Titanium obtaining through electrochemical reduction of titanium dioxide in chlorinated media.

Tarcolea, Mihail ; Cotrut, Mihai Cosmin ; Ciuca, Sorin 等

1. INTRODUCTION

A variant of FFC Cambridge process (Fray, 1998) for direct production of Titanium is presented. The declared intention is to replace Kroll process (Kroll, 1940).

The method consists in reduction of Ti oxide through an electro-deoxidation and electro-reduction process, with molten Calcium chloride as electrolyte.

The cathode is a pressed-sintered disc, consisting in Ti dioxide powder.

The established method is based on the following physical and chemical considerations:

--CaO solubility in Calcium chloride is very high (about 20% moles), making possible the electrolysis process and cathode deposited Calcium reduces the metal oxide that constitutes the cathode (Fray et al., 1998).

--decomposition voltage of CaO is less than that of Ca[Cl.sub.2], hereby only CaO electrolysis determines oxygen liberation in anode (eventually with C[O.sub.2] and CO forming), avoiding gaseous Chlorine liberation. Concomitantly, by Oxygen consuming during electrolysis, through dissolved CaO re-establishment (Ca solubility in Ca[Cl.sub.2] is about 4% mol), the chemical composition of the electrolyte maintains quasi-constant, with a favorably influence on the electrochemical process.

--high affinity of Ca for Oxygen allows the advanced reduction of the Oxygen content in cathode, towards level of ppm, enabling high quality alloys obtaining.

The procedure bases are the flowing electrochemical processes, expressed as characteristic reactions (Chen et al., 2000; Chen & Fray, 2000; J.F Suzuki et al., 2003, Suzuki, 2005):

--deoxidation of Ti[O.sub.2] through contained Oxygen ionization as a result of applied voltage on the electrodes.

1/2[O.sub.2] + [2.sub.e.sup.-] = [O.sup.2-] (1)

2Ca[Cl.sub.2] + [O.sup.2-] = 2CaO + 2[Cl.sup.-] (2)

[Me.sub.x][O.sub.y] + [2.sup.e-] = xM + y[O.sup.2-] (3)

--calciothermic reduction of the oxides through the action of deposited Ca on cathode, as a result of the dissolved CaO electrolysis in Ca[Cl.sub.2] electrolyte.

[FIGURE 1 OMITTED]

CaO = [Ca.sup.2+] + [O.sup.2-] (4)

2[O.sup.2-] [right arrow] [O.sub.2] (gas) + 2[e.sup.-] (5)

[O.sub.2] + C = C[O.sub.2] (or CO) (6)

yCa + [Me.sub.x][O.sub.y] = xMe + yCaO (7)

2. EXPERIMENTAL PROCESS EVOLUTION

The experimental installation for electro-reduction process (fig. 1) consists mainly in a crucible (Silicon and Aluminum oxy-nitride) that contains the electrolyte (Calcium and Sodium chloride), having an anode (super dense graphite) and a cathode (pressed/sintered disc of Titanium dioxide to be reduced). All of them are included in a heating furnace to assure the necessary thermal conditions.

The electrochemical process develops in two stages (fig. 2). In the first stage the electric potential is 2.6 V (less than CaO decomposition voltage); during this stage current intensity constantly decayed (fig. 2). In the second stage the voltage was increased to 3 V (less than Ca[Cl.sub.2] decomposition voltage, to avoid gaseous Chlorine appearance), with an initial current intensity of about 3.7 A. The time current intensity decay was more abrupt in the second stage (fig. 2). Entire process develops at 850[degrees]C. The main process parameters are listed in Table 1.

[FIGURE 2 OMITTED]

3. RESULTS

After electrolyte removal and disc extraction from cathode holder, disc was washed in a slightly acid solution (pH = 2+4) and physical, chemical and structural analyzed. Dimensional measurements of discs before the electrochemical reduction process are presented in Table 2.

Chemical analysis on samples assayed from plane surfaces of discs, in depth towards 1 mm, (Table 3), demonstrates the success of this direct obtaining method; the Titanium content can be raised further, by raising the cathode discs porosity.

SEM and EDAX analysis of the sintered cathode after electrolysis was performed to reveal the structural aspects (fig. 3) and to determine phase composition on the analyzed surface (fig. 4) for the sample C. It is also obvious the advanced reduction of Titanium oxide to metallic Titanium.

