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PARTICLE SIZE INCREASE IN PAD 4B OPERATIONS TO IMPROVE PERMEABILITY CONDITIONS DURING THE LEACHING PROCESS

By: José Luis Alvis, Senior Metallurgist, y Manuel Aragón, Manager of Shared Services/Center of Excellence, Sociedad Minera Cerro Verde.


Abstract 

Over the years, the ore sent to the leaching process at Cerro Verde has had an increase in clay content and, therefore, in fines. The main effect is the decrease in permeability in the heap. Different controls have been applied, such as blending mine ore, increasing the particle size of crushed ore, on-line monitoring of groundwater levels and others, to avoid stability problems due to low permeability in the leach heap. 

As part of the operational management, it was decided to increase the crushed ore P80 that goes to the heap, from 13 to 16 millimeters, in order to improve permeability conditions and also minimize the effects of decrepitation that was observed after the first leaching cycle; the fine fractions can be doubled after the first leaching cycle at 200 days.

In this paper, we will show several options that were managed to address the fines/clay content, applicable to Cerro Verde's operating conditions; as well as the work done in laboratory and on site to support the change in particle size, such as permeability and agglomeration tests, monitoring of ore decrepitation and the impact on Copper Recovery. 

Introduction

Cerro Verde Operations

Since 2006, with the commissioning of concentrator 1, Cerro Verde has been primarily a grinding operation. Hydro circuit production represents about 8 to 10% of its total copper production.

In terms of Mine Planning, the ore supply to the Hydro Process comes from temporary stocks, secondary sulfides, and the Cerro Negro pit, which is oxidized ore. 

Therefore, the relocation of scooptrams to improve the quality of ore supplied to the heaps due to the blending of faces is difficult to carry out.

Leaching Process

The leaching circuit at Cerro Verde is composed of ROM and crush leach operations. The average total PLS flow rate is about 20,000 gallons per minute. 

Ore for pad 4B is agglomerated with raffinate and sulfuric acid at rates between 5 to 9 kg of acid per mt of ore. The material is then placed on pad 4B using a radial stacker. Table 1 shows the main characteristics of pad 4B.

The P80 of the crushed leach ore was 13 mm and irrigation is with driplines at 0.13 liters / (min.m2) as irrigation rate. 

The regular leaching cycle is 220 days. At the end, another lift is stacked on top to continue operations.

The content of swelling clays in the ore delivered to the Hydro process has been increasing. S Clay have a huge impact on permeability in the Heap Leach operation due to their high capacity to absorb water and expand, decreasing porosity and vertical conductivity. The final effect is ponding in areas where clay content is high.

The high content of fines and clays also affects the crushing plant and conveyor belt operation due to plugging events in chutes, conveyor belt overload, etc.

The swelling clay content in the ore supplied for Cerro Verde has increased significantly since 2017 from an average of 3% to 4%. Figure 2.4 shows the current trend:

In April 2017, the crushing plant was shut down for several days due to the non-delivery of ore for the Hydro Process because of a lack of ore containing less than 3.5% S Clay. 

With this background, several alternatives were evaluated to increase the process clay content while minimizing the effect of permeability loss. 

This high clay and fines content can also lead to stability problems in the pad, as there are sectors of high saturation that may eventually even liquefy after a significant earthquake occurs.

In addition, according to data from the geotechnical Cone Penetration Test (CPT), areas with contractive behavior and dominated by fines have been identified, precisely these areas correspond to fine/clay sectors.

Geotechnical events may require costly remediation such as solution pumping wells, installation of vertical and horizontal drains, wick drains, slope reconfiguration, construction of buttresses, etc. Figure 7 shows an example of the buttress built in the face of pad 4B. In general, a buttress can cost $50 per ton of material used for construction. For this construction, about $4 million dollars were required.

Activities Developed and Implemented

Alternatives for treating ore with a high content of clays and fines

In general, to support a safe operation in permanent leach pads, the following alternatives are available to deal with clays and fines: 

ν Increased particle size with decreased copper recovery. According to tests, around 2% in copper recovery can be lost for each millimeter increase in P80 in the first leaching cycle. 

