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Effect of urea on decomposition of sodium aluminate solution
Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 815–822
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Effect of urea on decomposition of sodium aluminate solution N.K. Sahu a,b, C.K. Sarangi b, B.C. Tripathy a,b,*, I.N. Bhattacharya a,b, B.K. Satpathy c a
Academy of Scientific and Innovative Research (AcSIR), New Delhi, India CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, India c National Aluminium Company, Bhubaneswar, India b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 March 2013 Received in revised form 31 July 2013 Accepted 1 September 2013 Available online 10 October 2013
Aluminium hydroxide particles were synthesized from sodium aluminate liquor by urea decomposition method at elevated temperatures in presence and absence of seed. Different parameters such as alumina to caustic (A:C) ratio, temperature, time period of precipitation, urea concentration, etc. were studied. Characterisations of the products were also carried out using X-ray diffraction, scanning electron microscopy, TG-DTA, etc. In general, higher precipitation ratios were observed with the addition of urea in sodium aluminate solution. Temperature was found to have a significant role in precipitating aluminium hydroxide with higher yield enhancement ratio compared to that obtained at lower temperatures. Higher precipitation ratios were also obtained with urea at elevated temperatures under available supersaturation. In the presence of urea, the precipitation ratios on yield enhancement front were observed to increase with decrease in A:C ratio. It has been observed that the liquor concentration reached below the equilibrium solubility limit after 30 h of precipitation when 460 mmol/L urea is added. The synthesized aluminium hydroxide particles showed gibbsitic nature and globular morphology, with agglomerates of mostly hexagonal platelets, as revealed through XRD and SEM studies, respectively. Calcination of the product at 1000 8C results in weight loss similar to that observed with gibbsite. Endothermic peaks at 325 8C and 550 8C revealed through DTA study indicated boehmite and chi-alumina transformations, respectively. The possible mechanism of urea decomposition method is also discussed. ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Aluminium hydroxide Urea Gibbsite Sodium aluminate Equillibrium solubility Supersaturation
1. Introduction Both the Bayer and the sintering [1,2] processes are used for the production of alumina via decomposition of metastable sodium aluminate solution. Bayer process mainly utilizes gibb bauxite ores through NaOH digestion for the production of sodium aluminate solution. Whereas in sintering process low grade diasporic/ boehmitic bauxite or nepheline or other alumino silicates [1,3,4] are used by sintering it with sodium carbonate at high temperatures to form sodium aluminate solution after leaching the sintered mass. In the Bayer process sodium aluminate is decomposed through seeded precipitation method to obtain aluminium hydroxide whereas carbonization decomposition method is used for its recovery from leach solution. In Bayer method some of the sodium hydroxide transformed to sodium carbonate due to atmospheric CO2. This carbonate is in turn treated with lime to get back sodium hydroxide before being recycled for
* Corresponding author at: CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, India. Tel.: +91 9861085358; fax: +91 674 2581637. E-mail addresses: [email protected], [email protected] (B.C. Tripathy).
further bauxite digestion. Generally in industries during precipitation of gibbsite in Bayer circuit, the alumina concentration could not be brought down to equilibrium solubility limit. However, in carbonization decomposition method more than 90% alumina is precipitated. As a result, the alumina concentration in the liquor goes well below equilibrium solubility limit. To enhance the precipitation ratio in Bayer aluminate decomposition process various efforts were made [5–18]. Activation [5–9] of seed hydrate is one of such methods but not yet commercialized due to additional steps and higher energy consumption. Addition of different polymers, surfactants, inorganic and organic additives [10–18] did not show encouraging results. Moreover, these additives would spoil the solution to a great extent as unwanted cations and anions are introduced which accumulate with time due to the cyclic nature of the Bayer process. In this paper an attempt has been made to evaluate the effect of urea (NH2CONH2) in sodium aluminate liquor and the extent to which it can be utilized in decomposition of aluminate liquor under seeding condition. The decomposition of aluminate liquor is related to the supersaturation (Al2O3: Na2O, i.e. A:C ratio and temperature) level of the solution. However, the excessive super saturation, particularly greater than 1.05 (A:C) or so, leads to the instability of the solution. This results in precipitation of hydroxide
1876-1070/$ – see front matter ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jtice.2013.09.001
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at various inappropriate locations during the operation leading to the loss of alumina and damage to the various equipment [12]. Therefore, excessive supersaturation should be avoided. Further, the release of hydroxyl ion (OH) into the system during hydrolysis leads to the slowing down or finally stoppage of further hydrolysis. Consequently, NaOH concentration in the solution increases leading to low A:C ratio, i.e., supersaturation falls to a lower value. The following equation shows the decomposition of aluminate ion. AlðOHÞ4 ! AlðOHÞ3 þ OH
AlðOHÞ4 ! AlðOHÞ3 þ OH
(8)
Naþ þ OH ! NaOH
(9)
(1)
To enhance the hydrolysis of aluminate liquor, OH liberated should be consumed so as to continue the hydroxide precipitation process. Urea is generally used for precipitating spherical hydrated basic aluminium salts, oxides etc. and high pure specialty alumina by utilizing it as hydrolysing precipitant for aluminium salt solutions at elevated temperatures [19,20]. As it slowly releases NH4+, CO2 and hydroxyl ions at elevated temperatures, the pH of the solution increases and results in the precipitation of spherical basic aluminium salts. Use of urea as an additive for aluminium hydroxide precipitation would be advantageous in many respect; (i) the product would be free from any contamination; (ii) urea is cheap and available on plenty; (iii) urea is an ecofriendly chemical. In the present study at elevated temperature urea decomposes to give NH3 and CO2 and in this case CO2 is mainly responsible for hydrolysis of aluminate solution. This process may be analogous to carbonization decomposition process. But the later carbonization decomposition is utilized for total decomposition of the solution whereas the idea in this study is to find an extent to which urea can be utilized in addition to seed effect because in all the Bayer plants conversion of sodium carbonate to sodium hydroxide is carried out routinely. 1.1. Conceptual mechanism of urea assisted aluminium hydroxide precipitation The carbonization decomposition process [21,22] is one of the major steps in sintering process for producing aluminium hydroxide. In carbonization decomposition process CO2 gas is introduced into sodium aluminate solution to precipitate aluminium hydroxide. There are various mechanisms of precipitation of aluminium hydroxide in carbonization process discussed by Li [2]. The carbon dioxide when added to aluminate liquor either neutralizes (Eq. (2)) sodium hydroxide present in the solution thereby increasing the A:C ratio which leads to continuous precipitation or direct decomposition of sodium aluminate to aluminium hydroxide (Eq. (3)). 2NaOH þ CO2 ¼ Na2 CO3 þ H2 O
As the concentration of OH decreases, the polymerization of aluminate species increases and forms aluminium hydroxide precipitates. As regards seeded precipitation method, sodium aluminate solution decomposes under seeded condition where seed provides the surface onto which new particles nucleates and grows. The hydrolysis reaction taking place as follows, producing Al(OH)3 and NaOH.
(2)
Generation of NaOH as per the above reaction (9) reduces the A:C ratio continuously, thus decreasing the super saturation level to such an extent where further precipitation is stopped. In case OH is consumed by some other means, the supersaturation level stabilizes producing continuous precipitation. The use of urea to precipitate aluminium hydroxide from sodium aluminate liquor was not reported earlier. It has been discussed in literature [23–25] that urea dissociation in aqueous system produces ultimately NH3 and CO2 but various mechanisms occur before the formation of NH3 and CO2. As our system is consisting of sodium aluminate, sodium hydroxide and H2O, the reaction pathway may not follow a simpler rule like in pure aqueous system. At neutral pH, activation energy for urea decomposition varies from 28.4 to 32.4 kcal/mol [26] and in alkaline medium the activation energy calculated was 22 kcal/mol [27]. Therefore under alkaline condition decomposition of urea is easier than in neutral pH. In aqueous system urea decomposes at an elevated temperature to cyanate and ammonium ions [28–30]. This cyanate ion then hydrolyses to carbamate anion which subsequently hydrolyses to bicarbonate ion [31] as per the following equations. NH2 CO NH2 ! CNO þ NH4þ
(10)
CNO þ H2 O ! NH2 CO2
(11)
NH2 CO2 þ H2 O ! NH3 þ HCO3
(12)
HCO3 then reacts with OH ion to produce CO3= as follows HCO3 þ OH ¼ H2 O þ CO3 ¼
(13)
2Naþ þ CO3 ¼ ¼ Na2 CO3
(14)
2NaAlðOHÞ4 þ CO2 ¼ Na2 CO3 þ 2AlðOHÞ3 þ H2 O
(3)
In another opinion [2] CO2 first forms carbonic acid which then decomposes sodium aluminate to from aluminium hydroxide and sodium carbonate. 2NaAlðOHÞ4 þ H2 CO3 ¼ 2AlðOHÞ3 þ Na2 CO3 þ 2H2 O
(4)
2NaOH þ H2 CO3 ¼ Na2 CO3 þ 2H2 O
(5)
According to Besuzin as mentioned by Li [2] carbon dioxide first reacts with OH ion and reduces the activity of OH ion as follows, OH þ CO2 ¼ HCO3
(6)
HCO3 þ OH ¼ H2 O þ CO3 ¼
(7)
Thus OH concentration in the solution decreases with enhanced aluminium hydroxide precipitation ratio along with the formation of sodium carbonate. 2. Experimental 2.1. Chemicals and apparatus The chemicals used in this study are of analytical grade and obtained from Merck, India. Supersaturated sodium aluminate solution was prepared by dissolving measured quantity of aluminium granules in sodium hydroxide solution. In the present study the mass ratio of Al2O3 to Na2O (A:C) was varied from 0.9 to 1.0. During the precipitation study, temperature was maintained between 608 to 80 0.1 8C. The gibbsite seed (d50 = 62.2 mm) used for this study was obtained from NALCO, Bhubaneswar, India.
