The work is devoted to the development of technologies for the processing of anthropogenic residues of growing single crystals based on cesium and sodium iodides into high-purity raw materials for growing single crystals that are environmentally safe.
For the processing of anthropogenic residues of single crystals based on cesium iodide, a method of multiple low-temperature mixed crystallization (MLTMC) of aqueous solutions of cesium iodide obtained from anthropogenic residues was developed, and the effect of temperature on the efficiency of cleaning solutions from impurities of alkali metals, thallium, copper, sulfates, etc. was studied.
Carrying out repeated low-temperature mixed crystallization of cesium iodide solutions, which includes mass crystallization and crystallization of water-salt eutectic (WSE), is a simple and convenient method for significantly reducing the concentration of almost all anthropogenic impurities, including isomorphous ones (Na, K, Rb). However, in the process of crystallization the product is enriched with an impurity of thallium, which is included in the crystallized part of the solution.
It was established that when MLTMC is carried out at a temperature of -14 °C, sodium impurity interferes with the effective removal of other impurities from cesium iodide during crystallization. When the concentration of sodium ions decreases to (5÷9)·10-2 mas. % intensity of displacement of other impurities increases significantly.
The purification coefficients for the main impurities in the process of mixed crystallization of cesium iodide were evaluated. For Na+, the effective purification coefficient β is in the range of 0.6-0.7 for the first three crystallization cycles and then decreases to 0.5, the corresponding β values for potassium are 0.74 and 0.5-0.53. For rubidium, the β value is almost the same throughout all MLTMC cycles and is within 0.65-0.75. For copper, the β value is close to unity, for thallium β>1, which makes it impossible to purify the cesium iodide solution from Tl and Cu impurities by crystallization methods.
As the degree of supercooling of the solution increases from -14 °C to -19 °C, the value of β for isomorphic impurities (Na, K, Rb) approaches 1 (0.87-0.89) and practically does not depend on the radius of the impurity cation.
For the purification of anthropogenic solution of cesium iodide, 20 cycles of low-temperature mixed crystallization were carried out. Starting from the 13th cycle of low-temperature mixed crystallization the level of impurities of heavy metals, potassium and rubidium in cesium iodide becomes lower than the requirements of Technical conditions of Ukraine 24.13.31331736-002-2004.
After carrying out 25, 30 and 35-fold low-temperature mixed crystallization (LTMC), experimental samples of raw material - cesium iodide were selected and experimental growth of cesium iodide single crystals was carried out. Analysis of the raw material showed that rubidium is removed from cesium iodide better than potassium. Carrying out 30-fold crystallization makes it possible to completely, below the detection limit, get rid of rubidium impurities, the concentration of potassium is at the level of 1.5∙10-5 mas.%.
The developed MLTMC method makes it possible to purify anthropogenic aqueous solutions of cesium iodide to a level of purity that corresponds to the raw material for obtaining scintillation single crystals with a much higher yield of the final product than with three-fold mass crystallization according to standard technology: after 20 cycles of low-temperature mixed crystallization, the direct yield of CsI is 53 % versus 20-25% after three-fold mass crystallization.
The production capabilities of the MLTMC method are 100-150 kg/year of cesium iodide of high purity based on one "BEKO HSA 40520" type freezer.
The developed method makes it possible to obtain after 10-12 stages of MLTMC a product that meets the Technical Conditions for especially pure cesium iodide with a reduced content of potassium and rubidium - impurities that cause their own radioactive background in scintillation single crystals. It was established that the MLTMC method does not make it possible to clean the remains of single crystals of cesium iodide from copper and heavy metals, and the concentration of thallium impurity even increases. These impurities are particularly undesirable when growing non-activated cesium iodide single crystals.
The research results were implemented at the experimental production of the Institute of Scintillation Materials of the National Academy of Sciences of Ukraine.
