Learning Unit 5_PROCESS CONTROL.pptx

Vaal University of Technology
Department of Metallurgical Engineering
MINERAL PROCESSING IV
PROCESS CONTROL IN MINERAL PROCESSING
Kentse Thubakgale

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• Mineral processing plants consist of networks of control systems that
vary from alarm signals to complex adaptive control strategies.
• The control strategies work within a distributed control system (DCS).
• Data collected from different points of the plant is fed to a centralized
control room for viewing by operations supervisors.
• Operators on the plant have the responsibility of monitoring and
supervising specific unit operations.
• The operators either respond to signals from local control loops or take
commands from the operations supervisors.
Introduction

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• Process control is an essential part of concentrator operations – it
provides vehicle for improving operation economics by increasing
revenues and reducing costs.
• Process control is the technology required to obtain information in real
time on process behaviour and then use that information to manipulate
process variables with the objective of improving metallurgical
performance of the plant.
• Process control will not correct inherent design / flowsheet problems.
Introduction

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• In operating plants, considerations may be:
o Is your feed stable?
o Are your instruments calibrated and performing?
o Are you aware of wireless instruments (including vibration)?
o Is your control system up to date and stable?
o Are you in manual or auto control?
o Are your operators acting on alarms or are they nuisance?
o Do you understand and accept your process variability?
o Are you operating within the design targets (set points) and
process constraints?
Introduction

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o Are you using your surge capacity or running tight level control?
o Are you at optimum and are the controls robust?
o Are you benefiting from asset management systems?
o Are failure/fault detection systems implemented?
o Can you make the same product for less energy, or material
consumption?
o Do you operate with or without a Control Engineer?
o Does your Plant/Process or Processes run as well as your car?
Introduction

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• Process control technology includes:
o Process measurement and monitoring
o Interfaces between the process, the operator and control system
• When introducing process control in mineral processing plants:
o What are the key process variables that should be controlled?
o Is there an economic justification for control?
o What can be controlled?
o What control philosophy can be used?
o Will the reliability of the proposed control system be high enough?
Introduction

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• Control objectives
o Minimize ore inventory by treating ore brought to the surface as
quick as possible.
o Measure the amount of metal in the feed ore being processed.
o Maximize mill throughput.
o Improve recovery of the valuable minerals.
o Improve head grade to downstream processes.
o Reduce operating costs by saving reagents.
o Use plant personnel more productively.
• When metal demand is high objective is to maximize throughput.
• When demand is low objective is to improve recovery and/or lower
operating costs.
Control objectives

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• Objectives lead to performance optimization (achieving and
maintaining the desired steady-state operating point).
• Control objective for an overall process is to keep physical
variables as close as possible to the desired values or set points.
• Physical variables: Flows, concentrations, densities, levels,
temperatures, pressures and speeds.
• Control systems should be able to manipulate process inputs to
achieve steady-state even in the presence of disturbances.
• Disturbances and uncertainties are the major reasons for using
control systems.
• Clear control objectives are key for the optimization of plant
economic performance.
Control objectives

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• Input variable
• Shows the effect of the surroundings on the process.
• Refers to the factors that influence the process.
• E.g. the flow rate of the steam through a heat exchanger that
would change the amount of energy put into a process.
• Types of input
• Manipulated inputs: variable in the surroundings can be
controlled by an operator or the control system in place.
• Disturbances: inputs that can not be controlled by ah operator
or control system, can be measurable and immeasurable.
Classification of process variables

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• Output variable
• Also known as control variable.
• Variables that are process outputs that affect the
surroundings.
• E.g. The amount of CO2 gas that comes out of a combustion
reaction.
• These variables may or may not be measured.

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o Category 1 – Outputs
o Kept as close as possible to their set-points
 Measured outputs
 Unmeasured outputs – control objectives properties that
cannot be measured
o Category 2 – Inputs
o When changed, outputs also change
 Control inputs – changed in order to steer outputs to desired
set points
 Disturbance inputs – affect outputs in any other way

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Grinding circuit process variables (Craig, 2014)

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• Two approaches used for control system structuring
o Top-down approach
o Bottom-up approach
Control system structuring

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• Top-down approach
o Step-wise design/decomposition
o Starts with the big-picture and breaks it down to into smaller
fragments
o Breaking down the system to get insight into its compositional sub-
systems
o Formulate an overview of the system and specify first-level sub-
systems
o The subsystem will then be detailed until the point of base
elements
o Specified with the assistance of “black-boxes”

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• Bottom-up approach
o Piercing together of systems to build complex systems
o Original systems become subsystems of the emergent system
o Information system of incoming data from the environment to form
a perception
o Individual base elements of the system are first specified in
details and linked to form larger subsystems until a complete top-
level is achieved.

