SILICA & DUST EXPOSURE
CONTROL HANDBOOK
(Rev. 1)
NIELSEN
C O N S U L T I N G
Prepared on: 20-February-2014
Revised: —
Prepared for:
Alberta Sand & Gravel Association
Alberta Roadbuilders & Heavy Construction Association
Prepared by:
Nielsen Consulting
201 Ridge Road
Bolton, Ontario
L7E 4V8
© Svend G. Nielsen
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Table of Contents
Page
1.0 INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2.0 BACKGROUND: WHAT IS SILICA & HOW IS IT HARMFUL?. . . . . . . . . . . 4
2.1 What is Silica?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 What Kinds of Silica are Harmful?.. . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 Health Effects of Silica Exposure.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4 Silica Regulation in Alberta. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.0 GENERAL APPROACHES TO SILICA EXPOSURE CONTROL. . . . . . . . .
3.1 Risk Factors.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Exposure Profiles.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Hierarchy of Controls. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4 Management Programs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
16
17
19
21
4.0 ENGINEERING CONTROLS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1 Debris & Spillage Prevention. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.1 Sources of Spillage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.2 Controlling Spillage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Fugitive Dust Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.1 Sources of Fugitive Dust. . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.2 Fugitive Dust Containment.. . . . . . . . . . . . . . . . . . . . . . . . .
4.2.3 Fugitive Dust Prevention & Suppression. . . . . . . . . . . . . . .
4.2.4 Fugitive Dust Capture. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
25
26
28
34
34
35
39
45
5.0 WORKER ISOLATION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1 Environmental Cabs.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Control Rooms & Towers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Ancillary Work & Rest Areas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
49
49
50
52
6.0 WORK TASK CONTROLS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
6.1 Grounds & Plant Cleanup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
6.2 Routine Maintenance & Inspection. . . . . . . . . . . . . . . . . . . . . . . . . . 55
7.0 ADMINISTRATIVE CONTROLS.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
8.0 PERSONAL PROTECTIVE EQUIPMENT. . . . . . . . . . . . . . . . . . . . . . . . . . 60
APPENDIX A – Glossary of Terms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
APPENDIX B – Web and Print Resources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
ii
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
ASGA & ARHCA
SILICA & DUST CONTROL HANDBOOK
1.0 INTRODUCTION
Occupational exposure to Respirable Crystalline Silica (RCS) is associated with the
development of adverse health effects including silicosis. Silicosis is a progressive,
irreversible, and potentially fatal lung disease.
Silica is the most common mineral in the earth's crust and therefore is present in
almost every process where natural minerals are handled. The metal and nonmetal
mineral processing industry involves the extraction, processing, manufacturing and
use of raw materials and products containing crystalline silica.
Aggregate
operations include a wide spectrum of processes (e.g. rock drilling, crushing,
conveying, etc.) and manual tasks (e.g. maintenance activities, housekeeping, QC
analysis, etc.) involving potential employee inhalation exposures to crystalline
silica-containing materials.
The Alberta Sand and Gravel Association, and the Alberta Roadbuilders and Heavy
Construction Association are committed to promoting a safe and healthful work
environment for employees and contractors of their member companies. This
commitment is demonstrated through the advancement of health and safety
practices and programs, including the establishment of practices as described in
this document.
The primary goal of this effort is the prevention of potential silica-related illnesses
in member companies through the implementation of an occupational health
programs that consist of hazard recognition, exposure assessment, risk
management and control.
1
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
A summary of the objectives include:
•
Disease prevention - Implementing a comprehensive management and
control program that limits occupational exposure to silica and respirable
dust in accordance with established occupational exposure limits.
•
Industry leadership – Building on existing occupational health and safety
programs to achieve industry leadership in occupational health programs for
silica.
•
Program efficiency – Implementing common programs applicable to various
product lines to ensure compliance with occupational exposure limits.
The elements contained in this guide reflect current industry-wide practices,
consensus standards, and regulatory standards in Alberta, that have been
developed to reduce the probability of harmful occupational exposures and disease
to workers in work environments containing silica. The guide includes the following
elements:
•
Silica Hazard Awareness
•
Site Exposure Control Plans
•
Occupational Exposure Limits
•
Hierarchy of Control Methods
•
Regulatory Requirements
•
Engineering Controls
•
Risk Factors
•
Worker Isolation
•
Exposure Mechanisms
•
Work Task Controls
•
Management Programs
•
Administrative Controls
•
Respiratory Protection
2
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Please note that for the remainder of this document, “respirable crystalline silica”
will simply be referred to as “silica”; and “respirable particulate matter, not otherwise
regulated” will be referred to as “respirable dust”.
3
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
2.0 BACKGROUND: WHAT IS SILICA & HOW IS IT HARMFUL?
2.1 What is Silica?
Silica is a naturally occurring mineral, found in all quarried stone, sand and gravel.
It is the most abundant mineral on the face of the earth. There exist three main
species of silica: Quartz; Cristobalite; and Tridymite.
The most commonly
encountered species of naturally-occurring silica at aggregate sites is Quartz. Of
note is that through more than 12 years of testing at aggregate sites throughout
North America, only one site had Cristobalite silica species present (a shale deposit
in British Columbia), and none of the tested sites showed Tridymite detected. All
other locations tested have had only Quartz present in the stone.
The amount of silica in the aggregate can vary greatly, depending on the location
of the site, the type of rock, and even the depth of extraction in the case of quarry
benches. Based on bulk samples collected at sites across North America, the
abundance of silica in the rock typically ranges from:
•
2% to 10% in limestone
•
25% to 60% in granite
•
25% to 85% in gravel
•
70% to 95% in sandstone
Alberta, in its sand and gravel deposits, has perhaps the highest silica levels found
in North America, when viewed over a large geographic region. Isolated sites in
other areas may have higher levels, but on an overall regional basis Alberta
consistently has the highest quartz content in crushed stone on the continent.
4
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Following are typical silica levels found in various regions of Alberta (based on
analysis of bulk samples collected by Nielsen Consulting):
Greater Calgary area – 50% to 65% in gravel
Canmore area – 1% to 8% in gravel and quarried limestone
Lethbridge area – 65% in gravel
Crowsnest Pass area – 1% in quarried limestone
Red Deer area – 75% in gravel
Greater Edmonton area – 55% to 95% in gravel
All aggregates, and therefore all operations processing or handling aggregates
regardless of product line, have some amount of silica present in the material being
handled. This also applies to demolition operations or processes which recycle
paving materials – the aggregate portion of such materials (sand, gravel) will
contain silica.
5
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
2.2 What Kinds of Silica are Harmful?
It is the physical shape and size of the silica particles which determines whether
they are harmful or not. All forms of silica are chemically the same, having the
chemical formula SiO2, (silicon dioxide), but it is the physical properties of the
particle which determine its potential impact on the body.
The two determining factors are:
(A) Crystal Structure – the particle must be a crystalline form of silica; and
(B) Respirable Size – the particle must be small enough to be respirable into
the deep lung area.
Crystalline Silica only is harmful to the lungs. The dust particle has a crystal
shape which has sharp edges and points which can cause permanent damage to
delicate lung tissue (see the following section for more on health effects).
Non-crystalline forms of silica (known as amorphous silica; eg. silica fume as used
as an additive in ready mix operations) are the less hazardous forms of silica, and
are typically considered to be nuisance dusts. Non-crystalline silica does not have
sharp edges or points on the particles, and cannot therefore do the same damage
to the lung tissue as crystalline silica can.
6
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Crystalline vs. Non-crystalline (Amorphous) Silica Particles
Note the sharp edges and points on the crystalline particle, compared to the
spherical shape of the amorphous particle. The amorphous particle in this image
is silica fume, an additive in ready mix. (Source: x-ray micrograph of a dust sample
taken at a ready mix plant)
Respirable Crystalline Silica only is harmful – that is, particles small enough to
enter the deep lung regions of the body. This means that only particles less than
a mean diameter of 5 microns (micrometers) can enter the deep lung passages, or
alveoli. Since silica is only damaging to this deep lung tissue (alveoli), only those
particles smaller than 5 microns (ie. respirable size) are of concern. Larger
particles are not able to reach the fine alveolar airways and are deposited in the
mouth, nose, throat, or bronchial (large) passages of the lungs, from which they are
eventually cleansed from the respiratory tract.
