There are several ways to solve this; here are three.
1. Potassium chloride is soluble in water; diamond is not. Regardless of the size of the particles, diamond and potassium chloride both have a higher density than water, so they should sink. Being soluble, the potassium chloride will dissolve, but the diamond will not.
Mix them into an amount of water, and wait for the diamond to settle on the bottom. Remove the diamond, such as through a filter, and add to a fresh container of water, repeating this process until the potassium chloride concentration has been virtually eliminated. You could also switch to less soluble liquids, like acetone. Boiling off the solvent will leave the diamond.
2. A smarter version of the idea above would be to dissolve the powder in water, then use electrolysis to separate the ions.
2. One of the unique properties of diamond is its ability to withstand very high temperatures. The mixture could simply be exposed to very high heat, until the potassium and chlorine liquify or even boil, while the diamond remains solid.
You could also use a hydrocyclone, which is a sort of centrifuge that separates materials based on density.