Waste Avoidance & Recycling

Preventing waste by using a new pretreatment method in galvanizing plants for the galvanization of parts

A separate acid management instituted in the new plant results in pure iron or zinc containing spent pickle liquors, which can be further utilized as raw materials. A dynamic cleaning process was used to lower the hydrochloric acid concentration during the degreasing and in the pickle bath. This resulted in low entrainment rates in subsequent baths as well as lower chloric acid emissions (HCI). The waste gas from the pickling and galvanizing baths are collected and purified. The powdery emissions within the purified waste gas reach only 1/25 of the permissible upper limit of 10 mg/m³; the values are even lower than the detection limit of 0,0002 mg/m³ for cadmium (detail information in German).

 

Summary

A new hot-dip galvanizing plant was conceived in such a way that no mixed pickles, which must be disposed of as special refuse, result when derusting the workpieces but only pure iron and/or zinc containing spent pickle liquors, which can be used as raw materials in the chemical industry. To this end the pickling bath for the zinc-afflicted workpieces (false-coated parts, zinc-afflicted racks) had to be separated spatially and organizationally from the production line degreasing, pickling, fluxing, galvanizing. By the separation of the pickling processes it is also possible to adjust the respective optimal pickling conditions.

The exhaust gases of the pickling and galvanization baths are collected and cleaned. The washed components HCl and NH3 are admitted again to the flux bath as ammonium chloride (NH4Cl). The powdery emissions in the cleaned exhaust gas only reach 1/25 of the permissible upper limit of 10 mg/cbm, for cadmium the values are even lower than the detection limit of 0,0002 mg/m³.  The total investments for the project amounted to approximately DM 5 millions (€ 2,6 million), the promotion means of the federation amounted to approximately DM 1 million (500.000 €).

Innovative pretreatment techniques in a hot-dip galvanizing plant

Environmental measures also brought economic advantages 

In hot-dip galvanizing plants metal parts are coated with a thin zinc surface by immersion into a liquid zinc bath. In preparation of the actual galvanization procedure the parts are submitted to a pretreatment (cleaning), which usually consists of the work procedures "defatting" and "pickling".

An effective defatting is necessary to obtain a clean material surface and to prevent that the following treatment baths are contaminated with oils and fats.  A transport of oils and fats into the galvanising boiler would lead to increased TOC contents (oil, fat, dioxins) of the filter dust, which would make the utilization of the filter dust more difficult. 

Pickling is an acid treatment by which a pure, metallic surface necessary for the following coating process is produced. In hot-dip galvanizing plants:

  • raw workpieces (material: iron),
  • false-coated parts (material: zinc and iron),
  • racks, at which the workpieces are hung up and transported (material: zinc and iron)
  • are pickled. 

At the conventional hot-dip galvanizing plants all pickling processes take place in a pickling plant, which has the consequence that the pickling acids are loaded with zinc and iron. These so-called mixing acids accumulate in Germany in an order of magnitude of 30,000 tons per year and are largely used in chemical-physical treatment plants for neutralization precipitation or emulsion splitting. The secondary resources contained in them (above all zinc) are lost to the economic cycle.

As a result of the relocation of its metal production it became necessary for the firm Otto KG in Kreuztal to establish a new galvanising workshop. The firm used this opportunity to realize an air, water and waste optimized enterprise under consideration of the economic feasibility by production-integrated environmental technology. The project (total investment volume approximately DM 5 millions (€ 2,6 million) was accomplished as pilot project in co-operation with the Federal Environmental Agency, Berlin, and promoted by the Federal Ministry of Environment with an interest subsidy (approximately DM 1 million (500.000 €)) to a loan from the environmental program of the German Balance Bank (DtA).

The new galvanising workshop was to minimize the waste accumulation, produce usable wastes instead of wastes which have to be disposed of by conversions in the production if possible and offer improved conditions of work concerning room air and dust composition.

The new plant conception of the firm Otto KG plans operational masts at all process baths of the pretreatment line. By intermittent stroke movements of the workpieces this dynamic cleaning method allows a substantial decrease of the hydrochloric acid concentrations in the acid defatting and pickle in relation to the usual values.

Apart from a smaller transport rate in subsequent baths this also leads to smaller hydrogen chloride (HCl) emissions at the workplace.

Compared to conventional plants the rinsing technology was likewise improved. The wastewater-free operation usual for hot-dip galvanizing plants can also be ensured here. The pretreatment optimized in such a way has led to the fact that the dust resulting during the waste gas cleaning of the galvanization bath with the help of filtering separators can be regenerated without problems for the recuperation of fluxing agent salts.

The mixed acid problem described above could be avoided after now one year's operation by a consistently accomplished separate acid economy. The zinc-containing etches corresponds to the acceptance criteria of the recycler.  In order to ensure this also for iron-containing pickles (zinc content below three grammes per litre), following measures must be taken: 

  •   Use of binding wire or very well strip-paintable chains,
  •   cleaning of the cross beams of zinc splashes,
  •   motivation and control of the personnel.

The environmental measures also brought economic advantages for the enterprise. 

The savings per one ton of old acid to be disposed by avoidance of mixed acid accumulation amount to DM 300,-- (153 €). This leads to a calculated amortization period of 3.6 years for the expenditures for environmental protection measures that exceed those for a conventional plant. Thus the project could show how environmental impacts can be avoided by production-integrated environmental protection at justifiable financial expenditure.

Project Participants

Implementing Institution

Otto Kunststoffrecycling GmbH & Co. KG

Promotion

Promoting institutions

Federal Environment Agency (UBA)

More Project Informations

Project title:  Preventing waste by using a new pretreatment method in galvanizing plants for the galvanization of parts

Project number:  9036

Project period:  1992 - 1994

Project region:  Germany (North Rhine-Westphalia)

Project contact:

Frau Fischer

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+49 (340) 2103 3067

+49 (340) 2104 3067

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Source: Environmental library of the Federal Environment Agency (UBA)