Pulping process: 

In Kraft Pulping process, which is most common chemical pulping process, lignin reacts with caustic soda or sodium hydroxide (NaOH). The Na ion attach with lignin and make sodium lignate (Na-lignin). Sodium lignate dissolve in water and fibers are separated.

Na-lignate is burned in the furnace in presence of sodium sulphate (Na2SO4 known as make up chemical in paper industry). The carbon portion of Na-lignate burns and provide heat energy and Na ion oxidized to Sodium Carbonate (Na2CO3). Sodium sulphate reduced to Sodium Sulfide (Na2S).

Sodium Carbonate (Na2CO3) is made to react with lime or calcium hydroxide (Ca(OH)2. This reaction gives NaOH and CaCO3 (lime stone). Calcium carbonate is burned to give back calcium hydroxide (Ca(OH)2 and CO2. So the only new chemical getting in the system is Sodium Sulphate, which is very inexpensive as it is a by-product.

Bleaching. Process:

Residual lignin depending on the bleaching chemical used reacts and either provide a water soluble compound or a modified lignin which is colorless. If chlorine based bleaching agents (Cl2, ClO2, Na-hypochlorite etc.) are used, lignin-cl are formed and dissolve in water and are removed. If oxygen based bleaching agents (Ozone O3, Sodium Peroxide Na2O2 etc.) are used, lignin is modified and don't dissolve in water and stay with the pulp.

In acidic sizing (sizing is the process of making paper liquid resistant), Rosin (C19H29COOH), a natural resin from pine trees reacts with alum [Aluminum Sulfate {Al2(SO4)3}] to create Aluminum- Rosinate, a liquid resistant layer around fibers.

In basic or neutral sizing (pH range 7. - 9.5), hydrolyzed Alkenyl Succinic Anhydride (ASA) or Alkyl ketene dimer (AKD) form covalent bond with the hydroxyl groups of cellulose and form an ester bond (b-keto esters).

The processes of/ chemical reactions used in making paper