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Wet-process phosphoric acid obtained from Kola apatite. Purification from sulphates, fluorine, and metals
Separation and Purification Technology 28 (2002) 197– 205 www.elsevier.com/locate/seppur
Wet-process phosphoric acid obtained from Kola apatite. Purification from sulphates, fluorine, and metals R. Kijkowska *, D. Pawlowska-Kozinska, Z. Kowalski, M. Jodko, Z. Wzorek Institute of Inorganic Chemistry and Technology, Krako´w Uni6ersity of Technology, Warszawska 24, 31 -155 Krakow, Poland Received 7 August 2001; received in revised form 3 April 2002; accepted 5 April 2002
Abstract The purification of pre-concentrated up to 80 wt.% H3PO4 phosphoric acid (PA) obtained from Kola apatite in one of the Polish plants by the wet processing route was carried out by sulphate precipitation, desorption of volatile components (SiF4, HF) and liquid–liquid extraction method using 4-methyl-2-pentanone (MIBK). The experiment was carried out on a laboratory scale. The effects of the reagent grade Ca(H2PO4)2 · H2O, CaHPO4 · 2H2O, Ca3(PO4)2, and technical grade calcium oxide, the molar ratio of Ca2 + to SO24 − (0.8– 1.5), the temperature (343– 363 K), and the duration of precipitation time (1800–7200 s) on the extent of purification from SO24 − were determined. The most efficient precipitant was CaHPO4 · 2H2O. The precipitation using CaHPO4 · 2H2O purified phosphoric acid from the initial SO24 − concentration (1.5–1.8%) to a level of 0.1– 0.2 wt.%. The use of 100% excess of SiO2 over the stoichiometric ratio (in relation to SiF4), while air bubbling and very intensive stirring of the phosphoric acid at 403 K was carried out, allowed the fluorine concentration to decrease to a level below 0.005 wt.% of F. Purification from metals was carried out at room temperature using 1:1.22 mass ratio of PA to MIBK. The stripped phosphoric acid, with a concentration of about 50 wt.% of H3PO4 and 1.5 wt.% of MIBK, contained Fe and Al at a level of 0.01– 0.005 wt.% each, Pb, Th B1 ppm, Cr, Co, Ni B0.1 ppm, As, Cd— not detected. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Phosphoric acid; Purification; Sulphate precipitation; Fluorine; Solvent extraction
1. Introduction Wet-process phosphoric acid contains a number of undesirable ionic impurities, like fluorine, sulphate, aluminium, iron, and other metals originally present in the phosphate rock. The impurities interfere in the technological process of * Corresponding author. Fax: + 48-12-633-3374 E-mail address: [email protected] (R. Kijkowska).
making phosphoric acid and/or liquid fertiliser. They also precipitate while the acid is concentrated or stored. Without previous treatment, the acid cannot be applied directly to the production of some industrial and food grade phosphate derivatives, for example calcium, sodium or ammonium salts. A wide variety of miscellaneous applications for the food, beverage, and toothpaste markets, or cleaning markets, require high purity acid.
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The earliest known phosphoric acid purification methods are based on the precipitation of sparingly soluble salts. De-fluorination by (Na,K)2SiF6 or MgSiF6 · 6H2O precipitation are traditionally used methods [1– 5]. To purify from sulphate ion precipitation methods at an elevated temperature using barium or calcium salts are applied [5,6]. Some companies, for example, Chemische Fabric Budenheim and R.A. Oetker, combine precipitation of barium sulphate with iron. The excess of barium for sulphate precipitation is removed from the acid using cationite [6]. The precipitation of heavy metals as sulphides [7] constitutes one of the methods. The precipitation of inorganic salts, frequently applied in industry, purifies phosphoric acid to a level at which the impurities, due to their solubility, still remain in the acid. The degree of phosphoric acid purification is higher when extraction techniques with organic solvents are applied [8– 15]. A few purification processes using nonaqueous solvents have been put into commercial practice. The available published data concern purification of wet-process phosphoric acid, which is derived mostly from African or North American rock phosphates [8– 10,12 – 14]. This paper presents the purification of industrial phosphoric acid obtained from Kola apatite (PA) in one of the Polish plants. The PA differs from the acid derived from Moroccan or American rock phosphates. It contains no organic materials or uranium. Instead, it contains a fairly significant amount of Ti and Th. The acid was pre-concentrated up to 77– 80% of H3PO4 (56 – 58% P2O5). By weight, it contained about 2% of SO24 − , 0.1% F, 0.5% Al, 0.3% Fe, 0.15% Ti and some other metals at a lower level (Table 1). The purification was carried out on a laboratory scale in three steps: pre-purification by sulphate precipitation, purification from fluoride, purification from metals by solvent extraction. The wastes obtained from the purification experiments underwent neutralisation treatment. The obtained products were analysed in order to assess whether they may be considered as a phosphate-containing mineral fertiliser.
2. Experimental procedure
2.1. Sulphate precipitation As a precipitant reagent grade (POCH, Poland): Ca(H2PO4)2 · H2O, CaHPO4 · 2H2O, Ca3(PO4)2 and technical grade calcium oxide (88% CaO, 12% CaCO3) were used. The investigated parameters were: molar ratio of Ca2 + /SO24 − (0.8–1.5), time of precipitation (1800–7200 s), temperature of precipitation (343–363 K). The experiment was carried out in a glass reactor (0.0005 m3) with a mechanical stirrer. It was furnished with a heating jacket connected to a thermostat. Phosphoric acid (0.2 kg), was heated to the desired temperature, then the precipitant Table 1 Concentration of metals, (ICP method), in pre-concentrated, pre-purified, derived from Kola apatite acid before (b) and after purification using MIBK (c), (units: wt.% or ppm) Component (a)
Before purification (b)
After purification (c)
H3PO4 (P2O5) SO4 F Fe Al Ti Mg Ca Sr Ba Cr Mn Co Ni Zn Pb La Ce Nd Sm Dy Er Yb Y As, Cd Th
77.3 (56.0)% 0.036% 0.041% 0.35% 0.50% 0.15% 620 ppm 236 ppm 1 ppm 3 ppm 2 ppm 350 ppm 3 ppm 2 ppm 12 ppm 20 ppm 14 ppm 24 ppm 31 ppm 10 ppm 10 ppm 10 ppm 10 ppm 140 ppm Traces 20 ppm
50.0 (36.2)% 0.003% 0.003% 0.01% 0.005% 0.01% B1 ppm B10 ppm B0.05 ppm B0.05 ppm B0.1 ppm B10 ppm B0.1 ppm B0.1 ppm B0.5 ppm B1 ppm B0.1 ppm B0.1 ppm B0.1 ppm B0.1 ppm B0.1 ppm B0.1 ppm B0.5 ppm B1 ppm Not detected B1 ppm
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Fig. 1. The scheme of a laboratory desorption unit. (1) Vessel, (2) resistor heater, (3) power regulation device, (4) thermoregulator, (5) temperature sensor of thermoregulator, (6) temperature sensor of digital thermometer, (7) digital thermometer, (8) stirrer, (9) rotations reducer gear, (10) alternating-current motor, (11) stroboscope, (12) aeration tube, (13) rotameter, (14) reducing valve, (15) membrane pump.