7
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
2.3 Health Effects of Silica Exposure
The primary disease caused by inhalation of respirable crystalline silica is Silicosis.
When a crystalline silica particle of respirable size is inhaled, it may become
deposited in the alveoli, where it becomes lodged and cannot be expelled by the
lung’s defense mechanisms. Upon deposition, the sharp edges of the crystal
particle make micro-fine cuts in the delicate side wall tissue of the alveoli.
Healthy Alveoli
(deep lung airway)
This image shows an
unobstructed airway, ~3000x
magnification; note the thin,
delicate, blood-rich tissue of the
alveoli wall (the dark spots are
blood capillaries)
In response to the cuts in the alveoli, the lung’s healing process creates scar tissue
at the site of the wound. The scar tissue forms a small ball of hard, dense material
called a fibrotic nodule. The lungs will continue to wrap the silica particles in scar
tissue, and this process continues even if exposure to silica stops. The progression
of scar nodule formation, as more silica particles are inhaled and deposited, results
in a condition called Fibrosis.
8
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Alveoli Obstructed by Fibrotic
Nodule
This image shows a deep lung
airway obstructed by scar tissue
caused by wounding from a silica
crystal
Scar tissue in the lungs cannot respire gases in or out of the blood. In other words,
it cannot pass oxygen into the body nor expel carbon dioxide. Over many years the
lungs lose respiratory capacity as the scar tissue progression begins to take over
healthy lung tissue with fibrotic nodules. The lungs also lose their natural elasticity
(scar tissue is inflexible) and the normal expansion and contraction of the lungs
through breathing becomes difficult.
The end result is constant shortness of breath, even at rest, and painful breathing.
In severe cases, there is early death from cardiac failure due to lack of oxygen.
9
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Healthy Lung – Note the dense
spongy tissue; these are the
deep lung areas where the fine
air passages (alveoli) are, and
where the silica crystals become
lodged.
Silica Diseased Lung – The
alveoli have been overtaken by
silica-induced fibrotic nodules
(in this case, a coal miner’s lung;
the black coloration is from coal
dust containing silica)
Silicosis is IRREVERSIBLE – the lungs cannot remove or regenerate scar tissue
nodules into healthy tissue again. There is no medical treatment or procedure
which can reverse this condition.
10
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
2.4 Silica Regulation in Alberta
Silica is a regulated substance in all jurisdictions in North America, including
Alberta. In the United States, workplace exposure to chemical agents is federally
regulated. In Canada, each province has its own regulations concerning worker
health, and exposure to silica is part of each one. Concerning silica specifically,
these provincial regulations vary in scope from basic, generic requirements for
limiting exposure, to more prescriptive detailed standards which establish a
comprehensive set of responsibilities for assessing, controlling and managing this
risk.
In Alberta, the exposure of workers to silica is governed under the Occupational
Health and Safety Code (as updated from time to time), made under the
Occupational Health and Safety Act.
Part 4 of the OHS Code concerns “Chemical Hazards, Biological Hazards and
Harmful Substances”. Note that this section is presently under review by the
Alberta government, and the revised Code is scheduled for release in 2015.
Present silica requirements are summarized below. For full details of all regulatory
requirements the OHS Code must be referred to.
Employer Obligations:
•
An employer must ensure that a worker’s exposure to silica is kept as low as
reasonably achievable.
•
An employer must ensure that a worker’s exposure to silica does not exceed
its occupational exposure limit.
•
Where workers may be exposed to silica, an employer must conduct a health
hazard assessment.
11
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
•
Workers exposed to silica must be informed of the health hazards, the
results of exposure measurements, and exposure controls in place.
Worker Obligations:
•
Workers exposed to silica must use the provided training and control
procedures.
Under Part 4 of the OHS Code there is a requirement for a workplace “Code of
Practice” for silica. Both the ARHCA and ASGA are providing their members with
a respective generic silica code of practice. In summary, these codes define
employer and worker responsibilities, including, but not limited to:
•
Health assessments
•
Exposure control plans and procedures
•
Worker exposure monitoring requirements
•
Worker training
Further general provisions regarding silica control in the workplace include:
General Provisions Regarding Silica:
•
An employer must minimize the release of silica into the air as far as is
reasonably practicable.
•
An area where silica is present is to be considered a “Restricted Area” with
limited access, warning signs posted at access points, and provision of
protective clothing.
An employer must undertake a health assessment program for workers exposed to
silica, including the following elements (refer to Code for detailed requirements):
12
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Health Assessments, Silica:
•
Medical exam, including chest x-ray and spirogram;
•
Medical exam to include a history of occupational exposures to asbestos,
silica, coal dust or other industrial dusts in ALL workplaces, past and
present, and during non work-related activities;
•
Workers must undergo the health assessment within 30 days after the
worker becomes exposed to silica, and every two years thereafter.
Permissible Exposure Limits, Silica and Respirable Dust
In the present Alberta OHS Code (2009), worker exposure limits to chemical agents
are presented in Schedule 1, Table 2.
Under the Code, following are the
permissible limits for silica and respirable dust:
•
Respirable Crystalline Silica (Quartz, Cristobalite) – 0.025 mg/m3, TWA (time
weighted average), 8 hours; no short term limit or ceiling limit. Silica is also
designated as an “A2" or suspected human carcinogen.
•
Particulate Matter, Not Otherwise Regulated, Respirable – 3.0 mg/m3, TWA,
8 hours; no short term limit or ceiling limit.
Compensation for Extended Work Shifts
In Alberta, regulatory exposure limits are referenced to work shifts of 8 hours per
day. For all substances, the OHS Code incorporates an exposure limit reduction
for work shifts longer than 8 hours. This compensates for the shortened recovery
period experienced by workers who are exposed to chemical agents for longer than
8 hours, by lowering the allowable concentration limit to which the workers may be
exposed. (See Part 4, Section 18 of the Code).
13
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Note that the Alberta OH&S ministry has recently stated a position that
compensation for extended work shifts is no longer applicable for silica exposure
measurements in this province. Therefore the permissible limit for silica need not
be adjusted (reduced) for any extended work beyond the standard 8 hours.
The reduced limit criteria for extended work shifts still applies to worker exposure
to respirable particulate matter.
14
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
3.0 GENERAL APPROACHES TO SILICA EXPOSURE CONTROL
The controls presented here are a basic overview of possible options for application
at an aggregate plant. Some of the controls may only be suitable for stationary
plants, while others are feasible solely for portable crushers. Every plant is unique,
and the employer should apply those options which he/she feels will be effective in
reducing exposure, based on the unique and particular process components, plant
conditions, and exposure risks. It is highly recommended that each plant perform
a risk assessment with the objective of determining How, When and Where their
employees may be receiving exposures, so that the subsequent control plan can
effectively target those parts of the operation which are at the source of the
problem(s).
In order to adequately and permanently reduce worker exposures to acceptable
levels, most sites will likely need to apply at least two, or sometimes more, control
strategies. As a worker’s possible exposure mechanisms may originate from
multiple sources, it is usually not sufficient to employ only one type of control. The
overall control plan must be multi-faceted and use a variety of methods in order to
be effective.
If more detailed information is required than is contained in this handbook,
Appendix B contains a list of print and web resources, which will further assist in
selecting and designing specific controls.
Before undertaking a silica control effort, it is important to understand what factors
influence the level of risk to workers. With this knowledge, a proper assessment
can then be made of who is exposed, and how and where exposures are occurring.
This will then guide the subsequent control effort, and ensure that resources are
directed where they will have the most benefit.
