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Amorphous magnesium silicate — synthesis, physicochemical properties and surface morphology
Advanced Powder Technol., Vol. 15, No. 5, pp. 549 – 565 (2004) © VSP and Society of Powder Technology, Japan 2004. Also available online - www.vsppub.com
Original paper Amorphous magnesium silicate — synthesis, physicochemical properties and surface morphology ANDRZEJ KRYSZTAFKIEWICZ, LIDIA KAROLINA LIPSKA, FILIP CIESIELCZYK and TEOFIL JESIONOWSKI ∗ Institute of Chemical Technology and Engineering, Poznan University of Technology, Sq. M. Skłodowskiej-Curie 2, 60-965 Poznan, Poland Received 14 October 2003; accepted 2 December 2003 Abstract—Studies were undertaken on the precipitation of highly dispersed magnesium silicate. Solutions of sodium metasilicate and of magnesium salts [chloride, sulfate(VI) or nitrate(V)] were employed. The chemical reaction of silicate precipitation was corrected by supplementation of the system with diluted (5–15 wt%) solutions of sodium hydroxide. Optimum parameters of the precipitation process were established. The precipitated magnesium silicates were comprehensively examined. Their chemical composition and principal physicochemical properties were established, including bulk density, capacities to absorb water, dibutyl phthalate and paraffin oil as well as the sedimentation rate in linseed oil. Moreover, their morphology, microstructure and particle size distribution were studied, using scanning electron microscopy and dynamic light scattering technique. The obtained products manifested variable physicochemical properties. The presence of sodium hydroxide solution in the course of precipitation proved to significantly affect the quality of the obtained magnesium silicates. Keywords: Magnesium silicate; precipitation; scanning electron microscopy; particle size distribution.
1. INTRODUCTION
Silicate raw materials are of enormous technological importance. They are regarded to represent the most important mineral raw materials in the ceramic industry as well as, directly or indirectly, in other industries, like in the iron and steel, electrochemical, chemical, metallurgical, construction, radiochemical, and electronic industries. In the production of highly dispersed synthetic silicates a particularly important role is played by the technological conditions of the process. The conditions affect the physicochemical properties of the obtained products and, in addition, provide a chance for improvement of their properties due to surface modification [1– 3]. ∗ To
whom correspondence should be addressed. E-mail: Teofi[email protected]
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The interesting trend in the production of highly dispersed silicate involves precipitation of sediments, particularly in the reaction of sodium metasilicate solution with solutions of selected salts [4– 8]. Magnesium silicate may be used as a filler and pigment in dispersive paints, as an adsorbent in affinity chromatography, and as a component of anti-epileptic drugs. It could be used in the treatment of alimentary intoxication, indigestion, inflammatory conditions of the small intestine, gastric peracidity and peptic ulcers. Magnesium silicate is also used in the production of confectionery as an antiadhesive and anti-caking agent (molding powder or a component of anti-glitter paste). As far as whiteness is concerned, its white color may easily compete with titanate-based pigments, which permits us to eliminate partially or totally titanium dioxide. Magnesium silicate formed on the basis of sodium metasilicate and magnesium sulfate(VI) with the addition of the correcting substance, sodium hydroxide, manifests ideal values of individual physicochemical parameters, including low bulk density, low capacity to absorb water or dibutyl phthalate and high capacity to absorb paraffin oil [9]. Magnesium silicate exhibits strong sedimentation interactions in an organic medium such as linseed oil. The interactions increase with increasing amounts of applied modifying compounds, which the improve physicochemical properties of the product. The surface of magnesium silicate carries free hydroxyl groups (silanol groups), the most reactive groups on the surface. They provide the site for physical adsorption of organic particles and easily react chemically with multiple substituents. Being substituted with new atom groups, they provide potential for surface modification. The surface of magnesium silicate may be presented as illustrated by Fig. 1 [10– 12].
Figure 1. Magnesium silicate surface.
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2. EXPERIMENTAL
2.1. Materials For the precipitation of magnesium silicate, aqueous solutions of magnesium sulfate(VI), magnesium nitrate(V), magnesium chloride (POCH, Poland) and of sodium metasilicate (VITROSILICON, Poland) were used. On the other hand, modification of the magnesium silicate precipitation process took advantage of a correcting substance, i.e. a solution of sodium hydroxide (POCH, Poland). 2.2. Procedures and methods The precipitation was conducted in a reacting vessel of 0.5 dm3 capacity, which before the start of precipitation contained 50 cm3 water. The reactor was equipped with a top stirrer which secured intense mixing of the system (2000 r.p.m.). The ractor was placed in a thermostat for continuous temperature control. The precipitating agent, sodium metasilicate solution, as well as an appropriate magnesium salt [magnesium sulfate(VI), magnesium nitrate(V) or magnesium chloride] were fed to the reactor using a peristaltic pump. Application of the pump permitted dosing of the precipitating agent at a constant (desired) flow rate. Dosing of the precipitating agent was continued until complete precipitation of magnesium silicate from the reaction solution took place. Termination of the precipitation process was noted due to control of pH of the reactive mixture: at the end of the reaction the pH value approached 10. The reaction yielded a white sediment of magnesium silicate, which was separated from the reactive mixture by filtration under suction. The sediment was washed with a defined amount of water to wash off remaining salts [sodium sulfate(VI), nitrate(V) or chloride]. The sample obtained in this way was subsequently dried in a stationary dryer at 105 ◦ C for 48 h. Studies were also performed on the precipitation of magnesium silicate using a correcting substance. The studies aimed at determining the optimum quantity of the correcting substance in the course of precipitation of highly dispersed magnesium silicate. The course of the precipitation process with application of the correcting substance was the same as described above. The scheme of precipitating highly dispersed magnesium silicate in the presence of a correcting substance is presented in Fig. 2. The correcting substance was introduced to the reactor together with sodium metasilicate solution in the course of the precipitation process. Various amounts of NaOH were used in order to determine the effects of the correcting substance on the precipitation process and on the quality of the obtained product. Precipitation of magnesium silicate samples was conducted under the following conditions:
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Figure 2. Schematic of the precipitated magnesium silicate.