was slowly added. At intervals of 1800, 3600 and 5400 s, a small sample of slurry was taken out through a stub pipe located at the bottom of the reactor and filtered off. After 7200 s, the total precipitate was filtered off from the phosphoric acid, dried at 353 K and subjected to X-ray diffraction (XRD), using X’Pert Philips. The sulphate was analysed in the filter solution: the turbidity was measured at 490 nm for the low SO24 − concentration (0.01– 0.2%), while for the concentrations higher than 0.2 wt.%, the BaSO4 precipitation method was applied [16].
2.2. Fluoride desorption The scheme of a laboratory unit equipment is presented in Fig. 1. While slowly stirred, the acid (0.42 kg) was placed in a vessel (1) and heated at the maximum capacity of the heater. Within 8– 12 min, the temperature of the acid increased to 373–403 K. Then, the stirrer was turned off, the level of the solution was marked and SiO2 was introduced. While the solution was intensively stirred (37/s), the bubbling was produced by pumping the air (1.5 dm3/s) through tube 12. Every hour, a sample of the acid was taken out,
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cooled, and the fluorine content was determined. Before sampling, the stirrer was turned off, an amount of water was added to refill the volume of the solution to the level marked, and the solution was carefully stirred. The same procedures were repeated before each sampling. The fluorine content in the phosphoric acid was determined with the use of an adapted W.C. Hanson and D.J. Lloyd method [17]. The F content was measured by the potentiometric method using the multivariable instrument CX-742 (Elmetron, Poland) and a specific fluoride ion electrode (Orion, USA). The standard deviation (S.D.) of the used analytical method was 0.0025% F.
2.3. Sol6ent extraction The pre-purified (0.04% SO24 − ) or crude ( 2% SO24 − ) phosphoric acid underwent purification by the liquid-liquid extraction method using 4methyl-2-pentanone, 99% purity (MIBK-SIGMAALDRCH). Several sets of experiments were carried out in a periodic system. Each set included four cycles with solutions recycled as illustrated in Fig. 8. Each cycle was arranged to have several stages as presented below: (a) extraction stage and separation of the phases, (b) washing the MIBK phase (1st washing stage+2nd washing stage); the solution from the 2nd washing stage was used for the 1st washing, while the solution from the 1st washing was recycled to the extraction stage in the subsequent cycle, (c) re-extraction of purified phosphoric acid with water. In each stage, the mixture was intensively stirred for 30 min then left to sit for the next 60 min to get the phases separated. The proportions of the contacted aqueous and MIBK phases were determined on the basis of the phase equilibrium diagram in the H3PO4 –MIBK –H2O system published by Feki et al. [18]. The amounts of the recycled aqueous solutions were adjusted to keep the concentration of MIBK close to: 55% MIBK at the extraction stage, 52% MIBK at the 1st washing stage, 50% MIBK at the 2nd washing stage.
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To simplify the experimental procedure, it was assumed that almost all MIBK inlet passed through the experimental stages. The approximation was based on the procedure that the washing solutions from the previous cycle were returned to the next one, while the amount of MIBK taken away with the raffinate could be neglected. The adjustment was usually made between washing stages. If the amount of the 2nd washing solution was too large to recycle it to the 1st washing stage, some of the surplus was removed. If it was too low to meet the parameters at the 1st washing stage, some reagent grade phosphoric acid was added. To control the amounts of recycled solutions, the phosphate was analysed in all phases at each stage of the experiment. For the analysis, the spectrophotometric method with ammonium molybdate and 2,4-diaminophenol hydrochloride (amidol) was used. The extinction was measured at 660 nm.
increase in P2O5 concentration decreases the metastable solubility of HH. However, the data available are limited to acid concentrations below the level of 55% P2O5, (74.8% H3PO4), [1,19]. To prove this for more concentrated phosphoric acid, a supplementary experiment on CaSO4 · 0.5H2O solubility at 323–363 K, extending the concentration up to 60% P2O5, was carried out. The results presented in Fig. 4 indicate that an increase in P2O5 concentration up to 60% should bring the SO24 − concentration down to a level of 0.2 wt.%. Actually, the purification effect by sulphate precipitation obtained after 7200 s using CaHPO4 · 2H2O was even higher than the predicted solubility of CaSO4 · 0.5H2O (Fig. 2D and Fig. 3). The increased effect might have a kinetic origin but to explain it some extended research has to be done.
3. Results and discussion
3.1. Effect of sulphate precipitation Figs. 2 and 3 present selected results chosen from 89 phosphoric acid purification experiments. The most effective precipitant was CaHPO4 · 2H2O. At 353 K, using its stoichiometric amount (Ca2 + /SO24 − =1), the SO24 − concentration went down to a level of about 0.1 wt.% (Fig. 3). An excess of precipitant over the stoichiometric Ca2 + /SO24 − ratio did not give any significant decrease in SO24 − concentration (Fig. 3). Within the range investigated (343– 363 K), there was no significant effect of temperature on phosphoric acid purification from SO24 − ions. The purification effects can be explained on the basis of solubility of CaSO4 · 0.5H2O in concentrated phosphoric acid. It is well known that above 40% P2O5, anhydrous calcium sulphate (CaSO4) is a thermodynamically stable phase. However, calcium sulphate hemihydrate (HH), CaSO4 · 0.5H2O, which shows a high degree of metastability, is usually formed first [1,19]. The HH was also forming in our precipitations. An
Fig. 2. Plot of wt.% of SO24 − in phosphoric acid (initial concentration: 72.5% H3PO4, 1.75% SO24 − ) versus time after sulphate precipitation at 353 K. Molar Ca2 + /SO24 − ratio = 1. Precipitants: A, Ca3(PO4)2; B, Ca(H2PO4)2 · H2O; C, CaO (technical grade); D, CaHPO4 · 2H2O.
