Electrochemistry enables a cleaner production of white phosphorus

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.