Florida Institute of Phosphate Research 1855 West Main Street Bartow, Florida (863) Fax: (863)

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2 The Florida Institute of Phosphate Research was created in 1978 by the Florida Legislature (Chapter , Florida Statutes) and empowered to conduct research supportive to the responsible development of the state s phosphate resources. The Institute has targeted areas of research responsibility. These are: reclamation alternatives in mining and processing, including wetlands reclamation, phosphogypsum storage areas and phosphatic clay containment areas; methods for more efficient, economical and environmentally balanced phosphate recovery and processing; disposal and utilization of phosphatic clay; and environmental effects involving the health and welfare of the people, including those effects related to radiation and water consumption. FIPR is located in Polk County, in the heart of the central Florida phosphate district. The Institute seeks to serve as an information center on phosphate-related topics and welcomes information requests made in person, or by mail, , or telephone. Executive Director Paul R. Clifford Research Staff Research Directors G. Michael Lloyd, Jr. J. Patrick Zhang Steven G. Richardson Gordon D. Nifong -Chemical Processing -Mining & Beneficiation -Reclamation -Environmental Services Florida Institute of Phosphate Research 1855 West Main Street Bartow, Florida (863) Fax: (863)

3 COST-EFFECTIVE REAGENTS AS DEFOAMERS AND CRYSTAL MODIFIERS TO ENHANCE THE FILTRATION OF PHOSPHOGYPSUM FINAL REPORT Hassan El-Shall, Brij M. Moudgil and El-Sayed A. Abdel-Aal Principal Investigators Department of Materials Science and Engineering Engineering Research Center for Particle Science and Technology (ERC) UNIVERSITY OF FLORIDA Gainesville, Florida Prepared for FLORIDA INSTITUTE OF PHOSPHATE RESEARCH 1855 West Main Street Bartow, Florida USA Contract Manager: G. Michael Lloyd, Jr. FIPR Project Number: March 1999

4 PERSPECTIVE The manufacture of phosphoric acid by the wet process is often characterized as a method devoted to producing the optimum size and shape phosphogypsum crystals. The phosphogypsum crystal characteristics determine the speed of filtration that defines the production rate for the plant and the phosphogypsum cake-washing efficiency that is one of the more important components contributing to acceptable yields. Controlling and/or modifying the phosphogypsum crystals has been a long-time goal of all phosphoric acid producers. Over the years there have been a number of studies designed to define the role of various naturally occurring metals in the phosphogypsum crystal formation mechanisms. Organic crystal modifiers have also been investigated and while they have never been demonstrated to be cost-effective for the dihydrate process, they have found wide application in the hemihydrate process for phosphoric acid manufacture. Where crystal modifiers have been used in the dihydrate process they have often demonstrated variable performance over time that may have been a function of the changes in minor element content of the phosphate rock. There is little doubt that a cost-effective crystal modifier would find widespread application in the phosphoric acid industry and that a crystal modifier that would be consistently effective for the more highly variable rock that will be mined in the future in Florida would be a decided asset for the Florida phosphate industry. iii

5 ABSTRACT Phosphoric acid is an important intermediate product for production of fertilizers. It is mainly produced by the wet process, in which phosphate concentrate is leached with sulfuric and weak phosphoric acids to produce phosphoric acid. Calcium sulfate crystallization occurs as leaching is taking place. According to the process adopted, calcium sulfate dihydrate (gypsum) (CaSO 4.2H 2 O) or calcium sulfate hemihydrate (CaSO 4.0.5H 2 O) is crystallized. Filtration of gypsum to recover phosphoric acid is the bottleneck in this process. Therefore, the major goal of this project is to enhance filtration of phosphogypsum using cost-effective reagents. Clarification of the mechanisms controlling the effect of such additives is another important objective in this study. In order to achieve these goals, several tasks have been conducted including: (1) benchscale testing of effect of two different surfactants (Crysmod and HiFlo-S5*) on crystal modification and filtration of phosphogypsum produced from South Florida high magnesium phosphate concentrate, at low, medium, and high sulfate levels, (2) role of techniques and point of addition of surfactants, (3) cost-benefit analyses, and (4) basic studies to elucidate the mechanisms controlling the effect of surfactants under different conditions. The results suggest the following: Optimum filtration results could be obtained by addition of surfactant as a mixture with water during nucleation. Both surfactants can improve filtration rate at all sulfate levels. At any sulfate content, both P 2 O 5 recovery and reaction efficiency are higher with the addition of surfactants. Both surfactants decrease the induction time at different super-saturations. Also, larger pure calcium sulfate dihydrate crystals with higher mean crystal diameter are produced in the presence of surfactants. A gain of about $1.0 / ton P 2 O 5, could be realized if Crysmod is used. However, the gain will be about $4.0 / ton P 2 O 5 if HiFlo-S5 is used. Such gain is in addition to the increase in filtration rates due to use of these surfactants. * Patent pending. v

6 ACKNOWLEDGMENTS The Florida Institute of Phosphate Research (FIPR) is acknowledged for sponsoring this study (FIPR Grant # ). The Engineering Research Center for Particle Science & Technology (ERC) at the University of Florida, the National Science Foundation grant #EEC and the Industrial Partners of the ERC are acknowledged for their partial financial support. Also, the principal investigators wish to thank the IMC-Agrico Company for providing the phosphate concentrate sample used in this study. Efforts conducted by the following research team are gratefully appreciated: Robert Gaertner (graduate student) and Arlene Maya, Daniel M. Kuncicky, Amy Bowden, Kathryn Gacek, Caroline Dugopolski (undergraduate students). vi

7 TABLE OF CONTENTS Page PERSPECTIVE...iii ABSTRACT...v ACKNOWLEDGMENTS...vi EXECUTIVE SUMMARY...1 INTRODUCTION...7 METHODOLOGY...9 Materials Characterization...9 Apparatus...10 Procedure...10 Estimation of Filtration Rate...10 Calculation of Reaction Efficiency, P 2 O 5 Recovery and Washing Efficiency...11 Determination of Crystal Size Distribution of Produced Phosphogypsum...12 Preparation of Phosphogypsum Slurry...13 Surfactant Addition Techniques...14 RESULTS AND DISCUSSION...17 Study of Surfactant Addition Techniques...17 Addition of Crysmod Surfactant Before Nucleation...17 Addition of Crysmod Surfactant During Nucleation...20 Addition of Crysmod Surfactant After Nucleation...23 Testing of Surfactants at Different Sulfate Contents...25 Testing at Low Sulfate Content ( %)...25 Testing at Medium Sulfate Content ( %)...28 Washing With Simulated Pond Water...30 Effect of Retention time on Crystal Size Distribution...30 vii

8 TABLE OF CONTENTS (CONTINUED) Testing at High Sulfate Content ( %)...35 Correlation of Crystal Size Distribution of Phosphogypsum and the Filtration Rate...37 Gypsum Morphology...39 BASIC STUDIES: PRIMARY NUCLEATION OF CALCIUM SULFATE DIHYDRATE...45 Materials...45 Procedure...45 Turbidity Measurements and Estimation of Induction Time (T) Calculation of Crystal Growth Efficiency (E) Calculation of Super-saturation (S) Effect of Surfactants on Crystal Growth (Turbidity) at Different Super-saturation...48 Effect of Super-saturation on Induction Time and Crystal Growth Efficiency With and Without Surfactants...51 Effect of Surfactants Concentration on Induction Time and Crystal Growth Efficiency...51 Correlation between Super-saturation and Induction Time...52 Effect of Surfactants on Crystal Size Distribution of Gypsum...53 Effect of Surfactants on Gypsum Crystal Morphology...55 COST-BENEFIT ANALYSES...57 CONCLUSIONS AND RECOMMENDATIONS...59 REFERENCES...61 viii

9 LIST OF FIGURES Figure Page 1. Simple dihydrate process flow-sheet Volume % of fine (-10 microns) phosphogypsum with and without surfactant (before nucleation) Effect of surfactant/recycle acid mixture addition before nucleation on phosphogypsum crystal size distribution Volume % of fine (-10 microns) phosphogypsum crystals with and without surfactant (during nucleation) Effect of surfactant addition during nucleation on phosphogypsum crystal size distribution Volume % ( 10 microns) of phosphogypsum crystals with and without surfactant (surfactant in water added after nucleation) Effect of surfactant addition after nucleation on phosphogypsum size distribution Comparative size distribution of phosphogypsum crystals (at low sulfate content and after 3 hr) Comparative size distribution of phosphogypsum crystals (at medium sulfate content and after 3 hr) Volume % (-10 µm) of phosphogypsum crystals with and without surfactants (at medium sulfate content) Volume % (-38 µm) of phosphogypsum crystals with and without surfactants (at medium sulfate content) Comparative size distribution of phosphogypsum crystals (without surfactant and at medium sulfate content) Comparative size distribution of phosphogypsum crystals (with Crysmod surfactant and at medium sulfate content) Comparative size distribution of phosphogypsum crystals (with Hiflo-S5 surfactant and at medium sulfate content) ix

