MacDermid Envio Solutions is working on the industrial testing of a new approach for the in-situ treatment of zinc-nickel wastewater and trivalent chromium waste. The results are promising.
The dark era in the history of surface finishing: what sounds ominous refers to the time of the widespread use of cyanide, hexavalent chromium and cadmium in electroplating. In fact, however, many of these substances were relatively easy to use and dispose of, despite the known safety risks.
Advances in electroplating technology have led to technically advanced and commercially successful developments. They offer unsurpassed performance characteristics such as high corrosion resistance, uniform coating thickness and excellent coating distribution, properties that would have been unthinkable 20 years ago. These benefits are reflected in the durability and reliability of everyday items such as cars, electronics and household appliances. Without them, our modern world could not function in the way we are used to: It is essentially no longer necessary to have everyday objects constantly repaired or maintained.
However, many modern surface finishing processes also have disadvantages, for example the resulting waste water is often much more difficult to treat than its predecessors.
The treatment of zinc-nickel
A closer look at modern processes: Zinc-nickel coatings for corrosion protection or even trivalent chromium coatings for decorative applications are state-of-the-art coating processes that were developed primarily to meet the need for improved occupational safety and higher performance standards in coating. They excellently fulfill both objectives, but also present users with some of the most difficult waste treatment challenges.
In the case of zinc-nickel processes, the process contains significant amounts of complexing agents to keep nickel soluble even at pH levels that are normally well above the optimum range. Alkaline zinc-nickel is particularly challenging given this optimum solubility below pH 10: alkaline systems operate in a very high pH range and therefore need to contain strong complexing agents to ensure the solubility of the nickel ions. While these complexing agents are not a problem for the production environment, they are very difficult to handle in wastewater streams. Many users therefore choose to separate their zinc-nickel waste from the rest of their wastewater and have it treated by external companies. These companies usually incinerate the waste instead of treating it chemically.
On-site treatment usually involves the use of sodium hypochlorite to partially destroy the complexing agents and make the nickel less soluble. This makes further pH treatment possible. This produces nickel hydroxide sludge, which can be pressed into a filter cake and then deposited. Unfortunately, this method is slow and therefore time-consuming. The hypochlorite reaction is also slow, so this method can take a good 4-8 hours of treatment time and produce large quantities of waste sludge.
Acid zinc-nickel processes have very similar problems as similar concentrations of complexing agents are required to maintain nickel solubility. As with alkaline processes, external wastewater treatment is common - with some smaller users using hypochlorite.
For larger zinc-nickel users, there is also the option of investing in an on-site vacuum evaporation plant to minimize the amount of wastewater to be removed. This has the advantage that some of the zinc-nickel rinse water that is normally sent for treatment can be reused. However, in view of rising energy costs, the investment and operating costs for evaporation are increasingly exceeding the financial possibilities of most companies.
Outlook: A new approach
There are various groups working to solve this particular wastewater treatment problem. In Europe, a small number of companies have joined forces with a university to characterize the problem and work together on a solution. However, the work is still at a very early stage.
However, the best option would be to develop alternative chemicals with easier to destroy complexing agents. However, given the narrow chemical parameters within which zinc-nickel processes must operate to optimize alloy composition and distribution, this is unlikely to be successful. What is needed is an approach that combines chemical and wastewater engineering expertise.
And this is exactly what MacDermid Envio Solutions, a relatively new division of Element Solutions Inc, does. MacDermid Envio Solutions has the chemical expertise of MacDermid on the one hand, and on the other hand, the company has extensive knowledge of wastewater treatment thanks to the acquisition of DMP, a specialist in wastewater treatment plants in 2020. MacDermid Envio Solutions is currently working on the industrial testing of a completely new approach for the in-situ treatment of zinc-nickel wastewater. Industrial testing of the patent-pending concept began in September 2022 and is expected to be completed in the first quarter of 2023.
The new approach uses a modular arrangement of small (treatment) cells, each of which is designed for the stepwise neutralization of zinc-nickel complexes. Both rinse water and decanted concentrates can be treated in the self-contained system before the wastewater stream is fed back into the user's conventional chemical wastewater treatment system. Laboratory tests have shown that this approach releases 100% of the complexed nickel and zinc compounds so that the metals can be easily and completely removed by a standard pH precipitation. The system allows this process to be used by users of any size and ensures that any future production expansion can be easily retrofitted.
Trivalent chromium
Electroplating trivalent chromium presents a similar problem to zinc-nickel. The complexed chromium is firmly bound in the process chemistry to ensure efficient deposition. However, this makes waste treatment more difficult. Unlike zinc-nickel, chromium ions exist in multiple forms as complexed compounds, limiting the effectiveness of a single chemical treatment in waste treatment. While some of these substances respond to low pH treatment, others remain in the wastewater. Conversely, some chromium complexes respond to higher pH treatment, while others remain unaffected. By adjusting the pH in both high and low ranges, not all complexed chromium ions are released, which is why conventional treatments are often only partially effective. A further complication is that the trivalent chemicals from different suppliers can use a range of complexing agents, further complicating the chemical treatment.
Currently, a MacDermid Envio Solutions team in the US is working on a new method for treating trivalent chromium waste. By combining chemical and equipment approaches, significant advances in treatment can be achieved. This new system is to be tested on a large scale by several users in industrial production this year.
Outlook for future wastewater treatment
It is clear that advances in surface finishing will continue to challenge the brightest researchers to improve the safety of chemical processes and achieve the performance improvements demanded by end users. As the range of environmentally safe chemicals continues to shrink, the use of more complex chemical processes is surely the best way to achieve these goals. In turn, we need to keep pace with new approaches to wastewater treatment to avoid increasingly costly and energy-intensive off-site treatment. Finally, the water used in surface finishing must be returned to the environment in a clean and sustainable state.
