Crystallization cancrinite

Cancrinite Crystallization from Alkaline Aluminosilicate Systems Containing Large and Small Cations... 

Cancrinite crystallization, promoted by the joint presence in the reaction system of lithium and a large cation, has been thoroughly investigated, pointing out the specificity of the cationic couple for the synthesized phase. The products obtained have been characterized by means of X-ray diffraction, chemical, microscopy and thermal analyses. A hypothesis on the mechanism of cancrinite formation has been worked out, also on the basis of the preliminary results of structural analysis. 

The oxide batch composition tested for cancrinite crystallization was as follows ... 

It looks like Cs+ and Tl+ are somewhat interchangeable, so that an.optimal ratio between them for cancrinite crystallization is not easily deducible. It should be noted that runs with prevailing cesium give pollucite cocrystallization, while those with more thallium also give rise to the formation of Tl-C (see above) ... 

The relative products have been referred to as Li,Cs-C, CAN Li,Tl-C, CAN and Li,Tl,Cs-C, CAN, respectively. Figures 1 and 2 report in particular two scanning electron micrographs of the second ar.d the third samples, in which the exagonal cross section of the prismatic cancrinite crystals is well visibile. 

Figure 1. Scanning electron micrograph of cancrinite crystals. Sample Li,Tl-C (see synthesis section).

Cancrinite crystallization from mixed cationic systems is an example, certainly not generalizable, of the determinant role played by the cations in framework silicate crystallization. In the present case the specificity is even double, because either the small cation (Li+) or the large one (Cs+,T1+, or Rb+) are essential for the synthesis and a role for each of them has been hypotized. The completion of the structural analysis, now in progress, may confirm the hypotheses, explaining also some still unclear points. 

A low-temperature process for conversion of radioactive sodium salt wastes into solid, relatively insoluble, thermally stable sodium aluminosilicates is described. The reaction of the waste (in aqueous solution) with powdered clays such as kaolin, bentonite, halloysite, or dickite produces small crystals (0.5 /xm) of cancrinite. Salts, including radioactive ones, are trapped in the cancrinite crystal lattice. The approximate chemical formula of the cancrinite produced is 2(NaAlSi04) x salt y H2O, with X = 0.52 and y = 0.68 when the entrapped salt is NaNOa. The stoichiometry requires two moles of NaOH for each mole of cancrinite formed. 

The mechanical strength of the product can be improved by adding hardeners or binders. Various inorganic cements, including Portland cement, can be used to bind the cancrinite crystals to make a stronger product. Organic polymers can also be used. Binders appear to be most effective when the cancrinite is made from fired clays in which the clay structure has been destroyed. Various alternatives for the process are presented these depend on whether or not a binder is necessary and at what point in the process a binder is added. 

X-ray diffraction of the products from the above experiments show crystalline cancrinite in each sample from the bentonite reaction (the first sample was taken at 15 min). However, no cancrinite is observed in the kaolin reaction until 60 min has elapsed. The induction period for cancrinite crystal growth is, therefore, shorter with bentonite as the reactant. This may be associated with the rate of dissolution of the clays. 

Hardness. If the solid waste form is to be handled or transported, it must be strong enough to prevent chipping and dusting of small particles from the bulk. Pure cancrinite is quite hard (5-6 on Mohs scale). However the product of clay—waste reactions is a mixture of small cancrinite crystals and unreacted kaolin (2.0-2.5 on Mohs scales). The hardness of this product is, therefore, limited to that of kaolin. Also, the cancrinite product containing excess kaolin clay as a binder has been observed to soften somewhat when covered with water. If the cancrinite product contains excess bentonite clay, the product will swell and crumble when... 

Cancrinite crystals were used as aggregate in Portland type III cement mixtures to bind the crystals. Hard products were obtained only when the cancrinite was prepared from fired clays. It appears that any unreacted clay in the cement mixture will cause the cement to crumble when air dried. This may be due to shrinkage of the clay upon drying. 

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