Role of starch characteristics in the properties of low-cost ceramic membranes

ROLE OF STARCH CHARACTERISTICS IN THE PROPERTIES OF LOW-COST CERAMIC MEMBRANES M-M. Lo en e-Ayza a , M. J. O s a , V. Pé ez-He anz b , S. Mes e a,* . a Ins i u o de Tecnología Ce ámica. Uni e si a Jaume I. Cas ellón (Spain). b IEC G oup, Uni e si a Poli ècnica de València (Spain). *Co esponding Au ho ([email p o ec ed]) Abs ac S a ch is used as po osi y gene a o in memb anes and o he po ous ce amics. The e a e di e en p o ide s ha o e a wide a ie y o s a ches, which p oduce dis inc po e size dis ibu ions. A s udy was conduc ed o e alua e he e ec s o he cha ac e is ics o s a ches (impu i ies, pa icle size), ob ained om po a o, pea, maize and whea , on he p ope ies o mic o il a ion memb anes o mula ed wi h adi ional aw ma e ials ( he ce amic ma ix was composed o qua z, albi e and mulli e). The esul s indica ed ha he po e size dis ibu ion and he pe meabili y coe icien o he memb ane can be con olled modi ying only he pa icle size o s a ch. In his way, co ela ions we e ob ained be ween su ace mean diame e o s a ch and memb ane p ope ies (cha ac e is ic po e diame e s d 16 and d 50 , and pe meabili y coe icien ). Mo eo e , i is necessa y o use a s a ch ha ing a mean pa icle size g ea e han abou 50 mic ons in o de o ob ain a signi ican change in he po e size dis ibu ion and an inc ease in he pe meabili y o he memb ane. Keywo ds: memb ane, po osi y, pe meabili y, s a ch, pa icle size. 1 In oduc ion Ce amic po ous ma e ials ha e been a subjec o esea ch due o hei po en ial applica ions in di e en ypes o memb anes, as well as in o he ields as ca alys suppo s 1 , acous ic o he mal insula o s 2 , and bioma e ials 3 among o he s. The syn hesis o po ous ce amics equen ly includes a subs ance which disappea s du ing he he mal ea men o sin e ing (by decomposi ion, e apo a ion, mel ing o bu ning 4 ), gene a ing an addi ional ne wo k o po es ha modi y he memb ane po e size dis ibu ion ha could be ob ained wi h he ce amic composi ion by i sel . As examples, he mel ing and e apo a ion o poly(me hylme ac yla e) desc ibed by Zeng e al. 5 , he decomposi ion o u ea by Vijayan e al. 6 o he bu ning o lou by Slosa czyk e al. 7 . The ange o ma e ials employed o c ea e po osi y in ce amics is b oad, as he e iews abou his subjec ha e shown (Che alie e al. 4 ). The addi ion o a empe a u e sensi i e componen in o de o c ea e po osi y in he suppo is applied in he manu ac u e o memb anes based on ad anced ce amics (alumina, i ania, zi conia), and also o memb anes based on adi ional composi ions 8 o local aw ma e ials (clays 9,10 , pe li e 11 ). Nume ous ma e ials ha e been used as po e gene a o s, s a ing by chemically pu e subs ances (u ea 6 ), going h ough p ocessed subs ances (co n s a ch 11 ), and ending in na u al p oduc s (poppy seeds 2 , ice b an 10 ), o e en in was es (sawdus 8 , ly ash 9,12 ). S a ches a e b oadly employed as ma e ials o gene a ing po osi y in ce amics as hey p oduce po es du ing bu ning ou a ound 500ºC 13-15 . In addi ion, hey a e easy o bu n, cheap and en i onmen ally iendly 4 . Howe e , s a ch, as a subs ance de i ed om na u al sou ces (po a o, pea, co n, whea , e c.) and subjec ed o di e en p ocesses o ex ac ion and condi ioning, has a wide ange o cha ac e is ics ha can a ec he inal p ope ies o he esul ing memb ane. Usually, in each esea ch abou memb anes only one speci ic ype o s a ch is used as po e gene a o . The e o e, his speci ic ype o s a ch i is a ixed pa ame e along he in es iga ion. Howe e , he a ailabili y o di e en ypes o s a ch opens he doo o modi y he p ope ies o memb anes wi hou changing he aw ma e ial’s p opo ions o he p ocessing pa ame e s du ing he expe imen