I n d . Eng. Chem. R e s . 1987,26,2501-2505
methanol-water distillation columns.
Literature Cited Astrom, K. J.; Wittenmark, B. Computer Controlled Systems: Theory and Design, 1st ed.; Prentice-Hall: Englewood Cliffs, NJ, 1984; pp 32&331. Chimp, T.-P. Ph.D. Dissertation, Lehigh University, Bethlehem, PA, Nov 1985. Finco, M. V. 'Modeling and Control of Low Relative Volatility Distillation Columns", M.S. Thesis, Lehigh University, Bethlehem, PA, Jan 1987.
2501
Luyben, W. L. Process Modeling, Simulation, and Control for Chemical Engineers, 1st ed.; McGraw-Hill: New York, 1973. Moudgalya, K. M.; Luyben, W. L.; Georgakis, C. A . Dual Pulse Method for Modeling Processes with Large Time Constants, Research Progress Report 3, Sept 1986; Chemical Process Modeling and Control Research Center, Lehigh University. Thompson, P. M. Program CC, Integrated Systems Analysis Package; System Technologies: Hawthorne, CA, 1985.
Received for review February 26, 1987 Revised manuscript received August 6, 1987 Accepted August 17, 1987
The Digestion of Phosphate Ore in Phosphoric Acid Sietse van der Sluis, Yulia Meszaros, Wim G. J. Marchee, Hans A. Wesselingh, and Gerda M. van Rosmalen* Technical University o f D e l f t , Faculty o f Chemical Engineering, Delft, The Netherlands
Phosphoric acid is used for fertilizer applications. T o minimize environmental pollution by heavy metals, a new process is being designed. One of the important steps in this process is the complete digestion of phosphate ore in phosphoric acid. The rates of digestion of fluoroapatite particles in phosphoric acid were determined. The particle size was varied from 150 to 2000 pm, the phosphoric acid concentration from 30 to 50 w t % P205,and the temperature between 60 and 90 "C. The rate of digestion was found to be controlled by transport of calcium ions to the solution. A model for the digestion process was developed, based on the diffusion of calcium ions as the rate-limiting step. The mass-transfer coefficients, calculated by applying this model on the experimental data, are slightly m s-*. increasing with temperature and have values of 5-10 X 1. Introduction Phosphoric acid is a major component of many fertilizers. It is mainly produced by digestion of phosphate ore (fluorapatite) with sulfuric acid. This yields phosphoric acid as a product and hydrated calcium sulfate as a byproduct,. The ore also contains traces of heavy metal ions, of which cadmium and radium are the most worrisome. They can cause serious environmental problems (Becker, 1983). A new "clean technology" phosphoric acid (CTPA) process is therefore being developed to reduce these problems (van der Sluis et al., 1986). This process aims at producing a 40 wt % P20,solution, with less than 5 ppm cadmium and hydrated calcium sulfate with less than 1ppm cadmium. Contrary to the conventional processes, the ore is not digested in a mixture of sulfuric acid and recycled phosphoric acid but is digested only in recycled phosphoric acid. This allows almost complete digestion of the ore, without the simultaneous precipitation of all calcium sulfate. After separation of the insoluble ore residue and a minor amount of calcium sulfate, a clear solution is obtained, from which the cadmium can be removed by ion exchange (Tjioe et al., 1986). After removal of the cadmium ions, the precipitation of the calcium ions is performed by adding sulfuric acid. Large equipment is required, due to the very large recycle stream of phosphoric acid needed for complete digestion. For its design, reliable data on the digestion rates are a necessity. Several publications have been devoted to this subject, which are unfortunately contradictory. Three different rate-determining steps have been proposed: diffusion of calcium ions away from the particle (Serdyuk et al., 1982; Hufmann et al., 1957); diffusion of hydrogen ions toward the particle (Bloise et al., 1984); and chemical reaction of the acid with the ore particle (Ivanov et al., 1977). This inconsistency explains, why the subject was reinvestigated under the conditions expected in the CTPA process. 0888-5885/87/2626-2501$01.50/0
2. The Digestion Stage of the CTPA Process The phosphate ores used in wet processes today are mainly sedimentary phosphates or francolites. These ores consist of fluoroapatite, with part of the phosphate ions replaced by fluoride and carbonate ions. By beneficiation of the phosphate rock, large amounts of waste material are removed. The phosphate ore, however, still contains residual waste components. If phosphate ore is digested by phosphoric acid, the following reactions can take place (Elmore and Farr, 1940):
