Stock preparation
Integrated mill
Non-integrated mill
Refining
Broke system
Short circulation
Paper mill circulation water system

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Stock preparation

Stock preparation is the process located between the pulp mill and the paper mill.

Stock preparation consists:

 

Integrated mill

In cases where a pulp mill is integrated with a paper mill, pulp is pumped through a pipe from the pulp mill into the paper mill. This eliminates the need for a treatment and pulping system for pulp bales in the paper mill stock preparation department, but pipe stock is refined in accordance with the paper grade Also dried pulp can be used in addition or instead of pumped pulp.

 

Non-integrated mill

For a non-integrated mill, pulp is delivered in the form of pulp bales, which must be disintegrated (slushed) in a bale pulper. If needed, pulp is defibrated in a deflaker and finally, beaten in refiners.

 

Refining

Fibers that have been chemically and mechanically detached from wood have poor bond formation ability, since they are stiff with a smooth surface. They are in this form unsuitable ingredients for almost any kind of paper grade production.

In refining, the bonding ability of slurry fibers is improved by refining fibers mechanically between refiner bars . Fibers will get more flexible after the collapse of their structure and removal of the poorly soluble lignin-containing primary wall which made them stiff . Internal fiber bonds open and fibers start to swell, which increases their fiber bonding area. The target of refining is to improve the fiber bonding ability by refining slurry fibers mechanically .

Chemically separated fibers are more detached, more intact and contain less stiffening and poorly soluble lignin than mechanically separated fibers . This is why refining improves the bonding properties of chemically separated fibers more than those of mechanically separated fibers, which are for instance more easily shortened in refining. Refining normally refers especially to chemical pulp refining. Chemical pulp is refined prior to stock proportioning .

Refining is one of the most important papermaking sub-processes where the operator can contribute to fiber properties and thus to nearly all properties of the finished paper. Some of the paper properties are improved when increasing fiber refining (e.g. tensile strength), whereas other paper properties are impaired (e.g. opacity). Due to this type of critical property pairs, we must arrive at an advantageous compromise in refining, regarding the end product, its conversion and end user needs.

 

Broke system

Broke is paper that is discarded at the manufacturing or finishing processes inside of the paper mill. The purpose of the broke system is to form broke suitable for reuse in paper production. The goal is to prevent the reused broke from causing runnability problems with the paper machine or paper quality problems as well as to minimize the amount of fibers and filler discharged as reject from the broke system. As a rule, broke handling costs should be minimized by reusing broke as efficiently as possible.

Paper broke is formed in the following locations:

The mill broke system can be divided into two main parts:

The structure of the broke system depends on the paper machine and grade. A typical broke handling line consists of the following processes:

Mixed broke is dosed in correct proportion to other fiber components before reaching the mixing tank. An extremely simplified broke system might only consist of pulping and storage.

 

Short circulation

Short circulation is the part of the paper making process where water filtering through the wire is utilized for diluting the stock supplied to the head box. In the mill, the short circulation is located between stock proportioning and the head box of the paper machine .

Short circulation functions:

Air, dissolved gases, and  impurities are removed from the stock . Impurities heavier than fiber are separated in centrifugal cleaners. Screens are used to remove the impurities larger than the fibers (e.g., shives). Air is removed in the wire pit and, especially, in the deculator

Water rich in fiber and filler material (white water), which has been filtrated through the wire, is recirculated . The objective is to maintain the mass flow of the dry substance flowing through the head box approximately to correspond with the dry substance mass flow supplied to the paper machine from the pulping plant.

The particle size distribution on the web formed on the wire is maintained roughly the same as that in the pulp supplied from the pulping plant.

Pressure variations in the stock flow supplied to the head box are manifest in disturbances on the web formed on the wire . One function of the short circulation is to supply the diluted stock to the head box at maximally constant consistency and minimal pulsation.

A number of different additives and chemicals are added to the short circulation . A function of the short circulation is to secure correct proportioning and even mixing of the added materials into the stock under various operating conditions.

 

Paper mill circulation water system

The purpose of the paper mill circulation water system is to improve

The circulation water system operates on a counter-current principle - fresh, chemically-treated water is brought into the process via the hot water system, where it is preheated. Fresh water is used, for example, in the paper machine wire and press section showers as well as other processes requiring a high degree of purity.

In traditional processes, white water (filtrates and shower water) from the wire section is collected in the wire flume, in which it is returned to the wire pit. At the bottom of the wire pit stock to the paper machine is diluted with white water. So fiber and fillers in white water are recycled to the short circulation.

Wire pit overflow - the cleanest fraction from the wire pit - is recycled in the circulation water tank and returned to the circulation water system to be cleaned. Water in the circulation water tank is run through the fiber recovery filter, which separates solids from the water. The filtrates produced can be used in various applications at the mill.

The circulation water system also includes water towers, which serve as buffers during various running situations. In the event of a web break circulation water is required for break showers, when the consumption of water is greater than during normal running conditions.

Mill-wide circulation water system