Ind. Eng. Chem. Res 1989, 28, 1101-1103 df = diameter of funnel constriction, length K = overall mass-transfer coefficient before perturbation,
length/ time K * = perturbed value of the mass transfer coefficient, length/ time K * = mean overall coefficient in a packed tower, length/time K , = overall coefficient in a spray column, length/time At = duration of perturbation, time Literature Cited Bozorgzadeh, F. Study of Mass Transfer during Droplet Splitting using a New Optical Technique. Ph.D. Thesis, University of Newcastle upon Tyne, 1980. Garner, F. H.; Tayeban, M. The Importance of the Wake in Mass Transfer from both Continuous and Dispersed Phase Systems. An. Real SOC.ESP.Fis. Quim. Ser. B-Quim. 1960, LVl(B),479. Javed, K. H.; Thornton, J. D. Time Dependent Mass Transfer Rates in a Liquid-Liquid System exhibiting Interfacial Turbulence. Inst. Chem. Eng. Symp. Ser. 1984, 88, 203. Liddell, J. Liquid-Liquid Extraction Stuides in a Single Droplet Column. Ph.D. Thesis, University of Durham, 1963. Lindland, K. P.; Terjesen, S. G. Effect of a Surface Active Agent on Mass Transfer in Falling Drop Extraction. Chem. Eng. Sci. 1956, 5, 1. Mardous, N. G.; Sawistowski, H. Simultaneous Transfer of Two Solutes across Liquid-Liquid Interfaces. Chem. Eng. Sci. 1964, 19, 919. Rahman,M. Study of Mass Transfer into Droplets in Liquid-Liquid Systems using a new Optical Technuque. Ph.D. Thesis, University of Newcastle upon Tyne, 1977. Ramshaw, C.; Thornton, J. D. Droplet Breakdown in a Packed Extraction Column: Part 1. The Concept of Critical Droplet Size. Inst. Chem. Eng. Symp. Ser. 1967,26, 73.
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Rogers, D.; Thompson, P. J.; Thornton, J. D. Time Dependence of the Mass Transfer of Uranyl Nitrate between Nitric Acid and Tributyl Phosphate. Inst. Chem. Eng. Symp. Ser. 1987,103,15. Skelland, A. H. P.; Wellek, R. M. Resistance to Mass Transfer inside Droplets. AZChE J. 1964, 10, 491, 789. Thornton, J. D. Droplet Behaviour and Mass Transfer Rates in Liquid-Liquid Extraction Operations. Ind. Chem. 1963,39(12), 632; 1964, 40(1), 13. Thornton, J. D. Interfacial Phenomena and Mass Transfer in Liquid-Liquid Extraction. Chem. Ind. 1987, G(March 16), 193. Thornton, J. D.; Egbuna, D. 0.;Rahman, M. Simulation of Droplet Behaviour in Packed Liquid Extraction Columns. Proceedings of Solvent Extraction Symposium, University of Newcastle upon Tyne, 1976. Thornton, J. D.; Anderson, T. J.; Javed, K. H.; Achwal, S. K. Surface Phenomena and Mass Transfer Interactions in Liquid-Liquid Systems. AZChE J . 1985, 31(7), 1069. *Author to whom correspondence should be addressed at Battle Well, Greenhill, Evesham, Worcestershire W R l l 4NA, England.
William Batey Dounreay Nuclear Power Development Establishment UKAEA Thurso, Caithness KW14 7TZ, Scotland
John D. Thornton* Department of Chemical Engineering University of Newcastle upon Tyne Newcastle upon Tyne NE1 7RU, England Received for review October 17, 1988 Revised manuscript received March 2, 1989 Accepted March 26, 1989
Recovery of Uranium and Lanthanides from Ca(H2P04)2-Ca(N03)2-H20 and Ca(H2P04)2-CaC12-H20Systems Solutions containing Ca(H2P04)2-Ca(N03)2-H20and Ca(H2P04),-CaC12-H20 obtained by leaching phosphate rock in situ or in dumps can be treated for uranium and lanthanides recovery prior t o P205recovery as follows: In a nitrate system, uranium is first extracted by a mixture of bis(2ethylhexy1)phosphoric acid (DBEHPA) and tributyl phosphate (TBP) in hexane followed by the extraction of the lanthanides with TBP. In a chloride system, uranium is first extracted by the same mixture, DBEHPA TBP, followed by the extraction of lanthanides with D2EHPA in toluene.
