Untersuchungen zur Eignung von natürlich alterierten sowie mit Oxalsäure aktivierten Bentoniten als Bleicherde für Pflanzenöle

Bleaching earths have been utilised for the refining of vegetable oils for more than 100 years. They are manufactured during the reaction between bentonite and acid (acid activation process). In the chemical industry the product of this reaction is used as a catalyst, in carbonless copying papers, or as a clarification agent of beverages. The acid activated bentonite is both a catalytic and adsorptive reagent. Therefore, this material has an extensive range of applications. A major disadvantage of both technical bleaching earths or acid activated bentonite production, is the output of large quantities of waste acid which needs to be disposed. The aim of this study was to investigate the possibility of producing an environmentally friendly bleaching clay, without compromising its efficiency. Two ideas were formulated: The application of oxalic acid as a suitable agent for acid activation. With the ecological advantage that the acid can be decomposed of biologically and/or chemically. The possibility of managing natural acid clays from the island of Milos, which were generated by natural acid alteration systems. Through using oxalic acid and strong mineral acids in the technical acid activation process the major principles of the smectite-acid-reaction were depicted. This modification of a bentonite can be monitored by N2-ad/desorption-measurements, IR-spectroscopy, proton affinity distribution, chemical analysis and cation exchange capacity determination. The applicability of the products were then further analysed in bleaching tests. The octahedral layering of smectites is dissolved during acid attack. The remaining tetrahedral layer forms an x-ray amorphous, in a sodium-carbonate-solution soluble and porous phases which provides most of the activity. The natural acid alteration of the milosbentonites are comparable to a technical reaction. However, in the natural system, dissolved cations like Al and Fe may remain within the bentonite. The activated clay represents a thermodynamically unstable material because of its trend towards recrystallisation of the silicious phase. If dissolved Al remains within the system kaolinite (a.o.) can be formed. In natural acid clays SiO2-phases with low crystallinity were identified (CT-Opalum). This mineral forms throughout geological periods from the remaining tetrahedral layer. Within 1,5 years it was not possible to crystallize CT-Opalum from a bleaching clay. However, the effect of time could be monitored by IR-spectroscopy, pore size distribution and the solubility of SiO2. Mineralogical-chemical characteristics of the investigated natural acid milosbentonites are rather different. These materials were classified by an „alteration-recrystallization-model“ with respect to mineralogical-genetic aspects as well as applicability. Using bleaching tests a high decolourisation ability with respect to soybean oil of some natural acid clays was detected. These materials are regarded as applicable for particular oils or a colour reduction step before bleaching. However, they cannot fully compete against technical bleaching clays. Kapitel 1: Einleitung Seite 1