# Carbon Tubular Morphologies in Blast Furnace Coke

Authors: Stanislav S. Gornostayev; Jouko J. Härkki
Journal: Research Letters in Materials Science
 (2008)

---

## Abstract

The paper reports on the first occurrence of microscale carbon tubular morphologies (CMTs) in a blast furnace (BF) coke. The CMTs were probably formed as a result of the conversion of solid disordered carbon via liquid phase metal particles involving a gas phase containing a substantial amount ofN2 and O2. The presence of CMTs may lie behind the generation of the smallest fraction of fines in BF exhaust dust. If the amount of CMTs present in the BF exhausts gases at any particular metallurgical site proves to be substantial, it could become a subject of environmental concern.


---

## Body

## 1. Introduction

Carbon tubular morphologies are mostly known as
nanotubes and microtubes, implying internal diameters in the nanometer or micron range,
respectively. Carbon nanotubes, which can be single (SWNT) or multiwalled
(MWNT), have many
properties that make them potentially useful in a wide variety of
applications, including composites, electronics, pharmaceuticals, and optics. They are also known for the
environmental and health risks attached to them [1–6]. SWNTs can be synthesized by arc
discharge, laser oven, and chemical vapor deposition techniques, and one
feature common to all these procedures is that metal catalysts play an
important role in their growth [7]. The most popular model for SWNT formation is “the vapor-liquid-solid model, in
which carbon from the vapor phase is deposited onto liquid-phase metal carbide
particles, from which the nanotubes then grow” [8]. There is also a solid-liquid-solid (SLS) model, which suggests the conversion of solid disordered carbon into
nanotubes via liquid phase metal particles [9]. As for MWNTs, arc evaporation produces the best-quality tubes, by three types of
mechanism, referred to as “gas,” “solid,” and “liquid” phase models [8].
“The gas phase models assume that nanotube nucleation and growth occur as a
result of direct condensation from the vapor, or plasma, phase. In solid phase
models, the nanotubes  and nanoparticles do not grow in the arc plasma, but
rather form on the cathode as a result of a solid state transformation. A
variant of the solid state model envisages a “crystallization” of nanotubes
from extended assemblies of disordered carbon, while the liquid phase model
assumes that nanotubes nucleate within globules of liquid carbon deposited onto
the cathode” [8]. It was also found that heat treatments could produce micron-length
SWCNTs at about 1000–1300C° [8].The features and
mechanisms of formation of carbon microtubes (CMTs) are characterized in
literature to a lesser extent than those of SWNTs and MWNTs. They are known as “ideal
candidates for microfluidic applications such as micropumps, flow channels and
nano-/microreactors” [10]. CMTs with controlled conical angles can be
synthesized in a microwave plasma
CVD chamber at 600–1100 W and a reactor pressure of 40–90 Torr,
for example, by spreading of a thin
film of gallium onto a graphite substrate followed by dusting with molybdenum
powder using different N2/O2 dosages [10]. This paper
represents the first description of microscale carbon tubular
morphologies found in samples of
blast furnace coke and discusses possible mechanisms for their formation.


## 2. Samples and Methods

Metallurgical coke, a key
material for BF operation, is made from mix of several coals by
heating it
to1100–1200C° in coke
ovens consisting of two walls spaced about 40–45 cm apart. The coke
is charged into the BF along with the iron ores, various fluxes, and scrap and
takes part in solid-to-solid, solid-to-melt, and solid-to-gas reactions that
occur inside the BF. The coke located in a BF is referred to as BF coke. The temperature
at a tuyere level, where hot air is blown into the BF, exceeds 2000C°.A number of samples of BF coke from two drill cores (30424109 and 130303203) were studied. The samples were
obtained from the tuyere zone of an operating BF at Ruukki Steel Works in Raahe, Finland, using a mobile tuyere rig [11]. They were cut under dry conditions (no cooling
water used in sawing), preserving one original surface, and sections about 35–40 mm long, 20–25 mm wide, and
c. 5–7 mm thick were
then fixed to glass plates. These dry-cut sections were studied with a stereomicroscope
and then with a Jeol JSM-6400 scanning electron microscope (SEM) equipped with
an energy-dispersive spectrometer (EDS). Two of the samples which contained
CMT-like (CMT thereafter) carbon tubular morphologies are described here, the first selected from
a point 35 cm from the tuyere level (drill core 30424109),
and the other from 70 cm away (drill core 130303203).


