Waste paper processing system

A waste paper processing system in which a fibrous suspension of waste paper is cleaned and sorted. The fine cleaning of the suspension is carried out by means of one or more conical centrifuges in a subsidiary stream which branches off from the mainstream and is later recombined therewith. The portion containing the largest content of coarse substances is introduced from a vertical sorter located in the mainstream into the subsidiary stream in which it is sorted and cleaned. There is practically no need to process the waste from the conical centrifuges and the conical centrifuge unit can be designed to be substantially smaller and thus more economical in terms of energy because of the smaller flow of material therethrough.

The present invention relates to a system for the processing of waste paper 
in screen sorting devices for both coarse and fine cleaning, and in a 
hydrocyclone or hydrocyclones, for example, conical centrifuges or 
cleaners, in one or more stages for fine cleaning, wherein a suspension of 
waste paper is sorted and cleaned. 
Systems of this kind are generally known having various designs. One such 
system is described, for example, in H. Mack's book entitled 
Altpapieraufbereitung, 1967, which is incorporated by reference herein, at 
page 107 and the associated FIG. 91 on page 109. A similar system is 
described in "Wochenblatt fur Papierfabrikation", 1966, which is also 
incorporated by reference herein. 
In such systems, the waste paper is first disintegrated in a pulper and the 
fibrous suspension thereby obtained is passed through various processing 
stages, including, despecking devices, hydrocyclones and flat sorters for 
preliminary or coarse cleaning purposes. By means of such devices, coarse 
impurities of both heavy and large-area kinds are largely removed from the 
fibrous suspension. 
In the case of the system described in "Wochenblatt fur Papierfabrikation", 
1966, page 396, FIG. 8, after this coarse cleaning, the fibrous suspension 
first passes to a flotation unit before undergoing further cleaning in a 
screen sorting device, usually referred to as a vertical sorter, for fine 
cleaning purposes; and subsequently in an extensive unit made up of 
hydrocyclones (so-called "cleaners" or conical centrifuges) for further 
fine cleaning. In this manner, both heavy and large-area impurities, such 
as foils, are separated. In this system, the conical centrifuge unit is 
followed by a thickening device in which the consistency of the pulp is 
increased, and by a bleaching tower. The fibrous suspension is withdrawn 
in diluted form from the bleaching tower and is thickened to a higher 
consistency by repeated thickening steps in a further thickening device, 
for example, a band pressure filter, for storage for further processing 
such as final cleaning, by stacking in a stacking tower. 
As may be gathered from the previously mentioned publications, a flotation 
unit need not necessarily be provided and the fibrous suspension can be 
thickened during an earlier stage in order that it can be treated with 
chemicals in a treatment tower at a higher density of, for example, more 
than 15%. In such a tower, the material can be subjected to the action of 
bleaching chemicals and also of chemicals for flotation. 
It should also be added that the known systems described in general fall 
under the generic term waste paper processing. To these extensive waste 
paper processing systems there is normally connected the so-called 
constant part which is connected to the stacking tower previously referred 
to and which contains further conical centrifuges or fine cleaning, i.e., 
hydrocyclones of a specific construction. The conical centrifuge units 
generally comprise a plurality of stages, as can be seen from FIG. 8 of 
the previously mentioned "Wochenblatt fur Papierfabrikation" article. For 
large quantities of waste paper, a plurality of the individual conical 
centrifuges illustrated in this FIG. 8 is generally provided. However, in 
previous systems of this type it has been found to be a disadvantage in 
that these conical centrifuge units are quite expensive in their 
construction, resulting in a considerable increase in the cost of the 
overall system. In addition, the multistage construction of these conical 
centrifuge units has been found to have the effect that a very large 
amount of fibers has to be discarded with the dirt, or that a relatively 
large quantity of coarse dirt remains in the fibrous suspension which 
passes to the stacking tower. 
It is an object of the present invention to provide a system of the type 
illustrated in the article referred to which will produce a more pure 
fibrous substance in a more simple and therefore less costly manner, or, 
while maintaining the same degree of purity of the fibrous substance, 
substantially reduce the cost of the system. 
According to the invention, there is provided a system for the processing 
of waste paper in which a fibrous suspension produced from waste paper is 
sorted and cleaned in a plurality of screen sorting devices to effect both 
coarse and fine cleaning and in one or more hydrocyclones, arranged in one 
or more stages, to effect fine cleaning, wherein the screen sorting 
devices intended for fine cleaning are arranged both in a main processing 
stream and in a subsidiary stream branching off from said mainstream, and 
said hydrocyclone or hydrocyclones intended for fine cleaning are 
exclusively arranged in said subsidiary stream; wherein said hydrocyclone 
or hydrocyclones is or are arranged in said subsidiary stream upstream of 
at least one screen sorting device; and wherein said subsidiary stream is 
arranged essentially to carry the portion with the greatest content of 
coarse material from the screen sorting device or devices in said 
mainstream intended for fine cleaning of the suspension in said mainstream 
.

