Ion exchange between solid phases: material balance and mechanism of interaction of aluminium hydroxide gel in aqueous suspension with a strong acid cation exchange resin

Itydrometallurgy, 27 ( 1991 ) 151-167

151

Elsevier Science Publishers B.V., Amsterdam

Ion exchange between solid phases: material balance and mechanism of interaction of aluminium hydroxide gel in aqueous suspension with a strong acid cation exchange resin Tak Ching Wong and A. William L. Dudeney Department of Mineral Resources Engineering, Imperial College, Prince ConsoreRoad, London Sl4r7 2BP, UK (Received December 13, 1990; accepted February 23, 1991 )

ABSTRACT Wong, T.C. and Dudeney, A.W.L., 1991. Ion exchange between solid phases: material balance and mechanism of interaction of aluminium hydroxide gel in aqueous suspension with a strong acid cation exchange resin. Hydrometallurgy, 27:151 - 167. Ion exchange between aluminium hydroxide gel, prepared from aluminium sulphate and aqueous ammonia, and a strong acid cation exchange resin (Amberlite IR 120) was studied to provide fundamental data in order to underpin potential applications in water treatment or hydrometallurgical processing. The overall ion exchange was an acid-base reaction which approached completion relatively slowly (over several hours) because both of the main reactants were solid in form. Preliminary work showed that an increase in ionic strength from zero to 1.5 by stepwise addition of ammonium sulphate increased the rate of ion exchange while decreasing the proportion of aluminium loaded on the resin at equilibrium. Quantitative kinetic and equilibrium data describing the distribution of aluminium, hydrogen and ammonium ions over the gel, aqueous and resin phases were obtained on the basis of a new experimental and charge/material balance design requiring the minimum of analytical measurements on ( i ) the initial pH and concentration (and molecular formula ) of the hydroxide precipitate suspension in lhe mother liquor from precipitation; (ii) the initial hydrogen ion content of the resim and (iii) the variation with time o f p H and suspended and dissolved aluminium concentration in the aqueous phase. Resulting composite plots of mole distribution against time up to 24 h gave information on the mechanism occurring (several overlapping stages in the initial reaction over the first 20 min, followed by slower changes, governed by small, roughly steady-state, concentrations of hydrogen and aluminium ions in aqueous solution, up to pseudo equilibrium at 300 rain) and indicated that the overall reaction was 60-70% complete within 1-2 h and that the practical ion exchange capacity for aluminium at equilibrium was greater than 70% of the theoretical maximum. The results also indicated that aluminium ions hydrolyse similarly in both aqueous and resin phases.

INTRODUCTION

Aluminium hydroxide gel is frequently employed in industrial water treatment processes for removing dissolved and suspended matter from raw water. 0304-386X/91/$03.50

© 1991 Elsevier Science Publishers B.V. All rights reserved.

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TAK CHING WONG AND A.W.L. DUDENEY

Typically, aluminium sulphate solution (alum) is added to raw water together with calcium hydroxide to precipitate the gel at pH 4-6 and adsorb impurities [ 1-4 ]. After dewatering, the spent gel (aluminium hydroxide plus impurities) is normally discarded but recycling and reuse of the alum, and land-spreading of the organic residue as a soil conditioner, are possible. Various methods of recycling by acid or alkaline leaching and solvent extraction have been studied [ 5-9 ] and industrial processes have been operated from time to time [7,10-11 ]. Technical problems, outlined below, together with relatively low alum prices in recent years, have generally made recycling economically unattractive in terms of the (poorer) quality and (small) value of the recovered alum in comparison with recycling costs. However, increasing awareness of the toxic effects of aluminium from spillages and natural leaching of discarded materials [ 12-14 ], and the likelihood in the future of incurring penalties for uncontrolled releases to the environment, are providing additional incentives to develop improved processes. In the case of alum, satisfactory technical improvements should lead to economic recycling in which process costs are justified partly by the value of the recovered product and partly by avoidance of penalties. Freshly precipitated aluminium hydroxide is a reactive substance which dissolves readily in dilute sulphuric acid and may be regenerated by dissolution, solution purification and reprecipitation. Unfortunately, the industrial processes based on sulphuric acid leaching are prone to a build-up of colour (dissolved organics) and toxic elements (e.g., lead and mercury) in recycle streams. Efficient filtration of the leach solutions to remove fine-sized mineral and organic solids (originally suspended in the raw water) can be difficult and expensive [10-11]. Acid leaching, combined with conventional solvent extraction with alkyl phosphates, facilitates improved control of dissolved impurities but may still cause problems with solid-liquid separation when organic matter is a prominant impurity. Solvent-in-pulp extraction with alkyl phosphate extractants in acid form [ 15 ] avoids the need for both direct acid addition and filtration but may be hampered by the tendency for interfacial "cruds", typically emulsions of mixed organic solid, aqueous solvent and organic solvent phases, to form and spoil liquid-liquid separation. Strong acid ion exchange resins, such as Amberlite IR-120, might provide an improved means of aluminium recycling in water treatment applications but have received little attention. As has been found in uranium processing [ 16 ], resins are generally less selective than solvent extraction reagents but, when fabricated as well-defined 650-800 ~tm spheres, do not cause emulsification and are readily filtered from discard suspensions after loading with metal cations. The present paper reports a kinetic and material balance study of processes occurring between synthetic aluminium hydroxide gels and Amberlite IR-120, and aimed at providing new data and a fundamental under-

