Online moisture analysis in materials handling
Moisture analysis during materials handling and processing presents a number of challenges that can be solved with appropriate use of online techniques.
Measuring the moisture content of raw materials is a crucial step in manufacturing and process quality assurance. Whether the material is a food, ore, chemical or other material, its quality, its ability to be processed and, ultimately, its price depend on the moisture content.
Traditionally, moisture in dry granular processes is measured by taking a sample either manually or with an automated sampling system, then taking it to a site lab where it is weighed, dried and then weighed again. However, this procedure has a few challenges.
The first challenge in manually determining moisture content is in obtaining a representative sample. Standing ISO committees have spent years trying to codify the process and, typically, when they have done so, it is published in six volumes and almost impossible for most people to understand. The second challenge is to ensure that the sample does not lose moisture between the sampling and measurement. The final challenge is the process in the lab, which is often seen as the backwater of the manufacturing process. The overall result is that there is a considerable time lapse in obtaining the information.
An alternative method of measuring moisture during materials processing and handling is online moisture analysis. The advantages of this method are that it provides timely information and takes the environmental and human factors out of the measurement process.
Which applications can benefit?
A wide range of manufacturing, domestic and industrial activities generate dust, with the construction, agriculture and mining industries contributing most to atmospheric dust levels. This dust can cause a variety of problems. In addition to potentially damaging or impairing equipment and technology, it can cause health and environmental pollution problems.
The amount of dust emitted during manufacturing depends on the material and the way it is handled throughout the process. While some dust generation is generally unavoidable in bulk materials handling, excessive dust emissions in the workplace need to be managed.
In the mining industry, online analysis allows for the controlled addition of water to condition the ore to a stage where little dust is emitted during handling. Online moisture analysers continuously measure the moisture content of the ore, which is then communicated to the plant control system. The plant control system then determines how much water to spray onto the ore to prevent unwanted dust. Moisture levels in the ore are kept to a certain limit, known as the dust extinction moisture (DEM). If the water added exceeds the DEM, the ore tends to swell, becoming sticky, and can build up on chutes and screens. If yet more water is added, the material can become unstable and collapse, giving up free water and leaving an extremely compact ore. Typically in this situation there will be catastrophic build-up in chutes or crushers, or alternatively the product can solidify in rail cars and ships. In ships the ore can undergo liquefaction, resulting in the cargo shifting and potentially sinking the ship. Figure 1 shows an example of the changing character of goethitic iron ore with the addition of moisture.
The second major area for moisture control is in manufacturing processes that involve agglomeration and sintering (see Figure 2). These processes involve the build-up of fines to create larger lumps, which are dried or fused to have enough strength for further processing. Accurately determining moisture content during the process has a significant impact on plant productivity and quality.
Figure 3 shows the strong correlation between the moisture content and the bed permeability. If the material is too dry then the pellets will not be properly formed and the fines will tend to fill the spaces between the pellets (thus dropping the permeability). If the material is too wet then the pellets lose their green strength, collapse and, again, drop the sinter bed permeability.
The third major area for online moisture analysis is to understand the dry massflow of the product. This could be for stock control, batching, blending or mass balance ore transactional accounting.
A number of technologies have been used for continuous moisture analysis, with varying degrees of success:
- Conductivity or capacitance: The earliest online moisture analysers usually used trailing arm electrodes which trailed along the top of the material being conveyed on a conveyor belt. The key limitations of this method are its low accuracy and the requirement for a high level of maintenance and constant recalibration.
- Neutron moisture instruments: These have applications where infrared and microwave techniques cannot be used - such as measuring the moisture in coke and magnetite. The main drawbacks of using these instruments are that they are expensive and have significant occupational health and safety management issues requiring extensive regulatory control. One of their distinguishing features is that they measure the total moisture content, including water of crystallisation. Depending on the application this can be an advantage or disadvantage.
- Near infrared: NIR is a surface reflectance technology that only measures the surface moisture of the material. It has been particularly successful for use with food and organic materials, as the same technique can be used to measure other physical parameters such as protein and carbohydrate. The technique is susceptible to errors associated with changing reflectance levels of the material being measured. The reflectance can change due to colour, texture, grading and the impact of ambient lighting levels. The systems are best used in inside locations where the surface of the material is representative of the material - for example, where material is exiting a bin in a controlled manner via a feeder. There is a very large range of gauges in this class with varying performance levels.
