26.1 What is pulp?

• • Pulp is a commercial fibrous material obtained from bamboo, wood, bagass (waste material) etc. by mechanical and chemical means.

• Pulping means disintegration of bulky fibrous material to small fibres.

• There are mainly three modes of production of pulp:

 

• Mechanical

• Chemical

• Semi chemical

26.2 Sulfate (Kraft) Pulping Process (Figure 26.1)

• • Most popularly used process.

• This is an alkaline process.

• Na₂SO₄ is added to the cooking liquor. So its common name is sulfate process.

• The presence of sodium sulfide makes bleaching of pulp easier and the paper produced has better strength.

Chemical reactions involved

(i) Digestion (hydrolysis and solubilization of lignin)

             R-R’ + NaOH → R”COONa + ROH
             R-R’ + Na₂S → Mercaptans

(ii) Chemical recovery from black liquor

• (a) Smelting

              2NaR + air → Na₂CO₃ + CO₂
              (lignin)
             Na₂SO₄ + 2C  → Na₂S + 2CO₂
              (from R)    (white liquor)

         (b) Causticizing

            Na₂CO₃ (aq) + Ca(OH)₂ (s) → 2NaOH (aq) + CaCO₃ (s)
            (green liquor)                         (white liquor)

            CaCO₃ → CaO + CO₂
            CaO + H₂O → Ca(OH)₂

Figure 26.1 Flow sheet of manufacture of pulp manufacture