CHEMICAL ENGINEERING TRANSACTIONS
VOL. 61, 2017
A publication of
The Italian Association
of Chemical Engineering
Online at www.aidic.it/cet
Guest Editors: Petar S Varbanov, Rongxin Su, Hon Loong Lam, Xia Liu, Jiří J Klemeš
Copyright © 2017, AIDIC Servizi S.r.l.
ISBN 978-88-95608-51-8; ISSN 2283-9216
Assessment of Treatment Configurations through Process
Simulations to Improve Basic Oxygen Furnace Slag Reuse
Ismael Matinoa,*, Teresa Annunziata Brancaa, Valentina Collaa, Barbara Fornaia,
Lea Romaniellob
aScuola Superiore Sant’Anna, TeCIP Institute – ICT-COISP, via Moruzzi 1, 56124 Pisa, Italy
bILVA S.p.A. Taranto Works, SS Appia Km 648, 74123 Taranto, Italy
i.matino@sssup.it
The European steel sector aims at improving its material efficiency by increasing the by-products recycling rate.
This can lead to approach the ambitious “zero-waste” goal, by minimising the need for landfilling, saving raw
materials, reducing emissions and contributing to improve the economic sustainability of the production cycle.
The steelmaking process produce several by-products: slags are the main one and can be used to make
different products, including cement, fertilisers and asphalt. This paper presents a work aiming at obtaining
reusable fractions from the Basic Oxygen Furnace slag. Two Aspen Plus®-based models were developed and
exploited: a model of the slag treatments to make sensitivity analyses, providing information about different slag
treatment configurations, particularly on different magnetic separation solutions, and a model computing the
pellet composition according to the different inputs. Other models, developed through reMIND® software, are
material flow superstructures that implemented results from the treatment model. They allowed assessing the
best route for internal or external reuse of the slag fraction, taking into account process, environmental and
economic impacts and making optimization assessment. They are able to identify the slag quality that is more
suitable for the reuse. The developed models can provide significant improvements based on economic and
environmental sustainability (e.g. increase of by-products recycling, reduction of slag recovering in the internal
quarry, reduction of treatment costs, etc.) compared to the current use at ILVA Steelworks.
1. Introduction
The European steel industry is committed to increase the recycling rate of by-products produced in steelmaking
processes, such as discussed in (Peters et. al. 2015), also according to the ever more stringent European
regulations. Significant environmental and economic benefits are expected, such as prevention of waste
landfilling, reduction of CO2 emissions and natural resources exploitation. The main by-products of the steel
sector are slags, dusts, mill scales and sludges. Slags represent the 90 % by mass (World Steel Association,
2017) of the total produced by-products. Basic Oxygen Furnace (BOF) slag is a by-product of the integrated
steelmaking, resulting from dolomite and other fluxes addition in the furnace during the conversion process of
the pig iron into steel. For the Italian legislation BOF slag is considered as a non-hazardous waste, which can
be recovered in a quarry, according to legislation limits of leaching tests. The BOF slag composition includes
CaO, SiO2, Al2O3, FexOy, MnO, P2O5 and VOx , depending on raw materials qualities and produced steel grades.
Over the last few years, the slag produced in the BOF decreased thanks to new techniques. However, further
efforts must be applied to achieve even better results, in order to approach the “zero-waste” target and to
improve resources savings.
In order to increase the recycling rate of slags, their composition must be clearly identified and carefully
considered, so as avoid any negative effects on the environment (Dippenaar, 2005). Investigations on the slag
characterization and on possible slag pre-treatments have recently carried out (Matino et al., 2015). On this
subject, the optimization of the slag products and the identification of suitable downstream ad-hoc recovery
treatments can increase the recycling of slag.
