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Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978
Production of KH2P0, from KCI and H,PO, in an Organic Liquid Medium Elierer Rubin, Edward Srpruch, and Aluf Orell Department of Chemical Engineering, Technion, Israel Institute of Technology, Haifa, Israel
KCI and H3P04are the natural raw materials for production of KH2P04and will yield in aqueous media KH2P04and HCI. However, all the components are highly soluble in water. The possibility of producing solid KH2P04using an inert organic solvent (1-butanol with a small amount of dissolved water) which will shift the equilibrium to the right by removing the HCI was studied in the present work. The paper deals primarily with the reaction stage and not with recycle of solvent and separation of acids. The reaction was carried out isothermally in a well mixed batch reactor. The mechanism of the reaction was determined from microscopic observations and experimental data. The experimental variables studied were temperature, water concentration, initial H3P04concentration, and initial KCI concentration. One of the main conclusions concerning the mechanism of the reaction is that it has an ionic character and takes place in a microscopicallydispersed aqueous phase. Disappearance of this phase terminates the reaction. The mechanism of the reaction is described in detail, and the effect of the experimentally studied variables on reaction rate and on KH2P04yield in this complex multiphase system are presented and discussed.
Introduction KH2P04has important properties which are required from a good synthetic fertilizer: a high concentration of important elements in a form that can be easily consumed by plants, absence of undesired elements, ease of transportation and application. Other phosphate fertilizers such as super-phosphate and triple-super-phosphate contain calcium, and potash contains chlorine, which may be an unnecessary ballast or harmful element in certain cases. However, up till now, KH2P04has not been manufactured industrially as a fertilizer since there is no available process to produce it in large quantities at low enough cost for agricultural purposes. KC1 and H3P04 are the natural raw materials for production of KHPP04by the reaction KC1 + H3P04
water
K H ~ P O ~ (+S )HCl(1)
7 ’
(1)
It is not economical to produce KH2P04directly by this reaction because of the high solubility of all the components in the reaction liquid. The literature offers two main routes for production of KH2P04in order to overcome the solubility problem. One is based on fusion and the other on the use of organic solvents. Van Wazer (1966) describes a process whereby KH2P04is produced by direct neutralization of phosphoric acid with caustic potash. According to Waggaman (1952) KH2P04may be produced by decomposing KCl with H3P04at 250 “C until complete evaporation of HC1 is obtained. The final product is very acidic, and addition of KOH or K2C03 is needed for its neutralization. Fusion processes are also reported by Worthington et al. (1973) and Grizdovitch (1971). All these suggested methods are too expensive for large-scale industrial production. Reaction 1belongs to chemical reactions of the “double decomposition” which may be written in a general form Mx(1,s) + Hy(1) + Hx(1) + My(s)
(2)
where M is a cation and x and y are appropriate anions. The shift of equilibrium in the right direction, i.e., for
* To whom correspondence should be addressed. Presently Visiting Professor, Department of Chemical Engineering, University of California, Berkeley, Calif. 94720. 0019-7882/78/1117-0460$01.00/0
manufacture of My(s) or Hx(1) using polar organic solvents has brought about in recent years the development of new and efficient technologies some of which have already found industrial scale application (Le,, manufacture of KNOBfrom KC1 and “Os). The published information on these technologies is, however, very scarce and incomplete. The introduction of a polar organic solvent to the “double decomposition” reactions offers several possibilities: (a) the efficient removal of the acid product by extraction into the solvent; (b) reduction of the salt product solubility and its removal as a solid; (c) selective removal of the acid product by formation of an easily decomposed chemical bond with the solvent; (d) removal of the acid product by distillation of the solvent, whereby the acid product is removed together with the solvent. General explanations of the first two techniques have been published by Baniel and Blumberg (1957). They also state that reaction 1in the presence of isoamyl alcohol is very fast. However, no explanation is offered as to the reaction mechanism. This seems to be the general pattern in the literature, of which a substantial part is in the form of patents: no details on reaction mechanism. In a patent, Baniel and Blumberg (1956a) suggest the conduction of reaction 2 in an aqueous medium and removal of the acid product by extraction. As solvents they recommend aliphatic alcohols that contain one OH group and four or more carbons. The reacting salt Mx is added as an aqueous solution, and the addition of the organic solvent results in settling of the product My and extraction of Hx into the organic phase. Separation of the acids Hx and Hy is achieved by distillation of the organic phase. Baker (1972) uses a selective amine to extract the x- ion. A patent by the Onoda Cement Co. (1964) involves using two different solvents in preparation of MH2P04. The first is an amine which reacts with HC1 binding it chemically and causing the precipitation of MH2P04. Addition of the second organic solvent (ether, alcohol, or ketone) further reduces the solubility of MH2P04. The amine is recovered after distillation in the presence of MgO, CaO, or Ca(OH), which bind the HCl. Fillipescu (1972) suggests the use of a tertiary amine to bind the HC1 (i.e., C2H6N= HC1). Abe and Morgigama (1963,1970) react KC1 and H3P04in the presence of butanol a t low temperatures. In two patents, Baniel and Blumberg (1956b,c) suggested carrying out the reaction of Mx H3P04using organic solvents of limited
+
0 1978 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978 461
Q i
Q
Q
~ o l v e n Il H3W4
KH2POk ,301
d)
Figure 1. Schematic diagram of KHzPOl production process.
