In my last article, I explained how wet dust control systems can be very effective in controlling dust at mining operations as long as moisture analyzers were in use to provide accurate, real time, and cost effective measurements throughout the processing and bulk handling of ore and minerals. The calibrating of the analyzers, however, depends upon the circumstances and the goal of the operations.
Dalrymple Bay Coal Terminal (operated by DBCT P/L) is a global coal exporter located in Queensland, Australia. DBCT P/L is a key player in the world’s global coal export market and its success is critical to the economic prosperity of their country.
To achieve operational excellence, DBCT P/L insists on uncompromising dedication to the safety of its people and surrounding community — and that includes reducing the impact of dust emissions. In order to control the dust emission and minimize dust pollution, DBCT P/L decided to measure the amount of moisture in the various coal types transported on the conveyors, and add measured amounts of water. They installed Low Frequency Microwave (LFM3) moisture analyzers on the incoming and outgoing coal belts to help ensure optimum coal moistures were achieved for site dust management.
DBCT P/L made use of a closed loop system where water sprays were installed in several places — prior to the LFM analyser and in the transfer chutes — to ensure penetration of the moisture throughout the material burden. With this system in place, they hoped to maintain both optimum coal moisture and minimal dust.
Because 125 different types of coal are transported on these conveyors and each one has to be measured, the calibration requirements for the LFMs were arduous and at times seemed impossible. LFMs are more than 10 times more sensitive to moisture variation than to mineralogy variation, and inaccuracies can occur due to mineralogy changes in material measured. This dilemma was resolved by grouping the different coal types together, paying close attention to ensuring the various coals in the groupings had similar mineralogy.
DBCT P/L collected a comprehensive set of samples ranging in duration from around 15 minutes to 2 hours. These were taken from their out-loader conveyors using their sampling stations. This enabled good calibration according to standard sampling regime practice. The challenge with DBCT P/L was the extremely large number of different coal types that are loaded at the Port. With one coal type, the microwave response can be readily linked with moisture content whereas with multiple coal types it is critical to ensure that the calibration is acting on the variation in moisture rather than the differences between coal types. Thus it is necessary to group the coals appropriately, both into groups of similar moisture and similar mineralogy, in order to create a calibration that predicts the moisture (rather than changes in the coal type). The DBCT coal types were allocated to 6 groups and a calibration was created for each group on each of the three out-loading systems. See the below chart:
The next stage was to calibrate the three in-loading systems conveyors, for which sampling was not available. The transfer calibration process tested on the out-loading systems was then used to calibrate the in-loading systems. This calibration process leaves an offset that can be removed using a small number of samples. This offset reflects differences in loading and geometry between belts that cannot be accounted for theoretically. However on the in-loading systems, even the small quantity of samples required for offset calculation was not available, which is not uncommon.
To complete the calibration and remove this offset, a cross site correlation was created, tracking material types from in-loading to out-loading based on time in and out of the facility. Over a long period, this creates a good estimate of the average moisture seen on the in-loading systems and can be used to correct the offset and complete the calibration.
For DBCT, the primary purpose of the LFM moisture measurement is as a process control variable for optimum coal moisture control and also dust suppression. The calibrations created were not as accurate as could be achieved if each coal type was calibrated separately; however DBCT indicated that the accuracy is sufficient to provide the optimum coal moisture control they required. So, the process helped them meet their goal and it was financially more viable than individual calibrations for each coal type.
Editor’s Note: If you would like to see the 6 charts of the calibration results and accuracies of predicted vs lab sampled data, email us at firstname.lastname@example.org.