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Introduction

Road construction has been so far the classical application route of metallurgical slags. For a production of slag aggregates, liquid slag is tipped into slag beds and after a slow cooling process under ambient air the solid material is crushed and sieved. Slag aggregates show positive application-specific properties (e.g. abrasion resistance) and additionally natural primary resources can be saved in case of a slag utilization in the road construction. However, an increasing critical consideration of steelmaking slag utilization for road construction as well as due to the legal framework utilization routes are limited at the moment and the reachable financial benefit is low.

In the course of the currently used slag treatment process a part of the metallic iron is separated by means of coarse grained low intensity magnetic separation whereas the separated iron (Fe) fraction is reutilized in the steel plant (e.g. sintering process). However, the main part of iron and other valuable metals remain as oxides in form of finest crystal aggregates inside the slag matrix. Consequently, it is not possible to separate them from the silicate slag phase.

Based on the reachable saving potential concerning a replacement of primary resources, especially Fe-, chrome- and manganese-carriers, efforts were done in recent years to develop processes for a valuable metal recycling.

Objectives and Motivation

  • An endotherm reducing slag treatment route for a more or less complete recovery of valuable metals
  • A further development of the oxidizing slag treatment route - Determination of the correlations between chemical slag composition and the oxidation degree
  • The development of suitable mineral processing steps for valuable material separation
  • A recycling of the metal fractions into the steel plant
  • The utilization of the low metal fraction as input material e.g. in the construction industry (product development)

Lab-scale experiments represent the main part of the planned methodology in the current project 1.3. Concerning the oxidizing steelmaking slag treatment melting experiments with modified steelmaking slags will be performed in a first step whereas the slag probes are enriched e.g. with magnesium oxide (MgO). Possible aggregates are a Tammann-Furnace (equipped with an electrical resistance heating), a Top Blown Rotary Converter (TBRC, rotating vessel equipped with a lance injection, which is placed at the converter cap) and, if necessary an induction furnace respectively. A scientific project partner, the Montanuniversit√§t Leoben, Chair of Non-Ferrous Metallurgy (MUL-CNM), provides the above mentioned technicum aggregates. 

Beside this a possible reducing steelmaking slag treatment should also be investigated. The reduction behaviour of phosphor (P) and other valuable metals e.g. Mn in dependence on the Fe-reduction degree will be one main focus of the experiments. Furthermore fundamental knowledge should be generated, if P and the valuable metals remain in the metal or slag phase. The InduCarb reactor in the technicum of the Chair of Thermal Processing (MUL-TPT) represents one suitable aggregate for the planned reducing slag treatment campaigns.

The generated product fractions are treated by means of mineral processing technologies. By a determination of the susceptibility spectra a possible magnetic Fe-separation should be evaluated. Furthermore a flotation treatment of the slag products should be investigated for an enrichment of certain material fractions. A further scientific project partner, the Chair of Mineral Processing (MUL-CMP), will execute the magnetic and flotation experiments. 

A company partner in project 1.3 plans to perform dry granulation tests with steelmaking slags after a reduction of the valuable metals. This will be done by means of a lab scale plant, which is operated with the working principle of a rotating cup. In the course of these research activities the formation of rapid hardening cement phases will be investigated in order to utilize the products in the cement industry. Based on the experiments the product properties with regard to the granule shape and size distribution as well as the amount and oxidation degree of metals and the phases generated will be characterized. In addition to this the possibility of an energy recovery from the hot liquid slag during the granulation process will be analysed and evaluated. 

Scanning electron microscopic analysis is planned beside technicum scale  campaigns to investigate and quantify the slag product phases generated in the course of the oxidizing and reducing treatment. With the help of this the intergrowth structures of the crystals can be observed serving as basis for the discussion, if e.g. Fe can be separated by means of magnetic treatment.

Concerning the oxidizing slag treatment route it is expected that, in case of a sufficient oxidation of the bivalent Fe2+ to the trivalent Fe3+ during the slag oxidation, the formation of strong magnetic inverse spinel's, especially magnesioferrite, takes places. These components can be separated efficiently by means of a downstream magnetic treatment reaching a higher Fe-output. In context with the reducing slag treatment limits should be identified concerning a recovery of Mn with a simultaneous P-separation based on the generated knowledge of the reduction behaviour of the mentioned valuable metals.

Finally, after conclusion of the thermo-chemical and mineral processing treatment experiments it is expected to generate basic concept parameters for a steelmaking slag treatment plant. Suitable types of grinding, magnetic separation and flotation aggregates should be identified as well as possible utilization fields of the generated products should be defined.

Based on the results of the dry steelmaking slag granulation specific fundamental knowledge should be generated concerning a possible utilization of the products in the construction industry. Furthermore the energy recovery potential of the dry granulation process should be estimated and eventually further pursued.