Zeolite formation was investigated under low temperature conditions between 40°C - 25°C under insertion of a Si-rich filtration residue (FR) from waste water cleaning of the silane production. Syntheses were performed under open system conditions without stirring. The application of templates was excluded but gel formation was optimized by addition of citric acid. Beside a minimal consumption of process energy during synthesis under very low temperatures even FR was inserted without an expansive pre-treatment like calcination. Disordered intergrowths of FAU-EMT intermediates were obtained in each case, even at a temperature of only 25°C. The reaction products always occurred in two sequences, one at the bottom and another at the top of the reaction bin. A better crystallinity was detected for the latter products and the Si/Al ratio of these FAU-EMT members was close to 1.0 like in zeolite LSX. In contrast chemical analyses revealed an enrichment of Ca- and Mg- impurities from FR in the bottom products.
Investigations on the recycling of industrial rest materials are significant for reasons of the environment protection as well as the recovery of valuable ingredients. In this sense the objective of the present work is the experimental study of zeolite synthesis under insertion of a silica-rich filtration residue “FR” obtained from waste water reconditioning of the silane production. SiO2 rich wastes like slags, ashes, filtration residues and related materials are often suitable educts for zeolite synthesis, as demonstrated in a large number of former investigations [
In the present work, the synthesis of nanosized members of zeolites of the Faujasite family is reported, using a silica rich filtration residue (FR). The molecular sieve Faujasite finds extensive application as a catalyst for various industrially important processes such as cracking, alkylation, hydrocracking and reforming [
Unfortunately the growth of FAU remained improper during these former experiments as no pure phase product was obtained and temperatures not lower than 60˚C were necessary [
In the present paper we investigate possibilities of FAU zeolite formation by a modified superalkaline reaction system under insertion of the residue FR and citric acid to rule gel polymerization and to buffer the whole system in the very low temperature interval of 40˚C - 25˚C. Sodium hydroxide as mineralizator and sodium aluminate additive as Al-source were further inserted. Hydrothermal process guidance with sodium aluminate was already an object of experimental investigations on the recycling of autoclave aerated concrete to zeolite Na-A [
The Si-rich filtration residue (FR) was obtained from the waste water cleaning procedure of the silane production. The material (kindly provided from the Federal Institute for Materials and Testing, BAM-Berlin) was dried and finely grounded. The chemical analysis of FR was even performed at BAM [
An XRD investigation proved that FR is mainly X-ray-amorphous silica and contains only a very low portion of crystalline calcite. A SEM study of FR exhibited a fine-grained material from nano- to microparticles, and rare agglomerates of a size of max. 1 µm.
Beside FR the following chemicals were used for syntheses:
Sodium aluminate, Riedel-deHaën 13404; sodium hydroxyde, Merck 1.06467 and citric acid, Fluka 27490.
The experimental conditions of the syntheses are summarized in
Solution (1): 2 g FR + 30 ml H2O + 17.0 g NaOH were mixed by stirring for 45 min. at room temperature (RT);
Solution (2): 20 ml H2O + 4.2 g citric acid (CA), mixed by stirring for 15 min. at RT;
Solution (3): 50 ml H2O + 12.6 g NaOH + 2.3 g NaAlO2, mixed by stirring for 15 min. at RT.
The solutions were mixed in ascending order (1 + 2 + 3) in a beaker glass.
A total number of three of those complete mixtures were prepared (batch No. 1 - 3,
Chemical composition (Wt. %)* | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
SiO2 | Al2O3 | Fe2O3 | TiO2 | CaO | MgO | Na2O | K2O | SO3 | Cl− | Loss of ignition |
92.30 | 3.83 | 0.49 | 0.05 | 2.05 | 0.19 | 0.01 | 0.05 | 0.23 | 0.80 | 3.23 |
*standardized on 100%, without glowing loss.
7 | Temperature (˚C) | Time (h) | Products* | ||
---|---|---|---|---|---|
Qualitative phase analysis (XRD) | Si/Al-ratio** | Total amount (a + b) (g) | |||
1 | 40 | 48 | a) FAU-EMT and LTA | 1.2 | 1.30 |
b) FAU-(EMT) and LTA | 1.0 | ||||
2 | 30 | 72 | a) FAU-EMT and (LTA) | 1.2 | 1.20 |
b) FAU-EMT and (LTA) | 1.1 | ||||
3 | 25 | 96 | a) FAU-EMT | 1.3 | 1.10 |
b) FAU-EMT | 1.1 |
*a) Products on the beaker ground; b) products in the upper part of the beaker; (): very low amounts; **Si/Al-ratio of the FAU-EMT intergrowth.
dryer for different times and temperatures without stirring, as summarized in
The application of templates was excluded but gel formation was optimized by addition of citric acid. With the application of citric acid we followed the certain aims:
- The additional proton enrichment of the sol/gel in an alkaline environment reduces the polymerization and avoids a too fast flocculation of the gel [
- Due to the dissociation strength citric acid is a suitable buffer to rule the kinetics of the reaction course [
- Citric acid often removes actually many impurities from the filtration residue besides, the resulting citrate complexes remain dissolved and can be removed with the washing water.
Our former investigations further showed the positive influence of the acid by buffering the reaction system during zeolite syntheses [
All products were analysed by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS).
XRD was performed on a D4 Endeavor diffractometer (Bruker) with Cu Kα monochromatized radiation (graphite monochromator) at 40 kV and 40 mA. The samples were scanned in the range 3˚ - 65˚2 Theta, with a step wide of 0.02˚2 Theta and a measuring time of 3 seconds for each step. The powder patterns (
The crystal size, morphology and homogeneity of the products were determined by SEM, using a Jeol JSM-6390A and 30 kV acceleration voltage.
