Hydrometall Hydrom etallurgy urgy 85 (2007) 53 – 58 www.elsevier.com/locate/hydromet
Technical note
Increasing the filtration rate of phospho-gypsum using surfactant E.A. Abdel-Aal
a, ⁎
, M.M.H. Mahmoud a , H. El-Shall b , A.K. Ismail
a
a
b
Central Metallurgical R&D Institute, P.O. Box 87 Helwan, Cairo, Egypt Department of Materials Science and Engineering, Particle Engineering Research Center, University of Florida, Gainesville 32611, FL. USA
Received 27 January 2006; received in revised form 7 August 2006; accepted 7 August 2006 Available online 18 September 2006
Abstract
Phospho-gypsum is a by-product of processing phosphate rock for phosphoric acid production by acidulation with sulfuric acid. The size distribution of phosphor-gypsum is a major factor for the economics of the process as it greatly affects the filterability of the acid. Surface active agents proved to be very effective additive for growth of gypsum crystals. Two phosphate concentrates, one from Egypt (New Valley) and the other from USA (South Florida) were tested for phosphoric acid production with modification of gypsum crystals using non-ionic surfactant (CMR-100) containing a mixture of C 6–C22 sorbitan esters. The studies were carried out using a semi-continuous laboratory-scale unit simulating the dihydrate process conditions. The mean diameter of the phospho-gypsum crystals increases in the presence of surfactant for both phosphate concentrates. The surfactant leads to decreasing the viscosity and modification of gypsum crystals through decreasing the fine size fractions and attainment of large and uniform crystal shape. The mean diameter of New Valley phosphor-gypsum increases from about 30.0 μm to 36.6 μm while the mean diameter of South Florida phospho-gypsum increases from about 30.3 μm to about 38.4 μm. On the other hand, the average surface area of both New Valley and South Florida phosphor-gypsum were decreased from 4461 and 8069 cm2/g without surfactant to 3284 and 3995 cm 2/g with surfactant, respectively. In addition, the surfactant leads to an increase in P2O5 recovery of 1 –2%, which consequently improves plant productivity. © 2006 Elsevier B.V. All rights reserved. Keywords: Solid liquid separation; Filtration; Dihydrate process; Phospho-gypsum; Sorbitan ester surfactant; Crystal growth modifier
1. Introduction
reaction for the dihydrate process is as follows (Becker, (Becker, 1989): 1989 ):
Phosphoric acid is an importa Phosphoric important nt intermediate product for production of fertilizers. It is mainly produced by the dihydrat dihy dratee proc process ess in whi which ch phos phosphat phatee con concent centrat ratee [fl [fluor uor-apatite Ca10F2(PO4)6] is leached with sulfuric (H 2SO4) and weak phosphoric acids to produce phosphoric acid. A simple diydrate process flow sheet is given in Fig. 1. 1. Crystallization of calcium sulfate dihydrate (phosphogypsum) occurs as leaching is taking place. The primary
Ca10 F2 ðPO4 Þ6 þ 14H3 PO4
⁎
Corres Corr espond pondingauthor ingauthor.. Tel. el.:: +20202 501 501064 0642; 2; fax:+20 202 5010 5010639 639.. E-mail addr address: ess:
[email protected] (E.A. Abdel-Aal).
0304-386X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2006.08.004 doi: 10.1016/j.hydromet.2006.08.004
→
10CaðH2 PO4 Þ2 þ 2HF
CaðH2 PO4 Þ2 þ H2 SO4 þ 2H2 O þ CaSO4Á2H2 O
→
2H3 PO4
Ca10 F2 ðPO4 Þ6 þ 10H2 SO4 þ 20H2 O þ 10CaSO4Á2H2 O þ 2HF
→
6H3 PO4
Although the reaction time is fast (2 to 10 min), the retention time is very long and extends from 2 to 8 h (Becker, 1989). 1989). After leaching, the slurry is filtered and
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E.A. Abdel-Aal et al. / Hydrometallurgy 85 (2007) 53 – 58
Fig. 1. Simple dihydrate process flow sheet.