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initial phosphoric acid (a) (75–78% H3PO4) to MIBK (J) was 1:1.22, which resulted in 55% of MIBK at the extraction stage (Ex). The Ex is the point where the a–J line crosses with the line of 55 mass% of MIBK. The concentration of H3PO4 in the mixtures obtained was at a level of 34– 35%. The separated phases were an aqueous phase (raffinate) (b), with a 68–71% concentration of H3PO4 and about 2.5% of (Fe+Al), and MIBK phase (extract) (L). The MIBK phase underwent washing stages.
Fig. 3. Influence of precipitation time and molar Ca2 + /SO24 − ratio on SO24 − concentration in phosphoric acid (initial concentration: 74.2% H3PO4, 1.54% SO4). Precipitant: CaHPO4 · 2H2O, molar Ca2 + /SO24 − ratio: 0.8 –1.5, temperature 353 K.
3.3.2. First washing stage (Fig. 6) For the first washing, in the 1st cycle the reagent grade phosphoric acid was used, while in the 2nd, 3rd and 4th cycles the solutions (d) from the 2nd washing stage were used. The amount of the washing solutions was adjusted to keep the concentration of MIBK close to 52% (S in Fig. 6). If the amount of the solution from the 2nd washing stage was not sufficient to meet the proportions determined by S, some amount of reagent
3.2. Effect of defluorination To transform fluoride into SiF4 the SiO2 was added in the amount of up to 100% over the stoichiometric quantity. During SiO2 additions, phosphoric acid was heated to temperatures of 373, 383, 393 and 403 K, respectively, and bubbled with the air. The defluorination rate was strongly dependent on both temperature and reaction time. Higher temperature of desorption, resulted in lower F concentration in the acid (Fig. 5). After 2 h of reaction time at 403 K the concentration of F in the acid was lower than 0.01 wt.%.
3.3. Purification from metals by sol6ent extraction 3.3.1. Extraction The extraction carried out at a room temperature is illustrated in Fig. 6, which refers to the phase diagram of Feki et al. The mass ratio of the
Fig. 4. Solubility of CaSO4 · 0.5H O in reagent grade phosphoric acid at temperatures: A, 343 K; B, 353 K and C, 363 K.
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MIBK phase using between 0.15:1 and 0.2:1 mass ratio. This located the re-extraction system near the point (r).
3.3.5. Effects of phosphoric acid purification The results based on phosphate, Fe and Al analysis are presented in Fig. 8 for one of the sets of four-cycle experiments. Depending on the acid concentration and the amount of washing solution recycled, some scattering of Fe and Al concentrations from one cycle to another was observed. The numbers in the diagram indicate the highest and lowest values of the component obtained at individual stages of the experiment. To follow the effect of purification while the concentrations of impurities and the phosphoric acid were changing from one stage to the next (Fig. 8), the molar ratios of Al/P and Fe/P in the aqueous phases were calculated (Fig. 9). It was
Fig. 5. The fluorine content in phosphoric acid after SiF4 desorption with the use SiO2 at temperatures. , 373 K; , 383 K, , 393 K, 2, 403 K.
grade phosphoric acid was added. After separation, the 1st washing solution (c) (63 – 65% H3PO4) was recycled to the extraction stage of the next cycle, and the MIBK phase (Q) underwent the 2nd washing stage.
3.3.3. Second washing stage (Fig. 7) The MIBK phase from the previous washing stage (Q) was contacted with purified phosphoric acid (product) (g), recycled from the re-extraction stage, taking such an amount that was sufficient to approach a concentration of MIBK close to 50 wt.% (i in Fig. 7). After separation, aqueous (f) and purified MIBK phase (R) were obtained. 3.3.4. Re-extraction of purified phosphoric acid (Fig. 7) The amount of water used for the re-extraction of the acid depended on the composition of the purified MIBK phase (the position of the point R) and the designed concentration of the product. To release purified phosphoric acid with a concentration of 50% H3PO4, water was introduced into the
Fig. 6. Extraction and 1st washing of the extract approximated to the phase diagram of Feki et al. in the system: H3PO4 – MIBK– H2O. Concentration is given in wt.%. a —crude phosphoric acid, b — raffinate, c —solution from the 1st washing stage, recycled to the extraction stage, d —solution from the 2nd washing stage. Ex— the mixture at the extraction stage (55% MIBK). L —organic phase (MIBK-extract) coexisting with aqueous phase b (raffinate). S —the mixture at the 1st washing stage (52% MIBK). Q— MIBK phase coexisting with aqueous phase c at the 1st washing stage.
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the majority of the impurities, the coexisting aqueous-MIBK phases had compositions located adequately to the H2O–H3PO4 –MIBK system on binodal curves, close to the compositions determined by Feki connodal lines.
3.4. Potential waste The purification of phosphoric acid described above generates potential wastes: 1. the filter cake from sulphate precipitation and
Fig. 7. 2nd Washing stage and re-extraction of purified phosphoric acid. I— the mixture at the 2nd washing stage (50% MIBK). R — MIBK phase coexisting with aqueous phase (f) at the 2nd washing stage. Q as in Fig. 6. r— The mixture at the re-extraction stage. g —Purified phosphoric acid (product).
very characteristic that Al/P increased more than Fe/P in the raffinate with respect to phosphoric acid undergoing purification, while the molar ratios of Al/P and Fe/P in the purified phosphoric acid (product) were on the same low level. To estimate the effect of the final purification, a full analysis of the initial (PA) and purified phosphoric acid (product) is presented in Table 1. Referring to the diagram of the H2O – H3PO4 – MIBK ternary system described by Feki et al. [18], the compositions of the separated phases at the extraction stage were located outside the heterogeneous region (Fig. 6). The MIBK phase (L) was close to the binodal curve, while the aqueous phase, raffinate (b), was located in the region which could be reached by an extrapolation of the Feki binodal curve. The 1st and 2nd washings of the MIBK phase and the re-extraction of the acid led to compositions of the separated phases located inside the heterogeneous region. The deviation from the binodal curve observed at the extraction stage might be a result of the influence of the impurities present in the phosphoric acid. After separation of the raffinate, which took out
Fig. 8. Block flow diagram of phosphoric acid purification experiment. Mass ratio of the aqueous phase to MIBK at the extraction stage by weight is 1:1.22. Concentration in wt.%.
