equipment is still used extensively, but
hydraulic,  tire-mounted   hauling
equipment is predominant.
Processing.-Processing plants have
become larger and more efficient, in part
because of an increase in the use of
computerized    control   systems.
Programmable controllers and/or
computers are viewed today as valuable
tools that improve operating efficiency
and product quality. Primary crushing is
often done at or near the pit, usually by
jaw or gyratory crushers, but impact and
other special types of crushers are also
used for nonabrasive stone or stone that
tends to clog conventional crushers. The
cost effectiveness of permanent crushing
installations can be limited because the
raw material moves away from the
primary stage. It becomes increasingly
expensive to haul material from the
production face to the crushing plant.
Improved  crusher  technology  and
increased  capacity  toward  greater
equipment mobility are accelerating the
use of in-pit movable primary crushers.
There is an increased interest in the use
of portable crushers ranging in size from
500 tons per hour up to 5,000 tons per
hour. For secondary crushing, a variety
of comminution equipment is used,
depending on plant size, rock type, and
kind of material being produced. Cone
crushers and gyratory crushers are the
most common types used, but impact
crushers, including hammer mills, and
roll crushers are often used.  Belt
conveyors are being used extensively to
haul material within the quarry because
they provide continuous operation, low
maintenance costs, low  work force
requirements, and reduced noise levels.
Conveying systems are often associated
with in-pit crushers and consist of several
portable units and a stationary field
conveyor.   The portable conveyors,
usually in 30- to 60-meter lengths, make
the connection between the portable
crusher and the field conveyor and
provide a good range of motion. The
increased use of conveyors reduces
significantly the use of haul trucks in
quarries.
Screening is the single most important


part of the processing cycle of crushed
stone particles. A wide variety of screen
types exists, and their selection is a
function of the material processed as well
as the final product required. Inclined
vibratory screens are most commonly
used in stationary installations, while
horizontal screens are used extensively in
portable plants. For screening large sizes
of crushed stone, grizzly bars, rod decks,
and heavy punched steel or plastic plates
are used; for smaller sizes, woven wire,
welded wire cloth, rubber, or plastic
screens are used.
Processed crushed stone is usually
stored in open areas except for relatively
small quantities that go to the loadout
bins. In the more efficient plants, the
crushed stone is transported from the
storage piles on belt conveyors through
tunnels installed under the piles, thus
facilitating the blending of any desired
mixture of sizes from different piles.
Recycling. -As the Nation moves into
an age of increased environmental
consciousness, recycling of old concrete
and asphalt, used mostly in pavements, is
increasing. Waste disposal problems as
well as local scarcity or high cost of
construction aggregates in some areas are
making   recycling  more   attractive
economically. In some States, limited
recycling, especially in road construction,
is either encouraged or required by law.
The best utilization for recycled cement
concrete is as road base material, because
the matrix of concrete cannot be
converted back to cement, and also
because of the lower strength of concrete
compared with that of natural aggregates.
Sound crushed concrete can make good
coarse aggregate, but the fines produced
in crushing the concrete have a higher
absorption, are more friable, and will
require a higher amount of asphalt in
asphaltic concrete and a higher amount of
mixing water in portland cement.'2
The amount of asphalt roads recycled
is significantly larger than that of
concrete roads, mainly because asphalt
can be rejuvenated with oil additives and
used in pavements, significantly reducing
the cost of the finished road. Experience
has shown that about 35% of crushed


asphalt pavement can be recycled into
new asphalt mixtures. Proper testing
under engineering supervision in all
construction projects using recycled
materials as aggregates should be
performed to ensure that the technical
specifications are met.
Byproducts and Coproducts
Clay of good quality is sometimes
encountered in limestone overburden and
is mined as a coproduct. Gypsum is
often encountered beneath beds of
limestone and is mined as a coproduct.
Amphibolite rock often occurs as
formations paralleling spodumene-bearing
pegmatitic dikes and is mined
concurrently with these, notably in North
Carolina.  This coproduct of lithium
mining is marketed primarily as
roadstone.  Stone is also mined in
conjunction with many metallic ores and
nonmetallic minerals, but seldom does a
market exist for it.
Economic Factors
Crushed stone is a high-volume, low-
value commodity. The industry is highly
competitive and is characterized by
thousands of operations serving local or
regional markets, largely because stone is
an abundant mineral. Production costs
are determined mainly by the cost of
labor, equipment, energy, and water, in
addition to the costs of compliance with
environmental and safety regulations.
These  costs  vary  depending  on
geographic location, the nature of the
deposit, and the number and type of
products produced. Despite having one
of the lowest average per ton values of all
mineral commodities, the constant dollar
price of crushed stone has changed
relatively little over the past 20 years.
Increased productivity achieved through
increased use of automation and more
efficient equipment was mainly
responsible for maintaining the prices at
an almost constant level.
The delivered price of the product, in
addition to quality, is a particularly
important element in any major
transaction because of the low unit value


CRUSHED STONE-1993




I




1112