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.
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