solubility in water. The ratio of water to solvent has to be such that no separate aqueous phase will be formed. In two other patents (Baniel et al., 1967; IMI, 1965) they suggest distilling an organic solvent through the reactor in order to remove the undesirable acid. A similar approach is suggested in another patent (Menzer and Friebe, 1962). Moldavan and Merimela (1966) report that reaction 1 can be carried out in the presence of butanol, which enables precipitation of pure KH2P04 even from wet phosphoric acid which contains many impurities. In summary, it may be concluded from the literature survey that there is a fair number of publications suggesting the use of an organic solvent. However, none of them presents systematic data or elaborates on the details of the reaction mechanism. The present paper summarizes work we have done on the reaction KCl
+ H3P04
-
1-butanol KHzP04(s) water
+ HCl(1)
(3)
carried out in an organic medium consisting of a homogeneous solution of water in 1-butanol. Butanol plays two important functions, namely removal of HC1 from the reacting system and precipitation of KH2PO4as a solid product. The entire process may be summarized schematically as shown in Figure 1. In this paper we report work done only on the reaction part of the process. Earlier experiments in this laboratory indicated that the reaction is highly affected by whether the KC1 is introduced into the reactor in a solid form or as an aqueous solution. Accordingly, the first objective was to elucidate the reaction mechanism when the feed KC1 was in the form of a solid or an aqueous solution. An additional objective was the systematic evaluation of the effect of reactant concentrations, temperature, and water concentration in the organic phase on the efficiency of KH2P04production in a batch reactor. The results of the present work can supply enough information for development of a continuous reaction system as well as data for investigation of the solvent recovery stage of the process. Experimental Section The batch reaction system is shown schematically in Figure 2. All parts of the system were made from glass or Perspex. Selection of the materials of construction was based on their ability to withstand concentrated phosphoric acid and butanol. The reaction was carried out in a baffled glass vessel 125 mm in diameter and 155 mm high. The reactor contents were mixed with a speed-controlled stirrer. Stirring speed was set a t 650 rpm based on results of preliminary experiments of the effect of stirring speed on reaction rate. The reaction temperature was kept constant at a predetermined value via an external water bath whose temperature was controlled using a heating element (for temperatures above ambient) or cooling coil (for temperatures below ambient). The progress of the reaction was followed by determining the composition of the liquid phase using the sampling device shown in Figure 3. The bottom part of this sampler is made from porous glass
Figure 2. Experimental system: 1, reactor; 2, speed controller; 3, isothermal bath; 4, bath stirrer; 5 , heating element; 6, contact thermometer; 7, heater switch; 8, cooling coil; 9, cooling unit; 10, thermometers.
R
li
Fritted glass
Figure 3. Sampling device.
which prevents passage of solids during suction (using vacuum) of samples through an opening in the reactor cover. Samples were taken directly from the sampler with a pipet. The composition of the liquid (concentration of C1-, H+, and HzP04-) was determined by automatic potentiometric titration. The batch reactor was charged with two types of feed: “dissolved KC1 feed”, which was used in most of the experiments, and “solid KCl feed”. In “dissolved KC1 feed” experiments the feed added to lo00 cm3of butanol was a solution of phosphoric acid, KC1, and water. Special precautions were taken to keep the temperature constant at all times, e.g., introducing the feed at reaction temperature, cooling the butanol prior to feed introduction so that after evolution of the heat of dilution of concentrated phosphoric acid the temperature will be the desired one. Samples of liquid were taken by introducing the sampler into the reactor, pumping liquid with a hand pump into the sampler, opening the stopper at the sampler top, and pipetting an accurate small volume of liquid. The liquid in the sampler was returned to the reactor. This technique enabled us to keep the reaction liquid volume essentially constant throughout the reaction period. At the end of the reaction (up to 5 h in certain experiments), the solids in the reactor were separated from the liquid by filtration. The solids were dissolved in distilled water and analyzed by potentiometric titration. In “solid KC1 feed” experiments, 1000 cm3 of butanol was premixed with the predetermined quantities of phosphoric acid and water. When the reactor temperature reached the proper value, solid KC1 crystals were added. The mixer was operated at 650 rpm (above the minimum velocity at which there is no accumulation of solids at the bottom) in all experiments. The sampling technique and analysis were the same as for the “dissolved KC1 feed” experiments.
