Crushing / Screening and Conveying

Crushing / Screening and Conveying
Plant Design Construction and Operation Plant optimisation and energy efficiency considerations Basdew Rooplal Mining & Metallurgical Consultant

Contents Plant Design Construction and Operation
Bench scale and pilot scale design for comminution circuits Factors influencing the selection of comminution circuits Types and characterisation of crusher equipment and circuit flowsheet Selection and sizing of primary crusher Computer aided design of Jaw Crusher Selection and sizing of secondary and tertiary crushers Optimising the Eccentric speed of cone crusher Selection and sizing of High pressure roll crushers Advancement in Screening Technology.

Contents Plant optimisation and energy efficiency considerations
Characterisation – Understanding the ore body and the Metallurgy Ore dressing studies – what is involved. Blasting for improved mining and comminution productivity Production planning for the combined mine and comminution operation Optimising lumps to fines ratio in Iron Ore processing Reducing fines generation in Coal Mining Profit based comminution controls Increasing the energy efficiency of Processing

Bench scale and pilot scale design for comminution circuits
Bench scale and pilot scale design for comminution circuits

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

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 and Median 14.8

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.

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.

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.

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 – 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.

Grindability test JKTECH drop weight 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

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

Grindability tests JKTECH drop weight test
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

Grindability Tests JKTECH drop weight test
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. 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

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.

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.

Grindability Tests MacPhersons Autogenous grinding tests
Throughput rates Specific Energy

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.

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.

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

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

Crushability tests Dynamic Fragmentation French Abrasion
Gives an Abrasion and Crushability Index Mainly used to estimate hammer wear in the Impactor application Conducted for Impactor application Measures the friability of the material Dynamic fragmentation number will indicate if the Impactor is feasible for a particular application.

Discussion Points! Where can I apply Bench scale and pilot scale programs in my work environment?

Factors influencing the selection of comminution circuits
Factors influencing the selection of comminution circuits

Geological Interpretation of Drill core and Bulb Sample Mineralogical Analysis Chemical Analysis Physical Properties Circuit feed Parameters Sampling requirements Contiguous properties Feed and product Specification Bond work Indices, Abrasion Index, and specific power consumptions

Circuit selection Metallurgical efficiency Cost Consideration Water supply Fine Grinding Plant layout

Geological Interpretation of Drill core and Bulk Sample
Information Gained Effect on Circuit Selection Identification and relative abundance of Mineral content Degree of Dissemination Type of Lithology Types of Alteration Degree of Oxidation Geotechnical Competence Hardness Provides a guide to the types of circuit required and the types of samples required based on precedent Determines the necessity of separate plants to process sulphide ores Provides a guide to the selection of autogenous grinding

Mineralogical Analysis
Information Gained Effect on Circuit Selection Identification of ore and gangue minerals and middling association Liberation and Modal Analysis Quantitative analysis – QemScan Determine Ratios of reduction Feed and product size analysis in primary , secondary and regrind circuits

Chemical Analysis Effect on Circuit Selection Information Gained
Identification of metallic , non-metallic and acid generating constituents Determining the requirements of pre- washing the ore

Physical Properties Information Gained Effect on Circuit Selection
Hardness, Blockiness, Friability, Quantification of primary fines and clay content Specific gravity of mineral constituents Provides a guide to potential problems in Crushing Screening and Grinding the ore with respect to equipment selection and Over grinding and avoidance of slimes generation with respect to softer minerals.

Circuit feed Parameters
Effect on Circuit Selection Information Gained ROM top size parameters Primary crusher discharge size analysis Throughput requirements and schedules Mining Plans , Schedules, methods and equipment sizes Determines selection of primary crushers and necessity for pre-crushing can influence this selection by determination of the product size at the required throughput rate.

Sampling requirements
Information Gained Effect on Circuit Selection Preliminary drill core for resource definition and split for bond work indices Whole core for Autogenous Media Competency Index, Impact crusher work indices and fracture frequency Bulk Sample , large diameter drill core, open pit or underground for pilot plant testing Preliminary Assessment of grinding requirements and ore variability Power based methods for mill sizing using results from Bond , Impact and grinding work indices Assist in definition of Pilot plant test program and ore Variability Characteristics

Contiguous properties
Information Gained Effect on Circuit Selection Definition of equipment characteristics Determines the utility of equipment with respect to its Inherent operating behaviour, e.g. Autogenous grinding mills grinding to a natural grain size, SAG mills breaking across grain boundaries and rod mill minimizing the creation of fines

Feed and product Specification
Information Gained Effect on Circuit Selection Definition of requirements at each comminution stage Influence of “Mine to Mill” and choke feeding the primary crusher on subsequent stages Performance Maximum feed top size in relation to high aspect and low aspect primary mills Use of HPGR

Bond work Indices, Abrasion Index, and specific power consumptions
Effect on Circuit Selection Information Gained Calculation of specific power consumption at each comminution stage for different ore types and composites. Assessment of ore variability Checking on pilot plant test data Assessment of risk or contingency based on samples selected according to the mine plan Distribution of power Confirmation of specific power consumption and contingencies for Process design criteria Calculation of estimates for media and liner wear. Estimation of mill power requirements and distribution of power between equipment

Circuit selection Effect on Circuit Selection Information Gained
Assessment of Overall Power requirements and power efficiency for different circuit options Assessment of Overall Operating Availability for different circuit options Determination of unit power cost and demand for different circuit options Determination of the Most economic option on the basis of NPV of Capital and Operating cost and circuit availability for a fixed revenue rate. Power efficiency should be optimised in design for each circuit option considered.

