GB2082930A

GB2082930A - Magnetic stirring element

Info

Publication number: GB2082930A
Application number: GB8124810A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-08-27
Filing date: 1981-08-13
Publication date: 1982-03-17
Also published as: GB2082930B

Abstract

A magnetic stirring element (4) for use in magnetic stirring apparatus (2, 4, 6, 8), comprising a symmetrical body member of an inert material such as polytetrafluoroethylene in the form of a pair (12, 14) of opposed cones or a pair of opposed segments of a sphere having a common base which comprises the central plane of the element (4). A bar magnet (16) is centrally located in the body member, the arrangement being such that, in use and on rotation of a horizontal drive magnet (6) of the apparatus about a vertical axis (XY), the element (4) is spun within the vessel (2) about a central axis (PQ) extending perpendicular to said central plane of the element (4) and is also rotated around the vessel (2) about said vertical axis (XY). <IMAGE>

Description

SPECIFICATION Magnetic stirring element This invention relates to magnetic stirring elements for use in magnetic stirring apparatus to mix liquids and solutions.

Magnetic stirring apparatus is well known and commonly comprises a magnetic stirring element placed in a vessel containing the liquid to be mixed, and a drive magnet located below said vessel. On rotation of the drive magnet, said magnet couples through the base of the vessel with the stirring element, causing said element to rotate and so stir the liquid.

Stirring elements commonly comprise a bar magnet encapsulated in a disc of inert material such as plastic, which discs, in order to improve mixing efficiency, may be provided with vanes applied to their outer surfaces or, as detailed in my co-pending United Kingdom patent application no.

81 10659, with one or more passageways formed through the disc.

However, in all such arrangements, the elements tend to rotate about substantially vertical central axes to create turbulence in the liquid which moves radially outwardly towards the walls of the vessel. In certain circumstances, this turbulence may not reach the extremities of the vessel, with the result that, in for example dissolution applications, material may remain undissolved in areas of the vessel remote from the centre.

According to the present invention there is provided a stirring element for use in magnetic stirring apparatus, the element comprising a symmetrical body member of non-magnetic material having a central, circular plane therethrough forming the base for two opposed body halves each substantially in the shape of a cone or a segment of a sphere, and a permanent magnet disposed within the body member, said magnet having a central longitudinal axis lying in said central plane of the body member.

When such an element is located in a vessel containing a liquid to be stirred and the vessel is positioned over a horizontal drive magnet rotating about a vertical axis, the element itself undergoes what can be termed planetary or precessional motion, in that the element is spun about a central axis extending perpendicular to said central plane of the body member whilst at the same time the element is rotated in a horizontal plane about the vertical axis of rotation of the drive magnet.

The angular position of the central axis about which the element spins can vary between the near vertical to the near horizontal and is a function primarily of the precise geometry of the part-spherical or conical body halves. However, the provision of body halves of this nature substantially prevents the simple coupling -- i.e.

horizontal alignment -- between the magnet of the stirring element and the drive magnet as occurs in the prior art arrangements and which results in spinning of the element in a substantially horizontal plane about a substantially fixed vertical axis. Instead, the stirring element according to the invention undergoes a movement which produces a centre of turbulence in the liquid to be stirred which is offset from the centre of the vessel and which itself circulates within the vessel to ensure a mixing action even at the radial extremities of the vessel.

In a preferred embodiment of the invention, the body member is of a plastics material such as polytetrafluoroethylene, while the permanent magnet is of generally cylindrical shape with the poles of the magnet at the ends thereof.

The body member of the element may have one or more open-ended passageways symmetrically formed therethrough, the configurations of which passageways may conform substantially with those disclosed in my co-pending application no.

8110659.

By way of examples only, embodiments of the invention will now be described in greater detail with reference to the accompanying drawings of which: Fig. 1 is a schematic view of magnetic stirring apparatus incorporating a stirring element according to the invention; Fig. 2 illustrates the motion of the drive magnet and the stirring element of the apparatus of Fig. 1; Figs. 3, 4 and 5 are perspective view, plan view partly cut away and side view respectively of a stirring element according to the invention; Figs. 6, 7 and 8 are views of further stirring elements according to the invention, and Fig. 9 is a plan view of the element of Fig. 8.

