Furnace Method for Dehydrating Hydrated Halloysite

Furnace Method for Dehydrating Hydrated Halloysite
Student’s Name
Institutional AffiliationIntroduction
Clay mineral has faced numerous investigations when it comes to the dehydration process and the techniques for forming crystal structure which are associated with tubular halloysite (Joussein et al., 2005). Apparently, there exist various methods that have been supported by many researches when it comes to the dehydration of kaolin mineral. The use of variety of methods has spurred a debate on whether they form exact anticipated dehydrated halloysite during the process or not. As a result, techniques such as X-ray diffraction, Thermal analysis and Infrared and Raman Spectroscopies have been the top priority methods for analyzing the method of heating the hydrated halloysite. In particular, kaolin mineral are heated at a rate of 5 to 30℃/min to obtain a significant heat effects on the mineral. Therefore, endothermic and exothermic effect is associated with dehydration to form halloysite (7(A)) ̇ under intense heat on a furnace. The aim of this paper is to examine the furnace method employed in dehydration of kaolin mineral and the underlying reasons that makes it best for dehydration process as opposed to other methods such as vacuum. As a result, the paper will provide various temperature levels that are deemed suitable for the dehydration of kaolin mineral.
Justification
Recent researches have shown that there exist variety of methods of heating kaolin mineral to halloysite (7(A)) ̇ but furnace method has been the most extensive method due to the rigid amorphous SiO2 network on the crystallization of the Al2O3 (Keeling et al., 2017). Since Le Chatelier discovered thermal effect, there has been a numerous debate on the cause of heat transmission and effect on the clay (Insley & Ewell, 1935). As a result, the use of certain techniques has been based on removing impurities which are associated with the clay mineral. Unfortunately, selection of furnace method was enhanced by the need to eliminate large amount of impurities that involves three phase heat effect. The assumption was based on sample withdrawn from the furnace method when a constant temperature is applied for long time in different phases. According to Insley & Ewell (1935) mixture of different species of clay minerals requires attainment of constant temperature that are in phases to eliminate the existent of three minerals of the kaolin during the dehydration process that consists kaolin group; Nacrite, Dickite and kaolinite with the same chemical composition. As a result, the furnace method provides an interface where the hydrated halloysite is heated at controlled rates of temperature in order to discover the changes in structure of the dehydrated halloysite. Having a controlled rate of heat effect will enable the clay mineral to develop different chemical composition with distinctive crystal structures. According to Wang et al. (2011), artificial alumina-silica gels and X-ray patterns of kaolin minerals led to the following conclusion: 1) endothermic heat effect associated in thermal behaviour leads to the dissociation of kaolin mineral into amorphous SiO2, amorphous Al2O3 and into water vapour; 2) the exothermic heat effect leads to the crystallization of γ=Al2O3 from amorphous Al2O3. Finally, the resultant intensity of the heat effect produced when the high temperature is reached leads to a delayed in crystallization of γ~Al2O3 due to the restraining action of rigid amorphous SiO2 that is closely associated with Al2O3.Figure 1. Chemical analyse of kaolin mineral
Materials investigated
From the materials collected, 21 samples of tubular halloysite were obtained from New Zealand, China, Australia and USA to study their morphological changes during the dehydration process. The deposits of these clay minerals were cvollected from rocks but the final analysis of the tubular halloysite was from Eucla basin in South Australia. Most of the samples originated from halloysite deposits in China including Dafang region (3Ch, 4Ch). These kaolin mineral were suitable for chemical analysis of morphological structure, atomic structure and crystallization form after dehydration. The heat effects on various layers of these minerals results into different interlayer structure when heated in a furnace. Ideally, Thermal analysis technique provides a substantial outlook when it comes to dehydration process. According to Keeling et al. (2011), different techniques have different method of dehydrating kaolin minerals as well as different temperature of heating. From this research, the paper will focus on thermal analysis technique which dehydrated the kaolin minerals using furnace method.
