Synthesis of Crystalline Layered Sodium Silicate from Amorphous

Apr 1, 2000 - The synthesis of crystalline layered sodium silicates from amorphous sodium silicate has been investigated. Different synthesis process ...
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Ind. Eng. Chem. Res. 2000, 39, 1249-1255

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MATERIALS AND INTERFACES Synthesis of Crystalline Layered Sodium Silicate from Amorphous Silicate for Use in Detergents Antonio de Lucas, Lourdes Rodrı´guez, Paula Sa´ nchez,* and Justo Lobato Facultad de Ciencias Quı´micas, Departamento de Ingenierı´a Quı´mica, Universidad de Castilla-La Mancha, 13004 Ciudad Real, Spain

The synthesis of crystalline layered sodium silicates from amorphous sodium silicate has been investigated. Different synthesis process variables have been studied to synthesize products of a suitable quality for their use as builders in detergent formulations. Two different experimental processes have been used: a furnace (discontinuous process) and a pilot-plant rotary tubular oven (continuous process). The capacity to remove calcium and magnesium ions from water directly depends on the crystallinity and the δ phase content of the samples. The presence of about 20 wt % crystalline seeds and a reaction temperature of around 685 °C are needed to synthesize highly crystalline δ layered silicates. Differential thermal analysis measurements confirmed that amorphous silicate undergoes a crystallization process (exothermic step at 685 °C) which is attributed to the formation of the crystalline δ phase. The increase in the crystallization rate produced by agitation in the rotary tubular oven resulted in shorter reaction times than those for the furnace experiments. Scanning electron microscopic analysis was performed for samples obtained at different reaction times in order to establish a possible crystallization mechanism. Introduction Since the 1960s, the use of phosphates as builders in laundry detergents has been increasingly criticized, despite their excellent washing properties, because of their contribution to eutrophication of waterways.1 Existing legislation or voluntary agreements have led to a practical ban of phosphates for laundry detergents in several Western European countries, Japan, and North America.2 Builders in detergents have the ability to soften washing liquors by reducing Ca2+ and Mg2+ cation contents. This effect precludes the precipitation of surfactants as insoluble calcium and magnesium salts, which can be deposited on fabrics and on the heating elements of washing machines, thus shortening their lifetime. Synthetic zeolite 4A is routinely used as a phosphate substitute; however, its capacity to remove Mg2+ cations is quite low, especially at low temperatures.3 For this reason, other zeolites with a higher affinity for Mg2+, such as 13X, constitute an important complement to zeolite 4A.4,5 Disadvantages for zeolite use include insolubility and the required increase in surfactants and bleaching agents. More recent developments in builder manufacture involve multifunctional, highly ecological products, marketed as liquids or as compact detergents. In this context, crystalline layered sodium silicate has been recently used as a new multifunctional detergent builder. This new builder consists essentially of the δ phase of * To whom correspondence should be addressed. Tel.: +34 926 295 300. Fax: +34 926 295 318. E-mail: psanchez@ inqu-cr.uclm.es.

sodium disilicate Na2Si2O5, which has a polymeric layered bidimensional crystal structure, in addition to small R and β contents as impurities.6 The substancespecific properties of δ layered crystalline silicates are essentially based on the absence of structural water, the exchangeability of intermediate layer sodium, and its solubility. Because of these properties, layered silicates efficiently remove the water hardness, provide an adequate washing alkalinity, enhance the performance of surfactants and bleachers, have a corrosion-inhibiting action, and can be used in formulations of both liquid and highly compact detergents. Moreover, these silicates are inert from an ecological point of view and can be mixed with any other builder. The interesting properties of these materials have originated several patents7-9 from important detergent manufacturers. Just a few papers in the literature have focused on their properties and washing characteristics, with no extensive study about their synthesis. Synthesis of well-crystallized samples with a high δ phase purity (R and β crystalline phases have low Ca2+ and Mg2+ binding capacities) requires a very well-established production process. In this paper, the influence of different variables on the synthesis of crystalline layered sodium silicates has been investigated, by using both a discontinuous (furnace) and a continuous (rotary tubular oven) process. Scanning electron microscopy (SEM) and differential thermal analysis (DTA) from representative samples have been carried out and are also discussed.

