PLANT AND PROCESS FOR THE DRYING TREATMENT OF A VEGETAL MATERIAL IMPREGNATED WITH WATER

The present invention relates to a plant and a process for the drying treatment of a vegetal material impregnated with water, and particularly it relates to a plant and a process which can be advantageously used for drying exhausted wood residues originating from a previous tannin extraction process. The plant and the process according to the present invention can also be used for extracting tannin, with best yields, from virgin water-impregnated wood material. It is known that tannin is a dark-colored acid obtained from the bark and wood of plants such as oak, quebracho, mimosa, tara, chestnut-tree, fir-tree and acacia. It is principally used for tanning leather, but also in dyeing processes as a mordant, in the ink industry, in the paper industry as a dye or in medicine as an astringent, in addition to other new applications owing to tannin being a natural ecological extract. So far, the principal process used for extracting tannin comprises crushing wood in order to obtain the so-called wood splinters or fragmented wood, steeping the wood splinters in a water bath and subsequently extracting tannin from water.

The above described process brings about the accumulation of a huge quantity of biomass consisting in an exhausted wood residue, which requires a suitable disposal. Presently, said biomass is recycled through various processes, such as a process for the production of wood pellets which can be used as fuel material. To this aim said residue, which is impregnated with water when exiting the steeping bath, is subjected to a first drying step at high temperature in order to reduce the water content thereof to values between 8% and 14% as prescribed by the current relevant EC regulations. In the second step of the recycling process the wood residue is additionally broken up by means of a milling machine in order to make it homogeneous, whereas the third step consists in a compression in order to form the pellets.

However, the two above described known processes involve some remarkable drawback. In particular, the process for tannin extraction by steeping the wood splinters in water requires very large plants, since for example huge steeping tanks are necessary,

and besides it is not completely effective as the wood residue exiting the bath still has a tannin content of at least 2-3% which is not recovered.

Besides, in the recycling process of the wood residues, the processing of the dried residue by means of the milling machine proved to be problematic due to its particular nature. As a matter of fact, it is an extremely inhomogeneous material wherein also harder and bigger wood fragments, such as knots, or even small stones, chestnuts or other foreign objects, which can break the machine and consequently block the whole process, are often present.

Moreover, the recycling process has a very low output so that accumulations of biomass downstream of the extraction process are unavoidable.

In addition, said recycling process has very high running costs, which are bound to the high energy requirements of the plant and to the need of a frequent maintenance, which make it disadvantageous in view of the low commercial price of the pellets.

It is therefore an object of the present invention to provide a plant and a process for the drying treatment of a vegetal material impregnated with water, which is free from the above indicated disadvantages. Said object is achieved with a plant, whose main features are disclosed in the first claim and a process whose main features are disclosed in claim 9. Other features of the process and plant according to the present invention are disclosed in the remaining claims. A first advantage of the plant and process according to the present invention consists in that they allow obtaining a very high yield both in the drying of the treated vegetal material, and in the optional extraction of the natural substances which were originally present in said vegetal material.

In particular, the plant and process according to the present invention can be advantageously used for extracting tannin from chestnut wood, since they allow extraction in water of substantially 100% of the tannin present in the wood. In this case, the plant and process according to the present invention prevent accumulation of exhausted and water impregnated wood residue deriving from the extraction, since the vegetal material treated according to the invention is progressively dried during the extraction itself and at the end thereof the exhausted biomass is already sufficiently dry and ready for the subsequent pellets production.

Another advantage of the plant and process according to the present invention consists in that also the treatment of the wood residues originating from a previous traditional tannin extraction is allowed, hi this case, the process allows recovering the tannin which is present in the residues, and due to its high output capacity, rapidly disposing the wood residues accumulated downstream of the extracting process.

The process and plant according to the present invention can be used also for processing chestnut branches and leaves, which have the same tannin content as the wood. Besides, the process and plant according to the present invention can be advantageously used for extracting natural substances other than tanning and /or for extracting biomasses having a nature different from wood, even if these do not have the above described particular problems of the chestnut splinters. For example, the process and plant according to the invention can be used for drying the fibrous residues of sugar cane, maize plants (after panicle separation), sugar beets, the residues of the musts for vinification, the residues of the olives intended for oil extraction, or other biomasses from which it is possible to obtain, in a subsequent combustion, an economically significant heating value.

Another advantage of the plant according to the present invention consists in that it has very limited energetic requirements and needs very little maintenance.

Further advantages and features of the process and plant according to the present invention will become clear to those skilled in the art from the following detailed description of an embodiment thereof with reference to the attached drawings, wherein: figure 1 shows is a side schematic view of a plant according to an embodiment of the invention; figure 2 is a top schematic view of the plant of figure 1 ; - figure 3 is a cross-sectional schematic view of a pressing station of the plant of figure 1; and figure 4 is a cross-sectional side view of a grinding station of the plant of figure 1.

Referring to figures 1 and 2 there is shown that the plant according to the invention comprises an inlet station for the vegetal material formed of a hopper 1. In the present embodiment of the invention, a pair of reciprocally engaging toothed cylinders, which carry out a first grinding of the vegetal material impregnated

with water, is advantageously positioned at the outlet channel of the hopper 1. In this way, the first grinding station of the plant is incorporated in the inlet station for the material. However in other embodiments of the invention a first grinding station of the vegetal material can be positioned separately from the inlet station for the material. After said grinding step the vegetal material, for example chestnut wood splinters impregnated with water, is distributed on a conveyor belt 2 which delivers it to a first pressing station 3.

