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While the published research papers describe preparation of amorphous silica from water glass or using special methods (e.g. hydrolysis of alkoxides), the present study demonstrates a feasible solution for the cheaper synthesis of highly reactive silica using two-stage (acid and alkaline) leaching of serpentinite (Mg3Si2O5(OH)4). The alkaline leaching, the purification of sodium metasilicate solution and the effect of impurities on the precipitation of amorphous silica under the conditions of maximum sol stability using HCl, CO2 and (NH4)2CO3 as the precipitating agents were examined in more details. The prepared samples of silica were of high purity (99.4 % SiO2), did not contain residues of the original raw serpentinite, and the size and shape of the particles were given by the conditions of precipitation. The presence of impurities in the sodium silicate solution had a beneficial effect on the specific surface area - in all cases the values for SiO2 powders prepared from serpentine were higher than those of SiO2 prepared from a synthetic solution of Na2SiO3. The specific surface area of SiO2 samples synthesized under alkaline conditions has been much more affected by the presence of impurities if compared to that achieved by acidic precipitation.

Leaching of particulate dead-burned magnesite with hydrochloric acid at 45 °C was investigated with special regard to the effectof acid concentration (from 0.1 M to 4.8 M) on the rate of chemical dissolution of magnesium, calcium and iron. The leaching process wasfound to be mostly selective in the initial stage and the differences in dissolution rates decreased with an increase in the fraction of deadburnedmagnesite reacted.

Thorough kinetic analysis of the magnesia leaching based on measuring the changes in lixiviant composition revealed that there are differences between the actual leaching behaviour and predictions obtained using the non-porous shrinking particle model (NSPM). A detailed study of the morphology of the solid during the leaching process using SEM showed that observed deviations from the NSPM model depend on the particle size; chemical (mineralogical) composition and the polycrystalline nature of magnesia.

Slovakia holds 4th to 5th position in the world with respect to the supplies and mining output of magnesite. The most important product of natural magnesite processing is magnesia. While dead-burnt magnesia is used for producing basic refractory materials for metallurgy and cement industry, caustic calcined magnesia (CCM) founds new applications in industries, agriculture and environment protection. Use of CCM, due to its alkaline nature, as a reactive material for remediation of wastewaters, can provide all advantages of methods based on neutralization (pH adjustment) by lime. In this study, CCM was experimentally tested as a reactive material for the removal of cations Cu2+, Zn2+ and Ni2+ from wastewaters. The effects of chemical composition of model wastewaters tested and water-to-CCM weight ratio were investigated. The results have shown that CCM is a suitable material for the removal of heavy metals, especially copper, zinc and nickel, from acid wastewaters. The efficiency of cation removal up to 100% has been observed.