Extraction of Silver, Lead And Magnesium

ChemistryGeneral Principles And Processes Of Isolation Of ElementsFor JEE aspirants

The extraction of silver, lead and magnesium uses three different ideas. Silver and gold are leached from their ores with sodium cyanide and air, then recovered with zinc. Lead is freed from galena by self-reduction or by carbon reduction, much like copper and iron. Magnesium is too reactive for either, so fused magnesium chloride from sea water or carnallite is electrolysed. The extraction of silver, lead and magnesium is tested in JEE Advanced through the cyanide process and refining methods.

On this page1Choosing a route2Silver3Gold4Lead5Magnesium6Mind map7Solved examples

Key Formulas - Quick Reference

★ Must learnCyanidation: $4Ag + 8NaCN + 2 H_{2} O + O_{2} \to 4Na [Ag (CN)_{2}] + 4NaOH$ (gold likewise, with KCN)
Argentite: $A g_{2} S + 4NaCN ⇌ 2Na [Ag (CN)_{2}] + N a_{2} S$ ; air oxidises the $N a_{2} S$
★ Must learnRecovery with zinc: $2Na [Ag (CN)_{2}] + Zn \to N a_{2} [Zn (CN)_{4}] + 2Ag ↓$
Pattinson: slow cooling, pure Pb crystallises (eutectic 2.6 % Ag, 303 °C). Parkes: Ag dissolves in molten Zn; the Zn-Ag crust is skimmed off
Cupellation: $2Pb + O_{2} \to 2PbO$ (litharge) in a bone-ash cupel
★ Must learnCarats: $% Au = \frac{carat}{24} \times 100$ ; 22 carat = 91.7 % gold
★ Must learnLead by self-reduction: $PbS + 2PbO \to 3Pb + S O_{2}$ ; $PbS + PbS O_{4} \to 2Pb + 2S O_{2}$
Betts refining of lead: impure Pb anode, electrolyte $PbSi F_{6}$ + $H_{2} Si F_{6}$
★ Must learnMagnesium (Dow): $M g^{2 +} + Ca (OH)_{2} \to Mg (OH)_{2} ↓ + C a^{2 +}$ ; fused $MgC l_{2}$ : $M g^{2 +} + 2 e^{-} \to Mg$
Pidgeon: $2 (MgO \cdot CaO) + Si \to 2Mg ↑ + C a_{2} Si O_{4}$ (about 1473 K, vacuum)

1. Choosing the Extraction Route

Silver, gold, lead and magnesium sit on three different branches of metallurgy. What decides the route is how reactive the metal is and what kind of ore it forms.

Metal	Chief ore	Deciding property	Main method
Silver	argentite $A g_{2} S$ ; native	noble; forms a very stable cyanide complex	cyanide process (hydrometallurgy)
Gold	native; tellurides	the most noble; also forms a cyanide complex	cyanide process; chlorination
Lead	galena $PbS$	sulphide of a fairly unreactive metal	self-reduction, or roasting and carbon reduction
Magnesium	carnallite, magnesite, sea water	very reactive: $E^{\circ} (M g^{2 +} /Mg) = - 2.37$ V	electrolysis of fused $MgC l_{2}$ ; silicon reduction

Figure 1 turns this into three questions you can ask about any metal.

Figure 1: Choosing the extraction route. Ask the three questions in order; the first "yes" gives the method. Silver, lead and magnesium each land on a different branch.

Key idea

The ore and the reactivity decide the route: noble metals are leached with cyanide, lead reduces itself, and magnesium needs electrolysis.

2. Extraction of Silver

2.1 Occurrence and ores

Silver is rare, about 0.1 part per million of the Earth's crust, but it has been prized since prehistoric times. It occurs native, usually with copper, gold and platinum metals, and combined, mainly as sulphides.

Ore	Formula	Ag (% by mass)
Argentite (silver glance)	$A g_{2} S$	87.1
Pyrargyrite (ruby silver)	$A g_{3} Sb S_{3}$ ( $3A g_{2} S \cdot S b_{2} S_{3}$ )	59.8
Stromeyerite	(Cu,Ag)₂S	variable
Horn silver (chlorargyrite)	$AgCl$	75.3

Much of the world's silver is a by-product: it comes from silver-bearing (argentiferous) galena and from the anode mud of copper refining.

