Introduction
The
most basic glass processing system includes only a feed hopper and crushing
mechanism. However, for most
applications, certain ancillary equipment is required. Ancillary equipment may include infeed
conveyors with speed controls to regulate the amount of material flow into the
crushing mechanism, discharge conveyors, dust control, and vibratory screens or
trommel screens for control of particle size and debris removal. Other equipment may be included with more
sophisticated beneficiation systems.
Equipment
for large-scale glass processing operations may be found in existing industrial
mineral or glass beneficiation operations.
However, the purpose of this report is to describe potential small-scale
glass operations. Small-scale glass
processing can be established either by assembling individual components or
through purchase of a system manufactured specifically for glass crushing. In either case, this report describes some
important issues to consider and develops a model for cost estimating.
This
report is intended to describe issues related to small-scale processing, rather
than the details of glass processing equipment. For more detailed information on specific pieces of equipment,
see individual reports in Best Practices in Glass Recycling, published by the Clean Washington Center.
In
this report, “fines” refers to material passing through a standard 8 mesh
screen. “Small scale” means under two
tons per hour.
Equipment Availability
Some
equipment used for crushing rock or concrete has proven to work for crushing
glass for use as a coarse construction aggregate. However, processing glass to fines in standard aggregate
processing equipment can result in increased maintenance of wearing surfaces
because glass is both hard and has a relatively low specific gravity, reducing
the effectiveness of most impact equipment.
In
addition, equipment such as jaw crushers and cone crushers which rely on
abrasion crushing perform poorly with glass because the hardness of the glass
causes excessive wear of the crushing surfaces, while the type of crushing
action can cause the glass to fracture into shards. Shard-like shapes are less desirable than cubical particles in
most applications.
When
designing glass crushing systems, individual situations should be evaluated
carefully in terms of potential supplies, demands, quality, and markets.
Dedicated Small-Scale
Equipment
There
are a few small-scale systems now
on the market manufactured specifically for pulverizing glass into sand. Such equipment offers the advantages of the
knowledge and experience of the manufacturers in dealing with glass processing
issues. These small scale crushers (one
to two tons per hour) for producing glass sand may cost anywhere from $10,000
to $40,000, without any ancillary equipment.
Recommended
Processing Methods
When evaluating the
feasibility of using a recycled material in a new application, it is important
to first document baseline requirements for target markets. This baseline information can be compared to
data obtained from tests performed on the recycled materials. It is critical to test recycled materials
from known sources processed by known processing equipment, because the
physical characteristics of the processed material may vary widely depending on
the source and processing strategy.
Processing
post-consumer or post-industrial recycled glass into high-grade, industrial
quality glass sand generally consists of the following steps, described in
detail below:
·
Feedstock Acquisition
·
Feedstock Preparation
·
Drying
·
Pulverizing
·
Debris Control
·
Dust Removal
·
Sizing
·
Packaging/Storage
Feedstock
Acquisition
It is important to first assess the
characteristics and availability of recycled feedstock. Glass that is free of
ferrous metal and other debris is preferable, to reduce both the wear on
crushing mechanisms and the cost of debris removal. Suppliers of post-consumer and post-industrial glass should both
be considered. Any possible effects from chemical or
physical differences between types of glass should be taken into account for
each possible application. Feedstock
may also be obtained directly from generators, such as local drop box sites,
community recycling centers, local restaurants, company cafeterias, or other
heavy users of glass.
Feedstock
Preparation
In this report, “breaking” refers to
gross size reduction and “pulverization” refers to reduction to fine
sizes.
Before any crushing, the glass may be run
over a magnetic removal device to pull out any large pieces of ferrous
metal. A picker station following the
magnet is necessary for visual inspection and removal of other large
contaminants.
The glass is then fed through a glass
breaker and reduced to cullet. Most of the contaminants expected with recycled
glass (e.g., paper, aluminum, and plastic), are less friable than glass. Therefore, they tend to remain in larger
pieces than the glass during crushing.
The cullet is then passed through a 3/4" screen to remove gross
debris. Running the glass over a second
magnetic head pulley after the glass is broken into cullet should remove any
remaining ferrous metal that may have been attached to the glass.
More sophisticated debris removal
techniques, such as nonferrous metal removal systems, are probably not
economical at a small scale.
In conveyors, sealed ball bearing
troughing idlers are preferred over grease-type idlers, since grease-type
idlers can be damaged by glass dust contaminating the lubricants. If the cullet is wet or sticky, it may also
be preferable to use a belt without cleats, so that a belt scraper may be
installed on the underside of the return conveyor. The maximum incline of a cleatless conveyor is thought to be
about 18 degrees.