ν Improve mine blending: not so easy to implement considering that Cerro Verde mine area needs to handle about one million tons per day for its processes.

ν Improve blending in the stockpile from primary crushing. However, if only high clay and high fine ore is received at some point, there is no additional coarse material to blend.

ν Modify the crushing plant configuration to screen and separate the fine fraction of ore and send it to another process.

ν Install additional drainage systems - with increased operating costs and no guarantee that they will be effective throughout the entire pad. Recovery is also an issue due to PLS short circuit. 

Considerations for increasing P80 

ν Effects on Cu recovery: Column tests determined that increasing the size of P80 by 1 mm decreases the predicted copper recovery by 2%.

ν The maximum mineral particle size for optimum agglomeration: the best approach is to perform agglomeration tests.

ν Decrepitation and fines migration: it is critical to determine the benefit of increasing the P80 in reducing fines generation due to ore decrepitation. Figure 10 shows the particle size curves for several samples taken in pad 4B; the fine content can be seen to vary from 10 to 25%.

There is no major negative effect on the Cu recovery of the ore supplied to Pad 4B, due to the accessibility of the ore. In the coarse fractions (sizes 1/2", 3/8" and 1/4"), it was observed that the fracturing is due to rock alteration, forming slabs. In addition, accessibility to Cu- and Fe-bearing mineralogical species is 99.7%.

Inaccessible grains are very small (5 to 25 microns) embedded in quartz.

Agglomeration Tests

It was determined that the agglomeration stage (adhesion of fines to coarse particles) is optimal up to a content in the +3/4" fraction of 8% (45% +3/8").

Figures 12 and 13 show the quality of the agglomerated ore in terms of fines attached to coarse particles, after several tests performed manually at laboratory scale. 

In addition, a sampling campaign of the agglomerated and stacked ore in the pad at different depths (top, middle and bottom of the pile) was carried out, and an excellent overall uniform distribution of particles was found at these different locations in the slope. This confirms what was achieved in the Metallurgical Laboratory, achieving good agglomeration up to a fraction of +3/4 inches as top size.

Permeability Tests

Permeability measures the ease with which the leach solution moves through the ore heap; there is a relationship between permeability and the grain size distribution of the ore within the leach heap. Experimental studies aimed at investigating these relationships consist of the systematic variation of statistical parameter values (particle size and compaction pressure) and the consequent evaluation of sample permeability. Several graphs were developed relating permeability to particle size determining the compaction that occurs when top lifts of ore are stacked. 

The test developed to determine ore permeability under increased confining pressure was performed using the flexible wall method. The test apparatus consists of a 6-inch diameter latex cylinder, bottom porous stone, base plate with a drain, top porous plate, reaction frame, and a loading device. Figure 4.1 shows the test apparatus. A sample of ore is first weighed and then deposited in the latex cylinder. Its density and initial moisture content are recorded. At constant permeability in the head, the test is started without load, usually in accordance with the test method. The axial load is increased to such a pressure that it simulates heap heights or stacked elevations above. The permeability test is repeated under new load. 

Experience with this type of testing can be carried out for different particle sizes for both the head and final riprap of ore that is stacked on a permanent leach pad.

When results are plotted, it is observed that by increasing the fraction +3/8" from 35 to 45%, permeability increases by almost 100%. As shown in Figure 16.

One of the most critical aspects to consider for these evaluations is the swelling clays content in the ore delivered to the Hydro Process and specifically for Cerro Verde it has been defined that for 30 to 35% +3/8", the maximum S Clay content is 3.5%. 

Other tests indicate that increasing the +3/8-inch fraction by 10% also allows and confirms that it is possible to increase the S Clay content up to 4.5% in the stacked ore. It was noted that the total clay content should not be changed.

Figure 17 shows that by increasing the particle size by 10%, the permeability increases by almost 100%, and this also increases the Swelling Clay content from 4 to 4.5%.

