Dross Formation Pptaaaaaaaaaaaaaaaaaaa

8/15/2019 Dross Formation Pptaaaaaaaaaaaaaaaaaaa

1/28

Dross Formation in the

Galvanizing Kettle

By Bernardo Duran


2/28

Goal of Presentation

Discuss why dross forms in the

galvanizing kettle and methods todecrease its formation.


3/28

Overview

Denition of dross

Why dross formation matters

How dross forms

How to reduce dross formation andits negative eects on the nishedproduct


4/28

Denition of Dross

Byproduct of the galvanizing processwhich consists of loose iron

particles (iron salts that havemetallurgically reacted with zinc inthe galvanizing kettle.

Dross can contain more than !"#zinc ($# iron.


5/28

Why Dross Formation Matters

%esthetic concerns from drosspimples.

&teel can 'e reected for gross

dross inclusions.Dross formation accounts forappro)imately *+# of the zincusage in the galvanizing process.

Dross formation can 'e reduced.


6/28

How Dross Forms

,ree iron particles in the galvanizingkettle metallurgically react with

zinc to create dross particlesrather than the zinc reacting withthe steel to create a galvanizedcoating.


7/28

Types of Dross

Floating Dross- free particles ofdross that can oat throughout thekettle and usually consist of longintermetallic spikes interwoven in

clumps/ hollow0like structures

Bottom Dross- dross particles that

aggregate (settle to the 'ottom ofthe galvanizing kettle


8/28

Floating Dross


9/28

Dross Pimples


10/28

Dross Pimples


11/28

Gross Dross Inl!sion


12/28

Where the Free Iron Partiles"ome From

1ron salts formed 'y a reaction'etween pickling agents and steel

1ron salts formed 'y reaction'etween u) and steel

2inc3iron alloys formed 'y a direct

reaction 'etween steel and moltenzinc (loose iron particles on thesteel


13/28

Iron #alts from Pi$ling %gents

4ickling agents react with irono)ide and the steel to create ironsalts.

5reates loose iron salts that canthen 'e transferred to su'se6uenttanks if not rinsed properly.


14/28

Iron #alts from &eations withFl!'

,lu)ing agents such as ammoniumchloride and zinc chloride can form

iron salts when reacting with thesteel which can then 'e carriedover to the galvanizing kettle.


15/28

Free Iron from (in)#teel&eation

7oose particles of iron from theiron or steel that comes o while inthe kettle can go on to createdross particles.

8eactive steels can develop e)cesszeta layer formation which can

ake o crystals that create freeiron particles.


16/28

*ew Galvanizing Kettles

Dross formation can 'e high in newkettles until a layer of intermetalliclayers form on the kettle walls9which then usually inhi'its furtherdross formation.


17/28

&e+!ing Dross Formation

7imiting formation of iron saltsduring pickling operation

%de6uately rinsing iron salts afterpickling operation

:onitoring iron levels in picklingand u) 'aths9 and galvanizingkettle

;lemental additions to galvanizingkettle

%voiding large temperature swingsin galvanizing kettle


18/28

,imiting Iron #alt FormationD!ring Pi$ling

1nhi'itors can 'e used which limitthe attack of the acid on the steelwhile not aecting dissolvingaction of the iron o)ide and millscale on the steel

:onitor pickling time to ensure

steel is not over pickled


19/28

%+e-!ate &insing of Pi$ling#alts

%llow enough time for picklingsalts to drain 'efore transferringthe steel to the rinse tank

%llow enough time in rinse tank

1mplement a second rinse tank

when possi'le3practical


20/28

Monitoring Iron ,evels inPi$ling an+ Fl!' #ol!tions

,re6uently check iron levels andpH in pickling tank and rinse tanks

,re6uently check iron levels in u)(should 'e no greater than


21/28

Properly Maintain Fl!'#ol!tions

Dross formation can 'e higher inwet u) method than dry method

%gitation of the u) in the dryprocess can help to convert e)cessiron to sludge

Work with u) supplier to ndoptimal chemical levels andltering strategies

l l ++i i h


22/28

.lemental %++itions to theGalvanizing Kettle

&mall additions of lead (*# can reducedross formation9 'ut the industry ismoving away from lead use in kettles

=ickel additions (


23/28

Iron #ol!/ility in (in an+ (n0*i1"o!rtesy of Te$2

M i i i " i


24/28

Maintaining "onsistent Temperat!res in the

Galvanizing Kettle1ron solu'ility increases with increases inkettle temperature (see graph on ne)tslide

When temperature drops9 the ironprecipitates out of solution which is thenavaila'le to form dross

>ppermost level of zinc can 'e cooler thanlower levels of zinc and have lower ironsolu'ility

# l /ilit f I i (i ith


25/28

#ol!/ility of Iron in (in with3arying Temperat!res


26/28

&emoving Dross from Kettle

Bottom dross should 'e removedon a scheduled 'asis9 whether 'ytime or steel volume throughput

=itrogen can 'e 'u''led in zinc(ensure 'ottom dross is notdistur'ed to oat dross to the

surface where it can then 'escooped out


27/28

"onl!sion

8educing dross formation-1ncreases prots 'ecause less zincis consumed in the form of dross

:akes for happier customers sincethere are less dross inclusions onthe steel


28/28

%++itional &eso!res

Galvanizing Note: Skimmings &Dross

Troubleshooting Guideline:Reducing Dross Pimles

%?% 8esource 7i'rary

@our pickling and u) solutionsuppliers

Dross Formation Pptaaaaaaaaaaaaaaaaaaa

Documents

Transcript of Dross Formation Pptaaaaaaaaaaaaaaaaaaa