Section Conclusions

In this chapter the manner in which plant data are gathered and processed is discussed, as well as its completeness and internal consistency. Due to the fact that key variables and values are missing from the data, and that compositional data is not available at a high enough frequency, a complete process mass balance is not possible. Data validation is therefore primarily lumped with model validation. After the data‟s discussion, the model used in this project is described, focussing on the purpose, form and adaptions of it, after which its migration from the normal MATLAB workspace to Simulink is discussed. This follows into the validation of the model, which is done in 3 steps – as recommended by Sargent (2013): computer model verification, conceptual model validation and operational validation.

As part of the conceptual validation stage, several changes are made to the model. These include the addition of cooling for compartment 2 and 3, level control in the autoclave, pressure control, dead time, stream 23, non-leaching components, formic filtrate to stream 3 and water to stream 1, as well as calculation corrections and the correction of inventory sizes. It was noted in this stage that the oxygen flow rate in the data is much lower than what is required stoichiometrically, as calculated by the model. This can be the result of a wrong calculation in the model of the amount of oxygen required, the sparging of an insufficient amount of oxygen on the plant or the oxygen flow rate into the autoclave is split in such a way that more than 50% of it is sparged into the last compartment. After conceptual validation sanity checks are done, which serves as a qualitative comparison with experimental knowledge, as presented by Dorfling (2012). The model‟s performance was found to correspond well with what is expected.

A sensitivity analysis is done in order to determine which changes have the potential of solving the copper and nickel dissolution inconsistency. None of the tested changes have this potential, but the acid addition rate, pressure and the first stage leach residue‟s mineralogy have significant influences on the process. The effect of a recommended adjustment to the copper leaching reaction kinetics presents a solution to the temperature bias found in the validation plots, but should not be used outside this project.

During operational validation, certain input flow rates were imported from data into the model, in order to bypass the control system as much as possible and to view the model‟s response to real, dynamic process inputs. Due to inconsistent data, some flow rates had to be calculated – either explicitly or by a controller. Model outputs and data values were compared to each other, noting the errors qualitatively and quantitatively. The propagation of trends through the system – both in the case of the data and model output – are investigated and discussed. A number of conclusions were made from these comparisons, and they include the following:

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 Data values around 400-TK-10 and 400-TK-20 do not balance.

 The model‟s current structure (with function blocks in Simulink) does not execute for all input data combinations, due to values not converging. It is recommended that the model is migrated to a Simulink-only platform, with its structure reconsidered.

 There is a near-constant temperature offset between model and data values, due to compositional differences in the autoclave or an inaccurate correlation in the model between the flow rate of the flash recycle stream and the resulting energy loss. It is recommended that this is examined in a future project.

Moreover, the model is validated for the investigation and improvement of the structure of basic regulatory control, due to the following observations:

 The inventory contents change in a correct manner in response to adjacent flow rate changes.

 The model‟s temperatures respond qualitatively as expected (though with an offset) to the flow rates of the flash recycle stream, the water in the cooling coils and the steam into the last compartment, as well as to compositional changes.

 The autoclave pressure changes as expected to changes in the oxygen sparging rate.

It is noted that the quantitative discrepancies between the model and data values need to be resolved before a detailed design of the basic regulatory control structure can be done. Until then the model is validated for structural research only.

The model is validated for the investigation and improvement of the structure of compositional control. This is due to the observation that flow rate changes in the model lead to sensible changes in the acid concentration and solids fraction, despite offsets between model and data values. The model is, however, not validated for the development of improved supervisory control structures. This is due to the fact that the leaching behaviour of the plant is not well predicted by the model, with the copper leaching especially being under-predicted. The mean concentration of copper in the second stage leach product is approximately 46% less than in the data mean, for example. It is recommended that the reaction kinetics of the model be improved in a follow-up project before investigating supervisory control. Until then the model is only valid for structure investigations on regulatory control. For such an investigation it is recommended that more complete data is made available (in terms of variables measured/logged and the frequency at which it is done) to be able to better validate the data and model for this purpose.

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