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State of California, Department of Transportation - page 23 / 52

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Baghouse Dust Return

When baghouse dust is returned to the mix in a batch plant, it must be proportioned by mass and as required in Section 39-3.03 of the Standard Specifications. The typical system used for returning baghouse dust would include an auger that carries dust from the baghouse to the storage

silo, large storage silo, a (high speed) to transport

weigh pot, and a system the dust from the weigh

pot

to

the

pug

mill.

The

batch

controller

for

this

system must be plant controller.

interlocked to the primary batch The zero and batching tolerances

(interlocks) for this the same as the

type of dust return system are tolerances for “supplemental

fines”, can be found in Standard Specifications. shows a typical system proportioning.

Section 39-3.03 of the The adjacent photograph used for baghouse dust

Hot Aggregate Screening And Storage After the aggregates have been heated and dried, a hot elevator carries them to the screen deck. The screen deck is made up of several different sized vibrating screens. The top screen in the deck is always a scalping screen, which removes oversized

size from top

to bottom

follow the

top

screen. At the

bottom of

the deck is

the

rock.

Intermediate

screens,

decreasing

in

sand

screen.

will

be used

The number of hot bins that to store the different sized

aggregates usually screens in a deck.

determines

the

number

of

The screen deck separates the aggregates into specific sizes. For this process to work properly, the cold feed rates must be in balance with the screen deck capacity. When excessive amounts of aggregate are fed over the screen deck (blinding the screens), or the screen openings become plugged, smaller sized particles that should have passed through there respective screen opening, ride over the screens and drop into a bin designated for a larger sized aggregates. This is commonly referred to as "carry-over". On the other hand, if screens are worn or torn, oversized material will be deposited into bins intended for smaller sized aggregates.

In any case the screens should be inspected and cleaned regularly by the Contractor.

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