Introduction to Strontium
Strontium (Sr), an alkaline earth metal with atomic number 38, is a highly reactive element never found freely in nature. It exhibits chemical properties similar to calcium and barium.
Natural Occurrence and Extraction
Where Strontium is Found
Strontium is primarily found in mineral forms due to its high reactivity. The two main strontium-containing minerals are celestite (strontium sulfate, SrSO4) and strontianite (strontium carbonate, SrCO3). Celestite is the more abundant and commercially significant ore. Large deposits of celestite are located in various regions globally, including Mexico, Turkey, China, Spain, and historically, the United Kingdom (e.g., the Yate district in Gloucestershire). Strontianite is less common but has been found in localities such as Germany and the United States.
Industrial Extraction
Industrial production of strontium compounds primarily utilizes celestite ore. The most common method involves converting celestite into strontium carbonate, which serves as a precursor for other strontium compounds. One prominent technique is the black ash process, where celestite (SrSO4) is roasted with coal to reduce it to strontium sulfide (SrS). The strontium sulfide is then dissolved and reacted with carbon dioxide or sodium carbonate to precipitate strontium carbonate (SrCO3). Alternatively, celestite can be directly converted to strontium carbonate by reacting it with a hot solution of sodium carbonate. Pure metallic strontium is typically produced by the aluminothermic reduction of strontium oxide (SrO) under vacuum.
Everyday Applications of Strontium Compounds
Pyrotechnics and Signal Flares
Strontium compounds are extensively used in pyrotechnics to produce vivid red colors. Strontium nitrate (Sr(NO3)2), strontium carbonate (SrCO3), and strontium oxalate are frequently incorporated into fireworks compositions, creating the characteristic brilliant crimson hues seen in international celebrations like New Year’s Eve displays in Sydney, London, or during Chinese New Year festivities. These compounds are also critical components in marine distress flares and road safety flares, essential for signaling in emergency situations globally.
Cathode Ray Tube (CRT) Displays
Historically, strontium oxide (SrO) was a key ingredient in the glass faceplates of color cathode ray tube (CRT) televisions and computer monitors. Its inclusion was vital for absorbing X-rays generated by the electron beam, preventing harmful radiation from reaching users. While CRT technology has largely been replaced by modern flat-panel displays, millions of these devices were manufactured and used worldwide for many decades, demonstrating a significant historical everyday application of strontium.
Permanent Magnets
Strontium ferrite (SrFe12O19) is a widely utilized ceramic material for producing inexpensive permanent magnets. These “hard” ferrite magnets are anisotropic, meaning their magnetic properties are stronger in a specific direction. They are found in numerous common household items and industrial components, including small electric motors (e.g., in toys, automotive accessories like window lifters), loudspeakers, refrigerator door seals, and craft magnets. Their widespread use makes strontium ferrite magnets an everyday material globally.
Toothpaste for Sensitive Teeth
Certain toothpastes designed to alleviate tooth sensitivity incorporate strontium compounds as active ingredients. Strontium chloride hexahydrate (SrCl2·6H2O) or strontium acetate can help reduce discomfort by blocking microscopic tubules in the dentin, which connect to the nerves within the tooth. This action helps prevent external stimuli from reaching the nerves, offering relief to individuals experiencing sensitivity. Such toothpastes are readily available in pharmacies and supermarkets internationally.
Glow-in-the-Dark Materials
Strontium aluminate (SrAl2O4), often doped with europium and dysprosium (SrAl2O4:Eu,Dy), is a modern phosphorescent pigment known for its superior brightness and longer-lasting afterglow compared to older zinc sulfide-based pigments. This material is used in various “glow-in-the-dark” products, including children’s toys, luminous paints, emergency exit signs in public buildings (e.g., airports, theaters worldwide), and watch dials, providing light without the need for an electrical power source.