ION EXCHANGE BETWEEN SOLID PHASES

153

standing of exchanges between the two solid phases shown, which should be of relevance to both water treatment and hydrometallurgical processes involving aluminium. These processes can be represented by the simplified equation: A1 (OH) 3(gel in aqueous suspension ) + 3 ( R e s i n - ) H + = (Resin-)3A13+ + 3H20

( 1)

THEORY

Equilibria in aqueous medium Various hydrolysed monomeric and polymeric aluminium species can form in solution in addition to the simple unhydrolysed A13+ (aq) ion, depending upon the overall ion concentrations and final pH prevailing [4,17]. For the concentration range 1-10 m M and pH range 3-5, relevant to the present work, activity coefficients may be set equal to unity and species other than A13+ (aq), A1OH 2+ (aq) and A1 (OH)~- (aq) may be neglected. The concentrations and concentration rations of the three predominating species can then be calculated from eq. (2), in which Kl and K2 are hydrolysis constants and AIT is the total (analytical) aluminium concentration in the aqueous phase: [A13+ ] = [A1OH 2+ ] [H + ]/K1 = [AI(OH) + ] [H + ]2//£2 =A1T/(1 +K,/[H +1 +K2/[H +12 )

(2)

Precipitation When base is added to a solution of an aluminium salt, a gelatinous and/ or colloidal precipitate forms, depending on the conditions ofpH and concentration employed. At pH 4-6 and at moderate concentration, the result is a gel which is readily sedimented (particle size well above 1 gm). This precipitate is normally considered to be an amorphous hydroxide but invariably contains a proportion of salt anions which cannot be fully washed out of the system [ 18 ]. In the case of aluminium sulphate mixed with aqueous ammonia the following overall equation (neglecting water of hydration) applies:

0.5A12(804)3+ ( 3 - n ) N H 3 + ( 3 - n ) H 2 0 = A I ( O H ) 3 _ , , ( S O 4 ) 0 5 n + (3-n)NH~ +0.5(3-n)SO]

(3)

In this equation, n is the number of moles of hydroxide replaced by the anion per mol of precipitate. The final ionic strength of the medium is fixed by the stoichiometry of the reaction and is generally sufficient to ensure that the colloidal-sized first-formed particles are coagulated into a gel.

154

TAK CHING W O N G AND A.W.L. DUDENEY

Dissolution and resin ion exchange When acid for dissolution is provided by an ion exchange resin, a relatively complex mass distribution results in which the six cations mentioned equilibrate over the aqueous and resin phases: Al(OH)3_n(SO4)o.sn+ ( 3 - n ) N H g

+ (3-n-p)Res-H

+

= m{aA13+ + bAIOH 2+ + cA1 ('OH) J- } + ( 1 - m) {x(Res- )3A13+ + y ( Res- ) zA1OH 2+ + zRes- A1 (OH) ~- } + l ( 3 - n ) Res- NH + + (1-l)(3-n)NH~-

+{(3-n)-m(b+2c)-

(l-m)

(y+2z)}H20

+0.5nSO4z-

(4)

where: p = t h e a m o u n t by which the stoichiometric requirement for hydrogen ion starting on the resin is reduced as a result of the formation of hydrolysed aluminium species (and not just A13+ ); m = the proportion of the aluminium going to true solution ( 1 - m goes to the resin ); / = t h e proportion of a m m o n i u m ions going to the resin phase ( 1 - l remains in true solution ); a, b, and c = t h e proportions in which the species, AI 3+, A1OH 2+, and AI(OH) + form in the aqueous phase; x, y and z = the corresponding proportions (assuming hydrolysed species can exist there) in the resin phase. The complicated coefficient attached to water arises from mole balance.