The amount of calcium ions which can be dissolved strongly depends on the temperature as well as on the phosphoric acid concentration of the solution. These parameters also determine which type of calcium phosphate is formed as shown in Figure 1 (Elmore and Farr, 1940). The phosphoric acid used for the digestion is recycled product acid. This recycle stream unavoidably contains sulfate ions in an amount dictated by the operating conditions in the crystallizer. So part of the calcium sulfate, up to a maximum of 25 wt %, will precipitate during digestion. When a mixture of phosphoric acid and sulfuric acid is used for the digestion of the ore, two processes occur more or less simultaneously: digestion of the ore and precipitation of calcium sulfate hemihydrate. At high sulfate concentrations, the calcium sulfate tends to precipitate not only upon the hemihydrate crystals but also upon the ore particles. Such a coating is often called blinding (Becker, 1983). Since a high digestion rate as well as complete digestion should be obtained, the sulfuric acid concentration in the digestion stage has to be carefully selected to avoid 0 1987 American Chemical Society
2502 Ind. Eng. Chem. Res., Vol. 26, No. 12, 1987 -
'A u%CaO
0
3b
35
Figure 1. Solubility of CaO in phosphoric acid. Table I. Chemical Composition of Zin Phosphate Ore component concn component concn CaO 52.2 wt % moisture 1.5 w t % A1203 0.3 w t % P205 31.5 wt 70 C02 (total) MgO 0.3 w t % 5.6 wt % F 3.6 wt % FeaOs 0.2 wt 90 c1 380 mg kg-' so, 2.4 wt % SiO, (total) 1.7 w t % Cd 22 mg kg-'
blinding. In a continuous process, blinding can only be avoided by maintaining a high calcium concentration in the digestion stage to keep the sulfuric acid concentration sufficiently low (< 2 w t % H2S04)(van der Sluis et al., 1986). Under such condities, sulfate ions precipitate upon the hemihydrate crystals and the digestion process can be treated as if proceeding in pure phosphoric acid. In most of the work on the digestion process, as presented in the literature, the solid product is a coprecipitate of hydrated calcium sulfate and solid calcium (mono or di)hydrogen phosphate, since this product is often directly used as a fertilizer. The complexity of this combined precipitation, however, hardens the unraveling of the mechanisms of the individual steps. The calcium (mono or di)hydrogen phosphate can, however, also be produced as a solution by digestion of the phosphate ore in a large amount of phosphoric acid, at low sulfate concentration. In this case, the rate-determining step was found to be calcium diffusion both in diluted phosphoric acid (< 5 wt % P,O& a t 25 OC (Hufmann et al., 1957) and in 47 wt % P205between 45 and 85 OC (Serdyuk et al., 1982). It was, however, also reported to be a fourth-order chemical reaction in 24-47 w t % P205between 80 and 120 O C (Ivanov et al., 1977). The confusion became even larger when in 1984 a mathematical model was presented in which the rate-limiting step of the digestion step at 70 OC was assumed to be diffusion of hydrogen ions to the particle surface (Bloise et al., 1984). No unambiquous conclusion could therefore be drawn about the rate-determining step of the digestion process. 3. Experimental Section The phosphate ore used was Zin ore from Israel. Its chemical composition is shown in Table I. Four different size fractions (150-212,425-500,1000-1180, and 170-2OOO pm) were obtained by sieving through "Twente" sieves. To remove agglommerates of smaller ore particles from these fractions, 500 g of each size fraction was gently stirred twice with about 2 kg of distilled water and sieved again. Then the fractions were dried a t 60 "C for at least 24 h. Chemically pure phosphoric acid (85 wt % H,PO,) and distilled water were used to prepare the solutions containing 30, 40, and 50 wt % P205(i.e., 41, 55, and 69 wt % H3P04). Figure 2 shows the reactor used, a double-walled glass vessel with baffles. The temperature was kept at a con-
