Pulp & Paper

What is the paper industry?

The pulp and paper industry is a major industry that produces paper and paper products from natural resources such as wood fiber. This industry plays a central role in our daily lives, providing the basis for a wide range of products, from newspapers to packaging and hygiene products.

The production of paper requires complex manufacturing processes that take into account both technological innovation and environmental concerns. In this context, the industry is also focusing on sustainability efforts and the responsible use of resources.

What is black liquor?

Black liquor is a byproduct of the pulp and paper industry that is generated during the production of pulp from wood fibers. It is a highly alkaline solution containing various chemical compounds such as lignin, inorganic salts, and other organic substances. Due to its high alkalinity and the impurities it contains, black liquor is harmful to the environment and requires careful treatment.

What happens to black liquor?

The papermaking process consists of four main parts. The first is the cooking process, which physically and chemically treats the wood. Second, cellulose and pulp production, and third, paper production in the paper machines. The remaining fourth part of this production is the black liquor treatment, a complex and costly process to reduce energy and environmental costs and make the entire paper production process economical.

Approximately seven tons of black liquor must be processed for each ton of pulp. It is concentrated to a higher dry mass in an evaporation process. The concentrated liquor is then injected into the recovery boiler where it is burned. This is the technical heart of black liquor processing, producing high-pressure steam that is fed to turbogenerators to reduce steam pressure for use in the mill and to generate electricity; it also recovers most of the valuable chemicals in several recovery stages.

During the incineration process, anhydrous sodium sulfate (Na₂SO₄) is reduced to sodium sulfide (Na₂S) by thermochemical reduction (Na₂SO₄ + 2C → Na₂S + 2CO₂).

The melt discharged at the bottom of the boiler is further recycled in recausticizing processes where sodium carbonate (Na₂CO₃), calcium carbonate (CaCO₃) and calcium oxide (CaO) are reacted to regenerate sodium hydroxide (NaOH).

Non-Process Elements (NPE)

Remaining mineral residues from the combustion process, called non-process elements (NPE), which contain sodium, chloride and potassium compounds (Na₂SO₄, Na₂CO₃, NaCl, K₂SO₄, K₂CO₃ und KCl) must be separated to prevent corrosion and can be recycled to the preceding processes. The life of the recovery boiler is also extended by preventing deposits on the heat exchangers, thus avoiding unplanned downtime.

Soot blowers are used to clean and collect the mineral deposits from the heat exchangers. Electrostatic precipitators (ESPs) collect volatile salts from the air downstream of the combustion section that can clog the gas path heat exchangers of the recovery boiler. These mineral-rich streams can be treated in separate facilities.

ARC or CRP Mineral Recovery Processes

NPE recovery systems are either leaching or crystallization processes downstream of sootblowers and electrostatic precipitators to recover sodium carbonate (NaCO₃), chloride (Cl⁻) and potassium (K⁺) salts or to reduce their concentrations in the effluent stream.

Special ARC and CRP systems allow savings by recovering sodium carbonate (NaCO₃), anhydrous sodium sulfate (Na₂SO₄) or converting sulfates into saleable potassium sulfate (SOP oder K₂SO₄). SIEBTECHNIK TEMA pusher centrifuges or CONTURBEX centrifuges, usually made of high-alloy stainless steels, are used for the separation of 1000 kg/h to 30000 kg/h of NPE salts per machine, depending on the size of the recovery vessel.