10 LIST OF FIGURES (CONTINUED) Figure Page 15. Comparative size distribution of phosphogypsum crystals (at high sulfate content and after 3 hr) Example of turbidity data plotting and induction time estimation Effect of time on turbidity of calcium sulfate dihydrate (super-saturation 1.018, 100 ppm surfactant) Effect of time on turbidity of calcium sulfate dihydrate (super-saturation 1.222, 100 ppm surfactant) Effect of time on turbidity of calcium sulfate dihydrate (super-saturation 1.502, 100 ppm surfactant) Effect of time on turbidity of calcium sulfate dihydrate (super-saturation 1.979, 100 ppm surfactant) Relation between super-saturation and induction time of calcium sulfate dihydrate Linear relationship between super-saturation and natural log of induction time for calcium sulfate dihydrate Differential volume at a super-saturation of after 5 minutes (surfactant concentration = 100 ppm) x

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12 LIST OF TABLES Table Page 1. Effect of Adding Crysmod Surfactant/Recycle Acid Mixture Before Nucleation on Phosphogypsum Filtration at Low Sulfate Levels ( %) Effect of Crysmod Surfactant Addition during Nucleation on Phosphogypsum Filtration at Low Sulfate Levels ( %) Effect of Crysmod Surfactant Addition after Nucleation on Phosphogypsum Filtration at Low Sulfate Levels ( %) Filtration and Reaction Data at Low Sulfate Content ( %) Filtration and Reaction Data at Medium Sulfate Content ( %) Filtration and Reaction Data at High Sulfate Content ( %) Chemical Analysis of High Dolomitic Phosphate Concentrate Sample Sieve Analysis of High Dolomitic Phosphate Concentrate Sample Effect of Adding Surfactant / Recycle Acid Mixture Before Nucleation on Phosphogypsum Filtration at Low Sulfate Levels ( %) Effect of Adding Surfactant / Recycle Acid Mixture Before Nucleation on Reaction Efficiency, P 2 O 5 Recovery and Washing Efficiency Effect of Surfactant Addition during Nucleation on Phosphogypsum Filtration at Low Sulfate Levels ( %) Effect of Surfactant Addition during Nucleation on Reaction Efficiency, P 2 O 5 Recovery and Washing Efficiency Effect of Surfactant Addition After Nucleation on Phosphogypsum Filtration at Low Sulfate Levels ( %) Effect of Surfactant Addition After Nucleation on Reaction Efficiency, P 2 O 5 Recovery and Washing Efficiency xii

13 LIST OF TABLES (CONTINUED) Table Page 15. Filtration and Reaction Data at Low Sulfate Content ( %) Comparative Size Analyses of Gypsum Crystals (at Low Sulfate Content and After 3 Hr) Statistics of Phosphogypsum Crystal Size Distribution (at Low Sulfate and After 3 Hr) Filtration and Reaction Data at Medium Sulfate Content ( %) Comparative Size Analyses of Gypsum Crystals (at Medium Sulfate Content and After 3 Hr) Filtration and Reaction Data at Medium Sulfate Content ( %) Comparative Size Analyses of Gypsum Crystals (Without Surfactant and at Medium Sulfate Content) Comparative Size Analyses of Gypsum Crystals (With Crysmod and at Medium Sulfate Content) Comparative Size Analyses of Gypsum Crystals (With Hiflo-S5 and at Medium Sulfate Content) Filtration and Reaction Data at High Sulfate Content ( %) Comparative Size Distribution of Gypsum Crystals (at High Sulfate Content and After 3 Hr) Statistics of Phosphogypsum Crystal Size Distribution (at High Sulfate and After 3 Hr) Relationship between % Phosphogypsum Fines and Filtration Rate at Low Sulfate Content Relationship between % Phosphogypsum Fines and Filtration Rate at Different Sulfate Contents xiii

14 LIST OF TABLES (CONTINUED) Table Page 29. Amounts of Calcium Hydrogen Phosphate Monobasic and Sulfuric Acid for Different Super-saturation Experimental Conditions for Primary Nucleation Studies Calculation of Super-saturation Effect of Super-saturation on Induction Time (T) and Growth Efficiency (E) of Gypsum Crystals (Using 100 ppm Surfactant) Effect of Surfactant Concentration on Induction Time (T) and Growth Efficiency (E) of Gypsum Crystals (Using Super-saturation) Statistics of Size Distribution of Gypsum Crystals xiii

15 EXECUTIVE SUMMARY The major goal of this project is to enhance filtration of phosphogypsum using cost-effective reagents. Clarification of the mechanisms controlling the effect of such additives is another important objective in this study. In order to achieve these goals the following tasks have been conducted: Bench scale testing of effect of two different surfactants (Crysmod and HiFlo-S5*) on crystal modification and filtration of phosphogypsum produced from South Florida high magnesium phosphate concentrate, at low-, medium- and high- sulfate levels. Role of techniques and point of addition of surfactants; specific techniques involve adding surfactants: As a mixture with recycle acid or water, and Before, during, or after nucleation Cost-benefit analyses Basic studies to elucidate the mechanisms controlling effect of surfactants under different conditions. The results are summarized in the following paragraphs. ADDITION OF SURFACTANT BEFORE NUCLEATION Tests are carried out in which the Crysmod surfactant (1.5 kg/ton P 2 O 5 ) is mixed with recycle acid before nucleation for duration times of 30 and 5 minutes. The filtration data in Table 1 show that, in general, mixing of the surfactant with recycle acid and its addition before nucleation result in low filtration rates as compared to the baseline without surfactant. It may be attributed to an inhibition of regular crystal growth as indicated by the increase in the amount of fine crystals obtained in presence of surfactant as added before nucleation. Table 1. Effect of Adding Crysmod Surfactant/Recycle Acid Mixture Before Nucleation on Phosphogypsum Filtration at Low Sulfate Levels ( %). Item Baseline (without Surfactant) Surfactant Addition Technique To RA before Nucleation and Mixed for 30 Min. To RA before Nucleation and Mixed for 5 Min. Filtration rate, Ton P 2 O 5 /m 2.day Moisture % Change in filtration rate, % RA: Recycle acid * Patent pending. 1

16 ADDITION OF SURFACTANT DURING NUCLEATION Tests are conducted by mixing the Crysmod surfactant (1.5 kg/ton P 2 O 5 ) with either recycle acid or water and adding the mixture during nucleation. The results are shown in Table 2 below. Item Table 2. Effect of Crysmod Surfactant Addition During Nucleation on Phosphogypsum Filtration at Low Sulfate Levels ( %). Baseline (without Surfactant) Surfactant Addition Technique With RA* and Added With Water and Added During the Nucleation During the Nucleation for for 30 Min. 30 Min. Filtration rate, ton P 2 O 5 /m 2.day Moisture % Change in filtration rate, % * RA : Recycle acid The filtration data show that mixing surfactant with recycle acid during nucleation leads to a decrease in filtration rate. On the other hand, adding surfactant / water results in an increase in filtration rate, which corresponds to about 31% improvement over that obtained in the absence of surfactant. Such improvement is attributed to change in crystallization as indicated by a decrease in the amount of fine crystals produced in the presence of surfactant/ water mixture. ADDITION OF SURFACTANT AFTER NUCLEATION A test was carried out by mixing the Crysmod surfactant (1.5 kg/ton P 2 O 5 ) with water, then adding the mixture continuously for 30 minutes. Feeding the surfactant mixture started after addition of the phosphate concentrate and sulfuric acid. The filtration data in Table 3 suggest that filtration rate is slightly improved (8%) due to addition of surfactant after nucleation. Thus, it may be concluded that optimum filtration results could be obtained by addition of surfactant as a mixture with water after nucleation. 2

17 Table 3. Effect of Crysmod Surfactant Addition After Nucleation on Phosphogypsum Filtration at Low Sulfate Levels ( %). Item Baseline (without Surfactant) Surfactant Addition Technique With Water and Added for 30 Min. After the Nucleation by 30 Min. Filtration rate, Ton P 2 O 5 /m 2.day Moisture % Change in filtration rate, % * RA : Recycle acid EFFECT OF SURFACTANTS AT LOW SULFATE CONTENT ( %) Tests have been carried out with and without surfactants addition at low sulfate contents. The amount of applied Crysmod and Hiflo-S5 surfactants correspond to 1.5 and 0.75 kg/ton P 2 O 5 produced, respectively. The filtration and reaction data are given in Table 4. The filtration rate is 4.24 ton P 2 O 5 /m 2.day without surfactant and improved to 5.54 and 6.88 ton P 2 O 5 /m 2.day with the addition of Crysmod and Hiflo-S5 surfactants, respectively. The percentages of filtration rate improvement correspond to about 31 and 62%, respectively. It is also important to note that the reaction efficiency is increased with the addition of both surfactants. This may be attributed to decrease of both lattice (cocrystallized) P 2 O 5 and unreacted P 2 O 5 losses. The lattice (co-crystallized) P 2 O 5, generally, represent about 60% of the total P 2 O 5 losses. The increase in reaction efficiency is significant as it approaches 3.9% and 4.1% and the increase in P 2 O 5 recovery is 1.6% and 3.1% with Crysmod and Hiflo-S5 surfactants, respectively. Table 4. Filtration and Reaction Data at Low Sulfate Content ( %). Item Without Additive With Crysmod Surfactant With Hiflo-S5 Surfactant Filtration rate, Ton P 2 O 5 /m 2.day Moisture % Increase in filtration rate, % Reaction efficiency, % P 2 O 5 recovery, % Washing efficiency, %

