3
8
M. Ma et al. / Journal of Alloys and Compounds 420 (2006) 37–45
the past at $ 5 kg 1 Ti or lower, for example, when unexpected
−
Table 1
EDX determined contents of metallic elements in the original titanium-rich slag
and in the electro-reduction product at 3.1 V and 900 C for 12 h in molten CaCl2
supply emerged from countries of the former Soviet Union.)
Simple calculations show that, at a price of $ 0.05 (kWh) , the
electricity cost in the FFC Cambridge process can vary from $
.3 kg Ti to $ 1.5 kg Ti when the current efficiency changes
from100%to20%. Thetitaniamarketishoweverrelativelymore
◦
−
1
Elements
Fe
Ti
Si
Mn
Al
Ca
−1
−1
0
a
a
Original sample (wt.%)
Reduced sample (wt.%)
8.59
8.68
65.80
80.94
9.46
9.08
5.38
1.30
10.47
a
−
1
stable. The pigment grade TiO2 (rutile) costs at about $1.2 kg
a
The element was not detected in the sample by EDX.
−
1
TiO2 (or $ 2.0 kg Ti) but the various precursors used to pro-
duce the TiO2 pigment are much cheaper as discussed below.
These numbers show clearly that the cost of titania, which is
the feeding material, can be a major financial factor affecting
the industrial application of the FFC Cambridge process. This
fundamental understanding forms the basis of this work as one
part of a grand ongoing research programme.
2
. Experimental
2
.1. Materials and reagents
Metatitanic acid was collected from the vacuum dehydration stage of the
sulfate process in the Titania Plant of Panzhihua Iron and Steel Group Cor-
poration. The titania dust, a non-classified by-product from anatase produc-
tion, was collected from the floor near the rotary kiln in the same plant. The
as-received metatitanic acid and titania dust were labelled by the supplier
to contain only Ti, H and O, and the particle size was of sub-micrometers.
The titanium-rich slag, provided by Zunyi titanium plant, was manufactured
by reaction between ilmenite and coal in a closed electric arc furnace. The
composition of the slag was analysed by EDX and the results are given in
Table 1. The slag was milled for 4 h at a rotation rate of 250 r/min in a
planet ball-mill. The particle size of the obtained powder was around 1 m.
Anhydrous CaCl2 (AR grade) was purchased from Shanghai Chemical Regent
Company.
Except for a brief mention [1], all previous work [1,10–12,15]
has used the commercial pigment or reagent grade TiO2 powder
as the feeding material in the electro-reduction process, but these
TiO2 products are not sufficiently economical to meet with the
mass demand on an industrial scale. The well-crystallised TiO2
(
rutile and anatase), which has been widely used as pigment and
additives in industry, is mainly manufactured by two methods:
the chloride and sulfate processes [16]. In these processes, the
titanium concentrate (titanium enriched ilmenite, titanium-rich
slag or rutile) is first reacted with chlorine gas or sulfuric acid to
form TiCl4 or TiOSO4. The TiCl4 is subsequently converted to
TiO2 at 1300–1800 C, and the TiOSO4 is hydrolyzed to form
the metatitanic acid which is then washed, filtered, dehydrated
and calcined in a rotary kiln (900–1250 C) to form anatase or
2.2. Pellet preparation
◦
The titania precursor powder was manually pressed into small pellets (20 mm
in diameter and 2.0–3.0 mm in thickness). The titania dust and titanium-rich
slag pellets were sintered at 900 C for 2 h and gained sufficient strength for
◦
◦
rutile TiO2 depending on the operation conditions. The obtained
TiO2 isfurthertreatedtoobtainsomespecialphysicalproperties,
including dispersibility, wettability, brightness and opacity for
pigment and other applications. Obviously, further cost reduc-
tion in the FFC Cambridge process for titanium extraction is
expected by using one of the intermediate products from the
above processes as the feeding material, instead of the pigment
titania. (It should be mentioned that the sulfate route and its
intermediate products are not preferred for the Kroll process in
which the titania precursors must be first converted into TiCl4
by carbochlorination.)
handling. The porosity was 40–50%. In the same sintering process, however,
the metatitanic acid pellet cracked into small pieces, possibly due to rapid
dehydration (decomposition). However, when the sintering temperature was
raised slowly in a suitable programme, sufficiently strong pellets were pre-
pared from the metatitanic acid. A typical programme used in this work was
◦
as follow: (1) room temperature to 300 C in 1 h and holding for 4 h (to allow
◦
slow and complete dehydration), (2) 300–900 C in 2 h and holding for 2 h.
After sintering, the pellet lost weight and shrank. For example, after sintering,
the mass of a 1.50 g metatitanic acid pellet decreased to 1.14 g, the diameter
from 20 to 17 mm, and the thickness from 3.1 to 2.7 mm. The porosity was
∼60%.
2
.3. Electrolysis procedure
This paper reports our investigation on using three titania
precursors, all collected from the industry in western China
where over 0.9 billion tonnes of titanium mineral are stored, as
the feeding material to produce titanium metal or alloy via the
FFC Cambridge process (laboratory scale). The results reveal
that titanium metal or alloy can be directly extracted from
different TiO2 precursors, namely titania dust, metatitanic acid
and titanium-rich slag, disregarding the starting morphology
and crystal structure of the precursor. The oxygen and other
impurity concentrations in the obtained metals were analysed.
Of particular importance is the finding that some metallic
impurities in the titanium-rich slag, namely manganese and
aluminium, were present at much lower levels in the electrolytic
product. Typical electrolysis current–time curves are presented
and used for calculations of the current efficiency and energy
consumption, which provides the basis for in-depth discussion
in terms of an earlier proposal for improving the electrolysis
efficiency.
The sintered pellet was wrapped tightly with molybdenum mesh and wires.
This assembly was used as the cathode in constant voltage electrolysis with
a graphite rod anode (20 mm diameter, 200 mm length) in a graphite crucible
(
100 mm inner diameter and 230 mm height) containing 500 g anhydrous CaCl2
granules. The electrolytic cell was placed at the bottom of a sealable stainless
steel tube reactor in a vertical tube furnace. The electrolysis bath was heated and
◦
kept at ∼330 C in air for 24 h to remove moisture. Subsequently, high-purity
argon was admitted into the sealed reactor continuously and the temperature was
◦
increased to and kept at a prescribed electrolysis temperature (850–950 C).
Pre-electrolysis was performed at ∼2.6 V between an iron wire cathode and
the graphite anode for 4–5 h to further remove residual moisture, metallic and
other redox active impurities in the molten salt. Afterwards, the iron wire was
replaced by the titania cathode and electrolysis proceeded at 2.9–3.1 V. The
electrolysis was controlled by a computer, which also recorded simultaneously
the current–time curve. When the electrolysis was terminated, the cathode was
lifted from the molten salt, cooled in the argon stream, removed from the reactor,
washed in distilled water and dried in air. The morphology, structure and com-
position of the electrolytic products were analysed by SEM, EDX (HITACHI
x-650), XRD (SHIMADZU X-ray 6000), and Inert Gas Fusion Oxygen Analysis
(LECO RO416-DR).