• • • • • • • •
Temperature: 80 ◦ C. Silicate modulus of the applied sodium metasilicate solution: 3.3. Concentration of SiO2 in the sodium metasilicate solution: 5 wt%. Direction of precipitation: dosing of sodium metasilicate solution and of magnesium sulfate(VI) solution, magnesium nitrate(V) solution or magnesium chloride solution to the water-filled reactor. Rate of dosing the precipitating agent: 3.2 cm3 /min. Intensity of mixing: 2000 r.p.m. Final pH of the mixture: 10. Supplementation with correcting substances: 15 cm3 of 5, 10 or 15 wt% solution of NaOH (NaOH solution was administered together with sodium metasilicate solution).
Precipitation of magnesium silicate was conducted using three techniques. The first technique involved addition of the precipitating agent (sodium metasilicate solution) to a 5 wt% solution of a magnesium salt. Otherwise, a 5 wt% solution of a magnesium salt was dosed to a solution of sodium metasilicate. In both instances a gel-like sediment formed which was difficult to filter off. In the third technique sodium metasilicate solution and the appropriate magnesium salt solution were added in parallel to the 50 cm3 of water which filled the reactor. Precipitated magnesium silicates have to exhaust certain requirements which determine their future applications.
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Principal physicochemical parameters of the obtained magnesium silicates were established (bulk density, capacity to absorb water, dibutyl phthalate and paraffin oil) [13]. Moreover, the morphology and microstructure of the silicates were determined as well as sedimentation in linseed oil and pH of the 5 wt% water dispersion. In order to define the surface morphology of the obtained magnesium silicate, selected samples were examined by scanning electron microscopy (SEM). This permitted us to document a three-dimensional pattern of replicas of the solid rough surfaces, like fracture surface, surface structure and silicate agglomerates. In order to avoid accumulation of electric charge, the silicate preparations were obtained in tertiary butyl alcohol. For studies on morphology and microstructure, a Philips SEM 515 was used. Particle size distribution of selected samples of magnesium silicate was documented using ZetaPlus (Brookhaven Instruments, USA) with the dynamic light scattering technique. Selected samples of magnesium silicate were subjected to sedimentation studies: their behavior in organic medium (linseed oil) was examined. The experiments were performed in a graduated cylinder of 10 cm3 capacity. The sample of studied pigment (1.0 g) was appropriately mixed with linseed oil, and the volume occupied by the suspension of magnesium silicate particles was measured in the cylinder after 1, 2, 6, 9 and 12 days. Selected silicate samples were also subjected to chemical analysis.
3. RESULTS AND DISCUSSION
Using the first technique of magnesium silicate precipitation, a product of a high bulk density was obtained (sample 1 corresponds to 215 g/dm3 ). The capacity to absorb water amounted to 250 cm3 /100 g and the capacities to absorb dibutyl phthalate and paraffin oil were 350 cm3 /100 g and 500 cm3 /100 g, respectively. A very good sample (sample 3) was obtained, manifesting augmented physicochemical parameters, by parallel introduction of 15 cm3 10 wt% NaOH solution per 70 cm3 of the magnesium sulfate(VI) solution and the solution of sodium metasilicate in the course of the precipitation process. The obtained magnesium silicate demonstrated a capacity to absorb water of 250 cm3 /100 g, a capacity to absorb dibutyl phthalate of 350 cm3 /100 g, a capacity to absorb paraffin oil of 700 cm3 /100 g, while its bulk density was 163 g/dm3 . Sample 5 was obtained by precipitation of magnesium silicate from solutions of sodium metasilicate and magnesium nitrate(V). Also this sample demonstrated advantageous physicochemical parameters. Its capacity to absorb water was as high as 500 cm3 /100 g. On the other hand, the capacity to absorb dibutyl phthalate was 400 cm3 /100 g and the capacity to absorb paraffin oil was 700 cm3 /100 g. Its bulk density was low and was to 161 g/dm3 .
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Sample 6 was obtained by precipitation of magnesium silicate from solutions of sodium metasilicate and magnesium chloride. The sample demonstrated a slightly higher bulk density (181 g/dm3 ), and moderate capacities to absorb paraffin oil and dibutyl phthalate (550 cm3 /100 g and 300 cm3 /100 g, respectively). Physicochemical parameters of the magnesium silicates are given in Table 1. Table 1. Principal physicochemical properties of magnesium silicate obtained by precipitation Sample Substrates
Capacity to Capacity to absorb water absorb (cm3 /100 g) dibutyl phthalate (cm3 /100 g)
Capacity to Bulk NaOH absorb density solution paraffin oil (g/dm3 ) concentration (cm3 /100 g) (wt%)
Amount of NaOH additive (cm3 )
1
250
350
500
215
—
—
400
350
600
198
5
15
250
350
700
163
10
15
200
250
600
178
15
15
500
400
700
161
—
—
300
300
550
181
—
—
200
200
600
211
—
—
2
3
4
5
6
7
magnesium sulfate(VI), sodium metasilicate magnesium sulfate(VI), sodium metasilicate, sodium hydroxide magnesium sulfate(VI), sodium metasilicate, sodium hydroxide magnesium sulfate(VI), sodium metasilicate, sodium hydroxide magnesium nitrate(V), sodium metasilicate magnesium chloride, sodium metasilicate magnesium chloride, sodium metasilicate
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Determination of magnesium oxide and silica contents in selected samples of magnesium silicate permitted us to examine the effect of the various precipitation techniques on physicochemical parameters of magnesium silicate. Sample 1 manifested a low content of magnesium oxide. Sample 3 was obtained in the presence of the correcting substance, sodium hydroxide, which significantly augmented the content of magnesium oxide in the sample (MgO 36%). However, the supplementation decreased the content of silica in the sample. Sample 5 was obtained using magnesium nitrate as a substrate and sample 6 was obtained using magnesium chloride. In the latter two samples, the magnesium oxide content approached that noted in sample 3. Samples 5 and 6 contained higher amounts of silica. The results of chemical analysis are presented in Fig. 3. Studies on sedimentation of magnesium silicate in linseed oil permitted to test the suitability of the obtained magnesium silicate as a carrier and active filler or as a white pigment in dispersion and silicate paints. The studies were aimed at demonstrating differences in sedimentation behavior between magnesium silicate samples obtained in the presence of correcting substances. Therefore, the results of sedimentation of various magnesium silicate samples, presented in Fig. 4, were
Figure 3. Comparison of magnesium oxide and silica contens in selected magnesium silicate samples.
Figure 4. Comparison of sedimentation in linseed oil for selected magnesium silicate samples.