Drying
Glass must be dried prior to
pulverization if the sand is to be dry sized, especially if using mesh sizes
smaller than 1/8". The glass is
much easier to dry prior to the final pulverizer because it has a smaller
surface area from which to evaporate the water before final crushing. Drying also helps to prevent the wet glass
dust from plugging dust collection bags.
The most commonly used type of dryer is a
tumbling rotary drier, fueled by natural gas or propane. Information on the appropriate flow rate and
temperature is best obtained by performing trial runs because most dryer
manufacturers will not be familiar with the drying characteristics of crushed
glass.
Pulverizing
Multiple row hammermills, consisting of
multiple hinged, free swinging bars or “hammers” attached to pivots fixed to a
rotating shaft, seem to work well for producing glass sand, as the multiple
impacts tend to produce grains that are relatively uniform in size and not
shardy. The tip speed of the hammers and the retention time determine the
product size. Conventional single-pass
hammer mills have also been successfully used if the cullet is properly
prepared and the hammer tip speed and flow are controlled properly.
If the multiple row hammermill is used,
the angle at which the hammermill barrel sits should be adjusted in order to
optimize the retention time in the mill and to spread wear evenly over the
hammers, increasing the time between shutdowns for hammer replacement.
The distance between the hammers and the
mill walls also affects system performance.
Placing the hammers too close to the walls can cause excessive wear to
both the hammers and the walls. If the
hammer tips are too far from the walls, however, the impact efficiency is
reduced.
It is advantageous to operate the
hammermill at a rotor tip speed and feed rate that will pulverize the glass but
leave any remaining debris intact, so that it may be screened out later. The optimal speed for this purpose,
according to one professional, is 3,500 feet per minute.
Some hammermills recirculate oversize
material internally until it reduces to a size small enough to pass through a
discharge screen or orifice. This
configuration gives tighter gradation control than single-pass mills, but
cannot tolerate as much contamination as systems that allow contaminants to
exit the mill in larger pieces that are easier to screen out.
Further
information on these and other small-scale glass-to-fines processors may be
obtained by contacting the following manufacturers.
· Andela Tool & Machine, Richfield Springs, NY
· Glass Aggregate Manufacturing & Engineering (G.A.M.E.), Faribault, MN
Feed rate can be an important parameter
in any of these systems. Feed rate
effects the gradation of the final product and the load on the pulverizer
motor. For both of these reasons it may
be worth controlling the feed rate.
When pulverizers are new, they tend to make more fine material. As impact surfaces wear and tolerances
increase, slowing the feed rate may enable the processor to maintain consistent
product gradation. In addition,
overloading the pulverizer can cause the motor to stall or jam. To address both of these issues, some
systems monitor the electrical current load on the pulverizer motor and regulate
the speed of the infeed conveyor to maintain a constant load.
The most common pulverizers seen in
large-scale glass processing facilities are vertical shaft impactors (Keu-Ken,
Remco, Barmac,
Canica, Spokane, etc.). These mills reduce the glass to fines by flinging it
against the inside wall of a rotating casing.
The rotating casing becomes lined with glass to be crushed. The advantage of this configuration is that
glass functions as both the impact and the wearing surfaces. These impactors process upwards of 20 tph
and start at about $50,000.
“Powders”
Processing
Some potential markets for very fine
(smaller than 200 mesh) glass are under investigation. In many
of these potential markets, fine glass substitutes for other fine
silicas or dry clays. The equipment
used to process glass to that grade of fineness is not covered in this
report. Ball mills and vibratory mills have
been in use for many years for that sort of processing. The physics of reducing one hundred percent
of the input of a glass processing machine to 200 mesh material differ markedly
from those employed with the generation of the 1/8-inch material described in
this report. For that reason, although
some of the pulverizers described here may be adjusted to produce more or less
very fine glass, the process is not an efficient one and results in extensive
wear.
On the other hand, all pulverizers
produce some dust, as described below.
That dust, if properly captured and secondarily processed, can be
marketed as a by-product of the main process outlined in this report.
Dust
Control
It is necessary to control glass dust generated by the pulverizer. Ambient dust can be controlled by pulling a negative pressure on
the outlet of the pulverizer. The
dust is pulled first across a drop-out box or through a cyclone, where the
velocity slows down and the heavier pieces fall into a barrel. After the drop-out box, the fan draws the dust into a baghouse for
collection.
If possible, the fan should be placed at
the exit of the baghouse out of the stream of material, because glass flowing
over the blades will wear them down very quickly. Bags should be porous enough to prevent plugging and to maintain
a negative air pressure.
With additional processing, it may be
possible to recover marketable fractions from the very fine glass collected in
the dust control system.
