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Winter
working a reality for
Hard Rock Paving quarry
By Andy Bateman,
Engineering Editor
Strong demand
for its products has meant winter working several years in a row for
a southern Ontario quarry operation. 
A
recently fitted BTI TB725X secondary breaker deals with any oversize
in the feed to the Telsmith 25B gyratory primary crusher.
The
Law Quarry, located in the south-east corner of the Niagara Peninsula
between Port Colborne and Wainfleet, Ont. has run through each of the
last three winters, due to strong demand for aggregates from parent
Hard Rock Paving as well as suppliers of the major casino and hotel
development in nearby Niagara Falls. 2003 customers will include Hard
Rock's Highway 402/Queen Elizabeth Way (QEW) roadbuilding contract and
the company's quarry-based Port Colborne asphalt plant. Additional demand
will come from 2002 internal carryover contracts, the resurfacing of
Highway 20 and an area ready mix concrete plant. Operations manager
Keith Porter predicts that 2003 may be a record year for the quarry,
depending on overall activity demand generated by the casino development
and the Niagara Region.
From a production perspective, Porter explains
that the focus now is on building inventories of granular products,
in advance of a plant switch in March that will maximize the production
of clear sizes. This production switch also entails a switch of quarry
working as the limestone deposit here is worked in two faces. The 9.4
m high top bench is the source of most granular products, while the
11.6 m high bottom bench is the source of clear sized concrete and asphalt
aggregates. Products from the top bench include Granular A base material,
19 mm clear drainage stone and 12.5 mm minus shouldering rock, while
products from the bottom bench include 19 mm concrete stone, HL3 asphalt
stone and HL8 asphalt stone as well as some manufactured sand for asphalt
and screenings. Given the anticipated high demand for clear sizes from
the bottom bench, Porter's ongoing challenge is to keep the top bench
face far enough in advance of the bottom bench face to release sufficient
bottom bench reserves.
Raw feed is processed by the quarry's main
stationary plant and a new separate spread, with material fed to each
according to target product mix. The main plant is fed both top and
bottom bench material, while the new spread is fed bottom bench stone
only to maximize the overall yield of concrete and asphalt aggregates.
Porter notes that winter conditions have impacts
throughout the whole operation. From a safety perspective, the winter
months naturally present increased risk from slips or falls and plant
screenings are spread in pedestrian traffic areas to help reduce these
risks. Screenings are also spread on haul roads when necessary to provide
safer running surfaces and also fill in washboard or low areas where
water can accumulate. The winter also places increased emphasis on good
housekeeping as any objects hidden under snow, such as screen of conveyor
parts represent a trip or injury hazard.
This Link-Belt 4300 excavator was used to remove frozen build up
from the corners of the primary dump hopper, a common problem in cold
weather working conditions.
At the front end of the process, all necessary stripping is completed
ahead of the winter to avoid dealing with difficult frozen or muddy
conditions. Drilling and blasting continues, but can be a challenge,
particularly in snow, when blast holes are difficult to mark and charge.
Production blasts are timed to allow working, as far as possible, with
a fresh muck pile, as a muck pile that has been exposed to snow, rain
or freeze-thaw cycles for any length of time can be difficult to separate
and load efficiently. Porter emphasises that the primary loader operator
plays a key role in efficient winter running, by selecting newly shot,
well-graded material and avoiding the frozen lumps, snow, or wet material
that can soon cause headaches downstream.
At the plant itself, process equipment is run
empty for 20 minutes at the beginning of each shift to warm all conveyor
belts. Any frost left on cold belts often results in slippage around
drive pulleys, even if the pulley is lagged. To improve frost protection,
many of the plant's process belts are covered and Porter is looking
at covering all remaining exposed belts. Crushers and screens are also
warmed up, aided by immersion heaters in crusher oil tanks. Once production
begins, the plant feed rate is steadily ramped up to ensure material
is flowing freely through the plant. Work patterns are also adjusted
at the end of the shift, when plant storage bins that would be normally
be left full are emptied to avoid overnight freezing of material and
two plant labourers stay an extra hour to remove any material spilled
at transfer points.
Screening efficiency is also impacted by weather
conditions. On cold dry days, screen efficiency is good, but deteriorates
when the temperature is around freezing point. In these conditions,
screening of smaller sizes is difficult as "piggy back" fines adhere
to the surface of larger size stone pieces. Porter adds that good results
have been obtained in these marginal conditions with four 4 mm opening
Flex-Mat screens from Major Wire Industries that have been in operation
for 12 months and reportedly doubled output.
Dust suppression becomes a challenge in cold
weather, mainly from vehicle tires and process locations such as crusher
discharges and conveyor transfer points where material free falls. Porter
is currently reviewing the set up of a number of transfer points with
a view to reducing dust by enclosure, reduction of free fall height
or a combination of the two. The quarry's rural location helps somewhat
in this regard with no recorded complaints to date due to off site dust
emissions.
