Technology News

From the Bureau of Mines, United States Department of the Interior

Technology news describes tested developments from Bureau of Mines programs, tie published to encourage the
tansfer of this information to the minarals industry and its application in commercial practice. Mention of company

(of product names is for documentation only and does not imply endorsement af a specific firm or product.

Bureau of Mi

810 7th St, NW., Washington, DC 20241-0001.

No. 437, May 1994

research Is performed and reported under mandate of the Unit
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tad States Congress, For a free
fer, Bureau of Minos, MS 820,

Total Mill Ventilation System for Mineral
Processing Facilities

Objective

Reduce the dust exposure of workers in large process-
ing buildings at mineral processing operations, using a
total mill ventilation system,

Background

Many mineral processing operations have difficulty
meeting Fedcral standards for respirable dust. All mineral
processing operations use some type of engineering con-
trols to address their most significant dust sources. En-
gineering controls include such things as baghouse-type
dust collectors, scrubbers, and electrostatic precipitators.

Although the most serious dust sourees are being ad-
dressed at these operations, there are usually a number of
minor, less significant dust sources that are not addressed.
Some of these minor dust sources do not lend themselves
to being controlled by standard engineering techniques.
These minot dust sources include such things as product
residue on walls, beams, and equipment, which becomes
airborne from the vibration of the processes and cquip-
ment operating within the mi roduct on walkways and
access areas, which becomes airborne as workers walk
throughout the mill; dust released or generated from im-
proper housekeeping practices; and product rcleascd from
screens or other milling processes when covers or lids are
opened.

Jo mineral processing operations (hat have many minor
dust sources, dust levels gradually increase as the day or
shift progresses. In some cases, the combined effect of
these minor sources can cause mill workers to exceed

Federal compliance standards for respirable dust. One
cost-effective supplemental dust control method to reduce
the cffect of these minor dust sources is the use of total
mill ventilation systems.

A total mill ventilation system can lower mill-wide dust
concentrations, hence reducing respirable dust exposures
of workers in mineral processing operations. The system
uses clean air drawn in at the base of the building to dilute
and remove dust from contaminated areas within the mill
structure. This dust-laden air is then exhausted out ncar
the top of the building,

How It Works

A total mill ventilation system draws dust-free makeup
air in near the base of the mill and then uses this air to
provide a gradual purging of the building, thus clearing
some of the dust-laden areas throughout the mill structure.
Wall louvers are installed to control where the makeup air
is brought into the structure. The makeup air must be un-
contaminated or it could increase dust levels within the
structure.

The air is discharged at or near the top of the structure,
away from plant personnel working outside. This total
mill ventilation system uses axial fans positioned at various
locations either across the roof or very high on the walls
of the structure, The quantity of air exhausted is normally
in the range of 10 to 30 air changes per hour based on the
volumetric capacity of the building. Dust levels in the
structure, height and competence of the outside walls of
the structure, and weather conditions are a number of the
factors that account for the variation in the ventilation

This document was prepared by the U.S. Bureau of Mines. Neither the United States Government nor any person acting on behaif of the United
Slates Government assumes any liabiity resulting from the usc of the information contained in this document, or warrants that such use be free from.

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range, In addition, thermodynamic effects from heat gen-
erated by mil! equipment produce a chimney effect, thus
assisting the basic flow pattern from the bottom to the top
of the structure,

Without this exhaust system, dust can exit the building
through open doors, windows, etc., and contaminate out-
side plant workers. It can also be recirculated back into
the mill building, causing additional contamination prob-
lems, A total mill ventilation systcm is not a stand-alone
technique, but a supplemental technique to assist other
localized dust control systems.

Approach and Results

Total mill yentitation systems were installed and eval-
uated at two mineral processing operations. Both systems
proved (o be very cost effective at reducing respirable
dust levels throughout the structure and reducing worker
exposure.

‘A 25,000-cubic-foot-per-minute (cfm) system installed
at a clay processing mill provided approximately 10 air
changes per hour at this 150,000-cubic-foot (f°) structure.
This system reduced respirable dust concentrations by ap-
proximately 40 percent throughout the mill building. The
graph shows respirable dust concentrations recorded by a
RAM-1 instantaneous respirable dust monitor for times
when the ventilation system was on and off. The cost of
all material and installation for this system was approx-
imately $10,000.00,

The second evaluation was performed at a silica sand
operation. Tests were performed with $0,000 and 100,000

cfm of ventilation to the 177,000-ft? mil building, cor-
responding to 17 and 34 air changes per hour. Average
mill-wide respirable dust concentrations were reduced 36
and 64 percent, respectively, at a cast of only $6,000.00.

Not only did these systems reduce respirable dust con-
centrations and increase visibility throughout the mills,
they were also easy to install and require minimal main-
tenance. A total mill ventilation system provides a general
purging of the mill air; the system should be viewed as a
supplemental technique to assist other dust control systems
in operation. The total mill ventilation system ranks high
when compared with other process-specific dust control
techniques on a cost-benefit basis. Considering the cost
awareness of the industry and the new pending respirable
dust standards for metal/nonmetal operations, the total
mill ventilation system is a technique that deserves
consideration,

For More Information

Mote information is given in USEM Report of Investi-
gations 9469, "Reducing Respirable Dust Concentrations
at Mineral Processing Facilities Using Total Mill Venti-
lation Systems." For additional information, please contact
the principal investigator for this research: Andrew B.
Cecala, U.S. Bureau of Mines, Pittsburgh Research
Center, P.O. Box 18070, Cochrans Mill Road, Pittsburgh,
PA. 15236-0070, Telephone: (412) 892-6677.

TM—T T T T

i
Ventilation off ——————-———-. Ventilation on

—T —— T

¢

Ventilation on-

DUST CONCENTRATION, RAM-1 units

Ventilation off

I I ri
‘90 100 iTfe) 120

130

140 150 160 170

TIME, min

Figure 1--Respirable dust concentrations during evatuation of total mill ventilation system.