Optimizacion de Circutos de Molienda

XXVIII Convención Minera Internacional, AIMMGM AC, Veracruz, Ver., 28 al 31 de octubre de 2009

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A CASE STUDY: FORECASTING PRODUCTION THROUGHPUT FOR AN OPTIMIZED GRINDING CIRCUIT DESIGN CARLOS LOZANO* y DAVE BULLED SGS Minerals Services (Toronto), (*[email protected])

RESUMEN

El estudio presentado aquí, muestra como el simulador CEET2®, una herramienta de diseño desarrollada por SGS MinnovEX y también un componente clave del diseño basado en la modelación geometalúrgiga, fue usado para producir el diseño óptimo de una planta de molienda capaz de tratar 29 t/h de mineral seco durante los primeros tres años de operación, aumentando la capacidad a 48 t/h de mineral seco a partir del cuarto año de operación, mediante la adición de un segundo molino de bolas. Este trabajo describe el diseño y dimensionamiento de un circuito de molienda SAG (diseño Conceptual) en base a los resultados de pruebas metalúrgicas realizadas con 15 muestras de barrenos de aproximadamente 15 a 20 kilogramos cada una más el uso de la tecnología CEET2®. La información presentada también muestra como el simulador CEET2® se usa para proyectar la operación del diseño, permitiendo al usuario no solo diseñar, sino que también le permite comprobar la solidez del diseño para cada una de las muestras, asegurando de esta manera que el diseño resultante será el óptimo

ABSTRACT

The paper presented shows how the CEET2® Simulator, a technology developed by SGS MinnovEX and a key factor of the geometallurgical modeling, is used to create an optimized SABC grinding circuit design to treat an average of 29 t/h of dry ore during the first three years of operation, increasing to 48 t/h of dry ore from the fourth year of operation with the addition of a second ball mill. The work describes the conceptual design and mill sizing of a grinding circuit based on the results of metallurgical testing done with 15 drill core samples of approximately 15 to 20 kg each and the use of the CEET2® technology. The information disclosed also shows how the CEET2® Simulator is used to forecast the throughput, allowing the user not only to design but also to check the robustness of the design sample by sample, ensuring that the final result will be optimized for the ore body.

INTRODUCTION

During the design stage of any metallurgical concentrator one of the most important questions (and also the most difficult to answer) is whether the designed plant will be able to fulfill the requirements of the project (throughput and product size, P80 or grade and recovery) at any time.

The main problems at this stage are the quantity, quality and the degree to which the samples selected represent the ore body Are they adequate for the metallurgical testing that generates data to properly characterize the ore body and allow the design of the most reliable and the most suitable operation to fulfil the project requirements.

Often, projects are designed based on large composite samples selected from large volumes of the deposit (so called “representative” samples) and do not take into consideration the ore variability. Variability is a key factor and perhaps the most important that must be taken into consideration when designing a plant. Knowing the ore variability allows the identification of areas that are more or less susceptible to the metallurgical process (SAG or HPGR grinding or ball milling or flotation etc). If knowledge of the ore variability is complete enough, it can be combined with the mining plan to isolate the ore characteristics for each and any of the different periods of operation (Bennett et al, 2004). Subsequently it will be possible to optimize the design, and more importantly show whether or not the design fulfils the project requirements at all times. If it does not, a correction could be developed well ahead of time.


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The uncertainty of the design will depend mainly on the quantity and quality of the samples used to perform the metallurgical testwork. This divides the design strategy into the following three stages:

Conceptual (requires few samples and no production forecasting is done)

Prefeasibility (requires more samples and includes a general production forecasting)

Feasibility (requires a large amount of samples and includes a detailed production forecasting (year by year) plus a complete uncertainty and risk analysis of the circuit design).

To ensure that the samples are representative of the ore deposit, complete geometallurgical work including domain delineation followed by a proper sample selection method must be conducted.

This paper shows the steps followed to design a SAG grinding circuit at the level of a conceptual study for the Matoush project to treat 29 tph during the first 3 years with expansion to treat 48 tons per day as from the 4th year. The design was performed by using the SGS – MinnovEX CEET2® simulator.

GRINDING TESTSWORK

The database used to perform the conceptual design for the grinding circuit was created from the results of metallurgical testing on 15 drill core samples sent to our laboratory in Canada and summarised in Table 1.

The SAG Power Index test (SPI®) is a test developed by SGS - MinnovEX and is a measure of the hardness of the ore from the perspective of semi-autogenous milling. The Ball mill Bond Work index grindability tests (BWI) is a measure of the hardness of the ore from the perspective of the Ball Mill milling. The crusher index Ci is a measure of breakage produced during sample preparation for SPI® testing; it is used within the CEET2® program to calculate the SAG mill feed size.

Figure 1 and 2 show that the hardness profile (cumulative frequency distribution) of the SPI® and the Bond work index results are too steep compared with other deposits that are based on much larger result databases, and the variability is not completely defined yet. This behaviour is typical of a project in the preliminary stage of development that may be using composite samples. It is indicative that a proper sample selection method, additional sampling and metallurgical testing should be completed to ensure a reliable representation of the ore body, before continuing with further work such as prefeasibility or feasibility studies.

Currently there is only enough data to perform a conceptual design study.