Overall charge and material balance The overall equation ( e q . ( 5 ) ) is the sum ofeqs. (3) and (4): 0.5A12 (SO4)3 + ( 3 - n ) N H 3 +

+ +{m(b+2c)

(3-n-p)Res-H

+ ( 1 - m ) (y+2z)}H20=m{aAl 3+ + bAIOH 2+ + cA1 ( O H ) ] } + ( 1 - m) {x(Res- )3A13+ + y ( R e s - )2AIOH 2+ + zRes-A1 (OH) J- } + l ( 3 - n)Res-NH~- + ( 1 - l ) ( 3 - n ) N H + + 1.5SO4z-

(5)

The corresponding charge and material balances are given in eqs. (6-8): Charge: O=3ma+2mb+mc+ ( 3 - n ) ( 1 - l ) - 3

(6)

Aluminium: 1 =ma+ mb+mc+ ( 1 - m ) x + ( 1 - m ) y + ( 1 - m ) z

(7)

Resin: ( 3 - n - p ) = 3 ( 1 - m ) x + 2 ( 1 - m ) y +

(1-m)z+l(3-n)

(8)

ION EXCHANGE BETWEEN SOLID PHASES

155

To determine the coefficients in these equations and the distribution of ions with time under chosen experimental conditions it is necessary to have the following available as a minimum: (i) A suitable experimental design such that, taking account of hydrolysis of dissolved aluminium species and removal of solid hydroxide and dissolved species through essential sampling for analytical purposes, the overall acidbase reaction occurring is complete (all of the hydroxide and hydrogen ions will just be consumed at equilibrium ) and therefore the hydrogen ion content of the resin can finally be set equal to zero, and an overall value of ( 3 - n - p ) in eqs. (4), (5) and (8) can thus be established. (ii) Analytical determinations of the initial total concentration and number of moles (D) of aluminium and the molecular formula of the precipitate (from which a value of n in eqs. ( 3 ) - ( 6 ) and (8), and therefore p in (i) above, are deduced), the initial number of moles (E) of ammonium ion in solution before ion exchange (from stoichiometry), and the initial hydrogen ion content in moles (I) on the resin in hydrogen form. (iii) Analytical determinations of the variation with time of the system pH and aluminium content in the solid hydroxide and solution phases--for which purpose the total and dissolved concentrations (O and S, respectively), originally in terms of parts per million of aluminium are converted to (P-T) = (O-S)B/(2.698X 107) tool solid hydroxide and T=SB/(2.698X 107) mol dissolved aluminium where B~ 102 (the ratio of actual system volume to original system volume) accounts for total material losses resulting from sampling. (iv) Data, such as that cited [ [4], to estimate the ratio of hydrolysed aluminium species in the aqueous phase in eq. (2) and the coefficients a, b and c in eqs. (6) and (7), and therefore, by overall material balance, the proportion of hydrolysed aluminium in the resin phase. The following material balance equations (augmented by a suitable computer spreadsheet) then facilitate calculation of the other elements of the distribution after certain time intervals of reaction. The symbols B, D, E , / , O, S, P and T defined above and L, AO, AG, AP, A J, BC, and BD, given below, are conveniently the same references as used in the spreadsheet. Mol hydrogen ion consumed,

(AO) = ( 3 - n - p )

Mol hydrogen ion remaining on the resin, Mol aluminium ion on the resin,

( D - A G - ( P - T) )

(AP) = I - L - A O

(A J) = D - P

(9) (10)

( 11 )

Mol ammonium ion on the resin,

(BC) = I - A P - ( 3x+ 2y+ z)AJ/ (x+ y+z) Mol ammonium ion in solution,

(BD) =2.72D(B/lO 2) - B C

(12) (13)

156

TAK CHING WONG AND A.W.L. DUDENEY

Proportion of aluminium in solution versus that on resin,

(m)= T/ ( T+AJ)

(14)

where:

( D - A G - ( P - T ) )=the mol aluminium hydroxide reacted with acid after set time intervals from the start of reaction; AG (actually a cumulative quantity ) = the mol solid hydroxide removed from the system by sampling and therefore not available for reaction with acid; ( P - T), which already accounts (as in (iii) above) for the dissolved sampling losses of the aqueous medium, = the mol solid hydroxide remaining in the system. At equilibrium, AP and ( P - T ) are zero and the value of the constant ( 3 - n - p ) is found as follows: (3 - n - p ) = (I-Lequi, )/(O-AGequil)

( 15 )

The term ( 3 x + 2y+z)AJ/(x+y+z) is the "equivalent mol version" of AJ. As regards site occupancy on the resin, xAJ/(x+y+z) tool A13+ is equivalent to 3 xAJ/(x+y+z) mol of a singly-charged cation such as NH +, while yAJ/(x+y+z) tool A1OH 2+ and zAJ/(x+y+z) mol AI(OH)J- are correspondingly equivalent to 2yAJ/(x+y+z) and zAJ/(x+y+z) mol NH~-. Equation ( 12 ) does not require a knowledge of individual values of x, y and z because, from eq. (7), (x+y+z) and (a+b+c) are both unity and, from eq. ( 8 ), (3x + 2y + z) takes the same value as { (3 - n - p) - l(3 - n ) }/ ( 1 - m ), the terms of which may be found via the analytical formula of the hydroxide precipitate in eq. (4), together with eqs. (6), (14) and ( 15 ). Individual values of x, y and z are considered (at the end of the paper) by substitution of the resulting values of/, m, n and p into eqs. (7) and (8). As these equations still contain three unknowns, an assumption is necessary about the relationship between y and z. EXPERIMENTAL

