Extraction of zinc

The extractive metallurgy of the Zinc is the whole of the operations allowing the metal manufacture of zinc starting from the ore.

The ore (generally sphalerite) is beforehand treated share floatation in order to separate, the sulfide of its gangue (Silicate, Carbonate etc). One obtains a concentrate of sulfide of lead zinc and sulfide.

There exist two ways of treatments then:

pyrometallurgical the
the hydrometallurgic follow-up of an electrometallurgy operation

Economic situation

In 2002, the worldwide production of zinc metal was of 9502 kT. The production of metal starting from the ore accounts for approximately 70% of the production, the remainder being provides by recycling.

History of the production of zinc

At the beginning India

According to the archeologist Paul Cradock, the Indians had invented at the 12th century a process of extraction of zinc. This process was described by well-read men in various works like the Rasaratnassamuchchaya (14th century). The process is based on the condensation of the zinc vapor. It presents similarities with the modern industrial processes.

The ore called now the Blende (zinc sulfide) was roasted to obtain zinc oxide (see the paragraph Grillage of the sulfide ). The oxide thus obtained mixed with various organic matters (source of Carbon) and of the Dolomie was placed in a lengthened ground crucible of form. This pot was called `' eggplant''. A wood rod was planted on in the mixture in order to creating a conduit by which the gas will circulate during heating. The crucible was closed by a lid provided in its center with a long conduit which will be used as condenser.

Several eggplants were placed in a furnace, the condenser directed downwards. The load thus made up was heated by a fire which burned above. Heat resulting fire caused the vaporization of zinc. This one condensed in the conduits of the condensers located in the low part of the device (thus colder).

China in XVIe century

Although there exist various assumptions, the production of zinc in China is proven at the 16th century. The process is described in one entitled book Tien-kong-kai-or published in the beginning of the 17th century.

Contrary to the Indian process, the Chinese use the Smithsonite often called calamine (Carbonate of zinc of ZnCO₃ formula). The ore mixed with Charcoal is placed in a pot. Zinc is collected in a crucible placed above the mixture. The circulation of gas is ensured by a tube. The pot is closed by an iron plate. It is heated and zinc condenses in the crucible in top of the pot.

Europe at the XIXe century

Various tests were led to the 18th century to extract zinc from its various ores. In 1742, the Swedish chemist Anton von Swab distilled zinc starting from calamine then in 1744 starting from the sphalerite. Independently, Andreas Sigismund Marggraf reduced calamine in 1746. It also showed that it could be extracted the Sphalerite.

It is William Champion (1709-1789) which will make the first industrial attempt at production of zinc using a condenser resembling the Indian condensers starting from calamine. It installed its factory in 1743 with Bristol in England. Its process was based on vertical crucibles prolonged in the bottom of condensers soaking in water to cool them. It took sixty ten hours to produce 400 kg of zinc. The annual production of the factory is estimated at approximately 200 ton S per annum. The process consumed much energy. A big change was carried out by Johan Ruberg (1751-1807) which developed starting from 1798 in Silesia a process with horizontal crucibles. This process allowed a loading and an unloading without cooling the installation what improved the energetic efficiency clearly. The process of Ruberg was at the origin of the creation of many factories in Europe and in particular in Belgium in the area of Liege where Jean-Jacques Daniel Dony (1759-1819) created a first factory in 1805 then in 1810 in the Saint-Léonard suburb of Liege. Dony used the principle of the horizontal crucibles but by modifying the design. At the time, Belgium was under French domination, it obtains Napoleon i, the concession of the Belgian layer richest in zinc. The Belgian factories are at the origin of the Société of the Mines and Foundry of the Old Mountain (in 1837) which quickly became the leader of the production of zinc.

The American production started in 1850 by using the process of Dony.

In 1907, the production was of 737500 ton, of which 31% came from the United States, 31% of Germany, 21% of Belgium.

Extraction by pyrometallurgy

The pyrometallurgical process is consisted of the following operations:

*grillage of sulfide to obtain a zinc oxide,

*réduction of the oxide

*affinage by liquation and distillation

Fit latticework on sulfide

Is to transform sulfide into oxide. Obtaining oxide ZnO zinc is carried out at a temperature ranging between 910 and 980 °C. The oxide obtained is called calcines it.