Metallurgical efficiency
Information Gained Effect on Circuit Selection Definition of Optimum comminution configuration Definition of feed rate variation Selection of grinding media Determination of necessity for stage grinding and stage concentration to optimise mineral liberation and recovery. Quantify the effect of feed rate variations on the metallurgical efficiency of down stream processes.

Cost Consideration Effect on Circuit Selection Information Gained
Definition of Largest practical equipment size and design Differences between comminution options Effect of efficiency on crushing and grinding equipment E.g. Separation of screening plant from crushing plant. Feed arrangement requirements Choke feeding crushers

Water supply Effect on Circuit Selection Information Gained
Definition of Process alternatives Determination of plant location Namely, Mine location, Applicability of dry grinding, Pre- concentration and use of sea water.

Fine Grinding Effect on Circuit Selection Information Gained
Determination of test requirements, batch and / or Pilot scale tests Determination of Optimum location of Fine grinding application within the circuit and definition of the types of machines used.

Plant layout Effect on Circuit Selection Information Gained
Definition of Geographic location, Climatic conditions, Accessibility Definition of relative location of Mine vs. Plant Definition of Operating schedules and manpower requirements Definition of expansion potential Determination of wet and dry processes Determination of Physical sizes of equipment and foot print of the plant Determination of built-in contingencies that allow for future expansion Consideration for the addition of equipment lines in the case of larger plants.

Discussion Points! Comments on pertinent factors that was involved in the selection of your plant system. The pros and cons of the current system, bottle necks, etc.

Types and characterisation of crusher equipment and circuit flowsheet
Types and characterisation of crusher equipment and circuit flowsheet

Introduction Standard Equipment New Equipment
Crushing flowsheet and equipment are selected to prepare ore for downstream purposes. Standard equipment for the minerals industry has been : Jaw crushers Gyratory crushers Cone crushers Water flush cone crushers Vertical and horizontal impactors High pressure grinding rolls

Factors Affecting crusher selection
Plant throughput, ore delivery schedules Size of feed Desired product size for down stream processing Ore characteristics: Hard rock Clay Gravel Variability Climatic conditions Down stream processes

Plant throughput and ore delivery schedules
E.g. A primary Jaw crusher will be better suited for a conventional underground mining operation because: Tonnages are typically lower Feed material size is smaller Less headroom and a smaller excavation is required. Forms the base line for flowsheet design and equipment selection Size type, number of stages and number of crushers per stage for an application can be identified.

Feed size The crusher selected must be sized for throughput as well as top size expected from the mine. Smaller the crusher the smaller the dimension of the feed material that can enter the crusher chamber. A balance between the plant capacity and the size of the crusher must be reached. In multi stage crushing circuits the products of the preceding stage will be the determining factor in the selection of the size of the crusher and the crusher liner configuration.

Product size The target product size required from the crushing circuit will determine the number of crushing stages and types of crushers to be used for a specific application. E.g.. To produce a coarse product a single stage crusher may be required. To produce a 15 mm product a two stage crushing may be required. The ability to crush finer has been required for specific application. For fine product sizes in dry process application flowsheet have incorporated vertical shaft impact crushers operated in closed circuit with vibrating screens.

Ore Characteristics When selecting equipment for inclusion in a crushing flowsheet the following factors should be considered: Hardness Toughness Abrasiveness Moisture content mineralisation Geologists should provide info with regards to: Rock types Abundance of various rock types LOM Short and long term delivery schedules should then be provided mining to adapt circuit configuration for LOM

Climatic Conditions A dry warm climate will allow for an unenclosed installation. Colder wet climates will require enclosures for operator protection and moisture problems. An enclosed crushing plant also posed dust extraction challenges.

Downstream processes Heap Leaching Milling
Crusher product size will be specified for optimum recovery Milling Type of grinding circuit will influence the number of crushing stages.

Application Primary Crusher Purpose
To reduce the ore to a size amenable to secondary crushing, SAG mill feed or heap leach product Usually operated in open circuit. Typical crushers used are Jaw Gyratory Horizontal impactors Rotary breakers Ratio of reduction 8:1 Some form of scalping screen may be installed in the case of Jaw and Impact crushers

Application Secondary Crushers Crusher types:
Purpose To produce an intermediate or final product Feed Size – typically between 200 & 75 mm depending on primary crusher Vibrating screen may be installed ahead to remove product size material. Crusher types: Standard cone crusher – traditionally Horizontal Impact crusher as alternative HPGR recently for diamond and iron ore

Application Tertiary Crushers Purpose: Produce the final product
Feed : 37 mm Product : 12 mm Crusher type: Short head cone crusher Longer crusher chamber and more even size distribution Usually operated in closed circuit with a vibrating screen HPGR and Nordberg Water Flush crushers have also been used.

Application Quaternary Crushing
Purpose: To produce fine dry product for downstream processing Vertical Impact Crusher has been used at Newmont’s heap leaching operation in Uzbekistan. High speed crusher that used high speed impact to effect particle reduction Nordberg’s Gyradisc crusher uses a combination of impact and attrition to effect particle size reduction. Applied in the industrial minerals and sand industry to produce finished products to microns.

