Q. Ye et al. / Journal of Alloys and Compounds 613 (2014) 102–106
103
to prepare high quality reduced iron powder in the microwave
field [19–22] considering the fact that mill-scale, coal and wood-
charcoal all have a good property of microwave absorbing.
100
80
60
40
20
0
2. Experimental
2.1. Materials
The compositions of the mill-scale and wood-charcoal are shown in the follow-
ing Tables 1 and 2, respectively. The particle size distribution of the material and
wood-charcoal used is represented in Fig. 1. All these materials are milled by the
ball grinding mill. The percentages shown in full paper are weight percentages.
Mill scale
Wood charooal
2.2. Methods
A self-made microwave tube furnace, which utilizes a single-mode continuous
controllable power is utilized for all experiments and shown in Fig. 2. The micro-
wave frequency is 2.45 GHz, while the output power is controlled within the max-
imum of 1500 W. The activation temperature is controlled by varying the input
microwave power. The activation temperature is measured by nickel chrome–
nickel silicon armor type thermocouple which is in contact with the material. The
thermocouple has dimension of length of 1000 mm, 3 mm diameter, with the tem-
perature range of 0–1250 °C, and a measurement precision of 0.5 °C.
Distribution: the materials are divided into three layers. At the bottom layer is
the mixture of wood-charcoal and calcium carbonate, and at the mid-layer is the
mixture of wood-charcoal and mill-scale while at the upper layer is wood-charcoal.
Note: a, b, c and d denote the mass of the corresponding materials in Fig. 3.
Wood-charcoal at the mid layer is called inner distribution wood-charcoal and
can be defined as:
0
5
10
15
20
Particle size/um
Fig. 1. Particle size distributions of mill-scale and wood-charcoal.
M0 is the mass of a sample, M1 is the mass of Fe, M2 is the mass of zero valent
iron. Determination of total iron and metallization ratio is measured using potas-
sium dichromate: Redox indicators.
3. Results and discussion
b
inner distribution wood ꢀ charcoal ¼
ꢁ 100%
ð1Þ
The total iron of the reduced iron powder obtained (with a hold-
ing time 50 min at 1150 °C) in experiment 1 was 98.56%, and the
metallization ratio was 99.25%. Based on the analysis of XRF
(Shimadzu, XRF-1800 Sequential WDXRF), the detailed composi-
tion of iron powder is listed in Table 3. From the table, it can be seen
that the chemical composition of the reduced iron powder meets
the specification of the HY100.23 first-class iron powder standard.
Fig. 4 shows the gas composition produced by wood-charcoal at
different temperature under microwave heating in experiment 2.
Fig. 5 shows the off-gas composition produced at different temper-
ature in the microwave field under the condition that the ratio of
both the inner and outer distribution wood-charcoal is 10% in
experiment 1.
Fig. 6 is the SEM of the mixed raw material with 10% of wood-
charcoal. Wood-charcoal corresponds to the darker color, with
smooth surface and clear profile, while the rest of the portion cor-
responds to the mill-scale.
Figs. 7 and 8 are the SEM of the reduced iron powder with an
inner distribution wood-charcoal of 10%, outer distribution
wood-charcoal of 10%, and reduction temperature of 1150 °C. But
the holding time is 5 min and 50 min, respectively.
a þ b
Wood-charcoal at the bottom is named as outer distribution wood-charcoal and
can be defined as:
b þ c
b
outer distribution wood ꢀ charcoal ¼
ꢁ 100% ꢀ
ꢂ 100%
ð2Þ
a þ b þ c
a þ b
Combined distribution of wood-charcoal means the combination of the inner
distribution wood-charcoal and outer distribution wood-charcoal. If c = 0 g, d = 0 g
and the reductant is carbon, the stoichiometric content of reductant required for
reduced completely of mill-scale is 15.78% [20].
2.3. Design of experiments
Experiment Process: Charge as Fig. 2. Subsequently, pass N2 for 30 min, start the
microwave, stop passing N2. Then start the experiment according to the process.
Experiment 1: a = 27 g, b = 3 g, c = 3.75 g, d = 2.5 g. Namely, both the percent-
ages of the inner distribution wood-charcoal and the outer distribution wood-char-
coal are 10%. The gas released at 550 °C, 850 °C, 950 °C, and 1150 °C was collected
by displacement of water and the gas composition was analyzed by gas chromatog-
raphy. The reduction temperature was set at 1150 °C and the holding time were
5 min and 50 min, respectively. Then the products were examined by SEM, and
the total iron and metallization ratio of the sample reduced by 50 min were
measured.
Experiment 2: a = 0 g, b = 30 g, c = 0 g, d = 0 g. This means that the material used
in this experiment is only the reduced agent, wood-charcoal. The components of the
gas released by wood-charcoal during the heating process were collected and
measured.
The reduced iron powder was prepared in microwave field by
combined distribution of wood-charcoal with the holding time
50 min (using Höganäs process-ring-like charging method, heavy
oil was used as fuel and the reduction period was 172 h). The
chemical composition of the products meets the HY100.23 first-
class iron powder standard. There are four reasons accounting for
this.
M1
M0
total iron ¼
ꢁ 100%
ð3Þ
ð4Þ
M2
M1
metallization ratio ¼
ꢁ 100%
Table 1
Compositions of mill-scale (total iron 74.25%) (%).
FeO
Fe2O3
36.36
Fe3O4
1.61
SiO2
MnO
0.30
P
S
CuO
0.12
SnO2
0.09
CaO
Cr2O3
0.029
61.31
0.096
0.014
0.017
0.032
Table 2
Compositions of wood-charcoal (%).
Fixed carbon
72.58
Volatile organic matter
22.26
H2O
4.67
CaO
0.40
FeO
MgO
S
K2O
0.069
Na2O
0.01
0.014
0.0028
0.0005