GB2268426A - Bead mill - Google Patents

Abstract

The mill, for wet grinding e.g of ink, paint, comprises a stationary casing 12, 14, 16, a shaft 40 rotatable within the casing and having a plurality of bead rotating discs 52 connected to it. The mill has a closeable opening 32 through which a mass of beads can be introduced, an inlet 30 through which a liquid to be ground can be pumped, and an outlet 38 through which ground liquid is delivered after passing through a screen 34. At least some of the discs have a triangular profile truncated at every corner, which traps beads, this causes the mass of beads to swirl around relative to cylinder 24 of the casing whereby rotational movement is imparted to the individual beads relative to one another; and/or by causing the beads to be packed closely into the outlet end of the mill by the force of the flow of liquid, and/or by making the ratio of the minimum radial clearance between the discs and the cylinder to the nominal diameter of the beads not greater than 10:1. As shown discs have slots 76 to enhance flow. The truncated triangular profile for the discs may be retained towards the outlet end of the mill. Cooling water enters a jacket 18; a hole 10 therein indicates leakage past seals 68. Mill may be shut down automatically if leakage occurs. Devices for monitoring temperature and pressure within cylinder may be provided. <IMAGE>

Description

Lead Mill This invention relates to a bead mill. for the wet grinding of ink, paint and like llqulds. o the type COmprising a stationary cylinder a rotor mounted co-axially within the cylinder for agitating a mass of hard beads contained therein. a closeable opening. through which the mass of beads can be introduced irto the cylinder, an inlet at one end of the cylinder through which a liquid to be ground can be pumped, and an outlet at the other end of the cylinder through which ground liquid is delivered after passing through a screen.The rotor normally comprises a shaft rotateable co-axially within the cylinder and having a plurality of axially-spaced agitator discs fixed on lt.

various agitator disc profiles have been employed, including a circle with four small flats equally spaced around its circumference. an oval with its centre of rotation on its major axis near its iarger end, and a star with all its points recurved in the same angular direction. Bead mills var considerably in size and capacity, but are generally large, say of 40 to 60 litres capacity in order to give an acceptable delivery rate.

Regardless of their size, however, they all contain as a grinding medium a mass of small beads having a nominal diameter of around lmm. to 3mm. The ratio of the minimum radial clearance between the agitator discs and the cylinder to the nominal diameter of the beads has been as large as SO. 1.

The object of the present invention is to provide a bead mill with a significantly increased delivery rate relative to its capacity.

According to one aspect of the invention, in a bead mill of the type hereinbefore referred to the rotor imparts rotational movement to the individual beads relative to one another.

Preferably, the rotor imparts rotational movement to the individual beads by trapping beads by virtue of its shape and thus causing the mass of beads to swirl around relative to the cylinder.

The rotor preferably comprises at least one bead-rotating disc having a triangular profile truncated at ever corner.

Preferably1 the or each bead-rotating disc is provided with at least one aperture.

Preferably, also, three apertures in the form of arcuate slots are provided in the or each bead-rotating disc.

The rotor preferably comprises a plurality of axially-spaced bead-rotating discs.

Preferably, the screen comprises a grid which co-acts with the beads to assist grinding.

According to another aspect of the invention, in a bead mill of the type hereinbefore referred to the rotor causes the mass of beads to swirl around relat^5.te to the cylinder.

According to a further aspect of the invention, in a bead mill of the type herelnbefore referred to the ratio of the minimum radial clearance between the bead-rotating discs and the cylinder to the nominal diameter of the beads is not greater than 101.

According to yet another aspect of the invention, in a bead mill of the type hereinbefore referred to the rotor comprises at least one bead-rotatin disc having a profile, and a minimum radial clearance from the cylinder, together adapted to trap beads and thus cause the mass of beads to swirl around relative to the cylinder whereby rotational movement is imparted to the individual beads relative to one another.