Crusher types Jaw Gyratory Horizontal shaft impact crushers
Rotary breakers Roll Crushers Cone crushers Gyradisc crushers Vertical impact crusher

Flowsheet – Two stage Crushing (Fine Product)
Figure 1 Two stage crushing (Fine Product) Figure 2: Two stage crushing (coarse product) Figure 3: Three stage crushing

Two Stage Crushing (Coarse product)

Three Stage Crushing

Two Stage with Water flush Crusher

Three Stage Crushing – Gold Heap Leach

Three stage crushing and Water flush Crusher

Water flush Crushing

SABC configuration

Three Stage Crushing with Vertical Shaft Impactors

Discussion Points! What are the Problem areas of current equipment installation?

Selection and sizing of primary crusher
Selection and sizing of primary crusher

Introduction Family of primary crushers
The rock / ore determines the type of crusher The plant capacity determines the size of crusher Gyratory Double toggle Jaw Single toggle Jaw High speed roll crusher Low speed sizer Impactors Hammer mill Feeder breaker

History

Mechanical Reduction Methods
Four basic ways to reduce a material Impact Attrition Shear Compression

Compression Should be used when Done between two surfaces
Gyratory and double toggle jaw uses this method Material is hard and tough Material is abrasive Material is not sticky Uniform product with a minimum of fines is desired The finished product is relatively coarse > 38 mm Material will break cubically

Impact Conditions Refers to sharp , instantaneous impingement of one moving object against another Two types Gravity Dynamic Cubical particles are needed Finished product must be well graded Ore must be broken along natural cleavage lines When material is too hard and abrasive or high moisture content

Attrition Conditions Scrubbing material between two hard surfaces
Hammer mills operate with close clearance between hammers and screen bars and reduce by attrition combined with shear and impact reduction. When material is friable and non-abrasive When top size control is not desired When maximum of fines is required.

Shear Conditions Consists of trimming or cleaving action
Exploits the fact that the ratio of compressive strength to tensile and shear strength in the majority or rocks is approximately 10 : 1 Low speed sizers break the rock in tension and shear by chopping action When the material is somewhat friable and has low silica content When material is soft to medium hardness For primary crushing with a reduction ratio of 6 : 1 When a minimum of fines is desired When a relative coarse product is desired > 38 mm

Primary Gyratory crushers
The main capacity advantage offered is centred around the Archimedes principal They found that the crushing chamber provides more effective volume than a rectangular volume The shaft grating speed adds a third dimension to crushing as opposed to two dimensional crushing

Gyratory crusher Advantages Disadvantage
Designed for direct dump from trucks Lowest maintenance per ton processed of any designed crusher Can handle crushing ore hardness up to 600 mPa Easy handling of tramp material with hydraulic reiief system Highest installed capital cost of any crusher design

Jaw Crusher Animation Video 1
Jaw Crusher Animation Video 1

Working principles of the Jaw Crusher Video 2
Working principles of the Jaw Crusher Video 2

Double toggle design The swing Jaw of the Standard DT crusher pivots from an overhead shaft . A Pitman hung from an eccentric shaft transmits motion through a pair of toggles at the bottom of the swing Jaw Swing Jaw motion is greatest at the discharge opening. The hinge pin is located behind the centreline of the crusher zone and it causes the swing Jaw to move perpendicular to the fixed Jaw. This arrangement provides twice the force in crushing Typical duty is 350 MPa

Double toggle Jaw Advantages Disadvantages
Lower installed cost than a Gyratory crusher Can handle high abrasion with low maintenance Can handle tough crushing application upto 600 MPa nickel ores, iron ores, etc. Same capacity limitations as the single toggle aw crusher Substantially higher installed cost than a single toggle Jaw crusher Same crushing size limitation as single toggle Jaw crusher

Single toggle Jaw crusher
The rotation of the eccentric shaft causes the swing Jaw assembly to move in an elliptical path. Maximum movement of the swing jaw assembly occurs at the top of the crushing chamber with minimum movement at the discharge opening At all points in the crushing chamber the crushing action has both vertical and horizontal components. Due to the rubbing action of this type of jaw, jaw plate wear is accelerated and power efficiency is lowered because the swing jaw is lifted on every stroke.

Single toggle Jaw crusher
Advantages Disadvantages Lower installed cost than a double toggle Lower power usage than a double toggle Can handle sticky, muddy ore easier than a double toggle or Gyratory Normal economic maximum capacity is 750 MTPH Duty of crusher is for light or medium hard material Does not handle high abrasive material as well as DT Requires feeder Primary crushing only

Low speed sizers The low speed sizing principle is the combination of high torque / low roll speeds. The interaction of tooth, spacer and roll set up a “sized void” which in turn sizes the material Used for non-abrasive sticky type material bet MPa Application Medium hard limestone, bauxite, kimberlite, gypsum, clay, shale and gold ore.

Low speed sizers Advantages Low reduction ratio
Can handle high tonnages – MTPH Low installation cost and minimum head room required Low fines production Low power consumption Easy rejection of oversize feed – using discharge gates Low reduction ratio Peak power loading up to 8 times installed power Not economic for low tonnage unless the material is very difficult to handle

Single toggle vs. Double toggle
ST has a larger angle of nip, the larger the nip angle the harder to grip the material.. ST – greatest movement at the top DT – greatest movement at the bottom ST – Movement of jaw is in downward rolling direction which gives a force feed action assists in handling sticky material Life of Jaw in ST is less than DT

Impact crushers Utilized in soft, non- abrasive application
Crushing availability and maintenance can economically offset against capital cost

Operation of an impact crusher Video 3
Operation of an impact crusher Video 3

Impactor Animation video 4
Impactor Animation video 4

Impact Crusher Advantages Disadvantages
Can handle larger size reduction 1000 : 75 High reduction ratio compared to investment cost Provides a high degree of fines Can handle up to MTPH Requires feeder Cannot handle tramp metal Higher power consumption as more fines are produced High wear due to higher silica content + 8%

Feeder Breakers Are utilised in soft to medium hard application
Coarsely break material for belt conveying Frequently used for overburden and underground duty

Feeder Breaker Advantages Disadvantage Very low reduction ratio
Avoids costly site preparation and civil work Can transfer and crush material in a single machine Handles wet material with ease Very low headroom Can handle upto 2000 MTPH Very low reduction ratio Crushing takes place in breaker bars and chains which causes wear.

