White water circuit system

Stock usually the best dewatering stock component used at the paper machine is selected in order to Hmit the size of the disc filter. In addition to cloudy and clear filtrate, disc filter save-aU applications also produce a superclear filtrate with very low solids content This superclear filtrate is used as a fresh water substitute for spray apphcations in the paper machine. The clear filtrate is stored in a buffer tank. The cloudy filtrate is usually fed directly back to the inlet of the disc filter, the used sweetener, including the recovered stock, is fed back to the thick stock in the approach flow system. DAF (dissolved air flotation) type save-alls are used in older machines and nowadays when a certain degree of ash and fines removal from the process is demanded. In this case, the sludge of the DAF is rejected. The advantages of a disc filter save-all compared to a DAF save-all are higher filtrate quahty, no chemical consumption and less space requirements (see Sections 4.2.62 and 4.2.9). 

There are two reasons why there is always an excess of water present in the paper machine loop. First, the paper machine loop is continuously fed with fresh water, used for spraying and chemical dilution. Secondly, the incoming water content of the stock is higher (consistency 12-30%) than the water content in the sheet after the press section (consistency up to 50% and more). This excess water is sent, in the form of clear filtrate, from the save-all unit backwards, as make-up water, to the stock preparation, thus following the counter current principle. This make-up water is often additionally treated in a DAF with coagulation and flocculation chemicals in order to remove detrimental substances before feeding it backwards. 

---

Attention has to be paid to the chemical and physical conditions of the white water circuit and to the measurement and control of the effectiveness of various chemical additives (Table 3.12). On-line measurements and integrated process control give the best and constant performance of multifunctional chemical combinations. Of similar importance are the methods and positions for metering chemical additives to the paper production system as well as the sequence of their addition (Fig. 3.28). 

---