DOI: 10.3303/CET1761086
Please cite this article as: Matino I., Branca T.A., Colla V., Fornai B., Romaniello L., 2017, Assessment of treatment configurations through
process simulations to improve basic oxygen furnace slag reuse, Chemical Engineering Transactions, 61, 529-534 DOI:10.3303/CET1761086
529
In order to obtain a Fe-richer product, which is suitable for the sintering process, slag, after discharging into the
slag yard, is cooled and subject to magnetic separation and iron recovering (Horii et al., 2012). The slow cooling
application to the BOF slag can produce a fraction with high iron and low phosphorus content (Wang et al.,
2012). The cooling conditions are important for mineral formation and crystallization. In particular, slow cooling
promotes the good distribution of crystals into the slag (Wang et al., 2012). After magnetic separation, BOF slag
can be internally recovered as raw material or used in different field of applications (World Steel Association,
2010), such as in overlays, bearing layers in mixed concrete, according to (European Regulation 305/2011),
road foundations and in concrete mixtures cement production and restoration of marine environments (Zhang
et al., 2011), according to the national legislation. It can be also reused in ironmaking and steelmaking processes
as raw material (thanks to the high content of valuable elements like iron). The not magnetic fraction can be
used for soil fertilization or amendment considering the good results in terms of crop yields and quality obtained
after long-term field or lysimeter trials (Branca et al., 2014) and the negligible effects in terms of groundwater
pollution (Morillon et al., 2015); while the magnetic fraction can be employed in pellets production (Kawatra et
al., 2002). It has been shown that the application of wet magnetic separation helps to obtain low impurities in
the fraction. By coupling the weak magnetic separation with the selective size screening, the recycling rate of
slag can be improved (Ma and Houser, 2014). On the other hand, the application of a strong magnetic field can
allow to separate FetO matrix in the crushed BOF slag (Yokoyama et al., 2007), by allowing the recovered slag
of higher quality, which can be further improved by repeating the separation practice (Kubo et al., 2010).
Previous studies underlined the slag enrichment through magnetic separation and the consequent effects on
the sinter quality (Bölükbaşı and Tufan, 2014).
As technical, economic and environmental aspects can affect the recycling rate, the use of process models and
simulations can help to investigate them in order to assess processes improving material and energy efficiency,
which can improve the environmental and economic sustainability in the steel sector, such as discussed in
(Matino et al., 2015a). In this regard, some recent studies have been carried out. For instance, as steelmaking
plants are huge energy consumers, energy efficiency can be significantly improved by applying the total site
approach using a "Total Site Profile (TSP) analysis”, based on pinch technology (Matsuda et al., 2012). Modeling
of different sub-plants in an integrated steelworks allowed the calculation of mass and energy balances in
different scenarios of operation in order to optimize the process gas network (Porzio et al., 2014). A recent
study, focused on improving the sustainability of electric steelworks, exploited a simulation model developed by
Aspen Plus® in order to investigate the correlations among electric energy consumption, steel grade, slag
quantity and composition (Matino et al., 2016a).
Process simulation is also the topic of the present work, which was carried out within the European project,
entitled ‘‘Removal of Phosphorus from BOF slag’’ (PSP-BOF). The work aimed at optimizing by-products reuse
scenarios at ILVA Steelworks in Taranto (Italy), with a particular focus on BOF slag reuse. As this by-product is
currently recovered in internal quarry, in order to increase its internal recycling through the pellets production,
some process models were developed. Several information were used: experimental information, the achieved
results on the evaluation through Aspen Plus®-based model of pre-treatments of other by-products (Colla et al.,
2015), that are potentially reusable in pellet production together with BOF slag or can be directly reused and the
preliminary results obtained for BOF slag with empirical MS Excel® – based model (Matino et al., 2015b). The
developed models aimed at considering slag treatment and processing as well as production of internal reusable
products. Two Aspen Plus®-based models are proposed: a model for different BOF slag treatment
configurations, taking into account different solutions for magnetic separation and a model computing pellet
composition, which takes into account different inputs. Models developed by reMIND® software, whose features
are described in (reMIND tremind) and in (Karlsoon, 2011), consist in material flow superstructures that
implement the results achieved in the treatment models, in order to assess the route which mostly allows the
reuse of by-products (especially BOF-slag) and gets a high-quality secondary raw material (e.g. pellets to feed
sinter plant). The work developed by Matino et al. (2016b) was improved and deepened. Process, environmental
and economic impacts are considered during the optimization assessment. Moreover, the models allowed
comparing the novel analysed applications to the current use of by-products at ILVA Steelworks.
