Annual report 2006-2007 - Taranaki Regional Council

3. Taranaki Galvanizers
3.1 Process description
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The galvanising plant of Taranaki Galvanizers is situated at the corner of State
Highway 3 and Monmouth Road, about 1 km north of Stratford. The plant was
owned and operated by Union Galvanizers 1995 Limited until September 1998, at
which time that company ceased operating. The plant remained idle until May 1999,
when Taranaki Galvanizers (2003) Ltd took it over. During the reported period
ownership changed once again, to Taranaki Galvanizers Limited.
A hot-dip galvanising process is used. Under Union Galvanizers, operations
consisted of stripping/degreasing steel articles in hot sodium hydroxide/water, a
water rinse, a cold hydrochloric acid wash, a water rinse, pre-fluxing with zinc
ammonium chloride, then hot-dipping in molten zinc. There is a separate galvanising
stripping tank containing hydrochloric acid/water for articles that have previously
been galvanised.
The galvanising process employed by Taranaki Galvanizers is essentially the same as
before. Minor changes have been made to improve product quality such as greater
heating of the caustic degrease solution (with gas). Particular care is taken to ensure
that flux solution dries on articles before they are dipped into the zinc kettle. This
minimises fume generation and sputtering. In the 2003-2004 monitoring period the
Company switched from the hosing of articles after caustic and acid treatment, to
using a rinse tank. This resulted in a decrease in the volumes going to the trade
waste tanks.
Both dilute and strong liquid process wastes are produced. These are treated and
disposed of separately. Dilute wastes from rinsing of articles in the galvanising
process, condensation in steam-jacketed vessels, and floor washing are collected in a
series of concrete tanks set within a concrete bunded area. The stormwater collected
inside the bund is also pumped into the tanks. The tanks are used to pH neutralise
and settle the wastewaters. The treated wastewater is then transported by road
tanker from the galvanising plant to Stratford oxidation ponds after testing for zinc
and pH under the supervision of the Stratford District Council.
Site stormwater discharges to an unnamed tributary of the Kahouri Stream.
Stormwater from the roof catchment is stored and utilised as process makeup, and is
not discharged under the stormwater discharge permit unless this tank overflows.
Strong wastes (comprising spent caustic cleaning and acid pickling liquors) are
rarely stored on site. Plastic tanks situated in a bunded area lined with acid/caustic
resistant resin are utilised entirely for emergency storage and have not been required
for this purpose since 1995.
The Kahouri Stream tributary originally began in the area where the galvanising site
is situated and is now culverted from its origin to where it appears on the other side
of SH3. The tributary downstream of the galvanising plant has been monitored
regularly since it was discovered that spent acid containing a high concentration of
zinc was disposed of in a bore on the galvanising plant site in mid-1987. Zinc leached
through the ground and appeared in the drain, which runs under the galvanising