9 DISCUSSION. 9.1 Long Fusion Times: Equilibrium Phases. Scheme 9.1: Reaction observed at 600 and 650 C when 2 mol of sodium hydroxide were

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1 9 DISCUSSION 9.1 Lng Fusin Times: Equilibrium Phases Lng fusin times were investigated t determine which equilibrium phases wuld frm as a functin f temperature and cmpsitin. Fusin experiments were cnducted fr 336 hurs, at different temperatures and varius NaOH:ZrSi0 4 ml ratis. The cnclusins drawn belw are based n the interpretatins f the XRD spectra listed in Appendix A and tabulated in Table 8.2. When 1 ml f zircn is fused with 2 ml f sdium hydrxide, zircn and Na2Zr03 are the main reactin prducts at 600 C and 650 C. The XRD spectra als reveal the presence f minr amunts f crystalline sdium metasilicate. The bserved prduct spectrum fr these temperatures is cnsistent with the reactin shwn in Scheme 9.1 belw. ZrSi04 + 4NaOH -+ Na2Zr03 + Na2Si03 Scheme 9.1: Reactin bserved at 600 and 650 C when 2 ml f sdium hydrxide were fused with 1 ml f zircn fr 336 hurs The XRD peaks fr Na2Zr03 are mre intense fr the 650 C fusin than the 600 C reactin. This implies imprved cnversin f zircn t sdium metazircnate at the higher reactin temperature. At a fusin temperature f 700 C, Na2ZrSiOs appears as a new phase in the XRD spectrum. This suggests the ccurrence f an additinal reactin, as shwn in Scheme 9.2. ZrSi04 + 2NaOH -+ Na2ZrSiOs Scheme 9.2: Reactin bserved abve 700 C when 2 ml f sdium hydrxide were fused with 1 ml f zircn fr 336 hurs Abve 750 C, ZrSi04, Na2Zr03 and Na2ZrSiOs are the majr equilibrium phases. Althugh sdium zircn ate appears as a majr phase at 750 C and a 2: 1 stichimetry, its peak intensities are clearly reduced cmpared with thse at the 700 C fusin temperature. This indicates that the frmatin f sdium zircnium silicate is favured ver sdium zircnate at lw ml ratis and high temperatures. 65

2 At 850 C, the XRD spectrum f the AFDZ prduced by fusin f 2 ml f sdium hydrxide with 1 ml f zircn fr 336 hurs shws that Na2ZrSiOs is virtually the sle equilibrium phase frmed. Only traces fna2zr03 and zircn remain. As expected, increasing the rati f sdium hydrxide t zircn increases the sdium zircnate yield. The apparent presence f Zr02 (as reprted n Spectrum 10 in Appendix A) may be an artefact f sdium carbnate as the XRD peaks f these tw cmpunds shw cnsiderable verlap (see Spectrum 12 in Appendix A). The phases bserved at 850 C using the 4:1 stichimetry indicate that the prducts may be explained by reactins 9.1 and 9.2 ccurring simultaneusly. The verall reactin is shwn in Scheme 9.3. ZrSi04 + 4NaOH xna2zr03 + (2x-l)Na2Si03 + (l-x)na2zrsios + (1- x)nsi04 Scheme 9.3: Reactin bserved at 850 C when 4 ml f sdium hydrxide were fused with 1 ml f zircn fr 336 hurs The frmatin f sdium rthsilicate (NSi04) can be explained by the reactin shwn in Scheme 9.4. ZrSi04 + 6NaOH Na2Zr03 + NSi04 Scheme 9.4: Reactin explaining the frmatin f sdium rthsilicate A key bservatin is that sdium zircnium silicate des nt frm when fusing belw 700 C. This implies that sdium zircnate is the equilibrium phase at lw temperatures Phase diagrams Figures 9.1 and 9.2 shw simplified phase diagrams previusly reprted by Manhique et al. [3]. They were cnstructed frm the phases bserved in the XRD spectra fr 336-hur fusins. The vertical dtted line in these figures shws the lcus f increasing the reactant rati f NaOH t zircn used in the current experiments. Nte that the phase NZr2Sh0I2, reprted by D' Ans et al. [62] was nt bserved in this study. Hwever, it was bserved by Kwela [16] when fusing 4 ml fnaoh with 3 ml f zircn at 850 C. 66