18 EFFECT OF SURFACTANTS AT MEDIUM SULFATE CONTENT ( %) To cover the range of sulfate contents applied in industry, tests at medium sulfate content ( %) are carried out with and without addition of surfactants. At medium sulfate content, the filtration data are given in Table 5. The filtration rate has increased by 25% and 43% with the addition of Crysmod and Hiflo-S5 surfactants respectively. This is attributed to modification and growth of crystals upon surfactant addition as confirmed by the gypsum crystals size distribution. It is most interesting that, at medium sulfate content, both P 2 O 5 recovery and reaction efficiency are higher with the addition of surfactants. The increase in reaction efficiency is significant as it approaches 2.8% with Crysmod surfactant and 1.9% with Hiflo-S5 surfactant as well as the increase in P 2 O 5 recovery is 1.4% with Crysmod surfactant and 1.8% with Hiflo-S5 surfactant. Table 5. Filtration and Reaction Data at Medium Sulfate Content ( %). Item Without Surfactant With Crysmod Surfactant With Hiflo-S5 Surfactant Filtration rate, tonp 2 O 5 /m 2.day Moisture % Increase in filtration rate, % Reaction efficiency, % P 2 O 5 recovery, % Washing efficiency, % EFFECT OF SURFACTANTS AT HIGH SULFATE CONTENT ( %) The excess of sulfuric acid in the reaction slurry is the most effective factor governing crystallization quality of phosphogypsum. Its effect is not only on the crystal shape and size, but also on co-crystallized P 2 O 5 losses and the unreacted P 2 O 5 losses. Therefore, it was decided to test the effect of adding surfactants at high levels of sulfate. The results are given in Table 6. It is interesting to note that, in this case also, the filtration rate has improved in the presence of surfactants. The reaction efficiencies and P 2 O 5 recoveries in the presence and absence of surfactants are given in the same table. It is most interesting that, even at high sulfate content, both P 2 O 5 recovery and reaction efficiency are higher due to the addition of surfactants. 4

19 Table 6. Filtration and Reaction Data at High Sulfate Content ( %). Item Without Additive With Crysmod Surfactant With Hiflo-S5 Surfactant Filtration rate, Ton P 2 O 5 /m 2.day Moisture % Increase in filtration rate, % Reaction efficiency, % P 2 O 5 recovery, % Washing efficiency, % Basic studies were performed to understand the effect of surfactants on the nucleation of pure calcium sulfate dihydrate (gypsum). The results show that both surfactants decrease the induction time at different super-saturations. Also, larger pure calcium sulfate dihydrate crystals with higher mean crystal diameter are produced in the presence of surfactants. Most importantly, the cost-benefit analyses suggest a gain of about $1.0/ton P 2 O 5, if Crysmod is used. However, the gain will be about $4.0/ton P 2 O 5 if HiFlo-S5 is used. Such gain is in addition to the increase in filtration rates due to use of these surfactants. 5

20 INTRODUCTION Phosphoric acid is an important intermediate product for production of fertilizers. It is mainly produced by the wet-process, in which phosphate concentrate is leached with sulfuric and weak phosphoric acids to produce phosphoric acid. Calcium sulfate crystallization occurs as leaching is taking place. According to the process adopted, calcium sulfate dihydrate (gypsum) (CaSO 4.2H 2 O) or calcium sulfate hemihydrate (CaSO 4.0.5H 2 O) is crystallized. A simple dihydrate process flowsheet is given in Fig. 1. The primary reaction for the dihydrate process is given below (Becker 1989): Ca 10 F 2 (PO 4 ) H 3 PO 4 10Ca(H 2 PO 4 ) 2 + 2HF 10Ca(H 2 PO 4 ) H 2 SO H 2 O 20H 3 PO CaSO 4 2H 2 O Ca 10 F 2 (PO 4 ) 6 +10H 2 SO H 2 O 6H 3 PO CaSO 4 2H 2 O+ 2HF After leaching, the slurry is filtered and counter-current washed to separate the acid from the phosphogypsum cake. Filtration operation represents a bottleneck in the wet-process phosphoric acid industry. Using the same filter area, the production capacity with lower operating costs can be achieved if the filtration rate is increased. It is known that the filtration rate depends on the characteristics of the filter cake such as crystal size, size distribution and morphology of the crystals. In other words, large, spherical and narrow size distribution crystals give better filtration rate. Recycle Acid 20% P2O5 Phosphate Concentrate Sulfuric Acid Product Acid 28% P2O5 W I REACTION FILTRATION WASHING I Wash Water W II Gypsum Cake WASHING III WASHING II W: Wash liquor W II 5%P2O5 Figure 1. Simple Dihydrate Process Flowsheet. W I 10%P2O5 7

21 High magnesium phosphate concentrate from South Florida was tested for phosphoric acid production with and without surfactants at different sulfate contents. The additives are used as crystal modifiers for phosphogypsum. In this regard, there are a limited number of published reports about industrial crystallization of phosphogypsum using surfactants. The available data in literature is about nucleation and crystal growth of gypsum from pure chemicals (Ibragimova 1986; Klima 1987; Witkamp 1990; Hatakka 1997). Also, the influence of different inhibitors on the growth rate of calcium sulfate dihydrate crystals has been studied (Ibragimova 1986; Klima 1987; Witkamp 1990; Hatakka 1997). In addition, many research papers are published explaining the effects of additives and impurities on crystallization of calcium sulfate (Davey 1982; Botsaris 1982; Budz 1986; Van Rosmalen 1987; Hasson 1990; Martynowicz 1996). There are, however, numerous reports about improving the filtration rate using polymers (Cody 1991; Liu 1973a; Liu 1973b; Liu 1975; MacCartney 1958; Sarig 1975; Smith 1970; Smith 1971; Taylor 1989; Shoulian 1996; Moudgil 1995). At the same time, studies dealing with effect of surfactants on filtration of phosphogypsum are limited (Rocha 1995; Schroeder 1975; Kopyleva 1987). Even in these studies, crystallization of calcium sulfate dihydrate from pure chemicals is reported but no data was available about surfactant addition technique. South Florida phosphate concentrate has a relatively high impurities content (MgO + Fe 2 O 3 + Al 2 O 3 + Na 2 O + K 2 O = 4.82 %). Generally, some of these impurities lead to decrease in the filtration rate during the processing of this particular phosphate using the dihydrate process. Also, excessive foaming is experienced during acidulation due to high carbonate content. To study this concentrate, dihydrate process conditions were simulated (Abdel-Aal, 1984, 1989) and two different surfactants were tested. The size distributions of the gypsum crystals were studied to determine the effect of the surfactants on crystal growth at different sulfate levels. The main aim of this study is to enhance the filtration rate of phosphogypsum using cost-effective reagents. In addition, clarification of the mechanisms controlling the effect of such additives is another important objective in this study. Achieving these goals will allow the following advantages: High filtration rate (high plant productivity) High P 2 O 5 recovery from the phosphate concentrate (high plant efficiency) High P 2 O 5 concentration in the filter acid, leading to energy savings during the concentration Low total P 2 O 5 losses in the phosphogypsum by-product. 8

22 METHODOLOGY MATERIALS CHARACTERIZATION IMC-Agrico Company provided the phosphate concentrate sample (high magnesium). Chemical and sieve analyses of this sample are given in Tables 7 and 8. As seen in Table 7, the phosphate concentrate contains high MgO content (1.58%). High MgO contents increase the acid viscosity, consequently decreasing the filtration rate. The accepted limit in commercial phosphate concentrates is less than 1.0%. The sieve analysis of the sample is given in Table 8. It contains 51.8% -100 mesh and 29.7% -200 mesh particle size, which are suitable for the dihydrate process. Most of the sample weight (80.5%) falls in the Fm size range. Pure (95.5%) sulfuric acid of g/ml specific gravity is used for digestion. The recycle (return) acid is accumulated while carrying out the tests. It is adjusted to the required P 2 O 5 content before recycling. For the first test (start-up), the required amount of recycle acid (about 18% P 2 O 5 ) is prepared by double-stage acidulation, the first with water and sulfuric acid while the second with about 10% P 2 O 5 phosphoric acid produced from the previous run and sulfuric acid. The surfactants used are Crysmod (a mixture of C 6 -C 22 Sorbitan esters) supplied by Polimeros Sinteticos, Mexico City, Mexico and Hiflo-S5 (patent pending) surfactants. Table 7. Chemical Analysis of High Dolomitic Phosphate Concentrate Sample. Constituent % Constituent % P 2 O Al 2 O CaO Na 2 O 0.73 MgO 1.58 K 2 O 0.12 Fe 2 O Insolubles 8.05 Table 8. Sieve Analysis of High Dolomitic Phosphate Concentrate Sample. Particle Size Cumulative wt. % Passing Mesh* Inch Mm * Tylor standard 9

