Georgi Kirov

Thermal decomposition products of hydrotalcite-like compounds: low-temperature metaphases

Synthetic hydrotalcite-like samples with composition [M1 − x2+Alx3+(OH)2](CO3)x/2 ·nH2O, where M = Mg, Ni, x = 0.25 and 0.33, n = 2x and 1.5x, are studied by XRD, DTA, TG and IR spectroscopy after heating in the temperature interval 120–260 °C and rehydration in air and water. Structural models of the two metaphases obtained are proposed. Metahydrotalcite-D (HT-D) is formed at 140–180 °C by the reversible dehydration of the interlayer. Metahydrotacite-B (HT-B) is formed at temperatures 240–260 °C as a result of the dehydroxilation of a part of OH groups of the brucite-like layer and inclusion of two oxygenes from the CO3-group in the same layer. The HT-B has a specific crystal structure. Rehydration does not restore the initial structural state but leads to the formation of a phase (HT-B-r) characterized with increased thickness of the interlayer, high content of water and a low temperature of dehydration. The properties of HT-B depend on the ratio M2+: Al of the initial sample. The substitution Mg ↔ Ni does not influence the properties of the metaphases in the low temperature region.

Cation composition during recrystallization of layered double hydroxides from mixed (Mg, Al) oxides

Abstract Changes in cation composition and M 2+ /M 3+ ratio during hydrotalcite regeneration were studied. Regenerated hydrotalcites were obtained by recrystallization of mixed (Mg, Al) oxides in solutions of divalent (Mg, Zn, Co, Ni, Cu) or trivalent (Al, Fe) cations. Heating Mg–Al–CO 3 hydrotalcites with Mg/Al=2, 3 and 3.7, at 600 °C for 2 h yielded periclase-like mixed (Mg, Al) oxides (HT-P). The hydrotalcite structure was restored by dispersing oxides 48 h in water or aqueous solutions of different cations. The presence of Mg 2+ , Zn 2+ , Ni 2+ , Co 2+ , Cu 2+ salts or of low soluble hydromagnesite increased the M 2+ /Al ratio, reaching a maximum value of 3.8. An incorporation of Zn 2+ , Ni 2+ , Co 2+ and Cu 2+ cations in the newly formed hydrotalcite was detected, while Mg 2+ remained in solution. In the presence of soluble Al salts or freshly precipitated Al(OH) 3 , the M 2+ /Al ratio approximated the minimal possible value of 2. The Mg/Al ratio of a hydrotalcite crystallized from a mixture of two HT-P samples with different Mg/Al ratios is equal to the weighted average value. The results obtained support the conception of the dissolution–crystallization mechanism of hydrotalcite regeneration from mixed (Mg, Al) oxides contrary to the widely accepted concept of topotactic processes.

Thermal evolution of Mg-Al-CO3 hydrotalcites

Abstract The thermal decomposition of hydrotalcite (HT), with chemical composition Mg1-xAlx(OH)2(CO3)x/2·(1-3x/2)H2O, (0.20 < x ≤ 0.33), is a complex sequence of dehydration, dehydroxylation and decarbonization and leads to the formation of a series of metaphases: HT-D (dehydrated HT), HT-B (partially dehydroxylated HT) and MO (mixed oxides with periclase-like structure). The evolution of water and CO2 in natural and synthetic hydrotalcites (a Mg/Al ratio between 2:1and 3.7:1), heated to 800°C, was investigated by differential thermal analysis, thermogravimetry and evolved gas analysis. At least six endothermic and two exothermic effects were established by computer-aided resolving of the curves. The formation of each HT metaphase was related to the release of a discrete number of water molecules depending on the Al content in the samples and each appeared as a corresponding endothermic peak in the DTA curves. The exothermic processes associated with the crystallization of HT-B and MO metaphases were specified by decomposition of DTA curves. The evolution of CO2 during the thermal decomposition of the carbonate groups was found to be different for the samples studied. The preservation of CO3 even at high temperatures was established for synthetic samples with a high Al content. The release of volatile H2O and CO2 (which comprise ~40% of the sample mass) provokes fine cracking both along and across the layers.

Alteration processes and products of acid pyroclastic rocks in Bulgaria and Slovakia

Abstract The genesis of the products of the alteration of acid pyroclastic material is discussed and interpreted on the basis of the distribution, sedimentation conditions, post-sedimentation activity, mineral and chemical compositions of pyroclastic deposits in Bulgaria and Slovakia. It is found that the disordered nonequilibrium nature of the volcanic glass induces a diagenetic devitrification of the tuffs and formation of clinoptilolitic, adularia-cristobalite and bentonite rocks. With increasing temperature, the volcanic glass tends forward a stable state through a series of zeolite mineral associations: clinoptilolite-mordenite-analcime-feldspar. The change in mineral composition of this series of rocks occurs without a change in the chemical composition of the rocks, which could be explained by the closed nature of zeolite systems. The formation of bentonites is associated with the removal of alkaline ions under diagenetic conditions, while the formation of halloysite rocks is caused by hydrothermal activity.

Structural interpretation of anion exchange in divalent copper hydroxysalt minerals

Abstract Minerals with the general formula Cu4(OH)6 A2/nn ± pH2O (A = Cl-, NO3-, SO42-) were synthesized and their behaviour following treatment with chloride, nitrate and sulfate solutions was studied by powder XRD and SEM. Two types of transformation were found: (1) an ion-exchange reaction manifested by preservation of both the precursor’s morphology and structural type; (2) a dissolution-crystallization mechanism characterized by changes in the structural type and the morphology. The results were considered by simultaneous application of the binary presentation of the structures, a bond valence approach and the ion-exchange ability. It was found that the structures of minerals with ion-exchange properties are built from similar layered structural unit of edge-shared and corner-shared Jahn-Teller square frameworks and different interstitial complexes of exchangeable ions, water molecules and cation-water groups. On the basis of their structural features the position of the investigated minerals in mineralogical classifications is also discussed.

Matching of the water states of products and zeolite during contact adsorption drying

ABSTRACT The mixing of dehydrated zeolites with product particles leads to drying of the product, owing to the decrease in relative humidity of the environment and increase the temperature of the mixture, due to the release of adsorption heat. The analysis of the conventional one-stage contact drying process showed a significant mismatching in the water states of the product and the zeolite during drying. A two-stage drying process is proposed for harmonization of the water state of both product and sorbent. The significant advantage of the two-stage drying over one-stage regarding the degree of drying of the product, the utilization of the zeolite adsorption capacity and the duration of the process were experimentally confirmed.