- Microwave: These analysers fall into two classes - antenna resonance systems and antenna transmission systems.
Microwave moisture analysers
Microwave resonance systems have been popular in concrete batch applications where they are installed in bins and measure free-flowing, low-attenuation material. The transmission systems are now used widely in conveyor-based applications and are emerging in filter and centrifuge applications.
Transmission microwave moisture analysers work on the principle that water has a very high dielectric constant compared to most other materials. When microwaves intersect with water molecules within the material they slow down (and hence change phase) and weaken (attenuate) as the energy is transferred to the material.
A typical analyser system transmits a beam through the material on a conveyer belt. The signal is received by a receiver patch located below the conveyer belt (see Figure 4) and compared to the transmitted signal for phase and amplitude change. Software then calculates the moisture content based on these changes.
The advantage of this technology is that it can analyse large samples of material being carried on the conveyor in real time. This allows for analysis of 100% of the material so that variations in moisture are identified throughout the entire batch. Since phase shift and attenuation also depend on the amount of material present, a measurement of the mass loading on the belt is required to compensate or normalise the microwave measurements. This is generally provided by a beltweigher, scanning laser and radar or ultrasound bed height indicators.
One of the drawbacks of online microwave moisture analysers is that, as materials become more highly attenuating (for example, iron ores and concentrates), it can become difficult to measure them with microwave technologies. This problem is compounded by higher capacity belts and by the higher moisture contents associated with wet beneficiation or extraction below the watertable.
It should be noted that most online moisture analysers need to be calibrated and validated against site moisture samples and that an online moisture analyser cannot be more accurate than lab samples on which the calibration is based. However, some of the better instruments have the potential for a ‘factory calibration’, usually based on customer-supplied material samples.
Low frequency microwave moisture analysis
Intalysis has developed a low frequency microwave (LFM) moisture analyser that is designed to address the problems of highly attenuating materials and high moisture contents. To do this the system uses a lower-frequency microwave signal. This allows the signal to penetrate through ores that have higher dielectric constants and still produce a significantly high signal power at the receiving antenna. While it is normally difficult to measure phase change in a lower-frequency signal, the LFM technology has superior phase resolution capability and a focused antenna to minimise the leakage of the signal around the conveyor belt. The instrument also has potential in low attenuation applications where the signal loss is relatively low and high precision is required, such as in organic materials like woodchips and bagasse.
Originally developed by the CSIRO, the LFM moisture analyser has become common in the Australian iron ore industry, with more than 60 being used in iron ore applications in Australia, South Africa and South America.
In the iron ore industry, conveyors are also now increasingly moving to 45°with bed depths up to 500 mm. This leads to high attenuation of the microwave signal and difficulties in moisture measurement. At the other end of the scale, minerals processing plants need to know the moisture in 50 mm filter cake, which may or may not be highly attenuating. This has led to the development of more flexible, modular LFM analyser designs, where different antennas, installation, compensation (depth, temperature, material profile) and plant communications are used for different materials and bed depths, allowing the analyser to handle beds of up to 900 mm.
The modular design, along with a digital signal processing system replacing the previous analog circuitry, leads to more accurate measurements and easier calibration. DSP also simplifies the tuning of antennas to RF circuitry, allowing for more rapid repairs in the event of RF circuit failure. Extremely low power emission of less than 30 nW ensures EMC compliance in all countries.
So why do it?
Online, continuous moisture analysis in materials handling and processing has a number of important advantages over manual measurement:
- It allows for 100% sample analysis in real time.
- It improves efficiency, especially considering the increased maintenance costs for critical equipment affected by moisture content.
- It reduces energy consumption by reducing the effect of moisture content on roasting, drying, calcining and sintering.
- It reduces material loss while improving process quality.
# From O’Dea, McGown and Gu, Evaluation of the Low Frequency Microwave Moisture Analyser for Sinter Plants
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