The invention relates to a method for drying liquid-bearing material according to the preamble of claim 1. Furthermore, the invention relates to an apparatus for drying liquid-bearing material according to the preamble of apparatus claim 1.

It is previously known to dry matters or materials in many different ways, starting from very traditional open-air drying. Typically, manufacturing utilises drying plants where moisture is evaporated from the matter or material by heat and airflow and moist air is removed from the drying space. These drying plants are characterised by high energy consumption. Also, the quality of the drying result generally requires improving. When pursuing a good drying result, drying must be done quite gently, which simultaneously implies long drying time.

In addition to a bottleneck, drying has been discovered a considerable cost factor for manufacturing. Hence, many different drying methods have been contemplated, based on various physical phenomena. Underpressure, overpressure and their alteration in the drying space have been utilised. Electricity, microwaves, ultrasound, and various types of radiation have also been suggested and even tested for drying matters and materials. There have been more than enough tests, but only a few of the methods have made it to industrial application.

Loose matter or loose material refers here to all kinds of loose matters and/or loose materials, which bear moisture or liquid. Loose matter or loose material refers to relatively small-sized, possibly fine-grained matter or material which can be compressed e.g. by an extruder. Such matters or materials are e.g. tree bark, wood chips, forest-processed chips, sawdust, hay, predried sludge, peat, corn, forage, building materials, many municipal and industrial wastes, and mixtures of the above etc., even foodstuffs. The effective drying of such loose matter or loose material is often awkward. An example of this is the drying of bark material. The wood industry uses tree bark as a fuel, which is even in that usage somewhat problematic. A considerable part of the problem is due to the high moisture of bark. It is known that, by removing liquid with sole mechanical compression, the residual moisture of bark is about 40%.

The object of the invention is to introduce a method and apparatus by means of which the drying of loose materials occurs more quickly and more cost-effectively than previously. The object of the invention is also to provide a method and apparatus, which is well integratable in various manufacturing lines as well as in industrial and municipal waste management.

The object of the invention will be achieved with a method and apparatus the characteristic features of which are described in more detail in the enclosed claims.

Many liquid-bearing materials are conductive i.e. liquid in the material can conduct electricity, and this property can be utilised in drying in accordance with Ohm's law. The invented method is based on using liquid included by loose matters or loose materials to conduct an electric current through the loose matters or loose materials after the loose material has been advantageously compacted to ensure its adequate conductivity. In the method according to the invention, liquid- bearing loose matter or loose material is compacted and advantageously compressed compact enough in order for the electric current to be able to transfer within it and advantageously through it. The electric current is connected on different sides of the compacted loose matter or loose material, whereby through- flowing electricity heats the liquid in the loose matter or loose material and changes the liquid into vapour the pressure of which pushes liquid out of the matter or material. Finally, vapour only comes out and it will stop when the electric resistance of the material increases as the material is drying. It should be noticed that quite a part of liquid exits the material capillarily as liquid, whereby a considerable amount of energy is saved compared to total vaporisation required by many previous methods.

Electrodes are advantageously perforated in order to enhance the exit of vapour and liquid. To assure the flow of electricity, the contact surface of the electrodes against the material can be provided with a spongy surface and a moisturising apparatus of electrodes. If required, it is also possible to use liquid more electrically conductive, such as e.g. saline water, for moisturising.

If wishing to increase the exit of liquid from the material at a specific end of the drying pipe, the electrode can be divided into several successive part-electrodes the power supply of which is arranged as successive impulses as a 'continuous wave' or the like.

If there is extra or inexpensive heat energy available, the material being dried can be pre-heated to some extent, which reduces the electricity costs of the actual drying apparatus. The efficiency of the apparatus is expected to be excellent, because no unnecessary heating of spaces takes place and the drying is only exerted on the material being dried. Besides, no heat is transferred in pipelines in the factory area.

It has become apparent that the method is well applicable to the drying of diverse materials as long as the material is in a form compact enough for water or other liquid included by it be able to conduct electricity.

Next, the invention will be described with reference to the accompanying drawing, which schematically shows the principle of an apparatus for applying the invention.

The apparatus includes electrodes, which are generally designated with number 1, and which electrode 1 is divided into part-electrodes Ia, Ib and Ic which are engineered to be set in contact with loose material being dried and in which an electric current is led successively in order as a continuous wave, a power source 2, loose matter or material being dried 3, a liquid-collecting trough 4, a drying pipe 5 manufactured of non-conductive material, a feed pipe 6, and a feed screw 7. The electrodes 1 are permeable to vapour and/or liquid.

The apparatus according to Fig. 1 can be used e.g. for drying tree bark material. The drying takes place in the drying pipe 5 into which the tree bark material i.e. loose material is fed by means of the feed pipe 6 and the feed screw 7. At the same time, the feed screw operates as the compacting device and compactor of the bark material when compressing it in the drying pipe into desired compactness. The structure of the feed pipe 6 and the feed screw 7 can be a structure conventional as such and, by means of their geometry, compression can be controlled in a way known per se. Due to structural plainness and electrical safety issues, it has been seen warranted to keep the feed screw 7 and the feed pipe 6 separate from actual high-voltage parts, such as shown in Fig. 1. The electrodes 1 are electrically insulated from the feed pipe 6 and the feed screw 7 by means of the drying pipe 5, which is of insulating material.

The structure of the drying pipe 5 is permeable to liquid and vapour, advantageously porous and/or perforated. As it is further non-conducting and resists wear and quite high temperatures, it is fairly demanding as a component. Fig. 1 shows only three electrodes, Ia, Ib and Ic. In practice, their number can be larger. At the beginning, in the vicinity of the screw 7, the bark is wetter, whereby the voltage between the electrodes Ia and Ib should be relatively low in order to avoid breakdowns. Between the electrodes Ib and Ic, it is possible to use higher voltage. At least two voltage levels are used in drying. If there are more electrodes, the voltage can still be raised to compensate the increase of resistance caused by the drying of bark. Using a continuous wave is also a possibility in power supply.

Moisture in the loose material is a prerequisite for the operation of the method and apparatus, because it is the liquid, which constitutes a route for electricity through the loose matter or material. The drying result is controlled by means of the voltage of the power source 2 and the power usage of drying is self-controlled and decreases as the material is drying. When compressed tree bark material dries, its electric resistance increases and voltage thus automatically decreases. Liquid in the bark material partially exits as liquid which falls in the liquid-collecting trough 4 below the drying pipe.

In principle, the drying event is very similar with all loose materials: compressing the loose material in such compactness that an electric current is able to travel through the material without sparking, using electricity and removing water partially as water into the liquid-collecting trough, partially as vapour and, after the drying, transferring the matter or material out of the apparatus pushed by the screw as notably dry.

The drying event is continuously-operating, as that is typical for a screw-feeding apparatus.

As already described above, the invention can be implemented in many different ways. It is still possible to consider using the invention together with other drying methods known previously. Thus, the invention is not limited to the above- described examples, but it can vary within the scope defined by the claims.