Introduction to Strontium
Strontium is a soft, silvery-white alkaline earth metal with atomic number 38. It is highly reactive, readily tarnishing upon exposure to air, and reacts vigorously with water. Due to its high reactivity, strontium is always found in nature combined with other elements, never as a free metal.
Natural Occurrence and Indian Reserves
Strontium constitutes approximately 0.037% of the Earth’s crust by weight, making it the 15th most abundant element. The primary minerals from which strontium is commercially extracted are Celestite (strontium sulfate, SrSO4) and Strontianite (strontium carbonate, SrCO3). Celestite is the more common and economically significant of the two.
In India, significant deposits of Celestite are found. The Pulivendula area in the Cuddapah district of Andhra Pradesh is particularly noted for its Celestite reserves, which contribute to the global supply of strontium-bearing minerals.
Extraction and Industrial Processing
The industrial extraction of strontium primarily begins with Celestite (SrSO4). The process typically involves converting strontium sulfate into more usable compounds. One common method, known as the “black ash process,” involves heating Celestite with coal in a rotary kiln. This reduces the strontium sulfate to water-soluble strontium sulfide (SrS):
SrSO4(s) + 2C(s) → SrS(s) + 2CO2(g)
The strontium sulfide solution is then further processed. It can be reacted with carbon dioxide to precipitate strontium carbonate (SrCO3), or with hydrochloric acid to produce strontium chloride (SrCl2). Strontium carbonate is a key intermediate compound used in many industrial applications. The pure metal can be obtained by electrolysis of molten strontium chloride or by reducing strontium oxide with aluminium.
Everyday Applications of Strontium
Strontium compounds find diverse applications across various industries due to their unique chemical and physical properties.
Pyrotechnics and Fireworks
One of the most visually striking applications of strontium compounds is in pyrotechnics. Strontium salts, particularly strontium nitrate and strontium carbonate, are responsible for producing the brilliant crimson red colour in fireworks, signal flares, and tracer ammunition. The characteristic red light is emitted when strontium ions are heated in a flame, exciting their electrons to higher energy levels. This property is widely utilized during festive celebrations like Diwali in India, where fireworks display a spectrum of colours, including distinct reds from strontium.
Cathode Ray Tube (CRT) Displays
Historically, strontium was an essential component in the glass used for the faceplates of colour Cathode Ray Tube (CRT) televisions and computer monitors. Strontium oxide was incorporated into the glass to absorb X-rays generated by the high-energy electron beams within the CRT. This absorption was crucial for protecting viewers from potentially harmful radiation, making these devices safer for widespread use. While CRT technology has largely been replaced by flat-panel displays, this was a significant past application.
Ceramics and Specialty Glass
Strontium carbonate is used in the manufacturing of various ceramic products and specialty glass. In ceramics, it acts as a flux, lowering the melting point and improving the glaze’s texture. It is also a key ingredient in the production of ferrite ceramic magnets (strontium ferrites), which are widely used in small electric motors, microphones, and loudspeakers due to their excellent magnetic properties and cost-effectiveness. In glass production, strontium can enhance durability and optical clarity.
Medical Applications
A specific strontium compound, strontium ranelate, has been utilized in medicine for the treatment of osteoporosis, a condition characterized by brittle bones. Strontium ranelate works by promoting bone formation and simultaneously reducing bone resorption (the breakdown of bone tissue). It helps to strengthen bones and reduce the risk of fractures in affected individuals. This application highlights the element’s unique interaction with biological systems, particularly bone metabolism.
Metallurgical Uses
In metallurgy, strontium plays a role in the refining processes of certain metals. For instance, it is used in the electrolytic refining of zinc to help remove lead impurities. Small additions of strontium can also improve the castability and mechanical properties of aluminium alloys, making them stronger and more workable for various industrial applications.