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3.14 Water Recycling In the Galvanic Metals Industry Technical

Description

Plating requires a large amount of water, much of which can be provided through the reuse of wastewater. With use of new techniques during the plating stage, combined with recycling of process waters, it has been possible to reduce water consumption by up to 90% of the original volume of water used. The general method comprises various possible technical solutions which can be applied within the galvanic metals industry to specific processes. The more efficient use of water by recycling is important not only because of the reduction in the volume of wastewater discharged, but also because of the potential for industrial effluents to lower the treatment efficiency of municipal wastewater treatment processes. For example, one of Hungary's largest metal plating factories is located in Miskolc in the northern part of the country. This plant has sophisticated wastewater treatment facilities which provide the level of treatment necessary for discharge into a natural river. Notwithstanding, one part of this system comprises a recycling system which is able to recycle washing water. The recycled water is treated in an ion-exchange system at a rate of 8 to 12 m³/hour. The system has acid and base collecting reservoirs as well as neutralization basins, and the polluted wastewater from each technological process is segregated depending on its metal content. NaOH, HCl, NaSO3 and NaOCl are used to neutralize these wastes before filtration and pH adjustment prior to reuse. The water circuit of the galvanic plant is completely closed.

This same plant also uses a batch-type, multiple counter-current rinse system. In the first tank, the water is static and does not flow out. This tank is used for the first, "rough" cleansing of the "work pieces" or items being manufactured. During this rinsing phase, the metal content in the water increases by up to 10% to 20% over that of the working solution. Once this concentration threshold is reached, the water is either recycled back into the processing solution used during the plating process (to supplement losses due to evaporation and spillage when the plated metals are removed from the tank). In the counter-current tanks, the work pieces are submerged in the water and moved against the water flow. In this way, the work pieces come into contact first with the most polluted water and, finally, is removed from the cleanest water. This is achieved, in practice, through the use of a cascade system. The basin is separated into compartments, with the cleaner water placed at the highest levels so it flows down into the progressively more polluted compartments. The disadvantage of this type of technology, however, is that the concentrations of the dissolved substances can become very high, which can make the treatment of wastewater difficult. If the main dissolved substances are sulphate- and phosphate-ions, use of CaCO3 for neutralization instead of NaOH can reduce the dissolved solids content; in other cases, the best solution is the use of raw water with a low salt concentration.

Boiler water condensate is also recycled after it is mixed with desalinated water. It is reused in the gas boilers as supply water. Because the condensate must contain minimal hardness and few suspended solids, the recycled water is routed through an anthracite filter at 900oC and softened with ion-exchange resin. The resin itself may be regenerated with a 100 g/l concentration NaCl.

Extent of Use

These technologies are used in recently modernized plating and metallurgical factories. Examples of closed water cycles in the plating industry may be found in Hungary, Poland, and Ukraine.

Operation and Maintenance

Once installed, the recycling system can be controlled automatically or manually. However, a specially designed installation is required and technically trained staff are needed to operate the system.

Level of Involvement

This technology is implemented at the company level. Financial assistance from industry or governmental organizations is a important factor in offsetting the relatively high capital costs of installing recycling technologies.

Costs

The investment and maintenance costs are difficult to identify because the original plant has been rebuilt in several phases over many years. Different treatment technologies are used for various plating processes according to the customers' orders.

Effectiveness of the Technology

This is an highly effective group of technologies. It is possible to save up to 90% of the original volume of water used by plating industries.

Suitability

These technologies are best suited for use in the plating and metallurgical industries.

Advantages

The primary advantages of these technologies are the overall water savings, enhanced environmental compliance, and reduced usage of chemical agents.

Disadvantages

The disadvantage of these types of water saving technologies is the high concentration of dissolved substances in the wastewater, which makes wastewater treatment difficult. However, these difficulties can be minimized by other process modifications.

Cultural Acceptability

This technology is culturally acceptable.

Further Development of the Technology

Future developments should provide for less expensive means of materials processing.

Information Sources

Petèr Kovac and Dr Kornèlia H. Kocsis, Felsö - Tisza - Vidèki Környezetvèdelmi Felügyelösèg, 4400 Nyiregyhàza, Szèchenyi u.19, Hungary, Tel. (36-42) 310 155, fax: (36-42) 310 713.

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