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Computer simulation of the Sherritt nickel-copper matte acid leach process
Hydrometallurgy, 29 ( 1992 ) 261-273
261
Elsevier Science Publishers B.V., Amsterdam
Computer simulation of the Sherritt nickelcopper matte acid leach precess M.D. Faris a, M.J. Moloney b* and O.G. Pauw a "Process Control and Automation, Sherritt Gordon Ltd., Fort Saskatchewan, Alta., Canada bKenwalt Pry Ltd., Rivonia, South Africa (Revised version accepted December 18, 1992 )
ABSTRACT Faris, M.D., Moloney, M.J. and Pauw, O.G., 1992. Computer simulation of the Sherritt nickel-copper matte acid leach process. In: W.C. Cooper and D.B. Dreisinger (Editors), Hydrometallurgy, Theory and Practice. Proceedings of the Ernest Peters International Symposium. Hydrometallurgy, 29: 261-273. The Sherritt acid leach process has been used -n several locations for the recovery of nickel and copper from high-grade mattes, in order to produce a concen~raled platinum group metals residue. Process steps include matte grinding, atmospheric leaching, pressure leaching, residue filtration, selenium removal, copper electrowinning, and nickel sulphate crystallization. Since 1987, computer simulation of the acid leach process has been utilized for process design, operability and control studies, control system testing, and persomld training. Material balances and dynamic simulations were generated on an IBM compatible personal computer, using flowsheet simulation software capable of communicating with other computers. A PC-based process control software package was also used to develop an operator console tbr the simulator, as a tool for personnel training. This paper presents an overview of the development and application of computer simulation for this process.
INTRODUCTION
The Sherritt acid leach process has been used by a number of South African firms to recover nickel and copper from high-grade mattes to produce a residue that is rich in platinum group metals. A flowsheet of the process in its most recent state of development is shown in Fig. 1. Brugman and Kerfoot [ 1 ] provide an overview of the development and application of the process up to 1986. Since then, two more plants have been built, one by LP Refineries at Brakpan which operated between 1989 and 1991, and one by Northam Platinum Ltd. as part of their mining complex, which at time of writing is in Correspondence to: M.D. Faris, Process Control and Automation, Sherritt Gordon Ltd., Fort Saskatchewan, Alta., Canada. *Currently with JKTech, Julius Krutschnitt Mineral Research Centre, Brisbane~ Australia.
0304-386X/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
M.D. FARIS ETAL.
262 Ground Matte H20 I H2SO4
02
Atmospheric Leach
H2SO4
H20 ]
02
Pressure Leach Steam Crystallization
I
PGM Residue
SO2 Selenium Removal
NiSO4.6H20 Crystals
Spent Electrolyte Recycle
Se Residue
^,r
t Copper Winning
Cu Cathode
Fig. 1. Simplified flow diagram of the Sherritt nickel-copper matte acid leach process (adapted fi'om Brugman and Kerfoot [1 ] ).
the final stages of construction. Most of the computer modelling work described in this paper was carried out on these two plants. In all plants referred to above, Sherritt was deeply involved in laboratory testing, process development, process design and plant commissioning. Recognizing the increasing role and future potential of computer simulation in each of these activities, Sherritt proceeded in 1987, with the assistance of Kenwalt (Pry) Ltd., to introduce simulation technology into various phases of project execution.
COMPUTER SIMULATION OF THE SHERRITT NICKEL-COPPER MATTE ACID LEACH PROCESS
263
MODELS, EQUIPMENT AND PROCEDURES
The acid leach process In the acid leach process (Fig. 1 ), nickel-copper matte is ground and then treated in an atmospheric leach with spent copper electrolyte. Some of the nickel is solubilized in this step, and copper is taken out of solution in a metathesis leach reaction. Solution from the atmospheric leach is sent to a crystallizer to produce a nickel sulphate product, while the leach residue is sent to a pressure leach autoclave. In the pressure leach, essentially all of the remaining nickel and copper are leached into solution, leaving the bulk of the platinum group metals in the leach residue. The residue is removed by pressure filtration, and the solution proceeds to a selenium removal step and copper electrowinning. Spent copper electrolyte is returned to the atmospheric and pressure leaches.
Computer software and hardware All simulation work was carried out on SysCAD (a trademark of Kenwalt (Pty) Ltd. ), a software package which allows the user to configure a flowsheet model by drawing a process flow diagram on a graphics screen, and entering the parameters for each unit model [2 ]. The software runs on an IBM PC/AT compatible computer with an 80286 or 80386 processor and math coprocessor. Until 1990, the simulator used 3 megabytes of extended memory to store program overlays. Since 1990, an INMOS T800 transputer has been used to enhance the speed and size of the simt~lations in the PC. SysCAD makes use of both a monochrome monitor and a colour graphics monitor for the simultaneous display of data and flowsheet graphics (see Fig. 2a). An operator console for personnel training was configured using control console software called FactoryLink (a trademark of United States Data Corporation) on an IBM PC/AT compatible computer with an 80386 processor, and a colour graphics monitor.
