CA2363648A1 - A method for the continuous electrowinning of pure titanium metal from molten titanium slag, ilmenite and other semiconductive titanium oxide compounds - Google Patents

Abstract

This invention relates to a method for the continuous electrowinning of pure titanium metal from titanium slag and other electrically semiconductive titanium mixed oxide compounds particularly ilmenite. The method involves crushing and grinding a mass of crude titanium slag, separating magnetically the major impurities in the ground slag, melting the purified and conductive titanium slag at elevated temperature, pouring the molten titanium slag at a bottom of an electrolytic cell to form a pool of liquid cathode covered with a layer of molten salt electrolyte. Reducing cathodically by direct current electrolysis at high temperatures the molten titanium slag to produce droplets of pure liquid titanium metal that fall by gravity at the bottom of the electrolytic cell forming after coalescence a pool of liquid titanium metal. The liquid titanium metal is continuously siphoned or tapped under an inert atomosphere and cast into a dense, coherent, and pure ingot.

Description

A METHOD FOR THE CONTINUOUS ELECTROWINNING OF PURE
TITANIUM METAL FROM MOLTEN TITANIUM SLAG, ILMENITE AND
OTHER SEMICONDUCTIVE TITANIUM OXIDE COMPOUNDS
TECHNICAL FIELD
This invention relates to a method for the continuous electrowinning of pure titanium metal from titanium slag and other electrically semiconductive titanium mixed oxide compounds particularly ilmenite.
BACKGROUND ART
Titanium metal has been produced and manufactured on a commercial scale since the early 1950s for its unique set of properties : (i) high strength-to-weight ratio, (ii) elevated melting point, and (iii) excellent corrosion resistance in various harsh chemical environmentst. Actually, about 55% of titanium metal produced worldwide is used as structural metal in civilian and military aircraft and spacecraft such as jet engines, airframes components, and space and missile applicationsz.
Titanium metal is also employed in the chemical process industries (30%), sporting and consumer goods ( 14%), and in a lesser extend power generation, marine, ordnance, architecture, and medical3. Titanium sponge which is the primary metal form of titanium is produced industrially by the Kroll's process which is a metallothermic reduction of gaseous titanium tetrachloride with magnesium.
Potential huge markets such as automotive parts are still looking forward to seeing the cost of the primary metal to decrease by 50-70%. Nevertheless, this cost is only maintained high due to the expensive metallothermic process used to win the metal.
Even if the Kroll's process was greatly improved and modernized since its first industrial introduction it still exhibits several drawbacks : ( 1 ) it is performed under ~ CARDARELLI, F (2001 ) - Materials Handbook : A Concise Desktop Reference. -Springer-Verlag, London, New York, pages 115-135.

2 GAMROGI, J. - Titanium and Titanium Dioxide - from - Mineral Commodity Summaries.- U.S. Bureau of Mines (1995) p.180.

3 GAMBOGI, J. - Annual Report : Titanium-1992 - U.S. Bureau of Mines ( 1993) p batch conditions leading to expensive downtimes, (2) the inefficient contact between reactants leads to slow reaction kinetics, (3) it requires the preparation, purification, and use the volatile and corrosive titanium tetrachloride (TiCl4) as the dominant feed with its associated health and safety issues, (4) the process can only accept expensive rutile or ruble substitutes (e.g., upgraded titanium slag, synthetic ruble) as raw materials, (5) the magnesium and chlorine must be recovered from reaction products by electrolysis in molten salts accounting for 6% of the final cost of the sponge, (6) the specification of low residual oxygen and iron content of the final ingot requires expensive and complex refining steps (e.g., vacuum distillation, and/or acid leaching) of the titanium sponge in order to remove entrapped inclusions accounting for about 30% of the final cost of the ingot, finally (7) it only produces dendritic crystals or powder requiring extensive reprocessing before usable mill products can be obtained (i.e., remelting, casting, forging) and wastage of 50% is common in fabricating titanium parts. For all these reasons, since the early 1970s there is a strong commitment of the titanium industry in synergy with several academic institutes for actively researching new routes to produce titanium metal with a focus on developing a continuous process to produce high-purity and low-cost titanium powder for metallurgical applications. Although a plethora of alternative methods have been examined beyond a laboratory stage or have been considered for preparing titanium crystals, sponge, powder, and alloys, none have reached the industrial production. Included in those processes were : (i) gaseous and plasma reduction, (ii) tetraiodide decomposition, (iii) calcio- and aluminothermic reduction, (iv) disproportionation of TiCl3 and TiCl2, (v) carbothermic reduction, and (vi) electrowinning in molten salts. Most were considered by the authoring National Materials Advisory Board committee (NMAB)4 panel to be unlikely to progress to production in the near future except electrowinning which seemed to be the most promising alternative route.
Actually, 4 NATIONAL MATERIALS ADVISORY BOARD Committee on Direct Reduction Processes for the Production of Titanium Metal. 1974. Report # NMAB-304, National Academy of Sciences, Washington, DC.