Experimental method
Thermal effect was measured in different thermocouple method which constitutes a platinum wire resistance furnace. From this experiment, a divided platinum cylinder and a K potentiometer were used to carry out the experiment. According to Kogure et al. (2011), the nature of relationship of dehydrated halloysite and hydrated halloysite are separated by mineral phase and the chemical structure. Therefore, exposing such a mineral in a considerate amount of heat changes the atomic structure and the morphological structure. Also, he argues that hydrated and non-hydrated halloysite states are member of different degrees and can be regarded as two forms of a single structure. On the other hand, X-ray diffraction has been used in investigative role but it has failed to show basal spacing at either end of the dehydration series. Therefore, interlayer water content could be still traced in the X-ray diffraction process that involves vacuum method to dehydrate kaolin mineral. However, curves for a halloysite during dehydration were found to have similar shape as attributed in the metahalloysite phase. Consequently, application of furnace method as a way of dehydrating kaolin mineral does not have environmental effect and change of basal spacing in kaolin mineral. According to Massaro et al. (2017), Artificial alpha-alumina which is known to show no measurable heat effects provides a reference material that can normalize the resistance of heat loss. Recent research has argued that the furnace temperature can be increased at a constant rate without affecting the crystal structure of the dehydrated halloysite as opposed to other techniques. They retaliated that the constant rate of manually decreasing the resistance in series at the furnace at regular interval enables time intervals to be adjusted while keeping the kaolin mineral normal. This implies that the normal heat rate selected at 6℃ but rates between 1 and 25℃ is essential in determination of the crystalline compounds present. However, the readings are significant in temperature adjustment and reading the two side of the cylinder that are made of different intervals. The determination of the compounds in the cylinders under specific temperature in the samples enables comparison of patterns. In supplementary analysis, the examination are always carried out with petrographic microscope but in most cases the materials might be so fine grained making it difficult or not conclusive.
Temperature levels
When the samples of kaolin mineral are heated at a constant rate of 6℃, the experiments constitute of three heat effects which are observed. Ideally, large endothermic effects occurs, sharp and intense exothermic effect occurs and small exothermic heat effects occurs which is very difficult to detect. According to Massaro et al. (2017), the heat effects associated with thermal analysis in a furnace are irreversible due to the reactions causing the heat effects.
The endothermic heat effect
The endothermic effect occurs at a wide temperature ranging from 450 to 580℃ in clay mineral. In particular, this heat loss is associated with water loss to enable the basal interlayer spacing to release water molecules during the heating process. According to Lvov et al. (2016), few experiments conducted by the elite scientist to examine the change of morphological structure under endothermic heat effect provided diverse response. The result interpreted was that the reflection of low angles decreases relative to that higher angle of dehydration. This implies that the fully hydrated and fully dehydrated phases could not account for intermediate water contents. This result led to the adoption of furnace method with different heat effect to examine the atomic structure of the dehydrated halloysite under intense heat. In this stage, the dehydration showed that with a considerate temperature, the dehydration takes place until at a constant temperature where there will be no expulsion of water from the dehydrated halloysite (Lázaro, 2015). This method has been tapped intrinsic due to the dissociation pressure that is involved in the removal of water content in different temperatures. Also, the true equilibrium of furnace method depicts water loss of Neurode pholerite at significantly higher temperature to enable the different crystalline structure to be formed. Moreover, the uniform coarse grain for measurement of dehydration temperature relies on the nature of the product sample such as kaolin minerals and a temperature at 6℃/min. The only direct crystalline material that form dehydrated halloysite from the china clay mineral was heated to temperature between 650℃ and 850℃ that resulted into a diffraction pattern. The pattern was attributed as crystalline material which was called ‘alpha’ and stated as anhydrous alumina-silicate (Al2O3.2SiO2). According to Kohyama (1978), he describes that the only phase of endothermic effect that exist change in temperature is during the phase change at 870℃ which has a solely thermal-expansion curve of clay mineral.Figure 2. Heating curves of dickite and kaolinite.