N.K. Sahu et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 815–822
The apparatus used for the precipitation studies was made from 304 stainless steel of 500 mL capacity which was fitted with a digital RPM controller. An anchor type impeller was used for the agitation of the solution. The rotation of the impeller was set at 190 2 revolutions per minute (RPM). The temperature of the system was maintained through a constant thermostatic bath circulator (JULABO, Germany). 2.2. Method All the experiments were conducted using 250 mL of sodium aluminate solution. In all the cases the caustic concentration was kept at 150 g/L and the requisite A:C ratio was maintained by varying Al2O3 concentration only. Dilute solutions were prepared by maintaining A:C ratio of 0.95 with Na2O concentration of 50 g/ L and Al2O3 concentration of 47.5 g/L. The time period of the precipitation study was maintained according to the requirement of the experiment. The seed and other additions were made once the desired temperature was attained. After each experiment the slurry was filtered and the residue was washed several times before it was dried in an oven at 105 8C. The aluminium hydroxide yield was calculated in g/L, yield in g/L = [(wt. of the precipitate wt. of the seed)/experimental volume of the solution] 1000.
Yield in gL
1
¼
wt: of the precipitate wt: of the seed 1000: experimental volume of the solution
In this study three different precipitation ratios were analyzed and calculated as shown below
Precipitation ratio ð%Þ ¼
Yield ðgL1 Þ Initial AlðOHÞ3 concentration in solution ðgL1 Þ
100
817
2.3. The equilibrium solubility The equilibrium solubility of sodium aluminate solution is the saturation point of the solution below which level precipitation is improbable at a particular temperature; only available supersaturation can practically be precipitated under seeded precipitation process. The equilibrium solubility of alumina in the sodium aluminate solution was measured using well known Rosenberg & Healy [32] formula as follows: " Aeqm ¼ 0:96197 C 1 þ
pffi pffi 3=2 #1 10½ðð9:2082 IÞ=ð1þ IÞÞþð0:2149I Þ expðDG=RTÞ
where, ionic strength I = 0.01887 C + 0.01937 SC; where C and SC are caustic and carbonate concentrations respectively, both in g/L of Na2CO3; DG = Gibbs free energy of solid formation = 30.96 kJ/mol; R = universal gas constant = 8.3145 J/K/ mol; T = precipitation temperature (K). Using the above equation, the equilibrium solubility of alumina at 75 8C (A75) was calculated to be 88 g/L when Na2O concentration was 150 g/L. Therefore, at A:C = 1.0, where the initial Al2O3 concentration in the solution is 150 g/L, The available supersaturation = 150– 88 = 62 g/L as Al2O3 or 95 g/L as aluminium hydroxide. Similarly, at A:C = 0.95, where the initial Al2O3 concentration in the solution is 142.5 g/L, the available supersaturation = 142.5 88 = 54.5 g/L as Al2O3 or 83.35 g/L as aluminium hydroxide. The calculated values of equilibrium solubilities at different temperatures with aluminium hydroxide concentrations at different A:C ratios have been given in Table 1. Some of the figures (Figs. 3 and 6) are plotted taking these data for calculation of precipitation rate under available supersaturation. 3. Results and discussion
Yield enhancement ratio ð%Þ ¼
Yield ðgL1 Þ with urea yield in blank ðgL1 Þ Yield in blank ðgL1 Þ 100
Precipitation ratio under available supersaturation ð%Þ ¼
Yield ðgL1 Þ Super saturation level ðgL1 Þ
The effect of urea on decomposition of aluminate liquor has been discussed in this work. The parameters varied are alumina:caustic ratio, temperature, time period of precipitation and urea concentration. The precipitation ratio depends mainly on decomposition of urea. The morphology of the precipitated product has also been discussed in this chapter. The main parameters measured in this study are precipitation ratio, precipitation ratio under yield enhancement and precipitation ratio under available supersaturation at different temperatures, A:C ratio and urea
100
*‘‘blank’’ specifies the condition when precipitation was carried out in the absence of additives. It should be kept in mind that yield under blank condition changes with solution concentration, temperature and A:C ratio. X-ray diffractograms were recorded for the aluminium hydroxide powders using PANalytical diffractometer (PW 1830, Philips, Japan) with Cu Ka radiation, l = 1.54056 A˚. The scans were recorded in 2u range 0–808. Fourier transform infrared (FTIR) spectrographs were recorded on a Nicolet 6070 spectrophotometer in the frequency range 400–4000 cm1. A scanning electron microscope (SEM) (JEOL JSM 6510, Japan) was used to examine the surface morphology of the aluminium hydroxide. Brunauer Emmett Teller (BET) surface area measurements were carried out using Autosorb-iQ (Quantachrome, USA). TG-DTA study was carried out using TGA/SDTA851e (Mettler-Toledo, USA).
Table 1 Calculation of available super saturation using Rosenberg and Healy method. (A) Equilibrium solubility at different temperatures (Na2O concentration = 150 g/L) Temperature (8C)
[Al(OH)3] (g/L) (x)
60 65 70 75 80
96.35 108.6 121.6 134.6 148.05
(B) Aluminium hydroxide concentration at different A:C ratios. A:C ratio
[Al(OH)3] (g/L) (y)
1.0 0.97 0.95 0.93 0.90
229.41 222.53 217.94 213.35 206.47
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818
60
Precipitation ratio, %
650 mmol
40
460 mmol 65 mmol
Blank
20
Yield enhancement rate, %
80 1000 mmol
1000 mmol 650 mmol
40
460 mmol 325 mmol 65 mmol
0
0.90
0.92
0.94
0.96
0.98
0.92
1.00
A:C ratio Fig. 1. Variation of precipitation ratio with A:C ratio at different urea concentrations, temperature 75 8C, time 24 h.
concentrations. Supersaturation is the measure of alumina or aluminium hydroxide concentration in excess of equilibrium solubility. In case equilibrium solubility of aluminium hydroxide at a particular temperature and soda concentration is x and initial hydroxide concentration in the liquor is y, then the available super saturation is equal to (y x). Equilibrium solubility is the minimum level of concentration up to which precipitation is possible under standard condition in regard to seeded precipitation process. 3.1. Effect of Al2O3: Na2O (A:C) ratio Fig. 1 shows the effect of A:C ratio on precipitation ratio when different quantities of urea were added at 75 8C. It has been shown that at higher quantities of urea the aluminate liquor decomposition is increased at every level of A:C ratio. This is due to availability of more urea decomposition products. Further, it has been observed that the slope of the curves are almost equal with upward inclination indicating more precipitation at higher A:C ratios. Considering the individual plots, as same amounts of urea decomposition products are available at different A:C ratios, the low supersaturation level did not produce same quantity of aluminium hydroxide particles as produced in highly supersaturated solutions. As in high supersaturated solution, the quantity of aluminate ions are more compared to that of low supersaturated solution and soda concentration is same in both the cases, the urea decomposition products mostly reacted with OH1 released during aluminate ion decomposition process. The HCO31, the urea decomposition product, reacted with OH1 forming CO3= and water molecule as shown in equation 13. When aluminium hydroxide yield enhancement ratio was plotted against A:C ratio (Fig. 2) it has been observed that yield enhancement decreased with A:C ratio in general at all urea concentration levels. The exception found at 65 mmol/L may be due to the fact that at such low concentration the effect of urea is not significant. In this case at low A:C ratio the curves are wider than at A:C ratio of 1.0. If we take two extreme cases when 1000 mmol/L urea was added, the enhanced yield ratios were found to be 65.0 and 81.16% for A:C ratios of 1.0 and 0.9, respectively. More than 16% yield enhancement was observed at lower A:C ratio than at higher ratio. Similar increased yield enhancement ratios were observed at other urea quantities at lower A:C ratios. This happened due to the fact that at higher A:C ratio under blank condition the yields are more than at lower A:C ratio. Therefore yield enhancement is lower at higher A:C ratios. Further, when precipitation ratio under available supersaturation was plotted against A:C ratios (Fig. 3) at different
0.94
0.96
0.98
1.00
A:C ratio Fig. 2. Variation of yield enhancement ratio with A:C ratio at different urea concentrations, temperature 75 8C, time 24 h.
