How Are PTFE Exchangers Deployed in Zinc Electrowinning Cell Cooling Systems?

Zinc electrowinning, like copper, employs a sulphuric acid electrolyte and creates considerable ohmic heat. But zinc is more susceptible to temperature and the presence of fluoride impurities from zinc concentrates presents another corrosion problem for cooling equipment.

Zinc Electrowinning
In zinc electrowinning zinc is deposited from a solution of sulphuric acid, often in the 150-200 g/L H2 SO4 range. The electrolyte temperature has to be maintained in a limited window of 35-40°C to prevent the undesired evolution of hydrogen gas and to achieve a good deposition rate. Any variation from this temperature range can have a substantial effect on the operation of the electrolytic cell, reducing zinc recovery and increasing energy consumption.

Cooling systems are used to control temperature and to maintain optimum performance. These devices are key to keeping the electrolyte in the target temperature range and to successfully handling the ohmic heat produced during the electrowinning process.

Special Challenge of Fluoride Contaminants
The introduction of fluoride ions (usually in the zinc concentrates) into the electrolyte makes the zinc electrowinning cooling process more difficult. Fluoride ions are quite hostile to most metals, and can cause serious corrosion in traces (a few ppm to tens of ppm). Fluoride ions also react with many other common materials used in cooling systems, causing them to rapidly deteriorate and fail.

The choice of the material for heat exchangers should be very resistant to sulphuric acid and fluoride ions due to the harshness of the electrolyte. PTFE (Polytetrafluoroethylene) is the best material for such use due to its excellent chemical resistance qualities. PTFE is completely resistant to fluoride attack at the temperatures typical in zinc electrowinning (35-40°C) and is therefore ideal for heat exchanger applications in these settings.

PTFE Configuration Heat Exchanger
The most popular cooling system in zinc refineries is the shell and tube heat exchanger. PTFE tubes are utilised for the internal fluid path and cooling water flows around the shell side. This design allows for effective heat transfer and reduces the danger of corrosion and material degradation. The PTFE tubes also exhibit good integrity over time, especially in the presence of fluoride-containing sulphuric acid, which ensures the reliable functioning of the cooling system.

The shell-and-tube construction with PTFE tubes is used because it provides flexibility to handle diverse flow rates and temperature differentials, while being corrosion resistant. The outer shell is usually made of stainless steel or other corrosion resistant alloys and the PTFE tubes are selected for their resistance to the aggressive character of the electrolyte.

Note on Material Compatibility: PTFE versus Other Materials
Material Resistance to Sulphuric AcidTeflon Fluoride Resistance Application in Zinc ElectrowinningExcellent Excellent The best choice, since it is resistant to both sulphuric acid and fluoride ions.
Stainless Steel Good Poor Susceptible to corrosion by fluoride ions and not appropriate for long term usage in fluoride bearing settings.
Titanium Excellent Poor Good against Sulphuric Acid, but subject to attack by Fluoride. Not recommended for fluoride containing electrolyte.
Graphite Moderate Moderate Graphite is resistant to sulphuric acid, but can wear down with exposure to fluoride, therefore is not always suitable for these uses.
PTFE heat exchangers are unsurpassed in their resistance to the corrosive effects of both sulphuric acid and fluoride ions in zinc electrowinning cooling systems.

Conclusion 
PTFE heat exchangers are ideally suitable for cooling zinc electrowinning solution because they are resistant to both sulphuric acid and fluoride ions. The electrolyte contains fluoride ions. The materials used must be able to resist chemical and temperature stressors without affecting the integrity of the system. PTFE exchangers have been proved to be the optimum solution to manage the electrolyte temperature, ensuring efficient zinc deposition and limiting hydrogen evolution.

Material choice is a vital part of hydrometallurgical operations, especially when dealing with trace contaminants like fluoride. Choosing the proper chemical compatibility of materials such as PTFE ensures the maximum lifetime and dependability of the cooling system, which in turn contributes to the overall efficiency of the zinc electrowinning process.