Published Jan 21, 2016

Video: Building the world’s largest HPGR

Metso’s HRC™3000 is a new generation high-pressure grinding roll that eliminates issues found in traditional HPGRs, allowing the mining industry to reach a new level of energy efficiency.

Facts and figures
The ore at Morenci is a porphyry copper deposit that has a Bond work index ranging from 11.5 – 19 kWh/t and an ore hardness with an Axb value of 45 - 67.
The Metcalf concentrator started operation in May of 2014. The plant is designed for an average of 63,500 tpd (70,000 stpd) , increasing the total Morenci concentrator complex production to a nominal 113,400 tpd (125,000 stpd).
The HRC™ HPGR is fed with the secondary cone crusher product and has a F80 of 26 mm and a top size of 50 mm. The HRC™ HPGR is in a closed circuit with an 8 mm wet screen, and feeds two 7.3m (24 ft) ball mills.

Due to diminishing ore grades and rising energy costs, the mining industry has been driven to find more energy efficient solutions. Freeport-McMoRan Inc. (FMI) is no exception. In developing their new Metcalf concentrator at the Morenci mine in Arizona, USA, FMI collaborated with Metso to create a revolutionary comminution circuit that challenges traditional expectations.

Aiming for maximum energy efficiency

Crushing circuits are known to be more efficient than traditional SAG milling circuits. However, in large operations, crushing circuits tend to have a greater number of lines and are more challenging to operate. On the other hand, SAG milling, while less efficient, tends to be simpler to operate and require less ancillary equipment.

The goal at the Metcalf concentrator was to develop a highly efficient high-pressure grinding roll (HPGR) crushing circuit capable of processing the total plant capacity. In addition, this HPGR would not simply be a larger scale version of what was currently available to the market, but would seek to eliminate some inherent concerns typically associated with traditional HPGRs, including skewing and edge effect.

The design concept came to fruition in the Metso HRC™3000, the largest HPGR in the world with a total installed weight of 900 tons. The HRC™3000 includes 3.0 m diameter by 2.0 m wide rolls and a total installed power of 11,400 kW. Depending on the application, the total capacity of this machine can exceed over 5,400 tph of ore.

Arch-frame eliminates skewing

Prior to the design of the HRC™3000, the Metso design team was tasked with developing an HPGR to meet the specific needs of hard rock mining applications. After a review of the existing technology, it became clear that certain inherent design problems such as roll skewing, edge effect and uneven roll wear needed to be eliminated to ensure the concept was successful.

The initial concept of the HRC™ HPGR began with what would become the patented Arch-frame, which mechanically absorbs unbalanced loads in order to eliminate downtime caused by skewing. Skewing is a condition where the axes of the rolls do not stay parallel due to uneven feed distribution. With the HRC™ HPGR, the two sides of the Arch-frame are mounted into the base frame with pins. Hydraulic cylinders at the top of the frame apply the crushing force. The cylinders only need to apply roughly half of the required force at the rolls due to the mechanical advantage of the pivoting Arch-frame. This idea was based on how a nutcracker uses a mechanical lever to multiply the crushing force.

In addition to the Arch-frame eliminating downtime caused by skewing, this feature also allows for the use of a flanged roll design. With this design, one roll includes a set of flanges, which are bolted onto the side of the roll. The flanges are designed to combat edge effect, a problem with traditional HPGRs in which comminution is reduced at the edge of the roll. Because the flanges are bolted onto the roll, the flanges move in the direction and speed of the ore and therefore pull material into the crushing zone. This is in contrast to a traditional cheek plate arrangement in which stationary cheek plates are mounted near the edge of the rolls.

Flanges enable even pressure distribution

Metso performed a series of tests on a lab scale HPGR which was fitted with pressure sensors embedded in the roll. When operating with traditional cheek plates, the pressure at the edges was much lower than the pressure at the center. This corresponds to the region of the roll which would generally produce a coarser product. Conversely, when flanges were installed, a much more consistent pressure profile across the full width of the roll was observed, indicating the full width of the roll is utilized for crushing.

It is important to note, that when crushing ore there is an optimum pressure for a given feed. Below this optimum pressure, less breakage will occur, while above the optimum pressure the energy efficiency will decrease. Therefore, it is important to have consistent pressure across the full width of the roll, so that an optimal pressure can be applied to the full bed of material. In the case of the traditional cheek plate design, the total pressure to the system is commonly raised to increase the amount of breakage at the edges of the roll. However, this results in higher pressure being applied to the center of the roll, leading to wasted energy and added wear in the center area of the roll. In addition, the higher localized pressure associated with the cheek plate design needs to be considered when selecting the stud hardness and composition in order to prevent stud breakage. The results of this lab test gave the engineering team the confidence to proceed with the flanged HPGR design on the pilot scale unit.