23 APPARATUS The reaction is carried out in a cylindrical 1L reactor of 10 cm diameter. It is fitted with a Teflon-coated stirrer and placed in a water bath adjusted to 80 o C. The impeller tip speed is adjusted at 1.44 m/s (550 rpm). The phosphate concentrate is added continuously using a vibrating rock feeder. The sulfuric acid is added continuously using a peristaltic pump with Viton tubing. The surfactant/water suspension is continuously added using a small graduated separating funnel. Filtration is performed using Buchner type filter of 4.6 inch diameter. Polypropylene filter cloth of 80 mesh aperture size is used. A vacuum pump is used for filtration. Coulter Laser Diffraction Analyzer model LS230 is used for determination of size distribution of the produced phosphogypsum crystals. JEOL Scanning Electronic Microscope model JSM-6400 was used for investigation of crystal morphology. PROCEDURE 1. Add the required amount of the return (recycle) acid R.A. to the reactor. 2. Wait until the temperature of R.A. goes up to 80 o C. 3. Add the phosphate concentrate, sulfuric acid, and the surfactant/water suspension, continuously, for 30 minutes. 4. Continue the reaction for another 2.5 hours. 5. After the required retention time, the slurry is weighed and poured into the filter. 6. Apply vacuum and record the time from the moment of vacuum application to just before the surface appears dry to avoid the effects of channeling and cracking (wall effect). 7. Wash with the required amount of wash I (~10 % P 2 O 5 ) and repeat step Repeat step 6 with wash II (~5% P 2 O 5 ), and wash water or simulated pond water. Then allow the cake to filter for 5 seconds to dry (taking off as much as possible of the residual liquor from the cake). 9. Weigh the wet cake and determine its moisture by drying at 60 o C for 24 hr. 10. After 0.5, 1.0, 2.0 and 3.0 hours from the start-up of the test, take 3 ml slurry and disperse in 100 ml methanol. Then sieve using 106 Fm screen and determine the size distribution using Coulter laser diffraction analyzer. Estimation of Filtration Rate In industry the filtration rate is expressed in tons P 2 O 5 produced per square meter per day. So the same expression was used. The filtration rate was calculated applying the following equation: F.R. = SW * SC * F/ T Ton P 2 O 5 /m 2.day 10

24 where: F.R: Filtration rate, ton P 2 O 5 /m 2.day SW: Slurry weight, g SC: Solid content, % F: Filtration factor T: Total time of filtration, washing and drying, sec The filtration factor was calculated according to the following equation: F = 94 * 56 * * 10 4 * * 94 * q500/381 = * 172 * * 10 6 * * 100 where: 94% : is the % CaSO 4.2H 2 O in solids (gypsum purity) 56 and172 : are the molecular weights of CaO and CaSO 4.2H 2 O and : are the % of P 2 O 5 and CaO in phosphate concentrate 10 4 : is the conversion factor from cm 2 to m : is the conversion factor from sec to day 94% : is the P 2 O 5 recovery q500/381 : is the pressure difference correction factor 10 6 : is the conversion factor from gm to ton : is the filter area in cm 2 After the required retention time, the produced phosphogypsum slurry was filtered and washed under the following conditions: Type of filter = Buchner-type funnel Diameter of filter = 4.6 inch ( cm) No. of cloth layers = 1 (single) Filter area = cm 2 Type of filter cloth = Polypropylene Pressure Difference = 15 inch. Hg (381 mm. Hg) Temperature of Slurry = o C Number of washing = 3 Wash liquors temperature = o C Calculation of Reaction Efficiency, P 2 O 5 Recovery and Washing Efficiency Reaction efficiency (digestion or process efficiency, % of extraction or conversion) is defined as the % of P 2 O 5 removed from the phosphate concentrate into solution, because some of the P 2 O 5 is lost with an incompletely washed gypsum. The reaction 11

25 efficiency is calculated from the following equation that obtained form IMC-Agrico Company (Moudgil, 1995): where: Reaction efficiency = *(A - B)*C/(D*E) A: % Total P 2 O 5 in gypsum cake, B: % Water-soluble P 2 O 5 in gypsum cake, C: %CaO in phosphate concentrate used to make the acid, D: % P 2 O 5 in phosphate concentrate used to make the acid, E: %CaO in gypsum cake. Also, P 2 O 5 recovery (overall or plant efficiency, or P 2 O 5 yield) is defined as the % of P 2 O 5 passing from the phosphate concentrate into the produced phosphoric acid. It is calculated as follows (Moudgil 1995): P 2 O 5 recovery = (94*A*C)/(D*E) 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 (filtration) efficiency is defined as the % of water-soluble P 2 O 5 passing from the slurry into the produced phosphoric acid. It is calculated applying the following relation (Moudgil 1995): Washing efficiency = (94*B*C)/(D*E) Determination of Crystal Size Distribution of Produced Phosphogypsum Different tests are carried out to find out the best methodology for sampling gypsum crystals for Laser diffraction size analyzer including: The first test is conducted using about 1.0 g dry gypsum cake. Drying is performed at 60 o C for 24 hours. The sample is, then dispersed in 100 ml methanol. The second test is carried out using about 1.0 g air dried gypsum cake and dispersed in 100 ml methanol. In the third test, a 3.0 ml slurry sample is withdrawn at different time intervals and dispersed in 100 ml methanol. 12

26 . The fourth test is performed by taking 3.0 ml slurry sample at different time intervals, and dispersed in 100 ml methanol, then screened using 106 m sieve before conducting size analyses by Laser Diffraction Technique. In this report, size distribution analyses have been done as outlined in the fourth test above. Preparation of Phosphogypsum Slurry Phosphogypsum slurry is prepared at 80 o C reaction temperature for 3 hours retention time. Recycle acid (~18% P 2 O 5 ) is one of the three main starting materials fed to the reactor with concentrated sulfuric acid (95.5%) and phosphate concentrate. Preparation of the recycle acid from the same concentrate is very important to simulate the continuous industrial unit. The level of the impurities in the filter acid depends entirely on its level in recycle acid. Utilization of pure phosphoric acid or recycle acid produced from another phosphate concentrate would produce erratic results for the filtration rates and level of impurities in both filter acid and phosphogypsum. The technique of recycle acid preparation comprises the following two stages; namely preparation of about 10% P 2 O 5 phosphoric acid and preparation of about 18% P 2 O 5 phosphoric acid (recycle acid). Amounts of reacted materials are calculated using a material balance computer program (Abdel-Aal 1984 and 1989). Sulfuric acid and water are used in the first stage. After the required retention time, the slurry is filtered and the cake is washed one time with wash water. The same test is repeated under the same conditions except that the already prepared first filtrate was used instead of water and the gypsum cake is washed twice, one with the previously produced wash liquor (about 5% P 2 O 5 ) and the second with wash water or simulated pond water. It should be noted that the amount of wash liquor used is also calculated using the computer program which depends on the water balance all over the process. Sufficient amounts of recycle acid, wash I and wash II were accumulated from several tests. During the phosphogypsum slurry filtration, recycle acid, wash I and wash II are produced and recycled in the next run. For preparation of phosphogypsum slurry the following data were used for the calculation of feeding amounts of reactants: Operating parameters: P 2 O 5 in phosphate concentrate = % CaO in phosphate concentrate = % SO 3 in phosphate concentrate = 1.44 % Moisture in phosphate concentrate = 1.30 % H 2 SO 4 concentration = 95.5 % 13

27 Retention time = 180 min P2O5 in recycle acid = % Free sulfate in filter acid = 2.0 % P2O5 in filter acid = 27.5 % Assumptions: Overall reaction efficiency = 98 % Gypsum in solids (purity ) = 94 % Solids in slurry = 35% P 2 O 5 recovery = 94% Moisture in gypsum = 25% Slurry density = 1.50 g/ml Volatilization rate = 0.2 kg/kg rock Feeding amounts and operation factors: Weight of phosphate concentrate = 240 g Weight of sulfuric acid, 95.5% = g Volume of sulfuric acid, 95.5 % = ml Weight of recycle acid = g Volume of recycle acid = ml Washing factor (water /slurry ratio) = Filtration factor = Surfactant Addition Techniques From the industrial point of view, it is very important to add the surfactant without negative effects on process performance. Thus, different methodologies for surfactants addition were studied. For example, effect of surfactant addition to the reactor after its mixing with recycle acid was tested. Also, mixing of surfactant with water instead of recycle acid was studied. It should be mentioned that, the slurry solid content and amount and P 2 O 5 concentration of recycle acid have been kept similar to that of the baseline (without surfactant). In these tests, the dosage of Crysmod surfactant is 1.5 kg/ton P 2 O 5. The following is a detailed description of these techniques: Addition of Surfactant Before Nucleation: Mix surfactant with recycle acid for 30 minutes in the reaction vessel before addition of reactants (phosphate concentrate and sulfuric acid). Then, add reactants continuously for 30 minutes. 14

