Magnetisch abtrennbare Mikroartikel mit einer silikatischen Hülle, Verfahren zu ihrer Herstellung sowie ihre Verwendung

The invention relates to magnetically separable particles, each comprising a plurality of nanoparticles with reversible magnetic properties and a matrix having an inorganic portion between 70 and 100 wt%, relative to the weight of the matrix, wherein the inorganic portion of the matrix consists of 80 to 100 wt% SiO2, characterised in that a. the particles have an average diameter in at least one direction of at least 5 mum, b. the particles have a surface area (according to BET measurement) of at least 1 m2/g and a (cumulative) pore volume of less than 0.195 cm3/g (ml/g), c. there is a loss of material from the nanoparticles with reversible magnetic properties of less than 2 wt% when the magnetically separable particles are stirred for 12 hours in acidic and alkaline solutions with a pH value of 1 and 12 respectively. These particles can be produced by a method comprising the following steps: a. provision of a fluid magnetic sol which was peptised by lowering the pH value to a value of not more than pH 2.5 by adding an acid; b. stabilising the sol by adding an organic complexing agent; c. adding ammonia or a compound having at least one amine function, or releasing such a function under the effect of thermal energy, to the stabilised sol in a quantity such that - optionally after the effect of the necessary thermal energy - a pH value of at least 9 is achieved; and d. effecting a precipitation of the magnetically separable particles by adding a silicon dioxide-forming agent to the stabilised sol, wherein the organic complexing agent is selected from monomeric and oligomeric hydroxycarboxylic acids with 1 to 3 hydroxy and 1 to 3 carboxylic acid functions, di-or oligocarboxylic acids and bis- or oligoalcohols with at most 8 functional groups suitable for complex formation, or the organic complexing agent is selected in such a manner that, after addition of the ammonia in accordance with step c., a measured hydrodynamic radius of the particles present in the sol in the range from 200 to 2000 nm, preferably 500 to 2000 nm results.