1
S a egies o imp o ing economic and en i onmen al
sus ainabili y o we chemical phospho us eco e y om sewage
sludge ash
L. Esposi o*, G. Bonia di**, D. El Chami***, R. Canziani* and A. Tu olla*
* Poli ecnico di Milano, Dipa imen o di Ingegne ia Ci ile e Ambien ale (DICA), Piazza Leona do da Vinci 32,
20133 Milano, I aly (E-mail: [email p o ec ed], obe [email protected] , [email p o ec ed])
** The Uni e si y o Queensland, School o Chemical Enginee ing, 4072 S . Lucia, Queensland, Aus alia (E-mail:
g.bonia [email protected])
*** TIMAC AGRO In e na ional, 27 a . F anklin Roose el , 35408 Sain -Malo, F ance (E-mail:
daniel.elchami@ imacag o.com)
Abs ac
Phospho us (P) eco e y om sewage sludge ash (SSA) ep esen s a p omising solu ion o P supply
conce ns, wi h a key challenge consis ing o he poo ci cula i y o cu en echnologies. This
esea ch explo es he po en ial o an inno a i e wo-s ep (TS) we chemical P eco e y p ocess om
SSA, assessing he use o indus ial by-p oduc s as seconda y eagen s and in es iga ing he euse
o he leacha e om a con en ional one-s ep (OS) p ocess as an ex ac an o a subsequen P
ex ac ion s age. Lab-scale TS es s we e conduc ed by simul aneously employing seconda y
ex ac an s (H2SO4 and HCl) and p ecipi an s (lime milk), compa ing p ocess pe o mances o a
con en ional OS p ocess pe o med wi h p ima y eagen s.
Seconda y eagen s we e e ec i e in eco e ing P, wi h a e age ex ac ion and p ecipi a ion
e iciencies o 86% and 99%, espec i ely. The second ex ac ion s ep success ully inc eased P
concen a ion in he leacha e by 33% o 43%, indica ing i s po en ial o euse. The TS p ocess
gene a ed high- alue P- ich p oduc s, wi h a P con en up o 18% when using H2SO4. Howe e , he
o e all eco e y e iciency o he TS p ocess was up o 27% lowe han ha o he OS p ocess,
indica ing he need o u he op imiza ion o he second ex ac ion s ep.
Keywo ds
Acid leaching ex ac ion; Bio-based e ilize s; P ocess op imiza ion; Seconda y eagen s
INTRODUCTION
Phospho us (P) plays a pi o al ole in sus aining human socie y, wi h a ound 89% o global P
p oduc ion dedica ed o e ilize s manu ac u ing (Desmid e al., 2015). Cu en ly, P is p ima ily
ex ac ed om phospha e ocks (PR), a non- enewable esou ce. In he las decades, in ensi e
ag icul u e p ac ices, popula ion g ow h and apid u baniza ion ha e led o o e exploi a ion and
quali y decline o PR ese es. In his scena io, eco e ing P om al e na i e and enewable sou ces
became c ucial o add ess he u u e P demand (Ca illo e al., 2024).
Sewage sludge ash (SSA) eme ged as a p omising al e na i e P sou ce due o i s high P con en (4 –
15.7%), compa able o medium/low-g ade PR (Esposi o e al., 2024). Be ween he a ious P eco e y
p ocesses om SSA, we chemical leaching gained signi ican a en ion in he li e a u e due o i s
lexibili y, low ene gy consump ion and scalabili y (Canziani e al., 2023). Despi e hei po en ial,
we chemical p ocesses emain economically un iable, being cha ac e ized by es ima ed ope a ing
expense (5 – 6 €/kg P eco e ed) signi ican ly highe han he a e age p ice o PR (1.1 €/kg P) and
con en ional P-based e ilize (2.2 €/kg P; Esposi o e al., 2024). Mo eo e , he p oduc ion o p ima y
eagen s, especially H2SO4, signi ican ly con ibu es o p ocess en i onmen al impac , in e ms o
CO2 oo p in and embodied ene gy (Fahimi e al., 2021). The e o e, iden i ying s a egies o enhance
he ci cula i y o we chemical P eco e y p ocesses is c ucial o imp o e hei compe i i eness
agains adi ional P p oduc ion me hods.