15
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
3.1 Risk Factors
The four main factors which influence the level of risk for a group of workers are:
1) Silica Content – higher silica content in aggregate = higher exposure risk
2) Fugitive Emissions – primary emissions of dust directly from the process
3) Work Task Emissions – dust disturbed by manual or mechanized work
(shoveling, skid steer, sweeping)
4) Duration of Exposure – length of time workers are exposed to fugitive dust
and/or work task emissions
Having a basic understanding of all four of these factors is important to in order to
properly design and implement a control plan. That is, knowing the silica content
of stone being handled; whether the workers at a site are receiving the majority of
their exposure from fugitive emissions, and/or from performing work tasks which
generate dust; and how long their exposures are to both sources, are all necessary
facts to be learned. A basic “exposure profile” can then formulated which will then
help guide the selection of appropriate controls.
16
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
3.2 Exposure Profiles
In order to understand how a worker receives his/her exposure, the following
illustrates the determinants of how much dust a worker may be exposed to during
a shift:
Dose = C x T
where:
Dose = exposure dose of a worker, averaged over a full shift
C = average concentration of airborne respirable silica
T = time (duration) of exposure
(Note: this formula is for illustrative purposes only, and is not used for actual exposure calculation)
Therefore, we can see that in order to reduce the exposure dose of a worker, a
reduction is required of: (i) the concentration “C” of the dust to which he is
exposed; or (ii) the time “T” which he is exposed to the dust; or (iii) both “C” and “T”.
Most successful control strategies focus on reducing both the exposure
Concentrations and the Time to which a worker is exposed to the emissions.
An example of what a typical exposure profile of an aggregate plant ground
operator over a normal work day may look like, is illustrated in the following graph:
17
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Ground Operator Exposure Profile (example)
Minimal exposure controls, 12 hour shift
We can see that this worker had a 40% overexposure to silica (based on a limit of
0.025 mg/m3), and that he was receiving his exposure from two main sources: (1)
work task emissions from clean up in an open skid steer and by shoveling; and (2)
fugitive emissions from the process through time spent in the plant. In order to
effectively reduce his exposure to an acceptable level, it is clear that both the work
task emissions and the fugitive emissions will have to be controlled. These controls
can address either the amount of emission (concentration, C), or the time spent in
that emission (time, T), or preferably both.
18
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
3.3 Hierarchy of Controls
A range of control strategies is recommended for eliminating, reducing, or
maintaining employee exposures to silica below established exposure limits. The
control measures are ranked as to their reliability and effectiveness in eliminating
or controlling health hazards. The hierarchy of control methods identified in the
following table is based upon the principles established within consensus standards
(ANSI®, CSA), professional associations (ACGIH®, AIHA) and the government (U.S.
and Canada OSH regulatory agencies). When airborne silica and respirable dust
cannot be controlled through engineering design or other effective means (or as an
interim measure until such controls can be implemented) a respiratory protection
program should be implemented.
When decisions are to be taken about appropriate selection of controls, the table
should form a guide to prioritizing the most preferred and effective controls.
19
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
HIERARCHY OF EXPOSURE CONTROLS
Most
Effective
1. Engineering Control - Elimination or Substitution
Eliminate or substitute silica-containing materials
or processes (e.g. non-silica media for abrasive
blasting activities)(Note: this control is otherwise
not feasible for aggregate plants).
Most
Preferred
2. Engineering Control - Exhaust Ventilation
Application of collective protective measures at
the source of the risk such as adequate
ventilation and appropriate organizational
measures (eg. local ventilation).
3. Engineering Control - Control Source of Exposure
Design appropriate work processes and
engineering controls and use of adequate
equipment and materials in order to avoid or
minimize the release of silica into the atmosphere
(eg. process enclosure or isolation).
4. Administrative Controls – Work Practices
Implementation of procedural controls including
worker rotation, limited exposure period, or
modifying work practices or posting work areas to
prevent or reduce employee exposure to silica.
5. Respiratory Protection (PPE)
Least
Effective
PPE is only be used if other control measures:
cannot sufficiently reduce exposures, are not
feasible, or as an interim control until other
controls can be implemented.
20
Least
Preferred
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
3.4 Management Programs
In addition to the potential health impact on exposed workers, there are several
risks and potential losses to companies arising from the uncontrolled exposure of
their workers to airborne silica dust. Tangible risks include:
•
Compliance action through regulatory enforcement, including fines, loss of
production, management effort to handle the cases; etc.;
•
Workers compensation costs through increased premiums and penalties;
•
Health support costs for affected workers;
•
Direct costs associated with affected worker reassignment to low-risk duties,
replacement worker training, loss of skilled employees in key roles.
Less tangible risks may include worsened relations with workers, the health and
safety committees, and/or unions, if worker health issues such as silica exposure
risks are not dealt with in a comprehensive and transparent manner. Worker
cooperation with company initiatives and worker productivity may suffer, which
cause indirect losses to the company.
In order to effectively minimize the potential impact of such risks, it is recommended
that ASGA and ARHCA member companies establish internal corporate
management and control programs as part of their overall health and safety efforts.
Note that some of the basic elements of such a program form the requirements of
silica exposure control under the OHS Code in Alberta, and the establishment of
a program will help a company to meet those obligations.
An internal corporate silica and respirable dust management program should have
the following recommended elements. This is presented with the understanding
that each company is unique and has differing needs with respect to the scope of
21
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
such a program, depending on size of workforce, number and type of operations of
concern, etc..
Silica Management Program Elements
•
Exposure Standards and Action Limits:
• Exposure standards will follow OHS Code limits, and kept updated with any
changes;
•
Exposure Monitoring Criteria:
• Minimum requirements to be followed by persons conducting worker silica
exposure monitoring;
• Partly dictated by the OHS Code, respecting sampling/analysis methods.
•
Worker Hazard Awareness Training:
• Required by the OHS Code, this is to educate workers on exposure risks,
safe work practices, and company policies;
• Should include ongoing training, new-hire training, and regular tool-box
refresher talks.
•
Medical Surveillance:
• Required by the OHS Code;
• Includes biannual medical exams (as per the OHS Code); worker exposure
history at present and past employers, as well as non-occupational
exposures; and record keeping requirements.
•
Preferred Controls:
• Internal standards regarding preferred control strategy (see Section 3.3,
Hierarchy of Controls);
• The OHS Code states basic requirements regarding permanent controls vs.
PPE.
•
PPE Requirements:
• A core part of all exposure reduction methods, this establishes minimum
standards to be followed so that adequate protection is used according to
the severity of the exposure at a site;
• May also include establishment of specific tasks, conditions, areas, or
scenarios where respiratory protection is mandatory.
22
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
•
Site-specific Control Plans:
• A minimum requirement of the OHS Code, which requires that a Code of
Practice be developed and implemented for all sites where silica poses an
exposure risk;
• Plans should consist of several elements, including establishment of
restricted areas; PPE policy; safe work practices; minimum worker exposure
controls to be implemented and maintained (engineering, work task,
administrative controls).
•
Record Keeping and Worker Communication:
• Maintenance of worker exposure records, as required by the OHS Code;
• Communication of exposure monitoring results with workers and the Health
and Safety Committee.
•
Corrective Action Management:
• Establishes the mechanism by which overexposure findings are dealt with,
including communication with workers, implementation of interim and
permanent controls, and case-closure protocols such as follow-up monitoring
and assessment.
As stated above, it should be the goal of all member companies of ASGA and
ARHCA to implement some form of silica management and control program, in
order to prevent adverse permanent health impact for its workers, to meet the
obligations established by the Alberta OHS Code, and to minimize associated costs
and other forms of loss arising from this risk.
As assistance to the member companies achieve part of this goal, the following
sections are intended as a general guide to controlling exposure of workers to
airborne respirable particulate and crystalline silica dust. They cover four main
control strategies that can be applied to solving exposure issues:
(1) Engineering controls
(2) Work task controls
(3) Worker isolation
(4) Administrative controls
23
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Respiratory protection will also be discussed, as it is an important tool in reducing
exposure as an interim measure until permanent controls can be implemented, or
as the main control for certain conditions and tasks.
Note that Elimination or Substitution as an engineering control is not part of this
document. Silica is an integral part of the materials being handled and processed
at ARHCA and ASGA member-company sites, and cannot therefore be substituted
or eliminated from the operations. The exception to this is abrasive blasting, where
substitution is a valid control for any sites which conduct abrasive blasting in
aggregate
or
non-aggregate
operations
(eg.
precast
panel
finishing;
equipment/machinery refurbishing, etc.). In such cases substitution with non-silica
abrasive media would be appropriate. Other examples of substitution or elimination
of silica may also exist, but will not be highlighted here.
