Page 209 - Materials Chemistry, Second Edition

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180 Waste Management Practices: Municipal, Hazardous, and Industrial

x + y
1 1
x + y o Separator
o
x + y 2
2

FIGURE 7.9 A binary separa-
x = desired product tor receiving a mixed feed of x
y = reject and y . 0
0
This equation, however, does not take into account purity of the product. If the separation
device is not operational, then all the input (both the desired product as well as the reject) will pass
through. In other words, x x with the result that R 100%. A second requirement, the purity
0 1 (x1)
of the product, is therefore necessary. The purity of the extract stream is defined as

P (x )/(x y ) 100 (7.4)
(x1) 1 1 1
There are also difficulties with using purity alone as a descriptor of separator performance. For
example, it might be possible to extract a small amount of x in a pure state, but the recovery (R )
(x1)
will be very small. It is therefore necessary to describe the operation of a materials separation device
by incorporating both the recovery and purity. Binary separator efficiency can be determined as
(Rietema, 1981)

E (x /x )(y /y ) 100 (7.5)
(x,y) 1 o 1 0

EXAMPLE 7.3
An eddy current separator (see below) is to separate aluminum product from an input stream of
shredded MSW. The feed rate to the separator is 1500 kg/h. The feed is known to contain 55 kg of
aluminum and 1445 kg of reject. After operating for 1 h, a total of 65 kg of material is collected in
the product stream. On close inspection it is found that 46 kg of product is aluminum. Calculate the
percent recovery of aluminum product, the purity of the product, and the overall efficiency of the
separator.

R x /x 100 46/55 100 83.6%
(x1) 1 0
P x /(x y ) 100 46/65 100 70.8%
(x1) 1 1 1
E (x /x )(y /y ) 100 46/55[(1500 65) (55 46)]/1445 100 82.5%
(x,y) 1 0 1 0

7.4 MATERIALS SEPARATION AND PROCESSING AT THE MRF

7.4.1 HAND-SORTING
The most basic and simple method for the separation of materials from MSW is hand-sorting.
Workers take positions along a conveyor belt, either on one or both sides (Figure 7.10). Sorting
takes place after the bags have been opened in a trommel screen or simple shredder. At a clean MRF
the material may arrive already in loose form.
At the MRF, picking can occur at several points along the system, and the workers have two pri-
mary functions. First, they recover any items of potential value that do not need to be processed.
Items such as metal and PET bottles are set aside in bins or chutes. Their second responsibility is