10.1021/ie990724n CCC: $19.00 © 2000 American Chemical Society Published on Web 04/01/2000

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Experimental Section Synthesis. Two different processes performed the synthesis of layered crystalline silicates: under static conditions (discontinuous process) and in a pilot-plant rotary tubular oven (continuous process). Static synthesis experiments were carried out in an air atmosphere by using a temperature-controlled furnace. Amorphous silicate (10 g) was first placed in a 100 mL porcelain cup and then introduced at the selected temperature for different reaction times. The rotary tubular oven consists of a tubular alumina reactor with 0.072 m internal diameter and 1 m length, surrounded by an electric heating system. The experimental installation permits one to adjust a number of synthesis variables such as reaction temperature, amount of feed, tube speed rotation, and tube angle inclination. In all cases, an amorphous sodium silicate with a molar ratio SiO2/Na2O ) 2 and a particle size in the range 0.10.038 mm, supplied by I.Q.E. (Industrias Quı´micas del Ebro) was used as the raw material. Products obtained were milled to a particle size lower than 0.038 mm, before their characterization. Characterization. X-ray powder diffraction (XRD) patterns were recorded in air at room temperature using a Philips PW-1700 diffractometer with monochromatic Cu KR radiation and a nickel filter. The crystallinity for each sample was determined as the ratio between the sum of peak intensities at 2θ ) 22.4° (hkl ) 120) and 2θ ) 27° (hkl ) 140), which are representative of δ and R phases, respectively, and the sum of intensities for a standard commercial layered silicate, considered to be 100% crystalline. The δ phase content (percent weight) for the assynthesized samples was calculated by least-squares regression of peak intensities at 2θ ) 22.4° for samples prepared with different percentages of δ phase. This characterization parameter was not calculated for samples with crystallinities lower than 50% because it was affected by a high experimental error. Samples used for the calibration as pure δ and R phases (100 wt % δ phase and 0 wt % δ phase, respectively) were also crystalline commercial layered silicates. It has to be emphasized that, even for the pure δ phase, weak reflection characteristics of the R phase are detected, indicating the unfeasibility of synthesizing an absolutely pure δ phase. Calcium and magnesium binding capacities were determined by titration of solutions resulting from ion exchange with 0.01 N CaCl2 and MgCl2 aqueous solutions, at 20 °C for 15 min. DTA was performed using a TGA-92 SETARAM equipment with a heating rate of 10 °C min-1 from 20 to 1000 °C under a dry argon flow and with alumina as the reference. SEM micrographs were obtained with a Philips XL30-CPDX4i scanning electron microscope on Au metalled samples. Results and Discussion XRD analysis of all of the samples indicated that, under the synthesis conditions described, δ and R phases of sodium silicate were the only crystalline phases obtained. XRD patterns for the R phase and a typical synthesized sample containing mainly δ phase are given in Figure 1. The structural buildup of these two phases are shown in Figure 2, where different folding, more corrugated in the case of the R phase, can be seen.

Figure 1. XRD patterns characteristic of R and δ crystalline sodium silicate.

Figure 2. Layer structure of (a) δ-Na2Si2O5 and (b) R-Na2Si2O5. Tetrahedra represent SiO4 units and spheres represent exchangeable sodium cations.

Furnace Experiments. (a) Effect of the Seed Content. To study the influence of the seed content, different synthesis experiments were carried out at 650 °C for 60 min, using as the seed a standard crystalline δ phase milled to a particle size identical with that of the raw amorphous silicate (0.1-0.038 mm). Figure 3a shows the effect of the seed content on both crystallinity and the δ phase content of products. Almost no crystallization occurs when crystalline material is not present in the raw material, as can be seen for the sample obtained with no seed (0 wt % seed content). Crystallinity and the δ phase content increase with the seed content up to 20 wt %, above which both properties remain constant. The observed increase of these two parameters are quite low when considering that the initial addition of the highly crystalline δ phase seed causes by itself an increase on the crystallinity as well as on the δ phase content of samples. Therefore, the presence of crystals in the amorphous starting mixture,

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Figure 3. Furnace experiments. Effect of the seed content. (a) Crystallinity and the δ phase content. (b) Ca2+ and Mg2+ binding capacities.

