PROJECT DESCRIPTION
BACKGROUND
In 1997, 5.7 million tonnes of stainless steel was produced in Europe. The main producers are located in Italy, Spain, Finland and Germany. During the manufacturing of stainless steel, solid residues of filter dust and slag are generated. The filter dust amounts to 1 – 3 % of the input (120,000 tonnes/yr); it is treated in plasma furnaces in the presence of coke. The main products are recycled metals and a material which is used as rock fill. Stainless steel production generates two types of slags: EAF (Electric Arc Furnace) slags and AOD (Argon Oxygen Decarburization) slags. These slags contain a high amount of chromium as well as some nickel and manganese. The slag amounts to 15 - 20 % of the input (app. 800,000 – 1,100,000 tonnes/yr). There are few practical applications for the slag due to a risk of heavy metal leaching and water pollution. The transmission of hexavalent chromium into the environment poses a risk of cancer in human beings and animals and adversely alters the genetic diversity of species. Moreover, the AOD slag is normally disintegrated into a powder due to the volume expansion that occurs during cooling. This not only limits its application as a construction material, but it also creates an unpleasant working environment and represents a loss of valuable raw materials. Thermochemical treatment in an arc furnace would enable a separation of the different fractions.
OBJECTIVES
The goal of this LIFE project, entitled RECARC, was to recover completely the heavy metal fraction of the residue (slag) resulting from the production of stainless steel and to convert the mineral fraction to a high-value building material. This was to be achieved by using a thermochemical treatment in an arc furnace. The objectives of the project were: 1.) Demonstration on a technical scale of thermochemical treatment of residues (slags) from steel and stainless steel production. 2.) Production of a pure metallic fraction for recycling. 3.) Production of a slag with low concentrations of heavy metals, so that the mineral fraction can be used as a building material.
RESULTS
The RECARC project successfully demonstrated a complete recycling of AOD slag generated from stainless steel production using a thermochemical treatment process. The highly innovative project resulted in the separation of two fractions: a chromium-rich metal fraction and a mineral fraction. In total, 5 tonnes of AOD slags were used for tests during the project. They were treated thermochemically in the beneficiary’s 300 kW alternating current arc furnace under “reducing conditions”. The high temperature process facilitated the separation between the molten slag and the liquid metal fraction, which sinks down due to its higher density. For the separation of both fractions the mineral phase is decanted, while the metal phase remains in the furnace. To optimise the reduction of the chromium compounds, the beneficiary investigated different melting conditions with (1) long arc, (2) short arc and (3) resistance melting, in each case with and without reducing agent. The project results showed that the reduction efficiency is highest when melting in resistance. Here more than 90% of the bonded chromium will reduce to the elemental form with additional reducing agents like petroleum coke (carbon) or aluminium, resulting in Cr2O3-amounts in the mineral fraction of <2000 ppm. In contrast, the tests showed that no significant reduction takes place in arc melting modes. The quality of the slag was then assessed with the help of an X-ray-fluorescent analysis instrument. To ensure that the analysis instrument was working correctly, the beneficiary produced chrome-standards with a defined chrome content as reference samples. In addition, the samples (800 samples were prepared, analysed and evaluated during the project) were also sent to the project partner for further analyses. Both resulting fractions can be used for further applications: - The mineral fraction can be used as a raw material in road or railway construction or for the cement industry in place of naturally-occurring materials that are usually required to be extracted from large, open-cast mines. This requires the addition of adequate additives such as quartz sand or alumina (to enable the slag to solidify into a glasslike material) and a “granulation” (to enable quick cooling by swirling the slag with air, water or both) to the slag. Thereby the mineral fraction can be improved to become a substance which is similar to typical cement components. - The chromium-rich metal fraction, with a very high value of about 50% chromium content, can be used for the production of steel that contains chromium as an alloying constituent. Using the thermochemical treatment process, approximately 40 kg of chromium can be produced from 1 tonne of slag. By separating the chromium-rich metal fraction and the mineral fraction from AOD slag, the project allows the complete recycling of AOD slag. This could benefit the environment enormously, since (a) natural resources of chromium can be saved and (b) the current practice of partially landfilling the slag can be reduced. The RECARC technology can be applied on an industrial scale and high environmental and economic benefits are expected: The complete recycling of AOD slag by way of thermochemical treatment can (a) reduce the hazardous potential of the highly contaminated steel, (b) save chromium as a natural resource and (c) reduce the high costs for the disposal of the slags. This project has been selected as one of the 21 "Best" LIFE Environment projects in 2007-2008
