Ind. Eng. Chem. Res. 1989,28, 225-230
225
Phase System Fe203-K20-P205-H20at 25 "C A. W. Frazier,* K. R. Waerstad, Y. K. Kim, and B. G. Crim Division of Research, Tennessee Valley Authority, National Fertilizer Development Center, Muscle Shoals, Alabama 35660
The solubility isotherms a t 25 "C in the system Fez03-Kz0-Pz05-Hz0 were determined for the acid region above 1.11% PZO+ Saturation compositions, optical and X-ray powder diffraction data, and solid-phase chemical analyses are given for the following ferric compounds: Fe(H2PO4I3-A,Fe(H2P04)3-B,Fe3H15(P04)8.4H20,Fe3KH14(P04)8.4H20,Fe3H9(P04)6-6H20,Fe3KH8(P04)6.6Hz0, FeH3(P04)z.4H20,FeH3(P04)2.H20,and FePO4-3Hz0. The data for the ferrous compounds FeH10(P04)4and Fe(H2P04)2.2H20also are included. The application of these data to the physical and chemical stability of commercial wet-process phosphoric acid is discussed. Ferric iron in wet-process phosphoric acid (WPA) is a troublesome cation that postprecipitates as a sludge component in shipping grade (54% P205)products (Lehr et al., 1966). Several representatives of the Florida WPA industry have attempted to eliminate this problem by producing a shipping acid of 60% P205. This product provides three advantages over the 50-54% shipping acid a more economical shipping cost due to the higher analysis, a Pz05concentration where Fe3KH14(P04)8-4H20 Will not precipitate, and an acid concentration suitable for producing ammonium polyphosphate products directly in a pipe reactor. Preliminary results from industrial sources were encouraging since several tank cars of 60% P205 WPA had been shipped in satisfactory condition. However, in a few tank cars, serious precipitation problems were encountered under conditions that were seemingly identical with those that previously had produced stable car loads of acid product. Ferric iron was identified as the problem, and two different unknown iron phosphates were characterized. Several phase system studies have been published (Jameson and Salmon, 1954; Kobayashi, 1970; Abduragimova and Gasanova, 1983), and d'Yuoire (1961) provides an excellent report describing Fe3Hg(P04)6.6Hz0and Fe3H15(P04)8-4H20. The potassium form, (Fe,Al),KH,,(P04)8.4H20,was reported by Lehr et al. (1967) and patented by McDonald (1951). However, the solid-phase identifications and saturation compositions of d'Yuoire's two acid compounds were not given, and the solids could not be identified from his report.
Experimental Design The solubility data for the system Fez03-K20-P205H 2 0 were determined to clarify the literature results, determine the relationship of the two unknown iron phosphates in the Fe203-P205-H20phase system, and determine the effect of potassium on this system. The solid phases were monitored by optical microscopy after the stable solid compounds had been isolated, characterized, and identified. The invariant points containing three stable precipitates were determined, and a few tie-line compositions were obtained to define the shape of the isotherm between the invariant points. Supersaturation in the more acidic region (above 70% P205)made equilibrium determinations difficult because the high concentration of Fe203caused the liquids to be extremely viscous and greatly restricted precipitation.
Results The solubility isotherms were determined at 25 "C and were unlike any of those referenced above. The saturation compositions for the three-component system (0% KzO)
Table I. Compositions in System Fe20S-K20-P206-H20at 25 OC solution anal., wt%
FelOl 0.69 0.55 0.87 1.20 3.71 4.00 4.60 5.77 6.01 7.00 4.70
P,O, 69.50 68.60 65.00 65.20
K,O 0.00 0.00 0.00 0.50
57.30 0.00 56.50 CO.01 56.20 0.00 53.10 0.00 52.80 0.00 51.40 0.00 43.00 o, moderate biaxial (-), 2V = 32O, Y = b, OAP = (010).X A e = 25- in obtuse 8 0 = .10; LC = 2.25; dispersidn u > moderate biaxial (+), 2V small; X = b; ddc = 2.27 (r,
biaxial (-), 2V = 59O;Y A c = 38" in scute 8; LO = 97*,d,. = 2.48 biaxial (-), 2V = 8 2 O ; OAP = (010).X A c = 26' in acute 8, LB = 102'. Y = b; d,, = 2.46
I
n
Figure 6. Modified rhombic hipyramids of FeH3(P0,ir4H,0.