r Figure 2. Experimental reactor used for digestion of phosphate ore in phosphoric acid: 1 = thermostated reactor; 2 = polypropylene baffles; 3 = six-bladed polyvinylidene fluoride stirrer; 4 = thermometer; 5 = polypropylene lid with silicon rubber seals; 6 = syringe; 7 = reflux cooler; the sampling hole is not shown. Table 11. CaO Content of Zin Phosphate Ore anal. of wt % CaO (ICP) mean particle size, um 1 2 3 181 55.3 52.9 53.0 463 53.0 1090 50.8 50.6 51.3 1850 52.2
stant value of 60, 75, or 90 "C by a thermostat. To prevent evaporation of water during the experiment, the vessel was equipped with a polypropylene lid with a reflux cooler. A 50-mL plastic syringe was used to feed the ore suspension in water to the reactor. The power input, provided by the six-bladed turbine stirrer, was maintained at about 5 kW/m3 (1000 rpm). This power input is sufficient to keep even the largest particles suspended. An amount of 20 or 40 g of an ore fraction was suspended in 8 or 16 g of water and 1-3 g of antifoaming agent in the plastic syringe. This suspension was added to 500 or lo00 g of phosphoric acid at the selected concentration and temperature. In this way, an instantaneous suspending of the ore particles in solution was obtained. The digestion process was followed by taking 3-mL samples from the reactor with a "Finn" pipet. Sampling intervals varied between 15 s at the start to 15 min toward the end of the experiment. The samples were immediately and quickly filtered over Teflon filters, provided with "Millipore AP 2002500" filter cloth. The filtrate was accurately weighted. The CaO content of the filtrate was analyzed by ICP (Klok et al., 1985) and/or a potentiometric titration (Jordan and Monn, 1967; Pribil and Vesely, 1966; Szekeres et al., 1965). 4. Results The results are shown in Figures 3,4, and 5 for various concentrations of the phosphoric acid (30, 40, and 50 wt % P205),temperature (60, 75, and 90 "C), and mean particle sizes (181,463,1090, and 1850 pm). Since the Zin phosphate ore used is a mineral, it is not homogeneous. As a consequence, the CaO content of each particle size fraction is different as shown in Table 11. The final CaO concentration reached in the solution is therefore used to estimate the CaO content of the ore particle size fraction, used in that particular experiment.
Ind. Eng. Chem. Res., Vol. 26, No..l2, 1987 2503 B
-
TEMPERATURE
-++
60DC 7 5 Y -0-
(d
90'C
-
300
1800
300
zlr-= +
E
1800
-TIMEIS1
D
C
TEMPERATURE 60°C
TEMPERATURE 6 0 e C -K-
IS'C
-0-
90'C
--c "
0
+
_I_r
I
0
1800
300
1800
300 IS1
-TIME
IS1
-TIME
+
-0-
90°C -a-
TIMEIS1
W%CaO
60'C 75'C
Figure 3. Digestion curves for phosphate ore in phosphoric acid containing 30 w t % P205 at three temperatures. Mean ore particle diameters: A = 181,B = 463,C = 1090,and D = 1850 pm.
TEMPERATURE 60'C -X7 s o c -a90-c t 1
0
-
iaao
300
TIMEIS!
1
if
TEMPERATURE 6 0 ° C -K754c
-c-
90'C
-0-
& ;
1800
300 -TIME
IS1
T E M P ERATURE 60'C X 7 5 ' C -0-
7 5 ' C -D9 0 V t
90'C
-8-
B
T E MP E R A T U R E 60'C -K75oc
-a-
90'C
t T-
O
300
-TIME
is1
1800
0
300
-TIME
-
1800
IS1
TEMPERATURE
C
0
-+-
60'C
IS'C
-c-
90'C
t
I
300
1800
T I M E IS1
probably due to the increase in viscosity with increasing phosphoric acid concentration as shown in Table 111. The viscosity is an important parameter because of its influence on the diffusion rate. The decreasing digestion rate with increasing acid concentration, which is most clearly revealed by the curves for the largest particles, is a strong argument against a hydrogen ion diffusion limitation. Moreover, diffusion of hydrogen ions is commonly considered to proceed very fast. Either the diffusion of calcium ions or the chemical reaction therefore seems more likely to be the rate-controlling step in the digestion process. 5.2. Influence of the Temperature. The effect of the temperature can also be observed from Figures 3-5 and in particular from the curves for the largest particle sizes. In all cases, the time needed to reach complete digestion decreases with increasing temperature. The differences between the digestion rates for 60, 75, and 90 OC are, however, rather small. By use of only the slope at t = 0 of the digestion curves presented in Figures 3, 4, and 5, an almost constant energy of activation of the digestion process, E,, has been calculated for each curve. Its value was about 15 kJ/mol, independent of particle size and phosphoric acid concentration. This rough value of E , indicates that the digestion rate is not controlled by a chemical reaction, which normally is more sensitive to the temperature.
