Water pollution from smelting, metal plating & metal finishing.pptx

8/12/2019 Water pollution from smelting, metal plating & metal finishing.pptx

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Water pollution from smeltersmetal plating & metalfinishing industry


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Contents

i) Smelting Industryii) Metal finishing & plating Industry


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Smelt in

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The process of smeltingSmelting is the process of separating the metalfrom impurities by heating the concentrate to a hightemperature to cause the metal to melt.Smelting the concentrate produces a metal or a

high-grade metallic mixture along with a solidwaste product called slag.Metal processing plants and smelters are facilitiesthat extract various metals from ore to create more

refined metal products.Smelting specifically involves heating the ore with areducing agent such as coke, charcoal or otherpurifying agents.


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LOCATION NAME OF COMPANY PLANT CAPACITYPER

YEAR(KT/YEAR)

Korba(Chhattisgarh) Bharat aluminium co.BALCO

345

Alupuram Hindustan aluminiumco.HINDALCO

14

Renukoot( UP ) Hindustan aluminiumco.HINDALCO

345

Mettur Madras aluminium co.MALCO

40

Hirakud Hindustan aluminiumco.HINDALCO

150

Angul National aluminium co. 460


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IntroductionMetal finishing processes involve treatment of ametal work-piece in order to modify its surfaceproperties, impart a particular attribute to thesurface, or produce a decoration.

Plating is a subset of such finishing operations thatinvolves putting a coating of metal over a basemetal substrate to give various desirable propertiesto the object.

Metal coating is another subset of such finishingoperations and involves the application of a paintor powder coating to a metal work-piece.Products from metal finishing operations can range

from structural steel to jewellery.


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Metal finishingThe reason(s) for carrying out metal finishing caninclude:

decoration,protection against corrosion,providing resistance to oxidation, hightemperatures, or UV radiation,imparting mechanical properties, such asresistance to fatigue, improvement of ductilestrength, or longevity,resistance to the use of abrasives,imparting electrical & thermal properties such assemi-conduction, thermal resistance, fireresistance, etc.


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Plating: electroplating of various types of metals onto metalsurfaces.Other metallic coating: including hot dipping (such asgalvanising) and mechanical plating (such as the peeningprocess used for Dublins spire).

Organic and other non-metallic coating: covers organic andother non-metallic coating and includes powder and liquidpaints, resins and enamels. The coatings that have beenapplied are subsequently dried. This can be by leaving todry in ambient air or assisted drying using an oven.

Stripping: used to remove previous metallic coatings fromparts or to remove coatings from articles that have to bereworked.


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Industry descriptionPhysical, chemical and electrochemical processes are allused to finish metal workplaces.The processes may simply polish the surface to provide abright appearance or apply another metal to change thesurface properties or appearance. Physical processes used in

the metal finishing industry-such as buffing, abrasive blasting,grinding, tumbling, and polishing-do not generate as muchwaste as chemical and electrochemical processes .Physical processes involve the use of a solid material (orabrasive) to change the surface characteristics of workpiece,

and the waste generated contains the abrasive and thematerial removed from the surface.The use of sand for paint stripping operations is an exampleof a physical finishing process


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Metal plating processPlating is a process of forming a thin coat of one metalover the surface of other metal or non-metal in order to

Alter the surface properties of the other metalImprove its appearanceImprove its resistance to corrosion & tarnishing,chemical attack etc.Increase hardness

Coating of chromium, cadmium, copper, zinc, silver, goldetc. can be given by electroplating, in which these metals

are made to form complexes with cyanide ion in order toincrease their throwing power.Typically double cyanide bath are used, one containingcyanide of the metal to be plated & the other containingexcess of alkaline cyanide.


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Continued . . . . .Cleaning :this is achieved by using org. solvents liketrichloroethylene or perchloroethylene or an emulsionmade from kerosene, a wetting agent & an alkali etc.the surface to be cleaned is made the anode.Rinsing :chemicals used in the cleaning operation areremoved by rinsing with water.Chemical cleaning : for this the surface is dipped in acidor alkali, depending on whether the earlier step hasused alkali/acid as a component of the cleaning

solution.Rinsing : the surfaces are rinsed with water to removereacting chemicals & products of the earlier reactions.Plating : the objects to be plated are made the cathode,

while the metal, which is to plate the objects, is madethe anode.


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If the objects to be plated are small, they are placed in abarrel, which is dipped in the plating bath. This is calledbarrel plating.

Draining : after the desired time period, the plated objectsare lifted out of the plating bath & are held over the bath toallow draining of the bath contents.Rinsing & drying : the plating objects are then rinsed withgood quality water(preferably demineralized water) & aredried with hot air. When the cleansing power of the rinsewater is exhausted, it is topped up with the necessaryplating chemicals & returned to the plating bath.