2.2 The cyanide process (MacArthur-Forrest process)

This is the modern method for argentite and native silver (Figure 2). It rests on two facts: silver and its compounds dissolve in sodium cyanide solution in the presence of air, forming the complex $Na [Ag (CN)_{2}]$ ; and zinc precipitates silver back from that complex.

Concentration by froth flotation.
Cyanidation. The concentrate is ground very fine in a ball mill and stirred with a dilute (0.4-0.6 %) solution of NaCN while air is blown through.
Filtration to separate the complex solution from the gangue.
Precipitation. Zinc dust is added; silver comes down as a black, amorphous precipitate and zinc goes into solution as a complex.
Fusion of the precipitate with $KN O_{3}$ , which oxidises base metals and gives a compact mass of silver.
Refining by cupellation or electrolysis.

The reactions of cyanidation are:

A g_{2} S + 4NaCN ⇌ 2Na [Ag (CN)_{2}] + N a_{2} S

4N a_{2} S + 5 O_{2} + 2 H_{2} O 2N a_{2} S O_{4} + 4NaOH + 2S

4Ag + 8NaCN + 2 H_{2} O + O_{2} 4Na [Ag (CN)_{2}] + 4NaOH

AgCl + 2NaCN Na [Ag (CN)_{2}] + NaCl

The first reaction is reversible. Air oxidises the sodium sulphide as it forms, removing a product, so the equilibrium keeps shifting to the right (Le Chatelier's principle). Air also oxidises any native silver. Zinc then recovers the metal:

2Na [Ag (CN)_{2}] + Zn N a_{2} [Zn (CN)_{4}] + 2Ag ↓

Figure 2: The cyanide (MacArthur-Forrest) process. Cyanide plus air dissolves the silver as a complex; zinc gives it back as metal.

2.3 Why cyanide and air work together

Oxygen in water cannot oxidise silver on its own, because $E^{\circ} (A g^{+} /Ag) = + 0.80$ V is higher than $E^{\circ} (O_{2} /O H^{-}) = + 0.40$ V. Cyanide changes this by binding $A g^{+}$ into a very stable complex (Figure 3).

Figure 3: Why cyanide and air work. Complexing drops the silver and gold couples below the

O_{2} /O H^{-}

couple, so oxygen can oxidise the metals; zinc sits lower still and recovers them.

JEE Advanced

Complexing lowers the reduction potential. For $[Ag (CN)_{2}]^{-} + e^{-} \to Ag + 2C N^{-}$ , $E^{\circ} = - 0.31$ V, far below +0.80 V for free $A g^{+}$ . With oxygen as oxidant:

E_{cell}^{\circ} = 0.40 - (- 0.31) = + 0.71 V

Δ G^{\circ} = - n F E^{\circ} = - 4 \times 96500 \times 0.71 \approx - 274 kJ per mol O_{2}

For gold, $E^{\circ} ([Au (CN)_{2}]^{-} / Au) = - 0.60$ V, so $E_{cell}^{\circ} = + 1.00$ V. The solution is kept alkaline with lime or NaOH, because acid would turn cyanide into volatile, poisonous HCN.

2.4 An older route: amalgamation

In the old patio process the ore was mixed with copper(II) chloride, which converts silver sulphide to silver chloride, and then with mercury, which reduces the chloride; the silver dissolves in the excess mercury, which is later distilled off:

A g_{2} S + CuC l_{2} 2AgCl + CuS

2AgCl + 2Hg 2Ag + H g_{2} C l_{2}

2.5 Silver from argentiferous lead

Lead from silver-bearing galena contains too little silver to cupel directly, so the silver is first concentrated by the Pattinson or the Parkes process, and the remaining lead is then removed by cupellation.

Pattinson's process uses the lead-silver phase diagram (Figure 4). A eutectic containing about 2.6 % silver melts at 303 °C, while pure lead melts at 327 °C. When molten argentiferous lead is cooled slowly, crystals of pure lead separate first and are removed with perforated ladles. The melt left behind grows richer in silver until it reaches about 2.6 % Ag.

Figure 4: Pattinson's process. Cooling a melt with little silver makes pure lead crystallise first, so the liquid left behind is enriched in silver, up to about 2.6 %.

Parkes' process, now the usual method, uses zinc (Figure 5). It depends on three facts: molten zinc and lead hardly mix; silver is much more soluble in zinc than in lead; and the zinc-silver alloy is lighter than lead and solidifies at a higher temperature. Zinc is stirred into the molten lead, and on cooling a crust of Zn-Ag alloy forms on top and is skimmed off. The zinc is distilled off (b.p. 1180 K) and the silver-rich residue is cupelled.

