INTERNATIONAL CLEANER PRODUCTION INFORMATION CLEARINGHOUSE

CASE STUDY #165

1. Headline: In-process Measures to Cyanide-free Zinc Baths

in a Steel Furniture Factory Reduce Metals in Effluent

2. Background: See below.

3. Cleaner Production Principle: Material substitution,

improved operating practices

4. Description Cleaner Production Application:

Technology Principle: The technology involves in-line

process measures such as cyanide-free process baths, water

reuse, filtration and monitoring, longer drip times, spray

rinsing, closed cooling, and wastewater treatment.

Case Study Summary

Process and Waste Information: The company operates two

lines which both contained cyanides originally. The first

line is for cyanide-free nickel and zinc plating and blue

passivation with chrome (VI), following a preparation

line. It is an automated drum line but is not used much

since it is very old. The second line contains an

electrolytical zinc sulfate bath free of cyanides and a

bath for blue or black passivation with chrome (VI). The

other baths in the line are for degreasing, staining,

rinsing (static and cascade), and acidic dipping. The

zinc sulfate bath has a concentration of 6-7 g/l.

The in-line process measures included 1) changing to

cyanide-free process baths, 2) partial return of water

from the static bath to the process bath, 3) continuous

filtration and monitoring of the zinc bath liquor, 4)

longer dripping times, 5) spray rinsing over the process

bath, 6) closed cooling system, and 7) installation of a

water treatment system.

In the original process, wastewater was not treated but

was directly sewered. In the new process, the treatment

system involves use of separate storage tanks for

alkaline, acid and chrome-containing rinsewater streams

and concentrates, final purification in a DND

installation, and sludge dehydration in a filter press.

Overflow from a sedimentation tank is sewered.

The new technology results in a decrease in the amount of

metals released in the effluent. New feedstocks include

sodium bisulfite for detoxification of Cr(VI),

neutralization with lime, pH adjustment with hydrochloride

or lye, and polyelectrolytes for flocculation. The

resulting sludge is 25% dry matter containing iron, lime,

and about 10% zinc.

Scale of Operation: Plant capacity is 12 million work

pieces per year, equivalent to 700,000 m2 of area or 500

tons of deposited metal per year. The plant employs 8

workers in the electroplating department.

Stage of Development: The technology is fully

operational.

Level of Commercialization: All necessary components are

widely available.

Material/Energy Balances and Substitutions:

Material Category Quantity Before

Quantity After

Waste Generation

(kg/yr):

Metals in effluent 945

37 Sludge, 25% dry 0

80,000 matter

Feedstock Use:

Lime(ton/yr) 0

14 HCl(m3/yr) 0

7 NaOH (m3/yr) 0

7 Sodium bisulfite 0

14 (ton/yr)

Flocculation 0

N/A chemicals

Water Use (m3/yr): 330,000

20,000*

*Includes 3,500 m3/yr of non-contaminated cooling water

used to prevent foaming in the end control pit.

5. Economics* It is difficult to make cost estimates of the

in-line measures since the rinse baths and other equipment

were installed at the time the lines were constructed.

Therefore, costs presented are for the water treatment

system.

Investment Costs: Costs for the DND system were Dfl

545,000 for 1980 - 1982. Capital costs were 95,400

Dfl/yr.

Operational & Maintenance Costs: Costs are as follows:

Labor 53,000 Dfl/yr

Chemicals 35,000

Energy 17,000

Sludge removal 29,000

Payback Time: Savings were estimated as 78,475 Dfl/yr

from pollution taxes with costs estimated at 229,400

Dfl/yr. Net costs would be 150,925 Dfl/yr.

6. Advantages: The change to cyanide-free sulfate baths was

inspired by safety considerations. The other measures

were taken to meet public standards. The economic benefit

is decreased taxes on heavy metal pollution, at a rate of

53.64 Dfl/kg or 78,475 Dfl/yr.

7. Constraints: It is more difficult to maintain the low zinc

concentration at a constant level than in a cyanide bath.

8. Contacts and Citation

Citation:

Wastewater problems in the metal industry: results of

interviews in 48 companies. Dr. Ir W.H. Rulkens, TNO,

Maatschappelijke Technologie, postbus 342, 7300 AH

Apeldoorn, Netherlands. Tel: 31-55-773344.

Industry/Program Contact and Address:

H.W. du Mortier

VOM

Jan van Eycklaan 2

Postbus 120

3720 AC Bilthoven

Netherlands

Tel: 31-30-287111

Fax: 31-30-287674

9. Keywords: the Netherlands, metal, iron and steel,

electroplating, process modification, cyanide, filtration,

spray rinsing, blue passivation, chromium, hexavalent

chrome, zinc, nickel, electrolysis, plating, ISIC 2080.

10. Reviewer's Comments: This case study was originally

compiled by the UNEP IE Working Group on Metal Finishing.

It underwent a UNEP IE funded technical review in 1994 for

quality and completeness. It was edited for the ICPIC

diskette in July 1995.

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