- Email: [email protected]
Characterization and dewatering of Australian alumina trihydrate
International Journal o f Mineral Processing, 16 (1986) 2 6 3 - 2 7 9 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
CHARACTERIZATION AND DEWATERING ALUMINA TRIHYDRATE
263
OF AUSTRALIAN
S.J. PUTTOCK, M.S. WAINWRIGHT*, J.W. McALLISTER, A.G. FANE, C.J.D. F E L L and R.G. ROBINS School of Chemical Engineering and Industrial Chemistry, University of New South Wales, Kensington, N.S. W. 2033, Australia (Received September 10, 1984; revised and accepted February 28, 1985)
ABSTRACT Puttock, S.J., Wainwright, M.S., McAllister, J.W., Fane, A.G., Fell, C.J.D. and Robins, R.G., 1986. Characterization and dewatering of Australian alumina trihydrate. Int. J. Miner. Process., 16: 263--279. This paper reports a study of the dewatering characteristics of Bayer alumina from three different processing plants. Particle characterization reveals that the surface area, exterior morphology, particle charge, and also the degree of hydration and temperature of dehydration, are similar for all three samples. However, significant differences in the pore size distribution of packed beds of particles are observed. The mercury intrusion porosimeter is shown to be a useful and versatile instrument for the simultaneous determination of surface area, particle size distribution and pore size distribution. The vacuum dewatering of alumina trihydrate filter cakes is shown to be influenced both by the particle size distribution of the feed slurry, and by the addition of anionic, cationic or nonionic surfactants to the cake wash water. Significant reductions in filter cake moisture content are obtained using all types of surfactant. The use of surfactants at concentrations much higher than typical industrial practice is also shown to result in further improvements in dewatering. The experimental results are interpreted in terms of the Laplace-Young equation for drainage from a saturated capillary.
INTRODUCTION Australia currently produces over 25% of the world's total supply of a l u m i n a , p r e d o m i n a n t l y f r o m t h r e e c o m p a n i e s w i t h p r o c e s s i n g p l a n t s in W e s t e r n A u s t r a l i a , Q u e e n s l a n d a n d in t h e N o r t h e r n T e r r i t o r y . I n r e c e n t y e a r s there has been a tendency at such Bayer plants to produce alumina having "sandy" rather than "floury" appearance and properties. This change has b e e n m a d e in o r d e r t o i m p r o v e t h e o p e r a t i n g e f f i c i e n c y o f a l u m i n u m s m e l t ers. A l t h o u g h i t is h i g h l y d e s i r a b l e t o p r o d u c e a l u m i n a w i t h s i m i l a r c h a r a c t e r i s t i c s f r o m all p l a n t s , i t is a c k n o w l e d g e d t h a t v a s t d i f f e r e n c e s c a n a n d d o exist. *To whom correspondence should be addressed.
0301-7516/86/$03.50
© 1986 Elsevier Science Publishers B.V.
264 The properties of the product alumina are largely reflected in the alumina trihydrate slurries leaving the crystallization process. Therefore, it is somewhat surprising that there have been few fundamental studies of the properties and processing of these slurries. After the crystallization step in the Bayer process, the product alumina trihydrate crystals are filtered from the caustic mother liquor, and subsequently washed to remove adherent liquor. It is important that the crystals in the feed slurry to the filter have suitable characteristics such that they will filter and dewater readily. In the washing/ dewatering procedure it is desirable to minimize retained moisture in order to avoid product contamination and to lessen the load in the downstream drying/calcination process. The incentive to reduce filter cake moisture content is appreciable in that a 1% reduction in moisture decreases energy demand by more than 2 X 1013 J per annum for a large (4.4 million tonnes/ annum) producer. In studies of other mineral systems, it has been shown that the physical properties of the feed slurry, such as particle charge (Dolina and Kaminskii, 1974), particle size distribution (Henderson et al., 1957), particle shape (McCall and Tadros, 1980), and feed slurry concentration (Rushton et al., 1980), can all influence filterability. Knowing these parameters, it may be possible for the processor to make suitable modifications to achieve improved filter performance. It has also been shown that the addition of a surfactant to the wash water of an alumina trihydrate filter cake, can lower the moisture retention in the washed cake (Puttock et al., 1985). Indeed, it is current practice in Australian alumina plants to add surfactant to cake wash water, although the extent or nature of such addition may not be optimal. This paper compares the physical properties and dewatering characteristics of alumina trihydrate particles from three Australian alumina plants. It also reports the influence of the addition of anionic, cationic or nonionic surfactants to the wash water, on residual cake moisture content after dewatering. The aim of this investigation is to achieve a better understanding of the role of bulk particle properties, and of surfactants in the dewatering of alumina trihydrate, with the view of making possible improvements in current operating practice in the washing/dewatering step of Bayer processing. EXPERIMENTAL Materials
Samples of alumina trihydrate used throughout this work were supplied by Alcoa of Australia Limited (Perth, Western Australia), Nabalco Limited (Gove, Northern Territory) and Queensland Alumina Limited (Gladstone, Queensland). The three materials used were spot samples and although probably representative of current operating practice at each plant, may
265 not necessarily be so. The samples have been randomly designated A, B and C in order that the sources of particular samples are not identified.
Particle characterization Particle size and size distribution measurements were made b y the following three methods: dry screening, optical image analysis using the Zeiss Microvideomat 2, and by sedimentation using the Sedigraph 5000D particle size analyser. Inter- and intra-particle pore size distributions were obtained using a Micromeritics 910 mercury penetration porosimeter, the bed of particles being initially evacuated overnight at ambient temperature. The BET surface areas were calculated from three point nitrogen adsorption data measured at --195°C using a Micromeritics 2100 E Orr surface area, pore volume analyzer. The samples were degassed for 12 hours under vacuum at 150~C. Scanning electron micrographs were recorded on a Cambridge SR4 stereoscanner, the specimen being prepared by evaporation of a gold film onto the particles. Particle charge was measured using a Micromeritics zeta potential analyzer, in which the mobility of particles is determined from the rate at which they migrate into a sample cell under a known applied field strength. Thermal analysis of the samples was carried o u t using a Du Pont 951 thermogravimetric analyzer and 910 differential scanning calorimeter. In all cases, samples (50 mg) were heated at a rate of 20~C min -1 from ambient temperature to 800°C in a flow (50 cm 3 min -1) of dry air.
Filtration procedure The filtration apparatus consisted of a 0.1-m diameter Buchner funnel fitted with Whatman 41 filter paper. Vacuum was applied to the filtrate flask by means of a rotary vacuum pump, and the vacuum level measured b y a mercury m a n o m e t e r and regulated using a by-pass valve on the pump. This unit has been demonstrated (Puttock et al., 1984) to adquately represent the filtration conditions prevailing on the rotary table filters commonly used in the alumina industry. Tests were performed by reconstituting the alumina trihydrate (350 g) with distilled water (230 g) to give a slurry containing 60% A1203" 3H20 b y weight. The pH of the slurry was adjusted to 12.0 by the addition of sodium hydroxide, and the thoroughly mixed slurry poured onto the filter deck and vacuum applied to the flask. Washing of the filter cake was achieved by passing 0.25 1 of water containing varying amounts of surfactant, at constant vacuum level (8 kPa). This volume of wash water represents approximately 1.5 times the void volume of the cake. Wash water addition was made such that the t o p surface of the cake was always covered with liquid. Two levels of pH (7 and 12) of the wash water were used in order to investigate the effect of increased pH on the dewatering process. The
266 higher value corresponds to the pH value normally encountered in industrial practice. Dewatering was continued for a period of four minutes after the disappearance of water from the upper surface of the filter cake. Residual cake moisture was determined by weighing before and after overnight drying at ll0°C, with moisture content being expressed as: Moisture (wt.%) =
wet weight -- dry weight ] wetet~ i J x 100%
(1)
This figure was reproducible between individual runs to + 0.2 wt.%. The surface tension of the wash water liquor was measured at 22+2°C using an Analite surface tension meter which employs the Wilhemy plate method (Princen, 1970). R eagen ts
The anionic surfactants [sodium dodecyl benzene sulphonate sodium salt of alpha olefin sulphonate, sodium dodecyl sulphate (ethoxylated)] and cationic surfactants [bis (2 hydroxy-ethyl) cocoamine, methyl bis (2 hydroxyl-ethyl) oleyl ammonium chloride, polyethoxy (15) tallow 1,3 diamino-propane] were supplied by Harcros Chemicals Pty. Ltd. The nonionic surfactants (ethoxylated coco alcohol, ethoxylated nonyl phenol) were supplied by ICI Aust. Operations Pty. Ltd. RESULTS Particle characterization Particle size m e a s u r e m e n t s
Cumulative particle size distributions of sample A determined by dry screening, image analysis, sedimentation, and mercury porosimetry are presented in Fig. 1. Median and mean particle sizes calculated from these data are presented in Table 1. The results obtained using optical image analysis are reported on the basis of the diameter corresponding to the projected volume of the particle. This is analogous to the weight percent value in the screening measurement. The sedigraph data are plotted as cumulative weight percent as a function of Stoke's diameter. This technique is limited to the measurement of particles having diameters less than 100 pm. Rootare et al. (1979) have described a simple mathematical procedure for determining the particle size of powders from mercury porosimetry measurements. Particle diameter (pm) is given as:
where P* = the breakthrough pressure (calculated from bed porosity), and P = the actual applied pressure on the sample.