s, as shown G ego o a e al. 16 . One o he mos in e es ing p ope ies o a memb ane is he pe meabili y coe icien ha is ela ed wi h he geome y o he memb ane’s po e ne wo k. Many models ha e been p oposed, based on di e en app oxima ions, in o de o ela e he pe meabili y o a po ous solid wi h he cha ac e is ics o i s own po e ne wo k. Some models a e simple like he Hagen-Poiseuille o he Kozeni-Ca man equa ions 17 . Howe e , he complexi y o he model g ows as he desc ip ion o he po e ne wo k geome y became mo e igo ous (as examples, he applica ion o g ain models 18 o digi ized s uc u e models 19 ). In consequence, he phenomenological app oaches emain o unique use ulness o he quan i ica ion o memb ane’s inal p ope ies 20 . This esea ch was ocused on analyzing he e ec s o di e en ypes o s a ches, wi h dis inc cha ac e is ics, on he p ope ies o low-cos ce amic mic o il a ion memb anes. The aim was o ex end he ange o memb ane ea u es (po osi y, pe meabili y), wi hou modi ying he p opo ions o he aw ma e ials o he p ocessing. Addi ionally, an a emp was made o ela e memb ane’s pe meabili y wi h he pa icle size o he speci ic ype o s a ch used as po e gene a o h ough a simple model. 2 Expe imen al The aw ma e ials o he ce amic memb anes we e clay (UA-50, Mine a ia, Spain), mic onized sodium eldspa (cou esy o Pamesa, S.A. Spain) and eldspa ic sand (AFS- 125, Ime ys, Spain). They we e p opo ioned by weigh in 40:40:20 a ios espec i ely. As a esul , he global mix u e was app oxima ely 72.0 SiO 2 , 17.6 Al 2 O 3 , 4.2 Na 2 O, 1.5 K 2 O, 0.6 TiO 2 , 0.5 Fe 2 O 3 , 0.3 CaO and 0.2 MgO, wi h a loss on igni ion o 2.9 (w %). Six di e en s a ches we e selec ed as po e gene a o s: S1 (po a o s a ch, Roque e F e es S.A., F ance), S2 (po a o s a ch, Sigma-Ald ich Co. USA), S3 (whea s a ch, Roque e F e es S.A., F ance), S4 (pea s a ch, Roque e F e es S.A., F ance), S5 (pea ibe L50M, Roque e F e es S.A., F ance), and S6 (maize s a ch ex a pu e, Fishe Chemical, USA). The abo e aw ma e ials we e p ocessed as ecei ed, in powde y s a e. In addi ion, a supplemen a y po e gene a o was p epa ed by sie ing he S6 s a ch h ough a 200 mic ons mesh ha was named S7. The pa icle size dis ibu ion o he s a ches was ob ained by d y lase di ac ion (mas e size 2000, Ma e n Ins umen s L d. UK) and he cha ac e is ic diame e s D 10 , D 50 , D 90 , D V and D S we e calcula ed. The pa ame e s D 90 , D 50 and D 10 a e he cu o pa icle size below which 90%, 50% and 10% o he o al pa icle olume lies. The pa ame e s D V and D S a e espec i ely he olume mean diame e and he su ace mean diame e . The humidi y was ob ained om he weigh loss a e d ying a 110 ºC in an elec ical o en (kg o wa e by 100 kg o d y solid). The ue densi y o he d ied s a ches was measu ed by helium pycnome y (Ul apycnome e 1000, Quan ach ome Inc., USA.) and he ash con en was de e mined by ea ing e e y s a ch a 1000 °C. Finally, he chemical analysis o he ashes was pe o med by EDX (Genesis 7000 SUTW, EDAX, USA), connec ed o a FEG-SEM (Quan a 200F, FEI Co, USA). The ce amic aw ma e ials we e p opo ioned by weigh and hen d y mixed in a blade mill (Mul i io, Moulinex In e na ional, F ance). Once homogenized, he chosen s a ch was added g adually o he blade mill o a oid he o ma ion o la ge agglome a es. All expe imen s we e pe o med using mix u es calcula ed o con ain 85 w % o ce amic ma e ial and 15 w % o d y s a ch (Table 1), excep he e e ence mix u e wi hou s a ch (whose ue densi y was measu ed by helium pycnome y). The olume ac ion o s a ch was be ween 23 % and 25 % in all he aw ma e ial mix u es, and he e o e beyond he pe cola ion h eshold o 18 % indica ed by G ego o á e al 21 . The eigh mix u es we e mois ened up o a wa e con en o 5.5 kg H 2 O/100 