the extraction and preparation of pure metals from ores using an electrolytic process is known as electrowinning. This process is based on the electrochemical reduction of metal cations present in a suitable electrolyte by electrons supplied by a negative electrode (i.e., cathode, -) according to the Faraday's law of electrolysis, while at the positive electrode (i.e., anode, +) an oxidation reaction occurs (e.g., anode dissolution, gas evolution, etc.). Today among the current industrial electrolytic processes several utilize an aqueous electrolyte to electrodeposit the metal (e.g., Cu, Zn, Ni, Pb, Au). Unfortunately, aqueous electrolytes which exhibit a narrow electrochemical span are unsuitable for preparing highly electropositive metals such as titanium. Actually, when cathodic (i.e., negative) potentials are applied to the electrode, the competitive process of the electrochemical reduction of protons occurs accompagnied with the evolution of hydrogen gas. This main parasitic reaction consumes the major part of the reduction current decreasing drastically the overall current efficiency. Despite the availability of cathode materials exhibiting a large hydrogen evolution overpotential (e.g., Cd, Hg, Pb), it is quite impossible to electrodeposit such metals despite numerous attempts reported in the literature about the electrodeposition of titanium metal from aqueous solutionss 6 7 g 9. prganic electrolytes were also testedlo II I2 but despite their wide decomposition potential limits, organic solvents in which an appropriate supporting electrolyte has been dissolved have not yet been used industrially owing to their poor electrical conductivity which increases ohmic drop between electrode 5 KOLTHOFF, M., and THOMAS, J. - J. Electrochem. Soc. 9( 1964) 1065.

6 SINHA, N.H., and SWARUP, D. -Indian Mining J. Spec. 1(1975)134.
KUDRYATSEV, V.N., LYAKHOV, B.F., ANUFRIEV, W.G., and PEDAN, K.S. - Hydrog.
Met. Proc. Int. Cong.
2nd. Pergamon Press, Oxford 1977, page 5.
g SOFRONKOV, A.N., PRVII, E.N., PRESNOV, V.N., and SEMIZOROV, N.F. -Zh. Prikl.
Khim. 51(1978)607 9 BRIBIESCA, S.L., CONTRERAS, E.S., and TAVERA, F.J. -Electrowinning of Titanium from Sulfuric Acid Titanium Solutions. -Proc. Titanium'92, The Minerals, Metals, and Materials Society 1993, pages 2443-2444.
~° BIALLOZOR, S., and LISOWSKA, A. -Electrochim. Acta 25(1980)1209.
~ ~ LISOWSKA, A., and BIALLOZOR, S. - Electrochim. Acta 27( 1982) 105.
~Z ABBOTT, A.P., BETTLEY, A., and SCHIFFRIN, D.J. -J. Electroanal. Chem.
347(1993)153-164.

gap, the low solubility of inorganic salts, their elevated cost and toxicity.
By contrast, molten salt based electrolytes were already used industrially since the beginning of the 1900s in the electrolytic preparation of important structural metals (e.g., Al, Mg), and in a lesser extend for the preparation of alkali and alkali-earth metals (e.g., Na, Li, and Be). Actually, fused inorganic salts exhibit numerous attractive features 13 14 over aqueous electrolytes, these advantages are as follows ( 1 ) they produce ionic liquids having a wide electrochemical span between decomposition limits (i.e., high decomposition potential) allowing the electrodeposition of highly electropositive metals such as titanium. (2) Based on the Arrhenius law, the high temperature required to melt the inorganic salt promotes fast electrochemical reaction kinetics suitable to increase hourly yields. (3) The faradaic efficiencies are usually close to 100%. (4) Due to their ionic state molten salts possess a high electrical ionic conductivity which minimizes the ohmic-drop and induces lower energy consumption. (5) The elevated solubility of electroactive species in the bath allows to utilize high solute concentrations allowing to operate at high cathodic current densities. Therefore, the best route for electrowinning titanium is to develop an electrolytic process conducted in molten salt electrolytes.
However, despite the numerous attempts performed until today there are still no current electrolytic processes in molten salts for producing titanium metal industrially. In order to reach industrial success the new electrochemical route must solve the major issues of the expensive and labor intensive Kroll's process and also overcome the pitfalls that have lead to unsuccess until today and abandon of all industrial electrolytic pilot plants.
Actually, several attempts were made in the industry to produce primary titanium metal by an electrolytic process. First of all, in 1956 at the former U.S.
Bureau of Mines (USBM) in Boulder City, Nevada a small pilot was built to ~3 DELIMARSKII, Iu.K., and MARKOV, B.F. (1961) - Electrochemistry ofFused Salts. - Sigma Press Publishing, New York.
" LANTELME, F., INMAN, D., and LOVERII~1G, D.G. (1984) - Electrochemistry-I, in Molten Salt Techniques, LovERING, D.G., GALE, R.J. (Eds.) Vol. 2., Plenum Press, New York, pp. 138-220.

