Selenium

Electron Configuration: [Ar] 4s² 3d¹⁰ 4p⁴

Properties:

Name of Element: Selenium
Element Classification: Non-Metal
Symbol: Se
Atomic Number: 34
Atomic Mass: 78.96 amu
Melting Point (°K): 490.15
Boiling Point (°K):  958.05
Crystal Structure: Hexagonal
Density (g/cm3): 4.79
Specific Heat (@20°C J/g mol): 0.321 (Se-Se)
Fusion Heat (kJ/mol): 5.23
Evaporation Heat (kJ/mol): 59.7
Pauling Negativity Number: 2.55
First Ionizing Energy (kJ/mol): 940.4
Oxidation States: 6, 4, -2
Lattice Structure: Hexagonal
Lattice Constant (Å): 4.360

Description:  Selenium can be prepared with either an amorphous or crystalline structure.  Crystalline monoclinic selenium is deep red; crystalline hexagonal selenium, the most stable variety, is a metallic grey. 

Separation:  Most selenium is made as a byproduct of copper refining.  It also accumulates in the residues from sulphuric acid production.  Extraction of Selenium is a complex process and the specific technique employed varies depending on the other compounds and elements that are present.  Typically, the first step involves an oxidation in the presence of sodium carbonate (soda ash):

                Cu₂Se + Na₂CO₃ + 2O₂ → 2CuO + Na₂SeO₃ + CO₂

The selenite Na₂SeO₃ is acidified with sulphuric acid.  Any tellurites precipitateout leaving selenous acid, H₂SeO₃, in solution. Selenium is liberated from selenous acid by SO₂ addition:

                H₂SeO₃ + 2SO₂ + H₂O → Se + 2H₂SO₄

References:

"Mineral Yearbook 2007: Selenium and Tellurium". United States geological Survey. http://minerals.usgs.gov/minerals/pubs/commodity/selenium/myb1-2007-selen.pdf.  

"Selenium",  Los Alamos National Laboratory, 2003-12-15,  http://periodic.lanl.gov/elements/34.html

Coutts, T.J. ; Kazmerski, L.L. ; Wagner, S., "Copper indium diselenide for photovoltaic applications", http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=5189733

Noufi, Rommel; Ken Zweibel. "HIGH-EFFICIENCY CDTE AND CIGS THIN-FILM SOLAR CELLS: HIGHLIGHTS AND CHALLENGES". National Renewable Energy Laboratory. http://www.nrel.gov/pv/thin_film/docs/wc4papernoufi__.doc.