kg d y solid. Cylind ical es specimens o 50 mm diame e and 3-4 mm hickness we e o med by uniaxial d y p essing a 300 kg·cm -2 and d ied in an o en a 110 ºC no less han 24 hou s. The bulk densi y o he g een samples, and la e o he sin e ed ones, was measu ed by me cu y displacemen . The g een specimens we e sin e ed in wo s eps ( igu e 1). Ini ially, he s a ch was oxidised in a mu le u nace wi h a slow ea men cha ac e ised by a maximum empe a u e o 500 ºC and a soaking ime o 1 hou (K60L, Nanne i Spa. I aly). Finally, he specimens we e sin e ed in a as elec ic kiln (pi ome ol S.A. Spain). This las he mal s ep was designed o balance he po osi y and mechanical s eng h in he sin e ed memb anes, and was cha ac e ized by a soaking ime o 1 hou a 1100 ºC. The po e size dis ibu ion o he memb anes was measu ed by me cu y in usion po osime y (Au oPo e IV 9500, Mic ome i ics Ins umen s Co, USA), and he open olume o po es and cha ac e is ic po e diame e s (d 16 , d 50 , and d 84 ), we e calcula ed. The wa e up ake was measu ed by he boiling wa e imme sion me hod and he pe meabili y coe icien o wa e was ob ained wi h a liquid pe meame e (LEP101-A, PMI, USA). Addi ionally, he ue densi y o he memb anes was measu ed by helium pycnome y o milled samples, he mine alogical composi ion was ob ained by XRD (D8 Ad ance, B uke Co, USA), and he mic os uc u e o some memb anes was analyzed by FEG-SEM. 3 Resul s and discussion 3.1 Cha ac e iza ion o s a ches The humidi y o he s a ches co e ed a ai ly wide ange (Table 2). In h ee o hem, humidi y was a ound 15 w %, while in o he was clea ly abo e, and in he wo emaining, humidi y was below. The ue densi y o he s a ches S1, S2, S3, S4 and S5 was almos he same conside ing he unce ain y o measu emen (Table 2), while his physical magni ude was signi ican ly lowe o s a ch S6. In spi e o hese di e ences, he p ocessing o he aw ma e ial mix u es was accomplished as in he C0 composi ion. The ash con en o he s a ches was less han 1 w %, excep in he case o S5 which was ma kedly highe (Table 3), possibly due o i s own p oduc ion p ocess, which inco po a es g ea e p opo ion o impu i ies. By con as , S2, S3 and S4 s a ches s and ou o i s low ash con en . The ash con en combined wi h EDX analysis p o ided he ype and con en o impu i ies. The esul s indica ed ha he s a ch S5 con ains he g ea es amoun o impu i ies, ollowed by he s a ch S6 a a g ea dis ance. By con as , s a ch S2 con ained he lowes amoun o impu i ies. The elemen s ounded in g ea e p opo ions in he ashes we e po assium, sodium, phospho us and calcium. These elemen s could ac as luxes du ing he sin e ing o samples. Pa icle size dis ibu ions o s a ches co e ed a a he b oad ange (Figu e 2). The ines s a ches we e S3 and S4, he coa ses we e S5, S7 and S6, while pa icle size dis ibu ions o S1 and S2 occupied an in e media e posi ion. The shape o he pa icle size dis ibu ions was symme ical, excep o S5, S6 and S7, which possessed a ail in he in e al o lowe diame e s. The cha ac e is ic diame e s indica ed ha he se en selec ed s a ches spanned an o de o magni ude in pa icle diame e , aken as D 50 (Table 4). F om hese esul s, i can be in e ed ha s a ch S3 should gene a e po es wi h diame e app oxima ely one en h o s a ch S6. Va ious op ions exis ed o D 50 alues less han hal o he co esponding o S6 s a ch, bu a gap in D 50 alues was de ec ed be ween he ob ained o S6 sample and he ollowing comme cial s a ch in size (S5). In o de o co e his gap in D 50 alues, he s a ch S7 was p epa ed. 3.2 Memb ane cha ac e is ics The ue densi y o s a ches and he mix u e o ce amic aw ma e ials (2.64±0.02 g·cm - 3 ), plus he da a o bulk densi y o he g een memb anes allowed he es ima ion o hei po osi y (ε G ) (Figu