LTU - EX - - 10/168 - - SE - Sök - Luleå tekniska universitet

Abstract
The largest residue in tonnes from the steel production at Ovako in Hofors is the slag from the
electric arc furnace. Until recently the residues were deposited in landfills, but the permit for
the old landfill ran out and instead of building a new one Ovako Hofors has adopted a zero
waste vision. The purpose of the zero vision is to find areas of use for all residues. A possible
use for the electric arc furnace slag is as aggregates in asphalt because the slag has excellent
properties in this field.
The steel production is scrap based, and some of the chromium content in the scrap is
transferred to the slag. It is important to minimize leaching of chromium to be able to use slag
as construction material in an environmentally friendly way. Ovako Hofors is actively
working on decreasing the leaching of chromium. So far progress has been made by
introducing dolomite in the slag forming agents.
Phases capable of stabilizing chromium are spinels and solid solution with MgO. Chromium
in other phases is capable of being leached. If the ratio between CaO/MgO is high enough a
primary solid solution of CaO is formed instead of the wanted primary solid solution of MgO.
With an increased content of FeO it takes a higher CaO/MgO ratio to form the primary solid
solution with CaO according to thermodynamic calculations.
The aim of this thesis is to investigate if it is possible to prevent leaching of chromium by
stabilizing the solid solution of MgO. An increased concentration of iron oxide stabilizes the
solid solutions of MgO according to research done at Luleå University of Technology. The
aim is to achieve a ratio between FeO/MgO in the solid solution of at least 50/50.
Thermodynamic calculations show that it is primarily FeO that helps the stabilization of the
solid solution of MgO.
The rate of solidification is important because FeO needs time to be integrated into the solid
solution of MgO before the melt solidifies. When the slag is solidified FeO cannot continue to
be incorporated in the solid solution at the same rate. According to thermodynamic
calculations a higher grade of FeO increases the rate of which FeO can be integrated into the
solid solution of MgO. If this is true the rate of solidification becomes less important with
increasing FeO content.
Three different methods were tested to increase the iron oxide content of the slag. In the first
method a new effective oxygen burner was installed and ran during all experiments. The other
methods used were addition of iron oxide material and additional oxygen. Samples of slag
were taken from both the slag pot and from the floor. The slag from the floor solidifies
quicker than the slag from the pot. For reference measurements old slags were used.
To evaluate the results XRD, SEM, leaching and chemical analysis were done. The oxygen
burner increased the iron oxide content while the other two methods did not show any further
influence on the iron oxide content. In most of the samples the leaching of chromium were
under the detection limit of 0,04 mg/kg. Only a few of the samples have leaching of
chromium which indicates that higher iron oxide content does affect the leaching of
chromium in a wanted way.
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