Referring to Fig. 1 there is shown magnetic stirring apparatus comprising a vessel 2 containing liquid to be mixed and in which is located a magnetic stirring element 4. The vessel 2 is located over a drive magnet 6 rotatable in a clockwise direction, shown by arrow A, about a vertical axis by means of a motor 8.

The element 4, shown in more detail in Figs. 3 to 5, comprises a body member including a central disc-shaped portion 10 the flat outer surfaces of which effectively form the bases of a pair of opposed, symmetrical conical portions or body halves 12, 14. The body member is moulded from a plastics material such as polytetrafluoroethylene around a cylindrical bar magnet 16, the magnet being centrally located within the body member with its longitudinal axis extending parallel with the central plane of the disc-shaped portion 10 and having its poles one at each end of the magnet.

The configuration of the element 4 is such that, on location of the element in a vessel 1 2, it is difficult for the magnets 6, 1 6 to align with one another to give the simple magnetic coupling between two parallel magnets as occurs in prior art arrangement. The result is that, with the element 4 in a non-central position in the vessel 2 and on rotation of the drive magnet 6 in the direction of arrow A in Fig. 1 --- i.e. about the vertical axis X-Y in Fig. 2 in the direction of the arrows A', A" in said figure - - the magnets 6, 16 are at an angle to one another whereby the element 4 spins in the direction of arrow B about a central axis PQ extending perpendicular to the length of the magnet 1 6 whilst at the same time rotating in the direction of arrow C around the periphery of the vessel 2 about the vertical axis XY.

In Figs. 1 and 2, the longitudinal axes of the magnets 6 and 1 6 are shown to be perpendicular to one another, but the angle between these two axes can vary within the range of near 0 to near 900. This angle is a function of several factors but primarily of the geometry of the portions 12 of the element 4 and also the configuration of the walls of the vessel 2.

Thus it will be appreciated that the motion imparted to the element 4 is somewhat planetary in nature with the result that the mixing action achieved arises from the formation of a centre of turbulence in a region offset from the centre of the vessel, said centre of turbulence moving around the vessel in a circular fashion. Such motion results in a much more thorough mixing of the liquid than with established arrangements, particularly in the regions of the vessel radially outwardly of the centre of the vessel and adjacent the periphery of the vessel.

The precise nature of the motion of the element 4 in a given vessel 2, as mentioned above, depends to a great extent upon the geometry of the element, Fig. 7 showing an alternative element 4 in which the body halves 12, 14 are in the form of segments of a sphere instead of being conical.

However such an element also undergoes the planetary-type motion detailed above, although the element of Fig. 7 precesses more readily in vessels with curved walls, such as roundbottomed flasks, while elements with conical body halves as shown in Figs. 3 to 5 are more suited to vessels with straight sides and bottoms such as beakers and flat-bottomed flasks.

As detailed in my co-pending application no.

81 10659, mixing of the liquid in a vessel 2 can be improved by providing one or more open-ended passageways symmetrically formed in the body member of a disc-like stirring element. Such passageways can also be formed in the partconical or part-spherical body members of the elements 4, for example as shown at 1 8, 20 in Figs. 6, 8 and 9. Clearly many other configurations for the passageways 18, 20 are possible, substantially equivalent to those shown in the Figures accompanying application no. 8110659.

The use of polytetrafluoroethylene as the material for the body member of the element 4, which material has a very low coefficient of friction, results in relatively unimpeded rotation of the elements in the liquid to be mixed.

Claims

1. A stirring element for use in magnetic stirring apparatus, the element comprising a symmetrical, body member of a non-magnetic material having a central circular plane therethrough forming the base for two opposed body halves each substantially in the shape of a cone or a segment of a sphere, and a permanent magnet disposed within the body member, said magnet having a central longitudinal axis lying in said central plane of the body member.

2. A stirring element as claimed in claim 1 in which the body member is moulded from polytetrafluoroethylene.

3. A stirring element as claimed in claim 1 or claim 2 in which the permanent magnet is of generally cylindrical shape with the poles of the magnet at the ends of said magnet.

4. A stirring element as claimed in any one of claims 1 to 3 in which one or more open-ended passageways are symmetrically formed through the body member thereof.

5. A stirring element substantially as described with reference to and as illustrated by the accompanying drawings.