+
Solutions containing Ca(H2P04)2-Ca(N03)2and Ca(H,P04)2-CaC12 are obtained when phosphate rock is leached by dilute HNO, and HC1, respectively. This is the basis of a new process proposed for leaching phosphate rock in situ or in dumps to decrease material handling problems especially when dealing with deposits like those in Central Florida, which is one of the largest phosphate rock producers in the world. The mineralogical analysis of these deposits is approximately 28% calcium phosphate, Ca3(PO4I2(Bone phosphate of lime), 36% silica sand (Si02), and 34% clay minerals. To get 1 ton of a commercial product, 4.2 tons overburden must be removed to expose 3.36 tons of ore, which is processed by physical and mechanical means to get the concentrate (1 ton), and to reject 1.4 tons of tailings and 0.95 ton of slime waste (clays). Due to the excessive material handling problems facing this technology (rejection of 7.56 tons of waste material per ton of product, plus enormous amounts of water) and the loss of about onethird of the phosphate values, the techniques known as in situ and dump leaching were pr0posed.l When using these techniques, which are applied in the uranium, gold, and copper industries,* the leach solution would be either injected in the bed containing the phosphate values or 0888-5885/ 89/ 2628- 1101$01.50/0
sprayed on the phosphate rock dumps with minimum material handling and efficient phosphate value recovery. It would be necessary in these cases to leach the rock with 10% HC1 or 20% HN03 to prevent the blocking of the beds with insoluble dicalcium phosphate. Also, H2S04, which is the cheapest acid, cannot be used because of the blocking of the bed with insoluble gypsum. Under the conditions of the proposed process, a solution of monocalcium phosphate was obtained, from which the double salts CaC1H2P04.H20or Ca(NO3)H2PO4-H20 were crystallized depending on the acid used.l The crystals were then separated and decomposed a t 200-250 "C to get dicalcium phosphate product (CaHP04). Nitric and hydrochloric acids used for this purpose, although they are expensive, have the advantage of solubilizing rapidly not only P205content of the rock but also uranium, lanthanides, and radium; hence their recovery or disposal (in the case of radium) can be c ~ n d u c t e d . ~In - ~ case of H2S04 leaching in conventional plants, only uranium can be recovered because lanthanides and radium remain in the gypsum. Uranium in the rock is about 0.015%e and the lanthanides about 0.5%.' Considering the large tonnage of rock treated annually, which is about 150 X lo6 short tons, 0 1989 American Chemical Society
1102 Ind. Eng. Chem. Res., Vol. 28, No. 7 , 1989 Table I. Analysis of Leach Solutions 10% HCl, g/L 20% HNOB, g/L pZ05 66.25 76.84 Ca 65.2 81.61 Mg 0.66 0.56 Fe 0.82 0.90 A1 3.33 4.41 F 1.52 0.37 U 0.03 0.05 Ln203 1.07 1.31
Table 11. Extraction of Uranium from HCl and HNOS Leach Solutions by Mixed Organic Solvents in Hexane Diluent. Three-Stage Extraction at Organic/Aqueous Phase Ratio 1:4 uranium extraction,
recovering these metals as byproducts during the production of fertilizers represents an important benefit. The purpose of this report is to describe methods found for their recovery.
the double salts Ca(N0,).H2P04.H20 and CaCEH2P04.H20 were obtained depending on the acid used. These crystals are then decomposed at 200-250 "C to get dicalcium phosphate (40% P,OB), suitable for shipment. All the uranium and lanthanides present in the rock were present in this product. The recovery of these metals, however, was possible from the solutions before evaporation as follows: HNO, System. When HNO, was used for leaching, the solution contained Ca(H2P04)2and Ca(N03)2. It was not possible to extract uranium from this solution by TBP alone, but a mixture of D2EHPA and TBP diluted with hexane was effective," as shown in Table 11. No lanthanides were coextracted. The concentration of the extractant in the organic phase was critical since dilute solutions in hexane were less effective. The loaded organic phase was scrubbed with the least amount of distilled water at the aqueous/organic phase ratio 1/10 in three stages to remove all the coextracted impurities. The organic phase was then stripped with 10% ammonium carbonate solution to form ammonium uranate precipitate. The organic solvent was then recycled to the extraction stage. The extraction of lanthanides was possible by TBP alone at the natural pH of the leach solution (pH = 0.3), and at the organic/aqueous phase ratio 111 in one stage, it was found that 100% of lanthanides was transferred into the organic phase. The lanthanides were stripped from the organic phase by dilute HNO, acid in two stages at the aqueous/organic ratio 1/1and then selectively precipitated by 12% oxalic acid solution. The oxalate precipitate was then calcined a t 1200 "C for 1 h to yield a concentrate analyzed as 12% Ln203and 88% CaO. The weight of product represents 2% of the original weight of the phosphate rock leached. HCl System. When HC1 was used for leaching, the solution contained Ca(H,P0J2 and CaC12. In this system too, it was not possible to extract uranium with TBP alone, but a mixture of D2EHPA and TBP diluted with hexane was effective, as shown in Table 11. No lanthanides were coextracted. The concentration of the extractant in the organic phase was also critical. Stripping of uranium and its recovery from the strip solution was possible as indicated earlier in the HNO, system. The extraction of lanthanides from solution after the removal of uranium was not possible by TBP, and also it was not effective to make an oxalate precipitation because excessive calcium also precipitated as CaHP04. It was also not possible to effectively precipitate the hydroxides by adding CaO, as shown in Table 111; while 95% of the lanthanides was precipitated, their content in the precipitate was only 1%. The problem was solved by using D2EHPA diluted with toluene; in this case, all the lanthanides were extracted.