Figure 5: Parkes process. Silver prefers zinc to lead; the light Zn-Ag crust is skimmed off, the zinc distilled away and the last lead removed by cupellation.

Pattinson's process

Physical: fractional crystallisation.
Pure lead crystallises; silver stays in the melt.
Limit: melt reaches about 2.6 % Ag (the eutectic).
Slow, many crystallisations; now obsolete.

Parkes' process

Uses a second metal: zinc.
Silver moves into the zinc, which floats and sets first.
Zinc is recovered by distillation.
Fast and thorough; the method used today.

Cupellation (Figure 6). The silver-rich alloy is heated in a shallow, porous dish of bone ash (a cupel) in a blast of air. Lead is oxidised to litharge, PbO, which is blown away or soaks into the cupel; noble silver is not oxidised and is left as a bright bead.

2Pb + O_{2} Δ 2PbO

Figure 6: Cupellation. Air oxidises the lead but not the noble silver; the litharge leaves as fume or sinks into the cupel.

2.6 Refining of silver

Crude silver may contain lead, copper and gold. Lead is removed by cupellation. For the purest silver, electrolytic refining is used: impure silver is the anode, a thin sheet of pure silver the cathode, and silver nitrate solution acidified with nitric acid the electrolyte. Copper dissolves and stays in solution; gold falls as anode mud.

Ag anode A g^{+} + e^{-}

A g^{+} + e^{-} cathode Ag

Exam Trick

Cyanide, Air, Zinc. Dissolve with cyanide, oxidise with air, drop with zinc. If a question asks for the oxidising agent, it is $O_{2}$ ; the reducing agent is Zn; the complexing agent is $C N^{-}$ .

Key idea

Silver dissolves in cyanide only because complexing lets air oxidise it; zinc, a stronger reducing agent, gives it back.

3. Extraction of Gold

3.1 Occurrence

Gold occurs mostly native: as vein gold in quartz rock and as alluvial (placer) gold in river sands. Small amounts are combined, as tellurides such as calaverite $AuT e_{2}$ and sylvanite (Au,Ag)Te₂, and in maldonite (bismuth aurite), $A u_{2} Bi$ . In India, gold is mined at Hutti in Karnataka.

3.2 The cyanide process for gold

The ore is concentrated by froth flotation and roasted to remove tellurium, arsenic and sulphur as volatile oxides. It is then leached with dilute KCN or NaCN in the presence of air, and the gold is recovered with zinc dust:

4Au + 8KCN + 2 H_{2} O + O_{2} 4K [Au (CN)_{2}] + 4KOH

2K [Au (CN)_{2}] + Zn K_{2} [Zn (CN)_{4}] + 2Au ↓

3.3 Plattner's chlorination process

The roasted ore is moistened and placed in wooden vats with perforated false bottoms, and chlorine is passed through it. Gold forms gold(III) chloride, which is leached out with water and reduced to the metal with iron(II) sulphate or hydrogen sulphide:

2Au + 3C l_{2} 2AuC l_{3}

AuC l_{3} + 3FeS O_{4} Au ↓ + FeC l_{3} + F e_{2} (S O_{4})_{3}

2AuC l_{3} + 3 H_{2} S 2Au ↓ + 3S + 6HCl

3.4 Parting

Crude gold often contains silver and copper. Removing them is called parting: the alloy is heated with concentrated sulphuric acid (or nitric acid), which dissolves Ag and Cu but not Au:

Cu + 2 H_{2} S O_{4} CuS O_{4} + S O_{2} + 2 H_{2} O

2Ag + 2 H_{2} S O_{4} A g_{2} S O_{4} + S O_{2} + 2 H_{2} O

3.5 Purity of gold: carats

Pure gold is too soft for jewellery, so it is alloyed with silver or copper. Purity is stated in carats: the number of parts of gold in 24 parts of alloy (Figure 7). Pure gold is 24 carat; 22 carat, used for most Indian jewellery, is $\frac{22}{24} \times 100 = 91.7 %$ gold; 14 carat contains 14 parts gold and 10 parts other metal, $\frac{14}{24} \times 100 = 58.3 %$ .

