It is well known that in the paper industry some kinds, of paper, particularly those of lower grade, are made using paper pulps obtained recycling waste paper.

The preparation of such paper stock is carried out in proper plants substantially comprising kneaders which blend waste paper with water and make a fibrous stock which is the filtered to remove the contraries.

As a matter of fact the stock coming out from the kneaders contains foreign matter of various kind and sizes that must be removed before the stock is used in the subsequent paper manufacturing phases.

A known embodiment of said plants has a plurality of kneaders with cylindrical tank provided with a helical impeller, each kneader being provided downstream with a rotary basket filter to remove the contraries from the fibrous stock.

The plants of such kind have the limitation that the operation of each couple consisting of kneader and filter is discontinuous because after a predetermined operation time, both the kneader and the filter must be stopped to allow discharge of the separated contraries.

In : an'attempt to overcome such limitation, equipments are made having a single kneader essentially comprising a cylindrical container with horizontal axis inside provided with blades of various shapes carrying out kneading of waste paper when the cylinder is caused to rotate.

In the end part the kneader has a pre-filtering zone generally consisting of a foraminous surface retaining the bigger contraries that are thus separated before the fibrous stock comes out from the kneader.

The stock is then filtered again through a plurality of filters cascade arranged downstream the kneader, comprising a cyclone filter receiving the fibrous stock coming out from the kneader, followed by a plurality of rotary basket filters cascade arranged, each of them separating the polluting, substances from the paper stock received from the preceding filter.

The number of filters arranged downstream the kneader is varying according to the kind of stock and contraries as well as the degree of filtration to be obtained.

The just described plant has the advantage to allow a continuous treatment of

the paper stock but has still some drawbacks.

A first drawback consists in that the pre-filtration stage carried out in the kneader is too little selective and this involves an excessive working load on the filters arranged downstream that being too loaded do not succeed to carry out an adequate separation of the residual contraries.

Moreover as use of rotary basket filters notoriously causes fragmentation of the polluting elements, mainly the brittle ones such as polystyrene pieces and the like, if the quantity of contraries in the stock is rather high, the quantity of the fragmented polluting substances left in the stock after filtration is increased.

This involves the drawback that a greater quantity of polluting substances goes into the subsequent treatments with a consequent reduction of quality of the final product.

The present invention aims at overcoming said drawbacks.

More particularly a first object is to provide an equipment for preparation of paper stock improving the operation of the filters arranged downstream the kneader.

Another object of the invention is to provide an equipment reducing the fragmentation of the polluting substances during the filtration phase of the paper stock.

A further object of the invention is to provide an equipment having a continuous cycle operation.

A fast but not least object of the invention is to provide an equipment allowing to improve the quality of the paper stock in comparison with the quality that can be obtained with equivalent plants.

The foregoing objects are attained by an equipment for preparation of paper stock that according to the main claim comprises: -at least a kneader provided with a rotary cylinder with horizontal'axis adapted to treat waste paper in an aqueous environment to obtain said paper stock ; at least a collection tank for said paper stock coming out from said at least a kneader; pumping means adapted to transfer said paper stock from said collection, tank to filtration means, and is characterized in that said filtration means comprise at least a static filtration unit arranged immediately downstream said pumping means.

According to a preferred embodiment the static filtration unit comprises a container inside provided with a fixed foraminous plate and. scraping means slidingly cooperating with the filtering surface of the fixed foraminous plate.

The foraminous plate is generally arranged orthogonal to the direction of advancement of paper stock inside the container, in which the plate defines a first chamber communicating with the pumping means and adapted to contain the paper stock to be filtered and a second chamber containing the filtered stock.

The filtering surface of the foraminous plate is facing the first chamber in which the scraping means are also arranged.

The filtered stock contained in the second chamber is conveyed through second pumping means to other filters for further filtration while the polluting substances settled on the bottom of the first chamber are periodically discharged.

Advantageously the equipment reducing crushing of the polluting substances contained in the waste paper, allows to obtain a paper stock of better quality relative to the stock that can be obtained with equivalent plants of the prior art.

The foregoing objects and advantages will be better understood by reading the following description of preferred embodiments of the invention that are given as an illustrative but not limiting example with reference to the accompanying sheets of drawings in which: - Fig. 1 shows a first embodiment of the equipment of the invention ; - Fig. 2 shows a constructional variation of the equipment of the invention ; - Fig. 3 is a longitudinal section of the static filtration unit of the equipment of figs. 1 and 2; and - Fig. 4 is a front view of the filtering plate and the scraping means of the static filtration unit of fig. 3.

As shown in fig. 1, the equipment of the invention generally indicated with reference 1 comprises a kneader generally indicated with 2, treating waste paper in an aqueous environment to obtain the paper stock; a collection tank generally indicated with 3 for the paper stock coming out from the kneader 2 and the pumping means generally indicated with 4, adapted to transfer the paper stock from the collection tank 3 to filtering means generally indicated with 5.