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due to the physicochemical properties of the examined samples, e.g. of their bulk density, dispersion extent and the correcting effect exerted by NaOH on the precipitation process. As indicated by the studies, sample 6 of magnesium silicate showed the lowest sedimentation rate in linseed oil: after 2 days the dispersion amounted to 5.2 cm3 /10 cm3 of the cylinder volume, after 12 days it amounted to 4.7 cm3 . A similar effect could be obtained for samples 2, 5 and 7, only. Sample 2 was obtained in the presence of 15% NaOH solution, used as a correcting substance in the course of the precipitation process. A sedimentation volume of the sample was 4.6 cm3 after 2 days and 4.3 cm3 after 12 days. The poor sedimentation rate of the sample reflected first of all the low supplementation with the correcting substance (5 wt% NaOH). The sample demonstrated high water absorbing capacity (400 cm3 /100 g) and moderate bulk density (198 g/dm3 ). Sample 5, obtained using magnesium nitrate (in the absence of correcting substances) manifested a very high capacity to absorb water (700 cm3 /100 g) and a very low bulk density (161 g/dm3 ). Sedimentation volumes for the sample were 4.6 cm3 after 2 days and 4.5 cm3 after 12 days. On the other hand, sample 7 showed a very low capacity to absorb water (200 cm3 /100 g) and a high bulk density (211 g/dm3 ). The sedimentation volume of the sample was 4.4 cm3 after 2 days and 4.3 cm3 after 12 days. Greater propensity to sediment was manifested by samples 1, 3 and 4. Sample 1 showed a low capacity to absorb water (250 cm3 /100 g) and a low capacity to absorb paraffin oil (500 cm3 /100 g). The sedimentation volume of the sample was 4.0 cm3 after 2 days and 3.8 cm3 after 12 days. Sample 3 showed ideal values of individual absorbing capacities and of bulk density. The sedimentation volume of the sample was 4.4 cm3 after 2 days and 3.4 cm3 after 12 days. A very high propensity to sediment was shown by sample 4, which exhibited very low capacities to absorb water and dibutyl phthalate, a very high capacity to absorb paraffin oil, and a very high bulk density (Table 1). The sedimentation volume of the sample was 3.2 cm3 after 2 days and 2.6 cm3 after 12 days. Therefore, the type of interaction of the sample in the organic medium was evident and the sedimentation rate was the highest. Studies on sedimentation in linseed oil confirmed that the interaction of magnesium silicate particles with an additionally modified outer surface increased in an organic-type medium in parallel with increasing amounts of the applied modifying substances, which improved the physicochemical properties of the final product. The surface of magnesium silicate obtained using magnesium sulfate(VI) and sodium metasilicate, which in parallel were fed to 50 cm3 water in the reactor (sample 1), is presented in Fig. 5a. The silicate showed the presence of large primary agglomerates and demonstrated a tendency to form numerous secondary agglomerates. Also, numerous primary particles of low diameters could be observed. Primary particles featured a smooth surface and no sharp edges could be noted. On the other hand, primary agglomerates of low diameter exhibited a tendency to acquire spherical shapes.
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(a)
(b) Figure 5. SEM (a) and multimodal particle size distribution (b) of magnesium silicate (sample 1).
The particle size distribution for magnesium silicate precipitated from solutions of magnesium sulfate(VI) and sodium metasilicate (sample 1) is presented in Fig. 5b. In the distribution, as many as four bands of various intensity could be noted. Two bands of the highest intensity corresponded to primary agglomerates. The first band ranged between 128 and 238 nm (maximum intensity of 40 corresponded to agglomerates of 204.1 nm in diameter), while the other band occupied the range of 279–607 nm in diameter (maximum intensity of 100 corresponded to primary agglomerates of 444.5 nm in diameter). The numerous bands pointed to low homogeneity of the precipitated magnesium silicate. The silicate structure contained also secondary agglomerates. They manifested within two ranges of diameters: 1544– 2108 nm (maximum intensity of 21 corresponded to agglomerates of 1804.3 nm in
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(a)
(b) Figure 6. SEM (a) and multimodal particle size distribution (b) of magnesium silicate (sample 2).
diameter) and 2878–3930 nm (maximum intensity of 36 corresponded to agglomerates of 3363.1 nm in diameter). A few secondary agglomerates were also present, which manifested high diameters, between 7325 and 8558 nm (intensity of the band was as low as 2). The surfaces of magnesium silicates which are presented in Figs 6a–8a were precipitated using solutions of magnesium sulfate(VI) and sodium metasilicate with supplementation of 15 cm3 5, 10 or 15 wt% sodium hydroxide, added in parallel to 50 cm3 water. Samples 2 and 3 demonstrated a slightly higher particle size than particles of sample 4, and a greater tendency to form numerous primary and secondary agglomerates (Figs 6a and 7a). The primary particles showed a rough surface and sharp edges. The sample 4 (Fig. 8a) demonstrated small particles and
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(a)
(b) Figure 7. SEM (a) and multimodal particle size distribution (b) of magnesium silicate (sample 3).
formed also primary particles, but of a markedly lower size. The surface pattern of the sample clearly proved that surface modification of the magnesium silicate using the correcting substance (15 wt% NaOH) in the course of the precipitation process exerted an advantageous effect on the development of the outer surface. The primary particles were characterized by a rough surface and rugged edges. Augmentation in the amounts of the correcting substance was noted to exert an advantageous effect on magnesium silicate surface, in particular on its morphology and microstructure, and it lowered the tendency to form agglomerates. Lower concentrations of the correcting substance (5 or 10 wt% solution of NaOH) failed to induce such advantageous effects on the magnesium silicate surface.
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(a)
(b) Figure 8. SEM (a) and multimodal particle size distribution (b) of magnesium silicate (sample 4).
The surface of magnesium silicate obtained with the use of magnesium nitrate(V) and sodium metasilicate solutions added in parallel to 50 cm3 water (sample 5) is presented in Fig. 9a. The sample contained numerous primary particles which exhibited, however, a much more variable size. Primary particles of average or small diameters prevailed. Similar properties were manifested by samples 6 and 7, presented in Figs 10a and 11a. In the photographs, primary particles manifested a rough surface and variable (sharp or rugged) edges. In the case of samples 5–7, a significant tendency could be noted to formation of spherical primary particles and of primary agglomerates of low diameter.
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(a)
(b) Figure 9. SEM (a) and multimodal particle size distribution (b) of magnesium silicate (sample 5).
