sampling and analysis for feasibility studies and mineral processing

Basdew Rooplal

Mining & Metallurgical consultant

http://mineralprocessingconsultant.com/



SAMPLING -Mineral Processing Deposits & Process Plants, Bench Scale

testwork with a view for flowsheet development.




CONTENTS

• Sampling Introduction

• Sampling for feasibility studies

• Types of sampling and samplers

• Types of Analysis

• Bench scale testwork

• Flowsheet development and ore dressing studies

• Ore characterisation and understanding the ore body




SAMPLING -INTRODUCTION

• The objective of sampling in mineral processing is to estimate grades and contents of sampling units in an unbiased manner and with an acceptable and affordable degree of precision.

• Mineral deposits are sampled for several reasons including:

• Resource evaluation

• Determination of the physical and chemical characteristics of material,

• And process amenability.




SAMPLING

• Generally sampling is the process of selecting a part of a whole such that the measured value for the part is an unbiased estimate for the whole.

• In mineral processing a whole is referred to as a sampling unit such as mill feed, dewatered concentrate or bullion.

• A sampling unit is classified as a dynamic stochastic system when sampled during transfer, and as static stochastic system when sampled whilst stationery.




DEFINITIONS: ACCURACY

• A generic term that implies closeness of agreement, between a single measured value or the central value of a set (arithmetic mean or weighted average), and the unknown true value of a stochastic variable.

• This definition reflects that accuracy is an abstract concept.

• By contrast a lack of accuracy can be measured and quantified in terms of a bias or a systematic error.

• Webster defines accuracy as free from error.




DEFINITIONS: BIAS

• A statistical significance difference between a single measured value or the central value of a set, and an unbiased estimate of the unknown true value of the stochastic variable.

• Testing for absence or presence of bias is an essential part of

• sampling in mineral processing.

• Terms such as random error or error without adjuncts or adjectives, will not be used to avoid confusion with randomly distributed variations for which the variance is the fundamental and unambiguous measure.




DEFINITIONS: PRECISION

• A generic term that refers to the magnitude of randomly distributed variations (random variations) in the measurement procedure applied to estimate the central value of the stochastic variable of interest.

• Quantitative measures of precision are:

• Confidence intervals in absolute values or

• Relative percentages

• Symmetric and asymmetric confidence ranges in absolute values,

• Derived from the variance of the central values for the variable




DEFINITION: SAMPLE

• A part of a sampling unit or sample space selected such that a measured value for the part is an unbiased estimate for the sampling unit or the sample space.

• A sample is often referred to as a representative part of a population or a whole.




SAMPLING FOR FEASIBILITY STUDIES

• Sampling for plant design during the feasibility phase of a project can be critical to the overall success of the project but this is all too often minimized.

• Sampling at this stage is usually carried by geologists and mining engineers for

• Identification and quantification of ore reserves.

• Sampling for comminution and the amenability of the ore for processing is left for later bulk sampling programs.




SAMPLING FOR FEASIBILITY STUDIES

• Precise quantification of the processing characteristics of the plant feed material does require the additional sampling, much information can be obtained from the initial feasibility sampling program that can guide and even determine

• The testwork program and even final design.

• Due to budget constraints the initial team is limited to geologist, mining engineers, geotechnical engineers and environmentalists.




TYPES OF SAMPLES

• Core Drilling

• Reverse circulation drilling

• Auger Drilling

• Other drilling methods

• Channel Sampling

• Trench sampling

• Geochemical and environmental Sampling

• Water samples

• Run of mine ore feed

• Crusher product sampling

• Sampling of pulps

• Sampling of loaded carbon, slags flue dust, etc.

• Sampling of bullion

• Sampling of liquids/




CORE DRILL SAMPLES

• This is the preferred method of recovering material from within the ore body and is also the most expensive.

• Generally between 27 mm to 100 mm in diameter.

• Cores are generated using a circular drill bit that allows a cylinder of rock to rise

• Within the interior of the drill steel as the drill progresses.

• The resulting drill core generally has a smooth surface and is consistent in diameter along its length.




DRILL CORE SAMPLES




BAUER LARGE DIAMETER DRILL RIG




REVERSE CIRCULATING DRILLING

• Operates in the same manner as holes drilled for general purpose viz. boreholes

• Less expensive than core drilling

• The samples from each drill interval are logged separately as they come from the hole.

• RC is not recommended when the exact transition from one rock to another is required viz. coal.




