Rhenium, represented by the chemical symbol Re, is the element defined by 75 protons in its nucleus. This heavy transition metal is silvery-white and holds the distinction of being the last stable, naturally occurring element discovered on Earth in 1925. Rhenium plays an important role in modern technology due to its unique and extreme physical properties. It is also one of the rarest elements in the Earth’s crust, making its procurement and application a significant industrial challenge.
The Defining Characteristics of Rhenium
Rhenium is positioned in Group 7 and Period 6 of the periodic table, placing it directly below manganese and next to tungsten. Its atomic structure gives it a wide range of chemical versatility, exhibiting oxidation states that vary from -3 up to +7, with +7, +4, and +3 being the most common forms observed in its compounds. This ability to form diverse chemical bonds is a factor in its extensive use in catalytic processes.
The metal possesses the third-highest melting point of all elements, around 3,180 degrees Celsius. Only tungsten and carbon (which sublimes at standard pressure) have higher melting points, making rhenium invaluable in high-temperature environments. Rhenium is also one of the densest elements, with a density of approximately 21.02 grams per cubic centimeter, exceeded only by platinum, iridium, and osmium.
Its hexagonal close-packed crystal structure contributes to its exceptional hardness, wear resistance, and high-temperature stability. When alloyed, rhenium increases the high-temperature strength and creep resistance of other metals, reducing the likelihood of deformation under prolonged stress at high heat. This combination of high melting point, high density, and chemical stability defines its utility in demanding engineering applications.
Extreme Rarity and Global Sources
Rhenium is one of the rarest stable elements in the Earth’s crust, with an estimated average concentration of less than one part per billion (ppb). This scarcity is the primary reason for its high cost and careful management in industrial use. The element does not naturally occur in a concentrated form or as a distinct mineral deposit of its own.
It is found dispersed as a trace element, primarily substituting for molybdenum within the mineral molybdenite (\(\text{MoS}_2\)). The extraction of rhenium is almost entirely a byproduct of processing porphyry copper ores, which also contain molybdenum. Molybdenite concentrates from these operations contain rhenium in concentrations ranging from 100 to 3,000 parts per million.
The recovery process is complex and energy-intensive, beginning with the roasting of the molybdenite concentrates. During this step, rhenium oxidizes into rhenium heptoxide (\(\text{Re}_2\text{O}_7\)), a compound volatile at high temperatures that passes up the flue stack with sulfur gases. The rhenium is captured from these flue dusts and gases by scrubbing them with water. Further chemical separation steps are then required to isolate the metal, often resulting in the commercial form of ammonium perrhenate (\(\text{NH}_4\text{ReO}_4\)).
Critical Roles in High-Performance Technology
Rhenium is indispensable for two major industrial applications that account for most of its global consumption. Approximately 80 percent of the world’s rhenium is used in the production of nickel-based superalloys for aerospace components. The addition of rhenium, typically at concentrations of 3 to 6 percent, increases the alloy’s creep resistance, allowing it to withstand permanent deformation under extreme temperatures and stress.
These superalloys are used to manufacture turbine blades, combustion chambers, and exhaust nozzles in advanced jet engines and industrial gas turbines. By incorporating rhenium, engineers can design engines to operate at temperatures exceeding 1,000 degrees Celsius, boosting efficiency, performance, and durability. The second and third generations of single-crystal turbine blades, for instance, utilize up to 3 percent and 6 percent rhenium, respectively, enabling their use in high-performance military aircraft like the F-22 and F-35.
Rhenium’s second major application, accounting for about 10 percent of its consumption, is in catalysis for the petrochemical industry. It is a component in platinum-rhenium bimetallic catalysts used in the process of catalytic reforming, which is essential for converting low-octane naphtha into high-octane, lead-free gasoline. Rhenium-containing catalysts offer high resistance to chemical poisoning from elements like nitrogen, sulfur, and phosphorus, which increases the catalyst’s lifespan and efficiency in the refinery. Beyond these primary uses, specialized alloys of rhenium with tungsten or molybdenum are used to create high-temperature thermocouples and filaments in mass spectrographs due to their stability and low vapor pressure.