As complementary information, several drilling campaigns have been developed in pad 4B at different depths with different particle size distributions and the effect on permeability at different fine contents (-200 Mesh) has been recorded. By increasing the minus 200 mesh fraction from six to ten percent, the permeability decreases by almost 80 percent. This is shown in Figure 18, considering the fine fraction up to 22%, where the permeability is almost zero corresponding to a compaction pressure of five lifts of stacked ore.

Finally, it has been observed that the influence on permeability with respect to total clay content. For Cerro Verde, the maximum total clay content allowed to enter the process is 8.5%.

Decrepitation Follow-Up

During the leaching cycle, due to acid attack, ore decrepitates, i.e., fine content increases at a variable rate, depending on the mineralogy, accessibility, porosity in the ore. It was observed in pad 4B that the decrepitation rate is almost 100% in the fines fraction (-100 and -200 M), which means that, for example, if at the beginning of the Leach Cycle the -200 M is 9%, after completing the first Leach Cycle, the fines content in this screen is in the range of 18 to 20%.

Figure 20 shows the decrepitation rate comparing the particle size distribution at zero days and after 240 days of leaching.

Several auger samplings on the heap showed that the fines content, -200 M (-74 microns) after the first leaching cycle, almost doubles. The same effect was also verified for the -100M (-150 microns) fraction, and for the coarser screens, 3/8" for example, a decrease between 20-30 % was observed. 

For Cerro Verde's leaching operations, the KPI for -200M size is 9.5% at the beginning of the leaching cycle.

After the size increase from 30 to 40-45% at fraction +3/8", the decrepitation decreased by almost 50% as shown in Figure 22, which is quite lower than the records at lower values of P80.

This increase in P80 was achieved by opening the tertiary screening circuit, and particle size control was performed using an automatic sampling system in the final conveyor that feeds the leach pad.

Another important conclusion is that decrepitation in the ore starts between the agglomeration and stacking process, mainly due to the addition of acid in the agglomerate and the transport by conveyor belts.  This effect is lower in the ore already deposited in the heap.

Figure 23 shows that the main decrepitation effect occurs in the early stages of the leaching cycle, and samples obtained at higher leaching cycles are not significantly affected. 

Heap Leach Recoveries

Copper recoveries in pad 4B, at lift 15, average 1.7% less than the metallurgical model. Starting at lift 11, the particle size of the ore increased from 35 to 45 % +3/8".

It is important to consider that Cu inventory may be lost due to this increase in size. As shown in the previous section, recovery can be affected by 8 to 9 % when the particle size is increased by 10 %, so future work would involve carefully reviewing the best economic scenario for the P80 parameter in Leaching Operations.

Coarse/Fine Ratio - Control of Fines and Clays Content

The coarse/fine ratio, which is the ratio of tons of fines content referred to -M200 and -M100 mesh to tons of coarse represented in the +3/8" fraction, is the ore stacked permeability monitoring index in the pad. Based on the different tests described above, the following KPIs were established:

Conclusions

1. It is possible to achieve a good quality of agglomeration at 8% +3/4".

2. Permeability increases significantly when the +3/8" fractions go from 30 to 40-45%.

3. With an ore content of up to 5% swelling clay (SC) the permeability remains the same when the coarse fraction (3/4" and 3/8") is increased.

4. At 30 % +3/8" decrepitation almost doubles the fine fraction after the first leaching cycle.

5. When the coarser fractions are increased to 8% +3/4", or 45% fraction +3/8", the decrepitation in the -200 M size decreases by an average of 50%.

6. Decrepitation occurs mainly after agglomeration and when the ore is stacked on the pad.

7. Daily recording of the coarse/fine ratios allows the monitoring of permeability in the heap and identification of zones that could present percolation problem events.

8. Total copper recovery is affected by 1.7% after the first leaching cycle. However, the effect on Cu inventory losses should be assessed to define the best economic scenario for the leaching process.

Acknowledgements

Special thanks to Guillermo Canchis, Casey Clayton and Bill Sircy for all the support in the activities described in this document.

References

Internal reports of the Hydromet Area, 2017- 2021. 

Internal Reports of the Geotechnical Area 2017-2021.

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