Ion exchange resin P r e m i u m quality Amberlite IR- 120--a strong acid gel-type cation exchange resin with sulphonate (-SO~- H + ) functional groups and a matrix of a copolymer of styrene with 8% divinylbenzene as a crosslinking agentwwas used in the form of 600-850 ~tm diameter spherical beads. New batches of resin were conditioned [ 19] as a slurry in distilled water by several cycles of contact successively with 1M sodium hydroxide, distilled water, 1. I M hydrochloric acid and distilled water by upflow (0.1 bed v o l u m e s / m i n ) in a glass column having a sintered glass disk at the base to retain the beads. Conditioned batches

ION EXCHANGE BETWEEN SOLID PHASES

157

(or batches for regeneration after use) were contacted in the column with four times the theoretical quantity of 1.1 M hydrochloric acid, washed with distilled water until the washings were neutral to methyl orange indicator, wet-screened to obtain the 600-850 g m size fraction and finally stored under distilled water. Immediately prior to use, the resin was centrifuged at 700 m i n - 1 to remove physically adhering water. About 3-5 g (accurately weighed) of the centrifuged resin was dried at 110 °C for 16 h. The residual mass after cooling gave the original moisture content as 51%. The wet volume of 10 g screened and centrifuged resin was found to be 13.6 ml by direct measurement in a graduated cylinder. The ( m a x i m u m ) cation exchange capacity of the resin was determined [20] by contacting about 1 g (accurately weighed) of centrifuged hydrogen form resin with 200 ml 0.1 M sodium hydroxide containing 10 g sodium chloride in a 250 ml conical flask. Three 50 ml aliquots of the supernatant were titrated with 0.1 M hydrochloric acid (standardised against sodium carbonate ) to phenolphthalein and the average titre used to calculate the ion exchange capacity of the resin as 7.15 m eq/g dry resin ( 3.51 m eq/g wet resin).

A luminium hydroxide suspension Aluminium hydroxide gel suspensions ( 1000 or 450 mg/l, 37.1 or 16.7 mM, with respect to the aqueous phase) were prepared by adding the calculated quantity of finely divided aluminium sulphate (A12 (SO4) 3" 16H20 ) to about 500 ml dilute aqueous a m m o n i a in a volumetric flask and making up to 1 1. The final pH was in the range 4.7-5.6, over which the concentration of alum i n i u m in true solution (by atomic absorption spectrometry (AAS)) was normally less than 5 mg/l. The fresh hydroxide precipitates had particle sizes (by Malvern 3600 Particle Sizer) in the range 28-87 g m and (by Malvern 4700c Sub-Micro Particle Size Analyser) colloidal-sized particles could not be detected in the supernatant solution after centrifuging. However, as expected, the gels were actually loose aggregates of such colloidal particles: mechanical agitation and washing, see below, to reduce the ionic strength both led to the detection of smaller particles. The precipitation method described gave a product having the same appearance and reactivity as that precipitated conventionally by mixing preprepared solutions of both reagents and avoided the disadvantage--of gel adhering to the neck of the flask--encountered in the more conventional approach. To reduce the ionic strength of the medium, when required, by removing the other product of reaction ( a m m o n i u m sulphate), the gel suspension was washed in eight, 120 ml aliquots up to seven times with distilled water, a MSE GF-8 centrifuge being used at 3200 m i n - 1 for interstage solid-liquid separation. The effects of washing were followed with the aid of aluminium (AAS)

158

TAK CHING WONG AND A.W.L. DUDENEY

and sulphate (tubidimetry [21 ] ) determinations. After the third washing the suphate to aluminium mole ration became approximately constant. The average value obtained for the gel phase of 0.14 _+0.02 gave the term n as 0.28 in eqs. (3-5) and (8) and yielded the corresponding molecular formula: A1(OH)2.72 (SO4)0.14"nH20, where n is in the range 1 10-120. Nail and White [ 18 ], employing different experimental conditions, reported a similar formula: A1 (OH) 2.3o(SO4)o.35. To increase the m e d i u m ionic strength over the stoichiometric value, the calculated quantity of a m m o n i u m sulphate was added before dilution to the mark. The gel suspensions were either used immediately or left to age for ! week prior to use.

