General Principles and Processes of Isolation of Elements:
Principles and methods of extraction and refining
The earth’s crust contains metals and their compounds which are always mixed with other substances and impurities.Metals mostly exist as metal oxides or metal sulphidesas part of ores which are excavated from the earth. There is crushed then ground to increase their concentration,the following methods may be used:
- Gravity concentration: The ore is shaken by a jolting box, or similar, while wet. Particles will separate into layers depending on weight, the metal may be removed as a layer.
- Flotation: The fine particles of ore are swirled around in large tanks with air blown in and flotation agents added. The particles attach to the bubbles and float to the surface where the froth is skimmed off.
- Magnetic separation: In the separation of ores that have magnetic properties, the ore is ground and passed over a rotating drum. The drum has a magnet inside it which holds the magnetic ore particles as the waste falls outside the screen.The ore held on the drum can be released or scraped off.
Metal can be removed from its oxide by splitting the metal ion from the oxygen ion. This process is called REDUCTION.Different metals have different reactivities. Metals high up on the activity series make more stable oxides than the once lower in the series. Therefore, metals with stronger reactivity are more difficult to split up than those lower down.A more reactive metal will remove oxygen from the oxide of a less reactive metal when a mixture of the two is heated.
|Reactivity||Element||Extraction method||Reaction with acid|
|Strong||K (Potassium)||Metals above carbon in the reactivity series must be extracted using electrolysis. Electrolysis can also be used to purify copper.||Too dangerous|
|Ca (Calcium)||Releases hydrogen and dilute hydrochloric acid.|
|Zn (Zinc)||Metals less reactive than carbon can be extracted from their ores by reduction using carbon, coke or charcoal.|
|Cu (Copper)||Platinum, gold, silver and copper can occur native and do not need to be extracted.||Doesn’t release hydrogen and dilute hydrochloric acid.|
Refining (purifying) the element is carried out by one of the following methods:
This entails the immersing of a positively and negatively charged electrical poles to a molten or aqueous solution of a substance. The passage of the current causes positively charged ions to be attracted to the negative pole and negatively charged ions to be attracted to the positively charged pole. While at the Negative pole, positively charged ions gain electrons to become neutral atoms and are deposited on the negative pole or fall to the bottom as a precipitate. The same occurs to positively charged ions at the negative pole. The overall effect is that the constituents of the substance are separated and usually are deposited on the Positive Pole or Anode and the Negative pole or Cathode or it may sink to the bottom of the container as a precipitate
Reduction with carbon
To obtain the metal from its oxide, the oxygen must be removed.The removal of oxygen from a substance is called reduction while gaining oxygen is called oxidation.
Carbon is most commonly to extract metals by reduction because it is cheap and readily available.
In the reaction above, the carbon removes oxygen from lead oxide. This occurs because carbon is more reactive than lead.Here, carbon gained an oxygen ion by oxidation.
- Distillation is used to refine metals with relatively low boiling points.
- Zone refining exploits freezing point depression.