简介概要

Technological conditions for purification of hafnium by iodide process

来源期刊：Rare Metals2020年第2期

论文作者：Li-Sen Tian Yan-Xi Yin Zhi-Fang Hu Hong-Lin Jiang Zhong-Qi Li Li-Jun Wang

文章页码：200 - 204

摘要：In nuclear industry,the purity of hafnium control rod is usually significant,and the process parameters will affect the impurity contents.In this study,the four key factors(filament temperature,retort materials,feeds,and iodide reaction time) were investigated.It is found that the impurity contents of Fe,Ni in the hafnium crystal bar decrease with the K value rising.Both the retort materials and feed types have obvious influence on the impurity contents of hafnium crystal bar.The iodide reaction time has little effect on the impurity contents of hafnium crystal bar.

稀有金属(英文版) 2020,39(02),200-204

Technological conditions for purification of hafnium by iodide process

Li-Sen Tian Yan-Xi Yin Zhi-Fang Hu Hong-Lin Jiang Zhong-Qi Li Li-Jun Wang

Division of Rare Metals Materials&Metallurgy,General Research Institute for Nonferrous Metals

作者简介：*Yan-Xi Yin,e-mail:yinyanxi@gmail.com;

收稿日期：11 October 2013

基金：financially supported by the National Technology Support Project of China (No.2012BAB10B10);the China-Russia Corporation Program Ministry of Science (No. 2010DFR50800);

Technological conditions for purification of hafnium by iodide process

Li-Sen Tian Yan-Xi Yin Zhi-Fang Hu Hong-Lin Jiang Zhong-Qi Li Li-Jun Wang

Division of Rare Metals Materials&Metallurgy,General Research Institute for Nonferrous Metals

Abstract：

In nuclear industry,the purity of hafnium control rod is usually significant,and the process parameters will affect the impurity contents.In this study,the four key factors(filament temperature,retort materials,feeds,and iodide reaction time) were investigated.It is found that the impurity contents of Fe,Ni in the hafnium crystal bar decrease with the K value rising.Both the retort materials and feed types have obvious influence on the impurity contents of hafnium crystal bar.The iodide reaction time has little effect on the impurity contents of hafnium crystal bar.

Keyword：

Hafnium; Purity; K value; Iodide;

Received： 11 October 2013

1 Introduction

Hafnium,a by-product in the zirconium industry,finds application as a control rod material in the nuclear industry [ 1, 2, 3] .This use stems from the fact that hafnium possesses high neutron absorption cross section,excellent high-temperature corrosion resistance,good ductility,and machinability.With the rapid development of aviation,aerospace and nuclear industry,hafnium sponge prepared by Kroll method could not satisfy these applications due to high impurity contents [ 4, 5, 6, 7] .In spite of using improved techniques,such as high-purity HfCl₄ prepared from the thermal decomposition of chloro-hafnate eutectic and the introduction of high-purity sodium with magnesium in the Kroll reduction,it was not possible to produce high quality sponge routinely for direct fabrication.Therefore,further refining of Kroll sponge is necessary to meet the performance requirements [ 8, 9] .The iodide refining process is a very effective refining method.At low temperature,iodine reacts with Hafnium sponge to produce the volatile hafnium iodide which is subsequently diffused to an electrically heated hafnium filament and decomposed on it.A layer of hafnium is deposited on the filament to produce a hafnium crystal bar,and released iodine to repeat the diffusion cycle [ 3, 10, 11, 12, 13, 14, 15, 16, 17] .Purification is effective in leaving behind interstitial impurities,such as oxygen,carbon,and nitrogen.Metallic impurities generally transport poorly,depending on the behavior of inpidual iodides,so the hafnium crystal bar is much purer than the sponge hafnium [ 8] .However,the systemic research of hafnium crystal bar impurity contents by the iodide process was rarely reported.The four important factors which might affect hafnium crystal bar impurity contents are as follows:the hafnium filament temperature,retort materials,feeds types,and iodide reaction time under the iodide process.The hafnium filament was deeply put in airtight retort,which was difficult for real-time monitoring,so in practice,hafnium filament temperature was characterized commonly by K value.K=UI^1/3,where U is the voltage across,I is the current passing through the filament,and K is a constant decided by filament length and temperature [ 18, 19, 20] .Hereafter,the hafnium filament temperature is referred to K value.Early related studies show that the better iodide effect can be received when the feed temperature is approximately 300℃ [ 7, 9] .In this article,the influences of four important factors on the impurity contents of hafnium crystal bar under iodide process were studied,and the results were proved by experiment.

