CHMT4011-PROCESS CONTROL

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School of chemical and metallurgical Engineering

CHMT4011-PROCESS CONTROLProject Part 1

Machethe Lebogang Given-6862115/4/2015

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Contents1.Project (Simulation) Description Summary12.Simulation Analysis33.Appendices43.1Appendix A: Nomenclature and numerical constants43.2Appendix B: List of equations53.3Appendix C: Process Flow Diagram-Milling Circuit63.4Appendix D: Simulink Model Map7

1. 2. Project (Simulation) Description SummaryThis project involves the simulation of a grinding circuit that consists of a ball mill, sump and a hydro-cyclone in a closed loop (see figure 3.2.1) using Simulink. For convenience and better overview, the model has been divided into 3 major sections, namely control, calculation and report sections. The control section is where most simulation parameters can be found and manipulated to the interest of the person modeling. The calculation section contains all the equipment model calculations of the most important process variables such as ball mill residence time, slurry S.G, percent oversize material etc. Lastly, the report section encloses the scope display blocks and this is where simulation results can be viewed after each simulation run. The scopes are intended to give stream properties (mass & volumetric flow-rates and specific gravities) while the displays show the steady state values.Equations for modeling the circuit included material balance equations coupled with empirical equations as listed in appendix C. Parameters of interest in this simulation are the ball mill residence time and the oversize material mass fraction in hydro-cyclone product stream (overflow). The circuit capacity was also tested by determining the maximum amount of dry ore the system can operate with before the capacities of individual equipment are exceeded (especially the sump which has a fixed height of 4m).It was found that under normal conditions, with the provided parameters, the slurry level in sump is approximately 6m (overflowing), mill residence time was 1129 min () and the percentage of solids in the hydro-cyclone overflow was found to be 50% while the fraction of oversize material was very small at 0.2%.For reference, the Simulink model can be accessed from: http://1drv.ms/1IDqNsJ

Important Assumptions1. DensityDensity is necessary for the conversion of mass flow rate to volumetric flow-rate; therefore, after calculating specific gravity (using equation 3 in appendix B), it needs to be translated into density through a relationship.By definition, specific gravity is a ratio of densities between fluid of interest and a reference fluid (usually water at and 1 atm). Mathematically it can be written as follows:

The reference density of water was taken as 1000 kg/m3and assumed constant throughout and densities of slurries were calculated from the relation above.3. Oversize material concentrationFor this simulation, the oversize mass concentration (or fraction) is taken and used on wet basis; i.e. if Dc represents the mass fraction of oversize material in stream D, then this would be the fraction of oversize material in the whole slurry. This distinction is important because the dry and wet basis mass fractions will always be different excerpt in a slurry with no water (e.g. Fresh ore feed).4. Hydro-cyclone operationFor the purpose of making hydro-cyclone material balance solvable, it is assumed that the solids in the underflow (recycle) contain only the oversize material. This is a fair assumption since it is highly desired to recycle the least amount of finer particles as it would be useless to recycle fines.5. Initial slurry level in the sumpTo make it possible to solve the dynamic volume balance over the sump, an initial height of 1m was taken as basis. The simple reason for this assumption is that at , volumetric efflux, given by will not be computable if h=0; i.e. the simulation will return an error since square-root of zero is not defined. This has negligible effect at the start of simulation.6. Parameter changeThe Kc value given was not compatible with the hydro-cyclone operation assumption above and had to be changed to 0.01 in order to give sensible answers. This can be viewed as reducing the amount of oversize material being recycled. 1 2. Simulation Analysis2.1 Maximum and Minimum feed that the circuit can cope with.The minimum dry ore feed that the system can cope with is approximately 22.5 kg/min. This minimum is the feed rate required to maintain slurry level in sump at an initial value of 1m. The maximum that can be achieved before sump overflows is 45kg/min of dry ore feed; above this value the slurry level exceeds the height of 4m (the tank height).2.2 Effect of water step input (sump dilution water) to the circuit.An introduction of water disturbance of which is equivalent to has the following effects on the circuit. Increased steady state slurry level in sump-figure 2.1 Dilute sump discharge (small fraction of solids) Dilute hydro-cyclone exit streams (overflow and underflow) Increased recirculation load and overflow and eventually mill feed stream Reduced mill residence time (due to decrease increase in volumetric flow into mill)-figure 2.2

Figure 2.1: Effect of step input in sump dilution water to the slurry level in sump

Figure 2.2: Effect of step input in sump dilution water to the mill residence time2.3 Suggestions on aspects to put under automatic control Sump level control to prevent overflow Mill & Sump water setting for control of slurry S.Gs (For desired S.Gs of slurries); S.Gs are parameters to control in order to match pump requirements involved in pushing the slurries around the circuit.Recycle rate used as process variable. Hydro-cyclone portioning (oversize recycle) Kw- A parameter that dictates the amount of water being recycled back into mill. This parameter must be automatically controlled to maintain amount of water in cyclone overflow at constant value. Usually it is desired to minimise water in cyclone overflow to prevent downstream solid-liquid separations of fine particles from water.

3. Appendices6.1 Appendix A: Nomenclature and numerical constants

SymbolsF: Dry Feed rateW1: Mill Feed Dilution Water Feed rateW2: Sump Feed Dilution Water Feed rateT: Mill Feed rateD: Mill Discharge rateCF: Sump Discharge rateR: Recycle rateP (OF): Product (Cyclone Overflow) rateS: Selection Function: Residence Time of Mill contentsh: Tank heightMSIS: Specific Gravity of Slurry M OS: Dry Solids Specific Gravity%J: Mill FillingSubscriptsm: Mass Flow ratev: Volumetric Flow rate%: Solids Mass Percentagec: Mass Fraction of Oversize material (Dry Basis)ConstantsKL=0.72KW=0.28KC=5 (changed to 0.01)=1000 kg/m3

6.2 Appendix B: List of equations..Equation 1............Equation 2Equation 3.Equation 4....Equation 5..Equation 6.Equation 7.Equation 8

.6.3 Appendix C: Process Flow Diagram-Milling Circuit

Figure 3.2.1: Mill CircuitRecirculation Load (R)Mill Water (W1)New Feed (F)Total Feed (T)Hydro-cycloneSump Water (W2)Product (OF)Cyclone Feed (CF)Discharge Sump Ball Mill

6.4 Appendix D: Simulink Model Map

Figure 3.3.1: Model Overview: shows the different areas of the model (feed, mill, sump, and hydro-cyclones systems). It also shows the areas where parameters and simulation results can be found.

Figure 3.3.2: Parameters found inside the control section system

Figure 3.3.3: Subsystems found inside the model calculations system

Figure 3.3.4: Subsystems found inside the report scopes system (these subsystems contain scopes for viewing results on each stream including variables such as mill residence time and sump slurry level)