Airborne Emissions from Si/FeSi Production
Airborne Emissions from Si/FeSi Production
IDA KERO 0 1
SVEND GRA˚ DAHL 0 1
GABRIELLA TRANELL 0 1
0 1.-SINTEF Materials and Chemistry , Alfred Getz vei 2, 7465 Trondheim , Norway 2.-Norwegian University of Science and Technology , Alfred Getz vei 2, 7491 Trondheim, Norway. 3.-
1 Kero , Gra ̊ dahl, and Tranell
The management of airborne emissions from silicon and ferrosilicon production is, in many ways, similar to the management of airborne emissions from other metallurgical industries, but certain challenges are highly branch-specific, for example the dust types generated and the management of NOX emissions by furnace design and operation. A major difficulty in the mission to reduce emissions is that information about emission types and sources as well as abatement and measurement methods is often scarce, incomplete and scattered. The sheer diversity and complexity of the subject presents a hurdle, especially for new professionals in the field. This article focuses on the airborne emissions from Si and FeSi production, including greenhouse gases, nitrogen oxides, airborne particulate matter also known as dust, polyaromatic hydrocarbons and heavy metals. The aim is to summarize current knowledge in a state-of-the-art overview intended to introduce fresh industry engineers and academic researchers to the technological aspects relevant to the reduction of airborne emissions.
Elemental silicon is often referred to as ‘‘silicon
metal’’ although it is not a true metal but a
semimetal (metalloid). ‘‘High-silicon alloys’’ typically
denote silicon-containing alloys in which silicon
dominates the behavior in the production furnace.
This normally includes metallurgical grade silicon
(MG-Si) with 96–99% purity and ferrosilicon (FeSi)
with a silicon content of 65–90%.1–3
Table I shows the world production of silicon
alloys in recent years. In 2015, an estimated 68% of
the global high-silicon alloy production was
produced in China.4
In Europe, emissions and waste associated with the
production of silicon and ferrosilicon are regulated at
both national and EU levels, and there are similar
divisions between state/territorial and federal
legislations in the USA, Canada and Australia. Some of the
national environmental agencies offer guidelines and
recommendations, published to complement and detail
the legal requirements of this industrial branch.6–10
Throughout this article, Norwegian legislation and
practices are often cited because they are among the
most stringent and comprehensive sets of rules. In some
countries, emission data are made publically available
through annual publications in open databases.11–14
The management of airborne emissions from
silicon production is, in many ways, similar to the
management of airborne emissions from other
metallurgical industries, such as other smelters,
foundries and electrochemical metal production.
Nonetheless, certain challenges are specific to the
silicon alloy-producing industry, for example, the
specific dust types and the management of NOX
emissions. A commonly encountered difficulty in the
mission to reduce emissions is that information on
emission types and sources as well as abatement
and measurement methods is often scarce,
incomplete and scattered. Much progress in emission
abatement has been achieved in the industry itself
over the last decades, but such work is rarely
published. At best, it may be partially presented at
industry-specific conferences or in confidential
reports, producing little or no documentation
available through database search engines. When the
industry cooperates with research institutions,
journal articles are published within a vast range of
different scientific fields, such as atmosphere and
aerosol physics, chemistry, process metallurgy,
occupational hygiene and environmental
monitoring. The sheer diversity and complexity of the
subject presents a hurdle, especially for new
professionals in this field.
aFerrosilicon production only.
The aim of this literature review is to summarize
current knowledge on emission types and
concentrations, as well as suitable measurement
techniques developed in and relevant to the Si- and
FeSi-producing industry. It is an attempt to create a
state-of-the-art overview, which can introduce
researchers, engineers and others to the relevant
technological aspects. The focus of this article is the
airborne emissions of particular significance to the
silicon industry, including greenhouse gases (GHG),
nitrogen oxides (NOX), particulate matter (PM),
polyaromatic hydrocarbons (PAH) and heavy
Silicon Production and Emissions
The submerged arc furnace (SAF) is the core
process for silicon production. Figure 1
schematically illustrates the production process steps and
The primary raw material for silicon production is
quartz. The reductants include coal, charcoal, wood
chips and sometimes coke. In addition, iron pellets
or sinter are included in the raw materials for
ferrosilicon (FeSi). The raw materials and
reductants are crushed and weighed before they are
charged to the furnace. The high-temperature
process continuously consumes the carbon-based
electrodes. Both ferrosilicon (FeSi) and metallurgical
grade silicon (MG-Si) are typically produced this
way and the product is hereafter simply referred to
as the ‘‘silicon alloy’’.