Gel-resin ion exchange Two batch methods were used. In the first method (employed mainly for preliminary determinations of the effects initial concentrations, ionic strength, resin bead size and degree of cross-linking, gel aging, rates of agitation and temperature on the rate and equilibrium position of gel-resin ion exchange) selected quantities (masses equivalent to 37-92% of that calculated to be required theoretically for complete reaction with the sample of gel suspension taken) of the centrifuged hydrogen form resin were contacted with selected aliquots (normally 100 ml, 15.9-37.1 m M total aluminium, 0-2.0 ionic strength, freshly prepared or aged for 1 week) of gel suspension in 250 ml conical flasks, covered with parafilm, and shaken in groups of up to 40 flasks in an Lh Fermentation Incubation Shaker at the desired temperature (normally 20 or 25°C) and rate (200-360 min-~, but normally 200 min -l ) for up to 24 h. At set time intervals (normally every hour initially but left undisturbed overnight) at least 3 flasks were removed from the shaker together. The pH values of the contents were measured. The ion exchange resin was separated from the residual aqueous suspension by decantation and thorough washing with distilled water. Each washed resin sample was placed on a W h a t m a n 541 filter paper, air dried for 10 min, and transferred to a 250 ml conical flask containing 100 ml 1 M sulphuric acid. The flasks and contents were equilibrated in the shaker and the aluminium concentration determined by AAS. In those cases where the aluminium contents in true solution and in suspension were required separately, the supernatant solution was first separated from the gel with the aid of the centrifuge. In the second batch method (employed mainly for determination of the distribution of aluminium, a m m o n i u m and hydrogen ions between the hydroxide, aqueous and resin phases) the general conditions were as above but a different sampling m e t h o d was used to facilitate more frequent collection of analytical data, at time intervals as short as 5 min. The initial concentrations of aluminium (in the 100 ml aliquots taken) were 1000 or 450 mg/1

ION EXCHANGE BETWEEN SOLID PHASES

159

( 37.1 or 16.7 m M ) at the stoichiometric ionic strength. These were contacted with about 85% of the theoretical, approximately 100% of the practical (2.7 or 1.2 g, respectively) of the resin. Instead of separating the ion exchange resin from the aqueous phase after each set time interval, an 0.5 ml volume (in duplicate) of the residual aqueous suspension was transferred by pipette to a separate flask, acidified and analysed, as above, for aluminium both in solution and in suspension. Corrections were made during later calculations for the volume changes (up to 11% of the total volume) resulting from successive sampling operations. RESULTS AND DISCUSSION

General considerations Equations ( 1 ), (4) and (5) represent reactions between a strong acid and a reactive base which were expected to go to completion, though at the relatively slow rate because of their "solid-solid" character. Notwithstanding this character, it was clear that all species were actually exchanged via the true solution phase. Gel-type ion exchange resins, such as Amberlite IR 120, do not formally contain open pores and, although the aluminium hydroxide gel particles are much smaller (by more than two orders of ten) than the resin particles, they are much larger (by several orders of ten) than the molecular dimensions necessary for effective diffusion into the resin structure. Hydroxide gel particles may approach the resin surfaces closely (although simple microscopic examination of resin beads contacted with gel suspensions failed to reveal any physical association). However, it is expected that the particles will always be separated by at least a thin liquid film, through which species are transported in true solution [ 22 ]. Preliminary experiments showed that the rate and extent of reaction was affected to a variable degree by the ionic strength of the aqueous medium, stirring speed, temperature, resin bead size, degree of cross linking and gel aging. Ionic strength or, more precisely, the concentration of the other cation of reaction (ammonium ion) which competed with dissolved aluminium for resin sites, had by far the largest effect. Only this effect is considered below. Figure 1 gives a family of curves of percent attained of the maximum cation exchange capacity ( 3.51 meq/g wet resin) for aluminium versus time at different ionic strengths from essentially zero (washed gel having a calculated ionic strength of 0.0007), through 0.147 (stoichiometric), 0.290, 0.582 and 0.87l to 1.501. (As a consequence of the computer package employed, the points are joined by straight lines. However, smooth curves through the points would represent physical reality better). As can be seen, reaction was generally fastest initially (becoming progressively slower as equilibrium was approached over 5-24 h). In terms of the percentage attained of the final

160

T A K C H I N G W O N G A N D A.W.L. D U D E N E Y 100

0.0007 0.147

80'

0.290 0.582

0.871

~- ~" 4/)~

20

a.