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P e ious wo ks ha e p ima ily ocused on op imizing he P ex ac ion s ep, assessing he pe o mance
o di e en ex ac an s – including ino ganic acids, o ganic acids, alkaline and chela ing agen s – in
e ms o P and me als/hea y me als/me alloids ex ac ion e iciency (Bonia di e al., 2024a; Luyckx
e al., 2020). Selec i e P p ecipi a ion was a ely in es iga ed, wi h only one s udy explo ing he use
o an inno a i e p ecipi an de i ed om a low-g ade magnesium oxide mining by-p oduc (Bonia di
e al., 2024b). Howe e , o he bes o he au ho s’ knowledge, no esea ch in es iga ed he easibili y
o pe o ming we chemical P eco e y om SSA using solely seconda y eagen s, such as was es o
by-p oduc s om o he indus ial p ocesses.
Building upon his gap, his esea ch e alua es he po en ial o an inno a i e wo-s ep we chemical
P eco e y p ocess, assessing he simul aneous use o indus ial by-p oduc s as seconda y ex ac an s
(H2SO4 and HCl) and p ecipi an s (lime milk – LM) and explo ing he po en ial euse o he leacha e
om a con en ional one-s ep p ocess as an ex ac an o a subsequen P ex ac ion s age. These
app oaches aim o dec ease eagen consump ion and inc ease P concen a ion in he leacha e,
enhancing he ci cula i y o we chemical P eco e y p ocesses.
MATERIALS AND METHODS
We chemical phospho us eco e y
The wo-s ep (TS) P eco e y p ocess in es iga ed in his wo k was es ed a he lab-scale ollowing
he con igu a ion illus a ed in Figu e 1.
Figu e 1. Two-s ep (TS) P eco e y p ocess es ed in his wo k.
The es s employed ly ash sampled om he elec o- il e s o he We dhölzli mono-incine a ion ull-
scale plan (Zu ich, Swi ze land). De ails on he physical and chemical cha ac e iza ion o he
employed SSA a e p o ided in Bonia di e al. (2021). The i s ex ac ion s ep (ex ac ion1) was
pe o med unde he ollowing ope a ing condi ions: 1 N H2SO4 o HCl; Liquid- o-Solid a io (L/S1)
o 10 mLacid solu ion/gSSA; con ac ime ( 1) o 30 min. These alues we e selec ed based on p ac ical
conside a ions, as explained in Bonia di e al. (2024b). The employed H2SO4 (95%w/w) and HCl
(13%w/w) samples esul ed om he ea men o S- o Cl- ich was es and we e p o ided by Nuo a
Solmine S.p.A (I aly) and Eco92 S. .l (I aly), espec i ely. The second ex ac ion s ep (ex ac ion2)
was ca ied ou using a ac ion o he leacha e om ex ac ion1 (leacha e1) and applying he ollowing
ope a ing condi ions based on he applied ex ac an : L/S2 o 15 mLleacha e1/gSSA (H2SO4) o 6
mLleacha e1/gSSA (HCl); 2 o 20 min (H2SO4) o 30 min (HCl). These alues we e de e mined based on
p elimina y i a ion es s o leacha e1 wi h SSA, a ge ing a pH o 2. This pH le el was chosen o
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dec ease he eagen consump ion in he subsequen p ecipi a ion s ep while p e en ing ea ly
p ecipi a ion o Al- and Fe-phospha es om he leacha e, ypically occu ing in he pH ange o 2 –
4 (Bonia di e al., 2024b). Leacha es om bo h he ex ac ion s eps (leacha e1 and leacha e2,
espec i ely) we e subjec ed o p ecipi a ion (p ecipi a ion1 and p ecipi a ion2, espec i ely) by
adding a 2%w/ LM solu ion ill a a ge pH o 7 was eached. Ta ge LM concen a ion and pH we e
selec ed in line wi h he a ailable li e a u e (Bonia di e al., 2021; Bonia di e al., 2024b). The LM
sample (27%w/ ) esul ed om he ace ylene p oduc ion p ocess and was supplied by Hyd oconsul ing
