Beneficiation, desilication and selective precipitation techniques for phosphorus refining from biomass derived fly ash

Abstract

The supply of affordable phosphorus (P), a biocritical element, is at great risk due to the utilization of limited natural phosphate minerals at an unprecedented scale. The need of phosphorus recovery to sustain the anthropogenic P-cycle was recog- nized and fly ash, a waste product from the combustion of renewable biofuels, was selected as the secondary resource. Increasing supply of fly ash in the future due to growing use of forest biofuels in energy production also contributes to its suitability for P-recovery. Fly ash comprises of silicates, oxides, carbonates, phosphates and hydroxides of the elements that potentially represent the entire periodic table. This only augments the complexity of the recovery process, since most of the interfering matrices need to be removed prior to the extraction of phosphorus. Methods for separation of impurities and extraction of the phosphorus were developed and optimized. The establishment of a relationship between the particle size and mineral content enables an elevation of the phosphorus level by a factor of 1.38, while the silicon (Si) content is reduced by a factor of 3.8 for a circulatory fluidized bed derived fly ash. This was achieved by the use of sieving beneficiation with a sieve size of 125 µm. Despite the feasibility of fly ash leaching with a lower molar concentration of acid for efficient dissolution of phosphorus, higher concentration of HCl at 8 M was employed. Aging of this leachate for 5 h facilitates the removal of silica with an effi- ciencyof99%fromtheleachatesolutionbyprecipitationofSi-particlesand/orSi-gel formation. The Si removal rate was found to vary proportionally with temperature and the H+ and Cl– ion concentration. Chemical equilibrium calculations (in the Medusa & Hydra software) were used to identify the phosphate species in the leachate and to draft the P-recovery strate- gies. Selective precipitation of AlPO4 was identified as a feasible method to recover phosphorus from the leachate. Interference of Fe was limited by the use of chelation- based masking with ethylenediaminetetraacetic acid (EDTA) at [EDTA]/[Fe]=1 and pH of 4. An amorphous white precipitant was obtained that comprises 18.1 wt% of P, 16.3 wt% of Al and 2.18 wt% of Fe as major components and also trace elements in low concentrations. AlPO4 was further converted to struvite; a slow releasing fertilizer. The process involved dissolution of solids with 0.5 M phosphoric acid, removal of Al and metal ion impurities with 0.6 g/mL of Amberlite IR120H+ (a strongly cation exchange resin), addition of Mg2+ and NH4+sources and precipitation at pH 9.5. Prospects of leachate recirculation for leaching were also investigated to lower the use of fresh phosphoric acid. Granulated white-pure crystalline struvites containing 12.5 wt% of P were obtained. The combination of refining and extraction technologies, employed in this study, contribute to elevate the P content in the final product by a factor of the order of 10. Procuring struvite. from biomass fly ash is seen as a sustainable P recycling method which promotes conservation of the natural resources used for fertilizer production.