urea concentrations, it has been found that precipitation ratio remains almost same at different urea concentrations though it has increased substantially from low concentration urea to higher concentration of urea. The graph shows more than or equal to 100% precipitation under available super saturation when higher quantities of urea was added, which indicates that the precipitation ratio under available supersaturation goes beyond equilibrium solubility range with the additional urea. This is because of the availability of more urea decomposition products. But at low urea concentration, precipitation ratio under available super saturation is less than equilibrium composition of aluminium hydroxide. At A:C ratio 1.0 (Table 1) the available super saturation at 75 8C is 94.81 g/L (229.41–134.6) of aluminium hydroxide, whereas precipitation obtained was around 112 g/L, this is therefore 18% more than the equilibrium solubility level. In nutshell, the effect of A:C ratio on precipitation of aluminium hydroxide can be explained as follows; it is obvious that when A:C ratio is increased, the hydroxide yield (or precipitation ratio) will also increase because of higher supersaturation level which is reflected in Fig. 1 (blank). When urea was added precipitation ratio also increased in all levels of A:C ratio because of decomposition of urea at 75 8C. Whereas when yield enhancement is concerned, the yield enhancement is increased with decrease in A:C ratio baring the case of 65 mmol/L urea concentration. The precipitation ratio under available supersaturation is remained almost same at higher urea concentrations which is almost 100% or beyond with 650 mmol/L or more addition of urea.
120
Precipitation ratio under avilable supersaturation, %
0.90
1000 m m ol 650 m m ol
100 460 m m ol 325 m m ol
80
65 m m ol Blank
60
0.90
0.92
0.94
0.96
0.98
1.00
A:C ratio Fig. 3. Variation of precipitation ratio under available supersaturation with A:C ratio at different urea concentrations, temperature 75 8C, time 24 h.
N.K. Sahu et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 815–822
Precipitation ratio under available supersaturation, %
Precipitation ratio, %
60
1000 mmol 650 mmol
40
325 mmol 65 mmol Blank
20
819
160
1000 m m ol
650 m m ol 120
325 m m ol 65 m m ol 80 Blank
40 60
65
70
75
80
85
Temperature,O C 60
65
70
75
80
85
Fig. 6. Variation of precipitation ratio under available supersaturation with temperature at different urea concentrations, A:C ratio 0.95, time 24 h.
O
Temperature , C Fig. 4. Variation of precipitation ratio with temperature at different urea concentrations, A:C ratio 0.95, time 24 h.
The effect of temperature was studied within the range 60– 80 8C, as urea decomposition rate depends on temperature. The effect of temperature on the variation of precipitation ratio at different concentration of urea was found to be very interesting (Fig. 4). It was found that in the case of urea, generally the precipitation ratio increased at all the concentrations however the increase in the ratio is more at higher urea concentrations. It has also been observed in the case of blank solution that with increase in temperature the precipitation ratio decreased continuously as at higher temperature supersaturation level is low. Similarly in the presence of various concentrations of urea the precipitation ratio decreased continuously but at 80 8C it showed an increasing trend. This is due to the fact that urea decomposition rate is very high at higher temperature leading to generation of higher amount of precipitates. As regards yield enhancement ratio (Fig. 5) is concerned it showed substantial enhancement at higher temperatures but at low temperature only around 1–13% enhancement was noticed at different levels of urea addition. Further it has been found that at higher temperature (80 8C), 43–142% enhancement was noticed when urea concentration was increased from 65 to 1000 mmol/L. This enhancement is substantial. Similar behaviour was noticed when precipitation ratio under available super saturation was plotted against temperature at different urea
160
Yield enhancement ratio, %
1000m m ol 120 650m m ol
200 180 Blank
160 140
Urea
Equilibrium solubility
120 100 0
10
20
30
40
50
Time, h Fig. 7. Variation of aluminium hydroxide concentration in the solution with time, A:C ratio 0.95, temperature 75 8C.
20
40
60 U re a - 1 0 0 0 m m o l
intensity,a.u
3.2. Effect of temperature
Aluminium hydroxide, g/L
220
B la n k
80 325m m ol 65 m m ol
40
0 60
65
70
75
80
85
Temperature,O C Fig. 5. Variation of yield enhancement ratio with temperature at different urea concentrations, A:C ratio 0.95, time 24 h.
20
40 o Position 2 Theta
60
Fig. 8. X-ray diffractogram of aluminium hydroxide obtained under urea added at 1000 mmol and blank conditions.
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N.K. Sahu et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 815–822
concentrations (Fig. 6). The general trend under blank condition is reduction of precipitation ratio with temperature but at 80 8C a stiff jump in precipitation is noticed at all levels of urea concentrations. At 80 8C, 60–147% precipitation was observed under available super saturation from blank to 1000 mmol/L of urea addition respectively but at 60 8C under similar condition only 83–94% precipitation was observed. At low temperature as the precipitation ratio is higher, the enhancement in the yield was less. Thus it has been observed that the precipitation of aluminium
hydroxide in aluminate liquor under seeded condition with the addition of urea, the role of temperature is more significant than any other parameter. This is due to the fact that urea decomposition rate is faster at elevated temperature. 3.3. Effect of time variation Fig. 7 shows the variation of aluminium hydroxide concentration with time during the progress of precipitation period. As it is
Fig. 9. Scanning electron micrographs of aluminium hydroxides obtained at different conditions.
N.K. Sahu et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 815–822
160
Transmitance, %
noticed that under blank condition aluminium hydroxide concentration continuously decreased with time and it reached almost to a constant value after about 36 h of precipitation. The precipitation stops well above equilibrium solubility level of aluminium hydroxide indicating further possible precipitation. Alternatively, when 460 mmol/L urea was added into the system, it has been observed that at about 30 h it crosses the equilibrium solubility line indicating more than 100% precipitation under available supersaturation. Therefore to avail the equilibrium solubility level certain amount of urea addition is required. The effect would have been much more pronounced at still higher temperature.
821
Urea-1000 mMol
80
Blank
3.4. Characterization 0
The characteristics of the precipitated products were analyzed using XRD, SEM,TG-DTA, FTIR and BET surface area measurement tools. The XRD data (Fig. 8) of products showed completely gibbsitic type of precipitates similar to that are generally obtained under seeded precipitation conditions in Bayer facilities. The figure shows the XRD pattern of precipitates obtained under the case when urea was added. In all the other cases also the XRD patterns showed gibbsitic nature. Even in 8 h study the precipitates obtained are gibbsitic in nature. This indicates that using urea the product characteristics does not change which implies that downstream operations of such precipitates would be the same as in the plants. The scanning electron micrographs (SEM) showed (Fig. 9) highly coarser gibbsite precipitates in all the cases. The products obtained are compact and showed agglomerates of hexagonal platelets. The precipitates showed globular morphology of about 80 to more than 100 mm size. Similar observation was also made when only urea (Fig. 9d) was used for the decomposition of aluminate solution in absence of seed particles. The BET surface areas of particles were very low of about 1.76 m2/g indicating compactness of the particles with minimum voids. The BET surface area under all other conditions also showed similar results. The TGDTA (Fig. 10) data showed 34.6 wt.% loss of the precipitates at 1000 8C obtained under urea decomposition condition which is equivalent to wt. loss calculated for gibbsite particles. It has been observed that wt. loss is drastic between 300 and 340 8C. This drastic wt. loss is due to the escape of water of hydration from the gibbsite lattice. From DTA it is clear that at around 325 8C a minima or endothermic peak is observed as because the sample becomes cooler than the reference material as a consequence of heat absorbed by the process. This is also accompanied by phase transformation at this temperature. Calculating the wt. loss value it
16 8 15 6 4
13
2
12 11
0
10
-2
DTA
TGA
14
-4
9 0
200
400
600
800
1000
O
Temperature, C Fig. 10. TG-DTA plot of aluminium hydroxide precipitated at 75 8C under urea concentration of 1000 mmol.
