METHOD AND APPARATUS FOR PREPARING PARTICLES

A method for preparing a target substance in particulate form comprises introducing a solution or suspension of a target substance and a compressed anti-solvent fluid into a particle formation vessel through separate first and second inlets so as to allow the anti-solvent fluid to extract the target substance from the solution or suspension and form particles of the target substance. The compressed anti-solvent fluid is introduced into the particle formation vessel at a sonic, near-sonic or supersonic velocity. The velocity of the anti-solvent fluid may be in the region of Mach 0.8-1.5 and the anti-solvent fluid may be introduced into the particle formation vessel as a single stream through a convergent nozzle. The anti-solvent fluid may be super-critical or near-critical and it may have a temperature and pressure sufficiently higher than the particle formation vessel as to ensure that Joule-Thomson cooling of the anti-solvent fluid as it enters the particle formation vessel does not lower its temperature to a level which affects the particle formation process. There may be two or more fluids carrying two or more target substances which are combined at the point of particle formation. The solution or suspension of the target substance and the anti-solvent fluid may meet, at an angle of 70{-110{, and mix immediately downstream of the point of anti-solvent fluid entry into the particle formation vessel. The particle formation vessel may be agitated. The particles formed may have a mean diameter of less than 1 žm and a standard deviation of 2.5 or less. An apparatus suitable for use according to this method comprises a particle formation vessel 1 with first 23 and second 21 inlets, the solution or suspension of the target substance and the compressed anti-solvent fluid entering the particle formation vessel at different points and meeting immediately downstream 22 of the second inlet.