The recovery process is an important one as it enables us to re-use the chemicals and water from our kraft pulping process. The system we use goes beyond recycling to re-use by applying an advanced technique to effectively dispose of dissolved wood substances, reducing our environmental footprint. Our goal is to consistently operate the chemical recovery system as efficiently and environmentally sound as possible.
The process begins in the digesters. The wood chips are separated from the lignin by using a cooking chemical called white liquor. The spent liquid containing the broken-down lignin is called black liquor. When it leaves the digesters, it is quite diluted – about 85% water. The water is evaporated out, condensed and purified to re-use in the mill. The remaining lignin and cooking mixture are burned in a specialized tank called a recovery boiler. The recovery boiler generates steam that turns into electricity, which is then used to power the Millsite and approximately 15,000 Albertan homes. After burning the black liquor, all that remains is inorganic chemicals, which are dissolved in a tank, becoming green liquor.
In the recaust process, lime is added to the green liquor causing a chemical reaction. The reaction products are solid calcium carbonate and white liquor. The solid calcium carbonate is separated from white liquor by filtering. The separated calcium carbonate, – much like mud, is washed and fired in a kiln to produce lime for re-use. The white liquor is then ready to begin the pulp processing again in the digesters — the cycle of white to black to green and back again to white liquor cycles repetitively. The sizeable intricate process is not only environmentally responsible, but it is also cost-effective.
Water is vital in the kraft pulping process; it is used to cool machinery, clean and inside the boilers – it generates steam. At Al-Pac, we continuously recycle water throughout the mill. Water is taken and returned from the same source, the Athabasca River. It is only returned after being cleaned, filtered and purified through a complex water treatment system. The activated sludge biological effluent treatment uses naturally occurring micro-organisms to digest traces of lignin or other components in the effluent before being returned to the source. The advantages of treating wastewater this way is that we achieve high removal efficiencies, the process is highly controlled and monitored, and the micro-organisms are adapted to the receiving wastewater.
Our Stack Heat Project is Another Example of how we Reduce our Environmental Impact