Collect Information about Extraction of Metals of low reactivity silver, platinum and Gold

Collect information about extraction of metals of low reactivity silver platinum and gold and prepare a report.

Low Reactive Metals
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Metallurgy Extraction Of metals from ores
Extraction of Metals from Ores

Extraction of metals at the bottom of the activity series (Ag, Hg etc)

Metals at the bottom of the activity series are often found in free state. Their reactivity with other atoms is very low. The oxides of these metals can be reduced to metals by heat alone and sometimes by displacement from their aqueous solutions.

For example When cinnabar (HgS) which is an ore of mercury, heated in air, it is first converted into (HgO) then reduced to mercury on further heating.

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Cinnabar To Mercury
SSC Text Book

Here Ag2S is dissolved in say KCN solution to get dicyanoargentate

(I) ions. From these ions Ag is precipitated by treating with Zn dust powder.[1]

Extraction of Silver by cyanide process

Silver also occurs both in combined state as well as in free state. The important ores of silver are: Argentite (Ag2S), Copper silver glance, Horn silver, Ruby silver. The silver ores are found along with gold ores in some parts of India.

Extraction

Silver is extracted from the ore-argentite (Ag2SAg2S). The process of extraction of silver is called as cyanide process as sodium cyanide solution is used. The ore is crushed, concentrated and then treated with sodium cyanide solution. This reaction forms sodium argento cyanide Na[Ag(CN)2]Na[Ag(CN)2].

AgS+4NaCN⇌2Na[Ag(CN)2]+Na2S(1)(1)AgS+4NaCN⇌2Na[Ag(CN)2]+Na2S

The solution of sodium argento cyanide combines with zinc dust and forms sodium tetra cyanozicate and precipitated silver. This precipitated silver is called spongy silver.

Zn+2Na[Ag(CN)2]→Na2[Zn(CN)4]+2Ag(2)(2)Zn+2Na[Ag(CN)2]→Na2[Zn(CN)4]+2Ag

The spongy silver is fused with potassium nitrate to obtain pure silver. Then the silver obtained is purified by electrolytic process.[2]

Extraction of Gold From Gold Ore

Gold is usually found alone or alloyed with mercury or silver, but can also be found in ores such as calaverite, sylvanite, nagyagite, petzite and krennerite.

Most gold ore now comes from either open pit or underground mines. The ores sometimes contain as little as 5/100 of an ounce of gold per ton of rock.

In all methods of gold ore refining, the ore is usually washed and filtered at the mine, then sent to the mill. At the mill, the ore is ground into smaller particles with water, then ground again in a ball mill to further pulverize the ore.

Cyanide

Several processes can then be used to separate the gold from the ore. The most common techniques in the United States use cyanide in various ways. In one, the ground ore is put into a tank containing a weak cyanide solution and zinc is added. The zinc causes a chemical reaction which separates the gold from the ore. The gold is then removed from the solution with a filter press.

For the carbon-in-pulp method, the ground ore is mixed with water before cyanide is added. Then carbon is added to bond with the gold. The carbon-gold particles are put into a caustic carbon solution, separating out the gold.

In heap-leaching, the ore is placed on open-air pads and the cyanide sprayed over it, taking several weeks to leach down to an impervious base. The solution then pours off the pad into a pond and is pumped from there to a recovery plant where the gold is recovered. Heap-leaching helps recover gold from ore that would otherwise be too expensive to process.[3]

Extraction of Platinum from its Ore

Platinum, along with the rest of the platinum-group metals, is obtained commercially as a by-product from nickel and copper mining and processing. During electro refining of copper, noble metals such as silver, gold and the platinum-group metals as well as selenium and tellurium settle to the bottom of the cell as “anode mud”, which forms the starting point for the extraction of the platinum-group metals.

If pure platinum is found in placer deposits or other ores, it is isolated from them by various methods of subtracting impurities. Because platinum is significantly denser than many of its impurities, the lighter impurities can be removed by simply floating them away in a liquid. Platinum is paramagnetic, whereas nickel and iron are both ferromagnetic. These two impurities are thus removed by running an electromagnet over the mixture. Because platinum has a higher melting point than most other substances, many impurities can be burned or melted away without melting the platinum. Finally, platinum is resistant to hydrochloric and sulfuric acids, whereas other substances are readily attacked by them. Metal impurities can be removed by stirring the mixture in either of the two acids and recovering the remaining platinum.

One suitable method for purification for the raw platinum, which contains platinum, gold, and the other platinum-group metals, is to process it with aqua regia, in which palladium, gold and platinum are dissolved, whereas osmium, iridium, ruthenium and rhodium stay unreacted. The gold is precipitated by the addition of iron(II) chloride and after filtering off the gold, the platinum is precipitated as ammonium chloroplatinate by the addition of ammonium chloride. Ammonium chloroplatinate can be converted to platinum by heating. Unprecipitated hexachloroplatinate(IV) may be reduced with elemental zinc, and a similar method is suitable for small scale recovery of platinum from laboratory residues. Mining and refining platinum has environmental impacts.[4]

Sources: This Project was made by Bajrai Online Solutions by using following Sources


[1]  SSC Physical Science Textbook page No. 245

[2] Chemistry LibreTexts

[3] Sciencing

[4] Wikipedia

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