Text 17

Hafnium

Interest in hafnium has increased owing to improved methods for its separation from zirconium and the possible use of zirconium and hafnium in light- weight refractory materials for aircraft construction.

The chemistry of hafnium began about 75 years ago. Coster and Hevesy decided to look for element 72 in minerals containing quadrivalent zirconium. Accordingly, the residues remaining after Norwegian and Greenland zircons had been leached in boiling acid, were placed on a copper anticathode and an X ray spectroscopic analysis undertaken. Coster and Hevesy announced the discovery of element 72, proposing the name hafnium in honour of the city in which the discovery was made. This name is generally accepted in France.

Occurrence

Hafnium occurs in nature in small to moderate amount associated with zirconium in all types of zirconium - bearing mineral. The ratio of hafnium to zirconium has been estimated to be about 0.02. Zircons from granite rocks are reported to have higher Hf/Zr ratios than minerals from alkalic ones. Zircons, which may contain up to 7% hafnium, may have their hafnium content determined by radioactivity measurements. Inasmuch as hafnium is isomorphous with uranium and thorium, a constant ratio between hafnium and these radioactive elements is obtained. It is estimated that there are 4 parts per million of hafnium in the earth's crust water-cooled nuclear reactors. In addition, hafnium is known to be used for making special glasses. It has been applied as a filament in incandescent lights, as a cathode in X - ray tubes, and as an electrode in high pressure discharge tubes. Hafnium - titanium alloys may be used as getters in evacuated or gas filled devices such as lamps, radio tubes, and television tubes.

Extraction

Hafnium and zirconium are always extracted together, and the separation of hafnium from zirconium is an important step in the production of reactor - grade zirconium. Thus any technique employed to separate the two elements necessarily must be fractional one.

Extraction of hafnium containing mineral is complicated by the fact that following the fusion of the mineral with ammonium hydrogen fluoride, the fused product may contain a considerable amount associated impurities and as a consequence is not easily dissolve. This necessitates repeated extractions, which result in the accumulation of a large volume in solution. The main problem in the production of hafnium is its separation from zirconium, inasmuch as no chemical reaction is known which is exhibited by one of these elements but not by the other. It is evident that any method used to open zirconium minerals will result in the inclusion of both zirconium and hafnium in the solution.

Physical, mechanical and chemical properties

The atomic number of hafnium is 72, the boiling point is known to be 54000C. Metallic hafnium has a brilliant luster. It is harder and less easily worked than zirconium. After melting in an arc furnace under argon1, hafnium is considered to be hot - rolled in air at 8400C. It can be cold - rolled into sheet, swaged or drawn into wire or rod. If the metal is cold reduced in thickness more than 30%, the sheet will fracture if bent. Cold - worked hafnium , if annealed in vacuum or in inert atmosphere, recrystalllizes between 7000 and 8000C.

Hafnium has excellent mechanical properties and is extremely corrosion resistant. The machineability of hafnium resembles that of stainless steel. Being a gas sensitive metal, traces of gases ruin its malleability, increase its electrical resistance, and decrease its temperature coefficient of resistance; its electrical conductivity being, at best, 6% that of copper.

The chemical properties of hafnium resemble those exhibited by zirconium very closely.

Hafnium freshly prepared in vacuum is so reactive that no dioxidizers for it are known. The rate of penetration of oxygen into metallic hafnium is lower than that for zirconium.

Applications

Hafnium had few commercial uses because of its limited supply and high price, which were due to the difficulty in obtaining it free from its ores.

In recent years, however, this metal has become somewhat more readily available as by-product of reactor-grade zirconium 2, and considerable interest has been aroused in its potential usefulness as a control material in water -cooled nuclear reactors. In addition, hafnium is known to be used for making special glasses. It has been applied as a filament in incandescent lights, as a cathode in X - ray tubes, and as an electrode in high pressure discharge tubes.

Hafnium - titanium alloys may be used as getters in evacuated or gas -filled devices such as lamps, radio tubes, television tubes.