267 TABLE 1 Characteristic properties of alumina trihydrate samples Property
Sample A
B
C
Median particle size (am) (a) Screening Image analysis Sedimentation Porosimetry
100 133 87 78
78 112 68 66
Mean particle size (screening) (urn)
102
84
72
30.2
40.8
% Particles -- 70 pm (screening) Porosity (cm 3 g-l)(b) Total void volume Intra particle porosity
10.6
0.47 0.44 0.45 0.014 0.018 0.022 (at rp = 44A) (at rp = 44A) (at rp = 37A)
Surface area (m ~ g-l) B.E.T. Porosimetry Zeta potential (mV) pH = 7 pH = 11 Thermal analysis Transition Temp. - TGA(°C) Transition Temp. - DSC(°C) Weight loss (%) - TGA Weight loss (%) - Theory (c)
74 113 65 68
0.31 0.33
--30 --180 260 235
330 325 34.8 34.6
0.61 0.51
--15 --164
265 265
340 330 32.2 34.6
-0.54
--32 --186
250 250
325 330 33.3 34.6
(a)Median refers to particle size at 50% cumulative mass oversize. (b)rp is pore radius. (c) based on the loss of 3H20 from A1203.3H20. T h e i n t r u s i o n v o l u m e versus applied pressure d a t a were i n t e r p r e t e d using this m e t h o d a n d used t o calculate t h e particle d i a m e t e r s p r e s e n t e d in Fig. 1. It is a p p a r e n t f r o m the size analysis results t h a t t h e r e are differences in t h e particle size d i s t r i b u t i o n s o f t h e t h r e e samples. Sample A has a coarser d i s t r i b u t i o n w h e n c o m p a r e d t o samples B and C. T h e differences in particle size d i s t r i b u t i o n s w o u l d be e x p e c t e d t o have a p r o f o u n d e f f e c t o n t h e filtrat i o n o f slurries o f these materials. Inter- and intra.particle p o r e size m e a s u r e m e n t T h e d i f f e r e n c e s in particle size d i s t r i b u t i o n s observed f o r t h e t h r e e alum i n a t r i h y d r a t e samples s h o u l d give rise t o differences in p a c k i n g characteristics o f t h e particles, and h e n c e to d i f f e r e n t bed voidages. L o w pressure m e r c u r y i n t r u s i o n d a t a have b e e n used t o d e t e r m i n e t h e overall p o r e v o l u m e s o f t h e beds o f particles (Table 1), and t h e p o r e v o l u m e s c o r r e s p o n d i n g t o
268 100--
• "~-~',-,
\.\\
"\
90 -
Xa
-
\
E
A
G
o
•
O O tO.
,= '5
0
L 50
I
I"i"~n
100 Particle diameter
I
150 (pro)
200
Fig. 1. Particle size analysis o f s a m p l e A. • = d r y screening ( p o i n t s r e p r e s e n t m e a n o f 180, 1 5 0 , 125, 106, 90, 71, 63 ~ m sieve sizes); • = o p t i c a l image analysis; • = s e d i m e n t a t i o n ; and • = mercury porosimetry.
•
•
A
A
O O v
E _= .os, o O a.
2
•
,-,-
&
6
8
----3 10
12
1&
16
18
Pore radius
(IJm)
Fig. 2. Pore size analysis b y m e r c u r y p o r o s i m e t r y .
20
22
2&
, 26
__" 28
30
269
capillaries of various sizes (Fig. 2). As expected from the particle size distribution results, beds of samples B and C contain a higher proportion of finer capillaries, as well as having smaller overall pore volumes than for sample A. Analysis of pore radii has been used on a number of occasions (Lloyd and Dodds, 1972) to obtain approximate values for the pressures required to drain saturated capillaries in packed beds of particles. The Laplace-Young equation {Young, 1805) relates the pressure (or vacuum) necessary to drain a capillary, to surface tension parameters (TLA and 0 LS ), for a given capillary radius (r). Ap =
2 7 L A COS 0 Ls
(3)
r
Table 2 gives examples of h o w eqn. (3) can be used to relate capillary radius to the capillary pressure AP, at k n o w n surface tension ~/LA (assuming cos ~ = 1). The effect of reducing the surface tension of the wash water from that of pure water ( ~ L A ---- 0.073 N m -1) to that which can be obtained using a surfactant (~/LA = 0.030 N m -1 ) is illustrated. Related experiments in this laboratory show that the packing mode of the sample in the porosimeter cell compartment is similar to that in a typical filter cake. It is evident therefore from Table 2 that it is the smaller pore size (< 5 ~m) in the filter cake which will prove the most difficult to drain. Typical industrial practice employs vacuum levels of 0.4--0.5 bar (including pressure drop across the filter medium). The figures in Table 2 show that by reducing the surface tension of the wash water from 0.073 to 0.030 N m -1, an improvement in filter cake drainage should result, for a constant applied vacuum. It is also of interest to note from eqn. (3), that any increase in the receding contact angle 0 us and hence a decrease in cos 0 LS, would similarly be expected to result in improved dewatering, even at constant ~LA. The use of surfactants to aid dewatering of beds of alumina trihydrate TABLE 2 E f f e c t o f p o r e size o n capillary r e t e n t i o n f o r c e Pore size (urn) 30 15 5 4 3 2 1
Capillary pressure (bar) ~'LA 0.05 0.10 0.30 0.38 0.50 0.75 1.5
=
0.073 N m -I
~'LA = 0.030 N m -1 0.02 0.04 0.12 0.16 0.20 0.31 0.62
270 particles, with capillary sizes within the range given in Table 2, is discussed in a later section of this paper. The mercury intrusion data were obtained up to a pressure of 2500 bar in order to determine the pore volume and mean radius of pores present in the particles. These data (Table 1) show that there is very little internal porosity, supporting the low surface areas measured b y nitrogen adsorption. S u r f a c e area m e a s u r e m e n t
BET surface areas measured by nitrogen adsorption are presented in Table 1. Also included in Table 1 are the surface areas calculated from the mercury intrusion data using a method described b y Rootare and Prenzlow (1967). This method uses numerical integration of the pore volume and pore radius data, and the assumption of cylindrical pores. Surface area is given as: V
0.02253 f p ~ a x
A - -
-
m
Pdvp
(4)
0
where: m = mass of sample; Vp = pore volume, and P = applied pressure. Surface areas obtained by the t w o methods show good agreement. The larger surface areas obtained for samples B and C reflect their greater percentages of fine particles. The absolute values of the areas are low, in accordance with the small internal pore volumes measured by mercury porosimetry. Scanning electron microscopy (S.E.M.)
Figure 3 shows electron micrographs of particles of sample A. No major differences were observed in the exterior morphology of the particles from the three different plants. It is immediately apparent that the particles are highly irregular, and in terms of dewatering, would be expected to provide a significant area for the presence of trapped lenses of water which would not drain under normal experimental conditions. Misra and White (1971) have made a study of the crystallization of Bayer alumina trihydrate, observing that particle growth takes place by the enlargement of hexagonal prisms and plates, this type of geometry being clearly shown in Fig. 3. Particle charge
Zeta potentials of the three samples of alumina trihydrate are given in Table 1. It is clear from these results that the charge on the alumina trihydrate particles shows a very strong pH dependence. This effect is rationalized in terms of the k n o w n presence of adsorbed humic material on the particle surface (Jayaweera, 1981), and the strong ionization of the corresponding surface functional groups at high pH (Schnitzer and Khan, 1972, pp. 29--54). The effect of pH on the dewatering characteristics of alumina trihydrate has been described in a previous paper (Puttock et al., 1985).
271
Fig. 3. Scanning electron micrographs of sample A.
272
Thermal analysis Thermal analytical techniques have been used to confirm the degree of hydration of the samples (thermal gravimetric analysis, TGA) and to monitor the temperature ranges at which the material begins to undergo dehydration (differential scanning calorimetry, DSC). Any differences in the crystal structure of the three hydrated alumina samples would be expected to be manifested by variations in the DSC traces. The thermal analysis results are summarized in Table 1 and no major differences are evident between the three samples. These results confirm that all three samples contain three moles of water, and dehydrate at similar temperatures, inferring similar crystal structures.
