Rhenium

Typical wastewaters

• molybdenite roasting (scrubber liquor)
  perrhenate ReO₄⁻ in the flue dust scrubber liquor of molybdenum roasters, from which it is recovered as perrhenic acid or ammonium perrhenate on strong base resin
  a recovery liquor rather than a discharge

1 · Identity

Symbol, number

Re, 75

Oxidation states in water

+7 as perrhenate ReO₄⁻ in oxic water, mobile and conservative; +4 as insoluble ReO₂ and as rhenium taken into sulfide and organic rich sediment under reducing conditions. The lower states are not stable in water.

Note

The element chapter covers the metal, the superalloy and catalyst markets and recovery from molybdenite roaster dust as ammonium perrhenate. This chapter is about ReO₄⁻ as a conservative tracer and technetium analogue.

2 · Occurrence in water

Natural sources

Oxidative weathering of black shales and of pyrite in basalts releases rhenium as perrhenate (Rahaman 2012); it stays dissolved through estuaries and the oxic ocean and is removed only into anoxic sediment, which is why the marine rhenium budget is used to read past ocean anoxia.

Anthropogenic sources

Indian Peninsular rivers carry a large anthropogenic rhenium load that accounts for most of their rhenium and about 70 percent of the total rhenium supply from Indian rivers to the sea, most of it in the Godavari (Rahaman 2012); the study does not name the industries. Molybdenite roasting sends rhenium to flue dust and scrubber liquor from which it is recovered as perrhenic acid or ammonium perrhenate (element chapter); molybdenum and copper mine and mill waters are the expected point sources.

3 · Speciation

In oxic water rhenium is the tetrahedral oxyanion perrhenate ReO₄⁻, the conjugate base of the strong acid HReO₄, so it carries a single negative charge at any pH, forms no hydroxide, is not sorbed by oxides or clays and travels with the water. Under sulfidic or organic rich reducing conditions it is reduced to Re(IV) and fixed in sediment, the same behaviour as technetium, uranium and molybdenum. The element chapter gives the formation of perrhenic acid from the heptoxide; no stability constants were read.

Solubility

Perrhenates of the alkali metals and ammonium are soluble; ReO₂ is insoluble (element chapter).

Hydrolysis

None; ReO₄⁻ is a stable oxyanion.

Complexation

Negligible; the anion does not pair strongly with the major cations.

Precipitates

None in natural water; ammonium perrhenate crystallises from concentrated recovery liquors.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Rhenium is conservative and easy to keep in solution; filter and acidify as for any trace metal. The risk is contamination from tungsten and molybdenum alloys in sampling hardware.

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

No known biological role (element chapter); no drinking water assessment exists.

Identity

Name and symbol

Rhenium, Re

Atomic number

75 protons

Position

group 7 · period 6 · d-block · transition metal

CAS number

7440-15-5

Atomic structure

Atomic mass

186.207 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d⁵
[Xe] 6s²⁴f¹⁴⁵d⁵

Electrons per shell

2, 8, 18, 32, 13, 2

Valence electrons

7 ns and (n-1)d

Physical properties

State at room temperature

Solid

Melting point

3,459 K (3,185.85 °C)

Boiling point

5,869 K (5,595.85 °C)

Density

20.8 g/cm3

Appearance

silvery-grayish

Thermal conductivity

48.0 W/(m·K)

Electrical resistivity

193 nΩ·m at 20 °C

Electrical conductivity

5.18 MS/m

Crystal structure

hexagonal close packed

Molar heat capacity

25.48 J/(mol·K)

Chemical properties

Oxidation states

+7, +6, +4

Electronegativity

1.9 (Pauling Scale)

Ionisation energy

7.88 eV
1st 760, 2nd 1,260, 3rd 2,510 kJ/mol

Electron affinity

0.15 eV

Atomic radius

empirical 151, covalent 151, van der Waals 217 pm

Ionic radius

Re⁴⁺ 63; Re⁵⁺ 58; Re⁶⁺ 55; Re⁷⁺ 53 pm

Reactivity

A group 7 metal resembling manganese and technetium that takes every oxidation state from -3 to +7, the +7 perrhenates being the commonest form; the bulk metal resists air, water and the non-oxidising acids at room temperature and is attacked only by oxidising conditions.

with water

Does not react with water.

with oxygen, air

Resists oxidation at room temperature; heated in air or oxygen it oxidises to the volatile yellow heptoxide: $\ce{4 Re + 7 O2 -> 2 Re2O7}$, which sublimes off the metal.

with acids

Resists hydrochloric acid, sulfuric acid and aqua regia in bulk, but hot nitric acid oxidises it to perrhenic acid: $\ce{Re + 7 HNO3 -> HReO4 + 7 NO2 + 3 H2O}$, and hydrogen peroxide does the same.

with halogens

Fluorine gives the +7 heptafluoride: $\ce{2 Re + 7 F2 -> 2 ReF7}$, and chlorine on heating gives the pentachloride: $\ce{2 Re + 5 Cl2 -> 2 ReCl5}$, the highest chloride reaching only Re(VI).

