Paper-Recycling System: Part 2

Paper fiber starts its way toward consumers at harvesting. An increasing part of pulp-logs is gained as a by-product of forest thinning. Today a major part of the felling and limbing is still based on manual technologies even though highly automated and powerful harvesting machines are occupying more space in the harvesting market. The part of the wood biomass in logs leaving forests for pulp mills is about 65% of the total biomass of of the trees. The rest, made up of roots, branches, and leaves, normally remains in the forests where it decomposes into methane and carbon dioxide in from five to ten years.

Primary wood fiber is extracted from logs in pulp mills. There are two principal families of technologies for the extraction: chemical and mechanical. Both families comprise several different technologies. There are also technologies utilizing a combination of these principal technologies characterized as semi-chemical or chemi-mechanical. These, however, represent a small minority of the overall pulp production capacity.

The two main technologies differ from each other in several ways. First, in chemical pulping, lignin and other impurities are separated from the cellulose fibers by a chemical-cooking treatment. In mechanical processing, practically all the wood material, except bark, is utilized for pulp. Second, chemcial processing is practically energy self-sufficient due to utilization of the heat potential of extracted lignin and bark. Mechanical pulping is fully dependent on external energy. In addition, the specific electric energy demand for chemical pulping is roughly one-third of that for mechanical pulping. Third, the pulp yield from mechanical processing is roughly twice as much as the yield from chemical processing; the larger energy consumption of mechanical processing is compensated by smaller raw wood material consumption. Fourth, currently chemically produced pulp has more permanent optical and strength properties than mechanically produced pulp. Therefore, chemical pulp can be used for more purposes; mechanical pulp is normally limited to products with a relatively short lifetime.