At Petrie, coal and gas fired boilers heat demineralised water from the on-site water treatment plant to produce up to 80 tonnes of steam per hour. Steam is primarily needed to dry the cartonboard in the production process but before the steam is piped to the Mill it passes through a turbine generator which produces 50% of the electricity requirements of the Mill.
This method of electrical production is energy efficient as well as environmentally sound. A condensing section on the turbines allows the boilers to be functioning at full capacity regardless of the steam demand on the paper machine. The Mill uses 15 to 16 megawatts of electricity, which is equivalent to the electrical power requirements of a small city such as Caloundra.
Local sawmills chip the waste off-cuts after sawing round logs into square timber.
The pine woodchips are first softened under steam pressure with a minimal amount of chemicals.
The softened chips are then conveyed into the pulp plant where refiners grind them between two grooved rotating disks - much in the same way as coffee beans are ground.
The separated cellulose fibres are washed and de-watered before being further processed for cartonboard manufacture.
Amcor and its forebears have built a number of paper mills over the last 100 years based largely on recycled fibres. In the early fifties, when Petrie Mill was built, it was specifically designed to process waste paper.
The waste paper now used at Petrie consists of 10% pre-consumer off-cuts from printers and converters, 25% office waste and 65% old corrugated cardboard containers.
There are additional costs in collection, sorting and cleaning the waste paper compared with just using virgin pulp.
In the first stage of stock preparation, conveyors transport the waste paper up to the hydrapulpers where it is slushed in water. A large funnel shaped screw or rotating disk in the bottom of the pulpers churns the water. This fast swirling action (700 rotations per minute) breaks the waste paper into individual fibres. Contaminants such as plastic, rubber, rags, rope and wire are pulled out of the pulper after being caught on a wire trawl called a ragger, which is a self generating rope with a barbed wire core. The separated fibres are then sucked through holes in the extraction plate and pass through a multi stage cleaning process.
To achieve optimum stiffness in the cartonboard, the fibres are sorted and separated into long and short fibres in a fibrefractor. The long fibres are subsequently used in the outer layers of the board.