Simulator configuration Typical hardware configulations for the various applications are shown in Fig. 2. Material balances and simplified dynamic flowsheets were prepared using the simulation software on a stand-alone basis. For control system testing and operator training at the Brakpan plant, the simulation computer was connected to a Siemens PLC (Programmable Logic Controller) through a serial communications link. Either a PLC programming panel or an operator console was also connected to the PLC. A self-contained operator training
264
a)
M.D. FARIS ET AL.
Monochrome Text Display INMOST800 Transputer
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Colour Graphics Display
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b)
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OperatorConsole
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TrainingConsole
SimulationComputer
Fig. 2. Computer hardware configuration for: (a) material balances and simplified dynamic simulations; (b) PLC logic testing and operator training; and (c) operator training with a "look alike" operator console.
system was prepared by connecting a simple operator training console to the simulation computer with a serial communications link. This system allows a trainee to interact with the simulator through an interface which resembles a
COMPUTER SIMULATION OF THE SHERRITT NICKEL-COPPER MATTE ACID LEACH PROCESS
265
control system console.
Levels of modelling Each application of the simulator required a different level of detail in the flowsheet model. The simple example of a hypothetical feed tank is shown in Fig. 3, where feed material and a recycle stream flow ~.t a set rate, and a reagent is added to achieve a desired concentration in the tank discharge. In a material balance flowsheet, the tank is represented by a simple mixer block that blends a number of input streams to produce a single output stream (Fig. 3a). A simplified dynamic flowsheet for control and operability studies includes a surge volume for the tank, with a controller config,~,~redto maintain a constant tank level (Fig. 3b). Pumps, valves, agitators, etc. are not necessary at this level of modelling, since the main objective is to determine plant flow rates and overall dynamic behaviour. A detailed dynamic flowsheet contains models for all of the essential equipment in the plant design. If the simulator is to be used to test the PLC code, the controller functions reside in the PLC, and the flowsheet model contains measurement elements and actuators which correspond to control system inputs and outputs in the real plant (Fig. 3c). However, if the simulator is connected directly to a training console, all controllers are configured in the flowsheet model, and controller setpoints and outputs are relayed to the console for perusal and adjustment by the trainee (Fig. 3d ). The representation of manual operations differs between applications as well. In the feed tank example, the process design calls for manual sampling and analysis of the discharge stream, with manual adjustment ~" the reagent flow setpoint. In both the material balance and simplified d~namic flowsheets, the mass flow of the reagent stream is adjusted by a feedback controller to obtain the specified discharge reagent concentration. In the flowsheet used fur PLC testing, the value of the discharge concentration must be read manually from the simulator, since there is no corresponding signal in the real control system. In the stand-alone training system, the value is sent from the simulator to the operator console and placed on a display which corresponds to a laboratory log sheet. The reagent flow setpoint i~ adjusted manually in both of the last two cases.
Scope of simulation The process scope encompassed by the simulator also varied between applications. Material balances and simplified dynamic flowsheets were used to simulate large areas of the plant in order to study the process as an integrated system (Fig. 4). Detailed dynamic flowsheets were used to represent smaller
M.D. FARISETAL.
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M.D. FARIS ET AL.
circuits when a closer study of the interactions of the plant equipment was in order. In these flowsheets, inputs from other circuits were usually kept constant, but could be perturbed manually or madeto follow a predefined input profile if necessary.
Mathematical models Material balance flowsheets were solved by the software in a sequential modular fashion [ 3 ], that is, calculations at each node in the process network were carried out one at a time, for several iterations until the flowsheet had converged. The recycle streams were identified by the software, with recycle convergence carried out by the Wegstein method [ 4 ]. The tolerance for variations in the stream flows between iteratiens was adjustable by the user. Chemical reaction rates were set by the user as a fixed flow of a particular reactant, or as a fraction of incoming reactant. In more recent work, control loops have been configured to adjust the reaction rates automatically to achieve the desired results. The dynamic flowsheets were also solved by evaluating each unit model sequentially. Flows in the streams were either determined by a momentum balance, or were set by the user. Pressure drops were calculated in all pipes, and the elevations of the plant equipment were taken into account. The size of the integration time step which occurred between iterations was user configurable, and typically ranged from l s to 60 s. By adjusting the time step, the speed of th~ simulation could be varied, although a practical upper limit was imposed on the time step by time integration stability and the accuracy of certain calculations, especialiy vapour flow rates. In most cases, simulations ran faster than real time by a factor of between 2 and 40. In the dynamic reactor models, chemical reaction rates were governed by simple models which reflected the fundamental aspects of the system. For example, water evaporation rates were calculated to maintain vapour-liquid equilibrium in each time step. In the pressure leach autoclave, a simplified shrinking particle model [5] was used to calculate the leaching rate. The number of parameters in each model was kept to a minimum, which sacrificed some accuracy Jander abnormal operating conditions, but which facilitated the setting of parameter vah.|es to match model performance to laboratory and operating data, arid which made for efficient use of computer resources, m/~ Chemical co onents were configured in the same manner for both material balance and dynamic flowsheets. The software allowed a total of 50 components to be represented in the system, with solid, liquid and vapour phases available. Information on elemental make-up, density, heat of formation, and heat capacity were kept for a.|l process streams and surges, giving the user a very detailed picture of chemistry of the process. Some of the calculations