Figure 7. Highly modified tablets mf FeH,(I'O,),.H,O.
1 shows that the stability fields for the two potassium complexes are limited to the undersaturated region of the Fez03-Pz06-H,0 systems and restricted by the isotherms for the pure acid iron phosphates, but not necessarily by their own isomorphous hydrogen forms. For example, one or the other of the potassium analogues can form a stable salt pair with any of the iron phosphates. Although structurally the potassium and hydrogen forms are the same (the crystallographic data are alike), the chemical properties are vastly different. Thus, a partial hydrogen substitution for potassium can only occur a t high iron concentrations (up to 7% Fez03) along the acid iron phosphate isotherms, whereas the fully substituted potassium form can saturate a solution as low as 0.54% Fez03 or 0.2570, as reported by Lehr et al. (1967). A similar
phenomenon exists where sodium substitutes for potassium-some sodium can substitute for potassium as this compound; however, the stable sodium salt under similar conditions is FeNaH,(P0,)3.Hz0 (Lehr et al., 1967). Table VI1 lists the ferric phosphates encountered in this study. Most of the iron phosphates either have not been identified by previous phase system reports or were inadequately described. The most acidic compounds from strong phosphoric acid are a t the top, with the least acidic at the bottom. The last compound is a trihydrate, as opposed to the naturally occurring form, a dihydrate. Even though stable in its mother liquor a t 25 "C, the trihydrate, after cleanup, decomposes rapidly a t ambient conditions to the dihydrate. The iron pyrophosphate, FeHPzO,, a t the top of the list was found in equilibrium with the second
228 Ind. Eng. Chem. Res., Vol. 28, No. 2, 1989 Table IV. X-ray Powder Diffraction Data for Ferric Phosphates" d, A
I/Io
d, A
1/10
d, A 2.646 2.613 2.440 2.289 2.238 2.209 2.181 2.129
1/10 d, A FeqHo(POa)c.6Hp0 i4 2.b80 31 2.047 16 2.010 21 1.983 2 1.969 9 1.948 15 1.910 8 1.875
2.959 2.919 2.826 2.718 2.645 2.595 2.567 2.545 2.462 2.398
FeH3(P04),.H,0 7 2.382 31 2.350 2 2.305 1 2.195 3 2.161 13 2.026 2 2.020 1 1.993 12 1.941 10 1.890 FeH,(P04),.4H,0 11 2.089 7 2.078 2 2.046 5 2.031 8 2.012 9 1.985 1 1.961 2 1.937 1 1.877 2 1.852 2 1.818
I
7.94 7.18 5.78 4.58 4.21 4.024 3.972 3.867
32 80 30 100 2 12 25 3
3.723 3.591 3.242 3.100 3.001 2.954 2.888 2.827
4 26 42 53 16 34 19 55
_
8.80 7.43 6.52 6.47 5.83 5.64 4.703 4.358 4.163 3.975
15 100 5 18 33 7 12 8 18 12
3.719 3.659 3.545 3.464 3.401 3.338 3.314 3.239 3.095 3.013
10 45 13
7.22 5.76 5.56 5.23 4.86 4.57 4.090 3.798 3.603 3.444 3.356
100 15 44 38 6 20 4 19 18 2
3.318 3.257 3.210 3.092 2.977 2.955 2.920 2.880 2.822 2.775 2.641
45 14 31 8 6 2 1 2 43 12
2.616 2.449 2.430 2.404 2.363 2.294 2.262 2.241 2.216 2.151 2.125
8.85 7.53 6.11 4.69 4.25
6* 2 100 38 12
4.02 3.749 3.638 3.360 3.276
7 5 65 18 2
3.060 3.019 2.921 2.410 2.344
Fe(HzP04)3-A 50 2.321 8 2.037 42 1.952 5 1.833 7 1.821
11.18 9.07 8.83 7.76 5.77 4.715 4.627 4.580 4.201