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cake or the raffinate were treated with technical grade calcium oxide using its stoichiometric amount with respect to monocalcium phosphate equivalent to the amount of H3PO4 in the potential waste. The heat from the exothermic reaction raised the temperature of the treated mixture up to 353 K, and some amount of water and/or MIBK evaporated. The product obtained was dried and subjected to chemical analysis and XRD. No analysis of MIBK content was carried out. From the equilibrium diagram (Fig. 6), the amount of MIBK in the raffinate may be about 2 wt.% and should decrease during water evaporation when the phosphoric acid neutralisation is carried out. The characteristics of the product obtained, with respect to the requirements of Polish National Standards [20–22], were comparable or even better than those for single superphosphate. Fig. 9. Molar ratio of Al/P and Fe/P in the aqueous phases at each stage of the phosphoric acid purification using MIBK.
4. Conclusions
2. the raffinate from the extraction stage. The filter cake obtained at temperatures higher than 343 K was well crystallised CaSO4 · 0.5H2O. It was also readily filtered off. Some amount of Ca(H2PO4)2 · H2O was found in the precipitate obtained below 333 K. To avoid transformation of the originally formed calcium sulphate into another form, the filter cake was not washed off. That resulted in the presence of phosphoric acid in the precipitate. The estimated amount of the filter cake from sulphate precipitation is 35– 50 kg per 1000 kg of pre-purified phosphoric acid. The raffinate, an aqueous phase from the extraction stage, was a green oily liquid. During storage, an easily filtered solid precipitated. The main component of the solid was an iron compound identified by XRD as Fe3(H3O)H14(PO4)8 · 4H2O accompanied by some amount of CaSO4 · 0.5H2O. The estimated amount of the raffinate with a concentration of about 70% of H3PO4 is 20 kg per 100 kg of purified phosphoric acid (50% H3PO4). To assess, whether the potential waste might be considered as a by-product for phosphate containing mineral fertiliser production, the filter
1. CaSO4 · 0.5H2O precipitation at temperatures 353–363 K is an efficient pre-purification method of pre-concentrated (above 75% of H3PO4) wet-process phosphoric acid. The application of CaHPO4 · 2H2O as a precipitant using the stoichiometric molar (Ca2 + /SO24 − = 1) ratio enables purification from the initial (1.5–1.7%) SO24 − concentration to 0.2–0.1 wt.% in purified phosphoric acid. 2. The desorption of volatile fluorine compounds (SiF4, HF), at a temperature of 403 K, using an excess of SiO2 over the stoichiometric quantity required to transform F into SiF4, defluorinates phosphoric acid to a level of 0.001–0.002 wt.% of F. The rate of defluorination is higher than 95%. 3. Pre-purified from sulphate and fluorine phosphoric acid was purified from metals by liquid–liquid extraction method using 4-methyl-2-pentanone. The purified product was phosphoric acid (50% of H3PO4) with fluorine and sulphate at a level of 0.003 wt.% each and other impurities as indicated in Table 1, column c. To balance the amounts of the
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washing solutions with the mass of the main stream passing from the extraction to the reextraction stages, it was necessary to keep the acid concentration at a level of about 34.5– 35% of H3PO4 at the extraction and 1st washing stages. Some dilution of the system may result in an excessive amount of the 2nd washing solution. The surplus might be withdrawn as a phosphoric acid (56– 65% of H3PO4 in Fig. 8) with Fe and Al at a level below 0.1 wt.%. This intermediate level of purification might be sufficient, for example, for tripolyphosphate use in detergent production.
References [1] A.V. Slack, in: M. Dekker (Ed.), Phosphoric acid, New York, 1968. [2] Procede de purification d’acide phosphorique (Societe de Prayon), Patent France No. 2216226 (1974). [3] M.N. Nabiev, K. Gafurov, B.I. Kosteleva, Sposob ochistki extactionnoi fosfornoi kisloty ot ftoristikh solei, Khim. Prom. 6 (1983) 348. [4] H. Ressel, H. Nierman, Verfahren zum Enthfluorieren von Phosphorsaure, (Knapstack AG), Patent West Germany No. 2221745 (1973). [5] D. Goldstein, Phosphoric acid purification (FMC Corp.) Patent USA No. 3819810 (1974). [6] K. Beltz, K. Frankenfeld, K. Gotzman, Verfaren zur Entfernung von Eisen aus Phosphorsaure, Patent West Germany No. 2047261 (1975).
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[7] K. Hirayama, C. Kimura, J. Minagawa, Phosphoric acid refining using hydrogen sulphide (Mitsubishi Kasei Koge) Patent Japan No. 49-37038 (1974). [8] I.L. Bradford, B.F. Ore, Extraction process for purification of phosphoric acid, Patent USA No. 4053564 (1977). [9] P.T. Chiang, J.D. Nikerson, Solvent extraction of H3PO4, Patent USA No. 3867511 (1975). [10] I. Toshimitsu, S. Fuji, T. Nakajima, Phosphoric acid purification, Patent USA No. 3978196 (1974) and Patent USA No. 3919396, (1975) and Patent USA No. 3920797 (1975). [11] M. Takahara, Production of phosphoric acid of high purity, Patent USA No. 3917805 (1975). [12] Pure phosphoric acid by Alblright & Wilson’s new process, Phosphorus & Potassium, No. 71 (1974) 35. [13] T.A. Williams, Purification of phosphoric acid. Patent UK No. 1436114 (1976). [14] T.A. Williams, F.M. Cussons, Purification of phosphoric acid, Patent UK No. 1436113 (1976). [15] B. Wojtech, K.P. Ehlers, W. Scheibitz Verfahren zur Reinigung von Phosphorsaure mit Hilfe organischer Losungsmittel, Patent West Germany No. 2334019 (1975). [16] Polish National Standard: Norma PN-93/C-84300/11. [17] W.C. Hanson, D.J. Lloyd, Direct determination of fluoride in phosphoric acid and calcium sulphate using a specific ion electrode, Chem. Ind. 7 (1972) 41. [18] M. Feki, M. Fourati, M.M. Chaabouni, H.F. Ayedi, Purification of wet process phosphoric acid by solvent extraction liquid – liquid equilibrium at 25 – 40 °C of the system water – phosphoric acid – methylisobutylketone, Can. J. Chem. Eng. 72 (1994) 939. [19] M. Sullivan, J.J. Kohler, J.H. Grinstead, Solubility of a-calcium sulphate hemihydrate in 40, 45, 50 and 55% P2O5 phosphoric acid solutions at 80, 90, 100 and 110 °C, J. Chem. Eng. Data 33 (1988) 367. [20] Polish National Standard: Norma PN-85/C-7008. [21] Polish National Standard: Norma PN-85/C-87015. [22] Polish National Standard: Norma PN-85/C-87020.