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• Bottom-up approach
• Feedback control (Closed loop)
• Most widely used building block for
process control systems design
• The principle is to reduce the effect of
disturbance input by first measuring its
effect on a process output and then
calculating the necessary correcting
input.
• Disturbances have to enter the system
in a well defined way.

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• Control inputs have to be adjusted
when entering the system in the
presence of the disturbance input.
Advantages
• Incorporates an inherent self-
correcting action.
• Does not require detailed information
on the characteristics of the process.
• Can operate with immeasurable
disturbance input.

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• Two-cascade control configuration -
Ball mill grinding circuit control
scheme
• Disturbance inputs immeasurable
• Maintaining a constant cyclone feed
water: solids ratio

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• Feedforward control
• Control input that attempts to
counteract the effect of the
disturbance is generated from the
measured disturbance input
• Measurable disturbance inputs
• Not as robust as the feedback
control system
• Used in conjunction with a feedback
control loop in order to improve its
robustness for unmodelled effects

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• Top-down approach – e.g. Grinding circuit
• Top-down approach involves careful analysis of control objectives in
order to select control principles to be implemented
• Typical control principles:
o Cyclone inlet solids flow control
o Cyclone underflow density control
o Mill power maximum-seeking control
o Variables to be controlled are the identified
• For each control principle selected, variables are identified
• If the variables cannot be measured, related measurable outputs must
be chosen
• Suitable control inputs must also be selected
• Control inputs and related outputs are then grouped into subsystems

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• Process-measurement
Properties to be measured may include:
1. Flow rate of solids on conveyor belts
2. Flow rate and density of dirty liquids and pulps in pipes and
launders
3. Particle size and size distribution of mill products in pulps
4. Flow rate and concentration of reagents in pulps

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• Process-measurement
Operation of an industrial instrument depends on:
1. Process stream sampling devices
2. Sensing element
3. Signal transmission cable
4. Data presentation device
5. Auxiliary electricity, air and water supplies
6. Trained personnel
7. Equipment needed to maintain and calibrate the measurement
system

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• Process-measurement terminology
• Measurand – Physical property to be measured
• Process medium – a channel or system of communication
• Sensing element – produces an output that varies as a function of
measurand
• Signal conditioning – provides more accurate sensor measurements
• Signal processing – modification of time-series data for analysis
• Transmission – convey coded message

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• Basic Physical Measurements
1. Solids flow rate
2. Liquids flow rate
3. Liquid level in a vessel
4. Pulp water content
5. Online particle size measurement
6. Basic chemical measurement

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Process control strategies
• Mineral processing plants have control systems that work within a
dedicated Distributed Control System (DCS).
• The systems focus on a centralised control room into which all data are
fed for viewing by an operations supervisor.
• On the plant floor, operators and helpers are responsible for monitoring
and supervising specific unit operations.
• This is in response to signals on local control loops or to commands
from the centralised system or supervisor.

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Process control strategies – Crushing plants
• Mill system
• Crushers
• Screens
• Feeders
• Conveyor belts
• Magnets
• Metal detectors
• Chutes
• Hoppers and bins
• Pumps and sumps
• Pipes and water flow rate controllers
• Grinding mills
• Classification units (cyclones or classifiers)
• Assorted head tanks and pressure control systems

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• Crushing circuit control system:
1. Control of feed ore by vibratory or rotating feeder
2. The set point to the system derives from the power draw from the
crushing unit or from a level sensor mounted within the crusher
chamber.
3. Vibrating screens in the crushing plants are used to control the
product size and to ensure that coarse material is retained within
the circuit for re-processing.
The feed to the crushers is controlled to match the overall plant
feed and maintain steady conditions with respect to bin levels and
power draw.
4. Conveyors have metal-detecting sensors to protect the crusher
mantle and bowl from receiving uncrushable (steel or wood)
material.