3 Figure 9.1: Simplified phase diagram fr alkali decmpsitin f zircn at 650 C Figure 9.2: Simplified phase diagram fr alkali decmpsitin f zircn at 850 C 67

4 Figure 9.1 applies fr fusins cnducted at 650 C, as well as at 600 C. Figure 9.2 indicates the equilibrium prduct spectrum fr reactins cnducted at higher temperatures. The latter is similar t the phase diagram accepted in the literature fr fusing zircn with sdium carbnate at temperatures abve 1 OOO C [62] Effect ffusin time n zircnia yield The effect f fusin time n zircnia yield was studied. Fusins were cnducted ver fusin times f 1, 2, 4, 24 and 336 hurs, using 2 and 4 ml f sdium hydrxide per ml fzircn in the fusin. The results are shwn graphically in Figures 9.3 and ,---<)--2:1750 2: , Fusin Time, h Figure 9.3: Effect f time n zircnia recvery at 750 and 850 C, using 2 ml f sdium hydrxide per ml fzircn. A lcal maximum in the yield is bserved at 2 hurs ffusin 68

5 The zircnia yield curve at 850 C shws an unexpected lcal maximum (f abut 63%) at the 2-hur fusin pint. Silica recvery als shws a lcal minimum f abut 22% here. A minimum ( 49%) in the zircnia yield is bserved at 4 hurs f fusin. This rughly crrespnds t the maximum in the silica recvery. Similar behaviur is bserved at 750 C (Figures 9.4 and 9.5). 80 <>.. 2:1 750 ;-0-2:1850 ;, ' e ll) > 0 Q Q ṟ rzj 40 t? 0, Fusin Time, h Figure 9.4: Silica recvery at 750 and 850 C using 2 ml f sdium hydrxide per ml f zircn sand Silica recvery reaches a minimum at 850 C fr 336 hurs fr the 2 ml stichimetric rati. This is cnsistent with the frmatin f Na2ZrSiOs, which is stable with respect t water, i.e. the cmpund des nt hydrlyse in pure water. Cnsequently, sdium cannt be leached with water washing. The XRD results cnftrmed the frmatin f this phase (see Spectrum 9 in Appendix A). Fr the fusin at 750 C, the silica recvery at 336 hurs is still as high as 19% (Figure 9.4). This suggests the presence f sluble sdium silicates in the prduct 69

6 spectrum and therefre, by implicatin, als NazZr03. This is cnfirmed by Spectrum 7 in Appendix A. Nte that the sdium silicates d nt crystallise easily and it is easier t infer their presence by indirect means in the XRD. Fr 4 ml, similar behaviur is bserved, i.e. at 2 hurs and 850 C there is a lcal maximum in the zircnia yield f ca. 76% (Figure 9.5). Figure 9.6 shws the silica recvery using 4 ml fsdium hydrxide per ml fzircn : : Fusin Time, h Figure 9.5: Effect f time n zircnia recvery at 750 and 850 C using 4 ml f sdium hydrxide per ml fzircn sand 70

7 " <> --4: 'l-q-4:1 850 '#. 60 > 0 (.) ttl... -(.) CZl 40,- <> 20, Fusin Time, h Figure 9.6: Silica recvery at 750 and 850 C using 4 ml f sdium hydrxide per ml f zircn sand 9.2 Optimisatin fzircnia Yield at Tw Hurs Fusin Time Cmmercially, shrt fusin times wuld be preferred. Based n this fact and the yield maximum bserved with the 2-hur fusins, further experiments were limited t this fusin time. The effects f fusin temperature and stichimetry n zircnia yield were studied and the results are presented in Figure 9.7. The zircnia yield shws a mntnic increase with temperature and ml rati. 71