The pressing station comprises two rollers having different diameter, positioned one above the other with horizontal axis of rotation. As clearly shown from figure 3, the roller positioned above, or upper roller 3a, is smaller than that positioned below, or lower roller 3b, which is provided with groovings, not shown, suitable for allowing the aqueous solution pressed from said vegetal material to down flow.

Said groovings are circular and are positioned perpendicularly to the axis of rotation of the roller, that is on a parallel plane with respect to the direction of movement of the vegetal material. Preferably, in order to allow a suitable down flow of the aqueous solution, lower roller 3b is provided with at least four groovings.

The aqueous solution, containing the active substance pressed out of the vegetal material, flows inside the groovings of the lower roller and drips inside a collecting tank, not shown in the figures, which is positioned under the pressing station. In this way, the solution taken from said collecting tank can be used for extracting the natural substances deriving from the vegetal material.

The rollers of the pressing station of the plant are arranged at the lower possible distance suitably determined in order to allow the vegetal material to easily pass through them and to ensure at the same time a suitable compression and squeezing of the same material. Preferably, the pressure between the two rollers is included between 30 and 80 tons/cm ² .

Ih addition, the pressing station is preferably provided with scrapers, not shown, which are suitable to maintain clean the roller surface, and with guides, not shown, which canalize the material between the two rollers. The material discharged from the first pressing station is delivered, through a conveyor belt 4, to a further grinding station 5.

As shown in figure 1, belt 4 pours the vegetal material into a further hopper, having a lower size than that of the inlet station 1, inside which two toothed cylinders 6 are arranged, which are contra-rotating and engaging each other and can be better seen in figure 4. The cylinders are arranged at the outlet channel of the hopper and allow grinding the vegetal material into small pieces of a homogeneous size.

In order to avoid corrosion of the plant by tannin or other substances involved in the extraction, and also in order to bear the working pressures , it is necessary that the toothed cylinders and the rollers of the grinding and pressing stations are made in special hardened and cemented steel. The grinded material is discharged from the hopper on a further conveyor belt 7 which transports the vegetal material to the following stations of the plant.

Preferably, in order to allow a suitable drying of the processed vegetal material the plant according to the present invention comprises at least three grinding stations and, reciprocally alternated with these, at least three pressing stations of the vegetal material.

The plant according to the present embodiment of the invention further comprises a final drying station 8 of the vegetal material , which consists in a tunnel internally provided with a hot air fan. The hot air produced by the fan can be regulated by means of a thermostat and a hygrometer at the end of the tunnel regulates the relative moisture of the material depending on the need.

The progression of the material inside the tunnel is carried out by means of a feed screw arranged inside the tunnel with axis parallel to that of the tunnel. The feed screw raises the vegetal material and drops it after a rotation of 180°, thus facilitating its drying. Preferably, also the rotation velocity of the feed screw can be regulated. At the exit of the final drying station 8, the dried vegetal material is discharged from the plant by means of a conveyor 9 provided with an adjustable-height support.

The plant shown in the figures comprises horizontally arranged inlet, grinding and pressing stations, but alternative embodiments of the invention are possible wherein said stations have another spatial arrangement, for example they could be vertically arranged.

The above described plant can be used for carrying out the process for drying

treatment of a vegetal material impregnated with water according to the invention. Said process essentially comprises the grinding and pressing steps of said vegetal material, besides an optional final drying step in a hot air current.

Preferably, in the process according to the present invention the grinding and pressing steps are repeated in an alternate way for at least three times. As a matter of fact it has been observed that such a sequence of the grinding and pressing steps allows that not only the water impregnating the vegetal material but also the natural substances originally present in the vegetal material are completely recovered.

For instance, when the vegetal material subjected to the process according to the invention is chestnut wood which was not previously extracted, extraction of tannin can be carried out from the aqueous solution collected in the tank with yields of nearly 100%. On the contrary, when the vegetal material loaded into the plant is formed of a wood residue deriving from a previous tannin extraction in water, the aqueous solution recovered can be eliminated or extracted in order to recover the residual tannin. Also the yield of the process relevant to the drying of the vegetal material is very satisfying, as a matter of fact when the process is applied to chestnut splinters having a water content of between 60% and 62%, said content can be easily reduced to a value lower than 8% at the exit of said final drying step, as shown in the following example.

Example 1 Chestnut splinters having a moisture content varying from 58% to 62% were fed into a plant such as the one shown in figure 1, in quantities of 15 tons/hour. Hopper 1 distributed the material onto conveyor belt 2, with varying velocity by means of an electrical motor which serves as a measuring device. Said electrical motor had a power of 2 KW. The grinding stations of the plant were operated by electrical motors having a power of 5 KW, whereas the pressing stations were operated by electrical motors having a power of 10 KW. The pressure between the two rollers of the pressing stations was about 40 tons/cm ² .

The conveyor belts of the plant were activated by means of a motor of 1 KW. The material laying on belt 2 was transported to pressing station 3 and there poured onto lower roller 3b with a homogeneous thickness of about 20 mm. The

thickness of the material on the roller was leveled by a suitable rake. The liquid pressed out from the vegetal material flowed inside four groovings provided on the lower roller and was collected in the collecting tank below said roller in order to be conveyed, by means of a pump, in a reservoir. The vegetal material was subjected to grinding and pressing in all the stations of the plant. After the third pressing station the moisture content was measured, which was about 20%.

Said material was conveyed by means of the suitable conveyor belt to the final drying station. The tunnel was operated by an electrical motor of 10 KW which controlled the movement of the feed screw inside it. The hot air fan inside the tunnel had a power of 3 KW.

The material coming out from the tunnel had a moisture content of about 8%.