18* 6 12* 100 15 3 16 2 2
4.170 3.875 3.802 3.740 3.590 3.519 3.318 3.280 3.247
13 31 2 38* 20 44 45 4 3
3.218 3.100 3.023 2.994 2.880 2.799 2.763 2.604
Fe(H2P04)3-B 21 2.584 22 2.519 5b 2.409 12 2.353 10 2.324 9 2.292 6 2.250 15 2.195
15.23 7.62
100 70
6.15 5.09
10 10
4.33 4.21
FeP04.3Hz0 50 3.81 5 3.67
4
4
5 24 20 6 1
8
n
III,
d, A
1/10
d, A
I/&
3 14
1.841 1.809 1.779 1.730 1.722 1.695 1.669 1.619
2 6 8 34 14 7 4 13
1.600 1.539 1.526 1.493 1.475 1.435 1.405 1.322 1.270
11
1.551 1.499 1.490 1.472 1.459 1.419 1.323 1.310 1.281
2 2
3 2 2 3
I
2 2
I 8 5 8
3 15 9 3 3 5 4 12
1.860 1.828 1.807 1.743 1.725 1.678 1.664 1.627 1.618 1.573
30
1.800 1.789 1.722 1.680 1.658 1.631 1.621 1.602 1.571 1.541
5 5 4 4 3 3 6 3 2 1
1.507 1.490 1.479 1.446 1.410 1.394 1.355 1.329 1.283 1.177
1.728 1.697 1.570 1.564 1.539
10 5 13 3
1.433 1.341 1.328 1.312
6 5 3 2
4
3 9 4 2
1.719 1.703 1.626 1.550 1.524 1.493 1.474 1.418
4 1 1
8 6 2
2.178 2.040 2.010 1.973 1.921 1.835 1.811 1.786
5 2
3.54 3.19
5 35
3.00 2.77 2.57
10 5 5
12 1 2 3 1
2 2 1 2 2 14
9 5 6 2 1 1 1
3
4
1 1 2 1 4 1
3 3 2
1
2 2 1
2 3 2
2 4 2 2 1 1
3 12 25 3
9 2 8 11 4
3 1
2
3 2 2
3 2 1 1 1
'Copper K a radiation, h = 1.541 78 A. d-space corrections based on tungsten (a, = 3.165 16 A) as internal standard. Intensities measured as peak heights above background and expressed as percentage of the strongest line. bIntensity uncertain due to hygroscopic nature of sample.
compound, Fe(H2P0,)3-A,at 68% P2OS,indicating that solid iron orthophosphates do not exist above 68% P205 at 25 "C and iron polyphosphates are not stable below 68% p205.
The second and third compounds, Fe(H2P04)3-Aand Fe(H2PO4I3-B,have been identified in commercial 60% shipping grade WPA. The compound Fe(H2P04)3-Aforms in a more acidic environment and was confirmed by chemical analysis as a nonpolyphosphate species. The compound Fe(H2P04)3-A,reported as FeP-1 in Table 11, precipitates in the same concentration range as that reported by Kobayashi (1970) for FeH9(P04),. However, Abduragimova and Gasanova (1983) report FeP03 (metaphosphate) as the stable compound for this region, while
Jameson and Salmon (1954) give Fe2P,Ol3 as metastable with Fe(H2P04)3.Our dimorph B compares with Jameson and Salmon's Fe(H2P04)3,while our dimorph A is stable in the composition range for their Fe2P4OI3. Several well-developed, homogeneous, crystalline samples of these compounds were analyzed to determine the correct formulations of these two dimorphs. The presence of Fe(H2P0,)3-A in 60% P205commercial acids gives an indication of the effect that other impurities have on the solubility fields of the acid iron phosphates in WPA products. Microscopically, the crystals of these compounds vary from highly modified prisms and rhombic bipyramids for Fe(H2P0,),-A to thin needles and plates for Fe(H2PO,),-B; each is readily recognized in its respective liquid