investigate the electrowinning of titanium's. It consisted of a 12-inch cylinder vessel lined with pure iron containing the molten electrolyte made of a mixture of LiCI-KC1 approximately at the eutectic composition with TiCl2 added. Three equally spaced openings in the cell top accomodated : (i) the replaceable anode 5 assembly, (ii) the titanium tetrachloride feed unit, and (iii) the cathode.
Three slide valves combined with air-locks allowed the quick and easy introduction or removal of assemblies without contaminating the cell. The desired solute (i.e., TiCl2) was produced in-situ either by the chemical reduction of stoichiometric amount of TiCl4 with titanium metal scrap or by direct electrochemical reduction of TiCl4 at the cathode. Actually, TiCl4 which is a covalent compound does not ionize and must be converted to a ionic compound such as TiCl2. The concentration was then increased by operating only the feed cathode and anode and feeding one mole of TiCl4 per two faradays of charge. In all cases gaseous TiCl4 was introduced into the bath close to the cathode with a feed nickel tube plated with molybdenum and dipped below the surface level of the melt. In order to avoid the oxidation of the newly formed Ti2+ and dragout of the dissolved TiCl4 with the chlorine evolved at the anode, a porous ceramic diaphragm made of alundum~ (i.e., 86 wt.% A1203-12 wt.%Si02)~6 surrounded the immersed graphite anode forming distinct anolyte and catholyte compartments. The optimum operating conditions identified were : ( 1 ) an operating temperature above 500°C to prevent the precipitation of K2TiF6, and below 550°C to avoid severe corrosion of the alundum diaphragm, usually 520°C, (2) a solute content comprises between 2 and 4 wt.% TiCl2, (3) a cathodic current density of 1 to 5 kA.lri Z, while the anodic current density was comprised between 5 and 10 kA.rri 2, (4) a diaphragm current density of 1.5 kA.rri 2. By conducting experiments with the above conditions USBM claimed that high-purity titanium was electrowon with a Brinell hardness as low as 68 HB and a current efficiency of ~5 LEONE, O.Q., KNUDSfiN, H., and COUCH, D.E. - High-purity titanium electrowon from titanium tetrachloride. -J. O. M. 19(1967)18-23 60%. However frequent failures of the diaphragm that became periodically plugged or loaded with titanium crystals proved troublesome. As the titanium content increased, the ceramic diaphragm became conductive and then acted as a bipolar electrode and had to be removed rapidly from the bath. In 1972, the same authors"
build a larger rectangular cell containing 226.8 kg (i.e., 500 1b.) of bath in order to use assess the actual performances of two kind of diaphragm materials : (i) solid materials composite diaphragms, and (ii) loose fill materials composite diaphragms.
For solid diaphragms it was observed that alundum coated nickel screen showing no deterioration but however it was subject to the same current density limitation as the porous alundum diaphragm. On the other hand, cemented coated nickel screen with loose fill material such as alumina was the best material in terms of strength, flexibility, resistance to corrosion, and low replacement of titanium (0.2 to 1.0 wt.%).
In 1968, Priscut8 of the Titanium Metal Corporation (TIMET) has designed and operated a new electrowinning cellt9 in Henderson, Nevada. This electrolytic cell was a unique pilot based on a non diaphragm basket cathode type. The cell used a suspended central metal basket cathode with sixteen anodes peripheral to the basket. The central basket cathode was a cubic box with the four sides made of perforated steel plates, while the bottom and top were blind plates. Four steel rods were used in the basket to act as cathode collectors while TiCl4 was fed using a tube positionned at the center of the basket. TiCl4 intially was fed at a low rate into the center of the basket walls. This porous sidewall deposit served as a diaphragm to keep the reduced TiCl2 inside the basket while a mechanical system for '6 COUCH, D.E., LEONE, O.Q., LANG, R.S., and BLUE, D.D. - Evaluation of diaphragm materials for electrowinning high-purity titanium. Proc. Extractive Met. Div. Symp., Met.
Soc. RIME Chicago IL
December 11-13 1967, pp. 309-323.
" LEONE, O.Q, and COUCH, D.E. (1972) - Use of Composite Diaphragms in the Electrowinning of Titanium.
- Report Investigation #7648, U.S. Dept. of Interior, Bureau of Mines, Washington D.C.
'8 PRISCU, J.C. - Symp. on Electrometallur~, Proc. AIMS Extractive Metallurgy Div, Cleveland Ohio, December 1968, page 83.
'9 TIMET - An electrolytic cell for electrolysis of titanium tetrachloride. -French Pat. 1,496,806 Aug. 24, 1966.

withdrawing the large cathode deposits into an inert-gas-filled chamber, installing a new cathode, and reclaiming the inert gas for reuse. The average valence of dissolved titanium canons was maintained very low generally no greater than 2.1 to obtain the electrodeposition of premium-grade titanium metal. TIMET claimed that later models of pilot-plants have produced until 363 to 408 kg (i.e., 800 to 900 1b.) of titanium metal in one cathode deposit.
Later in 1971, Hashimoto et al. have published a series of three comprehensive articles2° ZI z2 regarding the electrowinning of titanium from its oxides or mixed oxides. Titanium solute was introduced in a molten fluoride bath, as a solid compound such as Ti02, CaTi03, FeTi03, or MgTi03. The melts tested were CaF2, MgF2, BaF2, NaF and their mixtures. The first electrolysis study was conducted at temperatures above 1600°C with a graphite anode and cathode. Only in the cases of the CaF2-Ti02 (1-10% wt.) and CaF2-CaTi03 (10% wt.) systems molten titanium was obtained but largely contaminated by carbon and oxygen (2-wt.%). In other cases, fine titanium powder was obtained. After the preliminary results, they focussed on the electrowinning of titanium from pure Ti02 carried out in molten salt baths made of CaF2, BaF2, MgF2, CaF2-MgF2, CaF2-NaF, CaF2-MgF2-NaF, CaFz-MgF2-NaF2, and CaF2-MgF2-SrF2 at 1300-1420°. The titanium electrodeposited in CaF2 and BaF2 baths was considerably contaminated by carbon owing to graphite electrodes. In NaF-containing fused salts such as CaFz-NaF
and CaF2-MgF2-NaF, only fine powdery deposits were obtained due to simultaneous sodium reduction. In the baths of MgF2, CaF2-MgF2, CaF2-MgF2-BaF2, and CaF2-MgF2- SrF2, dendritic deposits were obtained. They pointed out that best result was obtained in the CaF2-MgF2 bath, but the purity of the deposit was not as high as z° HASHIMOTO, Y.; URII'A, K.; and KOIVO, R. - Electrowinning of titanium from its oxides. Part I. Fused salt electrolysis at temperatures above the melting point of the metal. - Denki Kagaku 39(6) (1971)516-522.
z' HASI-IImoTO, Y. - Electrowinning of titanium from its oxides. Part II.
Influences of fluoride salt baths on fused-salt electrodeposition of titanium metal from titanium dioxide. - Denki Kagaku 39(12) (1971)938-943.
z2 HASHIMOTO, Y. - Electrowinning of titanium from its oxides. Part III.
Electrowinning of titanium from titanium dioxide or calcium titanate in calcium fluoride-magnesium fluoride molten salt baths. - Denki Kagaku 40(1) (1972) 39-44.

