The facilities provided by this project consist of:
The gas-fired high-temperature (over 1100 C) rotary Trefoil kiln is at the heart of the facility. The name Trefoil relates to the internal shape of the kiln, which, unlike conventional cylindrical kilns, is similar to a three-leaf clover. This shape, when rotated, allows a bed of pellets to gently cascade from leaf to leaf with every revolution of the kiln to ensure even distribution and mixing. The super alloy steel lining of the kiln wall is shaped to allow a greater proportion of the radiant heat emitted from the wall to be directed back to the bed of pellets resulting in an even temperature throughout and more efficient heating. Insulating fibres surround the kiln wall to further minimise heat loss. The resulting low thermal mass gives this lightweight kiln excellent temperature control permitting rapid heat-up and cool-down times. This is not possible with traditional kilns lined with refractory bricks.
The following seven stages are integrated within the pilot plant:
Up to three dry powdered resource materials can be proportioned by mass within this stage using three metering screws, each having a variable speed drive for calibration with any suitable material.
The metering screws discharge the resource materials into a sealed high-speed mixer. The mixer incorporates a shaft controlled to rotate up to 1450 rpm. The shaft is fitted with paddles that can be adjusted to control the powder retention time within the mixer to ensure complete homogenisation. The mixer discharges the finished blend to a screw, which transfers the material to the pan pelletiser.
The homogenised raw material is fed at a predetermined rate into a pan pelletiser and sprayed simultaneously with a prescribed amount of pressurised water through atomising nozzles. The pelletiser consists of a rotating inclined pan with controllable angle of inclination and rotational speed. The combination of rotating action, inclination, raw material blend, water content and retention time causes the powder to agglomerate to form pellets of the required size and density.
After pelletising, pellets not conforming to the required size are removed by an inclined vibrating screen, fitted with chutes attached to collection bins for reprocessing. The 'green' pellets of the correct size are fed by conveyor to the coater/drier.
The moisture content of the 'green' pellets must be reduced prior to high temperature kilning to prevent them disintegrating by steam under pressure. The 'green' pellets can also be coated with a prescribed quantity of powdered coating material to enhance their performance. Moisture is removed by rotating the pellets at about 150 C in the inclined gas-fired cylindrical drier, which also gives the pellets a hard outer shell, and coating to increase their strength for high temperature kilning.
Pellets are fed into the Trefoil kiln at a controlled rate by a vibrating feeder. The kiln can operate continuously for periods of at least 10 hours at the required temperature and residence time, the latter being controlled by varying the speed of rotation and pellet feed rate. At a sufficiently high temperature any combustible constituents within the pellets are burnt and any expandable materials are 'bloated'. Both actions reduce pellet density. The high temperature melts the glassy constituents to form a vitrified interior and exterior structure thereby reducing the penetrability of the finished pellets. The precise temperature control required during kilning is achieved by controlling the gas burner via a non-contact thermometer continuously monitoring the temperature of the pellet bed. This avoids undesired melting of the pellet surface and consequential agglomeration. The fired pellets exit the rear of the kiln via a chute for cooling.
The fired pellets are cooled rapidly to 'freeze' their structure before storage in sealed containers.
Of 105 funded projects funded by the RMC Environment Fund in three years, 18 projects have been granted Beacon Status. This project was awarded Beacon Status at the 2002 Celebration Event. Tim Pinder of the RMC Environment Fund and Environment Manager (Aggregates and Waste) of RMC (UK) Limited said We are delighted to have been able to support a project with such wide-reaching environmental benefits, and one which complements the aims of the fund. This pilot plant is an example of best practice in how the purpose of the Landfill Tax can and should be fulfilled through the Landfill Tax Credit scheme.
© 2002
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