Mineral Process Chemistry
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4 Processes used to concentrate ores in preparation for extraction of metals etc: study guide
Chapter 4 The processing of ores
Minerals seldom occur pure in nature. The part of an ore which is wanted is called ‘values’ ; the unwanted material is called ‘gangue’ . This chapter is about how the values and the gangue are separated.
Section 4.1 Liberation and separation of mineral grains
This section is concerned with the process of comminution, the crushing and grinding of the ore down to its liberation size. You should become familiar with each of the following:
• Cone-crusher
• Ball mill
Section 4.1 is also concerned with separation methods which follow from comminution, and which depend on density differences between values and gangue:
• Shaking table
• Spiral separator
http: //www.cfd.com.au/cfd_conf99/cyclones_and_spirals.htm From here you can download an academic paper about spiral concentrators. The early sections contain a good simple description and a useful photograph.
Separation can also be effected by means of
• Froth flotation
In this case, you should be aware of the way the detergent used waterproofs the grains of metal-containing compound.
Your GCSE course might have included the idea that a detergent is a compound with an ionic end which tends to dissolve in water (hydrophilic), and a hydrocarbon end which will not mix with water (hydrophobic). The ionic end of the detergent molecule is attracted to the ions in the metal-containing compound and coats the surface of it. This leaves the hydrophobic hydrocarbon ends of the detergent molecules sticking outwards. These coated grains are attracted to air bubbles by van der Waals forces.
The ‘values’ grains are thus carried up into the froth at the surface of the mixture.
Warning and apology: about one third of the way down the left hand column of page 23, where the text is explaining the action of detergents, there is a reference to the Students’ Book. This reference is to the 3rd edition of main the book. The subject of detergents was omitted from the 4th edition you are using because it is now so often in GCSE courses.
The diagram in figure 4.7 is useful. Experiment 4.2 part 2 is rather less useful, because froth flotation is difficult to imitate in the laboratory.
Section 4.2 Units operations and flow diagrams
This introduces the idea of unit operations, the individual steps in a succession of processes undertaken to isolate values from gangue.
There are two good examples of separation in figures 4.11 (this is actually in section 4.1) and 4.12. You should study diagrams carefully, and make sure you understand what goes on in each unit operation.
There are two more good examples of separation in the next chapter at figures 5.2 and 5.4, and a further one at figure 6.2 in Chapter 6.
Experiments in Chapter 4
This set of experiments is the longest and most complex of the Special Study. There is some very good chemistry in them, with various points of contact with the main course.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Experiment 4.2 Physical separation
The point of these experiments can be seen simply by reading the instructions. You should realise that, not only are a few transition elements attracted to a magnet, but so are some of their compounds.
Surface chemical separation
Froth flotation is notoriously difficult to simulate in the laboratory. There is a good account in the text (p23) and another on a website. See www.elmhurst.edu/~chm/vchembook/332flotation.html for an account of froth flotation with some useful photographs.
Bulk chemical processing
There are several points to be appreciated here:
• A sulphide ore reacts only slowly with any of the possible reagents designed to make the metal into a soluble compound.
• Roasting in air converts the sulphide into an oxide which reacts much more rapidly
e.g. 2CuS(s) + 3O2(g) -> 2CuO(s) + 2SO2(g)
• Acid would convert copper oxide into e.g. copper chloride which is soluble but
• A ‘leach’ solution containing ammonia and ammonium carbonate works much better, giving a deep blue solution containing complex ions such as Cu(NH3)42+.
Experiment 4.3 Quantitative analysis of an ore
This is a cleverly-designed experiment which does three things, starting from a partially oxidised (‘weathered’) copper and zinc ore.
• It shows that you don’t extract much metal from the sulphide ore.
• It shows that you can get much more metal if you roast the ore first.
• It works out the percentage of both copper and zinc in the ore.
In ‘a’ you leach out the copper and the zinc ions as their soluble complexes.
In ‘b’ you filter off the ‘gangue’ (waste solid) giving you a clear blue solution of the complex copper and zinc compounds.
In ‘c’ you look at the dried residue and (probably) detect signs that not all of the copper has been extracted.
In ‘d’ you evaporate the solution to dryness. The residue contains copper(II) oxide and zinc oxide.
In ‘e’ you dissolve these oxides in sulphuric acid, converting them into copper(II) sulphate and zinc sulphate. You neutralise any remaining acid, and make up the solution to a known volume in a standard (volumetric) flask.
In ‘f’ you treat a sample with potassium iodide which liberates iodine according to the equation
2Cu2+(aq) + 4I-(aq) -> 2CuI(s) + I2(aq)
The iodine is titrated with sodium thiosulphate
2Na2S2O3(aq) + I2 (aq) -> Na2S4O6(aq) + 2NaI(aq)
(Revise from this Topic 6 of the Nuffield course.)
You can use starch as an indicator (blue -> colourless).
YOU DON’T THROW AWAY THE SOLUTION.
In ‘g’ you use the solution from ‘f’ and titrate with edta. (Revise edta from Topic 19 of the Nuffield course). The hexamine/xylenol orange mixture acts as an indicator; you do not need to know how it works.
The zinc ions react in 1:1 ratio with edta.
From the results of ‘f’ you work out the percentage of copper in the ore and from the results of ‘g’ you work out the percentage of zinc in the ore.
You should find that if you leach the unroasted ore you get much less metal out of it because sulphides do not react at all quickly with the leach solution.
One of the clever features of this experiment is that you analyse for the copper first and then the zinc. This is for several reasons:
• Copper compounds are coloured which would mask the indicator used in ‘g’
• The final solution from ‘f’ is colourless.
• Edta reacts with both copper(II) and zinc ions.
• Acids react with sodium thiosulphate, and the solution in ‘g’ involves acids.
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updated: 01 March 2006
