1

Grinding Improvements at Mineral Park

Grinding Improvements at Mineral Park

Abstract

The Mineral Park Mine is an open pit copper-molybdenum mine located in northwestern Arizona, approximately twenty miles northwest of Kingman, Arizona. The concentrating facility has been regarded, with the expansion in 2011, as a plant capable of treating fifty thousand short tons of feed per day for the mixture of supergene and hypogene ores available. The grinding circuit consists of two 32’ x 14’ SAG mills and four 20’ x 28’ ball mills. As ores became harder with greater depth in the adjacent pit areas, however, the SAG mills became severely throughput limited.

The traditional treatment for similar properties is the installation of a recycle pebble crushing circuit. Because the cost of such an installation is quite elevated, and preliminary testing and research was inconclusive regarding the predicted throughput increases, management and technical personnel set out to find another solution to the problem. The result entailed a redesign of SAG lifters/liners and a nearly unique circuit change which was, at best, a difficult step that defied current grinding logic. This paper describes these changes, events leading up to them, and the results.

Introduction

The Mineral Park property, owned by Mercator Minerals Ltd., is an open pit copper- molybdenum mine situated approximately twenty miles northwest of Kingman, Arizona. Previous owners were Duval Corporation, Cyprus Mining Company, and Equatorial Mining. Mercator purchased the property from Equatorial in 2003.

A copper and molybdenum (moly) flotation concentrator to treat 25,000 tons per day was designed in 2007-2008. Construction was completed, and startup occurred in November and December of 2008. An expansion that approximately doubled the concentrator capacity was completed in October of 2011.

The plant now has two 72” jaw crushers (located at different points near the pit edge), two 32’ x 14’ SAG mills, four 20’ x 28’ ball mills, twenty-two 9,000 cubic foot rougher flotation cells, and a complete moly concentrating facility. Copper concentrate is shipped off the property in bulk by truck, and the molybdenum concentrate is shipped by truck in tote sacks.

Recent changes to the grinding circuit to achieve design throughput for the plant diverge considerably from traditional grinding methods. The methods of improving plant performance, reducing costs, and the ability to operate without a pebble crusher are summarized herein. The whole goal at Mineral Park mine is to maintain a successful plant operation. This is achieved by examining the economic role

2

of the grinding media and how it plays a critical role in the costs.

Throughput Issues

The current grinding circuit is made up of two phases. One SAG mill headlines each phase and in turn feeds a vibrating screen. Until March of this year, the undersize of the screen was split and pumped to two clusters of 26” cyclones, one cluster for each ball mill. The configuration is shown in Figure 1. The oversize from the vibrating screen is returned by conveyor belt to the feed of the SAG mill.

Figure 1.

It was commonly known by operating personnel that a few specific areas in the pit that exceeded the cutoff grade were somewhat harder to grind than normal, and at times the tonnage rate through the SAG mills would decrease dramatically when these ores were treated. In response to this, geology and metallurgical personnel began a program to predict ore hardness from blast holes so that ores could be blended for hardness in addition to metal value. The

hardness for blast-hole cuttings was measured using a shortened Bond Work Index test for ball mills. Hardness of the ores are measured in Kwhr/ton where it represents how many kilowatts per hour (energy) is used to break down one ton of ore. Upon original startup, the SAG had ore hardness values between 7-9 Kwhr/ton. Since the phase expansion and changes in the ore body, the hardness has increased to an average of 11 Kwhr/ton. The current goal is an ore blend with an average hardness of around 11 Kwhr/ton.

As the database with this information grew, then ore hardness could be predicted for each day, and SAG throughput could be easily correlated to the hardness information from each area of the pit. However, as mining proceeded, and mining depths increased, the various ores generally exhibited a higher work index than expected and were more difficult to grind. In just a few months, the mining department had little soft ore to blend with the harder material so that reasonable throughput could be maintained. The relationship of measured ore hardness to the average SAG mill throughput is shown in Figure 2.

