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Potential for potassium recovery as K-struvite Bennett, Aline Miriam

Abstract

Crystallization of NH₄-struvite (MgNH₄PO4:6H₂O) pellets has proven to be a successful method of recovering nitrogen and phosphorus from wastewaters. Thus far, little work has been done on potassium recovery since it is not considered a water pollutant, nor do we face potassium shortages. However, potassium is an essential plant macronutrient and we are seeing worldwide imbalances in nutrient and fertilizer use, as well as a need for a slow release potassium fertilizer. Development of a full complement NPK fertilizer with NH₄-struvite and K-struvite components may have great potential. Given this, research into potassium recovery through crystallization of K-struvite (MgKPO₄: 6H₂O) is relevant and complements previous work done with NH₄-struvite. The goals of this research were to develop fundamental understanding of K-struvite formation as the first step to recovering potassium, and eventually produce a full complement NPK slow-release fertilizer from wastewaters. This required the determination of new solubility product values for K-struvite at different temperatures, followed by bench-scale experiments to assess K-struvite synthesis under various solution conditions. A model to simulate each batch experiment and to predict optimal supersaturation conditions for K-struvite precipitation was developed using PHREEQC, aqueous equilibrium modelling software. Finally, initial experiments in the UBC fluidized bed reactor (UBC-FBR) were undertaken to assess the pelletization potential of K-struvite. New solubility product values for K-struvite indicate that it is less soluble than previously reported, and the values determined at 10, 25 and 35°C fit the Van’t Hoff model. Optimal Mg:K:P molar ratio for synthesis of pure K-struvite was found to be approximately 3:50:1 in a wastewater matrix with pH 8, P-PO₄ concentration of 8 mM and a Mg:P ratio of 3:1. These concentrations were used in the UBC-FBR to assess the pelletization potential of K-struvite. These initial reactor runs were inconclusive due to an inability to stabilize the reactor without seeding. It would be recommended to seed the reactor during start-up in order to be able to compare process performance with the NH₄-struvite crystallization process in the UBC-FBR. This study showed that formation of pure K-struvite is possible given the right supersaturation conditions in solution, requiring high potassium concentrations.

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