**4. Conclusions**

Using various analytical techniques, we determined the chemical composition of several soda lime samples: fresh sample, exhausted sample after use in hospital and three samples after carbon dioxide absorption under conditions imitating semi-closed circuit apparatus. Thermogravimetric and XRD analyses comprehensively described the thermal properties and decomposition ways of the investigated samples. This product decomposed in a stable manner, releasing water and carbon dioxide. It is possible to recycle and reuse soda lime in different forms; however, the calcination process would require relatively high temperatures. On the other hand, high temperatures would ensure the biological neutrality of recycled soda lime. Calcium oxide itself could be reused in many different areas, e.g., in absorption and desiccation, in the construction industry or in the manufacturing of chemicals.

Soda lime is a fairly efficient carbon dioxide absorbent that has been used for a long time. It has, however, some limitations and drawbacks that require further investigations, as it is a product used in environments where dependability is a factor of grea<sup>t</sup> importance. One of the issues that should be addressed is possible interactions between absorbent and anesthetic gases, which can lead to the release of harmful compounds. Another limitation is the speed of carbon dioxide absorption, which is the highest at the beginning of the process and slows down relatively fast. On the surface of the soda lime granules, water forms a film less than three molecular layers thick, and the reaction rate is reduced [21]. As the carbonation proceeds, the product particles precipitate on the surface of Ca(OH)2 and cover it a thicker, porous deposit layer, which inhibits the exchange of reacting species between the surface of calcium hydroxide and bulk solution. Therefore, the di ffusion rate of reacting species is an important factor a ffecting the final stage of carbonation. The carbonation of Ca(OH)2 was observed to stop before one hour. However, carbonation may go on by di ffusion through the covering layer, but its rate is too slow to be detected in the range of carbonation time used [28]. Thorough research on soda lime properties is an important step in the evaluation of its performance as an agen<sup>t</sup> responsible for preventing carbon dioxide poisoning. Lack of data concerning the kinetics of this process causes this problem to be still very interesting and important in both anesthetic and medical science. Such data would provide a reliable tool to compare di fferent types of absorbents and thus would allow proper absorbent choice taking into consideration all other aspects of the environment or apparatus. Our research, only to some extent, covers the main problems of soda lime use. Additional further studies must be performed in order to ensure the required level of safety and e fficiency and to determine the best recycling method.

**Author Contributions:** Conceptualization, A.C., P.A., P.S. and B.R.; methodology, A.C., P.A., P.S., B.R.; validation, A.C., P.A. and P.S.; formal analysis, P.A, B.R.; investigation, A.C., P.A., P.S., B.R.; resources, A.C., P.S.; data curation, P.A., B.R.; writing—original draft preparation, P.A., B.R.; writing—review and editing, A.C., P.A., P.S., B.R.; visualization, P.A., B.R.; supervision, A.C.; project administration, A.C., B.R; All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