that of the common grade titanium sponge required in the industry. In the third article, electrowinning of titanium was carried out in CaF2-MgF2 (50-50 wt.%) molten salt baths at 1020-1030°C in an argon atmosphere by using a completely enclosed cell. In electrowinning from Ti02, the form of the electrodeposited metal changed from crystaline to spongelike with an increase in current density, or cell voltage, but when CaTi03 was used, deposits were spongelike. The hardness of the deposits was influenced by the bath temperature of the last stage of electrolysis and by the duration of electrolysis at the given temperature. The material yield of titanium was superior to 95 wt.% but do not meet the requirements of commercial sponge. Moreover, Hashimoto used a graphite basket to contain the titanium compounds at the anode because it was unable to make a conductive soluble anode.
Later in 1980, the Dow Chemical Company in a close working relationship with the HOWMET group (i.e., subsidiary of the French Pechiney Ugine Kuhlmann Group) founded the D-H Titanium Company for producing continuously high-purity electrolytic titanium at Freeport, Texas. Cell design, operating procedure, metal quality, proposed production, and economic projections have been described by Cobel et a1.23. The technology utilized was based on the cell designed by the U.S. Bureau of Mines, and on the previous work done at Dow by Juckniess et a124. Actually, an alleged major cell improvement in the D-H Titanium design was the fabrication of a metal screen diaphragm that was electroless-plated with cobalt or nickel to give the required electrical and flow characteristics. The cell operated with argon blanketed LiCI-KC1-TiCl2 (ca. 2 wt.% TiCl2) at 520-600°C
molten salt electrolyte. TiCl4 was fed continuously into a pre-reduction cathode compartment where reduction to dichloride TiCl2 takes place at a separate feed cathode within the cell. Final reduction to metal is continuously done on separate deposition cathodes which are periodically removed hot into an inert atmosphere z3 CosEl., G.; FISHER, J.; and SNYDBR, L.E - Electrowinning of titanium from titanium tetrachloride: pilot plant experience and production plant projections 1969-1976. - Conference:
Titanium '80, Science and Technology, Vol. 3, Kyoto, Japan, 19-22 May 1980 TMS/AIME, Warrendale, Pa.

stripping machine and then replaced within the cell. Metal-working cathodes were individually pulled, stripped, and replaced in the cell, in an argon atmosphere, by a self positioning and automatically operated mechanical device. A sealed, argon-sheilded hopper containing the titanium crystals and entrained electrode was cooled before being opened to discharge its contents. Crystalline metal and dragout salts were crushed to 3/8-inch size and leached in dilute 0.5 wt.% HC1 solution.
Then the spent solution was neutralized with a mixture of Li2C03 and KOH in a ratio equivalent to that used in the electrolyte. Dragout of electrolyte varied with the titanium crystal sizes to about 1 kg per kg of fine titanium for coarse washed metal.
Dragout was dried and passed over a magnetic separator, and metal fines were removed by screening to about 80 mesh (177 gm). They claimed that the sponge produced exhibited both a low residual oxygen, nitrogen, iron and chlorine content, had a Brinell hardness of 60 to 90 HB and excellent melting characteristics.
According to Cobel et a1.25, the direct current required for electrowinning ( 17.4 kWh/kg) appears to be only about half that required for the Kroll process.
Although titanium sponge of excellent purity was claimed to be produced in relatively small pilot-plant cells with a daily titanium capacity of up to 86 kilograms per day the electrowinning of titanium was far from the industrial scale. Unfortunately, in December 30th, 1982, according to the American Metal Market (AMM), the expenses for completing the joint program and the economic climate at that time have forced the dissolution of the D-H Titanium Company. With the breakup each company (i.e., Dow and Howmet) Dow has continued some research and development work on the electrolytic process but without success while Howmet apart some work done in France and applied for patent coverage2627 have later focused in the metal fabrication area.
za JucKNIESS, P.R., and JotINSON, D.R. - Method for Electrowinning Titanium. -U.S. Pat. 4,118,291;
October 3 (1978).
zs MAY COBEL G., FISHER J., and SNYDER L. (1980) - Electrowinning of Titanium from Titanium Tetrachloride. - 4th International Conference on Titanium, May 1980, Kyoto, Japan.
zs A~ANO, M. - Process for the Preparation of Titanium by Electrolysis. - U.
S. Pat. 4,381,976, May 3 ( 1983).