Figure 7: Carat means parts of gold per 24 parts of alloy, so

% Au = \frac{carat}{24} \times 100

3.6 Properties of gold

Gold is yellow, soft, very malleable and ductile, and a good conductor. It is not attacked by air, water or single acids, but dissolves in aqua regia (3 parts concentrated HCl to 1 part concentrated $HN O_{3}$ ), where nitric acid oxidises the gold and chloride ions hold it as a complex. Its main oxidation states are +1 and +3.

Au + 4 H^{+} + N O_{3}^{-} + 4C l^{-} [AuC l_{4}]^{-} + NO + 2 H_{2} O

Exam Trick

Carat over 24. Divide the carat number by 24: 22/24 = 91.7 %, 18/24 = 75 %, 14/24 = 58.3 %. "14 carat" never means 14 %.

Quick Recall: tap to check

What does air do in cyanidation of argentite?

It oxidises

N a_{2} S

(shifting the equilibrium) and oxidises metallic silver into the complex.

Which metal precipitates silver and gold from their cyanide complexes?

Zinc dust:

2 [Ag (CN)_{2}]^{-} + Zn \to [Zn (CN)_{4}]^{2 -} + 2Ag

What is removed in cupellation, and as what?

Lead, as litharge PbO, blown off or absorbed by the bone-ash cupel.

What percentage of gold is 18 carat gold?

18/24 × 100 = 75 %.

4. Extraction of Lead

4.1 Ores of lead

Lead does not occur free. Its ores are:

Ore	Formula	Pb (% by mass)
Galena	$PbS$	86.6 (chief ore)
Cerussite	$PbC O_{3}$	77.5
Anglesite	$PbS O_{4}$	68.3
Lanarkite	$PbO \cdot PbS O_{4}$	-
Crocoite	$PbCr O_{4}$	-

4.2 Concentration

Galena is concentrated by froth flotation. When zinc blende is also present, NaCN is added as a depressant so that only PbS floats. Two routes then lead to crude lead (Figure 8).

4.3 Self-reduction (air reduction) in a reverberatory furnace

Stage 1. The ore is heated in a limited supply of air at a moderate temperature, so that only part of the galena is oxidised to PbO and $PbS O_{4}$ :

3PbS + 5 O_{2} Δ 2PbO + PbS O_{4} + 2S O_{2}

Stage 2. The air supply is cut off and the temperature is raised. The unchanged sulphide now reduces the oxide and the sulphate, and molten lead runs out:

PbS + 2PbO Δ 3Pb + S O_{2}

PbS + PbS O_{4} Δ 2Pb + 2S O_{2}

4.4 Carbon reduction in a blast furnace

Alternatively the galena is roasted almost completely, and the roasted ore (mainly PbO with some $PbS O_{4}$ ) is smelted with coke, limestone and a little silica in a blast furnace:

PbO + C Pb + CO

PbO + CO Pb + C O_{2}

2PbS O_{4} + 2Si O_{2} 2PbSi O_{3} + 2S O_{2} + O_{2}

CaC O_{3} CaO + C O_{2}

CaO + PbSi O_{3} CaSi O_{3} + PbO

Lime frees the lead from lead silicate, so it can be reduced too, and calcium silicate leaves as slag.

Figure 8: Lead from galena. Self-reduction (like copper) or carbon reduction (like iron) both give crude lead, which is refined in stages.

Self-reduction

Reverberatory furnace; no carbon.
Partial roast, then air cut off.
PbS reduces PbO and $PbS O_{4}$ .
Suits rich galena; same idea as copper.

Carbon reduction

Blast furnace with coke and limestone.
Full roast to PbO first.
C and CO reduce PbO.
Suits poorer, mixed ores; same idea as iron.

4.5 Refining of lead

Crude lead contains Cu, Ag, Au, Sn, As, Sb and Bi, which make it hard. It is refined in stages:

Drossing: the melt is cooled close to its melting point, and copper separates as a solid crust (dross) that is skimmed off.
Softening: air is blown over the molten lead in a reverberatory furnace; Sn, As and Sb are oxidised and removed as dross, making the lead soft.
Desilverisation by the Parkes process (section 2.5).
Electrolytic refining (Betts process): impure lead anodes, thin pure-lead cathodes, and an electrolyte of lead fluorosilicate $PbSi F_{6}$ with hydrofluorosilicic acid $H_{2} Si F_{6}$ and a little gelatin for a smooth deposit. Bi, Ag, Au, Sb and As collect in the anode mud; the lead is 99.99 % pure.

