A R C H I V E S O F M E T A L L U R G Y A N D M A T E R I A L S

Volume 55 2010 Issue 4

DOI: 10.2478/v10172-010-0020-6

Y. PONTIKES∗ , P.T. JONES∗ , D. GEYSEN∗ , B. BLANPAIN∗

OPTIONS TO PREVENT DICALCIUM SILICATE-DRIVEN DISINTEGRATION OF STAINLESS STEEL SLAGS

MOŻLIWOŚCI ZAPOBIEGANIA ROZPADOWI SIARCZANU DWUWAPNIOWEGO Z ŻUŻLI STALI NIERDZEWNYCH

In this paper, a short overview is given on the strategy for high dicalcium silicate stainless steel slags stabilisation along
with some results from work done and currently being performed at the K.U.Leuven.
Keywords: slags, stainless steel, production, solidification, microstructure, utilization

W niniejszej pracy, znajduje się krótki opis strategii stabilizacji żużli stali nierdzewnych zawierających krzemian dwu-
wapniowy, także niektóre wyniki dokonanych i obecnie przeprowadzanych prac w K.U. Leuven.

1. Introduction internal stresses finally cause the disintegration of the

slag.
The potential routes to avoid the formation of
Slags are essential in high temperature metallurgical γ-dicalcium silicate explored in this work are: a) chemi-
processing to purify molten metal at competitive prices. cal stabilization by additions, b) change in slag chemistry
Large volumes are produced annually, leading to impor- and c) fast cooling.
tant economical and ecological issues regarding their af-
terlife. To maximise the recycling potential, slag process-
ing has become an integral part of the valorisation chain 2. Chemical additions to stabilise the high
[1]. temperature phases
Production of steel slags in Europe was 15 Mt in
2004 [2], of which, stainless steel slags constitute a more 2.1. Boron additions
than proportional fraction due to the higher slag to met-
al ratio in stainless steel production. Stainless steel slags In metallurgy, the option of inhibiting the β to γ
are currently used for a number of applications and some transformation of C2 S was first elaborated in 1986 by
alternatives have been suggested in literature [3]: aggre- Seki and co-workers [6], who developed a borate based
gates, additives or raw materials for cement making, car- stabiliser for stainless steel decarburisation slag. At the
bon sink, as well as fertilisers and additives for soil treat- time, it was already known that borates stabilise the
ment are only a few of the potential routes. Nonetheless, higher temperature polymorphs of pure C2 S to ambient
disintegration of stainless steel slags hinders the valori- temperatures by forming a solid solution. Seki proved
sation and increases considerably the landfilling cost. that by adding borates to the high temperature slag, C2 S
It is well established that this kind of disintegra- grains in the cooled slag can also be stabilised. The
tion is driven by the presence of dicalcium silicate crystallographic mechanism is believed to be the partial
(2CaO·SiO2 or C2 S) in the slag [4]. This mineral un- replacement of SiO4− 3−
4 units by BO3 units [7]. Because
dergoes several phase transformations from one poly- of the large difference in ionic radius between Si4+ and
morph to another when the slag is cooled. As the ather- B3+ , this replacement suppresses the Ca2+ migrations
mal, martensitic-like transformation of the monoclinic and SiO4−4 rotations required for the β to γ transforma-
β-polymorph to the orthorhombic γ-polymorph [5] is ac- tion, even with only 0.13 wt% of B2 O3 [8] (although this
companied by a volume expansion of about 12%, high theory should be juxtaposed with the other suggested
∗
DEPARTMENT OF METALLURGY AND MATERIALS ENGINEERING, KATHOLIEKE UNIVERSITY OF LEUVEN, KASTEELPARK ARENBERG 44, B-3001, HEVERLEE (LEUVEN), BELGIUM
1168

