Above 10 000 tons of fiber was considered to be a huge operation. Recovery of

the pulping chemicals and treatment of the effluent was not usual [4], and for the

Sulfite process in some regions this approach was maintained for almost 100

years. The cheap pulping chemicals – limestone and sulfur – did not require

Expensive recovery, and consequently all wood compounds dissolved during pulping

Were discharged into the rivers. Assuming a pulping yield (on wood) of less

than 50%, more dissolved material such as carbohydrates and sulfonated lignin

Was discharged than fiber produced.

This situation was different in kraft pulping, however, as the rather expensive

caustic soda made recovery economical. The process acquired its other name –

sulfate pulping – from the addition of sodium sulfate as sulfur make-up ahead of

The reduction and combustion steps. As the processes changed slowly to provide

Better recoveries, huge amounts of organic material were removed from the effluent.

Today, in sulfite pulping magnesium is the cation of choice. Following evaporation

And combustion of the dissolved wood material, it is recovered as MgO

From the dust filters. The combustion gases contain SO2, which is recovered by

Absorption by a magnesium oxide slurry. The small number of calcium sulfite

Mills remaining in operation also include an evaporation stage, and produce different

Types of lignosulfonates as valued byproducts. The sulfur dioxide remaining

In the combustion gases is recycled and used to prepare fresh pulping acid solution.

Therefore, sulfite mills do not contribute as excessively as in the past to the

problem of “acid rain”, and sulfur dioxide emission using the best available techniques

(BAT) is now running at 0.5–1.0 kg t–1 pulp (as sulfur) [2].

Environmental Aspects of Pulp Production

In alkaline pulping, the intensity of pulping chemical recovery has been

Increased significantly. This had a pronounced impact on the color of the effluent.

In general the effectiveness of brownstock washing and measures to avoid spills

Were intensified. Special basins are required to handle spills. The use of an oxygen

stage as an “extension” of the pulping process has become common practice, as

This permits to combust additional amounts of lignin dissolved by the oxygen

Treatment. This requires a higher evaporation and boiler capacity, but has the

Advantage of reducing the amount of dissolved organic material from the bleach

Plant.

Initially, the bleaching of chemical pulp was limited to treatment with hypochlorite

Дата добавления: 2015-10-21