Image Analysis And Modeling Of The Dynamic Mixing Process Of The Material In Rotary Drums

Rotary kilns, ball mills and other process equipments with rotating cylinder asmain component are widely used in chemical, metallurgical, mining and otherindustries. However, the running efficiency and automation level of those processequipments is not satisfactory compared with those in developed countries. This ismainly caused by the complexity of the coupled sub-processes occurring within therotating cylinder, including material mixing, gas flow, gas flow, chemical reaction,heat and mass transfer, among which the mixing rate and mixing cont ent of thematerial is determinative to the heat transfer efficiency and product quality. Hence, itis theoretically and practically meaningful to investigate the dynamic mixing processof the material and establish an appropriate model to describe the pro cess.Up to now, the main methods used to study the material mixing process in arotating cylinder is physical experimental method and the so called Discrete ElementMethod(DEM). DEM is based on the discrete dynamics theory and is advantageousfor simulate the track of individual particles and visualization of the tracks. However,the computation capacity of DEM is rather limited and simulation results are sensitiveto input parameters, so that the results must be validated by experimental data.Therefore, physical experiments are still is an important approach to study thedynamic mixing process of the material. Image-based measuring and analysistechnologies are especially welcome in the experimental study due to its availability,low costs and convenience.In this work, the dynamic mixing process of the material is investigated on a coldstate rotating cylinder. Image-processing technology is applied to measure the localconcentration variation of the particle as well as the contact pixels between differe ntlycolored particles. The time dependence of the concentration variation and contractpixels are measured and compared to each other. A least-square curving fitting methodis used to establish a model for the mixing process. It is found that the mixing p rocesscan be well described by one-order system. Based on the model, the mixing time andmixing rate are defined quantitatively and equations are given to calculate these twofeature variables. The model is scaled and tested by data both from our ownmeasurements and literature data. The influence of the rotating speed and fillingdegree of the cylinder is also studied. Under the experimental conditions of this work, the mixing time shows little increases as the filling degree increases, but it decreaseswith increasing rotating speed.Additionally, the relationship between the material mixing process and heattransfer process inside an outer-heated rotating drum is investigated. The temperaturechanges of the material of different depths is measured using radio wirelesstemperature measuring system, and is then compared to the profiles of the materialmixing process. Results show that, as the rotating speed increases, the mixing rate isimproved and the material temperatures approach faster to the steady state. The imageanalysis methods and the material mixing model proposed in this work is meaningfulto the design and operation optimization of cylinder-type process equipments.