Pyrochemical Separation of Zirconium and Hafnium Tetra- Chlorides Using Fused Salt Extractive Distillation Process

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 from zirconium. 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 Γ ι * si ^ ζ! ΪΙ j· £ Π π ο S Ο ΰ \ 3 V * d Ξ σ is sfS is CeOs u . g δ, 7 A ο I Ι UJ Si Β -I 5 δ ι χ u ς U UJ 8 U53L] i Μ ο Si 1? ο ί! ο ΰ ο U Λ •a u 8 pi. sä I I Π, Ζ" ^ 2 -ttr gis ο äfi!_ α •οβ inχ! £ »i *-? <8 δ δ δ Ζ Ρ < i * I 5 5 ϊ υ s α g Ik. JTT ud. si 5 ο. & s si <r 8 IL υ Π 1 ι*» rv δ -1 I 3 U. — ρ δ II s c Csf ω ώ i UJ J Ζ 2 - # Β U 2 8 ? 2 id uι δ 3 ζ δ Η (Λ 2 5 IuS ® η a uj ο g u. ijjguS u TT1. ο fih* "I lift »5 < SM 3 hin C5 5-s1 - !Γ δ 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 from zirconium. 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.

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