Refinement of 4,6-diaminoresorcinol by recrystallization in hydrochloric acid

4,6-Diaminoresorcinol is the most important monomer used for the synthesis of poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers. Method of recrystallization to get highly purified 4,6-diaminoresorcinol dihydrochloride (DRDH) was studied. The laser was used to measure the solubility and supersolubility of the DRDH in hydrochloric acid of different concentration and temperature. The analysis of the thermodynamic characteristics of the DRDH in hydrochloric acid is the theoretical guidance for its crystallization techniques. Furthermore, the effects of factors, such as the volumetric ratio of hydrochloric acid to crude crystalloid, the recrystallizing time and temperature, the cooling scheme and the stirring speed on the process of recrystallization were studied, and the suitable technical conditions for the recrystallization are found as following: the proper concentration of the hydrochloric acid used as solvent is 3.5 mol·L^-1, the volumetric ratio of hydrochloric acid to crude crystalloid is 20:1, the beginning crystallizing temperature is controlled above 40°C and is gradually decreased to -20°C slowly, the stirring time is maintained 10 minutes with a speed of 80 r·min^-1, the crystallizing time is prolonged for 9 h and then recrystallized once more. According to above suitable conditions, the highly purified DRDH can be gotten.