Preferably, the force of the flow of liquid to be ground is sufficient to pack the beads closely into the outlet end of the mill.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings of which : Figure 1 is a sectional side elevation along the axis of a bead mill; Figure 2 is a view of the right-hand end as seen in Figure 1 of a main body part of the mill; Fire 3 is a section on the line 3-3 in Fir 2; and Fiure 4 is an end view of one of the bead-rotating discs.

Referring now to the drawinge, a bead mill includes a stationary casing fixed to an electric motor 1-0 with a maximum speed of, say, ,300revolut ions per minute. The casing comprises a mount in flange 12 secured to the motor 10 by set-screws, an end plate 14 and a stainless steel main body part 16 secured to the mounting flange 12 by common set-screws, a cylindrical stainless steel water jacket 18 welded to the part 16, a stainless steel end ring 20 welded to the water jacket 18, and a stainless steel end cap 22 welded to the ring 20. Conf ned between the part 16 and the ring 20, and sealed by O-rings, i a stainless steel mill cylinder 24.A helical stainless tee 1 member (not shown) of rectangular cross-section is welded along almost the full length of the outer periphery of the cylinder 24. its radial thickness effectively closing the gap between the cylinder 24 and the jacket 18 so that cooling water entering the jacket 18 through an inlet 26 is constrained to follow a helical path around the cylinder 24 before leaving the jacket 18 through an outlet 28.The main body part 16 is provided with an inlet 30 through which a liquid to be ground can be pumped, and with a closeable opening 32 shown in Figures 2 and 3 through which a mass of beads compesed, for example, of glass. zirconium, steel, or a ceramic material such as steatite can be ntroduced into the cylinder 24 Ground liquid passes thrush a screen 34 positioned by a pacer 36 and is delivered througn an outlet 38 formed in said spacer and In the ring . The screen 24 is provided to prevent the beads escaping from the cylinder 24 together with the liquid, arid comprises Cs a grid which co-acts with the beads to assist grinding.

The bead mill also includes a rotor consisting of an @ssembly rotated by the drive shaft 40 uf the electric motor 10. The assembly comprises a coupling sleeve 42 dr veably connected to the shaft 40 by a key 44. a stain mess steel shaft 46 drlveably connected to the sleeve 42 by a key 48 and also connected to said sleeve by a set-serew 50, a plurality of bead-rotating discs 52 driveably connected to the shaft 46 by respective keys 54.

polyprepylene spacers 56 interposed between the discs 52 to space them axially apart, a flanged polypropylene spacer 56 for spacing that disc 52 adjacent the main body part 16 axially from said part and a stainless steel retainer 60 for the discs 52 and the spacers 56 and 58 which retainer is connected to the shaft 46 by a set-screw 62. The coupling sleeve 42 is rotatably mounted in the part 16 in two ball bearings 64 separated by a spacer 66, and a seal arrangement 68 of lip or mechanical type is disposed between the sleeve 49 and the part 16.A hole 70 formed in the water jacl:et 18 and the main body part 16 communicates with a Space In said part between the bearings 64 and the seals 68 for the purpose of indicating any leakage past said seals of the liquid being ground, such leakage being indicated simply by observing seepage from the hole 70 or by fitting said hole with a pressure sensor wh ch automatically shuts down the mill if leakage and a consequent pressure increase in said hole should occur.

As shown in Figure 4, each of the bead-rotating discs 52 has a triangular profile truncated at every corner.

More specifically, the profile of each disc 52 consist of three chords 72 which alternate with three arcs 74, the angle subtended at the centre of the disc 52 bv each chord 72 and by each are 74 being 60 degrees. The chords 72 trap beads and thus cause the mass of beads to swirl around relative to the cylinder 24. Each disc 52 also has three apertures in the form of arcuate slots 76 the angular centre-lines 78 of which co-incide with the angular centre-lines of the arcs 74. These slots 70 enhance flow of the liquid through the mill even though beads are trapped within theme and increase swirling of the beads which occupy, say, from 50 to 90 per cent of the space within the cylinder.The bead ill and its motor 10 are mounted on top of a control cabinet (not shown) which contains the pump for the liquid, devices for monitoring the temperature and pressure within the cylinder, means for automatically shutting down the mill if the temperature and/or the pressure become excessive, and manual contrel means for varying the speed of the motor 10 and the delivery rate of he pump in accordance with the kind of liquid being ground.

