Chemical Recovery Process

After cooking the wood chips, the cellulose fibers that are released become the pulp used for making paper.

Pulp goes through a series of washers including the pressure diffusion washer, the atmospheric diffusion washer and a rotary drum displacement washer.

The cellulose, or wood fibers, are cleaned and separated from the cooking chemicals and dissolved organics (lignin) before going on to the bleaching stage.

The black liquor is now at a higher concentration – at roughly 68% solids – so that it can be burned in the kraft chemical recovery furnace, more commonly known as a recovery boiler.

The water and steam mixture eventually flows into the steam drum where the steam exits and flows to the superheater sections. The liquid water re-circulates to eventually be made into steam. In the superheater sections the temperature is raised to a point suitable for use in electricity generating turbines.

The steam system is quite complex. Purified water enters the boiler through the economizer first where it picks up the last of the heat produced by the combustion process. This allows the boiler thermal efficiency to be maximized. The water then flows into a series of drums and eventually flows through piping to feed the lower furnace water walls picking up additional heat and forming a steam/water mixture.

In addition to providing electricity, the steam generated in the process is used in the form of thermal energy in the pulp mill to heat the evaporators, the washing cycle, the digester, the slaker, and is also used in many other parts of the paper making process.

The two primary components of green liquor are sodium sulfide (Na2S) and sodium carbonate (NaCO3).

Green liquor contains the chemicals (in the form of sodium salts) that were originally used to cook the pulp and have been recovered from the bottom of the recovery furnace.

The recovery of lime is also a closed loop. Heating the recovered lime in the kiln converts calcium carbonate back to calcium oxide which then becomes reusable for its role in creating white liquor. At Sappi’s Somerset and Cloquet mills, carbon dioxide (CO2) from the lime kiln stack gases is consumed in a separate process to produce precipitated calcium carbonate (PCC). This fine material is sent to the paper mill as a key component of the coatings used in the paper manufacturing.

The white liquor has now made a complete loop as it gets pumped back into the digester to start the whole wood pulping process over again.

Calcium solids now in the form of calcium carbonate (CaCO3) are settled out of the white liquor clarifier and are sent to the lime kiln.

While the recovery process involves the constant reuse of inorganic chemicals and generates renewable energy from black liquor, there is a need for some fossil fuels. In order to reach a temperature that is hot enough, the lime kiln requires either fuel oil or natural gas.