Vacuum filtration of alumina trihydrate The residual moisture contents of typical filter cakes of the three samples A, B and C after washing with distilled water (~/LA = 0.073 N m -1 at 22 + 2°C) were 9.1, 9.5 and 10.3%, respectively. The higher moisture contents of samples B and C are consistent with the higher percentage of fine particles present in those samples. Sieve analysis (Table 1) shows that samples A, B and C have minus 70 pm percentage of 10.6, 30.2 and 40.8, respectively. These results are similar to those reported by Henderson et al. (1957) for the vacuum filtration of iron ore concentrates.
Effect of anionic surfactants Figure 4 shows the results of standard filtration tests using three anionic surfactants at both neutral pH (7) and at high pH (12). In all cases it was found that increasing the concentration of a particular surfactant (20-50100 ppm) at constant pH, resulted in a reduction in filter cake residual moisture content. This is consistent with the lowering in surface tension that occurs [TLA in eqn. (3)]. The proposal of a direct relationship between surface tension and moisture content has two major inadequacies. Firstly, for a number of cases, in going from 50 to 100 p p m concentration in the wash water, the surface tension changes very little whereas the moisture content falls rapidly. Secondly, any possible relationship between surface tension and moisture content only holds for a particular surfactant at constant pH and hence, two surfactants, at the same surface tension, may exhibit very different dewatering performances. From experimental observations of this type, such factors as t h e ability of a surfactant not only to change 7LA in eqn. (3), b u t also the contact angle 0 LS must be considered. For all surfactants, the higher pH conditions resulted in an increase in wash water surface tension, accompanied by an increase in residual moisture content. However, the sulphuric acid ester salt (sulphated polyether alcohol) suffered relatively little loss of surface activity at high pH and gives the best
273 10 Pure wash w a t e r
0
(U
O
7"
(J O) L. -,-,
6"
O
:E 5-
3'o
3'5
~o
S u r f a c e t e n s i o n of w a s h w a t e r
is
""
7'~
(raN/m)
Fig. 4. I n f l u e n c e o f surface t e n s i o n o n residual m o i s t u r e c o n t e n t o f filter cakes o f s a m p l e A, u s i n g a n i o n i c s u r f a c t a n t s : s o d i u m d o d e c y l b e n z e n e s u l p h o n a t e - - ; s o d i u m salt a l p h a olefin s u l p h o n a t e . . . . ; a n d s o d i u m d o d e c y l s u l p h a t e ( e t h o x y l a t e d ) - - - - - . C o n c e n t r a t i o n : . , ~, o = 20, 50, 1 0 0 p p m a t p H 7; e, A , . = 20, 50, 100 p p m at p H 12. o refers t o p u r e w a t e r wash o n l y ( n o s u r f a c t a n t ) .
overall performance among the anionic surfactants tested. This latter surfactant would be expected to have a high degree of nonionic character due to its p o l y e t h o x y l a t e d structure.
Cationic surfactants The dewatering results corresponding to the performance of cationic surfactants are given in Fig. 5. In a similar fashion to the anionic surfactant results, it is apparent that an increase in the concentration of a cationic surfactant in the wash water, reduces both the surface tension and the residual moisture c o n t e n t of the filter cake. The results show that concentrations in excess of 50 ppm are required to obtain a dewatering performance equivalent to t h a t of a typical anionic surfactant at 20 ppm. This observation would infer a mechanism involving significant adsorption of the cationic surfactant from wash water onto the highly negatively charged alumina trihydrate surface. It must be assumed that any increase in the contact angle 6 LS caused by this adsorption process, is outweighed by the corresponding loss in surface tension of the wash water. All the cationic surfactants evaluated in this work showed good stability in wash water of higher pH.
274 10 9-
0
/'/
•
Pure
~
8.
~
7-
0 0 m,. =
I ./r
!/i
//x i
; I
6-
•;
0
5-
o"
I'
/ 5
•
r
3'o
water
/i I
i
/
wash
3's
4'0
S u r f a c e t e n s i o n of w a s h w a t e r
""
¢
7'2
(mN/m)
Fig. 5. I n f l u e n c e o f surface t e n s i o n o n residual m o i s t u r e c o n t e n t o f filter cakes of s a m p l e A using c a t i o n i c s u r f a c t a n t s : bis (2 h y d r o x y - e t h y l ) c o c o a m i n e - - ; m e t h y l bis (2 h y d r o x y e t h y l ) oleyl a m m o n i u m c h l o r i d e . . . . ; oleyl t r i - m e t h y l a m m o n i u m c h l o r i d e - - - - ; and p o l y e t h o x y ( 1 5 ) t a l l o w 1, 3 - d i a m i n o - p r o p a n e - - " --. C o n c e n t r a t i o n : o, a, o = 20, 50, 100 p p m a t p H 7; m, A, • = 20, 50, 1 0 0 p p m at p H 12. o refers t o p u r e w a t e r wash o n l y ( n o surfactants).
Nonionic surfactants Figure 6 shows the results of standard dewatering tests using ethoxylated nonionic surfactants, all used at a concentration of 100 p p m in the filter cake wash water. It is apparent that the nonionic surfactants chosen for performance evaluations are generally superior in terms of dewatering efficiency to the anionic or cationic types. In all cases the surface activity of the wash water is not diminished appreciably in the higher pH conditions. For both the alcohol and the alkylphenol ethoxylates, an o p t i m u m length of the e t h o x y chain, at which filter cake moisture content is minimum is noted. It would be envisaged that fine tuning of this chain length could be used to further improve the dewatering performance of such ethoxylated nonionic surfactants. Care must be taken in this respect, since in these industrial samples the length of the e t h o x y chain (n number) is not constant for each surfactant molecule, but represents an average figure comprising a Poisson t y p e distribution (Mayhew and Hyatt, 1952). It is also evident from Fig. 6 that the increase in wash water pH appears to shift the length of the e t h o x y chain corresponding to a minimum moisture content, to a slightly higher value. It might be expected that for the nonyl phenol ethoxylate at neutral pH, the point corresponding to a value of n = 5.5 (M = 4.3%), represents a minimum in moisture content which would increase again for
275 10
9-
29.8 e-
O o 61
y317
2~'t*~~
,//
& ]].6
/
•
~o.2
O
I
J
!
h
\'.!
~
I
8
I
12
ii6
Length of ethoxy chain (n)
Fig. 6. Influence of e t h o x y chain length on residual moisture c o n t e n t of filter cakes of sample A using n o n i o n i c surfactants (all at 100 ppm): e t h o x y l a t e d c o c o alcohol - - ; e t h y o x y l a t e d n o n y l p h e n o . . . . ; at pH 7 (o, ~) and pH 12 (e, A). N u m b e r i n g refers to wash water surface tension (mN m -~ ).
values of n less than 5.5. This pH dependence in the minimum moisture c o n t e n t may be due to the known adoption of a small degree of cationic character in such nonionic surfactants at high pH.
Surfactants at high dosage rates The previous work involving the use of surfactants has dealt exclusively with a study of the dewatering performance of anionic, cationic and nonionic surfactants over the concentration range 20--100 ppm, this being typical of industrial practice. Figure 7 shows the results using anionic, cationic and nonionic surfactants in the filter cake wash water at concentrations from 100 to 3000 ppm. It is immediately apparent t h a t the residual moisture content of the filter cake can be significantly reduced by the use of these higher dosage rates. From an industrial standpoint, such an improvement is offset by the increased cost of surfactant, and perhaps more importantly, by the modified physical state of the dewatered cake. At concentrations of either anionic or cationic surfactant above about 400 ppm, it was observed that the filter cake became progressively stickier and hence more unmanageable. This effect was far less pronounced with the use of nonionic surfactants at high concentrations. For each surfactant, the surface tension 7LA of the wash water was approximately constant (+ 1 mN m -1) throughout this high concentration
276
7.