Microscopic studies confirmed that addition of the correcting substance in the form of 15 wt% sodium hydroxide solution was sufficient to correct the morphology and outer surface of magnesium silicate surface. Silicate samples precipitated from solutions of magnesium sulfate(VI) and sodium metasilicate in the presence of supplements in the form of various amounts of NaOH exhibited a much more uniform character. The silicate precipitated in the presence of 5 wt% NaOH solution (sample 2) exhibited two bands of different intensity in the particle size distribution (Fig. 6b). The band which reflected the presence of primary agglomerates (aggregates) spanned the range of 209–293 nm (maximum intensity of 100 corresponded to agglomerates of 256.1 nm in diameter). On the other hand, secondary agglomerates formed a band of lower intensity, present in the range of
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(a)
(b) Figure 10. SEM (a) and multimodal particle size distribution (b) of magnesium silicate (sample 6).
926–1299 nm (maximum intensity of 81 corresponded to secondary agglomerates of 1060.5 nm in diameter). The size distribution of magnesium silicate agglomerates precipitated in presence of 10 wt% solution of NaOH was quite similar to that described above. The distribution manifested the presence of two bands (Fig. 7b). The more intense band could be ascribed to the presence of primary agglomerates (aggregates) which ranged in diameter from 174 to 333 nm (maximum intensity of 100 corresponded to primary agglomerates of 241.2 nm in diameter). On the other hand, the band of definitely lower intensity could be ascribed to secondary agglomerates and spanned the range of 1036–1980 nm (maximum intensity of 61 corresponded to secondary agglomerates of 1432.1 nm in diameter).
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(a)
(b) Figure 11. SEM (a) and multimodal particle size distribution (b) of magnesium silicate (sample 7).
Magnesium silicate precipitated in the presence of 15 wt% NaOH solution (sample 4) demonstrated a particularly homogenous character (Fig. 8b). The particle size distribution documented a very intense band in the range of 205– 367 nm (maximum intensity of 100 corresponded to the aggregate diameter of 258.6 nm). Secondary agglomerates formed a band of low intensity in the range of 1053–1681 nm (maximum intensity of 24 corresponded to the secondary agglomerate diameter of 1330.1 nm). The particle size distribution documented also
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the presence of primary particles in the range of 80–100 nm (intensity of the band was around 5). Magnesium silicate precipitated from solutions of magnesium nitrate(V) and sodium metasilicate exhibited a slightly less uniform character (Fig. 9b). The agglomerate size distribution documented the presence of two bands of various intensity. The intense band in the range of 185–235 nm reflected the presence of primary agglomerates (maximum intensity of 100 corresponded to the agglomerate diameter of 208.4 nm). The band of lower intensity also represented primary agglomerates and was positioned in the range of 623–845 nm (maximum intensity of 68 corresponded to the agglomerate diameter of 704.0 nm). Thus, the silicate contained small primary agglomerates of around 700–800 nm in diameter. Silicates precipitated from solutions of magnesium chloride and sodium metasilicate (Figs 10b and 11b) demonstrated a much less uniform character. The agglomerate size distribution (sample 6, Fig. 10b) manifested three bands of a variable intensity. Two of the bands could be ascribed to primary agglomerates and they showed a similar intensity. The first band in the range of 124–265 nm was more intense (maximum intensity of 100 corresponded to the primary agglomerate diameter of 195,7 nm). The next band, which also reflected the presence of primary agglomerates, fitted the range of 485–889 nm. Secondary agglomerates formed a band of a very low intensity in the range of 6352–10 000 nm (maximum intensity of 15 corresponded to agglomerates of 8596.0 nm in diameter). The size distribution of magnesium silicate agglomerates (sample 7, Fig. 11b) contained two bands. The intense band in the range of 225–464 nm reflected presence of primary agglomerates (aggregates). The maximum intensity of 100 corresponded to the agglomerates of 300.1 nm in diameter. On the other hand, the band reflecting the presence of secondary agglomerates spanned the range of 1482– 3062 nm (maximum intensity of 44 corresponded to the agglomerate diameter of 1981.0 nm).
4. CONCLUSIONS
Application of a correcting substance, sodium hydroxide solution, permitted us to augment the content of magnesium oxide in the obtained products. In the unmodified sample, the MgO content was 20%, while in the modified sample it was 36 %. A new technique was proposed out of precipitating highly dispersed magnesium silicate in the presence of 5–15 wt% NaOH solutions as correcting substances, added in the course of the precipitation process. The supplementation permitted us to obtain products with very good physicochemical properties. SEM studies on morphology and microstructure permitted us to demonstrate the development of the outer surface of magnesium silicate particles. Sample 4 manifested a very low size of particles due to application of the correcting substance (15 wt% NaOH solution). The decrease in correcting substance concentration in the
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course of the precipitation unfavorably affected the morphology of the magnesium silicate surface. Testing of particle size distribution for selected silicates allowed us to demonstrate a significantly higher homogeneity of samples obtained using the correcting substance (5–15 wt% NaOH solution) as compared to unmodified samples. Sample 4 demonstrated the presence of a very intense band in the range of 205–327 nm (maximum intensity of 100 corresponded to primary agglomerates of 258.6 nm in diameter). The secondary agglomerates were characterized by a low intensity band in the range of 1053–1681 nm (maximum intensity of 24 corresponded to the secondary agglomerate diameter of 1330.1 nm). Primary particles were also present in the range of 80–100 nm. Acknowledgements This work was supported by Poznan University of Technology research grant DS no. 32/115/2003.