Primary Crusher selection Criteria
Will it produce the desired product size at required capacity Will it accept the largest feed size expected What is the capacity to handle peak loads Will it choke or plug Is the crusher suited to the type of crushing plant design Is the crusher suited for underground or in-pit duty Can it handle tramp material without damage How much supervision is required How does the crusher resist abrasive wear What is the power consumption

Primary Crusher selection Criteria
Does the crusher operate economically with minimum maintenance Does the crusher have an acceptable parts replacement cost Does the crusher have easy access to internal parts How does the initial cost of the machine compare to the long term operating cost.

Primary crusher selection - Capacity

Primary Crusher Selection – Feed Size

Primary Crusher selection – Product size

Primary Crusher Selection – Compressive Strength

Primary Crusher Selection – Abrasion Index

Primary crusher selection – Clay content

Primary Crusher Selection – Underground Application

Primary Crusher Selection – Mobile Plants

computer Aided Design of Jaw Crusher
computer Aided Design of Jaw Crusher

Components of a Jaw Crusher

Material for components of Jaw Crusher

Kinematic Analysis of Jaw Crusher
The geometry of the moving Jaw results in a movement change which has a great effect on the crushing action and particle breakage. Based on the analysis of the moving jaw movement, the squeezing process and the crushing force distribution, the jaw plate wear on a macroscopic scale level aiming to predict the wear distribution on the jaw plate can be studied.

Swinging Jaw movement The reciprocating jaw MN driven by the eccentric shaft AB does kind of a periodic plane swing movement. Jaw crusher can be considered as a four bar mechanism in which link AN is the crank and OA is the fixed link

MN is the moving jaw and OM is the toggle bar.
In the analysis we are intended to find out the displacement, velocity and acceleration of various points on the swinging jaw plate.

Data extracted from standard Jaw crusher
Length AN = 172 cm Length MN = 1085 cm Length OM = 455 cm Co-ordinates of A (45.3 , ) Crank angle rotates from 0 to 360 degrees anticlockwise.

Crank angle vs. angle made by moving jaw

Crank angle vs. Angle between moving Jaw and Y axis
The graph shows as the moving Jaw approached its counterpart which is stationary it tends to be vertical i.e. the angle between the moving Jaw and the Y axis decreases as a result the crushed product slips downwards.

Vertical Displacement Vs. Horizontal displacement

Horizontal displacement Vs. Crank angle

Displacement Vs. crank angle

Points on the moving Jaw
Every point on the moving Jaw follows an elliptical path When it moves towards the fixed Jaw, it goes vertically down and in the return stroke it moves vertically up.

Vertical Velocity Vs. Crank angle
The rate of change of vertical velocity is greater for the topmost point and decreases downwards

Horizontal Velocity Vs. Crank angle
The rate of change of horizontal velocity is greater for the bottom most point and decreases upwards

Velocity Vs. Crank angle
The maximum rate of change of final velocity is greater for the points away from the crank.

Horizontal acceleration Vs. Crank angle
With progress from 0 to 360 degrees crank angle rotation the horizontal acceleration first increases then decreases

Vertical Acceleration Vs. Crank Angle
With progress from 0 to 360 degrees crank rotation the vertical acceleration first decrease then increases

Acceleration Vs. Crank Angle
The maximum acceleration is observed for the points farthest away from the crank angle

Effect of sliding motion on Jaw wear
Breakage Analysis 3 types of Fracture mechanisms are observed Abrasion Cleavage Shatter

Breakage Analysis The particle fracture mechanism in the Jaw crusher chamber is a mixture of cleavage and abrasion. The abrasion fracture is caused with the localised too much energy input to the area directly under the loading points and the Friction between the Jaw plates and the particle. The induced tensile stress results in the cleavage fracture.

Crushing Process Theoretically a particle inside the crusher is crushed when it is compressed and fails in tensile stress. In practice the particles also undergo slipping motion between the jaw plates The forces acting on the element during the crushing process is shown below

Crushing Process As the horizontal and vertical velocities of the moving jaw changes during the crushing process, the forces on the particle varies at different times. When the component of the vertical velocity is greater than the components of the horizontal velocity the forces on the particle is shown in Fig. 3.3 (a)

Crushing process When the component of the vertical velocity is less than the components of the horizontal velocity the forces are shown in Fig (b)

Crushing process By a resolution of forces acting on the particle as shown in figure 3.3. it can be proved that conditions for the particle to slip against the fixed jaw plate is much greater than with the moving jaw plate. Condition for slide between the particle and the fixed jaw plate is unavoidable The chance for the particle to slide is greater with the fixed jaw than the moving jaw. Due to vertical motion irregular geometry of particles, a classification process before the particle fracture may exist during close process in which the particle adjustment may take place.