AUGER DRILL




AUGER DRILLING

• When material is unconsolidated Auger drilling method is used.

• Soils, placer deposits of river gravel, and previously placed material such as mine waste dumps and impoundments.

• The sample may consist of the entire amount of material extracted from the hole or a hollow tube in the middle of the drill stem can be used to collect materials from only desired depths.




OTHER DRILLING METHODS

• Other specialized drilling methods are also used to recover samples of material and to make holes within which in situ measurement can be made.

• Chip and mud samples are collected from ordinary rotary drilling

• Which are less expensive but also less reliable and are used for indicative purposes only.

• Specialized drills for collection of large pieces of rock such as placer sampling are also used.




CHANNEL SAMPLING

• Where the material to be sampled is exposed to the surface or in underground workings, channel sampling is used.

• In this procedure a channel of dimensions similar to the diameter of the core or RC hole is excavated.

• The resulting samples are chips similar to those recovered through RC drilling.

• Channel samples have the advantage over RC samples in that the material can be geologically logged prior to excavation.




TRENCH SAMPLING

• Where the material to be sampled is close to the surface, trenches are excavated to gain access to the material and samples are taken either as channel samples within the trenches or as selected samples of the excavated material.

• This results in broken material




GRAB SAMPLING

• When material is exposed on surface or underground, it is possible to grab samples on a random basis as it is mined or transported.

• Such samples maybe useful for obtaining overall averages for large amounts of material but not

• Useful for identifying material characteristics at a specific location.




STOP BELT SAMPLING FOR ROM MATERIAL

• The feed belt is stopped, a former placed on it and all the material within the former is removed into a bucket before restarting the belt.

• This method is recognized by certain national and international standards as the reference sampling method when checking for bias in automatic samplers.




AUTOMATIC SAMPLING OF RUN OF MINE ORE

• Due to problems occurring during stop belt sampling the process can be automated.

• Primary increments from the discharge of the head pulley are taken and increments and sub-divisions are further processed by crushing.

• Automatic samplers have now been developed where the samples are taken in the direction of the material flow.




PRIMARY CROSS BELT SAMPLERS

• Easily – mounted on conveyor stringers along length of conveyor

• Guaranteed not to damage the conveyor belt when correctly installed




CRUSHER PRODUCT SAMPLING

• Rotary Plate Divider

• For continuous or timed dry material splitting

• Suitable for primary sampling and secondary division

• Must be consistently fed by vibratory feeder or feeder conveyor

• Ideal sample divider for bulk materials handling systems




SAMPLING OF PULPS

• Manual sampling is usually employed but is not reliable:

• There is no guarantee that the samples will be unbiased

• Precision of the measurement is much lower than can be obtained with mechanical methods.

• The mechanical systems are usually of two types, viz.

• Those with cross stream cutters

• And the arc type rotary cutters.




SLURRY SAMPLING

• Primary Rotary Samplers • Vezin Samplers & Launder Samplers




TYPES OF ANALYSIS: IN SITU MEASUREMENTS

A down hole survey is carried out in order to locate the position of the sample that has been taken since drill holes are seldom straight over long distances.

Other types of probes that are inserted into open drill holes include:

callipers to measure the diameter of

• The hole along its length

• Resistance sensors to detect the presence of water, carbonaceous material, and gamma sources,

• And detectors to measure density and normal radiation levels,




TYPES OF ANALYSIS: IN SITU MEASUREMENTS

• Expandable cylinders to measure rock strengths and stresses,

• Gauges to measure ground water levels

• .Sophisticated tomographic techniques use sensors placed in multiple drill holes to provide detailed information on the material between the drill holes.

• Work is underway to develop sensors that will be able to detect the presence of metals.




TYPES OF ANALYSIS: GEOLOGIC LOGGING

• Drilling samples both core and chips are logged by geologists in order to record the material characteristics along the drill hole.

• Samples are logged for:

• Rock types

• Alteration types

• Association of various minerals

• And estimated content of the desired commodity

• Core samples are also logged for:

• fracture spacing and intensity,

• Rock quality designation

• And fracture orientation

• Photographs are also taken of the core.




TYPES OF ANALYSIS: CHEMICAL ANALYSIS

Representative samples of the core are subjected to chemical analysis.

The analysis includes:• moisture content• Total metal content• Soluble metal content• Content of non-metal

commodities• Heat content for coal and

oil shale• Ash content after

combustion

• In some cases multiple samples are combined to give composite metal values.




