Ind. Eng. Chem. Res. 1993,32, 1645-1650
1645
MATERIALS AND INTERFACES Use of Spanish Natural Clays as Additional Silica Sources To Synthesize 13X Zeolite from Kaolin Antonio de Lucas,' M. Angeles Uguina, Ignacio CoviBn, and Lourdes Rodriguez Department of Chemical Engineering, Universidad Complutense, 28040 Madrid, Spain
The use of Spanish natural clays as additional silica sources to synthesize 13X zeolite from kaolin with adequate specifications to be used as a detergent builder has been investigated. A bentonite has been selected as the most appropriate, and the best calcination conditions for its use have been determined. The process variables for the different synthesis steps have been optimized and compared with those obtained using other additional silica sources. The scale-up to a pilot plant and the recovery of mother liquors has been verified.
Introduction The use of synthetic zeolites as partial or total substituents of the highly contaminant sodium tripolyphosphate (STPP)(Kurzendofer et al., 1987;Ainsworth, 1992) may be explained taking into account that many countries have or are developing legislative or voluntary regulations for the reduction or the total ban of phosphates in laundry detergents (Schwugerand Liphard, 1989;Ainsworth, 1992). This fact leads to an important increase of zeolite production and, at the same time, a decrease of sodium tripolyphosphate production during the 1980s (Greek, 1990). However, the use of zeolites as builders in detergents requires a competitive price. For this reason, zeolites must be synthesized from economical raw materials such as Si02 and A1203sources (Barrer et al., 1959;Borthabur et al., 1979; Bosch et al., 1983). In a previous investigation developed in our laboratory (Costa et al., 1988a), the synthesis of 4A zeolite, the most used STPP substituent, from different Spanish clay minerals has been investigated. Three Spanish kaolins were selected, based on their optimum SiOz/A1203 mole ratio, high purity, and easy pretreatment by calcination. The maximum concentration allowed for the impurities in the raw kaolins (CaO, MgO, FezO3), and their best calcination conditions were also established. In alater work (Costaet al., 1988b)the process variables of the different synthesis steps have been optimized in order to obtain 4A zeolite from the three selected kaolins at a lower price and with the established specifications for its use in detergent formulation (crystallinity, particle size distribution, exchange capacity, whiteness). Recently, it has been demonstrated that 4A zeolite may be complemented or substituted by 13X zeolite due to its low capacity to remove Mg2+ ions, an aspect specially important when hard water is used. 13X zeolite with a Si/Al ratio close to 1 showed a capacity to remove Mg2+ clearly higher than 4A zeolite, especiallyat low temperature (Kuhl and Sherry, 1980; Burzio and Pasetti, 1983; Costa et al., 1987; Schwuger and Liphard, 1989). Kaolins are also the most adequate natural clays to synthesize 13X zeolite with low silica content (TatiE and Driaj, 1985). However, they do not have a Si/A1ratio high enough, and
* Address correspondence to this author at Department of ChemicalEngineering, Universidad de Castilla-LaMancha, 13071 Ciudad Real, Spain. Q888-5885f93/2632-1645$04.00f0
the addition of silica from another source (sodium silicate, active silica, or another clay with higher silica content such as bentonite or sepiolite) is necessary. This silica addition allows the formation of pure 13X zeolite (Breck, 1974). The synthesis, at low price, of 13X zeolite from a Spanish kaolin (kaolin P) and different silica sources for its use in detergents have been investigated for the past five years in our laboratory. In a former work (Lucas et al., 19921, the synthesis of 13X zeolite from kaolin P and sodium silicate was investigated. The process variables for the different synthesis steps were optimized in order to produce the zeolite at a lower price for its use as builder. An economical evaluation of this process was carried out with an estimated price for the zeolite produced of 58 pesetas ($0.56)/kg, a price really competitive with that of STPP. In a second work (Lucas et al., 19931, a similar study was developed using active silica as an additional source of silica. In the present work, natural clays with SiOz/A1203ratios higher than those of kaolins are used as additional sources of silica. The experimental results are investigated and compared with those obtained in previous works.
