SAG Mill Success—Start with the Basics

SAG Mill Success—Start with the Basics
New products and techniques proliferate for maintaining and improving mill performance,
but careful planning at installation may be the most effective tool of all

SAG mills are often flexible enough to accomplish the same size reduction provided
by two or three stages of crushing and screening, a rod mill, and some or all of the
work of a ball mill.

With units available in ratings up to 35,000 hp, capable of supporting a grinding line that can output more than 100,000 mt/d, SAG mills have played a steadily growing role in mineral processing. To get an idea of the extent to which this technology has been accepted by the mining industry, it’s only necessary to look at a brief sampling of recent contract awards from major mill suppliers.
• FFE Minerals Brazil was awarded a contract to supply a 26 x 16-ft, 6,500-hp SAG mill—as well as a 21 x 32.5-ft, 9,600–hp ball mill—for CVRD’s Paragominas bauxite mine in northern Brazil.
• Metso Minerals provided a 5,400-kW SAG mill along with technical services, spare parts and supervisory services for erection and commissioning to Kunming Iron and Steel Group Co. Ltd (KISCO) for its Dahongshan mine in Yunnan province, China. Delivery was completed in the second quarter of 2006. Metso Minerals more recently announced that it will supply grinding and flotation equipment to the Gaisky copper mine of Ural Mining and Metallurgical Co. The order, valued at more than €20 million, will be completed by the end of 2007 and comprises two 5,000 kW SAG mills, two 4,100-kW ball mills and 17 RCS (Reactor Cell System) flotation cells, complemented by installation and commissioning support services.
• Wedgetail Exploration purchased a SAG mill from Thyssen Krupp Engineering Polysius Division for installation at its Nullagine gold project in Western Australia. The mill, which measures 6.1 m in diameter by 7 m long, is powered by a 3,500-kW drive.
• Outokumpu Technology was selected to provide a 32-ft-diameter, 20-ft-long SAG mill with 12,000 kW of installed power to First Quantum Minerals’ Frontier project in the Democratic Republic of Congo. The trunnion-supported mill features variable speed capability, allowing greater production flexibility and the ability to cope efficiently with large ore variations. The Frontier mill is notable because it highlights an approach toward over-coming what may be SAG technology’s major challenge—load stabilization and control. A credible argument can be made that the flexibility offered by SAG mills may also be their greatest weakness: A mill that is touted to be capable of grinding anything that is fed to it, might therefore be fed a mix of everything—ranging from vastly different chunk sizes to variable
rock hardness. Feed variations such as this, as well as over or underloading a mill, can result in less-than-optimum performance and make it difficult to achieve satisfactory mill control. In fact, at least in part because of the attention that must be given to the care and feeding of SAG mills, producers have begun to look increasingly favorably at highpressure grinding rolls as a viable option for grinding circuit design, either in combination with SAG mills or as a replacement, particularly when dealing with lowgrade, abrasive ore. There are a number of laboratory and software tools that can provide both preand post-installation solutions to SAG mill power and feed problems. As an example, Dawson Metallurgical Laboratories, based in Salt Lake City, Utah, USA, in association with the SAG Design Consulting Group, offers SAGDESIGN test work and consulting. The SAGDESIGN test is performed with a specially designed laboratory SAG mill and uses 7- to 9-kg (depending on ore specific gravity) of –38 mm ore. The SAG product is then used as feed to a Bond ball mill work index test. The results can be used to determine or confirm SAG (or AG) and ball mill energy requirements. Other engineering and laboratory firms offer similar services.

Variable Drive = Mill Efficiency
According to Outokumpu Technology, the advanced technology utilized in the Frontier mill’s Hyper SER Drive ensures that the mill can operate efficiently both above and below the synchronous speed of the motor, thus allowing better management of variable feed rate and feed competency. The design also is claimed to facilitate optimized handling of any short- or long-term variations in ore. Variable speed offers optimized trajectory for the charge during mill operation, minimizing unnecessary wear of the mill lining, according to Oskar Gustavson, Outokumpu Technology’s global technology manager for grinding. “The beauty of this customized system is that it is designed for optimal operation and can cope with pretty much anything. This is critical, especially at remote sites which can be prey to challenging situations such as voltage variations and thus unavoidable downtime. The Hyper SER Drive can transfer to fixed speed mode during under-voltages, then transfer back to variable speed when the supply voltage stabilizes, ensuring no costly mill stops occur.” SAG mill breakdowns are costly events for producers. As an example, early in 2006, Cia. Minera Doña Ines de Collahuasi had to make temporary repairs on the drive on the largest and newest of its three SAG mills at the Collahuasi copper mine in northern Chile, and then in mid-January 2007, finally began full replacement of the motor stator on the 21-MW gearless drive for the 40-ft-diameter mill. The company expected the mill to be out of action for approximately 65 days, representing about 30,000 mt of lost output. The need to monitor and control these mills has led to the growth of a significant sub-sector in mineral processing technology and product development.

Metso Minerals helped Anglo American’s Lisheen mine improve mill uptime by developing
SAG mill linings that can be replaced without halting production.

