Paper plays an important role in our daily life. It is one of the greatest inventions in human history. Paper can be made from pulp of wood, grass, waste paper or other materials.
Paper manufacturing processes are as follows:
- The pulping process, it separates and cleans the fibers
- Refining process would be followed after the pulping process
- Dilution process to form a thin fiber mixture
- Formation of fibers on a thin screen
- Pressurization to enhance the materials’ density
- Drying to eliminate the moisture in the materials
- Finishing process to provide a suitable surface for usage
Paper and pulp are made from cellulosic fibers and other plant materials. Some synthetic materials may be used to impart special qualities to the finished product. Paper is made from wood fibers, but rags, flax, cotton linters, and bagasse (sugar cane residues) are also used in some papers. Used paper could also be recycled, and after purifying and sometimes deinking, it can be blended with virgin fibers and reformed again into paper. Products such as cellulose acetate, rayon, cellulose esters that are made from cellulose will be used for packaging films and explosives.
The pulping process is aimed at removing lignin without loosing fiber strength, thereby freeing the fibers and removing impurities that cause discoloration and possible future disintegration of the paper.
Hemicellulose plays a key role in fiber-to-fiber bonding in papermaking. It is similar to cellulose in composition and function. Several extractives such as waxes, oleoresins are contained in wood but they do not contribute to its strength properties; these too are removed during the pulping process.
The fiber extracted from any plant can be used for paper. However, the strength and quality of fiber, and other factors complicate the pulping process. In general, the softwoods (e.g., pines, firs, and spruces) yield long and strong fibers that contribute strength to paper and they are used for boxes and packaging.
Hardwoods produce a weaker paper as they contain shorter fibers. Softwoods are smoother, transparent, and better suited for printing. Softwoods and hardwoods are used for paper-making and are sometimes mixed to provide both strength and print ability to the finished product.
Steps involved in the Pulp and Papermaking Process:
Preparation of raw materials
Wood that has been received at a pulp mill can be in different forms. It depends on the pulping process and the origin of the raw material. It may be received as bolts (short logs) of round-wood with the bark still attached, as chips about the size of a half-dollar that may have been produced from sawmill from debarked round wood elsewhere.
If round wood is used, it is first debarked, usually by tumbling in large steel drums where wash water may be applied. Those debarked wood bolts are then chipped in a chipper if the pulping process calls for chemical digestion. Chips are then screened for size, cleaned, and temporarily stored for further processing.
Separation of Fiber
In the fiber separation stage, several pulping technologies will be diverged. The chips are kept into a large pressure cooking digester, into which is added the appropriate chemicals in Kraft chemical pulping.
The chips are then digested with steam at specific temperatures to separate the fibers and partially dissolve the lignin and other extractives. Some digesters operate continuously with a constant feed of chips (furnish) and liquor are charged intermittently and treat a batch at a time.
After the digestion process, the cooked pulp is discharged into a pressure vessel. Here the steam and volatile materials are tubed off. After that, this cooked pulp is returned to the chemical recovery cycle. Fiber separation in mechanical pulping is less dramatic.
Debarked logs are forced against rotating stone grinding wheels in the stone ground-wood process. Refiner pulp and thermo-mechanical pulp are produced by chips. These chips are ground by passing them through rapidly rotating in both processes.
In the second stage after refining, the pulp is screened, cleaned, and most of the process water is removed in preparation for paper making.
Paper Pulp Bleaching Process
Raw pulp contains an appreciable amount of lignin and other discoloration, it must be bleached to produce light colored or white papers preferred for many products. The fibers are further delignified by solubilizing additional lignin from the cellulose through chlorination and oxidation. These include chlorine dioxide, chlorine gas, sodium hypochlorite, hydrogen perioxide, and oxygen.
Sodium Hydroxide, a strong alkali is used to extract the dissolved lignin from fibers surface. The bleaching agents and the sequence in which they are used depend on a number of factors, such as the relative cost of the bleaching chemicals, type and condition of the pulp.
Mechanical pulp bleaching varies from chemical pulp bleaching. Bleaching of mechanical pulp is designed to minimize the removal of the lignin that would reduce fiber yields.
Chemicals used for bleaching mechanical pulps selectively destroy coloring impurities but leave the lignin and cellulosic materials intact, These include sodium bisulfite, sodium or zinc hydrosulfite (no longer used in the United States), calcium or sodium hypochlorite, hydrogen or sodium peroxide, and the Sulfur Dioxide-Borol Process (a variation of the sodium hydrosulfite method).
Bleached or unbleached pulp may be further refined to cut the fibers and roughen the surface of the fibers to enhance formation and bonding of the fibers as they enter the paper machine.
Water is added to the pulp slurry to make a thin mixture normally containing less than 1 percent fiber. The dilute slurry is then cleaned in cyclone cleaners and screened in centrifugal screens before being fed into the ‘wet end’ of the paper-forming machine. The dilute stock passes through a head-box that distributes the fiber slurry uniformly over the width of the paper sheet to be formed.