While the overall carbothermic reduction reaction
of quartz in the furnace may be expressed as;
SiO gas are produced at temperatures around
2000 C through various sub-reactions, giving
different stoichiometric versions of the overall
ð1 þ xÞSiO2 þ ð2 þ xÞC ! Si þ xSiO þ ð2 þ xÞCO
In the outer zone, SiO ascending from the inner
(lower) hot zone reacts with carbon materials
SiO þ 2C ! SiC þ CO ð3Þ
and condenses, depending on temperature,
according to either:
2SiO ! SiO2 þ Si
3SiO þ CO ! 2SiO2 þ SiC
Furnace operation and raw material properties
will determine the Si yield, i.e. how much SiO gas
leaves the furnace. This will, in turn, affect the
composition of the furnace off-gas. The raw silicon
alloy produced in the furnace hot zone is tapped
from the furnace and refined in a slag process before
it is cast in molds for cooling. The solidified product
is also crushed, sized, weighed and packed at the
plant before it is sent to the customer.1,15–17 The
most commonly considered emission types, emission
points and their origin, as discussed in this article,
are listed in Table II.
While most gases are generated in the furnace
itself, dust is generated in almost every step of the
silicon production process. The transport and
handling of raw materials, reductants and products at
ambient conditions generates PM through
mechanical impact. Hot processes on the other hand, such
as tapping, refining and casting, are sources of
thermally generated fumes. Most of the processes
Fig. 1. High silicon alloy production process and its primary emission sources.
Reductants, electrodes, carbon paste
Reductants, electrodes, carbon paste
All raw materials
Solid material handling
Liquid alloy in contact with air, SAF charge top
are equipped with ventilation systems and the
collected off-gases are typically transported through
a system of heat exchangers and filters before
escaping through the chimney. The furnace dust
collected in the filters is commercially termed
‘‘micro-silica’’ and is used in a variety of
applications, such as concrete filling.
‘‘Fugitive’’ or ‘‘diffuse’’ emissions are emissions
which do not pass through a stack, chimney, vent,
duct or other functionally equivalent opening.
Typical fugitive emissions in the metallurgical industry
are gases and PM leaking into the working
atmosphere from closed or encapsulated processes, where
the hoods are not capable of capturing 100% of the
Major Greenhouse Gases (GHG)
In the production of silicon alloys, the
carbonaceous reductants are usually coal and coke, but
biocarbon (charcoal and wood chips) may also be used.
Table III. Typical compositions of coal, coke and electrode material used for FeSi and Si production and
their calculated emission factors
aMay be lower.bMay be higher.23
The carbothermic reduction of the quartz to Si will
create CO gas through the overall oxide reduction
SiO2 þ 2C ! Si þ 2CO
The CO gas will oxidise to CO2 at the furnace
charge top in an open or semi-closed furnace.
Methane (CH4) and volatile hydrocarbons are also
generated in the combustion of the carbonaceous
materials and electrodes (pre-baked for MG-Si
production and Søderberg-type for FeSi production).
Iron-bearing raw materials added as oxides in FeSi
production are reduced to metallic iron through the
CO gas and the volatile hydrocarbons in the
furnace, hence generating CO2. In the top part of the
furnace, the Boudouard reaction may also
contribute to CO2 emissions:
The extent of GHG emission is highly dependent
1. Type of alloy Reduction of quartz requires more
energy (i.e. carbon and electricity) than iron
oxides, so the higher the Si content, the higher
the GHG emissions.
2. Carbonaceous material mix The levels of fixed
carbon and volatile matter depend on the choice
of carbon materials, which in turn affect GHG
emissions. In national emission inventories,
only emissions from fossil carbon are accounted
for and therefore the use of charcoal lowers the
reported specific CO2 emissions.
3. Furnace operation Furnace operation and charg
ing method strongly influence the emissions, in
particular of CH4 and NOX. More even charging
will generally reduce emissions as compared to
The tapped alloy will contain some of the added
carbon, mainly in the form of carbides, as the
solubility of carbon is generally low compared to
other ferroalloys (typically in the order of 0.005–
0.02% at 1400–1600 C, depending on the alloy Si
content). Typical GHG emissions from this industry
Coal for FeSi and Si
Type of alloy
Ferrosilicon 45% Si
Ferrosilicon 65% Si
Ferrosilicon 75% Si
Ferrosilicon 90% Si
MG-Si (>98% Si)
Generic emission factor
have been assessed and documented by a number of
authors.18–20 The 2006 IPCC Guidelines for
National Greenhouse Gas Inventories21 and Lind
stad et al.22 present a summary of pre-2006 work in
terms of general, operation-based and reductant
materials-based emission factors. These emission
factors are still in use for national GHG inventories.
Typically, most production emissions are reported
on the basis of raw material type/use and production
tonnage. Then, control measurements are carried
out to verify the calculated numbers. Coke and coals
are the main contributors to the GHG emissions,
but the carbon-based electrodes and electrode paste
will also contribute substantially. Typical
compositions are shown in Table III which is a summary of
data from Lindstad et al.22 Recent, personal
communications with the Norwegian industry indicate
that the total %C in coal and electrode paste may be
approximately 81% and 94%, respectively.23
Based on the above summarised raw
materialand production-based emissions, generic (average),
total emission factors for different high Si alloys are
summarised in Table IV. Note that CH4 emissions
based on semi-closed furnaces with the sprinkle
charging and off-gas temperatures >750 C are used
as default values in these factors.
