Hazardous Waste Treatment

Investment program for treatment of pickling acid

In hot-dip galvanisation processes the metal construction parts are coated with negatively electric metals, chiefly zinc. The surface of the metal being subjected to this process must be bright. To achieve this the parts are first pickled in a hydrochloric acid bath. In the course of time the hydrochloric acid becomes spent and has to be disposed of. The HYC-process recovers from the waste mixture valuable raw materials (iron chloride and zinc chloride) which can then be reintegrated in the processing cycle. A multiphase process removes the secondary constituents fouling the product or interfering with the separation process. The liquid is passed through an activated charcoal adsorption unit to remove the grease, inhibitors etc. from it. In a two-phase carburizing process heavy metals introduced through the steel being galvanised and the zinc used for this purpose are precipitated. The core of the unit consists of a zinc specific liquid-liquid extraction in which the iron chloride and zinc chloride are separated from one another. After passing through several retreatment phases in an evaporation unit both the FeCl2 and the ZnCl2 solutions have reached the concentration desired. 

 

Summary

A very severe drawback of metal construction parts made of iron and steel is their susceptibility to corrosion. There are a number of processes for attaining long-term material protection. In the case of hot-dip galvanisation the parts are coated with negatively electric metals, chiefly zinc. The surface of the metal being subjected to this process must be bright. To achieve this the parts are first pickled in a hydrochloric acid bath. In the course of time the hydrochloric acid used for the pickling becomes spent as rust, hammer scales, iron and so on are deposited in the solution. This waste mixture has to be disposed of. In the techniques most commonly used up to now (oil-emulsion separation, CP units) the waste mixture is neutralized/precipitated and subsequently deposited in a special dump. The HYC-process recovers from the waste mixture valuable raw materials (iron chloride and zinc chloride) which can then be reintegrated in the processing cycle. A multiphase process removes the secondary constituents fouling the product or interfering with the separation process. The liquid is passed through an activated charcoal adsorption unit to remove the grease, inhibitors etc. from it. In a two-phase carburizing process heavy metals introduced through the steel being galvanised and the zinc used for this purpose are precipitated. The core of the unit consists of a zinc specific liquid-liquid extraction in which the iron chloride and zinc chloride are separated from one another. After passing through several retreatment phases in an evaporation unit both the FeCl2 and the ZnCl2 solutions have reached the concentration desired.

Utilization of waste pickles from hot-dip galvanisation

Project of the program "Investments to Reduce Environmental Burdens" terminated

The firm Hydro Chemicals Germany has established a plant for the utilization of waste pickles from hot-dip galvanisation at its location in Oberhausen.

Starting situation

Waste pickles arise in the hot-dip galvanising workshop when pickling steel parts and zinc-afflicted racks in hydrochloric acid. If these pickling procedures were accomplished in the same pickling bath, so-called mixed pickles develop, which apart from free hydrochloric acid primarily contain zinc and iron in higher concentrations. A materials recycling of this mixture is not possible without separation of zinc and iron. The disposal of these mixed pickles traditionally takes place by treatment in chemical/physical treatment plants and sedimentation of the developed neutralization sludges. This procedure is to be classified as waste disposal.

New procedure developed 

Long before the commencement of the recycling economy and waste act (KrW/AbfG) the firm Hydro Chemicals Germany had developed a procedure, with which secondary raw materials can be manufactured from unexploitable wastes. From the zinc and iron containing mixed pickle usually contaminated with toxic heavy metals such as cadmium, lead, and chrome ferric chloride and zinc chloride are won as solutions of high purity in a multi-level procedure. These solutions can be used in various processes of the chemical industry, where they replace primary raw materials.

The decision for the building of the plant held a high business risk both in technical and financial regard. In the run-up a coordination with the hot-dip galvanising workshops had to be achieved first, by which it was guaranteed that no materials disturbing the preparation process were brought into the waste pickles on the part of the waste producers.

After the process-related development had been terminated, a further investment obstacle arose: The goal to develop an equipment technology with which the utilization could be offered for the same price as the removal seemed attainable. It could not to be guaranteed, however, that the waste acids suitable for utilization actually arrived at the utilization plant. It was rather feared that they would be transported to cheaper removal plants.

Only after the adoption of a sample administrative regulation for the avoidance and utilization of wastes from hot-dip galvanisation, in which the materials recycling of mixed pickles is explicitly required, the Hydro Chemicals Germany could give the green light for the building of the processing plant. 

The building of the plant was promoted in the context of the Investment Program to Reduce Environmental Burdens with means of the Federal Ministry for Environment. (The technical support was carried out by the Federal Environmental Agency). 

Project Participants

Implementing Institution

Hydro Chemicals Deutschland GmbH

More Project Informations

Project title:  Investitionsvorhaben Beizsäureaufbereitung

Project number:  6024

Project period:  1993 - 1996

Project contact:

Frau Fischer

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+49 (340) 2103 3067

+49 (340) 2104 3067

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Source: Federal Environment Agency (UBA)