Wear Analysis Squeezing and sliding are the two principal factors affecting the Jaw plates wear Squeezing plays the main role at the top of the crusher and the wear is small. As the particles move down the crusher the probability of slip increases and the wear becomes more pronounced. At the middle lower part of the crusher where the ratio of the vertical distance to the horizontal stroke reaches a maximum value resulting in maximum wear of the crusher. The slide between the fixed Jaw and particle is greater compared to the moving jaw hence the wear is dominant in the fixed jaw.

Discussion Points! What are the flaws of the current primary crusher installation? Where can we improve?

Selection and sizing of secondary and tertiary crushers
Selection and sizing of secondary and tertiary crushers

Introduction Modern crushers have increased in performance
Evolved to focus greater on the quality of desired product More stringent requirements are being placed in terms of shape and gradation. Proper size reduction results in better recoveries In milling feed preparation, the generation of fines and total top size reduction results in maximum mill productivity. Proper understanding of crusher capabilities will minimize both installation and operating capabilities.

How the symons cone crusher works video 5
How the symons cone crusher works video 5

New Generation of Cone crushers video 6
New Generation of Cone crushers video 6

Cone Crushers Modern Cone crushers New cone crushers
Safer more reliable hydraulic clamp and clearing system to protect the crusher from uncrushables and overload conditions Adaptation of hydraulic setting adjustment system in the cone crusher design improves overall efficiency of crushing operation New cone crushers Increased performance capabilities More power capabilities Larger in size Higher capacities Better product shape Higher percentage of final product yield

New cone crushers New generation of cone crushers provide
ease of operation Simple maintenance Uniform production throughout the liner life High availability Technology has evolved to include computer controls to maximize and optimize crusher performance based on application requirements Modern devises provide real time feedback : Power draw, cavity level, crushing force, temperatures, pressures, etc.

Cone crusher selection criteria
Information required Material characteristics Capacity required with consideration for expected availability Expected gradation and product size Specific gravity Bulk density Impact work index Moisture content Abrasion index How the material breaks Small scale lab tests and full scale pilot tests

Cone crusher design limits
Volume limits Power limits Force Limits

Design Limits - Volume Defining variables
Maximum rate of feed to the cone crusher without overfilling the cone crusher feed hopper Function of Speed of the crusher Closed side setting CSS Head angle Material density Feed gradation Crusher chamber configuration Transport of material through the crusher cavity Fragmentation characteristics

Design Limit - Power Power limit is reached when average power draw kW exceeds the installed motor power of the crusher. Ore of high impact work index or strong resistance to fragmentation tend to reach or exceed the power limit easily. Pilot scale test work can provide information regarding power consumption

Design Limit – Force factor
The force limit of a crusher is reached when the combined forces exerted during crushing exceeds the force available on the machine to hold the desired closed side setting. Force limits may be exceeded due to uncrushables material entering the crushing chamber Operating at a small closed side setting Packing of wet sticky material High power draws Incorrect crushing cavity design

Cone crusher sizes and Capacity ranges

Secondary Cone Crusher Selection
Example Ensure the feed material does not exceed the acceptable maximum size for the crusher Determine the capacity requirements at a given closed side setting based on a 4/6:1 reduction ratio. maximum feed material 200mm Capacity 500 tph Table 1 : HP 300 At 32 mm CSS the crusher is unable to achieve a minimum of 500 tph Table 1 : HP500

Secondary Cone Crusher Selection
Correct cavity configuration The cavity configuration has to suit the feed gradation so that the maximum crushing performance and liner utilisation is achieved Several cavity configurations are available for cone crushers to maximise performance. An improper liner configuration applied can create high crushing forces leading to adjustment ring movement , exceeding crusher force limit.

Case Study: HP700 replacing Hammer mill
Copper mine in Portland Results Hammer mill used to prepare rod mill feed Hammer mill replaced by HP700 cone crusher 20% gain in energy efficiency By reducing the rod mill feed from 80% passing 30 mm to 80% passing 14 mm.

Case Study: HP700 replacing Hammer mill

The Pre-crusher option
The most recent evolution for pebble crushing finds a basis in the presumption that the most appropriate primary mill feed contains a minimum amount of critical size material. The initial feed of the primary mill should dominantly consist of fine and coarse material. Coarse material serves as impact media and fines as transport medium for down stream processing. Pre-crushing targets to convert the middling to fine fraction.

Case Study: Pre- Crushing
Troilus Mine Kidston Mine 150 – 50 mm is pre- crushed using an HP 700 cone crusher Production increase and operating cost decreased. All primary crusher ore is pre -screened to remove fines All +50 mm oversize is crushed at maximum reduction ratio to deliver maximum fines. Proved effective in boosting milling productivity and lowering operating cost.