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The reaction progress could be determined directly or indirectly. In the direct determination the concentration of H2P04- as well as total H+ was measured. H2P04-in the organic phase originates only from dissolved H3P04 since KH2P04is insoluble in the organic phase. H+ originates from HC1 and H3P04. The ratio of concentration [(H+) - (H2P04-)]/ (H+) expresses the instantaneous conversion of H3P04. Because of the high acidity only KH2P04and HC1 are the reaction products (no K2HP04 and K3P04are formed). In this research, molar conversion can be expressed relative to KC1 or H3P04which were not fed in stoichiometric ratios of 1:l. During the progress of the research it was found that above a certain level the initial KC1 concentration does not affect the reaction. It was therefore decided to relate conversion to the phosphoric acid introduced as feed into the reactor. The indirect determination of reaction progress involved determination of C1- ions in the liquid phase. The source for C1- is almost exclusively HC1, since KC1 is practically insoluble in butanol. From considerations based on convenience and accuracy of analytical determination it was decided to use in most cases the C1- concentration as an indication of reaction progress. The final yields of reaction batches (total amount of K H 2 P 0 4 produced) were determined on the basis of analysis of the final solid product and liquid obtained. The solid was separated from the liquid by filtration, then dissolved in water and analyzed.
Experimental Results and Discussion In accordance with the research objectives the work was divided into two parts: (a) determination of reaction mechanism and (b) investigation of relevant parameters in accordance with the proposed mechanism. Determination of Reaction Mechanism. The proposed reaction mechanism in this complex multiphase system was based on a microscopic study of the reaction progress and on conclusions arrived at from determination of the concentration of reactants and products in the stirred batch reactor under various conditions. The conclusions of the microscopic observations were valid because of the different crystalline form of KC1 and KHzP04and were arrived a t after careful observations by optical and scanning electron microscopes. K H 2 P 0 4 crystallizes basically in a tetragonal structure. When crystallized from butanol one obtains bi-tetragonal pyramide or a prism with tetragonal pyramides attached to both ends. KC1 crystallizes in a basically cubic form. From butanol one can obtain cubes or plates building one on top of the other. Careful observations were made of samples taken out of the reactor at different times as well as following experiments using “solid KC1 feed”, conducted in Petri dishes. In the latter experiments, KC1 crystals were placed in a Petri dish containing H3P04and H2O dissolved in butanol. Since there was no mixing, the reaction was slow and the same KC1 crystals could be followed microscopically as they disappeared and KH2P04 crystals formed. Generally, the microscopic observations indicated that the reaction takes place in thin aqueous films surrounding the KC1 and KH2P04crystals which form quickly when feed is added into the butanol. Below a certain initial water concentration, these aqueous films disappear a t the end of the reaction. Series of experiments were conducted with “solid KC1 feed” and “dissolved KC1 feed” a t 25 and 40 “C. In these experiments, the concentration of H3P04and the amount of water in the feed were kept constant, whereas the amount of KC1 in the feed was varied. The experimental results indicated that the yield using “dissolved KC1 f e e d
0
0.2
0.1
0.4
0.3 K C I in t i e d 2 mole
Figure 4. Effect of initial amount of KCl on KHzP04yield: dissolved KC1 feed method. Feed composition: 1.08 mol of H3P04/Lof butanol; 95 g of H20/L of butanol.
”0.
I
0
40
I
I
I
80
I
120
I
I
160
I
I 200
T m e , mln
Figure 5. Effect of feed composition on the kinetics of the dissolved KC1 feed method. Feed composition: 0,standard; X, H3P04added to butanol; 0 , water added partially to butanol; A, 0.1 mol of HC1 added to butanol; Q 0 . 2 mol of HC1 added to butanol; +, 0.1 mol of HC1 added to water.
is higher than with “solid KC1 feed” under identical conditions. For “dissolved KC1 feed” the KH2P04yield increases with a decrease in temperature whereas the initial amount of KC1 (above a certain concentration) does not affect the results. On the other hand, the yield in “solid KC1 feed” experiments is much lower and increases somewhat with increase in temperature and initial KC1 concentration. Figure 4 shows the effect of initial KC1 concentration on the final KH2P04 concentrations in “dissolved KC1 feed” experiments. It can be clearly seen that above a certain initial KC1 concentration the excess KC1 has no effect on the final results. Similarly, additional experiments indicated that above a certain critical amount of KC1 in the feed, its initial concentration has no effect on the reaction rate. Figure 5 summarizes the effect of feed composition on the reaction kinetics. It can be clearly seen that: (a) There is practically no effect on the form of H3P04introduction. In other words, H3P04can be fed together with KC1 and water, or directly into the butanol with the same experimental results. (b) Initial transfer of part of the water into the organic phase accelerates the reaction at the initial stages. (c) The addition of HC1 into the feed solution (water, H3PO4, KC1) or into butanol a t the beginning of the reaction has an identical effect on the results.
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978
463
Table I. Relative Composition of t h e Solid during a “Dissolved KC1 Feed” Experiment (wt %)a time, min 51.5 48.5
73 27
KCl KH,PO,
8
20
40
70
110
200
660
40.5 59.5
36.8 63.2
35.7 64.3
32.0 68.0
29.0 71.0
28.4 71.6
28.4 71.6
3
1
a Experimental conditions: temperature, 1 0 “ C ; feed, 1000 c m 3 of butanol, 1.0 mol of H,PO,, 0.402 mol of KC1, 100 g of water. T i m e , min
I
0 I
40 I
1
80 1
I
120 I
I
I
Time, m,n
Figure 6. Effect of water addition after reaction terminationdissolved KC1 feed method: 0 , water added 36 min after start of reaction; water content, initial, 45 g/L of butanol; final, 100 g/L of butanol; A , regular experiment, 100 g of H20/L of butanol.
