Thick stock

A low consistency suspension of fibres, pigments and chemical additives (thin stock) flows on to the moving wire mesh filtration medium of the paper machine where the wet web is formed. The water which drains away in this process (white water) is then recycled as far as it is practical to do so and is used to dilute the incoming high consistency suspension (thick stock). Retention can be considered either in overall terms or in terms of a single pass of the thin stock across the machine wire (first pass retention). For efficient operation, paper makers aim to achieve as high a first pass retention as possible, which they do by the use of retention aids. This reduces material losses and also assists in minimising the level of suspended solids in the effluent. 

Mineral fillers should be added at points which promote good mixing. A typical thick stock addition point is the suction side of the machine chest pump. Addition of filler in thick stock will give increased residence time and may improve ash retention nominally depending on the addition points and residence time of the retention chemistry. A typical thin stock addition point is the primary or secondary fan pump, which yields quick response to elevated levels of minerals, which is useful for mills that make several significant grade changes. 

Opinions vary as to the optimnm point to add a dry strength resin, and indeed this can differ from one paper machine to another. It will also to some extent depend upon the specific objectives for nse. The most commonly nsed point of addition is to the thick stock, after the last refining has been carried ont. Strazdins [13] reported a laboratory study on a European secondary fibre fnmish, where the benefits of resin addition to the long fibre fraction were demonstrated. Good mixing is essential to obtain the best performance from the product, so addition to a weU-stirred chest or the down leg of the thick stock level box is ideal. If the dilnte resin is added to a pipeline, optimum mixing will be achieved if a mnltipoint injection system is nsed. 

When two dry strength components are added to achieve the synergy mentioned above, both components must be added after refining, but apart from each other. Typical addition points might be before and after the machine chest feed pump. This adds the resins to the thick stock, but ensures good mixing between each component. The further apart the two components, typically the better the results. 

The quality of the water in the wet-end of the papermaking system can have a large bearing on the performance of the PAE resin. Whilst dilution of the resin from the storage tanks to the chosen point of addition requires fresh water, recycled water is used for preparation of the thick stock and subsequent dilutions to the thin stock prior to formation of the wet web. The properties of the recycled water can determine the effectiveness of the PAE resin. 

From DDJ results it was concluded that the retention of a cationic polymer in a paper web will be maximised by allowing sufficient time and agitation for the dissolved polymer molecules to contact and become adsorbed on the surfaces of fines and fibres. Such conditions favour the use of these polymers as dry strength additives. The reverse is true in regard to their use as retention and drainage aids, i.e. thick stock addition for strength, thin-stock for retention drainage [9]. 

When both anionic and cationic dyes are being used to produce a particular shade, these colours cannot be added at the same point or via the same dye addition due to co-precipitation of the dyes. In such circumstances the anionic dyes, which have lower substantivity, would be added first to the thick stock, whilst the cationic dyestuffs, due to their high substantivity, can be added later, even after dilution to the lower consistency required for sheet preparation. 

Thick stock ash control is another promising solution. An ash sensor either based on dilution of the blend chest stock or an inline sensor in various thick stock locations - coated broke, uncoated broke and blend chest pump enable control of thick stock filler addition or ratio control of incoming pulp streams. 

The key to microflocculation is that after addition of the high molecular weight, cationic polymer to the thick stock, the resultant floes are deliberately sheared down prior to being coagulated with a treated bentonite clay having an anionic character. The mechanism is illustrated in Figure 2.11. The key benefits of this system are ... 

Another commercially available test, the chlorophenol red (CPR) method, can measure residuals over the concentration range of 0.2 to 2.0 ppm, with a detection limit of 0.1 ppm. This method is used in water and waste treatment plants, but its accuracy in paper systems is unknown. Monochloramine and chlorate ions do not interfere with CPR, but chlorite ion concentrations above 1.5 ppm do interfere. In mills that use chlorine dioxide for freshwater treatment, 1.5 ppm levels of chlorite may not be reached, making it possible to measure free CIO2 residuals. High levels of chlorite can occur in whitewater if the chlorine dioxide is being applied to the thick stock, eliminating the use of this test for process water applications. Free chlorine will also interfere with the CPR method. 

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). 

Drainage is opposed by the resistance to filtration, which depends on the degree of beating, chemical treatment, and type of stock, as well as on the amount of fines and fillers present. The dry content after the wire section in most cases is about 18-20%. The water removed in the filtration process (white water) carries away fibers, fines and fillers. The percentage of solids of the suspension retained on the wire, also called retention, can be increased by the addition of retention aids. The white water is reused to dilute the thick stock in the stock approach flow system. Figure 6.42 shows the filler distribution in the z-direction (across the web thickness) for dewatering the stock to only one side and symmetrically to both sides. 

Uses Pitch dispersant, pitch control agent for use in thick stock to limit pulp defects and stock system deposition, for kraft and newsprint furnish systems Properties Cl, pumpable liq, si, odor sol. in water sp.gr. 1.18 pH 7 (1%)... 

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