A new approach to the prediction of particle separation by sieving in flour milling

Abstract

Sieving is probably the oldest and most widely used method for solid-solid separation. It is relatively rapid and simple and, therefore, a popular method, but it has not received the scientific attention it deserves. This could be due to the fact that in this unit operation the variables involved are numerous and, as a result, their interactions are so complex that no satisfactory method of evaluating and predicting the sieving process has yet been developed. This has led to the inefficient operation of industrial sieving equipment. It is, therefore, important to have a better understanding of the process to define the optimum operating conditions for a sieve or for a sifter cascade. It is also evident that the kinetic aspects of sieving have been particularly neglected, even though they govern the efficiency of the separation and the economy of the screening process at industrial scale. In practice, sieving is performed based more on experience and intuition rather than theoretical foundation. In flour milling breakage of particles is always followed by separation, and sieving is one of the main unit operations within the process. In this work the kinetic aspects of sieving are studied, and a new methodology based on mass balances is developed to predict the sieving performance. The main aim is to develop a graphical technique able to predict the quantity of undersize removal for a given period of time, or the sieving time and area necessary for a required undersize removal. The method uses mass balances in parts of the sifter sieves of the same aperture size, and is validated in a batch sifter.