28 . Mix surfactant with recycle acid for 5 minutes before nucleation. Then add the reactants continuously for 30 minutes. Addition of Surfactant During Nucleation: Mix surfactant with a volume of recycle acid equivalent to 10% of the total volume of recycle acid. The suspension is then added continuously during the nucleation for 30 minutes. Mix surfactant with water (volume of water is 10% of the total volume of recycle acid). The suspension is then added continuously during the nucleation for 30 minutes. Addition of Surfactant After Nucleation: Mix surfactant with a volume of water (equivalent to 10% of the total volume of recycle acid). The suspension is then added continuously starting 30 minutes after nucleation for 30 minutes addition time. In these experiments, it is assumed that, the nucleation begins with the start-up of the reaction. 15

29 RESULTS AND DISCUSSION South Florida phosphate concentrate (high magnesium) was tested for phosphoric acid production with and without Crysmod and Hiflo-S5 surfactants. Surfactant addition techniques are studied in details (before nucleation, during nucleation and after nucleation) using Crysmod surfactant. During industrial crystallization of phosphogypsum, the presence of free (excess) sulfuric acid in the reaction medium is the most effective factor governing crystal quality. Its effect is not only on the crystal shape and size, but also on co-crystallized P 2 O 5 losses and the unreacted P 2 O 5 losses (Becker 1989). Free sulfate concentration affects the super-saturation, which is the driving force of both nucleation and crystal growth. At low sulfate content, the super-saturation is low and the rate of both nucleation and crystal growth are low and vice versa. The range of free sulfate industrially applied is % (Becker 1989). However, sulfate as low as 1.5% and as high as 3.5% can be encountered. Consequently, South Florida phosphate concentrate is tested at different sulfate contents (1.5%-3.5%). The results of filtration rates, reaction efficiencies, P 2 O 5 recoveries, washing efficiencies and size distribution of the phosphogypsum crystals are obtained. SEM photomicrographs of the phosphogypsum crystals are given. Basic studies are also given. STUDY OF SURFACTANT ADDITION TECHNIQUES Addition of Crysmod Surfactant Before Nucleation Tests were carried out in which the Crysmod surfactant (1.5 kg/ton P 2 O 5 ) is mixed with recycle acid before nucleation for duration times of 30 and 5 minutes. The data are given in Tables 9 and 10 and in Figures 2 and 3. The results suggest, in general, mixing of the surfactant with recycle acid and its addition before nucleation result in low filtration rates as compared to the baseline without surfactant. It may be attributed to an inhibition of regular crystal growth as indicated by the increase in the amount of fine crystals obtained in presence of surfactant as added before nucleation. Figures 2 and 3 show that % fine crystals of produced gypsum (<10 Fm) after 3 hours are increased to about 83% and 69% for tests of high mixing time (30 min) and low mixing time (5 min), respectively as compared to lower value of 64% for the baseline. The reasons for formation of very fine crystals are not yet understood. It may be attributed to retardation or inhibition of regular crystal growth by surfactant/recycle acid mixture. On the other hand, addition of surfactant has resulted in an increase in reaction efficiencies, P 2 O 5 recoveries and the washing efficiencies as compared to the baseline, where surfactant is not added (see Table 10). 17

30 Table 9. Effect of Adding Surfactant/Recycle Acid Mixture Before Nucleation on Phosphogypsum Filtration at Low Sulfate Levels ( %). Item Time, sec Surfactant Addition Technique Baseline (without Surfactant) To RA* Before Nucleation and Mixed for 30 Min. To RA* Before Nucleation and Mixed for 5 Min. Filter acid Recycle acid Wash I Wash II Drying Total time (in sec) Filtration rate, Ton P 2 O 5 /m 2.day Moisture % Change in filtration rate, % * RA: Recycle acid Table 10. Effect of Adding Surfactant/Recycle Acid Mixture Before Nucleation on Reaction Efficiency, P 2 O 5 Recovery and Washing Efficiency. Surfactant Addition Technique Reaction Efficiency, % P 2 O 5 Recovery, % Washing Efficiency, % Without surfactant (Baseline) To RA* before nucleation and mixed for 30 min To RA* before nucleation and mixed for 5 min * RA: Recycle acid 18

31 Volume % of Phosphogypsum Crystals, -10 microns Without Surfactant 30 min Mixing With Surfactant 5 min Mixing With Surfactant 60 After 1 hr. After 2 hr. After 3 hr. Figure 2. Volume % of Fine (-10 Micron) Phosphogypsum Crystals With and Without Surfactant (Surfactant Was Mixed With Recycle Acid Before Nucleation). Figure 3. Effect of Addition of Surfactant/Recycle Acid Mixture Before Nucleation on Phosphogypsum Crystal Size Distribution. 19

32 Addition of Crysmod Surfactant During Nucleation Tests were conducted by mixing the Crysmod surfactant (1.5 kg/ton P 2 O 5 ) with either recycle acid or water and adding the mixture during nucleation. The surfactant/liquid suspension is added continuously for 30 minutes with the reactants (during nucleation). The amount of either recycle acid or water is equivalent to 10% of total volume of recycle acid. The results are given in Tables 11 and 12 and Figs. 4 and 5. The filtration data in Table 11 show that mixing surfactant with recycle acid during nucleation gives low filtration rate. On the other hand, adding surfactant/water mixture during nucleation results in a higher filtration rate (5.54 ton P 2 O 5 /m 2.day). This increase in filtration rate corresponds to about 31% improvement. Data in Figs. 4 and 5 show that mixing surfactant with recycle acid results in an increase in the amount of fine crystals. On the other hand, less fines are produced due to adding surfactant/water mixture. The reasons of formation of very fine crystals upon the addition of surfactant /recycle acid mixture during nucleation are not yet understood. However, it may be attributed to reaction of the surfactant with the recycle acid leading to formation of some compounds that inhibit crystal growth. The reaction efficiencies, P 2 O 5 recoveries and the washing efficiencies of the phosphate concentrate with surfactant additions during nucleation are given in Table 12, together with the data obtained without surfactant addition (baseline). It is clear that the reaction efficiency is increased with the addition of surfactant. This may be due to decrease of lattice (co-crystallized) P 2 O 5 losses, which represent the majority of the total P 2 O 5 losses. 20

33 Table 11. Effect of Surfactant Addition During Nucleation on Phosphogypsum Filtration at Low Sulfate Levels ( %). Item Time, sec Surfactant Addition Technique Baseline (without Surfactant) With RA* and Added During the Nucleation for 30 Min. With Water and Added During the Nucleation for 30 Min. Filter acid Recycle acid Wash I Wash II Drying Total time (in sec) Filtration rate, Ton P 2 O 5 /m 2.day Moisture % Change in filtration rate, % * RA : Recycle acid Table 12. Effect of Surfactant Addition During Nucleation on Reaction Efficiency, P 2 O 5 Recovery and Washing Efficiency. Surfactant Addition Technique Reaction Efficiency, % P 2 O 5 Recovery, % Washing Efficiency, % Baseline With RA and added during the nucleation for 30 min With water and added during the nucleation for 30 min * RA: Recycle acid 21

34 Volume % of Phosphogypsum Crystals, -10 microns Without Surfactant With Surfactant in R.A. With Surfactant in Water After 1 hr. After 2 hr. After 3 hr. Figure 4. Volume % of Fine (-10 Micron) Phosphogypsum Crystals with and without Surfactant (During Nucleation). Figure 5. Effect of Surfactant Addition, During Nucleation, on Phosphogypsum Crystal Size Distribution. 22

35 Addition of Crysmod Surfactant After Nucleation A test was carried out by mixing the surfactant (1.5 kg/ton P 2 O 5 ) with water then adding the mixture continuously for 30 minutes. Feeding the surfactant mixture started after addition of phosphate concentrate and sulfuric acid. The amount of surfactant corresponds to 1.5 kg/ton P 2 O 5 and the amount of water is equivalent to 10% of total volume of recycle acid. The results are given in Tables 13 and 14 and Figures 6 and 7. The filtration data in Table 13 show only 8% improvement in the filtration rate. Results in Figures 6 and 7 suggest that the amount of fine gypsum crystals (<10 Fm) is decreased to about 37% upon addition of surfactant with water after nucleation. Note that -10 micron fraction in the baseline without surfactant is 64%. The reaction efficiencies, P 2 O 5 recoveries and the washing efficiencies obtained with surfactant addition after nucleation are given in Table 14 together with those achieved without surfactant (baseline). In this case also, the reaction efficiency has increased with the addition of surfactant. This may be attributed to decrease of lattice (coprecipitated) P 2 O 5 losses as mentioned above. Table 13. Effect of Surfactant Addition, After Nucleation, on Phosphogypsum Filtration at Low Sulfate Levels ( %). Item Surfactant Addition Technique Baseline (without Surfactant) Time, sec With Water and Added for 30 Min. after the Nucleation by 30 Min. Filter acid Recycle acid (RA*) Wash I Wash II Drying 5 5 Total time (in sec) Filtration rate, Ton P 2 O 5 /m 2.day Moisture % Change in filtration rate, % * RA : Recycle acid 23

