Tantalum

Typical wastewaters

• secondary tantalum recovery (capacitor scrap, tantalum sludge and tantalum alloy scrap leaching) dissolved and fine particulate tantalum in the acid leach and rinse water, the tantalum powder acid wash water and the leaching scrubber water, together with copper, lead, nickel and zinc from the scrap 40 CFR 421.283 sets a tantalum limit as a daily maximum only, in mg per kg of tantalum powder produced: 103.8 for tantalum alloy leach and rinse, 9.09 for capacitor leach and rinse, 92.39 for tantalum sludge leach and rinse, 0.158 for tantalum powder acid wash and rinse, 2.196 for leaching wet air pollution control; no monthly average and no concentration in mg/L is given
• primary columbium-tantalum hydrometallurgy (hydrofluoric acid digestion, MIBK solvent extraction, K₂TaF₇ precipitation and reduction to metal) spent hydrofluoric acid raffinate, ammonia precipitation filtrate, tantalum salt drying and tantalum powder wash water: a fluoride and ammonia matrix in which tantalum is the process metal but not a regulated pollutant 40 CFR 421.113 limits lead, zinc, ammonia and fluoride per kg of concentrate digested or salt handled (solvent extraction raffinate: fluoride 324.0 and ammonia as N 1233 mg/kg daily maximum; tantalum powder wash: fluoride 715.2 and ammonia as N 2724 mg/kg); EPA names the raffinate containing the spent hydrofluoric acid and the scrubber liquors as wastewater; no tantalum concentration in any stream was read

1 · Identity

Symbol, number

Ta, 73

Oxidation states in water

+5 only; in seawater the neutral hydroxo complex Ta(OH)₅ (Ueki 2023). No redox chemistry in water.

Note

The element entry covers the metal, coltan and the capacitor industry. This chapter says only how little tantalum water carries and why.

2 · Occurrence in water

Natural sources

Weathering of columbite-tantalite and of tin ore, which carries it (element entry). In the subarctic North Pacific dissolved tantalum is uniform with depth at about 0.015 pmol/kg, unlike hafnium, which rises with depth (Ueki 2023).

Anthropogenic sources

No effluent figure was read. The plausible sources are coltan concentrators and tin smelter slag processing, tantalum chemical plants making potassium heptafluorotantalate, and capacitor manufacture; the fluoride route keeps tantalum dissolved as fluoro complexes until the fluoride is stripped.

3 · Speciation

Ta(V) hydrolyses fully at natural pH; the dissolved species in seawater is the neutral Ta(OH)₅ (Ueki 2023). The pentoxide is insoluble and inert to every acid but hydrofluoric, which dissolves it as fluoro complexes such as TaF₇²⁻ (element entry). The near uniform depth profile means tantalum is neither strongly scavenged nor regenerated once dissolved, but the absolute level is so low that the element is effectively absent from water.

Solubility

Tantalum pentoxide is insoluble at natural pH; no solubility product was read.

Hydrolysis

Complete at natural pH; the dissolved remainder is the neutral pentahydroxo complex.

Complexation

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

Precipitates

Ta₂O₅ and hydrous tantalum oxide.

4 · Role in treatment

Not relevant or not given for this element.

5 · Removal and control

6 · Analytics

Sampling pitfalls

Tantalum adsorbs to container walls and particles at these levels; filter and acidify at once, and keep fluoride out of the sample train unless it is meant to be there.

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

Non toxic, no biological role, no immune response, which is why it is implanted in the body (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 source statements are qualitative.

Gaps

• No source read gives tantalum in groundwater, rivers, municipal or industrial wastewater.
• EU DWD 2020/2184 Annex I was not read this session; its absence of a tantalum parameter is not asserted.
• No hydrolysis constants or solubility product for tantalum were read; the speciation is taken from the seawater species named by Ueki 2023.
• No tantalum 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

Tantalum, Ta

Atomic number

73 protons

Position

group 5 · period 6 · d-block · transition metal

CAS number

7440-25-7

Atomic structure

Atomic mass

180.947 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d³
[Xe] 6s²⁴f¹⁴⁵d³

Electrons per shell

2, 8, 18, 32, 11, 2

Valence electrons

5 ns and (n-1)d

Physical properties

State at room temperature

Solid

Melting point

3,290 K (3,016.85 °C)

Boiling point

5,731 K (5,457.85 °C)

Density

16.4 g/cm3

Appearance

gray blue

Thermal conductivity

57.5 W/(m·K)

Electrical resistivity

131 nΩ·m at 20 °C

Electrical conductivity

7.63 MS/m

Crystal structure

body-centered cubic

Molar heat capacity

25.36 J/(mol·K)

Chemical properties

Oxidation states

+5

Electronegativity

1.5 (Pauling Scale)

Ionisation energy

7.89 eV
1st 761, 2nd 1,500 kJ/mol

Electron affinity

0.322 eV

Atomic radius

empirical 170, covalent 170, van der Waals 217 pm

Ionic radius

Ta³⁺ 72; Ta⁴⁺ 68; Ta⁵⁺ 64 pm

Reactivity

A group 5 refractory metal that is almost completely immune to chemical attack below about 150 C because a dense, stable Ta2O5 film covers it; in aqueous chemistry only the +5 state exists, and the metal becomes reactive only at high temperature.

with water

Does not react with water; the oxide film protects it.

with oxygen, air

Passivated by a thin oxide film at room temperature; on heating in air the metal oxidises to the pentoxide: $\ce{4 Ta + 5 O2 -> 2 Ta2O5}$, the stable dielectric oxide.

with acids

Attacked only by hydrofluoric acid, by acid solutions containing fluoride ion and by free sulfur trioxide; hot concentrated sulfuric acid and hot alkalis corrode it slowly, and aqua regia below 150 C does not.

with halogens

Reacts with the halogens on heating to the pentahalides: $\ce{2 Ta + 5 Cl2 -> 2 TaCl5}$, a volatile solid, and with fluorine to TaF5.