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Waste characteristicsMost stripping baths are acidic in nature and consist of solutions ofsulfuric, nitric, hydrochloric acid but alkaline baths containing sodiumsulfide, cyanide & hydroxide may also be used.The total liquid wastes in the plating industry are not voluminous, butare extremely dangerous because of their toxic content. Thecharacter & the strength of plating wastes vary considerably,depending on plating requirements and type of rinsing used.So the contents of the plating bath must be destroyed(especiallycyanides and other heavy metals such as chromium, cadmium etc.)or their concentrations must be reduced to safe limits.


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PRETREATMENT STANDARDS FOR EXISTING SOURCES (PSES)ELECTROPLATING CATEGORY [courtesy EPA]Facilities Discharging ~


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Facilities Discharging >38,O00 liters (I0,000 gallons) perday

Pollutant Daily Maximum Maximum4 Day Average(mg/l) (mg/l)

Cadmium (T) 1.2 .7Chromium (T) 7.0 4Copper (T) 4.5 2.7Lead (T) 0.6 0.4Nickel (T) 4.1 2.6Zinc (T) 4.2 2.6Silver (T) 1.2 0.7Total Metals 0.5 6.8Cyanide, T 1.9 1.0Total Toxic Organics 2.13 ----


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Installation of a chemical(or physical) treatment plant todestroy or remove toxic & objectionable materials in theeffluent-

Such Treatment are designed primarily to accomplish 3objectives:

1) Removal of cyanides2) Removal of chromium3) Removal of all other metals, oil and greases


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Removal of cyanides All acidic waste streams should be separated fromcyanide bearing streams to avoid even accidentalformation of HCN, a highly toxic chemical.Various method of cyanide bearing wastes are:

Alkaline chlorination(most commonly used)Ozonation (costly)

Acidification & natural aeration by lagooning (in remoteareas like gold mine where HCN can be dissipated in airor absorbed in caustic soda)

Acidification followed by forced aeration into a tall stackTreatment with ferrous sulphatesElectrolytic oxidation in the presence of chloride ionTreatment with addition products


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Alkaline chlorination(continuous & batch)

R d i & i i i f C


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Reduction & precipitation of Cr.-

This involves reducing Cr(VI) aschromic acid & chromates in thewaste to Cr(III) with reducingagents such as iron sulfate, sulfurdioxide or NaHSO 3.Sufficient mineral acid should alsobe present to combine with the

reduced Cr. And to maintain a pHof 3 or lower, which will ensurecomplete reaction.When the reduction is complete,an alkali (usually lime slurry) is

added, to neutralize the acid andprecipitate trivalent chromium.


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Advanced techniques for Cr.Combination of membrane bioreactor & ion exchange.Low cost adsorbents(coconut fibre etc. ) have also beenusedRice husk, treated with sulfuric acid, followed by CO2activation.

Activated aluminaFor other heavy metals:Metals can be recovered from filter cake.Colloid flotation

Electrolytic recoveryLead plating & cadmium plating is done in fluoborate bathwhich produces effluents having 100-250 mg/l of lead & 7-50mg/l cadmium.it can be removed by coconut shell, treated with

sulfuric acid in the presence of potassium persulphate/hydrogen peroxide.


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NeutralizationTreatment of other metal, oil &grease bearing wastes byNeutralization & precipitationUsually involves recombining thWastes with previously oxidised

cyanide and reduced chromiumwastes for subsequent andfinal treatment.


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Recovery

Chrome acid recovery by vaccum evaporation


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Current best practices to savewaterImprove Operating Procedures -This section sets outimprovements that can be made in operating practices.Reducing Drag-out -Drag-out is a term which refers to theremoval of process solution from a bath as a result of itsadherence to the work-pieces and the jig or barrel. If the amount

of drag-out formed in the first place is reduced, this willconsequently reduce rinsing requirements, effluent treatmentrequirements and prolong the life of process baths. There are anumber of ways that drag-out can be reduced.Better positioning of work-pieces on racks: parts should be

placed on the rack so that the largest surface or plane is nearlyvertical and the longer dimensions are horizontal. The loweredge should be inclined, so that run-off will occur at a cornerrather than the entire edge. Avoid positioning parts directly overone another where possible.

Provide static rinses instead of running rinses to the platesurfaces.


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Continued ..

Using countercurrent washing system, inwhich the plate object is passed throughstatic rinsing tanks from left to right, whilethe rinse water is supplied only to the lastrinsing tank.


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References

Water.epa.gov.inInfohouse.p2ric.orgwww.envirocentre.ie


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Thank you

Water pollution from smelting, metal plating & metal finishing.pptx

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