Pressing and sintering of Ti[O.sub.2] powder cathode used to obtain titanium by electrochemical reduction.

Roman, Costel ; Carcea, Ioan ; Chelariu, Romeu 等

1. INTRODUCTION

Titanium and especially titanium alloys find usage in different industries such as aeronautics, chemical, medical, metallurgical, naval etc. At the present days, the applications development of titanium-based metallic materials is limited due to its high cost (Nagesh& Ramachandran 2007, Nie et al. 2006).

Now, the most used commercial process to produce titanium is Kroll process (Qiu et al. 2008), which is very expensive one with low efficiency. Thus, the long-term success of the titanium-based materials remains to find new efficient solutions to produce titanium with a minimum impact on the environment. Taking into consideration this, in the last years there have been tested, especially in laboratories, new extraction processes of the metallic titanium such as: Ti[Cl.sub.4] electrolysis in alkaline chloride mixtures or directly from Ti[O.sub.2] through the electrolysis in melted salts, usually calcium chloride (Liu et al. 2007). The most important variants of the electrochemical reduction are FCC, OS, QIT, MER Corporation, BHP Billiton process, respectively. Among these, FCC process seems to have the most success chances at commercial level (Wang & Li 2004).

This paper describes the pressing/sintering process of Ti[O.sub.2] powder to make the cathode. Afterwards, the cathode will be used into an electrochemical reduction process that is presented hereinafter.

The experimental procedure for titanium extraction uses an installation whose principle scheme is shown in fig. 1. It consists of electrolysis cell, resistance electric furnace, DC source, inert gas injection system, evacuation device of the melt salt. The resistance electric furnace (l) with a power of 4,5 kW and the chamber made of ceramic provides a maximum heating temperature of 1200[degrees]C. The electrolysis cell is mounted inside the heating furnace. The crucible is made of silicon and aluminum oxinitride, a resistant material to high temperatures and chemical degradation of the melted electrolyte. It has the inner diameter of 68 mm and a height of 100 mm. At the bottom of the ceramic crucible was made a circular hole for mounting cathode's support (7) made of refractory steel. In order to insulate, the space delimited by metallic shell-ceramic crucible-cathode support (2) it was filled with a mixture made of oxides and silicon nitride.

[FIGURE 1 OMITTED]

Cell's anode (5) made of high density graphite has a cylindrical shape with the dimensions of [PHI] 54 mm and height of 80 mm. Into the anode there are two horizontal and one vertical holes in order to assure a good eviction of the anodic gases and to improve electrolyte's circulation. Anode bar (3) is made from two stainless steel concentric tubes jointed by a tronconic bush. Thus, it allows adjusting anode-cathode distance and to collect and evict anodic gases and the heat insulating material. The used electrolyte is a mixture of Ca[Cl.sub.2] + NaCl + CaO. Before to be used, the Ti[O.sub.2] powder cathode have been passed through a pressing/sintering process. It has a cylindrical shape with [PHI] 30 mm and height between 5-10 mm. Dimensions are adequate to the used electrochemical cell.

Beside electrolyte's properties, we consider that the geometrical shape and cathode's characteristics are equally important for the success of the electrochemical reduction process. That is why, in the paper, we present some aspects regarding the implications of the pressing/sintering process on apparent density and electric resistivity of the Ti[O.sub.2] powder cathode.

2. MATERIALS AND METHODS

The cathodes were made of Ti[O.sub.2] powder (Merck, Germany) having chemical reagent purity and the granulation between 0.1-10.0 um. The stages of pressing/sintering process are given in the following. In order to eliminate the possible humidity, Ti[O.sub.2] powder was dried in air at a temperature of 200[degrees]C for 2 hours. After weighting the necessary mass, the Ti[O.sub.2] powder was pressed. The die used to make the cathodes with the desired dimensions and shape, was designed to bear forces till 500 kN and to remove cathode after pressing operation.

A press with maximum pressing force of 750 kN assured the pressing force. The press is drove with a hydraulic driving system having the maxim pressure of 1000 mbar, and it is equipped with force and displacement transducers making possible pressing process monitoring.

To record parameters values of the pressing process one computer with interface for data acquisition was used.