COMPUTER SIMULATION OF THE ~HERRITT NICKEL-COPPER MATTE ACID LEACtt PROCESS
269
were simplified to gain simulation speed, such as the density calculations, which took the overall density as the sum of the contributed densities of all components, ignoring any effects of concentration and temperature. The heat capacity of each component was also treated as a single value, independent of temperature. These inaccuracies only caused problems where thermodynamics had a noticeable impact on the mass fows in the system. For example, independent heat balances had to be carried out to determine some of the mass flows in design material balances, and the dynamic model for the restricting orifice on the pressure leach autoclave discharge line required more accurate modelling of the system thermodynamics. In general, however, the simplified calculations represented the system with sufficient accuracy to allow the successful application of the simulator. RESULTS AND DISCUSSION
Material balances Computer material balance calculations were carried out for the design of the Northam acid leach plant. The flowsheet model has been developed to facilitate frequent changes to input parameters by the user, allowing plant metallurgists to use the material balance on a routine basis. The software has a facility for transferring the results to a spreadsheet program, which allows the production of custom reports and/or subsequent data transfer to other drafting software packages. A number of benefits arise from the use of a computerized material balance. Time savings in the design office permit the generation of several balances in order to refine the results. The material balance also provides operating personnel with a powerful tool for training, production planning, and process monitoring.
Control and operab,!lity studies A simplified dynamic flowsheet for the Northam plant (see Fig. 4), which includes the atmospheric leach, pressure leach and copper electrowinning, was used to study the overall dynamic behaviour of the plant. In general, this exercise served to confirm knowledge gained by Sherritt through several years of process development, design and commissioning. However, some water and sulphur balance characteristics were demonstrated on the simulator which should assist in clarifying plant startup and operating procedures. The use of simulators for operability testing has the greatest economic potential for new processes, or processes where the cost of learning 0y trial and error is prohibitive. In the defense industries, for example, simulation is used extensively in the conceptual and design stages of a new weapons system to minimize ~.hecost of development [ 6 ]. As equipment development proceeds,
270
M.D. FARIS ET AL.
a greater emphasis is placed on laboratory and field testing, with some testing being carried out to validate simulation results and enhance computer models. Throughout the development and deployment cycle, simulation plays an important role in focusing the efforts of the various disciplines involved. Careful control over the development process must be maintained to ensure that the degree of accuracy of the simulation is appropriate for a particular stage of development. A similar scenario could be envisaged for the development of a new hydrometallurgical process. This was not borne out by the current work owing to a large body of previous experience in the development of the acid leach process.
Control system testing The PLC logic for the Brakpan plant was configured at the offices of Kenwalt in the latter half of 1988. A dynamic simulator was used to test each loop as it was configured, thereby ensuring a high quality of design and delivery of control code. Most conceptual and typographical errors were detected at the testing stage, prior to installation in the plant, including some major problems with the operation of the feedback control loops. Early correction of these errors provided for smoother plant commissioning, since most of the errors that arose in the plant were minor data addressing problems that could be corrected quickly in the field. The economic benefits of the testing are difficult to quantify, although it has been reported that in complex cases, "it is 20 times faster to correct a program bug on a simulator than in the field" [ 7 ]. In the case of the Brakpan plant, the problems with the feedback control loops would have hampered plant commissioning if they had been present in the installed control system. Testing enhanced other aspects of the system, such as alarm and status displays, and control system recovery from power failures. At any rate it has been found practical to spend up to 25% of the PLC programming time on simulator testing [ 7 ].
Personnel training The simulator used for testing the PLC logic for the Brakpan plant was taken to the plant a few weeks prior to commissioning, and was used for operator training. Trainees went through the startup and operating procedures for matte grinding, atmospheric and pressure leaching, selenium removal, and electrowinning circuits. The actual control system console was used (see Fig. 2b), improving the "keyboard literacy" of the operators. Most of the plant equipment was in place at the time, so manual field functions could be incorporated into the training sessions using the actual equipment. In some cases, this included using radios as if to contact field operators. This gave the train-
COMPUTER SIMULATION OF THE SHERRITT NICKEL-COPPER MATTE ACID LEACH PROCESS
271
ees some familiarity with the plant facilities, as well as a deeper knowledge of the operating proce,lures. A total of four shift supervisors and three senior operators took part in the training. Trainees typically worked in pairs, and each spent about 30 h in the training sessions. The supervisors and operators were monitored for reaction to fault conditions and preventive decision making. One advantage of using the training simulator prior to plant commissioning is that upset conditions or emergency situations could be staged, to give the trainees exposure to rare situations. The overall result was a higher level of operator confidence and improved decision making skills. This certainly had a positive impact on plant productivity, although the benefits are difficult to quantify. The material balance and simplified dynamic plant flowsheet developed for the Northam plant will serve as training tools for plant metallurgists. By adjusting the model parameters and observing the responses, users can gain an appreciation of the interactions between the various circuits within the acid leach process. The self contained training simulator (Fig. 2c) has so far only been developed for the pressure leach autoclave in the Northam plant. It was designed to provide the operators with all of the major process control features that would exist in a control system console, as well as simulating access to some manually operated plant equipment (valves, push buttons, etc. ).