Experimental Section The phosphate rock sample was obtained from Central Florida, analyzed as 18.22% P205,which corresponds to 40% calcium phosphate and 60% insoluble gangue minerals (clays and silica sand). Since mining the rock is conducted by high-pressure water jets, the sample was wet and represented the actual granulometry of the mined rock. The sample was therefore dried at 80 "C overnight to represent as much as possible the original rock. It was composed mainly of lumps about 5 mm in size with about 20% fines. Reagent-grade HC1 and HNO, were diluted with distilled water to the desired concentration. The organic solvents used were tributyl phosphate, (C4HJ3P0, bp 180-183 "C (Aldrich), abbreviated as TBP; and bis(2ethylhexy1)phosphoric acid, abbreviated as D2EHPA (Albright & Wilson Inc.), diluted with hexane or toluene. Procedure. Dried phosphate rock samples were leached in open vessels using 10% HC1 and 20% HNO, acid to simulate solutions obtained by in situ and dump leaching performed earlier.' Batches (400g) of phosphate rock were dissolved in 680 mL of 20% HNO, and 840 mL of 10% HC1. These were the quantities required to produce monocalcium phosphate solution. The reaction mixture was agitated for 1 h a t room temperature and then left overnight. The mixture was filtered to remove the siliceous gangue and clays and the volumes of the solution, and the washings were adjusted to 700 and 800 mL, respectively. The analysis of the leach solution of HNO, and HC1 is shown in Table I. Solvent extraction and stripping experiments were conducted by agitating the organic and aqueous phases in open vessels with magnetic stirrers for 10 min and then transferring to a separating funnel to effect the separation of phases. Extraction was always conducted in three stages unless otherwise stated. (1) Uranium was determined spectrophotometrically using arsenazo 1.8Nitric acid was first added to the leach solutions of HNOBand HC1 before extraction with tributyl phosphate to form uranium nitrato complexes. (2) Lanthanides were determined after oxalate precipitation by titration with ethylenediaminetetracetic acid disodium salt, EDTA, using arsenazo 1 as i n d i ~ a t o r . ~ (3) P20, was determined by precipitation as phosphomolybdate followed by titration with 0.5 M NaOH.1° Results and Discussion In the proposed process, phosphate rock was leached with dilute HN03 or HC1 to get monocalcium phosphate solution: Calo(P04)6F+ 14H+
-
6H2P04+ 10Ca2++ 2HF
When the leached solution was evaporated to near dryness,
70
solvents 0.04 M D2EHPA + 0.03 M TBP 0.4 M D2EHPA + 0.3 M TBP
HC1 solution 84 99
HN03 solution 33.3 100
Conclusions When phosphate rock is leached in situ or in dumps, it is possible to recover its uranium and lanthanide contents
Ind. Eng. Chem. Res. 1989,28, 1103-1106 Table 111. Precipitation of Lanthanides by CaO Addition to the Leach Solution in the HCl System (50 mL of Leach Solution; 50 g of Rock) precipitate CaO
added, 1.0 2.0 3.0
final uH 1.2 1.8 5.0
wt,
g
1.1 4.9 9.1
LnZO3O content, % 0 0.2 1.0
Ln203
recovery. % 0 19 95
Ln = lanthanides.
(nearly 100%) from the leach solutions prior to P205recovery. The solutions obtained containing monocalcium phosphate can be treated as follows: (1)In the HNO, system, uranium is first extracted by a mixture of D2EHPA and TBP in hexane followed by extraction of lanthanides by TBP. (2) In the HCl system, uranium is first extracted by a mixture of D2EHPA and TBP in hexane followed by extraction of lanthanides by D2EHPA in toluene.
Acknowledgment We are grateful to H. J. Cheek of Albright & Wilson Inc. for kindly supplying us with a sample of bis(2-ethylhexyl) phosphoric acid. Registry No. DSEHPA, 298-07-7; TBP, 126-73-8; Ca(H2P04)2, 7758-23-8; Ca(N0&, 10124-37-5;CaC12, 10043-52-4;U, 7440-61-1.
