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Industrial waters 105

Table 3.7 Comparison of mechanical and chemical pulping processes (mesa, 1990)
Mechanical pulping Chemical pulping
Groundwood CTMP“ Sulphate Sulphite
Process log is pressed wood chips are Wood chips are Wood chips are boiled
against a vapour-heated boiled in in sulphuric acid
grindstone and chemically caustic
treated before soda
grinding
Lignin Much lignin left in pulp, turns dark Little lignin left, remains white after
when exposed to light bleaching
Fibre make-up Yellow torn fibres, Slightly yellow, Dark brown Light brown whole
very weak more whole whole fibres, fibres, strong, soft
fibres, very strong
fairly strong
Pulp yield 0.9-0.95 t pulp/t wood 0.45-0.5 t pulp/t wood
Bleaching Hydrogen peroxide Chlorine/chlorine dioxide. sometimes
oxygen. hydrogen peroxide
Electrical 2000 kWhlt 1150kWhjt 1000 kWh/tonne
energy
Steam energy nla 5200 kWh/t 4400 kWhjt
Water 10 000-1 5 000 gallons/t 35 000-45 000 gallons/t

* Chemothermomechanical pulping.

(TAPPI, 2000). At the mill site the recovered, sorted paper is stored, and the
paper is re-pulped, screened and cleaned. The printing ink and “stickies” (sticky
materials like glue residue and adhesives) are removed in the de-inking process.
After de-inking the pulp is refined and, if needed, bleached using hydrogen
peroxide, chlorine dioxide, and/or oxygen.
Paper manufacturing involves three stages: stock preparation, pressing, and
thermal drying. During stock preparation, wood fibre is mixed with a 1% solution
of minerals in water and the water drained from the stock to form a web of paper.
In the next step, pressing, the paper is squeezed between rollers and a felt (a large
mat of nylon and polyester filaments) to produce a material generally containing
40-50% water. In the third step water is removed by thermal drying to produce
paper containing 2-6% water. If size (a starch or starch-like substance used to
modify the surface properties of the paper) is to be used a size press coating
consisting of starch and minerals is put onto the paper next. Following this the
paper is dried once again before finally being wound into a large roll. The surface
of the paper is often modified by coating to improve the print quality: the paper is
passed under a blade that deposits a homogeneous coating onto its surface.
Different kinds of pigments and adhesives are used for different grades of paper.
Supercalendering, used for some grades to make the paper glossy and smooth,
employs a number of large rollers to simultaneously press and heat the paper.
Sheet converting is the final stage of papermaking, in which the rolls of paper may
be cut into individual sheets.
Pulp and paper mills can be either discrete production units or form an integral
process. The water consumption differs between non-integrated and integrated