All hydrocarbon emissions are highly dependent on
both alloy type and operation, which in turn lead to
high variations and uncertainties of reported data.
Lindstad et al.24 estimated typical CH4 emissions for
different alloys and furnace operations in the 1990s.
Comparing these estimates to the reported CH4
Fig. 2. Temperature dependence of NOX formation mechanisms compared to process temperatures in Si production. (Adaptation of original
figure by DeNevers28).
emission values from Norwegian Si and FeSi
smelters (available at norskeutslipp.no12 for the period
2002–2014), the discrepancy is of the order of a factor
of 10. The reported values are lower than those
calculated using emission factors for a given
production tonnage. With such large divergences of data,
there is undoubtedly room for improvement.
Nitrogen Oxides, NOx
Nitrogen oxides (NO and NO2; often referred to as
NOX) are important emissions due to their role in
the atmospheric reactions creating fine particles
and ozone smog. NOX emissions also contribute to a
suite of year-round environmental problems,
including acid rain, eutrophication (stimulated growth of
algae and bacteria) and bronchial suffering.
Figure 2 illustrates the temperature dynamics of
the three main NOX formation mechanisms
compared to typical processes and operation
temperatures for silicon alloy production. The fuel and
thermal formation mechanisms are the dominant
mechanisms in electric arc furnaces producing
ferrosilicon and silicon. Fuel NOX is formed by
oxidation of the nitrogen components present in the
solid fuel, while thermal NOX is formed by direct
oxidation of nitrogen (from the air) at temperatures
above 1400 C. Such temperatures are frequently
observed in the furnace hood.25–27
Combustion of gaseous SiO above the charge
surface and in the tap-hole may locally increase the
temperature. The amount of SiO(g) released from
the charge will therefore also influence NOX
formation, while any SiO reducing measures also
seem to reduce NOX emissions. The Norwegian
Ferroalloy Association (FFF), SINTEF and the
Norwegian University of Science and Technology
(NTNU) have collaborated on NOX-reducing
strategies for over 20 years and the investments have
proven successful.27,29,30 While some of this work is
available through international journals and
conference publications, a significant part of the results
and achievements remain unpublished. For this
paper, we have had the opportunity to read and
evaluate some unpublished work in conjunction
with the published papers, and we have, with
permission from the authors and industrial
partners, chosen to include brief summaries of some of
the major, unpublished findings and conclusions in
Research initiatives have both focused on waste
gas dynamics in general31 and NOX emissions in
particular (see also Table V; Fig. 4).32 Efforts to
understand the NOX formation have shown that
furnace design and furnace operating procedures,
such as stoking and charging, heavily influence
NOX emissions.33–39 Reported NOX emission values
vary greatly, with typical values ranging from 500
to 1500 ton per site and year.12 The NOX production
is inversely proportional to the silicon yield (low Si
yield, high SiO losses), at least up to a certain level
of silica fume formation. NOX also forms during
tapping, when an oxygen lance is used to open up
the tapping channel to increase the metal flow out of
the furnace.26,40,41 The general correlation between
SiO and NOX emission is illustrated in Fig. 3.
The NOX formation is also correlated to the
moisture content of the furnace gas.42,43 Moisture
is introduced through the raw materials and, hence,
will vary throughout the materials charging cycle. It
is well documented that the injection of water vapor
into a combustion engine increases the heat
capacity (CP) of the off-gas, so that the temperature
cannot exceed the limit for thermal NOX
production.44–46 Although not validated, it is likely that
this effect, in part at least, explains the observations
made in high silicon alloy production.27,42,43
The effect of the furnace hood design and the inlet
for false air on the NOX emission has been
thoroughly studied and modeled by Kamfjord26 and
others,47–49 but most of this work is not published.
The main conclusions from the unpublished reports
are that the amount of air and its flow path
throughout the hood determines when, where and
whether oxygen and nitrogen are mixed for a
sufficiently long time in a sufficiently hot zone to
produce NOX. Optimization of furnace hood designs
is very complex and the trial-and-error approach is
both time- and cost-consuming. Therefore, modeling
capabilities are extremely valuable. A modeling
concept for predicting turbulent flows, heat
transfer, combustion and NOX formation in the furnace
hood of a typical submerged arc furnace where
silicon or ferrosilicon is produced has been
developed. Currently, it is not accurate enough to
calculate the true NOX emissions, but it can predict
whether it increases or decreases when changes are
made in the design or process operations.50–55
Primary strategies for NOX reduction includes
modifications to the furnace operation, process
management and/or the SAF system itself.56–58
For silicon alloy production, this means:40,59
Reducing the combustion temperature through
active cooling of the primary flame zone.
Avoiding the ‘‘blowing’’ of SiO-rich gas up
through the charge surface.
