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.
DOCNO: DOCUMENT NOT AVAILABLE *****10-07