Studies of Heavy Metals

Open-hearth (Siemens–Martin) process

In this process, steel is made from pig (cast) iron in a shallow hearth heated by burning producer gas. The furnace uses the regenerative system of heating, which keeps the temperature at about 1500–1600°C.

Raw materials (charge): pig iron, steel scrap and a calculated amount of haematite ($Fe_2O_3$), which acts as the oxidising agent. Limestone is added when the lining is basic.

Furnace and lining

• The hearth is lined with silica (acid lining) when the pig iron has little phosphorus.
• It is lined with calcined dolomite, $CaO \cdot MgO$ (basic lining) when the pig iron has a lot of phosphorus.
• Below the hearth there are four chambers (regenerators) filled with a checker-work of fire bricks. Two of them heat the incoming producer gas and air, while the other two are heated by the outgoing hot waste gases. The direction of flow is reversed every 20–30 minutes, so the heat of the waste gases is not lost.

        gas + air in                     waste gases out
             |                                ^
             v      ___________________      |
          ---->   /    molten charge    \  ---->
                 |______________________|        <- hearth (lined)
        +--------+--------+  +--------+--------+
        | regen. | regen. |  | regen. | regen. |   checker-work
        | (air)  | (gas)  |  | (air)  | (gas)  |   chambers
        +--------+--------+  +--------+--------+
          (heated by waste gas; flow reversed every 20-30 min)

Reactions: the oxygen of $Fe_2O_3$ oxidises the impurities of pig iron.

$$Fe_2O_3 + 3C \rightarrow 2Fe + 3CO\uparrow$$ $$2Fe_2O_3 + 3Si \rightarrow 4Fe + 3SiO_2$$ $$Fe_2O_3 + 3Mn \rightarrow 2Fe + 3MnO$$ $$5Fe_2O_3 + 6P \rightarrow 10Fe + 3P_2O_5$$ $$2Fe_2O_3 + 3S \rightarrow 4Fe + 3SO_2\uparrow$$

The oxides form slag:

$$MnO + SiO_2 \rightarrow MnSiO_3 \quad (\text{slag})$$ $$CaO + SiO_2 \rightarrow CaSiO_3; \qquad 3CaO + P_2O_5 \rightarrow Ca_3(PO_4)_2 \quad (\text{basic lining, Thomas slag})$$

Finishing: samples are drawn from time to time and analysed. When the carbon content reaches the required level, a calculated amount of spiegeleisen (an Fe–Mn–C alloy) is added. It supplies the needed carbon and manganese, and the Mn removes dissolved oxygen as slag. The steel is then tapped off.

Advantages over the Bessemer process

• The temperature is controlled externally, so scrap iron can also be used.
• The process is slow (8–10 hours), so samples can be tested and the composition controlled exactly. This gives steel of better quality.
• Heat is economised by the regenerative system, and larger batches can be made.

a) Metal and ore

The metal is iron (Fe) and the ore is haematite, $Fe_2O_3$.

b) Blast furnace

The blast furnace is a tall steel tower (about 25–30 m) lined inside with fire bricks. The charge (calcined ore + coke + limestone in about 8 : 4 : 1) enters at the top through a cup-and-cone arrangement. Hot air is blown in near the bottom through tuyeres.

                 charge (ore + coke + limestone)
                          |
                    [ cup & cone ]
   waste gases <==  |            |  ==> waste gases (CO, CO2, N2)
                   /              \
                  /   ZONE OF      \     400-700 °C
                 /    REDUCTION     \    Fe2O3 + 3CO -> 2Fe + 3CO2
                /                    \
               /   ZONE OF SLAG       \  about 1000 °C
              /    FORMATION           \ CaO + SiO2 -> CaSiO3
             |                          |
             |   ZONE OF FUSION         | 1200-1300 °C
             |  (heat absorption)       | CO2 + C -> 2CO ; Fe melts
              \                        /
  hot air ==>  \  ZONE OF COMBUSTION  / <== hot air   1500-1600 °C
  (tuyere)      \  C + O2 -> CO2     /     (tuyere)
                 |~~~~ slag ~~~~~~~~|---> slag outlet
                 |#### molten iron #|---> iron (tap hole)
                 +------------------+
                       hearth

Molten iron collects at the bottom of the hearth, with lighter molten slag floating on it, and each is tapped off separately. The iron obtained is pig iron.

c) Slag formation reactions

Limestone decomposes to lime, which combines with the silica gangue (and other acidic impurities) to form fusible slag:

$$CaCO_3 \xrightarrow{\Delta} CaO + CO_2$$ $$CaO + SiO_2 \rightarrow CaSiO_3 \quad (\text{calcium silicate, slag})$$ $$CaO + Al_2O_3 \rightarrow Ca(AlO_2)_2 \quad (\text{calcium aluminate})$$

(a) Metal and ore

M is zinc (Zn), atomic number 30. Its main ore is zinc blende, $ZnS$. Calamine, $ZnCO_3$, is another ore.