acting as crystallization nuclei, is needed for a quick crystallization of layered crystalline silicates. The growth of crystals increases with the number of crystalline nuclei up to a limit at about 20 wt % seed. The effect of seed on the calcium and magnesium binding capacities of products (Figure 3b) was the same as that described for the crystallinity and the δ phase content. This result indicates a direct relation between the quality of silicates and their crystallinity and purity in the δ phase. In fact, neither the amorphous starting silicate nor the standard R silicate showed binding capacities above 20 mg of CaO/g of solid. The reason of this behavior is due to the layered structure characteristic of crystalline silicates. Layers, formed by tetrahedral SiO4 units, are bonded to one another via Na+ ions, which are exchangeable by other ions such as Ca2+ or Mg2+.6,10 Although both δ and R phases have a similar structure, tetrahedral layers are more corrugated for R modification11,12 (see Figure 2), which seems to be related to the difficult Na+ exchangeability observed. Moreover, 23Na MAS NMR studies show that two types of sodium atoms (surrounded by five and by six oxygen atoms respectively) exist in the interlayer space of δ silicate, while only sodium atoms surrounded by six oxygen atoms were detected in R crystalline silicate.13 These results give rise to the suggestion that sodium atoms surrounded by five oxygen atoms are easier to exchange than those surrounded by six atoms, thus explaining the high Ca2+ and Mg2+ binding capacity

Figure 4. Furnace experiments. Effect of the reaction temperature. (a) Crystallinity and the δ phase content. (b) Ca2+ and Mg2+ binding capacities.

exhibited by the δ phase. On the other hand, ion exchange is not feasible when layers are not formed, which would explain the poorly crystalline samples obtained for amorphous silicate. From all of the considerations above, a seed of 20 wt % was selected to study the influence of the rest of crystallization variables. (b) Effect of the Reaction Temperature. The influence of the reaction temperature was investigated in the range 400-750 °C for 1 h of reaction time by adding a 20 wt % crystalline δ phase as the seed. As shown in Figure 4a, crystallinities lower than 35% were observed at temperatures under 550 °C. Both crystallinity and the δ phase content increase with temperature from 550 to 685 °C. Samples obtained at temperatures higher than 685 °C exhibited a further increase in crystallinity, whereas their δ phase content decreased. These results indicate that temperatures in the range 650-685 °C promote the formation of the crystalline δ phase and temperatures over 700 °C favor the crystallization of the R phase. The highest Ca2+ and Mg2+ binding capacities (Figure 4b) were obtained for the sample synthesized at 685 °C, as a result of its highest values of both crystallinity and the δ phase content. Again, samples not well-crystallized (obtained at 700 °C) showed a lower capacity for removal of hardness from water. DTA measurements (Figure 5) demonstrated that amorphous silicate undergoes a crystallization process (exothermic step at 685 °C) which is attributed to the formation of the crystalline δ phase. A second endothermic step, explained by the melting of the silicate, is observed at 850 °C. The absence of an exothermic step at about 700 °C suggests that the R phase could be formed at the expense of the δ phase through a transformation of phases. In fact, DTA analysis of the crystalline δ phase (Figure 5) shows an endothermic step at 730 °C that could be due to the above-mentioned transformation. The DTA analyses are also in agreement with the influence of the reaction temperature described above. Thus, a temperature of 685 °C was selected as the most adequate for the synthesis of layered crystalline δ silicate. (c) Effect of the Reaction Time. A set of experiments was carried out at 685 °C and 20 wt % seed at different reaction times. Figure 6 shows the influence of time on both crystallinity and the δ phase content of products as well as on their calcium binding capacity. It can be seen that crystallinity and the δ phase content increase with reaction time, reaching an almost constant value from 100 min on both characterization parameters, being always under 90%. Long reaction times (>60 min) do not improve the quality of silicates; on the contrary samples synthesized after 250 min showed a slight decrease of crystallinity as well as the δ phase. As explained for samples obtained at high temperatures (>700 °C), a transformation from the δ phase to a more stable R phase seems to occur for long reaction times. Ca2+ binding capacity (see Figure 6b) exhibits the same trend as crystallinity and the δ phase content (see Figure 6a) along the reaction time studied, because of the close relation of these parameters. To study the effect of the reaction time on morphology and the crystallization mechanism of silicates, repre-

Figure 6. Furnace experiments. Effect of the reaction time. (a) Crystallinity and the δ phase content. (b) Ca2+ binding capacity.