REFERENCES 1. P. van der Voort and E. F. Vansant, Silylation of the silica surface a reviev, J. Liq. Chromatogr. Rel. Technol. 19, 2723–2752 (1996). 2. G. Vigl, Z. Xu, S. Steinberg and J. Israelachvili, Interactions of silica surfaces, J. Colloid Interf. Sci. 165, 367–385 (1994). 3. M. W. Daniels and L. F. Francis, Silane adsorption behaviour, microstructure and properties of glycidoxypropyltrimethoxysilane-modified colloidal silica coatings, J. Colloid Interf. Sci. 205, 191–200 (1998). 4. R. Werner, A. Krysztafkiewicz and T. Jesionowski, Efect of silane coupling agents on properties of sodium–aluminium silicate P-820, Pigment Resin Technol. 29, 277–287 (2000). 5. A. Krysztafkiewicz, R. Werner, L. K. Lipska and T. Jesionowski, Effect of silane coupling agents on properties of precipitated sodium–aluminium silicates, Colloids Surf. A 182, 65–81 (2001). 6. A. Krysztafkiewicz, Modified zinc silicate — an active rubber filler, J. Mater. Sci. 23, 2407–2414 (1988). 7. A. Krysztafkiewicz, Modified calcium silicate as active rubber filler, J. Mater. Sci. 22, 478–482 (1988). 8. H. Endriß, Inorganic Coloured Pigments Today. Curt R. Vincentz Verlag, Hannover (1998). 9. S. Suda, T. Tashiro and T. Umegaki, Synthesis of MgO–SiO2 and CaO–SiO2 amorphous powder by sol-gel process and ion exchange, J. Non-Cryst. Solids 255, 178–184 (1999). 10. R. F. G. Brown, Ch. Carr and M. E. Taylor, Effect of pigment volume concentration and latex particle size on pigment distribution, Progr. Org. Coat. 30, 185–194 (1997). 11. T. Kobayashi, Pigment dispersion in water-reducible paints, Progr. Org. Coat. 28, 79–87 (1996). 12. M. Wang, S. Deb and W. Bonfield, Chemically coupled hydroxyapatite–polyethylene composites: processing and characterisation, Mater. Lett. 44, 119–124 (2000). 13. T. Jesionowski and A. Krysztafkiewicz, Preparation of the hydrophilic/hydrophobic silica particles, Colloids Surf. A 207, 49–58 (2002).
Original paper Amorphous magnesium silicate — synthesis, physicochemical properties and surface morphology ANDRZEJ KRYSZTAFKIEWICZ, LIDIA KAROLINA LIPSKA, FILIP CIESIELCZYK and TEOFIL JESIONOWSKI ∗ Institute of Chemical Technology and Engineering, Poznan University of Technology, Sq. M. Skłodowskiej-Curie 2, 60-965 Poznan, Poland Received 14 October 2003; accepted 2 December 2003 Abstract—Studies were undertaken on the precipitation of highly dispersed magnesium silicate. Solutions of sodium metasilicate and of magnesium salts [chloride, sulfate(VI) or nitrate(V)] were employed. The chemical reaction of silicate precipitation was corrected by supplementation of the system with diluted (5–15 wt%) solutions of sodium hydroxide. Optimum parameters of the precipitation process were established. The precipitated magnesium silicates were comprehensively examined. Their chemical composition and principal physicochemical properties were established, including bulk density, capacities to absorb water, dibutyl phthalate and paraffin oil as well as the sedimentation rate in linseed oil. Moreover, their morphology, microstructure and particle size distribution were studied, using scanning electron microscopy and dynamic light scattering technique. The obtained products manifested variable physicochemical properties. The presence of sodium hydroxide solution in the course of precipitation proved to significantly affect the quality of the obtained magnesium silicates. Keywords: Magnesium silicate; precipitation; scanning electron microscopy; particle size distribution.
1. INTRODUCTION
Silicate raw materials are of enormous technological importance. They are regarded to represent the most important mineral raw materials in the ceramic industry as well as, directly or indirectly, in other industries, like in the iron and steel, electrochemical, chemical, metallurgical, construction, radiochemical, and electronic industries. In the production of highly dispersed synthetic silicates a particularly important role is played by the technological conditions of the process. The conditions affect the physicochemical properties of the obtained products and, in addition, provide a chance for improvement of their properties due to surface modification [1– 3]. ∗ To
whom correspondence should be addressed. E-mail: Teofi[email protected]
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A. Krysztafkiewicz et al.
The interesting trend in the production of highly dispersed silicate involves precipitation of sediments, particularly in the reaction of sodium metasilicate solution with solutions of selected salts [4– 8]. Magnesium silicate may be used as a filler and pigment in dispersive paints, as an adsorbent in affinity chromatography, and as a component of anti-epileptic drugs. It could be used in the treatment of alimentary intoxication, indigestion, inflammatory conditions of the small intestine, gastric peracidity and peptic ulcers. Magnesium silicate is also used in the production of confectionery as an antiadhesive and anti-caking agent (molding powder or a component of anti-glitter paste). As far as whiteness is concerned, its white color may easily compete with titanate-based pigments, which permits us to eliminate partially or totally titanium dioxide. Magnesium silicate formed on the basis of sodium metasilicate and magnesium sulfate(VI) with the addition of the correcting substance, sodium hydroxide, manifests ideal values of individual physicochemical parameters, including low bulk density, low capacity to absorb water or dibutyl phthalate and high capacity to absorb paraffin oil [9]. Magnesium silicate exhibits strong sedimentation interactions in an organic medium such as linseed oil. The interactions increase with increasing amounts of applied modifying compounds, which the improve physicochemical properties of the product. The surface of magnesium silicate carries free hydroxyl groups (silanol groups), the most reactive groups on the surface. They provide the site for physical adsorption of organic particles and easily react chemically with multiple substituents. Being substituted with new atom groups, they provide potential for surface modification. The surface of magnesium silicate may be presented as illustrated by Fig. 1 [10– 12].
Figure 1. Magnesium silicate surface.
Amorphous magnesium silicate
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2. EXPERIMENTAL
2.1. Materials For the precipitation of magnesium silicate, aqueous solutions of magnesium sulfate(VI), magnesium nitrate(V), magnesium chloride (POCH, Poland) and of sodium metasilicate (VITROSILICON, Poland) were used. On the other hand, modification of the magnesium silicate precipitation process took advantage of a correcting substance, i.e. a solution of sodium hydroxide (POCH, Poland). 2.2. Procedures and methods The precipitation was conducted in a reacting vessel of 0.5 dm3 capacity, which before the start of precipitation contained 50 cm3 water. The reactor was equipped with a top stirrer which secured intense mixing of the system (2000 r.p.m.). The ractor was placed in a thermostat for continuous temperature control. The precipitating agent, sodium metasilicate solution, as well as an appropriate magnesium salt [magnesium sulfate(VI), magnesium nitrate(V) or magnesium chloride] were fed to the reactor using a peristaltic pump. Application of the pump permitted dosing of the precipitating agent at a constant (desired) flow rate. Dosing of the precipitating agent was continued until complete precipitation of magnesium silicate from the reaction solution took place. Termination of the precipitation process was noted due to control of pH of the reactive mixture: at the end of the reaction the pH value approached 10. The reaction yielded a white sediment of magnesium silicate, which was separated from the reactive mixture by filtration under suction. The sediment was washed with a defined amount of water to wash off remaining salts [sodium sulfate(VI), nitrate(V) or chloride]. The sample obtained in this way was subsequently dried in a stationary dryer at 105 ◦ C for 48 h. Studies were also performed on the precipitation of magnesium silicate using a correcting substance. The studies aimed at determining the optimum quantity of the correcting substance in the course of precipitation of highly dispersed magnesium silicate. The course of the precipitation process with application of the correcting substance was the same as described above. The scheme of precipitating highly dispersed magnesium silicate in the presence of a correcting substance is presented in Fig. 2. The correcting substance was introduced to the reactor together with sodium metasilicate solution in the course of the precipitation process. Various amounts of NaOH were used in order to determine the effects of the correcting substance on the precipitation process and on the quality of the obtained product. Precipitation of magnesium silicate samples was conducted under the following conditions:
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Figure 2. Schematic of the precipitated magnesium silicate.
