1. Classical Methods (Benchmark Methods)
This method offers high precision and is often used as a benchmark for refereeing or calibrating other methods.
1. Drying Oven Method (Gravimetric Method)
This is the most classic and fundamental standard method, and is currently the most commonly used in laboratories.
Principle: The moisture in the chromite ore sand is evaporated by heating, and the moisture content is calculated based on the difference in mass before and after heating.
Procedure:
Take a representative chromite ore sand sample and accurately weigh its initial mass (recorded as m₁).
Place the sample in a constant-weight drying dish and flatten it.
Place the drying dish in a forced-air drying oven and dry it at 105±5°C.
After drying for a specified period of time (usually 2-4 hours, or until constant weight is reached), remove the sample and place it in a desiccator to cool to room temperature.
Accurately weigh the mass of the dried sample (recorded as m₂).
Calculation formula:
Moisture (%)=m1−m2/m1 ×100%
Advantages: Simple equipment, low cost, accurate and reliable results.
Disadvantages: Time-consuming, typically taking several hours, and not suitable for rapid on-site monitoring.
2. Karl Fischer Method
This method is primarily used to determine trace amounts of moisture in substances. this method is considered when extremely high accuracy is required or when different forms of moisture need to be distinguished.
Principle: A chemical method that utilizes the quantitative reaction of iodine and sulfur dioxide with water in the presence of pyridine and methanol.
Advantages: Very high accuracy, capable of measuring to the ppm level.
Disadvantages: Complex operation, expensive reagents, high operator skill requirements, and unsuitable for routine testing of high moisture content. Therefore, it is not commonly used in routine moisture testing of chromium ore.
2. Rapid Methods (On-site/Online Methods)
To meet the speed requirements of production process control (such as sand mixing and molding), various rapid measurement methods have been developed.
1. Rapid Moisture Meter (Halogen/Infrared Lamp Heating Method)
This is currently the most widely used rapid method, integrating a weighing and heating unit.
Principle: Similar to the drying oven method, both are based on loss-in-temperature methods. However, it integrates a balance with a high-efficiency halogen lamp or infrared heating source. Using a built-in program, the instrument automatically heats and weighs the sample in real time, and the final moisture content is automatically calculated and displayed.
Advantages:
Extremely Fast: Results are typically obtained within minutes.
Easy to Use: Virtually one-touch operation reduces human error.
Portable: Some models can be carried on-site.
Disadvantages: Slightly lower accuracy than the standard drying oven method, and regular calibration using the drying oven method is required. The instrument itself has a certain cost.
2. Online Moisture Detector
Used for continuous, real-time monitoring of production lines.
Principle: Indirect measurement using the physical properties of neutrons, microwaves, infrared light, and other sensors interacting with moisture.
Neutron Method: Hydrogen atoms moderate neutrons. By measuring the number of moderated neutrons, the hydrogen content is estimated, and thus the moisture content is determined. While highly accurate, the equipment is expensive and radioactive, requiring strict management.
Microwave Method: Moisture absorbs microwaves differently from dry sand, and the phase shift is measured to calculate moisture.
Infrared Method: Water molecules absorb infrared light of specific wavelengths, and the absorption intensity is measured to determine moisture content.
Advantages: Fully automated, with real-time feedback, making it ideal for process control.
Disadvantages: The equipment requires significant investment and professional installation and maintenance. Furthermore, the measurement results are easily affected by factors such as material particle size, density, and composition, requiring frequent calibration.