4.6 Uses of lead

Lead storage batteries, cable sheathing, radiation shields, solder and bullets. Its compounds were once used as pigments (red lead $P b_{3} O_{4}$ , white lead $2PbC O_{3} \cdot Pb (OH)_{2}$ , chrome yellow $PbCr O_{4}$ ) and as the anti-knock agent tetraethyl lead, which has been phased out of petrol because lead is toxic.

Quick Recall: tap to check

Why is the air supply cut off in the second stage of self-reduction?

So that the remaining PbS is not oxidised; it is needed to reduce PbO and

PbS O_{4}

to lead.

Name the electrolyte in the Betts process.

Lead fluorosilicate

PbSi F_{6}

with

H_{2} Si F_{6}

(and a little gelatin).

What is softening of lead?

Oxidising Sn, As and Sb out of molten lead with air, which makes the lead soft.

Key idea

Galena reduces itself: part of it is roasted into the oxidant for the rest, exactly like copper glance.

5. Extraction of Magnesium

5.1 Sources

Source	Formula	Mg (% by mass)
Magnesite	$MgC O_{3}$	28.8
Dolomite	$MgC O_{3} \cdot CaC O_{3}$	13.2
Carnallite	$KCl \cdot MgC l_{2} \cdot 6 H_{2} O$	8.75
Kieserite	$MgS O_{4} \cdot H_{2} O$	17.6
Sea water	$M g^{2 +}$ (as chloride and sulphate)	about 0.13

5.2 Why electrolysis?

Magnesium is very reactive: $E^{\circ} (M g^{2 +} /Mg) = - 2.37$ V. Its oxide is so stable that carbon could reduce it only above about 2000 K, and in water the cathode would give hydrogen instead of magnesium. So magnesium is made by electrolysing a dry, fused chloride, or by reducing its oxide with silicon under vacuum.

5.3 Dow's process (from sea water)

Oyster shells or limestone are calcined and slaked to give slaked lime: $CaC O_{3} \to CaO + C O_{2}$ , $CaO + H_{2} O \to Ca (OH)_{2}$ .
Slaked lime added to sea water precipitates magnesium hydroxide, which is less soluble than calcium hydroxide: $MgC l_{2} + Ca (OH)_{2} \to Mg (OH)_{2} ↓ + CaC l_{2}$ .
The hydroxide is filtered off and dissolved in hydrochloric acid: $Mg (OH)_{2} + 2HCl \to MgC l_{2} + 2 H_{2} O$ .
The solution is evaporated and $MgC l_{2} \cdot 6 H_{2} O$ crystallises.
The crystals are partly dehydrated by heating in a current of dry HCl gas.
Fused $MgC l_{2}$ , mixed with NaCl and $CaC l_{2}$ , is electrolysed at 973-1023 K.

Dry HCl is essential. Heated in air, hydrated magnesium chloride hydrolyses to magnesia, which is useless for electrolysis:

MgC l_{2} \cdot 6 H_{2} O Δ MgO + 2HCl + 5 H_{2} O

Figure 9: Dow's process. Magnesium is pulled out of sea water as the insoluble hydroxide, turned into chloride and electrolysed; the chlorine goes round again as HCl.

5.4 The electrolytic cell

The cell is a steel pot that acts as the cathode; a graphite anode dips into the melt inside a porcelain hood (Figure 10). NaCl and $CaC l_{2}$ lower the melting point and raise the conductivity. Molten magnesium (m.p. 923 K) is lighter than the fused chlorides, so it rises and floats; it is ladled out under a stream of inert gas so that it does not burn. The hood keeps the chlorine away from the magnesium, and the chlorine is converted into HCl for step 3.

M g^{2 +} + 2 e^{-} cathode Mg

2C l^{-} anode C l_{2} + 2 e^{-}

Figure 10: The magnesium cell. Unlike aluminium, molten magnesium is lighter than its electrolyte, so it floats; the hood stops it meeting the chlorine.

5.5 From carnallite

Carnallite is dehydrated (again in HCl gas) and the fused KCl- $MgC l_{2}$ mixture is electrolysed in the same kind of cell; the potassium chloride acts as the melting-point-lowering additive.