36 Table 14. Effect of Surfactant Addition After Nucleation on Reaction Efficiency, P 2 O 5 Recovery and Washing Efficiency. Surfactant Addition Technique Reaction Efficiency, % P 2 O 5 Recovery, % Washing Efficiency, % Baseline With water and added for 30 min after addition of phosphate concentrate and sulfuric acid Volume % of Phosphogypsum Crystals, -10 microns Without Surfactant With Surfactant in Water 20 After 1 hr. After 2 hr. After 3 hr. Figure 6. Volume % ( 10 Microns) of Phosphogypsum Crystals With and Without Surfactant (Surfactant in Water Added After Nucleation). 24

37 Figure 7. Effect of Surfactant Addition, After Nucleation, on Phosphogypsum Size Distribution. TESTING OF SURFACTANTS AT DIFFERENT SULFATE CONTENTS Testing at Low Sulfate Content ( %) Tests have been carried out, with and without surfactants addition at low sulfate contents. The amount of applied Crysmod and Hiflo-S5 surfactants correspond to 1.5 and 0.75 kg/ton P 2 O 5 produced, respectively. The results are given in Tables and Figure 8. The filtration and reaction data are given in Table 15. The filtration rate is 4.24 ton P 2 O 5 /m 2.day without surfactant and improved to 5.54 and 6.88 ton P 2 O 5 /m 2.day with the addition of Crysmod and Hiflo-S5 surfactants, respectively. The percentages of filtration rate improvement correspond to about 31 and 62%, respectively. This is attributed to modification and growth of produced crystals upon surfactant addition. The size distributions of gypsum crystals (given in Tables 16 and 17 and Fig. 8) show that the volume percentages of fine crystals (<10Fm) is decreased to about 31% and 9% upon addition of Crysmod and Hiflo-S5 surfactants, respectively, while it is as high as 64% without surfactant addition. Also, the mean of crystal size increases with addition of surfactants. The data for reaction efficiencies, P 2 O 5 recoveries and washing efficiencies with and without additives are given in Table 15. It is important to note that the reaction 25

38 efficiency is increased with the addition of both surfactants. As mentioned before, this may be attributed to decrease of both lattice (co-crystallized) P 2 O 5 and unreacted P 2 O 5 losses. The lattice (co-crystallized) P 2 O 5 losses, generally, represent about 60% of the total P 2 O 5 losses. In this regard, it has been reported that, presence of a surfactant has resulted in reduction of P 2 O 5 in gypsum and this is attributed to the surfactant effect in reducing the capture of phosphate ions from solution by gypsum crystals. Regarding the decrease of unreacted phosphate, it was found that addition of surfactant increases the phosphate concentrate solubility and consequently increases reaction efficiency. In another study, it has been found that the presence of surfactant increases the solubility of calcium phosphate in thermal and wet-process phosphoric acid of 10-40% P 2 O 5 concentration and at temperature range of o C. The increase in reaction efficiency is significant as it approaches 3.9% and 4.1% and the increase in P 2 O 5 recovery is 1.6% and 3.1% with Crysmod and Hiflo-S5 surfactants, respectively. Table 15. Filtration and Reaction Data at Low Sulfate Content ( %). Item Without Additive With Crysmod Surfactant With Hiflo-S5 Surfactant Filtration rate, Ton P 2 O 5 /m 2.day Moisture % Increase in filtration rate, % Reaction efficiency, % P 2 O 5 recovery, % Washing efficiency, % Table 16. Comparative Size Analyses of Gypsum Crystals (At Low Sulfate Content and After 3hr). Size, Cumulative Volume % Passing µm Mesh* Without Additive With Crysmod Surfactant With Hiflo-S5 Surfactant * ASTM standards 26

39 Table 17. Statistics of Phosphogypsum Crystal Size Distribution (At Low Sulfate and After 3 Hr). Item Phosphogypsum Crystal Size Distribution, µm Mean, µm *d x d 10 d 25 D 50 d 75 d 90 Without surfactant With Crysmod surfactant With Hiflo-S5 surfactant *d x = diameter of crystals passing x% by volume. Figure 8. Comparative Size Distribution of Phosphogypsum Crystals (At Low Sulfate Content and After 3 Hr). 27

40 Testing at Medium Sulfate Content ( %) To cover the range of sulfate contents applied in industry, tests at medium sulfate content ( %) were carried out with and without addition of surfactants. At medium sulfate content, the obtained filtration data are given in Table 18. The filtration rate is increased by 25% and 43% due to the addition of Crysmod and Hiflo-S5 surfactants, respectively. This is attributed to modification and growth of produced crystals upon surfactant addition as confirmed by the size distribution of gypsum crystals given in Table 19 and Figure 9. It is clear that the volume percentage of coarse phosphogypsum crystals (>75Fm) is increased with surfactants addition. On the other hand, the volume percentage of fine phosphogypsum crystals (<38Fm) is decreased with surfactants addition. The reaction efficiencies and P 2 O 5 recoveries in presence and absence of surfactants at medium sulfate content are given in Table 18. It is most interesting that, at medium sulfate content, both P 2 O 5 recovery and reaction efficiency are higher with the addition of surfactants. The increase in reaction efficiency is significant as it approaches 2.8% with Crysmod surfactant and 1.9% with Hiflo-S5 surfactant as well as the increase in P 2 O 5 recovery is 1.4% with Crysmod surfactant and 1.8% with Hiflo-S5 surfactant. Table 18. Filtration and Reaction Data at Medium Sulfate Content ( %). Item Without Surfactant With Crysmod Surfactant With Hiflo-S5 Surfactant Filtration rate, tonp 2 O 5 /m 2.day Moisture % Increase in filtration rate, % Reaction efficiency, % P 2 O 5 recovery, % Washing efficiency, %

41 Table 19. Comparative Size Analyses of Gypsum Crystals (at Medium Sulfate Content and After 3 Hr). Size, Cumulative Volume % Passing µm mesh* Without Additive With Crysmod Surfactant With Hiflo-S5 Surfactant * ASTM standards Figure 9. Comparative Size Distribution of Phosphogypsum Crystals (At Medium Sulfate Content and After 3 Hr). 29

42 Washing with Simulated Pond Water. Two tests were carried out to study effect of using simulated pond water on the filtration rate without surfactant and with Hiflo-S5 surfactant. The simulated pond water has specific gravity of 1.02 g/ml and contains 1.5% P 2 O 5 and saturated with phosphogypsum. The results are reported in Table 20 and compared with the results using wash water. Filtration rate was slightly decreased upon washing with simulated pond water with and without surfactant. It is interesting to notice a slight decrease in filtration rate whenever pond water is used for washing instead of tap water. Table 20. Filtration and Reaction Data at Medium Sulfate Content ( %). Item Without Surfactant With Hiflo-S5 Surfactant Washing liquor Water Pond water Water Pond water Filtration rate, ton P 2 O 5 /m 2.day Moisture % Change in filtration rate, % Effect of Retention Time on Crystal Size Distribution. Slurry samples were taken from the reactor for crystal size distribution at different time intervals (after 0.5, 1, 2 and 3 hours) with and without surfactant addition. The results are reported in Tables and Figures The results show that, the volume percentages of fine crystals (-38 µm) are highly decreased with increasing the retention time for all the samples. In addition, the amount of fine crystals is less in presence of surfactants as compared to the tests without surfactant at all tested retention times. Table 21. Comparative Size Analyses of Gypsum Crystals (Without Surfactant and at Medium Sulfate Content). Size, Cumulative Volume % Passing µm mesh* After 0.5 hr After 1 hr After 2 hr After 3 hr * ASTM standards 30

43 Table 22. Comparative Size Analyses of Gypsum Crystals (With Crysmod and at Medium Sulfate Content). Size, Cumulative Volume % Passing µm mesh* After 0.5 Hr After 1 Hr After 2 Hr After 3 Hr * ASTM standards Table 23. Comparative Size Analyses of Gypsum Crystals (With Hiflo-S5 and at Medium Sulfate Content). Size, Cumulative Volume % Passing µm Mesh* After 0.5 Hr After 1 Hr After 2 Hr After 3 Hr * ASTM standards 31

44 Volume % of Phosphoypsum Crystals, -10 microns Without Surfactant With Crysmod Surfactant WithHiflo-S5 Surfactant 0 After 0.5 hr After 1 hr After 2 hr After 3 hr Figure 10. Volume % (-10 µm) of Phosphogypsum Crystals With and Without Surfactants. 32

45 Volume % of Phosphogypsum Crystals, -38 microns Without Surfactant With Crysmod Surfactant With Hiflo-S5 Surfactant 0 After 0.5 hr After 1 hr After 2 hr After 3 hr Figure 11. Volume % (-38 µm) of Phosphogypsum Crystals With and Without Surfactants. Figure 12. Comparative Size Distribution of Phosphogypsum Crystals (Without Surfactant at Medium Sulfate Content). 33

