Pyrochemical Separation Of Zirconium And Hafnium Tetra- chlorides Using Fused Salt Extractive Distillation Process

D. Sathiyamoorthy, S.M.Shetty, D.K.Bose and C.K.Gupta

Materials Group, BARC, Mumbai-400 085, India.

(Received May 20, 1998; final form July 27, 1998)

ABSTRACT (1.05 χ 10"26 m2 per atom) for thermal neutron as compared to that of zirconium (1.8 χ 10"29 m2 per The paper presents studies on a nonaqueous atom). The two major commercial ores of zirconium pyrochemical process for the separation of tetra- metal are zircon and baddeleyite. Zircon is an chlorides vapour of zirconium and hafnium during its orthosilicale ore corresponding to the formula ZrSi04 contact with a mixture of molten KC1-A1C13 salt which and baddeleyite is essentially an impure zirconium acts as a solvent for preferential absorption of zirconium dioxide. Both these ores contain hafnium anywhere tetrachloride. The results of the experiments carried out between 0.5 to 3.5 percents. In fact, on account of the on a 100 mm sieve plate extractive distillation column configuration of valence electron 4d2 , 5s2 and 5d2 , 6s2 and the experience on the operating the system are and identical ionic radii, physicochemical similarity of presented. Experimental findings revealed the feasibility these two metals is so close that zirconium metal when of separation of hafnium tetrachloride up to fifty percent extracted will have always some amount of hafnium. in a twelve stage sieve plate distillation column. Also Materials containing hafnium would seriously reduce presented in this paper are some of the critical design the neutron flux in the thermal nuclear reactors. Hence it aspects for a successful operation of a demonstration is imperative to separate hafnium from zirconium for plant. use in thermal nuclear reactors. A variety of routes have been proposed and 1. INTRODUCTION employed for the separation of hafnium fromzirconium . However, only a few processes have received Nuclear power stations all over the world are importance and are accepted in the industries. These generally light and heavy water cooled reactors. In these processes are (i) multiple crystallization of potassium reactors, the zirconium base alloys are essential for zirconium fluoride, (ii) liquid-liquid extraction using as fuel clads and structural materials. Pressure involving preferential extraction of zirconium using tubes in pressurised heavy water reactor should have either TBP in nitric acid media or n-octylamine in requisite properties such as low absorption cross-section sulphuric acid media or chloride media as thiocyanate for thermal neutrons, resistance to corrosion in boiling / complex using ΜΠ3Κ as the solvent to extract hafnium pressurised water, high temperature strength and high in the organic phase, and (iii) distillation of chlorides. melting point. Table 1/1,2/ lists some of the properties Fig. 1 shows the different flow sheets which are of zirconium metals which are of interest to nuclear widely accepted and operated or in operation in the engineers and scientists. The specifications of zirconium different parts of the world for the separation of metal and alloys for application in nuclear reactors are hafnium from zirconium Among these multiple stringent. The current specifications for zirconium and crystallization involves sintering of zircon sand with zircalloys are given in Table Π /3 /. As can be seen from KjSiFg and KCl. The sintered product K^ZrCH^Fg is the Table Π, hafnium content in the zirconium must be leached with water. The leached K^ZrFg is then less than 0.01 percent Hafnium is not a desirable metal crystallised from a hot leach solution by cooling it to in zirconium since it has high absorption cross-section room temperature. As the potassium hafnium fluoride is

213 Vol. 18, No. 4, 1999 Pyrochemical Separation of Zirconium and Hafnium Tetrachlorides Using Fused Salt Extractive Distillation Process

Table 1 Physical and chemical properties of zirconium and hafnium

PHYSICAL PROPERTIES Zr HF NUCLEAR PROPERTIES Zr Hf

Atomic No. 40 72 Neutron Cross-Section (barn) : 0.18 105 Atomic Weight 91.22 178 Scattering Cross-Section(barn): 8+1 8±2

