The present invention relates to a method for removing liquid from bark, for example in order to increase the dry solids content of wet bark or other bark generated in wood debarking, and a drum-type press.

A drum-type press mainly comprises a rotary external screen drum and a press roll mounted rotatably in the interior thereof. Further the press has e.g. a frame/support structures supported by which the screen drum rotates, a drive mechanism or drive mechanisms and feed devices for introducing the material to be pressed into the press. The material from which the liquid is required to be removed is fed into the screen drum, wherein it is passed into a throat formed by the screen drum and the press roll and is pressed between the screen drum and the press roll, whereby liquid is removed from the material. The dewatered material is discharged from the press after the pressing, e.g. by means of a screw type conveyor or by plowing.

Drum-type presses are used in various object of application, among other in the chemical pulping or saw industry for dewatering bark. The purpose of pressing bark is to improve the heat value of the bark or to facilitate further drying for some other process by removing as much water from the bark as possible. The aim in bark pressing is the highest possible dry-solids content value. The bark generated in wood debarking is used almost solely as fuel and every additional percentage unit in the dry-solids content means, even at a medium-size power plant, savings amounting to hundreds of thousands of euros at an annual level compared to producing the amount of energy corresponding to increase in the dry-solids content by means of some other fuel, for example. As the capacity of new production plants increases, the significance of the final dry-solids content of pressed material also increases. The dry-solids content of bark after pressing is nowadays commonly just slightly over 40%, and even this value is not always reached. Thus, there is a need to reach a higher dry solids content than reached by the present press solutions. Additionally, the capacity of drum-type presses of prior art remains e.g. in the chemical pulping industry low compared to the debarking capacity of new and bigger production plants. As an example of this kind of prior art solution, the solution presented in publication Fl 70542 can be mentioned. In the solution according to the publication is a dewatering drum and inside it eccentrically a press roll. Material is pressed between the dewatering drum and the press roll, whereby water is removed through the dewatering drum and the pressed bark is removed from the apparatus by means of an exit conveyor. In the solution according to the publication, like in other prior art, the problem is low capacity and low dry solids content of the pressed bark.

Thus, the common problem with the prior art solutions is low capacity, low final dry solids content and fluctuation of the final dry solids content. Additionally, in the present solutions a large amount of solid matter is passed into the pressed liquid.

The purpose and solution of the invention

A novel solution has been developed for removing liquid from a material. The purpose of the invention is to provide a drum-type press that is remarkably more productive than before and to provide the most wide and versatile solution possible for solving the problems present in the prior art solutions.

In the solution according to the invention, a drum-type press has inside its screen drum two press rolls, which together with the screen drum form a first and a second pressing stage. The pressing is performed in two stages so that the first pressing stage is carried out between the screen drum and the first press roll and the next stage between the screen drum and the second press roll.

In the first pressing stage, an even solid material mat is formed from the bark being pressed, which material mat is transferred on the inner surface of the screen drum as such to the second pressing stage. The second pressing stage can utilize ready-formed material mat, from where a portion of the liquid has been removed. A ready-formed material mat at a higher dry solids content resists a higher pressing pressure. Additionally, a ready-formed material mat acts as a filtering mat, which decreases the passing of solid matter into the exiting liquid. Further, a ready-formed bark mat decreases material packing into the dewatering holes.

In a preferred embodiment of the drum-type press according to the invention, the screen press includes flanges that rotate along with it. The flanges are arranged substantially at the ends of the screen drum and they extend in the radial direction of the screen drum at least towards the interior. The press rolls can rotate between the flanges and a suitably small gap remains between the flanges and the press rolls. The height of the flanges is arranged such that the pressing takes place outside the inner circumference of these flanges, whereby material being pressed cannot pass in the axial direction out of the screen drum end and no separate end plates are needed for preventing the exit. According to a preferred embodiment of the invention, the press additionally has guide plates, which are used to guide the bark being pressed between the screen drum flanges. The guide plates extend between the screen drum flanges, overlapping with them in the radial direction of the screen drum.

For intensifying liquid removal and for increasing the capacity of the apparatus, in a preferred embodiment of the invention liquid is also removed from the press rolls, in addition to the screen drum. When liquid is removed from the bark through both the screen drum casing and the perforated casing of the press roll, the pressing is intensified, since liquid is removed in the thickness direction of the material bed in both directions. Liquid is removed, in addition to the screen drum, from all press rolls, whereby the pressing result is further improved. The press rolls, from which liquid is removed, are at their ends provided with openings, through which the liquid is led from the interior of the press roll to the outside of it.

