2
Metals And Minerals
2.1 What to test for?
Minerals and ores are the rocky materials extracted from the ground, and will be commercially useful
if they are rich is a particular type of metal. For example, bauxite is the main source of aluminium.
Almost always, the metal will exist as an oxide (e.g. iron and aluminium) or sulfide (e.g. lead, copper,
zinc), but some of the noble metals such as gold and platinum can be found in the metallic state.
Regardless of chemical form, the majority of the rock will be commercially useless
Minerals and ores will be most commonly tested for the level of the commercially interesting
metal(s) in the rocky material. Sometimes this can be as low as 1-2%, and sometimes as high as 30%
depending on the source and type of rock. The refined metals will have different testing
requirements.
CLASS EXERCISE 2.1
Why will testing requirements be different for refined metals?
2.2 How to test?
The great majority of chemical tests on metals and minerals will be elemental. The exceptions will be
where the mineral itself is the valuable commodity, eg gemstones. Even metallic compounds which
are commercially valuable, such as titanium dioxide, will be manufactured from the refined metal,
rather than extracted as is. We will concentrate on the elemental analyses.
The elements of interest in the minerals and refined metals divide into two basic categories on
the basis of how they can be analysed:
•
metals
•
non-metals and metalloids
Metallic elements
The analysis of metallic elements has passed through a number of stages over time. Initially, before
the age of spectroscopic instrumentation, titrimetric and gravimetric methods were developed for
most elements. Some of these still remain as a useful backup for the modern spectroscopic
techniques. Obvious examples are the analysis of manganese or chromium by redox titration (the
analysis of chromium in steel by potentiometric titration remained a current Australian Standard until
quite recently).
2. Metals and Minerals
The first major spectroscopic instrument used in the minerals and metals industry was the emission
spectrograph, which used a high voltage electrical spark to vaporise, atomise and excite the sample.
The radiation emitted, which is of course characteristic of the different elements and proportional in
intensity to their concentration was collected by an array of up to 30 detectors, each positioned to
measure a particular element. The major advantage of this instrument was that it did not require a
solution sample, and could comfortably cope with powders and solids. Its disadvantage was matrix
interference. These are minimised by the use of matrix-matched standards, which can be either
purchased or manufactured and validation by other methods. Because metal alloys are made with a
consistent matrix, and small variations in alloying elements, sets of appropriate standards will serve
very well.
The flame AAS and more recently, the ICP emission spectrophotometer require solutions, and
this makes them less useful for the analysis, unless the matrix is unusual.
Therefore the spark emission spectrograph remains the major method for analysing alloying
elements in metals. Now portable instruments are available (as shown in Figure 2.1), which provide
great flexibility in the testing of materials. Samples need not be taken; the device can be taken to the
ingot (for example).
FIGURE 2.1 Portable spark spectrometer
Non-metallic elements
These include sulfur, carbon, nitrogen, phosphorous, boron, arsenic, oxygen and hydrogen.
Sometimes these are desirable components, at other times they are contaminants. Some of these can
be analysed by ICP (eg arsenic), but generally some other specialised technique will be required.
Combustion analysis is the normal method for the analysis of C, H, N, O and S. Specialised
equipment, variously known as elemental or combustion analysers, are available, which oxidise the
sample under controlled conditions in a small electric furnace. The gas stream produce contains the
oxides of the various elements, eg CO2, H2O, NOx. The presence and concentration of these gases are
measured by either a specialised infrared photometer, designed to detect the characteristic
frequencies of these gases, or the effect on the thermal conductivity of the carrier gas.
Moree information on these elemental analysers will be provided when we look at the coal
industry, where these analyses are even more important than in the metals sector.
Industrial Products (Testing)
2.2
2. Metals and Minerals
CLASS EXERCISE 2.2
You should not only be able to perform a wide range of chemical analyses, but also have a basic
understanding of how a particular method works, ie the chemistry and the reasons behind the various
steps.
Chromium in steel was for many years analysed by potentiometric titration as a checking
procedure for spark emission. It remains a viable option. You may will have done this analysis in an
earlier subject. Provide a simple explanation of each step.
Step
Procedure
1
Add conc HCl to sample; heat until
reaction ceases
2
Add conc. HNO3 and boil until
brown gas is evolved
3
Dilute with 300 mL of water
4
Add silver nitrate solution
5
Add potassium persulfate and heat
until purple colour develops
6
Add conc. HCl until the purple
colour disappears
7
Cool in icebath
8
Set up potentiometric apparatus
using Pt & SCE electrodes
9
Titrate with standardised Fe(II)
until endpoint
10
Repeat with standard steel
Reason
What You Need To Be Able To Do
•
•
•
list basic testing requirements for metals and minerals
outline the basic methods used to analyse metals and minerals
explain the purpose of each step in a procedure
Industrial Products (Testing)
2.3