(b) Why zinc is not a transition element

A transition element must have a partly filled d-subshell in the atom or in one of its common ions. Zinc has a completely filled $3d^{10}$ subshell in the atom and also in its only ion, $Zn^{2+}$ ($[Ar]\,3d^{10}$). So it shows no variable valency and forms colourless, diamagnetic compounds. It is not a true transition element.

(c) Vertical retort and reduction

Roasted ore ($ZnO$) is made into briquettes with coke and fed into the top of a tall vertical fireclay retort. It is heated externally by producer gas to about 1400°C. Zinc vapour passes out at the top and condenses into molten zinc (spelter) in a condenser. The spent residue is removed from the bottom.

          briquettes (ZnO + coke)
                  |
                  v
             +---------+------> Zn vapour + CO
             |         |            |
  producer   |         |            v
  gas  ===>  | retort  |  <=== [ condenser ] --> molten Zn
  (heating)  | (fire-  |
  chamber    |  clay)  |
             |  ZnO+C  |
             | ~1400°C |
             +---------+
                  |
                  v
           spent residue out

Reduction reaction:

$$ZnO + C \xrightarrow{1400^\circ C} Zn\uparrow + CO\uparrow$$

(d) Zinc exposed to air for a long time

Its surface becomes dull. Zinc reacts with oxygen, carbon dioxide and moisture of the air to form a thin, firm layer of basic zinc carbonate. This layer protects the metal from further corrosion:

$$4Zn + 2O_2 + 3H_2O + CO_2 \rightarrow ZnCO_3 \cdot 3Zn(OH)_2$$

(a) Flux

A flux is a substance added to the ore in metallurgy to combine with the infusible gangue and form a fusible mass. This fusible product is called slag (so slag is the product, not the reactant 'A'). For example, $CaO + SiO_2 \rightarrow CaSiO_3$ (slag).

(b) $Hg_2O$

When mercury comes in contact with ozone, it is oxidised to mercurous oxide: $2Hg + O_3 \rightarrow Hg_2O + O_2$. The oxide dissolves in the mercury, which then loses its convex meniscus and sticks to glass, leaving a "tail". This is called tailing of mercury.

(b) Sn and Cu

Bell metal is a bronze containing about 80% copper and 20% tin. It is hard and gives a ringing sound, so it is used to make bells and gongs.

i) Preparation of white vitriol ($ZnSO_4 \cdot 7H_2O$)

Zinc metal, zinc oxide or zinc carbonate is dissolved in dilute sulphuric acid:

$$Zn + H_2SO_4\,(\text{dil.}) \rightarrow ZnSO_4 + H_2\uparrow$$ $$ZnO + H_2SO_4 \rightarrow ZnSO_4 + H_2O$$

The solution is filtered, concentrated and cooled. Colourless crystals of $ZnSO_4 \cdot 7H_2O$ separate out.

ii) Heating to 800°C

On heating, white vitriol first loses its water of crystallisation. Above about 800°C, the anhydrous salt decomposes into zinc oxide, sulphur dioxide and oxygen:

$$ZnSO_4 \cdot 7H_2O \xrightarrow{100^\circ C} ZnSO_4 \cdot H_2O \xrightarrow{280^\circ C} ZnSO_4$$ $$2ZnSO_4 \xrightarrow{800^\circ C} 2ZnO + 2SO_2 + O_2$$

iii) Double salt

A double salt is formed when two simple salts crystallise together in a fixed ratio. It exists only in the solid state; in water it dissociates completely into all its simple ions. It does not give a complex ion.

Example: Mohr's salt, $FeSO_4 \cdot (NH_4)_2SO_4 \cdot 6H_2O$, or potash alum, $K_2SO_4 \cdot Al_2(SO_4)_3 \cdot 24H_2O$.

iv) Lithopone from white vitriol

A solution of zinc sulphate is mixed with barium sulphide. A white precipitate of $ZnS$ and $BaSO_4$ forms together; this mixture is lithopone, a white pigment:

$$ZnSO_4 + BaS \rightarrow \underbrace{ZnS + BaSO_4}_{\text{lithopone}}$$

v) Zinc as a non-typical transition metal

Zinc ($[Ar]\,3d^{10}4s^2$) has a completely filled d-subshell both as the atom and as its only common ion, $Zn^{2+}$ ($3d^{10}$). So it does not show the typical transition properties: it has no variable oxidation state, forms colourless and diamagnetic compounds, and has a low melting point. It is placed in the d-block only because its last electron enters the d-subshell, so it is called a non-typical transition metal.

i) Open-hearth process

It is a process (Siemens–Martin) for making steel from pig iron. A mixture of pig iron, steel scrap and haematite is heated to about 1500–1600°C on a shallow hearth by burning producer gas, using the regenerative system of heat economy. The oxygen of haematite oxidises the impurities (C, Si, Mn, P, S) of pig iron, which are removed as gases or slag.

