J. Am. Ceram. Soc., 98 [12] 3666–3669 (2015)
DOI: 10.1111/jace.13987© 2015 The American Ceramic Society

JournalRapid Communication

Iron Deposition from Aluminosilicate Containing Trace Amount of Iron Oxideby Borate-Enhanced Hydrogen Reduction
Isao Tsuyumoto†Department of Applied Chemistry, College of Bioscience and Chemistry, Kanazawa Institute of Technology, 7-1 Ohgigaoka,
Nonoichi, Ishikawa 921-8501, Japan
conditions in details, and the analysis of the metallic irondeposit.

Aluminosilicate containing 0.50 wt% of iron oxide as impurity
yields metallic iron deposit by heating with sodium borate(3Na2OB2O3) in H2 at 950°C, while it remains unchanged byheating without the borate under the same condition. The SEMand EDX measurements indicate that the metallic iron deposit
is formed on the sodium borate glass layer. We first find thatthe reduction in minor constituents in oxides is enhanced by
the addition of borate as a flux.

I.

M

II.

Experimental Section(1) Preparation
Sodium borate at Na/B molar ratio of 3.0 was prepared bydissolving 11.0 g of NaOH and 5.7 g of H3BO3 into 1 L ofwater and drying it at 115°C in air for 24 h. The dried residue consisted of amorphous sodium borate, NaBO22H2O
and Na2CO3H2O, and these act as a sodium borate ﬂux
above its melting point around 600°C–700°C. The powder
sample of AS was prepared by crushing and grinding a crucible boat (CB, Nikkato, Osaka, Japan) in a tungsten carbide
mill. The composition from X-ray ﬂuorescence (XRF) analysis is shown in Table I. The XRD pattern of AS indicatesthat it consists of mullite (3Al2O32SiO2, PDF-4 00-006-0259)
and an amorphous phase. A slight amount of iron oxide at
0.50 wt% was considered to exist as a solid solution in AS.
Ordinary portland cement (OPC; The Japan Cement Association, Tokyo, Japan) was also used for the sample containing
a slight amount of iron oxide as a solid solution. The ironoxide at 2.00 wt% in OPC was contained in the cement mineral phase, C4AF (4CaOAl2O3Fe2O3), as a solid solution.
The iron oxides in these samples are considered to be notintentionally added in production processes, but originallyderived from their raw materials. The powder samples ofsodium borate and AS were mixed at a weight ratio from 1:3
to 3:1 in agate and mortar, and heated at 950°C in 100% H2
for 2 h. In the heating process, we used a crucible boat madeof alumina (Al2O3) with high purity 99.7% (ACE-997; TGK,Tokyo, Japan) in order to avoid contamination.

Introduction

ETAL deposition by reduction in metal oxides has beena fundamental and essential process in metallurgy
from ancient times. Ore reﬁning is generally performed above
the fusion temperatures of the metal and gangue,1 and thus
it would be desirable for energy conservation to produce
metals below their melting points. The reduction to metals
thermodynamically occurs even below the melting points on
the condition that the reactants and products satisfy ΔG < 0
at the temperature, but kinetically the reaction rates in actual
reactions are extremely small below the melting points
except for the case with ﬁne particles.2 This is because thedeposition occurs only locally in the vicinity of grain surfaces and remains without aggregation.3 Reduction kinetics
of iron oxides by H2 has been extensively studied for theenergy conservation and CO2 emission reduction,4–7
While many reports have appeared on the reduction inpure iron oxides such as hematite, magnetite,8–10 there havebeen no reports on the reduction in trace iron oxides in solidsolutions. It is highly desirable to enhance the reduction
kinetics of trace elements in solid solution matrix, and thiswill lead not only to the energy conservation in ore reﬁning,
but also to selective reduction in trace elements existing as asolid solution in metal complex oxides. This will be beneﬁcial
for separation and recovery of valuable metals from urban
waste and for extreme puriﬁcation to remove some trace
metals.11
In this study, we ﬁrst found that the metallic iron wasformed from 0.50 wt% of iron oxide existing as a solidsolution in aluminosilicate (AS) by heating with sodium
borate (3Na2OB2O3) in H2 at 950°C. Heating AS without
borate under the same condition brought about no changeon the sample except for a slight color change, andthus the reduction from solid phases was enhanced by theaddition of borate as a ﬂux. We report here the reducing(2) Characterization
The powder XRD patterns were measured with a diﬀractometer (XD-D1; Shimadzu, Kyoto, Japan) using graphitemonochromatized CuKa radiation (k = 0.15406 nm). The Xray proﬁles were collected between 5° and 120° of 2h angles
with a step interval of 0.02°. The morphology and element
distribution were observed by a scanning electron microscope(SEM) in back - scattered electron (BSE) mode equippedwith an energy dispersive X-ray spectrometer (EDX; Swift
ED3000, Hitachi, Tokyo, Japan) without carbon coating.
The compositions were determined by a wave-dispersive
XRF spectrometer (XRF-1700; Shimadzu). Resistance wasmeasured coarsely at a distance of 2 mm by the DC two probe measurement.

A. Makaya—contributing editor

III.

