A combined process for the conversion of solid starting particles into solid intermediate particles and reducing the median diameter of the intermediate particles to obtain product particles. This process involves flowing a suspension of starting particles through a series of at least two conversion vessels, thereby converting at least part of the starting particles into intermediate particles, adding a supercritical fluid to one or more of the conversion vessels, thereby forming a supercritical suspension, and releasing pressure from the supercritical suspension, thereby expanding the suspension and converting the intermediate particles into product particles.