Recovery of pickling acids in the production of stainless steel using a dual drying / pyrohydrolysis process

This innovative technique enables the recovery of acids used in stainless steel production for pickling (e.g. mixed acids: HF/HNO3, HF/H2SO4 mixtures, single acids like HF, HNO3, HCl) with very high recovery rates while significantly reducing pollutant emissions (e.g. NOx) and production costs. The technology was developed by SUSTEC GmbH, part of a Horizon 2020 project (REGMAX). The technology is now implemented full scale in 3 plants in Europe. It is commercialised under the name REGMAX®.

TECHNICAL DESCRIPTION
This innovative technique was developed to address one of the stainless-steel industry's toughest acid recovery challenges which is to develop a process designed for total recovery of all acid components used in pickling, eliminating the need for waste disposal, thereby creating a circular economy system which transforms waste acids into valuable and reusable resources.
Most of the existing alternative technologies address only the recovery of the free unused acids from the pickling bath and do address the reacted acids (about 50%) or have otherwise huge losses in the regeneration process (e.g. HNO3). Furthermore, these technologies come with a small operation window for the pickling process to function efficiently limiting the pickling process conditions.

The REGMAX® process tackles the key issues in today’s mixed acid pickling by enabling complete recovery of all waste acid components.

The REGMAX® process operates through a two-step methodology that fundamentally transforms how waste acids are handled in stainless steel pickling operations. The first step involves a gentle drying process of the liquid waste acids, which carefully removes water content while preserving the chemical integrity of the acid component. The waste acid is transported with low pressure to the top of a spray dryer, where it is atomised into fine particles by means of pressurised air. The fine drops are mixed with the preheated air stream. Heat is consumed to bring the liquid and acids in a mild way into the gas phase, while the used acids are transformed into a dry metal salt powder. This gentle approach is crucial for maintaining the quality of the recovered materials and ensuring optimal regeneration efficiency. It is also an essential step for chemical stability of sensitive acids or for treating complex mixtures. The heat used for drying can largely come from neighboring annealing waste off gases, internal heat recovery and the remainder from generated process heat (e.g. natural gas, hydrogen). An advanced control system can accommodate variations in annealing furnace exhaust gas volumes/temperatures while maintaining stable drying process conditions.
While the gaseous acids go the absorption step the salts are separated from the gas stream and transported to the second treatment step.
The second step employs an advanced pyrohydrolysis reaction, where the dried metal salts undergo a chemical transformation process. During this phase, the metal salts are converted into gaseous acids and metal oxides through controlled thermal treatment. The gaseous acids are then captured and absorbed back into usable acid form, while the metal oxides are recovered as valuable by-products that can be reused in steel production. Today, it is collected in a storage bin and brought to a reduction process to be transferred to pure metals. While direct smelting of the metal oxides is technically feasible, early customers have selected an intermediate conversion process. This dual-recovery approach ensures that virtually all components of the waste stream are transformed into valuable resources.

The technology ensures that no harmful by-products are released into the environment, protecting local ecosystems from contamination. Additionally, the process significantly reduces the demand for newly produced acids, which themselves require energy-intensive manufacturing processes that generate substantial greenhouse gas emissions.

The environmental benefits extend beyond waste elimination and include also substantial reductions in water consumption and energy usage. By avoiding the need for traditional waste treatment processes and reducing the production of fresh acids, REGMAX® contributes to a decrease in the overall environmental footprint of stainless steel production. The system also enables the recovery of valuable metals like chromium, nickel and iron, reducing the need for mining these resources and preserving natural ecosystems. By transforming all waste acid components into valuable resources, the technology creates a truly circular system where nothing is wasted.

APPLICABILITY TO OTHER INDUSTRIAL SECTORS
With ongoing advancements, this technology may also be applicable to other potential industrial sectors seeking sustainable acid management solutions (e.g. Battery-Industry, Chip Industry, Solar Industry). The battery industry sector is most advanced and already in pilot stage testing. For next generation batteries, nano-porous silicon anodes will play an important role. Their production relies on a wet-chemical etching step using hydrofluoric acid that selectively dissolves a metal-silicide phase from directionally solidified Si–metal composites. The resulting waste is a complex mix of concentrated HF and dissolved metal ions that must be recycled to enable a process meeting circular economy objectives as well as financial feasibility. The REGMAX® technology can separate the metal salts from such HF-rich streams, regenerating virtually all the acid, and returning purified HF to the etch bath while delivering metal oxides ready for recovery. Pilot trials have confirmed that the system can be adapted to the high-fluorine and high-metal loads characteristic of silicon-anode etching processes. In this application, integrating REGMAX® could therefore close the acid loop, eliminate hazardous waste, and reduce fresh-acid demand in next-generation battery manufacturing facilities.



DEGREE OF MATURITY
REGMAX® is implemented at full industrial scale in three commercial plants in Europe. Based on demonstrated performance and full scale operation, this technology could potentially be considered as BAT candidate in a future revision of the FMP BREF document.
Across these plants, long term operation under real production conditions has been achieved, evidencing market readiness and robust performance in stainless steel pickling acid recovery. A fourth REGMAX® plant with 18.3 million litres / year capacity is under development in Europe, reflecting continued scale up and market adoption in the stainless steel pickling sector.
The first industrial installation was carried out in Germany, it was a turnkey integration project with pickling line. The next two plants commissioned in Belgium were significant scale up projects.

Basic information about the technique

Participant Companies