In 1988, the Italian company Elettrochimica Marco Ginatta S.p.A. (EMG) owned by the Italian scientist and business man Marco Vincenzo Ginatta2g applied for patent on a new electrowinning process largely inspired from the three previous pilots of USBM, TIMET, and DH29. This new upgraded process for the electrolytic 5 preparation of titanium uses always the dissolution and cathodic reduction of titanium tetrachloride in an electrolyte made of alkali or alkaline-earth metal halides and the electrodeposition of the dissolved titanium cations. The process was supported by RMI Titanium, and the company built a pilot plant. Ginatta claimed that the current production capacity of this plant reached 70 tonnes per year in 10 19853°. Unfortunately, in 1990 RMI closed the plant owing to inability to solve "engineering issues". More recenty, the same company, now renamed Ginatta Torino Technology (GTT) applied for patent on a new process for electrowinning titanium based on the recovery of the molten metal using a pool cathode like for aluminium. The main idea of Ginatta is to avoid common dendritic electrodeposits by producing the electrodeposited titanium metal in the liquid state such as for aluminium. Marco Ginatta continues to pursue chloride process and has built a pilot in Torino, Italy.
Simultaneously, in the period 1997-2000 Kawakami et a1.31 have proposed an electroslag remelting process. The main idea was to avoid common dendritic electrodeposits by producing the electrodeposited titanium metal in its liquid state.
Direct electrowinning of liquid titanium metal was the investigated techniques by using a direct current Electro-Slag Remelting (i.e., DC-ESR) apparatus. A
small 2' ARMAND, M. - Novel Apparatus and process for the TiCl4 feed to electrolysis cells for the preparation of titanium. - U.S Pat. 4,396,472, August 2 (1983).
z8 GINATTA, M. V.; ORSELLO, G.; and BERRUTI, R. - A method for the electrolytic production of a polyvalent metal and equipment for carrying out the method. - PCT Int. Appl., 33 pp. WO
8910437 (1990).
Z9 GINATTA, M.V.; and ORSELLO, G. - Plant for the electrolytic production of reactive metals in molten salt baths. - Eur. Pat. Appl. EP 210961 Apr. 02, 1987.
3o pIMARIA, E. - RMI Gets License to Make New Type of Titanium. - Metalworking News, February I S'., 1988.
3~ KAwAIC.4MI, M.; OOISHI, M.; TAKE1JAKA, T.; and SUZUKI, T. - The possibility of electrowinning of liquid titanium using ESR apparatus. - Proc. Int. Conf. Molten Slags, Fluxes Salts '97, Sth (1997) 477-482 Iron and Steel Society, Warrendale, PA.

scale DC-ESR unit of 110 mm inner diameter was operated in d.c. reverse polarity mode, where a graphite rod was used as anode and a steel or a copper base-plate was used as cathode. The used slag was Ca0-CaF2-Ti02 mixture. The current was approximately 1.5 kA. Under certain experimental conditions, some amount of titanium was electrodeposited in the metal pool. From the view point of heat balance, the sufficient heat was supplied by the Joule heating in molten slag phase.
It can be seen from these results that the present process is possible in principle but unfortunately most of the deposit was obtained as TiC and the current efficiency for the reduction was only 1.5%. In 1999, the process was improved32, the current efficiency for the reduction was up to 18% with the proper distance between the electrodes. Some amount of titanium was electrodeposited on the base-plate though its state changed with the electrolytic condition. Pure titanium metal pieces were obtained in the solidified salt after the run with the bigger distance. It was concluded that the electrowinning of liquid titanium metal by the present process was possible if sufficient heat to form a metal pool can be supplied at the bigger distance between the electrodes. The DC-ESR process was patented in 1988 and reconducted in 2000, and then recently presented at ECS33.
Recently in 2000, based on early results obtained by Fray, Farthing, and Chen34 3sat the Dept. of Materials Science of the Cambridge university, and later on early trials that were conducted and patented36 3~at the Defence Evaluation and 32 TAKENAKA, T.; SUZUKI, T.; ISHIKAWA, M.; FUKASAWA, E.; and KAWAKAMI, M. -The new concept for electrowinning process of liquid titanium metal in molten salt. -Electrochemistry 67(6) (1999) 661-668.
33 TAKENAKA, T.; ISHIKAWA, M.; and KAWAKAMI, M. - Direct electrowinning of liquid titanium metal by using direct current electro slag remelting apparatus. - Proc. Electrochem.
Soc. (Molten Salts XIL) (2000) 99-41 578-584.
3a CHEN, G.Z, FRAY, D.J., and FARTHING, T.W - Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride - Nature 407 (2000) 361-364.
ss FRAY, D.J., FARTHING, T. W., and CHEN, G.Z. - Removal of Oxygen from Metal Oxides and Solids Solutions by Electrolysis in a Fused Salt. - British Pat. Appl. WO 99/64638 16 December 1999.
36 WARD-CLOSE, C. M., and GODFREY, A.B. - Electrolytic reduction of metal oxides such as titanium dioxide and process applications. - U.S. Pat. Appl. W001/62996, 20 Feb. 2001.
3' WARD-Cl.osE, C. M., and GODFREY, A.B. - Method of Manufacture for Ferro-Titanium and Other Metal Alloys Electrolytic Production. - U.S. Pat. Appl. W001/62994, 19 Feb. 2001 Research Agency (DERA) at Farnborough (Hampshire, U.K.) a new company British Titanium (BTi) has been formed to commercialize the newly discovered process of refining metallic titanium3g. The inventors have proposed an entirely novel electrolytic route, that the scientific litterature has already called the Fray's or FFC process, the process claims to avoids conversion of the titanium dioxide to the tetrachloride or the dissolution of the feedstock into a molten electrolyte.
The inventors have demonstrated at the laboratory scale that the reduction reaction proceeds at 950°C from a cathode made originally of solid Ti02 while oxidation of oxygen anions occurs at the graphite anode with evolution of carbon dioxide.
The molten salt electrolyte selected is made of pure calcium chloride (CaCl2) bath which exhibits both a high solubility for oxygen and excellent migration transport properties for oxygen anions. According to inventors, the process for the production of pure titanium metal consists of the following sequences of operations. The titanium dioxide powder is mixed with an appropriate binder to form a past or slip, and cast into a rectangular shape cathodes using one of the techniques common in the ceramic industry, such as rolling or slip casting.
The green cathode will be then fired in an air kiln to initiate sintering in order to produce a solid ceramic material. After sintering the shapes give solid cathodes.
Reduction of titanium occurs in an enclosed electrolytic cell with inert gas filling.
The cell is designed for continuous operation with cathodes at different stages in their cycles being inserted and removed through an automated air lock. By controlling the cathode potential, oxygen can be removed from titanium dioxide allowing to leave behind a high purity metal which is morphologically similar to the Kroll's sponge. The cell voltage is roughly 3 V, which is just below the decomposition voltage of CaCl2 (3.25 V at 950°C), avoiding chlorine evolution at the anode but well above the decomposition voltage of Ti02 (1.85 V at 950°C).
Sufficient overpotential is necessary to reduce the oxygen content of the titanium 38 Financial Times December 21 st, 2000, page 12.