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• Crushing circuit control system:
• Control systems must respond to:
• Changes in ore conditions (feed size and hardness)
• Changes in the amount of fines and/or moisture – “clay minerals”
• Presence of wood and/or metal (proper protection leads to trip-outs)
• Trip-outs of other equipment
• Hang-up of material within bins and chutes
• High and low level alarms on bins and on power draw

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Primary sensing element, accuracy and calibration
• Accuracy of a measurement is the closeness of the displayed or output
value to the true values of the measurand.
• It is quantified in terms the measurement error or inaccuracy i.e. the
difference between the measured value and the true value.
• True value of a variable is the measured value given by a standard
instrument of ultimate accuracy.
• Repeatability is the ability of the measurement system to give the same
output

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Process measurements
• Basic physical measurements
1. Flow rate of solids
2. Flow rate of liquid
3. Liquid level in a vessel
4. The water content of a pulp
5. On-line particle size measurement
6. Basic chemical measurements

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Actuators and final control elements
• A controllable process requires sufficient number of actuators and final
control elements.
• Controllability of a process requires that actuators to should allow
process variables to be changed over a sufficient range without
excessive change in the gain.
• Actuators used in ore treatment plants fall into three man categories:
1. Control valves
2. Material feeders
3. Variable speed drives

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1. Control valves
• Main types of valves used in mineral processing plants include Globe,
Ball valves, Diaphragm, Pinch valves.

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1. Control valves
• Globe and ball valves are suitable only for use on clean or slightly
dirty fluids.
• Small ball or needle valves are often used for reagent flow control
duties.
• Diaphragm valves are the traditional means of manipulating the flow
rate of pulp in a pipe line. They have the advantage of simplicity and
robustness, but the disadvantage of having a large mass and a high
actuator power requirement.
• Pinch valves consist of a length of rubber pipe which is pinched
between bars or, preferably, by a pneumatic or hydraulic sphincter.
• The rubber pipe has significant hysteresis, and a valve that provides a
pull action as well as a pinch action may be required to overcome this.

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1. Control valves
• Actuators for control valves may be electrical, pneumatic and electro-
hydraulic.
• Pneumatic actuators are popular because they offer they have a high
power to mass ratio, simplicity, low-cost and a well-defined fail-safe
action.
• Actuators assist in operating valves in infinitely variable and fully
open/fully shut modes.
• Electrical valve actuators powered from the main electricity supply are
often unreliable as a results of power failures.
• Electro-hydraulic actuators overcome the problem by providing a
reserve accumulator to provide definite failure action. These actuators
do not require air supply but can be expensive as a result of high-
precision construction requirements.

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2. Material feeders
• Vibrating and belt feeders are the commonly used types of automatic
control feeders.
• The vibrating feeder is a flexible device that can be used for feeding
fine powder to ROM rock.
• Advantage: High throughput capability, low cost and low
maintenance requirements.
• Disadvantage: the actual mass feed rate varies erratically when
the material changes through the range from wet fines to dry rock.
• Belt feeders are expensive and require high maintenance but have the
advantage that a belt weighing action can be directly incorporated to
give a so-called weighfeeder.

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3. Variable-speed drives
• Two types are available: Hydraulic control and solid-state
electronic control.
• Hydraulic variable-speed drives are based on a variable fluid
coupling placed between a conventional a.c. induction motor and its
load.
• Electrical variable-speed drives can be subdivided into a.c. and eddy
current types. A.C. drives consist of a solid-state rectifier/inverter for
control of frequency and voltage supply. They are expensive but
reliable.
• Eddy-current have a variable magnetic coupling placed between a
conventional a.c. induction motor and its load. They are reliable and
easy to maintain.

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Process control system design
philosophies and architectures
• Process control systems basically gather data from various process
instrumentation subsystems and present the data in a suitable form to the
process operating personnel (act as interface between the process and the
operator).
• Another require is to implement an automatic safety shut-down logic from
alarm monitoring and reporting tasks.
• Control systems also have the task of data presentation, storage and
processing. This is important in presenting data for process study,
evaluation and problem investigation purposes.

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• Centralized computer control systems
• Previously, computer process control systems were arranged in such
a way that individual signals from all field transmitters were wired into
one central computer.
• The computer carried out all the process control tasks including
information display, continuous control, alarm monitoring and safety
shut-down logic functions.
• Recently, control systems have been built such that process control
tasks are distributed among simpler separate hardware modules. The
separate modules are computer-based.

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• Centralized computer control systems
• Modern systems have a number of programmable logic controllers
(PLCs) connected to a mini computer in a star arrangement.
• The PLC modules perform scanning and pre-processing of the sensor
inputs as well as logic control and PID control tasks.
• The minicomputer provides information display, historical data logging,
report generation and process study tools.