Melting Point °C 1852 2222 Boiling Point °C 3850 5400 Density g/cc 6.4 13.09

Transition Temp. °C : 862 176

Table 2 Current ASTM specification for nuclear grade zirconium metal

ELEMENT ppm ELEMENT ppm Al 75 Mo 50

Β 0.5 N2 50 C 50 Ni 70 Cd 0.5 02 1400 Co 20 Mn 50 Cr 200 Si 120

Cu 30 Cl2 1300 Fe 1500 Ti 50 Hf 100 w 50 (U total) 3.5

about 1.5 times more soluble than potassium zirconium MIBK, impose conditions on the use of special fluoride, the recrystallised salts that contain zirconium equipments. In the TBP-HN03 system, zirconium is will have less hafnium than the input salt in each stage. extracted by organic stream and hence gives relatively a About 9-10 recrystallization steps are found to be pure product containing less than 100 ppm of hafnium. adequate to achieve nuclear pure zirconium inter- The major disadvantages of this process are (i) high mediate. The mother liquors become progressively consumption of chemicals, (ii) generation of large enriched with hafnium. It is a simpler and easier amounts of toxic pollutant, and (iii) poor recovery of method, but it is a batch process. nitric acid from the lean solution In the case of MEBK thiocyanate process, hafnium In recent years, one more solvent extraction process gets extracted in the organic phase. The solvent ΜΠ3Κ based on the preferential extraction of zirconyl sulphate has very high separation factor (~ 80). However, as by aliphatic tertiary amine (alamine -336) has come into zirconium is extracted in to the raffinate along with the prominence. Like the TBP-HN03 process, zirconium other impurities, further purification process is essential goes into organic phase and hence a high purity product to get nuclear pure zirconium intermediate. The low is obtained. Lower cost of operation and reagents are the flash point, high toxicity and corrosive nature of the other major advantages. However, poor loading factor

214 D. Sathiyamoorthy et al. High Temperature Materials and Processes

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215 Vol. 18, No. 4, 1999 Pyrochemical Separation of Zirconium and Hafnium Tetrachlorides Using Fused Salt Extractive Distillation Process and larger size of the plant are some main distillation at atmospheric pressure /10, 11/. Of these disadvantages. processes, the distillation of the chlorides using All the processes discussed hitherto are the KCI-AICI3 as a solvent /ll/ has been developed to a combination of hydro and pyrometallurgical steps and production stage and is currently practised on a tonnage they work generally with poor conversion efficiency. scale by M/s. Cezus in France. Furthermore, they generate a large amount of toxic This nonaqueous process eliminates hydro- pollutant. In view of this, the current interest is to metallurgical operations and involves just three vital develop an all-nonaqueous process. In this context, a steps to obtain hafnium free tetrachlorides of zirconium new route named as a pyrochemical process has starting from zircon The consumption of chemicals and emerged and it seems to be attractive to separate the merits and demerits of various processes including hafnium tetrachloride from its mixture with the the nonaqueous one are presented in Table ΠΙ and IV tetrachloride of zirconium The end products after respectively. These data clearly show that consumption separation are zirconium tetra chloride / hafnium tetra- of chemicals and energy is much lower in the case of chloride which can be directly reduced by the Kroll molten salt based pyrochemical route as compared to reduction. A few nonaqueous processes have been any of the processes. studied by various research groups. In the Newnham Research work has been carried out at the Materials process IM a mixture of tetrachlorides of zirconium and Processing Division, BARC to study the non aqueous hafnium is subjected to heating with some reducing process that involves fused salt extractive distillation agents at selected temperatures. Zirconium The main objective was to investigate and establish the tetrachlorides is preferentially reduced to ZrCl3 while engineering parameters and to develop a flow sheet for hafnium tetrachloride remains unaffected. Separation is the production of hafnium free (<100 ppm) zirconium finally achieved through the sublimation of more tetrachloride starting from zircon sand. The unit volatile hafnium tetrachloride leaving behind a operations involved are (i) direct chlorination of zircon nonvolatile ZrCl3. Zirconium trichloride so obtained is sand, (ii) purification of raw chlorides by fused salt converted to ZrCl4 by disproportionation. Chandler 151 scrubbing and (iii) extractive distillation of the chloride employed a chloride-oxide exchange process in the mixture to separate HfCl4 from its associate ZrCl4. separation of hafnium fromzirconium . By this method a Among these steps, the most important step is the reaction between HfCl4 in the feed and Zr02 in the bed extractive distillation and it is discussed in detail in the yields a nonvolatile Hf02 and ZrCl4 vapours as per the followings. following reaction. 2. THEORETICAL CONSIDERATIONS