metal. The inventors claim39 that stoichiometric mixture of other metal oxides with Ti02 into the original cathode are also concurrently reduced to metal leading to the possibility to produce valuable titanium alloys although the microstructure may not be the same. The process has been demonstrated in a bench-scale reactor (i.e., kilogram per day). Whilst there are obvious difficulties to be overcome in scaling-up the vessel, primarily in reducing the diffusion path for oxygen through the metal leading to several hours to completely reduce a porous pellet made of sintered Ti02, and increase the extremely low energy efficiency. It is a single-stage process, since the waste CaCl2 can be removed of the titanium by water leaching after the completion of the reaction. They claim that perating costs will be somewhat reduced by lower labour requirements, and by the elimination of the vacuum distillation stage. Unfortunately, the specific energy consumption of the process is not provided and the process requires pigment-grade TiOz or synthetic Ti02, such as upgraded titanium slag, though the impurities will report to the finished metal; or on mixtures of oxides.
The Fray's process4° claimed that it overcomes several of the issues encountered by its predecessors but its poor energy efficiency, its extremely low hourly yield related to the slow diffusion kinetics of oxygen into titanium metal, and the mandatory use of a graphite anode are important pifalls for a future commercial development. Actually, the electrodeposited of titanium metal at the surface of the cathode impedes the diffusion of oxygen anions and represent the rate determining step in the overall process. Moreover, the use of a graphite anode leads to an elevated anode overpotential, leading to high cell voltage and thus elevated specific energy consumption. Finally, the anodic oxidation mechanisms leads to severe corrosion of the graphite with contamination of the melt by floating 39 FRAY, D.J., FARTHING, T. W., and CHEN, G.Z. - Removal of Oxygen from Metal Oxides and Solids Solutions by Electrolysis in a Fused Salt. - British Pat. Appl. WO 99/64638 16 December 1999.
ao CHHN, G.Z, FRAY, D.J., arid FARTHING, T.W - Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride - Nature 407 (2000) 361-364 flakes of graphite leading to short circuits between electrodes. In addition the evolution of gases such as carbon dioxide and chlorine gives the so-called "anode effect" at elevated anodic current densities.
Therefore, the present invention would seek to resolve most if not all of the previous issues related to the electrolytic production of the titanium metal by using an anode and a cathode made of titanium slag such as the Sorelslag~ and Richard's Bay Slag~ which are produced at QIT and Richard's Bay by the reduction of ilmenite with anthracite by smelting into an electric arc furnace (EAF).
Actually, titanium slag is mainly composed of sub-stoichiometric titanium oxides with the typical Andersson-Magnelli crystal structure4~ having the global chemical formula Ti02_X, (e.g., Ti305, Ti40~, and Ti509) and its melting range is comprised between 1640-1660°C. This composition was already confirmed by the work of Desjardins42. Hence, these oxides exhibit both a good corrosion resistance and a low electrical resistivity similar to that of pure graphite (i.e., as low as 630 ~S2.cm) and their mechanical and electroconductive properties can be compared to those of the electrode material named Ebonex~43 and produced commercially by the British company Atraverda Ltd. since the 1980s.
The major benefits to use a soluble anode made of titanium slag are : ( 1 ) the anodic dissolution of the titanium slag feed continuously providing the electrochemical reactor with titanium electroactive species avoiding the tedious introduction of a solute (i.e., the electrode acts as an electroactive species reservoir) and hence preventing the anodic evolution of chlorine or carbon dioxide. (2) The cell operating voltage being maintained extremely low resulting in a low specific energy consumption. (3) Finally, the more active impurities (e.g., Si, Ca, Mg) 4~ ANDERSSON, S., COLLEN, B., KUYLENSTIERNA, U., and MAGNELLI, A. Acta Chem.
Scand. 11(1957)1641 az pESdARDINS, J. F. ( 1986) - Study of Insoluble Formation in Richards Bay Slag. - QIT Internal Report No R-I1-86.
as CARDARELLI, F (2001 ) - Materials Handbook : A Concise Desktop Reference. -Springer-Verlag Ltd., London, pages 329-330 contained in the titanium slag remaining dissolved in the bath while the more noble (e.g., Fe, V, Mn, and Cr) not being dissolved and being recovered in the sludge at the bottom of the cell (i.e., electrorefinning). While the major benefits of a molten titanium slag cathode is its low viscosity allowing the droplets of liquid titanium 5 metal to fall by gravity to the bottom of the cell and forming a pool of liquid titanium acting as a current collector and that does not impedes the oxygen diffusion allowing rapid reduction of the titanium slag. In addition conducting the electrolysis in inorganic fluoride melts which are strong complexing ligands solves the issue of disproportionation of titanium species and stabilizes the high valence 10 states of titanium in the form of the T1F62~. Other benefits provided by fluoride melts that are utilized in this invention are44 : ( 1 ) wide decomposition potentials, (2) low dynamic viscosities, (3) low melting range when used in mixtures with the eutectic composition, (4) elevated ionic conductivities, and (5) capability to dissolve large amount of solute leading to operate at elevated cathodic current 15 densities. Therefore, the soluble titanium slag anode and molten titanium slag cathode overcomes all the issues of the prior art FFC process such as the poor hourly yield and the low energy efficiency and utilization of crude feedstock materials.
DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic flow sheet of the entire process.
Figure 2 is a schematic illustration of the electrochemical reactor.
MODES OF CARRYING OUT THE INVENTION
Generally speaking, This invention relates to a method for the continuous electrowinning of pure titanium metal from titanium slag and other electrically semiconductive titanium mixed oxide compounds particularly ilmenite. The '4 CARDARELLI, F., T.axiL, P., and SAVALL, A. - Tantalum Protective Thin Coating Techniques for the Chemical Process Industry : Molten Salts Electrocoating as New Alternative -Int. J. Refractory Metals &
Hard Materials 14(1996)365-381.