The pressing stages and their nips can be adjusted separately. A nip refers to a zone between the screen drum and the press roll, wherein the radial distance between their casings is the smallest. Especially in the first pressing stage the thickness of the nip varies as the screen drum and the press roll rotate, e.g. if the moisture content of the material being pressed changes. The nip can be controlled, if desired, by adjusting the pressing force. If, for instance, the thickness of the nip decreases, the pressing force can be decreased for increasing the thickness of the nip. For the adjustment, the press rolls are provided with a sensor system measuring the position and force. By means of the sensor system, data is obtained from each pressing stage, such as e.g. the thickness and pressing force of the nip. The obtained data is utilized in controlling the pressing stage in question and the subsequent pressing stage.

Advantages of the present invention include e.g.:

- higher capacity with only one apparatus,

- higher dry solids content of the pressed material, whereby the heat value of the material increases and in possible subsequent process stages advantages are obtained, such as e.g. significant energy saving in further drying, when there is remarkably less liquid remaining in the bark, which is to be removed e.g. by evaporating. By means of the method and apparatus according to the invention, a dry solids content of up to 50% can be obtained,

- better usability and more even final dry solids content, as the apparatus is less sensitive to the moisture content and fluctuation of the material being fed into the press,

- smaller amount of solid matter in the removed liquid. In the following, the invention is disclosed in more detail with reference to the appended drawings, of which

Fig. 1 illustrates a drum press roll according to a preferred embodiment of the invention seen from the direction of the end, and

Fig. 2 illustrates a detail of a solution according to a preferred embodiment of the invention seen from the lateral direction of the press. Fig. 1 illustrates, according to a preferred embodiment of the invention, a drum- type press 10, which comprises a feed screw 1 1 , substantially a horizontal screen drum 20, flanges 40 at the ends of the screen drum, substantially a substantially horizontal first press roll 30, substantially a horizontal second press roll 31 , members 38 for leading the once-pressed bark to a subsequent pressing stage, and devices 12 for discharging the pressed bark from the press. The bark being pressed is fed into press 10 with suitable members, such as the feed screw 1 1 shown in the figure. The flanges 40 are arranged substantially at the ends of the screen drum 20, extending radially at least towards the interior, and press rolls 30 and 31 are arranged on the inside of these. The pressing stage occurs within the area between the flanges 40 of the screen drum 20. The first press roll 30 and screen drum 20 comprise the first nip between them, and the second press roll 31 and screen drum 20 comprise the second nip. The screen drum 20 and first press roll 30 are positioned so that the bark being pressed is guided by gravity into the space between screen drum 20 and first press roll 30 before the first nip, where the bark is pressed in the first pressing stage. The axles of screen drum 20 and press rolls 30 and 31 are substantially horizontal. Press rolls 30 and 31 are positioned so that the liquid being removed from the material can be led by gravity out of the press, without allowing the removed liquid to flow back into the already pressed material or unpressed material. Consequently, no separate vacuum source is needed to discharge from the press the liquid removed from the bark. In order to ensure that no liquid flows back into the already pressed bark or unpressed bark, both press rolls 30 and 31 are positioned so that both nips, the first and the second, are positioned underneath the rotating shaft of the screen drum 20. Furthermore, press rolls 30 and 31 are positioned inside the screen drum 20 so that the press roll rotating shafts are on both sides of the vertical axis of the screen drum as seen from the end of the screen drum 20. In a preferred embodiment of the invention, the press rolls are positioned so that the first nip is lower than the second nip. In a preferred embodiment of the invention, the material mat pressed once in the first pressing stage is led along the inner surface of the screen drum 20 to the next pressing stage in the zone between the screen drum 20 and press roll 31 . The members 38 can be used to lead the material mat, as shown in the figure.

In a preferred embodiment of the invention, the drum-type press 10 comprises guide plates 45, which are used to lead the bark being pressed between the screen drum 20 flanges 40. The guide plates 45 extend between the screen drum 20 flanges 40, overlapping with them in the axial direction of the screen drum.

Liquid is removed from the screen drum 20 as well as from both press rolls 30 and 31 in order to intensify pressing. The press rolls are at their casings provided with holes and with openings in the ends, through which the liquid is led out. Fig. 2 shows a detail of the screen drum 20 and press rolls 30, 31 , according to the invention. The figure shows how a pressing space 4, where the material is pressed, is defined against the screen drum 20, the press rolls 30, 31 and the flanges 40 of the screen drum, and the pressing of the material takes place in this space and material being pressed cannot pass in the axial direction out of the end of the screen drum 20.

Although the above description relates to embodiments of the invention that in the light of present knowledge are considered the most preferable, it is obvious to a person skilled in the art that the invention can be modified in many different ways within the broadest possible scope defined by the appended claims alone.