Extracting copper from its ore.

EXTRACTING GOLD FROM ITS ORE.

Crushing and Grinding. This reduces the chunks of gold to fine grains. A crusher first commences to break the fragments to the size of road gravel. A cyanide solution is then mixed with the fragments. A rotating ball mill reduces the fragments to form a pulp. Any coarse particles will be returned to the mill to be reground.

Flotation - the slurry is mixed with water - air is blown through. This has the effect of sending the lighter particles to float to the top. The lighter particles are the gold containing particles. This concentrate can be separated from the remaining mixture, as the concentrate will float. The water can now be removed and recycled. This will form thick slurry.. Once the slurry is mixed with water, bacterial oxidation can then take place

The slurry contains the gold particles, which are encapsulated inside a matrix (a framework) of minerals - for example Arsenopyrites, (FeAsS).

To extract the gold, the minerals are oxidised, and made water-soluble so that they are able to dissolve in water, leaving behind the insoluble gold particles.

The oxidation is carried out by Sulpholobus acidocalderius in the presence of O2 and H2O at a low pH. Firstly S. acidocalderius catalyses formation of Fe, As and sulphur compounds.

FeAsS --> Fe (II) + As (III) +S (VI)

The As and S have been oxidised in this reaction.

Secondly further oxidation of Fe (II) and As (III) occurs:

Fe (II) -->Fe (III)

As (III) --> As (V)

All the products are water-soluble.

2FeAsS + 7O2 +4H+ + 2H2O --> 2Fe3+ +2H3AsO4 + 2HSO4-

The gold is still solid after separation from liquid (which contains Fe, As and S) The pH is raised by lime, then the gold is ready to be extracted using the method of Cyanidation. Ions form a soluble complex of gold, which is suitable for electrolysis to isolate the pure gold.

Advantages And Disadvantages Of Using Bacterial Leaching.

Biologically, leaching is cheaper, however delay due to the slow process means it is expensive. The old method involved smelting ores at high temperatures. It is true that this is much faster, however it is an extensively polluting and energy demanding process.

Bacterial leaching is relatively environmentally friendly -the leaching solutions are recycled. Also underground mining means there are no environmental eyesores. The water pollution problems that result from acid mine drainage and acid rock drainage are very difficult to clean up. Acid mine drainage will go on for thousands of years once the chemical and microbial processes that create acid mine drainage in exposed overburden are set into motion.

Bacterial leaching is considered second for copper for the fact that it is not able to be performed on a large scale mass. Also it is an extremely slow process. For gold however it proved to be cost effective -as it recovered almost all the gold. In 1998, $2 to $3 billion worth of gold was extracted by this process annually. [Sulpholobus acidocalderius, website]

Developments of a New process.

Geography concerning the location - Firstly it must be considered where exactly the ore in question is situated. It must be within reasonable grounds for access by workers. A good supply of transport will also be required either by train, road, or rail links. Should the proposed area lack transport then it must be acknowledged in the additional costs of building such means of transport accessibility.

Market for the trade - There should be a good level in demand for the product. Otherwise no profit will be made and the venture will be fruitless ending up in debt. Once again referring back to location - the site of the plant must be in within a reasonably close proximity to a market. The cost of transporting the material vast distances can otherwise be a disadvantage.

Other raw Materials and Machinery - The materials needed to extract the product must also be considered. Such essentials such as water may prove to be vital therefore the availability of possible raw materials must be available in large quantities. Machinery must be available and cost effective.

Effect on the environment and the people surrounding the site. The effect of setting up a plant will increase employment and will attract people to the location. In some cases this may not be accepted - such as in a quiet country village.

Stages

. A chemist first discovers and invents a process in the lab.

2. A pilot scheme can then be set up to see how successful the idea is in practise.

3. The cost of the operation must then be put to consideration.

4. If all these targets can be satisfied then the procedure may begin.

Bibliography

[1] Chemical Technology - Kirk, Othmer.

[2] Advanced Chemistry - Loughton

[3]Via Internet: " Google "- 'Bacterial Leaching' - [no particular address given].