method involves crushing and grinding a mass of crude titanium slag, separating magnetically the major impurities in the ground slag, melting the purified and conductive titanium slag at elevated temperature, pouring the molten titanium slag at a bottom of an electrolytic cell to form a liquid cathode pool covered with a layer of molten salt electrolyte. Reducing cathodically by direct current electrolysis at high temperatures the molten titanium slag to produce droplets of pure liquid titanium metal. Owing to the slightly higher density of the liquid titanium compared to that of the molten titanium slag or molten ilmenite, the droplets fall by gravity at the bottom of the electrolytic cell forming after coalescence a pool of liquid titanium metal. The liquid titanium metal is continuously siphoned or tapped under an inert atomosphere and cast into a dense, coherent, and pure ingot.
The first step consists in comminuting (i.e., crushing and grinding) the crude titanium slag (i.e., 78-82 wt.% Ti02) or other semiconductive titanium compounds such as ilmenite to a final particle size comprised preferably between 0.075 mm and 0.42 mm (i.e., 40 and 200 mesh Tyler), but most preferably between 0.105 mm and 0.150 mm (100 and 150 mesh Tyler). The comminution step is necessarily required in order to render easier the removal of inert minerals present in the crude titanium slag (e.g., silicates, sulfides) and facilitate the removal of associated chemical impurities (e.g., Fe, Si, Ca, Mg). The removal of these impurities is suitable for both improving the electrical conductivity of the titanium slag or ilmenite and for enhancing its titanium concentration.
Secondly, the finely ground titanium slag or ilmenite produced undergoes a common magnetic separation step. The strong ferromagnetic phases such as for instance free metallic iron entrapped in the titanium slag during the smelting process and the intimately binded silicate minerals are efficiently removed using a low magnetic induction of 0.3 tesla and separated with the magnetic fraction which is discarded or rerouted. Then the remaining materials undergoes a second magnetic separation conducted with a stronger magnetic induction of 1 tesla.
The non magnetic fraction that contains all the diamagnetic mineral phases (e.g., silica, silicates, etc.) is also discarded. The remaining material consists of a finely purified ground titanium slag (i.e., 85-88 wt.% Ti02) or finely ground ilmenite mainly composed of semiconductive titanium oxide with the Andersson-Magnelli crystal structure (i.e., titanium oxides having the general stoichiometry Ti02_X, such as Ti305, Ti40~, and Ti509).
Thirdly, the purified ground titanium slag or ilmenite is melted above 1700°C during at least 2 hours. Once totally liquid the molten purified titanium slag or molten ilmenite is introduced at the bottom of an electrolytic cell and covered by a layer of molten salt electrolyte.
The electrolytic cell which is designed for continuous operation consists of a closed reactor vessel set into a high temperature furnace. with soluble titanium slag anodes or consumable graphite anodes that can be inserted and removed from the electrochemical reactor at different stages in their cycles being without any entries of air and moisture through airtight locks which are closed by means of large gate valves. The electrochemical reactor is heated by Joule effect during electrolysis.
The inert atmosphere within the cell is performed by an argon stream purified by passing it through a water and oxygen traps (i.e., getter).
The electrolytic bath is made of inorganic salts or their mixtures preferably selected from the group consisting of M"Xm wherein M = Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba and X = F-, Cl-, Br , I', SO42-, N03-, CO32-, B032-, P043-or mixtures thereof, preferably alkali-metals and alkali-earth metals halides, but more preferably alkali-metals and alkali-earth metals chlorides or fluorides with a final preference for CaCl2, CaF2, or the following mixtures of salts with the eutectic composition (e.g., CaF2-CaCl2, CaF2-CaCl2-CaO, CaF2-LiCI-Ca0).