Modelling considerations There were some differences between dynamic flowsheet model results and known or expected plant dynamic behaviour. These differences were the result of simplified kinetic models, manual control loops modelled as automated loops, and differing responses of modelled and real automated control loops. There are also some inherent difficulties in the mathematics of modelling a continuous process with a discrete solution method [ 8 ]. However, these differences did not prevent the successful application of simulation technology, although in some instances there were additional development costs involved in compensating for the discrepancies. When developing and implementing a process model, the cost of implementation must be weighed against the needs of the end user of the simulator. A very high level of reality can be simulated with today's computer technology [ 9 ], but such costly development is not required for typical process plant applications. Ideally, the level of modelling should be just sufficient to complete the job at hand. For example, a material balance, an operability simulator and a training simulator all require a fairly accurate representation of the process chemistry, whereas a PLC testing simulator can, in many cases, rely on a very simple representation of material flows in the process. Often during operator training, the main objective is to teach the trainee the layout
272
M.D. FARIS ET AL.
of the control system, process terminology, and basic operating procedures. Although a very high degree of fidelity is desirable, it is usually not cost-effective, and is seldom essential in these situations.
Knowledgegained As a closing comment, the authors agree with J.W. Evans in his conviction that the primary purpose of" modelling is "knowledge, not numbers" [ l 0 ]. Although the numbers are instrumental in preparing, validating and applying a model, the ultimate objective is to use those numbers to impart knowledge about a process. Numbers obtained from even the most accurate model for design purposes must be applied with sound engineering judgement. When a process model is used within a framework of simulation software and hardware, knowledge can be gained that ranges from a detailed understanding of equipment performance, to an appreciation of overall system dynamic behaviour. In the applications described in this paper, knowledge about the process was gained, or at least refined, in the design office. Knowledge was gained and utilized by control system engineers regarding the effectiveness and quality of the logic in a control system, and knowledge was gained by operating personnel about control system functions and plant operating procedures. CONCLUSIONS
Computer simulation has made a significant contribution in recent years to the commercialization of the Sherritt nickel-copper matte acid leach process. Because this technology was introduced at a mature stage of process development, applications have focused on process design, control system testing and personnel training, rather than on process conceptualization. The application of simulation has been made cost-effective by the increasing power and falling cost of desktop computers, and by the availability of software that facilitates the rapid generation of flowsheet models. It was found that models for reaction kinetics did not have to be elaborate, as long as they reflected the major process phenomena, and as long as the steady state results of the simulations were in close agreement with laboratory and plant experience. ACKNOWLEDGEMENTS
The authors wish to thank Impala Platinum Limited, Gold Fields of South Africa Limited, Kenwalt (Pry) Ltd., and Sherritt Gordon Ltd. for permission to publish this paper. Messrs. R. Du Plessis, E. Mendes, and A. Hunt each played a major role in carrying out the work described in this paper.
COMPUTER SIMULATION OF THE SHERRITT NICKEL-COPPER MATTE ACID LEACH PROCESS
273
REFERENCES 1 Brugman, C.F. and Kerfoot, D.G., Treatment of nickel-copper matte at Western Platinum by the Sherritt acid leach process. In: Proceedings of Nickel Metallurgy, Volume I. Can. Inst. Min. Metall., 1986, pp. 512-531. 2 Garner, K.C., Peberdy, N.J. and Moreton, C.N., Process and process control design using dynamic flowsheet simulation. Miner. Metall. Process., 3 (1986 ): 41-45. 3 Biegler,L.T., Chemical process simulation. Chem. Eng. Prog., 85(10) (1989): 50-51. 4 Franks, R.G.E, Modeling and Simulation in Chemical Engineering. Wiley, New York (1972), pp. 26-28. 5 Levenspiel, O., Chemical Reaction Engineering. 2nd Ed., Wiley, New York (1972), pp. 368-373. 6 Atkinson, J.H., Sr., Modeling and simulation in the test and evaluation process. Simulation, 54 (3) (1990): 127-132. 7 Pritchard, K., Applying simulation to the control industry. Control Eng., 36( 1989): 7072. 8 Smith, J.M., Mathematical Modeling and Digital Simulation for Engineers and Scientists. Wiley, New York, 2nd Ed. (1987), pp. 107-149. 9 Geber, B., Simulating reality. Training, 27 (1990): 41-46. 10 Evans, J.W., Mathematical modelling: expectations, failures and successes, In: J. Szekely, L.B. Bales, H. Henein, N. Jarrett, K. Rajamani and I. Samaresekera (Editors), Mathematical Modelling of Materials Processing Operations. Metali. Soc., Warrendale, Pa. ( 1987 ), pp. 9-21.