6
I0~0400 I0'00
20'00
30'00
Concentration of surfactant (ppm)
Fig. 7. Influence of surfactant concentration on residual moisture content of filter cakes of sample A. Surfactants used: sodium dodecyl sulphate (anionic) e, cetyl trimethyl a m m o n i u m bromide ( c a t i o n i c ) v , ethyoxylated (n = 8) nonyl phenol (nonionic) =.
range, suggesting that the observed variation in moisture content was not a function of the filtrate surface tension. The reason for the observed decrease in moisture content with surfactant dosage is believed to be due to the increased mass transfer of the surfactant to the particles during the very short contact times prevailing. A consequence of this may be a beneficial reduction in cos 0 LS which would be predicted [eqn. (3)] to improve dewatering. DISCUSSION
Characterization of alumina trihydrate particles This initial study of some of the important physical characteristics of alumina trihydrate particles from three Australian alumina plants at widely different locations, has demonstrated many similarities between the samples. Surface areas, exterior morphology and particle charge, along with the degree of hydration and temperature of dehydration, have all been shown
277
to be similar for all samples. The particle sizing results from dry screening, optical image analysis, sedimentation and mercury porosimetry, all confirmed that sample A has larger particles than either B or C. Such size differences are important in the filter cake dewatering step of Bayer processing. The different sizing results illustrate the ambiguity which arises when quoting particle size data, since each size clearly depends on the m e t h o d used for its measurement and as such would not be expected to be directly comparable. This is shown in Table 1 where the median particle sizes vary from one technique to another, although it is evident that the ratio of particle sizes from sample to sample are similar for all four sizing techniques (ration A:B = 1.23 -+ 0.06, A:C = 1.24 +- 0.10). Lloyd and Ward (1977) discuss the filtration applications of particle characterization illustrating that a particle may have a number of equivalent sphere sizes and show that particle size analysis using different techniques can be correlated by the use of a shape factor. The importance of having a good knowledge of particle size distribution is evident from two recent papers (Creehan, 1984; Zwicker, 1984) describing techniques investigated for improved sizing of alumina trihydrate particles. An important aspect of the mercury intrusion porosimetry technique is that it gives information on inter- and intra-particle pore size distributions, particle size distribution, and surface areas. Values of the latter two properties calculated from the intrusion volume data, are in close agreement with those obtained from other techniques. The mercury intrusion porosimeter is therefore perceived to be a versatile instrument for use in the alumina industry.
The use of surfactants for improved dewatering The addition of either anionic, cationic or nonionic surfactants to the wash water of alumina trihydrate filter cakes, has been shown in all cases to result in improved dewatering. Generally, the anionic surfactants tested in this work (Fig. 4) were more susceptible to loss of surface activity at higher pH than either the cationic or nonionic types (e.g. 20 ppm sodium dodecyl benzene sulphonate, 7LA = 34.0 and 42.1 mN m -1 at pH 7 and 12, respectively). However, the inherently higher surface activity of the anionic surfactants generally resulted in better dewatering than the cationics, even in the higher pH wash water. The anionic surfactant of the sulphuric acid ester salt type showed greater tolerance to the alkaline conditions o f the wash water, than the sulphonic acid types, and performed better in the dewatering tests. The sulphated polyether alcohol, which has a considerable degree of nonionic character, showed good potential as a dewatering aid. The cationic surfactants (Fig. 5), showed good tolerance to the higher pH conditions and in m a n y cases exhibited good surface activity [e.g. 100 ppm bis(2-hydroxyethyl)cocoamine, 7LA = 28.3 mN m - ' ] . Nevertheless, despite this high level of surface activity, the cationic surfactants generally did not
278
exhibit exceptional dewatering performance, especially at low concentrations. Nicol (1976) reported the use of anionic and cationic surfactants in the dewatering of fine coal, the anionic surfactant reducing the filter cake moisture content appreciably more than the cationic type. The poorer performance of the cationic surfactant was explained in terms of surfactant adsorption from the wash water onto the coal surface and consequent loss of surface activity in the bulk filtrate. Overall, the nonionic surfactants performed very well in the dewatering tests (Fig. 6), exhibiting good surface activity at both neutral and high pH, in all cases resulting in the formation of filter cakes with low residual moisture contents. It would be anticipated from these results that further detailed work involving a study of the effect of varying both the alkyl and the e t h o x y chain lengths, will lead to further improvements in dewatering. A recent patent (Yama, 1982) has reported a study involving the dewatering of aluminium hydroxide using surfactants of the t y p e R-0-(AO)n-X (where R refers to the alkyl chain, AO to the ethylene, butylene or propylene grouping, n varies from 3 to 1O0, and X is a sulphate group). The authors report that in all tests, the best dewatering performance was achieved for values of n=6, for example in the surfactant C12H2sO(C2H40)6SO3Na. Another important area w o r t h y of further research involves the use of surfactants at concentrations much higher than that used typically in industrial practice (Fig. 7). For anionic, cationic and nonionic surfactants, very significant reductions in filter cake moisture content were observed, although for industrial application any cake "stickiness" problems with ionic surfactants must be avoided. Of course due consideration must be given to the effect of higher levels of surfactants, which will ultimately be circulated to other parts of the Bayer process circuit through the filtrate. CONCLUSIONS
The results reported in this paper highlight three major points of interest to both the Australian and to the worldwide alumina processing industry. {1) Particle characterization studies reveal that mercury intrusion porosimetry is a valuable tool in the characterization of alumina trihydrate particles. (2) Anionic, cationic and nonionic surfactants can all be used to enhance filter cake dewatering. The effectiveness of a particular surfactant to reduce moisture content appears to be difficult to predict, b u t involves its ability to reduce wash water surface tension ~LA, and possibly to modify the receding solid/liquid contact angle 0 Ls. Overall, ethoxylated nonionic surfactants appear to have the most suitable characteristics for use as a dewatering aid. (3) The use of surfactants at high concentrations has been shown to result in further improvements in filter cake dewatering. Reductions in filter cake moisture content at constant wash water surface tension, confirm that the
279
exact mechanism of dewatering is complex and again may involve changes in the-contact angle 0 LS. ACKNOWLEDGEMENTS
The authors gratefully acknowledge financial assistance from the Australian Research Grant Scheme. We wish to thank Alcoa of Australia Ltd., Nabalco Ltd. and Queensland Alumina Ltd. for the alumina trihydrate samples, and Akzo Chemie (Nederland B.V.), Harcros Chemicals Pty. Ltd. and ICI Australia Operations Pty. Ltd. for the surfactants. The assistance of Mr. A. Deacon in the particle characterization studies is much appreciated. REFERENCES Creehan, J.T., 1984. Improved particle size analysis method for precipitation control. Light Metals, 1: 39--51. Dolina, L.F. and Kaminskii, V.S., 1974. Influence of the electrokinetic charge potential of particles on dewatering processes. Coke Chem., U.S.S.R., 1: 8--11. Henderson, A.F., Cornell, C.F., Dunyon, A.F. and Dahlstrom, D.A., 1957. Filtration and control of moisture content on taconite concentrates. Trans. AIME, 204: 349--355. Jayaweera, L.D., 1981. The effect of organic impurities on the precipitation of alumina trihydrate in the Bayer process. Ph.D. thesis, University of New South Wales. Lloyd, P.J. and Dodds, J.A., 1972. Liquid retention in filter cakes. Filtr. Sep., 9: 91--96. Lloyd, P.J. and Ward, A.S., 1977. Filtration applications of particle characterisation. A.I.Ch.E. Symp. Ser., 73: 6--12. Mayhew, R.L. and Hyatt, R.C., 1952. The effect of mole ratio distribution on the physical properties of a polyoxyethylated alkyl phenol. J. Am. Oil Chem. Soc., 29: 357-362. McCall, M.T. and Tadros, M.E., 1980. Effects of additives on morphology of precipitated calcium sulphate and calcium sulphite -- implications on slurry properties. Colloids Surf., 1: 161--172. Misra, C. and White, E.T., 1971. Crystallisation of Bayer aluminium trihydroxide. J. Crystal Growth, 8: 172--178. Nicol, S.K., 1976. The effect of surfactants on the dewatering of fine coal. Proc. Australas. Inst. Min. Metall., 260: 37--44. Princen, H.M., 1970. Advantages and limitations of the grooved Wilhelmy plate. Aust. J. Chem., 23: 1789--1799. Puttock, S.J., Fane, A.G., Fell, C.J.D., Robins, R.G. and Wainwright, M.S., 1985. Role of surface effects in the dewatering of alumina trihydrate. A.I.Ch.E.J., 31: 1213-1216. Rootare, H.M. and Prenzlow, C.F., 1967. Surface area from mercury porosimeter measurements. J. Phys. Chem., 7 1 : 2 7 3 4 - - 2 7 3 6 . Rootare, H.M., Powers, J.M. and Spencer, J., 1979. Particle size of abrasive powders from mercury porosimetry data. Proc. Int. Powder and Bulk Solids Handling and Processing, Philadelphia, Pa., pp. 129--143. Rushton, A., Hosseini, M. and Hassan, I., 1980. The effects of velocity and concentration on filter cake resistance. J. Separ. Process. Technol., 1 : 35--41. Schnitzer, M. and Khan, S.V., 1972. Humic substances in the Environment. Marcel Dekker Inc., New York, N.Y., pp. 29--54. Yama, M. (to Kao Soap Co., Ltd.), 1982. Filtration-dewatering aids for aqueous slurries. Japanese Patent 57-84708. Young, T., 1805. An essay on the cohesion of fluids. Philos. Trans. R. Soc., 15: 65--87. Zwicker, J.D., 1984. Particle size analysis in Bayer plant control. Light Metals, 1: 173-193.
