Electrochemistry enables a cleaner production of white phosphorus

Significance 

Phosphorus is an element of life. Not only is a key nutrient for plants, phosphorus is also indispensable in modern industry. As the most common allotrope of elemental phosphorus, white phosphorous is the starting material for various critical chemicals, extensively applied in semiconductor or pharmaceutical industry. For example, the ingredient of covid-19 vaccines contains several kinds of fine phosphates. These valuable phosphates can’t be produced without white phosphorus. Consequently, its global demand has been on the rise in recent years; however, phosphorous-producing industries have been fast vanishing. This is due to the fact that white phosphorous production process is highly energy intensive and environmentally unfriendly. Specifically, production of white phosphorus exclusively by carbothermic reduction of phosphate rock can thus be only carried out in countries with low electricity power prices, or where there are loose environmental regulations. Moreover, quality control is difficult because of easy contamination by impurity elements at the high temperatures involved. Per se, technical defects have posed challenges to the downstream sectors relying on phosphorus-derived chemicals. Therefore, technological innovations to achieve a cost-effective and clean production is urgently needed for the last surviving phosphorus-making plants and their customers.

The current white phosphorous production technique was invented over a century ago but has since failed to evolve with time; particularly, to match current stringent environmental regulations. To bridge this gap, Professor Xiao Yang at the Westlake University in China together with Professor Toshiyuki Nohira at the Kyoto University in Japan developed a new concept for producing white phosphorus by electrochemistry. Their work is currently published in the research journal, ACS Sustainable Chemistry and Engineering.

Phosphate can be dissolved in a solvent and electrochemically converted to elemental phosphorus. In their approach, this concept was demonstrated by conducting experiments in the molten salt system of CaCl2-Ca3(PO4)2. Basically, Ca3(PO4)2 dissolves in molten CaCl2 to form a solution containing diffusible and electrochemically reducible phosphate ions. Since CaCl2 is an excellent high temperature electrolyte widely used for materials synthesis and Ca3(PO4)2 is soluble in molten CaCl2, continuous formation of white phosphorus in a gas form was realized by electrolyzing the molten bath of CaCl2-Ca3(PO4)2.

The authors reported that Ca3(PO4)2 dissolved in molten CaCl2 to form a solution containing diffusible phosphate ions, which were confirmed to be electrochemically reducible. In addition, cyclic voltammetry conducted in molten CaCl2-Ca3(PO4)2 (2 mass%) showed that phosphorus could be extracted at potentials more negative than −2.10 V (vs Cl2/Cl) at 850 °C. Accordingly, electrolyzing the molten bath at a constant potential of −2.48 V (vs Cl2/Cl) was reported to result in continuous formation of phosphorus vapor on the cathode and carbon oxides on the carbon anode.

In summary, the Yang-Nohira study demonstrated a novel concept for producing white phosphorus by electrolysis of phosphate containing molten salt. Remarkably, a sustainable process to produce white phosphorus from phosphate-bearing precursors including phosphate rock and sewage sludge ash by electrolysis in molten CaCl2 was reported. Ideally, producing white phosphorus by electrolysis showed substantial advantages over the conventional carbothermic reduction technology in simplicity, energy efficiency, and cleanness. In a statement to Advances in Engineering, the authors said their work will lead to a paradigm shift of the phosphorus-making industry toward a cleaner and more sustainable production.

Electrochemistry enables a cleaner production of white phosphorus - Advances in Engineering

About the author

Xiao Yang is an Assistant Professor in School of Engineering, Westlake University. Dr. Yang is an extractive metallurgist with years of research experience from both academia and industry. He was born in Pingxiang, Jiangxi Province (China). He received his bachelor’s degree in materials engineering at Beihang University in 2003 and master’s degree in chemical engineering at Chinese Academy of Sciences in 2006. He was a recipient of the Japanese Government MEXT scholarship. After obtaining his Ph.D. in metallurgical engineering from The University of Tokyo in 2009, he successively held the posts of a Research Associate at National Institute of Advanced Industrial Science and Technology in Japan (2009-2010), a Research Engineer at ShaGang Group (2010-2012), a Research Assistant Professor at Kyoto University (2012-2016), a Research Fellow at The University of Texas at Austin (2016-2017), and later an Assistant Professor at The University of Tokyo (2017-2020). He was a Visiting Professor in the Department of Materials Science and Engineering at Yonsei University in 2019.

He joined Westlake University in 2020 and launched the Laboratory of Resource Extraction & Process Tailoring as a Principle Investigator. His laboratory commits to developing sustainable chemical processes for resources extraction or utilization. Exploring novel approaches for producing or recycling phosphorus is one of his recent primary focuses.

About the author

Toshiyuki Nohira is a Professor in the Advanced Energy Utilization Division of the Institute of Advanced Energy at Kyoto University, where he has been since 2015. He was an Associate Professor during 2007-2014 and an Assistant Professor during 1998-2006 in the Graduate School of Energy Science at Kyoto University. He was a Visiting Scientist in the Department of Materials Science and Engineering at Massachusetts Institute of Technology in 2007. He received the degree of Doctor of Engineering from Kyoto University in 1998 for a thesis entitled “Electrochemical studies on hydride ion and hydrogen absorbing alloys in molten salt systems”, which was supervised by Professor Yasuhiko Ito. Professor Nohira’s research has focused on electrochemical energy conversion and electrochemical material production utilizing molten salt and ionic liquid electrolytes. He has been researching and developing new production methods of solar-grade silicon using molten salt electrolysis. He is also interested in new electroplating methods of silicon, titanium, and tungsten from molten salts. Furthermore, he has been working on the development of new Na-ion batteries and K-ion batteries using amide ionic liquids as electrolytes.

He published over 200 peer reviewed papers, 15 books and over 40 patents in the field of electrochemistry and inorganic chemistry. He received Scientific Achievement Award of The Electrochemical Society of Japan in 2020. He was awarded Molten Salt Prize from the Molten Salt Committee of the Electrochemical Society of Japan in 2019. He was also awarded The Young Scientists’ Prize, The Commendation for Science and Technology by the Minister of Education, Culture, Sports, Science and Technology of Japan in 2009. He was awarded the Excellent Paper Awards from The Electrochemical Society of Japan in 2000, 2006, 2010 and 2014.

Reference

Xiao Yang, Toshiyuki Nohira. A New Concept for Producing White Phosphorus Electrolysis of Dissolved Phosphate in Molten Chloride. ACS Sustainable Chemistry and Engineering 2020, volume 8, page 13784−13792.

Go To ACS Sustainable Chemistry and Engineering

Check Also

Robust Solid-Electrolyte Interphase (SEI) Enables Near-Theoretical Capacity of Graphite Battery Anode at Four Times Faster Rate in Propylene Carbonate-Based Nonflammable Liquid Electrolyte - Advances in Engineering

Robust Solid-Electrolyte Interphase (SEI) Enables Near-Theoretical Capacity of Graphite Battery Anode at Four Times Faster Rate in Propylene Carbonate-Based Nonflammable Liquid Electrolyte