ii) Chemical reactions in the open-hearth furnace

Removal of impurities by haematite:

$$Fe_2O_3 + 3C \rightarrow 2Fe + 3CO\uparrow$$ $$2Fe_2O_3 + 3Si \rightarrow 4Fe + 3SiO_2$$ $$Fe_2O_3 + 3Mn \rightarrow 2Fe + 3MnO$$ $$5Fe_2O_3 + 6P \rightarrow 10Fe + 3P_2O_5$$ $$2Fe_2O_3 + 3S \rightarrow 4Fe + 3SO_2\uparrow$$

Slag formation:

$$MnO + SiO_2 \rightarrow MnSiO_3$$ $$CaO + SiO_2 \rightarrow CaSiO_3; \qquad 3CaO + P_2O_5 \rightarrow Ca_3(PO_4)_2 \ (\text{basic lining})$$

iii) Why spiegeleisen is added

Spiegeleisen is an alloy of iron, manganese (about 20–30%) and carbon.

• It adds the required amount of carbon (and Mn) to the decarburised iron, which turns it into steel of the desired grade.
• Its manganese acts as a deoxidiser: it removes dissolved oxygen/FeO ($FeO + Mn \rightarrow Fe + MnO$), which passes into the slag. This prevents blow-holes and makes the steel sound and hard.

iv) Composition of stainless steel

(b) $CuO$

The blue salt is $CuSO_4 \cdot 5H_2O$ ($Cu^{2+}$ is in group II). With NaOH it gives a blue precipitate of $Cu(OH)_2$, which on boiling loses water and forms black cupric oxide:

$$CuSO_4 + 2NaOH \rightarrow Cu(OH)_2\downarrow\,(\text{blue}) + Na_2SO_4$$ $$Cu(OH)_2 \xrightarrow{\Delta} CuO\,(\text{black}) + H_2O$$

i) Zinc with hot conc. NaOH

Zinc dissolves to form sodium zincate and hydrogen gas:

$$Zn + 2NaOH \xrightarrow{\Delta} Na_2ZnO_2 + H_2\uparrow$$

ii) Corrosive sublimate ($HgCl_2$) with excess KI

First a scarlet-red precipitate of mercuric iodide forms. It dissolves in excess KI to give a colourless solution of potassium tetraiodomercurate(II):

$$HgCl_2 + 2KI \rightarrow HgI_2\downarrow\,(\text{red}) + 2KCl$$ $$HgI_2 + 2KI \rightarrow K_2[HgI_4]$$

iii) $HgCl_2$ with $SnCl_2$

Stannous chloride first reduces $HgCl_2$ to a white precipitate of calomel. With more $SnCl_2$, this is reduced further to grey/black mercury:

$$2HgCl_2 + SnCl_2 \rightarrow Hg_2Cl_2\downarrow\,(\text{white}) + SnCl_4$$ $$Hg_2Cl_2 + SnCl_2 \rightarrow 2Hg\downarrow\,(\text{grey}) + SnCl_4$$

Corrosive sublimate is mercuric chloride, $HgCl_2$.

Methods of preparation

1. Direct combination: mercury is heated in a current of dry chlorine.

$$Hg + Cl_2 \xrightarrow{\Delta} HgCl_2$$

2. From mercuric sulphate (commercial method): mercuric sulphate is heated with common salt and a little $MnO_2$ (which oxidises any mercurous salt). $HgCl_2$ sublimes and is collected, which is why it is called corrosive sublimate.

$$HgSO_4 + 2NaCl \xrightarrow{\Delta} HgCl_2\uparrow + Na_2SO_4$$

(It can also be made by dissolving mercuric oxide in HCl: $HgO + 2HCl \rightarrow HgCl_2 + H_2O$.)

a) Action with excess of KI

A scarlet-red precipitate of mercuric iodide forms first. It dissolves in excess KI to give a colourless soluble complex, potassium tetraiodomercurate(II). An alkaline solution of this complex is Nessler's reagent.

$$HgCl_2 + 2KI \rightarrow HgI_2\downarrow\,(\text{scarlet red}) + 2KCl$$ $$HgI_2 + 2KI \rightarrow K_2[HgI_4]\,(\text{colourless})$$

b) Action with $NH_3$

Ammonia gives a white precipitate of mercuric amidochloride, known as infusible white precipitate:

$$HgCl_2 + 2NH_3 \rightarrow Hg(NH_2)Cl\downarrow\,(\text{white}) + NH_4Cl$$

(a) $\mathrm{[Cu(NH_3)_4]^{2+}}$

$NH_4OH$ first gives a blue precipitate of $Cu(OH)_2$. This dissolves in excess ammonia to give the deep blue tetraamminecopper(II) ion, in which copper stays in the +2 state with coordination number 4:

$$Cu(OH)_2 + 4NH_3 \rightarrow [Cu(NH_3)_4]^{2+} + 2OH^-$$

Rusting

Rusting is the slow corrosion of iron in the presence of moisture and oxygen (air). A reddish-brown, flaky layer of hydrated ferric oxide, called rust ($Fe_2O_3 \cdot xH_2O$), forms on the surface. Dissolved $CO_2$, salts and acids in water speed it up.

