Process and Apparatus for Treating Waste Paper

The invention relates to a process for treating waste paper of the type employing a beater for rough-pulping the waste paper, followed by a first sorting stage from which foreign matter separated out is processed further in a subsidiary circuit which incorporates a vibratory sorter for lightweight foreign matter and a hydro-cyclone for heavy foreign matter, the fibers separated out during this further processing being returned again to the treatment process.

Until now, the impurities present in waste paper have been separated out in most cases in the main stream, during the treatment process, in sequence in speck-removing devices and sorting devices. However, this results in heavy contamination of the purifying equipment. Moreover, there is the problem that the foreign matter left in suspension for increasingly long periods is gradually reduced to such a small size that its removal becomes increasingly difficult and under certain circumstances, no longer possible at all. Consequently, this has had a detrimental effect on the quality of the treated raw material.

It is therefore already known from German Pat. No. 2,514,162 to provide as the first sorting stage, a so-called lightweight material pulper, in which a large part of the foreign matter to be separated out is removed. However, so that the fiber losses remain within bounds, the mesh of the strainer in the vibratory sorter located in the subsidiary circuit has to be relatively coarse. This means that relatively large pieces of foreign matter also pass through the strainer mesh and so re-enter the treatment process, which leads to an undesirable concentration of the suspension with foreign matter and, with repeated circulation, these pieces of foreign matter can be broken down undesirably small so that finally they pass through small holes of a half-stuff strainer in the lightweight material pulper and remain in the fibrous suspension.

An object of the present invention is therefore to provide a treatment process for waste paper, by means of which a better quality pulp can be produced with an acceptable level of capital outlay on equipment.

According to one aspect of the invention a process for treating waste paper of the type described above is characterized in that, in the first sorting stage, a sufficiently rigorous separating process is carried out to remove the greater part of the foreign matter, together with un-pulped lumps of fibers, which is then passed to the subsidiary circuit in which it and half-stuffs, which have been sorted in the hydro-cyclone and in the vibratory sorter and still contain some foreign matter, are subjected together to a multi-stage further sorting and speck-removing process.

With the process according to the invention the foreign matter present in the waste paper is separated from the main stream with the pulped fibers right at the beginning of the treatment process in the thick stock range in the first sorting stage, and is separated from the fibers which still adhere to it in a separate subsidiary circuit. The speck-removing and cleaning equipment usually arranged in the main stream for treating the fibers therefore becomes superfluous. In addition, the foreign matter is prevented from being entrained any longer than is absolutely necessary. Due to the almost complete removal of the foreign matter in the subsidiary circuit, a concentration or a circulation of foreign matter in the treatment process is substantially prevented.

Provision is made in a further development of the invention for the half-stuff from the hydro-cyclone to be delivered to the vibratory sorter. In this way, joint further processing of the foreign matter which is to be removed can be carried out.

According to another aspect of this invention a plant for carrying out a process of the type referred to above, includes a first sorting stage which consists of a rotationally symmetrical light material pulper arranged with a rotor therein at one end and to provide an over-pressure, and having in its peripheral wall an outlet hole for heavy foreign matter and in a central region of its end wall opposite to the rotor another outlet hole for lightweight foreign matter, and also having a strainer arranged in front of the half-stuff outlet, and is characterized in that the strainer has such small holes that, in general, only pulped fibers can pass through it, and in that in the subsidiary circuit behind the vibratory sorter and the hydro-cyclone there is a second sorter, the half-stuff outlet duct from which emerges again in the treatment process.

In the second sorter, which can be constructed so that speck removal may be carried out and which is connected to the half-stuff outlet duct from the hydro-cyclone and in the vibratory sorter, further pulping of the lumps of fibrous material and removal of the foreign matter can be carried out.

Provision is made, in a further development of the invention, for the outlet ducts for the separated-out foreign matter from the second sorter to lead to a further sorter with smaller holes than the first vibratory sorter. In this instance, according to the invention, provision is made for the additional sorter to be a vibratory sorter. Naturally, within the scope of the invention, a sorter operated by pressure may also be used.