In operation, the individual beads are retated relative to one another by the rapidly rotating discs 52, and the relative motion of said beads grinds the liquid to a smuoth consiztency as it is pumped through the cylinder 24. The bead mill illustrated has a total capacity of beads p us liquid of 1.3 litres, and the radial clearance between the arcs 74 of the bead-rotating discs 52 and the cylinder 24 is, say, 6mm.Using beads with a nominal diameter of 1mm, the ratio of said clearance to said diameter is therefore 6.1. A deliver', rate of 00 litrez per hour has been achieved psin a liquid once only through the bead iil Illustrated and grinding to an ultra-smoothness of less than 5 microns on the Hegman scale, compared uith a rate of 5 to 10 litres per hour typically achieved by a single pass through known mills of the same capacity grinding to the same smoothness.The significant increase in delivery rate is attributeable to the profile of the bead-rotating discs 52 which traps beads as aforesaid thus causing the mass of beads to swirl around relative tc the zylinder 24 whereby rotational movement is Irrarted to the ndividual beads relative tC one another; and/or to the beads being packed closely into the outlet end cf the mill by the force of the flow of liquid: and/or to the ratio of the minimum radial clearance between the discs 52 and the cylinder 24 to the nominal diameter of the beads being not greater than 10:1 all of which features tend to agitate the beads very much more vigourously and effectively than hitherto.

In a modification, the bead-rotating discs 52 d not all have the same profile but the truncated triangular profile is retained towards the outlet end of the mill.

The scope of this invention is not restricted to the features set out in the foregoing Statements of invention, but may include any other inventIve feature or features of the bead mill ereinbefore described and/or illustrated.

Claims (1)

1. What we cli i:-

   1. A bead mill of the type hereinbefore referred to, wherein the rotor imparts rotational movement to the individual beads relative to one another 2. A bead mill according to claim 1, wherein the rotor imparts rotational movement to the individual beads by trapping beads by virtue of its shape and thus causing the mass of beads to swirl around relative to the cylinder.

   3. A bead mill according to either of the preceding claims, wherein the rotor comprises at least one bead-rotating disc having triangular profile truncated at every corner.

   4. A bead mill according to claim 3, wherein the or each bead-rotating disc is provided with at least one aperture.

   5. A bead mill according to claim 4, wherein three apertures In the form of arcuate slots are provided in the or each bead-rotating disc.

   6. A bead mill according to any one of claims 3 to 5, wherein the rotor comprises a plurality of axially-spaced bead-rotating discs 7. A bead mill according to any one of the preceding claims, wherein the screen comprises a grid which co-acts with the beads to assist grinding.

   8. A bead mill of the type hereinbefore referred to, where in the rotor causes the mass of beads to swirl around relative to the cylinder.

   8. A bead mill of the type here-inbefore referred to, where in the ratio of the minimum radial clearance between the bead-rotating discs and the cylinder to the nominal diameter of the beads is not greater than 10:1.

   10. A bead mill of the type hereinbefore referred to, wherein the rotor comprISes at least one bead-rotatinc disc having a profile, and a minimum radial clearance from the cylinder, together adapted to trap beads and thus cause the mass of beads to swirl around relative to the cylinder whereby rotational movement is Imparted to the individual beads relative to one another.

   11. A bead mill according to claim lO, wherein the force of the flow of liquid to be ground is sufficient to pack the beads closely into the outlet end of the mill.

   l2. A bead mill constructed, arranged and adapted to operate substantially as hereinbefore described with reference to, and as illustrated by, the accompanying drawings.