Generation of solids from the liquid phase by precipitation requires a contact between reagents at molecular scale. This is achieved in different types: of precipitators: batch, semi-batch (combined with single- or double jet feeding), or continuous. It is shown that the competition between mixing and primary nucleation is the main phenomenon, which influences both crystal concentration and size. Via two sensitivity-to-mixing indicators, simple general rules for the choice and design of precipitators are derived. This sensitivity is enhanced by the use of semi-batch devices, unbalanced flow rates or feed tank volumes and premixing.