AU697677B2 - Attrition mill II - Google Patents

Description

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AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT 0 *0.0 a a a a a *0 C a. I a o a j I a a a

ORIGINAL

Names of Applicants: MOUNT ISA MINES LIMITED A.C.N. 009 661 447 Names of Applicants: MOUNT ISA MINES LIMITED A.C.N. 009 661 447 and ERICH NETZSCH GmbH CO HOLDING KG Actual Inventors: Peter WOODALL and Udo ENDERLE Address of Service: SHELSTON WATERS MARGARET STREET SYDNEY NSW 2000 Invention Title: ATTRITION MILL II Details of Associated Provisional Application No. PN0172 dated 19th December, 1994 The following statement is a full description of this invention, including the best method of performing it known to us:- ~i -2- This invention relates to an attrition mill and more particularly to an attrition mill for use in grinding ore.

The invention is of particular value for concentrating sulphide ores which are finely distributed in shale or silica and is herein described in that context but is not limited to that use.

In outline, in traditional processes for concentrating sulphide ores, the ores are first crushed through primary jaw crushers and secondary cone crushers to yield a product 80% finer than about 3mm. The crushed ore is then separated from low-grade material. Low-grade material is separated by a heavy medium process and the heavier high-grade ore is then ground to 90% passing 70-75 microns by means of rod or ball mills. The rod or ball mill discharge is then subjected to further separation from r gangue in flotation cells. In the case where lead and zinc sulphide ores are used, the lead ore is floated first and the slurry then conditioned, e.g. with copper sulphate, prior to the zinc being floated. The lead and zinc concentrates so obtained are subsequently de-watered and transported to smelters. This process and variations of it are well known in the art.

Up to a decade ago, flotation feeds were ground down to 75-100 microns.

Over the past decade, a plant has been developed which enables flotation feeds to be around 40 n icrons. This feed is achieved in tower mills having a nr minal capacity in the range of from 10 to 100 tonnes per hour. A typical tower mil uses a screw agitator driven at 60 to 160 rpm and employs large balls (greater than 6mm diameter) as a grinding medium. Although it has been claimed that tower mills may be effective with lead and zinc concentrates to reduce particles so that 80% are less than microns, in practice tower mills cannot economically grind better than 80% less than 20 microns because of excessive energy, medium costs and wear costs.

There are many deposits (for example, those at the McArthur River) in which sulphide ores (for example, galena, pyrites) are finely distributed in a host gangue (for -3example, shale and/or silica) and which cannot be economically concentrated by known methods.

It is known to use attrition mills to grind foodstuffs, paint pigment, and such like high value materials. The term "attrition mill" is herein used to include mills used for ultra fine grinding for example, stirred mills in any configuration such as bead mills, colloid mills, fluid energy mills, ultrasonic mills, petit pulverizers, and the like grinders.

In general such mills comprise a grinding chamber, and an axial impeller having a series of radially directed arms or discs, the inmpeller being rotated by a motor via a gearbox. Such mills are usually provided with grinding media and the mterial to be ground is fed to the mill as a slurry.

Attrition mills have the capability of producing a fine product, and readily reduce 80% of particles for example to less than 10 microns, or even to less than 6 microns. However attrition mills commonly have a small capacity and are therefore 15 expensive to operate. Moreover as such mills operate at high rotor speeds in comparison with tower mills, the costs of grinding medium and component wear are generally higher. Their use is thus confined to grinding expensive materials where the marginal benefits of fine grinding justify the marginal costs. Attrition mills have not S" been generally used for ore beneficiation.

Hitherto one of the largest successful attrition mills has been the Netzsch LME 500 bead mill which has a capacity of 500 litres. The Netzsch LME 500 has a power of 160 Kw and in trials with ores this mill had a throughput rate of 2.5 TPH of product. For practical and economical use in ore beneficiation a throughput preferably in the order of 100 TPH would be required and this is not achievable with current technology. To be commercially viable for use in ore beneficiation an attrition mill would need to be constructed with a significantly greater capacity, for example greater than 1000 litres and more preferably greater than 2000 litres.