Experimental Section The experiments were carried out in a laboratory reactor of 1-Lcapacity and in a pilot plant reactor of 50-L capacity, previously used in other works (Costa et al., 1988b; Lucas et al., 1992). Both experimental setups are basically constituted by a spherical, stirred reactor, hermetically sealed and equipped with a heating system and temperature and stirring speed measure and control. The reactants used were kaolin P (CaosilS.A.) as a source of Si02 and AhO3and the following natural clays, selected from the moat important Spanish deposits: five bentonites, named as A, T, B, G, and C, one smectite, one sepiolite, and one atapulgite. Table I shows the chemical compositions of these clays. Syntheses were carried out following seven sequential steps (Ruiz, 1986; Covian, 1991): previous kaolin pretreatment, gel formation, aging, crystallization, filtration, washing, and drying. The kaolin pretreatment (crushing and screening to particle diameters lower than 40 pm and calcination at 900 "C for 30 min) was carried out in the conditions established in a previous investigation (Costa et al., 1988b). 0 1993 American Chemical Society
1646 Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 Table I. Chemical Composition of Kaolin P and the Eight Different Natural Clays Used kaolin P bentonite A bentonite C eemectite sepiolite atapulgite bentonite B bentonite T bentonite G 55.80 58.00 62.50 62.00 60.35 56.53 64.84 49.00 62.00 Si02 (wt %) 14.42 18.84 2.20 2.20 1.90 10.50 9.69 19.50 37.00 &os (wt %) 20.06 3.50 4.95 19.60 22.10 23.50 8.90 0.46 5.30 MgO (wt %) 0.40 0.50 1.10 0.91 1.83 1.10 1.00 5.50 0.25 CaO (wt %) 0.60 3.40 3.41 2.40 3.87 3.20 2.90 0.77 0.12 Fez03 (wt %) 0.16 4.84 1.50 0.50 0.09 0.80 0.30 2.53 0.17 NazO (wt % ) 1.20 0.70 1.98 0.13 1.40 0.20 0.48 0.90 0.48 KzO (wt %) 0.00 0.00 0.00 0.03 0.08 0.01 0.50 0.00 0.06 MnzOa (wt %) 12.50 8.60 9.20 20.97 7.00 10.30 10.10 12.80 8.50 calcination loss (wt %) 10.03 10.58 6.66 6.10 56.00 44.96 10.95 5.41 (SiOz/A1203)(mole ratio) 2.25
The gel was prepared by dissolvingthe pretreated kaolin (55 % Si02-42.26 % A12031 in a NaOH solution and adding the clay used as additional source of silica. After an aging time, the gel was crystallized under the desired conditions. Two gels with different compositions,selected in a previous work (Lucas et al., 1992), were used: a gel with low reactivity and high concentration, gel 1 (NazO/SiOz I 1 and H20/Na20 I 34), and a gel with high reactivity and low concentration, crystallized at low temperature, gel 2 LT (NazO/SiOz L 1.5 H20/Na20 L 60 and T I 80 "C). The crystallization product was filtered under vacuum, washed with distilled water (1mL of water/g of zeolite), and dried at 120 "C for more than 12 h. All the experiments were carried out using a constant volume of water in the initial gel: laboratory experiments, V = 0.5 L; pilot plant experiments, V = 30 L. The following variables were modified Gel formation: time, tD; temperature, TD;stirring speed, ND. Aging time: time, tJ&temperature, TE. Crystallization: temperature, TR; time, tR; stirring speed, NR;gel composition (SiOd A1203, H20/Na20, Na2O/SiO2, and K20/(K20 + Na2O)); mole ratios; seeding, I. The solid present in every crystallization sample as well as the final product were analyzed to determine the following properties: Crystallinity, binding capacity, Si/ A1 ratio, particle size distribution curve (PSD),and whiteness. The methods used to characterize the solids have been largely described in previous works (Costa et al., 1988b; Lucas et al., 1992).
