Semichemical Pulping

Semichemical pulping consists of treating with a mild chemical to remove part of the lignocellulose, then refining mechanically to complete the fiber separation. Yields between 70 and 85 percent are possible, with strengths equal to fully-cooked pulps. Chips can be cooked in a neutral sodium sulfite solution, a sulfate liquor adjusted to concentrations of sodium hydroxide, and sodium sulfide, acid sulfite, or soda. Fibers in the semichemical process have good bond strength due to fibrillation by mechanical refining and chemical removal of some lignin. Strength values for semichemical yields of 70 to 81 percent for 29 hardwoods and 10 softwoods are given. Soft wood semichemical pulps are stronger than hardwood pulps, but both are weaker than completely delignified pulps. Semichemical pulps produce hard, brittle sheets with low opacity. A yield of 50 to 60 percent produces maximum strength; strength decreases rapidly at yields greater than 80 percent. Neutral sulfite and sulfate semichemical pulps produce higher strength products than soda or acid sulfite. Bleaching increases the pulp strength, but more chlorine is required to bleach semichemical pulps. Semichemical pulp has uses in production of corrugating board, wrapping paper, insulating board, roofing felt, linerboards, and newsprint. Bleached semichemical pulp has uses in manufacture of ground wood paperbooks, towel papers, and bond papers.

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