Conical filtering centrifuge

Abstract

 A conical filtering centrifuge for separating solids and liquids comprises a basket (1) having a baseplate (2) and a frusto-conical sidewall in the form of a plurality of leaves (3) each of which is pivotally attached to the baseplate (2), and each of which partially overlaps adjacent leaves (3) so as to constitute a frusto-conical basket (1) the cone angle of which can be varied whereby different feedstocks can be efficiently separated.

Description

[0001] This invention relates to a conical filtering centrifuge, in particular for the separation of solids from liquids.

[0002] A conical filtering centrifuge may be used to separate solid and liquid fractions of a feedstock. The most common design of conical centrifuge comprises a basket having a circular base, and a frusto-conical side wall tapering outwardly from the base to define an open-ended vessel. The side wall is perforated to allow liquid to pass through it. A filter cloth may be fixed within the basket to lie against the side wall, the filter cloth, if present, being secured by a fixing plate which is attached to the base of the basket. In use, the basket is rotated about its central longitudinal axis, and the feedstock is introduced into the basket through a feed pipe which opens close to the base of the basket.

[0003] The feedstock, on entering the rotating basket, is distributed by centrifugal force against the filter cloth and by the same force the liquid fraction is forced through the filter cloth, leaving the solid fraction behind. Since the centrifugal force is greatest at the mouth of the basket, there is a secondary force acting on the solids which cause them to flow towards the open mouth of the basket, and hence out of the basket.

[0004] The filter cloth may be eliminated, perforations or slots in the side wall itself of the basket effecting the desired filtering.

[0005] The above types of device have the advantage of simplicity and an ability to operate continuously. Furthermore, the flow of material within the basket is commonly known as plug flow, this meaning that material leaves the basket in substantially the same order as it entered the basket.

[0006] This type of centrifuge however has a disadvantage that the cone angle (which is defined as the angle between the side wall of the basket and the base of the basket minus 90°) is critical to the operational efficiency of the device. If the angle is too wide the residence time of the feedstock in the basket is too short for complete separation to take place, whereas if the cone angle is too narrow the solid fraction is prevented from discharging from the basket. In practice a specific basket must be designed for each different application, and even a slight change in some characteristics of the feedstock, for example increase in the percentage of solids, particle size, or viscosity, may cause the centrifuge to function unreliably or not at all. Hence the application of this type of centrifuge is limited to cases where the nature of the feedstock is well known and predictable.

[0007] The aim of the present invention is to ameliorate or overcome the problems of known centrifuges and preferably to present a fully automatic and self regulating conical filtering centrifuge which can handle any suitable feedstock.

[0008] Accordingly, the present invention provides a conical filtering centrifuge, particularly for the separation of solids from liquids, comprising a basket having a baseplate, and a plurality of leaves each of which is pivotally attached to the baseplate, and each of which is arranged partially to overlap adjacent leaves so as to form a frusto-conical basket in which the cone angle is variable.

[0009] In specific embodiments of the invention the basket is lined with a filter cloth. The filter cloth may be attached to the baseplate of the basket by a fixing plate.

[0010] The leaves may be either perforated or have drainage grooves therein in order that liquid, separated from feedstock that is fed into the basket whilst the basket is rotating at high speed, may be removed.

[0011] In a preferred embodiment of the present invention the leaves may be secured at a specific angle relative to the baseplate by a strap or other suitable locking arrangement that is adjustable so that the basket can be used for a variety of feedstocks with different characteristics, eg. viscosity, percentage of solids, etc.

[0012] In more preferred embodiments, each leaf is extended below the point at which it is attached to the baseplate and has attached thereto an arm which extends in a generally radially outwardly direction and is adapted to carry one or more counterweights. These counterweights may be fixed at different distances from the leaf, and this can be achieved by having notched or threaded arms, along with suitably adapted counterweights. The purpose of the counterweighted arms is to present a counterforce to the force that tends to open up the basket ie. increase the cone angle, when it is spinning, and these counterweighted arms enable the centrifuge to find the correct angle for operation whilst in use.

[0013] Embodiments of the invention will now be described in detail by way of example with reference to the accompanying drawings in which:

Figure 1 is a side view of a basket of an embodiment of the present invention;

Figure 2 shows a leaf which forms part of the basket shown in figure 1;

Figure 3 is a schematic view of the baseplate and leaf arrangement of an embodiment of the present invention; and

Figure 4 shows a cross-section of a conical filtering centrifuge which is an embodiment of the present invention.

[0014] Referring to figures 1, 2 and 3, a basket 1 of a conical filtering centrifuge is illustrated that may be used for centrifugal filtration of, for example, organic and inorganic crystals, powders, granulates and similar solids in liquid suspensions. The frusto-conical basket has a baseplate 2 and a sidewall that consists of a plurality of leaves 3. Each leaf 3 is fitted to the baseplate 2 by a hinge 6, and is of generally truncated triangular shape and perforated with holes 5.

[0015] The arrangement of the overlapping leaves is illustrated in figure 3 from which it can also be seen that the baseplate 2 is of generally circular shape the circumference of which is of stepped configuration to provide a series of angularly offset notches 2a therein, one for each leaf 3. This arrangement enables the cone angle of the basket to be varied, as illustrated by the two positions 3a and 3b of the leaves shown in figure 1.

