Software Modeling Cuts SAG Energy Consumption at Cortez Gold
By AbdulMajeed Aziz and Mike Mosser



Cortez Gold site location map
Ore size reduction in mining operations is a significant contributor to energy consumption and mineral production costs. In fact, more than 50% of the total energy consumed in production of copper, gold and other mined commodities is spent on size reduction, and this energy intensity is exacerbated as plants process higher tonnage of ores of lower head grade. Semi-autogenous grinding (SAG) mills account for a large portion of the overall grinding energy input in mineral processing.

Operation of existing SAG mills can be improved by conducting a plant sampling campaign using a flowsheet simulator to examine the flow rates and size distribution of various streams and implementing these findings in the plant. Many plants also routinely apply optimizing control strategies to improve mill operation. A new method that examines the design of SAG mill internals and improves the design through process simulation and modeling is promising large energy savings in SAG mill operations.

The U.S. mining industry operates approximately 80 SAG mills domestically. Depending on the mill size, SAG mills draw from 2,680 hp to 22,800 hp. The product from the mill is further reduced in size using pebble crushers and ball mills. Typical gold and copper ores require energy between 2 and 7.5 kWh/t of ore to reduce the particle size. It is not uncommon for a SAG mill operation to draw ±50% of nominal energy over a weekly period. Such variation in energy consumption is attributed to ore geology, as SAG mills are expected to adapt to changes in ore geology, posing a constant obstacle for mill operators.

A research team led by Dr. Raj Rajamani and Dr. Sanjeeva Latchireddi at the University of Utah is pursuing improved energy efficiency of grinding in SAG mills by means of new shell and pulp lifter design, and process optimization through simulation and modeling of the SAG mill process.


Figure 1: Cortez SAG mill crash-stop photos. Packing and slurry pool formed by old design (l) decreased in new lifter design (r).
The Mining Portfolio at the U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy (EERE) has been supporting this research project since 2003 for the specific purpose of improving energy efficiency and productivity in SAG mills. The research project is conducted in partnership with the U.S. mining industry and includes the participation of Cortez Gold Mines, Kennecott Utah Copper Corp., Process Engineering Resources Inc., and others.


Figure 2: Millsoft charge flow simulation for Cortez Gold’s SAG mill. Charge motion with
26 pairs of existing low-lifters (l) showing a very high charge motion profile, and new lifter
design (r) showing a robust, yet, nondamaging motion profile with the use of fewer lifters.
In 2003, Cortez Gold Mines, located in northeastern Nevada, USA, encountered problems of increasing plant work index, SAG-limiting mill feed, and everincreasing maintenance requirements of the SAG mill causing excessive mill downtime. Cortez actively participated in the university’s project by allowing the research team to gather operating data,apply it to their pilot mill to optimize parameters, and conduct a full-scale test on a 26-ft SAG mill to increase energy efficiency and their bottom line.

Julius Stieger, mill superintendent at Cortez Gold Mines, played a key role by facilitating between Cortez’s mill operations and the research team. According to Stieger, the main problems encountered in Cortez’s SAG mill operation were the number of unscheduled shutdowns required to repair broken liners, resulting in fluctuations in mill throughput. In the first quarter of 2004, the research team did a number of crash-stop studies at Cortez to understand the slurry transport and grinding performance of the mill (Figure 1). In particular, the ability of the pulp lifter to fully discharge all of the fine pulp generated in the mill was examined in several ways.


Figure 3: SAG mill power and operating work index trends.
The University team used two software packages, Millsoft and FlowMod, to tackle ore breakage and slurry transport issues. Millsoft is capable of simulating charge motion in the mill, and FlowMod calculates the slurry flow through the grate and pulp lifters. Additional information on slurry flow through grate and pulp lifters was provided by a pilot-scale grinding mill study conducted at the University of Utah. Based on these data, the two models were fine-tuned to fit the Cortez SAG mill (Figure 2).

In the summer of 2004, a new shell lifter design was presented to Cortez and ultimately installed in the SAG mill in September. According to Stieger, “At first we were back on the learning curve…we learned how to operate the SAG mill with the new set of lifters. It was not easy, but it paid off.” The SAG mill was drawing approximately 236 kW less power—a 10% energy reduction— while maintaining the same level of production by December 2004.