(I

I 5

[0

15 TIME / HOUR

20

1.501 25

Fig. 1. Variation of aluminium ion exchangewith time and ionic strength in the range 0.00071.501. equilibrium position (rather than the percentage attained of the m a x i m u m theoretical cation exchange capacity (CEC) shown), the reaction was also faster at higher ionic strength (particularly over the interval from zero to the stoichiometric value). The extent of exchange at equilibrium was reduced strongly by increasing ionic strength, although the percentage of theoretical CEC at the stoichiometric value was still greater than 70%. Each point on the graphs represented the average of triplicate samples having an uncertainty of about 0.5-2% of the values of percent theoretical capacity attained. This uncertainty arose largely from variations in the water content of individual resin samples, which was difficult to control routinely. In terms of any eventual practical water treatment or hydrometallurgical process, however, the trends were clear enough to deduce that little would be gained, on either kinetic or equilibrium grounds, in varying the ionic strength from the stoichiometric value. On the other hand, the reactions were faster, at least initially, than was originally anticipated. This observation merited more detailed investigation. To obtain quantitatively more reliable kinetic data dealing with all the species likely to be present, the material balance technique outlined in the theory section was adopted. In this method, samples (of the aqueous m e d i u m only) were taken successively for essential analyses from a single system of resin and aqueous suspension (stoichiometrically equivalent in acid-base capacity). This avoided the problems experienced with capricious variation of water content found in multiple resin samples but necessitated careful corrections for the loss of material through sampling. Table 1 gives the values of coefficients calculated (taking K1 and 1£2 in eq. (2) to be 10 -4.97 and 10 -93, respectively, [4 ] and assuming (see l a t e r ) y = z ) . Eqation (5) then becomes eq. (16): 0.5A12 (SO4)3+ 2.72NH3+ 2 . 6 0 R e s - H + + 0 . I 19H20 =0.261A13+ +0.022A1OH 2+ + 0.008A1 ( O H ) f + 0.654 (Res-)3A13+ +0.027 (Res-)2A1OH 2+ +0.027Res-AI(OH)~- + 0 . 5 5 5 R e s - N H 2 + 2 . 1 6 5 N H ~ + 1.5SO24-

(16)

161

I O N E X C H A N G E B E T W E E N S O L I D PHASES

Figures 2-4 show the results obtained in terms of equivalent moles of aluminium, hydrogen ion and ammonium ion against time for 1000 mg/1 fresh unwashed aluminium hydroxide suspension. Results for the 450 mg/1 system and for both systems after ageing were substantially similar qualitatively and will not be considered further here. Distribution of ions on the resin

Figure 2 shows the distribution of ions on the resin and is considered in five parts: ( 1 ) time zero, (2) 0-5 min, (3) 5-20 min, (4) 20-300 min and (5) 5-24 h. (1) At time zero the aluminium was essentially all in suspension as the solid hydroxide (suspension concentration 37.1 m M relative to the aqueous phase at pH 5.45); ammonium ions were essentially all in true solution (nominally 2.72 × 37.1 m M with an exact concentration depending upon the TABLEI Calculated coefficients Coefficient

Value

n p a b c m / x y z

0.28 0.12 0.896 0.076 0.028 0.292 0.204 0.924 0.038 0.038

0.010 TOTAL 0.008

ALUMINIUM

i-, 0.006 < 0.004

AMMONIUM O

O----O

~ 0.002 ' HYDROGEN

.

0.000 50

100

150

200 250 300 TIME / MIN

,,-1.

350

, r/..~

400

[ 344

Fig. 2. Variation o f m o l balance with time for aluminium, hydrogen and a m m o n i u m ions in the resin phase (see text for explanation).

162

TAK CHING WONG AND A.W.L DUDENEY

precise amount of a m m o n i a added during the precipitation of the aluminium hydroxide) and hydrogen ions were all in the resin phase (9.48X 10 -3 mol and equivalent to 2.60 (3.71 - 0 . 0 6 5 ) X 10-2M--where 0.065 is the final cumulative value of AG in eq. ( 9 ) - - w h e n exchanged into the aqueous phase). Initially, therefore, the mole contents of aluminium and a m m o n i u m ions on the resin were both zero while that of hydrogen ion was a m a x i m u m (horizontal dotted line in the figure). (2) During the first 5 min, the main change was a rapid exchange of hydrogen ions by a m m o n i u m ions, some 41% of the original a m m o n i u m ions appearing on the resin. As a result of dissolution of aluminium hydroxide by the exchanged hydrogen ions, aluminium ions (about 24% of the total) also appeared on the resin during this time. Of course, these changes were too rapid to be shown in detail on the scale adopted in the figure but were readily followed by separate in situ pH measurements. (3) From 5 to 20 min, aluminium ions replaced hydrogen and a m m o n i u m ions on the resin, because of their increasing abundance and presumably, in the case of a m m o n i u m ions, because of their greater selectivity for sites [ 22 ]. (4) After about 20 min the a m m o n i u m ion content became roughly constant and from 20 to 300 min the main change was a slow exchange of hydrogen ions with aluminium ions. During this period the aluminium and hydrogen ion curves exhibited distinct "kinks". These are unexplained but may be partly the result of minor experimental error propagated by the mass balance technique. However, when equilibrating an ion exchanger with a multicomponent solution, it is possible that the concentrations of certain species in either the ion exchanger or the solution go through a m a x i m u m or m i n i m u m before attaining the equilibrium value because of the different mobilities of the ions diffusing through the resin mass [22 ]. (5) After 300 min the system seemed to be at equilibrium with an equivalent mole ratio of aluminium to a m m o n i u m ions of abouts 2.55:1 and alum i n i u m to (original) hydrogen ions of about 0.72: 1. The position of equilibrium, which coincided with the disappearance from the system of the solid hydroxide phase, is indicated on the figure by the vertical dotted line. Distribution o f ions in true solution

Figure 3 shows the distribution of ions in the solution phase and is explained in terms of the same time intervals. ( 1 ) Initially, the dissolved aluminium and hydrogen ion contents were essentially zero (at the starting pH (5.45) they were both too small to be shown clearly on the figure), while a m m o n i u m ions were at a m a x i m u m (as indicated by the left hand side of the "TOTAL" line--which denotes the total equivalent moles in the solution phase, decreasing with time as a result of sampling).

