Pulping Processes

Mechanical mill types

Mechanical - Stone groundwood (SGW): This type of mill uses large grindstones to mechanically break down wood into pulp, often used for making newsprint and other lower-quality papers. It consumes a lot of energy but produces high yields of pulp from softwood fibers.

Mechanical - Pressure groundwood (PGW): Similar to stone groundwood, this process operates under higher pressure to enhance fiber breakdown, often producing pulp with stronger properties. It is primarily used in the production of lightweight coated papers.

Mechanical - Refiner mechanical (RMP): This process grinds wood chips between metal discs to produce pulp, offering a higher yield compared to chemical processes. The pulp is used in products like boxboard and printing papers, though it has shorter fiber length.

Mechanical - Thermomechanical (TMP): Wood chips are steamed before being mechanically pulped in refiners, enhancing fiber flexibility and strength. The pulp is commonly used in newsprint and magazine papers, providing better quality than traditional mechanical pulps.

Chemical mill types

Chemical - Kraft (KP): The Kraft process uses chemicals, such as sodium hydroxide and sodium sulfide, to break down lignin in wood, yielding strong pulp from both hardwood and softwood. The process allows for the recovery of chemicals and is highly efficient in producing strong, durable pulp for a wide range of products like linerboard and unbleached kraft.

Chemical - Sulfite (SP): This method uses sulfurous acid and bisulfite ions to dissolve lignin, creating a pulp with a smooth texture, often used for specialty papers and high-grade printing. It is less common today due to environmental concerns and lower yield compared to Kraft.

Chemical - Soda (Soda): The soda process uses sodium hydroxide as the main chemical agent to break down non-wood raw materials like straw and bagasse into pulp. It is less efficient for wood fibers but is useful in countries with limited forest resources.

Chemi-mechanical (CMP): In this hybrid process, wood chips are chemically pre-treated before being mechanically refined, offering a balance between fiber strength and pulp yield. It is often used for products requiring both bulk and smooth surface properties, such as tissue and packaging papers.

Chemi-Thermomechanical (CTMP): This process combines chemical pre-treatment with mechanical refining under heat, resulting in improved fiber quality and strength compared to traditional mechanical pulps. CTMP is widely used for products such as tissue, board, and printing papers where both strength and bulk are required.

Recycling mill types

Recycling - General Repulping: In this process, recovered paper is mechanically and sometimes chemically repulped to break it down into usable fibers. Unlike deinking, it focuses less on contaminant removal and more on general fiber recovery, making it ideal for producing lower-quality products such as cardboard, packaging, and tissue. This approach prioritizes efficiency and fiber yield over brightness or surface quality.

Recycling - Deinking: This process involves cleaning recovered paper to remove ink, adhesives, and other contaminants. It typically uses flotation, washing, and chemical treatments to produce clean recycled pulp suitable for high-quality paper products, such as printing and writing papers. Deinking improves the brightness and cleanliness of the recycled fibers, making it essential for applications requiring smooth and uniform surfaces.