46 Figure 13. Comparative Size Distribution of Phosphogypsum Crystals (With Crysmod Surfactant at Medium Sulfate Content). Figure 14. Comparative Size Distribution of Phosphogypsum Crystals (With Hiflo-S5 Surfactant at Medium Sulfate Content). 34

47 Testing at High Sulfate Content ( %) The excess of sulfuric acid in the reaction slurry is the most effective factor governing crystallization quality of phosphogypsum. Its effect is not only on the crystal shape and size, but also on co-crystallized P 2 O 5 losses and the unreacted P 2 O 5 losses. Sulfate concentration affects super-saturation of the medium. At high sulfate content, the super-saturation is high and the rate of both nucleation and crystal growth are high. The amounts of applied Crysmod and Hiflo-S5 surfactants correspond to 1.5 and 0.75 kg/ton P 2 O 5 produced, respectively. The results obtained are given in Tables and Figure 15. It is interesting to note that the filtration rate (6.77 ton P 2 O 5 /m 2.day without additive) has improved to 8.33 and 8.98 ton P 2 O 5 /m 2 day with the addition of Crysmod and Hiflo-S5 surfactants, respectively. The increase in filtration rate is related to the growth of crystals upon surfactants addition and formation of high volume percent of coarse crystals (>75 µm). The size distribution of phosphogypsum crystals given in Tables 25 and 26 and Figure 15 shows that, the volume percentages of coarse crystals (>75 µm) are increased to about 15% and 24% upon addition of Crysmod and Hiflo-S5 surfactants, respectively while it is as low as 10% without surfactant addition. Also, the mean of crystal size increases with addition of surfactants. The reaction efficiencies and P 2 O 5 recoveries in presence and absence of surfactants are given in Table 24. It is most interesting that even at high sulfate content, both P 2 O 5 recovery and reaction efficiency are higher with the addition of surfactants. The increase in reaction efficiency is significant as it approaches 3.9% and 4.1% and the increase in P 2 O 5 recovery is 1.6% and 3.1% with Crysmod and Hiflo-S5 surfactants, respectively. Table 24. Filtration and Reaction Data at High Sulfate Content ( %). Item Without Additive With Crysmod Surfactant With Hiflo-S5 Surfactant Filtration rate, Ton P 2 O 5 /m 2.day Moisture % Increase in filtration rate, % Reaction efficiency, % P 2 O 5 recovery, % Washing efficiency, %

48 Table 25. Comparative Size Distribution of Gypsum Crystals (At High Sulfate Content and After 3 Hr). Size, Cumulative Volume % Passing µm Mesh* Without Additive With Crysmod Surfactant With Hiflo-S5 Surfactant * ASTM standards Table 26. Statistics of Phosphogypsum Crystal Size Distribution (At High Sulfate and After 3 Hr). Item Phosphogypsum Crystal Size Distribution, µm Mean, µm *d x d 10 d 25 d 50 d 75 D 90 Without surfactant With Crysmod surfactant With Hiflo-S5 surfactant *d x = diameter of crystals passing x% by volume. 36

49 Figure 15. Comparative Size Distribution of Phosphogypsum Crystals (at High Sulfate Content and After 3 Hr). Correlation of Crystal Size Distribution of Phosphogypsum and the Filtration Rate The relationship between amount of phosphogypsum fines and the filtration rates are given in Tables 27 and 28. These results show that, at each sulfate level, higher filtration rates are obtained as the fine crystals (-10 Fm) decrease, and the mean and the median of crystal size distribution increase. 37

50 Table 27. Relationship Between % Phosphogypsum Fines and Filtration Rate At Low Sulfate Content. Surfactant Addition Technique To RA before nucleation and mixed for 30 min With RA and added for 30 min during the nucleation To RA before nucleation and mixed for 5 min Baseline (without surfactant) With water and added for 30 min after addition of phosphate concentrate and sulfuric acid With water and added for 30 min during the nucleation RA: Recycle acid Phosphogypsum Fines (-10 Fm), % Mean Diameter, µm Median, µm Specific Surface Area, cm 2 /g Filtration Rate, Ton P 2 O 5 /m 2. Day

51 Table 28. Relationship Between % Phosphogypsum Fines and Filtration Rate at Different Sulfate Contents. Item Phosphogypsum Fines (-10 Fm), % Mean Diameter, µm Median, µm Specific Sur. Area, cm 2 /g Filtration Rate, Ton P 2 O 5 /m 2. Day Low Sulfate Content: Without surfactant With Crysmod With Hiflo-S Medium Sulfate Content: Without surfactant With Crysmod With Hiflo-S High Sulfate Content: Without surfactant With Crysmod With Hiflo-S Gypsum Morphology One of the most important factors affecting filtration rate is gypsum morphology (size and shape of the crystals). For best filtration and washing, crystals of uniform sizes are most desirable. At low sulfate content, shape of the crystals is changed from tabular crystals to clusters by addition of the surfactants. On the other hand, at high sulfate content large clusters are formed upon using the surfactants as shown in SEM photomicrographs, Plates 1-8. These clusters are composed of many small crystals while small clusters are formed without addition of surfactants. This may be due to adsorption of surfactants on crystal surfaces rendering it hydrophobic. Thus, clusters may form due to hydrophobic interactions between crystals. Similar findings have been reported in literature (Sirianni and others 1969). 39

52 Plate 1. SEM Photomicrograph of Phosphogypsum without Surfactant (Magnification 500). Plate 2. SEM Photomicrograph of Phosphogypsum without Surfactant (Magnification 2000). 40

53 Plate 3. SEM Photomicrograph of Phosphogypsum with Crysmod Surfactant (Magnification 200). Plate 4. SEM Photomicrograph of Phosphogypsum with Crysmod Surfactant (Magnification 500). 41

54 Plate 5. SEM Photomicrograph of Phosphogypsum with Crysmod Surfactant (Magnification 2000). Plate 6. SEM Photomicrograph of Phosphogypsum with Hiflo-S5 Surfactant (Magnification 200). 42

55 Plate 7. SEM Photomicrograph of Phosphogypsum with Hiflo-S5 Surfactant (Magnification 500). Plate 8. SEM Photomicrograph of Phosphogypsum with Hiflo-S5 Surfactant (Magnification 2000). 43

56 BASIC STUDIES: PRIMARY NUCLEATION OF CALCIUM SULFATE DIHYDRATE Several investigators have studied primary nucleation of calcium sulfate dihydrate using pure chemicals. However, no data have been reported under simulated industry conditions. Therefore, these data cannot be extended to explain crystallization of phosphogypsum. Thus, in our studies, pure chemicals, including phosphoric acid, are reacted under industrially simulated conditions of temperature, P 2 O 5 and free H 2 SO 4 acid concentrations. The primary nucleation of calcium sulfate dihydrate with and without surfactant is followed by turbidity measurement. The primary nucleation occurs when there are no solids in the medium and it continues until the solids content becomes as high as 10% by volume as described by Becker (1989). In addition, the induction time (the time elapsed between achievement of super-saturation by addition of reactants and appearance of nuclei, turbidity) is measured. The reaction that takes place can be written as:: CaH 4 (PO 4 ) 2 H 2 O + H 2 SO 4 + H 2 O 2H 3 PO 4 + CaSO 4 2H 2 O MATERIALS Pure chemicals including phosphoric and sulfuric acids, and calcium hydrogen phosphate monobasic (from Fisher Scientific) are used to prepare the following solutions: Solution # 1: 27.5 % P 2 O 5 and 2.5 % H 2 SO 4 Solution # 2: 20% P 2 O 5 Solution # 3: 32.5 % H 2 SO 4 PROCEDURE 1. Add 500 ml of solution # 1 in 800 ml beaker and heat to 80 C using a constant temperature water bath. 2. Then add simultaneously the following solutions (see Tables 29-30): Calcium Hydrogen Phosphate Monobasic (which is prepared by dissolving the required amount in 100 ml of solution #2 and the required amount of sulfuric acid, solution #3, and 50 ml of deionized water or water/surfactant solution. 3. Keep reaction at 80 C with constant agitation. 4. Measure turbidity of the resulting suspension at different time intervals during the course of the reaction using a HACH 2100A Turbidimeter. 5. Take samples (5 ml) from the pregnant liquor for particle size distribution analysis after 1 minute, 5 minutes, 15 minutes and at the end of the experiment (when the turbidity value exceeds 1000 NTU). 45

57 6. For size distribution measurements, disperse the samples in about 50 ml of methanol and measure the size distribution using a Coulter Laser Size Analyzer. Table 29. Amounts of Calcium Hydrogen Phosphate Monobasic and Sulfuric Acid for Different Super-saturation. Calcium Hydrogen 32.5% H 2 SO 4 Acid, ml Super-saturation Phosphate Monobasic, g Table 30. Experimental Conditions for Primary Nucleation Studies. Experiment No. Supersaturation Surfactant Used Surfactant Concentration, ppm Amount of Surfactant, g Without Crysmod Hiflo-S Without Crysmod Hiflo-S Without Crysmod Hiflo-S Without Crysmod Hiflo-S Crysmod Hiflo-S Crysmod Hiflo-S Crysmod Hiflo-S Crysmod Hiflo-S