$\ begin {matrix} & \ \ ZnS + 3/2 O_2 & \ overrightarrow {\ qquad} & ZnO + SO_2 & {\ rm \ Delta \ H} = - 445 \ {\ rm KJ/mole \ of \ ZnS \ between \ 800 \ and \ 1000^ \ circ \ C} \ \ \ end {matrix}$

The reaction is Exothermique.

Reduction of oxide

To reduce zinc oxide, it should be heated with an higher temperature at the temperature of Vaporisation of zinc (907 °C). All these processes are based on the heating of oxide to obtain zinc in gas form, by Carboréduction. Zinc is recovered by Condensation of this gas.

Process out of vertical crucible

One mixes calcines it with fine of coal the whole is mixed closely with a binder. The mixture after coking is placed in a silicon carbide crucible of ten meter in height and of a rectangular section of approximately 2 m by 0,3 Mr. The crucible is supplied continuously by the top.

This last is heated between 1280 and 1320° C. the zinc oxide is reduced in contact with zinc metal carbon.

$\ begin {matrix} & \ \ ZnO + C & \ overrightarrow {\ qquad} & Zn + CO \ \ \ \ end {matrix}$

With resulting from the reaction, zinc is gas. The gas mixture zinc, carbon monoxide is recovered in top of the crucible. Zinc is condensed using a turbine.

Process out of horizontal crucible

The crucible is out of refractory ground of a relatively reduced capacity. It consists of a room in which a mixture will be placed calcines, coal (in excess). It is prolonged by a condenser which will be used to condense the vapors of zinc and of a lengthening-piece which will collect gases and in particular the Carbon monoxide which will be burned at the exit.

Taking into account the low capacity of the crucibles, they are associated out of battery. The crucibles are heated throughout one one to two days with 1200°C. The output of this process is relatively weak compared to other process.

Imperial process smelting

This process is used when one wants to extract from other metals (copper, but, antimony, bismuth, money) present in the ore. The process comprises similarities with that of the extraction of the cast iron in a Haut-fourneau. One mixes in the furnace of calcines and the coke. This mixture runs out from top to bottom. The combustion of coke in bottom of the produced furnace of the Carbon monoxide. This reducing gas assembles and reduces zinc metal zinc oxide.

$\ begin {matrix} & \ \ ZnO + CO & \ overrightarrow {\ qquad} & Zn + CO_2 \ \ \ \ end {matrix}$

Zinc is gasified and is collected in top of the device whereas liquid lead runs out with a slag containing various oxides: SiO, Al₂O₃, CAD, SO₂. While running out, lead takes along other metal elements which can be developed.

The gas mixture collected in top of the furnace contains approximately zinc 8%; carbon monoxide 25% and 11% of Carbon dioxide. The gas crosses a pulverization of lead droplet in fusion at a temperature of 550°C. One will recover 1 ton of zinc for 400 tons of lead. On the outlet side of the condenser, the mixture lead zinc is treated by liquation in order to separate lead from the zinc which contains lead 1,5% more.

Refining: liquation, distillation

The zinc obtained at the time of the preceding operations still contains lead and other impurities (iron, cadmium in proportions of about 0,1%). To increase the zinc title of metal, it is refined by two operations: liquation and distillation.

Liquation is based on a difference of miscibility between lead and zinc at a temperature ranging between 430 and 440°C. In the same way, the solubility of iron strongly decrease when one cooled the mixture iron zinc.

By treating zinc resulting from the preceding operations in a Reverberatory furnace at a temperature ranging between 430 and 440°C during one to two days one separates: the zinc which contains lead 0,9% more of what is called the `' zinc matte'' container of lead, 5 to 6% of zinc and a ferrous compound of FeZn₁₃ composition.

To obtain a pure zinc, it is necessary to pass by an operation of split distillation which makes it possible to separate different the component metal while exploiting their melting points. For that, one heats the mixture of metals to make it gas. Using various fractionating columns one separates metals by condensing them.

Extraction by hydrometallurgy

The hydrometallurgic process is composed of the following operations:

* leaching

* cementing

*électrolyse.

Leaching

By this operation one deals either with directly calcines resulting from the operation of netting or zinc sulfide. This last process being more recent than the treatment of zinc oxide.