1000
2000
Wave length,cm
3000
4000
-1
Fig. 11. FTIR spectroscopy of aluminium hydroxide under blank and urea conditions, A:C ratio 1.0, temperature 75 8C, time 24 h.
is confirmed that two water molecules are escaped from the gibbsite matrix leaving only one water molecule. This situation is exactly similar to boehmite (Al2O3H2O) conditions where only one molecule will be attached to the alumina matrix. A second endothermic peak was observed at 550 8C, which confirms the escape of other water molecule thereby forming a new phase of chi-alumina. Earlier results [33] also showed similar behaviour of gibbsitic materials. The FTIR data showed in Fig. 11 is similar to gibbsite particles. The OH-stretching bands are found to be at around 3400–3600 cm1, OH-bind mode at around 1000 cm1 and Al–O stretching bands at around 750 cm1. The data above showed the formation of gibbsite precipitates under urea addition conditions which are similar to Bayer precipitated gibbsite structure. 4. Conclusions The aluminium hydroxide precipitation ratio was substantially improved with the addition of urea in the seeded precipitation process in sodium aluminate solution. Temperature was found to have significant effect in aluminate solution decomposition. It is clear that the excess precipitation will take place due to the subsequent actions of urea decomposition products on aluminate solution. At higher temperature urea decomposition rate is faster and attends to early decomposition. This leads to more precipitation due to urea at higher temperature than lower temperatures. It should be kept in mind that at low temperature under blank condition more yields are obtained as supersaturation is higher, therefore availability of alumina to be precipitated as gibbsite is more. Therefore one has to compare the precipitation ratios at different temperatures on the basis of enhancement of gibbsite yield. In such cases at 80 8C yield enhancement ratio was found to be 142% compared to blank condition. Similar comparison at other temperatures showed yield enhancement ratios lower than 100%. Similarly under available supersaturation at 80 8C much higher precipitation rates were obtained at all concentrations of urea. The results obtained under A:C ratio variation showed interesting outcomes. The more the ratio generally more precipitation rates are obtained under blank condition (i.e., only seed addition). It has been observed therefore that precipitation ratio increased with A:C ratio from 22 to 29% under blank condition and 40 to 47% under 1000 mmol/L urea condition for 0.9 and 1.0, respectively. But yield enhancement ratios were increased with decrease in A:C ratio. It was found that at A:C ratio of 0.9 the precipitation ratio was 81%
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whereas 65% at 1.0 A:C ratio under 1000 mmol/L urea concentration. At other urea concentrations also similar trends are obtained. The aluminium hydroxides obtained under different conditions were gibbsitic in nature. The morphology showed globular type of precipitates consisting of agglomerates of hexagonal platelets. These are of typical gibbsitic morphology. TG analysis revealed about 34.6 wt. loss which is equivalent to gibbsite particles. DTA analysis showed an endothermic peak at 325 8C which is accompanied by escape of two water molecules from the gibbsitic matrix and therefore transforming to boehmite phase. A second endothermic peak was also observed at around 550 8C, which is accompanied by complete removal of hydration water and transforming to chi-alumina. When precipitation period was increased to 48 h under seed and urea condition, the concentration of aluminium hydroxide in sodium aluminate solution went beyond the equilibrium solubility range whereas under blank condition the aluminium hydroxide concentration in sodium aluminate solution remained constant much ahead of equilibrium solubility line. The mechanism of urea decomposition on the precipitation process was also discussed. Acknowledgements The authors acknowledge M/s Nalco, Bhubaneswar, for the partial financial support. Authors are grateful to the Director, CSIRInstitute of Minerals and Materials Technology, Bhubaneswar, for his keen interest and allow publishing this work. One of the authors NKS thanks Department of Science and Technology for providing him INSPIRE fellowship. References [1] Li Y, Zhang Y, Yang C, Zhang Y. Precipitating Sandy aluminium hydroxide from sodium aluminate solution by the neutralization of sodium bicarbonate. Hydrometallurgy 2009;98:52–7. [2] Li X. A preliminary discussion on the carbonation decomposition process. Light Met 1988;1:135–43. [3] Kirby C, Barley JA. Alumina from non-bauxite resources. Trav Com Int Etude Bauxite Alumine Alum 1981;79:15–20. [4] Abramov VYu, Alekseev AI, Badal’yants KhA. Complete processing of nepheline apatite raw material. Moscow: Metallurgiya; 1990. 392. [5] Hector Juarez MJ, Merced Martinez RJ, Manuel Ruvalcoba L, Oriano A, Vargas P, Juan Serrato R. Aluminium oxide and hydroxide from non-bauxite sources. Am Ceramic Soc Bull 1997;76:55–9. [6] Anjier JL, Breuer RG, Butler HL. Utilisation of partially calcined alumina as precipitation aid in the Bayer process, US Patent No. 4568527. [7] Halfon A, Kaliaguine S. Alumina trihydrate crystallisation. Part 1, Secondary nucleation and growth rate kinetics. Can J Chem Eng 1976;54:160–7. [8] Zhang B, Li J, Chen Q, Chen G. Precipitation of Al(OH)3 crystals from supersaturated sodium aluminate solution irradiated with ultrasonic sound. Miner Eng 2009;22:853–8. [9] Jishu Z, Zhoulan Y, Qiyuan C. Intensification of precipitation of gibbsite from seeded caustic aluminate liquor by seed activation and addition of crown ether. Hydrometallurgy 2007;89:107–16.
[10] Roe WJ, Perish JL. Use of polymers in alumina precipitation in Bayer process of bauxite beneficiation. US Patent No. 4608237. [11] Owen DO, Davis DC. Use of surfactants in alumina precipitation in Bayer process. US Patent No. 4737352. [12] Gontijo GS, de Araujo ACB, Prasad S, Vasconcelos LGS, Alves JJN, Brito P. Improving the Bayer process productivity – an industrial case study. Miner Eng 2009;22:1130–6. [13] Zeng JS, Yin ZL, Chen QY. Effect of tetracarbon additives on gibbsite precipitation from seeded sodium aluminate liquor. J Cent South Univ Technol 2008;15: 622–6. [14] Zhang Y, Zheng S, Du H, Xu H, Wang S, Zhang Y. Improved precipitation of gibbsite from sodium aluminate solution by adding methanol. Hydrometallurgy 2009;98:38–44. [15] Lu BL, Chen QY, Yin ZL, Hu HP. Effects of Na4EDTA and EDTA on seeded precipitation of sodium aluminate solution. Trans Non-Ferr Met Soc China 2010;20:37–41. [16] Barakat MA, El-sheikh SM, Farghly FE. Removing Al and regenerating caustic soda from spent washing liquor of Al etching. JOM 2005;1:34–8. August. [17] Barakat MA, El-sheikh SM, Farghly FE. Regeneration of spent alkali from aluminium washing. Sep Purif Technol 2005;46:214–8. [18] El-Katatny EA, Halawy SA, Mohamed MA, Zaki MI. A novel synthesis of high area alumina via H2O2-precipitated boehmite from sodium aluminate solution. J Chem Technol Biotechnol 1998;72:320–8. [19] Michael DS, Tseung-YT, Soo YL. Thermal decomposition of spherical hydrated basic aluminium sulphate. Am Ceramic Soc J 1984;63:301–10. [20] Subrt J, Stengl V, Bakardjieva S, Azatmary L. Synthesis of spherical metal oxide particles using homogenous precipitation of aqueous solutions of metal sulfates with urea. Powder Technol 2006;169:33–40. [21] Pyzalski M, Wojcik M, Urbanski R. Production of aluminium hydroxide and oxides with elevated purity in the direct, complex and without wastes treatment of the aqueous sodium aluminate solutions. Light Met 1992;1: 131–7. [22] Wojcik M, Pyzalski M. Factors effecting on the properties of Bayerite obtained in the carbonization process af alkaline aluminate solutions. Light Met 1990;1:161–5. [23] Soler-Illia G, Jobbagy M, Candal RJ, Regazzoni AE, Blesa MA, Dispers J. Synthesis of metal oxide particles from aqueous media: the homogenous alkalinisation method. Sci Technol 1998;19:207–28. [24] Lagana V. Process for ammonia production through urea hydrolysis. US Patent No. 5985224, November 1999. [25] Mavis B. Homogeneous precipitation of nickel hydroxide powders. Ames, IA: Iowa State University; 2003 (Ph.D thesis). [26] Shaw WHR, Bordeaux JJ. The decomposition of urea in aqueous media. J Am Chem Soc 1955;4729–34. [27] Lynn KR. Kinetics of base-catalised hydrolysis of urea. J Phys Chem 1965;69: 687–9. [28] Alexandrova AN, Jorgensen WL. Why urea eliminates ammonia rather than hydrolyses in aqueous solution. J Phys Chem 2007;111:720–30. [29] Kieke ML, Schoppelrei JW, Brill TB. Spectroscopy of hydrothermal reactions The CO2–H2O system and kinetics of urea decomposition in an FTIR spectroscopy flow reactor cess operable to 725K and 335 bar. J Phys Chem 1996;100: 7455–62. [30] Okazaki M, Funazukuri T. Decomposition of urea in sub- and supercritical water with/without additives. J Mater Sci 2008;43:2316–22. [31] Ebrahimian V, Nicolle A, Habchi C. Detailed modeling of the evaporation and thermal decomposition of urea-water-solution in SCR system. AICh E J 2012;58:1998–2009. July. [32] Rosenberg SP, Healy SJ. A thermodynamic model for gibbsite solubility in Bayer liquor. In: Proceedings of the fourth alumina quality workshop; 1996. p. 301–10. [33] Bhattacharya IN, Das SC, Mukherjee PS, Paul S, Mitra PK. Thermal decomposition of precipitated fine aluminium trihydrate. Scand J Metall 2004;204, 33:211–9.