Electrochemical theory of rusting

According to this theory, rusting is an electrochemical process. The surface of iron is not uniform (impurities, strain), so tiny galvanic cells form on it. A film of water containing dissolved $CO_2$ and $O_2$ acts as the electrolyte.

$$H_2O + CO_2 \rightarrow H_2CO_3 \rightleftharpoons 2H^+ + CO_3^{2-}$$

At the anode (where iron is in contact with water), iron is oxidised:

$$Fe \rightarrow Fe^{2+} + 2e^-$$

At the cathode (where more oxygen is available), the electrons flowing through the metal reduce oxygen:

$$O_2 + 4H^+ + 4e^- \rightarrow 2H_2O \quad \text{or, in neutral water,} \quad O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$$

Overall cell reaction:

$$2Fe + O_2 + 4H^+ \rightarrow 2Fe^{2+} + 2H_2O$$

Formation of rust: the $Fe^{2+}$ ions move through the water and are further oxidised by air to hydrated ferric oxide:

$$4Fe^{2+} + O_2 + 4H_2O \rightarrow 2Fe_2O_3 + 8H^+$$ $$Fe_2O_3 + xH_2O \rightarrow Fe_2O_3 \cdot xH_2O \ (\text{rust})$$

         air (O2)                    air (O2)
   ~~~~~~~~~~~~ water drop (H2O + CO2 + O2) ~~~~~~~~~~~~
   cathode: O2 + 4H+ + 4e- -> 2H2O     rust Fe2O3.xH2O
          ^                               |
   =======|======== iron ===========  anode: Fe -> Fe2+ + 2e-
          <------- e- flow through iron -------

So rusting needs both water and oxygen, and it is faster in salty or acidic water, which is a better electrolyte.

Methods of prevention of rusting (any two)

1. Painting, oiling or greasing the surface keeps out moisture and air.
2. Galvanization: coating iron with zinc gives sacrificial (cathodic) protection.
3. Tin-plating, electroplating with Cr/Ni, or alloying (e.g. stainless steel).
4. Cathodic protection: joining the iron to a more active metal such as Mg or Zn, which corrodes instead.

(d) Zn-plating

Galvanization is coating iron with a thin layer of zinc, usually by dipping it in molten zinc. Zinc is more electropositive than iron, so even if the coating is scratched, zinc corrodes in preference to iron (sacrificial protection).

(b) Granulated zinc

When molten zinc is poured into cold water, it solidifies suddenly into small, irregular granules. This granulated zinc has a large surface area and is used in the laboratory, e.g. for preparing hydrogen with dilute acids.

Calomel is mercurous chloride, $Hg_2Cl_2$.

(a) Preparation (any two)

1. By heating mercuric chloride with mercury. Calomel sublimes:

$$HgCl_2 + Hg \xrightarrow{\Delta} Hg_2Cl_2$$

2. By precipitation: dilute HCl or NaCl is added to a solution of mercurous nitrate, which gives a white precipitate:

$$Hg_2(NO_3)_2 + 2HCl \rightarrow Hg_2Cl_2\downarrow + 2HNO_3$$

3. By partial reduction of $HgCl_2$ with $SnCl_2$:

$$2HgCl_2 + SnCl_2 \rightarrow Hg_2Cl_2\downarrow + SnCl_4$$

(b) Reactions

(i) With ammonia solution: calomel turns black, because a mixture of mercuric amidochloride (white) and finely divided mercury (black) forms. This is a test for mercurous ion and the origin of the name calomel (beautiful black).

$$Hg_2Cl_2 + 2NH_3 \rightarrow \underbrace{Hg(NH_2)Cl + Hg}_{\text{black}} + NH_4Cl$$

(ii) With stannous chloride: calomel is reduced to grey/black metallic mercury:

$$Hg_2Cl_2 + SnCl_2 \rightarrow 2Hg\downarrow + SnCl_4$$

(iii) With aqua regia: the nascent chlorine of aqua regia oxidises calomel to soluble mercuric chloride:

$$HNO_3 + 3HCl \rightarrow NOCl + 2H_2O + 2[Cl]$$ $$Hg_2Cl_2 + 2[Cl] \rightarrow 2HgCl_2$$

Overall: $3Hg_2Cl_2 + 6HCl + 2HNO_3 \rightarrow 6HgCl_2 + 2NO + 4H_2O$

The metal M is copper (Cu), $[Ar]\,3d^{10}4s^1$.

(a) Chief ore

The chief ore of copper is copper pyrites (chalcopyrite), with molecular formula $CuFeS_2$ (i.e. $Cu_2S \cdot Fe_2S_3$).

(b) Diagram for smelting

The roasted ore is mixed with coke and a little silica and smelted in a water-jacketed blast furnace. Hot air is blown in through tuyeres near the bottom.

              charge (roasted ore + coke + silica)
                            |