-4- To date it has not been possible to construct a satisfactory mill for ore beneficiation of capacity greater than 500 litres because the tip velocity of the impeller in such a mill at, for example, 1000 litre capacity would exceed 13 metres per second and in trying to construct such a mill it was found that the rate of wear in the mill was very rapid. Even hard chrome plated steel surfaces were worn to destruction within a few hours when grinding an ore slurry in a 1000 litre mill utilizing tip speeds greater than about 13m/s.

It is an object of the present invention to provide an attrition mill which avoids or at least ameliorates some of the problems of the prior art. It is an object of preferred embodiments of the invention to provide an attrition mill having a capacity in excess of 1000 litres and which is able to grind an ore so that at least 80% of

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S particles have a size less than 15 microns with an acceptable rate of wear of mill parts.

i According to one aspect the invention consists in an attrition mill comprising a mill body and an impeller characterized in that wear prone surfaces of the mill body are lined with an elastomeric composition having a hardness of less than 60 shore A, and preferably a hardness of from 30 to 45 shore A, most preferably from 35 to shore A.

In a preferred embodiment the rotor of the mill has a tip velocity in excess of 13 metres per second and more preferably in excess of 15 metres per second.

Desirably the wear prone surfaces of the rotor are also covered with an elastomeric composition having a hardness of from 30 to 60 shore A, and preferably from 30 to shore A.

It has been hitherto believed by those skilled in the art that maximum wear resistance in an attrition mill could be obtained by using hard chrome plated steel or ceramic linings on the wearing surfaces. It has also been proposed to coat wearing surfaces with a polyurethane resin bu, the polyurethanes used had a hardness of approximately 80-90 shore A and were believed to be less wear resistant than ceramic L l -u PUICL*I~I~- I i II i linings. Although it has not hitherto been practiced to provide attrition mills surfaces with a rubber composition to reduce wear, rubber compositions have been used to reduce grinding wear in other grinding equipment but such compositions were much harder (in excess of around 60 shore It was therefore surprising that by using a significantly softer rubber in an attrition mill sufficient reduction in wear was possible to allow rotor tip speeds in excess of 13 metres per second, and to enable mill capacity to be increased beyond 1000 litres.

The invention will now be more particularly described by way of example only with reference to an embodiment.

With reference to figure 1 there is shown a schematic drawing of a mill a, t according to the invention, partly sectioned.

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The mill is viewed in side elevation and comprises a mill body 1 having a generally cylindrical side wall 2 provided with an inlet port 3 and having inlet flange 4 Sadjacent inlet 3. At the opposite end of the mill body is a discharge flange 5 adjacent an outlet 6. An impeller shaft 10 extends axially through inlet flange 4 at gland 11 and is supported by bearings 12 and 13. The shaft 10 is rotated by a motor via a drive It f train (both not shown).

a a Shaft 10 is fitted with a series of radially directed impeller disks 14 each of a which, when viewed in the axial direction (see Figure is provided with equiangularly spaced openings Figure 2 shows a detail of the outlet end of the mill which in the embodiment shown includes a cylindrical displacement body 20 having an outer end plate 21 and displacement body end plate 22, defining an outlet chamber 23 communicating with outlet 6 and having a perforated or slotted screen 24.

The impeller has a rotor disk 16 closely spaced adjacent displacement body end plate 22 and rotor fingers 26 which extend axially from rotor disk 16 adjacent to and spaced from screen 24. The parts described are generally of steel construction.

-6- Although not visible in the drawings, the interior surface 25 of side wall 2 is coated with a rubber composition having a shore A hardness of from 30 to 45, and more preferably 35 to 40. A composition of hardness approximately 60 shore A is used to coat the exposed surfaces of each of grinding disks 14, rotor disk 16 and rotor fingers 26. Slotted screen 24 is also desirably coated with below 60 shore A rubber composition provided that satisfactory openings are maintained in the slotted screen.

The interior surface of the inlet and outlet pipe may also be rubber coated.

