Ind. Eng. Chem. Res. 1997, 36, 2657-2661
2657
Effects of the Impurities on the Habit of Gypsum in Wet-Process Phosphoric Acid Li Jun,* Wang JianHua, and Zhang YunXiang Department of Chemical Engineering, SiChuan Union University, ChengDu, China 610065
There is a poor understanding of the effect of impurities in phosphate rock on the crystal habit of gypsum during wet-process phosphoric acid production. The present work studies the effects of Al3+, SiF62-, and F- on the habit of gypsum in an MSMPR crystallizer. It is found that the gypsum crystal shape changes little in the presence of Al3+ alone. In the presence of both Al3+ and F-, the shape of the gypsum crystal changes greatly and forms sphere-shaped aggregates instead of the elongated needles formed in the presence of Al3+. In the presence of F-, Al3+, and SiF62- ions, the gypsum crystal shape is similar to that in the presence of both F- and Al3+ ions. In addition, the effects of Al3+, SiF62-, and F- on the crystallization kinetics of gypsum are also investigated. 1. Introduction The effects of impurities in phosphate rock on gypsum crystallization are quite complicated during H3PO4 production by the wet-process. Although considerable work on the effects of ions had been carried out for many years, there were no identical results. The effects of F-, SiF62-, and Al3+ ions on gypsum crystallization are different according to the work by different researchers. For example, Gilbert (1966) studied the effects of Al3+ on gypsum crystallization. He found that the crystal size of gypsum reduced in the presence of Al3+. The effect was more pronounced in the solution containing 27.7% P2O5. Orenga (1983) examined the effect of Al2O3 on gypsum crystallization. He found that the length of a needlelike crystal of gypsum decreased, but the width did not increase. Becker (1989) pointed out that Al3+ was conducive to the formation of rhombic crystals. In summarizing previous work on the effect of ions, David (1990) studied the effect of Fe3+ and Al3+ ions on the crystallization of gypsum. He found that Fe3+ and Al3+ exerted similar effects at the same molar concentration. At low concentrations the ions increased the average size of the gypsum crystals. At high ion concentrations the average crystal size decreased. Jerzy et al. (1986) studied the effect of Al3+ on gypsum crystallization from water solution. They found that the gypsum aggregates occurred and the average crystal size increased in the presence of Al3+. Sarig and Mullin (1982) examined the effect of Al3+ and F- on the precipitation of gypsum from water solution. The results showed that the induction time increased with AlF3 concentration, indicating that the nucleation rate decreased, but when the AlF3 concentration was increased to a certain value, the induction time decreased with AlF3 concentration, indicating that the nucleation time increased. Moreover, they also studied the effect of Al3+ on the gypsum crystallization; the results obtained were similar to those in the presence of both Al3+ and F- ions, but the crystal shape was different. Slack (1968) said that the gypsum crystal shape changed greatly and its size was smaller in the presence of great amounts of Al3+. One of the aims of the present * To whom correspondence should be addressed. S0888-5885(96)00422-8 CCC: $14.00
Figure 1. Experimental apparatus: (1) mixed solution feed of sulfuric and phosphoric acid; (2) monocalcium phosphate feed; (3) variable-speed masterflex peristaltic pump with two heads; (4) regulated power supply; (5) thermostatical controlled water bath; (6) 1.4-L MSMPR crystallizer; (7) gas-liquid separator; (8) slurry receiver; (9) vacuum pump; (10) electrical relay; (11) platinum electrode.
