Hafnium

Typical wastewaters

• primary zirconium and hafnium production (zircon sand chlorination and MIBK thiocyanate separation plants) hafnium in the filtrate left after the separated hafnium is precipitated as hydroxide, and in the MIBK iron extraction steam stripper bottoms; the hafnium filtrate is reused for its zirconium content or sent to evaporation ponds, so 40 CFR 421.333(h) gives it a zero allowance for chromium, cyanide, lead and nickel (a no discharge stream) the streams that are discharged (sand drying and chlorination scrubber waters, zirconium filtrate, MIBK stripper bottoms, acid leachate and leaching rinse from zirconium metal and alloy production) are limited for chromium, cyanide, lead, nickel and ammonia in mg per kg of zirconium dioxide plus hafnium dioxide produced (zirconium filtrate: chromium 14.35, cyanide 7.758, lead 10.86, nickel 21.33, ammonia as N 5171 mg/kg daily maximum); hafnium itself is not a limited pollutant and no hafnium concentration in any of these streams was read; EPA describes the zirconium filtrate as treated by ammonia steam stripping, chemical precipitation and sedimentation at 37,640 to 39,900 L per tonne of oxide

1 · Identity

Symbol, number

Hf, 72

Oxidation states in water

+4 only; in seawater the anionic hydroxo complex Hf(OH)₅⁻ (Ueki 2023). No redox chemistry in water.

Note

The element entry covers the metal, the zirconium twin problem and the nuclear use. This chapter says only how little hafnium water carries and why.

2 · Occurrence in water

Natural sources

Weathering of zircon and baddeleyite, in which hafnium rides with zirconium at about 1 part in 50 (element entry). Dissolved hafnium in the open ocean is depleted at the surface and rises with depth, a scavenged profile that follows dissolved silicon (Ueki 2023).

Anthropogenic sources

No effluent figure was read. The plausible sources are zirconium and hafnium chemical plants, nuclear fuel fabrication and heavy mineral sand processing, all of which discharge the element as particulate oxide.

3 · Speciation

Hf(IV) is a small, highly charged cation that hydrolyses fully above pH 2 to 3; in seawater the dissolved species is the pentahydroxo anion Hf(OH)₅⁻ (Ueki 2023). Solid HfO₂ and hydrous hafnium oxide are insoluble and inert to everything but hydrofluoric acid (element entry), so the element partitions to particles and is scavenged from the water column. Fluoride is the only common ligand that keeps it in solution.

Solubility

Hafnium(IV) oxide and hydroxide are insoluble at natural pH; no solubility product was read.

Hydrolysis

Complete at natural pH; the dissolved remainder is anionic hydroxo complex.

Complexation

Hydroxide and fluoride; carbonate and organic complexes were not sourced.

Precipitates

HfO₂ and hydrous hafnium oxide, in practice a trace within zirconium oxide solids.

4 · Role in treatment

Not relevant or not given for this element.

5 · Removal and control

6 · Analytics

Sampling pitfalls

Hafnium adsorbs to container walls and to any particle; filter and acidify at once, and expect fluoride in the sample to change the answer.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

Low chemical toxicity and no biological role (element entry); no drinking water assessment exists.

Bioaccumulation

Not addressed in the sources read.

Ecotoxicity

Not addressed in the sources read.

Flags

• The seawater range is one Pacific study; the element entry's PubChem figure is inconsistent with it and is flagged above.
• No industrial effluent or fresh water concentration was read; the sources statements are qualitative.

Gaps

• No source read gives hafnium in groundwater, rivers, municipal or industrial wastewater.
• EU DWD 2020/2184 Annex I was not read this session; its absence of a hafnium parameter is not asserted.
• No hydrolysis constants or solubility product for hafnium were read; the speciation is taken from the seawater species named by Ueki 2023.
• No hafnium removal study was read; the removal row is by analogy with any hydrolysed metal.
• No GCC discharge standard was read.

Sources

Identity

Name and symbol

Hafnium, Hf

Atomic number

72 protons

Position

group 4 · period 6 · d-block · transition metal

CAS number

7440-58-6

Atomic structure

Atomic mass

178.486 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d²
[Xe] 6s²⁴f¹⁴⁵d²

Electrons per shell

2, 8, 18, 32, 10, 2

Valence electrons

4 ns and (n-1)d

Physical properties

State at room temperature

Solid

Melting point

2,506 K (2,232.85 °C)

Boiling point

4,876 K (4,602.85 °C)

Density

13.3 g/cm3

Appearance

steel gray

Thermal conductivity

23.0 W/(m·K)

Electrical resistivity

331 nΩ·m at 20 °C

Electrical conductivity

3.02 MS/m

Crystal structure

hexagonal close packed

Molar heat capacity

25.73 J/(mol·K)

Chemical properties

Oxidation states

+4

Electronegativity

1.3 (Pauling Scale)

Ionisation energy

6.825 eV
1st 658.5, 2nd 1,440, 3rd 2,250 kJ/mol

Electron affinity

0 eV

Atomic radius

empirical 175, covalent 175, van der Waals 212 pm

Ionic radius

Hf⁴⁺ 71 pm

Reactivity

A group 4 metal whose chemistry is almost identical to zirconium's because the lanthanide contraction gives Hf4+ the same radius as Zr4+; a thin protective oxide film makes the bulk metal very corrosion resistant, but the powder is pyrophoric and the hot metal combines with most non-metals.