Historically, the SAG mills performed more efficiently, and general throughput was higher when the mills had new liners and lifters despite the fact that the volume inside the mills was increased with worn liners. It was assumed that the ball/ore combination actually did much better at grinding hard material when lifted higher and dropped during the process. The worn liners/lifters gave more of a “rolling” effect. Evidence of this was visually displayed by pebbles on

3

the recycle belts that were more rounded when lifters were substantially worn. Conversely, recycle pebbles from a newly lined mill were essentially angular, resulting in evidence of improved grinding efficiency.

The pebble recycle load at this point averaged 5 - 10% of the mill feed tonnage. The traditional reaction to improve throughput would be the installation of a pebble crushing circuit with the thought of replacing the pebble tonnage plus some excess with new feed.

Two consultant groups, one with a successful computer model and the other with years of first- hand experience, were contacted and presented with the Mineral Park dilemma. Each came back with predictions that a ten to fifteen percent increase in throughput could be expected with a properly engineered and constructed pebble-crushing circuit. Although this information was attractive, it was decided to conduct some preliminary tests to confirm that at least the recycle pebble tonnage could be regained.

For the test period, the tonnage of pebbles rejected could not be completely replaced by new ore, as shown in Table 1. It became obvious that rapid ore changes could certainly have biased the results but, nevertheless, the actual benefit of a pebble crusher was in question to a point that a sizable investment was not attractive.

Table 1. (Pebble Drop Test)

Lifter/Liner Design

The SAG mills are constant speed, and each is powered by two - three thousand horsepower motors. Each mill rotates at 76% of critical speed. The lifter design that was used for several years displayed a taper of fifteen degrees from the vertical. (See figure 3.) This was projected to give plenty of lift to the ball/rock combination to maximize impact grinding.

As experience was gained with the Mineral Park ores, however, it was determined that a portion of the load lifted in this manner actually overshot the bulk of the overall mill load. In fact, some liner breakage issues were experienced at one point. The lifters were then redesigned with a thirty degree

Phase 1

Phase 2

Date

TPH Feed

TPH Pebbles

% of Feed

TPH Feed

TPH Pebbles

% of Feed

Pebbles In

Mar 1-4

906 63.6 7.0 981 58.2 5.9

Pebbles Out

Mar 6-11

893 54.1 6.1 915 42.7 4.7

4

taper and made thicker (16” for the 30⁰ versus 12” for the 15⁰.) This necessitated that each lifter be incorporated as part of the liner to match the bolt hole pattern of the mill shell. The intention of this redesign was to (1) direct impact of the lifted load back onto the entire mill load to maximize impact grinding, and (2) prolong the life of the lifter. Figure 3 displays the differences in liner/lifter configuration as well as the difference in action from the fifteen degree lifters to the thirty degree lifters.

Figure 3a. (15 degree lifter)

Figure 3b. (30 degree lifter)

Circuit Enhancement

The logical step, because the pebble crusher alternative was not attractive and excess power was available in the fine grinding circuit, was to advance material forward in the circuit. Thus, the SAG mill discharge was screened at a coarser size. This not only advanced material to the ball mills but also drastically reduced the pebble recycle tonnage.

It was possible to accomplish this because (1) the transfer pump was capable of handling up to 2-1/2” rock, (2) the ball mills could be charged with a coarser ball load, and (3) the screen undersize could be routed to the ball mills before being pumped to the cyclones. A new splitter box for each phase was fabricated, and the screen underflow transfer pump lines were routed to the splitter box on each phase.

Figure 5 shows the revised circuit.

5

Figure 5.

Discussion of Results & Conclusion

So far the changes to the grinding circuit at MPI have shown significant increases in the throughput per each phase. In fact, the economies of scale have improved since the changes in the grinding circuit and lifters/liners have been made. Table 2 below illustrates the total increase in throughput per phase.

