Paper is a thin sheet typically produced from cellulose pulp, derived from wood and other lignocellulosic materials like cotton, rice, or wheat straw, for writing, printing, and packaging purposes. It’s believed that paper originated in China in the 2nd century as a writing material, replacing silk. Paper production was introduced to Europe in the 12th century. Approximately 200 years of mechanized paper production led to significant changes in information networks worldwide.

Stages of Paper Production

Pulp and paper are made from cellulose fibers and other plant materials. Some synthetic materials may be used to impart special qualities to the final product. Paper is primarily made from wood fibers, but linen, cotton fibers, and bagasse (sugarcane residue) are also used in some papers.

Used papers are also recycled; after purification and sometimes de-inking, they are often combined with virgin fibers and converted back into paper. Products like cellulose acetate, rayon, and cellulose esters are used for packaging films and explosives. Paper manufacturing industries, both in Iran and globally, must follow these stages:

• Pulping method for separating and cleaning fibers.
• Refining method after pulping processes.
• Dilution process to form a thin fiber mixture.
• Fiber formation on a thin screen.
• Pressing to increase material density.
• Drying to remove material density.
• Finishing method to provide a suitable surface.

Stages Related to the Pulp and Papermaking Process

Raw Material Preparation

At the beginning of paper manufacturing, the wood received at the paper mill can be in various forms, depending on the pulping process and the origin of the raw material. It might be received as bolts (short logs) of round wood with bark still attached, or as chips that may have been produced elsewhere from debarked round wood at a sawmill.

If round wood is used, it is first debarked, usually by tumbling in large steel drums where wash water may be applied. If the¹ pulping process requires chemical digestion, the debarked wood bolts are then chipped in a chipper machine. The chips are then screened for size and cleaned, and temporarily stored for further processing.

Fiber Separation

In the fiber separation stage of paper manufacturing, several paper pulping technologies are employed. In chemical pulping (Kraft process), chips are placed in a large digester, and appropriate chemicals are added. The chips are then cooked with steam at specific temperatures to separate the fibers and partially dissolve lignin and other extractive materials. Some digesters operate continuously with a constant feed.

After the digestion process, the cooked pulp is discharged into a pressurized vessel. Here, steam and volatile materials are piped away. Subsequently, this cooked pulp returns to the chemical recovery cycle. Fiber separation in mechanical pulping is less dramatic. In the second stage after purification, the paper pulp is screened, cleaned, and most of the process water is removed in preparation for papermaking.

Bleaching Process

Following the initial processes in paper manufacturing, raw pulp contains a significant amount of lignin and other discolorations. To produce bright-colored or white papers, which are preferred for many products, it must be bleached. Fibers are further separated by dissolving additional lignin from the cellulose, through chlorination and oxidation. These include chlorine dioxide, chlorine gas, sodium hypochlorite, hydrogen peroxide,² and oxygen.

Chemicals used to bleach mechanical pulps selectively remove colored impurities but leave lignin and cellulosic materials intact. These include sodium bisulfite, sodium or zinc hydrosulfite, calcium or sodium hypochlorite, hydrogen or sodium peroxide, and sulfur dioxide process.

Papermaking Procedure

Continuing the paper manufacturing process, the bleached or unbleached pulp may be refined to cut the fibers and roughen their surface to enhance fiber formation and bonding when entering the paper machine.

Water is added to the pulp slurry to obtain a thin mixture, typically containing less than 1% fiber. The diluted slurry is then cleaned in cyclone cleaners and screened in centrifugal screens before entering the paper-forming machine. The dilute stock passes through a headbox, which distributes the fiber slurry evenly across the width of the paper sheet to be formed.