Transformation of heulandite type natural zeolites into synthetic zeolite LTA
Introduction
Natural zeolites of the heulandite (HEU) class are considered one of the most abundant occurring types globally (Armbruster, 2001). Heulandite crystal structures are normally described as monoclinic and belonging to the space group C2/m. The heulandite class of zeolite can be divided into two mineralogical phases, clinoptilolite and heulandite, which are usually distinguished from each other based on their thermal stability or the Si/Al ratio: Si:Al 4.0 as clinoptilolite and Si/Al < 4.0 as heulandite (Coombs et al., 1998). General petrological differences between clinoptilolite and heulandite have been noted (Boles, 1972); however, based on the unit cell and chemical formula from X-ray diffraction, they are considered to be indistinguishable from one another (Bish and Boak, 2001).
The HEU zeolite family has been used in applications such as: (1) ammonium nutrient removal and recovery from water/wastewater (Liang and Ni, 2009); (2) heavy metal remediation (Rodríguez-Iznaga et al., 2018); (3) gas sorption (Ackley et al., 2003); (4) catalysis (Kusuma et al., 2013); (5) geopolymers (Villa et al., 2010) and (6) as a resource for making higher value zeolites (Ngapa et al., 2016). Key properties include ion exchange capacity, surface area, pore size/shape/volume, and thermal stability. However, the uptake of natural zeolites has been hindered due to a range of circumstances. For example, synthetic zeolites can exhibit significantly greater cation exchange capacity (cf. 500 meq/100 g) (Mackinnon et al., 2010, Mackinnon et al., 2012) compared to natural HEU zeolite (ca. 120 meq/100 g). Therefore, a key research direction is the promotion of HEU zeolites to improve performance. One strategy is the enhancement of natural zeolite physical properties by thermal pre-treatment. Dong et al. studied a Chinese clinoptilolite material and reported that heat treatment increased the cation exchange capacity from 129 to 171 meq/100 g (Dong et al., 2017b). As a consequence, the activated natural zeolite exhibited an increase in ammoniacal nitrogen removal from solution from 40.6 to 93.5%. In contrast, Christidis et al. did not observe any increase in the cation exchange capacity value upon heat treating natural zeolites (Christidis et al., 2003). Instead, the disappearance of zeolitic characteristic peaks by X-ray diffraction (XRD) was noted upon heat treatment to ca. 600 °C and concomitantly the presence of an amorphous phase was recorded (Dabbebi et al., 2018, Duvarci et al., 2007, Nikolov et al., 2020). Analogous to the heat activation of clays to form metakaolinite (Abdullahi et al., 2017, Hartati et al., 2020, Johnson and Arshad, 2014), calcination of natural zeolites can form metazeolite, the structurally collapsed version of, for example, HEU (Nikolov et al., 2020).
The formation of an amorphous phase upon heating of natural zeolite opens up possibilities for conversion of natural zeolite into higher value synthetic zeolites, geopolymer, or catalysts. In this instance, natural zeolite was regarded as an inexpensive and abundant source of aluminosilicate material. For example, Nikolov et al. found that clinoptilolite activation at 900 °C facilitated the creation of geopolymers due to the structural collapse of the natural zeolite (Nikolov et al., 2020). Similarly, Dabbebi et al. calcined HEU zeolite at 600, 700 and 800 °C prior to alkali activation (Dabbebi et al., 2018). It was reported that heating at 700 °C was sufficient to transform the natural zeolite to amorphous metazeolite. Indeed, heating the zeolite material at 800 °C was suggested to decrease performance.
