One of the main tasks of process monitoring, system control and quality assurance in various branches of the industry is to determine the grain size distribution of granular material. The range of tasks extends from pharmaceutics and food production to chemistry, as well as to construction and mining industry. There is a central role to processing of mineral raw material, particularly in mining, since here the processes of crushing, sorting and grading, which are directly linked to the distribution of grain size of the material, can be optimized, supervised and controlled. The capturing the grain spectrum of a material mixture before or after each process step is usually done through sampling and analysis through well-established screening machines in the laboratory, often preceded by preparation of the specimen, for example drying.
Since it is difficult to determine grain size of dust and ultrafine grains, laser-based analysers, which are integrated in the laboratory or partly in the process chain, have been developed. However, the time-consuming and costly sampling from the process cycle with subsequent laboratory analysis, as well as the limited range of application and the sensitivity of optical methods call for the development of a robust measuring system for use in a broad range of application for the online analysis of the grain size distribution of granular material. The development and application of a new method to determine grain size is based on pulse-induced vibration (Fig. 1).
During its transport in various media collisions of granular material with solid bodies generate impulses, that evoke oscillations in those solids. The characteristics of the oscillations are directly proportional to the pulse intensity, and in case of the same density of the granulate material, they are directly proportional to the grain size. The correlation of the inter-class time-bound intensities of the signals results in a distribution, which is directly proportional to the distribution of the grain spectrum that has generated vibrations in the analysed timeframe. A pro-active one-time calibration of the system allows for the assignment of signals to the respective grain categories, making an online grain size analysis possible.
The new development is available for on-line particle size analysis in bulk flow on belt conveyors and in hydraulic or pneumatic tube conveying.
The device is used for process monitoring and control as well as quality assurance for all processing operations such as crushing, sorting, classifying and refining. Currently several plants are in industrial operation:
• Plant control in the quartz sand processing - hydraulic tube conveyor
• Control of processing plants of a sand-and-gravel operation - conveyor belt (Fig. 2)
• Laboratory analysis of sand and gypsum products - pneumatic tube conveyor
The new development offers a variety of technical and economic benefits and is also used for humanization of the workplace by improving occupational safety and health.