From Minerals To Elements

M.1. Chemicals from the sea.

The Dead Sea – The lowest point on earth.

·        Approx. 400m below sea level.

·        In a Rift valley between E. Africa & Syria

It acts like a vast evaporating basin;

·        Water flows in from river Jordan – but no outflow

·        Very hot (desert conditions)

·        H₂O evaporates in heat (a thick haze is formed – reduces visibility)

·        Over 1000’s of years huge accumulations of salts formed.

·        Hence Dead Sea has an unusually high density.

Dead Sea water contains approx. 350 g/dm³ of salts compared to 40 g/dm³ in ordinary sea water. (Table 1, pg 46. Compares concentrations of various ions in Dead sea water and typical ocean water)

·        43 billion tonnes of salt estimated to be present in the Dead sea

·        Including a relatively high proportion of Bromides

·        A large chemical industry has developed around Dead sea in Israel

·        World’s largest exporter of Bromine cmpds.

·        Over 230 000 tonnes per annum exported.

M2 – Copper From Deep In The Ground.

          Cu was discovered over 11,000 years ago – potters used glazes containing coloured copper minerals – These were reduced to Cu metal accidentally by the hot carbon fire.

e.g.     2CuO   +    C      ------------   2Cu     +     CO₂   (Cu²⁺  +  2e  -----  Cu)

          Cu is durable, attractive and easily shaped (malleable) so it was found to be useful for jewellery, ornaments, pots, utensils……etc.

          Cu can also be alloyed with tin to make bronze (remember the BRONZE age)

Where did it come from?

Cu, Ag, Pb, Fe, Zn, Mb,W and Sn minerals are formed in hydrothermal deposits.i.e. they were formed from hot water !!

          Underground water is at very high pressure – so it’s boiling point is increased – it becomes superheated. (I.e. it is above 100 C, but the pressure prevents it from boiling.)

          These high temps encourage dissolving and normally insoluble compounds will dissolve to quite high concentrations.

·        These hot solutions are left over when magma cools under the Earth’s surface. (Magma is a mixture of rock, water and crust components – crustal soup!)

·        Approx. 300 million years ago upheavals in the Earth’s crust forced magma under the existing rocks (see fig. 18 pg. 54). Most of the magma solidified to form ‘granite intrusions’ leaving the hot mineral rich solutions near the top.

·        As the rocks cooled they cracked and the solutions flooded into the cracks. (cracks 1mm to 1m across)

·        As the solutions cool the minerals crystallise out e.g. Cassiterite (SnO₂), Pyrite (FeS₂), Chalcopyrite (CuFeS₂), Sphalerite (ZnS) and Galena (PbS).

·        A vein of minerals which fills a crack in rock is called a Lode.

SL – M2 Ancient Miners.

Chalcopyrite – CuFeS₂- is the principal copper containing mineral in the hydrothermal lodes. When these lodes are exposed to air and water at the surface the sulphide is oxidised to oxides, hydroxides and carbonates e.g.

Cuprite – Cu₂O                Malachite – Cu₂(OH)₂C0₃

Tenorite – CuO                Azurite – Cu₃(OH)₂(CO₃)₂

·         In early times the richest deposits from the surface were mined and reduced with hot carbon. The sulphide materials were not processed due to lack of knowledge.

·         When surface supplies were depleted miners extracted the minerals from further underground. They found they could roast these ores in air.

CuS      +      1.5O₂    ----------    CuO   +    SO₂

The metal oxide could then be reduced to the metal. (Today roasting techniques give the metal directly)

The Biggest Hole in the World.

·        Demand for Cu rose rapidly towards end of 19^th. Century due to copper being used in electrical applications.

·        The rich deposits of Cu were depleted. All that was left was the 0.5 – 1.0 % ore – This had to be used.

·        This was profitable at Bingham Canyon USA – and here Cu has been extracted since 1896.

·        Even though %Cu by mass is very low, the total amount of Cu available is vast.

·        A process was sought which could handle low grade material in large quantities ( we will look at the current process used in more detail )

5 000 000 000 Tonnes of ore have been removed from the canyon so far !!!

SL M2 Cont…. Rock eating bacteria.

                10% of all Cu produced in USA comes from bacteria which ‘feed’ off chalcopyrite. The bacteria make use of the Fe²⁺ and S^2- leaving the Cu behind.

E.g.’s of bacteria

                                Thiobaciliius Ferro-oxidans (Fe)

                                Thiobacillus thio-oxidans (S)

A series of reactions involved;

                                Fe²⁺  ®   Fe³⁺   +    e^-

                                S^2-  +   4H₂O    ®     SO₄^2-   +  8H⁺   +  8e^-

The electrons produced enter the cells of the bacteria and are used for biochemical processes.

Overall  bacteria convert CuFeS₂ to a solution of Fe(SO₄)₃ and CuSO₄

                4CuFeS₂ +  17H₂O   +  4H⁺  ®   4Cu²⁺   +   4Fe³⁺   +  8SO₄^2-   +   2H₂O

                This process is cheap and non-polluting but unfortunately very slow. It is used on the waste dumps at Bingham canyon.

Ideal conditions for bacteria

q     20 – 25 °C

q     pH 2

q       Acidified water is sprayed onto surface of dump, it slowly seeps into rocks. The bacteria are already present in the rock (they occur naturally).

q       The CuSO₄(aq) produced is dilute and impure – must be concentrated and separated from other metal ions.

q       Scrap iron is used to displace the copper

Fe    + Cu²⁺     ®      Cu    +    Fe²⁺

q       The copper is then further purified by electrolysis.