Zr02(s) + HfCl4(g) 1222k—> Hf02(s) + ZrCL, (1) Extractive distillation in principle is a technique by which the relative volatility of a component is altered by

Separation of hafnium by selective reaction of HfCl4 adding a fluid component known as solvent. This with alkali metal chlorides was studied by Flengas and alteration in volatility of the components is desired in Dutrizac 161. Both the chlorides of zirconium and view of the close values of the vapour pressures of the hafnium form hexachloro complexes with alkali metal feed components (e.g. ZrCl4 and HfCl4). chlorides. Zirconium chlorocomplexes are less stable The separation of the vapours of ZrCl4 and HfCl4 is than the corresponding hafnium chloro complexes. governed by a number of their physicochemical

Hence ZrCl4 gets more easily removed than HfCl4 from properties. The essential parameters to be considered for its chloro complexes. Some interesting nonaqueous the separation process are (i) the selection of a suitable methods are: fractional distillation process 111 at solvent melt, (ii) the operating temperature and (iii) a atmospheric pressure, formation of additional reliable material of construction. In addition to these compounds with their tetrachlorides followed by parameters there are some practical limitations refluxing of these compounds /8/, fractional distillation associated with the handling of hygroscopic chloride of tetrachlorides at high pressures 191 and molten salt salts and the circulation of the molten salt.

216 D. Sathiyamoorthy et al. High Temperature Materials and Processes

Table 3

Pattern of chemical consumption to produce pure ZrCl4 for various processes

Sr. CHEMICALS TBP- AMINE- MIBK- PYRO- No. NITRATE SULPHATE THIOCYANATE CHEMICAL 1 NaOH 4.50 4.50 0.99 1.30

2 HNO3 (58%) 18.00 — —

3 HCl — — 3.2 —

4 H2S04 0.35 4.00 1.35 —

5 Solvent 0.20 0.30 0.17 —

6 Diluent 0.20 0.30 0.17 —

7 NH4CNS — — 0.13 —

8 ( NH4 ) 2 CO3 — 0.50 — —

9 Na2CC>3 0.50 — — —

10 Filter Aid 0.25 — — —

11 NH3 1.80 1.50 1.60 — 12 Pet. Coke 0.22 0,22 0.52 0.40

13 Cl2 1.60 1.60 3.50 2.60

14 Starch 0.10 0.10 0.10 —

3. SELECTION OF SOLVENT 4. OPERATING TEMPERATURE

The selectivity for the solvent should be high; it The close proximity of sublimation temperatures of should be cheaper and less volatile. Several molten salt these metal chlorides of zirconium and hafnium systems have been examined for use as solvent Some of preclude the separation of chlorides by simple the recommended molten salt baths for extractive distillation route. Hence extractive distillation has been distillation are given in Table V /12/. It has been a favoured choice for the separation of these chlorides observed from this table that potassium salts are using KC1:A1C13 melt The lower limit of the preferred to the sodium salts in view of their ability to temperature for extractive distillation is fixed by the dissolve more of Zr / Hf chlorides. The molten salt melting temperature (234°C) of the KC1:A1C13 mixture mixture of KCl- FeCl3 and NaCl - FeCl3 systems as shown in the phase diagram /13/ (Fig. 2) and the appears to be the best choice. But these salts are highly upper limit is set by the stripping temperature (550°C) corrosive, and have a lot of problems with regard to the of the ZrCl4 from the melt Considering all these factors stripping of dissolved content of zirconium tetra and from the point of view on the better fluidity, and chloride. The stripping characteristics from iron high loading factor, a temperature range of 325 to chloride melts are poor as compared to aluminium 350°C has been selected for operating the distillation chloride melts. Some of the parameters on the stripping column. of dissolved chlorides of Zr and Hf from different salt baths are presented in Table VI. Results clearly indicate that KCI-AICI3 is the most suitable melt for stripping of 5. SELECTION OF MATERIAL OF CONSTRUCTION