5.6 Pidgeon (silicothermic) process

Calcined dolomite ( $MgO \cdot CaO$ ) is mixed with ferrosilicon, pressed into briquettes and heated to about 1473 K in a steel retort under vacuum (Figure 11):

2 (MgO \cdot CaO) + Si 1473 K, vacuum 2Mg ↑ + C a_{2} Si O_{4}

On its own this reduction is uphill, but two things pull it forward: lime ties up the silica as calcium orthosilicate, and the vacuum removes magnesium vapour as fast as it forms. The vapour condenses as crystals at the water-cooled end.

Figure 11: Pidgeon process. The vacuum sweeps magnesium vapour away and lime ties up the silica, which pulls an otherwise uphill reduction forward.

Electrolytic route (Dow, carnallite)

Raw material: sea water or carnallite.
Needs dry fused $MgC l_{2}$ .
Energy as electricity; $C l_{2}$ recycled.
Mg floats on the melt.

Pidgeon process

Raw material: dolomite.
Reducing agent: silicon (ferrosilicon).
Heat and vacuum drive it forward.
Mg distils off as vapour.

Exam Trick

Magnesium floats, aluminium sinks. In the magnesium cell the metal is lighter than the chloride melt and is skimmed from the top; in the Hall-Héroult cell aluminium is heavier than cryolite and collects at the bottom.

Quick Recall: tap to check

Why is

MgC l_{2} \cdot 6 H_{2} O

dehydrated in dry HCl gas?

Heating in air hydrolyses it to MgO:

MgC l_{2} \cdot 6 H_{2} O \to MgO + 2HCl + 5 H_{2} O

Which electrode is the steel pot in the magnesium cell?

The cathode.

What drives the Pidgeon reduction forward?

Lime binds silica as

C a_{2} Si O_{4}

and the vacuum removes Mg vapour.

Key idea

Magnesium is won only by electrolysing a dry fused chloride, or by silicon under vacuum; carbon and aqueous electrolysis both fail.

6. Summary and Mind Map

Metal	Ore	Extraction	Refining
Silver	argentite $A g_{2} S$ ; argentiferous lead	cyanide process; Parkes or Pattinson + cupellation	cupellation; electrolysis ( $AgN O_{3}$ + $HN O_{3}$ )
Gold	native; tellurides	cyanide process; Plattner's chlorination	parting with conc. $H_{2} S O_{4}$ ; electrolysis
Lead	galena $PbS$	self-reduction; or roasting + C/CO	drossing, softening, Parkes, Betts
Magnesium	sea water, carnallite, dolomite	electrolysis of fused $MgC l_{2}$ ; Pidgeon	sublimation or remelting

Figure 12: Mind map of the four metals. Two noble metals leach with cyanide, lead reduces itself like copper, and magnesium needs electrolysis or silicon.

7. Solved Examples

Solved Example 1

Lead can also be obtained by reducing the roasted ore with coke. Outline the process.

Solution:

Galena is roasted in air to lead monoxide with some lead sulphate:

3PbS + 5 O_{2} 2PbO + PbS O_{4} + 2S O_{2}

The roasted ore is smelted with coke, limestone and a little silica in a blast furnace. Coke and CO reduce the oxide, and lime frees lead from lead silicate:

PbO + C Pb + CO

PbO + CO Pb + C O_{2}

2PbS O_{4} + 2Si O_{2} 2PbSi O_{3} + 2S O_{2} + O_{2}

CaO + PbSi O_{3} CaSi O_{3} + PbO

Calcium silicate slag floats off; molten lead is tapped from the bottom.

Solved Example 2

In the cyanide process for silver, zinc acts as
(A) an oxidising agent
(B) a reducing agent
(C) a complexing agent
(D) a flux

Solution:

Answer: (B). Zinc is oxidised to Zn(II) and reduces silver(I) in the complex to silver metal: $2 [Ag (CN)_{2}]^{-} + Zn \to [Zn (CN)_{4}]^{2 -} + 2Ag$ . Cyanide is the complexing agent and oxygen the oxidising agent.

Solved Example 3

Why is air blown through the solution during the cyanidation of argentite?

Solution:

For two reasons. First, $A g_{2} S + 4NaCN ⇌ 2Na [Ag (CN)_{2}] + N a_{2} S$ is reversible; air oxidises the $N a_{2} S$ , $4N a_{2} S + 5 O_{2} + 2 H_{2} O \to 2N a_{2} S O_{4} + 4NaOH + 2S$ , so the equilibrium moves forward. Second, oxygen oxidises any metallic silver into the complex: $4Ag + 8NaCN + 2 H_{2} O + O_{2} \to 4Na [Ag (CN)_{2}] + 4NaOH$ .