It is advantageous if the half-stuff from the additional sorter is delivered to a hydro-cyclone which has an outlet hole for heavy sludge, an outlet for lightweight sludge arranged in a central zone of the hydro-cyclone and a drainage duct for half-stuff arranged coaxially around the outlet for lightweight sludge, the drainage duct for half-stuff emerging again in the treatment process. A hydro-cyclone by means of which very lightweight foreign matter can be separated out as well as heavy sludge is described, for example, in German Pat. No. 1,442,503.

By employing these measures, almost complete removal of the troublesome foreign matter is achieved in the subsidiary circuit. Certain lightweight foreign matter, particularly bits of latex, adhesive particles and the like, may still be present in the half-stuff which comes from the last sorter in the sequence. This foreign matter is now removed in the said hydro-cyclone.

One process according to the invention will now be described with reference to the accompanying drawing, which is a schematic showing of the basic process and plant, and further characteristics of the invention will be found in this description.

Waste paper to be treated arrives in the form of bales which are fed to a beater 1 for rough pulping, and is conducted from there at a stock density of 4 to 5% via a pump 2 into a rotationally symmetrical lightweight material pulper 3. This lightweight material pulper is of known design with a rotor 21 arranged therein at one end and in which an over-pressure prevails (see, for example, German Pat. No. 2,514,162). The half-stuff, i.e., the pulped fibers, is drawn off through a strainer 22 at a stock density of 4 to 4.5%, and is conducted to an intermediate vat 4, from which it is pumped via a pump 5 for further processing.

From the lightweight material pulper 3, lightweight foreign matter is delivered to a vibratory sorter 7 via a duct 6. Heavy foreign matter, which accumulates around the periphery of the lightweight material pulper 3, is delivered to a hydro-cyclone 9 via a duct 8, at a stock density of 1 to 1.5%. The amount of separated-out foreign matter, together with water and fibers (fibrous lumps) is about 20% of the amount supplied to the lightweight material pulper 3. The foreign matter separated out in the hydro-cyclone is removed, whilst the suspension which arrives in a half-stuff duct 10 is also delivered to the vibratory sorter 7. The vibratory sorter 7 has a strainer with a relatively large hole width, so that not too much lumpy fibrous material is lost. The half-stuff from the vibratory sorter 7 is delivered via a pump 11 at a stock density of 1.5 to 2% to a second sorter, such as a lightweight material pulper 12, for example. The half-stuffs which end up behind the strainer 23 of this sorter 12, i.e., recovered fibers, are conducted away in quantities of around 70% of the amount supplied to the sorter 12, via a duct, and deposited in the intermediate vat 4. The lightweight and heavy foreign matter separated out in the second sorter 12 is delivered via ducts 14 and 15 to a second vibratory sorter 16 having a strainer with correspondingly smaller hole width. From the half-stuff duct of the vibratory sorter 16 the suspension is delivered via a pump 17 to a hydro-cyclone, at a stock density of 0.5 to 2%. Heavy foreign matter separated out in the hydro-cyclone 18 in the form of sludge is removed as waste at its lower end, as is certain very lightweight foreign matter (or sludge) which accumulates in a central duct 19 of the hydro-cyclone. The recovered fibers are returned to the beater 1 via the half-stuff outlet duct 20 of the hydro-cyclone 18.

The hole-width in the lightweight material pulper 3 is chosen so that about 10-30% of the suspension arriving in the lightweight pulper is drawn off into the subsidiary circuit for removal of the foreign matter. By an appropriate choice of the design and arrangement of the sorting and de-specking appliances in this subsidiary circuit, it is ensured that any fibrous material which is present in largely recovered, whilst any foreign matter is removed.

While this invention has been described as having a preferred design, it will be understood that it is capable of further modification. 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.