Wood pulping by-products

The types of by-products vary from one pulp mill to another. Many of these by-products would undoubtedly have value as animal feed, but this aspect has been poorly investigated so far.

• Screen rejects, which are partially pulped but not bleached, have a dry matter digestibility of 44%.
• The ray cell and vessel fines, also a partially pulped by-product, have a dry matter digestibility of 72%.
• Centrifugal cleaner fines are a fully pulped and bleached by-product with the same digestibility as the cellulose.
• The by-product from mills producing synthetic fibre from wood is rich in digestible hemicellulose and has been used experimentally in sheep rations without any obvious harmful effects in amounts of up to 0.6 kg per day.
• Lignin sulphonates are produced by concentrating sulphite pulping liquors until they contain 50-55% solids, after which they are handled like molasses or are spray-dried.
• Ammonium, calcium, magnesium or sodium sulphonates can be produced, but the ammonium lignin sulphonates are probably the most important as they are a good source of both crude protein and energy. Commercially, they are mixed with molasses for use as a binding and pelleting agent or for feeding as a low-level energy source.