24
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
4.0 ENGINEERING CONTROLS
The engineering controls discussed here will focus on two main aspects: (1)
preventing debris and spillage from falling onto plant structures and the ground; and
(2) preventing fugitive emissions. Both are important for reducing plant worker
exposures.
Prevention of spillage will greatly reduce work task emissions from cleanup jobs
(skid steer, shoveling) and reduce the amount of time spent in the plant by
minimizing the need to perform these tasks.
Reducing fugitive emissions will reduce the concentration of dust present in the
ambient environment of the plant to which workers are exposed.
Further control measures will be presented in Section 6.0, Work Task Controls,
which are aimed at reducing emissions and time spent on cleanup, maintenance
and inspection tasks.
4.1 Debris & Spillage Prevention
The objective of preventing debris from falling to the ground is to avoid requiring
worker cleanup with an open skid steer or shovel. Labourers may be exposed to
high levels of dust from such cleanup. The emissions from the material disturbance
are very localized, being concentrated in the immediate work area of the cleanup
job.
25
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
4.1.1 Sources of Spillage
•
Crusher infeeds – caused by poor targeting of feed belt; poor containment
by feed chute
Inadequate containment at
infeed = spillage
•
Crusher discharge to receiving belt – caused by poor sealing around the
edge of the discharge chute
•
Belt heads – caused by material sticking to the belt and being flung off at the
underside of the head or at downstream return rollers
26
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Spillage from a belt
head;
and from return rollers
•
Belt tail loading areas – caused by poor targeting alignment of feed belt; poor
containment by loading area flashing or transfer chutes; material drop height
too high (bounce off; wind capture);
•
Screen decks – caused by material bouncing off; poor containment at feed
belt
Spillage on decking
from an overhead
screen infeed
•
Feeders – caused by poor sealing around the edge of a feeder at the loading
area
27
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
4.1.2 Controlling Spillage
There are several straightforward ways to contain material on the belts and in the
chutes and feeders. Some of these involve installing containment devices, others
require removing and capturing stuck-on material off the belts, and still others
involve belt adjustment.
Belt Cleaners: these are scrapers installed on belt heads at transfer points where
carry-over and debris is being flung off below the belt head. The scrapers remove
the stuck-on material at the apex of the head so that it will not fall off further
downstream.
Scraped material falls down to a dribble chute (see below).
Secondary scrapers downstream of the head are sometimes needed if the primary
scraper cannot remove all material.
Belt scraper with dribble chute
- this is a primary scraper at a
belt head with a dribble chute
below
*Note the damage to the blade
from the belt clips. Belts with
scrapers should be vulcanized.
(welded) to prevent blade
Secondary scraper with dribble chute
- this blade is set back several feet
from the belt head to remove material
not caught by the primary scraper
- the dribble chute directs scrapings
back onto a belt below
28
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Dribble Chutes: are installed at transfer points below the belt scrapers, and direct
the scrapings down onto a receiving belt, so that they do not end up on the ground
and require cleanup. (See photos above).
Return Rollers (Idlers): the use of disc-type return rollers instead of the solidcylinder type can help reduce the amount of spillage below the belt. The lesser
contact area of these rollers against the belt underside means that less stuck-on
material will be dislodged. Alternatively, the use of a return belt tension bar which
replaces almost all the return rollers, may be considered for some applications.
This effectively eliminates belt underside contact with any roller or guide,
preventing residue from being released as spillage (see picture, below).
Return belt tensioning bar
- no rollers req’d, therefore
minimal debris drops to ground,
esp. if used in combination with a
scraper at the head
29
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Feeder Skirt Boards & Dust Seals: seals (flashing) along the back and sides of
crusher outfeeds, feeder boxes, and at the loading area at the bottom of a transfer
chute. The most commonly-found side seals are vertical hard rubber, but these do
not offer effective sealing due to their poor edge contact with the belt – any belt
sagging or roller misalignment will create gaps and leakage. Rapid wear of this
type also creates gaps unless frequently adjusted. A better design are the flexible
side contact skirts (see picture, below) which have a much larger contact patch and
are able to contour to an uneven belt for effective sealing. Shaped backspill seals
at the rear (tail end) of crusher outfeeds and feeder boxes are contoured to fit the
curve of the belt, and are sometimes double-layered to ensure a good seal. Seals
are also used for fugitive dust control (see Section 4.2).
Bolt-on side seals
- these seals are angled where
they contact the belt, which is
better than vertical skirting for
improved sealing
This flexible sidecontact skirt contours to
an uneven belt for a
good seal
Note the large contact
patch of this flexible skirt
30
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Contoured back spill seal
- note the steep trough
angle of the belt; this
greatly improves
containment and prevents
fugitive emissions
Belt Trough Angle: the angle of belt trough can be increased, if possible, below
crusher outfeed and feeder box loading zones in order to better contain the material
and prevent spillage. This must be done in combination with side and tail flashing
(see above). Increased idler angle at key loading zones serves to concentrate
material to the center of the belt, raises the side height of the belt to better contain
spillage and dust, and creates a better seal at the flashing. (See photo above).
Rollers, Loading Zone: adding extra rollers at
loading zones below crusher outfeeds and
feeder boxes prevents belt sagging and ensures
that the side flashing will seal properly. Rollers
must be aligned on a uniform plane to ensure a
tight fit for the dust seals. Loading zone impact
beds serve a similar purpose.
Belt Tension: belt tension should be optimized to prevent sagging and ensure
smooth running at the loading zone to ensure a good dust seal at crusher discharge
and feeder side skirting. Insufficient tension may cause sagging, belt flapping or
vibration, leading to dust and debris leakage.
31
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Feeder Boxes & Chutes: are installed at transfer points at the tail end loading
zone of a receiving belt. The box and chute direct material to the center of the
target belt and contain material on the belt by preventing bounce-off and overflow.
They can be very useful in a tight location were head-to-target belt alignment is
difficult and material must be deflected or directed to the loading zone. The box
can be designed to have sufficient height to partially enclose the supply belt head.
Dust seals should be used at the sides and ends of the feed box to prevent
leakage.
Feeder box on a transverse belt
- the tight location necessitates the use
of a feeder box, otherwise extensive
spillage would result
- note the rubber dust seals at back and
sides
Screen Deck Covers & Skirts: prevent spillage by enclosing the entire deck with
a cover attached to tight-fitting side skirts at the perimeter of the deck. The cover
may be flexible or rigid, depending on the application. Alternatively, a heavy
“blanket” may be laid directly on the upper part of the deck to suppress stone
bounce.
This is often made from old
conveyor belting lain in wide strips down
the top of the deck; the aggregate feeds
beneath the blanket, which is configured to
allow material to flow down the screen (see
photo at right). If deck covers or blankets
are not used, then raised skirting installed
at the side of the screen deck helps to keep
material on the screen, and also shields
from wind-blown dust entrainment.
32
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Raised skirts at the side of this
screen deck keep stones from
bouncing off and keep wind out
- note the feed dampening curtain at
the head to suppress aggressive
material feed
This rigid screen deck cover tightly
encloses the deck keeping all
material and dust inside. This
application is for an indoor plant
where good dust and spillage
control is very important
Belt Tracking: should to be optimized for all belts in order to avoid downstream
targeting problems and spillage off the belt.
Below-Belt Self-Cleaning Trough: for stationary plants with difficult to control
spillage problems, such as fine material like stone dust, the use of a self-cleaning
trough mounted below a belt may be
appropriate. These troughs catch all spillage
along the full length of the belt, and have an
integral screw conveyor which continuously
cleans the fallen debris. Such systems are
fully enclosed, including top covers and side
skirting
to
prevent
spillage
and
dust
emissions.
33
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
4.2 Fugitive Dust Control
Controlling fugitive dust is essential if
there
are
workers
who
spend
extended time periods in a plant on
foot. Even at low visible dust levels in
the plant environment, if silica content
in the stone is high, then worker
exposures can exceed acceptable
limits with only brief exposure times.