Typical compounds

• Re₂O₇ rhenium(VII) oxide
  yellow volatile heptoxide, dissolves in water to perrhenic acid
• NH₄ReO₄ ammonium perrhenate
  white salt reduced with hydrogen to rhenium metal
• HReO₄ perrhenic acid
  strong acid formed when the heptoxide dissolves
• ReO₂ rhenium(IV) oxide
  dark insoluble +4 oxide
• ReF₇ rhenium heptafluoride
  the only stable heptafluoride of a metal
• Re₂(CO)₁₀ dirhenium decacarbonyl
  entry point to organorhenium chemistry

Occurrence, production and use

Crustal abundance

7×10-4 milligrams per kilogram

Oceanic abundance

4×10-6 milligrams per liter

Occurrence and sources

Rhenium does not occur free in nature or as a compound in a distinct mineral species. It is, however, widely spread throughout the earth's crust to the extent of about 0.001 ppm. Commercial rhenium in the U.S. today is obtained from molybdenum roaster-flue dusts obtained from copper-sulfide ores mined in the vicinity of Miami, Arizona and elsewhere in Arizona and in Utah.

Some molybdenum contains from 0.002% to 0.2% rhenium. More than 150,000 troy ounces of rhenium are now being produced yearly in the United States. The total estimated Free World reserve of rhenium metal is 3500 tons. Rhenium metal is prepared by reducing ammonium perrhentate with hydrogen at elevated temperatures.

• trace substitution in molybdenite of porphyry copper-molybdenum deposits
  Chile, Arizona, Montana, Peru, Mexico; recovered from roaster flue dust
• in copper minerals of sedimentary copper deposits
  Kazakhstan, Poland, Armenia, Russia, Uzbekistan; recovered from smelter residues
• crustal and oceanic abundance
  about 0.001 ppm ¹; 0.000188 ppm ²; 0.0007 mg/kg crust and 0.000004 mg/L seawater ³

Extraction, production

Recovery from molybdenite roaster flue dust as ammonium perrhenate or perrhenic acid

based on about 80 percent recovery of the rhenium in molybdenum disulfide concentrates; scrubbing and ion-exchange chemistry not stated by the sources

Hydrometallurgical recycling of superalloy scrap and spent reforming catalysts

about 25,000 kg worldwide in 2024; US capacity 18,000 to 20,000 kg per year

Uses

Rhenium is used in flash lamps for photography and for filaments in mass spectrographs and ion gages, but is most frequently used as an alloying agent in tungsten and molybdenum and as a catalyst for performing certain reactions to a type of hydrocarbon known as an olefin.

It is widely used as filaments for mass spectrographs and ion gauges. Rhenium-molybdenum alloys are superconductive at 10 K.

Rhenium is also used as an electrical contact material because it has good wear resistance and withstands arc corrosion. Thermocouples made of Re-W are used for measuring temperatures up to 2200C, and rhenium wire is used in photoflash lamps for photography.

Rhenium catalysts are exceptionally resistant to poisoning from nitrogen, sulfur, and phosphorus, and are used for hydrogenation of fine chemicals.

• Aerospace superalloys: nickel-base superalloys for single-crystal turbine blades and vanes
  about 80 percent of end use (usgs-mcs2025-rhenium, 2024)
• Petroleum refining catalysts: platinum-rhenium reforming catalysts for high-octane, lead-free gasoline; recycled in a closed loop
  about 15 percent of end use (usgs-mcs2025-rhenium, 2024)
• Chemicals
  • rhenium catalysts for hydrogenation of fine chemicals
  • ammonium perrhenate and perrhenic acid production
• Electrical and instruments: tungsten-rhenium and molybdenum-rhenium filaments, X-ray targets, thermocouples, electrical contacts, mass-spectrograph parts
• Mining: by-product of porphyry copper-molybdenum mining and molybdenite roasting

Safety, toxicity

GHS classification, signal word Danger

• H228 Flammable solid Flammable solids

Discovery and name

Discovered by

Masataka Ogawa

Discovered

1908

First isolated

Masataka Ogawa

Named by

Walter Noddack, Ida Noddack, Otto Berg

Origin of the name

after the river Rhine (German: Rhein)

The element is silvery white with a metallic luster; its density is exceeded only by that of platinum, iridium, and osmium, and its melting point is exceeded only by that of tungsten and carbon.

The usual commercial form of the element is powder, but it can be consolidated by pressing and resistance-sintering in a vacuum or hydrogen atmosphere. This process produces a compact shape in excess of 90 percent of the density of the metal.

Annealed rhenium is very ductile, and can be bent, coiled, or rolled. Rhenium is used as an additive to tungsten and molybdenum -based alloys to impart useful properties.