Table 11. Synthesis of 13X Zeolite from Kaolin P and Natural Clays. Selection of Calcination Conditions Operational Conditions pretreatment crushed and screened to dp< 40 M m calcination: TQ = 900 'c; tQ = 1 h gel formation tD = 6 h; TD= 70 "C; ND = 500 rpm aging t e e t E = 24 h; TE 20 'c crystahation TR= 80 O C ; NR= 250 rpm; SiOdAhOs = 2.9; HzO/NazO = 32; NazO/SiOz = 1; K20/(KzO + NazO) = 0;Z = 0% additional silica source
t R (h)
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tr A tr A tr 13X + tr A tr 13X + tr A tr 13X + tr A tr 13X + tr A tr 13X + tr A tr 13X + tr A 10 35 tr A tr A tr 13X tr 13X
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tr A
bentonite A bentonite B 3 6 3 6 3 6 3 6
bentonite C bentonite G bentonite T esmectite
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Results and Discussion Selection of the Best Natural Clay To Be Used as an Additional Silica Source. To select, among the eight Spanish clays mentioned above, the most appropriate to be used as an additional silica source, comparative syntheses were carried out keeping the clay under agitation for 6 h at 70 "C and adding the kaolin just 1h before the end of the gel formation step. The clays were calcined at the same time as the kaolin at 900 "C for 1 h. Table I1 summarizes the results of these experiments as well as the operation conditions used. These conditions were selected in previous works using other additional silicasources (Lucas et al., 1992,1993). It can be observed that 13X zeolite of a certain crystallinity mixed with 4A zeolite was obtained only when bentonite T was used. All the other clays used lead to 4A zeolite mixed with traces of 13X zeolite. This is probably due to the high amount of MgO in the composition of these natural clays. The MgO stabilizes the structure of the clays making their attack in the crystallization medium and therefore formation of 13X zeolite difficult. Taking into account these results, bentonite T was selected as an additional silica source for synthesizing 13X zeolite. (i) Pretreatment of t h e Bentonite. Four series of calcinations using temperatures in the range 600-900 "C, varying the calcination time, were carried out to determine
0.6
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ta( min) Figure 1. Pretreatment of the bentonite T. Calcination curves: (*)TQ 600 'c; ( 0 )TQ 700 'c; (A)TQ = 800 'c; ( 0 ) TQ = 900 OC.
the most adequate temperature and calcination time to transform to the amorphous from the crystalline structure of the clay. Figure 1showsthe amorphous grade (levelof destruction of the crystalline structure), X A , versus the calcination time, tQ, a t the different calcination temperatures (TQ)assayed. In all cases X A increases with TQ,decreasing with increasing values of this temperature the time needed to achieve the maximum amorphous grade. Also it was observed that
Ind. Eng. Chem. Res., Vol. 32,No. 8, 1993 1647 only when a calcination temperature of 900 "C was used, was the total destruction of the clay structure (XA = 100% ) achieved. When temperatures lower than 600 "C were used, no appreciable changes were observed in the crystallinity of the clay. From these results, a temperature of 900 "C and a time of 30 min were selected for the bentonite pretreatment. Using these calcination conditions, the total destruction of the clay was assured. Moreover, the selected conditions are coincident with those used in the pretreatment of kaolin P. This fact made possible the simultaneous calcination of both reactants if it was necessary. (ii) Gel Formation. In previous investigations carried out in our department about the synthesis of 4A, 13X,and Y zeolites (Costa et al., 1979, 1980, 1988b;Lucas et al., 1992)using pure and commercial reactants and kaolins, it has been established that an increase of H20/Na20 mole ratio above 30 produces a decrease of gel nucleation. Therefore, the variables of the gel formation step (temperature, time, and stirring speed) were studied using a value of 30 for the H20/Na20 mole ratio that assured high values of the nucleation process. All the other variables were maintained in the values optimized in previous works (Lucas et al., 1992,1993). ( a ) Temperature, T D , and Time, to: Experiments carried out varying the temperature of the gel formation step showed that when a temperature of 70 "C was used, 6 h was necessary to obtain mixtures of 4A and 13X zeolites with a maximum crystallinity of 45 % . That means that when this temperature was used, the bentonite was slowly destroyed and too-large reaction times were needed. Taking that into account, experiments