Wet-process phosphoric acid obtained from Kola apatite. Purification from sulphates, fluorine, and metals R. Kijkowska *, D. Pawlowska-Kozinska, Z. Kowalski, M. Jodko, Z. Wzorek Institute of Inorganic Chemistry and Technology, Krako´w Uni6ersity of Technology, Warszawska 24, 31 -155 Krakow, Poland Received 7 August 2001; received in revised form 3 April 2002; accepted 5 April 2002
Abstract The purification of pre-concentrated up to 80 wt.% H3PO4 phosphoric acid (PA) obtained from Kola apatite in one of the Polish plants by the wet processing route was carried out by sulphate precipitation, desorption of volatile components (SiF4, HF) and liquid–liquid extraction method using 4-methyl-2-pentanone (MIBK). The experiment was carried out on a laboratory scale. The effects of the reagent grade Ca(H2PO4)2 · H2O, CaHPO4 · 2H2O, Ca3(PO4)2, and technical grade calcium oxide, the molar ratio of Ca2 + to SO24 − (0.8– 1.5), the temperature (343– 363 K), and the duration of precipitation time (1800–7200 s) on the extent of purification from SO24 − were determined. The most efficient precipitant was CaHPO4 · 2H2O. The precipitation using CaHPO4 · 2H2O purified phosphoric acid from the initial SO24 − concentration (1.5–1.8%) to a level of 0.1– 0.2 wt.%. The use of 100% excess of SiO2 over the stoichiometric ratio (in relation to SiF4), while air bubbling and very intensive stirring of the phosphoric acid at 403 K was carried out, allowed the fluorine concentration to decrease to a level below 0.005 wt.% of F. Purification from metals was carried out at room temperature using 1:1.22 mass ratio of PA to MIBK. The stripped phosphoric acid, with a concentration of about 50 wt.% of H3PO4 and 1.5 wt.% of MIBK, contained Fe and Al at a level of 0.01– 0.005 wt.% each, Pb, Th B1 ppm, Cr, Co, Ni B0.1 ppm, As, Cd— not detected. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Phosphoric acid; Purification; Sulphate precipitation; Fluorine; Solvent extraction
1. Introduction Wet-process phosphoric acid contains a number of undesirable ionic impurities, like fluorine, sulphate, aluminium, iron, and other metals originally present in the phosphate rock. The impurities interfere in the technological process of * Corresponding author. Fax: + 48-12-633-3374 E-mail address: [email protected] (R. Kijkowska).
making phosphoric acid and/or liquid fertiliser. They also precipitate while the acid is concentrated or stored. Without previous treatment, the acid cannot be applied directly to the production of some industrial and food grade phosphate derivatives, for example calcium, sodium or ammonium salts. A wide variety of miscellaneous applications for the food, beverage, and toothpaste markets, or cleaning markets, require high purity acid.
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The earliest known phosphoric acid purification methods are based on the precipitation of sparingly soluble salts. De-fluorination by (Na,K)2SiF6 or MgSiF6 · 6H2O precipitation are traditionally used methods [1– 5]. To purify from sulphate ion precipitation methods at an elevated temperature using barium or calcium salts are applied [5,6]. Some companies, for example, Chemische Fabric Budenheim and R.A. Oetker, combine precipitation of barium sulphate with iron. The excess of barium for sulphate precipitation is removed from the acid using cationite [6]. The precipitation of heavy metals as sulphides [7] constitutes one of the methods. The precipitation of inorganic salts, frequently applied in industry, purifies phosphoric acid to a level at which the impurities, due to their solubility, still remain in the acid. The degree of phosphoric acid purification is higher when extraction techniques with organic solvents are applied [8– 15]. A few purification processes using nonaqueous solvents have been put into commercial practice. The available published data concern purification of wet-process phosphoric acid, which is derived mostly from African or North American rock phosphates [8– 10,12 – 14]. This paper presents the purification of industrial phosphoric acid obtained from Kola apatite (PA) in one of the Polish plants. The PA differs from the acid derived from Moroccan or American rock phosphates. It contains no organic materials or uranium. Instead, it contains a fairly significant amount of Ti and Th. The acid was pre-concentrated up to 77– 80% of H3PO4 (56 – 58% P2O5). By weight, it contained about 2% of SO24 − , 0.1% F, 0.5% Al, 0.3% Fe, 0.15% Ti and some other metals at a lower level (Table 1). The purification was carried out on a laboratory scale in three steps: pre-purification by sulphate precipitation, purification from fluoride, purification from metals by solvent extraction. The wastes obtained from the purification experiments underwent neutralisation treatment. The obtained products were analysed in order to assess whether they may be considered as a phosphate-containing mineral fertiliser.
2. Experimental procedure
2.1. Sulphate precipitation As a precipitant reagent grade (POCH, Poland): Ca(H2PO4)2 · H2O, CaHPO4 · 2H2O, Ca3(PO4)2 and technical grade calcium oxide (88% CaO, 12% CaCO3) were used. The investigated parameters were: molar ratio of Ca2 + /SO24 − (0.8–1.5), time of precipitation (1800–7200 s), temperature of precipitation (343–363 K). The experiment was carried out in a glass reactor (0.0005 m3) with a mechanical stirrer. It was furnished with a heating jacket connected to a thermostat. Phosphoric acid (0.2 kg), was heated to the desired temperature, then the precipitant Table 1 Concentration of metals, (ICP method), in pre-concentrated, pre-purified, derived from Kola apatite acid before (b) and after purification using MIBK (c), (units: wt.% or ppm) Component (a)
Before purification (b)
After purification (c)
H3PO4 (P2O5) SO4 F Fe Al Ti Mg Ca Sr Ba Cr Mn Co Ni Zn Pb La Ce Nd Sm Dy Er Yb Y As, Cd Th
77.3 (56.0)% 0.036% 0.041% 0.35% 0.50% 0.15% 620 ppm 236 ppm 1 ppm 3 ppm 2 ppm 350 ppm 3 ppm 2 ppm 12 ppm 20 ppm 14 ppm 24 ppm 31 ppm 10 ppm 10 ppm 10 ppm 10 ppm 140 ppm Traces 20 ppm
50.0 (36.2)% 0.003% 0.003% 0.01% 0.005% 0.01% B1 ppm B10 ppm B0.05 ppm B0.05 ppm B0.1 ppm B10 ppm B0.1 ppm B0.1 ppm B0.5 ppm B1 ppm B0.1 ppm B0.1 ppm B0.1 ppm B0.1 ppm B0.1 ppm B0.1 ppm B0.5 ppm B1 ppm Not detected B1 ppm
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Fig. 1. The scheme of a laboratory desorption unit. (1) Vessel, (2) resistor heater, (3) power regulation device, (4) thermoregulator, (5) temperature sensor of thermoregulator, (6) temperature sensor of digital thermometer, (7) digital thermometer, (8) stirrer, (9) rotations reducer gear, (10) alternating-current motor, (11) stroboscope, (12) aeration tube, (13) rotameter, (14) reducing valve, (15) membrane pump.