Typical Pre-Crusher installation

Track mounted cone crusher video 7
Track mounted cone crusher video 7 Selection and sizing of Secondary antertiary crushers

Case Study : Influence of Eccentric speed of Cone crusher production and operation Selection and sizing of Secondary antertiary crushers

Case Study : Pilot test program
The research was performed in Tampere, Finland using an HP 200 cone crusher The study can be separated into three groups of test: Base tests Fixed tonnage tests Feed size distribution tests The base tests were used to measure the crushers maximum performance for a given eccentric speed. The fixed tonnage tests simulated operating conditions where the feed rate to the crusher is limited below the maximum capacity based on the base eccentric speed and CSS Selection and sizing of Secondary antertiary crushers

Case Study : Pilot Test Program
A third set of tests utilized a different feed size in order to verify results as well as reducing the effect of top size particles possibly being inhibited to enter the crushing cavity. The tests in each group used the same homogenous feed of known characteristics with feed sample being taken every forth test for verification. Selection and sizing of Secondary antertiary crushers

Case Study: Pilot test results
Overall Base testing results Most of the data showed clear trends in capacity, power and discharge size distribution as the eccentric speed was varied. For the base testing where each test was operated at the optimal cavity level to develop a baseline for maximum production, the results matched theory. As the eccentric speed was increased the capacity decreased in a nearly liner manner. Selection and sizing of Secondary antertiary crushers

Case Study : Base Testing Results
On average, the total capacity tph fluctuated by 22.5% over a design speed range of 34%. The increase in capacity but decrease in reduction as the speed is lowered results in relatively low changes to power draw as shown in figure 2. Selection and sizing of Secondary antertiary crushers

Case Study: Base Testing Results
For a base case testing with a full cavity throughout, it was seen that there was slight benefits in throughput and energy efficiency when the crusher was operated at near the minimum design eccentric speed. The higher capacity outweighed the slight loss in reduction through the machine and the machine was more mechanically efficient at the lower speeds. It was best to operate at the low end of the speed range. Selection and sizing of Secondary antertiary crushers

Case Study: 32 mm CSS Production Vs. Specific energy
Selection and sizing of Secondary antertiary crushers

Case Study : 19 mm CSS Production Vs. Specific Energy
Selection and sizing of Secondary antertiary crushers

Case Study: Fixed Tonnage Test Results
The tests operated at a fixed tonnage were conducted to simulate a crushing application where the crusher is not the limiting equipment therefore the tonnage to the crusher is fixed by other plant limitations therefore the crusher cannot normally achieve a full choke condition. The power draw of the crusher dropped significantly as the speed decreased resulting in a lower kW/t specific energy through the machine. There was a major shift in reduction through the machine ‘ Selection and sizing of Secondary antertiary crushers

Case Study: Fixed tonnage test results
The tph of the mm product fell slightly as the eccentric speed reduced from the reference speed by 20% The phenomenon occurred at the point where the cavity level in the crusher could not fill up half of the crushing chamber and the discharge became coarser While operating with a higher cavity level was more efficient, the crusher was more mechanically efficient at the lower speeds Selection and sizing of Secondary antertiary crushers

Case Study : Fixed tonnage test results
For the fixed tonnage tests there was a marked improvement in the variation of power draw as the speed and cavity level increased. Selection and sizing of Secondary antertiary crushers

Case Study : Practical Application
There are a number of uses for these principles in a crushing plant. The main points are as follows: Changing the speed to find a more optimal setup than that supplied by the manufacturer. Manipulating the speed based on current static plant conditions, And dynamic control of eccentric speed in a control system. The optimization of eccentric speed may be beneficial where feed conditions and plant requirements change. Dynamically manipulating the eccentric speed using a variable frequency drive has not been widely used. Selection and sizing of Secondary antertiary crushers

Case Study : Practical Application
A dynamic control system can be used to vary the speed resulting in benefits to production and energy efficiency. E.g. When the throughput of the crusher is high it could be operated most efficiently in the lower speed range. If the throughput requirements drop for a short period of time it would be more productive and efficient to increase the speed of the crusher and operate with a fuller chamber. An underlining benefit for greater control of the crusher operation is maintaining a choke fed condition, which has benefit to production , operating cost and mechanical health of the crusher. Selection and sizing of Secondary antertiary crushers

Discussion Points! Choke feeding in your current application, the pros and cons. Selection and sizing of Secondary antertiary crushers

Selection and sizing of high pressure grinding roll crushers
Selection and sizing of high pressure grinding roll crushers

New crushers on the market Video 8
New crushers on the market Video 8

HPGR Introduction HPGR are well established in the cement industry for the grinding of clinker, limestone, slag and other relatively non-abrasive material. Minerals are 20 – 100 times more abrasive than cement raw materials. Acceptance by the minerals industry has required the development of special wear protection surfaces and rapid change out procedures for the rolls. Range of grinding Coarse < 75 mm To grinding of fine concentrate < 100 microns

HPGR Introduction Moisture content up to 12 %
Machines are available with capacities up to tph Installed power up to kW

HPGR Installed in Diamond and Iron Ore Industries

HPGR – L/D ratios Length to Diameter ratio
Is it more advantageous to design rolls with smaller diameters and larger widths or larger diameters and smaller widths?

HPGR – L/D Ratio The decision as to which approach to adopt is capital. It has an impact not only on the performance of the crusher but also major impact on the design of the individual components and on the general layout of the unit. The minimum roll diameter is prescribed by the outside diameter of the bearings and the thickness of the bearing block. The bearings are sized according to the installed grinding force.

HPGR – L/D Ratio The size of the bearing determines the shaft diameter and pre- determines the manner in which the gear box and shaft are to be connected. Larger rolls with low L/D ratios offer greater freedom in selecting the most appropriate bearings. The larger roll diameter makes the connection between the shaft and the gear box simple to execute. And allow large gear boxes to be located on one side to save space and facilitate maintenance.