261
Elsevier Science Publishers B.V., Amsterdam
Computer simulation of the Sherritt nickelcopper matte acid leach precess M.D. Faris a, M.J. Moloney b* and O.G. Pauw a "Process Control and Automation, Sherritt Gordon Ltd., Fort Saskatchewan, Alta., Canada bKenwalt Pry Ltd., Rivonia, South Africa (Revised version accepted December 18, 1992 )
ABSTRACT Faris, M.D., Moloney, M.J. and Pauw, O.G., 1992. Computer simulation of the Sherritt nickel-copper matte acid leach process. In: W.C. Cooper and D.B. Dreisinger (Editors), Hydrometallurgy, Theory and Practice. Proceedings of the Ernest Peters International Symposium. Hydrometallurgy, 29: 261-273. The Sherritt acid leach process has been used -n several locations for the recovery of nickel and copper from high-grade mattes, in order to produce a concen~raled platinum group metals residue. Process steps include matte grinding, atmospheric leaching, pressure leaching, residue filtration, selenium removal, copper electrowinning, and nickel sulphate crystallization. Since 1987, computer simulation of the acid leach process has been utilized for process design, operability and control studies, control system testing, and persomld training. Material balances and dynamic simulations were generated on an IBM compatible personal computer, using flowsheet simulation software capable of communicating with other computers. A PC-based process control software package was also used to develop an operator console tbr the simulator, as a tool for personnel training. This paper presents an overview of the development and application of computer simulation for this process.
INTRODUCTION
The Sherritt acid leach process has been used by a number of South African firms to recover nickel and copper from high-grade mattes to produce a residue that is rich in platinum group metals. A flowsheet of the process in its most recent state of development is shown in Fig. 1. Brugman and Kerfoot [ 1 ] provide an overview of the development and application of the process up to 1986. Since then, two more plants have been built, one by LP Refineries at Brakpan which operated between 1989 and 1991, and one by Northam Platinum Ltd. as part of their mining complex, which at time of writing is in Correspondence to: M.D. Faris, Process Control and Automation, Sherritt Gordon Ltd., Fort Saskatchewan, Alta., Canada. *Currently with JKTech, Julius Krutschnitt Mineral Research Centre, Brisbane~ Australia.
0304-386X/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
M.D. FARIS ETAL.
262 Ground Matte H20 I H2SO4
02
Atmospheric Leach
H2SO4
H20 ]
02
Pressure Leach Steam Crystallization
I
PGM Residue
SO2 Selenium Removal
NiSO4.6H20 Crystals
Spent Electrolyte Recycle
Se Residue
^,r
t Copper Winning
Cu Cathode
Fig. 1. Simplified flow diagram of the Sherritt nickel-copper matte acid leach process (adapted fi'om Brugman and Kerfoot [1 ] ).
the final stages of construction. Most of the computer modelling work described in this paper was carried out on these two plants. In all plants referred to above, Sherritt was deeply involved in laboratory testing, process development, process design and plant commissioning. Recognizing the increasing role and future potential of computer simulation in each of these activities, Sherritt proceeded in 1987, with the assistance of Kenwalt (Pry) Ltd., to introduce simulation technology into various phases of project execution.
COMPUTER SIMULATION OF THE SHERRITT NICKEL-COPPER MATTE ACID LEACH PROCESS
263
MODELS, EQUIPMENT AND PROCEDURES
The acid leach process In the acid leach process (Fig. 1 ), nickel-copper matte is ground and then treated in an atmospheric leach with spent copper electrolyte. Some of the nickel is solubilized in this step, and copper is taken out of solution in a metathesis leach reaction. Solution from the atmospheric leach is sent to a crystallizer to produce a nickel sulphate product, while the leach residue is sent to a pressure leach autoclave. In the pressure leach, essentially all of the remaining nickel and copper are leached into solution, leaving the bulk of the platinum group metals in the leach residue. The residue is removed by pressure filtration, and the solution proceeds to a selenium removal step and copper electrowinning. Spent copper electrolyte is returned to the atmospheric and pressure leaches.
Computer software and hardware All simulation work was carried out on SysCAD (a trademark of Kenwalt (Pty) Ltd. ), a software package which allows the user to configure a flowsheet model by drawing a process flow diagram on a graphics screen, and entering the parameters for each unit model [2 ]. The software runs on an IBM PC/AT compatible computer with an 80286 or 80386 processor and math coprocessor. Until 1990, the simulator used 3 megabytes of extended memory to store program overlays. Since 1990, an INMOS T800 transputer has been used to enhance the speed and size of the simt~lations in the PC. SysCAD makes use of both a monochrome monitor and a colour graphics monitor for the simultaneous display of data and flowsheet graphics (see Fig. 2a). An operator console for personnel training was configured using control console software called FactoryLink (a trademark of United States Data Corporation) on an IBM PC/AT compatible computer with an 80386 processor, and a colour graphics monitor.