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Table 1 Metallurgical grinding testwork Results Summary

Figure 1 SAG Power Index (SPI) Hardness Profile

Figure 2 Bond Work Index (BWi) Hardness Prof


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GRINDING CIRCUIT DESIGN SPECIFICATIONS

The circuit design was conducted using the CEET2® simulator (Dobby et al, 2001). CEET2® is internet-based simulator software developed by SGS – MinnovEX that offers two major advantages to the user:

It allows for the simultaneous handling of a large quantity of information (samples or mine blocks)

The CEET2® approach to variability incorporates the different bottlenecking situations when the plant is either SAG or ball mill-limited due to the ore’s hardness characteristics.

The combination of these two factors prescribes a proper size circuit design to ensure that the specified overall throughput and grind target averages are met.

The objective of the design is a plant capable of milling an average 29 t/h for the first three years and 48 t/h for year four and beyond, based on grindability test results of the fifteen samples received. The circuit specifications from the client and major assumptions made by SGS, based on experience from similar operations, were used as input parameters to the CEET2® program. The design criteria are summarized in Table 2 below. Throughput and P80 were allowed to vary by +/-10% from sample to sample, while maintaining the specified average values.

Table 2: Project Specified Design Criteria

DESIGN STRATEGY

The following considerations were included:

The basic circuit configuration will be a SABC grinding circuit as the circuit shown below in Figure 3.


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Figure 3: SABC grinding circuit configuration

• The expansion from the first period to the second will be done by adding more ball mill capacity.

• In order to facilitate maintenance and to reduce cost for the spare parts inventory, the overall design must be done in such a way that the expansion requirements will be achieved by adding a twin ball mill.

• Since there is no mining plan available, it is assumed that the same dataset is valid throughout the life of mine.

With these considerations in mind it was decided that the target of the design should be an operation that is balanced between SAG and Ball mill capacities as from the fourth year, making this period the grinding circuit design base case. Consequently the grinding circuit will comprise one SAG mill with pebble crusher and one ball mill for the first 3 years and the same SAG mill with pebble crusher followed by two ball mills as from the fourth year.

DESIGN SIMULATIONS

Several design simulations were conducted using an SABC circuit configuration. Combinations of SAG grates and vibrating screen apertures were investigated to examine the effect on throughput rate, grind size, and equipment sizes. The CEET2® program was first used to develop the base case SABC circuit configuration to achieve the specified throughput and product size target after year 4. Table 3 summarises the results of these CEET2® simulations.


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Table 3: Preliminary design results

The next step was to simulate the circuit and see if it can be run during the first three years with half of the ball mill capacity to treat the 29 t/h requested by the project specifications. In this regard the base case circuit design for year 4 was modified so that the throughput rate target specified for years 1-3 could be achieved. Table 3 summarises the results of the preliminary CEET2® simulations for the years 1 to 3 design, which indicate that the circuit designed to run after 4 years could not produce the targeted throughput of 29 t/h when running with only one ball mill, and required an adjustment to be able to generate the throughput specified for the first three years of operation.

The simulation conditions were now set to design only the ball mill at a fixed SAG Mill capacity (296 kW at the shell) to achieve the specified throughput and P80. The results of these runs are shown in Table 4 below. Examination of these results indicated that the required ball mill capacity to achieve the project targets was in the range of 305 to 315 kW at the mill shell depending on the grate screen size combination.

Table 4: Determination of ball mill size for the first three years

Consequently, a ball mill with a capacity equal to the average of the range defined above (310 kW at the shell) was selected. The CEET2® program was then used to predict throughput for a conceptual design of a SAG mill of 296 kW (at the shell) and a ball mill of 310 kW at the shell for the first three years and then with an additional mill of similar size for year 4 and thereafter, for a choice of two grate sizes and three screen sizes.


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Table 5: Forecasting of grinding circuit final design

The results from the forecasting simulations show that the circuit will perform to the target specified of 29 t/h and P80 of 100 microns during the first three years of operation. Table 5 shows that when the circuit is expanded with one additional ball mill it is able to achieve the increased throughput specification of 48 t/h and P80 of 100 microns from year 4 and above.

Table 6 below presents a typical block output from the CEET2® simulations showing how the designed circuit will perform when treating sample by sample.

Table 6: CEET Block output

CONCLUSIONS

The conceptual design recommendations including an expansion of the plant capacity as from the year 4 of operation are summarised as follows: For year 1-3, the selected SABC circuit design capable of treating 29 t/h to an average grind P80 of 100 µm comprises:

• One SAG mill of 13’ diameter by 6’ EGL drawing 296 kW at the shell. • One ball mill of 9’ diameter by 16’ EGL drawing 310 kW at the shell. • One 3’ diameter pebble crusher is required at all times. • The circuit uses 50 mm grate and 10 mm vibrating screen apertures.


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To achieve the specified throughput capacity of 48 t/h and P80 average of 100 µm, the selected SABC circuit for year 4 and onwards would require one additional ball mill of the same size.

ACKNOWLEDGEMENTS

The management of Strateco Resources for their permission to write and present this paper.

REFERENCES

Bennett C., Dobby G., Kosick G., Bulled D., 2004, Geometallurgical Modelling The New Approach To Plant Design And Production Forecasting/Planning and Mine/Mill Optimization, Canadian Minerals Processors Proceedings 2004, p. 227-240

Dobby G., Bennett C., Kosick G., 2001, CEET (Comminution Economic Evaluation Tool) for Comminution Circuit design and Production Planning, SME 2001, Denver, Colorado

Optimizacion de Circutos de Molienda

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