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Process engineers responsible for the design of a new plant must
at all stages of the project make efficient use of the results of
experiments carried out for the purpose of optimizing the
installation:
- After the initial laboratory tests, it is necessary to draw up
a general flowsheet of the future plant and evaluate approximately
its capital cost. At this stage it is important to complete
a preliminary project rapidly
- During the pilot plant tests, it is necessary not only to keep
the cost of the pilot operations as low as possible - especially
if the final investment decision has not yet been made - but also
to use the pilot results to dimension the future equipment of the
plant and to determine, from the possible flowsheet
configurations, the best solution to meet the technical and
economic requirements
For all such processing problems, recent advances have developed
new ways to address these problems. Computer methods of
steady-state simulation allow the mineral processing engineer to
assess a large number of possible solutions in a reasonably short
time, leading to much better designed and more optimized plants.
Steady-state mineral processing plant
simulation
A steady-state mineral processing plant simulator is a software
package capable of predicting plant operation according to the
characteristics of the ore feed and the circuit. The
prediction of water and ore streams of the plant operated at
steady-state under given conditions is called direct simulation,
and the back-calculation of parameters of plant configuration
(such as the required size of a piece of equipment) to obtain a
given operation of the plant is called reverse
simulation.
USIM PAC [Broussaud et al., 1988 through 1991, Guillaneau et al.,
1992] is an advanced simulator. It also offers many other
capabilities for data input, display of results, and data processing
including material balance computation, model parameter estimation,
direct, reverse and objective driven simulation, plant capital
investment estimation, etc. (See Figure 1).
Figure 1. Main functions of a
steady-state simulator
Steady-state simulation does not compete with dynamic simulation:
it is not a lower or higher level of simulation. Whereas
dynamic simulation is an essential tool for the design of
process control strategies and a key element of advanced process
control systems, steady-state simulation is an essential tool for
plant design and pre-control optimization: it is adequate to
optimize the circuit design and the dimensions of the units of
equipment before the implementation of a process control system.
Steady-state simulation is a very effective approach for plant
design since it enables:
- a great number of hypotheses to be considered regarding
flowsheet, equipment, etc.
- the processing equipment and the material handling equipment
units to be sized
- the configuration of an almost optimal industrial plant to be
attained in a very short time
It is also used as an operational aid for decision making in
existing plants, such as:
- improving the flowsheet or adapting to changes in the ore or
in the concentrate market
- choosing the settings or operating parameters of certain units
of equipment
The changes generated by the availability of mineral processing
plant simulators are very deep and the time has come when the role
of the experiments is being reconsidered. Whereas
experiments traditionally aimed at small-scale reproduction of all
process configurations considered for an industrial plant, the aim
in the near future will be to help in the selection of unit
operations models, to generate data to fit unit operation models and
to verify simulation results.
Unit operation models
The main components of a simulator are:
- the simulation software, per se, which enables communication
between user and simulator and coordination of calculations: as
this is the only component visible to the user, it is often called
"the simulator"
- mathematical models for unit operations (see Figure 2), which
constitute the core of the system, albeit they are buried inside
the simulator as subroutines
The USIM PAC software is delivered with an extensible library of
36 unit operation models. These models are classified into
four levels; Level 0 to Level 3, according to their increasing level
of complexity. This does not mean that Level 3 models are
better, per se, than other models, but only that each model level is
better adapted to some kinds of application.
Figure 2. Unit operation model
Level 0 models do not predict how units will perform: they simply
calculate output streams from a specification on unit performance
given by the software user. For instance, for a grinding mill,
a Level 0 model is a model for which the d80 of the particle size
distribution at the output of the mill and possibly its slope, are
specified. These models, sometimes called flowsheeting models,
are essential for the first steps of a new plant design project.
Level 1, 2 and 3 models are predictive models and most of them
are phenomenological models. They predict the particle size
distribution at the outlet of the mill and calculate the power draw
as a function of mill feed, size and load.
Level 1 models rely on Bond's energetic approach, perfected by
Rowland and Kjos [1978], whereas Level 3 models are based on the
population balance approach and an assumption on the relationship
between the selection function and the energy available both
developed by Herbst [1973]. Level 2 models [Broussaud et al.
1986 and 1988] are a simplified form of Level 3 models.
Level 3 models are the most accurate, but their application
requires more experimental work (e.g. determination of breakage and
selection matrices from laboratory experiments). Level 1
models are adequate to solve most preliminary design problems.
General features of USIM PAC
USIM PAC was not designed to be used by computer experts, but by
mineral processing specialists. It consequently offers the
following user friendly features:
The data input and the presentation of results make use of
graphics and a vocabulary familiar to industry personnel. The
flowhseet is shown in the traditional graphic form; most parameters
have a clear physical meaning: phase flowrate, component grades,
particle size and mineral distributions, volumes, mill diameter,
tank volumne, weight, rotation speed, etc.
Data entry is interactive, clear and simple. The user can at any
time modify data entered earlier on the same dialog box or a
previous one. Extensive data checking by the program reduces the
risk of problems due to typing errors. The data are stored in a form
which allows them to be displayed, reused, or modified easily. The
methodology used to develop USIM PAC contributes to its reliability
and robustness.
In order to satisfy the varied requirements of users, USIM PAC
may be used at three different levels:
Interactive user level: This is the "normal" mode of
use. It is the only level used by a mineral processing engineer to
solve a practical problem. At this level, the program is totally
"user friendly."
Setup level: It is possible to rapidly modify many of
USIM PAC's parameters to adapt it to a particular context, for
example, the inclusion of special jargon used in a plant, or
suppression or creation of the possibility of accessing a parameter
interactively. These modifications are implemented via configuration
options.
Programming level: The purchaser of USIM PAC can create
new icons to represent the devices on a flowsheet, or new equipment
simulation models. Models and icons are introduced in the form of
FORTRAN subroutines, which must respect a few simple, well-defined
rules. These subroutines must be compiled and linked with an object
code module supplied with the USIM PAC Development Kit. A FORTRAN
compiler and linker are provided with the USIM PAC Development
Kit.
This flexibility makes it possible to satisfy the need of some
USIM PAC users to insert their own models in the software, and to
provide a legal guarantee on the delivered object code.
The user may also insert into the program completely new
functions taking some or all of their data from USIM PAC
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