The chemical composition of the samples was determined by EDXS with an Xflash Detektor 410-M (Bruker).
The results of qualitative phase according XRD patterns of
The reaction products of all three syntheses accrued in two sequences:
- on the ground of the reaction glass: here fine disperse nanocrystalline zeolite material was deposited beside more or less amorphous aluminosilicate (samples 1a, 2a and 3a);
- and in the upper area of the reaction glass: here zeolite with somewhat higher crystallinity but even in polycrystalline form was found (samples 1b, 2b and 3b);
Hence, it was decided to describe these synthesis products in separate sections:
The X-ray powder pattern of the product 1a (shown in the 3˚ - 40˚2 Theta range for a better overview) in
The SEM picture of sample 1a, given in
The field analysis according EDXS proved the following chemical composition (Wt.%): SiO2: 46.2; Al2O3: 31.8; Na2O: 15.5; CaO: 5.6; MgO: 0.9. The Si/Al-ratio was ~1.2.
The XRD analysis of sample 1b (
The SEM image of product 1b shows mainly agglomerates/aggregates of FAU crystals and a few FAU crystals with octahedral habit (see
The chemical composition amounts (Wt.%): SiO2: 44.5; Al2O3: 36.3; Na2O: 18.8; CaO: 0.4. The Si/Al-ratio is 1.0. Due to this ratio the exact classification of the FAU-form is complicated and according to the chemical analysis it is probably the LSX-form.
A comparison of analyses data of sample 1b with 1a exhibits an enrichment of Si-, Ca- and Mg- in the bottom product 1a.
The X-ray powder pattern of sample 2a shown in the 3˚ - 40˚2 Theta range for a better overview in
The SEM image,
The chemical composition is (Wt.%): SiO2: 46.0; Al2O3: 32.6; Na2O: 13.8; CaO: 6.6; MgO: 1.0. The Si/Al-ratio is 1.2.
The XRD analysis of the product 2b (
On the SEM photo,
The chemical analysis (Wt.%) of the sample is: SiO2: 44.5; Al2O3: 35.4; Na2O: 19.8; CaO: 0.3. The Si/Al-ratio varies below 1.1 and again an exact classification of the FAU-form is complicated and according to the chemical analysis it is probably the LSX-form.
A comparison of analyses data of sample 2b with 2a again shows the enrichment of Si-, Ca- and Mg- in the bottom product 2a.
The X-ray powder pattern of product 3a shows wide and rather vague reflexes and the background contribution is high. The zeolite phase FAU-EMT (PDF
38-237/PDF 46-566 [
The EDXS field analysis at this area shows the following composition (Wt.%): SiO2: 47.1; Al2O3: 30.3; Na2O: 12.8; CaO: 8.8; MgO: 1.0. The ratio Si/Al is equal 1.3.
The powder pattern of product 3b points to FAU-EMT zeolite and as sample 3a even no LTA (
In the SEM investigation sample 3b shows close resemblance with the synthesis product 2b. In
The chemical analysis of the sample 3b represents (Wt.%): SiO2: 46.1; Al2O3: 35.2; Na2O: 17.8; CaO: 0.9. The Si/Al-ratio is 1.1. A comparison of the analyses data of sample 3b with 3a again shows the enrichment of Si-, Ca- and Mg- in the bottom product 3a.
Low temperature synthesis using an industrial waste material “filtration residue FR” from waste water reconditioning of the silane production as substitute for sodium silicate, mainly yields to disordered intergrowths of zeolite FAU-EMT. More or less amounts of zeolite LTA were observed as byproducts in the 40˚C and 30˚C synthesis. A product without parts of zeolite LTA could be synthesized at only 25˚C.
Syntheses were successfully performed under open conditions without stirring. The starting material FR was used without an energy consuming pre-treatment like calcination.
An optimization of gel formation was reached by the addition of citric acid. The latter caused an additional proton enrichment of the sol/gel in an alkaline environment and is thus reducing the polymerization and avoids a too fast flocculation of the gel [
The reaction products always occurred in two sequences, one at the bottom of the reaction vessel and another at the top with portions of amorphous aluminosilicate within both parts. In general a comparison of analyses data showed a higher crystallinity of the products formed at the top of the crystallization bin. The concentrations of Ca and Mg accumulated in the bottom products and even the Si/Al-ratio of the FAU-EMT products were always higher in the lower crystallization area, than at the top. In general a remarkable low Si:Al ratio was analyzed in the FAU-EMT intergrowths, especially in the products from the top of the reaction bin and a LSX like composition of the FAU parts of the intergrowth cannot excluded.
Future investigations on the mechanism of zeolite formation from industrial residues like FR at low temperatures are necessary. An improvement of the crystallinity of the samples by fine tuning of the preparation steps at 25˚C is an interesting topic for such an upcoming work. Even the characterization of the pore system according to BET-analysis and tests of sorption properties and thermal stabilities should be included in future experimental studies of the FAU-EMT zeolites.
The authors like to thank Dr. K. Rübner, BAM-Federal Institute for Materials and Testing Berlin, for kindly providing the FR raw material and for performing the chemical analysis of FR.
Petrov, V. and Buhl, J.-Ch. (2017) Low Temperature Synthesis of Zeolites of the Faujasite Family (FAU-EMT) Using Industrial Mineral Waste (FR) from Silane Production. Journal of Materials Science and Chemical Engineering, 5, 1-13. https://doi.org/10.4236/msce.2017.511001