                            v
        gases (SO2, CO2) <==+==> gases
                    +---------------+
                    |               |
   water jacket --> |  roasted ore  | <-- water jacket
   (cold water      |  + coke       |     (keeps walls cool)
    circulates)     |  + silica     |
                    |               |
   hot air blast => |   tuyeres     | <= hot air blast
                    |~~~~ slag ~~~~~|---> slag outlet (FeSiO3)
                    |#### matte ####|---> matte outlet (Cu2S + FeS)
                    +---------------+
                         hearth

(c) Reactions during smelting

Iron sulphide is oxidised to ferrous oxide:

$$2FeS + 3O_2 \rightarrow 2FeO + 2SO_2\uparrow$$

FeO combines with silica (flux) to form fusible slag, which floats on top:

$$FeO + SiO_2 \rightarrow FeSiO_3 \quad (\text{ferrous silicate, slag})$$

Any $Cu_2O$ formed is converted back to $Cu_2S$, because iron has a greater affinity for oxygen than copper:

$$2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2$$ $$Cu_2O + FeS \rightarrow Cu_2S + FeO$$

The FeO formed again goes into the slag as $FeSiO_3$. The molten mixture of $Cu_2S$ and a little $FeS$ that collects at the bottom is called copper matte, and it is tapped off separately from the slag. Coke burns to supply heat: $C + O_2 \rightarrow CO_2$.

(d) Cinnabar

Cinnabar ($HgS$) is the chief ore of mercury. Calamine ($ZnCO_3$) is an ore of zinc, haematite ($Fe_2O_3$) is an ore of iron, and calomel ($Hg_2Cl_2$) is a compound of mercury, not its chief ore.

(b) AgNO$_3$

Silver nitrate (lunar caustic) is reduced by organic matter such as skin, cloth or paper in the presence of sunlight. Finely divided black metallic silver is deposited, which leaves a black stain. That is why it is kept in dark bottles and used in marking inks.

Blue vitriol is copper(II) sulphate pentahydrate, $CuSO_4 \cdot 5H_2O$.

1. Preparation

(a) Laboratory: copper oxide, hydroxide or carbonate is dissolved in dilute $H_2SO_4$. The solution is concentrated and crystallised:

$$CuO + H_2SO_4 \rightarrow CuSO_4 + H_2O$$ $$CuCO_3 + H_2SO_4 \rightarrow CuSO_4 + H_2O + CO_2$$

(b) From copper and conc. $H_2SO_4$:

$$Cu + 2H_2SO_4 \xrightarrow{\Delta} CuSO_4 + SO_2 + 2H_2O$$

(c) Commercial method: scrap copper is treated with dilute $H_2SO_4$ while air is blown through:

$$2Cu + 2H_2SO_4 + O_2 \rightarrow 2CuSO_4 + 2H_2O$$

2. Properties

Physical: it forms blue triclinic crystals that are soluble in water. Its aqueous solution is acidic because of hydrolysis.

(i) Action of heat:

$$CuSO_4 \cdot 5H_2O \xrightarrow{100^\circ C} CuSO_4 \cdot H_2O \xrightarrow{230^\circ C} \underset{\text{white}}{CuSO_4} \xrightarrow{>720^\circ C} CuO + SO_3$$

(The $SO_3$ partly breaks down further: $2SO_3 \rightarrow 2SO_2 + O_2$.) Anhydrous white $CuSO_4$ turns blue again with water, so it is used to test for water.

(ii) With ammonium hydroxide: a pale blue precipitate forms, which dissolves in excess to give a deep blue solution:

$$CuSO_4 + 2NH_4OH \rightarrow Cu(OH)_2\downarrow + (NH_4)_2SO_4$$ $$Cu(OH)_2 + (NH_4)_2SO_4 + 2NH_4OH \rightarrow [Cu(NH_3)_4]SO_4 + 4H_2O$$

(iii) With KI: cuprous iodide (white) precipitates and iodine is liberated:

$$2CuSO_4 + 4KI \rightarrow Cu_2I_2\downarrow + 2K_2SO_4 + I_2$$

(iv) With KCN: cuprous cyanide forms, which dissolves in excess KCN to give a complex:

$$2CuSO_4 + 4KCN \rightarrow 2CuCN + (CN)_2 + 2K_2SO_4; \quad CuCN + 3KCN \rightarrow K_3[Cu(CN)_4]$$

(v) With $H_2S$: a black precipitate of CuS forms: $CuSO_4 + H_2S \rightarrow CuS\downarrow + H_2SO_4$

(vi) With potassium ferrocyanide: a chocolate-brown precipitate forms:

$$2CuSO_4 + K_4[Fe(CN)_6] \rightarrow Cu_2[Fe(CN)_6]\downarrow + 2K_2SO_4$$

(vii) With NaOH: blue $Cu(OH)_2$ precipitates, and it turns black (CuO) on boiling.

3. Structure

