Kraft Pulping ~ Chip to Dirty Pulp

The Beginning of the Pulp Mill

The History

First patented by C.F. Dahl in 1884, Kraft Pulping is an excellent way to pulp wood chips. High strength pulp is produced because of the dissolving lignin by sulfide cooking liquor. The process is economical due to its ability to recycle sodium carbonate (soda ash). Improvements came to the Kraft Pulping in the 1930s with the introduction of the Tomlinson recovery furnace and in the late 1940’s when Howard Rapson achieved bleached brightness levels comparable to sulfite pulps.

The Process

Chips are loaded in from the chip yard into digesters that cook the chips. In the digester, white liquor (sodium hydroxide and sodium sulfide) is then used to cover all the chips. The contents are then heated with air and other noncondensible gases relieved at the top. Digesters are heated to about 170 degrees Celsius which may take about an hour and then cooked for an additional two hours to complete the cooking reactions. Toledo runs on three hour cycles and have 11 batch digesters. This means that usually they are able to achieve about 90 cooks a day.

Hot and dirty pulp is then pumped into the three blow tanks where steam can be generated and the stock can depressurize.

The Chemistry

Wood chips are made out of cellulose, hemicelluose, and lignin. Paper gets its strength from hydrogen bonding between cellulose fibers. Lignin can block the hydrogen bonding. During Kraft Pulping the swollen lignin is chemically split into fragments by hydroxl and hydrosulfide ions present in cooking liquor. Lignin can also be dissolved as phenolate or carboxylate ions. The hydrosulfide ions prevent the dissolved lignin from condensation reactions which help remove lignin from the fibers.

The reaction is driven by the alkali concentration of the white liquor and the cooking temperature with the delignification rate doubling every 10 degree Celsius.

The Variables

• Species of Wood Chips
• Chip Quality
• Moisture Content
• Sulfidity
• Active Alkali or Effective Alkali
• Liquor to Wood Ratio
• Temperature Cycle
• H-Factor (Time/Temperature)
• Kappa Number (Degree of Delignification)
• Residual Alkali