Experiments were conducted in order to find the effect of addition of crystallization nuclei. It was found that the addition of KC1 nuclei has no effect on the reaction. Addition of KH2P04nuclei accelerates the initial reaction rate. When in addition to adding KH2P04nuclei part of the water is added into the butanol, a further initial acceleration of the reaction is obtained. Table I gives the relative composition of the solids in the course of a “dissolved KC1 feed” experiment. Initially, the solids are composed primarily of KC1. However, the amount of KH,PO, increases rapidly with time. Figure 6 shows the effect of addition of water to the final products mixture in a “dissolved KC1 feed” experiment after the reaction terminated. It can be seen that the addition of water caused the reaction to continue and the reaction rate at this second stage is identical with that of a regular experiment where all the water was added initially. Based on the experimental results and observations, it seems clear that the reaction is of an ionic nature with all stages taking place in a thin layer of an aqueous phase surrounding the crystals. However, under appropriate conditions, e.g., initial water concentration below a certain critical value, the aqueous phase disappears during the reaction, and only an organic liquid and solid phases are present at reaction end. It is also clear that the formation of KH2P04by this process involves, as far as mechanism is concerned, a very complex situation of multiphase equilibrium, solubility, and diffusion. The following are the postulated stages of the reaction. “Dissolved KC1 Feed”. (a) A very quick salting out of KCl and KH2P04crystals from the butanol. The first crystals formed are those of KC1. KH2P04crystallizes at a slower rate (See Table I). The fast rate of crystallization of KC1 also affects the crystal size. These crystals are very small relative to those of KHpP04(this was clearly seen under the microscope). The first stage lasts about 2 min. (b) A gradual dissolution of KC1 crystals in a surrounding
aqueous droplet and crystallization of KH2P04in this fast-forming, aqueous media. The organic phase supplies and removes HC1. This stage is much slower than the initial stage and continues almost to the end of the reaction. The aqueous phase disappears gradually as the reaction proceeds. (c) The reaction terminates because of conditions which do not enable further dissolution of KC1 (aqueous films disappear) and removal of HC1 (saturation of the organic phase). Finally, the remaining KC1 crystals are bare, Le., are not covered by layers of KH2P04. (This was verified microscopically.) The existence of the aqueous phase and its disappearance at the end is the key to the reaction mechanism. The formation of the aqueous phase is associated with the effect of K+ and C1- ions on the phase equilibrium. An experiment with the system butanol-water-KC1 indicated that the formation of an aqueous film around KC1 crystals occur at relatively low water concentration (about 60 g of water/1000 cm3 of butanol). The first aqueous film around KHzPOl crystals appears only at about 110 g of water/lOOO cm3 of butanol. Thus, the formation of aqueous films around KCl and KH2P04crystals is due primarily to KC1 and not KH2P04. Also, equilibrium data on the system KC1-HC1-H,O-butanol (Blumberg et al., 1960) indicate that the presence of KC1 causes a reduction in water solubility in butanol whereas the presence of HC1 increases it. “Solid KC1 Feed”. (a) Initial slight dissolution of KC1 in the homogeneous mixture of water-butanol and H3P04. The solubility of KC1 in butanol is very small. Initial formation of aqueous films around the KC1 crystals. (b) Following the slight dissolution of KC1 in butanol the aqueous films around KC1 continue to grow causing a faster dissolution of KC1. (c) H3P04penetrates into the aqueous films and KH2P04starts to crystallize. However, since there are no crystallization nuclei for KHZPO4it tends to crystallize on the available KC1 crystals. (d) The reaction terminates when KH2P04covers all the available KC1 crystals and the aqueous films disappear. Comparison between the “dissolved KCl feed” and “solid KC1 feed” reaction mechanisms, as well as reaction yields under identical conditions, indicates that the former leads to much higher yields and purity. As for the rate-determining steps: the reaction is not sensitive to excess KC1 or the rate of KC1 dissolution. Since experiments in which HC1 was added initially to the various phases did not affect the rate, it may be concluded that extraction of HC1 by the butanol is not a rate-determining step. Similar conclusions were arrived at with respect to The addition of KH2P04nuclei tends to increase the initial reaction rate. From Table I it is clear that the rate of crystallization of K H 2 P 0 4is slower than that of KC1. All these lead to the conclusion that the rate of KH2P04crystallization governs the reaction rate. Effect of Operating Variables on Yield and Conversion. The system studied is very complex. It involves the components KC1, H3P04, butanol, and water as feed and KHzP04 and HC1 as products of the reaction. A mixture of solid and liquid phases is involved: KC1 and KHzP04 crystals in the solid phase and butanol as the
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Chem. Process Des. Dev., Vol. 17, No. 4,
1
I
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t
I
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I 120
0‘
0
40
80
W o k - b u t a n o l ratio
1978
I
1 IO
grflit
Figure 7. Effect of water-butanol and H3P04-butanol feed ratios on KHBP04yield and final Cl- concentration at 30 “C. H3P04-butanol ratio, mol/L v,0,0.58; A, *, 1.10;m, +, 1.62; 0 , X, 2.29; ---,KH,P04; -, Cl-.