was slowly added. At intervals of 1800, 3600 and 5400 s, a small sample of slurry was taken out through a stub pipe located at the bottom of the reactor and filtered off. After 7200 s, the total precipitate was filtered off from the phosphoric acid, dried at 353 K and subjected to X-ray diffraction (XRD), using X’Pert Philips. The sulphate was analysed in the filter solution: the turbidity was measured at 490 nm for the low SO24 − concentration (0.01– 0.2%), while for the concentrations higher than 0.2 wt.%, the BaSO4 precipitation method was applied [16].
2.2. Fluoride desorption The scheme of a laboratory unit equipment is presented in Fig. 1. While slowly stirred, the acid (0.42 kg) was placed in a vessel (1) and heated at the maximum capacity of the heater. Within 8– 12 min, the temperature of the acid increased to 373–403 K. Then, the stirrer was turned off, the level of the solution was marked and SiO2 was introduced. While the solution was intensively stirred (37/s), the bubbling was produced by pumping the air (1.5 dm3/s) through tube 12. Every hour, a sample of the acid was taken out,
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cooled, and the fluorine content was determined. Before sampling, the stirrer was turned off, an amount of water was added to refill the volume of the solution to the level marked, and the solution was carefully stirred. The same procedures were repeated before each sampling. The fluorine content in the phosphoric acid was determined with the use of an adapted W.C. Hanson and D.J. Lloyd method [17]. The F content was measured by the potentiometric method using the multivariable instrument CX-742 (Elmetron, Poland) and a specific fluoride ion electrode (Orion, USA). The standard deviation (S.D.) of the used analytical method was 0.0025% F.
2.3. Sol6ent extraction The pre-purified (0.04% SO24 − ) or crude ( 2% SO24 − ) phosphoric acid underwent purification by the liquid-liquid extraction method using 4methyl-2-pentanone, 99% purity (MIBK-SIGMAALDRCH). Several sets of experiments were carried out in a periodic system. Each set included four cycles with solutions recycled as illustrated in Fig. 8. Each cycle was arranged to have several stages as presented below: (a) extraction stage and separation of the phases, (b) washing the MIBK phase (1st washing stage+2nd washing stage); the solution from the 2nd washing stage was used for the 1st washing, while the solution from the 1st washing was recycled to the extraction stage in the subsequent cycle, (c) re-extraction of purified phosphoric acid with water. In each stage, the mixture was intensively stirred for 30 min then left to sit for the next 60 min to get the phases separated. The proportions of the contacted aqueous and MIBK phases were determined on the basis of the phase equilibrium diagram in the H3PO4 –MIBK –H2O system published by Feki et al. [18]. The amounts of the recycled aqueous solutions were adjusted to keep the concentration of MIBK close to: 55% MIBK at the extraction stage, 52% MIBK at the 1st washing stage, 50% MIBK at the 2nd washing stage.
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To simplify the experimental procedure, it was assumed that almost all MIBK inlet passed through the experimental stages. The approximation was based on the procedure that the washing solutions from the previous cycle were returned to the next one, while the amount of MIBK taken away with the raffinate could be neglected. The adjustment was usually made between washing stages. If the amount of the 2nd washing solution was too large to recycle it to the 1st washing stage, some of the surplus was removed. If it was too low to meet the parameters at the 1st washing stage, some reagent grade phosphoric acid was added. To control the amounts of recycled solutions, the phosphate was analysed in all phases at each stage of the experiment. For the analysis, the spectrophotometric method with ammonium molybdate and 2,4-diaminophenol hydrochloride (amidol) was used. The extinction was measured at 660 nm.
increase in P2O5 concentration decreases the metastable solubility of HH. However, the data available are limited to acid concentrations below the level of 55% P2O5, (74.8% H3PO4), [1,19]. To prove this for more concentrated phosphoric acid, a supplementary experiment on CaSO4 · 0.5H2O solubility at 323–363 K, extending the concentration up to 60% P2O5, was carried out. The results presented in Fig. 4 indicate that an increase in P2O5 concentration up to 60% should bring the SO24 − concentration down to a level of 0.2 wt.%. Actually, the purification effect by sulphate precipitation obtained after 7200 s using CaHPO4 · 2H2O was even higher than the predicted solubility of CaSO4 · 0.5H2O (Fig. 2D and Fig. 3). The increased effect might have a kinetic origin but to explain it some extended research has to be done.
3. Results and discussion
3.1. Effect of sulphate precipitation Figs. 2 and 3 present selected results chosen from 89 phosphoric acid purification experiments. The most effective precipitant was CaHPO4 · 2H2O. At 353 K, using its stoichiometric amount (Ca2 + /SO24 − =1), the SO24 − concentration went down to a level of about 0.1 wt.% (Fig. 3). An excess of precipitant over the stoichiometric Ca2 + /SO24 − ratio did not give any significant decrease in SO24 − concentration (Fig. 3). Within the range investigated (343– 363 K), there was no significant effect of temperature on phosphoric acid purification from SO24 − ions. The purification effects can be explained on the basis of solubility of CaSO4 · 0.5H2O in concentrated phosphoric acid. It is well known that above 40% P2O5, anhydrous calcium sulphate (CaSO4) is a thermodynamically stable phase. However, calcium sulphate hemihydrate (HH), CaSO4 · 0.5H2O, which shows a high degree of metastability, is usually formed first [1,19]. The HH was also forming in our precipitations. An
Fig. 2. Plot of wt.% of SO24 − in phosphoric acid (initial concentration: 72.5% H3PO4, 1.75% SO24 − ) versus time after sulphate precipitation at 353 K. Molar Ca2 + /SO24 − ratio = 1. Precipitants: A, Ca3(PO4)2; B, Ca(H2PO4)2 · H2O; C, CaO (technical grade); D, CaHPO4 · 2H2O.