Fabricated by Norcast, the new single shell lifter unit weighed 3,000 lb more than double the weight of the previous design at 1,200 lb. Because of the added weight and other configuration and design differences from the conventional lifters in use at the time, the mine had exercised caution in the installation of the new lifters, and was even uncertain about the inching drive’s capability to handle the extra weight. The plant’s liner handler was upgraded from 3,000- to 4,000-lb capacity for safety and extra handling capacity and the research team and mill designer assured Cortez that the SAG mill could now handle an additional capacity of 65 tons. Calculations showed that although an extra 100 hp was needed to start the mill and motor energy consumption increased slightly, net energy savings outweighed these increases.

Although each lifter weighed 250% more than previously installed lifters, 33% less lifters were needed for installation. This was after Cortez’s decision in 2003 to eliminate every other lifter row to minimize packing and maximize charge lift and mill volume. The lifter relief angle provides for optimal ball strikes, which decreased noise around the mill area.


Figure 4: SAG mill speed before and after shell lifter replacement on Sept. 2004.
As shown in the accompanying table, results obtained from the tests conducted at Cortez indicate significant energy and cost savings.

In the first month, extreme cycling of feed rates to the SAG mill still occurred and operators had to fine-tune their parameters to optimize power consumption. After optimizing parameters, electric energy consumption, on average, decreased in the range of 0.3–1.3 kWh/t. Actual SAG mill power drawn, on average, decreased by 230–370 kW or 10% with the implementation of the new lifters (Figure 3).


Figure 5: SAG feed rate (t/h) from January 2004 to August 2005
The SAG mill ran slower following installation of the new shell liner. As seen in Figure 4, the mill operated at 11.3 rpm on average in the eight months leading up to lifter relining. After the new lifters were installed, average speed dropped to 10.5 rpm for the next seven months. The drop in rpm is conclusive proof that power savings were realized in the circuit.

Also, recirculation of material to the cone crusher was reduced by up to 10% as a result of more efficient grinding of critical size material in the mill. Cortez’s Stieger said the savings in operating costs totaled $0.023–$0.048 per ton milled.

The feed rate trend during the same period of operation is shown in Figure 5. It is not unusual for feed rate to decrease toward the end of liner life, and as a new set is installed, the feed rate starts to increase. In this case, Cortez operators had to learn the behavior of the SAG mill while operating with the radically new design of liners.


Figure 6: Net effect of SAG power reduction and tonnage in mill.
The net effect of the new lifter design on SAG power and tonnage through the mill is reflected in energy savings shown in Figure 6. The average energy consumed per month from January to September 2004 is 6.5 kWh/t compared to 6 kWh/t, the average consumption from October 2004 to June 2005. The energy savings amounts to 7.7% sustained over a nine month period.

After six months of operation, Cortez estimated that the new lifter design decreased steel degradation to 57 kg per day in the SAG mill, compared with 81 kg per day from the old design. The optimized particle trajectory and better locking of the charge between lifters prevents impact and abrasion damage. Due to these changes, downtime from cracked shell plates, broken lifters or leaky bolts was eliminated. The lifetime of this lifter set lasted 11 months (October 2004 to August 2005). At the end of set life, Cortez re-installed a new set of lifters and continued using this process.

SAG mill simulation and design optimization were successful for Cortez, where an estimated $1.5 million will be saved annually through reduced operating costs and unscheduled mill shutdowns. Due to the tremendous success of this research, Cortez developed a new pulp lifter design that was installed in August 2006. DOE regards this project as a significant success with potential cumulative energy savings of up to 11.6 x 1012 Btu over a 10-year period when applied to 50% of the installed SAG mills in the United States This figure could increase if more mines begin or restart production.

The case study performed at Cortez is applicable to SAG mills up to 40 ft in size. The design of shell and pulp lifters could be performed using Millsoft and Flowmod modeling software at a number of mine sites to reduce downtime, grinding steel losses and energy consumption. It is believed that energy savings could reach up to 20% in larger SAG mills.



AbdulMajeed Aziz is an energy consultant with BCS, Inc., a Washington, D.C.-based energy-focused management consulting firm, where he supports corporate and program strategy development and execution to EERE programs. Mike Mosser is a project manager for the U.S. Department of Energy's National Energy Technology Laboratory.