liquid phase. During the progress of the reaction a thin film of an aqueous phase is also present. The multicomponent mixture is not at equilibrium and its composition changes constantly during the reaction. The following simplified description gives an idea of the complexity of the system. At the beginning the mixture contains four components: KC1, H3P04, butanol, and water. If this mixture would have been at equilibrium then three phases could be expected: solid KC1, organic phase, and aqueous phase. Therefore, initially an aqueous phase in the form of a film surrounding the solid KC1 crystals is formed. As the reaction progresses, two additional components are formed: HC1 in the organic (and aqueous) phase and KH2P04in the aqueous phase. As the HC1 concentration in the organic phase increases, the solubility of water in this phase increases too. Toward the end of the reaction the water may dissolve completely in the organic phase. It was not feasible to prepare “phase diagrams” or similar general correlations which could describe completely this complex system. It was found, however, that by careful considerations and analysis of experimental data it is possible to have enough information that can be used to predict the behavior of the system under the effect of the more important experimental variables. Since the “dissolved KC1 feed” method results in a higher yield of KH2P04than “solid KC1 feed”, all the batch reaction experiments described in this section were conducted using the former technique. Also, the results presented in the previous section indicate that the main parameters affecting the conversion of H3P04 to KH2P0, are temperature, initial concentration of phosphoric acid, and the amount of the added water relative to butanol. It was also mentioned that when KC1 concentration in the feed is above a certain level, the effect of the excess KC1 on the yield is negligible. Based on the reaction mechanism, it can be concluded that the number of components is 5 , the number of phases is 3, and therefore the number of degrees of freedom is 4. Thus, at any given temperature and pressure, two concentrations have to be specified, e.g., the total concentration of H+ and water, in order to determine the system.
I
I
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I
80
40
I
lo
120
W a t e r - butonol r m o , grllit
Figure 8. Effect of water-butanol and H3P04-butanol feed ratios on KHpP04yield and final C1- concentration at 10 “C. H,PO4-butanol ratio, mol/L: V,0 0.58; A, *, 1.07; W, +, 1.62; 0 , X, 2.30; ---,KH2P04;
-, c1-.
I
I
I
I
I
I
30
25 -
;?
0
5
20-
8
0 “ 1 ”
15-
: 80 120
lo 0
40
woter- butanol r a t i o , g r f l i t
Figure 9. Effect of H3P04-butanol ratio on H3P04 conversion. H3P04-butanol ratio, mol/L: 0 , 0.58; 0, 1.10; m, 1.62; A, 2.29; -, 10 “C; - - -, 30 “C.
The variables studied were: temperature, 10, 20, and 30 “C; initial concentration of H3P04,0.57-2.35 mol/L of butanol; water in feed, 30-120 g/L of butanol. KC1 initial concentration was usually in the range where it has no effect on reaction yields, namely 215-402 mmol/L of butanol. The ranges of variables were determined on the basis of the reaction mechanism and physical limitation of the experimental system. Figures 7 and 8 show the effect of water-butanol feed ratio on final dissolved C1- concentration and KHlP04 obtained as a solid product at termination of the reaction (after about 2 h) at two different temperatures.
Ind. Eng. Chem. Process Des. Dev., Vol. 17,
No. 4, 1978 465
I
0.30
-
-
0.26 -
0
-
2
,
9
-2 c
i
0.22-
c Q
-
I
P c
0.18-
-0
-
401 0
80
40
Water-butanol
00
40
a0
120
w a t e r - butanol r o t l o , g r l l i t
120
ratio, g i i l i l
Figure 11. Final C1- concentration in the organic phase vs. water-butanol feed ratio. Temperature: 0 , 10 OC; A, 20 "C; 0,30 "C; 1.10 mol of H3P04/Lof butanol.
Figure 10. Solubility of KC1 in the organic phase. H3P04-butanol ratio, mol/L: 0, 0.58; X, 1.10; A , 1.62; +, 2.32.