• • • • • • • •
Temperature: 80 ◦ C. Silicate modulus of the applied sodium metasilicate solution: 3.3. Concentration of SiO2 in the sodium metasilicate solution: 5 wt%. Direction of precipitation: dosing of sodium metasilicate solution and of magnesium sulfate(VI) solution, magnesium nitrate(V) solution or magnesium chloride solution to the water-filled reactor. Rate of dosing the precipitating agent: 3.2 cm3 /min. Intensity of mixing: 2000 r.p.m. Final pH of the mixture: 10. Supplementation with correcting substances: 15 cm3 of 5, 10 or 15 wt% solution of NaOH (NaOH solution was administered together with sodium metasilicate solution).
Precipitation of magnesium silicate was conducted using three techniques. The first technique involved addition of the precipitating agent (sodium metasilicate solution) to a 5 wt% solution of a magnesium salt. Otherwise, a 5 wt% solution of a magnesium salt was dosed to a solution of sodium metasilicate. In both instances a gel-like sediment formed which was difficult to filter off. In the third technique sodium metasilicate solution and the appropriate magnesium salt solution were added in parallel to the 50 cm3 of water which filled the reactor. Precipitated magnesium silicates have to exhaust certain requirements which determine their future applications.
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Principal physicochemical parameters of the obtained magnesium silicates were established (bulk density, capacity to absorb water, dibutyl phthalate and paraffin oil) [13]. Moreover, the morphology and microstructure of the silicates were determined as well as sedimentation in linseed oil and pH of the 5 wt% water dispersion. In order to define the surface morphology of the obtained magnesium silicate, selected samples were examined by scanning electron microscopy (SEM). This permitted us to document a three-dimensional pattern of replicas of the solid rough surfaces, like fracture surface, surface structure and silicate agglomerates. In order to avoid accumulation of electric charge, the silicate preparations were obtained in tertiary butyl alcohol. For studies on morphology and microstructure, a Philips SEM 515 was used. Particle size distribution of selected samples of magnesium silicate was documented using ZetaPlus (Brookhaven Instruments, USA) with the dynamic light scattering technique. Selected samples of magnesium silicate were subjected to sedimentation studies: their behavior in organic medium (linseed oil) was examined. The experiments were performed in a graduated cylinder of 10 cm3 capacity. The sample of studied pigment (1.0 g) was appropriately mixed with linseed oil, and the volume occupied by the suspension of magnesium silicate particles was measured in the cylinder after 1, 2, 6, 9 and 12 days. Selected silicate samples were also subjected to chemical analysis.
3. RESULTS AND DISCUSSION
Using the first technique of magnesium silicate precipitation, a product of a high bulk density was obtained (sample 1 corresponds to 215 g/dm3 ). The capacity to absorb water amounted to 250 cm3 /100 g and the capacities to absorb dibutyl phthalate and paraffin oil were 350 cm3 /100 g and 500 cm3 /100 g, respectively. A very good sample (sample 3) was obtained, manifesting augmented physicochemical parameters, by parallel introduction of 15 cm3 10 wt% NaOH solution per 70 cm3 of the magnesium sulfate(VI) solution and the solution of sodium metasilicate in the course of the precipitation process. The obtained magnesium silicate demonstrated a capacity to absorb water of 250 cm3 /100 g, a capacity to absorb dibutyl phthalate of 350 cm3 /100 g, a capacity to absorb paraffin oil of 700 cm3 /100 g, while its bulk density was 163 g/dm3 . Sample 5 was obtained by precipitation of magnesium silicate from solutions of sodium metasilicate and magnesium nitrate(V). Also this sample demonstrated advantageous physicochemical parameters. Its capacity to absorb water was as high as 500 cm3 /100 g. On the other hand, the capacity to absorb dibutyl phthalate was 400 cm3 /100 g and the capacity to absorb paraffin oil was 700 cm3 /100 g. Its bulk density was low and was to 161 g/dm3 .
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Sample 6 was obtained by precipitation of magnesium silicate from solutions of sodium metasilicate and magnesium chloride. The sample demonstrated a slightly higher bulk density (181 g/dm3 ), and moderate capacities to absorb paraffin oil and dibutyl phthalate (550 cm3 /100 g and 300 cm3 /100 g, respectively). Physicochemical parameters of the magnesium silicates are given in Table 1. Table 1. Principal physicochemical properties of magnesium silicate obtained by precipitation Sample Substrates
Capacity to Capacity to absorb water absorb (cm3 /100 g) dibutyl phthalate (cm3 /100 g)
Capacity to Bulk NaOH absorb density solution paraffin oil (g/dm3 ) concentration (cm3 /100 g) (wt%)
Amount of NaOH additive (cm3 )
1
250
350
500
215
—
—
400
350
600
198
5
15
250
350
700
163
10
15
200
250
600
178
15
15
500
400
700
161
—
—
300
300
550
181
—
—
200
200
600
211
—
—
2
3
4
5
6
7
magnesium sulfate(VI), sodium metasilicate magnesium sulfate(VI), sodium metasilicate, sodium hydroxide magnesium sulfate(VI), sodium metasilicate, sodium hydroxide magnesium sulfate(VI), sodium metasilicate, sodium hydroxide magnesium nitrate(V), sodium metasilicate magnesium chloride, sodium metasilicate magnesium chloride, sodium metasilicate
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Determination of magnesium oxide and silica contents in selected samples of magnesium silicate permitted us to examine the effect of the various precipitation techniques on physicochemical parameters of magnesium silicate. Sample 1 manifested a low content of magnesium oxide. Sample 3 was obtained in the presence of the correcting substance, sodium hydroxide, which significantly augmented the content of magnesium oxide in the sample (MgO 36%). However, the supplementation decreased the content of silica in the sample. Sample 5 was obtained using magnesium nitrate as a substrate and sample 6 was obtained using magnesium chloride. In the latter two samples, the magnesium oxide content approached that noted in sample 3. Samples 5 and 6 contained higher amounts of silica. The results of chemical analysis are presented in Fig. 3. Studies on sedimentation of magnesium silicate in linseed oil permitted to test the suitability of the obtained magnesium silicate as a carrier and active filler or as a white pigment in dispersion and silicate paints. The studies were aimed at demonstrating differences in sedimentation behavior between magnesium silicate samples obtained in the presence of correcting substances. Therefore, the results of sedimentation of various magnesium silicate samples, presented in Fig. 4, were
Figure 3. Comparison of magnesium oxide and silica contens in selected magnesium silicate samples.
