Dissolution of Iron During Biochemical Leaching of Natural Zeolite

Natural zeolite, including clinoptilolite, often contains iron and manganese which decrease the whiteness of this sharp angular material.The biological treatment of zeolite enables its use as an substitute for tripolyphosphates in wash powders which have to comply with strict requirements as far as whiteness is concerned and rounded off grain content. Insoluble Fe3+ and Mn4+ in the zeolite could be reduced to soluble Fe2+ and Mn2+ by silicate bacteria of Bacillus spp. These metals were efficiently removed from zeolite as documented by Fe2O3 decrease (from 1.37% to 1.08%) and MnO decrease (from 0.022% to 0.005%) after bioleaching. The whiteness of zeolite was increased by 8%. The leaching effect, observed by scanning electron microscopy, caused also a chamfer of the edges of sharp angular grains. Despite the enrichment by fine-grained fraction, the decrease of the surface area of clinoptilolite grains from the value 24.94 m2/g to value 22.53 m2/g was observed. This fact confirms the activity of bacteria of Bacillus genus in the edge corrosion of mineral grains.Removal of iron and manganese as well as of sharp edges together with the whiteness increase would provide a product suitable for industrial applications

Study of the formation of acid‐based geopolymer networks and their resistance to water by time/temperature treatments

International audienceAcid-based geopolymers are known for their high temperature resistance and good mechanical performances. Some compositions also exhibit water resistance, which make them suitable for outdoor applications. This work aims to understand accurately the steps occurring during the geopolymerization process, and focuses on the type of networks forming at early stages of the reaction. The sample, prepared from metakaolin and phosphoric acid, has been analyzed at different durations and temperatures during its solidification, in order to identify which compounds would crystallize. The structural characterizations have been performed using Fourier Transform InfraRed spectroscopy and X-Ray diffraction. Several Al-O-P compounds and especially AlPO4 polymorphs crystallize in these conditions. They belong to three families of aluminophosphates, and are a reliable indication of the amorphous organization of the original sample. The first one is similar to the phases that appear in the phase transition sequences of silica, the second one is composed of AlPO4-based zeolites, and the last one is characterized by the presence of aluminum in an octahedral network. The transitions between the different phases are also discussed. Finally, the water resistance of the calcined samples is improved, and the release of acidic species in the water is limited even after a calcination at low temperature