counter-current washed to separate the acid from the phospho-gypsum cake. The basic objective of phosphoric acid manufacture is to obtain the highest concentration of phosphoric acid possible with the maximum yield (Becker, 1989; Slack, 1968). The yield is dependent on the completeness of the reaction of phosphate concentrate with sulfuric acid, the efficiency of separation of phosphoric acid from calcium sulfate and the quantity of wash water required to remove essentially all the P2O5 from the calcium sulfate during the filtration (Becker, 1989; Slack, 1968). Consequently, the characteristics of calcium sulfate crystals formed during the reaction are the most important fundamental factors in both phosphoric acid filtration rate and the economics of phosphoric acid production. Using the same filter area, the production capacity with lower operation (running) costs can be achieved if the filtration rate is increased. It is known that the filtration rate depends on characteristics of filter cake such as crystal size, size distribution and morphology of the crystals. In other words, large, spherical and narrow size distribution crystals give a better filtration rate (Becker, 1989; Slack, 1968). The temperature of the reaction mass and the phosphoric acid concentration are the controlling factors governing the degree of hydration of calcium sulfate in phosphoric acid (Becker, 1989; Slack, 1968). Many materials are tested as crystal modifiers for crystallization of calcium sulfate dihydrate e.g., aluminum sulfate, clay and calcined clay (Abdel-Aal, 1989), aluminum hydroxide, pearlite, active silica, active charcoal, and manganese dioxide (Ismail, 1997; Abdel-Aal et al., 2002), polymers (Amjad and Hooley, 1986; Kerr et al., 1991; Zhu, 1996), surfactants (El-Shall et al., 1999, 2000a; Mahmoud et al., 2004), phosphonates (El-Shall et al., 2002; Tardos and Mayes, 1997), foreign ions (Rashad et al., 2004; De Vreugd et al., 1994), carboxylic acids (Tardos and Mayes, 1997; Badens et al., 1999; Rashad et al., 2005), and other organic additives such as EDTA
and gelatin (Liu and Nancollas, 1973). To our knowledge, limited data are reported about the testing of surfactants for phosphoric acid production on industrial-scale (El-Shall et al., 2000b). Different surfactant addition methods are described (El-Shall et al., 2000b) as follows: addition of surfactant before nucleation; addition of surfactant during nucleation and addition of surfactant after nucleation. The present paper summarizes the results of phosphoric acid production from Egyptian New Valley phosphate concentrate and South Florida phosphate concentrate with addition of non-ionic surfactant aiming at increasing the filtration rate by 25% and more. Understanding the effects of the applied surfactant on filtration rates and crystal size distributions of formed phospho-gypsum crystals is another main objective. 2. Experimental
2.1. Materials characterization
The phosphate concentrates samples used in the study are from New Valley (Egypt) and from South Table 1 Chemical analysis of phosphate concentrates (on dry basis) Constituent, %
New Valley (Egypt)
South Florida (USA)
P2O5 CaO F SiO2 Fe2O3 Al2O3 MgO SO3 Na2O K 2O Moisture Insolubles
30.09 44.40 2.81 2.29 3.80 0.46 0.90 1.50 0.28 0.04 1.86 N.D.
27.22 42.79 N.D. N.D. 1.56 0.95 1.58 N.D. 0.73 0.12 N.D. 8.05
N.D.: Not determined.
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E.A. Abdel-Aal et al. / Hydrometallurgy 85 (2007) 53 – 58
Florida (USA). Chemical analyses of these samples are given in Table 1. As seen in Table 1 the phosphate concentrate from New Valley contains extremely high Fe2O3 content which in turn leads to production of phosphoric acid of high viscosity and consequently low filtration rate. The upper merchant-grade limit of Fe2O3 content in phosphate concentrate is correlated with Al2O3 and P2O5 contents as in the following ratio (Becker, 1989):
ðFe2 O3 þ Al2 O3 Þ=P2 O5 ¼ 0:08 0:1 −
On the other hand, South Florida phosphate concentrate contains high MgO content (1.58%). High MgO contents increase the acid viscosity and consequently decrease the filtration rate. The accepted industrially limit in commercial phosphate concentrates is less than 1.0% MgO. New Valley and South Florida phosphate concentrates contain 80% and 52% 100 mesh as well as 40% and 30% 200 mesh particle size, respectively. These sieve analyses are within the typical size suitable for the dihydrate process. Pure (95.5%) sulfuric acid of 1.835 g/ml specific gravity is used for the digestion. The recycle (return) acid (18–20% P2O5) is accumulated while carrying out the tests. It is adjusted to the required P 2O5 content before recycling. The surfactant used is a non-ionic Crystal Modifier Reagent (CMR-100) containing a mixture of C6–C22 Sorbitan Esters provided by University of Florida, USA. Coulter Laser Diffraction Analyzer model LS 230 was used for determination of size distri bution of the phospho-gypsum crystals produced. JEOL Scanning Electron Microscope model JSM-6400 was used for investigation of crystal morphology.
