Molybdenum Testing Methods for Ores, Concentrates and Mill Products
Feb 26, 2024
Deciding Factors Before Choosing a Technique
Molybdenum is measured across a wide range of concentrations, from trace levels in geological material to major content in concentrate, ferromolybdenum and molybdenum metal. No single technique covers that whole span, so laboratories hold more than one method and select by sample form, expected concentration and the accuracy the customer requires. Sample decomposition matters as much as the instrument: molybdenum can volatilise during fusion or acid digestion if the temperature is not controlled.
Spectrophotometric Determination
Spectrophotometry is based on the absorption of light by a coloured molybdenum complex at a defined wavelength. The sample is reacted with a complexing reagent to form a coloured species and the absorbance is measured against calibration standards. The technique is inexpensive, robust and well suited to low and moderate molybdenum content, which is why it remains the standard production method for ore and tailings. Its limitation is that other elements forming coloured complexes, or consuming the reagent, must be suppressed by masking or by extracting the complex into an organic solvent.
Atomic Absorption Spectrometry
Atomic absorption spectrometry measures the absorption of element-specific radiation by ground-state molybdenum atoms produced in a flame or an electrothermal atomiser. The sample is introduced as a solution, atomised, and the absorbed intensity compared with calibration standards to obtain concentration. It is a single-element technique with modest running cost and good selectivity. Flame atomisation suits the higher concentrations found in concentrate solutions, while electrothermal atomisation extends the working range downwards. Chemical interferences in refractory-element chemistry must be controlled with suitable matrix modifiers.
Spark Emission Spectrometry
Spark emission is used for solid samples, which makes it the routine technique for molybdenum metal, ferromolybdenum and alloy mill products. A spark is struck on the prepared surface at high temperature, exciting the molybdenum atoms to emit a characteristic spectrum, and the intensity and position of the emission lines are measured by the spectrometer to give the content. Because the sample is measured directly, digestion losses and reagent blank problems are eliminated and a result can be produced in minutes. Accuracy depends on the reference standards used to calibrate the instrument, which must bracket the expected composition.
Plasma Emission and Mass Spectrometry
| Technique | Typical sample | Strength | Limitation |
|---|---|---|---|
| Spectrophotometry | Ore, tailings | Low cost, stable colour | Single element, interference control needed |
| Atomic absorption | Solutions, concentrate | Selective, low running cost | One element per measurement |
| Spark emission | Solid metal and alloy | Direct, fast, no digestion | Needs matrix-matched standards |
| ICP optical emission | Digested samples | Multi-element, wide range | Digestion required |
| ICP mass spectrometry | Trace-level work | Very high sensitivity, multi-element | Higher capital cost, matrix effects |
Inductively coupled plasma optical emission spectrometry atomises and excites the sample in a high-temperature argon plasma and measures the emitted light; ASTM E1479 describes the practice of specifying and describing such instruments so that performance between laboratories is comparable. Inductively coupled plasma mass spectrometry ionises the sample in the same type of plasma, then separates and counts the ions by mass-to-charge ratio. It is extremely sensitive and selective and can measure several elements simultaneously, which makes it the method of choice for trace molybdenum and for impurity surveys in high-purity molybdenum metal.
Reporting Results
A laboratory result is only usable when the method, the sample preparation route and the digestion or fusion details are reported with it. Where molybdenum is a payable element, the assay should be supported by a certified reference material run in the same batch, and analytical results should be traceable to a documented calibration. Ferromolybdenum and molybdenum mill products are sold against composition limits in ASTM A132, ASTM B386 and ASTM B387 respectively, so the test method must be capable of resolving those limits rather than merely reporting a value near them.
Frequently Asked Questions
Q: Which method is best for a solid molybdenum metal sample?
A: Spark emission spectrometry, because it accepts a prepared solid surface directly and avoids digestion losses and reagent blanks.
Q: What is ICP-MS used for in molybdenum analysis?
A: Trace and ultra-trace determinations and multi-element impurity surveys, where its sensitivity and selectivity exceed those of emission techniques.
Q: Why is spectrophotometry still used for ore?
A: It is inexpensive, robust and accurate at the low concentrations typical of ore, and the coloured complex is stable enough for routine production control.
Q: Why must digestion conditions be controlled?
A: Molybdenum can be lost by volatilisation during high-temperature fusion or acid attack, which produces a low and unrepeatable result.
Q: Which composition limits are the results checked against?
A: ASTM A132 for ferromolybdenum, ASTM B386 for molybdenum plate, sheet, strip and foil and ASTM B387 for bar, rod and wire.