Simulator configuration Typical hardware configulations for the various applications are shown in Fig. 2. Material balances and simplified dynamic flowsheets were prepared using the simulation software on a stand-alone basis. For control system testing and operator training at the Brakpan plant, the simulation computer was connected to a Siemens PLC (Programmable Logic Controller) through a serial communications link. Either a PLC programming panel or an operator console was also connected to the PLC. A self-contained operator training
264
a)
M.D. FARIS ET AL.
Monochrome Text Display INMOST800 Transputer
D
O 111f:l!l:i/ I
Colour Graphics Display
--I
I.,~ I I]
IBM PC/AT(8O386) Compatible
SimulationComputer
b)
D RS-232 Serial Link
OperatorConsole
Programmable Logic Controller
Simulation Computer
ProgrammingConsole
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TrainingConsole
SimulationComputer
Fig. 2. Computer hardware configuration for: (a) material balances and simplified dynamic simulations; (b) PLC logic testing and operator training; and (c) operator training with a "look alike" operator console.
system was prepared by connecting a simple operator training console to the simulation computer with a serial communications link. This system allows a trainee to interact with the simulator through an interface which resembles a
COMPUTER SIMULATION OF THE SHERRITT NICKEL-COPPER MATTE ACID LEACH PROCESS
265
control system console.
Levels of modelling Each application of the simulator required a different level of detail in the flowsheet model. The simple example of a hypothetical feed tank is shown in Fig. 3, where feed material and a recycle stream flow ~.t a set rate, and a reagent is added to achieve a desired concentration in the tank discharge. In a material balance flowsheet, the tank is represented by a simple mixer block that blends a number of input streams to produce a single output stream (Fig. 3a). A simplified dynamic flowsheet for control and operability studies includes a surge volume for the tank, with a controller config,~,~redto maintain a constant tank level (Fig. 3b). Pumps, valves, agitators, etc. are not necessary at this level of modelling, since the main objective is to determine plant flow rates and overall dynamic behaviour. A detailed dynamic flowsheet contains models for all of the essential equipment in the plant design. If the simulator is to be used to test the PLC code, the controller functions reside in the PLC, and the flowsheet model contains measurement elements and actuators which correspond to control system inputs and outputs in the real plant (Fig. 3c). However, if the simulator is connected directly to a training console, all controllers are configured in the flowsheet model, and controller setpoints and outputs are relayed to the console for perusal and adjustment by the trainee (Fig. 3d ). The representation of manual operations differs between applications as well. In the feed tank example, the process design calls for manual sampling and analysis of the discharge stream, with manual adjustment ~" the reagent flow setpoint. In both the material balance and simplified d~namic flowsheets, the mass flow of the reagent stream is adjusted by a feedback controller to obtain the specified discharge reagent concentration. In the flowsheet used fur PLC testing, the value of the discharge concentration must be read manually from the simulator, since there is no corresponding signal in the real control system. In the stand-alone training system, the value is sent from the simulator to the operator console and placed on a display which corresponds to a laboratory log sheet. The reagent flow setpoint i~ adjusted manually in both of the last two cases.
Scope of simulation The process scope encompassed by the simulator also varied between applications. Material balances and simplified dynamic flowsheets were used to simulate large areas of the plant in order to study the process as an integrated system (Fig. 4). Detailed dynamic flowsheets were used to represent smaller
M.D. FARISETAL.
266
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circuits when a closer study of the interactions of the plant equipment was in order. In these flowsheets, inputs from other circuits were usually kept constant, but could be perturbed manually or madeto follow a predefined input profile if necessary.
Mathematical models Material balance flowsheets were solved by the software in a sequential modular fashion [ 3 ], that is, calculations at each node in the process network were carried out one at a time, for several iterations until the flowsheet had converged. The recycle streams were identified by the software, with recycle convergence carried out by the Wegstein method [ 4 ]. The tolerance for variations in the stream flows between iteratiens was adjustable by the user. Chemical reaction rates were set by the user as a fixed flow of a particular reactant, or as a fraction of incoming reactant. In more recent work, control loops have been configured to adjust the reaction rates automatically to achieve the desired results. The dynamic flowsheets were also solved by evaluating each unit model sequentially. Flows in the streams were either determined by a momentum balance, or were set by the user. Pressure drops were calculated in all pipes, and the elevations of the plant equipment were taken into account. The size of the integration time step which occurred between iterations was user configurable, and typically ranged from l s to 60 s. By adjusting the time step, the speed of th~ simulation could be varied, although a practical upper limit was imposed on the time step by time integration stability and the accuracy of certain calculations, especialiy vapour flow rates. In most cases, simulations ran faster than real time by a factor of between 2 and 40. In the dynamic reactor models, chemical reaction rates were governed by simple models which reflected the fundamental aspects of the system. For example, water evaporation rates were calculated to maintain vapour-liquid equilibrium in each time step. In the pressure leach autoclave, a simplified shrinking particle model [5] was used to calculate the leaching rate. The number of parameters in each model was kept to a minimum, which sacrificed some accuracy Jander abnormal operating conditions, but which facilitated the setting of parameter vah.|es to match model performance to laboratory and operating data, arid which made for efficient use of computer resources, m/~ Chemical co onents were configured in the same manner for both material balance and dynamic flowsheets. The software allowed a total of 50 components to be represented in the system, with solid, liquid and vapour phases available. Information on elemental make-up, density, heat of formation, and heat capacity were kept for a.|l process streams and surges, giving the user a very detailed picture of chemistry of the process. Some of the calculations
COMPUTER SIMULATION OF THE ~HERRITT NICKEL-COPPER MATTE ACID LEACtt PROCESS
