Planeación Preliminar

This is the first step in paper manufacturing in which raw wood and other materials are converted to pulp. Pulp from fibrous raw material is manufactured either by mechanical process or chemical process. About 80-85% pulping is carried out by chemical process and only about 8-10% by mechanical process.

Mechanical Process: Mechanically, wood pulp is obtained from pine and other soft wood such as spruce and balsam. The wood is first slashed and debarked. The debarking is usually carried out by two methods.

1) Continuous rotating cylindrical drums or stationary machines fitted with agitating cam are used. In this case, the wood is introduced into the upper end of a rotating drum immersed in a tank partly filled with water where it is tumbled. The bark is rubbed off and the clean wood discharged at the other end.

2) Mechanical friction i.e by applying high pressure water jets to individual logs by means of hydraulic barkers. Water jets are directed against the log by means of hydraulic barkers in such a manner that the bark is broken up and removed.

The debarked wood is then subjected to mechanical grinding in presence of water in grinders to remove the heat of friction and to float the extremely small pieces of fibers. The pulp and water mixture thus obtained contains all the lignocellulose of the original wood. The mixture from the grinders is withdrawn into a stock sewer, below the grinders and then conveyed to the silver screen. The fine particles, thus screened by these screens are then concentrated in thickeners to get commercial mechanical pulp. The larger particle retained by silver screen and the fine screen are treated in refiners and then returned to the screens again to recover more mechanical pulp.

The white water, overflowing from the thickeners contain about 15-20% of the original fibres. It is used in grinding and to aid flow in the stock sewer. The remaining fibres are strained from the white water before it is run to waste. The fibres so strained are conveyed to thickeners for concentration to mechanical pulp. As mechanical pulp cannot be completely bleached, it is employed in the manufacture of cheap grade of paper, such as required for newspaper and wrapping. Its colour can however, be considerably lightened by the use of bleaching agents, such as sodium or calcium bisulphite, hydrogen peroxide or sodium peroxide.

Chemical Process: This process is used to obtain high grade paper. Various types of pulps, such as sulphate pulp, soda pulp, sulphite pulp etc are prepared by chemical process.

Sulphite Pulp: The wood is slashed, debarked, cleaned and chipped. The disintegrated chips are then digested with an aqueous solution of calcium bisulphite or magnesium bisulphite and an excess of sulphur dioxide. The sulphite process involves sulphonation, and solubilizing of the lignin with the bisulphite and the hydrolytic decomposition of cellulose lignin complex.

The newer and technically more acceptable sulphite process is based on magnesium bisulphite instead of calcium bisulphite due to the following important reasons.

1) The waste liquor from the calcium bisulphite process cannot be used again as calcium bisulphite would not decompose to SO2, whereas magnesium bisulphite does. The magnesium bisulphite waste liquor is therefore recoverable and reusable.

2) The disposal of calcium bisulphite waste liquor is a serious pollution problem since more than half the raw material entering the process appears as dissolved organic solids. The magnesium bisulphite waste liquor, on the other hand, reduces pollution because of the fact that it can be used again and it is recoverable. Moreover, a large number of by- products such as lignin, vanillin, tanning materials, rod binders, portland cement, accelerator, plastics from the lignin present, and food yeast etc can be obtained from the waste liquor of magnesium bisulpite process.

In the process, sulphur is heated in a tank by the rotary burner, and the vapours of the sulphur are allowed to enter a combustion chamber, where they are re-oxidised by air to sulphur dioxide.

S + O2 → SO2

The amount of air is carefully controlled in other to prevent the further oxidation of SO2 to SO3. The hot sulphur dioxide thus formed is immediately cooled in coolers, surrounded by water cooled pipes. The cooled SO2 gas is then passed through a series of two or more absorption towers or acid making tanks containing magnesium carbonate. A fine spray of water trickles down the tower system and meets sulphur dioxide gas in a counter current manner (SO2 gas is blown up through the tower)

2SO2 + MgCO3 + H2O Mg (HSO3)2 + CO2

The final liquid containing a solution of magnesium bisulphite and some free unabsorbed sulphur dioxide is charged to the digester. It should be noted that free SO2 gas vented from the digesters is also bubbled through acid making towers. Hence, the amount of SO2 in the liquor is increased from time to time. The digester is filled with the chip and the acid cooking liquor is introduced through the bottom of the digester. To prevent corrosive action of the cooking liquor, the digesters are lined with crushed quartz or acid resisting bricks. The digester may be heated with steam, but it causes dilution of the liquor. In other to achieve a better temperature distribution pattern throughout the system and to prevent dilution of the liquor, the cooking liquor is heated outside stainless steel tube heater with forced outside circulation of steam and the heated liquor is then circulated through the charge by making use of pumps. The cooking is carried out at about 70-80^oC and takes about 6-12hours.

After the completion of the cooking process, the pulp is washed with fresh water in a large round tank having a false bottom. The cooking or red liquor is evaporated and burnt in boiler to MgO and SO2. The MgO is slaked into Mg (OH)2 and are pumped to the cooling and acid tower down from which SO2 gas (formed as a result of burning) is passed to produce fresh bisulphite liquor.

Mg (OH)2+ 2SO2 Mg(HSO3)2

The pulp is collected in a tank from where it is pumped to a series of screens. Knots and large lumps of fibre are retained by the screens and the desired fibre screened is conveyed to the centrifuge so as to remove foreign impurities. The pulp is then concentrated in a cylindrical thickener and retained on the screen and water passes through. The thickened pulp is bleached with chlorine or chlorine dioxide and then bleached pulp is treated with milk of lime to neutralize the mass. The resulting stock is again washed with water, thickened and passed

to the machine stock chest. The chest gives pulp laps containing about 30-35% dry fibre.

These laps are dried in a pulp dryer by means of steam heated rolls to get a product containing 80-90% dry fibre.

Sulphite pulp is a high quality pulp and used in the manufacture of some finest papers, including sunlit bond.