Solved Example 4

Using

4Ag + 8NaCN + 2 H_{2} O + O_{2} \to 4Na [Ag (CN)_{2}] + 4NaOH

, find the minimum mass of NaCN needed to dissolve 1.00 kg of silver, and the mass of zinc needed to recover it. (Ag = 107.9, NaCN = 49.0, Zn = 65.4)

Solution:

Moles of Ag $= \frac{1000}{107.9} = 9.27$ mol. Each Ag needs 2 NaCN, so NaCN $= 2 \times 9.27 \times 49.0 \approx 908$ g.

2Na [Ag (CN)_{2}] + Zn N a_{2} [Zn (CN)_{4}] + 2Ag

One Zn recovers two Ag, so zinc $= \frac{9.27}{2} \times 65.4 \approx 303$ g. In practice extra cyanide is used, because some is lost to other metals in the ore.

Solved Example 5

A chain sold as 22 carat gold weighs 10.0 g. How much gold does it contain? A 14 carat alloy contains only gold and copper; what percentage is copper?

Solution:

Gold in the chain $= 10.0 \times \frac{22}{24} = 9.17$ g.

14 carat is $\frac{14}{24} \times 100 = 58.3 %$ gold, so copper $= 100 - 58.3 = 41.7 %$ .

Solved Example 6

Parkes' process for desilverising lead works because
(A) silver is far more soluble in molten zinc than in lead, and the Zn-Ag alloy is lighter and sets first
(B) lead and silver form a eutectic with 2.6 % Ag
(C) silver is more volatile than lead
(D) lead is oxidised selectively in air

Solution:

Answer: (A). Option (B) is the basis of Pattinson's process, and (D) is cupellation. Silver is not volatile at these temperatures.

Solved Example 7

A magnesium cell works at 100 000 A for 1.00 hour. What mass of magnesium is produced, and what volume of chlorine (at STP, 22.4 L mol^-1)? (Mg = 24.3, F = 96 500 C mol^-1)

Solution:

MgC l_{2} Mg + C l_{2}

m = \frac{I tM}{n F} = \frac{1.00 \times 1 0 ^{5} \times 3600 \times 24.3}{2 \times 96 500} = 4.53 \times 1 0^{4} g = 45.3 kg

Moles of Mg $= \frac{45 300}{24.3} \approx 1865$ mol, and 1 mol $C l_{2}$ forms per mol Mg. Volume of $C l_{2}$ $= 1865 \times 22.4 \approx 4.18 \times 1 0^{4}$ L $= 41.8 m^{3}$ .

Solved Example 8

In Dow's process, sea water treated with slaked lime gives a precipitate A. A dissolves in hydrochloric acid to give B, which crystallises as C. Heating C in air gives a white solid D, but heating C in dry HCl gas gives anhydrous B, whose electrolysis gives magnesium and a gas E. Identify A to E.

Solution:

Label	Substance	Equation
A	$Mg (OH)_{2}$	$MgC l_{2} + Ca (OH)_{2} \to Mg (OH)_{2} + CaC l_{2}$
B	$MgC l_{2}$	$Mg (OH)_{2} + 2HCl \to MgC l_{2} + 2 H_{2} O$
C	$MgC l_{2} \cdot 6 H_{2} O$	crystallised from solution
D	MgO	$MgC l_{2} \cdot 6 H_{2} O \to MgO + 2HCl + 5 H_{2} O$
E	$C l_{2}$	$MgC l_{2} \to Mg + C l_{2}$ (electrolysis)