In $CuSO_4 \cdot 5H_2O$, four water molecules are coordinated to $Cu^{2+}$ in a square-planar arrangement. The fifth water molecule is hydrogen-bonded to the sulphate ion and a coordinated water: $[Cu(H_2O)_4]SO_4 \cdot H_2O$. This is why one water molecule is lost only at a higher temperature.

4. Uses

• As a fungicide (Bordeaux mixture = $CuSO_4$ + lime) and germicide
• In electroplating, electrotyping and electrolytic refining of copper
• In preparing Fehling's solution and Benedict's solution
• Anhydrous $CuSO_4$ to detect and remove water
• As a mordant in dyeing and calico printing

(a) $\mathrm{ZnS + BaSO_4}$

Lithopone is a white pigment made by mixing solutions of zinc sulphate and barium sulphide. Both products precipitate together:

$$ZnSO_4 + BaS \rightarrow \underbrace{ZnS + BaSO_4}_{\text{lithopone}}$$

(d) Wrought iron

Wrought iron contains only about 0.1–0.25% carbon, so it is the purest commercial form of iron. Steel has 0.25–2% carbon and cast iron 2–4.5%. Haematite is an ore ($Fe_2O_3$), not a form of iron metal.

Mercuric chloride ($HgCl_2$), also called corrosive sublimate.

Preparation

1. By heating mercury in dry chlorine: $Hg + Cl_2 \rightarrow HgCl_2$
2. Commercially, by heating mercuric sulphate with common salt (a little $MnO_2$ is added). $HgCl_2$ sublimes:

$$HgSO_4 + 2NaCl \xrightarrow{\Delta} HgCl_2\uparrow + Na_2SO_4$$

Properties

Physical: a white crystalline solid that is soluble in water and highly poisonous. On heating, it sublimes.

(i) With $SnCl_2$: a white precipitate of calomel forms, which turns grey/black (Hg) with excess $SnCl_2$:

$$2HgCl_2 + SnCl_2 \rightarrow Hg_2Cl_2\downarrow + SnCl_4$$ $$Hg_2Cl_2 + SnCl_2 \rightarrow 2Hg + SnCl_4$$

(ii) With KI: a scarlet-red precipitate forms, which is soluble in excess KI (Nessler's reagent base):

$$HgCl_2 + 2KI \rightarrow HgI_2\downarrow + 2KCl; \qquad HgI_2 + 2KI \rightarrow K_2[HgI_4]$$

(iii) With $NH_3$: a white precipitate forms (infusible white precipitate):

$$HgCl_2 + 2NH_3 \rightarrow Hg(NH_2)Cl\downarrow + NH_4Cl$$

(iv) With NaOH: a yellow precipitate of mercuric oxide forms:

$$HgCl_2 + 2NaOH \rightarrow HgO\downarrow\,(\text{yellow}) + 2NaCl + H_2O$$

(v) With mercury: on heating, calomel forms: $HgCl_2 + Hg \rightarrow Hg_2Cl_2$

(vi) With $H_2S$: a black precipitate forms: $HgCl_2 + H_2S \rightarrow HgS\downarrow + 2HCl$

Uses

• As an antiseptic and disinfectant (dilute solution) and for preserving wood and anatomical specimens
• For preparing calomel and other mercury compounds, and Nessler's reagent
• As a catalyst in the manufacture of vinyl chloride
• As a reagent in the laboratory (e.g. test for $Sn^{2+}$)

The metal with configuration $[Ar]\,3d^{10}4s^1$ is copper (Cu). It actually lies in group IB, and it is extracted mainly from copper pyrites, $CuFeS_2$. The roasted ore (a mixture of $Cu_2S$, $FeS$ and some $FeO$, $Cu_2O$) is mixed with coke and silica and smelted.

a) Blast furnace for smelting copper

The furnace is a water-jacketed blast furnace. Cold water circulates in the jacket so that the walls are not attacked, and hot air is blown in through tuyeres.

              charge (roasted ore + coke + silica)
                            |
                            v
        gases (SO2, CO2) <==+==> gases
                    +---------------+
                    |               |
   water jacket --> |  roasted ore  | <-- water jacket
   (cold water      |  + coke       |     (keeps walls cool)
    circulates)     |  + silica     |
                    |               |
   hot air blast => |   tuyeres     | <= hot air blast
                    |~~~~ slag ~~~~~|---> slag outlet (FeSiO3)
                    |#### matte ####|---> matte outlet (Cu2S + FeS)
                    +---------------+
                         hearth

b) Reactions during smelting

Iron sulphide is oxidised to ferrous oxide:

$$2FeS + 3O_2 \rightarrow 2FeO + 2SO_2\uparrow$$

FeO combines with silica (flux) to form fusible slag, which floats on top:

$$FeO + SiO_2 \rightarrow FeSiO_3 \quad (\text{ferrous silicate, slag})$$

