The manufacture of yellow board has two major stages. The first stage is pulp making and the second is paper or board making. The detail of the production process is presented below.
Stage I: Pulp Making
1. Pretreatment of Straw and Cooking
A. Pretreatment of Straw: The wheat straw, which has been collected and stored, is cut to a suitable uniform length by a cutter, and it is sent to the duster via a belt conveyor. The dusted straw is carried to the next process on another conveyor.
B. Cooking: The straw stored in a chip bin is put in a digester at the rate of about 2.5 tons of dry straw at a time. When the above operation is over, a fixed amount of lime solution is added, after which digestion continues for several hours (6-8 h/cycle) with steam under a pressure of 4-6 kg/cm2. The cooked material is blown into a blow pit, and waste liquid is washed away by a beater placed in the lower part of the pit.
2. Stock Preparation
The raw material made available from the above operation is fed into a dilution box with a pump. Fresh water is added to the box to provide consistency to suit the following beating process. Nodes and sheaves are eliminated by the inclined screen to drop the stock into the next beater, in which it is treated for an appropriate number of hours so as to make it suitable for paper machining, as well as to provide a good quality for the finished paper. Stock which has been subjected to the beater process is stored in the beater chest, out of which it is sent into the head box by the pump. The stock goes out of the above equipment into the selective screen, where is dusted, for transfer to the distributing box and head box.
3. Chemical Preparation
This is an independent section, in which the lime for digestion is dissolved to the stipulated concentration and stored. The process so far described constitutes the first half of the paper making process, namely the section for the manufacturing of raw materials.
Stage II: Paper Making
1. Paper machine- Wire, Press and Dryer Part
A. Wire Part: One or several cylinder moulds rotate in each vat. The stock which has been fed is scooped up by the rotating cylinder mould machine for formation of sheet. When cylinders go out of the stock liquid, the surfaces of the cylinders are covered with wet paper which constitutes fabrique. The number of cylinders is decided according to the thickness of paper to be made.
B. Press part: The bottom felts, which rotate endlessly, pass the crests of the cylinders one by one to pick up the paper layer formed there. After passage of the second press roll, felt and paper are separated, with felt resuming its advance to the cylinder after water has been squeezed out of the felt by the squeeze roll, and paper proceeding towards the next dryer part.
C. Dryer part: The paper which comes out of the final press roll has both smoothed by the smoother. It is dried when it comes into contact with the surfaces of the fourteen paper dryers (cylindrical, with steam inside), one after another. Canvass or dry felt is employed to insure good contact with cylinders.
The calendar comprises for chilled rolls, the surfaces of which are hardened. Paper gains glaze as it passes between the rolls through the slip action of heat and pressure.
Paper is finally cut into the required size by the cutter, thereby ending the continuous paper making process. When paper is specially required in a smaller size, it is cut by a guillotine cutter which is provided separately.
The alternative technological option refers to the use of manual labor in the drying process. In this method a blotter sheet is used to collect the water instead of canvass or dry felt. Then, the paper is hanged in open space and is left for a while until the moisture level is reduced to a level less than 6 percent. Although this option has the advantage of lower cost when compared to steam-heated dryer discussed earlier, it is not recommended for a scale and quality of production envisaged by the proposed plant.