theory of martensitic-like β to γ transformation). As bo- The addition of as little as 0.10 wt% of stabilizer
rates increase steel hardness and may lead to hot tearing or 0.04 wt% B2 O3 proved to be sufficient to avoid dis-
during rolling or forging, they need to be added to the integration. Although the minimal required level could
slag after slag/metal separation to avoid boron pick-up not be determined exactly, it is <0.04 wt% for the ex-
by the steel. Fortunately, the required level is so low perimental conditions used. Phase analysis proved to be
that the heat content of the slag is sufficient to melt very similar for the synthetic and industrial samples. The
and dissolve the stabiliser. Because of its effectiveness synthetic samples contain C2 S, C7 MS4 , C3 MS2 , C2 MS2 ,
and simplicity, borate stabilisation of air-cooled slags is and CS. As fluorine was added to the industrial slag to
widely implemented in industrial practice. increase its fluidity, cuspidine formed in the later stages
Results regarding the distribution of boron in the of solidification instead of C2 MS2 and CS. Furthermore,
slag matrix have been reported in a previous work [9]. the inevitable presence of CrOx in the industrial slag
Both industrial and synthetic slags were investigated. resulted in spinel formation. A Cx Sy Bz phase was found
For the synthetic slags a 200 g powder mixture with a in both types of samples. No sodium was detected by
composition of 52-wt% CaO, 39-wt% SiO2 , and 9 wt% EDS. It probably evaporated as Na2 O during equilibra-
MgO was made from dried CaO, MgO, and SiO2 pow- tion at high temperature. The borate distribution was
ders. This mixture was milled for 24 h in ethanol using determined using EPMA-WDS. The overall results are
zirconia balls for homogenisation. After drying, multi- very similar for both types of samples. Figure 1 shows
ple samples of 20 g were weighed. Sodium tetraborate the result for the industrial sample. C2 S and C7 MS4 show
decahydrate (Na2 B4 O7 .10H2 O), containing 37 wt% of small but clear boron peaks. C3 MS2 contains a borate
B2 O3 , was added as a stabilizer. The borate levels varied level close to the detection limit. Cuspidine holds some
from 0- to 1.83 wt% B2 O3 . The samples were put into a borates but the major fraction is found in the Cx Sy Bz
Pt–Rh crucible and loaded in a bottom-loading furnace. phase. The latter phase has not been fully characterised,
The samples were heated in air to 1640◦ C at 5◦ C/min for but there is indication that it is vitreous in nature. The
8 h and were subsequently cooled at 1◦ C/min. The indus- microstructure is depicted in Fig. 2.
trial sample was stabilized with 2 wt% of Na2 B4 O7 or From quenching experiments, EPMA measurements
1.4 wt% B2 O3 . The mineralogy of the slag samples was and FactSage calculations it could be determined that the
determined using quantitative X-ray diffraction (QXRD). distribution ratio of borate between the C2 S phase and
Spectra of samples mixed with 10 wt% ZnO as an in- the liquid corresponds to approximately 0.1. Based on
ternal standard were collected under ambient conditions this observation and a mass balance calculation and ad-
with CuKα radiation using a laboratory Philips PW1830 ditionally assuming a minimum level of borate necessary
Bragg-Brentano diffractometer. Quantitative results were to stabilize the C2 S phase of 0.1 wt% an estimate can
obtained by performing a Rietveld refinement using the be made of the level of borate necessary to stabilize the
“Topas Academic” software. For microstructural char- C2 S phase as a function of C2 S level. As the distribution
acterization samples were embedded, grinded, polished, ratio is smaller than one, the borate level decreases as
and carbon coated. Semi-quantitative analyses were per- the C2 S fraction increases (see Figure 3). If this line
formed with a JEOL 733 EPMA system equipped with is compared with experimental data, it is clear that the
an energy dispersive spectrometer (EDS), which can de- calculated level of borate is not in agreement with the ex-
tect elements from sodium onwards. The borate distri- periments (dotted line) that indicate a minimum fraction
bution was determined using an ARL SEMQ EPMA of about 20 wt% C2 S before any stabilisation with borate
system equipped with WDS and a multilayer Ni–C crys- becomes necessary as well as an increase in B2 O3 con-
tal. For each slag phase, three 7.6–5.8-nm wave scans tent necessary to stabilize the slag with increasing C2 S
were performed at different locations in the sample. levels. Evidently, chemical stabilisation of C2 S is not the
only factor and other physical or mechanical stabilisation
mechanisms need to be considered.
 1169

Fig. 1. EPMA-WDS wave scans for the different phases in the industrial stainless steel slag, net peak height correlates with B level

Fig. 2. Effect of B2 O3 additions to a C2 S containing slag. Left: The untreated slag contains fractured γ-C2 S grains and disintegrates during
cooling. Right: The treated slag contains stable β-C2 S and does not disintegrate

Fig. 3. Stability diagram as a function of B2 O3 content and C2 S fraction. The full line is the result of a mass balance and partition calculation,
the dotted line is based on experimental results
1170

