Thin 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. 

Other considerations include material support, especially when turning long thin stocks due to the flexibility of PTFE. Another issue is the characteristics of machined resin, which tends to be continuous and... 

Thin stock free drainage measurement, with and without addition of retention/drainage aid, can be carried out readily using a Schopper Riegler Beating and Freeness Tester or Canadian Standard Freeness Tester (see Fig. 3.5). 

Water quality - 99% or so of the thin stock is water. Water quality in terms of pH, temperature, solids, colloidal and dissolved materials has a significant impact on the wet-end chemicals and the performance of all additives. Changes in freshwater source and quality need to be monitored carefully as well as any change in system water closure. 

Simple stab-ins or quills have been the standard addition points for many years, but a new generation of addition points is emerging. Metso s Retamix system delivers the product into the stock flow at multiple points, while the Wetend Technologies Limited Trumpjet systems allow additive dilution water to be replaced by clarified water or thin stock. 

A further cause of deposits is a lack of alum. If sufficient is not added to the systan to precipitate the rosin, it will deposit in the whitewater and thin stock systan. 

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. 

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]. 

The development of micro-particle retention systems radically changes the chemistry. In these systems an overdose of cationic material is added to the thin stock, such that the system becomes cationically dispersed. An anionic micro-particle is later added to flocculate the stock. Under these circumstances a complete starch cook out may not be necessary - especially if the system is being over-charged with a polymer rather than starch. Cross-linked cationic starches that do not fully disperse may be used in such systems. The partially dispersed starch acts partly as a colloid and partly as a particle, having a larger active radius than a fully dispersed starch molecule. Benefits in starch retention are claimed. 

Stable ash content in the sheet usually is controlled with the dry ash scanner value dosing filler in the thin stock. Often this control strategy is limited when coated broke is used in varying amounts. Another source of disturbance can be the retention aid control loop. A retention aid controller reacts with increased polymer dosage by, for example, variation in anionic trash loads. This will quickly impact the ash control loop and a stable sheet ash cannot be maintained. 

Inclining the hearth increases the safe length. This is the principal reason why furnaces for heating thin stock have inclined hearths. Hearth inclination reduces pusher force in accordance with the equation... 

Flat and U-bend coupons may be supplied with an autogenous or butt weld [/3]. With a butt weld, the weld profile is retained and not machined ofi, to duplicate the condition of process equipment, except on U-bend coupons. Autogenous welding (application of a locally melted region on base metal that contains no filler metal) does not always simulate real-world conditions. If welding of the coupon with filler metal is not possible, for example with thin stock, a sensitizing heat treatment may be preferred. 

Atomic hydrogen welding, another important application, is particularly suitable for thin stock and can be used with practically all nonferrous metals and alloys, as well as with ferrous alloys. 

Use Level % - % gal/1000 gal of water in thin stock 3-4% (turning, milling, drilling) ... 

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