HPGR – Roll design Three different roll designs have been successfully applied: Solid rolls Rolls with tyres Rolls with segmented liners

HPGR – Criteria for selecting optimum design
The balance between operating and investment cost The acceptable lifetime and frequency of replacement The tolerable down time for liner replacement

HPGR - Comparison Tyres Segments Lower investment cost
No interfaces (joints) Longer lifetime Lower wear cost No pressure restriction Higher investment cost Joints between segments require more maintenance due to washouts Shorter lifetime Higher wear cost Only for low pressure

HPGR - Wear protection surfaces

HPGR - Wear protection of roll surfaces

HPGR – Key Parameters Throughput
Achieve the throughput requirements and to achieve the desired product fineness Function of roll dimension Type of roll surface Feed material properties For a given material and roll dimension the throughput is controlled by the roll speed.

HPGR – Key Parameters Product Fineness
Controlled by the grinding force applied to the material bed between the rolls. The grinding force creates the pressure in the material bed which causes micro-cracks and breakage of the particles. The correlation between particle breakage and grinding force required needs to be determined for each material Key parameters are Specific throughput rate Specific press force to be applied to achieved the desired comminution results

HPGR – Throughput rate vs. roll speed

HPGR – Feed moisture content vs. Throughput rate

HPGR – Throughput vs. Size distribution

HPGR – Product fineness

HPGR – Product of various ores

HPGR – Energy consumption vs. Force

HPGR – Energy Input vs. Roll Surface

HPGR - Energy Input for various ores

HPGR – Energy input vs. Grinding force

HPGR Wear Factors

HPGR – Roll Diameter Vs. Roll Speed

HPGR - Application

HPGR – Pebble Crusher

HPGR – Pre-Crusher

HPGR – Replacement of 3rd and 4th stage

Discussion Points! Is it possible to include HPGR in your circuit?

Ore Characterisation

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)

Highlands Valley 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 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.

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.

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

Metallurgical 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

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.

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.

Case Study: Antamina boosts throughput for hard ores
Case Study: Antamina boosts throughput for hard ores

Introduction The ore body that Compania Minera Antamina has been mining in Peru since contains two principal ore types, copper molybdenum ores and much harder copper zinc ores which exist about 70 : 30 ratio. Historically the copper zinc ores were processed at a far slower rate and it was clear that something needed to be done. A collaboration between Metso Process Technology and Innovation and the Mine began in which aimed to optimise the entire comminution process.

The team began by auditing the drill and blast practice as well as sampling the crushing and grinding circuit. This helped them to develop models that would reveal what each step was achieving and what could be tweaked to improve performance The mine and the processing plant was then benchmarked. The models were calibrated and then a number of scenarios of operating strategies for both mine and process plant were run. An in-depth review of existing practices were carried out.

The ore was categorised in varying groups of hardness. Blast practices were audited and blast fragments were measured which made it possible to benchmark existing practices, and to define the main constraints related to wall stability and control ore dilution and environmental aspects. Site specific models for the comminution process was created and it became evident that the largest potential gains to the blast could be found. The basic idea was to increase the powder factor using more explosives to create a finer ROM fragmentation so that downstream equipment would treat the ore with ease.

In the drilling process the drill pattern ( burden and spacing) was reduced By maintaining the same type and amount of explosives in each drill hole, the corresponding blast powder factor rose from kg/ton In addition switching to electronic detonators proved to be more reliable and ensured that blasts went off according to plan. A pebble crusher was also installed and modification to the pulp lifters were made.

Conclusion Mine to mill optimisation work increased throughput by 30 % Process plant improvements contributed 10 % increase in throughput Reduction in hardness of the copper zinc ore contributed 15% to the increase in throughput As of 2011 Antamina was processing copper zinc ores at an average rate of tons per hour, up 60 % from the performance prior to 2007 The copper – molybdenum ore also saw an increase to tons per hour.

Case Study : Batu hijau (Indonesia) production planning for the combined mine to mill operation

Case Study: Introduction
The Batu Hijau copper – gold operation commenced a mine to mill program in 2001 with the standard initial objective: To modify blast practice to improve SAG mill throughput. The work presented spans over 10 years of development.

Case Study Batu Hijau Using rock mass characterisation data, ore hardness and blast design data, simple regression models were developed which predicted SAG mill throughput. This was done for different zones in the ore body ultimately resulting in separate throughput predictions for 16 ore body domains. JKSimMet was used to enhance the initial regression models in order to more accurately predict the expected SAG mill throughput for the different domains.

Case Study : Batu Hijau Attention then turned to developing the best blasting practice for the domains to reduce fragmentation top size in order to improve loading rates in the pit and increase grinding circuit throughput. Different blast designs were developed for each domain. The modelling approach also provided a basis for ore scheduling and production forecasting

Case Study : Batu Hijau The second phase of the study was based on improving prediction of mill throughput based on improved orebody characterisation. Improving prediction of blasting performance and refining mill models. The other major advance has been the use of the modelling approach for both short and long term production planning.

Case Study : Batu Hijau In 2007 the equations linking mill throughput to measurable variables were coded into the mine block model so that throughput predictions became a direct output from the block models. As previously the throughput relations were based on regression models of the tph as a function of characterisation variables. In effect the models established a benchmark performance which can be expected when mining and processing ore from different domains.

