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| Raschman P. |
| UNCERTAINTY OF KINETIC LEACHING DATA INTRODUCED BY APPLIED EXPERIMENTAL PROCEDURE |
Abstract
Reliable kinetic data is important for an engineering, as well as for a scientific analysis of the leaching process. The present paper is focused on the methodology of measuring the kinetic data in reacting suspensions - chemical reactions of granular or powdered solids with liquids play a significant role not only in metallurgy and technical chemistry, but also in materials- and environmental engineering.
Since the overall rate of leaching is always a result of interactions between chemical reactions and transport phenomena, accuracy, validity and adequacy of the measured kinetic data are strongly affected by the factors related to the applied experimental procedure. Contacting of the phases in the reaction mixture and the liquid-phase hydrodynamics are determined by the choice of an appropriate reactor - simple, perfectly mixed reactors are therefore recommended. As far as the reaction conditions are considered, variation of the values of rate-controlling parameters (such as mass or heat transfer coefficients, or leaching agent concentration) should be under control - e.g. the values of kinetic parameters can be calculated much more easily when concentration of the leaching agent does not vary significantly during individual leaching tests. Though it is simple to measure time itself, some difficulties may arise when the actual reaction (leaching) time is to be measured - total time which is necessary to withdraw a sample from the reaction suspension and stop the reaction should be much shorter than the characteristic time of chemical reaction (e.g. half-time of reaction), otherwise it can become a source of a significant systematic error. The rates of intrinsic chemical reactions are very sensitive to temperature - temperature difference 1K, caused by the heat of reaction, may result in a relative error about 10%. The latter fact is also valid for the rates of chemical reactions taking place at the liquid-solid interface and isothermal conditions in the whole leaching system are therefore necessary to obtain accurate and valid kinetic data. The results of individual measurements are commonly presented in terms of conversion-time curves - adequate analytical methods should be therefore used to determine the compositions of liquid and solid phases before, during and after leaching.
Using experiments and mathematical modelling simultaneously is recommended. Adequate model equations can be used to analyse the effects of individual factors on the overall rate of leaching and to design leaching experiments - more accurate values of model parameters can be obtained more easily using this method, if compared with a "traditional" approach based on carrying out the measurements and consequent processing of the data collected.
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| Keywords: leaching|kinetics|experiment|measurement|sources of inaccuracy| |
| No 4 (1999), p. 267-274 |
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