S. .l. (I aly).
To p o ide a baseline o compa ison, a con en ional one-s ep (OS) P eco e y p ocess was conduc ed
a he lab-scale, applying he same ope a ing condi ions as in ex aci on1 and p ecipi a ion1. Fo his
pu pose, comme cial-g ade H2SO4 (96%w/w) om Ca lo E ba Reagen s S. .l. (I aly), HCl (37%w/w)
and Ca(OH)2 (powde ) om Sigma Ald ich (Ge many) we e used. Addi ional de ails abou he
me hodologies adop ed o each P eco e y s ep a e p o ided in Bonia di e al. (2021).
Phospho us eco e y pe o mance
To comp ehensi ely e alua e he pe o mance o he discussed p ocesses, he ollowing pa ame e s
we e analysed: P ex ac ion e iciency (PEE, %); P p ecipi a ion e iciency (PPE, %); P o e all
eco e y e iciency (PRE, %); P con en in he p ecipi a e (Pp , %). Pa ame e s o he i s s ep, second
s ep and o e all TS p ocess a e indica ed by subsc ip s 1, 2 and TOT, espec i ely. PEE and PPE
we e de e mined as in Bonia di e al. (2021) and Bonia di e al. (2024b), espec i ely. PRE was
calcula ed as he a io be ween he ou coming and incoming P mass ac oss he bounda ies o he
conside ed eco e y s ep.
RESULTS AND DISCUSSION
The pe o mances o he p ocesses analysed in his wo k a e summa ized in Figu e 2.
Figu e 2. P eco e y pe o mances o he H2SO4- (a) and HCl-based (b) con igu a ions o he one-
s ep (OS) and wo-s ep (TS) p ocesses es ed in his wo k.
No subs an ial di e ences we e no iced when compa ing PEE1 and PEE om TS and OS p ocesses,
wi h a e age alues be ween 85% and 87%. Mo eo e , ex ac ion2 success ully enhanced he P
con en in he leacha e, wi h an inc ease o 33% and 43% o H2SO4- and HCl-based ou es,
espec i ely. These ou comes indica ed he sui abili y o he employed seconda y acids o P
ex ac ion and highligh ed he po en ial o eco e ing addi ional P wi h leacha e1, possibly educing
he inpu o p ima y ex ac an and, he e o e, enhancing he ci cula i y o he p ocess. In ex ac ion2,
PEE2 was signi ican ly lowe han PEE1, p obably due o he lowe ex ac ing po en ial o leacha e1
compa ed o he ini ial acid solu ion.
PPE was consis en ac oss all eco e y p ocesses con igu a ions, wi h alues exceeding 98%. This
inding sugges ed ha he p ecipi a ion s ep may be una ec ed by he numbe o pe o med ex ac ion
(a)
(b)
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s eps o he choice o acid ex ac an .
In he TS p ocess, bo h he acids showed high and compa able PRE1 anging be ween 85% and 87%.
Howe e , he HCl-based con igu a ion exhibi ed a PRE₂ 25% lowe han ha o H2SO4-based one,
possibly due o he lowe obse ed PEE2. As a esul , he PRETOT o he TS con igu a ions was 74%
wi h H2SO4 and 59% wi h HCl, highligh ing he be e pe o mance o H2SO4 when applying he TS
p ocess. The PRETOT o he TS p ocesses was consis en ly lowe han ha o he OS p ocesses, wi h
di e ences up o -27% when employing HCl. This ou come, mainly a ibu ed o he lowe ex ac ing
po en ial o leacha e1 compa ed o he ini ial acid solu ion, unde sco ed he need o u he
op imiza ion o he ex ac ion2 ope a ing condi ions.
Finally, he Pp o each TS p ocess con igu a ion anged be ween 14% and 18%, sligh ly exceeding
ha o H2SO4- (11%) and HCl-based (17%) OS con igu a ions. This inding indica ed he sui abili y
o he TS p ocess o eco e ing high- alue P- ich p oduc s.
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