:-? 0
*
'1'
N s m-2) for Phosphoric Table 111. Viscosity Data (7 in Acid (Slack, 1968) viscosity for PzOs concn of temp, OC 30wt % 40 wt 90 50 wt % 2.8 5.47 60 1.69 75 1.37 2.30 4.21 1.88 3.31 90 1.14
0
300 -TIME
.
1800
I51
Figure 5. Digestion curves for phosphate ore in phosphoric acid containing 50 w t % Pz05at three temperatures. Mean ore particle diameters: A = 181,B = 463,C = 1090,and D = 1850 p m .
5. Discussion 5.1. Influence of the Phosphoric Acid Concentration. The effect of the phosphoric acid concentration on the digestion rate of the phosphate ore was studied at constant values of the stirrer speed, the particle size, the temperature, and the weight ratio of phosphoric acid and phosphate ore. Comparison of Figures 3A, 4A, and 5A up to 3D, 4D, and 5D shows that the time needed for complete digestion increases from 30 to 50 w t % Pz05. This is
6. Preliminary Conclusion From the results obtained, it is reasonable to assume that the diffusion of calcium ions from the surface of the reacting ore particles into the bulk of the solution is the rate-determining step of the digestion process. Preliminary Conclusion 7. A Kinetic Model of the Digestion Process In this section, a model will be developed for the digestion process of ore particles of one size, which is based on the following assumptions: the transport of acid to the surface of the particles proceeds fast; at the surface, the acid rapidly reacts, until the saturation concentration of Ca(HzPO& is reached; the transport of this calcium phosphate into the solution is rate limiting; the masstransfer coefficient, k , for its transport has a constant value; the particles of one size fraction are spherical and have the same initial radius, Ro, and they do not disintegrate during digestion; and the solution is well mixed. The rate at which calcium ions (taken as CaO) diffuse from a single particle into the solution is then given by
f dM/dt = kA(c, - C J
(7-1)
Since the particles are spherical, their mass, M , and external surface area, A , are given by
M = ps4/9~R~ A = 4rR2
(7-2) (7-3)
This leads to f
~
&/dt s = k(c, - ct)
(7-4)
2504 Ind. Eng. Chem. Res., Vol. 26, No. 12, 1987
b0
I Fr
Table IV. Calculated k Values m 8-l) k for mean diameters of the ore particle size cOncn of P,05 and fraction of temp 181 pm 463 pm 1090pm 1850 bm
a 60°Crc=0,994 "C rc= 0,999 OC
rc= 0,991
50
30 wt % 60 "C 75 "C 90 "C 40 wt % 60 O C
40
30
20
4 6
7
5 75 "C 6 80 "C 8 50 wt % I 100 10 50 60 "C 4 ----+ t i m e i s 1 75 "C 6 Figure 6. F(R*) as a function of time in 40 wt % P205at three 90 "C 7 temperatures for a mean ore particle diameter of 463 pm. 10
5
4 6 8
1
7 9
7 9 11
5
5
6
6
6 8
7
8
9
4 5 6
4 6
4 7
7
8
It is convenient to simplify this equation by introducing the ratios
This yields dR*/dt* = 1 - C*
(7-5)
A second relation is obtained from the total CaO balance:
Mo = M , + Lc,
(7-6)
2.1
After complete digestion, the final CaO concentration in the solution, cf, is obtained:
Mo = Lcf
(7-7)
Equations 7-6 and 7-7 are easily combined to give
M J M , = 1 - CJCf
(7-8)
or in dimensionless form
R*3 = 1 - C*/P
(7-9)
c* = p(l - R*3)
(7-10)
with p = cf/c,. So
Together eq 7-5 and 7-10 yield
dR*
CY
+ P R * ~= dt*
(7-11)
with a = 1- p. Integration of the left-hand side of eq 7-11 (Weast, 1976) leads to
F(R*) =
with y = (~u/p)'/~. In a digestion experiment, Ro and c, are known beforehand. The c, values used in this paper are those determined by Elmore and Farr (1940). If C, and cf are measured, R* can be calculated from eq 7-9 and also the corresponding value of F(R*). According to eq 7-11, a plot of F(R*) against time should give a straight line with a slope of (kc,)/(fR,p,). The mass-transfer coefficient, k , is then determined by the value of this slope. The density of pure fluoroapatite is 3200 kg/m3 (Weast, 1976). The real density of the Zin ore was estimated with a pycnometer to be about 2700 kg/m3. This value was used in the calculations.