ZrCl4 almost completely at a relativelylo w temperature. The material of construction for the processes involving high temperature and corrosive chemicals is

217 Vol. 18, No. 4, 1999 Pyrochemical Separation of Zirconium and Hafnium Tetrachlorides Using Fused Salt Extractive Distillation Process

Table 4 Merits and demerits of various processes to Zr/Hf separation

PROCESS/ TBP - AMINE- MIBK -THIO PYROCHEMICAL MERITS NITRATE SULPHATE CYANATE

1. Product Zr02 Zr02 Zr02 ZrCl4 2. Recovery 58% 70% 80% 98% (Zircon-

ZrCI4) 3. Energy Consumption Zircon to 90 KWH 70 KWH 65 KWH 40 KWH Reactor Grade Sponge (600 T/year) 4. Others 1. Technology 1. Process 1. Technology 1. Fewer unit known known known operations & hence 2. Low 2. Low 2. Cheaper high yield maintenance maintenance consumables& 2. Cheaper and lowest cost cost less reagent consumption of 3. Compact 3. Low consumption consumables plant consumption 3. High stream 3. Dry process 4. No Problem of cheaper concentration 4. Lower energy of material of chemicals 4. Hf is consumption construction 4. Low recovered in 5. Hf is recovered 5. High loading solvent loss organic phase 6. Lowest cost of 6. Solvent 5. Low cost of 5. High production TBP is production separation 7. Separation to any available factor extent possible by indigenously recycle 8. Temp. And flow controls are only needed. Hence online computer application for better quality assurance possible 9. No second stage chlorination 10. No byproduct 11. Ease of solvent regeneration

Table 5 Solubility of Zr chlorides in different molten salt bath

SOLVENT TEMPERATURE IN PRESSURE IN SOLUBILITY OF DISTILLATION DISTILLATION Zr/Hf CHLORIDE COLUMN COLUMN PER 100g INOC IN TORR SOLVENT

KCl - A1C13 345 750 42.3g 345 920 46.8g 500 750 5-2g 500 13 0.6g 345 750 31.0g NaCl - AICI3 345 920 35.lg KCl - FeCl3 345 750 112.5g

NaCl - FeCl3 345 750 91.5g

218 D. Sathiyamoorthy et al. High Temperature Materials and Processes

Table 6 Experimental results of studies on stripping characteristics of fused salt solvent

SALT SYSTEM LOADING FACTOR OF TEMPERATURE RESIDUAL ZrCl4 MOLE % Z1CI4 CONTENT IN OF STRIPPING IN AFTER TRIPPING 100g SOLVENT BEFORE °C PER 100g STRIPPING SOLVENT KCl : AICI3 45 500 2.5g 1 : 1.04 KCl : 110 550 16.2g

FeCl3 1 : 1 NaCl : AICI3 30 500 9.6g 1 : 1.04

NaCl : FeCl3 85 550 24.5g 1 : 1 600 17.5g

Alloys International Inc., Huntington, WV.) for distillation column and other components of the system,

6. EXPERIMENTAL SET UP

The studies on extractive distillation have been investigated using a 12 stage sieve plate column. Each stage was of 100 mm dia and 150 mm length fitted with a sieve plate and a down comer tube. A schematic of the experimental set up is shown in Fig.3. The complete assembly consisted of a distillation column, a screw feeder, a sublimer, a reboiler, a stripper, a feeder tank and two condensers to collect hafnium and zirconium tetrachlorides. A gas lift pumping system has been incorporated in this system to recirculate the molten salt The screw feeder was used to feed the solid