research is to find out the effect of Al3+, F-, and SiF62ions on the gypsum crystallization and study their effect on the growth rate, nucleation rate, and the crystal habit. 2. Experimental Section A schematic diagram of the experimental system is shown in Figure 1. The MSMPR crystallizer is made of Mo2Ti steel. Two feeds, consisting of an acidified calcium phosphate solution and a mixing solution of sulfuric acid and phosphoric acid, were continuously metered to the 1400 mL MSMPR crystallizer. The reaction volume of 1200 mL was maintained by the continuous removal of slurry by a vacuum pump controlled by electrical relay. Reagent-grade raw materials were used Al3+ was obtained by dissolving Al2(SO4)3‚ 18H2O in the sulfuric acid feed solution. Similarly, Fwas obtained by dissolving HF acid in the sulfuric feed, and SiF62- was obtained by dissolving HF acid and active SiO2 solution in the sulfuric acid feed. All runs were carried out under the following identical conditions in the reaction-crystallization volume: residence time, τ ) 30 min; temperature, T ) 60 °C; phosphoric acid concentration ) 22% by weight P2O5; gypsum slurry concentration ) 100 g/L slurry. © 1997 American Chemical Society
2658 Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997
where Li is the average crystal size and Vi is the volume fraction of crystals with size Li. The population density is given by
ni )
ViMt1010 kvFcQi∆Li
(2)
where Vi is the volume fraction of the crystal with Li, Mt is the suspending solid density, kv is the shape factor, Fc is the density of gypsum, and Qi is the volume of a single particle with the diameter of Li. Usually, the growth rate and the nucleation rate are obtained by the following equations (Randolph, 1971):
Figure 2. Variation of average size of gypsum crystal with Al3+ concentration.
ln ni ) n0 - Li/(Gτ)
(3)
B0 ) n0G
(4)
Since usually there is great error in the determination and calculation of ni and Li, so the values of n0 and G calculated from logarithmic coordinates are not very accurate. In this research, they are calculated by an improved method. Theoretically, the folliwng equation is always correct:
Mt ) Fckv
∫0∞nL3 dL
(5a)
by integrating it, it becomes
Mt ) 6Fckvn0(Gτ)4
(5b)
According to eq 5b, we can see that n and G are not independent of each other; so this limiting condition should be considered in the regression of eq 3. The regression model is Figure 3. Variation of growth and nucleation rates with Al3+ concentration.
After a run was carried out continuously for 10 times the residence time, two samples were taken out. One was for the determination of the suspending solid density; the other was for the determination of crystal size distribution and the SEM photographs of the gypsum crystals. The average size of the gypsum crystal is given by
Lav )
∑i LiVi
(1)
{
Fmin )
∑i [ni - n0 exp(-Li/(Gτ))]2
(Mtd - 6kvFcn0(Gτ)4)/Mtd e δ%
The constraint simplex method is adopted to solve this problem; thus n0 and G are obtained, and then B0 can be obtained from eq 4. 3. Results 3.1. Effect of Al3+ on the Crystallization of Gypsum. Figure 2 shows the measured change in average size of gypsum crystals with Al3+ concentration;
Figure 4. Effect of Al3+ on gypsum habit: (a) 0.0% Al2O3; (b) 0.737% Al2O3; (c) 1.84% Al2O3.
Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997 2659
Figure 5. Variation of average size of gypsum crystal with Al/F molar ratio.
Figure 6. Variation of growth and nucleation rates with Al/F molar ratio.
similarly, Figure 3 presents the measured change in G and B0 with Al3+ concentration. It can be seen that Al3+ exerts different effects on the crystal average size, nucleation rate, and growth rate at high and low Al3+ concentration levels. The nucleation rate increases with increasing Al3+ concentration at low levels but decreases with Al3+ concentration at high levels. The maximum nucleation rate is obtained at about 1.11% Al2O3. The effects of Al3+ on the crystal average size and growth rate are quite the reverse. SEM photographs of the
gypsum crystals obtained in the presence of different amounts of Al3+ ion are shown in Figure 4. Gypsum crystals show rhombic swallowtail twins in the absence of Al3+. The crystal size in the presence of Al3+ is smaller than that without Al3+, but the crystal shape has no significant changes. The morphological observations of crystals in this study are in agreement with those reported by Slack, but the change of length was smaller than that reported by Slack. 