e 3). The addi ion o s a ch causes a educ ion in he bulk densi y o he g een memb ane. This is he esul o lowe ue densi y o s a ch, oge he wi h he smalle compac ion du ing p essing, as he po osi y inc eases espec o he alue co esponding o composi ion C0. In addi ion, s a ches’ ue densi y da a allowed o discoun he olume occupied by e e y s a ch in he g een memb anes simula ing hei s a e once he oxida ion s ep o he he mal cycle has inished (ε GT in Figu e 3). I was ound ha ε GT was p ac ically independen o he s a ch employed. Acco dingly, he po osi y o he memb anes a he s a o he sin e ing s ep was nea ly he same o all composi ions excep C0. This ac acili a ed he assessmen o he e ec s o s a ch’s cha ac e is ics on he p ope ies o memb anes. A e sin e ing, memb anes we e ob ained ee o de ec s and wi h su icien s eng h o pe o m cha ac e iza ion es s. I should be men ioned ha o he me hods o p epa ing he mix u e o aw ma e ials caused de ec s in he memb anes and hus we e disca ded (speci ically, we mixing and g anula ion we e in es iga ed). The sin e ed memb anes we e composed by qua z, albi e and mulli e (Figu e 4), and showed a b oade ange o bulk densi ies han ha o he g een ones, which indica ed ha he e ec s o he oids le by s a ches du ing sin e ing had been di e en (Figu e 5). The po osi y and densi ica ion (de ined as he change o po osi y o he specimen as a consequence o sin e ing, di ided by i s ini ial po osi y 22 ) o syn hesized specimens we e calcula ed om he measu ed ue densi y o he sin e ed memb anes (2.59±0.02 g·cm -3 ). The esul s indica ed ha he addi ion o s a ches inc eased po osi y and educed densi ica ion in ela ion o he e e ence composi ion C0, bu di e ences exis ed depending on added s a ch. The eby, he e is an app oxima ely linea ela ionship wi h nega i e slope be ween densi ica ion and he pa icle size o s a ch ( aken as D 50 , Figu e 6), which is consis en wi h he inc eased di icul y in emo ing he la ges po es du ing sin e ing. On he o he hand, he impu i y con en o he s a ches did no seem o exe a signi ican e ec on sin e ing, which is consis en wi h he high p opo ion o luxes p o ided by he aw ma e ials, bu i could be an impo an ac o o memb anes based on high-pu i y oxides. In addi ion, SEM images ( igu e 7) showed ha he mic os uc u e o he suppo s conside ably changed depending on he speci ic added s a ch. As an example, he S2 s a ch (D 50 nea he lowe limi o he ange explo ed) gene a ed abundan ounded po es, and appa en ly wi h li le di ec in e connec ions (black a eas in he image), while he coa ses s a ch S6 p oduced bigge po es, bu less egula and appa en ly in e connec ed by openings o highe a ea. The o al po e olume, calcula ed om he bulk densi y o he ce amic ma ix and he ue densi y o he memb anes, showed a e y good co ela ion wi h he wa e up ake (Figu e 8). The alue o he slope, e y close o 0.01, indica es ha p ac ically all he po osi y o he memb anes was open. By con as , me cu y in usion po osime y always esul ed in lowe alues o he open olume o po es, indica ing ha a ac ion o he open po osi y was below he de ec ion limi o he equipmen used (0.005 mic ons). This ac ion o he open po osi y was also a unc ion o he added s a ch. Ob iously, he employed s a ch a ec s he po e size dis ibu ion o he memb ane modi ying, among o he ea u es, he ac ion o po es whose inle is below he limi achie able by me cu y in usion. B oadly, he ine s a ches end o inc ease he ac ion o po es unde ec able by me cu y in usion, bu a di ec co ela ion has no been iden i ied, sugges ing ha he mixing p ocess can a ec o some ex en he po e size dis ibu ion gene a ed in he sin e ed memb ane. Memb ane’s po e size dis ibu ion showed bigge di e ences han hose o po osi y as a consequence