ION EXCHANGE

0.010

163

BETWEEN SOLID PHASES

.......

TOTAL

.... ~ " + " +

"4" . . . , + - ..........................."4-........................+ . . . . . . . . . . . . . + .........................+ . . . . +

,.A 0.008 AMMONIUM

0 ~_ 0.0O6 k~ ,.4

O.O04

ALUMINIUM

~', 0 . 0 0 2 0.000 ,"":"",'' 50

•

. 100

.

.

. 150

.

.

~,

.

,rp

200 250 300 TIME / MIN

HYDROGEN . ,m. , r--/ m, 350

400

1344

Fig. 3. Variation of mol balance with time for aluminium, hydrogen and ammonium ions in true solution in the aqueous phase (see text for explanation). (2) During the first 5 min, the hydrogen ion content increased, passed through a maximum, and decreased again to near zero as ions were rapidly released from the resin and quickly consumed by reaction with aluminium hydroxide. These changes were too rapid to be shown satisfactorily on the figure but were clearly established by separate in situ measurements of pH. Also, during this time a m m o n i u m ions decreased to a m i n i m u m (about 59% of the original total) owing to exchange onto the resin, while aluminium ions increased to a m a x i m u m (about 75% of the original total) because dissolution of hydroxide was proportionately faster than removal from solution by ion exchange. (3) From 5 to 20 min a m m o n i u m ions partiallyreplaced aluminium ions in solution. The hydrogen ion concentration was constant (5.6 X 10-SM, pH 4.25 ), or more accurately at steady state because hydrogen ions were continually being consumed in dissolving aluminium hydroxide and replaced by exchange from the resin. (4) From 20 to 300 min hydrogen and aluminium ions were both at steady state. A m m o n i u m ions were at about 65% of their original concentration, but were slowly exchanging onto the resin, presumably largely at the expense of hydrogen ions. The aluminium to hydrogen ion ratio was about 420:1, reflecting the effects of a m m o n i u m ions in preventing full exchange of aluminium onto the resin: in the absence of a m m o n i u m ion this ratio must be the stoichiometric one (nominally 1 : 3 ). ( 5 ) From 5 to 24 h the system was apparently at equilibrium, this position being indicated by the vertical line on the figure.

Distribution of aluminium between gel, solution and resin phases Figure 4 gives the time dependence of the aluminium distribution throughout the three phases. The curves are consistent with Figs. 2 and 3 in that the

TAK CHlNG WONG AND A.W.L. DUDENEY

164 0.004

h~,~,

TOTAL IN THREE PHASES .............. i - . . 1 _ . . + . . . , ...+..........t- ........................+ ...................... + ........................+ ..........................+

0,003

RESIN PHASE

0,002 O

0.001

;TION PHASE O O

~HYDROXIDE ~

0.000 0

50

1O0

150

PHASE r7

200 250 300 TIME / MIN

r7

,r-/

350

400

m

1344

Fig. 4. Variation of mol balance with time for a l u m i n i u m species in the hydroxide, aqueous solution and resin phases (see text for explanation). TABLE2

Concentration ratios for aqueous aluminium species Equilibrium pH

[AI3+ ( a q ) ] / [A1OHZ+ ( a q ) ]

[A13+ ( a q ) ] / [ A I ( O H ) + (aq) ]

3 4 4.25 5

93 9.3 5.3 0.93

2.0X l03 20 6.3 0.20

quantity of hydroxide phase decreased rapidly and became zero at equilibrium after 300 min; the dissolved aluminium rose to a transient maximum and then fell to a steady value (29% of total aluminium) after 20 min; the aluminium on the resin initially increased rapidly, but later less and less rapidly until it became constant (at 63% of the total aluminium). The total varied, as before, because of sampling.

