Chemical Engineering August 2023 - 33

increase human exposure to chemical
and toxic respiratory hazards.
Evaluating risks
Fortunately, flowability, process and
safety risks can and should be identified
before selecting comminution
and screening equipment. Assessing
these risks can be broken down
into three steps:
FIGURE 8. Segregation of fine and coarse particles
can occur in gyratory screener
sive, dust explosivity risks can be
drastically increased if crushing leads
to increased dust generation. According
to a study conducted by the
U.S. Chemical Safety Board (CSB;
Washington, D.C.; www.csb.gov),
dust explosions pose a significant
safety hazard, resulting in 718 injuries
and 119 fatalities in the U.S. between
1980 and 2005 [3]. Additionally,
increased dust generation can
1. Evaluate the material's physical,
chemical and flow properties
2. Conduct a flowability and sizereduction
review
3. Perform preliminary safety
assessments
Material characterization is paramount
not only in selecting sizereduction
and
screening
equipment,
but also in the design of all
solids handling equipment. Just as
it is imperative that a car mechanic
run diagnostics tests to understand
how a car's engine and transmission
work together before beginning
repair work, it is critical to fully
evaluate the material being crushed
or screened before selecting comThe
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minution and screening equipment.
Unlike liquids and gases, you won't
find critical solids-flow property data
in textbooks or manuals. Rather,
flow property testing must be performed
to establish design criteria.
Common flow properties evaluated
include the following:
* Cohesive strength - Used to determine
hopper outlet sizing to prevent
cohesive arches and ratholes from
forming, as well as plugging, caking
potential and buildup
* Wall friction - Used to evaluate
solids flow along the inside surfaces
of the hoppers, transfer chutes, and
screens. Commonly used to determine
critical hopper angles to avoid
funnel flow pattern
* Compressibility - Used to establish
the relationship between consolidating
pressure and bulk density
Flow properties are not the only
characteristics needed before selecting
comminution, screening and
other solids-handling equipment. As
discussed earlier, most powders are
explosive, and if conditions align,
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AUGUST 2023
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Chemical Engineering August 2023

Table of Contents for the Digital Edition of Chemical Engineering August 2023
