STOCK PREPARATION

Scope and importance of stock preparation It is that section on paper industry where pulp property are changed into the desired property and quality to produce the required type of paper. Paper is made from fiber after suitable treatment and addition of specific chemical to import desired properties. The change of properties of paper based on mixing and blending of various Chemicals and combination of fibers.

Introduction of stock preparation section of the paper mills and common terms used therein PULPING: Dry pulp is dispersed into water to form a slush or slurry. The operation can be either batch or continuous. CONSISTENCY REGULATOR: For beating and refining a particular consistency have to be maintained therefore it is present before the refiner/beater to maintain stock consistency. BEATING AND REFINING: The fibers are subjected to mechanical action to develop their optimal papermaking properties with respect to the product being made. The operation is usually continuous, but some pulps are still treated batch-wise.

METERING AND BLENDING: The various fibrous and non-fibrous components are continuously combined and blended to form the papermaking furnish. A wide variety of chemicals and mineral additives are used to impart specific properties to the paper product or to effect economies in the papermaking process. Preparation is usually carried out batch-wise.   MACHINE CHEST: It is also called as “surge” chest because machine chest contains enough furnish that the rest of the stock preparation system can be shut down for a period of time and the paper machine will continue to run on furnish stored in machine chest.

STUFFBOX: It is used to maintain constant volumetric flow rate to basis weight valve. The basis weight valve would be located immediately after the stuffbox and serves to control the amount of furnish pumped to paper machine   JORDAN: it is simply a conical refiner that is configured to cut the fibres to a certain degree the shortening of the fibers improve their distribution in the formed web and also tends to slow down the water drainage rate on the forming section of paper machine. Before dilution step the pulp slurry in stock preparation system are all above 3 to 4 % consistency. After the dilution step the furnish consistency is 0.5 to 1.0%.

Description of different types of chest and agitators used in stock preparation Receiving chest Refiner chest Mixing chest Machine Chest

TYPES OF CHEST

Receiving Chest- It is responsible for receiving pulp after bleaching section. It feed the pulp to refiner. Refiner chest- It store the pulp after refining stage having a specific consistency. Mixing chest- It is used for mixing and blending the chemicals like wet strength additive, dry strength additive etc. Machine Chest- it receives the final stock it is an agitated storage chest that hold stock prior before being sent to the paper making process. Advantage keep the fiber agitated Ensure constant volumetric flow rate

TYPES OF AGITATORS Thorough blending of the stock furnish before the paper machine is essential towards avoiding operating upsets producing a uniform products. The ideal arrangement of Blending required large chest (3-4 hrs.) retention at a consistency below 4% at sufficient agitation.

Cowan Stock Agitator: It take the material from the center of the tank and distributed it to the Periphery

Propeller Agitator: It is shaped with tapering towards the shaft to minimize the centrifugal force and to produce maximum axial flow

Paddle Agitator: In this type the blades can reach up to the tank wall it is used to where an uniform laminar flow of liquids is desired

Theory of beating, refining and its effect on fiber structure and strength properties of paper BEATING AND REFINING: The terms “beating” and “refining” are used interchangeably. More precisely, beating refers to the action of rotating bars opposite a stationary bedplate as the fibers flow past perpendicular to the bars. This operation is exemplified in the batch Hollander beater, the original refining machine, which is till used in some paper mills (Fig. 2.0); especially for handling such difficult furnishes as jute, hemp, flax, and cotton.

Refining refers to the action carried out in continuous conical or disk-type refiners where the fibers flow parallel to the bar crossings. In all cases, the objective is to “develop” or modify the pulp fibers in an optimal manner for the demands of the particular papermaking furnish. Two major types of continuous refiners are used for stock preparation- disc refiners and conical refiners. The conical refiners can be further differentiated into low –angle type (and) high angle type

rotating disc opposite stationary disc, two opposing rotating discs,   In conical refiners, the rotating plug (rotor) and its housing (stator) are fitted with metal bars oriented lengthwise. The fibers flow parallel to the bars. The position of the plug determines the clearance of the bars and controls the amount of work done on the fibers. Disc refiners are a more recent development, and are available in a wide variety of designs and disc patterns. The following are the principal designs: rotating disc opposite stationary disc, two opposing rotating discs, rotating double-sided disc between two stationary discs.   

Conical Refiner

Mechanism of Refining

The disc-type refiners appear to offer substantial advantages over the conical designs. The major advantages are:   Lower no-load energy requirement. This advantage is becoming relatively more important as energy costs continue to rise. Higher stock consistency can be used. Higher load can be applied (also greater rpm). Greater versatility in tackle design. Experimentation in patterns and materials of construction is ongoing. Self-correction of wear patterns. There is always a uniform mating of high and low spots, in contrast to the conical refiner (Fig.18). More compact design. Less space is required. Easier and lower maintenance (this claim is not well documented). Lower capital investment per ton of production.

Variables affecting Beating/Refining:   Wood species, Pulping method, Degree of pulping, Bleaching treatment, Prior processing (e.g. drying, mechanical treatment) Fiber length distribution, Fiber coarseness, Blending before refining or after refining Early wood/ late wood ratio and Chemical composition (lignin, cellulose & hemicellulose).  

Operation of Hydrapulper Hydrapulper is a device which is used to recycle the waste paper or secondary fiber pulping. Construction: It is a big hollow cylinder contain water with a powerful agitator at the bottom. It help to cut the waste paper in small pieces. Working: Hydrapulper is used to recycle the paper. when the waste paper is come in mill it is convey or passed through a conveyor and put into the tank and make a thick paste this machine is called Hydrapulper. It operate automatically when disintegrated process is done it discharge pulp into large storage tank. it is used mainly to waste paper and many types of seperator are used to remove staple, dust, pins, clips, plastic etc.

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