Frequent stoking and semi-continuous charging.
Gra˚ dahl et al.25 found that it was possible to
reduce NOX emissions from poorly operated
furnaces by 50% if best practices were
Recycling the flue gas to reduce excess air above
Secondary strategies includes chemical reduction
treatments for the flue gas from the furnace, such as
selective catalytic or non-catalytic reduction with
ammonia (NH3) or urea (CO(NH2)2),60 as used in
steel production.61 To date, there is no literature on
the use of secondary methods for silicon alloy
production, and the effect of such chemical
treatments on the silica fume quality is therefore
SOX, Dioxin, and Other Gases
A great number of gases may be present in the
furnace flue gases, some of which are regularly
measured while some are more occasionally
detected and documented. Examples of such gases
include sulfur oxides (SOX) and other compounds
(sVuOchCa).s1,H622S and various volatile organic compounds
SOX emissions are often mentioned as a type of
gaseous emissions which occurs in the silicon alloy
industry, but very few authors seem to have
specifically studied these emissions. The origin of SO2 gas
is the sulfur content of the raw materials, primarily
reductants, and the reported emissions levels are
typically calculated based on material balances.
While abatement methods for post-filter cleaning of
SO2 are available, the current installation rate is
primarily inhibited by investment costs.59 Gra˚ dahl
et al.25 showed the correlation of SO2 and CO gas
emissions with certain furnace events called
‘‘avalanches’’ (collapse of charge burden near the
electrodes), the occurrence of which could be reduced by
use of semi-continuous charging procedures. The
reported SO2 emissions from silicon alloy
production are typically of the same order of magnitude as
the NOX emissions.1,12 Table V illustrate typical
values of NOX and SOX off-gas concentrations,
varying with furnace and product type. The values
presented in the Table are averaged means of
several measurements on different furnaces
performed irregularly over some 20 years (1995–2016).
Only a small fraction of the data has been
previously published.25,27 The measurement campaigns
were carried out by SINTEF, NTNU and FFF in
Norway, The results were compiled by S. Gra˚ dahl at
SINTEF for the sake of this article, and the data are
published with permission from FFF.
Fig. 3. Correlation between furnace emissions of NOX gas and SiO2 fume with simultaneous temperature measurements (reproduced with
permission from Gra˚dahl et al.25).
Dioxins are a class of persistent organic
pollutants (POPs) which are highly toxic to human
health. Like polycyclic aromatic hydrocarbons
(PAHs), dioxins may be both gaseous and
particlebound, depending on temperature. The generation
of dioxins in combustion and metallurgical
processes is, in a general sense, quite well
established.63,64,25 The destruction of dioxins and organic
compounds, such as furans and PAH’s, at high
temperatures allow for efficient reduction or even
elimination in modern, semi-closed SAFs. Furnace
design and operation are keys and can be optimized
for close control of the flue gas temperature, see
Fig. 4. Tveit et al.59 suggest that the use of a heat
exchange system (where off-gases are effectively
cooled, post-furnace, in a steam boiler) will allow for
higher off-gas temperatures and therefore have the
same reducing effect on this type of emissions.
PARTICLE-BOUND GASEOUS COMPOUNDS
Polycyclic Aromatic Hydrocarbons, PAH
Polycyclic aromatic hydrocarbons (PAH) consist of
organic structures having more than two joined
aromatic (benzene) rings. Anthropogenic PAHs are
typically formed by incomplete combustion of
organic materials like oil, wood, or garbage. The
lighter compounds, with few aromatic rings, are
gaseous at room temperature whereas the larger
molecular compounds are liquid or solid and
commonly adsorbed on particles, for example, soot.
PAHs belong to the Persistent Organic Pollutants
(POP), a group of airborne emissions which are
particularly resistant to degradation. Some of the
PAH compounds are linked to various forms of
cancer and the US Environment Protection Agency
(EPA) has identified 16 priority PAHs, based on
their potential to induce adverse environmental and
health effects. The main sources of PAH in silicon
alloy production is the combustion of reductants in
the furnace and the baking of electrodes. Typical
PAH and NOX emissions for different furnace
operations are listed/plotted in Fig. 4. Reported
PAH values from Norwegian plants range from 10
to 70 kg per site and year. PAH emissions from
industrial sites are estimated by use of emission
PAH formation is linked to soot formation which
in turn is influenced by furnace design and
operation and varies throughout the charging cycle.59 As
PAHs are destroyed at high temperatures,
emissions can be significantly reduced by increased
offgas temperatures as illustrated in Fig. 4 by Gra˚ dahl
et al.25 The reference case (A) represents a
traditional open furnace with batch charging. The second
case (B) is a semi-closed furnace, with feeding tubes
through which the raw materials were fed
semicontinuously (every minute). Case (C) represents
the semi-closed furnace with average off-gas
temperatures raised from 635 C to 812 C. The
increased temperature leads to the destruction of
PAH and dioxin but may also increase the formation
of thermal NOX.