Final
Debris Removal
Final debris removal is accomplished through
a well-designed combination of dust control and screening. A correctly sized dust control fan pulls a
negative pressure adequate to remove the finest mesh glass and light weight
contaminants, including paper and plastic.
After the dust control, a small rotating trommel screen with a carefully
calibrated mesh size will remove heavier contaminants that have remained larger
than the glass through the pulverizer.
Sizing
Screening systems for fine aggregates
have been developed over the decades and are available in every size and many
configurations. Two basic types of
screening systems for industrial aggregate applications are circular and tilted
bed rectangular vibratory screens.
Either type of screen can be built up to any number of layers for multiple
size gradations. Material to be
screened enters from the top and passes from deck to deck. The top screen has the largest opening
size. At each deck, the aggregate
larger than the screen size opening is removed from the flow.
In circular screens, the aggregate flows
in a circular course around the screens, with the oversize exiting through an
outlet hose at each level. In
rectangular screens, the aggregate enters at the elevated end of the screen and
flows across, with the finer aggregate dropping through the screen while the
oversize flows off the end. In general,
rectangular, tilted bed screens have greater capacities, while flat circular
screens do a better job of separating the material. Circular screens may be adequate for feed rates of up to two tons
per hour. Above that rate, a multiple
decked screen may be necessary.
Most manufacturers of screening systems
will perform test runs in their laboratories to provide data on flow rate and
efficiency of separation for a new material.
A perspective system buyer can have a barrel of glass processed in the
type of pulverizer he is considering sent to a screen manufacturer for test.
Packaging/Storage
Provisions need to be made for both
packing and weighing the final product.
Bagging, palletizing, bulk-bagging and wrapping requirements will depend
on product specifications and quantities required by the customer, as well as
the customary packaging expected in a particular industry.
There are many manufacturers of packaging
systems, many of which are listed in the Thomas Register. Research
should be performed to detemine the packaging requirements for the perspective
markets, then finding packaging systems to fulfill those requirements. The processor must respond to market
packaging expectations, rather than expecting the market to respond to the
processor’s convenience.
A Cost Model
The following categories of costs should
be considered when evaluating a prospective processing system.
Capitol
Cost
Capitol costs consist of the fixed costs
of purchasing equipment and the costs of mechanical and electrical
installation. As with any commodity
manufacturing process, return on investment is dependent upon economics of
scale. Templates for estimating capitol costs and production costs are
presented in Appendix A.
Production
Cost
Production costs include the costs of
labor, building and equipment rental, utilities, gasoline, oil, maintenance and
supplies, and the cost of dust and debris disposal.
Continuous labor is required to perform
maintenance tasks and replace parts, line up feedstock, change the dust
collector barrel, move bulk bags of product, and weigh the product. If the system is properly designed, however,
one full-time operator is probably adequate in a small-scale operation.
Gasoline, oil, lubrication, and rental
costs should be calculated for a forklift (for moving the product and bins) and
bobcat (for loading feedstock onto the infeed conveyor).
Building rental costs should be based on
the floor space required for equipment as well as the space required to
stockpile materials.
Actual line item production costs for a
pilot operation are summarized in appendix B.
Selling,
General, & Administrative Costs
Selling, general, and administrative
costs include management, office expenses, insurance, taxes, and
commissions. These line items are also
summarized in appendix A.
Forms to Record Trial Conditions
Forms
that have been developed to capture the pertinent data for evaluating prototype
or pilot systems are presented at the end of Appendix A. The following forms are included:
Economic Evaluation - Recycled Glass Sand
Processing System.
This
spreadsheet enumerates the costs detailed above, along with formulas for
duplicating the spreadsheet.
Equipment List
A
listing of the principle equipment in a glass processing system.
Recycled Glass Feedstock
Sourcing Report
Sourcing
quality material is a critical prerequisite to producing a quality
product. This form helps to track
incoming recycled glass and to link it to the production of final products.
Recycled Glass Daily
Operations Report
This
form tracks the overall production rate and efficiency of the system.
Operational Data &
Analysis
This
form tracks equipment performance data for comparison over time and can give an
early indication of performance deterioration.
Recycled Glass Sand Production/Inventory/
Shipping Report
Prototype
or pilot processing can be used to generate material for end-use testing. This form tracks inventory so that end-use
projects can be developed.
Production Cost Data
This
form is the first-cut for accumulating equipment operational data
Parameters for Equipment Evaluation
Appendix C is a listing of parameters to
be considered when evaluating a glass processing system. Many are quantitative factors that have been
integrated into the economic evaluation above.
Others are qualitative factors requiring consideration.
Acknowledgments
The support of this project by Don Freas,
of IMTEK, Inc. is gratefully acknowledged.