Zr(Hf)Cl4 into the sublimer which evaporated the chlorides and fed the vapour at the sublimation temperature into the distillation column. Nitrogen gas lift pump was used to pump the molten salt from the reboiler. The reboiler was maintained at 450°C. The stripper was used to remove or strip out the dissolved Fig. 2: Topical phase diagram for KC1-A1C13 system content of ZrCL in the molten salt

very important. Nickel and nickel base alloys, such as 7. FEED MATERIAL Inconel 600, have been found suitable as the material of construction for handling chlorine and chloride baths. In The feed sample materials, Zr (Hf)Cl4 were prepared the present work, inconel 600 has been selected as the by the direct chlorination of zircon sand in an construction material (.Inconel is a trade mark oflNCO electrothermal fluidized bed chlorinator. Hafnium

219 Vol. 18, No. 4, 1999 Pyrochemical Separation of Zirconium and Hafnium Tetrachlorides Using Fused Salt Extractive Distillation Process

GAS LIQUID SEPARATOR

Fig. 3: Extractive distillation assembly

tetrachloride mixed with nuclear pure ZrCl4 was also 9. AUXILIARY SYSTEMS used as the alternate feed material. The Zr(Hf)Cl4 was obtained from the Nuclear Fuel Complex, Hyderabad, Flexible cord heaters were provided for maintaining India uniform temperature in the distillation column, reboiler, sublimer and the stripper. A 30 channel programmable temperature indicator, controller cum recorder was used 8. SOLVENT MELT to monitor and record the temperature at various points. Flow/ pressure measuring devices for the measurement The solvent employed for the extractive distillation of flow and pressure in each stage of the column were in these studies was prepared by mixing potassium used. A photograph of the entire experimental set up is chloride (BDH make) and aluminium chloride (E'merk shown in Fig. 4. Finding a suitable pump for circulating make) in the mole ratio of 1 : 1.04. the molten salt, which is highly corrosive and at high

220 D. Sathiyamoorthy et al. High Temperature Materials and Processes

Fig. 4: Extractive distillation column

22-

s οΕ I 20 ja ο Ο α ο ra QI 18

u> 1ο α 16

14 1— 1 1 1— T X 0.005 0.010 0.015 0.020 0.025 Gas Flow kg / min

Fig. 5: Pressure drop vs gas flow rate: behaviour of gas lift pump used in the distillation column temperature is a big problem. In the present work, gas characteristics of the gas lift pump are shown in Fig. 5 lift pump has been used for the purpose. The flow and 6. Since the density and viscosity of the KCl: A1C13

221 Vol. 18, No. 4, 1999 Pyrochemical Separation of Zirconium and Hafnium Tetrachlorides Using Fused Salt Extractive Distillation Process

Fig. 6: Water flow rate vs nitrogen gas flow rate molten salt bath in the ratio of 1 : 1.04 are close to that of the chlorides entered into the column and contacted of water, these studies have been carried out using water with the molten salt mixture which circulated counter at room temperature. current to the ascending vapours. The more soluble

ZrCl4 was progressively carried down by the molten salt 10. EXPERIMENTAL stream whereas the rising vapour phase was enriched

Potassium chloride and anhydrous aluminium with HfCl4 and reached to the top of the column. The chloride in the mol ratio of 1:1.04 was dried and 30kgs ZrCl4 rich molten melt entered the reboiler at 350°C and of this mixture was charged into the feeder kept then into a stripper at 500°C. ZrCl4 depleted from HfCl4 preheated at 100°C. The mixture was subsequently was then stripped out at 500°C by sparging nitrogen gas melted at 234°C under a positive pressure of argon. and collected in a condenser. The top fraction enriched

Once the charge was molten, the temperature of the melt in HfCl4 was collected in a condenser at the top of the was maintained at 350° C and then transferred to the column. Separated chlorides were analysed for the reboiler. The molten salt was lifted to the top of the contents of both hafnium and zirconium column from the reboiler using a nitrogen gas lift pump. Nitrogen gas flow rate was kept at 2.5 1 pm. A pressure drop of 15kPa was obtained in the gas lift pump. Once 11. RESULTS AND DISCUSSION the circulation of the molten salt in the column was stabilised, a zirconium/hafnium tetrachloride mixture As reported in the literature and ascertained from was fed at 5 kg per hour by a screw feeder into a our experiments, molten salt of KCl and A1C13 mixed in sublimer which was connected to the fourth stage of the the mol ratio of 1:1.04 is the best solvent for the system column above the level of reboiler position. The The molten salt of KC1:A1C13 is a clear and colourless sublimer temperature was controlled at 500°C. Vapours liquid even with a substantial amount of ZrCl4 dissolved