The preferred rubber coating composition is obtained from Beltreco Pty Ltd is marketed under the trade name "Rema-line", and has a hardness of from 34 to 36 shore A.

The rubber coating has a thickness of from 20 mm to 40 mm and preferably r 30-35 mm and is adhered by means of a resin glue, by hot press forming around the impeller or other parts, or by any other suitable means.

In the preferred embodiment of the invention the mill body has a capacity of 3000 litres. The rotor is driven at an impeller tip speed in excess of 15m/sec and the mill is able to operate at a capacity in excess of 75 tonnes per hour when fed with ore in a water slurry to which is added a grinding medium having a particle size of less than 6mm. The grinding medium may for example be a silica grinding medium.

SCC Other successfully used grinding mediums are glass beads and the ore itself. Under those conditions the mill acts as an autogenous or semi-autogenous mill.

In comparison with a mill having chrome plated steel parts, wear in a mill coated internally with the comparatively soft (30 to 45 shore A) rubber as described herein, is subject to little wear.

As will be understood by those skilled in the art from the teaching hereof the invention is also applicable to mills having other form and construction.

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Claims (14)

1. An attrition mill comprising a mill body and an impeller characterized in that wear prone surfaces of the mill body are lined with an elastomeric composition having a hardness of less than 60 shore A.

2. A mill as claimed in claim 1, wherein the hardness is from 30 to 45 shore A.

3. A mill as claimed in claim 1 or claim 2 wherein the hardness is from 35 to shore A.

4. A mill as claimed in any one of claims 1, 2 or 3 wherein the rotor of the mill has a tip velocity in excess of 13 metres per second. S 5. A mill as claimed in claim 4, wherein the tip velocity is in excess of 15 metres per second.

6. A mill as claimed in any one of claims 1 to 5 having a capacity in excess of C 1000 litres.

7. A mill as claimed in any one of claims 1 to 6, having a capacity in excess of 2000 litres.

8. A mill as claimed in any one of claims 1 to 7, having a capacity in excess of 3000 litres.

9. A mill as claimed in any one of the preceding claims wherein the wear prone surfaces of the impeller are covered with an elastomeric composition having a hardness of from 30 to 60 shore A. A mill as claimed in claim 9, wherein the wear prone surfaces are covered with an elastomeric composition having a hardness of from 30 to 40 shore A. i 1 -8-

11. A mill as claimed in claim 9 or claim 10, wherein the wear prone surfaces include grinding disks, rotor disks, rotor fingers and slotted or perforated outlet screen.

12. A mill as claimed in any one of the preceding claims having inlet and outlet pipes, the interior surfaces of the pipes being coated with an elastomeric composition having a hardness of from 34 to 36 shore A.

13. A mill as claimed in any one of the preceding claims wherein the elastomeric composition lining is from 20 to 40 mm thick.

14. A mill as claimed in any one of the preceding claims wherein the elastomeric SO 10 composition lining is from 30 to 35 mr' o thick. A mill as claimed in any one of the preceding claims wherein the elastomeric composition is adhered to the mill body and/or other wear prone surfaces by a resin glue. .16. A mill as claimed in claims 1 to 14, wherein the elastomeric composition is 15 adhered to the mill body and/or other wear prone surfaces by hot forming around the mill body and/or other wear surfaces.

17. A mill as claimed in any one of claims 1 to 14 wherein the elastomeric composition is adhered to reinforcing steel work and bolted to the mill body and/or other wear prone surfaces. -9-

18. An attrition mill substantially as hereinbefore described with reference to the accompanying figures. DATED this 8th Day of December, 1995 MOUNT ISA MINES LIMITED and ERICH NETZSCH GmbH CO HOLDING KG Attorney: IAN T. ERNST FelloW Institute of Patent Attorneys of Austra. a of SHELSTON WATERS *C I c c 1 .Ci <C i_ ABSTRACT An attrition mill comprising a mill body and an impeller (10) wherein wear prone surfaces of the mill body are lined with an elastomeric composition having a hardness of less than 60 shore A. I i I