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initial phosphoric acid (a) (75–78% H3PO4) to MIBK (J) was 1:1.22, which resulted in 55% of MIBK at the extraction stage (Ex). The Ex is the point where the a–J line crosses with the line of 55 mass% of MIBK. The concentration of H3PO4 in the mixtures obtained was at a level of 34– 35%. The separated phases were an aqueous phase (raffinate) (b), with a 68–71% concentration of H3PO4 and about 2.5% of (Fe+Al), and MIBK phase (extract) (L). The MIBK phase underwent washing stages.
Fig. 3. Influence of precipitation time and molar Ca2 + /SO24 − ratio on SO24 − concentration in phosphoric acid (initial concentration: 74.2% H3PO4, 1.54% SO4). Precipitant: CaHPO4 · 2H2O, molar Ca2 + /SO24 − ratio: 0.8 –1.5, temperature 353 K.
3.3.2. First washing stage (Fig. 6) For the first washing, in the 1st cycle the reagent grade phosphoric acid was used, while in the 2nd, 3rd and 4th cycles the solutions (d) from the 2nd washing stage were used. The amount of the washing solutions was adjusted to keep the concentration of MIBK close to 52% (S in Fig. 6). If the amount of the solution from the 2nd washing stage was not sufficient to meet the proportions determined by S, some amount of reagent
3.2. Effect of defluorination To transform fluoride into SiF4 the SiO2 was added in the amount of up to 100% over the stoichiometric quantity. During SiO2 additions, phosphoric acid was heated to temperatures of 373, 383, 393 and 403 K, respectively, and bubbled with the air. The defluorination rate was strongly dependent on both temperature and reaction time. Higher temperature of desorption, resulted in lower F concentration in the acid (Fig. 5). After 2 h of reaction time at 403 K the concentration of F in the acid was lower than 0.01 wt.%.
3.3. Purification from metals by sol6ent extraction 3.3.1. Extraction The extraction carried out at a room temperature is illustrated in Fig. 6, which refers to the phase diagram of Feki et al. The mass ratio of the
Fig. 4. Solubility of CaSO4 · 0.5H O in reagent grade phosphoric acid at temperatures: A, 343 K; B, 353 K and C, 363 K.
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MIBK phase using between 0.15:1 and 0.2:1 mass ratio. This located the re-extraction system near the point (r).
3.3.5. Effects of phosphoric acid purification The results based on phosphate, Fe and Al analysis are presented in Fig. 8 for one of the sets of four-cycle experiments. Depending on the acid concentration and the amount of washing solution recycled, some scattering of Fe and Al concentrations from one cycle to another was observed. The numbers in the diagram indicate the highest and lowest values of the component obtained at individual stages of the experiment. To follow the effect of purification while the concentrations of impurities and the phosphoric acid were changing from one stage to the next (Fig. 8), the molar ratios of Al/P and Fe/P in the aqueous phases were calculated (Fig. 9). It was
Fig. 5. The fluorine content in phosphoric acid after SiF4 desorption with the use SiO2 at temperatures. , 373 K; , 383 K, , 393 K, 2, 403 K.
grade phosphoric acid was added. After separation, the 1st washing solution (c) (63 – 65% H3PO4) was recycled to the extraction stage of the next cycle, and the MIBK phase (Q) underwent the 2nd washing stage.
3.3.3. Second washing stage (Fig. 7) The MIBK phase from the previous washing stage (Q) was contacted with purified phosphoric acid (product) (g), recycled from the re-extraction stage, taking such an amount that was sufficient to approach a concentration of MIBK close to 50 wt.% (i in Fig. 7). After separation, aqueous (f) and purified MIBK phase (R) were obtained. 3.3.4. Re-extraction of purified phosphoric acid (Fig. 7) The amount of water used for the re-extraction of the acid depended on the composition of the purified MIBK phase (the position of the point R) and the designed concentration of the product. To release purified phosphoric acid with a concentration of 50% H3PO4, water was introduced into the
Fig. 6. Extraction and 1st washing of the extract approximated to the phase diagram of Feki et al. in the system: H3PO4 – MIBK– H2O. Concentration is given in wt.%. a —crude phosphoric acid, b — raffinate, c —solution from the 1st washing stage, recycled to the extraction stage, d —solution from the 2nd washing stage. Ex— the mixture at the extraction stage (55% MIBK). L —organic phase (MIBK-extract) coexisting with aqueous phase b (raffinate). S —the mixture at the 1st washing stage (52% MIBK). Q— MIBK phase coexisting with aqueous phase c at the 1st washing stage.
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the majority of the impurities, the coexisting aqueous-MIBK phases had compositions located adequately to the H2O–H3PO4 –MIBK system on binodal curves, close to the compositions determined by Feki connodal lines.
3.4. Potential waste The purification of phosphoric acid described above generates potential wastes: 1. the filter cake from sulphate precipitation and
Fig. 7. 2nd Washing stage and re-extraction of purified phosphoric acid. I— the mixture at the 2nd washing stage (50% MIBK). R — MIBK phase coexisting with aqueous phase (f) at the 2nd washing stage. Q as in Fig. 6. r— The mixture at the re-extraction stage. g —Purified phosphoric acid (product).
very characteristic that Al/P increased more than Fe/P in the raffinate with respect to phosphoric acid undergoing purification, while the molar ratios of Al/P and Fe/P in the purified phosphoric acid (product) were on the same low level. To estimate the effect of the final purification, a full analysis of the initial (PA) and purified phosphoric acid (product) is presented in Table 1. Referring to the diagram of the H2O – H3PO4 – MIBK ternary system described by Feki et al. [18], the compositions of the separated phases at the extraction stage were located outside the heterogeneous region (Fig. 6). The MIBK phase (L) was close to the binodal curve, while the aqueous phase, raffinate (b), was located in the region which could be reached by an extrapolation of the Feki binodal curve. The 1st and 2nd washings of the MIBK phase and the re-extraction of the acid led to compositions of the separated phases located inside the heterogeneous region. The deviation from the binodal curve observed at the extraction stage might be a result of the influence of the impurities present in the phosphoric acid. After separation of the raffinate, which took out
Fig. 8. Block flow diagram of phosphoric acid purification experiment. Mass ratio of the aqueous phase to MIBK at the extraction stage by weight is 1:1.22. Concentration in wt.%.
