UQ - Newcrest research outcome: Poor classification costing the minerals industry significant revenue - Sustainable Minerals Institute

Results of Newcrest funded PhD research conducted by Juan José Frausto González that aims to better understand the impacts of the poor classification efficiency of hydrocyclones on the performance of grinding and flotation circuits

Key learnings

Poor hydrocyclone efficiency results in unnecessary recycle of fines to underflow, reducing ball mill circuit capacity.
Hydrocyclones allow bypass of poorly liberated coarse composites to flotation, reducing flotation recoveries.
By replacing hydrocyclones with screens, in a lead zinc operation, classification efficiency was improved resulting in a 20% increase in throughput and 6% increase in flotation recovery.
Hybrid classification and semi inverted (SIV) hydrocyclones are alternative methods of improving classification which are easier to implement & operate than screening

Introduction

The process linking comminution with flotation is the size classification. It determines which particles are diverted back for additional grinding and which pass to downstream flotation. Hydrocyclones and screens are the most commonly used size classification units in the mining industry. Hydrocyclones do have some major advantages such as robustness, relatively small footprint for a high capacity and comparatively low maintenance, capital, and operating cost. They are, however, associated with some problems and classification efficiency can be poor, especially when one is trying to achieve a coarse cut size.

***Figure 1*** *– Typical Hydrocyclone* *efficiency curve compared to an ideal separator (after Lisso, 2020)*

A typical classification efficiency curve of a hydrocyclone, as compared to a perfect separator, is shown in Figure 1. Poor separation efficiency in a cyclone is caused by:

A high proportion of water in the feed bypasses to the underflow and this brings with it fines which should report to the overflow. Typical water recovery to underflow in a cyclone is 30 to 40%.
Cyclones are a hydraulic classifier, where separation is not only based on size but on particle density. Mean performance is an average of a feed that actually contains different minerals with different degrees of liberation which have different density.
To cut coarse, mining operations need to use large diameter hydrocyclones operating at high percentage of feed solids which are associated with poorer separation efficiencies.

Newcrest funded a PhD conducted by Juan José Frausto González (González, 2020) which aimed to better understand the impacts of the poor classification efficiency of hydrocyclones on the performance of grinding and flotation circuits. His project involved performing surveys of the hydrocyclones at Cadia and a comparison of hydrocyclones and screens at Minera Saucito’s lead zinc operation in Mexico.

Characterisation of cyclones at Cadia

Surveys of a hydrocyclone nest at Newcrest’s Cadia Valley Operation (CVO) T3 Concentrator (March 15^th, 2017) showed significant variability in the cyclone cut size and cyclone efficiency was poor, with a significant degree of fine particle misclassification due to high water recoveries to the underflow, as shown in Figure 2.

***Figure 2*** *– Cyclone performance measured in Cadia’s T3 Concentrator (after González, 2020)*

The recycle of fine particles both reduces capacity within the ball mill and increases the production of slimes, which have a poor flotation response due to their low probability of attachment. Improving efficiency of the hydrocyclone nest would improve throughput and energy use in the comminution circuit.

*Hydrocyclone Cluster Surveyed at Cadia*

Circuit survey results also suggest that improving cyclone efficiency and reducing the recirculating load in the comminution circuit will put more particles into the ideal flotation size range and result in increased flotation recoveries. This is because, in the surveys performed at Cadia, flotation feed P₈₀ was directly correlated with comminution throughput and coarser flotation feed was shown to result in poorer flotation recoveries across the T3 roughing circuit.

Simulations predict that improved cyclone efficiency would result in a 3 to 4% improvement in the copper concentrate production rate, because of improvements in circuit throughput and copper recovery.

Hydrocyclones vs screens at Minera Saucito

Screens, in comparison to hydrocyclones, have the advantage that they achieve better classification efficiency and do not suffer from a density bias during separation. The major challenge of using screens in an operation, however, is their large footprint and increased maintenance requirements relative to cyclones.

Minera Saucito is an operation which has the ability to switch between using hydrocyclones or screens as the classifier in its ball milling circuit. González (2020) analysed historical plant performance data, performed circuit surveys and did some exploratory batch flotation testing to investigate the effects of changing classifier on the upstream grinding and downstream flotation processes.