at temperatures higher than 70 "C (80 and 90 "C) were performed. However, to avoid the formation of zeolites other than 13X,only the attack of the clay was carried out a t these higher temperatures, decreasing after that the temperature to 70 "C, the temperature at which the nucleation of 13X zeolite is favored. At this moment, the kaolin was added and it was maintained under agitation for 1 h. Figure 2 shows the results of these experiments. It can be observed that when the temperature of the initial attack over the clay increases, the crystallinity of the zeolite obtained increases. When the temperature was maintained a t 90 "C for 2 h, 64% of crystallinity was achieved. All these experiments lead to 13X and 4A zeolite mixtures. From these results, the following conditions were selected for the gel formation step:
(b) Stirring Speed: From the results obtained varying the stirring speed from 125 to 750rpm, it can be concluded that when values of this variable higher than 500 rpm were used, the kinetic curves and the properties of the zeolites obtained were practically the same. When lower values were used, the crystallinity of the zeolites obtained decreased. Consequently, a stirring speed of 500 rpm was selected. ihese results agree with those obtained in previous wor s developed using sodium silicate and active silica as additional silica sources (Lucas et al., 1992,1993). (iii) Aging Time. Kinetic curves varying temperature and time of the aging step and keeping constant all the other variables were obtained. From the results achieved it was concluded that only when aging times higher than 12 h were used, was 13X zeolite the main phase in the synthesized product. In addition, aging temperatures above 20 "C decreased the crystallinity of the zeolite
13 X Zeo!ite Phases rnlxture
75
50 -
0
2
4
6
8
tR(
10
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Figure 2. Gel formation step. Influence of gel formation time. Gel formation: ND= 500 rpm; SiOdAl20~= 2.9; NazO/SiOz = 1; HsO/ Na2O = 30; K20/(Na~0+ KzO) = 0. Aging time: TE= 20 "C; tE = 24 h. Crystallization: TR= 80 "C;NR = 250 rpm; I = 0%. ( 0 )TD 80 (2 h)/70 "C (1 h); ( 0 )TD 90 (2 h)/70 "C (1 h); (A)TD =: 80 (3 h)/70 "C (1h); (*) TD = 90 (3 h)/70 "C (1h).
obtained, and Pc zeolite was observed in the final product. Figure 3 shows one of the SEM micrographs that indicated the presence of the PCzeolite over the 13X particles. These results, quite similar to those obtained using other additional silica sources (Lucas et al., 1992,1993),show that the aging of the gel must be done at low temperatures. Taking into account that an increase in the aging time clearly increases the cost of the installations needed to synthesize zeolites, an aging time of 12 h (minimum to assure the majority presence of 13X zeolite in the product) and a temperature of 20 "C were selected. Selection of the Crystallization Conditions. To select the most suitable conditions of the crystallization step, experiments using two different gels previously selected as the most adequate to obtain 13X zeolite (Lucas et al., 1992) were carried out. The gels used were gel 1 (low reactivity and high concentration) and gel 2LT (high reactivity, low concentration, and low temperature). (i) Gel 1. (a)Seeding: Experiments carriedout varying the percentage of seeding in the crystallization medium showed that when the seeding increases, the nucleation period decreases and only 13X zeolite with high crystallinity was obtained. At the same time, the PSD curves shift to higher values of d,, increasing the percentage of particles with d , > 10 pm. Hence, a seeding of 5% was selected to assure the synthesis of pure 13X zeolite with a percentage of particles with d , > 10 pm lower than 8%. ( b )Temperature and Stirring Speed When the effecta of the temperature and of the stirring speed over the crystallization step were investigated, the results obtained were completely coincident with those obtained using sodium silicate and active silica as additional sources of silica (Lucas et al., 1992,1993).That means the following: 1. When temperature increases, the nucleation and the growth periods decrease. A mixture of 13X and 4A zeolite was obtained at 85 "C. At the same time, the PSD curves obtained shift to higher d , values. 2. The homogeneity of the reaction system is reached using a stirring speed of 250 rpm. When higher values of this variable (750rpm) were used, traces of Pc zeolite were detected. The PSD curves did not show any important modification. Thus, a temperature of 80 "C and a stirring speed of 250 rpm were selected.