TYPES OF ANALYSIS: MINERALOGICAL

ASSESSMENT• Direct measurement of

mineral types is possible using QEM*SEM.

• QEM*SEM uses a computer controlled scanning electron microscope which can distinguish minerals and their attributes in individual ore particles.

• These images can also be processed to provide information on:

• modal abundance

• Grain size

• Mode of occurrence

• Liberation characteristics

• And quantity of mineral phase recoverable.




TYPES OF ANALYSIS: PHYSICAL CHARACTERISTICS

• Direct measurement of physical parameters of the materials sampled are also performed. These include:

• In situ density

• Porosity

• Permeability

• Compressive strength

• Compaction

• Grinding index

• In some commodities it is the physical characteristics that determine value e.g. kaolin where the value is determined by brightness and slurry viscosity.




METALLURGICAL TESTWORK -BENCH SCALE TESTWORK

Introduction

• The resistance of ore samples to breakage (or hardness) is measured through grindability tests.

• Several grindability tests have been developed over the years for different applications and each test has its own strengths and weaknesses

• Grindability tests are a compromise between test costs and its deliverables.

• The highest degree of deliverables and certainty is achieved in a pilot plant, which is also the most reliable test procedure to determine the resistance of ore samples to grinding or hardness and is also the most expensive.




SUMMARY OF GRINDABILITY TESTS

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GRINDABILITY TESTS

Bond Ball mill Grindability

• The AG/SAG mill or HPGR circuit products, which have non-standard particle size distribution.

• One of the keys of the Bond work index success over time has been its reliability and reproducibility.

• The figure below shows that the Ball Mill work index is normally distributed with AVG 14.6 and Median 14.8

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GRINDABILITY TESTS

Bond Rod mill work Index

• The rod mill work Index is also normally distributed with and average and median of 14.8kWh/t

• It is common to observe difference between the ball and rod mill caused by variation in ore hardness

• The test has been mainly used for the design of rod mill or primary ball mills.

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GRINDABILITY TESTS

Bond low energy impact test

• Consists of an apparatus with two pendulum hammers mounted on two bicycle wheels, so as to strike equal blows simultaneously on opposite sides of each rock specimen.

• The height of the pendulum is raised until the energy is sufficient to break the rock specimen

• The test is generally performed on 20 rocks

• One of the strengths of the test is to measure the natural dispersion in the sample.

• Another advantage of the test is the coarse size 2 – 3 inches which makes it unique in the series of tests.

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GRINDABILITY TESTS

SAG power index (SPI)

• SPI expressed in minutes , is the time T necessary to reduce the ore from P80 of 12.5mm to P80 of 1.7 mm

• The SPI has the advantage of requiring low weight and is suited for geometallurgical mapping of ore deposits

• SPI is widely used and deposits can be compared in terms of hardness and variability, see fig below.

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GRINDABILITY TESTS

JKTECH drop weight test

• Developed by JKMRC

• Divided into 3 components:• Test measures the resistance

to impact breakage of coarse particles in the range 63 – 13.2 mm

• Then evaluates the resistance to abrasion breakage in the range 53 –37.5 mm

• Finally the rock density of 20 particles is measured to asses the average ore density as well as its dispersion.

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GRINDABILITY TEST


• The test generates the appearance function –• E.g. the breakage pattern of

the ore under a range of impact and abrasion breakage conditions

• The appearance function can be used in the JKSimMet modelling and simulation package to predict the ore response to comminution process

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GRINDABILITY TESTS

JKTECH Drop weight test

• Also part of these procedure is the density determination of 20 rock samples, using water displacement techniques.

• Figure 5 shows an ore displaying a wide range of densities.

• The density distribution of the ore is important in AG/SAG milling because

• It affects the bulk density of the charge and associated power draw

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GRINDABILITY TESTS


• A great number of rock weight tests have been performed over the years which allows for comparison of ore types in a data base.

• The frequency distribution of the function ‘A x b’ from JKTech is depicted in Fig 6

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GRINDABILITY TESTS


• One of the interesting features of the drop weight test procedure is that it provides a variation in rock hardness by size from 13.2 to 63 mm.

• Fig 7 illustrates this at 3 different energy levels.

• 0.25 1.0 and 2.5 kWh/t

• For a very competent ore, the curve will be nearly horizontal, a non-competent fractured ore will show a high gradient with increasing size

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GRINDABILITY TESTS

SAG Mill comminution test

• This is an abbreviated drop weight test, which can be performed at low cost on small samples 19 – 22 mm or drill cores.