3.2. Effect of Al3+ and F- on the Crystallization of Gypsum. In order to research the cooperative action of Al3+ and F- on the crystallization of gypsum, the amount of F- added was calculated by the molar ratio of F- to Al3+ with 1:1, 2:1, 3:1, 4:1, 5:1, and 6:1, while the Al3+ concentration was kept constant at 1.11% Al2O3. The results are shown in Figures 5 and 6. When the Al3+ concentration is kept constant, Fexerts different effects on the nucleation rate, growth rate, and crystal average size below and above a critical F- concentration. The nucleation rate increases with increasing F- concentration at low levels, but decreases with F- concentration at high levels. The maximum nucleation rate is obtained at about 1.23% F-. The effects on the crystal average size and growth rate are reversed. SEM photographs of the gypsum crystal obtained in the presence of different amounts of F- and a constant amount of Al3+ are shown in Figure 7. The morphology of gypsum has been greatly changed after adding F-. The gypsum crystal becomes short in length and starts to aggregate. The numbers of sphere-shaped crystal aggregates increases with the increasing amount of F-. 3.3. Effect of F-, Al3+, and SiF62- on the Crystallization of Gypsum. In order to investigate the cooperative effects of F-, Al3+, and SiF62- ions on the crystallization of gypsum, the F- ion concentration was kept constant at 1.23%. The study was carried out by changing the molar ratio of aluminum to silicon. The results are shown in Figures 8 and 9. Similarly, there is a critical molar ratio of aluminum to silicon (about 2) in the presence of F-, Al3+, and SiF62- ions. The nucleation rate increased with the increasing molar ratio at low molar ratio levels but decreased with the augmenting molar ratio at high molar ratios. SEM photographs of the gypsum crystals obtained in the presence of F-, Al3+, and SiF62- ions are shown in Figure 10. It can be seen that the morphology in the presence of three ions is similar to that in the presence of both F- and Al3+ ions, but less regular.
Figure 7. Effect of Al3+ and F- on gypsum habit: (a) 1.11% Al2O3, 0.41% F-; (b) 1.11% Al2O3, 1.22% F-; (c) 11% Al2O3, 2.46% F-.
2660 Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997
Figure 8. Variation of average size gypsum crystal with Al/Si molar ratio.
SiO2 and Al2O3 are added to the phosphoric acid solution containing a constant F- concentration, the change of the Al2O3 to SiO2 ratio leads to similar gypsum morphology, but the clusters are less regular. Our additional studies showed that the separate presence of Al3+ or F- or SiF62- only affected the size of gypsum crystal but did not change the shape in the absence of impurities. On the basis of this, we consider that the most effective ions in the production of wet-process H3PO4 are F- and Al3+. They can form several complex ions (Akitt, 1971; Matwiyoff, 1970; Norwood, 1991), such as AlF2+, AlF2+, AlF52-, AlF3, AlF63-, [AlF2(H3PO4)3]+, and so on. The occurrence of clusters is due to selective adsorptions of the complex ions on different gypsum crystal planes. When both F- and Al3+ are present in solution, a dynamic equilibrium is constructed. If the equilibrium is corrupted, the amount of different complex ions will vary and cause the shape of the gypsum crystal to be changed. Actually, the role of addition of SiO2 is to change the complex equilibrium of F- and Al3+. Witkamp (1987) proposed that complex ion AlF52is the most effective, while Hapet (1977) considered it to be the AlF2+ ion. Since by far there is no information on the equilibrium of F- and Al3+ in the phosphoric acid solution, the authors suggest that there is a lack of evidence on which the complex ion is most effective. Our experiment cannot be related to the existing concrete form of F- and Al3+ in the phosphoric acid solution. Clearly, the mechanism by which fluorine and aluminum affected gypsum growth shape is still poorly understood and requires further investigation. Conclusion
Figure 9. Variation of crystal growth and nucleation rates with Al/Si molar ratio.
4. Discussion In the present study, Al3+ is observed to have fewer effects on the shape of gypsum crystal. Addition of Fleads to extreme changes in gypsum shape when the Al3+ concentration is kept constant. The length of the long needle-type crystals is gradually shortened, and sphere-like clusters occur. When various amounts of
This study has examined the effect of Al3+, F-, and SiF62- ions on gypsum crystallization in a simple model system, and it is found that in the presence of Al3+, the morphology of the obtained gypsum crystals changes little, but both their length and width are shorter. In the presence of Al3+ and F-, the morphology of the gypsum crystals changes greatly and forms sphereshaped aggregates instead of the elongated needles formed in the presence of Al3+. In the presence of Al3+, F-, and SiF62-, the shape of the obtained gypsum aggregates is similar to that in the presence of both Al3+ and F-, but its size is greater. The effect of impurities on the gypsum habit may be the result of selective adsorption of AlFx3-x on a different crystal plane.