1648 Ind. Eng. Chem. Res., Vol. 32, No. 8,1993
Figure 3. SEM micrograph. Zeolite Pc over particles of 13X zeolite 100
x&) 75
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tR(h) Figure 4. Optimization of crystallization conditions for gel 1. Influence of gel composition. Gel formation: To = 90/70 "C; t D = 2/1 h;ND = 500 rpm; KzO/(NazO + Kz0) = 0. Aging time: Te = 20 O C ; te = 12 h. Crystallization: TR= 80 "C; NR = 250 rpm; I = 5%. ( 0 )SiOdALO3 = 2.9. NazO/SiOz = 1.0, HzO/Na*O = 32. (A) SiOd A1203 = 2.8, NasO/SiOz = 1.0, HzO/NazO = 32. (0) SiOdA1203= 2.9, NazO/SiOz = 1.1,H%O/NazO= 32. (*)SiOdA1203 = 2.9, NarO/SiOl = l.O,HzO/NazO= 34. (a) SiOdAIz03 = 2.9,NazO/SiOz= 1.0, H20/ NazO = 36.
(e) Gel Composition: Figure 4 shows the kinetic curves and the PSD curves obtained varying slightly the SiOd AlzOa, NazO/SiO,, and H20/Na20 moleratios with respect to the values optimized in previous works using other additional silica sources (Lucas et al., 1992,1993).It can be observed that a decrease of the Si02/A1203 mole ratio and an increase of the NazO/SiOz mole ratio lead to mixtures of 13X and 4A zeolites. No effect was observed over the PSD curves. On the other hand, all the zeolites obtained using different H20/Na20 mole ratios fulfill the particle size distribution specification. Since the best change capacity wasobtainedusing HZO/NazO = 34,this value wasselected.
Thus, the gel composition selected was SiOz/A1203= 2.9,NazO/SiOZ = 1.0,and H20/Na20 = 34. ( d )Effect ofthe Potassium Addition: Since the use of 13X zeolite in detergent requires a change capacity as high as possible and taking into account that the partial substitution of NazO by KzO reduces the &/A1 ratio increasing this property (Lucas et al., 1992,1993),experiments were planed varying the KzO/(Na20 + K20) mole ratio in the range 0.05-0.2. In all these experiments only PC zeolite was obtained. These results show that the structure of the gel obtained when kaolin P and bentonite T were used is quite different from the one obtained using sodium silicate or active silica. The potassium addition favored the nucleation of Pc zeolite when bentonite was used. Consequently, the addition of potassium was discarded. (ii) Gel 2 LT. ( a ) Seeding: It was observed that the seeding increases the nucleation of 13X zeolite. However, no influence of this variable was observed on PSD curves. On the other hand, the properties of the zeolites synthesized using 5% and 10% seeding were similar. Their Si/ A1 ratios were lower than those of the zeolites synthesized using gel 1; however, their binding capacities were only 1.6 mmol of CaZ+/gof zeolite due to their low crystallinity. The particle size distribution and the whiteness were similar, too. For these reasons a seeding of 5% was selected. ( b ) Temperature of Crystallization: According to the results achieved in a previous work (Lucas et al., 1992) that showed too high nucleation times when this kind of gel was used, and in order to find a temperature profile that increases the nucleation rate, experiments modifying thetemperaturefrom60to80Tduringthecrystallization time were carried out. Figure 5 shows the kinetic curves obtained. It is observed that increasing the 60 "C period the nucleation and growth periods increase, and when the gel is maintained for 12 h at 60 "C and 5 h a t 80 O C , a 13X zeolite with 59% crystallinity, a Si/Al ratio of 1.14, a binding capacity of 1.6 mmol of Ca2*/gof zeolite and 0.5 mmol of Mgz+/gof zeolite, and a 93% of whiteness is obtained. On the other hand, the PSD curves are similar for all the experiments. There was a maximum over d , = 8.7 pm and
Ind. Eng. Chem. Res., Vol. 32,No. 8, 1993 1649 100
100
