Synthesis and Characterization of Ca and Na Zeolites (Non-Pozzolanic Materials) Obtained From Fly Ash–Ca(OH)2Interaction

What is it about?

The conventional method for alkali activation of fly ash utilizes Ca(OH)2 and NaOH for formation of pozzolanic material and fly ash zeolites, respectively. Sodium based fly ash zeolites (say Na- zeolite) mostly employ NaOH (the high grade mineralizer and alkali) for activation of fly ash before its application as an absorbent. However, the Na-zeolites as absorbents (in agro- and aqua-culture) result in sodicity (i.e., excess of Na, present as impurity in the zeolite), which in turn reduces its holding capacity of moisture, nutrient (i.e., NPK fertilizers), microorganisms (viz., microbial spores) or heavy metals and negatively affect the growth of plant and aquatic life. In order to resolve such problems, the present study is focused on synthesis of agro-grade blend (dominated by Ca-zeolite) of zeolites by using Ca(OH)2 as major alkali and two well established mineralizers, NaOH and NaCl, used in the trace quantity. In order to monitor activation of the fly ash in two different conditions, the synthesis of zeolites could be carried out by employing(1) conventional (the open hydrothermal system) and (2) autoclaving (the closed hydrothermal system) methods. The main attributes which control the entire study include temperature and reaction times for both methods. In addition, the present study demonstrates: (1) effectiveness of Ca(OH)2 in creation of blend of zeolites with considerable cation exchange capacity when an optimum chemical composition (comparable to a pure agro- and aqua-grade zeolite 4A) and (2) suitability of the conventional hydrothermal method over autoclave method in synthesizing the blend of Ca- and Na-zeolites possessing a cation exchange capacity up to 394 meq/100g. The formation of needle/star/spherule/small cube-shaped crystals (i.e., Na-P1, the Na-zeolite) and prismatic/cuboidal shaped crystals (i.e., heulandite, the Ca-zeolite), confirms suitability of the end product as a good sorbent and manure.

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Why is it important?

to ensure large scale production of the agro- and aqua-grade zeolites and economic scaling up of the pilot synthesis unit in future, the present study is focused to employ: (1) NaCl as a minor reagent, (2) high pH condition, by dosing reaction mixture with traces of NaOH and (3) Ca(OH)2, hydraulic lime, as major activating reagent. In addition, to establish a suitable process for low cost production of Ca- and Na-zeolites (the agro-grade zeolites), the present study is targeted to undergo a comparative evaluation of two different types of synthesis methods: (1) conventional hydrothermal activation under atmospheric pressure and (2) autoclaving the reaction mixture at autogenously set pressure and temperature under controlled conditions.