When rock leaves a crusher with different particle-size fractions, there is no place for sensitive, easily damageable processing equipment or sensors. That’s why the operator of a major lime works decided to use a radiometric system from VEGA for precise mass flow measurement.
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The Lhoist Group is a world leading company that specialises in burnt and unburnt lime products. Headquartered in Belgium, the company is present in 25 countries with more than 100 subsidiaries around the globe. Rheinkalk is a Lhoist Group company in Germany that operates the Flandersbach plant in Wülfrath, the largest plant of the Lhoist Group and the largest lime works in Europe. Around 7.5 million tonnes of limestone are mined there every year. The production of unburnt limestone products in Wülfrath amounts to approximately 4.0-million tonnes. The raw limestone is either used as grit for road construction or it is ground and further processed.
A special hallmark of the Flandersbach plant is the very high quality of its burnt lime. Some of this material is further processed there, but approximately 2.0-million tonnes of it leaves the plant every year as an end product. Lime is known mainly for its use as a flux in steelmaking processes. However, lime and limestone products are also an indispensable raw material in drinking water and sewage treatment, in construction and agriculture, and in flue gas cleaning.
The quality demands placed on these products have grown steadily over the years. For example, the grain size specified by the customer must be strictly adhered to. At the same time, delivery periods are getting shorter and shorter. These factors, as well as many others, make it necessary to measure the material quantities much more accurately than ever before.
Precision feeding
Lime processing begins with the extraction of limestone from the quarry. The material is transported on large haul trucks to a processing plant where it is pre-crushed, washed, pre-sorted according to size, and distributed to various storage areas. From there, the prepared material is further refined or fed into burners. With the help of rotary and shaft kilns, the raw minerals are converted into lime products. At the end of the production process, the fired products are ready for a wide variety of applications.
Numerous conveyor belts with different lengths ranging from a few metres to one kilometre ensure that the limestone, which can be in a wide variety of different processing states, is transported to the right place for further processing. Ambient conditions are harsh. Operating at speeds of up to 4.0 m/s, the belts transport rock with diameters from 1.0 to 250 mm at delivery rates up to
4 000 t/h. Moisture, dust and dirt are the order of the day here. Nevertheless, exact measurement of this mass flow is required to ensure that downstream processes are fed with the right amount of material.
To measure the quantities passing through, mechanical belt scales are installed at strategic points in the conveyor belt systems. These 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 conveyor 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, 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 overflow detector. Overall, mass flow and density measurement are its two main applications in this industry.
The system consists of a scintillation detector and a radioactive source safely enclosed in a source holder. A minimally radioactive isotope 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 applications 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
Radiation-based mass flow measurement
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.
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. 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 detector is thus proportional to the amount of material on the conveyor and is translated into an output signal from the detector.