222 D. Sathiyamoorthy et al. High Temperature Materials and Processes

in it This has been observed visually by tapping the Comparable results have been obtained from the molten salt mixture from the reboiler at 350°C. As per chlorides prepared either by direct chlorination of zircon

the prediction of Dutrizac and Flengas /14/, the stability or by blended pure chlorides. The stripped ZrCl4 2 of the chlorocomplexes varies with the ratio, (rm + rcl) / product has been found to contain 1.5% of HfCl4 as

qm, where rm is the ionic radius of the metal, rc] is the compared to the starting chloride of 3.1% HfCl4. The

covalent radius of chlorine and qm is the charge on the top product contained 8% of HfCl4. The analysis of metal ion. The relatively smaller size and the trivalency product as presented in Table VLH indicates that in a 12 of aluminium helps in removing zirconium tetrachloride stage column, hafnium content has been brought down more easily from the stripper. The data /15/ in Table V from the initial value by 50% in a single run. The and VI indicate that the loading of ZrCl4/HfCl4 in hafnium depleted ZrCl4 obtained from one run has been KCI-AICI3 bath is moderate (42%) at 345°C and near to recycled as feed for the next run and in this manner atmospheric pressure (750 Torr). Stripping of ZrCl4 is ZrCl4 of hafnium content less than 100 ppm can be almost complete. ZrCl4 in the solvent melt is brought achieved in 8 cycles. The method though seems to be down to 2.5 gm per 100 gram of Κ A1C14 at 500°C attractive for achieving the desired separation in about 8 under a pressure of one atmospheric. passes: each time when enriched feed is charged into the Apart from loading factor and stripping distillation column, the feed port need to be relocated characteristics, the circulation rate of molten salt is an due to the fact that for each subsequent pass feed important parameter which decides the effective composition is altered or enriched with respect to ZrCl4 separation of zirconium/hafiiiiun tetrachlorides. Con- content In view of the simplicity of the operation for tinuous circulation of the molten salt is essential in this achieving a product of consistent quality, the distillation process. In view of the high temperature coupled with column should have the required stages to achieve a the corrosive nature of the chlorides, conventional desired separation Our experimental results with a 12 pumps could not be used for lifting the molten salt from stage column showed that the top condenser had a mole the reboiler to the top of the distillation column Hence fraction of HfCl4(XD)0.084 and the bottom condenser a specially designed gas lift pump which works on the 0.015. Using the above data and the Fenske-Underwood principle of density difference across the two limbs of a /16/ equation, the relative volatility of HfCl4 in the U-loop has been incorporated for our present KCI-AICI3 melt system at 350°C is 1.161. The number experiments. Table VII gives some of the parameters for of minimum stages required to bring the hafnium operating the gas lift pump for KC1:A1C13 melt content down from 2.5% to 0.01% i.e. a separation ratio circulation in the present experiments. In view of the of 250 fold are 82 stages, if the relative volatility is practical limitations in terms of low efficiency, carry 1.161. This can be calculated using the Fenske- over of dissolved ZrCl4 and requirement of high purity Underwood equation for xD = 0.96 and xB = 0.00001 gas it may not be advisable to consider the use of the gas lift pumps for large scale pumping of molten salt. In (Nmin + 1) log =log(xD/l-xD)/(xB/l-xB) (2) a continuous operation, a gas lift pump may be replaced where xD and xB respectively are top and bottom by simpler van type pumps. A submerged pump also did concentration of HfCl4 in the distillation column, not work satisfactorily and its cost for this specific use whereas N^ is the minimum number of stages and is is exorbitant. the volatility factor. Fig. 7 shows a plot on volatility

Table 7 Molten salt pumping parameters

NITROGEN PRESSURE 1500 TO 2000 MM WC NITROGEN FLOW RATE 2.0 TO 4.0 1pm . SALT PUMPING RATE 8.0 TO 10.0 kg/hr.