Plant operators and labourers at many
sites may spend more than half of
their shift, or longer, on foot in the
plant directly exposed to fugitive dust.
4.2.1 Sources of Fugitive Dust
•
Crusher discharge, feeders – caused by impact of material; lack of enclosure at
base of chute/feeder; poor seal between chute/feeder and belt.
•
Conveyor transfer points – caused by
impact of material on receiving belt; wind
capture of fine dust from falling material
stream.
•
Screen decks – caused by agitation of material on deck; impact of material
stream on head of screen; impact of fine material falling through screen onto
receiving belt.
34
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
•
Loader and haul truck dumping – caused by sudden impact of large amounts of
material into receiving bin or vehicle.
4.2.2 Fugitive Dust Containment
Engineering controls specifically designed to contain fugitive dust are presented
below. Note that the focus is on dust containment, without the use of water spray.
These may be of particular value for those sites which can use only limited amounts
or no water (eg. winter operation; clear stone production). Dust suppression
methods using water sprays are discussed in Section 4.2.3.
Several of the same controls noted in the previous section on spillage prevention,
are also used to contain fugitive dust. All the following methods prevent fugitive
dust emissions and also control spillage:
•
side and end flashing (dust seals) on feeders and crusher discharges;
•
extra rollers at the loading zones;
•
proper belt tension;
•
screen deck covers and skirting; and
•
increased belt trough angle
Please refer to Section 4.1 for discussion of these controls.
Conveyor Head Enclosures & Chutes: are enclosures which cover the entire belt
head at a transfer point, including part of the upstream belt before the head, and
extend into an enclosed chute which carries the rock to the receiving belt or screen.
These are commonly installed in stationary plants. While not typical for portable
plants, compact versions can have useful application on portable conveyor rigs, and
full-size enclosures may be used within a mobile structure such as a short belt at
a jaw or cone crusher discharge. The transfer chute should be angled (approx. 45
deg. incline) to slow falling material and to provide it with momentum in the direction
35
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
of receiving belt travel. The enclosure should have a large enough air volume to
accommodate fast-moving, expanding bursts of dust – the extra space inside allows
the pressure to dissipate and prevents dust-laden air leakage out of the enclosure.
Transfer chute enclosure
- note the angled chute which
directs the falling material to
the direction of travel of the
receiving belt
- dust seals are installed at
the bottom of the chute to
prevent leakage
Downstream belts should be covered for approximately 10' to 15' to contain
emissions, and should have dust seals at the lower edges. Discharge end and
backspill dust seals must also be installed at the front and rear of the enclosure.
Conveyor head enclosures with
transfer chutes
This chute has vertical
material drop which
causes higher impact and
turbulence on the
receiving belt
This chute is inclined, which
eases the impact and changes
the direction of material flow
36
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Belt Covers: are installed over crusher discharge belts or downstream of a transfer
chute/enclosure to contain billowing dust at the loading zone. The covers extend
from crusher frame or enclosure wall, to usually
about 15 feet downstream so that the dust has
time to settle back onto the belt before exiting
the cover. Some covers enclose the entire belt,
from loading zone to head. All covers must
have side dust seals for their entire length, and
a flexible curtain seal at the exit opening. At the
feed end they are fitted tightly to the crusher frame or enclosure wall for a gap-free
installation. They must have sufficient air volume to accommodate expanding dust
plumes – ie. enough space inside to allow the pressure from air displacement to
dissipate, preventing leakage out of the enclosure.
Pressure Relief Vents: the crushing chambers and discharge chutes of some
crushers create significant positive air pressure which aggressively forces dust out
of any available opening, including any enclosures and downstream belt covers.
If the entire enclosure system has insufficient air volume to allow the dust-laden air
to expand, and/or is not well sealed, then this pressure should be vented through
a stack or relief vent. Dust exiting through the vent can be captured or suppressed
to prevent excessive fugitive emissions.
Wet Curtains: are thick, tightly spaced fabric strands that are hung across the
discharge openings of crusher or transfer chutes, and are used instead of flexible
rubber curtains. They are kept wet with a gentle, constant stream of water from
nozzles mounted to a dust suppression spray bar (see Section 4.2.3). Dust
adheres to the wet curtain and is contained within the chute. The water stream
keeps the curtain clean, although wash-down with a high pressure hose is
sometimes required. The curtain is mounted flush with the chute for a gap-free fit.
The water spray serves a dual purpose by also dampening the aggregate on the
37
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
belt, thereby suppressing dust emissions at downstream transfer points (see
Section 4.2.3).
Conveyor Head Elevations: all transfer points should have their conveyor heads
set to the minimum possible drop height. This serves two purposes: (1) the impact
of the material on the receiving belt is minimized; and (2) it reduces wind capture
of fine dust from the material stream by minimizing the exposed contact area. Low
drop height ensures that the material is still moving in a forward direction when it
hits the receiving belt, and is not in free fall. Less turbulence on impact means less
dust emission and spillage.
This conveyor’s drop height is
much too high. The material
is in vertical free-fall at the
loading zone, creating high
impact turbulence. Also note
the extensive wind-capture of
dust from the falling material.
Feeder Skirt Boards & Dust Seals: tightly-fitted dust seals are important for
fugitive dust containment at crusher discharges and feeders and transfer chutes.
See Section 4.1 for description.
Screen Deck Covers & Skirts: if a screen deck is a source of fugitive dust, then
a cover over all or part of the deck may be needed to contain or suppress the dust.
See Section 4.1 for description.
38
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
4.2.3 Fugitive Dust Prevention & Suppression
A well-designed water control system is one of the most common, least expensive,
and most effective means of preventing and suppressing fugitive dust emissions.
The following section will guide the proper placement and application of water
sprays. The controls will focus on two different approaches to dust control with
water sprays: (1) preventing emissions at transfer points and material feed areas
by adding moisture to the aggregate with coarse spray; and (2) suppressing
airborne dust using fine aerosol water spray.
Emission Prevention with Water Spray
In general, the objective of preventing dust emission using water spray is to make
the aggregate sufficiently damp so as to prevent fine particles from becoming
airborne when agitated, but not too wet so as to cause material flow or other
process problems. This is done using carefully calibrated coarse spray nozzles
which moisten the material. These are typically located at the start of a belt near
the loading zone, so that the water has time to soak in to the layer of aggregate as
it travels along the belt.
The water sprays should be configured in the following manner:
•
Wetting nozzles (coarse spray) are installed well upstream of transfer points,
locations are chosen based on assessment of major emission points.
•
Spray bars with multiple nozzles are mounted across the entire width of the
belts to ensure adequate coverage from edge to edge; higher volume nozzles
can be placed in the center of the bar if material layer is significantly thicker at
39
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
the middle of the belt – this adds more water to the deepest layer. Three or
more nozzles are typically needed for adequate coverage across a belt.
•
Soak-in time is maximized when spray bars are located immediately downstream
of the tail end loading area. This allows for the maximum water penetration as
the stone travels along the belt toward the transfer point.
Spray bar needed!
The dark wet stripe down
the middle of the stone is
from a single garden
hose sprayer. The stone
at the sides is dry, and
dust prevention at the
transfer point is poor. A
spray bar with multiple
coarse nozzles to cover
the width of the belt is
needed. However, this
sprayer is properly
located at the loading
zone.
•
Non-clogging nozzles (such as spiral nozzles) should be used to prevent failure
due to plugged openings. Alternatively, in-line water filters can be installed to
remove impurities.
•
Water pressure can be moderate, but pump capacity must be sufficient to supply
the necessary volume.
40
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
A spray bar located
across the feed end
of this belt would
greatly reduce the
fugitive dust at the
transfer point, and
several more
transfer points
downstream as well.
However, conveyor
drop height should
also be lowered.
A general rule when first deciding where to locate the sprays within the plant
material stream, is to start by placing them as close to the beginning of the process
flow path as possible where dry material begins emitting dust. Thus the maximum
number of downstream emission points can benefit from the spray. The effects of
the first spray bars should then be studied to determine how far along the material
path the added moisture is able to suppress the dust. Additional sprays can then
be located downstream, just before the next significant emission points.
