The overall technical objective of this project is to provide preliminary evidence for the feasibility of developing a waste valorization and management platform for the treatment of ammonium nitrate solution (ANSOL) generated during the production of energetic materials. In addition, a preliminary life-cycle and economic analysis of the process will reveal the costs, cost savings, and profit centers of the technology.
The process relies on the deployment of electrically conducting membranes (capable of inducing electrooxidation and electroreduction) that efficiently transform contaminants and extract valuable chemicals. Specifically, membranes will oxidize residual high energy materials (such as nitramine explosives) to harmless degradation products, and reduce Cr(VI) to Cr(III) that is readily removed; ammonium (NH4+) will either be transformed to ammonia (NH3) and extracted together with nitric acid (HNO3), or recovered as ammonium nitrate (NH4NO3) solids. As the electrochemical reactions can be completed across a wide temperature range, the project team will be able to test ANSOL treatment across this range, which will allow the project team to determine the economic viability of the process with a range of product endpoints (e.g., at low temperatures, solid NH4NO3 can be recovered, while at higher temperatures all constituents will be dissolved and processed by the membranes).
In the process, electrochemical reactions at the membrane/water interface have four specific purposes: i) on a gas-stripping membrane/cathode, NH4+ is transformed from its ionic to its neutral form (NH3) by electrochemically increasing the local pH (through water electrolysis) while extracting the NH3; ii) on a microfiltration membrane/anode (on the surface or within membrane pores) electrochemically-generated hydroxyl radicals can oxidize energetic compounds (such as hexahydro-1,3,5-trinitro-1,3,5-triazine, octahydro-1,3,5,7-tetranitro-1,3,5,7- tetrazine, and methylamine) with the reduced diffusion pathway within the membrane’s pores minimizing mass transfer limitations between target compounds and radicals; iii) protons, electrochemically generated on the membrane/anode, will serve as counter-ions to nitrate, forming a purified HNO3 solution; and, iv) on an ultrafiltration membrane/cathode, Cr(VI) is electrochemically reduced to Cr(III), which is efficiently removed by the membrane as solid Cr(OH)3.
An alternative extraction process will be evaluated where NH4NO3 will be extracted (via a temperature swing) after the solution is cleared from energetic compounds and Cr(VI). A constrained economic optimization model within a lifecycle analysis framework will be developed and used to evaluate the different treatment and extraction endpoints. Specifically, the ANSOL waste stream will be combined with inputs such as energy and capital to maximize the net benefits of ANSOL waste disposal given environmental/regulatory, chemical, technological, and economic constraints. Net benefits include profits from the valorization of transformed products (e.g., ammonia) less the costs of transforming and disposing of the remaining residual waste. Solutions will be identified and compared while maintaining materials and energy balance under varying assumptions regarding waste stream composition, market conditions, and technological factors.
The benefits of the process include the transformation of a waste product (ANSOL) that faces uncertain demand and large disposal costs into valuable commodity chemicals with predictable demand (NH3 and HNO3, or NH4NO3), while simultaneously eliminating contamination from residual high-energy molecules and Cr(VI). This project is responsive to the objectives articulated in the statement of need: to identify a more cost-effective process that transforms ammonium nitrate solution (ANSOL) from a waste product to several valuable commodities, while simultaneously destroying nitramine explosives and eliminating chromium leaching into the environment. Importantly, the waste valorization method relies only on electrochemical reactions and membranes and does not require any additional chemical additives.
The project will provide insight into electrochemistry-assisted separation processes, which have the potential of transforming other waste streams into valuable commodities. The modular and scalable nature of membrane processes enables their deployment and application across a large scale of processes and plants, including both small-scale (10’s of L/min) and large-scale (mega-liters/day) industrial wastewater treatment systems.