ELECTROPLATING INDUSTRY:SOURCES, EFFECTS AND TREATMENTOF EFFLUENTS

Introduction• production of a thin surface coating of one metal upon another by electro deposition.

• metal ions supplied by the dissolution of metal from anodes or other pieces, are reduced on the work pieces (cathodes) while in either acid, alkaline, or neutral solutions.

• Uses less water, so volume of wastewater is less, but very toxic due to metals.

Processes Involved• Cleaning• Stripping• Plating• Rinsing

Cleaning• Removal of Grease, due to machining,

stamping, polishing.• Organic grease – removed by

saponification with alkali• Petroleum & Mineral grease – benzene,

gasoline, carbon tetrachloride used Most commonly employed method is

emulsification with alkalisspent cleaning solutions are discharged

as waste water.

StrippingRemoval of rust and scaleFor articles made of iron, H2SO4 or HCl usedelectrolytic method is also being used

increasinglyFine sand particles remaining on the

surface due to sand blasting can be removed separately by hydrofluoric acid

spent acid solutions and rinse waters constitute the waste water

Platingpickled articles are placed in wooden or mild

steel vats with special liningOr in tanks made entirely of polymer materialmetal to be plated is made the cathode in an

electrolytic cell.Plating baths are acidic in nature and generally

contain H2SO4 , HCl or HNO3 Alkaline baths containing sulphide, carbonate,

cyanide and hydroxide are also used.concentration of chemicals normally used in

some of the common plating baths is shown

RinsingAfter plating, objects are rinsed with water.First dipped in stationary water baths,

which are allowed to drain.Then dipped in running water baths to

remove the adhering plating solution.Stationary baths are utilized to make

solution for the plating operations.Running water baths are discharged into

the drains.

Sources and Characteristics of Wastewater

Sources are generally two:

• Batch Solutions• Rinsing Waters

• Cleaning Solution• Spent Alkaline and Rinse waters• Acid Pickling and Rinse Waters• Cyanide Concentrates• Chromate Wastes• Metal wastes from Plating baths• Floor Washes• Regeneration Wastes from Ion-Exchange Columns• Other Wastewaters • Composite Wastewaters

Composite Wastewaters

Potential Environmental Problems• Toxicity to fish and other aquatic life

• Effects on sewers• Effects on biological sewage treatment processes

• Pollution of ground water and surface waters

Water Abatement TechniquesWaste Prevention• Installing a gravity-fed, non-overflowing emergency

holding tank for toxic• materials.• Eliminating breakable containers for concentrated

materials.• Provision for special drip pans, spraying rinses, and

shaking mechanism.• Reducing spillage, drag-out and leakage to the floor,

or other losses by curbing the area and discharging these losses to a holding tank.

• Using high pressure rinses rather than high volume water washes.

• Re-circulating valuable materials from concentrated plating bath wastes.

• Evaporating reclaimed wastes to desired volume and returning to plating bath at rate equal to the loss from bath.

• Re-circulating wet washer wastes from fume scrubbers.

• Use of counter-flow rinses to reduce the concentration of contaminants in the final rinse water.

• Recovery of metals from the wastes.

Cyanide TreatmentAlkaline Chlorinationi. pH = 10.5ii. 2NaCN + 5Cl2 + 12NaOH N2 +

2Na2CO3 +

10NaCl + 6H2O

Two methods are suggested:iii. Batch Typeiv. Continuous Type

Batch treatment for Cyanide wastes

Continuous treatment of Cyanide wastes

Ferrous Sulphate Treatmenti. pH = 7.5 to 9.0ii. Formation of dark blue sludgeiii. Large sludge and odor produced

6NaCN + FeSO4 Na2SO4 + Na4Fe(CN)6

Chromium TreatmentChromates, dichromates and chromic acid occur

in the waste waters generated from chromium plating, anodising, and other metal finishing operations.

Hexavalent chromium ions in these wastes are highly toxic even in very low concentrations.

treatment involves reduction to trivalent state and subsequent precipitation with an alkali.

Ferrous sulphate along with sulphuric acid is commonly used.

Maximum reduction occurs in the pH range 2.0 to 2.5.

H2Cr2O7 + 6FeSO4 Cr2(SO4)3 + 3Fe2(SO4)3 + 7H2O

+ 6H2SO4

Cr2(SO4)3 + 3Ca(OH)2 2Cr(OH)3 + 3CaSO4

Fe2(SO4)3 + 3Ca(OH)2 2Fe(OH)3 + 3CaSO4

Batch Treatment of Hexavalent Chromium Wastes

Continuous Treatment of Hexavalent Chromium wastes

Treatment of Other Metal bearing Wastes

common holding tank is normally used for all the metal wastes.

Precipitation occurs in the holding tank if the pH is on the alkaline side.

To prevent settling and to ensure proper mixing, agitation is provided.

Almost all the metals precipitate completely in the pH range of 9.5 to 10.5.

Batch Treatment of metal wastes not containing hexavalent chromium

Continuous treatment of metal wastes not containing hexavalent chromium

Metal Recovery by Soluble Concentration

In large electroplating industries, chrome, nickel and copper in acid type plating solutions are reclaimed from the rinse tank by evaporation in glass-line equipment or other suitable evaporators and the concentrated solution returned to the plating system. This process appears to be economically feasible, where chemical quantities involved are relatively large and would justify recovery

Overall Flow Sheet for treatment of various wastes from electroplating works

Standards by CPCB

Abhishek Koul13201001

M.E Environmental Engg

Thank You