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the percentage of particles with d, > 10 pm was very low (4%). The average size was 8.9 pm. Finally, although the Si/A1 ratio, the particle size distribution, and the whiteness of the zeolite synthesized are suitable for its use in detergents, the low crystallinity achieved led to very low binding capacities that do not fulfill the specification required. This low crystallinity may be due to the structure of the gel formed using bentonite that need higher temperatures to increase the nucleation and growth rates. Taking into account that the 13X zeolite synthesized using gel 1 fulfilled all the specifications for its use in detergents and the problems derived from the use of gel 2 LT, this second type of gel has been discarded. (iii) Scale-up and Recovery of the Mother Liquors. To check the scale-up of the 13X zeolite synthesized from kaolin P and bentonite T, two experiments were carried out in the pilot plant equipment at the selected conditions for gel 1,keeping constant the agitation power/volume of reaction ratio used in the laboratory. Under these conditions, 4.2 kg of zeolite was produced in each batch in approximately 17 h. The results obtained are compared in Figure 6 with the corresponding laboratory experiments. It is observed that kinetic and PSD data and properties of zeolites obtained were practically the same for the experimental setup. These results indicate that the scale-up criterion used is valid, at least in the range used. As is known, to obtain a zeolite at a competitive price for its use in detergents, the recovery of the mother liquors, which are concentrated NaOH solutions, is necessary. Consequently, experiments of synthesis were carried out at the optimum conditions for gel 1, each time using the mother liquor obtained from the previous experiment. The results obtained are shown in Figure 6 with the scale-up experiments. It can be observed that the kinetic curves and the properties of the zeolites obtained remained constant in all cases. I t could also be observed that the concentration of Fe3+ ions in the mother liquors remained practically constant, which indicates that most of the Fez03 of the kaolin was incorporated into the zeolites. Only a minimal proportion
4
6
8
10
tR(h)
tR(4 Figure 5. Optimization of crystallization conditions for gel 2 at low temperature. Influence of the crystallization temperature. Crystallization: TR= 60/80 OC; N R = 250 rpm; Z = 5%; KzO/(NaZO + KzO) = 0; SiOdAlzOs = 2.7;HzO/NazO = 70;NazO/SiOz = 1.5.(0) t ~ ( 6 O"e) 15 h; (A)t ~ ( 6 0"c)= 12 h; (0) t ~ ( 6 "C) 0 = 10 h.
2
Figure 6. Scale-up and recovery of mother liquors (conditions Gel 1 (laboratory); (*)gel 1 (pilot summarized in Table 111). (0) plant). Recovery of mother liquors: (0)2; (A)3. Table 111. Operational Conditions Selected and Properties of the 13X Zeolite Synthesized from Kaolin P and the Different Silicate Sources silicate sources sodium active bentonite silicate" silica* lSelected Conditions gel formation 500 500 500 ND(rpm) 70 70 90/70 TD("0 1 1 tD (h) 2/1 aging time 20 20 20 TE("0 12 12 18 tE (h) gel composition SiOz/AlzOs 2.9 2.9 2.9 (NazO + KzO)/SiOz 1.0 1.1 1.0 HzO/(NaZO + KzO) 32.0 30.0 34.0 KzO/(NaZO + KzO) 0.1 0.1 0.0 250 250 250 NR(rpm) 0 0 5 I (%) 80 80 80 TR("0 7.5 6.0 6.5 t R (h) t of total synthesis (h) 20.5 25.0 21.5 13X zeolite obtained (kg/50 L) 4.8 5.1 4.2 Properties of 13X Zeolite 93 95 85 X c (%) Si/Al 1.15 1.08 1.08 10.9 11.1 9.4 d, (pm) CI(mmol of Ca*+/gof zeolite) 2.4 2.3 2.1 CI(mmol of MgZ+/g of zeolite) 1.4 1.3 1.0 94 94 93 L (%) Lucas et al., 1992. Lucas et al., 1993. This work. (I
of Fe3+ ions remains in the solution, and the whiteness of the zeolites produced was not affected. From these resultsit must be concluded that the recovery of mother liquors is possible and that there is no change in the synthesis rate and in the properties of the zeolites obtained. Comparative Study. In Table I11 the operational conditions selected and the properties of the 13X zeolites synthesized using three different silica sources are compared. I t can be observed that, at the operational conditions selected, the total synthesis time for the three additional silica sources is between 20.5 h using sodium silicate and 25 h using active silica. The maximum production (0.23