with water

No reaction under ordinary conditions; the protective hafnium oxide film keeps water and steam off the bulk metal.

with oxygen, air

Forms a protective film of monoclinic HfO2 in air that stops further attack; heated or finely divided hafnium burns: $\ce{Hf + O2 -> HfO2}$, and the powder can ignite spontaneously.

with acids

Resists most acids and concentrated alkalis; it is attacked by hydrofluoric acid, which dissolves the oxide film and forms fluoro complexes, and by hot concentrated sulfuric acid.

with halogens

The halogens react directly, on heating, to give the volatile tetrahalides: $\ce{Hf + 2 Cl2 -> HfCl4}$, and the chloride and iodide are used in producing and purifying the metal.

Typical compounds

• HfO₂ hafnium(IV) oxide hafnia, high-melting insulator used as a gate dielectric
• HfCl₄ hafnium(IV) chloride volatile tetrahalide, precursor for hafnia films
• HfF₄ hafnium(IV) fluoride the fluoride, listed by Los Alamos
• HfC hafnium carbide the most refractory binary compound, melts near 3890 C
• HfN hafnium nitride the most refractory metal nitride, melts about 3305 C

Occurrence, production and use

Crustal abundance

3.0 milligrams per kilogram

Oceanic abundance

7×10-6 milligrams per liter

Occurrence and sources

• in zircon (ZrSiO4) at Zr:Hf about 50:1, and in baddeleyite every zirconium deposit; heavy-mineral sands of Australia, South Africa, Mozambique, Senegal, Florida
• crustal and oceanic abundance about 3 ppm (BGS figure via RSC); 3 mg/kg crust and 0.000007 mg/L seawater (PubChem)

Extraction, production

Separation from zirconium chemical intermediates, then reduction of hafnium tetrachloride with magnesium or sodium

$\ce{HfCl4 (g) + 2 Mg (l) -> Hf (s) + 2 MgCl2 (l)}$

RSC states the tetrachloride is reduced with sodium or magnesium; the sodium version is $\ce{HfCl4 + 4 Na -> Hf + 4 NaCl}$; the metal chloride co-products are implied by the reduction, not named; the zirconium-hafnium separation chemistry is not stated

Iodide decomposition to pure metal (1925)

$\ce{HfI4 (g) -> Hf (s) + 2 I2 (g)}$

thermal decomposition on a hot tungsten wire

Uses

Hafnium is a good absorber of neutrons and is used in the control rods of nuclear reactors. Hafnium is also used in vacuum tubes as a getter, a material that combines with and removes trace gases from vacuum tubes. Hafnium has been used as an alloying agent in iron, titanium, niobium and other metals.

Melting near 3890°C, hafnium carbide (HfC) has the highest melting point of any known two-element compound. Hafnium nitride (HfN) also has a high melting point, around 3305°C. Other hafnium compounds include: hafnium chloride (HfCl4), hafnium fluoride (HfF4) and hafnium oxide (HfO2).

Because the element not only has a good absorption cross section for thermal neutrons (almost 600 times that of zirconium), but also excellent mechanical properties and is extremely corrosion-resistant, hafnium is used for reactor control rods. Such rods are used in nuclear submarines.

Hafnium is used in gas-filled and incandescent lamps, and is an efficient getter for scavenging oxygen and nitrogen.

• Aerospace superalloys: hafnium in nickel-base superalloys; the leading use of the metal; alloys with iron, titanium and niobium
• Nuclear: neutron-absorbing control rods, including nuclear submarines; boron or cadmium-silver-indium alloys substitute
• Electronics and welding: hafnium oxide gate dielectric in microchips; plasma welding torch electrodes
• Chemicals: hafnium polymerisation catalysts
• Mining: hafnium content of zircon from heavy-mineral sand mining

Safety, toxicity

Finely divided hafnium is pyrophoric and can ignite spontaneously in air. Care should be taken when machining the metal or when handling hot sponge hafnium.

Exposure to hafnium should not exceed 0.5 mg/hr. (8 hour time-weighted average - 40-hour week).

GHS classification, signal word Danger

• H250 Catches fire spontaneously if exposed to air Pyrophoric liquids

Discovery and name

Discovered by

Dirk Coster and George de Hevesy

Discovered

1922

First isolated

not in sources

Named by

not in sources

Origin of the name

after Hafnia]]. Latin for: Copenhagen, where it was discovered

Hafnium is a ductile metal with a brilliant silver luster. Its properties are considerably influenced by presence of zirconium impurities. Of all the elements, zirconium and hafnium are two of the most difficult to separate. Although their chemistry is almost identical, the density of zirconium is about half of hafnium. Very pure hafnium has been produced, with zirconium being the major impurity.

Hafnium has been successfully alloyed with iron, titanium, niobium, tantalum, and other metals. Hafnium carbide is the most refractory binary composition known, and the nitride is the most refractory of all known metal nitrides (m.p. 3310C). At 700 degrees C hafnium rapidly absorbs hydrogen to form the composition HfH1.86.

Hafnium is resistant to concentrated alkalis, but at elevated temperatures reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon. Halogens react directly to form tetrahalides.