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Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Effect of urea on decomposition of sodium aluminate solution N.K. Sahu a,b, C.K. Sarangi b, B.C. Tripathy a,b,*, I.N. Bhattacharya a,b, B.K. Satpathy c a
Academy of Scientific and Innovative Research (AcSIR), New Delhi, India CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, India c National Aluminium Company, Bhubaneswar, India b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 March 2013 Received in revised form 31 July 2013 Accepted 1 September 2013 Available online 10 October 2013
Aluminium hydroxide particles were synthesized from sodium aluminate liquor by urea decomposition method at elevated temperatures in presence and absence of seed. Different parameters such as alumina to caustic (A:C) ratio, temperature, time period of precipitation, urea concentration, etc. were studied. Characterisations of the products were also carried out using X-ray diffraction, scanning electron microscopy, TG-DTA, etc. In general, higher precipitation ratios were observed with the addition of urea in sodium aluminate solution. Temperature was found to have a significant role in precipitating aluminium hydroxide with higher yield enhancement ratio compared to that obtained at lower temperatures. Higher precipitation ratios were also obtained with urea at elevated temperatures under available supersaturation. In the presence of urea, the precipitation ratios on yield enhancement front were observed to increase with decrease in A:C ratio. It has been observed that the liquor concentration reached below the equilibrium solubility limit after 30 h of precipitation when 460 mmol/L urea is added. The synthesized aluminium hydroxide particles showed gibbsitic nature and globular morphology, with agglomerates of mostly hexagonal platelets, as revealed through XRD and SEM studies, respectively. Calcination of the product at 1000 8C results in weight loss similar to that observed with gibbsite. Endothermic peaks at 325 8C and 550 8C revealed through DTA study indicated boehmite and chi-alumina transformations, respectively. The possible mechanism of urea decomposition method is also discussed. ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Aluminium hydroxide Urea Gibbsite Sodium aluminate Equillibrium solubility Supersaturation
1. Introduction Both the Bayer and the sintering [1,2] processes are used for the production of alumina via decomposition of metastable sodium aluminate solution. Bayer process mainly utilizes gibb bauxite ores through NaOH digestion for the production of sodium aluminate solution. Whereas in sintering process low grade diasporic/ boehmitic bauxite or nepheline or other alumino silicates [1,3,4] are used by sintering it with sodium carbonate at high temperatures to form sodium aluminate solution after leaching the sintered mass. In the Bayer process sodium aluminate is decomposed through seeded precipitation method to obtain aluminium hydroxide whereas carbonization decomposition method is used for its recovery from leach solution. In Bayer method some of the sodium hydroxide transformed to sodium carbonate due to atmospheric CO2. This carbonate is in turn treated with lime to get back sodium hydroxide before being recycled for
* Corresponding author at: CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, India. Tel.: +91 9861085358; fax: +91 674 2581637. E-mail addresses: [email protected], [email protected] (B.C. Tripathy).
further bauxite digestion. Generally in industries during precipitation of gibbsite in Bayer circuit, the alumina concentration could not be brought down to equilibrium solubility limit. However, in carbonization decomposition method more than 90% alumina is precipitated. As a result, the alumina concentration in the liquor goes well below equilibrium solubility limit. To enhance the precipitation ratio in Bayer aluminate decomposition process various efforts were made [5–18]. Activation [5–9] of seed hydrate is one of such methods but not yet commercialized due to additional steps and higher energy consumption. Addition of different polymers, surfactants, inorganic and organic additives [10–18] did not show encouraging results. Moreover, these additives would spoil the solution to a great extent as unwanted cations and anions are introduced which accumulate with time due to the cyclic nature of the Bayer process. In this paper an attempt has been made to evaluate the effect of urea (NH2CONH2) in sodium aluminate liquor and the extent to which it can be utilized in decomposition of aluminate liquor under seeding condition. The decomposition of aluminate liquor is related to the supersaturation (Al2O3: Na2O, i.e. A:C ratio and temperature) level of the solution. However, the excessive super saturation, particularly greater than 1.05 (A:C) or so, leads to the instability of the solution. This results in precipitation of hydroxide
1876-1070/$ – see front matter ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jtice.2013.09.001
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at various inappropriate locations during the operation leading to the loss of alumina and damage to the various equipment [12]. Therefore, excessive supersaturation should be avoided. Further, the release of hydroxyl ion (OH) into the system during hydrolysis leads to the slowing down or finally stoppage of further hydrolysis. Consequently, NaOH concentration in the solution increases leading to low A:C ratio, i.e., supersaturation falls to a lower value. The following equation shows the decomposition of aluminate ion. AlðOHÞ4 ! AlðOHÞ3 þ OH
AlðOHÞ4 ! AlðOHÞ3 þ OH
(8)
Naþ þ OH ! NaOH
(9)
(1)
To enhance the hydrolysis of aluminate liquor, OH liberated should be consumed so as to continue the hydroxide precipitation process. Urea is generally used for precipitating spherical hydrated basic aluminium salts, oxides etc. and high pure specialty alumina by utilizing it as hydrolysing precipitant for aluminium salt solutions at elevated temperatures [19,20]. As it slowly releases NH4+, CO2 and hydroxyl ions at elevated temperatures, the pH of the solution increases and results in the precipitation of spherical basic aluminium salts. Use of urea as an additive for aluminium hydroxide precipitation would be advantageous in many respect; (i) the product would be free from any contamination; (ii) urea is cheap and available on plenty; (iii) urea is an ecofriendly chemical. In the present study at elevated temperature urea decomposes to give NH3 and CO2 and in this case CO2 is mainly responsible for hydrolysis of aluminate solution. This process may be analogous to carbonization decomposition process. But the later carbonization decomposition is utilized for total decomposition of the solution whereas the idea in this study is to find an extent to which urea can be utilized in addition to seed effect because in all the Bayer plants conversion of sodium carbonate to sodium hydroxide is carried out routinely. 1.1. Conceptual mechanism of urea assisted aluminium hydroxide precipitation The carbonization decomposition process [21,22] is one of the major steps in sintering process for producing aluminium hydroxide. In carbonization decomposition process CO2 gas is introduced into sodium aluminate solution to precipitate aluminium hydroxide. There are various mechanisms of precipitation of aluminium hydroxide in carbonization process discussed by Li [2]. The carbon dioxide when added to aluminate liquor either neutralizes (Eq. (2)) sodium hydroxide present in the solution thereby increasing the A:C ratio which leads to continuous precipitation or direct decomposition of sodium aluminate to aluminium hydroxide (Eq. (3)). 2NaOH þ CO2 ¼ Na2 CO3 þ H2 O
As the concentration of OH decreases, the polymerization of aluminate species increases and forms aluminium hydroxide precipitates. As regards seeded precipitation method, sodium aluminate solution decomposes under seeded condition where seed provides the surface onto which new particles nucleates and grows. The hydrolysis reaction taking place as follows, producing Al(OH)3 and NaOH.
(2)
Generation of NaOH as per the above reaction (9) reduces the A:C ratio continuously, thus decreasing the super saturation level to such an extent where further precipitation is stopped. In case OH is consumed by some other means, the supersaturation level stabilizes producing continuous precipitation. The use of urea to precipitate aluminium hydroxide from sodium aluminate liquor was not reported earlier. It has been discussed in literature [23–25] that urea dissociation in aqueous system produces ultimately NH3 and CO2 but various mechanisms occur before the formation of NH3 and CO2. As our system is consisting of sodium aluminate, sodium hydroxide and H2O, the reaction pathway may not follow a simpler rule like in pure aqueous system. At neutral pH, activation energy for urea decomposition varies from 28.4 to 32.4 kcal/mol [26] and in alkaline medium the activation energy calculated was 22 kcal/mol [27]. Therefore under alkaline condition decomposition of urea is easier than in neutral pH. In aqueous system urea decomposes at an elevated temperature to cyanate and ammonium ions [28–30]. This cyanate ion then hydrolyses to carbamate anion which subsequently hydrolyses to bicarbonate ion [31] as per the following equations. NH2 CO NH2 ! CNO þ NH4þ
(10)
CNO þ H2 O ! NH2 CO2
(11)
NH2 CO2 þ H2 O ! NH3 þ HCO3
(12)
HCO3 then reacts with OH ion to produce CO3= as follows HCO3 þ OH ¼ H2 O þ CO3 ¼
(13)
2Naþ þ CO3 ¼ ¼ Na2 CO3
(14)
2NaAlðOHÞ4 þ CO2 ¼ Na2 CO3 þ 2AlðOHÞ3 þ H2 O
(3)
In another opinion [2] CO2 first forms carbonic acid which then decomposes sodium aluminate to from aluminium hydroxide and sodium carbonate. 2NaAlðOHÞ4 þ H2 CO3 ¼ 2AlðOHÞ3 þ Na2 CO3 þ 2H2 O
(4)
2NaOH þ H2 CO3 ¼ Na2 CO3 þ 2H2 O
(5)
According to Besuzin as mentioned by Li [2] carbon dioxide first reacts with OH ion and reduces the activity of OH ion as follows, OH þ CO2 ¼ HCO3
(6)
HCO3 þ OH ¼ H2 O þ CO3 ¼
(7)
Thus OH concentration in the solution decreases with enhanced aluminium hydroxide precipitation ratio along with the formation of sodium carbonate. 2. Experimental 2.1. Chemicals and apparatus The chemicals used in this study are of analytical grade and obtained from Merck, India. Supersaturated sodium aluminate solution was prepared by dissolving measured quantity of aluminium granules in sodium hydroxide solution. In the present study the mass ratio of Al2O3 to Na2O (A:C) was varied from 0.9 to 1.0. During the precipitation study, temperature was maintained between 608 to 80 0.1 8C. The gibbsite seed (d50 = 62.2 mm) used for this study was obtained from NALCO, Bhubaneswar, India.