The Exothermic heat effect
According to Kogure et al. (2008), exothermic heat effect encompass a rapid change of heat beginning at 925℃ with a heating rate of 6℃/min. The maximum temperature that the heat effect can reach during this stage is 985℃. With the condition of the present experiments, the increase of temperature may be as much as 30℃ depending on the sample of clay minerals and the characteristics of heat effect including intensity (Kohyama, 1978). He argues that the thermal analysis have a varying heating rate between 1 and 20℃/min of temperature which may experience evolution that does not exceed more than 4℃. However, the heat rate may evolve to a maximum temperature ranging from 957℃ for a heating rate of 1℃/min to 1,013℃ that has a heating rate of 20℃/min. this heat evolution is harnessed by the decomposition of the kaolin mineral into alumina, silica and water that constitutes the properties of amorphous alumina and dehydrated kaolinite. The attribution of these changes relies on the increased heat effect that separated different compounds from the clay mineral as it loss water to form dehydrated halloysite. In order to show these changes, the kaolinite were heated at 6℃/min just below the beginning of 925℃. From this heat effect, the exothermic showed no crystalline x-ray pattern when air-quenched was noticed during the maximum heat effect. However, a faint identical pattern of γ=Al2O3 was noticed as a crystalline pattern in the on-going heat effect. Recent research has shown that kaolin minerals have a constant temperature that is used to dehydrate kaolin mineral (Kogure et al., 2013). As a result, a constant temperature below 925℃ reduces the intensity of the exothermic heat effect. This implies that the conditions for exothermic heat effects are dependent of time of preheating and varying degrees of temperature. It is evident that other techniques such as use of vacuum to dehydrate kaolin mineral can be of great importance. However, it reduces the size of exothermic effect with increase in temperature. As a result, no strictly comparison can be used to in the size of exothermic heat effect to centre the thermocouple junctions in the sample. Interestingly, the exothermic heat effect is relevant during the pre-heating and the spread of intensity of heat that is subjected to the entire basal spacing range of the dehydrated halloysite.
The profile of this layers is enabled due to the partial alternation of the layers completely which varies in position throughout the basal spacing of dehydrated kaolin. Also, the samples heated at a constant temperature are air-quenched which is mainly consisting of the amount of acid-soluble alumina in heated treated samples. Upon reaching a given temperature, the intensity of the exothermic effect displays an appearance of γ=Al2O3.Figure 3. Crystalline phases of kaolin minerals.
Thermal behaviour of kaolin minerals
The corresponding heat effect on these kaolin minerals depicts the same intensity as other kaolin minerals from different regions. According to Keeling et al. (2011), the heating curves for the Al2O3.SiO2 gel and Al2O3.2SiO2 have a varying heat effect when exposed under intense temperature. He argues that the same temperature when applied in these samples has the same conclusion but the effect is slightly less intense in the case of 2Al2O3.SiO2 gel. However, it was noted that the exothermic heat effect is dependent on a constant temperature just below 925℃. Intuitively, this constant temperature resulted in reduced in the amount of heat effect for the kaolin minerals. On the other hand, X-ray patterns when observed for the exothermic samples which had been air-quenched showed a similar basal spacing range for the entire samples except 4Al2O3.SiO2. Previous research has shown that the exothermic heat effect provides only crystalline phase with increase in temperature as compared to endothermic heat effect (Kogure et al., 2008). Alternately, an advance experiment was conducted to vary the temperature of heating the kaolin mineral to certain temperature with the presence of phenol red as an indicator. The sample consisted amorphous Al2O3 which was heated at 6 ℃/min to certain temperature, x-rayed and air-quenched. The final result depicted from the experiment was the formation of faint patterns of γ-Al2O3 that was air-quenched between 600 and 700℃. In addition, stronger patterns of the x-ray were made from 800 and 700℃ in the samples of kaolin minerals (Keeling et al., 2011). Consequently, patterns of dehydrated halloysite can be formed with different temperature and under certain conditions involving indicators as a catalyst of the entire experiment.
