Spiral Jet Mill Research Articles

Modelling and control of a jet mill plant

Although jet milling is a very energy consuming grinding process it is increasingly used in industry because very fine grinding product with a narrow size distribution is attained without contamination as the milling occurs by inter particle collisions. At Delft University of Technology a project has been started to achieve a considerable energy reduction in jet milling processes. The grinding plant consists of a spiral jet mill in closed loop with an external classifier. Main feature of the system will be an operation control based on in-line particle size measurements using laser diffraction. In industrial practice the operating conditions are often determined by trial and error. To avoid off-spec material the mill is often tuned to lower risk. This results in a relatively large amount of overground material. With respect to the control strategy, the first step was real time particle size monitoring to explore the operating ranges of the jet mill. The controlability is studied in relation to several process inputs and process configurations. A dynamic model of the closed loop grinding plant is developed. Particle transport and size reduction inside the mill show a stochastic behaviour and are described by size and state dependent probability functions. Separate experiments are carried out to derive equations for the breakage kinetics of different materials. Numerical flow simulations are carried out to provide statistic data about the frequency and intensity of collisions between particles in relation to state conditions in the mill. A glass bottom plate will be placed on the mill to observe flow patterns. The influence of several process input variables on the dynamics of the grinding plant and the final product are simulated. Pilot plant experiments are carried out to verify and optimize the dynamic model by direct measurement of the PSD under actual system conditions. The ultimate objective of the dynamic model will be the implementation in a control system. The required setpoints of the mill/classifier system are predicted to obtain the desired product quality at minimum energy use. Furthermore the model can be used for scale up and plant design.

Micronization of pharmaceutical substances in a spiral jet mill

Many studies have been conducted to help understand the effects of the variables involved in jet milling. The first part of this work is an attempt to summarise the results published in the literature concerning horizontal and vertical jet mills. This focuses on the research of the optimal design of the mills and on the improvement of their performance. Several publications have been found, concerning mineral grinding. It seemed more interesting to present results on organic crystals' jet milling in the second part of this paper. The experiments concern three organic substances, and were run on three different spiral jet mills: Chrispro-Jetmill 50 and 100 and MicronMills 8″. The results are presented in terms of specific energy consumption with an adaptation of the correlation proposed by Sarma. These representations show that, within the operating energy range and above a critical energy value, the creation of specific surface area corresponds to an increase of fine particles production.

Effects of grinding parameters on product fineness in jet mill grinding

The influence of several grinding parameters on the product fineness in jet mill grinding was studied experimentally with a laboratory scale spiral type jet mill. The most significant variables in the jet mill grinding were feed rate, volumetric flow rate of grinding air and the height of an inside classification tube for grinding at constant pressure. Results showed that the height of the inner classification tube (vortex finder) is also a relevant variable in a spiral jet mill, and should be taken into account in optimising; the process. The finest product size was obtained with smallest material feed rate, highest grinding air flow rate and with the shortest classification tube.

Spiral jet mills: hold up and scale up

The comminution characteristic of the spiral jet mill is a super-position of the comminution as such and the sifting effect of the vortex dominating in the mill, thus resulting in a maximum of the load at mean specific milling energies. At higher specific energies the load in the mill decreases, leading to a broad particle size distribution. The residence time was measured with radioactive tracers and the load by the hold up of the mill. The decrease at low energies corresponds to instabilities of the slowly rotating vortex. The effects on the comminution are also evident from the ground particle size distributions. It is possible to monitor the milling process by measuring the pressure variation. Initial instability of the vortex corresponds to pressure variations, therefore the feed has to be reduced until stable operating conditions are achieved again. Based on the observations mentioned above, a scale up model which has already repeatedly proved effective in practise is introduced. It is based on the same cut size at the vortex finder diameter. The circumferential speed of the vortex has to determined experimentally, depending on the product properties.