Ind. Eng. Chem. Res., Vol. 28, No. 2, 1989 229 Table V. X-ray Powder d,, 8, d,, 8, 8.82 8.794 7.50 7.485 4.77 4.711 4.65 4.664 4.634 4.41 4.397 4.25 4.251 4.20 4.201 4.201 3.801 3.802 3.745 3.743 3.702 3.705 3.697 3.272 3.271 3.240 3.248 3.237 3.160 3.159 3.116 3.117 3.098 3.093 3.055 3.056 3.031 3.017 3.007 3.006 2.950 2.953 2.947
Diffraction Data with Corresponding hkl d,, 8, hkl I/I, d,, 8, 002 100 2.837 2.833 111 10 2.799 2.799 2 113 2.789 311 4 2.730 2.731 311 2.547 2.547 8 004 2.547 2 2.525 312 2.520 3 2.499 2.500 022 2.410 2.410 312 12 2.391 2.390 402 222 39 2.382 7 2.337 023 2.338 2.318 313 2.319 21 2.306 314 2.307 11 115 2.304 2.227 2.226 024 2 2.168 2.168 510 3 2.167 511 3 2.143 2.143 421 2.142 8 404 224 2.035 2.037 38 224 2.034 8 2.010 404 2.011 16 422 1.981 1.980 132 1.956 1.957 1.952
Reflections for FerHls(POJn 4H20n d,,A hkl I/I, d,,A 025 5 1.956 1.952 513 2 1.950 1.052 600 1.950 331 2 1.949 514 1 1.832 1.833 134 1.807 316 1 1.805 1.805 026 1 1.805 117 4 1.772 1.774 040 4 1.805 1.805 021 1.767 1.766 604 2 1.693 710 1 1.693 1.692 042 1 1.691 711 1.673 1.673 2 532 1.673 1.611 713 1 1.611 335 1.590 1.590 2 243 1.540 1.541 227 1.475 1.475 606 4 1.451 1.451 534 1.412 1.413 11 318 1.401 517 1 1.400 1.400 137 3 1.400 1.389 427 1.390
hkl
I/I,
517 443 715 228 716 153 246 716 642 352 824 825 247 717 519 155 736 339 262 752 753 464 358 066 936 557
3 4 1
6 4 3 2 2 2 2 1 2 1 1 2 2
"Copper K a radiation, A = 1.54178 A. d-space corrections based on tungsten (a, = 3.165 16 A) as internal standard. Intensities measured as peak heights above background and expressed as percentage of the strongest line.
Table VI. X-ray Powder Diffraction Data for Ferrous Phosphates' d, A
111,
5.88 4.98 4.713 4.513 4.108 3.819 3.645 3.421 3.345 3.182 3.028 3.000 2.941 2.825 2.777 2.660 2.587
15 9 19 100 11 4 7 25 5 30 10 33 3 2 4 3 6
8.90 7.95 5.89 4.72 4.52 4.116 3.827 3.651 3.750 3.424 3.347 3.184 3.020 2.945 2.910
12b lob
55 15 46 17 4 8 1
100 1 58 40 5 1
d, A 1/10 Fe(H2P04)z.2Hz0 2.505 4 2.490 4 2.356 18 2.290 5 2.256 26 2.233 3 2.180 6 2.133 5 2.082 5 2.070 4 2 2.043 2.027 1 1.967 8 1.887 2 1.853 9 1 1.794 1.764 4
d, 8,
I/I,
1.744 1.711 1.699 1.662 1.641 1.622 1.583 1.571 1.539 1.528 1.487 1.445 1.410 1.375 1.350 1.336 1.324
3 3 1 26 3 2 3 2 3 2
FeHl0(P04)~ 2.822 1 2.780 8 2.661 4 2.587 4 2.357 15 2.294 3 2.257 8 2.182 8 2.135 6 2.083 3 2 2.069 1.984 1 1.966 9 1.944 1 1.882 2
1.854 1.796 1.743 1.712 1.698 1.669 1.660 1.641 1.623 1.541 1.445 1.382 1.337 1.296 1.163
3 2 3 13 1 4 4 2
1 2 1 1 1 1 1
1 2 2 1 1 2 1
"Copper K a radiation, A = 1.54178 A. d-space corrections based on tungsten (a, = 3.16516 A) as internal standard. Intensities measured as peak heights above background and expressed as percentage of the strongest line. *Intensity uncertain due to hygroscopic nature of sample.
phases (Figures 2 and 3). All crystalline phases are easily identified from their optical data, as shown in Table 111.
Table VII. Acid Iron Phosphates compd stability region, % P z 0 6 FeHP20, >68% a t 25 "C Fe(HzP04)3-A >65% a t 25 "C Fe(H2P04)3-B 56-65% Fe3H15(P04)8-4H20 51-56% FeH3(P04)2-H20 54-56% a t 75 "C 29-51% a t 25 "C Fe3H9(P04)6.6H20 FeH3(P04)z.4Hz0 10-29% FeP04-3H20