Pressing time was 5 minutes for each cathode. After die removing, each cathode supported sintering process in two stages (T = 400[degrees]C, t = 2 h, T = 900[degrees]C, t = 8 h) by using Vulcan 550 resistance electric furnace. After furnace cooling at room temperature, the cathodes were weighted and dimensionally verified in order to determine apparent density. Apparent density was determined by calculation of the ratio between cathode's weight and the volume that corresponds to measured dimensions.

The resistivity was determined with relation [rho] = RA/l where R is electric resistance ([OMEGA]) of cathode, A is contact area ([cm.sup.2]), l is the length between measurement points of the resistance (cm).

3. RESULTS AND DISCUSSIONS

The pressing force, pressing time, cathode's weight, dimensions and apparent density are indicated in the table 1, and the dependency [D.sub.a] = f(F, m) are graphically showed in fig. 2.

The values of electric resistivity of the experimentally achieved cathodes at room temperature are indicated in table 2.

The dependence of electric resistivity of cathodes (6g weight) on apparent density both before and after sintering is graphically shown in fig. 3.

4. CONCLUSION

To keep the integrity of cathode to dry the Ti[O.sub.2] powder before pressing is necessary. Also, for the same reason two stages of the sintering process must be used. The pressing force of 75 kN is adequate to obtain minimum electric resistivity of sintered Ti[O.sub.2] powder, and a reasonable value of apparent density. The validation of this choice remains to be demonstrated by subsequent electroreduction experiments.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

5. REFERENCES

Liu, M., Lu, S., Kan, S., Li, G. (2007) Effect of electrolysis voltage on electrochemical reduction of titanium oxide to titanium in molten calcium chloride, Rare Metal, Vol. 26, no. 6, 547-551, ISSN 1001-0521.

Nagesh, CR., Ramachandran, CS. (2007). Electrochemical process of titanium extraction, Trans. Nonferrous Met. SOC. China, Vol. 17, no. 2, 429-433, ISSN 1003-6326.

Nie, X., Dong, L., Bai C., Chen D., Qiu G. (2006). Preparation of Ti by direct electrochemical reduction of solid Ti[O.sub.2] and its reaction mechanism, Trans. Nonferrous Met. SOC. China, Vol. 16, no. 3, 723-727, ISSN 1003-6326.

Qiu, G., Feng, X., Liu, M., Tan, W., Liu, F. (2008). Investigation on electrochemical reduction process of [Nb.sub.2][O.sub.5] powder in molten Ca[Cl.sub.2] with metallic cavity electrode, Electrochimica Acta Vol. 53, no. 12, 4074-4081, ISSN 0013-4686.

Wang, S., Li, Y. (2004). Reaction mechanism of direct electro-reduction of titanium dioxide in molten calcium chloride, Journal of Electroanalytical Chemistry, Vol. 571, no. 2, 37-42, ISSN 0022-0728.

Table 1. Variation of apparent density in terms of pressing
force and cathode's weight.

Pressing Cathode's Cathode's Cathode's Apparent
force, F weight, m diameter, height, density,
 [kN] [g] [cm] [cm] [D.sub.a]
 [g/[cm.sup.3]]

 50 6 2.95 0.49 1.77
 50 8 2.98 0.63 1.81
 75 6 2.98 0.45 1.92
 75 8 2.96 0.62 1.85
 100 6 2.97 0.45 1.95
 100 8 2.98 0.61 1.87

Table 2. Electric resistivity values of Ti[O.sub.2] cathodes

Pressing Cathode's Apparent
force, F weight, m density,
[kN] [g] [D.sub.a]
 [g/[cm.sup.3]]

50 6 1.77
50 8 1.81
75 6 1.92
75 8 1.85
100 6 1.95
100 8 1.87

 Electric resistivity,
Pressing [M[OMEGA] x cm]
force, F
[kN] Before After
 sintering sintering

50 77 [+ or -] 7 18 [+ or -] 2
50 86 [+ or -] 10 20 [+ or -] 3
75 43 [+ or -] 7 14 [+ or -] 1
75 68 [+ or -] 5 78 [+ or -] 12
100 51 [+ or -] 12 15 [+ or -] 2
100 76 [+ or -] 7 209 [+ or -] 50