Figure 4. Comparison of sedimentation in linseed oil for selected magnesium silicate samples.
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due to the physicochemical properties of the examined samples, e.g. of their bulk density, dispersion extent and the correcting effect exerted by NaOH on the precipitation process. As indicated by the studies, sample 6 of magnesium silicate showed the lowest sedimentation rate in linseed oil: after 2 days the dispersion amounted to 5.2 cm3 /10 cm3 of the cylinder volume, after 12 days it amounted to 4.7 cm3 . A similar effect could be obtained for samples 2, 5 and 7, only. Sample 2 was obtained in the presence of 15% NaOH solution, used as a correcting substance in the course of the precipitation process. A sedimentation volume of the sample was 4.6 cm3 after 2 days and 4.3 cm3 after 12 days. The poor sedimentation rate of the sample reflected first of all the low supplementation with the correcting substance (5 wt% NaOH). The sample demonstrated high water absorbing capacity (400 cm3 /100 g) and moderate bulk density (198 g/dm3 ). Sample 5, obtained using magnesium nitrate (in the absence of correcting substances) manifested a very high capacity to absorb water (700 cm3 /100 g) and a very low bulk density (161 g/dm3 ). Sedimentation volumes for the sample were 4.6 cm3 after 2 days and 4.5 cm3 after 12 days. On the other hand, sample 7 showed a very low capacity to absorb water (200 cm3 /100 g) and a high bulk density (211 g/dm3 ). The sedimentation volume of the sample was 4.4 cm3 after 2 days and 4.3 cm3 after 12 days. Greater propensity to sediment was manifested by samples 1, 3 and 4. Sample 1 showed a low capacity to absorb water (250 cm3 /100 g) and a low capacity to absorb paraffin oil (500 cm3 /100 g). The sedimentation volume of the sample was 4.0 cm3 after 2 days and 3.8 cm3 after 12 days. Sample 3 showed ideal values of individual absorbing capacities and of bulk density. The sedimentation volume of the sample was 4.4 cm3 after 2 days and 3.4 cm3 after 12 days. A very high propensity to sediment was shown by sample 4, which exhibited very low capacities to absorb water and dibutyl phthalate, a very high capacity to absorb paraffin oil, and a very high bulk density (Table 1). The sedimentation volume of the sample was 3.2 cm3 after 2 days and 2.6 cm3 after 12 days. Therefore, the type of interaction of the sample in the organic medium was evident and the sedimentation rate was the highest. Studies on sedimentation in linseed oil confirmed that the interaction of magnesium silicate particles with an additionally modified outer surface increased in an organic-type medium in parallel with increasing amounts of the applied modifying substances, which improved the physicochemical properties of the final product. The surface of magnesium silicate obtained using magnesium sulfate(VI) and sodium metasilicate, which in parallel were fed to 50 cm3 water in the reactor (sample 1), is presented in Fig. 5a. The silicate showed the presence of large primary agglomerates and demonstrated a tendency to form numerous secondary agglomerates. Also, numerous primary particles of low diameters could be observed. Primary particles featured a smooth surface and no sharp edges could be noted. On the other hand, primary agglomerates of low diameter exhibited a tendency to acquire spherical shapes.
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(a)
(b) Figure 5. SEM (a) and multimodal particle size distribution (b) of magnesium silicate (sample 1).
The particle size distribution for magnesium silicate precipitated from solutions of magnesium sulfate(VI) and sodium metasilicate (sample 1) is presented in Fig. 5b. In the distribution, as many as four bands of various intensity could be noted. Two bands of the highest intensity corresponded to primary agglomerates. The first band ranged between 128 and 238 nm (maximum intensity of 40 corresponded to agglomerates of 204.1 nm in diameter), while the other band occupied the range of 279–607 nm in diameter (maximum intensity of 100 corresponded to primary agglomerates of 444.5 nm in diameter). The numerous bands pointed to low homogeneity of the precipitated magnesium silicate. The silicate structure contained also secondary agglomerates. They manifested within two ranges of diameters: 1544– 2108 nm (maximum intensity of 21 corresponded to agglomerates of 1804.3 nm in
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(a)
(b) Figure 6. SEM (a) and multimodal particle size distribution (b) of magnesium silicate (sample 2).
diameter) and 2878–3930 nm (maximum intensity of 36 corresponded to agglomerates of 3363.1 nm in diameter). A few secondary agglomerates were also present, which manifested high diameters, between 7325 and 8558 nm (intensity of the band was as low as 2). The surfaces of magnesium silicates which are presented in Figs 6a–8a were precipitated using solutions of magnesium sulfate(VI) and sodium metasilicate with supplementation of 15 cm3 5, 10 or 15 wt% sodium hydroxide, added in parallel to 50 cm3 water. Samples 2 and 3 demonstrated a slightly higher particle size than particles of sample 4, and a greater tendency to form numerous primary and secondary agglomerates (Figs 6a and 7a). The primary particles showed a rough surface and sharp edges. The sample 4 (Fig. 8a) demonstrated small particles and
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(a)
(b) Figure 7. SEM (a) and multimodal particle size distribution (b) of magnesium silicate (sample 3).
formed also primary particles, but of a markedly lower size. The surface pattern of the sample clearly proved that surface modification of the magnesium silicate using the correcting substance (15 wt% NaOH) in the course of the precipitation process exerted an advantageous effect on the development of the outer surface. The primary particles were characterized by a rough surface and rugged edges. Augmentation in the amounts of the correcting substance was noted to exert an advantageous effect on magnesium silicate surface, in particular on its morphology and microstructure, and it lowered the tendency to form agglomerates. Lower concentrations of the correcting substance (5 or 10 wt% solution of NaOH) failed to induce such advantageous effects on the magnesium silicate surface.