Figure 10. Effect of Al3+, F-, and SiF62- on gypsum habit: (a) 0.162% Al2O3, 1.23% F-, 0.381% SiO2; (b) 0.250% Al2O3, 1.23% F-, 0.294% SiO2; (c) 0.423% Al2O3, 1.23% F-, 0.126% SiO2.
Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997 2661
Acknowledgment Thanks are due to the national nature science fund for its support. Nomenclature B0 ) nucleation rate, no./min G ) crystal growth rate, µm/min Vi ) volume fraction of crystals with size Li, dimensionless L ) crystal size, µm Lav ) average crystal size, µm n0 ) population density of embryo-size crystals, no./min n ) population density of crystals, no./min Mt ) suspended crystal density, g/mL Mtd ) determined solid content, g/mL Kv ) volume shape factor, dimensionless Qi ) single crystal volume with the diameter of Li, mL τ ) mean residence time, min Fc ) crystal density, g/mL
Literature Cited Akitt, J. W. Nuclear Magnetic Resonance and Raman Studies of the Aluminium Complexes Formed in an Aqueous Solution of Aluminium Salts Containing Phosphoric Acid and Fluoride Ions. J. Chem. Soc. A 1971, 2450-2457. Becker, P. Phosphates and Phosphoric Acid: Raw materials, Technology, and Economics of the Wet-Process, 2nd ed. Revised and expanded; Dekker: New York, 1989; pp 126-132. David, H.; Mensah, I. A. Filterability of Gypsum Crystallized in Phosphoric Acid Solution in the Presence of Ionic Ions. Ind. Eng. Chem. Res. 1990, 29, 867-875. Gilbert, R. L., Jr. Crystallization of Gypsum in Wet-Process Phosphoric Acid. Ind. Eng. Chem. Process Des. Dev. 1966, 5, 388-391.
Naret, A. A. Zh. Prikl. Khim. 1977, 50 (11), 2440-2443. Jerzy, B.; Jones, A. G.; Mullin, J. W. Effect of Selected ions on the Continuous Precipitation Calcium Sulphate (Gypsum). J. Chem. Technol. Biotechnol. 1986, 36, 153-161. Matwiyoff, N. A. Nuclear Magnetic Resonance Studies of Aluminium(III) Fluoride Ion Complexes in Aqueous Solutions. Inorg. Chem. 1970, 5 (9), 1031-1036. Norwood, V. M.; Kohler, J. J. Characterization of Fluorine, Aluminum, Silicon, and Phosphorus Containing Complexes in Wet-Process Phosphoric Acid Using Nuclear Magnetic Resonance Spectroscopy. Fertilizer Res. 1991, 28, 221-228. Orenga, M. Production of Phosphoric Acid from Phaloborwa Rock from the Pilot Studies to Experience of the Industrial Plants of Rhone Poulenec. Presented at the IFA Seminar, Raw Materials, in Johannesburg, South Africa, 1983; Part III, pp 45-55. Randolph, A. D.; Larson, M. A. Theory of particulate process; Academic: New York, 1971. Sarig, S.; Mullin, J. W. Effect of Trace Ions on Calcium Sulphate Precipitation. J. Chem. Technol. Biotechnol. 1982, 32, 525531. Slack, A. V. Phosphoric Acid; Marcel Decker: New York, 1968; Part I. Witkamp, G. J.; Van Rosmalen, G. M. Incorporation of Cadmium and Aluminium Fluoride in Calcium Sulphate. Ind. Crystallization 1976, 265-270.
Received for review July 19, 1996 Revised manuscript received February 7, 1997 Accepted February 10, 1997X IE960422A
X Abstract published in Advance ACS Abstracts, May 1, 1997.