Heavy metals enter the production process as
trace elements in the raw materials and electrodes
and are redistributed to metal, slag, fume and gas.
The concentrations depend on the alloy composition
and the process temperatures. At temperatures of
1600 C or higher, certain metals such as Zn, Pb, Cd,
Na, Mn and Fe go into the gas phase and may
escape as metal vapor. When the off-gas
temperature drops, the metal vapors are condensed and
therefore often collected with the dust. Myrhaug
Fig. 4. Off-gas emissions of PAH, dioxin and NOx for (A) a batch-fed, open SAF, (B) a semi-closed SAF with semi-continuous charging and (C) a
semi-closed SAF with semi-continuous charging and off-gas temperatures >800 C (reproduced with permission from Gra˚dahl et al.25).
and Tveit71,72 showed that a boiling point model can
be used to predict the redistribution of an element
in the furnace, as shown in Fig. 5. Naess et al.73
showed that the model is also applicable to the
refining ladle, with some modifications due to the
oxidation of elements, as shown in Fig. 6.
The Norwegian legislation for heavy metal
emissions appears to be one of the most rigorous in the
world, requiring emission control of 11 trace
elements for silicon alloy production facilities. These
trace elements are: As, Cd, Co, Cr, Cu, Hg, Mo, Ni,
Pb, Se and Zn. The European, USA and other
partners to the United Nations Environment
Program (UNEP) have put special emphasis on lead,
cadmium and mercury.74 The reported emissions of
trace elements are often based on material balances
and may vary greatly between plants, but an
example is shown in Table VI.59
Mercury constitutes a special case among the
airborne heavy metal emissions as international
legislation has long been stringent with respect to
this metal.75 In silicon alloy plants, the particulate
control devices (e.g., fabric filter or wet scrubber)
capture the particle-bound mercury. The more
volatile elemental mercury is emitted to the
atmosphere if no further gas treatment is applied. Hg
and Cd levels in the off-gas may be reduced by the
use of bag filters with an adsorbent injection (such
as activated carbon or lignite coke).8,76,77
Emission estimates to air through the filter
systems must cover both gaseous and
particlebound heavy metals, but a major challenge for the
estimation is the low concentrations of these
elements in the material flows. Mercury typically has
detection limits (DL) given in units of parts per
billion (ppb) whereas the other heavy metals have
DL of the order of magnitude of parts per million
(ppm). Hence, very significant measurement
uncertainties are introduced and it is often impossible to
‘‘close’’ the material balance for individual elements.
The total uncertainty for elements such as Co, Hg
and Mo is often around or above 100%. These
uncertainties may be lowered by continuous, on-line
measurements after the filter systems, but such
measurements are often practically challenging.
Additionally, large uncertainties are also related
to sampling and representability.73,78
Airborne PM is an important constituent in the
diffuse emissions escaping the plants and may not
only affect the air quality inside the plant but also in
the local, urban communities as well as the
environment at large. The PM may be harmful if inhaled
and exposure to high levels of particles has been
linked to cancer, pneumonia, chronic obstructive
pulmonary disease (COPD) and other respiratory
and cardiovascular syndromes.79–86
Almost all processes involved in silicon alloy
production produces PM in some form. In this
article, the terms particulate matter and dust are
used as synonyms and primarily used for solid
particles. The term aerosol includes both liquid and
solid particles and the term fume relates to
thermally generated aerosols. Table VII provides an
overview of the PM sources and a rough estimate
of their relative importance to indoor air PM
concentrations and to PM emissions from the
Fig. 5. Distribution of trace elements from SAF (reproduced with permission from Myrhaug and Tveit71).
Fig. 6. Distribution of trace elements from the gas-blown refining ladle (reproduced with permission from Naess et al.73).
The furnace generates most of the PM, through
combustion of escaping SiO gas from the furnace hot
zone to SiO2 above the charge surface. A typical
metal yield of between 80% and 90% means that up
to 10–20% of incoming Si-units in the furnace
escape as fumed silica. Modern ventilation and
filter systems have enabled efficient collection of
this type of dust and it even constitutes a
Share of emissions to outer
Share of indoor
air pollution (%)
Raw material transport pre-storage
Raw material handling, post-storage
Semi-closed furnace process
Casting and liquid alloy handling
Solid alloy product handling
Microsilica packing and handling
profitable by-product (microsilica). A typical
Norwegian PM emission limit is approximately 30 mg/
Nm3. The characteristics of microsilica have been
described in the literature as agglomerates of
amorphous silica spheres.71,87,88
The PM in the silicon alloy industry includes both
fine (FP) and ultrafine particles (UFP), i.e. particles
with aerodynamic diameters of <2.5 lm
and <0.1 lm, respectively. UFPs represent a rather
special case of particulate matter as the large
surface area implies higher reactivity and different
physico-chemical properties than the larger
particles.89–91 Current administrative norms as well as
other limits are established in mass concentrations,
but UFPs make little contribution to the mass
Measurements of dust and NOX above the furnace
charge and in the off-gas show strong correlations.
