MechChem Africa September-October 2023

⎪ Minerals processing and materials handling ⎪

The greater the loading or mass on the belt, the lower the radiation field at the detector. Conversely, the lower the loading or mass on the belt, the higher the radiation field at the detector. The amount of radiation seen at the de tector is thus proportional to the amount of material on the conveyor and is translated into an output signal from the detector. q wet, or dusty. What is more, it is mechanically robust and requires little maintenance. With radiometric measurement, level, density, mass flow, limit value and point level can be measured at a wide variety of locations: on conveyor belts or screw conveyors, or in bunkers, silos, basins or pipelines, for example. Although primarily designed for mass flow measurement, WEIGHTRAC 31 also works well as a position or limit switch and as an over flow detector. Overall, mass flow and density measurement are its two main applications in this industry. The system consists of a scintillation detec tor and a radioactive source safely enclosed in a source holder. A minimally radioactive iso tope emits focused gamma rays that penetrate the bulk material. The receiver, which is mounted below the belt, picks up this radiation. Because gamma rays are attenuated when penetrating matter, the receiver can calculate the rate of mass flow from the intensity of the incoming radiation and the speed of the conveyor belt. The speed can either be stored as a fixed value in the system or measured by an impeller. Thanks to its well-designed mounting frame, WEIGHTRAC 31 can easily be installed retroactively on conveyor belts. Since it is a non-contact measuring device, wear is not a problem. Radiometric measurement, which requires extremely little space compared to traditional mechanical belt scales, makes reliable measurement possible even in applica tions with fluctuating belt tension and strong vibration. It thus ensures exact balancing of the bulk solid material. The acquisition costs of a WEIGHTRAC measuring system are comparable to those of a belt scale. However, the maintenance costs of radiometric measurement are minimal after installation and commissioning. To be on the safe side, Rheinkalk first put the new measuring principle to test for several weeks on a running conveyor belt. The results were convincing, so the company decided to leave it in place. But that’s not the end of the story: three more radiometric systems from VEGA have been installed in the meantime and a fifth and sixth are to follow. q

The PROTRAC series from VEGA offers a variety of sensors that can be optimally adapted to any radiometric measuring task. They are used on conveyor belts or screw conveyors, bunkers, silos and pipelines.

The inclusion of a WEIGHTTRAC mounting frame makes it easy to retrofit the measuring system onto any conveyor system. In this application, the source is mounted above, and the detector mounted below.

scales are, however, susceptible to dirt and grit and require a lot of maintenance. They must be checked and cleaned every few weeks, at considerable cost. In addition, belt scales take up a lot of space, requiring pre-run as well as post-run sections, and they must be set up perfectly level. Many con veyor belt scales, therefore, do not function

optimally despite intensive maintenance.

The VEGAS WEIGHTRAC 31 In May 2014, VEGA recommended a trial measurement using WEIGHTRAC 31, a sensor based on the radiometric measuring principle. This sensor has proven to be particularly useful in applications that are heavily soiled,

A radiometric sensor consists of a sealed radioactive source in a source holder and a scintillation detector. The source and detector are mounted on opposite sides of the conveyor (belt, screw, drag chain, or vibrating). In some applications, the source is mounted above, and the detector is mounted below, while in other applications the detector is mounted above and the source below. Radiation-based mass flow measurement

In either case, a fan-shaped collimated beam of radiation is transmitted from the source through the process material and the conveyor to the detector. As radiation passes through matter, its field strength weakens. As the loading of the material, or total mass per square foot, on the belt or screw conveyor changes, the amount of radiation reaching the detector changes.

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