-+ 2.8
2.9
3.0
x1~3
Figure 7. In k as a function of 2'-' and the phosphoric acid concentration for a mean particle diameter of 463 pm.
8. Determination of the Mass-Transfer Coefficient The function F(R*) given by eq 7-12 was plotted against time for all experiments presented in Figures 3-5. For each experiment, a straight line was obtained, which could be fitted by the method of least squares with a regression coefficient (rx.) larger than 0.99. Examples of these fits are given in Figure 6. From the slope of the straight lines, the mass-transfer coefficients were calculated. These values are presented in Table IV. For a given temperature, the mass-transfer coefficients depend only slightly on the particle size. Their values are close to those predicted by existing correlations for the mass-transfer coefficients of particles in mixed vessels (Calderbank and Moo-Young, 1961). The influence of the phosphoric acid concentration can best be observed from the results obtained for the largest particle size fractions, because their digestion proceeds slowest. A slight decrease of the k values with increasing phosphoric acid concentrations is observed. This decrease is caused by the increase in viscosity, given in Table 111, and also follows from the existing correlations. The temperature also has a slight influence on the k values. Due to the decrease of the viscosity and the increase of the diffusion coefficient,there is a small increase of the k values with temperature. The influence of the temperature on the digestion process can be related to an activation energy, which can be obtained from an Arrhenius plot of In k versus T-I. In Figure 7 this is illustrated for one particle size fraction (463 pm) and three phosphoric acid concentrations (30,40, and 50 wt % P205). The energy of activation appeared to be independent of the particle size fraction and nearly independent of the phosphoric acid concentration. Its value lies between 13 and 23 kJ/mol, an order of magnitude expected for a
Ind. Eng. Chem. Res. 1987,26,2505-2508
diffusion-controlled process. 9. Conclusive Remarks Although Huffmann et al. (1957) also found that calcium ion diffusion was the rate-limiting step of the digestion process, their model did not take into account the considerable change in surface area of the ore with time. Their equation describing the digestion pocess in diluted phosphoric acid is therefore too simple and cannot be used to calculate the mass-transfer coefficients. The model presented by Serdyuk et al. (1982) was an empirical model, in which a coefficient of digestion and an undefined constant were introduced. Their model is thus only applicable within the limits of their experiments and cannot be used to calculate mass-transfer coefficients, because the relationship between the rate of digestion and the masstransfer coefficients is not well defined. The model used here does not suffer from these disadvantages. Since the digestion process is dominated by the diffusion of calcium ions from the surface of the ore into the bulk of the solution, no influence of impurities present in the process acid is expected. This was confirmed by using real product acid where the impurities did not hamper the digestion. Only if the impurities present in the acid (like excess sulfate ions) give rise to blinding of the ore particles can a reduction of the digestion process be expected. The influence of impurities in the ore can be large if the carbonate ions in the apatite structure are considered as impurities. A very low carbonate content of the ore can reduce the surface reaction step. From the foregoing, it can be concluded that the digestion of Zin phosphate ore in chemically pure phosphoric between 60 and 90 OC can be acid (30-50 wt % P205) described by a model in which calcium ion diffusion is the rate-limiting step.
Acknowledgment We are indebted to DSM for their financial support of the project.