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cake or the raffinate were treated with technical grade calcium oxide using its stoichiometric amount with respect to monocalcium phosphate equivalent to the amount of H3PO4 in the potential waste. The heat from the exothermic reaction raised the temperature of the treated mixture up to 353 K, and some amount of water and/or MIBK evaporated. The product obtained was dried and subjected to chemical analysis and XRD. No analysis of MIBK content was carried out. From the equilibrium diagram (Fig. 6), the amount of MIBK in the raffinate may be about 2 wt.% and should decrease during water evaporation when the phosphoric acid neutralisation is carried out. The characteristics of the product obtained, with respect to the requirements of Polish National Standards [20–22], were comparable or even better than those for single superphosphate. Fig. 9. Molar ratio of Al/P and Fe/P in the aqueous phases at each stage of the phosphoric acid purification using MIBK.
4. Conclusions
2. the raffinate from the extraction stage. The filter cake obtained at temperatures higher than 343 K was well crystallised CaSO4 · 0.5H2O. It was also readily filtered off. Some amount of Ca(H2PO4)2 · H2O was found in the precipitate obtained below 333 K. To avoid transformation of the originally formed calcium sulphate into another form, the filter cake was not washed off. That resulted in the presence of phosphoric acid in the precipitate. The estimated amount of the filter cake from sulphate precipitation is 35– 50 kg per 1000 kg of pre-purified phosphoric acid. The raffinate, an aqueous phase from the extraction stage, was a green oily liquid. During storage, an easily filtered solid precipitated. The main component of the solid was an iron compound identified by XRD as Fe3(H3O)H14(PO4)8 · 4H2O accompanied by some amount of CaSO4 · 0.5H2O. The estimated amount of the raffinate with a concentration of about 70% of H3PO4 is 20 kg per 100 kg of purified phosphoric acid (50% H3PO4). To assess, whether the potential waste might be considered as a by-product for phosphate containing mineral fertiliser production, the filter
1. CaSO4 · 0.5H2O precipitation at temperatures 353–363 K is an efficient pre-purification method of pre-concentrated (above 75% of H3PO4) wet-process phosphoric acid. The application of CaHPO4 · 2H2O as a precipitant using the stoichiometric molar (Ca2 + /SO24 − = 1) ratio enables purification from the initial (1.5–1.7%) SO24 − concentration to 0.2–0.1 wt.% in purified phosphoric acid. 2. The desorption of volatile fluorine compounds (SiF4, HF), at a temperature of 403 K, using an excess of SiO2 over the stoichiometric quantity required to transform F into SiF4, defluorinates phosphoric acid to a level of 0.001–0.002 wt.% of F. The rate of defluorination is higher than 95%. 3. Pre-purified from sulphate and fluorine phosphoric acid was purified from metals by liquid–liquid extraction method using 4-methyl-2-pentanone. The purified product was phosphoric acid (50% of H3PO4) with fluorine and sulphate at a level of 0.003 wt.% each and other impurities as indicated in Table 1, column c. To balance the amounts of the
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washing solutions with the mass of the main stream passing from the extraction to the reextraction stages, it was necessary to keep the acid concentration at a level of about 34.5– 35% of H3PO4 at the extraction and 1st washing stages. Some dilution of the system may result in an excessive amount of the 2nd washing solution. The surplus might be withdrawn as a phosphoric acid (56– 65% of H3PO4 in Fig. 8) with Fe and Al at a level below 0.1 wt.%. This intermediate level of purification might be sufficient, for example, for tripolyphosphate use in detergent production.
References [1] A.V. Slack, in: M. Dekker (Ed.), Phosphoric acid, New York, 1968. [2] Procede de purification d’acide phosphorique (Societe de Prayon), Patent France No. 2216226 (1974). [3] M.N. Nabiev, K. Gafurov, B.I. Kosteleva, Sposob ochistki extactionnoi fosfornoi kisloty ot ftoristikh solei, Khim. Prom. 6 (1983) 348. [4] H. Ressel, H. Nierman, Verfahren zum Enthfluorieren von Phosphorsaure, (Knapstack AG), Patent West Germany No. 2221745 (1973). [5] D. Goldstein, Phosphoric acid purification (FMC Corp.) Patent USA No. 3819810 (1974). [6] K. Beltz, K. Frankenfeld, K. Gotzman, Verfaren zur Entfernung von Eisen aus Phosphorsaure, Patent West Germany No. 2047261 (1975).
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[7] K. Hirayama, C. Kimura, J. Minagawa, Phosphoric acid refining using hydrogen sulphide (Mitsubishi Kasei Koge) Patent Japan No. 49-37038 (1974). [8] I.L. Bradford, B.F. Ore, Extraction process for purification of phosphoric acid, Patent USA No. 4053564 (1977). [9] P.T. Chiang, J.D. Nikerson, Solvent extraction of H3PO4, Patent USA No. 3867511 (1975). [10] I. Toshimitsu, S. Fuji, T. Nakajima, Phosphoric acid purification, Patent USA No. 3978196 (1974) and Patent USA No. 3919396, (1975) and Patent USA No. 3920797 (1975). [11] M. Takahara, Production of phosphoric acid of high purity, Patent USA No. 3917805 (1975). [12] Pure phosphoric acid by Alblright & Wilson’s new process, Phosphorus & Potassium, No. 71 (1974) 35. [13] T.A. Williams, Purification of phosphoric acid. Patent UK No. 1436114 (1976). [14] T.A. Williams, F.M. Cussons, Purification of phosphoric acid, Patent UK No. 1436113 (1976). [15] B. Wojtech, K.P. Ehlers, W. Scheibitz Verfahren zur Reinigung von Phosphorsaure mit Hilfe organischer Losungsmittel, Patent West Germany No. 2334019 (1975). [16] Polish National Standard: Norma PN-93/C-84300/11. [17] W.C. Hanson, D.J. Lloyd, Direct determination of fluoride in phosphoric acid and calcium sulphate using a specific ion electrode, Chem. Ind. 7 (1972) 41. [18] M. Feki, M. Fourati, M.M. Chaabouni, H.F. Ayedi, Purification of wet process phosphoric acid by solvent extraction liquid – liquid equilibrium at 25 – 40 °C of the system water – phosphoric acid – methylisobutylketone, Can. J. Chem. Eng. 72 (1994) 939. [19] M. Sullivan, J.J. Kohler, J.H. Grinstead, Solubility of a-calcium sulphate hemihydrate in 40, 45, 50 and 55% P2O5 phosphoric acid solutions at 80, 90, 100 and 110 °C, J. Chem. Eng. Data 33 (1988) 367. [20] Polish National Standard: Norma PN-85/C-7008. [21] Polish National Standard: Norma PN-85/C-87015. [22] Polish National Standard: Norma PN-85/C-87020.