The kneader 2 is a kneader of known type comprising a cylindrical container

2a rotating around a longitudinal axis X and provided with a loading inlet 2b to introduce te. waste paper to be treated in an aqueous environment, a pre-filter 2c adapted to carry out a filtration of the bigger contraries and arranged at the exit of the paper stock and a discharge outlet 2d arranged on the bottom of the container to remove the polluting substances separated by filtration.

According to the invention the filtering means 5 comprise at least a static filtration unit 6 arranged immediately downstream the first pumping means 4 for the paper stock to be filtered.

More particularly said static filtration unit 6 that can be seen in the longitudinal section of fig. 3 and in the inner front view of fig. 4, comprises a vessel 7 in which a fixed foraminous plate 8 divides the internal room defining a first chamber 9 provided with a flange 10 connecting the pipe 11 feeding the paper stock to be filtered and a second chamber 12 receiving the stock filtered by the fixed foraminous plate 8 and provided with a flange 13 connecting the exit pipe 14 for the filtered paper stock.

The fixed foraminous plate 8 is arranged generally orthogonal to the direction V of flow of the paper stock in the vessel 7. On the bottom of the first chamber 9 there is a hopper 15 provided with a flange 16 connecting to the pipe 17 to discharge the contraries separated by filtration.

A proper valve 18 to be seen in fig. 1, allows to discharge the contraries separated by the static filtration unit 6 on the conveyor belt 19 carrying them to a collection storage 20.

In the fixed foraminous plate 8 one can see the filtering surface 8a facing the first chamber 9, slidingly cooperating with scraping means generally indicated with 21 consisting as shown in fig. 4, of an impeller 22 provided with three scraping blades 23a, 23b, 23c each being in contact with the filtering surface 8a and having a crescent-like profile making the scraping action particularly efficient.

The peculiar feature of the static filtration unit 6 consists in that the impeller 22 which is being rotated by external driving means of known type indicated with M in fig. 1 to which it is connected through the axis 24, rotates at a very low speed.

Such a low rotation speed, together with the crescent-like profile of the blades reduces the likelihood of fragmentation of the substances polluting the paper stock to be filtered and therefore drastically decreases the quantity of

contraries that can reach the filtered bath and therefore the final product.

In this way the static filtering unit 6 carries out an intermediate filtration between the filtration effected by the pre-filter 2c of the kneader 2 and the fine filtration effected by the final filters generally indicated with 25 substantially consisting of a cyclone filter followed by rotary basket filters that are fed by the filtered paper stock coming out from the static filtration unit 6 through pumping means 26.

In order to optimize the equipment operation, it is important to keep constant the pressure inside the static filtering unit 6 and more particularly inside its vessel 7.

To this purpose pressure regulation and control means generally indicated with 27 are provided in the equipment.

According to the embodiment shown in fig. 1, said pressure regulation and control means 27 comprise a first tank 28 connected through a pipe 28a to the pipe 11 feeding the static filtering unit 6; a second tank 29 connected through a pipe 29a to the inlet of the collection tank 3; and a third tank 30 connected to the delivery side of the pumping means 4 preferably consisting of a pump 4a.

One can see that the third tank 30 is interposed between the first tank 28 and the second tank 29 and has a first overflow wall 31 separating from the first tank 28 and a second overflow wall 32 separating from the second tank 29.

The second overflow wall 32 is arranged at a height H2 relative to the ground line S, greater than the height H1 of the first overflow wall 31. In this way it is possible to keep at the inlet of the static filtration unit 6 a generally constant hydrostatic pressure, in the range of values comprised between a quantity H1 and H2, each corresponding to the minimum and maximum static level, respectively, that the paper stock to be treated reaches inside the first tank 28 and the third tank 30 respectively.

A constructional variation of the equipment of the invention is shown in fig. 2 where one can see that said equipment generally indicated with 40, is different from the previously described equipment for a different embodiment of the pressure regulation and control means inside the static filtering unit 6, generally indicated with 41.

Said means comprise particularly a feeding valve 42 for the paper stock to be filtered, interposed between the pumping means 4 and the pipe 11 feeding the static filtration unit 6 as well as a safety valve 43 interposed between the

pumping means 4 and the pipe 29a at the inlet of the collection tank 3.

An electriWelectronic control and driving board generally indicated with. 44 is provided and comprises: a first driving unit 44a connected to the actuator 42a of the feeding valve 42; - a second driving unit 44b connected to the actuator 43a of the safety valve 43; - a sensing unit 44c connected to a pressure gauge 45 arranged inside the vessel 7 of the static filtration unit 6 ; and a control and processing unit 44d connected to the driving units 44a, 44b and sensing unit 44c.

In view of the foregoing it can be seen that the equipment of the invention attains the intended objects in both embodiments hereinbefore described and illustrated.

More particularly variations may be provided especially as to the design of filters arranged downstream the static filtration unit 6 or the pressure regulating means inside the filtration unit that may be different from those illustrated'and described.

It is however to be understood that any modification that should be made to the equipment of the invention, when falling in the scope of the appended claims, is to be considered as covered by the present patent.