• 5 kg of sample is normally sufficient.

• The advantage of the SMC test is that it generates the energy versus breakage relationship with as small quantity of sample of a single size fraction.

• Because the test can be performed on small rocks, it is well suited for geometallurgical mapping.

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GRINDABILITY TESTS

MacPherson Autogenous Grindability tests

• This is a continuous test performed in a 46 cm semi-autogenous mill with an 8% ball charge.

• The pilot plant consists of a feed hopper, cyclone, screen and dust collector with a control system to regulate the charge volume and circulating load.

• 100 to 175 kg of sample is required with a top size greater than 25 mm.

• The test is run continuously for 6 hours.

• The importance of reaching a steady state in a grinding mill is widely accepted, this test is the only small scale test that offers the option.

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GRINDABILITY TESTS

MacPhersons Autogenous grinding tests

• Throughput rates • Specific Energy

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GRINDABILITY TEST

Media Competency test

• There has been some variations of media competency test developed over the years with the assessment of media survival in autogenous milling being the main objective.

• 104 to 165 mm rocks are subjected to a tumble test using 10 large rock in 5 size fractions.

• The surviving rocks are submitted to fracture energy test procedure.

• This provides the relationship between the first fracture energy requirement and rock size.

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GRINDABILITY TESTS

High Pressure Grinding Rolls

• HPGR are emerging as an energy efficient alternative to AG/SAG circuits.

• The traditional method for testing is processing large samples in a pilot scale.

• Several tests are performed to asses the effect of operating pressure and moisture content on HPGR performance

• The power input is recorded and presented below.

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CRUSHABILITY TEST

Impact Crushability

• Gives a WI that can be applied to 3 types of crushers

• Gyratory – WI can be used to determine the horse power.

• Impactors – WI is an indication of hardness

• Cone Crusher – rate the material to determine the duty of the crusher h

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CRUSHABILITY TESTS

Paddle Abrasion

• Results are in the form of Abrasion Index and chemical makeup of the material

• Tests are used to determine whether an Impactor or cone crusher is suitable.

• Can also be used to calculate the approximate liner life for the crusher

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CRUSHABILITY TESTS

French Abrasion

• Gives an Abrasion and Crushability Index

• Mainly used to estimate hammer wear in the Impactor application

Dynamic Fragmentation

• Conducted for Impactor application

• Measures the friability of the material

• Dynamic fragmentation number will indicate if the Impactor is feasible for a particular application.

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FLOWSHEET DEVELOPMENT - ODS

Introduction

• Ore dressing studies the characterisation of the ore body with respect to metallurgical properties.

• In conjunction with the project requirements, geologists and mineral resource management, a sampling program is compiled for the specific ore body.

• These samples are characterised with respect to various flowsheet and data obtained from the characterisation work is analysed and evaluated to improve the process recovery .

• This provides information with regards to risk minimisation, for both plant design envelopes as well as operational efficiency




ODS - KNOWLEDGE FLOW




ODS - IN AN ORE BODY DEVELOPMENT




ODS - GENERIC DIAGRAM FOR SAMPLE

CHARACTERISATION




ODS - COMMINUTION CHARACTERISATION

• Test work consists of a suit of laboratory and pilot plant scale tests

• Laboratory tests are typically rock mechanic tests as used by equipment manufacturers to provide performance guarantees for comminution equipment.

• These also include drop weight tests , a methodology used to determine the extent of breakage resistance due to impact and abrasion.

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ODS - COMMINUTION CHARACTERISATION

• Depending on the requirement of the specific ore dressing study, i.e. feasibility study , pilot scale tests can be conducted on various comminution equipment to validate laboratory scale test results and generate plant design information.

• Samples can also be provided to equipment manufactures to conduct their own tests

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ODS - DATA ANALYSIS AND INTERPRETATION

• The data generated from the characterisation tests is analysed and interpreted by process specialists.

• This is a collaborated effort amongst in-house specialists, proprietary and commercial software, research institutes, and equipment manufactures and suppliers.

• Interpretation in this context means that key metallurgical parameters are determined and operating envelopes are established.

• Also potentially problematic ore types are identified and process recommendations are made. h

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ODS - DATA ANALYSIS AND INTERPRETATION

• The output results in key plant design information.

• E.g. comminution characterisation predicts the product size distribution and mass balance via simulation for scrubbing and each of the crushing stages.