DESCRIPTION OF A PREFERRED EMBODIMENT 
In the figures, that part of the waste paper processing system which 
extends from the pulper in which the waste paper is disintegrated to the 
flotation system, if a flotation system is provided, has been omitted. As 
regards the design of this part of the system, reference may be made to 
known devices such as those earlier described. 
In the embodiment of FIG. 1, the section of the system illustrated 
commences with flotation unit 20 represented in dash-dotted lines. From 
this unit 20, the fibrous suspension is fed through pipe 8 into container 
7, whence it is conveyed by means of pump 16 to sorter 1 for fine 
cleaning. Sorter 1, and sorter 3 in the subsidiary stream, are of the type 
known by the technical name of "vertical sorters". These sorters 1, 3 are 
generally provided with a rotor which rotates about a vertical axis in a 
filter basket and which is provided with sorter vanes or strips (not 
shown). The size of the perforations in the filter basket is selected in 
accordance with the degree and nature of the impurities present and the 
degree of purity required after sorting. Sorters 1, 3 can be arranged in a 
plurality of stages, but generally in a maximum of two stages as explained 
in the book by Mack at pages 107 and 109. The construction of such devices 
is described, for example, in U.S. Pat. No. 4,276,159, which is 
incorporated by reference herein. 
High quality material from sorter 1 is fed forward in a mainstream pipe 21 
and, after thickening in a thickening device 11, for which known band 
pressure filters are used, is prepared for storage in a stacking tower 
(not shown). The overflow separated from sorter 1, which is enriched and 
abundant in coarse materials, is fed through outlet 9 into container 5 and 
thence conveyed by means of pump 19 as a subsidiary stream in subsidiary 
pipe 23 into conical centrifuge unit 2, which is schematically represented 
in the figures. The construction of conical centrifuge unit 2 corresponds 
to that shown in FIG. 8 of the article in "Wochenblatt fur 
Papierfabrikation" previously referred to, although each of the conical 
centrifuges (cleaners) represented therein can be replaced by a plurality 
thereof, if the quantity of waste paper to be processed is sufficiently 
large, which is generally the case. The construction of these conical 
centrifuges is described, for example, in German patent specification No. 
1,442,503, incorporated by reference herein. 
In FIG. 1, a broken line indicates that the light dirt portion is withdrawn 
through central collector pipe 22 which leads out of the top of conical 
centrifuge unit 2 or centrifuges 2. Previously, i.e., when the conical 
centrifuge unit 2 was arranged in the mainstream 21, it was necessary 
further to process this light dirt portion in order to prevent excessive 
further losses. In the system of the present invention, however, depending 
upon the quantity withdrawn from sorter 1, which can be adjusted by 
selecting the size of the perforations, i.e., the size of the holes or the 
slot width, of the filter basket, only an amount of at most 50% by volume 
of the total fibrous suspension is prepared in the subsidiary pipe 23 and 
thus in conical centrifuge unit 2. Generally the size of the subsidiary 
stream will be 20 to 30% by weight of the total fibrous suspension, and 
therefore for this reason along, a very much smaller amount of fibers is 
withdrawn with the light dirt portion from the hydrocyclones of the 
conical centrifuge unit. 
It is also an advantage that the subsidiary stream is used to process a 
portion which is heavily enriched and abundant with dirt from the total 
amount of fibers, so that the degree of separation of these conical 
centrifuge units 2 is also substantially greater for the light dirt, which 
means that in the system of the present invention the light dirt portion 
withdrawn from conical centrifuge unit 2 can readily by discarded since 
the fiber losses are low. Since the amount of fibers to be processed is 
thus substantially smaller, a conical centrifuge unit 2 can also be 
smaller than in previously known systems in which the conical centrifuge 
unit was arranged in the mainstream. This in turn results in a substantial 
reduction in the cost of the overall system due to the fact that these 
conical centrifuges have a very high power requirement since they must 
operate at a high input pressure. 
Downstream of conical centrifuge unit 2, the high quality material is 
collected and fed to sorter 3, similar to sorter 1 in mainstream 21. From 
sorter 3 the cleaned high quality material is collected for storage in the 
stacking tower after having been thickened in thickening device 12. 
The overflow from sorter 3 is fed through pipe 10 via flat filter sorting 
device 4, downstream of which the separated waste liquid is discarded and 
the high quality material is returned to container 5 at the beginning of 
the subsidiary stream. Thus in this embodiment, the subsidiary stream 
begins at overflow outlet 9 of sorter 1 of the mainstream. 
In FIG. 2, the upstream end of the system, i.e., elements 20, 8, 16, 1, 9, 
and 5, and their arrangement, is substantially the same as in the system 
of FIG. 1, and consequently, in FIG. 2 only the downstream end of the 
system has been illustrated. In FIG. 2, the main and subsidiary streams 
21, 23, respectively, from sorters 1 and 3 are combined in vertical sorter 
6 also similar to sorters 1 and 3, and then commonly run by way of 
thickening device 13 to the stacking tower (not shown). Naturally, it is 
also possible to combine the main and subsidiary streams 21, 23 in 
thickening device 13. 
The conical centrifuges have already been referred to as known through the 
above-mentioned German Patent specification. It should also be noted that 
these conical centrifuges, which in English are often referred to as 
"cleaners", have a very slim or narrow conical lower part, the cone 
opening angle of which is generally less than ten degrees. At the tip of 
the cone they possess a waste liquid discharge nozzle and mainly serve to 
clean the suspension from very fine impurities in particle form, for 
example, sand, fine splinters, bark particles, and metal particles. These 
conical centrifuges operate at the optimum when the consistency of the 
fibrous suspension is below about 1% and preferably below about 0.8%. The 
maximum consistency which avoids too much disturbance of the operation is 
about 1.5%. It is also possible to use these conical centrifuges to degas 
the fibrous suspension. The high quality material is withdrawn from the 
center of the cleaning body upwardly to an annular chamber which is formed 
in a pipeline arranged at this point containing a central discharge pipe 
for the light dirt fraction and for gases. 
While this invention has been described as having a preferred embodiment, 
it will be understood that it is capable of further modifications. This 
application is therefore intended to cover any variations, uses, or 
adaptations of the invention following the general principles thereof, and 
including such departures from the present disclosure as come within known 
or customary practice in the art to which this invention pertains and fall 
within the limits of the appended claims.