The paper is organized as follows: Section 2 presents the modeling phase; in Section 3 the achieved results
are discussed, while Section 4 includes some concluding remarks.
2. Materials and Methods
In order to assess BOF slag treatments, processing and production of reusable products as well as to identify
the best route for by-products internal or external reuse, two models based on the commercial software Aspen
Plus® V. 8.6 and two models developed by means of the reMIND® software were developed. The first ones are
flowsheet-based while the second ones are flow superstructures and they are linked as depicted in Figure 1A.
One of the two Aspen Plus® models is a “scenario data generator”, which allows considering and assessing
530
different process configurations and different BOF slag types, while the other one provides the composition of
one of the possible secondary products, i.e. the pellets, after that the mixture for their production was optimized.
The two reMIND models are mass flow superstructures: the former one identifies the best route for reusing BOF
slag (and other by-products), while the latter one points out the best BOF slag quality to be reused. The models
were setup and tuned starting from laboratory tests and industrial data collected during the project, especially
at ILVA. Moreover, also results of a previous project named REFFIPLANT (Matino et al., 2016b) funded by the
Research Fund for Coal and Steel were used. For instance, Particle Size Distributions (PSD), distribution of
slag component according to cooling procedure, BOF slag fraction compositions, separation efficiencies, etc,
were exploited.
The first Aspen Plus® model considers the main slag treatments and processing steps, such as cooling, grinding
and sieving, and magnetic separation. Some stream duplicator blocks included in the model allow the
simultaneous assessment of different treatment configurations or process units. The model computes the
following parameters:
• distributions of chemical compounds;
• PSD after grinding;
• composition of the main sieved fractions (e.g. ≤ 2mm and > 2mm);
• compositions of magnetic and non-magnetic fractions after different magnetic separations (manual
magnetic separation with neodymium magnet, Wet High Intensity– WHI - magnetic separation with
Davis Tube at 1.2T, Wet Low Intensity - WLI - magnetic separation with low intensity wet drum
separator and a magnetic field strength less than 1.2T, dry magnetic separation with rotatory
separation drum having a neodymium magnet and a magnetic field intensity of about 0.2 T at the
surface);
• approximate estimation of required energy in the grinding step based on Bond’s Law.
The model was tuned by exploiting the experimental results related to a selected BOF slag from ILVA
Steelworks. The comparison between results of the tuned model and results of the real case showed
insignificant errors. For instance, Figure 1B shows that the real and simulated slag fractions compositions after
grinding and sieving are very similar, and the same behaviour is observed also for other monitored parameters.
The results of the Aspen Plus® model were used to point out which was the best slag treatment route, by taking
into account the composition of the fractions resulting from treatment and by exploiting them for further economic
and environmental constraints. To this aim, two reMIND-based superstructures mass models were developed,
upgrading the reMIND superstructure developed within the REFFIPLANT project to optimise the reuse of
different by-products/wastes (Matino et al., 2016b). The first superstructure included the different magnetic
separations of BOF slags and the different fates of the BOF sludge and mill scales that were considered as
potentially suitable for pellets production. Each treatment was characterized by some normalized indicators
connected to the following parameters: final product qualities (depending on the iron or phosphorous content. It
is defined as the ratio between the minimum quality value and the quality related to the considered treatment
and for this reason lower its value, higher the quality), environmental impact, treatment costs, revenues (lower
this indicator, higher the revenues), efficiency of separation (lower this indicator, higher the efficiency). These
indicators constitute the objective functions considered in the optimization. A second more simplified reMIND-
superstructure was developed to identify the best BOF slag to be reused. This was done through another multi-
objective optimization by minimizing all the objective functions that were considered in the analysis previously
performed.