The SiO ‘‘combustion’’ is a highly exothermic
reaction which produces high-temperature zones locally.
In these high-temperatures zones, thermal
production is promoted (see the NOX section).26N,9O5X
It is clear from Table IV that the major sources of
PM, both inside the plant and escaping from the
plant, are those which involve the liquid alloy.
Tapping, refining, casting and other operations
where high-temperature liquid alloy is in contact
with air produces a silica fume which has many
similarities to the microsilica.26,73,78,92,93,96,97
Figure 7 shows an SEM picture and an ELPI particle
size distribution for thermally generated fume
particles from ferrosilicon tapping. Naess et al.96,98
studied the process by which this type of silica dust
forms, and concluded that the main dust formation
mechanism is the active oxidation of the liquid
silicon alloy, while a small fraction (<1%) of the dust
particles would form by splashing (droplet
expulsion).97 The active oxidation was found to occur in
two steps in which the silicon would first react with
oxygen to form SiO gas which would then oxidize
further to SiO2.97,99–101 The kinetics of this process
is governed by oxygen access to the alloy surface,
and therefore highly dependent on the dynamics of
the alloy surface exposed to the air.102,103
Depending on the gas flow rate, a refining ladle for MG-Si
generates 0.8–1.7 kg SiO2 per ton Si.
The dust from the handling and transport of solid
materials, such as the product and the raw
materials, is fundamentally different from the dust
generated by the active oxidation. It is typically
coarser, and the physical and chemical properties
depend on the material from which it was
generated. Raw material handling and transport can, for
example, produce airborne crystalline alpha-quartz
which is a health hazard in its own right. No
literature on the generation, collection and
reduction of the mechanically generated PM in high-Si
alloy smelters has been found.
EMISSION MEASUREMENT TECHNIQUES
Off-gas monitoring in MG-Si and FeSi production
is connected to a couple of specific challenges
compared to emission measurements in other
industries. The gas temperature in proximity of the
furnace is very high, as illustrated in Fig. 2 and
this constitutes a major difficulty, as described
below. Another difficulty is the high PM
concentrations before the filter. The wear on instruments
installed in particle-laden gas streams is
considerable, and material deposits on the instruments risk
completely off-setting the results obtained in such
Fig. 7. Fume from FeSi furnace tapping area. (a) Scanning electron micrograph of the fume particles. (b) Particle size distribution as measured
by an electrical low pressure impactor (reproduced with permission from Kero et al.93).
Fig. 8. Overview of the classification of measurement methods for
conditions. In addition, data handling and the
interpretation of results is made difficult by the
varying conditions caused by the industrial
The IPPC BREF documents10 offer some general
guidelines for emissions monitoring. Timing
considerations, such as averaging time and sampling/data
collection frequency, are of prime importance and
depend heavily on the processes. Hence, process
understanding is essential. Figure 8 gives an
overview of the variety of measurement methods
available for airborne emissions. A bordering field of
science is that of occupational hygiene, a topic which
is outside the scope of this article and will not be
covered here. Hence, only measurement methods
using stationary devices will be described.
The concentration of the detected pollutant is
read as a function of time with in situ,
directreading or in-line instruments. They operate in real
time and are often equipped with data logging.
Indirect instruments are samplers which collect the
pollutant over a certain time interval with
subsequent laboratory analysis. This is sometimes
referred to as ex situ analysis. Active or extractive
sampling refers to the use of a pump to draw the
polluted air into the instrument whereas passive
methods operate without alteration of the air
In addition to in situ and extractive
measurements, the materials balance (process- or
site-specific mass flow calculations) are often carried out to
estimate the emissions of, for example, heavy
metals and CO2.
To report the correct emissions of the different
components, representative flow measurement in
the off-gas channel is an essential complement to
correct concentration measurements. Different
measuring principles are used, like pitot tubes,
annubars, orifice plates, ultrasonic flowmeters and
thermal mass flow meters. Extractive measurement
techniques applied to off-gas ducts and pipes often
call for isokinetic sampling and/or dilution, which
can be extremely challenging in terms of practical
operation. Both procedures will also, inevitably,
introduce additional error sources and increase
uncertainty, especially under the non-ideal
conditions of industrial operations.105–107
Round-the-clock gas measurements are desirable,
but may be difficult to achieve in high-temperature
dusty gas streams. Most melting plants have no
chimney after the baghouse filter and, therefore, all
the gas measurements have to be done in the duct
before the filter. This is an extremely harsh
environment and continuous measurements are
therefore very challenging. Optical sensors are the first
choice, typically recommended for most continuous
industrial measurement applications, but their use
is often limited by high gas temperatures.