Detailed experimental procedure including reaction and filtration techniques, and determination of crystal size distribution as well as calculation of filtration rate, reaction efficiency, P2O5 recovery and washing efficiency are described elsewhere (El-Shall et al., 1999, 2000a; Mahmoud et al., 2004). 2.3. Calculation of reaction efficiency, P 2O5 recovery and washing efficiency
Reaction efficiency (digestion or process efficiency, % of extraction or conversion) is defined as the % of P2O5 removed from the phosphate concentrate into solution, because some of the P2O5 is lost with incompletely washed gypsum. The reaction efficiency is calculated according to the following equation (Moudgil, submitted for publication): Reaction efficiency ¼ 100 94*ð A BÞ*C =ð D* E Þ −
−
−
−
2.2. Apparatus and procedure
The reaction was carried out in cylindrical 1 L reactor of 10 cm diameter. It was fitted with teflon-coated stirrer and placed in water bath adjusted at 80 °C. The impeller tip speed was adjusted at 1.44 m/s (550 rpm). The phosphate concentrate was fed using vibrating rock feeder. Also, the sulfuric acid was pumped using a peristaltic pump with Viton tubing. The surfactant/water suspension was added using small graduated separating funnel at a dosage of 1.5 kg CMR-100 surfactant per 1 ton P2O5 produced. All the above feeding was done on continuous bases. Filtration was performed using Buchner type filter of 4.6 in. diameter. Polypropylene filter cloth of 80 mesh aperture size was used. A vacuum pump was used for filtration.
where: A: % total P2O5 in gypsum cake; B: % water-soluble P2O5 in gypsum cake; C: % CaO in phosphate concentrate used to make the acid; D: % P2O5 in phosphate concentrate used to make the acid; E: % CaO in gypsum cake. Also, P2O5 recovery (overall or plant efficiency, or P2O5 yield) is defined as the % of P2O5 passing from the phosphate concentrate into the produced phosphoric acid. It is calculated as follows: P2 O5 recovery ¼ 100 ð94* A*C Þ=ð D* E Þ −
The main objective of the washing operation is to extract, with wash liquor, as much as possible of the phosphoric acid held by the capillary forces in the interstices of the gypsum cake. Wash liquor displaces the impregnating phosphoric acid. The washing Table 2 Filtration and reaction data of New Valley and South Florida Item
Without surfactant With surfactant New Valley
Filtration rate, ton 3.5±0.3 P2O5/m2 day Moisture % 34.5 Increase in filtration rate, % – Reaction efficiency, % 95.3 P2O5 recovery, % 93.2 Washing efficiency, % 97.8
South Florida
New Valley
South Florida
5.1± 0.3 5.2± 0.6 6.4±0.4 28.3 –
95.8 95.0 99.2
37.9 + 48 96.8 95.4 98.6
Phosphate concentrates with and without surfactant.
27.8 + 25 98.6 96.4 97.9
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Fig. 2. Comparative size distribution of New Valley (A) and South Florida (B) phospho-gypsum crystals (after El-Shall et al., 2000a).
(filtration) efficiency is defined as the % of watersoluble P2O5 passing from the slurry into the produced phosphoric acid. It is calculated applying the following relation: Washing efficiency ¼ 100 ð94* B*C Þ=ð D* E Þ −
This is related to the formation of narrow size distribution phospho-gypsum crystals with low volume % of fines as confirmed by the size distribution of phospho-gypsum crystals given in Fig. 2A, B. It is clear that the volume percentage of fine phospho-gypsum crystals (b 10 μm) is decreased with surfactant addition from 26% to 18% for New Valley phosphate and from 22% to 10% for South Florida phosphate. Also, the volume percentage of fine phospho-gypsum crystals b 400 mesh (b 38 μm) is decreased with surfactant addition from 73% to 67% for New Valley phosphate and from 67% to 50% for South Florida phosphate. Also, the mean diameter of crystal size increases with addition of the surfactant. The statistics of phospho-gypsum crystal size distribution data show that the mean diameters of New Valley and South Florida crystals without surfactant are 30.0 μm and 30.3 μm, respectively, increasing to 36.6 μm and 38.4 μm with surfactant. Also, the average specific surface areas without surfactant are 4461 cm2/g and 8069 cm2/g, decreasing to 3284 cm2/g and 3995 cm2/g ∼
∼
3. Results and discussion
Abdel-Aal et al. (2004) investigated processing of South Florida phosphate under different free sulfate contents ranging from 1.5% to 3.5%. They showed that the optimum free sulfate content is 2.5% which gave the highest filtration rate with the highest mean diameter of the produced gypsum crystals. Ismail (1997) showed that the optimum free sulfate content for New Valley phosphate is about 2.0%±0.4% which gave the highest filtration rate. The surfactant type and dose are phosphate dependent (Becker, 1989). 3.1. Effect of phospho-gypsum size distribution on processing parameters
Tests at optimum free sulfate content (about 2.0%± 0.4%) are carried out using New Valley and South Florida phosphate concentrates with and without addition of the surfactant. The amount of applied surfactant corresponds to 1.5 kg/ton P2O5 produced. The obtained filtration and reaction data are given in Table 2. With addition of the surfactant, the filtration rates are increased by 48% and 25% with New Valley and South Florida phosphate concentrates, respectively.
