C2764 - Galvanising Services Ltd - Auckland Region

Summary
Galvanising Services has virtually eliminated the liquid effluent discharged to trade waste by implementing a Cleaner Production programme. The mechanical cleaning process has reduced acid consumption by 50%. Annual savings from projects implemented to date are approximately $40,000 with a maximum payback period of 1.5 years.

Business Profile
Galvanising Services galvanises steel products and nail components to provide an anti-corrosive layer. The main processes are metal cleaning and hot dip galvanising. The business employs 20 staff.

Type of Project
Self-regulating flow valves, recycling and filtration of rinse water from cleaning baths, replacement of caustic cleaning and acid pickling by mechanical rumbling, continuous preflux filtration, use of inhibitors, fume suppressants and surface wetting agents, removal and recycling of zinc dross and dust, temperature control and insulation of prefluxing bath, negotiating reduced power rates for bulk power consumption.

Reasons for Project
The company participated in the cleaner production project due to its social responsibility towards the environment and also to comply with trade waste regulations.

The Project

METAL CLEANING PROCESSES

Self regulating flow valves

Details	The objective of rinse water conservation is to use only as much water as is needed. Self regulating flow controls valves have been installed in the rinse tanks water inlet. The valves set the water flow at a constant rate (0.06 L/sec) independent of the back pressure in the mains. The flow control valves are low cost, easy to install and maintain, and reliable. The company saves 2000 m3 of water per year.
Economics	Annual water costs are reduced by $1,700.
Benefits	Reduced water consumption and costs.

Recycling and filtration of rinse water from cleaning baths

Details	Wastewater from rinsing tanks needs to be treated to remove zinc prior to discharge into the sewer. Galvanising Services previously treated the wastewater with caustic soda in a neutralisation tank. As a result the zinc hydroxides precipitated and the clean liquid was discharged as trade waste. As there was little control over the neutralisation reaction, however, suspended salts were discharged with the wastewater. In the new situation, spent diluted acid and rinse water are recycled as shown in the figure below. Spent weak acid is disposed of through a waste contractor. The spent dilute acid is filtered and then reused in the weak acid tank. Fresh acid is added to obtain the correct concentration. Fresh dilute acid is made using concentrated acid and filtered rinse water. Thus spent dilute acid and rinse water are recycled rather than dicharged as trade waste. The suspended iron particles are trapped in the filter. Water consumption has reduced by approximately 3,500 L per month. Quantities of wastewater and sludge discharged and associated cost were reduced. As the zinc levels in trade waste are reduced, the business now complies with trade waste bylaws.
Economics	Reduced water costs of $2,400 per year.
Benefits	Reduced water consumption. Compliance with environmental regulations through reduced zinc levels in wastewater. Reduced wastewater quantities and costs. Reduced quantities of sludge.

Replacement of acid cleaning by mechanical rumbling

Details	In the nail galvanising plant at Galvanising Services, the acid cleaning process has been replaced with abrasive cleaning by mechanical rumbling. Nails and sawdust are mixed in a rumbling barrel. The barrel rotation exposes the surface of the nails to the sawdust which absorbs the drawing soaps and the manufacturing oils. The impact of nail on nail removes any corrosion and the activation of the extractor removes both the contaminated sawdust and any corrosion dust. The process gives reliable surface finish so there is no need for pickling with hydrochloric acid, and the component can be directly processed in the prefluxing tank and the galvanising zinc pot. As a result consumption of 42,000 l/year of hydrochloric acid is eliminated.
Economics	Capital costs for the purchase and installation of the system are $45,000. The operating costs of the mechanical rumbler are nominal. Total estimated savings from elimination of the acid pickle and rinsing process are $30,000 and include: - reduced acid consumption $20,000/year; - reduced spent acid disposal $8,000/year; - reduced treatment $2,000/year. The payback period is 18 months.
Benefits	Reduced costs for acid consumption, disposal and treatment. Reduced water consumption and costs. Reduction of zinc ash and drost generation. Compliance with environmental legislation. Improved health and safety conditions resulting in improved staff morale.