2 Experimental

2.1 Materials and equipment

The hafnium crystal bar with low impurity contents was prepared by the iodide process using the raw hafnium(nuclear level hafnium sponge,electrolytic hafnium powder,and hafnium turnings),iodine (99.8%),and hafnium filament (99%) as raw materials.Hafnium sponge was bought from Ati Wha Chang,and the iodine was supplied by ACF Minera S.A.Based on the graphite crucible anode,stainless steel rods cathode,electrolytic hafnium powder was obtained by employing a mixture of alkali metal chloride as the electrolyte,and using hafnium compounds as feed for electrolysis.Hafnium powder particle size was about75-180μm.Before the iodide process,hafnium sponge (size of 3-18 mm) and hafnium powder should be acid-cleaned and water-rinsed to remove the impurities and the adhesion of molten salt separately,and put in drying oven for 10 h to remove the moisture adsorbed on the surface.Hafnium turnings were obtained by turning lathe processing from electron beam melting of the hafnium rods,and its size was(3-20) mm×(3-5) mm×(0.5-2.0) mm.The self-made iodide equipment was adopted,and the schematic illustration of iodide equipment is shown in Fig.1.The retort materials were 304 stainless steel,GH4109 nickel-base super alloy,and stainless steel with molybdenum cylinder liner.The retort material chemical components are shown in Table 1.If not specified,the process parameters were as follows:voltage 0-20 V,electric current 0-1,000A,vacuum degree<1×10^-2 Pa,retort temperature 300℃,iodine 100 g,hafnium sponge raw weight 5 kg,K value 15,molybdenum feed retainer as loading method,the retort material 304stainless steel,and the reaction time 12 h.

Fig.1 Schematic illustration of iodide device

2.2 Methodology

Single-variable method and inductive coupling plasma emission spectrometer were adopted in the experiment.Influences of K value,retort materials,feed types,and iodide reaction time on hafnium crystal bar impurity contents under iodide process were studied.The experimental parameters of K value were as follows:approximately 15,25,and 35,respectively.The retort material experimental parameters were as follows:304 stainless steel,GH4109nickel-base super alloy,and stainless steel with molybdenum cylinder liner.The feed type experimental parameters were as following:hafnium sponge,hafnium powder and hafnium turnings,molybdenum tray as loading method.The experimental parameters of the iodide reaction time were as follows:the first hafnium crystal bar (hafnium turnings) was used as the second iodide feed.The impurity contents of the hafnium crystal bar were analyzed by inductively coupled plasma emission spectrometer.

3 Results and discussion

3.1 Influence of K value on impurity contents of hafnium crystal bar

As shown in Table 2,compared with the sponge hafnium,all the three kinds of the hafnium crystal bars with different K values receive very good purification effect.The impurity contents of Fe,Ni,and Cr obviously decrease,and the other metal impurities are not introduced in the final hafnium products under iodide process.It also can be seen that the impurity contents of Fe,Ni in crystal bar decrease with the K value rising.In the iodide process of preparing hafnium crystal bar,the contents of metal impurity such as W,Mo,Mg,Al,and Si are very small due to the low vapor pressure of performing iodide.The low vapor pressure of performing iodide cannot be spread to or decomposed at the high temperature zone [ 9] .The iodine vapor can react with the device which is made of 304 stainless steel.The Fe,Ni impurities in crystal bars mainly come from the reaction between the iodine and Fe,Ni in the device.There are equal contents of performing Fe,Ni iodide due to the same retort,mass of iodine,and react temperature(300℃).Furthermore,the contents of iodide vapor and depositing Fe,Ni in the high temperature zone are nearly equal.However,according to the result studied in this laboratory by iodide process,the deposition rate increases with the K value rising,and the contents of hafnium crystal bar get bigger in the unit time.So,the contents of Fe,Ni in hafnium crystal bar decrease with the K value rising.

