Carl J Kamme reveals how mining applications can benefit from direct optical monitoring methods
Direct optical methods for monitoring of gases are in use in a wide range of applications. For mining smelters and sulphuric acid plants, these technologies provide reliable measurements of gases with real-time feedback to the operators.
Direct optical methods mean that gas analysis takes place in the gas stream, without extracting any samples. A beam of light probes the gas. A receiver collects the signal and a fibre optic guides it to the analyser spectrometer some distance away. The system digitises the spectral information with incredible resolution and speed. Then, in a fraction of a second, it calculates the result and puts it out to the plant DCS.
The result is a rugged and stable solution. Compared to traditional technologies based on sample extraction, transport and conditioning before measurement, there are significant differences. The sample handling in an extractive system causes a slow and flattened signal due to an integrating effect. Direct optical measures the exact content passing the light beam during sampling. The result is a true dynamic real-time signal.
In addition, since these methods exclude sampling handling used in traditional units, optical methods are less prone to failures, and the maintenance needed to operate such a system is limited.
For mining smelters, sulphuric acid and other process plants, these systems can deliver a high-performing, stable solution in demanding situations where gases may be hot, corrosive and contain high levels of vapours and dust. Applications include continuous emissions monitoring, measurement in raw gas to control a scrubber function and other process types of measurements.
Ultraviolet DOAS
UV-DOAS is the most dynamic tool for measurements of SO2 and at sulphuric acid plants and metal smelters. The same system can monitor any level of SO2 from low ppm levels to high percent per volume. It can also measure other components, e.g. NOx, Hg, CO2 and H2O.
Infrared FTIR
This technology is the most accurate alternative for measurements in the near-infrared, and will do a great job for measurements of HF, HCl, CO and CO2. It can also pick up SO3 in ranges from 0-1% Vol. and upwards.
Tunable laser diode (TDL)
The laser diode solution is the obvious pick for reactive gases such as HF, NH3 and HCl. The TDL is quick and versatile. It can deliver results every few seconds from multiple monitoring points. Additional parameter options include CO2, H2O and O2.
Applications
One example of positive sensation when exchanging an extractive type of system with a direct optical method comes from the copper industry in Australia. This is a very large industry and producer of several types of metals and sulphuric acid. When producing high-grade copper, they process the material in converters to remove sulphur, where it is heated and oxygenated. The converter off-gas is totally “raw” and unfiltered. The gas temperature is high, the concentration of dust is significant and the sulphur dioxide is at percent levels.
The operator is depending on the monitored SO2 data to determine when it is time to replace the matter in the converter. With the data from the UV-DOAS system, timing became so much easier and precise.
One system monitors the SO2 in the off-gas from four different converters. The optical systems can monitor multiple locations, up to six, by multiplexing the fibre optic cable.
Direct optical systems have applications at several positions within the sulphuric acid plant. These provide good examples of applications that are generally applicable at any type of smelter or production industry.
The SO2 level input to the sulphuric acid plant is an important measurement in order to get the concentration in the proper range for the converter process. Here the UV-DOAS can monitor SO2 before and after dilution. Typical range is 0-30% per volume.
Liquid SO2 production
The pressure of this process stream is high. UV-DOAS does the SO2 measurement in process streams up to 1 bar (g).
Measurements are made to track levels to indicate efficiencies in terms of converting SO2 to Oleum as well as levels after absorption stages as well as scrubbers. One system can make measurements of high range (0-20% Vol.), mid-range (0-1% Vol.) and low concentrations (0-20 ppm).
Continuous Emission Monitoring (CEM) System
The optical system monitoring technology is the choice for stack monitoring at mining and sulphric acid plants. Requirements may include reporting SO2, NOx, CO, CO2, H2O and other components. Systems will integrate with flow and dust/opacity measurements. The technology has accreditation from EU standards, and operates according to EPA protocols.
Rugged design makes the system cope with corrosive gases. In fact, these optical systems are exceptionally stable. A verification by TÜV in Germany included a two-year stability test of two optical UV-DOAS and IR-FTIR systems. TÜV concluded that the allowable calibration interval for this technology is one year. That is the longest interval awarded any type of gas monitoring system certified by TÜV. That is for the record. Local regulations still determine the frequency of calibration checks.
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