A diamond in level measurement
June 2017, Level Measurement & Control
Reliable level measurement under extremely harsh conditions is now possible, thanks to 80 GHz radar level sensors. This article explains how one operator at a diamond ore processing plant was able to gain significant advantages by switching to radar level measurement at 80 GHz.
The dense media separation (DMS) process is a special flotation process in diamond ore processing. Dust and dirt are, among other things, the major factors that adversely affect level measurement in the flotation tank.
The highest diamond mine in the world at 3200 metres above sea level is located in the Maluti Mountains in the Kingdom of Lesotho. The environmental conditions there are correspondingly rough: frequent snowfall, temperatures that fluctuate between -18°C and 20°C and strong winds, which intensify the low temperatures, are part of everyday life.
The conditions in the ore preparation process are also pretty rough. The mine transports the ore to the surface through two kimberlite pipes. These are vertical chimneys of volcanic origin that extend deep into the earth’s crust. The source rock is crushed and further processed to extract diamonds. This whole procedure is extremely laborious. Worldwide production of natural diamonds is now about 20 tonnes per year but covers only about 23% of industrial demand. The rest is industrially manufactured.
The two pipes in the Lesotho mine contain only a very small proportion of diamonds. Their yield is less than two carats per hundred tons of rock and a huge effort is required to get these diamonds. The mine, 70% of which belongs to Gem-Diamonds and 30% to the Lesotho government, processes 5,8 million tonnes of ore per year in two plants. An additional 1,2 million tonnes are mined and processed by a contractor at a separate plant. The combined tonnage produces approximately 100 000 carats per year.
Separating diamonds from kimberlite
In a DMS plant, ferrosilicon (an alloy of iron and silicon) in powdered form is suspended in water to obtain a fluid with the density of diamonds, about 3,52 g/cm³. To this is added the previously crushed diamond bearing material, in order to separate the heavier minerals from the lighter rock. The DMS process produces a concentrate which generally amounts to less than 1% of the original material fed into the plant at the beginning of the process. An alternative processing method is centrifugation, where the denser material is swirled at low and high speeds in cyclones. In the process, the diamonds and other dense minerals are pressed to the walls and then out the bottom of the cyclone. The waste water rises at the centre of the cyclone and is sucked out and screened to remove the remaining particles.
Both methods have their advantages and disadvantages. The investment costs for a DMS plant are 10 times higher than for a cyclone. The DMS plant, however, provides better yields. The water consumption and operating costs for a DMS plant are also significantly higher than is the case with centrifuge processing. However, the service life of kimberlite mining facilities is very long, which makes it worthwhile to build stationary infrastructure that, in the long run, leads to higher productivity of the overall process. Of course, the efficiency of a plant also depends on the skill of the operator and the applied technology. Decisive factors for the smooth operation of a DMS plant and, ultimately, the whole process, are, among other things, a high level of automation and measurement technology that delivers reliable values.
Turbulence and inlet tubes make measurement more difficult
In the flotation tank, the level of the flotation liquid containing the enriched material has to be precisely measured. However, this is far from easy because of the harsh environment and the internal components of the tank. The medium is fed into the flotation tank through pipes from different directions. These pipes cause extreme turbulence and water splashing inside the tank. An older radar sensor with a transmission frequency of 26 GHz, which was installed there a few years ago, always had problems. For example, it displayed the built-in pipes as the level, which was totally incorrect. Another difficulty was the accumulation of dust and debris on the antenna, which resulted in repeated false readings: although radar is a non-contact measuring method and therefore ideal for dirty environments, the sensor no longer worked optimally because of the extreme ambient conditions. Due to the resulting signal attenuation and interfering reflections, the measuring point could only be kept in operation through constant servicing.
80 GHz technology brings stable measurement
Last spring, when the first 80 GHz radar level sensor for liquids was introduced to the market, VEGA’s South African subsidiary quickly suggested replacing the existing technology with the new VEGApuls 64. The previous 26 GHz sensor, with its 80 mm antenna, had a beam angle of 10°. The narrower beam angle of VEGApuls 64, only 3°, promised a solution to the problems caused by the inlet pipes. This considerably tighter focusing of the radar beam made it possible to distinguish the actual measurement signal from the interference signals. The new radar sensor also has significant advantages because of its higher dynamic range of 120 dB. What is more, it provides higher accuracy, reproducibility and reliability in general within the application.
The measuring process itself is completely independent of process conditions, which is one of the greatest advantages of radar technology. Varying temperatures and pressures affect the measuring results just as little as the properties of the liquid to be measured, e.g. density or viscosity. This is important, especially in the inhospitable temperatures that prevail in the diamond mine: VEGApuls 64 measures under pressures from -1 bar to +20 bar and process temperatures between -40°C and 200°C. Despite the considerably shorter wavelength of the 80 GHz sensor, it is hardly affected at all by deposits or condensation. This is achieved mainly through special signal processing in the area close to the sensor. The distance-dependent dynamic adaptation reduces the effects of interference directly in front of the antenna system and at the same time allows a very high signal sensitivity at a greater distance. The measuring distance can be up to 30 m and measurement accuracy still remains at 2 mm.
Problems in the mud bath?
None whatsoever! Besides the exceptional stability of its measuring signal, the radar sensor is also characterised by mechanical robustness, i.e. it is virtually wear and maintenance free. Even if the sensor has to be freed of large quantities of mud now and then, the process can go on unhindered. Cleaning is fast and uncomplicated.
The extraction and processing of diamond ore definitely has nothing to do with the glitsy world where the diamonds later make their appearance. The environment in the mine is harsh and forbidding. But what really matters here is the efficiency of the process. For the mine operators, the very idea that a process would have to be interrupted just because of a defective measuring instrument is unacceptable. They are keenly aware that most of the mining and extraction processes are interconnected and depend heavily on each other.
The first 80 GHz radar measuring instrument for liquids proved to be a real godsend for the mine. Everything in the flotation tank has been running smoothly since VEGApuls 64 was installed.
For more information contact Chantal Groom, VEGA Controls SA, +27 (0)11 795 3249, firstname.lastname@example.org, www.vega.com