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ODS - INTEGRATION

• The role of the metallurgist is key in generating the flowsheet design knowledge package through the interaction with a variety of process specialists and process engineers.

• Important major ore related problem areas within a specific ore type are also highlighted.

• This means that such problem areas and solutions are integrated within the overall process design.

• Depending on the phase of the project the integration process also includes a level of simulation of the ore dressing study, and derived flowsheet options that resulted from the characterisation of the various ore types.

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ODS - INTEGRATION

• Simulation enables critical investigation of all system attributes, and the ability of the circuit design to deliver finished product with out recycling.

• Raw ore dressing information and knowledge is traded off against practical operational constraints, which leads to a fit-for-purpose design

• That has the best chance of maximizing recovery of minerals from in-situ resources.

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ODS - PLANT DESIGN

• To reduce the risk of selecting incorrect equipment from a vast array of possibilities a formalised set of tools to guide equipment selection and plant design have been developed

• These tools consist of commercially available as well as proprietary tools

• Process engineers are provided with basic flow diagrams and related metallurgical parameters.

• The process engineer will then expand on the original ore dressing flowsheet provided and develop a number of flowsheet based on the project requirements.

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ODS - PLANT DESIGN

• Completed ore dressing study assists the process engineer to rapidly evaluate scenarios using existing models and create an understanding of how the metallurgical envelope of characteristics develop through the ore body.

• An evaluation of proposed solutions against a background of knowledge derived from the study is then conducted.

• The knowledge derived from the study supports the engineer in the design phase and assists in reducing project risk and increases confidence in the approved flowsheet.

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ORE CHARACTERISATION -UNDERSTANDING THE ORE

BODY AND THE METALLURGY

• The best possible characterisation of the ore body will enhance the ability to extract better outcomes from a mine to mill application.

• The greater data, the better characterisation of the ore body Properties.

• This characterisation is important in developing extraction and processing strategies which enhance the productivity gains possible from a mine to mill application (JKMRC 1998)




CHARACTERISATION -UNDERSTANDING THE ORE


• At its simplest , characterisation is about developing the best possible understanding of the ore body , in particular its variability.

• One of the first comprehensive characterisation studies was reported by Simkus and Dance (1998) at the Highland Valley Mine

Highlands Valley

• Had developed a program mapping the hardness of different ore types, since the late 1970’s.

• By late 1990’s , drill monitors were being used to provide an estimate of ore hardness of subsequent blasted ore.

• Ore was then tracked to stockpiles using mine dispatch systems and movement through stockpiles was modelled.

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• An image analysis system was used to provide an estimation of the feed size distribution to the SAG mills.

• Relationships were developed between ore hardness, feed size and mill throughput.

• This approach provided a strong ability to predict expected mill throughput information which could then be utilised in process control.

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Rock Mass Properties

• Standard rock mass properties are usually obtained as geotechnical information from drill core and include:

• Rock Mass Rating

• Rock quality designation

• Point load Index

• Young’s Modulus

• Poisson’s Ratio

• Unconfined Compressive stress

• In-situ block size

• Joint spacing

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BODY AND THE METALLURGYMetallurgical Process Parameters

• These data typically include:

• Grades, including the grades of gangue minerals and minor elements

• Grindability data, principally related to ore hardness, as measured by bond work indices and JKMRC grinding model parameters,

• Flotation grade and recovery data as determined by laboratory flotation tests

• Mineral liberation

• Lithology

• Geological Alteration

• Acid forming potential of ore

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Predictive Models

• Models frequently used in mine to mill studies include

• Mine block models incorporating geotechnical and geometallurgical parameters.

• Blast fragmentation models

• Muck pile models

• Comminution models

• Models which predict the final stockpile shape resulting from open pit blast are increasingly useful when it is desirable to understand where material of different properties, notably grade, reside in the muck pile after blast.

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Conclusions

• The literature analysis suggests that the tools required to implement Mine to mill approach are available in acceptable form.

• Many of these hardware and software tools are provided by established suppliers and have been successfully implemented.

• Most tools are also subjected to research and further development

• The area of greatest need is the availability of tools to monitor mine to mill outcomes.

• To date these have been developed at individual sites

• More generic software tools would be useful.

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sampling and analysis for feasibility studies and mineral processing

Design

samplinggenerally sampling

single measured value

contents of sampling

mineral deposits

unbiased estimate

unknown true value

later bulk sampling

stochastic variable