269
were simplified to gain simulation speed, such as the density calculations, which took the overall density as the sum of the contributed densities of all components, ignoring any effects of concentration and temperature. The heat capacity of each component was also treated as a single value, independent of temperature. These inaccuracies only caused problems where thermodynamics had a noticeable impact on the mass fows in the system. For example, independent heat balances had to be carried out to determine some of the mass flows in design material balances, and the dynamic model for the restricting orifice on the pressure leach autoclave discharge line required more accurate modelling of the system thermodynamics. In general, however, the simplified calculations represented the system with sufficient accuracy to allow the successful application of the simulator. RESULTS AND DISCUSSION
Material balances Computer material balance calculations were carried out for the design of the Northam acid leach plant. The flowsheet model has been developed to facilitate frequent changes to input parameters by the user, allowing plant metallurgists to use the material balance on a routine basis. The software has a facility for transferring the results to a spreadsheet program, which allows the production of custom reports and/or subsequent data transfer to other drafting software packages. A number of benefits arise from the use of a computerized material balance. Time savings in the design office permit the generation of several balances in order to refine the results. The material balance also provides operating personnel with a powerful tool for training, production planning, and process monitoring.
Control and operab,!lity studies A simplified dynamic flowsheet for the Northam plant (see Fig. 4), which includes the atmospheric leach, pressure leach and copper electrowinning, was used to study the overall dynamic behaviour of the plant. In general, this exercise served to confirm knowledge gained by Sherritt through several years of process development, design and commissioning. However, some water and sulphur balance characteristics were demonstrated on the simulator which should assist in clarifying plant startup and operating procedures. The use of simulators for operability testing has the greatest economic potential for new processes, or processes where the cost of learning 0y trial and error is prohibitive. In the defense industries, for example, simulation is used extensively in the conceptual and design stages of a new weapons system to minimize ~.hecost of development [ 6 ]. As equipment development proceeds,
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a greater emphasis is placed on laboratory and field testing, with some testing being carried out to validate simulation results and enhance computer models. Throughout the development and deployment cycle, simulation plays an important role in focusing the efforts of the various disciplines involved. Careful control over the development process must be maintained to ensure that the degree of accuracy of the simulation is appropriate for a particular stage of development. A similar scenario could be envisaged for the development of a new hydrometallurgical process. This was not borne out by the current work owing to a large body of previous experience in the development of the acid leach process.
Control system testing The PLC logic for the Brakpan plant was configured at the offices of Kenwalt in the latter half of 1988. A dynamic simulator was used to test each loop as it was configured, thereby ensuring a high quality of design and delivery of control code. Most conceptual and typographical errors were detected at the testing stage, prior to installation in the plant, including some major problems with the operation of the feedback control loops. Early correction of these errors provided for smoother plant commissioning, since most of the errors that arose in the plant were minor data addressing problems that could be corrected quickly in the field. The economic benefits of the testing are difficult to quantify, although it has been reported that in complex cases, "it is 20 times faster to correct a program bug on a simulator than in the field" [ 7 ]. In the case of the Brakpan plant, the problems with the feedback control loops would have hampered plant commissioning if they had been present in the installed control system. Testing enhanced other aspects of the system, such as alarm and status displays, and control system recovery from power failures. At any rate it has been found practical to spend up to 25% of the PLC programming time on simulator testing [ 7 ].
Personnel training The simulator used for testing the PLC logic for the Brakpan plant was taken to the plant a few weeks prior to commissioning, and was used for operator training. Trainees went through the startup and operating procedures for matte grinding, atmospheric and pressure leaching, selenium removal, and electrowinning circuits. The actual control system console was used (see Fig. 2b), improving the "keyboard literacy" of the operators. Most of the plant equipment was in place at the time, so manual field functions could be incorporated into the training sessions using the actual equipment. In some cases, this included using radios as if to contact field operators. This gave the train-
COMPUTER SIMULATION OF THE SHERRITT NICKEL-COPPER MATTE ACID LEACH PROCESS
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ees some familiarity with the plant facilities, as well as a deeper knowledge of the operating proce,lures. A total of four shift supervisors and three senior operators took part in the training. Trainees typically worked in pairs, and each spent about 30 h in the training sessions. The supervisors and operators were monitored for reaction to fault conditions and preventive decision making. One advantage of using the training simulator prior to plant commissioning is that upset conditions or emergency situations could be staged, to give the trainees exposure to rare situations. The overall result was a higher level of operator confidence and improved decision making skills. This certainly had a positive impact on plant productivity, although the benefits are difficult to quantify. The material balance and simplified dynamic plant flowsheet developed for the Northam plant will serve as training tools for plant metallurgists. By adjusting the model parameters and observing the responses, users can gain an appreciation of the interactions between the various circuits within the acid leach process. The self contained training simulator (Fig. 2c) has so far only been developed for the pressure leach autoclave in the Northam plant. It was designed to provide the operators with all of the major process control features that would exist in a control system console, as well as simulating access to some manually operated plant equipment (valves, push buttons, etc. ).
