ENVIRONMENTAL PROBLEMS & SOLUTIONS FOR STAINLESS STEEL PICKLING
During pickling metal salts are formed and a signification portion of the acid is remaining as free acids. These components forming the so called “waste acid” and need to be neutralized with Ca(OH)2 .
The result is an aqueous solution and sludge. The aqueous solution will be sent to industrial waste water treatment, while the problematic sludge needs to be disposed.
This is particularly disadvantageous as the concentration of metals is very low in the pickling solution and a lot of free acid needs to be removed from the process. This is causing not only an environmental problem.
It is also a waste of resources as the disposed acids need to be purchased again for the next pickling campaign.
To solve the neutralisation problem, many studies have been made on processes for the regeneration and recovery of nitric and hydrofluoric acids from pickling waste acid. Results are two main groups of processes, one is purification and the other is regeneration.
PURIFICATION PROCESSES – RETARDA
In purification processes, like retarda systems, the metal fluoride and nitrate salts are partially removed from the pickling bath and free acid concentrations will be increased with feed of fresh acid. In this way the bath lifetime is extended.
A purification system will not eliminate the purchase of fresh acid; it will only reduce the quantity that is consumed. Neutralization of used bath and removed salts is still necessary.
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 salts and regenerated acid. Both products can be reused in steel production and pickling.
Sprayroasting – Pyrohydrolysis
Until today the most common process is spray roasting of the waste acids. The principle of this process is evaporating the waste acid and a simultaneous pyrohydrolysis reaction.
The formed gas is absorbed and metal oxides are gained. However significant portions of the nitric acid will decompose to nitric oxides (NOx), which needs to be neutralized at the end of the process with Denox systems.
Suppliers of this systems imply that it is possible to reduce the purchase of hydrofluoric and nitric acid with high recovery rates. In practice recovery rates are much lower than advertised, especially for HNO3 due to the NOx formation.
During evaporation following reactions are taking place:
(6) H2O (l) → H2O (g)
(7) HNO3 (aq) → HNO3 (g)
(8) HF (aq) → HF (g)
These three reactions are representing the evaporation of water, nitric acid and hydrofluoric acid. This is the first reaction after the acid is sprayed into the reactor.
The temperature in the reactor is between 300 and 700 °C. The consequence of this reaction conditions is, that large parts of the gaseous nitric acid will decompose according to reaction (9-10) to NOx.
(9) 2 HNO3 (g) → NO2 (g) + NO(g) + H2O(g) + O2(g)
(10) 2 NO2 ⇔ 2 NO + O2
Reaction (10) takes place in both directions, so that both NO and NO2 will be present after the reactor.
Nearly all present evaporated HNO3 will decompose to NO and NO2. In the absorption process downstream the spray roaster a certain amount of the NOx will react to HNO3 again – see reactions (11-12).
(11) NO2 (g) + H2O (l) ⇔ HNO3 (aq) + NO (g)
(12) NO + 0.5 O2 ⇔ NO2
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, causing additional operation costs.
The recovery rates are in the range of 99% for HF and 65% for HNO3.
SUSTEC has developed the REGMAX process to overcome all existing problems with mixed acid pickling and its regeneration.