Objective
The energy content of neutral lye, which to date has been discharged via cooling towers, is to be recovered using suitable heat exchangers and returned to the process.
It is to be expected that this process will lead to an annual energy saving of about 2000 toe at project level.
For the demonstration phase TWO spiral countercurrent heat exchangers, each with a energy potential of 2140 x 103 kcal/h, were installed. The encrustation from the neutral solution forms in a then layer inside the pipes and hence with the frequent swapping of medium sides, requiring a problematic program controlled switching of numerous isolation valves, the problem can be solved.
In continuous operation it was found that due to filtration problems in the following process steps, the planned final temperature of 67 deg.C for the neutral solution had to be raised to 70 deg.C. This resulted in more favourable conditions for avoiding encrustation, however it also resulted in lower than expected energy savings.
At the end of the demonstration the following results were obtained :
- The selected construction material (1.4563) is corrosion resistant to the heated dilute sulphuric acid, up to a corrosion inhibitor.
- The final temperature of 70 deg.C for the neutral solution can be achieved using only ONE heat exchanger. This is achieved by selecting correctly the mixing temperature of the uncooled and cooled solutions.
- The time interval between swapping of medium sides, for dissolution of crust formations, is so long that the task can be performed manually, without the need for a program controlled switch-over.
- Swapping the medium sides at the optimal point in time gave a heat exchanger efficiency above 90 %/.
By the end of the demonstration phase there were many large changes that had to be made so as to achieve the set targets. This resulted in the project being no longer economically viable and hence the heat exchangers were removed from the circuit.
From the results and the experience gained it is clear that for subsequent projects, with similar expectations, the capital investment costs can be considerably reduced since only one heat exchanger of the above capacity is required to heat the neutral solution.
The construction of a new plant, in view of experience now gained, would recover energy of about 1.190 t ROE/a. This is about 60 % of the expected savings. The amortisation period would be less that TWO years.
The electrolytic production of zinc includes a loop process for solutions. During electrolysis so-called neutral lye is partially dezincified under formation of sulphuric acid.
The solution discharged from the zinc electrolysis cells,which contains sulphuric acid, is called spent.
It is used in the leaching process to dissolve zinc from roasted material. Then enriched with zinc, it is returned to the electrolysis as so-called neutral lye.
The leaching and cleaning process takes place at a temperature of between 80 and 90 deg C to obtain higher yields. To date, the spent has been heated up to this temperature level by adding steam before the leaching process.However only cold solutions can be used for electrolysis.
The neutral lye therefore has to be cooled down to the required temperature level in atmospheric cooling towers. By means of spiral counterflow heat exchangers, which guarantee a high exchange flux, the heat content of the neutral lye is to be recovered and used for thedirect heating of the spent. This means that it will no longer be necessary to heat the spent with steam from the factory's steam network.
A suitable alternating flow between the inlet and outlet channels of the heat exchangers should prevent the accumulation of gypsum deposits in the narrow channels of the heat exchangers. The purpose of the measuring and demonstration programme which is planned after erection and commissioning is to provide information on the corrosion resistance of the heat exchanger materials and on the precipitation of solid matter in the heat exchangers. The latter will have a decisive effect on the operating efficiency and hence on the commercial efficiency of the project.
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DEM - Demonstration contractsCoordinator
45711 Datteln
Germany