It is apparent that the activation of natural zeolite by thermal treatment results in a wide range of outcomes. Notably, zeolitic transitions to amorphous material depend upon whether heulandite or clinoptilolite phases are present. For example, heulandite is indicated to decompose once heated to approximately 250 °C whereas clinoptilolite requires heating to ca. 600 °C (Armbruster, 2001, Bish and Boak, 2001, Esenli and Kumbasar, 1994). Another factor which may influence the formation of amorphous phases from thermal decomposition of natural zeolite is the material composition. Depending upon the location of the zeolite deposit not only does the SiO2/Al2O3 ratio vary (Esenli and Kumbasar, 1994, Stocker et al., 2017, Yang et al., 2010) but also the identity/amount of zeolite, quartz and feldspars present (Christidis et al., 2003). Feldspar and quartz are notably resistant to dissolution in strong alkali solutions (Kang and Egashira, 1997). A number of authors have considered the versatility of a fusion conversion technology for activation of natural zeolite (Wang et al., 2007) and coal fly ash (Berkgaut and Singer, 1996, Franus et al., 2014, Jiang et al., 2016). For example, prior to zeolite synthesis Shigemoto et al. fused coal fly ash with sodium hydroxide to form sodium aluminate and sodium silicate, respectively (Shigemoto et al., 1993). However, the benefits of this approach have not been extensively investigated for natural zeolites.
Literature evaluation revealed that minimal research has been completed relating to the transformation of natural zeolite to form synthetic zeolites. A lack of knowledge exists regarding the need for activation of natural zeolite and the preferred methodology: (1) thermal activation; or (2) alkali fusion; or (3) a combination of heat treatment and alkali fusion. Moreover, the impact of using material from different zeolite deposits upon the effectiveness of activation procedures has not been resolved. There is also a need to employ quantitative XRD of zeolite materials as this allows insight into the phases present including non-diffracting materials which is expected to be produced when natural zeolite is activated.
Therefore, the aim of this study was to develop a systematic approach to activate natural zeolite samples comprising majorly of heulandite and clinoptilolite in order to make high quality zeolite LTA. Zeolite LTA was chosen as it is the most widely used zeolite due its ability for water softening in detergent applications. The hypothesis was: “If activation strategies can be tailored to different natural zeolite deposits then a range of value-added zeolites can be made”. Research questions which were addressed to support the hypothesis included: (1) How do key physical properties of HEU zeolites differ? (2) What is the impact of zeolite identity upon the activation process? (3) Is the quality of zeolite LTA related to the natural zeolite type? The methodology involved use of three HEU zeolite materials which were characterisedby multiple techniques including quantitative XRD, in situ XRD, X-ray fluorescence, thermogravimetric analysis, and electron probe microanalysis. Alkali fusion on either heat treated or as received natural zeolite rock was investigated to investigate the relationship between amorphous material and the alkali activation process
Section snippets
Chemicals and materials
Two Australian natural zeolites were obtained from Zeolite Australia; Escott (0.7 to 1.2 mm) and Avoca (<0.7 mm). NextSand (0.7–1.2 mm) natural zeolite from the USA, was also supplied by Pacific Water Technology. Prior to use, the crushed zeolite samples were washed several times with deionised water to remove excess particulates and dried in an oven at 80 °C. Sodium aluminate (NaAlO2), 32% hydrochloric acid (HCl), sodium hydroxide (NaOH) mini pearls, ammonium chloride (NH4Cl) and potassium
Composition and properties of HEU zeolite material
Optical microscopy images of Escott, Avoca and NextSand zeolite samples revealed differences in grain size, angularity, morphology, and heterogeneous colouration; with a distinctive and pervasive pink/orange colouration in the Avoca sample [Fig. 1].
Bulk chemical compositions of the natural zeolite samples were collected using XRF [Table 2].
Silica and alumina were the dominant species as expected with smaller amounts of iron, sodium, potassium, magnesium, and calcium. The data was consistent
Conclusions
The classification of Heulandite type natural zeolites has been shown not to be straightforward. Probably the preferred means of classification is the differences in thermal stability; with heulandite type exhibiting low stability and clinoptilolite high stability. More detail was revealed by use of EPMA which allowed compositional data for individual zeolite grains to be collected. On this basis it was concluded that previous ideas relating to the Si/Al ratio in the context of heulandite (< 4)
CRediT authorship contribution statement
Katrina Wruck: Investigation, Methodology, Validation, Writing - original draft. Graeme J. Millar: Supervision, Conceptualization, Methodology, Writing review & editing. Tony Wang: Formal analysis, Investigation, Validation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was enabled by use of the Central Analytical Research Facility at QUT. This investigation was supported by the ARC Research Hub for Energy-efficient Separation with industry contribution from Zeolites Australia. Special thanks to Henrietta Cathey (IFE, QUT) for performing EPMA analysis and the helpful comments.
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