1. Introduction - Firenze University Press

In the electrolysis the ZnSO4-solution is transformed into Zn metal. This is one of the large energy
consuming steps. The recommendations from the process intensification study, have led to renewed
thinking on the electrolysis process.
3.2.4. Process control
The present process control system is mainly by hand and quasi static, i.e. the process is
automatically controlled on the level of a single process unit for which a desired value is manually
set, but the process is not automatically controlled for a combination of process units that interact
with each other. There is no feedback from the output to the input and the systems to be controlled
have a very complex interaction. It is suggested to apply a process controller that incorporates
feedback by application of an Advanced Process Controller. Additionally it is suggested to use a
planning system and a real-time optimizer to determine the optimal operation points of the plant.
In the electrolysis section the production capacity is proportional to the power requirement, and
therefore it is desirable to adjust the production capacity with the electricity price. Because of this,
the production capacity in the leaching and purification section may have to vary. The present
control systems have controllers which have relative large steps, which means that a small deviation
will lead to bigger changes in the input than required. This can be changed easily by using filters,
that allow the process to operate more smoothly.
3.3. Selected measures
From the proposed measures, twelve have been selected to be worked out in business cases. About
half of the measures were in the area of improved heat integration. The pay-back period for each
solution ranges between 4.5 and 30 years (including steam turbine), basically because heat
integration itself does not lead to energy efficiency, because heat is available in excess and no steam
turbine is present to convert the saved steam. It is demonstrated that the present system of direct
coupling between the steam turbine and SO2-blower is not ideal. When revision of the blowers is
required an investment in a steam turbine is strongly recommended. If the steam turbine would be
dimensioned for a larger steam flow than present, or alternatively is supported by a gas boiler, the
presence of the steam turbine will act as a catalyst for further reduction in heat consumption, as in
this case heat or steam will have a monetary value.
Other plans focused on the improvement of the roaster oven and electrolysis section. Interesting
opportunities could be created by improvement of the process control systems. These solutions
potentially could be used at more sites. Further study should show the feasibility of these ideas.
It is noteworthy, that the exergy losses in the gas cleaning system are significant, even though the
removal of SOx from the exhaust gas is state of the art. Further process development is required.
4. Conclusions
The integral approach, is a well-structured method for performing energy saving studies. The
relatively long analysis phase gives additional insight in the process regarding the loss of the quality
of energy, which is often a precursor to the actual energy loss. The exergy analysis and pinch
analysis were used on the level of process functions. The exergy analysis acted as a focus for more
detailed analyses. On the detailed level, a process intensification scan and a process control analysis
were performed. Exergy can be applied on the detailed level as well, and thereby indicate where the
losses are taking place, but in this study it was opted not to.
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