Description
Course Overview
Optimisation problems are problems where one seeks to maximise (or minimise) a function by changing variables
Excel Solver is a publicly available addin in Microsoft Excel that can be applied to optimisation problems. Whilst Excel Solver has strengths and weaknesses, it is suitable for simple optimisation problems.
The class of optimisation problems that are the focus of the course is ‘the mass balance problem’. The mass balance problem is a fundamental problem in mineral processing and chemical engineering – and it involves minimising the difference between original measured value and new estimated values subject to mass conservation constraints. The conventional approach is to treat the problem as a least squares problem (quadratic minimisation or nonlinear problem).
1 Section 1 Introduction to Mass Balancing
Section 1 is an introduction. Section 1 is split into three lectures:
1. An overview of the course,
2. A brief explanation of the Mass Balancing problem
3. A brief explanation of Least squares minimisation
1.1 Lecture 1 Overview
1.1.1 Objectives
You will understand what is covered in the course – which primarily consists of two concepts:
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Mass balancing
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Using Excel Solver
1.1.2 1.1.2 Description
Mass balancing is a technique used in mineral processing to reconcile plant data so that it is consistent. Mass Balancing is conventionally solved using least squares minimisation. Hence the problem is to identify a least squares objective function that is minimised subject to constraints.
Excel Solver is an addin that can solve a variety of optimisation problems including the nonlinear mass balance problem.
1.2 Lecture 2 A brief explanation of the mass balancing problem
1.2.1 Objective
You will understand the mass balance problem in mineral processing.
1.2.2 Description
The mass of what goes into a unit must equal what comes out. This is called mass conservation. There are a variety of mass balance problems – but here the core problem is that ore properties are measured before and after the unit. The measured ore properties are subject to sampling error; and therefore the measurements are generally not consistent with mass conservation. They are therefore adjusted to allow a consistent understanding of ore flow through units.
1.3 Lecture 3 Least squares minimisation
1.3.1 Objective
The mass balance problem is conventionally solved using least squares minimisation. Hence you will learn to apply least squares minimisation.
1.3.2 Description
Least square minimisation is an extension of maximum likelihood theory. You may recall least squares minimisation as the basis of regression.
There are two branches of least squares minimisation:
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Non-weighted least squares minimisation
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Weighted least squares minimisation
Non-weighted least squares minimisation simply means that the new estimates are as close as possible to the original estimates.
Weighted least squares means that we take into account the expected departure of the measurements from the actual value.
2 Section 2 Solver
Excel Solver is an addin freely available at all licensed Excel users. You will learn how to use Excel Solver and apply it to straightforward mass balance problems. The section is split into 6 lectures:
4. Using Excel Solver
5. Setting up Excel Solver
6. Applying Excel Solver (Ex1)
7. Two Products Exercise (Ex2)
8. Water Flow Exercise (Ex3)
9. Reducing Constraint Equations and variables
2.1 Lecture 4 Using Excel Solver
2.1.1 Objective
You will understand the advantages of using Excel Solver for optimisation problems. You will also discover some disadvantages of using Excel Solver.
2.1.2 Description
Excel Solver is an addin that is available to all Excel users. It is used to optimise (minimise or maximise) an objective function, represent by a formula in cell, subject to constraints, and by varying cell values (variables).
2.2 Lecture 5 Setting up Excel Solver
2.2.1 Objective
Although Excel Solver is available to all users not all users will know how to access it. You will learn how to gain access to Excel Solver.
2.2.2 Description
Excel Solver is made available by using the options in Excel.
2.3 Lecture 6 Applying Excel Solver
2.3.1 Objective
You will apply Excel Solver to a simple problem.
2.3.2 Description
Excel Solver minimises or maximises an objective. The objective is a function in an Excel cell. Excel Solver also requires variables that are to be adjusted. These variables are cells. It is common to add constraints.
The problem given is a simple mass balance problem. Harder problems will then be introduced into successive lectures.
2.4 Lecture 7 Two Products Exercise
2.4.1 Objective
You will solve a mass balance problem where there are two products coming from the unit.
2.4.2 Description
A more complex mass balance problem is constructed with two products. Excel Solver is applied to minimise the least squares error.
2.5 Lecture 8 Water Flow
2.5.1 Objective
You will mass balance both water flow and solid flow.
2.5.2 Description
Whilst mineral processors are most interested in solid flow (and of course assays), water flow is additional information which is important for both unit models and for improving the estimate of solid flow.
2.6 Lecture 9 Reducing Constraint Equations and variables
2.6.1 Objective
You will be able to reduce the number of constraining equations and variables used by the Solver algorithm.
2.6.2 Description
Solver has a limit on how many variables that can be adjusted.
The equations can be adjusted to reduce the number of variables thereby improving the possibility of convergence.
For more complex problems one therefore needs to have a good understanding of the problem to incorporate into the problem methods that simplify the problem.
3 Section 3 Mass Balancing extensions
By this stage in the course you should be competent in using Excel Solver. For the remainder of the course you will focus on mass balance extensions.
There are four lectures:
10. The 1D Mass Balance problem
11. The 2D Mass Balance problem
12. Estimating Solid Flows from Assays
13. Treatment of Remnant Minerals
There are of course other extensions, but this is sufficient in the context of using Excel Solver.
3.1 Lecture 10 The 1D Mass Balance Problem
3.1.1 Objective
You will be able to mass balance assay data.
3.1.2 Description
This is the first lecture which focuses on dimensionality of ore properties. Examples of 1D data are size distribution or assays – but not both (which is 2D).
3.2 Lecture 11 The 2D Mass Balance problem
3.2.1 Objective
You will be able to mass balance assay data within size-classes.
3.2.2 Description
In this lecture we extend the mass balance problem to 2D data (assays within sizes). The course does not go to more complex dimensions even though 3D mass balancing is also a valid subproblem. The problem complexity is limited because of the limitations of Excel Solver.
3.3 Lecture 12 Estimating Solid Flows from Assays
3.3.1 Objective
You will be able to estimate solid flows using assays.
3.3.2 Description
Mass balancing thus far has largely been used to adjust measured values. It can also be used to estimate unmeasured variables. In this case we estimate unmeasured solid flow values.
3.4 Lecture 13 Treatment of Remnant Minerals
3.4.1 Objective
You will learn the options on how to extend mass balancing of assays to include remnant minerals.
3.4.2 Description
If we have say copper (Cu) and lead (Pb) and we focus on these we can also consider the remnant mineral. We can do this by either adding a constraint or adding a variable; but the effect is to ensure that the sum of the focus minerals: Cu and Pb, does not exceed 1 or (100%).
4 Section 4 Closing
The closing section consists of the following lectures
14. Closing lecture
15. Bonus Lecture
4.1 Lecture 14. Closing Lecture
4.1.1 Objective
You will be able to summarise knowledge gained by the course. Acknowledgement to those who have contributed is included.
4.2 Lecture 15. Bonus Lecture
4.2.1 Objective
To discuss extension courses in preparation.
4.2.2 Description
The course was introduction only. The scope was limited to using Excel Solver.
There are extension courses and future courses being developed and planned.
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