1650 Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993
kg/50 L) was obtained using active silica due to the lowest HzO/(NazO + Kz0) mole ratio selected. The two other silica sources used lead to a similar producticn (0.20 kg/50 L). Related to the properties of the zeolites obtained, it can be observed that the zeolite synthesized using sodium silicate has the same Si/Al ratio as that obtained using active silica, having a higher exchange capacity due probably to the different occupation of the cationic positions. The third zeolite obtained has a &/A1 ratio lightly higher and a lower crystallinity that lead to a lower exchange capacity. The particle size distributions show that the 13X zeolite from bentonite has the lowest percentage of particles with d, > 10 pm and, therefore, the lowest average size (about 9 pm). The worst particle distribution was obtained using active silica with an average size of 11.1pm and 14% of particles with d, > 10 pm. On the other hand, the three zeolites fulfill the whiteness specification. Finally, to end the study carried out, an economic evaluation of the process proposed in this investigation, similar to that carried out in a previous work for the synthesis of 13X zeolite using sodium silicate as additional silica source, has been performed for a production of 30 000 metric tons/year. Considering a period of amortization of 10 years and a benefit of 15% of the total investment of 2060 X lo6 pesetas ($20 X 109, the resulting price for the zeolite obtained was in the same order as that resulting using sodium silicate as a silica source (about $0.5/kg of zeolite). Also, this price is comparable with other detergent builders. Taking into account all the considerations mentioned above, it can be affirmed that it is possible to use the three additional silica sources assayed to obtain 13X zeolite for use in detergents. However, it is difficult to decide which of these three possibilities is the most adequate. The lower crystallinity and exchange capacity lead us to think that the zeolite synthesized from bentonite may be discarded, but the abundance and the low price of this raw material, clearly cheaper than the other two silica sources used and the possible addition of higher amounts of charges (zeolite) to compensate for its worst properties, does not justify this decision. The higher percentage of particles with d, > 10 pm achieved using active silica makes this material less desirable for the zeolite synthesis; however, its price is lower than that of the sodium silicate. Consequently, in order to select the additional silica source that depending on the market conditions leads to the cheapest synthesis process, an economic study will be necessary. In any case it is important to remark that the change of additional silica source does not imply important modifications to the equipment used and in the operational conditions established.
Acknowledgment We acknowledge “Industrias Quhicas del Ebro” (Fundaci6n Universidad-Empresa, FUE 1112/90) and the “Comisi6n Internacional de Ciencia y Tecnologfa” (CICYT PA 850047) for financial support. Nomenclature CI:change capacity (mmol/g) d,: particle size (pm) Fp: fraction of particles with a d, size (%) I: amount of seeding (% ) L: Hunter index (%)
ND: gel formation stirring speed (rpm) NR: crystallization stirring speed (rpm) tD: gel formation time (h) TD:gel formation temperature (“0 t ~ aging : time (h) TE:aging temperature (“C) tQ: calcination time (h) TQ: calcination temperature (“C) TR:crystallization temperature (“C) t ~ crystallization : time (h) tr: zeolite traces XA: amorphous grade (%) x,: crystalline zeolite phase (% )
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Received for review November 2, 1992 Revised manuscript received April 5, 1993 Accepted April 14,1993