CHARACTERIZATION AND DEWATERING ALUMINA TRIHYDRATE
263
OF AUSTRALIAN
S.J. PUTTOCK, M.S. WAINWRIGHT*, J.W. McALLISTER, A.G. FANE, C.J.D. F E L L and R.G. ROBINS School of Chemical Engineering and Industrial Chemistry, University of New South Wales, Kensington, N.S. W. 2033, Australia (Received September 10, 1984; revised and accepted February 28, 1985)
ABSTRACT Puttock, S.J., Wainwright, M.S., McAllister, J.W., Fane, A.G., Fell, C.J.D. and Robins, R.G., 1986. Characterization and dewatering of Australian alumina trihydrate. Int. J. Miner. Process., 16: 263--279. This paper reports a study of the dewatering characteristics of Bayer alumina from three different processing plants. Particle characterization reveals that the surface area, exterior morphology, particle charge, and also the degree of hydration and temperature of dehydration, are similar for all three samples. However, significant differences in the pore size distribution of packed beds of particles are observed. The mercury intrusion porosimeter is shown to be a useful and versatile instrument for the simultaneous determination of surface area, particle size distribution and pore size distribution. The vacuum dewatering of alumina trihydrate filter cakes is shown to be influenced both by the particle size distribution of the feed slurry, and by the addition of anionic, cationic or nonionic surfactants to the cake wash water. Significant reductions in filter cake moisture content are obtained using all types of surfactant. The use of surfactants at concentrations much higher than typical industrial practice is also shown to result in further improvements in dewatering. The experimental results are interpreted in terms of the Laplace-Young equation for drainage from a saturated capillary.
INTRODUCTION Australia currently produces over 25% of the world's total supply of a l u m i n a , p r e d o m i n a n t l y f r o m t h r e e c o m p a n i e s w i t h p r o c e s s i n g p l a n t s in W e s t e r n A u s t r a l i a , Q u e e n s l a n d a n d in t h e N o r t h e r n T e r r i t o r y . I n r e c e n t y e a r s there has been a tendency at such Bayer plants to produce alumina having "sandy" rather than "floury" appearance and properties. This change has b e e n m a d e in o r d e r t o i m p r o v e t h e o p e r a t i n g e f f i c i e n c y o f a l u m i n u m s m e l t ers. A l t h o u g h i t is h i g h l y d e s i r a b l e t o p r o d u c e a l u m i n a w i t h s i m i l a r c h a r a c t e r i s t i c s f r o m all p l a n t s , i t is a c k n o w l e d g e d t h a t v a s t d i f f e r e n c e s c a n a n d d o exist. *To whom correspondence should be addressed.
0301-7516/86/$03.50
© 1986 Elsevier Science Publishers B.V.
264 The properties of the product alumina are largely reflected in the alumina trihydrate slurries leaving the crystallization process. Therefore, it is somewhat surprising that there have been few fundamental studies of the properties and processing of these slurries. After the crystallization step in the Bayer process, the product alumina trihydrate crystals are filtered from the caustic mother liquor, and subsequently washed to remove adherent liquor. It is important that the crystals in the feed slurry to the filter have suitable characteristics such that they will filter and dewater readily. In the washing/ dewatering procedure it is desirable to minimize retained moisture in order to avoid product contamination and to lessen the load in the downstream drying/calcination process. The incentive to reduce filter cake moisture content is appreciable in that a 1% reduction in moisture decreases energy demand by more than 2 X 1013 J per annum for a large (4.4 million tonnes/ annum) producer. In studies of other mineral systems, it has been shown that the physical properties of the feed slurry, such as particle charge (Dolina and Kaminskii, 1974), particle size distribution (Henderson et al., 1957), particle shape (McCall and Tadros, 1980), and feed slurry concentration (Rushton et al., 1980), can all influence filterability. Knowing these parameters, it may be possible for the processor to make suitable modifications to achieve improved filter performance. It has also been shown that the addition of a surfactant to the wash water of an alumina trihydrate filter cake, can lower the moisture retention in the washed cake (Puttock et al., 1985). Indeed, it is current practice in Australian alumina plants to add surfactant to cake wash water, although the extent or nature of such addition may not be optimal. This paper compares the physical properties and dewatering characteristics of alumina trihydrate particles from three Australian alumina plants. It also reports the influence of the addition of anionic, cationic or nonionic surfactants to the wash water, on residual cake moisture content after dewatering. The aim of this investigation is to achieve a better understanding of the role of bulk particle properties, and of surfactants in the dewatering of alumina trihydrate, with the view of making possible improvements in current operating practice in the washing/dewatering step of Bayer processing. EXPERIMENTAL Materials
Samples of alumina trihydrate used throughout this work were supplied by Alcoa of Australia Limited (Perth, Western Australia), Nabalco Limited (Gove, Northern Territory) and Queensland Alumina Limited (Gladstone, Queensland). The three materials used were spot samples and although probably representative of current operating practice at each plant, may
265 not necessarily be so. The samples have been randomly designated A, B and C in order that the sources of particular samples are not identified.
Particle characterization Particle size and size distribution measurements were made b y the following three methods: dry screening, optical image analysis using the Zeiss Microvideomat 2, and by sedimentation using the Sedigraph 5000D particle size analyser. Inter- and intra-particle pore size distributions were obtained using a Micromeritics 910 mercury penetration porosimeter, the bed of particles being initially evacuated overnight at ambient temperature. The BET surface areas were calculated from three point nitrogen adsorption data measured at --195°C using a Micromeritics 2100 E Orr surface area, pore volume analyzer. The samples were degassed for 12 hours under vacuum at 150~C. Scanning electron micrographs were recorded on a Cambridge SR4 stereoscanner, the specimen being prepared by evaporation of a gold film onto the particles. Particle charge was measured using a Micromeritics zeta potential analyzer, in which the mobility of particles is determined from the rate at which they migrate into a sample cell under a known applied field strength. Thermal analysis of the samples was carried o u t using a Du Pont 951 thermogravimetric analyzer and 910 differential scanning calorimeter. In all cases, samples (50 mg) were heated at a rate of 20~C min -1 from ambient temperature to 800°C in a flow (50 cm 3 min -1) of dry air.
Filtration procedure The filtration apparatus consisted of a 0.1-m diameter Buchner funnel fitted with Whatman 41 filter paper. Vacuum was applied to the filtrate flask by means of a rotary vacuum pump, and the vacuum level measured b y a mercury m a n o m e t e r and regulated using a by-pass valve on the pump. This unit has been demonstrated (Puttock et al., 1984) to adquately represent the filtration conditions prevailing on the rotary table filters commonly used in the alumina industry. Tests were performed by reconstituting the alumina trihydrate (350 g) with distilled water (230 g) to give a slurry containing 60% A1203" 3H20 b y weight. The pH of the slurry was adjusted to 12.0 by the addition of sodium hydroxide, and the thoroughly mixed slurry poured onto the filter deck and vacuum applied to the flask. Washing of the filter cake was achieved by passing 0.25 1 of water containing varying amounts of surfactant, at constant vacuum level (8 kPa). This volume of wash water represents approximately 1.5 times the void volume of the cake. Wash water addition was made such that the t o p surface of the cake was always covered with liquid. Two levels of pH (7 and 12) of the wash water were used in order to investigate the effect of increased pH on the dewatering process. The
266 higher value corresponds to the pH value normally encountered in industrial practice. Dewatering was continued for a period of four minutes after the disappearance of water from the upper surface of the filter cake. Residual cake moisture was determined by weighing before and after overnight drying at ll0°C, with moisture content being expressed as: Moisture (wt.%) =
wet weight -- dry weight ] wetet~ i J x 100%
(1)
This figure was reproducible between individual runs to + 0.2 wt.%. The surface tension of the wash water liquor was measured at 22+2°C using an Analite surface tension meter which employs the Wilhemy plate method (Princen, 1970). R eagen ts
The anionic surfactants [sodium dodecyl benzene sulphonate sodium salt of alpha olefin sulphonate, sodium dodecyl sulphate (ethoxylated)] and cationic surfactants [bis (2 hydroxy-ethyl) cocoamine, methyl bis (2 hydroxyl-ethyl) oleyl ammonium chloride, polyethoxy (15) tallow 1,3 diamino-propane] were supplied by Harcros Chemicals Pty. Ltd. The nonionic surfactants (ethoxylated coco alcohol, ethoxylated nonyl phenol) were supplied by ICI Aust. Operations Pty. Ltd. RESULTS Particle characterization Particle size m e a s u r e m e n t s
Cumulative particle size distributions of sample A determined by dry screening, image analysis, sedimentation, and mercury porosimetry are presented in Fig. 1. Median and mean particle sizes calculated from these data are presented in Table 1. The results obtained using optical image analysis are reported on the basis of the diameter corresponding to the projected volume of the particle. This is analogous to the weight percent value in the screening measurement. The sedigraph data are plotted as cumulative weight percent as a function of Stoke's diameter. This technique is limited to the measurement of particles having diameters less than 100 pm. Rootare et al. (1979) have described a simple mathematical procedure for determining the particle size of powders from mercury porosimetry measurements. Particle diameter (pm) is given as:
where P* = the breakthrough pressure (calculated from bed porosity), and P = the actual applied pressure on the sample.