Such coarse sprays are typically needed immediately after crushers (esp. cones),
which fracture the stone and expose the dry interior, producing fine dust. Emissions
at screen decks downstream of the crushers are often controlled as well.
Emission Suppression with Water Spray
Dust suppression with water differs from dust prevention in that it removes dust that
is already airborne using a very fine mist or aerosol. The objective is to suppress
the dust as close to the source as possible, and to provide a wind-free environment
so that maximum contact can be made between the fine water droplets and the dust
particles.
41
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
In order to knock the tiny dust particles out of the air, the sprays must produce a
greater number of airborne water droplets than there are dust particles. This can
only be achieved using very fine spray nozzles which “atomize” the water under
high pressure and create a large number of very tiny droplets, similar to a fog.
Because these tiny droplets are very susceptible to being carried away by air
currents, they should be shielded from wind.
A significant advantage of using fogging systems is that they add very little water
to the aggregate (typically 0.1% the weight of the aggregate produced). This can
be very important for controlling dust in a plant set up for clear stone production,
where adding soaking sprays to the belts would create quality control problems.
Water sprays (fogging systems) for dust suppression should be configured as
follows:
•
Mist/fog nozzles are installed inside feeder boxes, conveyor head enclosures,
transfer chutes, belt covers, and crusher discharge chambers.
•
Multiple nozzles are mounted on spray bars placed across the width of the
enclosures to ensure adequate coverage from edge to edge. Three or more
nozzles are typically needed for adequate coverage. Multiple spray bars may
be required for some enclosures to capture dust plumes traveling in different
directions at once – eg. one bar at the top of a transfer chute to capture rising
42
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
dust, and a second bar after the loading zone under a belt cover to capture dust
moving along the belt.
Conveyor head enclosure &
transfer chute
•
Fogging system
spray bar locations
Water filters prevent plugged nozzles by removing impurities. Individual filters
installed before each set of nozzles can be used, or a large central filter can be
installed on the main supply feed.
•
Water pressure must be high in order to produce fine mist. This can be
achieved with a high-pressure main supply pump, or with small booster pumps
at each set of nozzles.
Water Supply Systems
Water supply for stationary plants is typically not a concern. There is usually a well
or surface body of water to draw from. However, for portable plants, water supply
from wells or surface water is not always reliable or easily accessible. Having a
reserve supply close to the plant location is often needed, and this means that the
system must be transportable to move with the plant. For this reason, many plants
43
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
have elected to use large portable tanks with built-in pumps, which are filled
regularly with a water truck.
Small supply tank
with pump and
manifold, gravity fed
by large secondary
reserve tank
Depending on the spray bar applications, the tank or well may be fitted with a high
pressure pump to supply fog nozzles, or low pressure pump if only coarse spray
nozzles are used. To prevent nozzle clogging, inline filters at the source can be
installed to filter the whole system, or smaller filters can be used at the individual
spray locations.
A central supply manifold with individual valves to control branch flow is usually
installed. This can be placed at the tank (see picture), or at the control tower
operator’s booth for greater convenience. A pressure gauge is useful to monitor
pump performance.
Distribution manifold
44
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Water lines should be heavy duty hoses and/or pipes permanently attached to belt
frames and crusher rigs. Domestic garden hoses are not acceptable as a long term
solution.
Other Dust Control Methods using Water
Bank Watering: for quarries, the regular watering of the blast rock pile, using a
water truck sprayer or moveable sprinklers and hoses, is often very useful in
preventing dust emission early in the crushing process, such as at the primary
crusher and immediately downstream thereof. The blast rock pile at the active
extraction area should be kept damp at all times. This can be especially effective
in porous rock deposits, such as limestone or sandstone, where the rock can
absorb some moisture. In these cases, early watering for several hours before
extraction will allow some soak-in time.
Road Watering: if haul roads and access roads around the plant are heavily
traveled and produce significant dust emissions, then they should be kept damp
with regular water truck spray-down. Long term suppressants such as calcium
chloride can also be considered, especially for long haul roads.
4.2.4 Fugitive Dust Capture
Local Exhaust & Dust Collectors
Dust collectors and local exhaust inlets are typically only used at large stationary
plants. They have particular application at indoor crushing and screening facilities
where there is little natural draft ventilation for dust dilution, and control of dust at
the source is required. They are generally not considered feasible for portable
45
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
plants, although small discreet units are available for mounting on individual
crusher rigs.
Typical dust collector systems at aggregate plants consist of local exhaust hoods,
exhaust ducts, and a large central fan located downstream of a baghouse filter
enclosure. Large sites with several crushing and screening towers may have more
than one such system. In order for these systems to be effective, the hoods should
be located so as to draw air from within an enclosure such as at a crusher or screen
discharge, transfer chute, or a conveyor cover.
The enclosures contain the
billowing, expanding dust emission, allowing the exhaust hood to capture and
extract it.
Basic design parameters include: sufficient fan capacity to supply all hoods and
ducts with the needed static pressure, face and transport velocities; well-sealed
enclosures to maximize interior negative pressure; and properly balanced duct and
hood branches. The design of the enclosures (transfer points, crushers, belt
covers, etc.) is an important variable in the effective operation of a dust collection
system, and should be considered an integral part of the system. They must have
sufficient air space to allow the rapidly moving dust to expand, and the
fan/duct/hood combination must have sufficient capacity and power to keep the
enclosure under a strong negative pressure.
Proper maintenance is essential to keeping such systems running as designed,
System maintenance must not be neglected or the entire system will become
ineffective within a very short time. Regular service must be carried out diligently,
including baghouse clean out, torn or clogged bag changes, and fan maintenance.
To maintain proper exhaust capacity, damaged ducts must be promptly repaired,
and the integrity of all enclosures must be maintained, such as side and end dust
seals and curtains, belt tension and alignment, etc..
46
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Despite their high cost, dust collection systems can be a very effective means of
controlling fugitive dust at an aggregate plant, but only if properly designed and
kept well maintained.
QC Lab Ventilation
Technicians at quality control labs have been shown to have elevated risk of silica
and respirable dust exposures if they handle a high volume of samples in the lab,
and if the ventilation in the lab is ineffective or lacking. Sample handling in a typical
aggregate lab involves the transfer of dry material many times during a work day,
from pans, sieves, shakers, etc., and each transfer can create dust emission. Up
to two hundred or more dust emission events per day is typical. In order to reduce
lab technician exposure, engineering control in the form of local exhaust ventilation
is required.
Dust emissions must be controlled at the following locations in a typical lab:
Scaling bench – emissions are created at the scale and
during sieve column loading. They are controlled with
a horizontal draft hood mounted at the rear of the
bench, approx. 10 to 12 inches above bench-top.
Emissions are drawn horizontally away from the
worker’s breathing zone; airflow can be moderate and
still effectively capture emissions.
Gilson shaker – emissions are created when loading the shaker, and when running
if the shaker is not enclosed. The shaker should be located in a sound- and dustproof cabinet. Dust from loading is controlled with high velocity slot hood mounted
47
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
at the front/side of the shaker, either at the bottom or the top, depending on shaker
design.
Splitters – emissions are created when loading the
splitter; dust from loading a large splitter is controlled with
high velocity slot hood mounted at the side of the splitter,
or with a flexible, moveable exhaust arm positioned near
the splitter(s). Small splitters may simply be placed on
the scale bench and their dust controlled by the bench
hood (above).
Heated/conditioned make up air must be supplied to the lab to replace the air
removed by the dust control system, or the exhaust system will not function
properly.
48
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
5.0 WORKER ISOLATION
Isolation of the workers serves to separate them from the dust sources, either
through an enclosed and controlled work environment, or through simple physical
separation by moving their work area away from the emissions.
Isolated work environments include control room and tower booths, heavy
equipment cabs, and even lunch room trailers if they are located close to a major
fugitive dust source. All require efficient dust filtration systems and good sealing
of seams and openings (windows and doors). Booths, trailers and other such
enclosed work structures should be located as far away from fugitive dust sources
as possible, to minimize the concentrations of dust impacting thereon.