Results and Discussion

The powder mixtures of sodium borate and AS at a ratio
from 1:3 to 3:1 turned black by heating at 950°C in 100%

Manuscript No. 37158. Received July 5, 2015; approved September 19, 2015.
†Author to whom correspondence should be addressed. e-mail: tsuyu@neptune.
kanazawa-it.ac.jp

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Table I. Compositions of the Aluminosilicate (AS) and theOrdinary Portland Cement (OPC) Determined by the XRF
Analysis
Component

SiO2
Al2O3
K2O
Fe2O3
CaO
Na2O
MgO
TiO2
CoO2
SO3
P2O5MnO
ZnO

AS (wt%)OPC (wt%)

59.01
37.35
2.14
0.50
0.32
0.21
0.19
0.19
0.03
—
—
—
—

21.58
6.74
0.66
2.00
63.26
0.27
2.42
0.24
—
2.37
0.17
0.06
0.05

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(a)

(b)

(a)

Fig. 2. SEM-BSE images of the deposits with black color (a) andmetallic luster (b).

(b)

(c)

(d)

Fig. 1. Appearances of the products obtained by heating thepowder mixture of sodium borate and AS at a ratio of 1:1 (a), 1:2(b) and 1:3 (c) at 950°C in 100% H2 for 2 h, along with that byheating the AS alone (d) under the same condition.

H2 for 2 h, and furthermore, the mixtures at a ratio from 1:1
to 3:1 yielded silver - colored deposit with metallic luster(Fig. 1). The silver - colored products were formed among
black products on glass phase, which was newly generated
on the boat. The AS powder without the addition of sodium
borate showed only a slight color change from light gray togray when heated at the same condition. The silver-coloreddeposit showed a small resistance approximately from 2 to30 Ω, while the resistance of the black product varied widely,depending on the measurement point, from 30 Ω to over20 MΩ. The gray product from AS alone showed a resistance over 20 MΩ. These results indicate that the formationof the conductive deposit from the raw materials is enhancedby the addition of sodium borate.
Figures 2(a) and (b) show the SEM - BSE images of theconductive deposits obtained by heating the mixture ofsodium borate and AS at a ratio of 1:1. Figure 2(a) is theblack deposit with a resistance ranging widely from 1 k to32 kΩ, and Fig. 2(b) is the deposit with metallic luster witha resistance approximately from 3 to 14 Ω. Figures 3(b)–(g)are the element distribution maps for O, Na, Al, Si, Fe,and Co, respectively, along with the BSE image of thesame area in Fig. 3(a). Table II shows the EDX point analysis of the marked areas in Fig. 3(a). The conductive
deposits with the metallic luster proved to be Fe and Co,and these metallic deposits were considered to be formedon oxide glass phase containing Al, Si, Na, and B. The
resistance diﬀerence between the areas of Figs. 2(a) and (b)was due to the diﬀerence in the amount of the deposits,and the percolation of the deposits brought about by thelow resistance. The tungsten and carbon detected in themetallic deposit in Table II were derived from the tungsten
carbide used in the crushing and grinding process. Figure 4
shows the powder XRD pattern of the conductive deposit
removed with the glass phase from the boat. A sharp peak
due to 110 reﬂection of the metallic iron (Fe, PDF-4 04003-3884) was observed with a broad peak from the glass
phase, and this also conﬁrms the formation of metalliciron. In short, a small amount of oxide impurities, 0.35%
Fe and 0.02% Co, in AS was reduced and concentrated toa metallic deposit of 87.1% Fe and 0.4% Co by heatingwith sodium borate at 950°C in H2. We ﬁrst found thatthe sodium borate acts as a ﬂux and enhances the reduction in metal oxide solid solutions to metals. Furthermore,
the same phenomena were also observed for the OPC. The
powder mixture of sodium borate and OPC at a ratio from
3:1 to 5:1 formed a mixture of black powder and small
metallic grains by heating at 950°C in 100% H2 for 2 h,and the products showed a low resistance in the range
from 1.3 to 1700 Ω. The OPC without the addition of
sodium borate showed only a slight color change and aresistance over 20 MΩ after heating at the same condition.
Small amounts of iron oxide in a matrix of oxides, existing
as a solid solution, were easily separated from the matrix
by heating in H2 with the sodium borate ﬂux. This suggeststhat iron can be extracted from iron ores at 950°C much
lower than the conventional reﬁning processes and in gen-3668

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Fig. 3. EDX element distribution maps of the conductive deposit for O (b), Na (c), Al (d), Si (e), Fe (f), and Co (g), along with the BSE image (a).

eral that the reﬁning temperatures can be reduced by using
the borate ﬂux. It can be expected that the technique will
be applied to the other metal elements by controlling temperature and gas concentration based on Ellingham diagrams, which show the temperature dependence of ΔGof metals and metal oxides. Our ﬁndings will be usefulfor the separation of valuable or hazardous metals in a

recycling process as well as the reduction in the reﬁning
temperatures.

IV.

SummaryA small amount of iron oxide existing as a solid solution inaluminosilicate or OPC was found to deposit as metallic iron

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Table II. EDX Point Analysis of the Deposit. Point 1 isLocated at the Conductive Area and Point 2 at theNonconductive Base Material. Both Points are Marked inFig. 3(a)ElementCONaAlSiFeCoW

Point 1 (wt%)

Point 2 (wt%)

4.4
0
1.2
0.4
0.1
87.1
0.4
6.3

3.2
53.1
18.4
8.2
14.5
1.5
0
0.2

sodium borate ﬂux. We expect that this technique will beapplied to the ore reﬁning processes at lower temperatures, e.
g., at 950°C for iron, and to the separation of trace metals invarious processes such as recycling, puriﬁcation, and materials synthesis based on the thermodynamic control of thedeposition process.

Acknowledgments
The author is sincerely grateful to Mr. T. Mitsui, Mr. Y. Noto, Mr. T. Yamato, and Mr. K. Kameda for experimental support. This work was supported
by a Grant-in-Aid for Scientiﬁc Research (C), No. 25410244, from the Japan
Society for the Promotion of Science (JSPS).

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