267 TABLE 1 Characteristic properties of alumina trihydrate samples Property
Sample A
B
C
Median particle size (am) (a) Screening Image analysis Sedimentation Porosimetry
100 133 87 78
78 112 68 66
Mean particle size (screening) (urn)
102
84
72
30.2
40.8
% Particles -- 70 pm (screening) Porosity (cm 3 g-l)(b) Total void volume Intra particle porosity
10.6
0.47 0.44 0.45 0.014 0.018 0.022 (at rp = 44A) (at rp = 44A) (at rp = 37A)
Surface area (m ~ g-l) B.E.T. Porosimetry Zeta potential (mV) pH = 7 pH = 11 Thermal analysis Transition Temp. - TGA(°C) Transition Temp. - DSC(°C) Weight loss (%) - TGA Weight loss (%) - Theory (c)
74 113 65 68
0.31 0.33
--30 --180 260 235
330 325 34.8 34.6
0.61 0.51
--15 --164
265 265
340 330 32.2 34.6
-0.54
--32 --186
250 250
325 330 33.3 34.6
(a)Median refers to particle size at 50% cumulative mass oversize. (b)rp is pore radius. (c) based on the loss of 3H20 from A1203.3H20. T h e i n t r u s i o n v o l u m e versus applied pressure d a t a were i n t e r p r e t e d using this m e t h o d a n d used t o calculate t h e particle d i a m e t e r s p r e s e n t e d in Fig. 1. It is a p p a r e n t f r o m the size analysis results t h a t t h e r e are differences in t h e particle size d i s t r i b u t i o n s o f t h e t h r e e samples. Sample A has a coarser d i s t r i b u t i o n w h e n c o m p a r e d t o samples B and C. T h e differences in particle size d i s t r i b u t i o n s w o u l d be e x p e c t e d t o have a p r o f o u n d e f f e c t o n t h e filtrat i o n o f slurries o f these materials. Inter- and intra.particle p o r e size m e a s u r e m e n t T h e d i f f e r e n c e s in particle size d i s t r i b u t i o n s observed f o r t h e t h r e e alum i n a t r i h y d r a t e samples s h o u l d give rise t o differences in p a c k i n g characteristics o f t h e particles, and h e n c e to d i f f e r e n t bed voidages. L o w pressure m e r c u r y i n t r u s i o n d a t a have b e e n used t o d e t e r m i n e t h e overall p o r e v o l u m e s o f t h e beds o f particles (Table 1), and t h e p o r e v o l u m e s c o r r e s p o n d i n g t o
268 100--
• "~-~',-,
\.\\
"\
90 -
Xa
-
\
E
A
G
o
•
O O tO.
,= '5
0
L 50
I
I"i"~n
100 Particle diameter
I
150 (pro)
200
Fig. 1. Particle size analysis o f s a m p l e A. • = d r y screening ( p o i n t s r e p r e s e n t m e a n o f 180, 1 5 0 , 125, 106, 90, 71, 63 ~ m sieve sizes); • = o p t i c a l image analysis; • = s e d i m e n t a t i o n ; and • = mercury porosimetry.
•
•
A
A
O O v
E _= .os, o O a.
2
•
,-,-
&
6
8
----3 10
12
1&
16
18
Pore radius
(IJm)
Fig. 2. Pore size analysis b y m e r c u r y p o r o s i m e t r y .
20
22
2&
, 26
__" 28
30
269
capillaries of various sizes (Fig. 2). As expected from the particle size distribution results, beds of samples B and C contain a higher proportion of finer capillaries, as well as having smaller overall pore volumes than for sample A. Analysis of pore radii has been used on a number of occasions (Lloyd and Dodds, 1972) to obtain approximate values for the pressures required to drain saturated capillaries in packed beds of particles. The Laplace-Young equation {Young, 1805) relates the pressure (or vacuum) necessary to drain a capillary, to surface tension parameters (TLA and 0 LS ), for a given capillary radius (r). Ap =
2 7 L A COS 0 Ls
(3)
r
Table 2 gives examples of h o w eqn. (3) can be used to relate capillary radius to the capillary pressure AP, at k n o w n surface tension ~/LA (assuming cos ~ = 1). The effect of reducing the surface tension of the wash water from that of pure water ( ~ L A ---- 0.073 N m -1) to that which can be obtained using a surfactant (~/LA = 0.030 N m -1 ) is illustrated. Related experiments in this laboratory show that the packing mode of the sample in the porosimeter cell compartment is similar to that in a typical filter cake. It is evident therefore from Table 2 that it is the smaller pore size (< 5 ~m) in the filter cake which will prove the most difficult to drain. Typical industrial practice employs vacuum levels of 0.4--0.5 bar (including pressure drop across the filter medium). The figures in Table 2 show that by reducing the surface tension of the wash water from 0.073 to 0.030 N m -1, an improvement in filter cake drainage should result, for a constant applied vacuum. It is also of interest to note from eqn. (3), that any increase in the receding contact angle 0 us and hence a decrease in cos 0 LS, would similarly be expected to result in improved dewatering, even at constant ~LA. The use of surfactants to aid dewatering of beds of alumina trihydrate TABLE 2 E f f e c t o f p o r e size o n capillary r e t e n t i o n f o r c e Pore size (urn) 30 15 5 4 3 2 1
Capillary pressure (bar) ~'LA 0.05 0.10 0.30 0.38 0.50 0.75 1.5
=
0.073 N m -I
~'LA = 0.030 N m -1 0.02 0.04 0.12 0.16 0.20 0.31 0.62
270 particles, with capillary sizes within the range given in Table 2, is discussed in a later section of this paper. The mercury intrusion data were obtained up to a pressure of 2500 bar in order to determine the pore volume and mean radius of pores present in the particles. These data (Table 1) show that there is very little internal porosity, supporting the low surface areas measured b y nitrogen adsorption. S u r f a c e area m e a s u r e m e n t
BET surface areas measured by nitrogen adsorption are presented in Table 1. Also included in Table 1 are the surface areas calculated from the mercury intrusion data using a method described b y Rootare and Prenzlow (1967). This method uses numerical integration of the pore volume and pore radius data, and the assumption of cylindrical pores. Surface area is given as: V
0.02253 f p ~ a x
A - -
-
m
Pdvp
(4)
0
where: m = mass of sample; Vp = pore volume, and P = applied pressure. Surface areas obtained by the t w o methods show good agreement. The larger surface areas obtained for samples B and C reflect their greater percentages of fine particles. The absolute values of the areas are low, in accordance with the small internal pore volumes measured by mercury porosimetry. Scanning electron microscopy (S.E.M.)
Figure 3 shows electron micrographs of particles of sample A. No major differences were observed in the exterior morphology of the particles from the three different plants. It is immediately apparent that the particles are highly irregular, and in terms of dewatering, would be expected to provide a significant area for the presence of trapped lenses of water which would not drain under normal experimental conditions. Misra and White (1971) have made a study of the crystallization of Bayer alumina trihydrate, observing that particle growth takes place by the enlargement of hexagonal prisms and plates, this type of geometry being clearly shown in Fig. 3. Particle charge
Zeta potentials of the three samples of alumina trihydrate are given in Table 1. It is clear from these results that the charge on the alumina trihydrate particles shows a very strong pH dependence. This effect is rationalized in terms of the k n o w n presence of adsorbed humic material on the particle surface (Jayaweera, 1981), and the strong ionization of the corresponding surface functional groups at high pH (Schnitzer and Khan, 1972, pp. 29--54). The effect of pH on the dewatering characteristics of alumina trihydrate has been described in a previous paper (Puttock et al., 1985).
271
Fig. 3. Scanning electron micrographs of sample A.
272
Thermal analysis Thermal analytical techniques have been used to confirm the degree of hydration of the samples (thermal gravimetric analysis, TGA) and to monitor the temperature ranges at which the material begins to undergo dehydration (differential scanning calorimetry, DSC). Any differences in the crystal structure of the three hydrated alumina samples would be expected to be manifested by variations in the DSC traces. The thermal analysis results are summarized in Table 1 and no major differences are evident between the three samples. These results confirm that all three samples contain three moles of water, and dehydrate at similar temperatures, inferring similar crystal structures.