Figure 9 shows the percent conversion of H3P04as a function of the feed water and H,P04-butanol ratio at two different temperatures. Figure 10 shows the solubility of KC1 in the reaction liquid, calculated as the difference between C1- concentration in the reaction liquid and the amount of KH2P04 in the solid product. C1- Concentration in the Organic Phase. The determination of the relation between C1- concentration and the temperature, water, and H+ concentrations enables the prediction of the amount of KHZPO4produced. When the temperature, H+ concentration, and HzO concentration are determined, the concentration of C1- is also determined. The C1- originates from dissolved KC1 and HC1. The quantity of HC1 is identical with the quantity of KH2P04 (see eq 3). Thus the amount of KH2P04obtained can be determined through the composition of the organic phase. Generally, for kinetic studies, monitoring the C1- concentration in the organic phase gives a good indication as to the progress of the reaction and the amount of KH2P04 product since it originates primarily from HC1. Figure 11compares the equilibrium results obtained for a given initial H+ concentration (1.10 mol of H3P04/Lof butanol) a t different water-butanol feed ratios and temperatures. It can be seen that the relation between final C1- and water concentrations is practically linear [Cl-] = A + B[H20] (4) Figure 1 2 shows values of A and B as a function of temperature where feed H,PO,-butanol ratio is the parameter. Using this figure it is possible to predict the final concentration of C1- in the organic phase under conditions similar to those used in the present research. The increase in C1- concentration with increase in H3P0,-butanol ratio and HzO concentration in the organic phase can be explained by changes in properties of the solvent. Pure butanol has low polarity and is not a very good dissolving medium for polar compounds such as HC1 and H3P04.The addition of polar compounds causes an increase in its ability to absorb more C1- ions. The effect of temperature is opposite to that usually expected. However, one can find systems with similar temperature
1.2
0.81 0
I
1-
!
10
!
I
20
I
I
I
30
Temperature ,'t
Figure 12. Constants A and B for determining the final C1- concentration in the organic phase. H3POI-butanol ratio, mol/L: V, 0.58; 0,1.10; A, 1.62; 0 , 2.32.
effects. Finally it should be pointed out that the range of water concentrations was limited to the homogeneous region of liquid phase; i.e., only one stable liquid phase (organic) could exist. Beyond this range, Le., when a separate and stable aqueous phase can form, the picture can be completely different, as shown later. Solubility of KCI in the Organic Phase. Determination of KCl solubility in the organic phase is important for the design and operation of batch or continuous reactors, as well as for the solvent recovery system. The solubility of KC1 in pure butanol at ambient temperature is very low-0.6 mmol/L. It is much larger in pure water. The presence of dissolved water, as well as other ions, in the butanol affects its solubility. In the present work KC1 solubility was determined indirectly as the difference between C1- concentration in the organic phase and KHzP04 in the solid. Considering the accuracy of the ahalytical methods and the low solubility of KCl, this method of determining KCl concentration is not too ac-
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Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978
0' t
Table 11. Effect of Initial Amount of KCl o n Composition of the Final Solid and Liquid in "Dissolved KCI Feed" ExuerimentsQ initial KC' mmol/L of butanol 7
402 362 322 295 275 200 150
liquid composition
solid composition, mmol
H'
C1'
KCl
1.07
0.276 0.275 0.275 0.266 0.255 0.200 0.150
130 91 47 28 21 2 1
1.08
1.09 1.08 1.08 1.07 1.07
H $0 4 converKH-PO, sion. % 254 250 253 246 237 174 130
23.5 23.1 23.4 22.8 21.9 16.1 12.0
a Experimental conditions: temperature, 1 0 "C; time, 2 h; feed, 1000 cm3 of butanol; initial H,PO,, 1.080 mol; water, 95 g.
01 4 1
curate. This inaccuracy did not enable detection of the slight effect of feed H3P04-butanol ratio in Figure 10. Therefore only a single curve was drawn in the plots of this figure. However, the accuracy of the data for Figure 10 is good enough to warrant the use of these data for possible design work. The solubility of KC1 in the organic phase increases with water concentration. This increase is gradual until the stable two-liquid phase region is reached which was essentially the limit in water concentration used in our experiments. Figure 10 also indicates a decrease in KC1 concentration with decrease in temperature. This means that by decreasing the temperature the contribution of KC1 to C1- concentration decreases and this enables an increase in reaction yield. In addition, any increase in KC1 solubility may increase the loss in raw materials. Thus lower temperatures are preferable for reaction also from the "KC1 point of view". K H z P 0 4Yield and Conversion. The yield at equilibrium of KHZPO4can be predicted from Figures 7 and 8, as a function of water concentration in the organic phase when H3P04-butanol feed ratios and temperature are the parameters. The maximum yield in KHzP04is obtained when the water concentration approaches the stable two-liquid phase region. The transition to this two-phase region results in a sharp decline of the KH2P04yield. Figure 13 shows the yield of KH2P04 at various temperatures. It indicates the increase in KH2P04yield with decrease in temperature. This is explained by increase in C1- concentration and reduction in KC1 solubility in the reaction liquid. One of the objectives of the batch experiments was to determine the maximum amount of H3P04which reacts with KC1. The results indicate that the amount of KHzP04 produced increases with the initial amount of H3P04,but not linearly. From Figure 9 it can be seen that the relation between conversion in the reaction and the initial H3P04-butanol ratio is inversely related to the amount of K H 2 P 0 4produced and the initial H3P04-butanol ratio. The conversion of H3P04to KH2P04in the reaction was the lowest when the initial H3P04-butanol ratio was
0
80
40
120
Water-buianol ratio, g r l l i t
Figure 13. Effect of water-butanol ratio on KHlP04 yield. Temperature: 0 , 10 "C; A, 20 "c; 0,30 "c; 1.10 mol of H3P04/L of butanol.