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(a)
(b) Figure 8. SEM (a) and multimodal particle size distribution (b) of magnesium silicate (sample 4).
The surface of magnesium silicate obtained with the use of magnesium nitrate(V) and sodium metasilicate solutions added in parallel to 50 cm3 water (sample 5) is presented in Fig. 9a. The sample contained numerous primary particles which exhibited, however, a much more variable size. Primary particles of average or small diameters prevailed. Similar properties were manifested by samples 6 and 7, presented in Figs 10a and 11a. In the photographs, primary particles manifested a rough surface and variable (sharp or rugged) edges. In the case of samples 5–7, a significant tendency could be noted to formation of spherical primary particles and of primary agglomerates of low diameter.
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(a)
(b) Figure 9. SEM (a) and multimodal particle size distribution (b) of magnesium silicate (sample 5).
Microscopic studies confirmed that addition of the correcting substance in the form of 15 wt% sodium hydroxide solution was sufficient to correct the morphology and outer surface of magnesium silicate surface. Silicate samples precipitated from solutions of magnesium sulfate(VI) and sodium metasilicate in the presence of supplements in the form of various amounts of NaOH exhibited a much more uniform character. The silicate precipitated in the presence of 5 wt% NaOH solution (sample 2) exhibited two bands of different intensity in the particle size distribution (Fig. 6b). The band which reflected the presence of primary agglomerates (aggregates) spanned the range of 209–293 nm (maximum intensity of 100 corresponded to agglomerates of 256.1 nm in diameter). On the other hand, secondary agglomerates formed a band of lower intensity, present in the range of
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(a)
(b) Figure 10. SEM (a) and multimodal particle size distribution (b) of magnesium silicate (sample 6).
926–1299 nm (maximum intensity of 81 corresponded to secondary agglomerates of 1060.5 nm in diameter). The size distribution of magnesium silicate agglomerates precipitated in presence of 10 wt% solution of NaOH was quite similar to that described above. The distribution manifested the presence of two bands (Fig. 7b). The more intense band could be ascribed to the presence of primary agglomerates (aggregates) which ranged in diameter from 174 to 333 nm (maximum intensity of 100 corresponded to primary agglomerates of 241.2 nm in diameter). On the other hand, the band of definitely lower intensity could be ascribed to secondary agglomerates and spanned the range of 1036–1980 nm (maximum intensity of 61 corresponded to secondary agglomerates of 1432.1 nm in diameter).
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(a)
(b) Figure 11. SEM (a) and multimodal particle size distribution (b) of magnesium silicate (sample 7).
Magnesium silicate precipitated in the presence of 15 wt% NaOH solution (sample 4) demonstrated a particularly homogenous character (Fig. 8b). The particle size distribution documented a very intense band in the range of 205– 367 nm (maximum intensity of 100 corresponded to the aggregate diameter of 258.6 nm). Secondary agglomerates formed a band of low intensity in the range of 1053–1681 nm (maximum intensity of 24 corresponded to the secondary agglomerate diameter of 1330.1 nm). The particle size distribution documented also
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the presence of primary particles in the range of 80–100 nm (intensity of the band was around 5). Magnesium silicate precipitated from solutions of magnesium nitrate(V) and sodium metasilicate exhibited a slightly less uniform character (Fig. 9b). The agglomerate size distribution documented the presence of two bands of various intensity. The intense band in the range of 185–235 nm reflected the presence of primary agglomerates (maximum intensity of 100 corresponded to the agglomerate diameter of 208.4 nm). The band of lower intensity also represented primary agglomerates and was positioned in the range of 623–845 nm (maximum intensity of 68 corresponded to the agglomerate diameter of 704.0 nm). Thus, the silicate contained small primary agglomerates of around 700–800 nm in diameter. Silicates precipitated from solutions of magnesium chloride and sodium metasilicate (Figs 10b and 11b) demonstrated a much less uniform character. The agglomerate size distribution (sample 6, Fig. 10b) manifested three bands of a variable intensity. Two of the bands could be ascribed to primary agglomerates and they showed a similar intensity. The first band in the range of 124–265 nm was more intense (maximum intensity of 100 corresponded to the primary agglomerate diameter of 195,7 nm). The next band, which also reflected the presence of primary agglomerates, fitted the range of 485–889 nm. Secondary agglomerates formed a band of a very low intensity in the range of 6352–10 000 nm (maximum intensity of 15 corresponded to agglomerates of 8596.0 nm in diameter). The size distribution of magnesium silicate agglomerates (sample 7, Fig. 11b) contained two bands. The intense band in the range of 225–464 nm reflected presence of primary agglomerates (aggregates). The maximum intensity of 100 corresponded to the agglomerates of 300.1 nm in diameter. On the other hand, the band reflecting the presence of secondary agglomerates spanned the range of 1482– 3062 nm (maximum intensity of 44 corresponded to the agglomerate diameter of 1981.0 nm).
4. CONCLUSIONS
Application of a correcting substance, sodium hydroxide solution, permitted us to augment the content of magnesium oxide in the obtained products. In the unmodified sample, the MgO content was 20%, while in the modified sample it was 36 %. A new technique was proposed out of precipitating highly dispersed magnesium silicate in the presence of 5–15 wt% NaOH solutions as correcting substances, added in the course of the precipitation process. The supplementation permitted us to obtain products with very good physicochemical properties. SEM studies on morphology and microstructure permitted us to demonstrate the development of the outer surface of magnesium silicate particles. Sample 4 manifested a very low size of particles due to application of the correcting substance (15 wt% NaOH solution). The decrease in correcting substance concentration in the
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course of the precipitation unfavorably affected the morphology of the magnesium silicate surface. Testing of particle size distribution for selected silicates allowed us to demonstrate a significantly higher homogeneity of samples obtained using the correcting substance (5–15 wt% NaOH solution) as compared to unmodified samples. Sample 4 demonstrated the presence of a very intense band in the range of 205–327 nm (maximum intensity of 100 corresponded to primary agglomerates of 258.6 nm in diameter). The secondary agglomerates were characterized by a low intensity band in the range of 1053–1681 nm (maximum intensity of 24 corresponded to the secondary agglomerate diameter of 1330.1 nm). Primary particles were also present in the range of 80–100 nm. Acknowledgements This work was supported by Poznan University of Technology research grant DS no. 32/115/2003.
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