Pilot plant confirms lab findings

Prior to installation of the HRC™3000, a pilot scale plant was installed at the Morenci Concentrator to use as a proving ground for the HRC™ HPGR.

The pilot plant included an HRC™ HPGR with 750 mm x 400 mm rolls, a primary wet screen, and a secondary wet screen. The HRC™ HPGR was fed by the primary and secondary screen oversize. The primary and secondary screen undersize was then transferred to the downstream process. The capacity of the plant varied depending on the circuit configuration, but in most conditions it could process approximately 50 - 70 tph. The F80 to the HRC™ HPGR varied significantly and ranged from 11 mm to +16 mm.

A series of twelve process surveys were performed at the pilot plant to better understand how the flanged roll design affects the performance of the HRC™ HPGR circuit. These surveys, identified as the edge effect testing series, showed that in all conditions and pressures, the flanges clearly provided more breakage across the width of the roll and increased the throughput of the HRC™ HPGR relative to a traditional cheek plate HPGR design. On average, the flanges were shown to reduce circuit specific energy by an average of 13.5% and to lower circulating load by approximately 24%, while increasing the specific throughput of the machine by 19%.

In addition to test the design, the pilot plant testing aimed to provide a better understanding of the performance of the circuit, and to give the plant personnel experience operating and maintaining an HPGR circuit.

From problem-solving to successful installation

The HRC™3000’s capacity is approximately double the capacity of the largest preceding HPGR in operation. It was unknown what issues would arise when scaling up to a machine that is over 50 times larger than the design prototype. In order to ensure the proper design of the Arch-frame, a series of structural analyses were performed prior to the final design of the HRC™3000.

While the new machine concept eliminated some traditional HPGR design concerns, it did introduce new design considerations. For example, the interface between the pivoting Arch-frame/roll assembly and the stationary dust enclosure and feed chute needed to be carefully designed. An integrated tramp bypass system was developed in order to eliminate the requirement for additional infrastructure typical of HPGR installations.

Installation of the HRC™3000 at the Metcalf concentrator started in September of 2013. For the installation of the HRC™3000, Metso provided the installation crew and field service supervision. This allowed for very close communication between the engineering team and the installation team.

The size of the large components needed to be considered for the installation phase of the project. Transporting the shaft from the laydown yard to the Metcalf building required coordination with site personnel to arrange for the use of the truck needed to move the 97 ton component, as well as the use of the overhead crane needed to lift it.

A machine as large as the HRC™3000 requires a significant infrastructure to support it. In multiple cases, the actual dimensions between the structural steel and the machine were different than originally designed. In some cases, the remedy was not a simple solution. For example, when installing the hydraulic cylinders, the amount of room between the steelwork was not enough to install the cylinders as originally planned. A different lifting fixture had to be designed to move the cylinders into place. Moreover, maintenance had to be carefully considered in order to safely access and handle the large components. A 20 ton gantry crane was installed inside the steelwork tower to handle the hopper and feed guide plates. In addition, 0.5 ton robotic arms are used to handle the maintenance for the edge blocks and flange segments.

Promising results

The HRC™3000 is to date the largest HPGR ever put into operation. At the time of writing, it has operated for over 11,000 hours. During that time, a total of 46,100,000 tons have been crushed by the HRC™3000 and the HPGR circuit has processed over 30,600,000 tons of ore.

The operational benefits of the flanged roll design, first tested on the pilot plant, were shown to exceed predictions on the HRC™3000. Table 1 shows a comparison of the predictions based on the pilot plant surveys versus the actual operating results at the HRC™3000. The higher-than-predicted machine throughput and increased particle breakage resulted in higher circuit capacity and it decreased circuit specific energy. In addition, the flanges have provided an even pressure distribution across the width of the rolls, which has allowed for installation of harder studs and ultimately has increased the life of the rolls.

Table 1 – Operating results

 

Predicted (based on flanged pilot plant tests)

Actual*

Operating gap (mm)

99

95 – 125

Specific throughput (t∙s/m3-hr)

276

310 – 360

Circulating load (%)

58 – 85

40 - 60

Specific energy (kW∙h/mt)

1.5

1.10 – 1.25

* Values based on typical operating pressure of 3.0 N/mm2

Additionally the HRC™3000 provides circuit flexibility compared to a traditional milling circuit. Both the speed and pressure can be changed online to meet the changing needs of the downstream circuit. The pressure can be increased, for example, if the plant is processing a harder ore. The HRC™3000 is capable of running at 30% nominal speed, which effectively reduces its capacity by the same amount. Under normal plant operations, the single HRC™ HPGR feeds two 7.3m (24 ft) ball mills. During times when one of the ball mills is offline, the plant can still operate at reduced capacity by turning down the HRC™ HPGR and feeding only one of the ball mills.

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