References (1) Habashi, F.; Awadalla, F. T. In Situ and Dump Leaching of Phosphate Rock. Ind. Eng. Chem. Res. 1988, 27, 2165-69. (2) Habashi, F. Hydrometallurgy. In Principles of Extractive Metallurgy; Gordon & Breach Science Publishers: New York-London-Paris, 1970 (reprinted 1980); Vol. 2.
1103
(3) Habashi, F.; Awadalla, F. T.; Zailaf, M. The Recovery of Uranium and the Lanthanides from Phosphate Rock. J. Chem. Technol. Biotechnol. 1986, 36, 259-267. (4) Habashi, F.; Awadalla, F. T.; Yao, Xin-bao The Hydrochloric Acid Route to Phosphate Rock. J. Chem. Technol.Biotechnol. 1987, 37, 371-383. (5) Awadalla, F. T.; Habashi, F. The Removal of Radium During the Production of Nitrophosphate Fertilizer. Radiochim. Acta 1985,38, 207-210. (6) Habashi, F. The Recovery of Uranium from Phosphate Rock. Progress and Problems. In 2nd International Congress on Phosphorus Compounds Proceedings, Institut Mondial du Phosphate: Paris, 1980; pp 629-660. (7) Habashi, F. The Recovery of Lanthanides in Phosphate Rock. J. Chem. Technol. BiotechnoL 1985,35A(l), 5-14. (8) Awadalla, F. T.; Habashi, F. Determination of Uranium and Radium in Phosphate Rock and Technical Phosphoric Acid. 2. Anal. Chem. 1986,324, 33-36. (9) Habashi, F.; Zailaf, M.; Awadalla, F. T. Determination of the Lanthanides in Phosphate Rock. 2.Anal. Chem. 1986,325(1), 479-480. (10) Wilson, H. N. The Determination of Phosphate in the Presence of Soluble Silicates. Application to the Analysis of Basic Slag and Fertilizers. Analyst 1954, 75, 535-546. (11) Amer, S. Aplicaciones de la extraccion con disolventes a la hidrometalurgia. V Parte. Tierras raras, itrio y escandio. Rev. Metal. CENIM 1981, 17(4), 245-283.
* To whom
all correspondence should be addressed.
Farouk T. Awadalla, Fathi Habashi* Department of Mining & Metallurgy Lava1 University Quebec City, Canada G l K 7P4 Received for review October 24, 1988 Revised manuscript received March 17, 1989 Accepted April 11, 1989
Generalized Temperature-Dependent Parameters for the Peng-Robinson Equation of State for n -Alkanes Temperature-dependent parameters for the Peng-Robinson equation of state have been determined from saturated liquid and vapor volumes and vapor pressure data in the subcritical region and from P-V-T data in the supercritical region for a number of n-alkanes for which sufficient data were available. These parameters have been generalized in terms of reduced temperature and acentric factor, so they can be used for other hydrocarbons. While liquid volumes are generally poorly predicted with cubic equations of state and generalized parameters, we find t h a t with the use of the Peng-Robinson equation of state with the proposed, generalized parameters the predicted liquid volumes are much better than those obtained with the original parameters, while other properties, such as vapor pressure and vapor volumes, are of comparable accuracy. Equations of state are frequently used for predicting the volumetric and thermodynamic properties of fluids and vapor-liquid equilibria. One method of improving the quality of equation of state predictions is to use temperature-dependent parameters obtained by fitting experimental data for the fluid under consideration (Harmens, 1975; Hsi and Lu, 1971; Joffe et al., 1970). However, correlations or generalizations of these fluid-specific parameters are needed for calculations of vapor-liquid equilibrium and volumetric properties in design and in process and reservoir simulation involving compounds for which limited data are available. The generalization of the equation of state parameters follows from the work of van der Waals more than a century ago and more recently from Soave (1972) and Peng and Robinson (1976). Haman et al. (1977) and Yarborough (1979) generalized the parameters of the SRK equation of state in terms of reduced temperature and acentric factor, but limited their corre0888-5885/89/2628-1103$01.50/0
lations to the subcritical region; they then used the values of the two equation of state parameters determined at the critical temperature for all higher temperatures. Morris and Turek (1986) allowed the parameters of the SRK equation of state to vary with temperature both above and below the critical temperature, but they did not propose the generalization of their parameters. Further, the correlations of Yarborough and of Morris and Turek result in a discontinuity in the temperature derivatives of the parameters at the critical point, which affects derivative properties, such as enthalpy and heat capacity. Xu and Sandler (1987a,b) correlated the parameters of the Peng-Robinson equation of state as a function of reduced temperature in the subcritical and supercritical region, and in the near-critical region, they used the cubic-spline functions at reduced temperature, which are continuous and smooth a t the critical temperature. But their correlations are specific to each fluid. It should be pointed out 0 1989 American Chemical Society