The screens installed resulted in a coarser cut size than the hydrocyclones and thus a coarser feed P₈₀ (Figure 3a). This coarser transfer size and reduction in misclassification of fines resulted in, on average, 20% more throughput through the grinding circuit. What was not expected, however, was that with the screens and a coarser feed P₈₀, galena flotation recovery would also improve relative to the cyclones (from 82 to 88%). This increase in galena recovery was a consequence of improved recovery of coarse particles (Figure 3b).

**Figure 3** – (a) Particle size distribution of flotation feed; (b) lead recovery by size (González 2020)

The improved coarse particle recovery with screening was largely due to increased coarse particle liberation, an example of which is shown in Figure 4. The proportion of galena in the 90 – 100% liberation class by mass was substantially higher when screens were used. Perhaps what is more important is that screening resulted in much less galena in the 0 – 20% liberation class. The presence of this poorly liberated material in a flotation feed is detrimental to recovery. It is difficult to recover because it floats slowly and when recovered, brings a lot of gangue, which reduces grade.

**Figure 4** – Proportion of galena in different liberation classes of the screen undersize and hydrocyclone overflow in the 53 µm size fraction (González 2020)

This change in liberation profile was because separation in the hydrocyclone, in contrast to a screen, is density dependent, promoting the misclassification of coarse and poorly liberated galena-bearing particles to the overflow, while fine and liberated galena-bearing particles are recirculated to the ball mill due to their higher density. With screens, however, liberated and composite particles of the same size have equal probability of reporting to the screen undersize, the net effect of which is improved liberation of the flotation feed.

Alternative technologies for improving classification

Hydrocyclones have long been tolerated in mining operations even though they are inherently poor classifiers. As this work has demonstrated, improving classification efficiency should be more a priority in mining operations. Not only is this important in primary grinding but also regrinding where bypass of coarse composites would also be very detrimental

Work conducted by JKMRC researchers has found that hybrid classification (a combination of hydrocyclones and screens) significantly improves classification efficiency and reduces the water split to underflow and thus the misclassification of fines (Jankovic and Morrell, 1994). This would result in increased grinding capacity and more effective use of grinding energy. Subsequent JKMRC research has also shown that the inclusion of screens in a hybrid classification system can minimise the density bias and improve the liberation of coarse particles in the grinding product stream.

Screens when employed for hybrid classification, can have a coarser cut size and receive much less of the feed, than when used as a standalone classification device. Thus, the high maintenance requirements and the large installation footprint associated with screens are somewhat mitigated.

Other research being conducted at the JKMRC have shown that SIV hydrocyclones (Figure 5) result in an improved classification efficiency over a conventional hydrocyclone design, producing similar results to screening (Jokovic et al, 2020). Some of the other benefits of SIV hydrocyclones include;

Production of a coarse product even at lower feed percent solids
Reduced water split to underflow (50-65% less compared to vertical cyclones), reducing misclassification of fines
Increased throughput during closed circuit grinding and more efficient use of grinding energy

***Figure 5 –*** *(a) Schematic representation of SIV hydrocyclone; (b) vertical vs SIV hydrocyclones (Jokovic et al, 2020)*

SIV hydrocyclones thus have the potential to be a game changing technology, enabling a step change in performance in mining concentrators. In addition to use in primary and regrinding applications, other potential applications include;

Dewatering, with potential use within a dry-stacked tailing system
Production of products with narrower size distribution for coarse particle separation processes

Future direction

Newcrest is currently sponsoring Mussa Lisso, a PhD student being supervised by Dr. Vladimir Jokovic, with the objective of improving the efficiency of a hydrocyclone to realise the benefits identified in González’s PhD study. The work will focus on comparing the performance of vertical and SIV cyclones at a range of different design and operational conditions. Material to be used in the study includes synthetic minerals of different density so that conditions which minimise the density bias in a cyclone can be identified. What is hoped will come out of this work is a better understanding of the operation of a hydrocyclone and the benefits possible if operated in the inverted position. The aim is also to develop an improved model of the hydrocyclone (including angle of inclination) to aid in circuit design.

References

Frausto González, J., 2020. The impact of classification efficiency on comminution performance and flotation recovery. PhD Thesis, The University of Queensland, Brisbane.

Jankovic, A., Morrell, S., 1994. Evaluation of the performance of the KHD micro-screen. Julius Kruttschnitt Mineral Research Centre.

Jokovic, V., Morrison, R. and Alexander, D., 2020. Can the performance of semi-inverted hydrocyclones be similar to fine screening?. Minerals Engineering, 146, p.106147.

Lisso, M., 2020. “Hydrocyclone Optimisation”, Newcrest Workshop Presentation.