∼
∼
∼
∼
∼
∼
Table 3 Statistics of phospho-gypsum crystal size distribution Item
Phospho-gypsum crystal size distribution, μm
*d x
d 10
New Valley without surfactant South Florida without surfactant New Valley with surfactant South Florida with surfactant
3.7 20.5 71.9 30.0 4461 2.4 26.5 63.2 30.3 8069 5.1 26.3 80.9 36.6 3284 9.4 38.1 62.3 38.4 3995
d 50
*d x: diameter of crystals passing x% by volume.
d 90
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57
Plate 1. SEM photomicrograph of New Valley phosphogypsum (A and B) with 500 magnification and South Florida phospho-gypsum (C and D) with 200 magnification (after El-Shall et al., 2000a).
with addition of the surfactant, respectively (Table 3). It is clear that the specific surface area is significantly reduced for South Florida phospho-gypsum with surfactant due to agglomeration and forming large clusters. This is confirmed using SEM. There is difference in filtration rate with the two phosphate samples due to differences in phosphate nature (Becker, 1989). The obtained reaction efficiencies and P2O5 recoveries in the presence and absence of surfactant are given in Table 2. It is clear that, both reaction efficiency and P2O5 recovery are higher with the addition of the surfactant. This is attributed to the decrease of lattice P2O5 and unreacted P2O5 losses. The increase in reaction efficiencies are 1.5% and 2.8%; and the increase in P 2O5 recoveries are 2.2% and 1.4% with New Valley and South Florida phosphate concentrates, respectively. The obtained washing efficiencies are within the industrial limits from 97% to 99% (Becker, 1989).
3.2. Investigation of phospho-gypsum morphology
One of the most important factors affecting filtration rate is gypsum morphology. For the best filtration rates, large crystals of uniform sizes are most desirable. In this study, the aspect ratio of phospho-gypsum crystals from New Valley phosphate concentrate is increased as seen in SEM photomicrographs, Plate 1. In addition, the South Florida phospho-gypsum crystal shape is clearly changed from individual tabular crystals to clusters by addition of the surfactant. This change in morphology could be the reason for change in size distribution and, consequently, the enhancement in filtration rates. Fundamentally, these large clusters are composed of many small crystals. This may be due to adsorption of surfactants on crystal surfaces causing hydrophobic (attraction) interactions. Using pure chemicals and under industrially simulated conditions, it is found that surfactant increases the aspect ratio of the formed crystals, decreases the induction time, decreases
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the nucleation rate, increases surface energy and decreases the percentage of fine crystals (El-Shall et al., 2005). 4. Conclusion
The effect of the CMR-100 sorbitan ester surfactant on phospho-gypsum filtration rate and size distribution has been investigated and compared on two phosphate concentrates with an addition dose of 1.5 kg CMR-100 surfactant per 1 ton P2O5 produced. The results indicate that 48% and 25% increase in filtration rate were achieved with New Valley and South Florida phosphate concentrates, respectively. The observed improvements are attributed to changes in crystal size distribution. By obtaining narrower size distributions with higher mean diameters of the crystals sizes, the filtration rates are highly increased. In addition, a noticeable increase in reaction efficiencies and P2O5 recoveries are obtained. The increase in reaction efficiencies are significant as it ranges between 1.5 and 2.9% and the increase in P2O5 recovery is 1.4–2.2% depending on the phosphate concentrate. Acknowledgements
The Particle Engineering Research Center (PERC) at the University of Florida is acknowledged for carrying out particle size distribution analyses. Also, the authors want to thank New Valley Phosphate Project (Egypt) and IMC-Agrico Co. (USA) for providing the phosphate concentrates samples used in this study. References Abdel-Aal, E.A., 1989. Industrial Simulation for Wet Process Phosphoric Acid Production, Ph.D. thesis, Chemistry Department, Faculty of Science, Cairo University. Abdel-Aal, E.A., Ibrahim, I.A., Mahmoud, M.H.H., El-Shall, H., Ismail, A.K., 2002. Improvement of phosphoric acid production parameters from Egyptian phosphor-concentrate through inorganic and organic additives. Proc. Mineral Processing Technology Conference, Bangalore. Regional Research Laboratory, India, pp. 85–93. Abdel-Aal, E.A., Rashad, M.M., El-Shall, H., 2004. Crystallization of calcium sulfate dihydrate at different supersaturation ratios and different free sulfate concentrations. Cryst. Res. Technol. 39 (4), 313–321. Amjad, Z., Hooley, J., 1986. Influence of polyelectrolytes on the crystal growth of calcium sulfate dihydrate. J. Colloid Interface Sci. 111 (2), 496–503. Badens, E., Veesler, S., Boistelle, R., 1999. Crystallization of gypsum from hemihydrate in presence of additives. J. Cryst. Growth 198/ 199, 704–709.
