Calcium oxide (CaO or lime) is used in the kraft recovery process. First, some background about the process is in order.
Kraft pulp mills rely on chemical recovery systems to reclaim spent chemicals, which are then reused in the process. Without this system a significant amount of chemicals and resources would be wasted.
These alkaline chemicals are used to pulp the wood chips, and end up in the waste pulping solution, called
black liquor. This black liquor is evaporated to remove water. After evaporation, it is injected as fuel into a recovery boiler, which burns off the organic material leaving the chemicals behind for recovery.
These remaining chemicals are mixed with water to form
green liquor. This green liquor is sent on for further processing called “
causticizing” to bring it back to its original form (
white liquor) for re-use in the pulp mill digester.
The Thunder Bay Operation has one operational kraft mill (‘B’ mill) with one chemical recovery systems, and recausticizing department that converts green liquor into white liquor for reuse in the
digester.
Associated with this recaust process are large
Lime Kilns used to regenerate the lime (CaO). Lime Kilns are similar to cement kilns, and are large cylindrical equipment that operate at high temperatures (800º + Celsius). Due to the nature of re-burning lime mud (calcium carbonate) a large amount of dust is generated. The dust has to be removed before the exhausts from the kilns are released to the environment.
The Lime Kilns were originally equipped with
wet scrubbers to remove the dust. The removal efficiency of these scrubbers didn't meet current requirements. So as part of the Air Improvement Plan a new “
electrostatic precipitator” was installed in 2003. This new precipitator is a state-of-the-art pollution control that operates at extremely high efficiencies.
An electrostatic precipitator is similar to electronic air filters located in houses and restaurants.
Flue gas passes through a large chamber containing electrically charged wires and plates. The dust particles are charged with electricity and attracted to large metal plates. The clean gas is then discharged to the atmosphere.
When there is a sufficient buildup on a plate, a large rapper or hammer knocks the plate, forcing the heavy particulate to fall to the bottom of the precipitator for collection. The particulate collected from the precipitator is sent back to the mill for re-use.
A precipitator is considered a dry system (unlike a scrubber) and the resulting flue gas is dry, which results in no visible emissions during warm weather. Due to the hot air being discharged a vapour plume is visible when the weather is cold.