Detrimental substances, water circuits

In the production and processing of chemical, semichemical and mechanical pulps and recovered paper, various inorganic and organic substances are accumulated in dissolved or colloidal dissolved form. Other water-soluble substances enter with the fresh water, fillers, recycled uncoated and coated paper broke and also by chemical additives. As the process water circuits are increasingly closed, the concentration of these water-soluble and colloidal substances and finely dispersed particles increases considerably and an additional contaminant load is thus imposed on the waste water. These substances interfere with the production process by increasing build-ups and deposits, they reduce the efficiency of the chemical additives, and impair the quality of the produced paper. Therefore these substances are also classed as detrimental substances (Fig. 3.18). 

Inorganic dissolved substances, i. e. salts, are measured as increased conductivity. Salts are also detrimental to the process performance and potentially for the paper properties. Electrolytes reduce the swelHng potential of fibers and chloride especially leads to corrosion of machine parts [4]. The content of detrimental substances in paper miU water circuit systems depends on the input of raw materials, on the output by bleeding through waste water disposal as well as by the degree of transfer to the final paper, on the loop design, and on the presence of kidney technologies in the miU. 

All processing cycles in paper production are connected directly or indirectly by water loops. The objectives of the water circuit system are to offer the required volume rate and quaHty of water for each consumer and to treat and/or bleed out water containing detrimental substances. A water circuit system of a paper mill usually includes different water loops (Fig. 5.1) ... 

The system consists of three water loops, two in stock preparation (because of the combination of flotation - dispersion - flotation) and one in the paper machine. The water circuit design follows the previously mentioned principles of counter current flow and thereby allows the fresh water to be used to the maximum effect. In addition the save-all is followed by an additional DAF unit DAF 1 is used as a kidney for removal of detrimental substances, treating the make-up water from the paper machine (clear filtrate) together with part of the clear filtrate from the disc filter in loop II. DAF 2 treats the filtrates from reject and sludge dewatering. The clarified water is partly recycled back into loop I, the rest being sent to the effluent treatment... 

---