During production of stainless steel, unwanted metal oxides are being formed on the surface of the material. This site gives an overview of the chemical reactions taking place through pickling – a process to remove the formed oxides.
Traditional Steel Pickling processes.
The acids, which are used for pickling, are only consumed to a certain degree. The formed metal salts lower the reaction rate and start to precipitate in the pickling bath. As a consequence, the acid solutions have to be disposed at a time when still a large portion has not reacted yet. These waste solutions contain free unreacted acids, spent acids and dissolved metal components, forming the so called “waste acid”.
Need for Waste Acid Disposal
Before the waste acid can be discharged into the environment, it is typically diluted with water and neutralized with Ca(OH)2 forming calcium fluorides and calcium nitrates as well as metal hydroxides. The fluorides and the hydroxides precipitate and can be separated through a filtration process. As a result, toxic sludge and nitrate-containing wastewater is generated. The sludge needs to be transported to a landfill for hazardous waste. Depending on the environmental limits, the nitrate-containing wastewater needs to be processed further in a wastewater treatment plant or can be directly disposed into the environment.
Acid Neutralization Plant
The Retarda purification process.
Purification processes like Retarda systems often use ion resins beds to partially remove the metal salts from the pickling bath. A branch stream is taken from the pickling bath and the resin is loaded with metal salts from the waste acid. A fraction with less metals is sent back to the pickling bath, while the metal salts have to be cleaned off from the resin with water. This wastewater stream has to be treated in the neutralization process like in traditional steel pickling processes. The solution, which is sent back to the pickling baths, is enriched with fresh acids again in order to reach the desired concentrations for pickling.
Need for Waste Acid Disposal
A purification system will not eliminate the purchase of fresh acid; it will only reduce the quantity that is consumed. Neutralization of waste streams – with the need of waste water disposal and sludge landfilling – is still necessary as well as complete exchange of the bath from time to time.
Acid Regeneration Plant
A regeneration process is designed both to convert the metals salts back to free hydrofluoric and nitric acid and to recover the free acids. The products are metal oxides and regenerated acid. Both products can be reused in steel production and pickling.
Spray Roasting of Waste Acid
Until today the most common regeneration process is spray roasting of the waste acids. The principle of this process is evaporation of the waste acid and simultaneous pyrohydrolysis, where the metal salts are transformed to metal oxides. During this transformation, acidic gas is formed and metal oxides are obtained.
NOx Formation during Spray Roasting
However, significant portions of the nitric acid gas will decompose to nitrogen oxides (NOx). This nitrogen oxides need to be removed from the exhaust gas stream with with the use of DeNOx systems. This means that not only large portion of the acid is lost but also needs to be removed from the exhaust gas stream with a DeNOx system, consuming additional ressources and operation costs.
Our REGMAX® Process.
SUSTEC has developed the REGMAX® process to overcome all existing problems with mixed acid pickling and its regeneration. Neither sludge nor wastewater is generated and no DeNOx unit is needed.
Recovery rates are >99% for HF and >98% for HNO3.
It does not only lower your production cost but is also inline with 3 United Nations Sustainable Devlopment Goals.