[0016] The leaves 3 may be interconnected with one another to provide a particular cone angle to the basket. For example a link chain 7 or like fastening strap may extend around the base of the basket 1 as shown in Fig. 1. Alternatively short chains 8 or similar flexible retainers may be provided between adjacent leaves 3 as shown in Fig. 1 and, to a limited extent, in Fig. 3.

[0017] In use, the basket 1 is rotated about the axis 4 at speeds of typically between 4000 and 5000 rpm for an average-sized basket.

[0018] Referring to figure 4, a conical centrifuge with basket 11 is illustrated in cross-section. The basket 11 is of similar construction to the basket 1 shown in figure 1. The baseplate 12 is of a similar notched circle shape to the baseplate 2 in figure 3, and has attached to it, by means of hinges 16, a plurality of perforated leaves 13, and these together form a frusto-conical basket.

[0019] The leaves 13 in this particular embodiment extend below the baseplate 12, and each leaf 13 has attached to it an arm 15 that extends generally radially outwards perpendicular to its respective leaf 13. Each arm carries a counterweight 20, and the position of the counterweight on the arm 15 is variable in order that the force experienced by the upper part of the leaf 13 can be adjusted.

[0020] A filter cloth 17 lines the basket 11, and is held in place by a fixing plate 18 and nut 19, that attaches the basket 11 to a rotatable shaft 14.

[0021] The centrifuge is utilised by first setting the counterweights 20 in positions that will result in the cone angle of the basket to be approximately correct as known from previous use and experience. The basket is then rotated by the shaft 14, and feedstock to be separated is added by a feedpipe (not shown) which opens inside the basket near the centre of the baseplate. The feedstock is pushed against the leaves 13 of the basket 11 by centrifugal force, and liquid to be separated is forced through the filter cloth 17 by the same centrifugal force and drains through the perforations in the leaves 13.

[0022] With solids from the feedstock building up on the filter cloth 17, the force on the upper part of the leaves 13 increases, causing the leaves 13 to hinge outwards, thus increasing the cone angle of the basket and causing the solids to flow out of the basket 11; this happens because the centrifugal force at the top of the leaves 13 is greater than that nearer the baseplate 12 of the basket 11. When, resulting from this flow of solids, the amount of solids on the filter cloth 17 reduces, the counterweighted arms 15 cause the leaves 13 to hinge inwards and the cone angle of the basket 11 to narrow. In practice this causes the centrifuge to find automatically the cone angle at which the amount of solids entering the basket 11 in the feedstock and the amount of solids leaving the basket is approximately equal.

[0023] In both of the embodiments described above, the leaves 3, 13 may be manufactured from relatively weak materials such as plastics. In known conical centrifuges with continuous baskets, so-called self-stresses resulting from spinning the basket at high speeds eg. 3000 rpm, are by far the greatest limiting design factor, often being responsible for 60 to 70 per cent of the total stresses. Since, in the above embodiments of the present invention, the basket is formed from a plurality of individual leaves, the self-stresses are greatly reduced or eliminated, and the material from which the leaves is manufactured can be weaker.

[0024] All the embodiments of the present invention, like most machines that work using centrifugal forces, would operate equally well in the illustrated horizontal mode as in a vertical mode or indeed in an inverted mode.

[0025] It should be understood that the embodiments of the present invention have been described above by way of example only, and various alternative features or modifications from what has been specifically described and illustrated can be made within the scope of the invention, as will be readily apparent to persons skilled in the art.

Claims

1. A conical filtering centrifuge for the separation of solids and liquids comprises a basket (1) having a baseplate (2) and a frusto-conical sidewall, characterised in that the sidewall comprises a plurality of leaves (3) each of which is pivotally attached to the baseplate (2), and each of which is arranged partially to overlap adjacent leaves (3) so as to form a frusto-conical basket (1) in which the cone angle is variable.

2. A centrifuge as claimed in claim 1 in which the basket (1) is lined with a filter cloth (17).

3. A centrifuge as claimed in claim 2 in which the filter cloth (17) is attached to the baseplate (2) by a fixing plate (18).

4. A centrifuge as claimed in any one of claims 1 to 3 in which the leaves (3) are either perforated (5) or have drainage grooves therein in order that liquid separated from feedstock that is fed into the basket (1) while the basket (1) is rotating at high speed is removed.

5. A centrifuge as claimed in any one of claims 1 to 4 in which the leaves (3) are secured at a specific angle relative to the baseplate (2) by an adjustable locking arrangement (7).

6. A centrifuge as claimed in any one of claims 1 to 4 in which each leaf (13) is extended below the point (16) at which it is attached to the baseplate (12) and has attached thereto an arm (15) which extends in a generally radially outwardly direction and carries one or more counterweights (20).

7. A centrifuge as claimed in claim 6 in which the counterweights (20) are fixed at different distances from the leaves (13).

8. A centrifuge as claimed in claim 7 in which the arms (15) are notched or threaded, and the counterweights (20) are correspondingly shaped to co-operate with said arms (15).

Drawing