o he e ec o he s a ch (Table 5 and Figu e 9). In gene al, he monomodal dis ibu ion o e e ence memb ane C0 shi ed o la ge diame e s and showed a end o become bimodal o he memb anes syn hesized employing s a ch. This end culmina es in memb anes C6 and C7, whose po e size dis ibu ion is bimodal. Despi e his clea end, a di ec co ela ion was no de ec ed be ween pa icle size dis ibu ion o he employed s a ch and po e size dis ibu ion gene a ed in he memb ane. I p obably was due o di e ences in he physical basis o each measu emen (ligh di ac ion o pa icle size and me cu y in usion o po e size, and in his me hod he measu ed diame e co esponds o he po e inle and no he eal diame e o he po e). Howe e , some co ela ions we e iden i ied be ween he cha ac e is ic diame e s o pa icle and po e, which could be used o es ima e he a e age po e size ha a pa icula s a ch could gene a e in he memb ane. Speci ically, he bes esul s we e ob ained by ela ing D S and po e diame e s d 16 o d 84 wi h a quad a ic polynomial (Figu e 10). In addi ion, he D 50 diame e o s a ches also showed a quad a ic ela ion wi h he same cha ac e is ic po e diame e s, bu wi h sligh ly lowe eg ession coe icien s ( hey we e no included by his eason). This pa abolic end means ha o gene a e la ge po es in he memb ane, o mo e co ec ly la ge po es whose inle is also la ge, i is necessa y o use s a ches wi h la ge pa icle size ( o example, a s a ch wi h a D S o 60 mic ons would be equi ed o gene a e a memb ane wi h a d 50 o 5 mic ons). Howe e , he a e age pa icle size o mos comme cial s a ches is less han 40 mic ons, Table 3: Ash and impu i y con en s o he s a ches. Impu i ies (mg/kg o d y s a ch) S a ch Ash con en (w %) Na K Mg Ca Zn P Cl Cu Si Al S1 S2 S3 S4 S5 S6 0.71 0.17 0.37 0.29 3.53 0.83 7 9 52 74 368 274 48 37 28 46 1198 14 12 3 3 4 119 5 100 6 15 6 207 17 9 2 10 1 0 23 145 38 74 24 108 38 - - - - 41 38 12 2 15 2 - 26 2 1 1 3 9 2 - - - - 3 - Table 4: Pa ame e s o pa icle size dis ibu ions o he s a ches. S a ch D 10 (µm) D 50 (µm) D 90 (µm) D V (µm) D S (µm) S1 S2 S3 S4 S5 S6 S7 26 22 12 17 16 57 43 46 39 20 24 75 190 145 76 67 31 34 184 393 274 49 42 21 25 90 211 154 41 35 18 23 35 95 79 Table 5: Po e size dis ibu ion pa ame e s, wa e up ake and pe meabili y coe icien o memb anes. Re . d 16 (µm) d 50 (µm) d 84 (µm) Wa e up ake (%) K P ·10 16 (m 2 ) C0 C1 C2 C3 C4 C5 C6 C7 1.5 6.3 6.1 4.4 4.7 6.4 17.6 13.2 1.2 3.7 3.6 3.2 2.9 3.8 9.5 7.2 0.8 1.6 1.6 1.3 1.5 1.8 2.5 2.1 8.4 23.9 22.4 24.4 23.4 24.2 28.9 26.3 ≈ 0 1.6 ± 0.4 1.4 ± 0.2 2.29 ± 0.10 1.67 ± 0.12 2.55 ± 0.16 23 ± 2 13.8 ± 1.4 Figu e 1: The mal ea men employed o oxidizing he s a ch and sin e ing he memb anes. Figu e 2: S a ches’ pa icle size dis ibu ions. Figu e 3: Measu ed bulk densi y (ρ G ) and po osi y (ε G ) o g een memb anes, and also calcula ed po osi y excluding he s a ch (ε GT ). Figu e 4: Di ac og am o memb ane C0. Figu e 5: Bulk densi y (ρ S ) and po osi y (ε S ) o sin e ed memb anes. Figu e 6: Rela ion be ween densi ica ion o sin e ed memb anes and D 50 o he employed s a ch. Figu e 7: Mic os uc u es o he memb anes ob ained om s a ches S2 and S6. Figu e 8: Rela ions be ween memb anes’ wa e up ake and he open po e olume measu ed by me cu y po osime y, and also he calcula ed o al po e olume. Figu e 9: Di e en ial po e size dis ibu ions o he memb anes. Figu e 10: Rela ions be ween he cha ac e is ic po e diame e s o he memb anes and he su ace mean diame e o he employed s a ch. Figu e 11: Rela ions be ween he pe meabili y coe icien o he memb anes and hei cha ac e is ic po e diame e s. Figu e 12: Rela ions be ween he pe meabili y coe icien o he memb anes and wo cha ac e is ic pa icle diame e s (D S and D 50 ) o he employed s a ch.