58 TURBIDITY MEASUREMENTS AND ESTIMATION OF INDUCTION TIME (T) As mentioned above, turbidity of the solution is measured at different time intervals. A graph of time vs. turbidity is plotted in Figure 16. The time corresponding to the point of intersection of the two asymptotic lines represent the induction time (see example in Figure 16). In this particular figure, T = 130 min. Turbidity (NTU) Induction Time of Calcium Sulfate Dihydrate with Crysmod Surfactant Supersaturation Time (min) Figure (3). From the Figure Time = 130 min Figure 16. Example of Turbidity Data Plotting and Induction Time Estimation. CALCULATION OF CRYSTAL GROWTH EFFICIENCY (E) Induction times are calculated as described above for the data collected in presence and absence of surfactants. From the estimated values, crystal growth efficiency (E) is calculated by the following equation as reported by Georgiev and others (1995): E = (T 0 T 1 ) T 0 where: T 0 = Induction time in absence of surfactant T 1 = Induction time in presence of surfactant 47

59 CALCULATION OF SUPER-SATURATION (S) Super-saturation (S) is calculated (Tavare 1995) and listed in Table 31 as follows: M.W. CaO Mass CaO = Mass CaH 4 (PO 4 ) 2 H 2 O M.W. CaH 4 (PO 4 ) 2 H 2 O Mass CaO % CaO = 100 Total Mass % CaO x M.W. CaSO 4 2H 2 O c = Calcium sulfate dihydrate concentration = M.W. CaO c* = Calcium sulfate dihydrate (solute) solubility at the given temperature = 0.83 (Becker, 1989) c S = Super-saturation = c* Table 31. Calculation of Super-saturation. Calcium Phosphate % CaO C c* S Monobasic, g EFFECT OF SURFACTANTS ON CRYSTAL GROWTH (TURBIDITY) AT DIFFERENT SUPER-SATURATION Effect of surfactants on crystal growth (turbidity) at different super-saturation, is given in Figs The surfactant concentration used is 100 ppm. It is clear that, both Crysmod and Hiflo-S5 surfactants are crystal growth promoters, because in their presence the solution turbidity reaches the highest value of 1000 NTU faster than the control solution (without any surfactant). 48

60 1200 Turbidity (NTU) Without Surfactant Crysmod Surfactant Hiflo-S Time (min) Figure 17. Effect of Time on Turbidity of Calcium Sulfate Dihydrate (Super-saturation 1.018, 100 ppm surfactant) Without Surfactant Crysmod Surfactant Turbidity (NTU) Hiflo-S5 Surfactant Time (min) Figure 18. Effect of Time on Turbidity of Calcium Sulfate Dihydrate (Super-saturation 1.222, 100 ppm surfactant) 49

61 1200 Turbidity (NTU) Without Surfactant Crysmod Surfactant Hiflo-S5 Surfactant Time (min) Figure 19. Effect of Time on Turbidity of Calcium Sulfate Dihydrate (Super-saturation 1.502, 100 ppm surfactant) Turbidity (NTU) Without Surfactant Crysmod Surfactant Hiflo-S5 Surfactant Time (min) Figure 20. Effect of Time on Turbidity of Calcium Sulfate Dihydrate (Super-saturation 1.979, 100 ppm surfactant) 50

62 EFFECT OF SUPER-SATURATION ON INDUCTION TIME AND CRYSTAL GROWTH EFFICIENCY WITH AND WITHOUT SURFACTANTS Induction time and crystal growth efficiency are determined at different supersaturation with and without surfactants and given in Table 32. The surfactant concentration used is 100 ppm. These results confirm that Crysmod consistently lowers the induction time to a greater degree than Hiflo-S5 when 100 ppm of surfactant is used at different super-saturation. The induction time (determined from Figures 17-20) of the solution without surfactant is always higher than that with Crysmod or Hiflo-S5 surfactant. In all three cases, as the super-saturation is increased, the induction time is decreased. Table 32. Effect of Super-saturation on Induction Time (T) and Growth Efficiency (E) of Gypsum Crystals (Using 100 ppm Surfactant). Super-saturation Surfactant T* E T E T E T E None Crysmod Hiflo-S * in minutes EFFECT OF SURFACTANTS CONCENTRATION ON INDUCTION TIME AND CRYSTAL GROWTH EFFICIENCY Effect of surfactants concentration on induction time and growth efficiency was studied in the range of 1.2 to 100 ppm at constant super-saturation of The results are given in Table 33. It is clear that when the concentration of Hiflo-S5 is decreased from 50 ppm to 1.2 ppm, the induction time is decreased and the growth efficiency becomes higher. This may suggest that Hiflo-S5, could be more efficient since less surfactant concentration is required in the reacting system. The only exception to this trend is noticed when 100 ppm of HiFlo-S5 surfactant is used. Crysmod does not show, however, a similar trend with changing concentrations, possibly because Crysmod is actually composed of several different surfactants. However, more work is needed to confirm these data since the differences in induction time are small. 51

63 Table 33. Effect of Surfactant Concentration on Induction Time (T) and Growth Efficiency (E) of Gypsum Crystals (Using Super-saturation). Surfactant Concentration, ppm Surfactant T* E T E T E T E T E None Crysmod Hiflo-S * in minutes CORRELATION BETWEEN SUPER-SATURATION AND INDUCTION TIME The induction time without and with 100 ppm of surfactant is plotted as a function of super-saturation, Figure 21. When the natural log of the induction time is plotted against super-saturation in Figure 22, a linear relationship is observed. This confirmed that the plot of Figure 8 is indeed exponential. Note that the induction time is inversely proportional to the super-saturation. Also, Figure 22 shows that, at high supersaturation the efficiency of the surfactants is higher. It may be inferred from these data that in the presence of surfactants, retention times may be reduced leading to high throughputs. However, this needs to be confirmed on plant scale testing. Induction Time (min) Without Surfactant Crysmod Surfactant Hiflo-S5 Surfactant Without Surfactant Crysmod Surfactant Hiflo-S5 Surfactant Supersaturation Figure 21. Relation Between Super-saturation and Induction Time of Calcium Sulfate Dihydrate. 52

64 6 Without Surfactant 5 4 Crysmod Surfactant Hiflo-S5 Linear (Without Surfactant) Linear (Crysmod Surfactant) Linear (Hiflo-S5) ln t ind 3 2 Without surfactant: y= x With Crysmod: y = x With Hiflo-S5: y= x Super-saturation Figure 22. Linear Relationship Between Super-saturation and Natural Log of Induction Time for Calcium Sulfate Dihydrate. EFFECT OF SURFACTANTS ON CRYSTAL SIZE DISTRIBUTION OF GYPSUM Gypsum crystal size distribution analysis at a super-saturation of after 5 minutes is determined in the presence and absence of 100 ppm surfactants (Figure 23). The results show that both surfactants increase the gypsum crystal sizes. According to Table 34, the d 50 of the formed crystals is µm without surfactant while with Crysmod and Hiflo-S5 surfactants the d 50 values are and µm, respectively. 53

65 Figure 23. Differential Volume of Gypsum Crystals Obtained after 5 Minutes at a Super-saturation of (Surfactant Concentration = 100 ppm). Table 34. Statistics of Size Distribution of Gypsum Crystals. Item Gypsum Crystals Size Distribution, µm Mean Diameter, µm D x d 10 d 25 d 50 d 75 d 90 Without Surfactant With Crysmod Surfactant With Hiflo-S5 Surfactant *d x = diameter of crystals passing x% by volume. 54

66 EFFECT OF SURFACTANTS ON GYPSUM CRYSTAL MORPHOLOGY Gypsum crystal morphology was investigated without and with 100 ppm surfactant. The photomicrographs are taken using an Olympus BX60F5 microscope. The calcium sulfate dihydrate crystals (gypsum) in these photomicrographs are precipitated at a super-saturation of These photomicrographs confirm that Crysmod and Hiflo-S5 surfactants have significantly altered the morphology of the gypsum crystals. The crystals shown in Plate 9, formed in the absence of surfactant, are needle-like shape with high aspect ratio. Needle-like crystals give low filtration rate and maintain high amounts of liquid. On the other hand, Plates 10 and 11 show crystals that are much larger in size and tabular in shape with low aspect ratio. These crystals are expected to allow high filtration rates. It is important to mention that the results of the fundamental studies confirm the data obtained in the digestion work using phosphate concentrate. Plate 9. Photomicrograph of Gypsum Crystals Formed Without Surfactant. 55

67 Plate 10. Photomicrograph of Gypsum Crystals Formed with Crysmod Surfactant (100 ppm). Plate 11. Photomicrograph of Gypsum Crystals Formed with Hiflo-S5 Surfactant (100 ppm). 56