N.K. Sahu et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 815–822
The apparatus used for the precipitation studies was made from 304 stainless steel of 500 mL capacity which was fitted with a digital RPM controller. An anchor type impeller was used for the agitation of the solution. The rotation of the impeller was set at 190 2 revolutions per minute (RPM). The temperature of the system was maintained through a constant thermostatic bath circulator (JULABO, Germany). 2.2. Method All the experiments were conducted using 250 mL of sodium aluminate solution. In all the cases the caustic concentration was kept at 150 g/L and the requisite A:C ratio was maintained by varying Al2O3 concentration only. Dilute solutions were prepared by maintaining A:C ratio of 0.95 with Na2O concentration of 50 g/ L and Al2O3 concentration of 47.5 g/L. The time period of the precipitation study was maintained according to the requirement of the experiment. The seed and other additions were made once the desired temperature was attained. After each experiment the slurry was filtered and the residue was washed several times before it was dried in an oven at 105 8C. The aluminium hydroxide yield was calculated in g/L, yield in g/L = [(wt. of the precipitate wt. of the seed)/experimental volume of the solution] 1000.
Yield in gL
1
¼
wt: of the precipitate wt: of the seed 1000: experimental volume of the solution
In this study three different precipitation ratios were analyzed and calculated as shown below
Precipitation ratio ð%Þ ¼
Yield ðgL1 Þ Initial AlðOHÞ3 concentration in solution ðgL1 Þ
100
817
2.3. The equilibrium solubility The equilibrium solubility of sodium aluminate solution is the saturation point of the solution below which level precipitation is improbable at a particular temperature; only available supersaturation can practically be precipitated under seeded precipitation process. The equilibrium solubility of alumina in the sodium aluminate solution was measured using well known Rosenberg & Healy [32] formula as follows: " Aeqm ¼ 0:96197 C 1 þ
pffi pffi 3=2 #1 10½ðð9:2082 IÞ=ð1þ IÞÞþð0:2149I Þ expðDG=RTÞ
where, ionic strength I = 0.01887 C + 0.01937 SC; where C and SC are caustic and carbonate concentrations respectively, both in g/L of Na2CO3; DG = Gibbs free energy of solid formation = 30.96 kJ/mol; R = universal gas constant = 8.3145 J/K/ mol; T = precipitation temperature (K). Using the above equation, the equilibrium solubility of alumina at 75 8C (A75) was calculated to be 88 g/L when Na2O concentration was 150 g/L. Therefore, at A:C = 1.0, where the initial Al2O3 concentration in the solution is 150 g/L, The available supersaturation = 150– 88 = 62 g/L as Al2O3 or 95 g/L as aluminium hydroxide. Similarly, at A:C = 0.95, where the initial Al2O3 concentration in the solution is 142.5 g/L, the available supersaturation = 142.5 88 = 54.5 g/L as Al2O3 or 83.35 g/L as aluminium hydroxide. The calculated values of equilibrium solubilities at different temperatures with aluminium hydroxide concentrations at different A:C ratios have been given in Table 1. Some of the figures (Figs. 3 and 6) are plotted taking these data for calculation of precipitation rate under available supersaturation. 3. Results and discussion
Yield enhancement ratio ð%Þ ¼
Yield ðgL1 Þ with urea yield in blank ðgL1 Þ Yield in blank ðgL1 Þ 100
Precipitation ratio under available supersaturation ð%Þ ¼
Yield ðgL1 Þ Super saturation level ðgL1 Þ
The effect of urea on decomposition of aluminate liquor has been discussed in this work. The parameters varied are alumina:caustic ratio, temperature, time period of precipitation and urea concentration. The precipitation ratio depends mainly on decomposition of urea. The morphology of the precipitated product has also been discussed in this chapter. The main parameters measured in this study are precipitation ratio, precipitation ratio under yield enhancement and precipitation ratio under available supersaturation at different temperatures, A:C ratio and urea
100
*‘‘blank’’ specifies the condition when precipitation was carried out in the absence of additives. It should be kept in mind that yield under blank condition changes with solution concentration, temperature and A:C ratio. X-ray diffractograms were recorded for the aluminium hydroxide powders using PANalytical diffractometer (PW 1830, Philips, Japan) with Cu Ka radiation, l = 1.54056 A˚. The scans were recorded in 2u range 0–808. Fourier transform infrared (FTIR) spectrographs were recorded on a Nicolet 6070 spectrophotometer in the frequency range 400–4000 cm1. A scanning electron microscope (SEM) (JEOL JSM 6510, Japan) was used to examine the surface morphology of the aluminium hydroxide. Brunauer Emmett Teller (BET) surface area measurements were carried out using Autosorb-iQ (Quantachrome, USA). TG-DTA study was carried out using TGA/SDTA851e (Mettler-Toledo, USA).
Table 1 Calculation of available super saturation using Rosenberg and Healy method. (A) Equilibrium solubility at different temperatures (Na2O concentration = 150 g/L) Temperature (8C)
[Al(OH)3] (g/L) (x)
60 65 70 75 80
96.35 108.6 121.6 134.6 148.05
(B) Aluminium hydroxide concentration at different A:C ratios. A:C ratio
[Al(OH)3] (g/L) (y)
1.0 0.97 0.95 0.93 0.90
229.41 222.53 217.94 213.35 206.47
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818
60
Precipitation ratio, %
650 mmol
40
460 mmol 65 mmol
Blank
20
Yield enhancement rate, %
80 1000 mmol
1000 mmol 650 mmol
40
460 mmol 325 mmol 65 mmol
0
0.90
0.92
0.94
0.96
0.98
0.92
1.00
A:C ratio Fig. 1. Variation of precipitation ratio with A:C ratio at different urea concentrations, temperature 75 8C, time 24 h.
concentrations. Supersaturation is the measure of alumina or aluminium hydroxide concentration in excess of equilibrium solubility. In case equilibrium solubility of aluminium hydroxide at a particular temperature and soda concentration is x and initial hydroxide concentration in the liquor is y, then the available super saturation is equal to (y x). Equilibrium solubility is the minimum level of concentration up to which precipitation is possible under standard condition in regard to seeded precipitation process. 3.1. Effect of Al2O3: Na2O (A:C) ratio Fig. 1 shows the effect of A:C ratio on precipitation ratio when different quantities of urea were added at 75 8C. It has been shown that at higher quantities of urea the aluminate liquor decomposition is increased at every level of A:C ratio. This is due to availability of more urea decomposition products. Further, it has been observed that the slope of the curves are almost equal with upward inclination indicating more precipitation at higher A:C ratios. Considering the individual plots, as same amounts of urea decomposition products are available at different A:C ratios, the low supersaturation level did not produce same quantity of aluminium hydroxide particles as produced in highly supersaturated solutions. As in high supersaturated solution, the quantity of aluminate ions are more compared to that of low supersaturated solution and soda concentration is same in both the cases, the urea decomposition products mostly reacted with OH1 released during aluminate ion decomposition process. The HCO31, the urea decomposition product, reacted with OH1 forming CO3= and water molecule as shown in equation 13. When aluminium hydroxide yield enhancement ratio was plotted against A:C ratio (Fig. 2) it has been observed that yield enhancement decreased with A:C ratio in general at all urea concentration levels. The exception found at 65 mmol/L may be due to the fact that at such low concentration the effect of urea is not significant. In this case at low A:C ratio the curves are wider than at A:C ratio of 1.0. If we take two extreme cases when 1000 mmol/L urea was added, the enhanced yield ratios were found to be 65.0 and 81.16% for A:C ratios of 1.0 and 0.9, respectively. More than 16% yield enhancement was observed at lower A:C ratio than at higher ratio. Similar increased yield enhancement ratios were observed at other urea quantities at lower A:C ratios. This happened due to the fact that at higher A:C ratio under blank condition the yields are more than at lower A:C ratio. Therefore yield enhancement is lower at higher A:C ratios. Further, when precipitation ratio under available supersaturation was plotted against A:C ratios (Fig. 3) at different
0.94
0.96
0.98
1.00
A:C ratio Fig. 2. Variation of yield enhancement ratio with A:C ratio at different urea concentrations, temperature 75 8C, time 24 h.