The electrolysis is performed under galvanostatic conditions (i.e., at constant current) by imposing a direct current between the soluble titanium slag anode or consumable graphite anode (+) and the molten titanium slag or molten ilmenite cathode (-) by mean of an electric power supply or a rectifier . Usually cathodic and anodic current densities of 20 kA.rri Z are imposed with a cell voltage of less than 5 volts. The electric current is maintained until the desired electric charge has circulated. During the cathodic reduction of the titanium slag or of the molten ilmenite, owing to the high operating temperature which is slightly above the melting point of titanium metal (1660°C) and the slightly higher density of the liquid titanium (4110 kg.m-3) compared to that of the molten titanium slag (4000 kg.m-3), the electrodeposited titanium forms small liquid metal droplets that fall by gravity at the bottom of the electrolytic cell forming a pool of pure liquid titanium metal that also acts as an efficient current collector and hence it does not impedes the diffusion of oxygen from the titanium slag to the electrolyte. While at the anode dissolution of the titanium slag anode or combustion of the graphite anode occurs.
The level of molten titanium slag in the electrolytic cell is permanently adjusted in order to insure continuous electrolysis. The liquid titanium metal is continuously siphoned or tapped under an inert argon atmosphere and cast into pure, dense, and coherent ingot. The titanium ingot produced exhibited a high purity (i.e., 99 wt.%
Ti) and other characteristics that satisfies the grade EL-110 in accordance with the standard B299-99 from the AmeYican Society for Testing Materials (ASTM)45 such as a low residual oxygen, nitrogen, iron and chlorine content, a Brinell hardness of 60 HB. The electrowinning process exhibits a low specific energy consumption of 15 kWh/kg of titanium metal produced.
as ASTM B299-99 - Standard Specifrcation for Titanium Sponge. - American Society for Testing and Materials (ASTM) EXAMPLE
The electrolytic cell consists to a molten titanium slag or molten ilmenite acting as a liquid cathode (-) covered by a thick layer of a molten salt electrolyte such as CaF2-CaCl2-CaO. The anode (+) is either a consumable graphite anode or a soluble titanium slag anode. The electrolysis is performed at temperature of 1680°C under galvanostatic conditions (i.e., at constant current) by imposing a direct current between the electrodes by mean of an electric power supply or a rectifier .
Usually cathodic and anodic current densities of 20 kA.rri 2 are imposed with a cell voltage of less than 5 volts. The electric current is maintained until the desired electric charge has circulated. During the cathodic reduction of the titanium slag or of the molten ilmenite, owing to the high operating temperature which is slightly above the melting point of titanium metal (1660°C) and the slightly higher density of the liquid titanium (4110 kg.rri 3) compared to that of the molten titanium slag (4000 kg.rri 3), the electrodeposited titanium forms small liquid metal droplets that fall by gravity at the bottom of the electrolytic cell forming a pool of pure liquid titanium metal that acts as an efficient current collector and hence it does not impedes the diffusion of oxygen required by the reduction from the titanium slag to the electrolyte. While at the anode, the dissolution of the titanium slag or the combustion of the graphite anode forming carbon dioxide occurs. The level of molten titanium slag in the electrolytic cell is permanently adjusted in order to insure continuous electrolysis. The liquid titanium metal is continuously siphoned or tapped under an inert argon atmosphere and cast into pure, dense, and coherent ingot. The titanium ingot produced exhibited a high purity (i.e., 99 wt.% Ti) and other characteristics that satisfies the grade EL-110 in accordance with the standard B299-99 from the American Society for Testing Materials (ASTM)46 such as a low residual oxygen, nitrogen, iron and chlorine content, a Brinell hardness of 60 HB.
a6 ASTM B299-99 - Standard Specification for Titanium Sponge. - American Society for Testing and Materials (ASTM) The electrowinning process exhibits a low specific energy consumption of 15 kWh/kg of titanium metal produced.

Claims (12)

1. A method for the continuous electrowinning of pure titanium metal (Ti) from titanium slag and other electrically semiconductive titanium mixed oxides Ti x M y O z particularly ilmenite FeTiO3, in a molten salt electrolyte M n X m or a mixture of these salts.

2. A method according to claim 1, wherein Ti x M y O z is a semiconductor used as a soluble anode where M = Ti, Fe, Ca, Mn, Cr.

3. A method according to claim 1, wherein Ti x M y O z is a semiconductor used as a molten cathode where M = Ti, Fe, Ca, Mn, Cr.

4. A method according to claim 1, wherein the titanium slag is obtained by the direct reduction of ilmenite with anthracite by smelting into an electrical arc furnace such as Sorelslag® or Richard's Bay Slag®.

5. A method according to the preceding claim, wherein the titanium slag, was ground, purified by magnetic separation, and melted under an inert argon atmosphere.

6. A method according to the preceding claim, wherein the molten titanium slag is continuously introduced at the bottom of an electrolytic cell and used as a liquid cathode.

7. A method according to any preceding claim, wherein electrolysis is conducted at a temperature ranging between 1000°C and 2000°C, but preferably between 1640°C and 1700°C, and more preferably slightly above the melting point of the titanium metal.

8. A method according to any preceding claim, wherein the electrolyte is a molten inorganic salt M n X m wherein M = Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba and X = F-, Cl-, Br-, I-, SO4 2-, NO3-, CO3 2-, BO3 2-, PO4 3- or mixtures thereof, prefereably alkali-metals and alkali-earth metals halides, but more preferably alkali-metals and alkali-earth metals chlorides or fluorides with a final preference for CaCl2, CaF2, or the following mixtures of salts with the eutectic composition (e.g., CaF2-CaCl2, CaF2-CaCl2-CaO, CaF2-LiCl-CaO).

9. A method according to any preceding claim, wherein the titanium is obtained by cathodic reduction of the molten titanium slag.

10. A method according to any preceding claim, wherein titanium metal is electrodeposited in the liquid or solid state and owing to its greater density than molten titanium slag, it falls to the bottom of the cell forming a pool of liquid titanium metal acting as a current collector.

11. A method according to any preceding claim, wherein the electrodeposited titanium metal is collected continuously by tapping or siphoning and cast into a pure dense and consistent ingots.

12. A method according to any preceding claim, wherein specifications of the titanium metal ingot produced satisfies the grade EL-110 in accordance with standard B299-99 from the American Society for Testing Materials (ASTM).