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AUTHORS: Frank Baerhold, Jovan Starcevic and Klaus ReichertAndritz AG

A fter the hot rolling and annealing of stainless steel,the scale has to be removed from the surface with

a pickling process. A mixture of nitric and hydrofluoricacids, so-called mixed acid, is used as a pickling agent.The spent pickling acid contains metal fluorides andnitrates but not dissolved scale. The waste acid and rinsewater are treated in the waste water treatment plant,however, after neutralisation and filtration the fluoridesand metals are found in the filter cake and the nitratesremain in the effluent leaving the plant.

PYROMARS PROCESS (PYROLYTIC MIXEDACID RECOVERY)The first step in the reduction of nitrates is made withthe installation of the mixed acid recovery plant –PYROMARS. The process itself is a RUTHNER-type spray-roasting pyrohydrolysis process, adapted to theHF/HNO3 system. Figure 1 shows a flow sheet of theprocess and Figure 2 an installation. The waste pickleacid is fed into the pre-evaporator stage. The hot reactor

off-gas is quenched and the acid is pre-concentrated.The hot pre-concentrated acid is then sprayed by anozzle system into the top of the reactor, which isdirectly heated by tangentially installed burners. Whilethe liquid droplets fall down, water and the free acidsevaporate and the metal salts are decomposed into the corresponding metal oxides and acid gases. Thenozzle system generates fine droplets, which arehomogeneously distributed over the roaster cross-section, in order to guarantee sufficient evaporation ofthe liquid at a low temperature level to minimise thedecomposition of nitric acid.

The metal oxide, which is more than 99% pure, istransported from the reactor bottom to the oxide bin andfilled into big-bags.

The processing of stainless steels involves pickling in a mixture of hydrofluoric and nitric acids. Thecontamination of the resulting effluents with nitrates causes a big problem for waste watertreatment. A significant reduction can be achieved by installation of a waste acid recovery plant –PYROMARS. To remove the remaining nitrates in the rinse water, the new ZEMAP process wasdeveloped which makes it possible to have a nitrate-free, zero effluent mixed acid pickling process.

Reduction of nitrates in stainless steel pickling

r Fig.1 PYROMARS process flow sheet r Fig.2 Pyromars plant

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The following reactions take place:Reactor Evaporation H2O (l) p H2O (g) (1)

HNO3 (aq) p HNO3 (g) (2)HF (aq) p HF (g) (3)

Conversion 2 FeF3 + 3 H2O p Fe2O3 + 6 HF (4)2 HNO3 p NO2 + NO + O2 + H2O (5)NO2 p NO + 1/2 O2 (6)

AbsorberHNO3 (g) p HNO3 (aq) (7)HF (g) p HF (aq) (8)

Oxidation columnOxidation NO + 1/2 O2 p NO2 (9)Absorption 3 NO2 + H2O p 2 HNO3 + NO (10)

Catalytic de-nitrification2 NO2(g) + O2 +4 NH3(g) p 3 N2(g) + 6 H2O(g) (11)4 NO(g) + O2 + 4 NH3(g) p 4 N2(g) + 6 H2O(g) (12)

The recovered acid being formed in the absorber is fed to astorage tank. The gases leaving at the top of the absorberstill contain most of the NOx formed according to reaction(5). In an additional cooling and oxidation step these NOxgases are converted into HNO3, thus increasing the recoveryrate of the process substantially. A part of the condensatefrom the first spray cooler is fed to the rinse waterevaporator, leading to a reduced volume of regeneratedacid. A reduction of approximately 20%, compared to thevolume of waste acid, guarantees a closed water balance ofthe entire pickling and acid regeneration process.

Here, all the valuable components are contained in thespent mixed acid. Costly hydrofluoric and nitric acids arerecovered as a reusable mixed acid. The metals areconverted into oxides and recycled in the melting process– especially beneficial in view of the high nickel content.

For nitrate removal, PYROMARS either recovers nitrates inform of nitric acid or decomposes them via NOx to nitrogengases. The process was first installed in 1992 and nineplants now operate successfully throughout the world.

ZEMAP PROCESS (ZERO EFFLUENT MIXED ACID PICKLING)Although the PYROMARS process effectively reducesthe amount of metals and nitrates in the waste water asmall amount remains. This is caused by a certain carry-over of the mixed acid into the rinsing section and,depending on the strip dimensions, line speed and typeof rinse, the amount can vary from 10 to 30% of thetotal waste acid.

The new ZEMAP process consists of the followingsteps:

` Neutralisation of rinse water by ammonia ( NH3)` Pre-concentration by evaporation` Concentrate is introduced to PYROMARS` Condensate is reused for rinsing

During neutralisation with ammonia, ammonium salts(NH4F, NH4NO3) are formed. These salts aredecomposable by the PYROMARS process so that fromammonium fluoride (NH4F) hydrofluoric acid (HF) isproduced. The vapours leaving the evaporator contain onlya very small amount of ammonia and can, aftercondensation, be used for rinsing, so that demineralisedwater consumption can be reduced. A further advantage isthat the metals, which are in the rinse water, are separatedas oxides in the PYROMARS process and cannot be foundin the filter cake after waste water treatment.

The closed-loop operation of pickling, rinsing,evaporation and acid regeneration is shown in Figure 3.

CONCLUSIONSThe PYROMARS process significantly reduces the amountof nitrates in the effluent and the amount of sludge afterwaste water treatment. What is remaining is the mixedacid contamination of the rinsing water. With the newlydeveloped ZEMAP process this contamination can also betreated and the mixed acid part of the pickling linebecomes a zero effluent unit.

Demineralised water consumption is also significantlyreduced and the overall consumption of the freshhydrofluoric acid is reduced to almost zero. The amount ofsludge after treatment of mixed acid waste water, whichhas to be put to waste disposal, is reduced to almost zeroand the effluents from the pickling line are nitrate free. MS

Frank Baerhold is Head of R&D Carbon Steel, JovanStarcevic is Head of R&D Stainless Steel and KlausReichert is with Corporate Communications & InvestorRelations, all at Andritz AG, Graz, Austria.

CONTACT: ruthner@andritz.com

r Fig.3 The closed-loop operation of pickling, rinsing, evaporationand acid regeneration

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