urea concentrations, it has been found that precipitation ratio remains almost same at different urea concentrations though it has increased substantially from low concentration urea to higher concentration of urea. The graph shows more than or equal to 100% precipitation under available super saturation when higher quantities of urea was added, which indicates that the precipitation ratio under available supersaturation goes beyond equilibrium solubility range with the additional urea. This is because of the availability of more urea decomposition products. But at low urea concentration, precipitation ratio under available super saturation is less than equilibrium composition of aluminium hydroxide. At A:C ratio 1.0 (Table 1) the available super saturation at 75 8C is 94.81 g/L (229.41–134.6) of aluminium hydroxide, whereas precipitation obtained was around 112 g/L, this is therefore 18% more than the equilibrium solubility level. In nutshell, the effect of A:C ratio on precipitation of aluminium hydroxide can be explained as follows; it is obvious that when A:C ratio is increased, the hydroxide yield (or precipitation ratio) will also increase because of higher supersaturation level which is reflected in Fig. 1 (blank). When urea was added precipitation ratio also increased in all levels of A:C ratio because of decomposition of urea at 75 8C. Whereas when yield enhancement is concerned, the yield enhancement is increased with decrease in A:C ratio baring the case of 65 mmol/L urea concentration. The precipitation ratio under available supersaturation is remained almost same at higher urea concentrations which is almost 100% or beyond with 650 mmol/L or more addition of urea.
120
Precipitation ratio under avilable supersaturation, %
0.90
1000 m m ol 650 m m ol
100 460 m m ol 325 m m ol
80
65 m m ol Blank
60
0.90
0.92
0.94
0.96
0.98
1.00
A:C ratio Fig. 3. Variation of precipitation ratio under available supersaturation with A:C ratio at different urea concentrations, temperature 75 8C, time 24 h.
N.K. Sahu et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 815–822
Precipitation ratio under available supersaturation, %
Precipitation ratio, %
60
1000 mmol 650 mmol
40
325 mmol 65 mmol Blank
20
819
160
1000 m m ol
650 m m ol 120
325 m m ol 65 m m ol 80 Blank
40 60
65
70
75
80
85
Temperature,O C 60
65
70
75
80
85
Fig. 6. Variation of precipitation ratio under available supersaturation with temperature at different urea concentrations, A:C ratio 0.95, time 24 h.
O
Temperature , C Fig. 4. Variation of precipitation ratio with temperature at different urea concentrations, A:C ratio 0.95, time 24 h.
The effect of temperature was studied within the range 60– 80 8C, as urea decomposition rate depends on temperature. The effect of temperature on the variation of precipitation ratio at different concentration of urea was found to be very interesting (Fig. 4). It was found that in the case of urea, generally the precipitation ratio increased at all the concentrations however the increase in the ratio is more at higher urea concentrations. It has also been observed in the case of blank solution that with increase in temperature the precipitation ratio decreased continuously as at higher temperature supersaturation level is low. Similarly in the presence of various concentrations of urea the precipitation ratio decreased continuously but at 80 8C it showed an increasing trend. This is due to the fact that urea decomposition rate is very high at higher temperature leading to generation of higher amount of precipitates. As regards yield enhancement ratio (Fig. 5) is concerned it showed substantial enhancement at higher temperatures but at low temperature only around 1–13% enhancement was noticed at different levels of urea addition. Further it has been found that at higher temperature (80 8C), 43–142% enhancement was noticed when urea concentration was increased from 65 to 1000 mmol/L. This enhancement is substantial. Similar behaviour was noticed when precipitation ratio under available super saturation was plotted against temperature at different urea
160
Yield enhancement ratio, %
1000m m ol 120 650m m ol
200 180 Blank
160 140
Urea
Equilibrium solubility
120 100 0
10
20
30
40
50
Time, h Fig. 7. Variation of aluminium hydroxide concentration in the solution with time, A:C ratio 0.95, temperature 75 8C.
20
40
60 U re a - 1 0 0 0 m m o l
intensity,a.u
3.2. Effect of temperature
Aluminium hydroxide, g/L
220
B la n k
80 325m m ol 65 m m ol
40
0 60
65
70
75
80
85
Temperature,O C Fig. 5. Variation of yield enhancement ratio with temperature at different urea concentrations, A:C ratio 0.95, time 24 h.
20
40 o Position 2 Theta
60
Fig. 8. X-ray diffractogram of aluminium hydroxide obtained under urea added at 1000 mmol and blank conditions.
820
N.K. Sahu et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 815–822
concentrations (Fig. 6). The general trend under blank condition is reduction of precipitation ratio with temperature but at 80 8C a stiff jump in precipitation is noticed at all levels of urea concentrations. At 80 8C, 60–147% precipitation was observed under available super saturation from blank to 1000 mmol/L of urea addition respectively but at 60 8C under similar condition only 83–94% precipitation was observed. At low temperature as the precipitation ratio is higher, the enhancement in the yield was less. Thus it has been observed that the precipitation of aluminium
hydroxide in aluminate liquor under seeded condition with the addition of urea, the role of temperature is more significant than any other parameter. This is due to the fact that urea decomposition rate is faster at elevated temperature. 3.3. Effect of time variation Fig. 7 shows the variation of aluminium hydroxide concentration with time during the progress of precipitation period. As it is
Fig. 9. Scanning electron micrographs of aluminium hydroxides obtained at different conditions.
N.K. Sahu et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 815–822
160
Transmitance, %
noticed that under blank condition aluminium hydroxide concentration continuously decreased with time and it reached almost to a constant value after about 36 h of precipitation. The precipitation stops well above equilibrium solubility level of aluminium hydroxide indicating further possible precipitation. Alternatively, when 460 mmol/L urea was added into the system, it has been observed that at about 30 h it crosses the equilibrium solubility line indicating more than 100% precipitation under available supersaturation. Therefore to avail the equilibrium solubility level certain amount of urea addition is required. The effect would have been much more pronounced at still higher temperature.
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3.4. Characterization 0
The characteristics of the precipitated products were analyzed using XRD, SEM,TG-DTA, FTIR and BET surface area measurement tools. The XRD data (Fig. 8) of products showed completely gibbsitic type of precipitates similar to that are generally obtained under seeded precipitation conditions in Bayer facilities. The figure shows the XRD pattern of precipitates obtained under the case when urea was added. In all the other cases also the XRD patterns showed gibbsitic nature. Even in 8 h study the precipitates obtained are gibbsitic in nature. This indicates that using urea the product characteristics does not change which implies that downstream operations of such precipitates would be the same as in the plants. The scanning electron micrographs (SEM) showed (Fig. 9) highly coarser gibbsite precipitates in all the cases. The products obtained are compact and showed agglomerates of hexagonal platelets. The precipitates showed globular morphology of about 80 to more than 100 mm size. Similar observation was also made when only urea (Fig. 9d) was used for the decomposition of aluminate solution in absence of seed particles. The BET surface areas of particles were very low of about 1.76 m2/g indicating compactness of the particles with minimum voids. The BET surface area under all other conditions also showed similar results. The TGDTA (Fig. 10) data showed 34.6 wt.% loss of the precipitates at 1000 8C obtained under urea decomposition condition which is equivalent to wt. loss calculated for gibbsite particles. It has been observed that wt. loss is drastic between 300 and 340 8C. This drastic wt. loss is due to the escape of water of hydration from the gibbsite lattice. From DTA it is clear that at around 325 8C a minima or endothermic peak is observed as because the sample becomes cooler than the reference material as a consequence of heat absorbed by the process. This is also accompanied by phase transformation at this temperature. Calculating the wt. loss value it
16 8 15 6 4
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2
12 11
0
10
-2
DTA
TGA
14
-4
9 0
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400
600
800
1000
O
Temperature, C Fig. 10. TG-DTA plot of aluminium hydroxide precipitated at 75 8C under urea concentration of 1000 mmol.