Any $Cu_2O$ formed is converted back to $Cu_2S$, because iron has a greater affinity for oxygen than copper:

$$2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2$$ $$Cu_2O + FeS \rightarrow Cu_2S + FeO$$

The FeO formed again goes into the slag as $FeSiO_3$. The molten mixture of $Cu_2S$ and a little $FeS$ that collects at the bottom is called copper matte, and it is tapped off separately from the slag. Coke burns to supply heat: $C + O_2 \rightarrow CO_2$.

The matte is then taken to a Bessemer converter, where self-reduction gives blister copper: $2Cu_2O + Cu_2S \rightarrow 6Cu + SO_2$.

(b) Steel

In the Bessemer converter, a blast of air under pressure is blown through molten pig (cast) iron. This oxidises the excess C, Si, Mn and P, and the right amount of carbon is then added back (as spiegeleisen) to make steel.

(c) Copper with 1-2% impurity

Blister copper is the crude copper obtained from the Bessemer converter, about 98% pure. It has a blistered surface because dissolved $SO_2$ escapes as the metal solidifies.

(b) White vitriol

$ZnSO_4 \cdot 7H_2O$ is white vitriol. Blue vitriol is $CuSO_4 \cdot 5H_2O$, green vitriol is $FeSO_4 \cdot 7H_2O$, and oil of vitriol is concentrated $H_2SO_4$.

(d) $CuO$

Copper(II) oxide, $CuO$, is black. $Cu_2O$ is red, $Cu(OH)_2$ is blue, and $CuFeS_2$ is the ore copper pyrites.

White vitriol is zinc sulphate heptahydrate, $ZnSO_4 \cdot 7H_2O$.

Preparation

• In the laboratory, zinc, zinc oxide or zinc carbonate is dissolved in dilute sulphuric acid, and the solution is concentrated and crystallised:

$$Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2$$ $$ZnO + H_2SO_4 \rightarrow ZnSO_4 + H_2O$$

• Industrially, zinc blende is roasted in air, which converts part of the ZnS directly to $ZnSO_4$: $ZnS + 2O_2 \rightarrow ZnSO_4$. The product is leached with water and crystallised.

Properties

• It forms colourless, transparent crystals that are soluble in water and have a nauseating metallic taste.
• Its aqueous solution is acidic because of hydrolysis.
• On heating it loses water of crystallisation in stages to give the anhydrous salt. At red heat it decomposes: $2ZnSO_4 \rightarrow 2ZnO + 2SO_2 + O_2$.
• With NaOH it gives a white precipitate of $Zn(OH)_2$, which dissolves in excess alkali to give sodium zincate.
• With $BaCl_2$ it gives a white precipitate of $BaSO_4$, and with $H_2S$ in alkaline medium it gives white ZnS.

Uses

• Used as an eye lotion and an astringent in medicine.
• Used as a mordant in calico printing and dyeing.
• Used in the preparation of lithopone, a white pigment, and as a wood preservative.
• Used as an electrolyte in zinc plating and as a zinc micronutrient in fertilisers.

Copper pyrite is $CuFeS_2$. The ore contains much earthy gangue, so it is processed in the following steps.

1. Concentration (froth flotation) [2]

The ore is crushed and finely ground, then mixed with water, pine oil (frothing agent) and potassium ethyl xanthate (collector) in a flotation tank. Compressed air is blown through. The sulphide particles are wetted by oil, stick to air bubbles and rise as a froth, which is skimmed off. The gangue is wetted by water and sinks. The froth is dried to get the concentrated ore.

2. Roasting [3]

The concentrate is heated strongly in excess air in a reverberatory furnace (below its melting point). Moisture is driven off, sulphur and arsenic impurities escape as $SO_2$ and $As_2O_3$, and part of the sulphides is oxidised:

$$2CuFeS_2 + O_2 \rightarrow Cu_2S + 2FeS + SO_2$$ $$2FeS + 3O_2 \rightarrow 2FeO + 2SO_2$$

3. Smelting [3]

The roasted ore is mixed with coke and silica ($SiO_2$, flux) and heated in a blast furnace. Coke acts as the fuel and reducing agent. Iron(II) oxide combines with silica to form a fusible slag, which floats on the molten mass and is removed:

$$FeO + SiO_2 \rightarrow FeSiO_3\ (\text{slag})$$

The heavier molten layer is copper matte ($Cu_2S$ with a little $FeS$). The matte is then blown with air in a Bessemer converter with silica, giving blister copper:

