PART I PROJECT DEVELOPMENT
5. STEADY-STATE CONTINUOUS SIMULATION
There are a number of software packages that are available to the process engineer and have been customized to handle steady-state simulations. These
Process simulation and modelling 113
have a variety of specialist uses and some of these will be discussed later as they fit into an overall gold plant design.
5.1. Comminution and size separation
Successful recovery of gold from the ore is very strongly influenced by the size to which the ore is ground prior to processing (see Chapter 12). All gold ores vary, depending upon their location, weathering and other factors, and as such there is no ‘standard’ comminution circuit. However, Fig. 1 below shows a fairly generic comminution circuit that may be used to grind gold- bearing ore.
Mills are one of the most expensive items of equipment to install and operate on gold plants and the quality of the grind achieved in the commi- nution circuit is critical to recovery processes downstream. Therefore, it is vital that this area of the plant is designed and operated efficiently. Process simulation is now used extensively in both the design of new comminution plants and also in the optimization of existing operations.
When optimizing or designing a comminution circuit, the process engineer requires specialized software that can predict changes to ore breakage as the circuit flows and/or equipment configurations are changed. This is only pos- sible if a sufficient quantity of sample is tested and analysed to obtain breakage functions for the specific ore in question. Since no two ores are identical, it is not possible to use the results from similar operations and expect accurate, or even representative results. Therefore, when designing a new milling circuit it is essential that sufficient sample of representative ore is obtained for testing. The information obtained from the testwork is then used in the process simulation. It is useful here to repeat the modeller’s golden rule, Garbage In¼Garbage Out. Nowhere is this adage more true
Fig. 1. Example of a comminution circuit flowsheet.
H. Smith 114
than with simulation of comminution circuits. This means that it is essential that time and effort are expended in obtaining representative samples of ore and using these to obtain the required grinding and breakage characteristics required. Obtaining samples and/or data will be a lot more straightforward when optimizing an existing circuit. Nevertheless, this aspect of the optimi- zation must not be overlooked when embarking on a simulation of an op- erating plant.
A further item that must be defined before running the simulation is the actual required capacity of the operation. This is usually decided by ore reserve and economic factors and is quoted in tons per annum (tpa). How- ever, equipment sizing, and hence simulations, are normally based on tons per hour (tph) and hence the conversion betweentpaandtphmust be carried out. The plant availability is the factor that is required to carry out this conversion. This is no minor operation, as plant availability has traditionally been a subjective value. It is based on past experience, vendor guarantees, the location of the operation and amount of stand-by equipment installed, amongst other factors. Availability factors quoted for comminution circuits are normally between 93 and 97%. This must be clearly stated within the criteria for the simulation, together with any supporting data.
Once the required information has been obtained, the simulation of the comminution circuit can be run. There are a number of simulation tools available to the process engineer and these include the following options:
Vendor packages. The mill and crusher vendors have computer packages, which they will run to determine optimum equipment size and power re- quirements. This can be a useful starting point and will typically be used when the engineer discusses equipment and process guarantees with the manufacturer.
Speciality comminution software.These packages have been written specif- ically to simulate comminution and classifying equipment. The following specialized process models are typically available in the packages:
J Rod mill;
J Ball mill;
J Autogenous mill;
J Semi-autogenous mill;
J Cone crusher;
J High-pressure grinding rolls (HPGR) crusher;
J Two-rolls crusher;
J Jaw crusher;
J Single-deck screen;
J DSM screen;
J Hydrocyclones;
Process simulation and modelling 115
J Rake classifier, and
J Spiral classifier.
The mill and crusher vendors usually have a large amount of information on breakage functions and the behaviour of classification equipment. For a specified circuit, the simulation software can be used to predict the following information:
Solids and water flowrate in all streams in the circuit;
Size distributions for all streams in the circuit;
Power draw for crushers and grinding mills;
Operating pressure for hydrocyclones; and
Charge load in autogenous grinding (AG) and semi-autogenous grinding (SAG) mills.
These packages have been written as a tool for process engineers where modelling is only an occasional part of their job function. Therefore, they are reasonably user friendly with windows-based graphical interfaces and menu- driven options. They must be used with the same amount of caution as any other simulation package, and the results obtained from the modelling must be scrutinized with a critical eye.
One of the most widely used comminution software packages in Australia is JKSimMetTM, which is often the software package of choice of process engineers involved in comminution circuits. This package has been used successfully on many gold projects around the world.
5.2. Recovery of gold from ore
Once the gold ore has been milled to the required size, usually on the order of P80 of 70mm, it is generally thickened and pumped to the leach circuit.
The plant may be a carbon-in-pulp (CIP) or carbon-in-leach (CIL) followed by elution and then recovery of the gold via electrowinning or precipitation.
No matter which route is chosen for the hydrometallurgical recovery of the gold, the design or optimization of the plant will require some form of a mass balance.
The main process flowsheet for a gold plant is not overly complicated, with very few recycle streams, and hence a mass and energy balance of the process is not too arduous. This can be carried out using hand calculations or a simple spreadsheet and this may be sufficient for a first pass of the design.
However, this is not recommended for a truly robust design for a more complex circuit flowsheet.
A simulation model requires that all of the processes in the plant be defined and configured. This approach is very useful; to develop the model the
H. Smith 116
process engineer must include all of the area and unit details. This has sig- nificant advantages over hand calculations or spreadsheets because the model does not allow omissions and forces the engineer to state any assumptions used in defining the model.
With the widespread availability of reasonably priced steady-state simu- lation packages it is becoming accepted practice to use one of these to pro- duce the mass and energy balance. These packages are relatively user friendly and generally have extensive chemical and physical databases, allowing the process engineer to produce a simulation model with the minimum of fuss.
Once the model has been constructed and validated it can be used to optimize both the main process flowsheet and the utilities, such as water, reagents,etc.
There are many advantages of producing the mass and energy balance of the recovery plant using a steady-state simulation rather than via a spreadsheet:
The process can be easily audited and validated;
The underlying algorithms are not open to change, as would be the case in a spreadsheet;
Changes can be implemented relatively smoothly; and
The simulation model provides a graphical representation of the process.
In addition to all of these advantages, it is possible to extend the steady-state simulation model to the next stage of simulation – either full or pseudo- dynamic model. The advantages of this will be discussed in the following section. Some of the simulation packages that are used in the gold industry to produce steady-state models are:
Metsim – this package includes a chemical species database and is used throughout the world for mass-balancing purposes. Add-on modules also include a heap-leach capability, for example, which is useful for modelling this area of a gold plant, if required.
SysCAD– this package has the advantage of being both a steady-state and dynamic simulation package. Hence, it can be used to produce a mass and energy balance, which can be extended into the dynamic world, if required.
Ideas – this package is relatively new to the metallurgical industry.