Energy aspects of incorporating heavy metal companion minerals into clay masonry units via combustion-engineering measures in a tunnel kiln

Research project conducted by the research alliance of the Clay Brick and Tile Industry Regd. (FGZ)
Project number
AiF 16329 N
Project funded by
BMWi through the German Federation of Industrial Research Associations "Otto von Guericke" Regd. (AiF)
Implemented by
Brick Research Institute Essen regd. (IZF) Project manager
Dipl.-Ing. Eckhard Rimpel

1 Introduction and objectives

Uniform national provisions concerning the reuse of industrial by-products and recycled materials are currently lacking in Germany. The planned alternative building materials regulation is intended to close that legal loophole, hence creating nationally uniform recycling regulations. At present, the individual German states and licensing authorities go by heterogeneous sets of rules. The first working draft of the alternative building materials regulation contained criteria that were not based on the present standard leaching process (water/solid ratio W/S = 10:1). Instead, a new leaching process based on the column method pursuant to DIN E 19528, with a water/solid ratio of W/S = 2:1, was introduced. Corresponding material (eluate)-value requirement were imposed for recycled building materials.

Clays used for brick manufacturing purposes contain certain amounts of carbonates, sulphates and metals, usually in the form of companion elements in diverse minerals. In that sense, the new alternative building materials regulation, with its envisaged limits on recyclability, will harbour a measure of risk, since the altered methods of analysis will not be buoyed by reference values and robust data.

The purpose of this project was to elaborate firing options and/or raw-material-specific measures with the capacity to limit or even eliminate the mobility of such eluate contents as heavy metals, which must be regarded as critical, and sulphates. This is intended to assure the recyclability of heavy clay materials.

2 Experimental procedure and findings

For specifying the method by which to leach metals from heavy clay material, numerous specimens of diverse provenance were eluted and the respective eluates analysed, with the result that vanadium and chrome, in addition to anions of sulphate, emerged as the leachable substances of relevance. As a result of these pretrials, the main experiments were confined to those three materials. For many metallic elements, sulphates constitute the most important mineral compounds. The metals occur as trace elements in the terrestrial crust and, hence, as minor constituents in the raw materials.

The next series of experiments concentrated on influencing the materials‘ leaching behaviour with non-clay components and reductive additives during preparation of the raw materials. It was, however, observed that all reactions achieved in the course of preparation were subsequently cancelled out in the kiln during oxidation firing. The post-firing use of reductive additives altered the appearance of the finished product and therefore had to be ruled out.

It would be difficult to draw uniform conclusions on the leaching behaviour of heavy clay products, because, for one thing, the various raw materials used for manufacturing different heavy clay products are multicomponent mixtures in which innumerable reactions can occur with mutual interference and effects on the pH of the eluate. pH, however, is a critical parameter for mobility, i.e., for the leaching behaviour of the various metals. Ultimately, however, the numerous experiments yielded plethoric new information on the leaching behaviour of metals from heavy clay raw materials.

The only practicable way to exert a positive influence on leaching behaviour is to reduce the level of oxygen in the kiln atmosphere. It is not necessary to maintain a reducing atmosphere for the entire firing process or to keep the oxygen content low throughout. A controlled firing zone with temperatures above approx. 650° C and oxygen contents below 10 Vol.-% should suffice, as the experiments showed.

This project of the research alliance of the Clay Brick and Tile Industry Regd (IZF) was implemented by the Brick Research Institute Essen Regd (IZF). The collective industrial research project 16329 N of the Research Association of the Brick and Tile Industry Regd (FZG) was funded by Germany’s Federal Ministry of Economics and Technology pursuant to a resolution of the Federal German Parliament (Bundestag) through the German Federation of Industrial Research Associations (AiF) within the scope of the collective industrial research program. The 51-page final report is available from the Research Alliance of the Clay Brick and Tile Industry Regd (FZG) in Berlin.

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