Figure 1: A. Links between the Developed Models; B. Comparison between real and simulated composition of
slag fractions after grinding and sieving.
531
3. Results and Discussions
In order to reduce the amount of BOF slag recovered in internal quarry and to increase the recycling of by-
products, the production of pellets was analysed: BOF slag, BOF sludge and mill scale were considered, to the
aim of increasing the BOF slag reuse, as BOF sludge and mill scale are already used in the sintering process.
The final fractions of the three BOF slag (A, B and C), both magnetic and non-magnetic, were obtained after
treatments, such as slow cooling, grinding and sieving, magnetic separation for fraction with a PSD <= 2mm
and > 2mm, mixing of magnetic fraction and mixing of non-magnetic fraction. The treatment model provides
these results considering different magnetic separation techniques and extending the outcomes of real
experimentation. Table 1 and Table 2 show results as portion of magnetic and non-magnetic fractions and of
obtained compositions.
Table 1: Magnetic fractions and their compositions after different separation solutions and with different BOF
slags.
Magnetic Fractions
Manual WHI WLI Dry
A B C A B C A B C A B C
wt %
CaO 30.1 35.9 29.4 37.4 42.9 36.4 30.8 36.7 30.2 42.9 50 41.9
SiO2 7.9 8.5 8.8 11 11.4 12.2 7.9 8.4 8.7 12 12.3 13.2
Al2O3 2.1 2.2 2.3 1.8 1.9 2.0 1.8 2.1 2.2 2.1 2.1 2.3
P2O5 0.6 1.1 0.7 0.9 1.5 1 0.6 1.1 0.7 1.0 1.6 1.2
Fe(0) 0.6 0.5 0.6 0.7 0.6 0.7 0.4 0.3 0.4 0.7 0.5 0.7
FeO 25.9 20.6 25.6 16.6 12.7 16.3 25.5 20.2 25.2 15.8 12 15.6
Fe2O3 18.4 14.6 18.2 17.2 13.1 16.9 18.1 14.3 17.9 16.3 12.4 16.1
Other 14.4 16.6 14.4 14.4 15.9 14.5 14.9 16.9 14.7 9.2 9.1 9
Magnetic
Fraction
27 26 27 41 40 41 21 20 21 36 35 36
Table 2: Non-Magnetic fractions and their compositions after different separation solutions and with different
BOF slags.
Non-Magnetic Fractions
Manual WHI WLI Dry
A B C A B C A B C A B C
wt %
CaO 47.5 52.6 46.0 46.5 52.0 45.1 45.9 51.2 44.5 42.7 47.9 41.3
SiO2 14.4 14.3 15.7 13.7 13.8 15.0 13.9 13.9 15.2 13.0 13.1 14.2
Al2O3 2.1 2.1 2.3 2.3 2.3 2.5 2.1 2.1 2.3 2.1 2.1 2.3
P2O5 1.2 1.9 1.3 1.1 1.8 1.3 1.1 1.8 1.3 1.0 1.7 1.2
Fe(0) 0.3 0.2 0.3 0.1 0.1 0.1 0.4 0.3 0.4 0.2 0.1 0.2
FeO 16.4 12.1 16 20.6 15.3 20.2 17.2 12.8 16.9 20.7 15.5 20.2
Fe2O3 8.3 6.1 8.1 6.8 5.0 6.6 9.2 6.8 9.0 8.1 6.1 7.9
Other 9.8 10.7 10.3 8.9 9.7 9.2 10.2 11.1 10.4 12.2 13.5 12.7
Non-
Magnetic
Fraction
73 74 73 59 60 59 79 80 79 64 65 64
The results show that WHI magnetic separation leads to separate a bigger magnetic fraction, but with iron
compounds less concentrated compared to the magnetic fraction after Manual and WLI magnetic separations.