Practice Questions

A white metal A (specific gravity 10.5) dissolves in KCN solution in air to give a complex B, from which metal C gives back A. The nitrate of A, heated strongly, gives two gases D and E; D forms colourless dimeric crystals below about −9 °C. A dissolves in HCl in the presence of E to give its halide F. Identify A to F.Answer: A = Ag, B = K[Ag(CN)₂], C = Zn, D = $N O_{2}$ (dimer $N_{2} O_{4}$ ), E = $O_{2}$ , F = AgCl. $2AgN O_{3} \to 2Ag + 2N O_{2} + O_{2}$ ; $4Ag + 4HCl + O_{2} \to 4AgCl + 2 H_{2} O$ .
Why is the cyanide solution kept alkaline during cyanidation?Answer: In acid, cyanide forms volatile, highly poisonous HCN; lime or NaOH keeps it as $C N^{-}$ .
What is litharge, and in which process is it formed?Answer: Lead(II) oxide, PbO; formed in cupellation.
Name the anode, cathode and electrolyte in the Betts process.Answer: Impure lead anode, pure lead cathode, $PbSi F_{6}$ + $H_{2} Si F_{6}$ with a little gelatin.
Why does molten magnesium collect at the top of the electrolytic cell?Answer: It is lighter than the fused chloride electrolyte.
What is 18 carat gold, and how much gold is in a 5.0 g ring of it?Answer: 75 % gold; 3.75 g.
Why can magnesium not be obtained by electrolysing aqueous $MgC l_{2}$ ?Answer: Water is reduced to $H_{2}$ at the cathode, because $E^{\circ} (M g^{2 +} /Mg) = - 2.37$ V is far below that of water.

Common Mistakes to Avoid

Watch out

Calling zinc the complexing agent in the cyanide process. Zinc is the reducing agent; cyanide is the complexing agent.
Leaving out air. Without $O_{2}$ , silver and gold do not dissolve in cyanide.
Swapping the desilverisation methods: Parkes uses zinc; Pattinson uses slow cooling (the eutectic).
Saying cupellation oxidises silver. It oxidises the lead; silver is left behind.
Reading 14 carat as 14 % gold. It is 14/24 = 58.3 % gold.
Heating $MgC l_{2} \cdot 6 H_{2} O$ in air to make $MgC l_{2}$ . It hydrolyses to MgO; dry HCl gas is needed.
Thinking magnesium sinks like aluminium. Magnesium floats on its electrolyte.
Keeping air on throughout the self-reduction of galena. Air is cut off in the second stage.

Frequently Asked Questions

How is silver extracted by the cyanide process?

Argentite is concentrated by froth flotation and stirred with dilute sodium cyanide while air is blown through. Silver dissolves as sodium dicyanidoargentate, and air oxidises the sodium sulphide formed. Zinc dust then precipitates silver, which is fused with potassium nitrate and refined by cupellation or electrolysis.

Why is zinc used in the cyanide process?

Zinc is a stronger reducing agent than silver or gold, and it forms its own stable cyanide complex. Added as dust to the cyanide solution, it reduces the silver or gold complex to the metal, which precipitates, while zinc goes into solution as the tetracyanidozincate ion.

What is the difference between the Parkes and Pattinson processes?

Both concentrate silver from argentiferous lead. Pattinson's process cools the melt slowly so that pure lead crystallises out, enriching the liquid to about 2.6 percent silver. Parkes' process adds zinc, which extracts the silver as a light zinc-silver crust that is skimmed off; the zinc is then distilled away.

What does 22 carat gold mean?

Carat gives the parts of pure gold in 24 parts of alloy. So 22 carat gold contains 22 parts gold and 2 parts other metals such as copper or silver, which is 91.7 percent gold. Pure gold is 24 carat, and 18 carat is 75 percent gold.

How is lead extracted from galena?

Galena is concentrated by froth flotation. In the self-reduction route it is partly roasted to lead oxide and lead sulphate, then heated without air so that the remaining sulphide reduces them to lead. Alternatively it is fully roasted and smelted with coke and limestone in a blast furnace.

How is magnesium extracted from sea water?

In Dow's process slaked lime precipitates magnesium hydroxide from sea water. The hydroxide is dissolved in hydrochloric acid, and the chloride is crystallised, dehydrated in dry hydrogen chloride and electrolysed as a fused mixture with sodium and calcium chlorides. Magnesium floats on the melt; chlorine is recycled as hydrogen chloride.

Is the cyanide process needed for NEET?

Metallurgy was removed from the NEET syllabus in 2024, so the full cyanide process is not examined. NEET students still meet its chemistry in Coordination Compounds, where gold dissolving as the dicyanidoaurate complex in cyanide, oxygen and water, and its recovery with zinc, illustrate a use of complexes.

Which questions on silver, lead and magnesium appear in JEE Advanced?

JEE Advanced names the cyanide process for silver and gold in its syllabus. Questions ask for the roles of cyanide, air and zinc, the reactions of self-reduction of galena, the choice of refining method such as cupellation or electrolysis, and why magnesium and similar reactive metals need electrolysis of fused salts.

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