Vacuum filtration of alumina trihydrate The residual moisture contents of typical filter cakes of the three samples A, B and C after washing with distilled water (~/LA = 0.073 N m -1 at 22 + 2°C) were 9.1, 9.5 and 10.3%, respectively. The higher moisture contents of samples B and C are consistent with the higher percentage of fine particles present in those samples. Sieve analysis (Table 1) shows that samples A, B and C have minus 70 pm percentage of 10.6, 30.2 and 40.8, respectively. These results are similar to those reported by Henderson et al. (1957) for the vacuum filtration of iron ore concentrates.
Effect of anionic surfactants Figure 4 shows the results of standard filtration tests using three anionic surfactants at both neutral pH (7) and at high pH (12). In all cases it was found that increasing the concentration of a particular surfactant (20-50100 ppm) at constant pH, resulted in a reduction in filter cake residual moisture content. This is consistent with the lowering in surface tension that occurs [TLA in eqn. (3)]. The proposal of a direct relationship between surface tension and moisture content has two major inadequacies. Firstly, for a number of cases, in going from 50 to 100 p p m concentration in the wash water, the surface tension changes very little whereas the moisture content falls rapidly. Secondly, any possible relationship between surface tension and moisture content only holds for a particular surfactant at constant pH and hence, two surfactants, at the same surface tension, may exhibit very different dewatering performances. From experimental observations of this type, such factors as t h e ability of a surfactant not only to change 7LA in eqn. (3), b u t also the contact angle 0 LS must be considered. For all surfactants, the higher pH conditions resulted in an increase in wash water surface tension, accompanied by an increase in residual moisture content. However, the sulphuric acid ester salt (sulphated polyether alcohol) suffered relatively little loss of surface activity at high pH and gives the best
273 10 Pure wash w a t e r
0
(U
O
7"
(J O) L. -,-,
6"
O
:E 5-
3'o
3'5
~o
S u r f a c e t e n s i o n of w a s h w a t e r
is
""
7'~
(raN/m)
Fig. 4. I n f l u e n c e o f surface t e n s i o n o n residual m o i s t u r e c o n t e n t o f filter cakes o f s a m p l e A, u s i n g a n i o n i c s u r f a c t a n t s : s o d i u m d o d e c y l b e n z e n e s u l p h o n a t e - - ; s o d i u m salt a l p h a olefin s u l p h o n a t e . . . . ; a n d s o d i u m d o d e c y l s u l p h a t e ( e t h o x y l a t e d ) - - - - - . C o n c e n t r a t i o n : . , ~, o = 20, 50, 1 0 0 p p m a t p H 7; e, A , . = 20, 50, 100 p p m at p H 12. o refers t o p u r e w a t e r wash o n l y ( n o s u r f a c t a n t ) .
overall performance among the anionic surfactants tested. This latter surfactant would be expected to have a high degree of nonionic character due to its p o l y e t h o x y l a t e d structure.
Cationic surfactants The dewatering results corresponding to the performance of cationic surfactants are given in Fig. 5. In a similar fashion to the anionic surfactant results, it is apparent that an increase in the concentration of a cationic surfactant in the wash water, reduces both the surface tension and the residual moisture c o n t e n t of the filter cake. The results show that concentrations in excess of 50 ppm are required to obtain a dewatering performance equivalent to t h a t of a typical anionic surfactant at 20 ppm. This observation would infer a mechanism involving significant adsorption of the cationic surfactant from wash water onto the highly negatively charged alumina trihydrate surface. It must be assumed that any increase in the contact angle 6 LS caused by this adsorption process, is outweighed by the corresponding loss in surface tension of the wash water. All the cationic surfactants evaluated in this work showed good stability in wash water of higher pH.
274 10 9-
0
/'/
•
Pure
~
8.
~
7-
0 0 m,. =
I ./r
!/i
//x i
; I
6-
•;
0
5-
o"
I'
/ 5
•
r
3'o
water
/i I
i
/
wash
3's
4'0
S u r f a c e t e n s i o n of w a s h w a t e r
""
¢
7'2
(mN/m)
Fig. 5. I n f l u e n c e o f surface t e n s i o n o n residual m o i s t u r e c o n t e n t o f filter cakes of s a m p l e A using c a t i o n i c s u r f a c t a n t s : bis (2 h y d r o x y - e t h y l ) c o c o a m i n e - - ; m e t h y l bis (2 h y d r o x y e t h y l ) oleyl a m m o n i u m c h l o r i d e . . . . ; oleyl t r i - m e t h y l a m m o n i u m c h l o r i d e - - - - ; and p o l y e t h o x y ( 1 5 ) t a l l o w 1, 3 - d i a m i n o - p r o p a n e - - " --. C o n c e n t r a t i o n : o, a, o = 20, 50, 100 p p m a t p H 7; m, A, • = 20, 50, 1 0 0 p p m at p H 12. o refers t o p u r e w a t e r wash o n l y ( n o surfactants).
Nonionic surfactants Figure 6 shows the results of standard dewatering tests using ethoxylated nonionic surfactants, all used at a concentration of 100 p p m in the filter cake wash water. It is apparent that the nonionic surfactants chosen for performance evaluations are generally superior in terms of dewatering efficiency to the anionic or cationic types. In all cases the surface activity of the wash water is not diminished appreciably in the higher pH conditions. For both the alcohol and the alkylphenol ethoxylates, an o p t i m u m length of the e t h o x y chain, at which filter cake moisture content is minimum is noted. It would be envisaged that fine tuning of this chain length could be used to further improve the dewatering performance of such ethoxylated nonionic surfactants. Care must be taken in this respect, since in these industrial samples the length of the e t h o x y chain (n number) is not constant for each surfactant molecule, but represents an average figure comprising a Poisson t y p e distribution (Mayhew and Hyatt, 1952). It is also evident from Fig. 6 that the increase in wash water pH appears to shift the length of the e t h o x y chain corresponding to a minimum moisture content, to a slightly higher value. It might be expected that for the nonyl phenol ethoxylate at neutral pH, the point corresponding to a value of n = 5.5 (M = 4.3%), represents a minimum in moisture content which would increase again for
275 10
9-
29.8 e-
O o 61
y317
2~'t*~~
,//
& ]].6
/
•
~o.2
O
I
J
!
h
\'.!
~
I
8
I
12
ii6
Length of ethoxy chain (n)
Fig. 6. Influence of e t h o x y chain length on residual moisture c o n t e n t of filter cakes of sample A using n o n i o n i c surfactants (all at 100 ppm): e t h o x y l a t e d c o c o alcohol - - ; e t h y o x y l a t e d n o n y l p h e n o . . . . ; at pH 7 (o, ~) and pH 12 (e, A). N u m b e r i n g refers to wash water surface tension (mN m -~ ).
values of n less than 5.5. This pH dependence in the minimum moisture c o n t e n t may be due to the known adoption of a small degree of cationic character in such nonionic surfactants at high pH.
Surfactants at high dosage rates The previous work involving the use of surfactants has dealt exclusively with a study of the dewatering performance of anionic, cationic and nonionic surfactants over the concentration range 20--100 ppm, this being typical of industrial practice. Figure 7 shows the results using anionic, cationic and nonionic surfactants in the filter cake wash water at concentrations from 100 to 3000 ppm. It is immediately apparent t h a t the residual moisture content of the filter cake can be significantly reduced by the use of these higher dosage rates. From an industrial standpoint, such an improvement is offset by the increased cost of surfactant, and perhaps more importantly, by the modified physical state of the dewatered cake. At concentrations of either anionic or cationic surfactant above about 400 ppm, it was observed that the filter cake became progressively stickier and hence more unmanageable. This effect was far less pronounced with the use of nonionic surfactants at high concentrations. For each surfactant, the surface tension 7LA of the wash water was approximately constant (+ 1 mN m -1) throughout this high concentration
276
7.
