Elucidating the Process of Electrochemical Phosphate Reduction in Molten Salts

Morocco has the single largest supply of economically viable phosphate rock in the world, an estimated total of 50,000,000 tons, allowing a unique opportunity to pursue scientific advancements in phosphate processing. OCP S.A., a leading Morocccan company, is a prominent world supplier of phosphate rock, phosphoric acid and phosphate fertilizers from its mining operations that produce roughly 30,000 tons of phosphate rock per year.  However, production surpluses from suppliers like China, with an annual mine output of about 138 ,000 tons despite a total supply of only 3,100,000 tons, prevent OCP from attaining a global market share proportional to its supply. A promising strategy for OCP is to diversify its output by entering the markets for chemical- and food-grade phosphate feed stocks. But current methods for generating such products involve producing pure phosphorus as an intermediate step, which is energetically costly and dirty. Developing an electrochemical method to reduce phosphate via a less energetic phosphorus-bearing species could drastically lower these economic and environmental costs, giving Morocco a much-needed edge in a variety of world chemical markets.

  Comparison of existing thermo-chemical method of phosphate processing (left) with the proposed electro-chemical "half-reaction" method (right).

Comparison of existing thermo-chemical method of phosphate processing (left) with the proposed electro-chemical "half-reaction" method (right).

 

Progress

Over the past year, we have worked to develop a reliable, air-free system with which to characterize the electrochemical behavior of molten salt systems. This system (shown schematically at left) has allowed us to survey several high-temperature salt melts for reduction features corresponding to phosphorus-bearing species. To create it at bench scale, we modified the idea of a closed-gasket steel cell to suit our own system’s suspected gaseous electrolysis products (P4 and either O2 or Cl2). The completed cell (see Figure 1) fits snugly inside an upright crucible furnace and can readily be brought to an internal temperature of 800-900 ⁰C. Our promising results to date suggest that phosphate reduction in molten salts is observable and can be readily characterized by electrochemical techniques.

 Schematic of a molten carbonate electrolyzer for PCl3 production built

Schematic of a molten carbonate electrolyzer for PCl3 production built

 
  Electrolyzer cell as built for laboratory experimentation.

Electrolyzer cell as built for laboratory experimentation.

Ongoing efforts

Based on promising early results, we are pursuing a more targeted study of the phosphate reduction reaction in high-temperature molten systems. Through a combination of product characterization and electro-analytical techniques, we are discovering the critical aspects of the system required for a complete understanding of the mechanism required for eventual commercial upscaling at maximum efficiency. These efforts include:

  • Isolating and characterizing the solid and gaseous by-products of the electrolysis reaction.
  • Determining the selectivity of the reaction and its voltage dependency.
  • Defining the reference electrode equilibrium system to pin down exact phosphate reduction potentials.
  • Detailing the electro-kinetics of the molten polyphosphate-system in order to fine-tune them.
  • Finding the optimal electrode materials for the target reaction.