This was due to the bigger entrainment of non-magnetic fraction by the WHI magnetic separation; nevertheless,
in absolute terms the recovered iron fraction with WHI is bigger than with the other magnetic separation
techniques. On the other hand, the kind of magnetic separation poorly affects the iron and phosphorous
compounds content in the non-magnetic fractions. Moreover, slag A provides fractions containing higher amount
of iron compounds, while slag B provides fractions with higher amount of P2O5.
In order to achieve the best route for reusing BOF slag, BOF sludge and mill scale, the reMIND-based
superstructures were exploited. Different multi-objective optimizations scenarios were assessed, in order to
minimise different combination of previously defined indicators, such as Environmental Impact (OP1), Quality +
Environmental Impact (OP2), Costs and Revenues + Environmental Impact (OP3), the minimization of each
532
indicators (Global Optimization – GOP). Table 3 shows the list of results as by-products/wastes percentage
distribution.
Table 3: Results of reMIND best route-optimization
Agglomeration Pelletization
Fertiliser and Other
Use
Disposal or
Environmental
Recovery
wt %
OP1 OP2
OP3
&
GOP
OP1 OP2
OP3
&
GOP
OP1 OP2
OP3 &
GOP
OP1 OP2
OP3 &
GOP
BOF
Slag
N.a. N.a. N.a.
20.4
(WLI)
26.4
(M)
40.9
(WHI)
79.6
(WLI)
73.6
(M)
59.1
(WHI)
0 0 0
BOF
Sludge
0 0 0 100 100 100 N.a. N.a. N.a. 0 0 0
Mill
Scale
0 100 100 100 0 0 N.a. N.a. N.a. N.a. N.a. N.a.
Results achieved through the optimization could allow not only reducing the disposal of by-products but also
increasing their reuse in different identified routes. Moreover, the best solution consists in using the BOF slag B
and a little fraction of slag C. This provides relevant iron content in the pellets as well as higher P2O5 content in
the non-magnetic fraction, which makes it suitable to be used for the production of fertilisers. This result comes
from the second reMIND model which gives the optimized pellet mixture in the case of GOP: magnetic fraction
of BOF slag obtained through WHI (57 wt%: B – 99 wt% and C – 1 wt%), BOF sludge (35 wt%), lime (1 wt%),
cement (7 wt%).
Finally, a simulation aimed at finding the chemical composition of optimized pellet mixture was carried out
through the other Aspen Plus®-based model and by taking into account each simulated results and some
experiments carried out during PSP-BOF project. This last model is able to mix the magnetic fractions of the
three BOF slags, obtained with the treatment model, BOF sludge, mill scale, cement and lime. The data can be
passed from one Aspen Plus-based model to the other one as they are connected to an Excel-sheet through
the Aspen Simulation Workbook® and the two Excel sheets are linked together. This feature allows also an easy
model management. Table 4 shows the resulting pellet chemical composition.
Table 4: Resulting pellet chemical composition.
Component Fe_tot P2O5 CaO SiO2 MgO Al2O3 MnO2 C Other
wt% 34.38 0.89 32.08 8.65 7.90 1.66 2.42 2.80 9.22
4. Conclusions
The models described in the present paper aimed at obtaining the best solutions to treat BOF slag, the best
slag to be reused, the best reusing route and the obtained pellets composition. Although the achieved data need
further analyses and tests, the developed models show some innovative aspects. The joint application of the
two software solutions can be considered a useful tool to be exploited by industrial staff in order to analyse
possible scenarios on the BOF slag use. This can lead to improvement of the internal reuse for pellets production
by saving time and natural resources. Future work aimed at the implementation of the internal reuse of pellets
production will consist in the assessment of the economic viability of the proposed solutions.
Acknowledgments
The work described in the paper was developed within the project entitled ‘‘Removal of Phosphorus from BOF
slag’’ (PSP-BOF) (RFSR-CT-2013-00032) that have received funding from the Research Fund for Coal and
Steel of the European Union. The sole responsibility of the issues treated in the paper lies with the authors; the
Commission is not responsible for any use that may be made of the information contained therein.
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