**5. Summary conclusions**

Extensive resources of experimental work have been called on to elucidate the effects of solids grading and pipe inclination on settling slurry transport. If flowing in horizontal and inclined pipes, broadly graded slurries exhibit lower friction losses than narrowly graded slurries of the same mean particle size, mean flow velocity, and slurry density. The difference in friction loss is particularly important in slurries that are stratified. In broad-graded slurries, fine particles are able to reduce mechanical friction associated with sliding of coarse particles over a pipe wall by developing a thin layer effectively separating the coarse particles from the pipe wall. Also associated with flow stratification and with a presence of a sliding bed is the observed anomalously high friction loss in mildly negatively sloped flows. This anomalous friction loss reduces but does not disappear if the slurry is broadly graded. The reason for the loss reduction is the same as in horizontal flows.

The observed variations of friction loss with the degree of solids grading can be predicted with reasonable success by the 4-component model. A prediction of the observed variation of friction loss with solids distribution, as in the tested inclined flows, requires a layered model.