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• Distributed computer control systems
• The increasing availability of cheap digital computing power in the form
of microprocessors and difficulties associated with using a single
minicomputer for centralized computer control systems have lead to the
development of digital computer-based process control systems known
as distributed computer control systems.
• These control systems combine the advantages of hard-wired analogue
process control systems and centralized computer control systems
without incurring the disadvantages of either.
• A distributed computer control system consist of a collection of fairly
autonomous digital processing modules that communicate with one
another over a shared high-speed digital communication channel or
data highway.

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• Various process control tasks are distributed among separate modules
which are physically distributed along the data highway.
• This presents simplicity and fault tolerance through the use of standard
modules and lowered wiring costs.
• Data collation and processing capabilities of the digital computer are
retained. This allows for powerful centralized information display and
operator interface capabilities and the implementation of advances and
flexible forms of control.

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Application of process control in the mining industry
• Mining
o Ventilation airflow (and pressure) control, compressed air and
dewatering controls
• Milling
o Feed and tonnage control, water ratio control
o Pumpbox and surge tank control
o Cyclone pressure and density controls
o Flotation, pH, air flow and level controls for Grade/Recovery Control
o Reagent (chemical controls) and ratio controls for Grade/Recovery
o Thickener bed level, flocculant, density, for clear overflowa and final
moisture controls

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Application of process control in the mining industry

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Control strategies
• The development of a control strategy consists of formulating or identifying the
following.
1. Control objective(s).
2. Input variables—classify these as (a) manipulated or (b) disturbance variables;
inputs may change continuously, or at discrete intervals of time.
3. Output variables—classify these as (a) measured or (b) unmeasured variables;
measurements may be made continuously or at discrete intervals of time.
4. Constraints—classify these as (a) hard or (b) soft.
5. Operating characteristics—classify these as (a) continuous, (b) batch, or (c)
semicontinuous (or semibatch).
6. Safety, environmental, and economic considerations.
7. Control structure—the controllers can be feedback or feed forward in nature.

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Control strategies
• A common multivariable control problem that we face every day is taking a shower.
Control objectives: Control objectives when taking a shower include the following:
a. to become clean
b. to be comfortable (correct temperature and water velocity as it contacts the
body)
c. to “look good” (clean hair, etc.) d. to become refreshed
Input variables: The manipulated input variables are hot-water and cold-water
valve positions.
Disturbance inputs include a drop in water pressure (say, owing to a toilet flushing)
and changes in hot water temperature owing to “using up the hot water from the
heater.”

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Control strategies
Output variables: The “measured” output variables are the temperature and flow rate
(or velocity) of the mixed stream as it contacts your body.
Constraints: There are minimum and maximum valve positions (and therefore flow rates)
on both streams.
The maximum mixed temperature is equal to the hot water temperature and the
minimum mixed temperature is equal to the cold water temperature.
The previous constraints were hard constraints—they cannot be physically violated.
An example of a soft constraint is the mixed-stream water temperature—you do not
want it to be above a certain value because you may get scalded. This is a soft constraint
because it can physically happen, although you do not want it to happen.

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Control strategies
Operating characteristics: This process is continuous while you are taking a shower but
is most likely viewed as a batch process, since it is a small part of your day. It could easily
be called a semicontinuous (semibatch) process.
Safety, environmental, and economic considerations: Too high of a temperature can
scald you—this is certainly a safety consideration.
Economically, if your showers are too long, more energy is consumed to heat the water,
costing money.
Environmentally (and economically), more water consumption means that more water
and wastewater must be treated. An economic objective might be to minimize the
shower time. However, if the shower time is too short, or not frequent enough, your
clothes will become dirty and must be washed more often—increasing your clothes-
cleaning bill.

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Control strategies
Control structure: This is a multivariable control problem because adjusting either valve
affects both temperature and flow rate.
The measurement signals are continuous, but the manipulated variable changes are
likely to be discrete (unless your hands are continuously varying the valve positions).
Feedback control: As the body feels the temperature changing, adjustments to one or
both valves is made. As the body senses a flow rate or velocity change, one or both
valves are adjusted.
Feed-forward control: If you hear the toilet flush, you move your body out of the stream
to avoid the higher temperature that you anticipate. Notice that you are making a
manipulated variable change (moving your body) before the effect of an output
(temperature or flow rate) change is actually detected

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Control strategies
• Primary Ball Mill Circuits
• Improved product quality
• Increased throughput
• More stable operation
• Easier operation
• Orderly maintenance scheduling
• Timely management reports