Aluminium hydrolysis equilibria Values Of Kl and/£2, taken here to be 10 -4.97 and 10 -9"3, respectively [4], are reasonably well established and concentration ratios calculated from them (Table 2 ) should be reliable for the aqueous conditions employed. It can be seen from the table that, while A13+ (aq) generally predominates, the other ions are not negligible. Over the pH range 4-4.25 (for which the true solution part of the system may be considered to be at steady state) the concentrations of the two hydrolysed species are quite close in magnitude to each other. No corresponding information is available in the literature for hydrolysed aluminium species in the resin phase. However, as the expanded resin phase contains a large proportion of water, the presence of hydrolysed species is

ION EXCHANGE BETWEEN SOLID PHASES

165

expected and the results of this work indicate that such species are present. Unfortunately, it is not possible unambiguously to assign values to x, y and z because only two relevant equations (eqs. ( 17 ) and ( 18 ), derived from eqs. (7) and ( 8 ) ) are available:

x+y+z=l

(17)

3x+ 2y+ z = 2.89

(18)

Clearly A13÷ greatly predominates just as it does in weakly acidic aqueous medium: from the equations y + 2 z = 0 . 1 1 and x takes values between 0.89 and 0.95 (depending upon the actual value of the unknown ratio y/z). Although small and without significant effect on the main results of the material distribution, the indicated hydrolysis on the resin (some 8%) seemed real and too large to be accounted for by experimental error. However, it was affected in principle by the assumption made that the hydrogen ion concentration on the resin was finally zero, and, in the absence of independent evidence, should therefore be treated with caution. For the purpose of calculation of the coefficients in Table 1, the simple assumption was made that the concentrations of the hydrolysed aluminium species were equal in the resin phase. CONCLUSION

The new material balance methods introduced provided essentially complete distribution data for aluminium, hydrogen and ammonium ions with time from a minimum of analytical determinations. They showed that the overall reaction occurred in a number of stages at different rates and was too complex to be fitted usefully to a single rate expression. They also indicated that hydrolysis of aluminium ions occurred to roughly the same extent in both aqueous and resin phases. As far as the authors are aware such information has not previously been available. The unexpectedly large initial rate of reaction (some 50% of the total resin sites being occupied by aluminium after 20 min, perhaps five times faster than expected without the presence of the ammonium ion) was traced to an initial rapid exchange of ammonium ions with a proportion of the hydrogen ions on the resin, leading to more rapid dissolution and exchange of aluminium ions than could otherwise occur. The accompanying reduction in resin ion exchange capacity for aluminium, resulting from competition for resin sites by ammonium ions, was finally quite small when an overall stoichiometric ratio of aluminium to ammonium ions was employed (at equilibrium after 300 min some 28% of total resin sites being occupied by ammonium ions and 72% by aluminium) because of the greater selectivity of the resin for the higher charged ion.

166

TAK CHING WONG AND A.W.L. DUDENEY

The form of phenomenological model most suited to the system containing a stoichiometric quantity of ammonium ions is as below. This model considers the time scales involved more sensitively than the earlier descriptions based on inspection of Figs. 2-4 alone. ( 1 ) Aqueous ammonium ions diffuse to and into the resin matrix and exchange with an equivalent number of hydrogen ions, which are released into the aqueous phase (0-1 min ). (2) Hydrogen ions initially generated in the aqueous phase diffuse to the hydroxide particles and react there to produce aluminium ions in solution (0-10 min). ( 3 ) Aluminium ions initially generated by hydroxide dissolution diffuse to and into the resin phase and exchange with both hydrogen and ammonium ions (0-20 min). (4) Hydroxide dissolution, ion transport and ion exchange continue at decreasing rates with aqueous hydrogen ions at approximately steady state concentration (0.56-0.79 mM, 10-300 min) and aqueous aluminium ions also at approximately steady state concentration ( 11-9.7 mM, 20-300 min). Ammonium ions load marginally onto the resin by exchange with hydrogen ions (20-300 min). (5) Pseudo-equilibrium is reached based on complete dissolution of the hydroxide phase and exchange of hydrogen ion from the resin (300 min) with final aqueous concentrations similar to the steady state values and very minor changes continuing to occur in the aluminium to ammonium ion ratio in the aqueous and resin phases ( 5-24 h). Both hydroxide dissolution and resin ion exchange were seen to be intrinsically rapid processes, slowed (i.e., kinetically controlled) after the initial reaction primarily by the low steady state aqueous concentrations of hydrogen and aluminium ions established in the system. From the point of view of any eventual water treatment or hydrometallurgical application of ion exchange resins to the recovery of aluminium from gels such as those studied in this work, the ionic strength should not be altered from the stoichiometric value. Thus, as mentioned above, at the stoichiometric value the practical exchange capacity was greater than 70% of the theoretical maximum and the reaction was largely complete (60-70%) within 1-2 h. Other values of ionic strength reduced the rate of exchange (or the loading capacity) of the resin without significant corresponding gain in the loading capacity (or the rate of exchange). Although preliminary work with real systems (synthetic water treatment waste) showed that the behaviour was bound to be more complex, further study of the application of resin ion exchange of aluminium gels is considered to be warranted. ACKNOWLEDGEMENTS

One of us (TCW) wishes to thank Shell (Hong Kong), the ORS Awards Scheme and the Leche Trust for financial support during this work.

ION EXCHANGE BETWEEN SOLID PHASES

167

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