Control system for the mining and fertiliser industries

BATEMAN has designed and installed an advanced optimal control system on a phosphoric acid plant, significantly improving product quality and reducing waste product. This has been achieved with reduced operator intervention and more stable unit operation.

The challenges encountered in this installation were typical of solvent extraction plants in the minerals processing and fertiliser industries. There were no analysers available to measure the concentrations of materials in the circuits, there was a strong non-linearity between process variables and little operational data of similar plants to fall back on. BATEMAN’s experience in inorganic processes and minerals engineering successfully overcame these obstacles.

Early measurements on the plant reveal that the installation of the optimal control system will yield between 5 % and 10 % of additional income - a level of improvement similar to that obtained in the hydrocarbon processing industry which is very familiar with the use of optimal control systems. This additional income translates directly to net profit, since no additional costs are involved.

Optimal control is a strategy that aims to improve the operation of a process plant. It works as follows. The benefits are obtained by pushing the unit to its most profitable operational limits and product specifications. The resulting operating point is a compromise between operating limits, safety and costs, and product specifications and market prices. A modern advanced control system thus combines economic considerations with the dynamic behaviour of the plant.

Each process has to be analysed appropriately in order to extract the latent benefits. This usually starts with the product specifications. Because of quality fluctuations in most processes, plants are usually set to produce product which exceeds the required quality in order to minimise the fraction of under-quality product which has to be discarded or reprocessed. Through optimal control the process fluctuations will be less, reducing the fraction of over-quality product. This reduces production and reprocessing costs.

When installing an optimal control systems, the whole plant is considered, as tightening of the control over a ulti-parameter process will impact upon other aspects. A dynamic process model is established based upon measurements on the plant or other data on the process variables involved. The analysis performed for the first process is then repeated for the other processes.

The cost of implementing the strategy involves the work needed to create a dynamic process model and the multi-variable control software. A typical controller costs about $250 000. In most cases the return on this investment is usually less than a few months.

For more information, please contact Ehud Levy, General Manager, Chemical Technologies, on
email chemicaltechnologies@batemanengineering.com.

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