sentative samples were analyzed by SEM. As seen in Figure 7a, starting amorphous silicate shows a nondefined appearance with the particle size varying from 5 to 30 µm. After 5 min of reaction (Figure 7b) the amorphous silicate evolves into spherical particles showing holes, possibly due to a sudden loss of water at such high temperatures. Moreover, the particle size increases (50-120 µm), indicating an aggregation process during the first stage of synthesis. As crystallization proceeds, samples show a different morphology evidenced by the emergence of very small layers in some particles (Figure 7c). Well-crystallized samples exhibited the above-mentioned morphology, characterized by particles formed by aggregates of layers (Figure 7d). These results suggest a crystallization mechanism based in solid-solid transformations consisting of growth of crystals added as seed, which act as crystallization nuclei, from the amorphous silicate. Noteworthy is the presence of a second type of crystal, with needle morphology, that mainly appears in samples obtained at long reaction times (Figure 7e), which can be attributed to the R phase formation. However, after 480 min of reaction time, such morphology is not observed, with a different one probably due to the decrease of crystallinity being apparent. Rotary Tubular Oven Experiments. Experiments varying the reaction temperature and the reaction time were carried out in a rotary tubular oven to better simulate industrial continuous processes. The accept-

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Figure 7. Furnace experiments. SEM micrographs of samples synthesized at different times: (a) starting silicate, (b) 5 min, (c) 30 min, (d) 60 min, (e) 120 min, (f) 480 min.

able quality of crystalline silicates obtained in the furnace experiments supported the study of continuous synthesis by using a 20 wt % crystalline seed. (a) Effect of the Reaction Temperature. Figure 8 shows the effect of the reaction temperature for

experiments carried out at temperatures from 630 to 710 °C for 5 min of reaction time. Products become more crystalline as the temperature increases, whereas an almost constant δ phase content was observed, with a slight decrease for the sample synthesized at 710 °C.

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Figure 8. Rotary tubular oven experiments. Effect of the reaction temperature. (a) Crystallinity and δ phase content. (b) Ca2+ and Mg2+ binding capacities.

Figure 9. Rotary tubular oven experiments. Effect of the reaction time. (a) Crystallinity and the δ phase content. (b) Ca2+ and Mg2+ binding capacities.

Binding capacities according to crystallinity and the δ phase content of the samples were obtained. These results are in agreement with those obtained in the discontinuous furnace. Important to note is the different reaction time needed to reach a similar quality of silicates, more than 10 times less in the tubular rotary oven. This fact demonstrates the importance of a good agitation in the crystallization process (increased number of crystallization nuclei by breaking of silicate particles14) and of a good contact between amorphous and crystalline solids, which leads to a decrease in the crystallization time. A temperature around 685 °C turned out to be the most appropriate for the synthesis of high crystalline layered δ silicate in the rotary tubular oven and then for an industrial synthesis. (b) Effect of the Reaction Time. Both the tube speed rotation and tube angle inclination of the rotary tubular oven were varied for experiments at different reaction times (3.5-8.5 min). The reaction temperature was maintained at 685 °C. As shown in Figure 9, crystallinity increases with the reaction time with a typical sigmoidal crystallization curve pattern. A reaction time of 7 min was sufficient to reach the maximum crystallinity. Additionally, the δ phase content was constant under these conditions. For these experiments the short reaction time or more likely a good agitation prevented the decrease in the δ phase content observed for the furnace experiments. As could be expected, the

Table 1. Comparison between Continuous and Discontinuous Processes (685 °C) binding capacities

process

reaction time (min)

crystallinity (%)

discontinuous continuous discontinuous continuous

90 8.5 5 5

83 95 40 66

δ phase (%)

mg of CaO/g of solid

mg of MgO/g of solid

88 83

125 116 41 80

137 133 69 111

84

sample obtained after 8.5 min, having the highest crystallinity and δ phase content, also exhibited the highest removal of hardness from water. In Table 1, the most relevant results showing the influence of the reaction time for both continuous and discontinuous processes are compared. It can be seen that shorter reaction time is required for the continuous process to obtain similar silicate quality. When samples synthesized at the same reaction time (5 min) are compared, crystallization of δ silicate takes place by using a continuous process whereas almost no crystal of the δ phase is detected for the discontinuous process. Finally, the high capacity to remove Mg2+ exhibited by the silicate is noticeable, as illustrated by comparing binding capacities of the best crystalline layered silicate synthesized in this work (125 mg of CaO/g of solid and 137 mg of MgO/g of solid) with those exhibited by a

