Pulp & Paper
Typical applications of agitators in pulp & paper processes include
Lime Slaking & Causticizing
In this process, green liquor, the dissolved smelt of sodium carbonate, sodium sulfide and other compounds, from the recovery boiler is reacted with quicklime to convert inactive sodium carbonate to sodium hydroxide. The reaction proceeds to the causticizing process where the green liquor is mixed with lime in the slaker.
Dregs are solid impurities in green liquor which must be removed before causticizing or they will contaminate the lime. These impurities include carbon from incomplete combustion, iron compounds and even pieces of refractory lining from the furnace.
Agitators are used for dissolving the lumps of sodium carbonate in freshwater. The remaining dregs are discarded.
Lime Mud Storage
After causticizing the slurry is sent to a clarifier where the white liquor is separated from the lime mud. The mud is washed and then stored in large tanks prior to burning in the kiln where it is reconverted to calcium oxide quicklime.
The agitator is used to maintain a pumpable slurry into motion. Mild agitation is required for the homogenization of slurry.
Black Liquor Storage
Due to vast quantities of liquor being handled each day it is not uncommon for a mill to have several weak and heavy liquor storage tanks. Side entry agitators are used to prevent sludge from building up on the bottom, especially in the heavy liquor tanks. They also maintain temperature uniformity. Propeller design is the preferred impeller since it has high flow generation capacity.
Tall Oil Reactor
Tall oil or tallol is a viscous yellow-black odorous liquid obtained as a by-product in the wood pulp manufacture when pulping mainly coniferous trees. Soap which floats to the surface of weak black liquor storage tanks is skimmed off and pumped to a tall oil reactor where a mixture of sulfuric acid and sodium sulfate is reacted with it.
The agitator provides thorough contact of the acid, flocculants and soap to help produce a uniform product.