5.1 Environmental Cabs
Heavy Equipment Cabs: in newer mobile equipment (loaders, haul trucks, etc.) the
air filtration systems have been found to be very efficient, and the cabs well sealed.
Sampling for airborne silica in such cabs have shown concentrations so low as to
be almost unmeasurable. Older equipment should be tested to confirm filtration
efficiency and proper door and window sealing. If deficiencies are found, the air
system should be repaired or upgraded to meet new equipment standards.
The operator of this
machine had an
overexposure to silica
due to the open
window. Root cause
was lack of A/C in the
cab, req’g open
window in hot weather
49
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Elevated exposures occur when the operators work with windows or doors open.
It is recommended that all sites have a “windows and doors closed” policy for
mobile equipment operation when working around the plant or on dusty haul roads
or plant/stockpile grounds.
5.2 Control Rooms & Towers
Control Booths: repeated air monitoring surveys have shown that control tower
operators can be overexposed to fugitive dust, even if they remain in the booth for
their entire shift. Leaking door and window seals, leaking seams around crude air
conditioner cutouts, and porous inefficient ventilation filters contribute to dust
ingress into the booth. The most effective way to control this is with a positive air
pressure system that supplies positive pressure to the booth and is filtered for sub5-micron particles. The booth structure must have good integrity, and air leakage
prevented through properly sealed doors, windows, seams and other openings.
Improvements to the
integrity of the tower booth
envelope, air filtration, and
pressure differential were
needed to reduce this
control tower operator’s
silica exposure
If possible, the tower/booth should be located away from dust sources. Prevailing
wind direction should be considered when choosing a location, and the booth
placed upwind of the sources.
50
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
The booth interior should be kept clean through proper housekeeping practices, as
buildup of tracked-in dirt can be a source of dust exposure if disturbed. Further,
any ventilation supply ducts should not be located at floor level, as they will entrain
dust into the air from tracked-in dirt.
The rooftop A/C unit in this
tower was replaced by an air
handler mounted below the
control booth. The unit has a
high-efficiency filter, and
draws air from inside the
trailer, further shielding the
unit from dust outside. The
booth is under positive
pressure, and cooling is by a
separate recirculating A/C
unit. The floor-mounted supply
vent is not ideal, but a clean
booth prevents dust
entrainment.
51
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
5.3 Ancillary Work & Rest Areas
Lunch Rooms: portable plants often place the lunch/office trailers immediately
adjacent to the process equipment. If fugitive dust is a problem in such a plant,
then the trailer can have a significant amount of airborne dust impacting on it and
entering the interior through leakage, opening/closing of windows and doors, or
being actively drawn in by an air conditioning unit.
Air conditioners should be of recirculating type, or have highly-efficient filters
capable of removing sub-5-micron particles, door and window seals must be tight
fitting, and gaps in walls or around cutouts must be tightly sealed. Alternatively, the
trailers may be located well away from the plant and fugitive emission sources, in
order to minimize dust impingement.
Maintenance & Welding Shop: the maintenance shop/trailer, tool trailer, and the
welding area should be set up well away from any plant fugitive emission sources
in order to minimize the potential for fugitive dust impacting on the area. Prevailing
wind direction should be taken into account when choosing a location, so that the
areas can be placed upwind of the plant.
52
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
6.0 WORK TASK CONTROLS
The objective of work task controls is to change how the worker interacts with a
hazardous substance. In the case of controlling silica, the focus of changing work
tasks is to: (A) reduce the emission (concentration) of dust created by specific
tasks; and (B) reduce the time spent by the worker in the hazardous environment
by making their work more efficient and eliminating some tasks by automation or
mechanization. (See Section 3.2 for more information on exposure profiles).
The primary tasks which benefit from modification are:
•
Debris and spillage cleanup;
•
Routine inspection; and,
•
Maintenance.
6.1 Grounds & Plant Cleanup
Exposure from cleanup tasks has been discussed in previous sections, but to
summarize, ground workers may be exposed to both cleanup-generated dust from
shoveling or open skid steer, and to fugitive process emissions while doing this
task. Based on more than one hundred worker exposure studies at aggregate
plants in North America, ground operators and labourers are consistently found to
be at highest risk for silica exposure. The biggest contributor to this elevated risk
is almost always dust disturbed by manual cleanup work such as shoveling.
Controls for this worker group should therefore focus both on reducing the amount
of work task emissions and on reducing exposure time, through work task
modification.
Shoveling/Sweeping Eliminated: in order for the controls discussed below
(enclosed skid steer, plant wash down, and other cleanup controls) to be effective
53
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
in reducing exposure, no shoveling or other manual cleanup work should be
performed at an aggregate plant, if at all possible. To facilitate this, the plant must
be set up so as to allow skid steer and/or high pressure hose access to all plant
areas and under all structures. Other cleanup and spillage prevention measures
aimed at eliminating shoveling may also be adopted (see below), depending on
feasibility at an individual site.
Ground Cleanup with Enclosed Skid Steer: one of the best ways to reduce skid
steer operator exposure is to isolate the worker in the sealed cab of an enclosed
skid steer, fitted with filtered air supply and A/C (see Section 5.0 for discussion of
worker isolation). This should be used for all ground cleanup work. Newer models
of skid steer are being fitted with efficient filtration systems similar to those in heavy
equipment (loaders, etc.). To access hard-to-reach areas under the plant, the skid
steer should be fitted with a long rake or pusher.
This skid steer has an
enclosed, filtered cab and
a rake for pulling debris
from under low-lying rigs
and belts. No shoveling is
required at this plant.
Plant Decking & Structure Cleanup: the preferred method of avoiding plant
decking cleanup is to prevent the spillage from occurring in the first place (spillage
control measures are discussed in Section 4.1). However, if spillage does occur,
such as from accidental events or process upsets, then plant decking and
components (motors, guards, etc.) should be washed with a high pressure hose
fitted to a water truck or permanent water line. Alternatively, solid plate decking
upon which spillage builds up, can be replaced with perforated decking so that the
stones fall through the openings to the ground for cleanup with an enclosed skid
54
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
steer. The openings should be large enough to allow the size of stones processed
there to pass through cleanly without jamming. For this reason, perforated decking
is not feasible where medium to large stones are present, such as at primary or
secondary crushers, or primary screens.
Problem: accumulated
fine debris on solidplate decking
Solution: perforated decking
allows small stones and dust
to fall to ground
6.2 Routine Maintenance & Inspection
Routine inspection tasks pose a risk through extended exposure duration in areas
of the plant that have fugitive dust emissions. The following work task controls
focus on reducing exposure time in the plant, thereby reducing the worker’s overall
exposure dose.
55
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Maintenance tasks can involve both lengthy periods exposed to fugitive dust, and
cleanup-generated dust from the clearing of settled debris from equipment being
worked on, such as dust buildup on motor housings or guards. Exposure reduction
is accomplished by limiting time in the plant, as well as changing how debris
cleanup is done.
Daily Maintenance Shutdown: whenever possible, routine maintenance which
does not require the plant to be operating, should be performed during a daily
maintenance shutdown or before/after normal production hours. The exception is
some maintenance items which must be performed when the plant is running.
Auto-Lubricating Bearings: to reduce the time spent greasing bearings while the
is plant running, wherever possible self-lubrication systems should be installed on
all bearings presently being manually lubricated. Bearings which can be lubricated
with the plant not running should be serviced during daily maintenance shut down.
Those bearings which must be lubed when the plant is running and cannot be fitted
with self-lubing units, should have grease line extension hoses fitted that reach to
ground level (if possible) so that the operator can perform the service without
having to climb onto a rig. This reduces exposure contact time and proximity to the
emission sources.
Bearing Temperatures: to reduce the time spent in the plant exposed to fugitive
dust, the installation of remote sensing bearing temperature readers should be
considered for all bearings requiring routine temperature checks, especially those
which bring the worker into close contact with fugitive emissions. These function
by reading the bearing temperature and relaying the value to the control room, so
that the plant operator can continuously monitor bearing status. Excursions by
workers to conduct these readings in a fugitive dust environment are eliminated.
