Technological background

The invention relates to a sieve that can be used to separate solids from liquids. The sieve is for instance suitable for sorting products according to particle size or for removal of impurities.

Vibrating screens or pressure screens, for instance, are currently used for the separation of solid matter.

Vibrating screens involve the inconveniences of being untidy, of requiring vast space compared to their capacity and of a relatively short operating life.

Pressure screens comprise a screen net through which the liquid to be filtered is pressed. Pressure screens are closed, and hence relatively tidy. Pressure screens, however, have the drawback of requiring cleaning of the screen net. Mechanical scrapers are usually used for the cleaning.

Pressure screen systems are also previously known, in which the screen net is cleaned by pressing the filtrate at intervals back through the screen net. Such a system has been described for instance in EP Patent Application 688 593. This system comprises two devices coupled in parallel, the filtrate from one of the devices being usable for the cleaning of the other one.

Description of the invention

A sieve as claimed in claim 1 has now been invented. A number of preferred embodiments of the invention are set forth in the remaining claims.

The sieve of the invention comprises two sieve surfaces separated from one another such that, whenever desired, the one sieve can be cleaned by conducting the filtrate from the other sieve back through the sieve. The design is simple and compact.

A preferred embodiment of the invention is described in greater detail below. In the drawings of the description

- figure 1 shows the sieve in the filtering step and

- figure 2 shows the sieve in the cleaning step of the other sieve surface.

The sieve comprises a container 1 with a rectangular cross-section, and in the container, two sieve planes 2 parallel to the long sides and extending from one wall to the other. The sieve planes 2 are shorter than the container height. The upper ends and the lower ends of the sieve planes 2 are connected with a horizontal wall, so that a feed chamber is formed at the upper end and a reject chamber at the lower end. Beyond the sieve planes 2, sieving chambers are provided, with a filtrate chamber inbetween. The feed chamber is divided into two sections by means of vertical diaphragm 3 in the shape of a diagonal S, which extends from the front edge of the one sieve plane 2 to the rear edge of the other sieve plane 2. From the feeding line 4, a feeding pipe 5 provided with a feed valve 6 leads to each of the feed chambers. At the lower end there is a reject chamber, which similarly to the feed chamber is divided into two sections by means of a diaphragm 3'. From each section of the reject chamber, a reject discharge pipe 7 provided with a valve 8 leads to the reject discharge line 9. The upper end of the filtrate chamber is closed, and a filtrate discharge line 10 provided with a valve 1 1 leads from the centre of its lower end. The pipe of the filtrate discharge line 10 forms part of the diaphragm 3' of the reject chamber.

In the filtering step of figure 1, both the feed valves 6 are open, both the reject valves 8 are closed, and the filtrate valve 1 1 is open. In this manner, the liquid will pass from both the sieving chambers through the sieve planes 2 to the common, continuous filtrate chamber, from where it is removed along line 10. Solids are separated as a reject on the outer surface of the sieve planes 2.

In the cleaning step of figure 2, only one (the right-hand feed valve in the figure) feed valve 6 is open, only the opposite reject valve 8 is closed and the filtrate valve 1 1 is closed. In this manner, the liquid will pass from the one sieving chamber (the left-hand sieving chamber in the picture) through the sieve plane 2 to the filtrate chamber, and from this through the opposite sieve plane 2 to the opposite sieving chamber. The reject precipitated on the sieve net 2 of the latter sieving chamber will be detached and removed along with the liquid to the discharge line 9.

Cleaning can be performed at fixed intervals or whenever necessary. Control is preferably automatically performed.

The prototype illustrated in the figures, in which the dimensions of container 1 were 1200x200x500 mm, were used for test running various paper coating kaolin mixtures. A suitable cycle comprised for instance 120 s of filtering and 4 s of

cleaning for each net, with an approx. 67% diy matter content, a 0.075 mm sieve net mesh and an output in the range of 6800 to 9900 litres per hour.

A conventional cross-web net can be used as a sieve net, which enables costs to be minimised as already existing nets and stretching devices are utilised. If necessary, the components are made of acid-proof materials, for instance.

The device does not require separate mechanical cleaning devices, nor power sources for these. This also eliminates the risk of parts of a broken (e.g. plastic) scraper being admixed with discharged products. The cleaning of the sieve net is more reliable than with scrapers, thus yielding high output even with very fine sieves. The sieve net will have a long operating life, since it is not exposed to vibrating or abrasive stresses.

The use of a folded sieve net will further increase the filter area and thus the output.

With the sieves located in the same container, the construction will be simple.

A further advantage of the invention is the small space requirement relative to the output. The floor area is about half of the area required for a conventional vibrating screen, while the output is threefold.

The device is usable in the process industries, such as the wood-processing industry, in various applications. Among the objects of use, kaolin elutriating plants, coating kitchens, carbonate plants, PCC plants, lime and talc plants, latex factories and water purifying plants may be cited.