8 hurs 750 C -0-2 hurs 850 C '$ 60 d'... (I)... > d 'E. N 40 '...'-? Ml Rati Figure 9.7: Effect fthe stichimetry fthe reactin n zircnia yield after 2 hurs ffusin Silica recvery shws a minimum when 2 ml f sdium hydrxide are used at 850 C fr 2 hurs. This is attributed t the frmatin fna2zrsios (see Spectrum 8 in Appendix A). 72

9 hurs 750 C. -::t(- 2 hurs 8500C i 80 ' Ml Rati Figure 9.8: The effect f stichimetry n silica recvery at 750 and 850 C fusin temperatures 9.3 Efficiency fthe Prcess The efficiency f the prcess was tested by studying the zircnia yield relative t the amunt f sdium hydrxide cnsumed (mass per mass basis), at and 850 C, using 2, 4 and 6 ml at 2 hurs fusin time. The results are presented in Figure 9.9. The highest reagent efficiency is attained fr fusins cnducted at 850 C using 2 ml fsdium hydrxide. 73

10 1.0 Zircnia yield per NaOH cnsumed 0.8,,,,,,,,,,,,,, '. ----,, '0,.,, '0 '",, !:s:- 650 C C C NaOH/zircn Ml Rati Figure 9.9: Efficiency fthe prcess analysed fr the effects f fusin temperature, stichimetry fr 2-hur fusin times The prcess als allws the recvery falkali in the frm fa saleable sdium silicate prduct stream. The frmatin f Na2ZrSiOs exacerbates the generatin f waste, as it has t be hydrlysed using acid. This nt nly creates a salt-based waste stream, but als cnsumes additinal acid reagent. Figure shws that fusins cnducted at 850 C using 2 ml f sdium hydrxide als yield the lwest amunt fsalt waste. 74

11 ... C) -C) :) :::I 1:) e c N Q N C)... (J) a. (J) +-' 0.2 -t:r-650 C en rn 3: C c I 0.1 -D-850 C 0 rn Z NaOH/zircn Ml Rati Figure 9.10: Generatin f salt waste stream by the prcess, analysed fr the effects f fusin temperature, stichimetry fr 2-hur fusin times 9.4 Pssible Explanatin fr Nar03 Frmatin t the Detriment f NarSi05 Frmatin The 336-hur fusins at high temperatures shwed that Na2ZrSiOs is the equilibrium phase at a ml rati f 2: 1. The frmatin f this phase als guarantees the mst efficient use f alkali fr the liberatin f zircnia frm zircn. Thus the lcal yield maxima bserved at a 2-hur fusin time in Figures 9.3 and 9.5 are due t the initial frmatin f this phase. Hwever, as the fusin time is extended, the prprtin f this phase in the prduct tends t decline befre increasing again. This presents a paradx: Hw des ne explain the temprary disappearance f the equilibrium phase nce it has been frmed? An attempt is made belw t prvide a ratinalisatin f these bservatins. 75