highest. For example: at 10 "C and 100 g of water/L of butanol, the conversion at an initial H3P04-butanol ratio of 0.58 mol/L of butanol is 28.9%, and when the H3P04-butanol ratio was 2.30 mol/L of butanol it decreased to 16.0%. On the other hand, the absolute amount produced was 169 mmol of KH2P04at the low concentration and 369 mmol at the higher concentration. This means that it is possible to reach practically complete conversion of the feed KCl (i.e., obtain a solid containing only KHzP04) but it is impossible to obtain complete conversion of H3P04. This latter point is further demonstrated by Table 11, which shows that by reducing the initial KC1 concentration relative to H3P04concentration, a practically pure KHZPO4product can be obtained. The explanation lies at least partly in the fact that the solubility of H3P04in the organic phase is much larger than that of KC1. Limiting Concentration of Water in t h e Organic Phase. From the description of the reaction mechanism it is clear that water plays a key role in the reaction. It can be expected that increasing the water concentration wil! increase the reaction yield. There is, however, a limit above which a stable aqueous phase appears in addition to the organic phase at reaction end. According to the experimental results the maximum conversion is obtained when the water concentration in the organic phase approaches the two stable liquid phases region. The optimum operating conditions are on the border of the twoliquid phase region (on the homogeneous side). From Figure 9 it is clear that transition to the two-liquid phase region causes a drastic reduction in conversion. No experiments were conducted in this research aimed at direct visual determination of the transition to the twophase region. However, the transition point can be de-
Table 111. The Limiting Concentration of Water in the Organic Phase feed ratio, H,PO,/butanol, mol/L 0.58
OQ
a
temp, "C
g/L of butanol
wt%
30 20 10
193 197 201
19.7 20.0 20.6
According to Melian (1959).
g/L of butanol 90 90 100
1.62
1.10
wt% 9.5 9.5 10.5
g/L of butanol 90
95 105
wt% 9.0 9.6 10.4
g/L of butanol 90 100
110
2.32
wt%
g/L of butanol
wt%
8.8 9.6 10.4
95 100
8.6 9.1 9.8
110
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978 0.51
0.25
I
I
I
I
l
l
I
I
1
1
1
0.20
-..
: 0.150 I
e
I
-
0.10-
I
I
I
I
80
40
I
I
I
120
Time, m n
Figure 14. Effect of water--butanol ratio on the instantaneous C1concentration in the organic phase. H,O-butanol ratio, g/L: 0,40; 0 , 5 0 ;V,W,O, 70; X, 80, A,90; +, 100;0.58 mol of H3P0,/L of butanol.
termined indirectly from the experimental data. Table 111 shows the maximum water concentration in the organic phase before two stable liquid phases will form at various initial H3P04-butanol ratios and temperatures. These values were obtained from the maximum KH2P04produced as shown in Figures 7 to 9. It should be noted that these values are based on a system which contains also H3P04 and HCl whereas the values in the literature (Melian, 1959) are for pure water and butanol. From the values in the table it is clear that if water-saturated butanol comes in contact with H3P04 + KC1, a stable aqueous phase will form immediately. This stable phase will be detrimental to the process. For high yields one has to take care not to have more water in the butanol phase than indicated in Table 111. Rate of Reaction. The reaction mechanism involves two main stages. The first stage is very fast and results in direct precipitation of K H 2 P 0 4and KC1, whereas the second stage is much slower and involves dissolution of KC1 and further formation of KH2P04. As mentioned earlier, the progress of the reaction was followed by monitoring the C1- concentration in the organic phase. It may be assumed that the concentration of dissolved KC1 in the organic phase changes very little with time and therefore C1- concentration represents directly HC1 concentration which is identical with the amount of KH2P04 formed. Since the aqueous films surrounding the crystals are very thin it can be assumed that all HC1 is in the organic phase. Figure 14 shows typical variations of C1- concentration with time for various water concentrations in the system. It can be seen that when the water concentration is relatively low, the reaction is completed very fast (within about 2 min), the second and slower step of the reaction does not exist, and the yield of KH2P04is relatively small. When the water concentration is relatively high, the second stage of the reaction is in force, and the yield of KH2P04 increases appreciably. In all the studied cases the reaction is practically completed within 40 min. The complex physical nature of the process has hindered, so far, the formulation of theoretical expressions for the reaction rate. Empirical expression can, however, be derived by considering a two-step process.
