How many pages go out for every page that goes in? I would be amazed if it were higher than 50%.
The mass-in/mass-out can vary quite a bit, depending on the feed paper. For example, brown paper bags are almost entirely composed of Kraft pulp, which is generally long fiber from softwood. To provide better strength and moisture-resistance, the manufacturer may have added "sizing" compounds to the Kraft sheet during production. The sizing makes the Kraft sheet a bit harder to break down and recycle, but Kraft paper returns a high yield of usable fiber. Glossy magazine paper returns a lot less usable fiber per ton. It often has a blend of short hardwood fibers and long softwood fibers, but the real important factor is that the sheet contains lots of additives. Print quality is important, so these papers may include kaolin filler, calcium carbonate for brightness, titanium oxides for opacity, and coatings containing latex and other compounds that can be calendared to achieve a glossy finish. There is far less usable fiber per ton in such paper than in Kraft paper. Newsprint is often recycled, too. It is commonly made from ground-wood - a process in which wood is mechanically abraded. Such paper usually contains some additives, but far less by weight than magazine papers. When newsprint is recycled, it may be mixed with other fibers. For instance, if you are going to produce tissue, paper towel, toilet paper, etc, you would want a high percentage of long, strong fiber in the sheet. You can use recycled newsprint in the mix, but you have to have enough long fiber so that the sheet holds together during forming, pressing, drying, and creping.
Producing paper is very energy-intensive. The energy savings realized by making paper from recycled fiber instead of raw wood can be significant, and this economic advantage is usually the motivation for using recycled pulp vs virgin fiber.
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