| The DIN-Norm 51033 recommends the addition of anti-flocculants (e.g. pyrophosphate) to suspensions of inorganic raw materials to improve particle size determinations. The anti-flocculants adsorb onto the particle surface and increase the particle charge (zeta potential). The increased electrostatic repulsion of like charged particles leads to the elimination of particle aggregates and enhanced stability of the suspension. The recommended anti-flocculant concentration was reviewed with regard to its effect on particle charge and was found to be below the concentration required for optimum stability. The concentration for the creation of the highest possible charge also depends on the type of anti- flocculant and on the type of suspended material and no general recommendation as made in DIN-Norm 51033 can be given. The effect on the obtained particle size distribution was measured by laser diffractometry. Addition of anti-flocculants leads to the removal of aggregates. However it should be noted that in the manufacturing process when the raw materials are used no anti- flocculants are present. The size distribution measured after anti- flocculant addition gives therefore only little information about the actual size of the material in the production process. Comment A Malvern Zetasizer and Model 2600 particle size analyser are used to determine the optimum dosage of anti-flocculant for a variety of materials. It is suggested that the optimum quantity of anti-flocculant has to be determined for each material and anti-flocculant and that over dosing reduces the effectiveness of the anti-flocculant. 
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