Metal Recovery in Electroplating

Email Ltd - Kitchen and Bathroom Products Division

The Dorf Design division of Email Kitchen and Bathroom Products Pty. Ltd. have installed electrolytic recovery equipment on their drag-out tanks of nickel and gold plating lines, and replaced chrome with hydrogen peroxide in the bright dipping process. The total capital cost of these initiatives was $67,800, with annual savings of between $43,190 - $50,190 per annum. This represents a payback period of between 1.4 to 1.6 years.

Contact

Mr Ron Short

Project Engineer

Email Limited

Kitchen and Bathroom Products Division

PO Box 179

Clayton

VICTORIA 3167

Ph: 61 3 9242 3888

Fax: 61 3 9242 3991

Background

The Dorf Design division of Email Kitchen and Bathroom Products Pty. Ltd. specialises in the production of tapware and bathroom accessories. The majority of tapware is produced on-site from brass and is then given a range of surface treatments to provide the desired finish.

The Process

Dorf Design operates two electroplating lines, namely a metal plating line and an acrylonitrile butadiene styrene (ABS) plastic plating line. In both processes a coating of copper is plated onto the workpiece, followed by a coating of bright nickel. A final surface coating of chrome, gold or black nickel is then applied, depending on the particular product range. Approximately 12 million individual components are plated per annum, with approximately 60 per cent being metal components and the remaining 40 per cent being ABS.

Three separate waste water streams are generated by the electroplating operations, namely acid, alkali and chrome. The chrome stream is treated with sodium metabisulphite to reduce chrome VI to the less toxic chrome III, while the acid and alkali streams are neutralized. The three streams are then combined, final pH adjustment made and metal hydroxide sludge settled out. A number of problems were being experienced with the effluent treatment plant, due to constraints on expansion from a lack of available space, ie:

• a high hydraulic flow rate, causing flooding of the final treatment tank and carryover of metal hydroxides to the sewer;
• a perceived difficulty in meeting new chromium trade waste limits being proposed by the sewerage authority; and
• the mass discharge of nickel was approaching the statutory limit.

The Main Electroplating Line

The largest contributor of metals in the effluent was from overflows on the various rinse stages throughout the plating process. The on-site effluent treatment facility removes a large proportion of these metals as a sludge. The typical composition of waste produced by the plant was:

	Liquid Effluent	Sludge
Volume, per annum	61 ML	36 KL (at 3.5% wt/wt solids)
Comprising:
Copper	5 ppm	6000 ppm
Nickel	2 ppm	2300 ppm
Chrome	2 ppm	3370 ppm
Gold	-	4.1 ppm

Of the total waste water leaving the site, approximately 17.6 ML per annum (pa) is generated from the metal plating line, and 16.8 ML pa from the plastic plating line.

Cleaner Production Initiative

Due to the complex shape of some of the tapware and associated components (eg. shower roses), drag-out from the various plating tanks can be substantial, averaging 3 litres per rack. Drag-out losses were recovered by using static drag-out tanks following the plating tanks. The contents of these drag-out tanks were periodically recovered, either by evaporation (eg gold) or by using the contents to top up plating solutions (eg nickel). A major drawback with this system was that as the metal concentration in the drag-out tank increased over time, there was a parallel increase in the metal concentration in the overflow from rinse tanks. This was due to the drag-out of a more concentrated solution from the drag-out tank to the rinse tank. An intensive in-plant monitoring program identified that this drag-out was contributing to elevated metal levels in the final effluent.

In response to this, Dorf decided to install electrolytic recovery equipment to continuously recover metals from the gold and nickel drag-out baths. The nickel recovery unit maintains the nickel concentration in the bath at around 0.7 g/L. This compares to concentrations as high as 70 g/L in the former static drag-out bath. The result is a significant reduction in the amount of nickel discharged to the effluent via the final rinse stage. The system has achieved a similar reduction in gold discharged to the effluent.

The recovered nickel is reused in the plating process, however currently the supply of recovered nickel exceeds the in-plant demand and the excess is being stored pending sale to a third party.

The recovered gold is approximately 80 per cent pure, with the remainder being impurities such as cobalt, copper and nickel. The recovered gold is sent off site for purification and sale.

The Nickel Recovery Unit

The units chosen by Dorf Design for the nickel and gold drag out tanks were the BEWT Chemelec Cell recovery units, which utilise a fluidised bed of ceramic beads to enhance the metal recovery. The metal is deposited at the cathode plates and is easily removed as required.

Another cleaner production initiative has focused on chrome usage. Chrome was used in three areas: brass plating, ABS plating and chromate bright dipping of brass products. The chrome bright dipping process has now been replaced with a hydrogen peroxide-based process. This has resulted in a reduction in the quantity of chrome being discharged to the sewer and a 50 per cent reduction in sludge generation from the effluent treatment process. The consumption of sodium metabisulphite for chrome treatment has also been reduced.

Advantages of the Process

The advantages that have flowed from these cleaner production initiatives have included economic gains from gold recovery, and increased ability to meet nickel and chrome discharge limits.

Cleaner Production Incentive

Dorf Design implemented these cleaner production initiatives in response to the cost of gold losses and increasing concerns over the ability to meet the discharge limits allowed by the sewerage authority.

The total effluent volume being discharged by Dorf was approaching the volume limit specified in the trade waste agreement (TWA). The concentration of chrome was ranging between 8-10 ppm, with the TWA limit of 10 ppm, and the mass discharge limit for nickel was close to being exceeded.

ECONOMIC BENEFITS
Costs
Gold recovery unit	$24,500
Nickel recovery unit	$43,300
Chrome substitution with hydrogen peroxide	No cost
Total	$67,800
Annual Savings
Gold recovery	$15,000 - $22,500
Reduced sludge generation	$14,500
Reduced sodium metabisulphite usage for chrome treatment	$9,660
Nickel recovery	$3,530
Total Annual Savings	$43,190 - $50,190
Payback Period	1.4-1.6 years

Barriers

Dorf Design has experienced very few problems in implementing these initiatives, other than difficulties in obtaining accurate mass balances across the various electroplating processes. This has complicated the process of identifying the major sources of metals being discharged to sewer and subsequent targeting of these sources for cleaner production initiatives.

Case study prepared: May 1997 by ACCP

The Cleaner Production Case Studies Directory is part of EnviroNET Australia.

For more information contact:

Environment Australia
Environment Protection Group
PO Box E305
KINGSTON ACT 2604
AUSTRALIA
Email: cproduction@ea.gov.au
URL: http://www.environment.gov.au/net/environet.html