Cement
Manufacturing
Limestone
of differing chemical composition is freely available in the quarries
owned by CN. This limestone is carefully blended before being crushed.
Red mineral is added to the limestone at the crushing stage to provide
consistent chemical composition of the raw materials. Once these materials
have been crushed and subjected to online chemical analysis they are
blended in a homogenized stockpile. A bucket wheel reclaimer is used
to recover and further blend this raw material mix before transfer
to the raw material grinding mills.
Transport
belt conveyor transfers the blended raw materials to ball mills where
it is ground. The chemical analysis is again checked to ensure excellent
quality control of the product. The resulting ground and dried raw
meal is sent to a homogenizing and storage silo for further blending
before being burnt in the kilns.
The
heat required to produce temperatures of 1,800°C at the flame
is supplied by ground and dried petroleum coke and/or fuel oil. The
Petcoke is imported via the companies' internal wharf, stored and
then ground in dedicated mills. Careful control of the mills ensures
optimum fineness of the Petcoke and excellent combustion conditions
within the kilns system.
The
raw meal is fed into the top of a preheater tower equipped with four
cyclone stages. As it falls, the meal is heated up by the rising
hot gases and reaches 800°C. At this temperature, the meal dehydrates
and partially decarbonizes. The meal then enters a sloping rotary
kiln, which is heated by a 1,800°C flame, which completes the
burning process of the meal. The meal is heated to a temperature
of at least 1,450°C. At this temperature the chemical changes
required to produce cement clinker are achieved. The dry process
kiln is shorter than the wet process kiln and is the most fuel-efficient
method of cement production available.
The
clinker discharging from the kiln is cooled by air to a temperature
of 70°C above ambient temperature and heat is recovered for the
process to improve fuel efficiency. Some of the air from the cooler
is dedusted and supplied to the coal grinding Plant. The remaining
air is used as preheated secondary air for the main combustion burner
in the kiln. Clinker is analyzed to ensure consistent product quality
as it leaves the cooler. Metal conveyors transport the clinker to
closed storage areas.
Dedicated
electrostatic precipitators dedust the air and gases used in the
Clinker Production Line Process. In this way, 99.9% of the dust is
collected before venting to the atmosphere. All dust collected is
returned to the process.
Different types of cement are produced by mixing and weighing proportionally the following constituents:
- Clinker
- Gypsum
- Limestone addition
- Blast Furnace Slag
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