12 Zircn is a highly refractry cmpund with a melting pint abve C. The sdium hydrxide reagent melts at abut 318 0C. Thus it is reasnable t assume that a "shrinking cre" mdel applies t the decmpsitin reactin. During the fusin reactin, the liquid-like high-basicity uter layer etches away the surface f the slid zircn particle. In the prcess, the cmpsitin f the liquid phase changes. A prgressive change in basicity is expected as the reactin prceeds t cmpletin. With regard t the phases bserved in the XRD, the prgress f the reactin under ideal cnditins and fr 1 ml f zircn reacting with 2 ml f NaOH can be described in tenns fthe fllwing prduct spectrum: Initially: ZrSi04+ 2NaOH Intermediate: xna2zr03 + yna2si03 + znsi04 + unarsios + vzrsi04+ wnaoh Cmplete cnversin: Na2ZrSiOs This scheme is cnnected t the experimental results btained via the fllwing additinal assumptins: 1. The reactin des nt prceed t cmpletin. 2. The zircnia yield reflects the fnnatin fna2zr03 and Na2ZrSiOs. 3. The slid residue cnsists f unreacted zircn and silica nly. Thus the zircnium remaining in the residue can be calculated frm the zircnia yield. 4. Frm this, the silica cntent fthe residue can be detennined. 5. All the silica in the sdium silicate stream stems frm the sluble Na2Si0 3 and NSi04, accepting that the NSh07 is a linear blend f the previus tw species. 6. The ttal amunt f silica, i.e. the silica in the sdium slutin plus the silica in the residue, is equal t the amunt fzircnia, n a ml basis. Frm these assumptins, we can establish the fllwing mass balance equatins: Zircnia yield: Zry =x+ u Zircn in the residue: v =J-x-u Silica in the residue: SiR =u Silica in the sdium silicate stream: Siss y+ z Ttal ml f silicn equals ml f y + z + U + v =X + U + v zircnium: Therefre: y + z = x 76

13 Frm these relatinships, it can be deduced that: the ml fna2zrsios present (u) equals the ml fsilica in the residue; and The quantity fna2zr03 present equals the zircnia recvered minus the silica in the residue. Since all these quantities were measured, the change ill the prduct spectrum can be determined. The results are shwn in Figure [ Q., S 0.4 N c: '" -, "... "."... ", " 0.2 ". -IS:.t:.-. -'. ", Na2Zr03 " ':. 0.0 '6, I Fusin Time, h ':. Figure 9.11: Crrelatin ffractins fna2zr03 and Na2ZrSiOs, frm mass balance Frm the graph it can be seen that initially the phases Na2Zr03and Na2ZrSiOs were frmed in the same prprtin, at the I-hur fusin pint. At 2 hurs, the cncentratin fna2zrsios is higher but it then falls t a minimum at abut 4 hurs, after which its cncentratin again 77

14 increases. This bserved behaviur cnfirms the paradx: it is bserved that the final equilibrium prduct initially frms, then wanes and then refrms. The explanatin might be that the actual prducts in equilibrium with the melt depend n the cmpsitin fthe latter Zircnia yield -<>- Silica Recvery.,, ' Fusin Time, h 8 10 Figure 9.12: Zircnia yield and silica recvery btained fusing a mle fzircn with fur mles fsdium hydrxide at 850 C A reactin that may explain the cncmitant decrease in zircnia yield at intermediate fusin times is presented in Scheme 9.5. Na2ZrSiOs + Na2Si03 NSi04 + ZrSi04 Scheme 9.5: Reactin that prbably explains the drp in the zircnia yield 78

15 This reactin is cnsistent with the change in the prduct spectrum presented in Figure 9.10 and the sdium rthsilicate bserved in the infrared spectra by K wela [16]. He cncluded that the sdium attacks the silica tetrahedra in preference t the zircnia. 9.5 Direct Zircnia Synthesis Direct synthesis f zircnia frm the AFDZ yielded a zircnia with purity as high as 87 %. This required fusin fr 24 hurs at 650 C using 6 ml sdium hydrxide per ml f zircn. When fusing with 4 ml sdium hydrxide fr 8 hurs at 850 C a yield f 79 % was btained. A similar result was btained using fur mles fsdium hydrxide at 650 C fr 48 hurs. The crrespnding silica recveries fell in a narrw range varying frm a high f 65% t a lw f 50%. Hwever, 2-hurs resulted in pr silica recvery. The lw silica recveries are attributed t frmatin f sdium zircnium silicate; it is nt hydrlysed t hydrus zircnia by water alne. 79