Mineral Process Chemistry
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5 Methods of obtaining metals from ores: study guide
Chapter 5 The processing of concentrates
As the beginning of Chapter 5 shows, this is the culmination of all that has gone before. It is seldom that a purified mineral is saleable as such. Usually a metal has to be extracted from it. This chapter summarises how this is done.
The reactions involved in the reduction of purified metal ores to the metals themselves are very simple in principle. There are two things which you should bear in mind as you study this chapter.
1 Some very familiar reactions have names peculiar to the mineral industry which are seldom encountered elsewhere. As an ‘insider’ now that you are studying the subject, you should be familiar with these terms. A displacement reaction, for example, is here called ‘cementation’ and and the specific use of electrolysis for metal extraction becomes ‘electrowinning’.
2 There is an occasional reference to an energetic criterion for reaction called ‘free energy change’ denoted by the symbol DG . Considerations of free energy changes are no longer in the A-level specification, but you will be able to appreciate the sense of the references if you know that enthalpy change is a major part of free energy change. You will know from the main course that enthalpy changes may be used as a rough guide to reaction feasibility – if you can show by calculation that a given reaction is exothermic then the chances are high that it will be feasible. Where free energy changes are mentioned in this chapter, think ‘enthalpy changes’ instead and the remarks will make sense .
Experiment 5.1 Leaching of the wolframite concentrate
Experiment 5.2 Precipitation of hydrated tunsten(VI) oxide
A-level Chemistry courses do not normally include details of tungsten compounds. It helps if you remember that tungsten is a transition element, and that transition elements show a number of similarities.
There are two properties of tungsten which you should become aware of. In both cases, tungsten, although the element itself appears to be a metal, shows properties which we normally associate with non-metals; (manganese, chromium and many other transition elements are similar in this).
The two properties are
• WO3 , tungsten(VI) oxide, is acidic – it reacts with alkalis but not with acids
• Tungsten forms oxyanions such as WO42-, tungstate (VI). Compare, for example, carbonate, CO32-.
Just as you can react carbon dioxide with an alkali to give a carbonate and then react the carbonate with acid to produce carbon dioxide again, you can do similar things with tungsten oxide. These reactions are used in Experiments 5.1 and 5.2.
Experiment 5.1:
WO3 (s) + 2NaOH(aq) -> Na2WO4 (aq) + H2O(l)
This reaction is there by implication. The ore already has the tungstate ions in it; the existence of these is itself an indication of the acidic nature of tungsten(VI) oxide.
Experiment 5.2:
Na2WO4 (aq) + 2HCl (aq) -> 2NaCl(aq) + WO3(s)
Experiment 5.3 Cementation of copper
Experiment 5.4 Electrowinning of zinc
These experiments are about extracting copper and zinc respectively from solutions of their ions. There are no really new principles involved but you should read the instructions carefully.
Some of the questions asked can be answered only by reference to sources outside the course. These are interesting ‘follow-ups’ but are not examinable. You may find, however, that taking the time and trouble to do these ‘follow-ups’ will have beneficial effects on your understanding of the material which is actually in the specification.
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updated: 01 March 2006
