Study of the reaction between dolomite and nitric acid

This paper is focused on the kinetics of the reaction between natural dolomite and diluted solutions of nitric acid at various temperatures. All experiments were carried out in a semi-batch reactor with an approximately constant nitric acid concentration using the pH-stat titration method. The reaction was studied in the temperature range from 293 to 353 K and the nitric acid concentration in the range from 0.001 to 0.200 mol dm-3. The strong effects of both temperature and nitric acid concentration were observed. The determined fractional values of the apparent reaction order (0.39 to 0.75) indicate a very complex reaction mechanism. In the studied concentration range, the values of the apparent activation energy increase from 30 to 58 kJ mol-1. Based on these data it can be assumed that the reaction takes place in the transitional regime with the stronger influence of chemical reaction. This assumption was verified by calculation of the nitric acid concentration on the surface of the dolomite grain using Sherwood criterion equation. The change in the role of the rate-controlling step was found for highly diluted solutions (< 0.010 mol dm-3 HNO3)

Microstructural Comparison of Porous Oxide Ceramics from the System Al2O3–ZrO2 Prepared with Starch as a Pore-Forming Agent

In this paper we show examples of microstructures of porous oxide ceramics prepared by traditional slip casting (TSC) and starch consolidation casting (SCC) and present results obtained using different microstructural characterization techniques; Archimedes method (open and total porosity), shrinkage measurement, mercury intrusion porosimetry (pore size distribution) and microscopic methods – optical microscopy with microscopic image analysis (pore size distribution) and scanning electron microscopy (detailed investigation of the local microstructure). In particular, microstructures are compared for porous ceramics from the system Al2O3–ZrO2 prepared with rice and corn starch. It is shown that maximum values of the total porosity of porous ceramics prepared with starch as a pore-forming agent were approx. 50%. A major finding by using SEM with respect to starch-produced porous ceramics is the existence of pore fillings in the form of small sintered ceramic shell inside the pores, as a result of starch granule shrinkage during the drying and burn-out steps