Table 2. (Throughput WTPH)

Before Change

After Change

Phase 1 884 1,147Phase 2 885 1,214

The problem that MPI has encountered now is that the ore hardness is increasing with depth. In fact, the hardness has increased on average by 33% since startup. Table 3 illustrates the relationship to ore hardness.

.

Table 3. Ore Hardness Relationship

The operations have learned that by using a thirty degree lifter in the SAG mills, the grinding efficiency of the SAG mills is improved. Table 4. Illustrates the increase in tonnage with respect to the liner/lifter changes. The operations at MPI conclude that the 0 and 15 degree lifters caused the tonnage and steel balls to be projected against the side shell of the SAG mill. By implementing the 30 degree lifter, the grinding media falls more on the mass and continues to do so as the liners get worn down.

Table 4. (Liner/Lifter Changes)

Average TPH Before

Average TPH After

Phase 1 808 949Phase 2 849 927

Along with the liner design, opening up the grates and allowing bigger ore to pass on through the SAG vibrating screens have worked as an advantage. This allows MPI to reduce the load on the SAG, decrease the amount of recycled pebbles, while increasing the throughput on each phase.

6

Table 5 shows the total tonnage increase for liner/lifter, plus the grate openings, and how each change has contributed to the increase in wet tons per hour on each phase. Table 6 shows the difference in recycle pebbles before and after the entire circuit changes.

Table 5. (Liner/Lifter & Grate Opening)

Liner/Lifter Grate Opening

Total

Phase 1

141 122 263

Phase 2

78 251 329

Table 6. (Recycle,% of Total Feed)

Before Change

After Change

Phase 1 6.6 0.9Phase 2 5.5 1.0

Another advantage to the grinding changes has been the replacement of the hydrocyclones. A good metallurgist will tell you that by pushing the SAG discharge directly into the both ball mills, this has reduced the wear on the hydrocyclones and extending the life of them as well. All the while, the recirculating load has reduced dramatically from 464% to 198% on average per ball mill. This leads to improved cost effectiveness of the grinding stage. Most importantly, the throughput on average has increased by 30% with this change resulting in higher throughput.

With the load being directed into the ball mills, the efficiency of the hand grinds going to bulk froth flotation have improved as well. Especially in phase 1, hand grinds

showed lower than expected parameter measurements. MPI needs to achieve a grind of no less than 70% passing 105 microns. Since the circuit changes, as illustrated in Table 7, grinds have improved on phase 1.

Table 7. (%Hand Grind, 140 Mesh)

Before Change

After Change

Phase 1 65.3 69.2

Phase 2 70.8 73.8

Another impressive part of the entire circuit change has helped MPI operations increase their moly and copper recoveries. This increase in metals recovery is illustrated in Table 8 and Table 9 below.

Table 8. (Copper Recovery)

Copper Recovery

Before Change

After Change

Phase 1 78.6 84.8Phase 2 84.7 89.1

Table 9. (Moly Recovery)

Moly Recovery

Before Change

After Change

Phase 1 80.4 87.2

Phase 2 87.5 91.0

Looking Forward

As the operations continue to improve, there are many other challenges and considerations that MPI faces. The main

7

challenge facing MPI going forward is the increase in ore hardness with the change in pit depth. An ore hardness of around 13 Kwhr/ton is now commonly being fed to the SAG mills.

MPI personnel are continuously striving and exploring ways to increase tonnage in order to remain competitive because the comparatively low grade ore body necessitates higher throughput. Maintaining a throughput of at least 50,000 tpd is now a realistic target, but with the harder feed

material, a high efficiency crusher coupled with the opening of SAG mill grates is now much more economically attractive. This cone crushing circuit addition would allow crushed pebbles (say, -3/4”) to be reintroduced to the fresh SAG feed. The SAG mill grate panels could be opened further from 2” to 3”, allowing a much greater proportion of material to be recycled to the crusher. This combination would then allow a sizable portion of the SAG mill load to be transferred to the ball mill circuit at a reasonable size.