5. DISCUTION

In the first stage of the electrochemical process, the current intensity decay can be explained through the corresponding decay of dissolved Oxygen in melted electrolyte by its discharging at anode. The same phenomenon, in the second stage of the electrochemical process, can be explained through the raise of the ions quantity in the melted electrolyte, as a result of dissolved CaO decomposition.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The experimental data demonstrate the possibility to obtain high purity Ti through an electrochemical process of deoxidation--reduction of Titanium oxide as follows:

--deoxidation of metal oxide through Oxygen ionization, developed at voltages about 2.0 / 2.5 V,

--calciothermic reduction of the oxide through the action of Ca deposited at the cathode as a result of electrolysis of the dissolved CaO into electrolyte, at voltages of 2.7 / 3.0V.

Through successive deoxidation processes, starting from Oxygen ionization from oxide, followed by its reduction with Calcium originated from the electrolysis of dissolved CaO in the electrolyte, the duration of alloy obtaining is reduced to about 4 hours, comparing with minimum 15 hours necessary to obtain the alloy only by deoxidation (Chen et al., 2000).

Oxygen and Calcium diffusion into electrolyte bath and sintered oxides, respectively, can be accelerated by decreasing height of sintered cathode and increasing cathode porosity. These precautions can improve the reduction process kinetics, with its duration decreasing.

4. CONCLUSIONS

In contrast to classical technologies for Titanium (Kroll, 1940), this experimental process presents spectacular advantages: drastic adjustment of ample operations number, of process duration, and of production costs through decrease of energetic and raw materials consumption.

This new technology is ecological (the electrochemical process develops at voltages lower than 3.2V, corresponding to Ca[Cl.sub.2] decomposition potential), without insult emission (Chlorine and chlorinated compounds, dust, etc).

Our method is based on cathode porosity control. This is the major difference from other authors' experiments: the use of porous sintered oxide cathode allows improving Titanium purity, process duration minimizing and a better process parameters control.

This procedure can be used other refractory metals and alloys direct obtaining. We already produced Ti6Al4V, following a similar procedure starting form Ti, Al, and V oxides.

5. REFERENCES

Chen, G.Z., Fray, J.F., (2000), Novel Direct Electrochemical Reduction of Solid Metal Oxides to Metal using Molten Calcium Chloride as the Electrolyte, Euchem 2000 Proceeding, Denmark, pp 157-161

Chen, G.Z., Fray, D.J., Farthing, T.W., (2000), Direct electrochemical Reduction of Titanium Dioxide to Titanium in Molten Calcium Chloride, Nature, vol. 407, pp. 361-364

Fray, D.J., Farthing, T.W., Chen, G.Z., (1998), Removal of Oxygen from Metal Oxides and Solid Solutions by Electrolysis in a Fused Salt, UK Patent, PCT/GB 99/01781, Inter. Pub. No. WO 9964638

Kroll, W.J., (1940), The Production of Ductile Titanium, Trans. Am. Electrochemical Society, vol. 78, pp. 35-47

Suzuki, R., (2005), Calciothermic Reduction of TiO.sub.2 and in situ Electrolysis of CaO in Molten CaCl.sub.2, Journal of Physics and Chemistry of Solids, volume 66, pp. 461-465

Suzuki, R., Teranuma, K., Ono, K., (2003), Calciothermic Reduction of Titanium Oxide and in-situ Electrolysis in Molten Ca[Cl.sub.2], Metallurgical and Materials Transactions B, volume 34 B, pp. 277-285

Tab. 1. Process parameters for samples A, B, and C.

 Distance
Sample Stage Potential, Current A-C, Duration,
 V intensity, A mm min

 A I 2.6 3.20 / 0.20 20 160
 II 3.0 3.75 / 0.30 200

 B I 2.6 3.30 / 0.25 15 160
 II 3.0 3.80 / 0.35 190

 C I 2.6 3.45 / 0.25 10 160
 II 3.0 3.95 / 0.35 180

 Cathode
 (pressed/ Density,
 sintered Surface, g/ Porosity,
Ti[O.sub.2]) [cm.sub.2] [cm.sub.3] %

 A 6.25 2.33 44.20
 B 6.25 2.34 44.05
 C 6.20 2.38 42.85

Tab. 3. Chemical composition of cathodes after
calciothermic reduction process.

Sample Chemical composition, % weight

 Ti O Ca Cl Fe

 A 95,00 0,70 1,49 2,61 0,20
 B 95,30 0,75 1,23 2,46 0,26
 C 94,43 0,74 1,56 2,98 0,29