Analysis

After one or more samples are taken from an amount of ore passing through a material stream such as a conveyor belt, the samples are reduced to quantities suitable for further analysis. Analytical methods include chemical, mineralogical, and particle size.

Chemical analysis

Even before the 16th century, comprehensive schemes of assaying (measuring the value of) ores were known, using procedures that do not differ materially from those employed in modern times. Although conventional methods of chemical analysis are used today to detect and estimate quantities of elements in ores and minerals, they are slow and not sufficiently accurate, particularly at low concentrations, to be entirely suitable for process control. As a consequence, to achieve greater efficiency, sophisticated analytical instrumentation is being used to an increasing extent.

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In emission spectroscopy, an electric discharge is established between a pair of electrodes, one of which is made of the material being analyzed. The electric discharge vaporizes a portion of the sample and excites the elements in the sample to emit characteristic spectra. Detection and measurement of the wavelengths and intensities of the emission spectra reveal the identities and concentrations of the elements in the sample.

In X-ray fluorescence spectroscopy, a sample bombarded with X rays gives off fluorescent X-radiation of wavelengths characteristic of its elements. The amount of emitted X-radiation is related to the concentration of individual elements in the sample. The sensitivity and precision of this method are poor for elements of low atomic number (i.e., few protons in the nucleus, such as boron and beryllium), but for slags, ores, sinters, and pellets where the majority of the elements are in the higher atomic number range, as in the case of gold and lead, the method has been generally suitable.

Mineralogical analysis

A successful separation of a valuable mineral from its ore can be determined by heavy-liquid testing, in which a single-sized fraction of a ground ore is suspended in a liquid of high specific gravity. Particles of less density than the liquid remain afloat, while denser particles sink. Several different fractions of particles with the same density (and, hence, similar composition) can be produced, and the valuable mineral components can then be determined by chemical analysis or by microscopic analysis of polished sections.

Size analysis

Coarsely ground minerals can be classified according to size by running them through special sieves or screens, for which various national and international standards have been accepted. One old standard (now obsolete) was the Tyler Series, in which wire screens were identified by mesh size, as measured in wires or openings per inch. Modern standards now classify sieves according to the size of the aperture, as measured in millimetres or micrometres (10^-6 metre).

Mineral particles smaller than 50 micrometres can be classified by different optical measurement methods, which employ light or laser beams of various frequencies.