The very efficient action of B in conjunction with 2.3. Changing the chemistry
the need to couple waste streams and achieve their val-
orisation through this waste synergy strategy, was the Alternatively, slag disintegration can be averted by
incentive for the use of other boron sources. Already in modifying the slag composition in order to avoid the
the literature, a similar attempt was performed by Branca presence of C2 S. Already in 1942, compositional limits
et al. using boron containing glazing powders [10] with were defined for disintegrating slags .....[19], based on
promising results. the stability field of C2 S in the CaO-MgO-SiO2 -Al2 O3
The boron source used is a form of boron waste system, with an adjustment for the sulphur content (S)
originating in Turkey. These wastes are produced during in the slag:
the refining of the boron ores. Their volume amounts
CaO + 0.8 MgO 6 1.20 SiO2 + 0.39 Al2 O3 + 1.75 S
to 400.000 tons/y and are currently disposed in tailing
dams. More information regarding the wastes can be re- CaO 6 0.93 SiO2 + 0.55 Al2 O3 + 1.75 S
trieved elsewhere [11, 12]
The boron waste was mixed with a synthetic slag of with the compounds referring to their respective weight
basicity 2. Additions in the range of <1wt.% of boron fractions. However, in many cases, slags that meet these
waste have been proven successful in terms of C2 S sta- conditions do not have the appropriate high temperature
bilisation. The analysis of results is underway. metallurgical functionality. In stainless steelmaking, C2 S
free, low basicity slags cause rapid refractory degra-
dation and low chromium yields [20]. To avoid mak-
ing such compromises towards production cost, the slag
2.2. Non-boron additions composition must be adjusted after slag/metal separa-
tion. Adding a relatively large amount of silica seems
to be the best way to avoid C2 S. This was proven on
Borate additions are not the only possibility to avoid a laboratory scale by Sakamoto [21], who stabilised a
the expansive transformation of C2 S and the associated stainless steel decarburisation slag with 12 wt% of waste
slag disintegration of the slag [8, 13-15]. The crystal- glass, containing 70-75 wt% SiO2 . The authors have al-
lographic coordination number, the ionic radius and the so demonstrated the potential of this method in trials
ionic valence of the doping ion all affect the deformation with waste glass in the slag pot. However, an addition-
of the C2 S crystal and, as a consequence, the stabili- al slag treatment process is probably required in order
sation. Recently, a qualitative criterion based on ionic to dissolve the waste glass. The principle of this step
radius, ionic valence and crystallographic structure of can be similar to the method developed by Kühn et al.
the additive was developed [16], which is capable of [22, 23]. It permits the dissolution of a large amount of
predicting whether or not a compound will stabilise the quartz sand in a basic oxygen furnace steelmaking slag
β-polymorph. In practice, different oxides have been re- by co-injection of oxygen. The injected oxygen reacts
ported to stabilise the different polymorphs of dicalcium with FeO in the slag to form Fe2 O3 , whilst generating
silicate. The α and α polymorphs have been reported the additional heat required for the sand dissolution. This
to be stabilised by oxides such as MgO, A12 O3 , Fe2 O3 , method has been adopted by Thyssen Krupp Stahl AG
BaO, K2 O, P2 O5 and Cr2 O3 . The β polymorph can be for years. In the case of slags containing little or no FeO,
stabilized by the addition of Na2 O, K2 O, BaO, MnO2 , an alternative solution for the heat generation is required.
Cr2 O3 or their combinations [13]. The difference in the An alternative option, as described by Kitamura et
stabilising ability of each oxide provides a certain degree al. [24], is to mix stainless steel slag with cold or pre-
of flexibility if the goal is to avoid the formation of the heated non-ferrous fayalite slag. In this way, the basicity
γ phase. of the slag can be substantially reduced, avoiding the
Based on this knowledge, the effect of phosphate ad- formation of C2 S. The FeO from the non-ferrous slag is
ditions to disintegrating stainless steelmaking slags was used as an additional energy source (exothermic reaction
investigated. Satisfactory stabilisation was obtained [17], to Fe2 O3 results in additional heat to dissolve the SiO2 .
but compared to borate additions a significantly larger However, this method has only been shown to work on
amount of phosphates (∼ 2 wt%) is required to avoid a lab-scale level. To scale this up to the industrial lev-
disintegration. Working on a similar direction, Yang et el is not straightforward. Heat balance calculations by
al. [18] used a feed grade mono-calcium phosphate, orig- Kitamura et al. [24] have shown that a mixing ratio of
inating from iron ore processing. The P addition pre- maximum 15% (non-ferrous slag to stainless steel slag)
vented the formation of γ- C2 S and only β- and α-C2 S can be achieved. This would be insufficient to reduce the
were detected, for additions in the range of 0.46-0.69 C/S ratio to a low enough level. In addition, it is possible
wt%. that around the cold fayalite slag particles a solidified
 1171

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Received: 10 April 2010.