Becker, P., 1989. Phosphates and phosphoric acid: raw materials, Technology and Economics of the Wet Processes, 2nd. Edition. Marcel Decker Inc., New York. De Vreugd, C.H., Witkamp, G.J., Rosmalen, G.M.V., 1994. Growth of gypsum III. Influence and incorporation of lanthanide and chromium ions. J. Cryst. Growth 144, 70–78. El-Shall, H., Moudgil, B., Abdel-Aal, E.A., 1999. Cost Effective Reagents as Defoamers and Crystal Modifiers to Enhance the Filtration Rate of Phospho-gypsum. Florida Institute Phosphate Research (FIPR). Publication No. 01-14-162, March. El-Shall, H., Moudgil, B., Abdel-Aal, E.A., 2000a. Effect of surfactants on phospho-gypsum crystallization and filtration during wet-process phosphoric acid production. Sep. Sci. Technol. 35, 395–410. El-Shall, H., Moudgil, B., Abdel-Aal, E.A., Stana, R., 2000b. Role of surfactants as crystal modifiers for enhanced filtration of phosphogypsum: effect of method of addition. Paper Presented at 13rd Annual National Technical Conference of the American Filtration and Separations Society (AFS), USA, May. El-Shall, H., Rashad, M.M., Abdel-Aal, E.A., 2002. Effect of phosphonate additive on crystallization of gypsum in phosphoric and sulfuric acid medium. Cryst. Res. Technol. 37 (12), 1264–1273. El-Shall, H., Rashad, M.M., Abdel-Aal, E.A., 2005. Effect of cetyl pyridinium additive on crystallization of gypsum in phosphoric and sulfuric acids medium. Cryst. Res. Technol. 40 (9), 860–866. Ismail, A.K., 1997, Chemical Processing of Abu-Tartur Phosphate Concentrate for Phosphoric Acid Production, Internal Report, Abu-Tartur Phosphate Project, Ministry of Industry and Mineral Wealth, Aug. Kerr, E.M., Connelly, L.J., Roe, W.J., Vallowe, R.M., 1991. Crystal Modification in Wet Process Phosphoric Acid, U.S. Patent 5,009,873. Liu, S.T., Nancollas, G.H., 1973. The crystal growth of calcium sulfate dihydrate in the presence of additives. J. Colloid Interface Sci. 44 (3), 422–429. Mahmoud, M.H.H., Rashad, M.M., Ibrahim, I.A., Abdel-Aal, E.A., 2004. Crystal modification of calcium sulfate dihydrate in the presence of some surface-active agents. J. Colloid Interface Sci. 270, 99–105. Moudgil Brij M., submitted for publication. Enhanced Filtration of Phosphogypsum, Final Report to Florida Institute Phosphate Research, Bartow, Fl. Rashad, M.M., Mahmoud, M.H.H., Ibrahim, I.A., Abdel-Aal, E.A., 2004. Crystallization of calcium sulfate dihydrate under simulated conditions of phosphoric acid production in presence of aluminum and magnesium ions. J. Cryst. Growth 276, 372–379. Rashad, M.M, Mahmoud, M.H.H., Ibrahim, I.A., Abdel-Aal, E.A., 2005. Effect of citric acid and 1,2-dihydroxybenzene 3,5disulfonic acid on crystallization of calcium sulfate dihydrate under simulated conditions of phosphoric acid production. Cryst. Res. Technol. 40 (8), 739–745. Slack, A.V., 1968. Phosphoric Acid. Marcel Dekker Inc., New York. Tadros, M.E., Mayes, I., 1997. Linear growth rates of calcium sulfate dihydrate crystals in the presence of additives. J. Colloid Interface Sci. 72, 245–254. Zhu, S., 1996. Modification of Crystal Size Distribution for Enhanced Filtration of Phospho-gypsum using Poly (ethylene oxide), Ph.D. thesis, University of Florida, USA.
