| Usually the application of palygorskite needs pyroprocessing, so the heat-treatment is one of the most common modifications used for industrial and scientific purposes. Some concomitant changes in properties like dehydration, structure changes, surface properties changes occurred apparently with the increase of temperature, but the origin of these changes is still unclear in both scientific and industrial world. The sample is from Dongfeng mineral cooperation stope Mingguang City, Anhui Province, China. The present work investigated the structure and morphology changes of different typical types samples during calcinations using X-ray diffraction, scanning electron microscope, high resolution transmission electron microscope,27Al and29Si magic angle spinning nuclear magnetic resonance, and also investigate the effect of heat treatment on the relationship between structure and surface, textural properties of palygorskite. The present work investigated the effect of heat treatment on the relationship between structure and surface, textural properties of palygorskite by using BET surface area and pore width analysis, thermogravimetric analysis, Fourier transform infrared spectroscopic analysis, especially the in-situ mass spectrometry for detecting the surface acid-alkali properties of palygorskite through the adsorption-desorption of water, NH3and SO2, The main conclusions of the present work are showed as follow:1. Compared the XRD, TGA analysis with the desorption analysis of water, it can be concluded that:the adsorbed water on the surface of palygorskite and in the channel of the crystal is eliminated,even the structure remained unchanged when the heat-treatment temperature is lower than200℃; half crystal water expelled from the crystal structure adjusted and channels in the crystal folded at the heat-treatment temperature range of200-300℃; the other half crystal water expelled, the structure water in the crystal and at out-surface began to be expelled, also the periodicity of the structure was destroyed at the heat-treatment temperature range of300-500℃; the sample after further dehydration formed anhydride, but the partly order in C axis and the chain of Si-O tetrahedral were still exist after heated at temperature range of500-700℃.2. XRD and29Si MAS-NMR results showed that the palygorskite had became amorphous state after heated at temperature range of500-700℃, and then transformed into cristobalite at800℃3. After half crystal water expelled from the crystal structure, the original structure state could be not only recover to after rehydration, but also the diffraction peak intensity was strengthened, it can be concluded that the heat-treatment and rehydration cycle can improve the order of structure. But when the heat-treatment temperature is above350℃, the structure couldn’t be recovered after rehydration 4. The Al-coordination of sample from Mingguang Anhui is mainly AlⅥ, and a minor amount of Al IV can be found. No significant change appears when treated at the heat-treatment temperature lower than500℃. The peak of Al(Ⅳ) is strengthened and a signal of Al(Ⅴ) is detected when treatment temperature higher than500℃. There are two lattice positions existed at the same time, showing that the. orderly structure is not destroyed compeletly. The peak of Al(Ⅵ) decreases and Al appears as Al(IV) after treatment temperature higher than600℃, At800℃, Al(Ⅵ) and Al(V) disappear completely, and the peaks of Al(Ⅳ) turn to form one peak, indicating that all the Al(Ⅳ) exists in one chemical environment.5. TEM and SEM images showed that the fiber morphology when the heat-treatment temperature is lower than800℃, and it would become shrinked, bended like earthworm shape, even sintered when the heat-treatment temperature is higher than800℃, so that to palygorkit, the critical temperature for the use of carrier material is800℃.6. The specific surface area did not changed apparently when the the heat-treatment temperature is lower than800℃, but it decreased sharply when the heat-treatment temperature is higher than800℃. The structure of palygorskite was folded when partly crystal water expelled at300℃, and it was destroyed when structure water expelled at500℃, but the SSA was stable until700℃, we can concluded that the SSA measured by BET-N2adsorption-desorption method is the external SSA, the radius of N2molecule is too large to enter the channel of palygorskite. The reason of sharply decrease of SSA at800℃is that the palylygorskite fiber structure was shrink into a ball, and the sintering made the decrease of porosity.7. Under hydrothermal conditions, the existence of alkaline magnesium compound can destroyed the crystal structure of palygorskite, and promote the transformation of palygorskite into smectite or serpentine. Under hydrothermal conditions of70-200℃, the adding of neutral magnesium compound will benefit the growth of palygorskite crystal but the growth is slow. The adding amount of MgCl2affects the crystal growth apparently, while the acting temperature and time was slightly.8. The thermal decomposition temperature of dolomite-palygorskite started at500℃, the peak temperature is745℃, and completely decomposition temperature is780℃. While for ordinary dolomite the thermal decomposition temperaturest started at600℃, the peak temperature is797.6℃, and completely decomposition temperature is825℃. The decomposition of dolomite has two steps, and has calcite as intermediate products. The dolomite decomposition temperature in dolomint-palygorskite clay is50℃lower than ordinary dolomite, and exhibited an unusual thermal chemically active. There are two reasons of this unusual thermal chemically active:Firstly, the minerals in dolomite-palygorskite clay are nano scale with nanostructure, the nanometer effect causes the unusual thermal chemically active; secondly, palygorskite as a kind of nano mineral coexist with dolomite, and formed interdigitations microstructural in nano scale, while palygorskite as a kind of higher thermal chemically active reacted with dolomite which promote the decomposition of dolomite, and lower its decomposition temperature. Activated silica plays a greater part in carbonate thermal decomposition of calcite than dolomite, and this can be attributed to the siliceous component which has higher thermal chemically active reacted with the Ca ions in carbonate, and this will promote the thermal decomposition of carbonate.9. All types of palygorskite had no effect on the removal of phosphate before calcination, and there is a very different adsorptive property after calcinations, only calcined dolomite-palygorskite clay has good effect for phosphate adsorption, and it can be concluded that not all types of palygorskite can be made into phosphate adsorbent. The modification effect can be found only when the thermal modification temperature is higher than500℃, the best activation temperature is nearly600℃. Dolomite-palygorskite clay has an unique adsorptive property for phosphate, The phosphate removal mechanisms are so different by the samples calcined at different temperature, the phosphate removal mechanism of samples calcined at500℃and600℃can be attributed to the adsorption at the surface of modified dolomite, while the phosphate removal mechanism of samples calcined at700℃and800℃can be attributed to the precipitation of magnesium phosphate and calcium phosphate which caused by the low crystalline larnite and spurrite formed from thermal reaction of the dolomite and palygorskie. |
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