A method of separating solids, the method comprising: adding said solids to a suspension of particulate material comprising magnetic, or magnetised, particles in a liquid, and passing said suspension into a separation vessel having an inlet , an underflow and an overflow , the particles of said particulate material having a nominal coarseness determined as a function of at least one of the size of the separation vessel, the particulate material shape and type, the solids particle size and type, the feed pressure of the combined solids and suspension, and a desired specific gravity of said suspension in the absence of said magnetic field; locating the combined solids and suspension in a separation vessel such that rotation is imparted to the combined solids and suspension around a space bounded by an outer wall of the vessel to impart a centrifugal force on the solids; applying, during operation of said separation vessel, a magnetic field to said combined solids and suspension in said separation vessel; characterised by the steps of :- a. increasing the coarseness of the particles of said particulate material from said nominal particle size by a predetermined amount; b. determining the density differential as a function of the magnetic field and identifying the magnetic flux density required to reduce the density differential to a predetermined optimum value; c. determining the density cut point and error of separation at said magnetic flux density; d. further increasing the particle coarseness in predetermined steps and repeating steps (b) and (c) until the error of separation increases; and e. determining the maximum particle coarseness used before the error of separation increased and using said maximum particle coarseness for subsequently separating solids while applying a magnetic field to the combined solids and suspension in the separation vessel. adding said solids to a suspension of particulate material comprising magnetic, or magnetised, particles in a liquid, and passing said suspension into a separation vessel having an inlet , an underflow and an overflow , the particles of said particulate material having a nominal coarseness determined as a function of at least one of the size of the separation vessel, the particulate material shape and type, the solids particle size and type, the feed pressure of the combined solids and suspension, and a desired specific gravity of said suspension in the absence of said magnetic field; locating the combined solids and suspension in a separation vessel such that rotation is imparted to the combined solids and suspension around a space bounded by an outer wall of the vessel to impart a centrifugal force on the solids; applying, during operation of said separation vessel, a magnetic field to said combined solids and suspension in said separation vessel; a. increasing the coarseness of the particles of said particulate material from said nominal particle size by a predetermined amount; b. determining the density differential as a function of the magnetic field and identifying the magnetic flux density required to reduce the density differential to a predetermined optimum value; c. determining the density cut point and error of separation at said magnetic flux density; d. further increasing the particle coarseness in predetermined steps and repeating steps (b) and (c) until the error of separation increases; and e. determining the maximum particle coarseness used before the error of separation increased and using said maximum particle coarseness for subsequently separating solids while applying a magnetic field to the combined solids and suspension in the separation vessel. <EOS>