Removal and recycling of zinc dross and dust

Details	Details Zinc dross is a pasty solid comprising of 96% zinc and 4% iron and is produced by dragout form iron-containing pickle salts, the fluxin solution, products that are being galvanised, and iron and steel products reacting with molten zinc. Zinc ash is a mixture of zinc oxide and entrained zinc, which is often in excess of 80% of the total weight. Zinc ash is produced when zinc is in contact with air. Galvanising Services has reduced the formation of zinc dross and zinc ash by 18 tonnes per year by: improving work practices through good housekeeping; filtration of the fluxing solution; replacing acid pickling by mechanical rumbling (see previous case study).
Economics	Savings of reduced zinc dross and ash formation is $45,000 per year.
Benefits	Reduced zinc costs. Reduced zinc dross and zinc ash waste. Reduced emissions. Improved health and safety conditions for staff.

Continuous preflux filtration

Details	Details Iron levels in the zinc ammonium chloride preflux tank must be maintained at minimum levels of 1% as excess iron has a detrimental effect on the amount of waste produced. The majority of the iron is introduced to the preflux through dragout from the acid tanks. Because of the high iron content during pickling, rinsing is necessary before prefluxing. A continuous preflux filtration system has therefore been installed over the preflux to remove iron from the process.
Economics	As the preflux filtration system was already available only installation costs were made. Zinc ammonium chloride savings have not been determined.
Benefits	Reduced zinc ammonium chloride consumption.

METAL FINISHING PROCESSES

Use of inhibitors and fume suppressants

Details	Details Hydrogen chloride (HCl) is used to pickle clean steel (Fe) objects prior to galvanising and to strip old layers of zinc (Zn) from already galvanised objects. When hydrogen chloride is in contact with steel or steel oxides, it reacts as follows: Fe+ 2 HCl FeCl2 + H2(g) FeO + 2 HCl FeCl2 + H2O Fe2O3 + 6 HCl 2 FeCl3 + 3 H2O Fe3O4 + 8 HCl 2 FeCl3 + FeCl2 + 4H2O Similar reactions take place between hydrogen chloride and zinc: Zn + 2 HCl ZnCl2 + H2(g) ZnO + 2 HCl ZnCl2 + H2O These reactions cause three problems for metal cleaning businesses: the reactions with steel and zinc generate iron chloride and zinc chloride which end up in the wastewater stream; hydrogen chloride emissions are produced near the bath surface through contact with the air. The vapours are a health hazard for production staff; during the reactions hydrogen gas (H2) is formed which is trapped in the solution. The gas is potentially explosive. Galvanising Services has added inhibitors, fume suppressants and wetting agents to the pickling baths to solve these problems: inhibitors (steel pickling bath): the first reaction between hydrogen chloride and steel is suppressed by up to 94-98%. This reduces the formation of hydrogen gas and the production of iron chlorides; fume suppressants (both baths): these chemicals form a "foam" layer on the bath surface, preventing the emission of hydrogen chloride vapours; wetting agents (both baths): these chemicals improve the contact between workpieces and chemicals in the bath. As a result the pickling process is improved.
Economics	Economics $600 is spent each year on inhibitors. This is a slight increase in costs but this is outweighed by the benefits.
Benefits	Reduced metal corrosion. Reduced hydrogen chloride emissions. Improved health and safety conditions for staff and compliance with health and safety regulations. Improved wastewater quality and reduced trade waste costs.

For additional information contact:
Mr A. Lonsdale Cooper,
Galvanising Services Ltd,
PO Box 13181,
Onehunga,
Auckland
Ph 09-6366003
Fax 09-6365967

For further information about Cleaner Production in the Metal Industry see:

Opus International Consultants (September 1997): Cleaner Production Guide for the Metal Industry- Minimising waste, reducing costs and caring for the environment.