2505
cf = CaO concentration in the bulk of the solution after complete digestion of the ore, kg m-3 c, = saturation concentration of CaO in phosphoric acid, kg
m-3 ct = CaO concentration in the
bulk of the solution at time t ,
kg m-3
f = weight fraction of CaO in the phosphate ore k = mass-transfer coefficient, m s-l
L = mass of phosphoric acid, kg Mt = mass of the phosphate ore at time t , kg R, = radius of an ore particle (mean) at time t , m t = time, s Greek Symbol
density of the Zin phosphate ore, kg m-3 Registry No. H3P04,7664-38-2; Ca, 7440-70-2; fluorapatite, 1306-05-4.
pa =
Literature Cited Becker, P. Phosphate and Phosphoric Acid, Fertiliser Science and Technology Series; Marcel Dekker: New York, 1983; Vol. 3. Bloise, R.; Shakourzadeh, K.; Baratin, F. Ind. Miner. Tech. 1984,9, 721. Calderbank, P. H.; Moo-Young, M. B. Chem. Eng. Sci. 1961,16,39. Elmore, K. L.; Farr, T. D. Ind. Eng. Chem. 1940, 32, 580. Huffmann, E. 0.;Cate, W. E.; Deming, M. E.; Elmore, K. L. J. Agric. Food Chem. 1957,5, 266. Ivanov, E. V.; Zinyuk, R. Yu.; Pozin, M. E. Zh. Prikl. Khim. 1977, 50, 1193. Jordan, D. E.; Monn, D. E. Anal. Chim. Acta 1967, 37, 42. Klok, A.; Tiggelman, J. J.; Weij, P.; van Dalen, J. P. J.; de Galan, L. Proc. Colloq. Spectrosc. Int., 24th 1985, 98. Pribil, R.; Vesely, V. Talanta 1966, 13, 233. Serdyuk, V. V.; Tereshchenko, L. Ya.; Panov, V. P.; Chekreneva, G. M. Zh. Prikl. Khim. 1982,55, 2190. Slack, A. V., Ed. Phosphoric Acid, Fertiliser Science and Technology Series; Marcel Dekker: New York, 1968; Vol. 1. Szekeres, V. L.; Kardos, E.; Szekeres, G. L. J. R a k t . Chem. 1965,4, 113. Tjioe, T. T.; Weij, P.; van Rosmalen, G. M. Proc. World Congr. Chem. Eng., 3rd 1986,2, 925. van der Sluis, S.; Meszaros, Y.; Wesselingh, J. A,; van Rosmalen, G. M. Proc. Fert. SOC.1986, no. 249. Weast, R. C., Ed. Handbook of Chemistry and Physics, 57th ed; CRC: Cleveland, OH, 1976; pp A-116, B-241.
Nomenclature A = external surface area of the phosphate ore, m2
Receiued for reuiew March 11, 1987 Accepted July 28, 1987
Efficient Methods of Inducing Air Suction by Means of Secondary Rotational Air Charge Tetsuo Akiyama,*t Taketoshi Marui,?and Motomi Konof Department of Chemical Engineering, Shizuoka University, Hamamatsu 432, Japan, and Aco Company Ltd., Chiba 272-01, Japan
Means of increasing the rate of air suction that is induced by secondary rotationary air charge from nozzles have been explored experimentally. Two factors that affect the rate of air suction were examined. One was the position of the guide vanes relative to the nozzles. The other was an impingement object a t the fluid exit. The interrelation between the position of the guide vanes and the impingement object was also studied, along with the effect of nozzle angle. Experiments have indicated that it is more advantageous to install guide vanes in the downstream rather than in the upstream of the nozzles, and the impingement object can be effectively used in a limited parameter range. The swirl flow in the straight-through cyclone is usually generated by fixed vanes or impellers, but sometimes turbocompressors can be used. A list of studies (up to Shizuoka University. Aco Company Ltd.
1966) on swirling jets issuing from vane swirlers are listed in the work by Mathur and Maccallum (1967). Experiments were carried out on swirling air flow in a sudden expansion by Hallett and Gunter (1984). The method to produce swirling flow by introducing air tangentially into a cylindrical chamber (instead of using guide vanes) was
0888-5885/87/2626-2505~01.50/0 0 1987 American Chemical Society