Notes

1. under argon: trong khÝ quyÓn argon

2. reaction-grade zirconium: lß lµm s¹ch zirconi

Exercises

I. Answer the following questions:

1. When did the chemistry of hafnium begin?

2. Who was the first to discover the hafnium?

3. What does the word " hafnium " mean?

4. What can you say about the physical properties of hafnium?

5. What are the chemical properties of hafnium?

6. Is hafnium widely used nowadays?

II. Describe the process of hafnium extraction.

III. Translate the following sentences into Vietnamese paying attention to the words in bold type:

1. Inasmuch as hafnium is isomorphous with uranium and thorium, a constant ratio between hafnium and these elements is obtained.

2. The main problem in the production of hafnium is its separation from zirconium, inasmuch as no chemical reaction is known which is exhibited by one of these elements.

3. Inasmuch as the color of any oil product is practically always improve by acid, the color is to a certain extent taken as a criterion of the degree of refining.

4. Inasmuch as the rare earth metal are very difficult to prepare in pure form, these has been a lack of data on their physical properties.

IV. Select attributes(from the list given below) to the following words and translate them into Vietnamese:

Amount

Analysis

Atmosphere

Conductivity

Crust

Filament

Number

Tube

Vacuum

Zirconium atomic

earth's

deep electrical

high

hot-rolled

incandescent

inert

moderate

spectroscopic

X-ray

V. Give Vietnamese equivalent to the following words:

1. extract, extraction, extractive, extractor;

2. oxide, oxidize, oxidizer, deoxidizer, deoxidate, deoxidation, deoxidization;

3. nuclear, nucleate, nucleated, nucleation, nuclei, nucleonics, nucleus;

VI. State the key words in the following word combinations and translate them into Vietnamese:

arc furnace

cold-rolled hafnium

gas-filled device

gas-sensitive metal hafnium-containing minerals

reactor-grade zirconium

zircon-bearing minerals

water-cooled nuclear reactor

VII. Translate the following sentences into Vietnamese paying attention to the words in bold type:

1. It is known that elementary hafnium was first prepared by Hevesy by the reduction of potassium hexafluohafnate with sodium.

2. The rate of penetration of oxygen into metallic hafnium is lower than that for zirconium.

3. The machineability of hafnium resembles that of stainless steel.

4. The chemical properties of hafnium resemble those exhibited by zirconium very closely.

VIII. Replace subordinate clauses by complex subject:

Model: a) It is known that hafnium is used for making special glasses.

b) Hafnium is known to be used for making special glasses.

1. It was proved that the boiling point of hafnium was 54000C.

2. It is considered that hafnium is hot-roll in air at 8400C.

3. It is known that zircons from granite rocks have higher Hf/Zn ratios than minerals from alkalic rocks.

4. It has been estimated that the ratio of hafnium to zirconium is 0.02

5. It is reported that the main problem in the production of hafnium is its separation from zirconium.

IX. Translate into English:

1. Nh­ ®• biÕt, trong tù nhiªn hafni ®­îc t×m thÊy hÇu nh­ trong tÊt c¶ c¸c kho¸ng vËt chøa zirconi.

2. VÒ mÆt tÝnh chÊt c¬ häc, hafni gièng nh­ thÐp kh«ng gØ.

3. Ng­êi ta ®• x¸c ®Þnh ®­îc r»ng, hafni chÞu ®­îc qu¸ tr×nh c¸n nãng còng nh­ c¸n l¹nh.

4. Nh­ s• biÕt hafni võa ®­îc ®iÒu chÕ trong ch©n kh«ng cã kh¶ n¨ng ph¶n øng ho¸ häc h¬n bÊt cø mét chÊt khö ®• biÕt nµo.

5. Mét mÆt v× nh÷ng khã kh¨n trong viÖc ®iÒu chÕ hafni ë tr¹ng th¸i tù do, mÆt kh¸c v× gi¸ thµnh cao mµ ngµy nay viÖc sö dông hafni rÊt h¹n chÕ.