Scandium

Typical wastewaters

• titanium dioxide pigment production (chloride process spent iron chloride acid) Sc³⁺ at 10 to 140 ppm in the acidic iron chloride by-product solution, co-extracted with Zr, Ti and V on ion exchange
• titanium dioxide production waste acid and solid waste scandium in the waste acid used to leach scandium-bearing solid waste, recovered by P₂₀₄ and TBP solvent extraction after iron removal
• acid mine drainage (abandoned mine, San Luis, Argentina) trace Sc³⁺ measured by on-line preconcentration ICP-OES detection limit 4 ng/L, linear to 10 mg/L

1 · Identity

Symbol, number

Sc, 21

Oxidation states in water

+3 only, as Sc³⁺ hydrolysed and complexed: Klimpel and Bau (2023) state that scandium forms strong bonds with organic ligands and with OH⁻ and F⁻, and that these complexes usually dominate its speciation in natural water.

Note

A small, hard trivalent ion that behaves like a light rare earth but is scavenged by particles even more strongly.

2 · Occurrence in water

Natural sources

Trace substitution in more than 100 minerals (element entry); weathered into rivers bound to organic colloids and nanoparticles, then scavenged onto particles in the ocean. Dissolved scandium correlates with dissolved organic carbon in boreal rivers (r squared 0.67), so organic colloids carry it.

Anthropogenic sources

None documented in the sources read; scandium is a by-product of titanium dioxide, nickel laterite and uranium processing and any release is inside those effluents (element entry).

3 · Speciation

Sc³⁺ is hydrolysed and complexed by hydroxide, fluoride and organic ligands at natural pH and is strongly particle reactive; in rivers it rides on organic colloids and in the sea it is scavenged, giving picomolar concentrations. It decouples from the rare earths in organic rich rivers (Klimpel and Bau 2023). Not relevant to treatment.

Solubility

Not relevant; Sc(OH)₃ and ScPO₄ would control at concentrations never met in water.

Hydrolysis

Strong; the trivalent ion hydrolyses at low pH like the light rare earths.

Complexation

Organic ligands, hydroxide and fluoride dominate (Klimpel and Bau 2023).

Precipitates

None relevant in water treatment.

4 · Role in treatment

Not relevant or not given for this element.

5 · Removal and control

Not relevant or not given for this element.

6 · Analytics

Sampling pitfalls

Filter in the field; colloidal and particulate scandium dominate, so filtered and unfiltered results differ many fold.

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

Not addressed in the sources read; the element entry notes no biological role.

Bioaccumulation

Not addressed in the sources read.

Ecotoxicity

Not addressed in the sources read.

Flags

• The seawater ranges are quoted through Klimpel and Bau 2023, not from Parker and others 2016 directly.
• The scandium as internal standard remark is general laboratory practice, not from a source read.

Gaps

• No toxicity, ecotoxicity, discharge or industrial water figure exists in the sources read; the sections are left as not relevant.
• Parker and others 2016 (Geophysical Research Letters) was not read directly.

Sources

Identity

Name and symbol

Scandium, Sc

Atomic number

21 protons

Position

group 3 · period 4 · d-block · transition metal

CAS number

7440-20-2

Atomic structure

Atomic mass

44.955 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹
[Ar] 4s²³d¹

Electrons per shell

2, 8, 9, 2

Valence electrons

3 ns and (n-1)d

Physical properties

State at room temperature

Solid

Melting point

1,814 K (1,540.85 °C)

Boiling point

3,109 K (2,835.85 °C)

Density

2.99 g/cm3

Appearance

silvery white

Thermal conductivity

15.8 W/(m·K)

Electrical resistivity

α, poly: 562 nΩ·m

Electrical conductivity

1.78 MS/m

Crystal structure

hexagonal close packed

Molar heat capacity

25.52 J/(mol·K)

Chemical properties

Oxidation states

+3

Electronegativity

1.36 (Pauling Scale)

Ionisation energy

6.561 eV
1st 633.1, 2nd 1,235, 3rd 2,388.6 kJ/mol

Electron affinity

0.188 eV

Atomic radius

empirical 170, covalent 170, van der Waals 211 pm

Ionic radius

Sc³⁺ 75 pm

Reactivity

The first transition metal ([Ar] 3d1 4s2), chemically closer to yttrium and the rare earths than to aluminium or titanium; a soft, reactive metal whose chemistry is almost entirely that of the Sc3+ ion.

with water

Reacts with water, giving hydrogen and the hydroxide; solutions of Sc3+ are acidic by hydrolysis.

with oxygen, air

Develops a yellowish or pinkish oxide cast on exposure to air; turnings ignite and burn with a brilliant yellow flame to scandium oxide: $\ce{4 Sc + 3 O2 -> 2 Sc2O3}$

with acids

Dissolves slowly in most dilute acids to Sc3+ salts and hydrogen, but is not attacked by a 1:1 mixture of nitric acid and 48 percent hydrofluoric acid, probably because a passive fluoride layer forms.

with halogens

Forms the trihalides ScF3, ScCl3, ScBr3 and ScI3, Lewis acids of which only the fluoride is insoluble in water: $\ce{2 Sc + 3 Cl2 -> 2 ScCl3}$