According to Joussein et al. (2005), stability of HNTs occurs in different temperatures and further remains unchanged after a given temperature. It was discovered that different temperature below 400℃ for HNTs, interfere with the basal spacing range causing the stability of the atomic structure to remain unchanged. However, the tubular structure of halloysite have different characteristic as opposed to HNTs. For hydrated polymorph of kaolinite (Al2 (OH)4Si2O5.2H2O), the chemical composition seems to vary from dehydrated kaolinite. Therefore, dehydroxylation of the hydrated halloysite occurs at 500℃ to 900℃ according to Kogure (2011). Notably, he argues that endothermic peaks that range between 20-140℃ and 400-600℃ provides intrinsic rate for the absorbed water to be loss during the dehydroxylation of kaolinite. He implies that the disconnection of alumina and silica enable the process to be efficient due to weak basal spacing between the kaolinite in the tetrahedral and octahedral sheets.Figure 4. X-ray patterns of kaolin mineral after heat treatment.
Discussion of results
According to Hillier eta l. (2016) , the formation of crystalline material is aided by the dissociation of kaolin in γ-Al2O3. As far as temperature is the main factor, moderate rapid rates of 2℃/min increase the formation of crystalline material due to basal spacing range and water loss. On samples removed from furnace immediately, the exothermic effect beginning at 925℃ enable x-ray patterns to be carried out after the evolution of heat. This implies that in thermal analysis (furnace) different temperature level have different reactions to the chemical compound of kaolin mineral. If samples are heated for long period or lower than 50℃, the partial transformation of the Al2O3 is partial affected in the process. This implies that the varying endothermic and exothermic heat range is relevant for the formation of crystalline structure. According to Kogure (2011), he describes partial or complete elimination of amorphous Al2O3 can be attributed by evolution of heating the kaolin mineral above heat range of exothermic. He argues that furnace method (thermal heating) provides reliant heat range which takes place rapidly with a violent evolution of heat. The heat is significant for the transformation of amorphous forms of pure alumina crystallization over long period of interval. We may assume that the amorphous silica has a retarding action on crystallization which is relevant for the formation of pure amorphous alumina. As a result, it is justifiable to conclude that long temperature intervals and lower temperature intervals provide crystalline structure which does not interfere with the basal spacing range of the dehydrated halloysite. Ideally, the different activities of heating range provide consequently individual molecules that are responsible for the formation of crystalline form (Guggenheim et al., 2009). Similarly, lower temperature forms a rigid amorphous silica structure which varies with different clay minerals.
Some experiments have ascribed that stabilization of furnace temperature might fail to bring exact anticipated crystalline structure if procedure are not well followed. For instance, amorphous form of silica when heated at a temperature of 1,300℃ showed a resistance in the form of crystal structure even when heated to very high temperature (Hillier & Ryan, 2002). This process led to the formation of sluggishness in the process in which the silica was crystallizing when heated at high temperature than it was expected. Apparently, furnace method is the most applied technique for dehydrating kaolin minerals in different heating ranges. Despite the overlying impediment in the crystalline formation, furnace method is widely used due to the ability of adjusting temperature levels.
Conclusion
In conclusion, furnace method is the most proficient technique of dehydrating kaolin minerals when it comes to the crystallization formation and basal spacing of the dehydrated halloysite. It is evident that the considerate rates of heating the kaolin mineral are associated with intense endothermic and exothermic heat effect. Similarly, the lower broad of heating the kaolin mineral to loss water is associated with endothermic heat effect while the intense or higher heating is associated with exothermic heat effect. These heat effects are associated with the dissociation of amorphous silica and amorphous alumina. The evidence portrayed in this research showed that the kaolin minerals are heated at rates from 5 to 30℃/min to enable significant heat effect. Apparently, the nature of crystalline depends on the internal structure of the material and different heating range. Generally, the heat effect for the kaolin mineral is at a constant temperature of 925℃ for the exothermic heat effect while the endothermic has a constant temperature of 450 and 600℃.Reference
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