223 Vol. 18, No. 4, 1999 Pyrochemical Separation of Zirconium and Hafnium Tetrachlorides Using Fused Salt Extractive Distillation Process

Table 8 Analysis of zirconium chloride samples

ELEMENTS STARTING MATERIAL FROM MATERIAL FROM MATERIAL BOTTOM TOP CONDENSER in ppm CONDENSER in ppm in ppm Al 57 2900 5000 Β 0.9 4.4 0.9 Ca 100 110 100 Cr 1700 2700 170 Cu 55 90 25 Fe 1100 700 2500 Ni 300 600 1100 Sn 25 25 25

HP 3.1% Xb = 1.52% Xd =8.4%

* : The relative volatility, calculated using equation 1 and Xj, & Xj values shown in this table is 1.161

<υ 800 ω £ c Ο <1> TD ζ -δ •c Ο xz ICT i— υ 600 £ £ (0 Εg W Sl

1.2 1.3 1.4 Relative Volatility

Fig. 7: Relative volatility vs number of equilibrium stage for hafnium tetra chloride present in the potassium chloride - aluminium chloride melt

224 D. Sathiyamoorthy et al. High Temperature Materials and Processes factor versus the minimum number of stages as stages due to constraints of height in the lab and the cost determined by the equation. The plot indicates that as factors. However, our interest was to conduct a the volatility factor increases, the number of minimum feasibility study hitherto unavailable in open literature. stages required for separation decreases exponentially. The total number of actual stages required can also REFERENCES be determined by the McCabe-Thiele method /17/ based on the relative volatility factor. Taking a volatility factor 1. A.N. Zelikman, O.E.Krein and G.V.Samsonov, of 1.16 the number of stages arrived at will be 101. Metallurgy of Rare Metals, Metallurgiya, Moskva, The McCabe-Thiele graphical method is not a 213 (1964). comfortable one for determining for the separation of 2. Roland Tricot, Journal of Nuclear Materials, 189, 250 folds from its initial content of Hf to 2.5%. Hence, 277, (1992). in order to evaluate precisely the number of equilibrium 3. ASTM Annual Book, 349-379, 1977. stages and equilibrium Χ,Υ values and also to locate the 4. I.E. Newnham, US Pat 2,791,485 (1957). feed plate, a computer program (Fortran) was developed 5. H.W. Chandler, U.S. Patent 3,276,862, Oct4, and it is given in the appendix. From this program, the 1966. feed plate location for a 101 stages is determined at 55 6. S.N. Flengas, and J.E. Dutrizac, Metallurgical stages from the top of the distillation column. Trans., 8B, 377 (1977). 7. D.R Spink, and K.A Jonasson, Proc. Symp. 12. CONCLUSION TMS-AIMERef. Metals (eds) H.Y. Sohn, O.N. Carlson and J.T. Smith. New York: Am.Inst.Min. Tangri, Bose and Gupta /18/, in their study on and Met Petr. Engg, 1981; 297. vapour pressure of ZrCl4 and HfCl4 over molten 8. D.C. Bradley, and W.J. Wardlaw, J. Chem. Soc., KCI+AICI3 system, found the relative volatility factor 280 (1951). of HfCl4 to be an average of 1.5 times that of the ZrCl4 9. H. Ishizuka, Eur. Pat. (Applied) 45,270, (1982). in the composition range 16.5% to 23.4 mole % of 10. D.R Spink, U.S. Pat3, 966,458, (1976). th ZrCl4/HfCl4 or below. Considering this relative 11. P. Brun, P.B. Thouvenin, and L.Moulin, 6 Int. volatility factor of 1.5 and starting ZrCl4 containing 2.5 Conf. on Nucl. Appl. of Zirconium, Vancouver, mass % hafnium (on metal basis) and assuming for the ASTM, 1982. extractive distillation to result in a bottom product 12. P. Besson, J. Guerin, P. Brun, and M. Bakes, US containing 0.01 mass % of HfCl4 and the top product Pat., C.R Boston, 4, 021, 531, May 3, 1977. containing 96 mass % HfCl4, a graphical computation 13. Advances in Molten Salt Chemistry, Plenum Press, by the McCabe and Thiele diagram predicted 49 New York, 1, 129, 1977. theoretical stages. 14. John E. Dutrizac and Spero N. Flengas, Can. A relative volatility of 1.7 has also been reported in Pat.863258, (1968). the literature /19/ with number of minimum stages 42. 15. U.S. Patent 4,021,531, May3 , 1977. Cezus process is reported to have a distillation column 16. M.R Fenske, Ind. Eng. Chem., 24,482, (1932). of 50 m height with about 60 stages. Not much 17. Raymond E. Kirk and Donald F. Othmer, information is available as how this was obtained. We Encyclopaedia of Chemical Technology, 5, 156, have confined our experiments only to test the 1950. feasibility of separation and demonstrate it on 18. RP. Tangri, D.K. Bose and C.K.Gupta, Journal of engineering scale. We have accomplished this goal with Chemical and Engineering Data, 40,823, (1995). meaningful results which otherwise have been not 19 Robert L. Skaggs, Daniel T. Rogers and Don B. reported openly in literature. In the existing set up we Hunter, Information Circular 8963, United States could not carry out our experiments incorporating 101 Department of the Interior, Bureau of Mines, 1984.

