Refined kraft paper production process and common problems

Refined kraft paper produced from sulphate wood pulp as a raw material, because of its lighter color, good smoothness and strength, stable product quality, as a very popular packaging material, has already replaced natural kraft paper in a wide range. The production of refined kraft paper uses larch sulfate larch pulp as raw material, which is bleached, sifted, beaten, slashed, slag-removed, and pre-grid system processed and machined. After calendering, improve the sheet gloss and reduce the paper industry two sides. With hypochlorite bleaching, the whiteness of the pulp reaches 35%-40%. The method has the advantages of simple production process, low production cost and easy operation, and is widely adopted by enterprises.

1 Production process

The production of refined kraft paper adopts Pusul-Sulphate Larch Wood Pulp and a small amount of hardwood pulp as raw materials. After cooking, the Kappa number of pulp is 45-55. The bleaching process is as follows: pulp concentration 3.5%, temperature 30-40 degrees Celsius, bleaching time 3-3.5 h, whiteness 35%-40%. When the whiteness reaches the requirements of the production process, it is sent to the storage tank after being washed by the vacuum washer. The bleached pulp was beaten with 450 double discs, with a beating concentration of 3.0%-3.2% and a freeness of 28-32 SR. Pulp slurry after sizing, into the cone de-sanding device, after rotor selection on the machine copy.

Sulfate pulp process as shown below:

Larch Wood Chips → Cooking Pot → Sprinkler Pot → Pressure Washer → Washed Bath → High Frequency Vibrating Box Sieve → Cx Sieve → Rotary Net Concentrator → Pulp Pond → Screw Conveyor → Bleaching Tower → Vacuum Washer → Drift After slurry pool → 450 grinding → 1# slurry pool → 2# slurry pool → diluted slurry tank → 600 cone descaler → rotor screen → headbox → paper machine → calendering → curling → rewinding → packaging

(Note: Drifting liquid is injected into the screw conveyor; the slag from the high-frequency vibrating box screen is sent back to the cooking pot; the rosin gum is added into the 1# pulp pool; the alum is added to the 2# pulp pool.)

2 Frequently Asked Questions and Handling

The production of refined kraft paper by the hypochlorite half-bleaching process has many advantages in production. However, the bleaching process of hypochlorite itself also has certain defects, such as greater degradation of cellulose and hemicellulose, and waste after bleaching. Liquor test is used for processing, etc., and the refined kraft paper used for packaging has both whiteness requirements and high mechanical strength requirements. Therefore, common problems that exist in actual production should be given sufficient attention to ensure product quality. Stable and reliable.

2.1 Strictly control the bleaching process to avoid reduced intensity due to excessive bleaching

The hardness of the slurry directly determines the amount of bleach solution. In the determination of the amount of liquid at the same time, because the hardness of each pot will always be different, and the difference in the temperature of the different seasons, especially in the north, the temperature difference between winter and summer can reach 15 degrees Celsius or more. Therefore, the bleaching time of each pulp material is also different, up to 0.5 h or more, so we must strictly control the bleaching time, otherwise due to the prolonged bleaching time, although it can improve the whiteness of the pulp to some extent, but it will reduce the paper Mechanical strength, and will increase the chance of paper yellowing, quality instability. Usually, by controlling the bleaching temperature between 30-40 degrees Celsius, a good bleaching effect can be achieved and a good and stable mechanical strength can be achieved.

2.2 Strictly control the degree of pulp washing to avoid the formation of calcium deposits in pipes and equipment

The production of refined kraft paper requires sizing, and the small amount of calcium hypochlorite remaining in the pulp is generally not affected by the sizing. However, in the actual production operation, in order to ensure the whiteness requirement of the semi-bleached pulp, some factories add a small amount of bleaching liquid to the pulp tank after bleaching to ensure the whiteness of the pulp, or bleaching due to insufficient bleaching slurry washing. The post-slurry still contains a large amount of calcium salt bases, undergoes a sizing section, forms calcium rosinate precipitates with free rosin acid anions, and forms CaSO4 precipitates with sulfate ions in alum. This kind of precipitate is combined with rubber and resin, and has a certain viscosity. It is very easy to scale and precipitate in the pipeline and equipment. When there are a lot of sediments, it can directly affect the paper sheet making, and at the same time, it reduces the sizing effect and affects the quality of the paper. This phenomenon becomes very evident in the hot weather in summer, and the high calcium content of water in some areas can also cause similar problems. The fouling deposits on pipelines and equipment will be layered concentrically and layer by layer. When pipelines or equipment are violently shaken, they will randomly fall off and broken into 1-2mm after being removed by the sand remover and rotor screen. The light yellow particles that can reach a maximum of 3mm (above) can enter the headbox through the rotor screen due to their low density. After these particles were surfed together with the slurry, the sheets were crushed by the calender and formed holes of 1-2 cm2, causing severe paper disease. Therefore, the degree of cleaning of semi-bleached pulp should be strictly controlled during production, and the operation of adding bleaching liquid to bleached pulp pool should be eliminated to ensure the normality of sizing and avoid the occurrence of paper diseases such as holes in paper sheets.

2.3 Enhanced purification screening system to remove particulate impurities

Most of the particulate impurities can be removed after passing through a 600 cone descaler and rotor screen. Once a large number of light-yellow particles of calcium deposits are present, the deposition of the system has reached its limit. This phenomenon will last for a period of time, ranging from 3-5 days, and up to over 1 month, which will seriously affect normal production.

After the above problems occur, the first-stage two-stage 606 conical slag removal system can be solved.

2.4 Correctly understand the difference between pulp whiteness and paper whiteness, and it is suitable to reduce the whiteness of pulp.

Strictly controlling the whiteness of the semi-bleached pulp of refined kraft paper, on the one hand, it is the requirement of paper color, on the other hand, it is the requirement of paper strength, the whiteness is improved, and the mechanical strength of the paper is reduced, so the production should be reasonably controlled Semi-bleached whiteness, for the purpose of achieving maximum paper strength while achieving whiteness. The same pot of pulp from the whiteness of the sheet, the whiteness of the pulp and the whiteness of the sheet is not consistent, the whiteness of the pulp is generally called paper brightness is lower than the 2% -3%, especially the pressure Refined kraft paper is more noticeable after light. The reason for this phenomenon is that the smoothness of the surface is improved after the paper has been calendered, and the same pulp, because of the rough surface, has a low reflectivity, and the whiteness of the formed paper is higher than the whiteness of the pulp.

Production experience shows that the whiteness of the pulp can be appropriately lower than the whiteness requirement of the formed paper by 1%-2%. While ensuring the whiteness, the length of the formed paper can be increased by 2%-3%. At the same time, it can also shorten the bleaching time and reduce the production cost.

2.5 Moderate calendering to increase paper sheet gloss and whiteness

In order to increase the gloss of the sheet, the groove press is changed to a glossy press, and finally can be calendered by a 4-6 calender to improve the gloss of the sheet and reduce the difference between the two sides of the sheet, and at the same time, the paper can be properly improved. BaiDu. During production, it was found that the gloss of the paper sheet processed by the calendering press was increased, the difference between the two surfaces was reduced, and the whiteness of the paper was increased by 1-2%. The effect was very significant. Even more than six rolls of calenders, even with an 8-roll supercalender, have little effect on sheet gloss, two-sidedness, and paper whiteness.