sulfatsprocesser

Sulfatsprocesser, commonly referred to as the kraft process, are a family of alkaline chemical pulping methods used to convert wood into wood pulp. The process uses a cooking liquor primarily containing sodium hydroxide and sodium sulfide to break down lignin, the glue that binds cellulose fibers in wood. By dissolving lignin while preserving most of the cellulose, kraft pulps exhibit high strength and good dimensional stability, making them suitable for a wide range of paper grades. The method can process many wood species, especially softwoods, and supports high pulp yields.

Process overview

Wood is chipped and cooked in large digesters with white liquor (the active cooking chemicals). During cooking, lignin and some carbohydrates are dissolved into the liquor, separating from the cellulose fibers. The resulting pulp is separated from the black liquor, washed, and often further processed in bleaching stages to achieve the desired brightness. A key feature of the kraft process is the chemical recovery loop: the concentrated black liquor is burned in a recovery boiler to recover chemicals and generate energy; the resulting smelt is processed in a lime kiln to regenerate white liquor, completing the cycle.

Variations and bleaching

Modern kraft mills employ chlorine-free bleaching sequences (ECF or TCF) to reach target whiteness while reducing environmental impact. Pulp produced by the kraft process is typically used for packaging papers, magazines, and various consumer paper products due to its strength and versatility.

Environmental and economic aspects

Chemical recovery makes kraft pulping resource-efficient and energy-intensive, but mills must manage effluents and sulfur compounds to minimize odors and pollution. The kraft process dominates modern chemical pulping because of its robustness, yield, and ability to handle diverse timber stocks.