Agitation method, agitation mixer and feed pipe structure

An agitation method for mixing a solution and a solvent to precipitate a solid substance from the solution. The method includes preparing an agitation mixer 10 including an agitation vessel 11, an impeller 12 rotated in the agitation vessel, and a feed pipe 15 connected to the agitation vessel and having a multiple pipe structure including an outer pipe 20 and an inner pipe 21 arranged in the outer pipe. A shearing clearance C is formed in the agitation vessel between the impeller and the feed pipe. The method further includes shearing the solution and the solvent by rotating the impeller to precipitate the solid substance while feeding the solution and the solvent into the shearing clearance from the outer pipe and the inner pipe. The method is particularly suitable for mixing a solution and a poor solvent that precipitates a polymer composition, especially a phosphorylcholine base polymer, dissolved in the solution. In use, the outer pipe feeds one of the solution and the solvent to the shearing clearance and the inner pipe the other and the solution and the solvent initially come into contact with each other in the shearing clearance.