The goal of leaching is to put in solution zinc in the form of the ion Zn²⁺ by obtaining the transformation either of zinc sulfide resulting directly from the ore or the zinc oxide (calcines) from netting into zinc sulfate (ZnSO²⁺).

Treatment of zinc sulfide

The sulfide is treated by action of the sulphuric acid in oxidizing medium to obtain zinc sulfate and Soufre in solid form. The reaction with the sulphuric acid in the presence of oxygen is slow. One uses as oxidant the Ion Fe³⁺.

The total reaction is the following one:

$\ begin {matrix} & {} _ {\ rm {T \ > \ 90^ \ circ \ C}} & \ \ ZnS + Fe_2 (SO_4) _3 & \ overrightarrow {\ qquad \ qquad \ qquad} & ZnSO_4 + 2FeSO_4 + S \ (solid) \ \ \ \ end {matrix}$

Treatment of zinc oxide

Leaching hydrolizes

One places calcines it in the presence of a solution very diluted sulphuric Acid . This operation allows the setting solution of 80% of zinc in the form of zinc sulfate. Certain metal impurities are eliminated: Arsenic, Germanium and Indium by injecting ferrous ions (Fe2+) which precipitate in hydroxide of iron (Fe (OH) ₃) by involving these made up.

One separates by decantation the solution from ions zinc which will be treated by electrolyzed cementing then. Muds containing 20% more of zinc in the form of oxide are treated by `' acid leaching''.

Acid leaching

The setting in solution of the zinc 20% remaining is obtained by using concentrated sulphuric acid. Iron and copper are also solubilized.

The solution elutriates to separate the elements solubilized from residual solid waste from the ore and certain insoluble elements.

One extracts the iron of the solution containing zinc by precipitating it in the form of ferric hydroxide (Fe (OH) ₃). The zinc solution thus obtained will be reintroduced in the operation of leaching hydrolizes.

Cementing

The goal of the cementing of the solution resulting from the operation from leaching hydrolysis is to withdraw the elements: Cobalt, Nickel, Cadmium and Copper. These elements having been solubilized in the form of ions.

The principle is to put in contact the ion metal (Cu²⁺ example) with a metal having a more important reduction (less electronegative). Fine zinc powder here is used.

There is a reaction of the type:

$\ begin {matrix} & \ \ Cu^ {2+} _ {aqueous} + solid Zn_ {} & \ overrightarrow {\ qquad} & solid Cu_ {} + Zn^ {2+} \ \ \ \ end {matrix}$

The operation is done by several successive cementings. The difficulty in extracting the elements follows the following order by increasing difficulty: Copper, Cadmium, Nickel, Cobalt. One exploits the temperature in particular (between 45°C and 65°C for cadmium, 75°C and 95°C for cobalt).

The liquids and the solids are separated by filtration.

Electrolyze

Electrolysis is carried out while running an electrical current between two electrodes in the zinc sulfate solution obtained at the conclusion of the operation of cementing.

It brings into play the following reactions:

* Cathode out of aluminum:

$\ begin {matrix} & \ \ Zn^ {2+} _ {aqueous} + 2e^- & \ overrightarrow {\ qquad} & solid Zn_ {} \ \ \ \ end {matrix}$

and

$\ begin {matrix} & \ \ 2:00 ^+ + 2e^- & \ overrightarrow {\ qquad} & H_2^ {gas} \ \ \ \ end {matrix}$

* Anode out of lead:

$\ begin {matrix} & \ \ 2:00 _2O & \ overrightarrow {\ qquad} & O_2 + 4:00 ^+ + 4e^- \ \ \ \ end {matrix}$

The total reaction is:

$\ begin {matrix} & \ \ ZnSO_4 + H_2O & \ overrightarrow {\ qquad} & solid Zn_ {} + H_2SO_4 + 1/2O_2 \ \ \ \ end {matrix}$

The installation consists of cells comprising each one to eighty six cathodes (on installations known as Superjumbo). For an installation of this size that represents a surface of 3,2 m² D electrode. Zinc is recovered by mechanical action on cathodes.

The electrolyte is at a temperature of 30 with 40°C. The theoretical tension of balance is of 2,022 V. Taking into account different the phenomenon physiques, of the geometry of the installation, the real tensions are from 3,2 to 3,7 V for intensity varying from 400 to 700 A/m². Electricity consumption is from 2950 to 3500 kWh /t of produced zinc.

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