1
1
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1
467 150
488
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978
solution-another source for water. Due to the scope of the presentinvestigation it was not possible to arrive at appropriate conclusions pertaining to the butanol recovery stage, as well as to the acids Beparation. Consequently, the optimal concentration in the reactor feed cannot be specified as yet. The amount of KC1 feed may be dictated by H3P04concentration. When the amount of KC1 feed is somewhat lower than the limiting concentration (calculated according to C1- concentration in the organic phase at given temperatures and H3P04butanol and water-butanol ratios) it is possible to obtain a chloride free product, as demonstrated in Table 11. Literature Cited Abe, T., Morigama, T., Japanese Patent 7002652 (1970). Abe, T., Morigama, T., Asahi Garasu Kenkyu Hokoku, 13, 117 (1963). Baker, J. D., U S . Patent 3661 513 (1972). Baniel, A., Blumberg, R., Israeli Patent 9539; U S . Patent 2902341 (1956). Baniel, A., Blumberg, R., Israeli Patent 9660 (1956a). Baniel, A., Blumberg, R., US. Patent 2894813 (1956b). Baniel, A., Blumberg, R., Chem. Ind., 78, 327 (1957).
Baniel, A.. Blumbera. R., Melzer, P., Israeli Patent 21 072 (1967) Blumberg, R., Levi,P., Metzer, P., Bull. Res. Counc. Isr.,' 9A, 177 (1960). Filipescu, L., Rev. Chim. (Bucharest), 22 (6), 339 (1971). Filipescu, L., Rev. chim. (Bucharest), 23 ( I ) , 25 (1972). Grizdovitch, J., Gnizdovitch, V., Israeli Patent 26 190 (1971). IMI (Israel Mining Industry), Dutch Patent 6 503 927 (1965). Melian, I., "Handbook of Industrial Solvents", Part 111, p 167, Reinhold, New York, N.Y., 1959. Menzer, Friebe, w., German Patent 1 102711 (1962). Moldovan, I., Marinela, M., Rev. Chim. (Bucharest), 17 (3), 144 (1966). Onoda Cement CO. Ltd., French Patent 1362950 (1964). Raz, Chem. E ~ Q81, - 52 (June 10, 1974). Seaton, W. H., Geankopolis, C. J., J . Chem. Eng. Data. 12 (4), 494 (1967). u.N.. "New process fw the Production of phosptwic Fertilizers Using Hydrochloric Acid", U.N. Industrial Development Organization ID/SER.F/5, New York, N.Y., w . 9
1 . 9
1969. Van Wazer, in D. F. Othmer, Ed., "Encyclopedia of Chemical Technology", 3rd ed Vol. IX, pp 25-32, 1214-1216, Interscience, New York, N.Y., 1966. Waggaman, W. H., "Phosphoric Acid, Phosphates and Phosphate Fertilizers", 2nd ed, p 353, Reinhold, New York, N.Y., 1952. Worthington, R. E., Thompson, W.H., Somers, T. N. E.. Dreschsei. E. V., U.S. Patent 3767 700 (1973).
Received for review September 8, 1977 Accepted May 8, 1978
Pelletizing Waste Cement Kiln Dust for More Efficient Recycling Nancy J. Sell" and Fritz A. Fischbach College of Environmental Sciences, University of Wisconsin-Green
Bay, Green Bay, Wisconsin 54302
Waste cement kiln dust can be compressed by commercial pelletizing equipment into pellets capable of substantially withstanding the forces they would be subjected to in being fired into the flame or feed end of a kiln. These pellets, after undergoing the clinkering reaction in the kiln, form clinker chemically indistinguishable from the remainder of the clinker. The lack of significant degradation of the pellets, even if the pellets are injected through the chain section, makes them an ideal way in which to recycle the cement kiln dust to avoid any recirculating loads and any major modification of the flame characteristics.
Introduction The present methods for recycling cement kiln dust in wet process rotary kilns (slurry feed, scoops, and insufflation) have had only limited success in the amount of dust returned. Pelletizing the waste dust before its return appears to be a very promising new way to overcome many of the problems inherent in these other techniques. The pelletizing of cement kiln dust for recycling purposes is a recently developed idea, one which has been tried for the first time at the General Portland, Inc., Signal Mountain (Chattanooga, Tenn.) plant. In order to be a viable method of dust return, it is necessary to form very hard pellets, capable of withstanding fairly strong impacts, and these pellets must be readily converted to good quality clinker by the regular kiln process. This paper will explain many of the physical and chemical characteristics of these pellets and the commercial scale testing which has been conducted on using them as a dust return method. Background Until now, the most efficient method of dust return has been insufflation: blowing the dust into the kiln alongside the flame, roughly parallel to the kiln axis. Two major effects limit the amount of dust which can be returned in this manner: (1)the dust interacts with the flame, and both lengthens and cools it (Siegert, 1974), and (2) the dust 0019-7882/78/1117-0468$01 .OO/O
can fairly easily be blown out of the process again and can thus develop into recirculating loads. It was postulated that if the dust could be pelletized prior to its return, both of these limitations could be overcome. To eliminate any possible interaction of the pellets in the case of wet processes, and destruction of the pellets by the chain section, it seemed desirable to fire them in directly from the flame end of the kiln by a feeding device-a technique similar to that used in dust insufflation. Cement kiln dust is composed of very fine particles (generally