Typical compounds

• Ta₂O₅ tantalum(V) oxide dielectric in tantalum capacitors, high-index lens glass
• TaC tantalum carbide extremely hard, one of the highest melting materials
• TaCl₅ tantalum(V) chloride volatile halide, Lewis acid and precursor
• K₂TaF₇ potassium heptafluorotantalate the salt reduced with sodium to make the metal
• LiTaO₃ lithium tantalate ferroelectric crystal for surface acoustic wave devices

Occurrence, production and use

Crustal abundance

2.0 milligrams per kilogram

Oceanic abundance

2×10-6 milligrams per liter

Occurrence and sources

Tantalum ores are found in Australia, Brazil, Mozambique, Thailand, Portugal, Nigeria, Zaire, and Canada.

• columbite-tantalite (coltan) in pegmatites Congo (Kinshasa), Rwanda, Nigeria, Brazil, Australia, China, Mozambique, Ethiopia
• by-product of tin extraction (cassiterite concentrates and tin slags) tin belts of south-east Asia, Africa, Brazil
• crustal and oceanic abundance about 0.7 ppm (BGS figure via RSC); 2 mg/kg crust and 0.000002 mg/L seawater (PubChem)

Extraction, production

Separation of tantalum from niobium in coltan concentrates

RSC describes several complicated steps; USGS names potassium fluorotantalate and tantalum oxide as traded intermediates and tantalum powder, alloys and carbides as products; no reaction is stated

Recycling of new scrap from capacitor manufacture, carbide and superalloy scrap

may supply up to 30 percent of consumption by US primary processors

Uses

Tantalum is a strong, ductile metal that is nearly immune to chemical attack at room temperatures. It can be drawn into a fine wire that is used to evaporate metals, such as aluminum. It has a high melting point and is frequently used as a substitute for platinum, which is more expensive. Tantalum is used to make components for chemical plants, nuclear power plants, airplanes and missiles. Tantalum does not react with bodily fluids and is used to make surgical equipment. Tantalum also does not irritate the body and is used to make surgical sutures as well as implants, such as artificial joints and cranial plates. Tantalum is alloyed with steel to increase steel's ductility, strength and melting point.

Tantalum pentoxide (Ta2O5), one of tantalum's compounds, is a dielectric material and is used to make capacitors. It is also used to make a glass with a high index of refraction that is used in camera lenses. A composite consisting of tantalum carbide (TaC) and graphite is one of the hardest materials known and is used on the cutting edges of high-speed machine tools.

Scientists at Los Alamos have produced a tantalum carbide graphite composite material, which is said to be one of the hardest materials ever made. The compound has a melting point of 3738°C. Tantalum is used to make electrolytic capacitors and vacuum furnace parts, which account for about 60% of its use. The metal is also widely used to fabricate chemical process equipment, nuclear reactors, aircraft, and missile parts. Tantalum is completely immune to body liquids and is a nonirritating material. It has, therefore, found wide use in making surgical appliances. Tantalum oxide is used to make special glass with high index of refraction for camera lenses. The metal has many other uses.

• Electronics: tantalum capacitors for phones and portable devices; sputtering targets for chip fabrication; demand rose with data centres in 2024 one of the main uses (rsc-element-73); no percentage published
• Medicine: surgical implants: skull plates, nerve foil and wire, abdominal gauze
• Chemicals (corrosion-resistant equipment): equipment for handling corrosive materials; tantalum crucibles
• Aerospace and tooling: superalloy turbine blades, rocket nozzles, supersonic nose caps; tantalum carbide cutting tools; neon-light electrodes, rectifiers, special lens glass
• Mining: coltan mining and by-product recovery from tin (cassiterite) extraction

Safety, toxicity

GHS classification, signal word Danger

• H228 Flammable solid Flammable solids
• H252 Self-heating in large quantities; may catch fire Self-heating substances and mixtures
• H335 May cause respiratory irritation Specific target organ toxicity, single exposure; Respiratory tract irritation
• H302 Harmful if swallowed Acute toxicity, oral
• H320 Causes eye irritation Serious eye damage/eye irritation

Discovery and name

Discovered by

Anders Gustaf Ekeberg

Discovered

1802

First isolated

not in sources

Named by

not in sources

Origin of the name

after Tantalus, Greek mythological figure

Tantalum is a gray, heavy, and very hard metal. When pure, it is ductile and can be drawn into fine wire, which is used as a filament for evaporating metals such as aluminum. Tantalum is almost completely immune to chemical attack at temperatures below 150°C, and is attacked only by hydrofluoric acid, acidic solutions containing the fluoride ion, and free sulfur trioxide. Alkalis attack it only slowly. At high temperatures, tantalum becomes much more reactive. The element has a melting point exceeded only by tungsten and rhenium. Tantalum is used to make a variety of alloys with desirable properties such as high melting point, high strength, good ductility, etc. Tantalum has a good "gettering" ability at high temperatures, and tantalum oxide films are stable and have good rectifying and dielectric properties.