Typical compounds

• Sc₂O₃ scandium oxide scandia; the traded form, about 20 tonnes a year
• ScF₃ scandium fluoride insoluble fluoride; reduced with calcium to the metal
• ScCl₃ scandium chloride soluble, mostly ionic trihalide
• ScI₃ scandium iodide added to mercury vapour lamps for sunlike light
• Sc(OH)₃ scandium hydroxide amphoteric, dissolves in excess alkali to scandate

Occurrence, production and use

Crustal abundance

2.2×101 milligrams per kilogram

Oceanic abundance

6×10-7 milligrams per liter

Occurrence and sources

Scandium is apparently much more abundant (the 23rd most) in the sun and certain stars than on earth (the 50th most abundant). It is widely distributed on earth, occurring in very minute quantities in over 800 mineral species. The blue color of beryl (aquamarine variety) is said to be due to scandium. It occurs as a principal component in the rare mineral thortveitite, found in Scandinavia and Malagasy. It is also found in the residues remaining after the extraction of tungsten from Zinnwald wolframite, and in wiikite and bazzite.

Most scandium is presently being recovered from thortveitite or is extracted as a by-product from uranium mill tailings. Metallic scandium was first prepared in 1937 by Fischer, Brunger, and Grienelaus who electrolyzed a eutectic melt of potassium, lithium, and scandium chlorides at 700 to 800°C. Tungsten wire and a pool of molten zinc served as the electrodes in a graphite crucible. Pure scandium is now produced by reducing scandium fluoride with calcium metal.

The production of the first pound of 99% pure scandium metal was announced in 1960.

• trace Sc3+ substituted in more than 100 minerals widely dispersed through the lithosphere; crustal abundance greater than lead; resources identified in Australia, Canada, China, Finland, Guinea, Kazakhstan, Madagascar, Norway, the Philippines, Russia, South Africa, Ukraine and the United States
• thortveitite (scandium yttrium silicate) the rare collectable mineral of Scandinavia; the main historic source with uranium mill tailings
• by product streams titanium dioxide pigment waste, nickel laterite tailings and leach residues, uranium and rare earth processing, zirconium and iron ore streams; Australia reports about 37,000 tonnes of scandium in economic demonstrated resources

Extraction, production

By product recovery from process streams

No scandium is mined for its own sake; it is leached and precipitated from titanium dioxide, nickel laterite (high pressure acid leach), uranium and rare earth streams. In 2024 a Philippine nickel plant recovered about 11,000 tonnes of scandium oxalate for conversion to oxide in Japan, a Quebec complex made 3 tonnes per year of oxide from titanium dioxide waste, and a French project plans 21 tonnes per year from 2026. Global production about 40 tonnes of oxide equivalent in 2024. The hydrometallurgy is not given as a reaction by the source.

Metal by calcium reduction of the fluoride

$\ce{2 ScF3 + 3 Ca -> 2 Sc + 3 CaF2}$

Balanced from the reactants and products stated by the sources (scandium fluoride reduced with calcium metal). Fischer, Brunger and Grienelaus first made the metal in 1937 by electrolysing a eutectic melt of potassium, lithium and scandium chlorides at 700 to 800 degrees C with tungsten wire and molten zinc electrodes; the first pound of 99 percent metal was announced in 1960.

Uses

Alloys of scandium and aluminum are used in some kinds of athletic equipment, such as aluminum baseball bats, bicycle frames and lacrosse sticks. It is expected that scandium-aluminum alloys will be important in the manufacture of fuel cells.

Scientists have only studied a few compounds of scandium. About 20 kilograms (44 pounds) of scandium oxide (Sc2O3), also known as scandia, are used each year in the United States in the production of high intensity lights. Scandium iodide (ScI3) is added to mercury vapor lamps so that they will emit light that closely resembles sunlight.

About 20 kg of scandium (as Sc2O3) are used yearly in the U.S. to produce high-intensity lights. The radioactive isotope 46Sc is used as a tracing agent in refinery crackers for crude oil, etc.

Scandium iodide added to mercury vapor lamps produces a highly efficient light source resembling sunlight, which is important for indoor or night-time color TV.

• Aerospace and light alloys: aluminium scandium alloys for aircraft, bicycle frames and sporting goods; scandium aluminium master alloy is the main traded form aluminium scandium alloys and solid oxide fuel cells were the principal uses in 2024 (USGS, ranking only)
• Energy: solid oxide fuel cells, the use that with aluminium alloys drove the recent rise in consumption
• Lighting and electronics: scandium iodide in mercury vapour lamps for daylight quality studio lighting; ceramics, lasers and radioactive tracers (scandium-46 in oil refining and pipeline leak detection)

Safety, toxicity

Little is yet known about the toxicity of scandium; therefore it should be handled with care.

GHS classification, signal word Danger

• H228 Flammable solid Flammable solids

Discovery and name

Discovered by

Lars Fredrik Nilson

Discovered

1879

First isolated

W. Fischer, K. Brünger, H. Grieneisen

Named by

not in sources

Origin of the name

after Scandinavia

Scandium is a silver-white metal which develops a slightly yellowish or pinkish cast upon exposure to air. A relatively soft element, scandium resembles yttrium and the rare-earth metals more than it resembles aluminum or titanium.

It is a very light metal and has a much higher melting point than aluminum, making it of interest to designers of spacecraft. Scandium is not attacked by a 1:1 mixture of HNO3 and 48% HF.

Chemically it is one of the alkaline earth elements; it readily forms a white coating of nitride in air, reacts with water, burns with a yellow-red flame.