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Control strategies
• Generally accepted control objectives for primary ball mill circuits:
1. Maintaining a desired percent solids in the mill
(a requirement for proper operation of the ball mill)
2. Maintaining a desired measure of the particle size distribution at the
cyclone overflow
(a product specification requirement for proper flotation operation)
3. Maximizing throughput
(an expression of the desire to get the most out of the circuit, is the easiest to
evaluate in economic terms)

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Control strategies
Generally accepted control objectives for primary ball mill circuits:
• Maximizing throughput when circuit changes are not allowed
• It is no longer possible to increase fresh feed into the circuit due to
constraints within the circuit
• E.g. The sump overflows, the cyclone is roping at the underflow or the
ball mill inlet is rejection material at the inlet end
• Throughput is maximized when the control is able to counteract the effect
of the constraint continually.
• Different hydrocyclones and variable-speed drives on the sump pump are
ways of removing circuit constraints to obtain higher production rates.

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Control strategies
Control strategies for circuits with hydrocyclone classifiers:

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Control strategies
• Base level instrumentation and control are provided to set values of local
variables manually.
• In the absence of ore variations - and also under conditions not exceeding
capacity constraints (both high or low) on any piece of equipment at any time
– base level automation will provide a manually set throughput at a fairly
consistent product size.
• Over a range acceptable to the circuit, product size can be increased
(decreased) by increasing (decreasing) fresh feed rate.

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Control strategies
• Base level instrumentation and control consist of:
1. Control of fresh feed rate with manual set point.
2. Control of headwater with manual set point.
3. Control of tailwater with manual set point.
4. Control of sump level with manual set point by varying pump speed or
tailwater flow in the case of a constant speed pump.
• Manual set points can be determined from operating experience and expected
circuit performance.
• E.g. If a high sump level can be maintained, a higher throughput can be
obtained for the same product size with a constant speed pump.
• At the same speed, the pump can deliver more slurry with a high inlet
pressure, i.e., higher sump level.
• This can be realised where all equipment is operating within capacity limits.

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Control strategies
• If ball mill, pump and
hydrocyclone capacities
are not constraining, two
additional control loops
may be added.
• (1) Headwater flow rate
set point to fresh feed
rate
(2) Slurry density gauge
at hydrocyclone feed,
control its density by
manipulating fresh ore
feed set point.

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Control strategies

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Control strategies
• Control strategies may be added to address control objectives such mill discharge,
density, product size and maximize throughput.
• First additional control strategy is as follows:
• Mill discharge density cannot be easily measured directly, hence, it is calculated
based on mass balance relationships. This requires the measurements and
calculations of mass flow of water and solids in hydrocyclone feed.
• The subtraction of tailwater flow will allow the mill discharge density to be
determined.
• The calculated mass discharge density is then used as the measurement to mill
discharge controller.
• The controller input is summed with the output of the ratio between fresh ore feed
and headwater.
• The controller set point can be changed to determine optimal grinding efficiency
due to density effect through the mill.

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Control strategies
• Second additional strategy:
• Direct control of desired product size requires a hydrocyclone overflow
particle size measurement.
• Overflow particle size controller has a manual set point and the output is
the fresh feed rate set point (This loop replaces the hydrocyclone inlet
density control).
• Experiments have shown that the effect of tailwater on product size is
minimal. Tailwater is therefore not chosen to control particle size.
• Maintaining a desired particle size set point by manipulating fresh feed
rate achieves both control objectives of maintaining size and
maximizing throughput.
• Any further increase in throughput will also increase (coarser) size.
• Conversely, for the same ore, reducing (finer) size set point will reduce
throughput.

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Control strategies
• The control strategies above have not taken into account circuit equipment
capacity constraints.
• Sufficient direct and calculated measurements on the circuit make it
possible to incorporate additional constrain control on the circuit.
• Principle of constraint control:
• Consists of automatic detection of equipment reaching capacity
constraints and then notifying the operator or taking programmed
automatic action so that the constraint is not exceeded, but at the cost
temporarily relinquishing one or more of the control objectives.

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Control strategies
• Example: If it is determined by mass balance calculations that the
hydrocyclone underflow solids mass flow is too high and “roping” is
imminent, fresh feed rate can temporarily be reduced to a predetermined
amount to alleviate “roping” while giving up size control temporarily.
• If “roping” persists or recurs much to frequently, it generally means the
hydrocyclone sizing is wrong and must be corrected.

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• Source:
• Stanley, G. G. 1987. The Extractive Metallurgy of Gold in South Africa, The
South Africa Institute of Mining and Metallurgy Monograph Series M7,
Johannesburg, Vol 2.

Learning Unit 5_PROCESS CONTROL.pptx

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