Unique Challenges in Stainless Steel vs. Carbon Steel Pickling

Unlike carbon steel pickling, which produces mainly ferrous chloride (FeCl₂), stainless steel pickling results in a higher concentration of FeCl₃ in the spent acid. This difference becomes problematic during the regeneration process, particularly when spray roasting technology is used. The key issue is the behavior of FeCl₃ within the regeneration reactor. FeCl₃ has a lower boiling point than FeCl₂ and tends to transition more easily from the solid to the gas phase. As a result, FeCl₃ can volatilize and leave the reactor before complete hydrolysis. This premature volatilization results in incomplete conversion of FeCl₃ to HCl and iron oxide (Fe₂O₃). Over time, this phenomenon causes FeCl₃ to build up in the system.

Chemical Properties of FeCl₃ in Spray Roasting

The chemical properties of FeCl₃ play a critical role in this issue. At the temperatures typically used in spray roasting (600°C to 800°C), FeCl₃ has a higher vapor pressure than FeCl₂. The vapor pressure of FeCl₃ at 750°C is about 4 atm, while that of FeCl₂ is only about 0.1 atm. This significant difference in vapor pressure explains why FeCl₃ is more likely to volatilization before hydrolysis can occur. The hydrolysis reaction of FeCl₃ is represented by the following equation:

 2FeCl₃ + 3H2O → Fe₂O₃ + 6 HCl 

However, due to the rapid volatilization of FeCl₃, this reaction does not proceed to completion within the spray roasting reactor, leading to the observed up-concentration effect.

Need for Specialized Techniques

In conclusion, the high FeCl₃ content in spent HCl from stainless steel pickling is a significant obstacle for traditional spray roasting regeneration methods. The tendency of FeCl₃ to volatilize prior to hydrolysis leads to its accumulation in the system, reducing process efficiency and potentially causing operational problems. This problem highlights the need for specialized regeneration techniques designed to meet the unique challenges of stainless-steel pickling acid recovery.

SUSTEC’s REGMAX^CL: A Complete Solution to Acid Recovery

SUSTEC's goal is to eliminate all waste acids from stainless steel pickling. Initially starting with its REGMAX solution for mixed acids (HF and HNO3), SUSTEC's strategy from the outset was to also solve all the other associated acid consumption and environmental waste problems in the industry.

REGMAX^CL: Advancing HCl Recovery in Stainless Steel Processing

Today, SUSTEC's REGMAX^CL is able to treat the hydrochloric waste acids from stainless steel pickling. The acid is completely recovered and converted into fresh hydrochloric acids, while the dissolved metals are recovered as oxides for reuse in stainless steel production.

A New Standard for Hydrochloric Acid Regeneration

The recovery of spent pickling waste acid from stainless or carbon steel pickling containing FeCl₃ is no longer a problem. Furthermore, with SUSTEC REGMAX^CL it is possible to obtain a superazeotropic hydrochloric recovered acid – making it unique among all spent hydrochloric acid regeneration plants in existence today.

Expanded Acid Regeneration Capabilities with SUSTEC's REGMAX Plants

SUSTEC's REGMAX Acid Regeneration Plants have provided reliable solutions for mixed acids, specifically HF and HNO₃. Now, they also offer advanced regeneration for hydrochloric acid, broadening their industrial application.