68 March/April 2008

Heritage

Historical ball mill migrates to Australiaby Peter Tilyard, FAusIMM

A note from the Heritage Committee

This is the second in a series of Heritage articles intended to examine how operations, techniques or experiences from the past may have relevance in a modern context. In this article Peter Tilyard, FAusIMM refers to the evolution of gearless drive mills and reveals how a pioneering ball mill will deliver competitive performance well into its third decade and beyond. Peter is Group Metallurgist with Oxiana Limited. He was Mill Manager at Bougainville Copper in 1985 when the gearless drive No.13 ball mill was installed.

In his article Chasing Ghosts in our Jan/Feb edition, Paul Gardner AAusIMM examined the history of silicosis in Australian mining and asked, “Do these devils still exist, are there new devils to be faced in the future, or am I just chasing ghosts?” As details for Paul were omitted from that article, we note that Paul is a Principal Consultant with AMC Consultants and has over 40 years experience in underground mining, with ‘hands on’ expertise in all aspects of ventilation for underground mines and tunnels. He as conducted numerous mine ventilation courses for university undergraduate studies, as well as ‘tailored in house’ training for engineers and ventilation officers.

Articles for this series are being sourced by our reinvigorated Heritage Committee under the Chairmanship of The AusIMM President Peter McCarthy. The other members of the committee at present are Bob Kininmonth, Ray Boyle, Rod Elvish and John Dunlop. Members interested in joining the committee are invited to contact Peter at pmccarthy@amcconsultants.com.au.

The Editor intends to continue this series of regular heritage articles in future editions, and members are encouraged to submit material.

Gearless drives for large SAG and ball mills are now common place in grinding/flotation plants. The

Lumwana plant of Equinox Minerals Ltd, currently under construction in Zambia , is an excellent example1. The 18 MW SAG mill is designed to mill about 60,000 tonnes of copper ore per day.

The details of the Lumwana mills are as follows2:

• SAG Mill: 11.55 m dia.x 6.1 m (38’ x 20’) with 18MW gearless mill drive. Mill provided by FFE Minerals (now FL Smidth) and the gearless mill drive by Siemens.

• Ball Mill: 3.52 m dia.x 12.2 m (26.5’ x 40’) with 16 MW gearless mill drive. Mill provided by FFE Minerals (now FL Smidth) and the gearless mill drive by Siemens.

A key (metallurgical) feature of these drives is the variable speed which, at many operations, underpins some very sophisticated process control (such as the Metso Minerals CISA

OCS© Expert System). Fortuitously the acceptance of ring motors in wet grinding applications and the switch from the fine crushing/single stage ball mill circuits (typified by Bougainville Copper) to SAG mill circuits coincided. This applied especially to high tonnage copper operations. However at several of the earlier operations (OK Tedi for example) the SAG mills were fixed speed, with ring gear/pinion drives.

Above and right: Equinox Minerals’ Lumwana grinding circuit nder construction in Zambia showing gearless drives for both mills

In my office I have a photograph of the 6.5 m dia. x 7.3 m (18’ x 24’) No.13 ball mill at the former Bougainville Copper Limited (BCL) mill. This ball mill, commissioned in 1985, was

69March/April 2008

Heritage

Gearless drive ball mill at Bougainville Copper Limited (1985)

Original 1981 installation of Sydvaranger ball mill showing gearless drive

Installation of Sydvaranger ball mill at Century

notable for being the second only application of a gearless drive in a wet milling environment and the first in a porphyry copper operation. The record of its installation and the metallurgical benefits it bestowed are well documented3. The motor was small (6 MW) by today’s ring motor standards. Although something of a novelty in mainstream mineral processing in 1985, ring motors were common in the cement industry long before this; the first installation was at a cement plant in Rhordorf, Germany, in 19704.

The No.13 ball mill at BCL was preceded in a wet application by the gearless drive installation at Sydvaranger, an iron ore plant in Norway. This was an 8.1 MW motor on a 6.5 m dia. x 9.65 m (21’ x 32’) ball mill, which was commissioned in 1981. A 1995 publication describes this installation5 and has references to other sources of information6.

What prompted the writing of this article was the discovery by the author that the “historical” Sydvaranger ball mill had been purchased by Zinifex and shipped to Australia for installation at the Century mine. Zinifex have

undertaken a refurbishment and upgrade program of the mill and are installing the unit as a second primary ball mill in the existing SAG/ball mill circuit. The mill will be used to improve the primary grind product sizing which in turn will enhance flotation recovery. Its significant role in grinding circuit development over 25 years ago is probably not widely known outside Scandinavia. The mill is expected to be commissioned in early 20087. The photograph (right) was taken in November 20078. ■

1 Photographs courtesy of Equinox.2 Personal communication Mr Robert Rigo, VP Project Development, Equinox Minerals Ltd.3 Plavina, P. and Clarke, G. The selection, commissioning and evaluation of a large diameter, variable speed mill at Bougainville Copper

Limited. 13th Congress of the Council of Mining and Metallurgical Institutions and the Australasian Institute of Mining and Metallurgy, 1986.

4 www.industry.siemens.com.5 Milling Iron, World Mining Equipment, May 19956 Forsund, B. et.al.Sydvaranger’s 6.5 dia. X 9.65 ball mill. XVI International Mineral Processing Congress, Stockholm, 1988.7 Personal communication Jason Whittle, Group Manager Business Improvement, Zinifex Limited.8 Photograph courtesy of Zinifex.