Instruments based on extractive principles will
automatically decrease the gas temperature (as the gas is
drawn out of the duct) which enables more
straightforward detection of gas species. The high dust load
in the furnace off-gas will, however, be very
challenging for most of the commercially available
devices. Nonetheless, these types of instruments are
frequently used in the industry today, albeit not
often for continuous measurements. The
secondbest option is systematic measurements for some
hours at a time. This is the most common industrial
approach for gas component measurements in
offgas from silicon alloy production.
In situ Measurements
In situ gas analyzers measure the gas directly
inside a duct or across an open path (0–500 m) with
very short response times. The measuring principle
is usually some form of optical spectroscopy, often
ultraviolet (UV) or infrared (IR). In smelters, the
instruments are selected based on their ability to
operate in hot, dirty and dusty gas conditions.
The tunable diode laser (TDL) has become standard
instrumentation for NO measurements at most
Norwegian plants,25,108 and can be used in off-gas ducts
prior to the baghouse filter, despite the high
temperatures and high dust concentrations. It can detect
many different gas species including NO, H2O, NH3,
HCl, and HF in a gaseous mixture if coupled with laser
absorption spectrometry (TDLAS). The advantage of
TDLAS over other techniques for concentration
measurement is its ability to achieve very low detection
limits and very short response time.
Extractive Gas Sampling Devices
Extractive techniques are acceptable for
quantification of non-reactive gas species, such as NO, CO and
CO2, as they may be allowed to cool down before
detection. Species like HCl, SO2 or H2O, however,
have to be kept at a constant, high temperature which
may be achieved by an electrically heated sample hose.
Fourier transform infrared (FTIR) devices have
been tested and proven useful in ferroalloy
industries in spite of the harsh conditions.42,43 Several
gas species may be detected simultaneously as well
as solid or liquid aerosols, but components with a
symmetrical electron binding cannot be assessed.
Gra˚ dahl et al.109 showed that an FTIR (with
inhouse analysis software) can be used over an
openpath, such as a slag-pit, in ferroalloy smelters.
For gas components with a symmetrical electron
binding (such as H2, N2 and O2), mass spectrometry
(MS), gas chromatography (GC) or Raman
spectroscopy may be used, but these methods tend to
have slow response times. Promisingly, Kjos et al.110
demonstrated the industrial relevance of a
combined GC–MS instrument in flue gas from
aluminum electrolysis and was able to detect very low
While many different measurement techniques
offer the ability to characterize and quantify the
airborne particulate matter, very few of the
commercially available instruments are tested in
industrial melting plants or validated for the
specific types of PM encountered there. PM
characteristics, such as optical properties, sizes, shapes,
density, etc., may heavily influence the
measurements, and site-specific calibrations are typically
necessary to ensure reasonable accuracy.
Airborne particulate matter is often classified by
the aerodynamic diameter (Dp) of the particles, but
the terminology is far from unambiguous. In
occupational hygiene and medicine, exposure terminology is
based on how the aerosols may penetrate the body
through the respiratory system. It is then common to
distinguish between the inhalable, thoracic and
respirable fractions where the term ‘‘respirable’’
indicates that the aerosol may penetrate the body
all the way down to the alveolar region of the
lungs.105,107 Another common and more technical
terminology for airborne particulate matter is based
on the so-called PM standards. For example, PM10
refers to all aerosols with Dp < 10 lm and PM2.5
refers to the concentration (in total mass per unit
volume of air) of particles with aerodynamic diameter
<2.5 lm. PM2.5 is a subset of PM10 and is sometimes
referred to as fine particles.104,107,111 Yet other
terminologies exist and are used in parallel with the
aforementioned ones in the literature. For instance,
Preining112 defines the terms fine, ultrafine (UFP)
and nanosized (NP) as particles with Dp < 750 nm,
Dp < 100 nm and Dp < 25 nm, respectively.
In situ PM Measurements
Passive instruments are typically long-range
instruments for measuring across a gas stream in
a duct. These are typically laser-based instruments
and other optical sensors. Such instruments have
been used in the silicon alloy industry to assess
fuming rates,78,97 and to continuously monitor PM
emissions as well as workplace atmosphere.113 For
roof measurements, Gra˚ dahl et al.113 recommends
the use of directional anemometers (able to detect
not only wind speed but also direction) in
combination with long-range, open-path devices.
Unpublished reports,114–116 however, emphasise a
need for site-specific calibration which can be
performed using gravimetric filters. A number of
filter cassettes are then mounted on wires along the
laser line-of-sight, and the mass readings of the
lasers are compared and adjusted to the dust mass
collected by the filters as a means for calibration.
Several reports indicate that, without such
calibration, the optical instruments are not reliable for
concentration measurements but may still be useful
for relative measurements for improved process
understanding and control.