$$2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2$$ $$2Cu_2O + Cu_2S \rightarrow 6Cu + SO_2$$

Labelled diagram (blast furnace for smelting)

   roasted ore + coke + silica  (charge)
                 |
          +------v------+      waste gases (CO2, SO2)  <-- exit
          |   charge    |
          |  (reduction |
          |    zone)    |
 hot air  |             |  hot air
 -------> |  slag layer | <-------
          |  (FeSiO3)   |  --> slag tap
          |  matte layer|
          |  (Cu2S,FeS) |  --> matte tap
          +-------------+

(d) $Cu_2O$

Copper heated in air below about 1000 °C forms black CuO, but above 1100 °C the CuO decomposes to red copper(I) oxide:

$$4CuO \xrightarrow{>1100^\circ C} 2Cu_2O + O_2$$

a) Calcination and roasting [2.5]

b) Froth flotation and magnetic separation [2.5]

(b) 5

Blue vitriol is copper sulphate pentahydrate, $CuSO_4\cdot 5H_2O$, so it has 5 molecules of water of crystallization.

(b) $Cu + SO_2$

In the Bessemer converter, part of the sulphide is oxidised to oxide and the oxide then reacts with the remaining sulphide (self-reduction):

$$2Cu_2O + Cu_2S \rightarrow 6Cu + SO_2\uparrow$$

Metallurgy is the whole process of extracting a pure metal from its ore, including concentration of the ore, reduction to the metal, and refining.

(a) Pyrometallurgy and hydrometallurgy

(b) Carbon reduction and electrochemical reduction

i) Air. In dry air copper is unaffected at ordinary temperature. In moist air containing $CO_2$ it slowly becomes coated with a green layer of basic copper carbonate (verdigris):

$$2Cu + O_2 + CO_2 + H_2O \rightarrow CuCO_3\cdot Cu(OH)_2$$

On strong heating in air, it forms black cupric oxide, $2Cu + O_2 \rightarrow 2CuO$.

ii) Conc. $HNO_3$. Copper dissolves, giving blue copper(II) nitrate and brown $NO_2$ gas:

$$Cu + 4HNO_3(conc.) \rightarrow Cu(NO_3)_2 + 2NO_2\uparrow + 2H_2O$$

iii) Dil. $H_2SO_4$. There is no reaction, since copper lies below hydrogen in the electrochemical series and cannot displace $H_2$. (In the presence of air, it dissolves slowly: $2Cu + 2H_2SO_4 + O_2 \rightarrow 2CuSO_4 + 2H_2O$.)

(c) Galena

Froth flotation is used for sulphide ores. Galena ($PbS$) particles are wetted by the pine oil and carried up in the froth, while the earthy gangue is wetted by water and sinks. Haematite ($Fe_2O_3$) is an oxide, siderite ($FeCO_3$) and malachite ($CuCO_3 \cdot Cu(OH)_2$) are carbonates; these are concentrated by gravity or magnetic methods instead.

a. Identification

The black oxide of a group II basic radical is $CuO$, and roasting a sulphide ore gives the metal together with $SO_2$ gas (which turns acidified $K_2Cr_2O_7$ green). So:

• Metal M = copper (Cu)
• Ore X = copper glance (chalcocite), $Cu_2S$
• Black oxide W = $CuO$
• Gas = $SO_2$

b. Calcination of X (heating the ore strongly in air)

$$Cu_2S + 2O_2 \xrightarrow{\Delta} 2CuO\,(\text{black}) + SO_2$$

When X is roasted, part of the $Cu_2S$ is oxidised to $Cu_2O$, which then reacts with the remaining $Cu_2S$ to give the metal (self-reduction):

$$2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2$$ $$2Cu_2O + Cu_2S \rightarrow 6Cu + SO_2$$

c. Action of $SO_2$ on acidified $K_2Cr_2O_7$

$SO_2$ is a reducing agent. It reduces orange dichromate ($Cr^{6+}$) to green chromium(III) sulphate ($Cr^{3+}$):

$$K_2Cr_2O_7 + H_2SO_4 + 3SO_2 \rightarrow K_2SO_4 + Cr_2(SO_4)_3\,(\text{green}) + H_2O$$

d. Conversion of copper into its vitriol (blue vitriol, $CuSO_4 \cdot 5H_2O$)

Copper is heated with concentrated sulphuric acid:

$$Cu + 2H_2SO_4\,(\text{conc.}) \xrightarrow{\Delta} CuSO_4 + SO_2 + 2H_2O$$

On a large scale, scrap copper is treated with dilute $H_2SO_4$ while air is blown through:

$$2Cu + 2H_2SO_4 + O_2 \rightarrow 2CuSO_4 + 2H_2O$$

The solution is concentrated and cooled. Blue crystals of blue vitriol separate out:

$$CuSO_4 + 5H_2O \rightarrow CuSO_4 \cdot 5H_2O\,(\text{blue vitriol})$$