SOLID PROPELLANTS FOR ROCKETS: A METHODOLOGY TO OBTAIN HIGH PURITY KNO3 FROM AN INEXPENSIVE SOURCE

Solid propellants are mainly used in rocket propulsion applications. The performance of a rocket is directly affected by the design of the solid propellant used. The presence of impurities in the propellant formulation can reduce the average thrust, specific impulse and characteristic velocity of a rocket. Oxidizers are the main compounds in propellants formulation, which produce the high energy on combustion. One the most commonly used oxidizers in amateur rocketry is potassium nitrate. KNO3 is normally used because of its good characteristics that include compatibility with others ingredients and availability at low cost in fertilizer formulation. The main objective of this work is to propose a methodology to obtain higher purity potassium nitrate from a fertilizer, an available and inexpensive source that can be used as oxidizer in a solid rocket propellant formulation. The KNO3 crystals were obtained by recrystallization process using a commercial fertilizer. The properties of purified fertilizer were compared with the properties of KNO3analytical. The fertilizer, purified and analytical grade KNO3 samples were evaluated by means of morphology, pH of saturated solution, Fourier transform infrared (FTIR) spectroscopy, and differential scanning calorimetry (DSC). The results demonstrated that recrystallization process effectively remove impurities from the fertilizer. The purified KNO3 crystals were optically transparent with a planar shape, similar to higher purity KNO3. The FTIR and DSC results showed that some impurities, such as, sulfur and alkaline compounds were removed after recrystallization and the purified KNO3 presented a similar behavior than higher purity KNO3. The recrystallization process used in this work can successfully used to obtain KNO3crystals with high purity that can be used as oxidizer in solid propellants for rockets.