56
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Two temperature sensor types
- to the left is a wired thermocouple design
- to the right is a wireless transmitter which can be
fitted to a thermocouple
Debris Cleanup on Machinery: maintenance work on items which require cleaning
of debris or settled fine dust should be done only after a water wash-down, if
possible. Disturbance of fine dust on machinery should be avoided, and if water
wash down is not feasible then alternative methods should be considered, or PPE
should be worn as a mandatory measure for these procedures.
57
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
7.0 ADMINISTRATIVE CONTROLS
The objective of administrative controls is to reduce the exposure duration of a
group of workers. By limiting exposure time, the total dust inhalation dose received
by a worker can be significantly reduced, often by one-half or more. A further
advantage of administrative controls, is that there may be little or no capital
investment required for new equipment or engineering controls.
Job Rotation: where possible, job rotation should be considered for any workers
who spend a significant amount of time in the plant exposed to fugitive or work-task
dust emissions. Workers such as foremen, ground operators and maintenance
personnel can be rotated into a clean environment for a to ½ of their shift – for
example, into an air conditioned loader, haul truck, well-sealed control booth, or to
a maintenance area well away from the main plant.
Shift Length Reduction: high risk worker groups (eg. ground and plant operators),
should be considered for shortened shift length as a means of exposure time
reduction. Many plants have 10 to 12 hour standard shift lengths. Making the tasks
of high risk workers more efficient (by automation, for example, or reducing the
need for cleanup of spillage) may allow the same tasks to be done in less time,
enabling a reduced work day and consequent reduced exposure duration.
Foot Traffic Reduction: for all non-essential personnel, there should be no foot
traffic through a plant while operating, if significant fugitive dust is present.
Furthermore, all daily routine maintenance should be done after plant shut-down,
wherever possible, to reduce exposure time of the maintenance workers.
Hazard Awareness Training: all plant workers must have silica hazard awareness
training. This is a requirement of the provincial OHS Code. Information from the
58
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
core training should be reinforced with regular inclusion of silica-related topics in
weekly/monthly site safety meetings and/or daily huddles. Making workers aware
of the hazards that they are exposed to greatly increases the likelihood that they
will control and limit their own exposure to the hazard, and work in a much safer
manner. It is also more likely that they will become actively involved and engaged
in developing and implementing permanent controls for the site, and work with a
team approach to solving exposure control problems.
59
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
8.0 PERSONAL PROTECTIVE EQUIPMENT
The use of respiratory protection is to be considered a last resort, or used as an
interim measure until permanent controls can be put in place. However, there may
be specific, occasional work tasks, or an event of engineering control breakdowns,
which necessitate the use of PPE. In such cases, the respiratory protection
standard established by the company must be followed. The table on the following
page details the suggested types of PPE appropriate for various exposure
severities.
When an exposure assessment survey indicates that an employee exposure to
silica or respirable dust is greater than the exposure limit and respiratory protective
equipment is necessary to limit the employee's exposure to silica or respirable dust,
the employee should be enrolled into a respiratory protection program. This
program must meet OHS Code requirements.
As part of the respiratory protection program, a site must establish the following
criteria:
•
Restricted areas where PPE is mandatory; this may be the entire crushing plant,
in some cases;
•
Minimum PPE selection requirements for the restricted area(s), specific tasks
or conditions, based on exposure measurement information or qualitative
assessment;
•
Worker training in the use, care, and maintenance of PPE;
•
Fit testing, quantitative or qualitative, as appropriate, conducted on a regular
basis;
•
Enforcement policies associated with PPE requirements at the site, such as
clean-shaven policy, must meet the Code of Practice as established by the
employer.
60
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
APPENDIX A
Glossary of Terms
61
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Glossary of Terms
ACGIH – American Conference of Governmental Industrial Hygienists; a nonregulatory body which establishes worker exposure guideline limits for chemical,
biological, and physical agents; ACGIH limits are adopted by most Canadian
provinces.
AIHA – American Industrial Hygiene Association.
ANSI – American National Standards Institute.
CSA – Canadian Standards Association.
mg/m3 – milligrams per cubic meter; a unit of measure used to denote the
concentration of a chemical agent in air to which a worker may be exposed (ie.
milligrams of substance per cubic meter of air).
STEL – Short Term Exposure Limit; the permissible limit of worker exposure to a
chemical agent over a period of 15 minutes, expressed in terms of concentration
(eg. mg/m3; ppm; fibers/cc).
TWA – Time Weighted Average; used to denote worker exposures which are timeweighted over 8 hours for reference to full-shift permissible limits.
62
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
APPENDIX B
Web and Print Resources
63
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
Alberta Industry Associations
Alberta Roadbuilders & Heavy Construction Association, ARHCA
http://www.arhca.ab.ca/
Alberta Sand & Gravel Association, ASGA
http://www.asga.ab.ca/
Health Effects, Assessment and Prevention
Centers for Disease Control, CDC – health effects of occupational exposure to
silica
http://www.cdc.gov/niosh/02-129A.html
Mine Safety & Health Administration, MSHA – silicosis prevention
http://www.msha.gov/S&HINFO/SILICO/SILICO.HTM
National Institute for Occupational Safety & Health, NIOSH – mining safety and
health research site
http://www.cdc.gov/niosh/mining/
Canadian Center for Occupational Health and Safety, CCOHS – general
information on many topics, including dust exposures
http://www.ccohs.ca/oshanswers/
Occupational Safety & Health Administration, OSHA – Silica Advisor tool
http://www.osha.gov/SLTC/etools/silica/index.html
Occupational Safety & Health Administration, OSHA – Safety & Health Topics,
Silica
http://www.osha.gov/SLTC/silicacrystalline/index.html
Federal & Provincial Regulations
U.S. Occupational Safety & Health Administration, OSHA – standards for Silica
http://www.osha.gov/SLTC/silicacrystalline/standards.html
U.S. Occupational Safety & Health Administration, OSHA – standards for
Permissible Exposure Limits (PELs), including respirable dust:
http://www.osha.gov/SLTC/pel/standards.html
64
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
U.S. Mine Safety & Health Administration, MSHA – Title 30 CFR (refer to
Sections 56 and 57)
http://www.msha.gov/30CFR/CFRINTRO.HTM
British Columbia, Ministry of Energy, Mines and Petroleum Resources – Health,
Safety and Reclamation Code for Mines in British Columbia, 2003
http://www.em.gov.bc.ca/Subwebs/mining/Healsafe/mxready/mxcode01.htm
Alberta Human Services – Occupational Health & Safety Act, Regulations and
Code
http://humanservices.alberta.ca/working-in-alberta/307.html
Saskatchewan Ministry of Labour – Occupational Health & Safety Regulations,
1996
http://www.labour.gov.sk.ca/legislation/
Manitoba Ministry of Labour, Workplace Safety & Health Division – Workplace
Safety and Health Regulation 217/2006
http://www.gov.mb.ca/labour/safety/actregnew.html
Ontario Ministry of Labour – regulations under the Occupational Health & Safety
Act
http://www.labour.gov.on.ca/english/about/leg/ohsa_regs.html
Quebec CSST – acts and regulations under the CSST
http://www.csst.qc.ca/Portail/en/lois_politiques/index_loi.htm
New Brunswick Workplace Health, Safety and Compensation Commission
– Occupational Health & Safety Act
http://www.gnb.ca/0062/regs/o-0-2reg.htm
Nova Scotia Ministry of Labour – occupational health regulations under the
Health Protection Act
http://www.gov.ns.ca/just/regulations/regs/hpaohs.htm
Engineering for Dust Control
Martin Engineering – Fundamentals 3 book, dust and material control
http://www.martin-eng.com/webfm_send/691
NIOSH Mining – Handbook for Dust Control in Mining, Publication No. 2003-147
http://www.cdc.gov/niosh/mining/pubs/pubreference/outputid20.htm
65
NIELSEN
C O N S U L T I N G
Silica & Dust Exposure Control Handbook
Rev. 1, 20-Feb-2014
OSHA Mining – Dust Control Handbook for Mineral Processing
http://www.osha.gov/SLTC/silicacrystalline/dust/dust_control_handbook.html
66
NIELSEN
C O N S U L T I N G