225 Vol. 18, No. 4, 1999 Pyrochemical Separation of Zirconium and Hafnium Tetrachlorides Using Fused Salt Extractive Distillation Process

APPENDIX

FORTRAN Program to calculate the equilibrium stages in extractive distillation column.

C** DISTILLATION COLUMN *** C q= 0.i.e. feed is satd vap. *** using McCabe Thiele Method (Analytical)**** y(l)=xd C** Notations used are : b=residue flowrate , do 10n=l,100 C d= Distillate, f=Feed, r=reflux ratio, alfal=y (n) +alfa* (l-y(n)) C liq=liq. flow rate, alfa = relative volatility, x(n) = y (n)/alfal C P,q = liq. and vap. comp in the stripping write (4,*) -n\ η,' χ', x(n), γ, y(n) C dl=liquid density if(y(n).le.xf goto 20 C Section, y (n+1) = rl*x (n) + r2 C Χ, Y = liq & vap. comp, in enriching section. 10 continue 20 write (*,*)'n ·, n-1

DIMENSION X (100), Y (100) C** stripping section real liq m=n Read(*,*) 5 write (x,5) format CMnput your xf, xd, xb, f, dl, alfa, frliq/,0183, .9458, do30fc=m,100 $ 0.0000761, 21.29,0.4058, 1.16,0.0/ if (x(k).le.xb) goto 40 y(k+l)=r3*x(k)-r4 open (4, file = 'distl.dat', status = V) alfa2=y (k+1) + alfa* (l-y(k+l)) x(k+l)=y(k+l) / alfa2 C** Use UNDERWOOD eqn for R(min) when q=0. write(4,*)'kl,k+1' x', x(k+l)/y·, y(k+l) 30 continue 59 Rmin = alfa* (xd/xf) - (1-xd) / (1-xf) C** k = total no. of plates. Rmin = Rmin/ (alfa-1) - 1 R=1.5*Rmin 40 write (*,*)'k k write (4, *) 'rmin', rmin,' r", r total = k write (*,*) 'rmin', rmin,' r1, r write (4,*)' total', total rl=r/ (i+l) r2=xd/(r+l) close (4) d=f (xf-xb) / (xd-xb stop C write(*,*)'d d,' r*,r End hq=r*d b=(f-d) C write (*,*) liq,b r3=liq/(liq-b) r4=b/(liq-b)*xb write (*,*) rl,r2,r3,r4 C* Liq rate at the top is same as bottom as

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