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2000
Wave length,cm
3000
4000
-1
Fig. 11. FTIR spectroscopy of aluminium hydroxide under blank and urea conditions, A:C ratio 1.0, temperature 75 8C, time 24 h.
is confirmed that two water molecules are escaped from the gibbsite matrix leaving only one water molecule. This situation is exactly similar to boehmite (Al2O3H2O) conditions where only one molecule will be attached to the alumina matrix. A second endothermic peak was observed at 550 8C, which confirms the escape of other water molecule thereby forming a new phase of chi-alumina. Earlier results [33] also showed similar behaviour of gibbsitic materials. The FTIR data showed in Fig. 11 is similar to gibbsite particles. The OH-stretching bands are found to be at around 3400–3600 cm1, OH-bind mode at around 1000 cm1 and Al–O stretching bands at around 750 cm1. The data above showed the formation of gibbsite precipitates under urea addition conditions which are similar to Bayer precipitated gibbsite structure. 4. Conclusions The aluminium hydroxide precipitation ratio was substantially improved with the addition of urea in the seeded precipitation process in sodium aluminate solution. Temperature was found to have significant effect in aluminate solution decomposition. It is clear that the excess precipitation will take place due to the subsequent actions of urea decomposition products on aluminate solution. At higher temperature urea decomposition rate is faster and attends to early decomposition. This leads to more precipitation due to urea at higher temperature than lower temperatures. It should be kept in mind that at low temperature under blank condition more yields are obtained as supersaturation is higher, therefore availability of alumina to be precipitated as gibbsite is more. Therefore one has to compare the precipitation ratios at different temperatures on the basis of enhancement of gibbsite yield. In such cases at 80 8C yield enhancement ratio was found to be 142% compared to blank condition. Similar comparison at other temperatures showed yield enhancement ratios lower than 100%. Similarly under available supersaturation at 80 8C much higher precipitation rates were obtained at all concentrations of urea. The results obtained under A:C ratio variation showed interesting outcomes. The more the ratio generally more precipitation rates are obtained under blank condition (i.e., only seed addition). It has been observed therefore that precipitation ratio increased with A:C ratio from 22 to 29% under blank condition and 40 to 47% under 1000 mmol/L urea condition for 0.9 and 1.0, respectively. But yield enhancement ratios were increased with decrease in A:C ratio. It was found that at A:C ratio of 0.9 the precipitation ratio was 81%
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whereas 65% at 1.0 A:C ratio under 1000 mmol/L urea concentration. At other urea concentrations also similar trends are obtained. The aluminium hydroxides obtained under different conditions were gibbsitic in nature. The morphology showed globular type of precipitates consisting of agglomerates of hexagonal platelets. These are of typical gibbsitic morphology. TG analysis revealed about 34.6 wt. loss which is equivalent to gibbsite particles. DTA analysis showed an endothermic peak at 325 8C which is accompanied by escape of two water molecules from the gibbsitic matrix and therefore transforming to boehmite phase. A second endothermic peak was also observed at around 550 8C, which is accompanied by complete removal of hydration water and transforming to chi-alumina. When precipitation period was increased to 48 h under seed and urea condition, the concentration of aluminium hydroxide in sodium aluminate solution went beyond the equilibrium solubility range whereas under blank condition the aluminium hydroxide concentration in sodium aluminate solution remained constant much ahead of equilibrium solubility line. The mechanism of urea decomposition on the precipitation process was also discussed. Acknowledgements The authors acknowledge M/s Nalco, Bhubaneswar, for the partial financial support. Authors are grateful to the Director, CSIRInstitute of Minerals and Materials Technology, Bhubaneswar, for his keen interest and allow publishing this work. One of the authors NKS thanks Department of Science and Technology for providing him INSPIRE fellowship. References [1] Li Y, Zhang Y, Yang C, Zhang Y. Precipitating Sandy aluminium hydroxide from sodium aluminate solution by the neutralization of sodium bicarbonate. Hydrometallurgy 2009;98:52–7. [2] Li X. A preliminary discussion on the carbonation decomposition process. Light Met 1988;1:135–43. [3] Kirby C, Barley JA. Alumina from non-bauxite resources. Trav Com Int Etude Bauxite Alumine Alum 1981;79:15–20. [4] Abramov VYu, Alekseev AI, Badal’yants KhA. Complete processing of nepheline apatite raw material. Moscow: Metallurgiya; 1990. 392. [5] Hector Juarez MJ, Merced Martinez RJ, Manuel Ruvalcoba L, Oriano A, Vargas P, Juan Serrato R. Aluminium oxide and hydroxide from non-bauxite sources. Am Ceramic Soc Bull 1997;76:55–9. [6] Anjier JL, Breuer RG, Butler HL. Utilisation of partially calcined alumina as precipitation aid in the Bayer process, US Patent No. 4568527. [7] Halfon A, Kaliaguine S. Alumina trihydrate crystallisation. Part 1, Secondary nucleation and growth rate kinetics. Can J Chem Eng 1976;54:160–7. [8] Zhang B, Li J, Chen Q, Chen G. Precipitation of Al(OH)3 crystals from supersaturated sodium aluminate solution irradiated with ultrasonic sound. Miner Eng 2009;22:853–8. [9] Jishu Z, Zhoulan Y, Qiyuan C. Intensification of precipitation of gibbsite from seeded caustic aluminate liquor by seed activation and addition of crown ether. Hydrometallurgy 2007;89:107–16.
[10] Roe WJ, Perish JL. Use of polymers in alumina precipitation in Bayer process of bauxite beneficiation. US Patent No. 4608237. [11] Owen DO, Davis DC. Use of surfactants in alumina precipitation in Bayer process. US Patent No. 4737352. [12] Gontijo GS, de Araujo ACB, Prasad S, Vasconcelos LGS, Alves JJN, Brito P. Improving the Bayer process productivity – an industrial case study. Miner Eng 2009;22:1130–6. [13] Zeng JS, Yin ZL, Chen QY. Effect of tetracarbon additives on gibbsite precipitation from seeded sodium aluminate liquor. J Cent South Univ Technol 2008;15: 622–6. [14] Zhang Y, Zheng S, Du H, Xu H, Wang S, Zhang Y. Improved precipitation of gibbsite from sodium aluminate solution by adding methanol. Hydrometallurgy 2009;98:38–44. [15] Lu BL, Chen QY, Yin ZL, Hu HP. Effects of Na4EDTA and EDTA on seeded precipitation of sodium aluminate solution. Trans Non-Ferr Met Soc China 2010;20:37–41. [16] Barakat MA, El-sheikh SM, Farghly FE. Removing Al and regenerating caustic soda from spent washing liquor of Al etching. JOM 2005;1:34–8. August. [17] Barakat MA, El-sheikh SM, Farghly FE. Regeneration of spent alkali from aluminium washing. Sep Purif Technol 2005;46:214–8. [18] El-Katatny EA, Halawy SA, Mohamed MA, Zaki MI. A novel synthesis of high area alumina via H2O2-precipitated boehmite from sodium aluminate solution. J Chem Technol Biotechnol 1998;72:320–8. [19] Michael DS, Tseung-YT, Soo YL. Thermal decomposition of spherical hydrated basic aluminium sulphate. Am Ceramic Soc J 1984;63:301–10. [20] Subrt J, Stengl V, Bakardjieva S, Azatmary L. Synthesis of spherical metal oxide particles using homogenous precipitation of aqueous solutions of metal sulfates with urea. Powder Technol 2006;169:33–40. [21] Pyzalski M, Wojcik M, Urbanski R. Production of aluminium hydroxide and oxides with elevated purity in the direct, complex and without wastes treatment of the aqueous sodium aluminate solutions. Light Met 1992;1: 131–7. [22] Wojcik M, Pyzalski M. Factors effecting on the properties of Bayerite obtained in the carbonization process af alkaline aluminate solutions. Light Met 1990;1:161–5. [23] Soler-Illia G, Jobbagy M, Candal RJ, Regazzoni AE, Blesa MA, Dispers J. Synthesis of metal oxide particles from aqueous media: the homogenous alkalinisation method. Sci Technol 1998;19:207–28. [24] Lagana V. Process for ammonia production through urea hydrolysis. US Patent No. 5985224, November 1999. [25] Mavis B. Homogeneous precipitation of nickel hydroxide powders. Ames, IA: Iowa State University; 2003 (Ph.D thesis). [26] Shaw WHR, Bordeaux JJ. The decomposition of urea in aqueous media. J Am Chem Soc 1955;4729–34. [27] Lynn KR. Kinetics of base-catalised hydrolysis of urea. J Phys Chem 1965;69: 687–9. [28] Alexandrova AN, Jorgensen WL. Why urea eliminates ammonia rather than hydrolyses in aqueous solution. J Phys Chem 2007;111:720–30. [29] Kieke ML, Schoppelrei JW, Brill TB. Spectroscopy of hydrothermal reactions The CO2–H2O system and kinetics of urea decomposition in an FTIR spectroscopy flow reactor cess operable to 725K and 335 bar. J Phys Chem 1996;100: 7455–62. [30] Okazaki M, Funazukuri T. Decomposition of urea in sub- and supercritical water with/without additives. J Mater Sci 2008;43:2316–22. [31] Ebrahimian V, Nicolle A, Habchi C. Detailed modeling of the evaporation and thermal decomposition of urea-water-solution in SCR system. AICh E J 2012;58:1998–2009. July. [32] Rosenberg SP, Healy SJ. A thermodynamic model for gibbsite solubility in Bayer liquor. In: Proceedings of the fourth alumina quality workshop; 1996. p. 301–10. [33] Bhattacharya IN, Das SC, Mukherjee PS, Paul S, Mitra PK. Thermal decomposition of precipitated fine aluminium trihydrate. Scand J Metall 2004;204, 33:211–9.