Modelling considerations There were some differences between dynamic flowsheet model results and known or expected plant dynamic behaviour. These differences were the result of simplified kinetic models, manual control loops modelled as automated loops, and differing responses of modelled and real automated control loops. There are also some inherent difficulties in the mathematics of modelling a continuous process with a discrete solution method [ 8 ]. However, these differences did not prevent the successful application of simulation technology, although in some instances there were additional development costs involved in compensating for the discrepancies. When developing and implementing a process model, the cost of implementation must be weighed against the needs of the end user of the simulator. A very high level of reality can be simulated with today's computer technology [ 9 ], but such costly development is not required for typical process plant applications. Ideally, the level of modelling should be just sufficient to complete the job at hand. For example, a material balance, an operability simulator and a training simulator all require a fairly accurate representation of the process chemistry, whereas a PLC testing simulator can, in many cases, rely on a very simple representation of material flows in the process. Often during operator training, the main objective is to teach the trainee the layout
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of the control system, process terminology, and basic operating procedures. Although a very high degree of fidelity is desirable, it is usually not cost-effective, and is seldom essential in these situations.
Knowledgegained As a closing comment, the authors agree with J.W. Evans in his conviction that the primary purpose of" modelling is "knowledge, not numbers" [ l 0 ]. Although the numbers are instrumental in preparing, validating and applying a model, the ultimate objective is to use those numbers to impart knowledge about a process. Numbers obtained from even the most accurate model for design purposes must be applied with sound engineering judgement. When a process model is used within a framework of simulation software and hardware, knowledge can be gained that ranges from a detailed understanding of equipment performance, to an appreciation of overall system dynamic behaviour. In the applications described in this paper, knowledge about the process was gained, or at least refined, in the design office. Knowledge was gained and utilized by control system engineers regarding the effectiveness and quality of the logic in a control system, and knowledge was gained by operating personnel about control system functions and plant operating procedures. CONCLUSIONS
Computer simulation has made a significant contribution in recent years to the commercialization of the Sherritt nickel-copper matte acid leach process. Because this technology was introduced at a mature stage of process development, applications have focused on process design, control system testing and personnel training, rather than on process conceptualization. The application of simulation has been made cost-effective by the increasing power and falling cost of desktop computers, and by the availability of software that facilitates the rapid generation of flowsheet models. It was found that models for reaction kinetics did not have to be elaborate, as long as they reflected the major process phenomena, and as long as the steady state results of the simulations were in close agreement with laboratory and plant experience. ACKNOWLEDGEMENTS
The authors wish to thank Impala Platinum Limited, Gold Fields of South Africa Limited, Kenwalt (Pry) Ltd., and Sherritt Gordon Ltd. for permission to publish this paper. Messrs. R. Du Plessis, E. Mendes, and A. Hunt each played a major role in carrying out the work described in this paper.
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REFERENCES 1 Brugman, C.F. and Kerfoot, D.G., Treatment of nickel-copper matte at Western Platinum by the Sherritt acid leach process. In: Proceedings of Nickel Metallurgy, Volume I. Can. Inst. Min. Metall., 1986, pp. 512-531. 2 Garner, K.C., Peberdy, N.J. and Moreton, C.N., Process and process control design using dynamic flowsheet simulation. Miner. Metall. Process., 3 (1986 ): 41-45. 3 Biegler,L.T., Chemical process simulation. Chem. Eng. Prog., 85(10) (1989): 50-51. 4 Franks, R.G.E, Modeling and Simulation in Chemical Engineering. Wiley, New York (1972), pp. 26-28. 5 Levenspiel, O., Chemical Reaction Engineering. 2nd Ed., Wiley, New York (1972), pp. 368-373. 6 Atkinson, J.H., Sr., Modeling and simulation in the test and evaluation process. Simulation, 54 (3) (1990): 127-132. 7 Pritchard, K., Applying simulation to the control industry. Control Eng., 36( 1989): 7072. 8 Smith, J.M., Mathematical Modeling and Digital Simulation for Engineers and Scientists. Wiley, New York, 2nd Ed. (1987), pp. 107-149. 9 Geber, B., Simulating reality. Training, 27 (1990): 41-46. 10 Evans, J.W., Mathematical modelling: expectations, failures and successes, In: J. Szekely, L.B. Bales, H. Henein, N. Jarrett, K. Rajamani and I. Samaresekera (Editors), Mathematical Modelling of Materials Processing Operations. Metali. Soc., Warrendale, Pa. ( 1987 ), pp. 9-21.