Case Study : Batu Hijau Outcomes first phase
Productivity gains of 10% for loading rates in the pit and 10-15% increases in SAG mill throughput for the individual ore domains were reported. Some of the important requirements for the effective implementation of the Batu Hijau M2M Strategy included: The need for a dedicated team of involved staff from geology, mining, milling IT support Strong and on-going support of senior management Best possible orebody characterisation – an on- going requirement with continuing updating of the domain models

Case Study : Batu Hijau Outcome first phase
Accurate models of blasting and comminution to establish expected performance for each domain and the best balance in cost and effort between blasting and milling for each domain

Case Study : Batu Hijau Outcome second phase
The second phase of the Batu Hijau study provides the basis for a much wider range of M2M applications than just increasing SAG mill throughput. There is a demonstrated ability to predict mill throughput over the long term to +/-2% accuracy At the core of the latest developments is a greater ability to predict mill throughput with considerable accuracy for different ore sources.

Case Study : Batu Hijau The applications of that capability include:
The understanding of the expected or benchmark performance against which actual performance can be compared. Deviations from the expected can be identified and remedial action to regain performance can be better targeted The availability of a sound basis on which improvements in the grinding circuit can be identified, implemented and measured A tool which is an integral part of both long and short term production planning to achieve required production rates

Ore dressing studies – what is involved
Ore dressing studies – what is involved

ODS - What is involved 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.

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

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.

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.

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.

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.

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.

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.

Profit based grinding controls
Case Study : Sierrita Program controller : Duval Corporation Profit based grinding controls

Case Study : Sierrita Introduction
The comminution circuit represents the largest user of energy in the Mineral Processing Industry As the grades of ore reduces the economics of energy usage becomes more significant. Pertinent control theory for the control of comminution circuits has been known for a long time but it is of recent years that practical techniques and robust computer control architectures for these systems have become available

Case Study : Sierrita Description of the Plant
Wet grinding circuit treats stpd Sixteen x Allis-Chalmers overflow ball mills are operated in parallel in a conventional closed circuit wet grinding system. The very low grade ore, variable crusher product, and changes in ore hardness produces disturbances that upset the performance of the grinding circuit.

Case Study : Sierrita Instrumentation Variable feeder
Feeder Weight measurement system Feed water flow meter Sump water flow rate Sump level indicator Mill power draft Pump amperage Control valves to feed and sump water All the other variables are calculated using inferential techniques

Case Study : Sierrita Process Analysis study
Fig. 2 shows a schematic diagram of a ball mill / cyclone control system This diagram shows the instrumentation and the calculated variables use in the control strategy. From the process analysis study several objectives were established.

Case Study : Sierrita Objectives Reduction of mill feed size
Reduction of mill power consumption Extending mill transport conditions Investment in variable speed drives Identification of proper linking of manipulated variables with control variables Identification of inferred measurements and signal conditioning of the raw measurements

Case Study : Sierrita Disturbances
Mill feed particle size distribution due to bin segregation and crusher circuit operation Ore hardness and ore mineralogical structure and composition due to natural mining characteristics Pumping / classification limitations and equipment wear. Process analysis study showed that the calculations of the inferred calculated variables can provide adequate information for the development of control strategy

Case Study : Sierrita Design of the plant control strategy
A finer grind producers better recoveries but loss in throughput rate. Swings outside the given band produces losses Design of the plant control strategy Fig. 3 shows that for a given ore there is a unique milling rate to provide the grind size that will yield maximum profit under certain economic conditions A higher milling rate can be achieved with a coarser grind which is off set by losses in recovery due to poor liberation

Case Study : Sierrita Design of plant control strategy
The mill load constraint controller involves the changing mill transport constraint and sets the tonnage for feasible operation Design of plant control strategy Fig. 4 shows the control objectives and limiting conditions that the control system must overcome to produce a profit. The main controller is the mill load constraint controller for safe operation followed by the grind cut controller for profitable operation

Case Study : Sierrita Design of plant control strategy
The grind cut inferential controller maintains the optimal liberation, if process operational limits permit. The curves depicted in fig. 3 and 4 are not unique and are changing constantly. Thus the information must be handled on a timely basis in a computer system. The computer system will in turn provide for adapting values of the moving constraint and set points. This known as online adaptive decision making or control.

Case Study : Sierrita Grinding controls
The primary objective of the grinding controls system is to provide a flexible , adaptive, easy to use system to: Maintain an optimal throughput depending on the ore conditions. This will provide the downstream process with a constant size distribution for improved recovery. Or to maintain a stable operation while assisting the operator in maximizing the throughput, avoiding frequent upsets or spills and maintaining an adequate grind.

Case Study : Sierrita Grinding controls
Simplified function block control strategy Grinding controls

Case Study : Sierrita The four principle controllers
The ball mill load control system The grind index control strategy The ball mill transport index control strategy Sump level controller

Case Study : Sierrita The ball mill load controller
This is the main controller Any additional capacity of the ball mill depending on the grind setting is sensed by the ball mill load controller and the feed rate is increased.

Case Study : Sierrita Control Design

Case Study : Sierrita MAIN CONTROLLER WINDOW DISPLAY

Case Study : Sierrita Computer Architecture for plant management

Case Study : Sierrita Overall plant control strategy
The current objective of increasing the recovery / profit by running an optimal throughput can be enhanced by proper co- ordination of the plant activities. Fig. 8 shows the computer architecture used to integrate the distributed control system with process management activities Four process control units are networked to two operator interface units. The plant host computer is also used for engineering analysis of operating and lab oratory information with statistical modelling, process analysis and simulation and reporting software packages.

Case Study : Sierrita Conclusions
A profitability concept was transformed into a feasible mode of operation. The tonnage setting can be safely pushed up to 400 stph from 250 stph while maintaining metallurgical performance.

Crushing / Screening and Conveying

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