## 3. Results and Discussion

Three CMTs
were found in two samples, two in sample 30424109-35 and one in sample
130303203-70. Stereo microscopy revealed that the sample surfaces hosted a
number of droplets of molten iron of diameter from 0.1 to 5 mm, some of the
droplets being surrounded by a 0.5–1.5 mm zone of
graphite crystals. SEM/EDS studies indicated that the iron droplets were
covered by a thin carbon shell that caused a dull metallic luster in a
stereomicroscopic view.The first CMT, in sample 30424109-35,
was attached by its long axis to the (0001) face of a flake-like graphite
crystal (see Figure1) in which stacks of graphitic micro- and nanosheets were rotated along [0001]. The tube was c. 40 μm in length
and about 13-14 μm in diameter,
but with the diameter decreasing asymmetrically to end in a cone. The angle
between the end of the tube and its axis was about 26–30°.
The formation of a conical angle may reflect local fluctuations in the N2/O2 ratio in the air blown into the
tuyere zone of the BF, since variations in nitrogen and oxygen dosing are known
to have an effect on the conical angles of CMTs during their growth [10].The SEM image of CMT attached by its long axe to (0001) face of a flake-like graphite
crystal, sample 30424109-35.
(a)(b)The second CMT-like particle was again observed in sample
30424109-35, where it was also attached to the 0001 face of a flake-like
graphite crystal (see Figure2) representing part of a complex aggregate of such
crystals. This tube was about 30 μm in length and 10 μm in diameter and had a
“cut” cone on its top and a cylindrical base. The middle part of the tube and the
cone at its “bottom” had different orientations of their axes, probably also a
result of changes in the N2/O2 ratio during particle growth.
The particle was spatially associated with aluminosilicate spherules attached
to the (0001) face of the graphitic host.The SEM image of CMT-like particle with
a “cut” cone on its top and a cylindrical base, sample 30424109-35.
(a)(b)The third CMT was identified in sample 130303203-70, where
it was located between two graphite flakes with an undeveloped hexagonal prism
and had several aluminosilicate spherules attached to its surface (see Figure3). The
“low” end of the tube was conical in shape, but the other was obscured by the
graphite crystal. The diameter of the middle part of the tube was about 2 μm
and the length of the (visible part of the) tube was
about 12–14 μm.The SEM image of CMT located
between the two graphite flakes, sample 130303203-70.
(a)(b)The relationships between the CMTs and the graphite
crystals in the samples suggest that the CMTs are “younger” (formed later) than
their graphitic hosts. The close association of the CMTs with the graphite
crystals appears to be not only spatial but also genetic in character and may
be related to the catalytic effect of molten iron or some other metal present
in the primary coke and inherited from the coking coals. Details of the
appearance of the graphite and a model for its formation in the BF coke have
been discussed recently [12], together with data on earlier experimental
studies [13]. The formation of graphite in BF coke seems to be a result of iron
melt penetration through an unordered carbon matrix, initiating dissolution-precipitation
sequences which leave the well-ordered graphitic carbons behind. The
temperature estimates for this process were all around 1500C° [13],
well below graphitization temperatures, due to the catalytic effect of molten
iron. The present data are insufficient to allow an unequivocal explanation to
be given for the origin of the CMTs in these samples. Taking into account the
data on graphite growth in BF coke [12, 13], it would be reasonable to suggest
that the conditions of CMTs formation were comparable to those reported by Gorbunov et al. [9] for the conversion
of carbon into SWCNTs via liquid phase metal particles (the solid-liquid-solid model
discussed above). The authors of the model suggested that “the first stage
involves a molten catalyst nanoparticle penetrating a disordered carbon
aggregate, dissolving it and precipitating carbon atoms at the opposite
surface. These atoms then form a graphene sheet, whose orientation parallel to
the supersaturated metal-carbon melt is
not energetically favorable. Any local defect of this graphene sheet will,
therefore, results in its buckling and the formation of a SWCNT nucleus” [9]. Thus,
the “graphitic” model [13] and the model for CMT formation [9] have a number of
features in common, and it seems that graphite crystals and CMTs are products
of different  parts of (or stages in)
one metal-catalyzed process. Among the metals, which are known for their
catalytic effect for nanotube formation, are, for example, Fe, Ni, and
Co [8, 9]. As we have reported earlier, these metals are common constituents of
the BF coke together with Al, Mg, Ca, Zn, and Mn [14]. Many other metals,
including Co and Sn, could affect this process in the BF since these elements
present in complex iron ores and scrap used for ironmaking.The formation of CMTs may lie behind the generation
of the smallest fraction of fines in the BF
exhaust dust, in the
same way as we suggested earlier for the tiny graphite crystals formed in BF
coke [12]. This is because CMTs can easily be captured by the
circulating BF gas flows, thus contributing to the total amount of BF-generated
dust. If the amount of CMTs in the BF exhaust gases proves to be
substantial, it could be a
subject of environmental concern, since the BF process in general is a
large-scale process and this is still one of the most common methods of iron
production in the world and one of the major industrial polluters. Similar
reasoning can be applied to the combustion of other types of coke (pitch coke, petroleum coke). Successful examples of dust management in pyrometallurgical
processes by mechanical and magnetic separation have been published earlier [15],
but it has not been reported so far whether CMTs can be captured in this way.
In the case of the Ruukki Steel Works at Raahe, Finland, the average annual amount of all inhalable particles
(under 10 μm) in the air as monitored at three
stations near the factory and in the city of Raahe was in the range 16.1–19 micrograms/m3 in
2006 [16]. This is well below the average annual limit of 40 micrograms/m3 [16], and of course CMTs must
account for only a fraction of this amount. Nevertheless, further research is needed to clarify this
issue with respect to more representative set of samples of BF cokes from
different sites, since the ability to produce CMTs can depend on the reactivity
and composition of a particular coke in the BF process and the process
conditions themselves.


## 4. Concluding Remarks

(i) CMT-like carbon tubular morphologies can be formed in a BF coke.
(ii) They are associated with graphite crystals and are formed later than their
graphitic hosts. (iii) The appearance of CMTs and associated graphite seems to be the result of a metal-catalyzed process involving a gas phase
containing a substantial amount ofN2 and O2. (iv) The presence of CMTs may lie behind the generation of the smallest fraction of fines in BF
exhaust dust. If the
amount of CMTs present in the BF exhaust gases at any particular metallurgical site proves to
be substantial, it could become
a subject of environmental concern. Modern iron production facilities can control this form of pollution, for example, by installing monitoring stations
to measure the amounts of inhalable particles (under 10 μm) in theair.



---

*Source: 751630-2008-04-30.xml*