6
I0~0400 I0'00
20'00
30'00
Concentration of surfactant (ppm)
Fig. 7. Influence of surfactant concentration on residual moisture content of filter cakes of sample A. Surfactants used: sodium dodecyl sulphate (anionic) e, cetyl trimethyl a m m o n i u m bromide ( c a t i o n i c ) v , ethyoxylated (n = 8) nonyl phenol (nonionic) =.
range, suggesting that the observed variation in moisture content was not a function of the filtrate surface tension. The reason for the observed decrease in moisture content with surfactant dosage is believed to be due to the increased mass transfer of the surfactant to the particles during the very short contact times prevailing. A consequence of this may be a beneficial reduction in cos 0 LS which would be predicted [eqn. (3)] to improve dewatering. DISCUSSION
Characterization of alumina trihydrate particles This initial study of some of the important physical characteristics of alumina trihydrate particles from three Australian alumina plants at widely different locations, has demonstrated many similarities between the samples. Surface areas, exterior morphology and particle charge, along with the degree of hydration and temperature of dehydration, have all been shown
277
to be similar for all samples. The particle sizing results from dry screening, optical image analysis, sedimentation and mercury porosimetry, all confirmed that sample A has larger particles than either B or C. Such size differences are important in the filter cake dewatering step of Bayer processing. The different sizing results illustrate the ambiguity which arises when quoting particle size data, since each size clearly depends on the m e t h o d used for its measurement and as such would not be expected to be directly comparable. This is shown in Table 1 where the median particle sizes vary from one technique to another, although it is evident that the ratio of particle sizes from sample to sample are similar for all four sizing techniques (ration A:B = 1.23 -+ 0.06, A:C = 1.24 +- 0.10). Lloyd and Ward (1977) discuss the filtration applications of particle characterization illustrating that a particle may have a number of equivalent sphere sizes and show that particle size analysis using different techniques can be correlated by the use of a shape factor. The importance of having a good knowledge of particle size distribution is evident from two recent papers (Creehan, 1984; Zwicker, 1984) describing techniques investigated for improved sizing of alumina trihydrate particles. An important aspect of the mercury intrusion porosimetry technique is that it gives information on inter- and intra-particle pore size distributions, particle size distribution, and surface areas. Values of the latter two properties calculated from the intrusion volume data, are in close agreement with those obtained from other techniques. The mercury intrusion porosimeter is therefore perceived to be a versatile instrument for use in the alumina industry.
The use of surfactants for improved dewatering The addition of either anionic, cationic or nonionic surfactants to the wash water of alumina trihydrate filter cakes, has been shown in all cases to result in improved dewatering. Generally, the anionic surfactants tested in this work (Fig. 4) were more susceptible to loss of surface activity at higher pH than either the cationic or nonionic types (e.g. 20 ppm sodium dodecyl benzene sulphonate, 7LA = 34.0 and 42.1 mN m -1 at pH 7 and 12, respectively). However, the inherently higher surface activity of the anionic surfactants generally resulted in better dewatering than the cationics, even in the higher pH wash water. The anionic surfactant of the sulphuric acid ester salt type showed greater tolerance to the alkaline conditions o f the wash water, than the sulphonic acid types, and performed better in the dewatering tests. The sulphated polyether alcohol, which has a considerable degree of nonionic character, showed good potential as a dewatering aid. The cationic surfactants (Fig. 5), showed good tolerance to the higher pH conditions and in m a n y cases exhibited good surface activity [e.g. 100 ppm bis(2-hydroxyethyl)cocoamine, 7LA = 28.3 mN m - ' ] . Nevertheless, despite this high level of surface activity, the cationic surfactants generally did not
278
exhibit exceptional dewatering performance, especially at low concentrations. Nicol (1976) reported the use of anionic and cationic surfactants in the dewatering of fine coal, the anionic surfactant reducing the filter cake moisture content appreciably more than the cationic type. The poorer performance of the cationic surfactant was explained in terms of surfactant adsorption from the wash water onto the coal surface and consequent loss of surface activity in the bulk filtrate. Overall, the nonionic surfactants performed very well in the dewatering tests (Fig. 6), exhibiting good surface activity at both neutral and high pH, in all cases resulting in the formation of filter cakes with low residual moisture contents. It would be anticipated from these results that further detailed work involving a study of the effect of varying both the alkyl and the e t h o x y chain lengths, will lead to further improvements in dewatering. A recent patent (Yama, 1982) has reported a study involving the dewatering of aluminium hydroxide using surfactants of the t y p e R-0-(AO)n-X (where R refers to the alkyl chain, AO to the ethylene, butylene or propylene grouping, n varies from 3 to 1O0, and X is a sulphate group). The authors report that in all tests, the best dewatering performance was achieved for values of n=6, for example in the surfactant C12H2sO(C2H40)6SO3Na. Another important area w o r t h y of further research involves the use of surfactants at concentrations much higher than that used typically in industrial practice (Fig. 7). For anionic, cationic and nonionic surfactants, very significant reductions in filter cake moisture content were observed, although for industrial application any cake "stickiness" problems with ionic surfactants must be avoided. Of course due consideration must be given to the effect of higher levels of surfactants, which will ultimately be circulated to other parts of the Bayer process circuit through the filtrate. CONCLUSIONS
The results reported in this paper highlight three major points of interest to both the Australian and to the worldwide alumina processing industry. {1) Particle characterization studies reveal that mercury intrusion porosimetry is a valuable tool in the characterization of alumina trihydrate particles. (2) Anionic, cationic and nonionic surfactants can all be used to enhance filter cake dewatering. The effectiveness of a particular surfactant to reduce moisture content appears to be difficult to predict, b u t involves its ability to reduce wash water surface tension ~LA, and possibly to modify the receding solid/liquid contact angle 0 Ls. Overall, ethoxylated nonionic surfactants appear to have the most suitable characteristics for use as a dewatering aid. (3) The use of surfactants at high concentrations has been shown to result in further improvements in filter cake dewatering. Reductions in filter cake moisture content at constant wash water surface tension, confirm that the
279
exact mechanism of dewatering is complex and again may involve changes in the-contact angle 0 LS. ACKNOWLEDGEMENTS
The authors gratefully acknowledge financial assistance from the Australian Research Grant Scheme. We wish to thank Alcoa of Australia Ltd., Nabalco Ltd. and Queensland Alumina Ltd. for the alumina trihydrate samples, and Akzo Chemie (Nederland B.V.), Harcros Chemicals Pty. Ltd. and ICI Australia Operations Pty. Ltd. for the surfactants. The assistance of Mr. A. Deacon in the particle characterization studies is much appreciated. REFERENCES Creehan, J.T., 1984. Improved particle size analysis method for precipitation control. Light Metals, 1: 39--51. Dolina, L.F. and Kaminskii, V.S., 1974. Influence of the electrokinetic charge potential of particles on dewatering processes. Coke Chem., U.S.S.R., 1: 8--11. Henderson, A.F., Cornell, C.F., Dunyon, A.F. and Dahlstrom, D.A., 1957. Filtration and control of moisture content on taconite concentrates. Trans. AIME, 204: 349--355. Jayaweera, L.D., 1981. The effect of organic impurities on the precipitation of alumina trihydrate in the Bayer process. Ph.D. thesis, University of New South Wales. Lloyd, P.J. and Dodds, J.A., 1972. Liquid retention in filter cakes. Filtr. Sep., 9: 91--96. Lloyd, P.J. and Ward, A.S., 1977. Filtration applications of particle characterisation. A.I.Ch.E. Symp. Ser., 73: 6--12. Mayhew, R.L. and Hyatt, R.C., 1952. The effect of mole ratio distribution on the physical properties of a polyoxyethylated alkyl phenol. J. Am. Oil Chem. Soc., 29: 357-362. McCall, M.T. and Tadros, M.E., 1980. Effects of additives on morphology of precipitated calcium sulphate and calcium sulphite -- implications on slurry properties. Colloids Surf., 1: 161--172. Misra, C. and White, E.T., 1971. Crystallisation of Bayer aluminium trihydroxide. J. Crystal Growth, 8: 172--178. Nicol, S.K., 1976. The effect of surfactants on the dewatering of fine coal. Proc. Australas. Inst. Min. Metall., 260: 37--44. Princen, H.M., 1970. Advantages and limitations of the grooved Wilhelmy plate. Aust. J. Chem., 23: 1789--1799. Puttock, S.J., Fane, A.G., Fell, C.J.D., Robins, R.G. and Wainwright, M.S., 1985. Role of surface effects in the dewatering of alumina trihydrate. A.I.Ch.E.J., 31: 1213-1216. Rootare, H.M. and Prenzlow, C.F., 1967. Surface area from mercury porosimeter measurements. J. Phys. Chem., 7 1 : 2 7 3 4 - - 2 7 3 6 . Rootare, H.M., Powers, J.M. and Spencer, J., 1979. Particle size of abrasive powders from mercury porosimetry data. Proc. Int. Powder and Bulk Solids Handling and Processing, Philadelphia, Pa., pp. 129--143. Rushton, A., Hosseini, M. and Hassan, I., 1980. The effects of velocity and concentration on filter cake resistance. J. Separ. Process. Technol., 1 : 35--41. Schnitzer, M. and Khan, S.V., 1972. Humic substances in the Environment. Marcel Dekker Inc., New York, N.Y., pp. 29--54. Yama, M. (to Kao Soap Co., Ltd.), 1982. Filtration-dewatering aids for aqueous slurries. Japanese Patent 57-84708. Young, T., 1805. An essay on the cohesion of fluids. Philos. Trans. R. Soc., 15: 65--87. Zwicker, J.D., 1984. Particle size analysis in Bayer plant control. Light Metals, 1: 173-193.