For example, the technical program for SAG 2006--the 4th International Conference on Autogenous and Semiautogenous Grinding, held in Vancouver, B.C., in September 2006, listed roughly 100 papers devoted to research, product development, maintenance and operation of this type of grinding equipment. Products for the care and feeding of SAG mills range from preventive maintenance- oriented items such as Eriez Manufacturing Co.’s SAG mill ball separator, which employs magnets to remove worn/broken media from mills before thetramp iron can cause problems downstream; to real-time monitoring devices such as Metso Minerals’ Electric Ear, which listens to the sound level generated by a SAG mill and provides an indication of load conditions within the mill. The Electric Ear can warn the operator of a low load level that might result in liner damage, or an overload condition that could interrupt production. Facing the risk of losing a significant amount of production in case of mill failure or downtime, producers are constantly on the lookout for new products and methods that have the potential for improving mill reliability and availability. Metso Minerals, for example, helped Anglo American’s Lisheen mine in Ireland improve mill uptime by developing SAG mill linings that can be replacedwithout halting production of zinc and lead concentrates (E&MJ, “Sectional Mill Lining Helps Keep Lisheen at Full Production,” Sept. 2006). The resulting design, according to Metso, provides optimum performance while enabling the linings to be quickly replaced during scheduled shutdowns. Lisheen opened in 1999 and produces some 300,000 mt/y of zinc concentrates and 40,000 mt/y of lead concentrates. Material is crushed underground, stockpiled and then conveyed to a 6.1 x 4.1-m SAG mill. It is powered by a 2.3-MW motor and runs with 8%-12% of the mill’s volume filled with 125-mmdiameter steel grinding balls. Incoming ore varies in zinc and lead levels and hardness. The SAG mill therefore operates at variable speeds. Normally, it would take about 60 hours to replace the mill’s entire lining in a single job—too long for a monthly shutdown. A cooperative effort between Metso and a mine task group was able to develop an approach that optimizes the life of the linings and staggers the work so that replacement of the feed, central and discharge sections of the mill’s liner are timed so that they need replacing in different months. End sections take about 15 hours to fit while the central section takes 36. Maximum life is gained from each section by monitoring the rate of wear and carrying out the replacement only when it becomes necessary. As mills grow larger, so do their support components. To illustrate, Eaton Corp.’s Airflex division recently developed what is claimed to be the world’s largest air activated drum-style clutch for use with grinding mills. The Airflex 76VC measures 8 ft (2.4 m) in diameter and is capable of transmitting 7.3 million lb-in. of torque with 9,500 hp at 200 rpm. Three of the units were shipped to a Newmont Gold project near Battle Mountain, Nevada, USA, with another order in the works for a customer in Chile Johnson Industries, which has offices in Canada and the U.K., says its TL series toggle lever caliper brakes are the largest known spring applied, hydraulic released caliper brakes in the world and are well-suited to accommodate very high braking torque requirements in applications such as SAG or ball mill installations. Features include a stackable design allowing 1 to 4 calipers to be mounted in a common frame and spring force compensation by toggle mechanism resulting in constant braking torque. Caliper shoes are attached with stainless steel shoe pins that incorporate countersunk lubricating holes, tapped pin-pulling holes, and a pin retention groove. Caliper levers incorporate a unique toggle mechanism to compensate for spring force and ensure constant braking torque. The system is powered by a hydraulic power unit incorporating a manifold fitted with valves to accommodate brake set, creeping and inching modes.

The Airflex 76VC drum-style clutch was designed to
handle the largest grinding mills currently available.

Focusing on Installation
With all the tools available for optimizing SAG mill performance, it might be possible to overlook what could be the most important element of all in achieving successful long-term performance: a careful, accurate and well-planned installation sequence. The trend toward installation of large, gearless drive mills amplifies the importance of this operation. At SAG 2001, Doug Farnell and Stephen Thompson, both principals in Farnell-Thompson Applied Technologies, Montreal, Canada, presented a paper titled Coordination of Mill, Motor and Plant Engineering for Large Gearless Mill Projects. In their words, “The successful execution of a gearless drive mill project requires that the mill vendor, motor vendor and plant engineer coordinate their design efforts to a much greater degree than has been the case for most traditional grinding mill projects. This coordination is critical to the design of the three major elements of the system—the mill, the motor and the foundation—so as to end up with a system that can be assembled, commissioned, and operated with as few surprises as possible.” The authors divide the necessary tasks into three main categories— basic design, operating and control logic design, and installation coordination— and focus mainly on basic design elements such as coordination of interfacing geometry/layout; structural design/analysis of mill, motor and foundation; static and dynamic stability analysis of the overall system; and coordination of services such as water, oil, air and power. Although they recommend against assigning technical responsibility for overall design of the motor, mill and foundation to any one specific entity, they do offer specific guidelines for improving the technical coordination required to achieve a successful installation. These include:
• Joint review by mill and motor supplier of foundation requirements, maintenance access and clearances, and installation planning prior to submission to the project engineer;
• Early exchange of data between mill and motor suppliers so that foundation drawings from both reflect the same geometry;
• Improving the timeliness of the system studies such that there is the opportunity to make any necessary design changes prior to fabrication;
• “Closing the loop” involving the mill vendor, motor vendor, plant engineer, and owner on control logic implementation through joint review of the final PLC logic; and
• Improved planning of the installation sequence, particularly in the area of motor alignment vs. bearing adjustment so as to avoid repeating any tasks.