Natural Sources: Where Strontium is Found
Unlike gold or copper, pure metallic strontium is not found freely in nature due to its high reactivity with oxygen and water. Instead, it exists in compounds combined with other elements, predominantly in two mineral forms: celestite and strontianite.
Celestite (strontium sulfate, SrSO4) is the most abundant strontium ore and is the primary commercial source. It is often found in sedimentary rock formations around the world. Strontianite (strontium carbonate, SrCO3) is a less common but historically significant source, named after the Scottish village Strontian where it was discovered. These minerals are mined from various global deposits, with major producers including Iran, Spain, China, and Mexico.
Strontium is also a ubiquitous trace element present in small quantities throughout the environment.
- Water: It is naturally dissolved in surface and groundwater, with typical concentrations in drinking water being less than 1 milligram per liter.
- Soil and Rocks: Varying amounts are found in soil and rocks, with concentrations influenced by the underlying geology.
- Foods: As plants absorb strontium from the soil and water, it enters the food chain. Common dietary sources include grains, leafy vegetables, seafood, and dairy products.
Industrial Extraction and Production
The industrial process for producing pure strontium and its compounds involves complex chemical treatments to separate the metal from its ores. One of the most common methods starts with celestite (SrSO4) and transforms it into the more useful strontium carbonate (SrCO3) intermediate.
The Black Ash Method
- Reduction: The mined celestite ore is heated in a rotary kiln with coal or coke (carbon) at high temperatures. This process reduces the strontium sulfate ($SrSO_4$) into strontium sulfide ($SrS$).
- Leaching: The resulting 'black ash,' which contains strontium sulfide, is leached with water to form a strontium sulfide solution.
- Carbonation: The strontium sulfide solution is then treated with a soluble carbonate, such as sodium carbonate ($Na_2CO_3$), which precipitates the solid strontium carbonate ($SrCO_3$). This carbonate is the key intermediate for producing other strontium salts.
The Electrolysis Process
To produce pure strontium metal, one common technique is the electrolysis of fused strontium chloride.
- Strontium Chloride Preparation: Strontium carbonate is dissolved in hydrochloric acid ($HCl$) to form strontium chloride ($SrCl_2$).
- Electrolysis: The molten strontium chloride, often mixed with potassium chloride to lower the melting point, is electrolyzed. This process uses a liquid copper-strontium alloy as a cathode and graphite as an anode, yielding pure strontium metal and chlorine gas.
Comparison of Extraction Methods
| Feature | Black Ash Method | Electrolysis of Chloride | SREX Process (Radioactive) |
|---|---|---|---|
| Starting Material | Celestine (SrSO$_4$) Ore | Strontium Chloride (SrCl$_2$) | High-level radioactive waste |
| Primary Product | Strontium Carbonate (SrCO$_3$) | Pure Strontium Metal (Sr) | Purified Strontium-90 (90Sr) |
| Complexity | Multi-stage chemical conversion | Requires high-temperature molten salt electrolysis | Highly specialized and complex solvent extraction |
| Selectivity | Produces a key chemical intermediate | Isolates pure metallic strontium | Selectively separates 90Sr from other waste products |
| Primary Use | Commercial strontium chemical production | Niche alloying and research applications | Medical radioisotope production |
Key Applications for Strontium Compounds
The most common strontium products are compounds, not the pure metal. These compounds are used in a wide array of industrial and commercial applications.
- Pyrotechnics: Strontium salts, especially strontium nitrate and carbonate, are essential for producing the vivid red color in fireworks and signal flares. The intensity and stability of the red flame emission make strontium compounds ideal for this purpose.
- Ceramics and Glass: Strontium carbonate is used in ceramic glazes to improve strength and durability. In the past, it was a major component in cathode-ray tube (CRT) glass to block X-rays.
- Magnets: Strontium ferrite ($SrFe{12}O{19}$) is a hard ferrite magnet used in small motors and magnets.
- Toothpaste: Strontium chloride hexahydrate is used in some specialized toothpastes to reduce tooth sensitivity by blocking microscopic tubules in the dentin.
- Medical Uses (Radioisotopes): Radioactive isotopes, particularly strontium-89 ($^{89}Sr$) and strontium-90 ($^{90}Sr$), are used in medicine. $^{89}Sr$ is used in palliative care for bone cancer patients. $^{90}Sr$ is a source of beta radiation used for superficial radiotherapy and in radioisotope thermoelectric generators (RTGs).
Conclusion
To get strontium, whether for industrial purposes or for a minor presence in your diet, you are interacting with a mineral that is naturally present throughout the Earth's crust. From its primary mineral ores, celestite and strontianite, industrial processes have been developed to extract and refine it into various compounds and, less commonly, pure metal. These extraction methods, such as the widely used Black Ash process, enable a diverse range of applications, from creating the vibrant red in fireworks to manufacturing specialty medical isotopes. Ultimately, the process of obtaining strontium is a journey from the Earth's sedimentary layers to its end use in sophisticated technologies and products.
How can I get strontium supplements?
To get strontium supplements like strontium citrate, you can find them at many health food stores, pharmacies, or online retailers. The FDA does not regulate these supplements as strictly as prescription drugs, so the quality and dosage can vary. Always consult a healthcare provider before starting any new supplement regimen.
How is radioactive strontium-90 handled?
Radioactive strontium-90, a byproduct of nuclear fission, is treated with extreme caution due to its hazardous nature. It is contained within nuclear reactor waste and undergoes highly specialized solvent extraction processes, such as the SREX method, to separate and manage it safely for specific medical or industrial applications.
What are the primary commercial uses of strontium?
The primary commercial applications of strontium are the production of red colors in fireworks, manufacturing hard ferrite magnets, use in specialized toothpastes for sensitive teeth, and the production of glass and ceramics. Its use in cathode-ray tube displays has declined significantly.
Where is strontium naturally most abundant?
Strontium is naturally most abundant in the mineral celestite (strontium sulfate), which is the principal source for commercial mining. Significant deposits of this mineral are found in regions like Spain, Iran, and Mexico.
Does strontium pose any health risks?
Stable, naturally occurring strontium is not generally hazardous to health at typical environmental exposure levels. However, the radioactive isotope, strontium-90, is a serious health risk, particularly as it can accumulate in bone tissue and lead to conditions like leukemia and bone cancer.
Why is strontium used in pyrotechnics?
Strontium is used in pyrotechnics because its volatile salts produce a vivid, bright red color when they burn. This is a distinct characteristic of the element that is utilized to create red hues in fireworks and signal flares.
What is the role of strontium in human health?
Strontium is physically and chemically similar to calcium and can be absorbed into bones. While most strontium enters the body through food and water, specific compounds like strontium ranelate have been studied for treating osteoporosis by affecting bone density, although its use is now restricted due to cardiovascular risks.
