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Pulping chemicals can be used many times over. Many valuable chemicals, such as the sodium and sulphur compounds used in pulping, can be efficiently recovered and recycled. The black liquor formed during the cooking process contains pulping chemicals and dissolved material from the pulpwood. The organic material in this liquor is incinerated in recovery boilers. Residual chemicals are then recovered to be used again in the cooking process. Small quantities of fresh sodium hydroxide or sodium sulphate may be added to the cooking process to compensate for minor losses of chemicals.
Paper-making chemicals cannot be recovered at the production stage, since most of them are incorporated in the end product. Any small amounts of non-recoverable chemicals remaining in waste water are cleaned out at mills' waste water treatment plants.
Chemical Table
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