Alright, you’ve been waiting for it so here it is! There are two main types of pulping processes in the paper making process. Generally they are chemical pulping and mechanical pulping. For simplicity sake, we will focus here on the modern mechanical process. However, since many end paper products use a combination of different pulps, we will give you a general idea of the chemical process.
Chemical pulping can be divided into sulphite pulp and Kraft pulp. Both employ the use of digesters which are large pressure vessels in which the wood chips are bathed in various chemicals and “pressure cooked” which dissolves the lignin in the wood. Lignin is an organic polymer which acts as the binding agent in wood and it serves to hold the individual cellulose fibers together. This type of pulping produces a very long fiber which provides excellent “strength” characteristics in the end product. Also, with Kraft pulp, the brightness of the pulp is increased due to the specific chemicals used which aids in the brightness of the end product. Brightness is an important characteristic in many grades of paper. The main drawback of chemical pulping is left over chemicals and vent gases which are subjected to increasingly strict environmental laws around the world requiring the treatment of waste in secondary waste treatment plants, use of scrubbers, etc. which can be a substantial cost.
Mechanical pulping used to be dominated by “ground wood” pulping. This process employs large rotating stone wheels which the logs and lubricating water are fed into. The friction of the grinding process creates heat which softens the lignin and releases the cellulose fibers creating ground wood pulp. This process is still in use today but later in the 20th century a newer form of mechanical pulping began to emerge which employs the use of “refiners” rather than ground wood stones.
A refiner is a pressurized machine that is powered by a large electric motor. Inside the refiner casing, there are two disks which have various patterns of “teeth.” One disk remains immobile and is referred to as a stator. The other disk referred to as a rotor is powered by the electric motor via a drive shaft and spins at high velocity. These plates operate at very close tolerances creating a very narrow pressurized “refining zone” in which the wood chips are fed. The intense heat, pressure and mechanical shearing forces are what releases the cellulose fibers.
Keep a lookout for our next blog on the paper making process!