In this study, the blocking filtration laws introduced by Hermans and Bredée are applied to evaluate filter aid filtration data. A model system is utilized for the filtration experiments, consisting of barium sulphate as the model impurity and diatomaceous earth as the filter aid. Based on the analysis of the filtration data, the relevant process laws are initially identified as cake filtration for the separation of the diatomaceous earth and the so-called missing law for the separation of barium sulphate. The results are substantiated with the aid of X-ray microscopy to monitor the incorporation of the fine particles in the filter aid structure. They demonstrate that the separation of barium sulphate is characterised by both depth migration and accumulation within the growing filter aid structure. The blocking filtration laws derive from a general exponential relationship between blocking rate φ and hight-specific resistance r with the blocking exponent n. Hermans and Bredée presented first the solutions for the four blocking exponents n=2 (complete blocking), n=1.5 (standard law), n=1 (intermediate law) and n=0 (cake filtration). Gonsalves later supplemented the solution for n = 0.5 from a purely logical point of view and called it the missing law, as he could not find a model behind it himself. Thus, for the first time, this study assigns a separation principle to the missing law, which until now has only been discussed theoretically.