Extractive PM Measurements
Extractive measurements on hot, particle-laden
or otherwise dirty off-gases typically require
dilution of the gas stream before it enters the sensitive
instrument. Dilution may, however, be challenging
in terms of practical operation and also introduces
additional error sources with significant
implications for data treatment and calibration. The
purpose of the dilution is typically two-fold: first, to cool
the gas stream to temperatures that can be handled
by the instrument; and second, to dilute the particle
concentration to a level which can be detected and/
or quantified by the instrument and/or avoid
condensation or clogging inside the instrument. The
cooling in itself introduces error sources such as
potential condensation of gases into aerosols and
deposition of substances onto surfaces inside the
A number of extractive methods for PM
quantification have been tested in the silicon alloy
industry, including gravimetric filters, optical
devices, mobility sizers and impactors. Gravimetric
filters offer a robust, cheap and simple way to
assess PM weight concentration.104,113 A standard
optical particle counter (OPC) and a condensation
particle counter (CPC) appear to be less useful in
silicon plants as they detect too-low PM
concentrations.94,113 Mobility particle sizers 117,118 and
electrical low-pressure impactors (ELPI)92–94
appear better suited for PM measurements in
silicon plants, although these instruments are
larger, heavier and more cumbersome to operate.
Data interpretation is also more challenging,
especially for the ELPI.119–121
In this literature review, current knowledge
developed in, and relevant to, the Si- and
FeSiproducing industry has been summarized. The
article is primarily based on information available
in the open literature, but some previously
unpublished reports, of utmost relevance to the topics,
have also been summarized and included. It
contains state-of-the-art overviews for gaseous and
particle-bound airborne emissions. Relevant
technological aspects for the control and reduction of
GHG, (NOX), (PAH), heavy metals and PM are
A number of research areas that need prioritized
consideration have been identified:
Emissions of GHGs other than CO2, such as
hydrocarbons. For methane, the discrepancy
between the very limited reported emission data
and emissions calculated by standard emission
factors is of the order of a factor of 10.
The use of chemical NOX reduction treatments
for SAF off-gases and the potential effect of such
treatments on the silica fume quality.
The mechanical generation of dust from
handling and transport of raw materials as well as
solid products has not been studied. Effective
methods for the collection and reduction of such
dust are needed.
Most gaseous emissions are reported and
monitored by use of emission factors. The overall GHG
emissions from FeSi and MG-Si production are
reasonably well understood and quantified, with the
exception of hydrocarbons. The extent of GHG
emissions is highly dependent on the carbon and
electricity consumption (which in turn depends on
the type of Si/FeSi alloy), the carbonaceous material
mix, charging methods and furnace operation. The
furnace design, flue gas management and furnace
operating procedures, such as stoking and charging,
heavily influence NOX emissions. Measurements
show strong correlations between PM and NOX
formation above the furnace charge. Localized
temperature control can only be achieved by limiting
the extent of silica fume production through SiO(g)
Close flue gas temperature control is extremely
important for several reasons. One reason is the
delicate trade-off between PAH and NOX
management. PAHs are destructed at high temperatures,
and PAH emissions can be significantly reduced
when off-gas temperatures are kept above 800 C.
PAH and heavy metals are simultaneously
present as gases and particulate forms, and their
distribution is highly temperature-dependent. The
particle-bound compounds are often collected in the
particulate control devices (e.g., fabric filter or wet
scrubber). The more volatile compounds, however,
will risk being emitted to the atmosphere if no
further gas treatment is applied, for example, by the
use of bag filters with adsorbent injection to remove
Hg and Cd.
It is particularly important to consider
uncertainty parameters arising from every step of the
monitoring process, yet their estimation is often less
than trivial. Accuracy and sample representability
are often limiting the trustworthiness of the
obtained data. The use of material balances are,
for example, sensitive to representability issues,
and heavy metals assessment is highly uncertain
due to the detection limits of currently available
analysis methods. For flue gas measurements,
averaging time and frequency are of prime
importance, and such timing requirements always depend
heavily on the processes at hand. Solid process
understanding is therefore essential if useful data
are to be produced.
Round-the-clock gas measurements are desirable,
but may be difficult to achieve in high-temperature
dusty gas streams. Smelter flue gas ducts present
an extremely harsh environment where sampling is
very challenging and the available instruments
must be selected based on their ability to operate
under such conditions. Dilution and isokinetic
sampling requirements may present additional
difficulties and typically increase uncertainty values. PM
measurement principles often remain to be
validated for the specific types of dust encountered in Si
and FeSi smelters. Hence, site-specific calibrations
are recommended to ensure reasonable accuracy.
Funding was provided by Norges Forskningsra˚ d
(NO) (Grant No. 237738). This article was enabled
through funding from the Research Council of
Norway through the center for research-driven
innovation (SFI) Metal Production. The authors
wish to thank Dr. Edin Myrhaug and Dr. Nils
Eivind Kamfjord at Elkem AS for comments and
This article is distributed under the terms of the
Creative Commons Attribution 4.0 International
), which permits unrestricted use, distribution, and
reproduction in any medium, provided you give
appropriate credit to the original author(s) and the
source, provide a link to the Creative Commons
license, and indicate if changes were made.
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