下载原图

Table 1 Chemical and spectrochemical analysis of retort material(wt%)

下载原图

Table 2 Chemical and spectrochemical analysis of hafnium crystal bar fabricated by different K values (wt%)

3.2 Influence of retort material on impurity contents of hafnium crystal bar

As indicated in Table 3,the contents of impurity elements between sponge hafnium and hafnium crystal bar were compared under iodide process.Compared with the sponge hafnium,three kinds of the hafnium crystal bar with different retort materials have very good purification effect.The impurity contents of Fe,Ni,and Cr are significantly reduced,and the other metal impurities are not brought in the final hafnium crystal bar under iodide process.It also can be seen that the impurity contents of Fe,Ni,and Cr in crystal bar change with the retort material.The contents of Ni,Cr in hafnium crystal bar employed by GH4109 nickelbase super alloy retort are higher.The contents of Fe in hafnium crystal bar employed by 304 stainless steel retort are greater.The contents of Fe,Ni,and Cr in hafnium crystal bar employed by stainless steel with molybdenum cylinder liner retort are the less than those of others.In the iodide process of preparing hafnium crystal bar,the contents of metal impurity such as W,Mo,Mg,and Al,and non-metal impurity as Si are very small due to the low vapor pressure of performing iodide.The low vapor pressure of performing iodide cannot be spread to or decomposed at the high temperature zone [ 9] .Fe,Ni,and Cr impurities in crystal bars mainly come from reaction between iodine vapor and the device.The contents of Ni,Cr in the GH4109 nickel-base super alloy are higher than those of others,so the contents of Ni,Cr in hafnium crystal bar used by GH4109 nickel-base super alloy retort are greater.Similarly,the contents of Fe in hafnium crystal bar employed by 304 stainless steel retort are higher.The stainless steel with molybdenum cylinder liner retort weakens the iodine vapor corrosion of equipment,so the contents of Fe,Ni,and Cr in hafnium crystal bar employed by stainless steel with molybdenum cylinder liner retort are the least in the above three retort materials.

下载原图

Table 3 Chemical and spectrochemical analysis of hafnium crystal bar fabricated by different retort materials (wt%)

3.3 Influence of feed types on impurity contents of hafnium crystal bar

As indicated in Table 4,the results of the feed types and the impurity contents of crystal bar illustrate that the impurity contents of Fe,Ni,and Cr are up to the maximum when the feed is hafnium powder;while the impurity contents of Fe,Ni,and Cr are the minimum when the materials are hafnium turnings.The other metal impurities are nearly equal when three types of feed were used under the iodide process.According to the result studied in this laboratory by iodide process,with the same process conditions except for the feed types,the fastest deposition rate is obtained by the crystal bar when hafnium turnings are employed as feed.The slowest deposition rate is achieved by the crystal bar when hafnium power is used as feed.The faster the deposition rate is,the bigger the mass of hafnium crystal bar is in the unit time.So,the contents of Fe,Ni,and Cr in hafnium crystal bar decrease with the deposition rate rising.The crystal bar with hafnium turnings as feed has lower impurity contents.

下载原图

Table 4 Chemical and spectrochemical analysis of hafnium crystal bar fabricated by different feed types (wt%)

3.4 Influence of iodide reaction time on impurity contents of hafnium crystal bar

As shown in Table 5,the results of the iodide reaction time and the impurity contents of crystal bar illustrate that the Fe impurity contents increase slightly,and the other impurity contents scarcely change.According to the analysis of influence of retort material on impurity contents of hafnium crystal bar,Fe impurities come from the stainless steel.The stainless steel with molybdenum cylinder liner retort has the role of weakening the iodine vapor corrosion of equipment,so the other impurity contents scarcely change.

4 Conclusion

In this study,the hafnium crystal bars employed with different K values,retort materials,feed types,and iodide reaction time were investigated.Experimental results show that the impurity contents of Fe,Ni in hafnium crystal bar decrease with the K value rising.Both the retort materials and feed types have obvious influence on the impurity contents of hafnium crystal bar.The elements enriched in retort material tend to become the main impurity elements in the hafnium crystal rods.The contents of Ni,Cr in hafnium crystal bar employed by GH4109 nickel-base super alloy retort are great,and the contents of Fe in hafnium crystal bar employed by 304 stainless steel retort are also great.The contents of Fe,Ni,and Cr in hafnium crystal bar employed by stainless steel with molybdenum cylinder liner retort are less than those of others.The impurity contents are up to maximum when the feed is hafnium power,while the impurity contents are minimum when the materials are hafnium turnings.The iodide reaction time has little effect on the impurity contents of hafnium crystal bar.

下载原图

Table 5 Chemical and spectrochemical analysis of hafnium crystal bar fabricated by different iodide reaction time (wt%)