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different builder as zeolite 4A (136 mg of CaO/g of solid and 27 mg of MgO/g of solid). Despite the higher price of silicate compared with zeolites, its multiple properties would permit one to reduce the amount of bleachers and surfactants in the formulation of detergents. Furthermore, it is interesting to point out that crystalline silicates are totally compatible with the use of any other builder. Likewise, these silicates can be used in formulations for liquid and highly compact ecological detergents. Conclusions Crystalline layered sodium silicates suitable for use in detergent formulations have been successfully synthesized from a commercial amorphous silicate by using both discontinuous (furnace) and continuous (rotary tubular oven) processes. The capacity to remove hardness from water turned out to be directly related to the crystallinity and the δ phase content of silicates, because of their characteristic layered structure. The initial presence of a 20 wt % crystalline product is needed for an adequate crystallization of samples. A temperature of around 685 °C, corresponding to an exothermic DTA step, favors crystallization of the δ phase; higher temperatures produce a transformation from the δ to the R phase in furnace experiments. High crystalline δ silicates were obtained after 60 min by using a furnace, while a reaction time of 7 min is enough to obtain even better silicates in the rotary tubular oven. The effect of agitation turned out to be decisive on the crystallization process. A crystallization mechanism based in solidsolid transformations consisting of a growth of crystals added as seed from the amorphous silicate seems to govern the process studied. Acknowledgment Financial support from “Industrias Quı´micas del Ebro” is gratefully acknowledged. Literature Cited (1) Leaf, S. S.; Chatterjee, R. Developing a Strategy on Eutrophication. Water Sci. Technol. 1999, 39 (12), 307.

(2) Fischer, W. K.; Gerike, P. Ullmann’s Encyclopedia of Industrial Chemistry; VCH: Weinheim, Germany, 1987; Vol. A8, Chapter 10. (3) Costa, E.; Lucas, A.; Uguina, M. A.; Ruiz, J. C. Synthesis of 4A Zeolite from Calcined Kaolins for Use in Detergents. Ind. Eng. Chem. Res. 1988, 27, 1291. (4) Lucas, A.; Uguina, M. A.; Covia´n, I.; Rodrı´guez, L. Synthesis of 13X Zeolite from Calcined Kaolins and Sodium Silicate for Use in Detergents. Ind. Eng. Chem. Res. 1992, 31, 2134. (5) Lucas, A.; Uguina, M. A.; Covia´n, I.; Rodrı´guez, L. Use of Spanish Natural Clays as Additional Silica Sources To Synthesize 13X Zeolite from Kaolin. Ind. Eng. Chem. Res. 1993, 32, 1645. (6) Rieck, H.-P. New Horizons. In An AOCS/CSMA Detergent Industry Conference; Coffey, R., Ed.; FMC Corp.: Princeton, NJ, 1995; Chapter 3. (7) Schirmer, W.; Hachgenei, J.; Gu¨nther, J.; Wichelhans, W. Phosphate-free Dishwashing Detergents Containing Sodium Disilicate. DE 4102743 A1, 1992. (8) Delwel, F.; Thennissen, J. P.; Osinga, T. J.; Vranchen, J. M. Manufacture of Alkali Metal Silicates. EP 0526978 A2, 1992. (9) Lee, J. M.; Suh, J. K.; Jeong, S. Y.; Jin, H. K.; Park, B. K.; Park, C. H.; Park, J. H.; Kim, J. H.; Lim, C. W. A Process for Preparing Crystalline Sodium Disilicate Having a Layered Structure. EP 0745559A1, 1996. (10) Wilkens, J. Structure-property relationships of sodium disilicates. Tenside, Surfactants, Deterg. 1995, 32 (6), 476. (11) Pant, A. K.; Cruickshank, D. W. The Crystal Structure of R-Na2Si2O5. Acta Crystallogr. 1967, B24, 13. (12) Pant, A. K. A Reconsideration of the Crystal Structure of β-Na2Si2O5. Acta Crystallogr. 1968, B24, 1077. (13) Heidemann, D.; Hu¨bert, C.; Schwieger, W.; Grabner, P.; Bergk, K.-H.; Sarv, P. 29Si and 23Na Solid State MAS NMR Investigations of Modifications of the Sodium Phyllosilicate Na2Si2O5. Z. Anorg. Allg. Chem. 1992, 169. (14) Schimmel, G.; Kotzian, M.; Gradl, R. Manufacture of Crystalline Layered Sodium Silicates. EP 0436835 A2, 1991.

Received for review September 30, 1999 Revised manuscript received February 11, 2000 Accepted February 14, 2000 IE990724N