Sugar cane is one of main raw materials for sugar production. About 50% of the fiber can be used for papermaking after the sugar cane residue left after the sugar is squeezed. However, some of the cane pith have no interweaving force and should be removed before the pulping process. Bagasse fibers have a length of about 0.65-2.17mm and a width of 21-28μm. Although its fiber form is not comparable to that of wood and bamboo, it is slightly better than that of rice and wheat straw. The pulp can be blended into a portion of wood pulp, and then produce printing paper, cement bag paper, and the like.
Many countries have been using bagasse, which is waste material left after the crushing operations in the sugar mills producing sugar from sugar cane. Though bagasse is a kind of potential raw material for the paper industry, it has one serious drawback of pith associated with fibrous portion.
What is pith?
Pith is composed mainly of parenchyma cells. Pith can’t be converted into a satisfactory pulp despite its resemblance to fiber in chemical composition because of its small size non-fibrous physical nature and close association with dust. The presence of pith in bagasse has adverse effect in respect of lower pulp yield, high chemical consumption and related problems.
What is depithing process?
Depithing process have been mainly on methods which have used the dry/moist depithing or a combination of moist and wet depithing. The bagasse is mechanically abraded to break the clusters of pith away from fibrous portion.
Dry depithing process
In the dry depithing process, it is carried out on stored bagasse having a moisture content of less than 35%. Hammers and shredder are used in the separation of pith from bagasse . This method has several disadvantages like heavy wear and tear of the machine, loss of valuable fiber along with the pith. The efficiency of the process is quite low and not more than 40%. The depithed bagasse obtained by this process still contains 20% residual pith.
Moist depithing process
Moist depithing process is generally done when wet bagasse has about 50% moisture. These depithers are designed to break and open fiber bundles and to dislodge the pith by mechanical rubbing and mild disintegrating action. This method is more applicable at pulp mill for final cleaning and depithing just before bagasse enters the digester. The bagasse is fed to the hydro-pulper where it is thoroughly wetted and broken up at a consistency of around 2-2.5% which maintained by continuous recirculation of process water. Residual pith content is not less than 14-15% and depithing efficiency is around 70%.
Depither is a depithing machine. It can be divided into horizontal depithing machine and vertical depithing machine.
Horizontal depithing machine
Horizontal depithing machine has lower rotation speed, the bagasse transportation and the separation of cane pith rely on mechanical stirring force, and the bagasse stays in the machine for a longer time, which increases power consumption and brings mechanical damage to bagasse fiber. The effect is to increase the amount of cane pulp and reduce the efficiency of removing the pith.
Vertical depithing machine
Vertical depithing machine is to use rotary blade (hammer) that rotates at a high speed on a vertical shaft drum to generate centrifugal force of the feeding bagasse and support fibers by centrifugal effect, so that fibers rotate spirally on the surface of the screen. It utilizes the blast air of the drum generated by centrifugal force and the blade to separate cane pith. This process causes fiber bundles to move vertically on the screen, creating rubbing effect and thus reducing the mechanical damage of the fibers. At the same time, the bagasse is transported in the machine by its own gravity, thus reducing the power consumption of the conveying process.
Vertical depithing machine adopts semi-wet removal of the pith(usually 48-52% moisture) and wet removal of the pith (rely on secondary removal of residual sugar, more than 100% moisture), to be used for new preparation technology for fiber washing, wet conveying and storage.
Vertical depithing machine consists of the following six parts:
(1) Transmission part: it includes electric motor and transmission device. There are three types of general transmission structures: triangular tape, bevel gears and direct flexible coupling drives. They can be selected according to model size and design parameters.
(2) Bearing seat: a component used to support rotors and blades (hammers).
(3) Rotary drum: it includes turntable and blades (hammers), it is main part of the work. The rotors have two structures, namely blade type and drum type. The blades (hammers) are mounted on the rotors in movable type and fixed type. The movable blades can swing freely on the tips of the rotors. When they encounter metal or hard objects, they can play safety protection function. During working, the baggage is treated by the centrifugal force of the blades. However, due to easy wear between the holes and the tips of the blades, which affects the service life and the fitting clearance, a fixed structure is currently used.
(4) Screen and casing: they are used for separating fibers from cane pith. The cane pith is discharged through the screen, and the long fibers are left in the screen and discharged from lower portion. The hole size of the screen can be designed according to the requirements of bagasse shape and the removal rate of pith.
(5) Inlet and discharge trough: they are used for feeding and discharging of bagasse. The discharge hopper discharges fibers and pith inside and outside the screen separately.
(6) Bearing lubrication system and random accessories: they include bearing lubrication oil supply pump, oil pipe valve, oil cooler and screen and other accessories. In addition, in order to ensure the efficiency of the depithing machine, a baggage meter is required on the feeding trough to adjust the feed amount uniformly.
As a leading paper making equipment manufacturer, AGICO can provide you with high quality depithing machine and relevant information about depither in details.