and Y1 and Y stream with
more Y - X2 and Y2 (see diagram)
X1, Y1
Xo, Yo
Screen
X stream
X2, Y2
Y stream
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Parameter
Recovery
X
Y
Comments
Possible to have 100%
recovery of X but yet
have 100% of Y in X
stream. Some
manufacturer use a
different expression
called Overall
Recovery:
 X  Y1 
100
OR X1   1

 X 0  Y0 
Purity
Efficiency
Another representation
is Worrell's Equation
 X  Y 
E ( x , y )   1  2 100
 X o  Y0 
Both expressions are
rational of perfect
separation occurs, i.e. X1
= X0, Y2 = Yo, then
efficiency is equal to
100%.
5.2 Most common Screens for Separating MSW - Rotary Drum or Trommel
- inclined cylinder mounted in rollers with holes/slots in the side
-roll at slow speed of 10 to 15 rpm
- works by allowing refuse in the screen to tumble around until the smaller pieces find themselves
next to the apertures and fall through
- can visualize tumbling motion as consisting of three types:
- cascading
- cataracting
- centrifuging
- cascading - the charge is lifted up by the circular motion of the screen and then tumbles down on
top of the layer heading upward
- cataracting - speed sufficiently great to actually fling the material into the air where it drops
along a parabolic trajectory back to the bottom of the screen. Greatest turbulence is achieved and
machine should achieve good efficiency for materials separation.
- centrifuging - materials adhere to the drum and does not drop off, low recovery
Efficieny of trommel is dependent on: rotational speed, diameter, length, angle of inclination, feed
rate, particle shape, particle distribution, screen sizes or slots
- Trommels are defined by a critical speed.
- assume a particle within the trommel located at angle  from the vertical
- the forces acting on the particle are
- centrifugal force, C;
- weight of the particle, W,
- the opposing forces are W1 = W cos 
If C > W1
centrifuging, solid adheres
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If C < W1
If C = W1
particle will fall off, cascading or cataracting
then separation occurs and the particle will leave the surface
of the trommel at a given velocity - cataracting
Therefore C = W cos 
C = W (r 2)/g
where r = radius (cm)
 = rotational velocity (rad/s)
g = acceleration due to gravity (cm/s)
W cos  = W (r 2)/g
or
cos  = (r 2)/g
substitute  = 2  n
where n is the speed of the screen in rev/s
cos  = 4 2 n2 r /g
Critical point or last chance for the particle to separate and float in the air is
when  = 0 or cos  = 1
Then 1 = 4 2 n2 r /g
or
nc = ( g/(42r)1/2 = (1/2 )(g/r)1/2
This is the critical speed, nc. Rotational speed varies from 50 to 80% of the
critical speed (approximately 10 to 18 rev/min)
See Figures for effects of speed, angle of trommel
Diameter of screen needed is given by various empirical equations


11 .36 Q m
D

0.5
 d b FK v g tan  
Q
D  7.66

where
Q = capacity tons/hr
 = specific gravity of material
D = diameter of drum (ins)
0.4
Qm = trommel capacity (lb/s)
db = bulk specifc weight of MSW (lb/ft3)
 = trommel inclination
Kv = velocity correction factor (1.35 - 1.85
depending on )
F = fillage factor (0.25 - 0.33)
g = 32.2 ft/s
6. Air Classification
- separate the light, mostly organic materials from the heavy (mostly inorganic such as glass)
- basic premise - the light materials will be caught in an upward current of air and carried by the
air while the heavier fraction will drop down
- the light fraction is then separated from the air using a cyclone separator (see Figures for typical
types of air classifier)
- can be a vertical column or one with a zigzag column with 60 or 90 o turns
- creates turbulence at the corners, causing further separation or breaking of clumps
- See Results - Figures attached.
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