A caterpillar 988B wheel loader is shown loading a Caterpillar 771D
haul truck at the Law quarry. The loader operator plays a key role in
efficient winter runnign by selecting newly shot, well-graded material
and avoiding the forzen lumps, snow, or wet material that can slow production.
At this operation, dewatering operations are
relatively unaffected by cold weather, with two Flygt 152 mm pumps running
year round to dewater the quarry.
Winter working also means that production of
granular base finished product is a fine balance between making sufficient
product and avoiding unnecessarily large stockpiles. Unprotected stockpiles
absorb water from snow and rain, resulting in hard to work icy piles
and potentially dangerous steep faces when product is finally shipped
from the centre of a large stockpile, sometimes months after it was
produced. For MTO work, the quarry has elected to use the process chart
method to monitor granular product quality, thereby avoiding the building
of 4000-tonne lots of product that require testing and approval prior
to shipment.
In terms of the overall impact on productivity,
Porter reports that the plant's average production rate drops from 375
tonnes per available operating hour in warmer weather to about 325 tonnes/h
in the winter, reflecting the accumulative effect of the morning warm-up
period, ramp-up period and other weather induced delays. On the day
of Aggregates & Roadbuilding's visit, for instance, production was halted
for about 30 minutes while a rental Link-Belt 4300 excavator removed
frozen build up out of the corners of the primary dump hopper.
Primary loading and hauling at the Law quarry
is completed by Caterpillar fleet including a 988B loader fitted with
a 5.3 m3 bucket, and two Caterpillar haul trucks, 771D and 769D units
having 40.7 and 37.4 tonnes maximum payload capacities, respectively.
These units haul to the primary dump hopper where feed to the primary
crusher is regulated by a 1.8 m x 4.5 m Simplicity pan feeder. The primary
itself is a Telsmith 25B gyratory primary crusher with a recently fitted
BTI TB725X hydraulic rock breaker.
The radial primary surge pile allows top and
bottom bench stone to be stockpiled separately. From there, two 1524
mm x 914 mm Syntron feeders regulate the flow of material to a 5x16
Tyler double-deck scalping screen. The scalping screen separates the
feed into three sizes. Oversize is first reduced by a Kleemann Reiner
impact crusher and then screened over 6x16 ElJay and 4x8 Tyler screens.
Material larger than 19 mm from these screens is returned back to the
initial scalping screen, while 19 mm x 6 mm concrete stone is stored
in product bins under the screens and 6 mm minus is pulled off via an
overflow chute for blending into granular base. Meanwhile, material
retained on the bottom deck of the scalping screen is reduced by a 41/4
Nordberg cone crusher, and screened over four 5x16 Tyler screens. Here,
19 mm plus material is directed to a 41/4 Nordberg Short Head tertiary
cone crusher, while 19 mm clear and HL3 asphalt aggregates are sorted
in product bins and 6 mm minus screenings are stockpiled separately.
The smallest product from the scalping screen, 25 mm minus, is stockpiled
as granular A finished product during the winter months. During the
construction season, the 25 mm screen cloths on the on the bottom deck
of the scalping screen are replaced by 50 mm opening cloths and 50 mm
minus passing this bottom deck becomes the feedstock for the new plant.
On arrival at the new plant, this 50 mm minus is first screened over
a 6x16 ElJay screen. 50 mm x 25 mm oversize is directed to an Allis
H3000 Hydrocone crusher, while 19 mm minus concrete aggregate passes
through a Ward Industrial twin-screw 914 mm diameter coarse material
washer and 6 mm minus is passed through a Ward fine material screw washer.
Support equipment at the operation includes
a Caterpillar 972G materials handling loader, a Bobcat 763 skid steer
on clean up and a full-time water truck (in the summer months) for dust
suppression.
Drilling and blasting at the Law quarry is
contracted out to Grand Valley, Ont.-based St Lawrence Explosives Inc.,
who in turn subcontracts drilling to Gibbons Contracting Ltd.. Gibbons
utilises a Gardner-Denver Hydratrac rig to drill 101 mm diameter production
holes. The Law Crushed Stone Division is part of The Hard Rock Group.
Hard Rock Paving Co. Ltd. has owned the Law quarry since 1978.
Keith Porter is operations manager - Materials
for Hard Rock Paving's Law Quarry, asphalt plants at Port Colborne and
Fort Erie, portable crushing operations and the company's CCIL approved
laboratory.
January/February
2003 issue
Aggregates
and Roadbuilding Magazine
4999 St Catherine Street West. Suite 315
Westmount, Quebec H3Z 1T3
Tel: (514) 487-9868 Fax: (514) 487-9276
EMail: rocktoroad@sympatico.ca
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