Bamboo Raw Materials for Pulp and Paper Making

China is rich in bamboo resources and known as the “kingdom of bamboo”. Bamboo is a fast-growing plant with the advantages of fast growth, easy reproduction and high yield. Therefore, it can be used continuously. Nowadays the paper-making wood fiber raw materials will still be lacking for a long time in China, and non-wood fiber raw materials are still an important replacement for pulp and paper making.

Bamboo fiber belongs to medium and long fiber, and its length is between softwood pulp and hardwood pulp. It has good characteristics of pulping and paper making, so it is one of the better raw materials for non-wood papermaking.

1. Fiber properties and chemical composition

Bamboo cells mainly include fibroblasts, parenchyma cells, stone cells, ducts and epidermal cells. Bamboo fiber cells account for about 60%-70% of the total cell area, which is lower than that of softwood and higher than that of common grasses. The bamboo fiber is slender and has a spindle shape with sharp ends. The average length is generally between 1.5 and 2.1 mm, and the average width is about15μm, its aspect ratio is 110-200. This is better than that of hardwood. The wall thickness of the bamboo fiber is about 5μm, the inner and outer walls of the fiber are relatively smooth, the cell wall is very thick, and the cell cavity is small. The comprehensive morphological characteristics of bamboo fiber are slender fiber, small wall thickness, large specific gravity, relatively stiff fiber and high transparency.

2. Storage and preparation

Due to the seasonality of bamboo cutting, it is necessary to store a certain amount of bamboo raw material for a long period of continuous production of the factory. Bamboo raw material can be usually used for 3 to 6 months. As we know, the raw materials need to be kept for a while for air drying and de-greening. The bamboo bundles can be stacked by a mobile belt conveyor. After they enter factory yard, the bamboo raw material will be sent to the cutting machine and be cut into bamboo pieces. Their length is 10 to 25mm. The cutting machine commonly used includes the knife-roller cutting machine, the cutter-disc cutting machine and the drum chipper. Generally, the quality of the drum chipper is better than that of the knife-roller cutting machine and the cutter-disc cutting machine.

The position of the open storage yard of bamboo chips should be arranged according to the principle of “stacking after screening” in the whole production process, which can make the sawdust in bamboo chips removed first by screening, avoiding the fermentation and mildew phenomenon of bamboo chips during the storage process.

3. Cooking

Bamboo chips are generally cooked by the sulfate method. This method is characterized by high cooking yield, high pulp strength, less environmental pollution, good fiber softness and the mature process of alkali recovery. The sulfate method of bamboo is easier than that of wood (Especially softwood) because bamboo contains less lignin. It is easier to remove lignin.

Most of the equipment used for cooking are steaming balls in small paper mills. For the medium paper mills, they often use vertical digester or horizontal tube continuous digester. Modern large-scale bamboo pulp mill often uses Kamyr continuous cooking digester (ITC or CC) with diffusion washing or use Super-Batch or RDH. The equipment has the features of high production efficiency, stable product quality, high yield and good dewatering performance. The operating conditions are improved, the automation gets higher.

Cold spray cannot only recycle waste heat but also prevent waste gas from being completely discharged during the cooking process. This can eliminate the pollution of waste gas generated by the discharge process and prevent the steam from being carried away with the pulp and reduce the steam consumption. Moreover, the pulp does not arouse high temperature cracking reaction and the fiber damage is less. Thus it can help increase the fiber strength. Therefore, the continuous digester with cold spray plays a role in improving pulp quality, reducing the steam consumption and eliminating the pollution of waste gas to the environment.

4. Washing and screening

The production practice proves that bamboo pulp fiber is relatively long, the water drainage is good and it is easier to be washed than other grass pulp, even easier than the hardwood pulp. The equipment used for bamboo pulp washing generally include vacuum washer, pulp press washer and horizontal belt washer. The large bamboo pulp production line can adopt twin roll displacement press washer or a DD washer.

Screening and purification mainly refer to remove coarse slag, dust and sand. The amount of coarse slag in bamboo pulping is relatively more. Coarse slag is mainly uncooked knots with bigger miscellaneous cells that cannot be used for cooking. Otherwise, paper quality will be affected.

5. Bleaching

5.1 Process characteristics

Unlike wood fiber, bamboo fiber has few and small pores. In the bleaching process, the bleaching liquor must penetrate from the tapered end of the fiber, while the unbleached bamboo pulp has a large amount of hemicellulose that expands during bleaching, which makes bleaching liquor get difficult to penetrate. About 25% of condensed lignin is not easily dissolved in the bleached lignin. Therefore, it is difficult for bamboo pulp to get bleached to 90% (ISO) whiteness without damaging fiber strength.

Modern bamboo pulp production lines should use ECF or TCF bleaching process. The TCF bleaching process is higher in cost, so the ECF bleaching process is used more. The ECF bleaching can adopt ODEOPD process or ODO (E/O) D1D2 procedure. Waste liquor from Oxygen delignification (O-stage) can be directly counter-currently recycled to the washing section and into the alkali recovery system, After two stages of oxygen delignification, The lignin rate can be up to 60%. It is with the advantages of less environmental pollution, higher pulp yield and viscosity, stable whiteness, good dewatering performance and low bleaching cost.

6. Paper making

Bamboo pulp can replace some long-fiber pulps to be used for various medium or high-grade paper making. In order to meet different requirements of paper varieties and more economics, wood pulp, bamboo pulp and other grass pulp can be generally mixed and homogenized in different proportions for paper making.

Because the fiber wall of bamboo pulp is thick and hard and easy to be cut horizontally, and bamboo pulp has high hemicellulose content, it is easily hydrated when beating, and beating degree is increased easily. If the free beating is mainly used for cutting, the paper wet strength and forming is not ideal. Therefore, bamboo pulp fiber should be kept as long as possible during the beating.