Distance and the risk of crashes

Mining is fraught with hazardous processes and staying in control of the processes is a constant challenge for engineers and mine operators. The operational magnitude, sheer weight and mass of ore, dust and underground environments creates risks to personnel, mine infrastructure and equipment every passing moment.

One such hazardous process within a mine is the 'ore pass'. These are shafts, sometimes up to 150m in depth. Ore is deposited by trucks into the top of the pass (known as the mouth) and held within the ore pass by gates which are closed and locked in the lowest section of the shaft. Below these gates is a rail line. Upon opening the gates, the ore falls progressively into rail cars waiting underneath for transportation via an underground rail line for processing.

The risk is that sometimes, several tonnes of ore can remain 'stuck' in the ore pass upon filling the rail cars. A major hazard to personnel and plant equipment exists should the ore loosen and crash to the bottom of the ore pass after the train has departed. Ore pass operators must somehow ensure that any ore that is deposited through the mouth of the pass is not left in the shaft after the last rail car has been filled.

One of Australia's largest underground copper/uranium/gold mines came to Krohne Australia to help solve this problem. Krohne's solution was to provide a non-contact level radar control device, the BM70A, in these passes, which addresses all the problems above, plus increases the mine's efficiency for this process. Due to the highly susceptible environment, it was discovered that Krohne was the only radar manufacturer able to perform this measurement.

The BM70A is a frequency modulated continuous wave (FMCW) radar. The FMCW radar uses a high frequency signal sweep from 8.5 to 9.9 GHz. The signal is emitted, reflected from the product surface and received after a delay. For further signal processing the difference is calculated from the actual transmitted frequency and the received frequency. The difference is directly proportional to the distance ie, a large frequency difference corresponds to a large distance and vice versa.

The frequency difference is transformed via a Fourier transformation (FFT) into a frequency spectrum and then the distance is calculated from the spectrum.

Compared to the simple pulse radar technology, the use of FMCW radar offers a higher bandwidth of the microwave signal leading to better reflection separation and reliable reduction of noise. The higher transmitting frequency also means a much smaller angle and thus fewer interference reflections, as well as a smaller antenna for the same measuring angle compared to pulse radars.

In these ore pass operations, a BM70A level radar has been (flange) mounted at the top of each ore pass and is used to monitor the level of the ore in the pass. The mine operators can 'see' the level of the ore gradually decrease when the rail cars are being filled. The BM70A will detect any ore that remains stuck in the ore pass allowing plant operators to take necessary precautionary action. Using the BM70A has further reduced the requirement to send inspection crews down the shaft to investigate the site of the blockage - thus preventing unnecessary risks to mine personnel.

Reflections caused by struts, weld seams and tank intervals are identified as fixed targets and can be 'blanked out' provided the useful signal reflected from the product surface is greater than the interference reflection. Further, sporadic interference signals from falling deposits or side-streams of the main filling are blanked out by the microprocessor-controlled signal evaluation.

The BM70A has been successfully installed for thirteen ore passes within this mine. The level detection range for these ore passes is up to 120 m. Further, remote programming of the BM70A and viewing of the process is made possible for mine operators through Hart communications on the 4-20 mA loop using Krohne's PC CAT software.