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BASICS ABOUT THE SILICATE AND CHLORINE-ALKALI INDUSTRY

• Dávid Havasi

• Gábor Stelén

• Content:

• About the silicon manufacturing

• Raw materials of the silicate industry

• Binder materials

• Glass

• Bricks and refractories

• Manufacturing of fine ceramics and glazes

• General informatiion about the chlorine-alkali industry

• The mercury cell, the diaphragm cell and the membrane cell process – and their comparision

About the silicon

Atomic number: 14; symbol: Si; metalloid It is not very reactive (more reactive than germanium), but has great chemical affinity for oxygen. Silicon is the eighth most common element in the universe by mass, but very rarely occurs as the pure element in the Earth's crust. It is most widely distributed in dusts, sands, planetoids, and planets as various forms of silicon dioxide (silica) or silicates. Over 90% of the Earth's crust is composed of silicate minerals, making silicon the second most abundant element in the Earth's crust (about 28% by mass) after oxygen. Uses in compound form: silicate industry Uses in pure form: steel refining, aluminium casting, fine chemical industries, semiconductor electronics (integrated circuits)

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Production of Si

Quartz, or silica, consists of silicon-dioxide. Sand contains many tiny grains of quartz. Silicon can be artificially produced by combining silica and carbon in electric furnace. The process gives polycrystalline silicon (multitude of crystals).

2SiO2(l) + 3C(s) Si(l) + 2CO2(g) + SiC(s)

2SiC(s) + SiO2(l) 3Si(l) + 2CO(g)

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By reduction silicon-dioxide (silica) 96 - 99% clean Si can be produced.

Reduction is accomplished in plasma pot.

Reduction is running with excess of SiO2 to avoid higher formation of silícium-carbide (SiC). :

Crystallizing of Si

Practical integrated circuits can only be fabricated from single-crystal material

Czochralski Process is a technique in making single-crystal silicon.

A solid seed crystal is rotated and slowly extracted from a pool of molten Si.

Requires careful control of to give crystals desired purity and dimensions.

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• The Silicon Cylinder is Known as an Ingot

• Typical Ingot is About 1 or 2 Meters in Length

• Can be Sliced into Hundreds of Smaller Circular Pieces Called Wafers

• Each Wafer Yields Hundreds or Thousands of Integrated Circuits

• The Silicon Crystal is Sliced by Using a Diamond-Tipped Saw into Thin Wafers

• Sorted by Thickness

• Damaged Wafers Removed During Lapping

• Etch Wafers in Chemical to Remove any Remaining Crystal Damage

• Polishing Smoothes Uneven Surface Left by Sawing Process

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Elements of the silicate industry

1. Binder materials

2. Brick and tile industry

3. Silicate-based light building materials

4. Glass industry

5. Refractories

6. Fine ceramics

7. Glaze industry

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Raw materials Clay Clay consists of tiny crystals often in flat, plate-like forms with a diameter of 1 µm to 10 µm. When mixed with water the crystals can easily slide over each other (like a pack of cards), and this phenomenon gives rise to the plasticity of clays. Clays are formed from rocks which have been weathered by physical and chemical action, e.g. orthoclase, a mineral found in granite, reacts with H2O and CO2 from the atmosphere as follows: K2O.Al2O3.6SiO2 + 2H2O + CO2 → Al2O3.2SiO2.2H2O + 4SiO2 + K2CO3

orthoclase feldspar orthoclase kaolin clay Kaolin clay is the clay most commonly used for pottery-making and, in common with all clays, the clay particles or crystals have a special layer structure. The silicon ions are in fourfold coordination with oxygen, and the vertices of all the SiO4 tetrahedra point in one direction. These apical oxygens link the tetrahedral layer to another sub-layer, called the octahedral layer, which is formed by Al ions in six-fold coordination with O and OH. The oxygens form a hexagonal ring of approximately the same size as the SiO rings. A kaolinite crystal consists of a stack of many layer with adjacent units linked by hydrogen bonds. This structure is broken down when the clay body is fired.

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Raw materials Silica Silica, SiO2, is mixed with clay to reduce shrinkage of the ware while it is being fired, and thus prevent cracking, and to increase the rigidity of the ware so that it will not collapse at the high temperatures required for firing. Silica is useful for this purpose becasue it is hard, chemically stable, has a high melting point and can readily be obtained in a pure state in the form of quartz. However, in the ceramic industry, silica is often obtained from sandstone, which consists of lightly bonded quartz grains. Feldspar Feldspars are alumino-silicate minerals found in nearly all igneous rocks; they have very similar chemical compositions, e.g. potash feldspar or orthoclase K2O.Al2O3.6SiO2, soda feldspar or albite Na2O.Al2O3.6SiO2, lime feldspar or anorthite CaO.Al2O3.2SiO2

Feldspars are widespread in nature and most commercial supplies are recovered from pegmatites, which are coarsely crystalline rocks formed in the later stages of crystallisation of a magma. Feldspars are used as a flux in the firing of ceramic ware. When a body is fired, the feldspar melts at a lower temperature than clay or silica, due to the presence of Na+, K+ or Ca2+ ions, and forms a molten glass which causes solid particles of clay to cling together: when the glass solidifies it gives strength and hardness to the body. The molten glass also reacts with the silica and clay particles

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Raw materials

Limestone Limestone is a sedimentary rock, composed mainly of skeletal fragments of marine organisms such as coral, forams and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). About 10% of sedimentary rocks are limestones. The solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years. Most cave systems are through limestone bedrock. Limestone has numerous uses: as a building material, an essential component of concrete (Portland cement), as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a chemical feedstock for the production of lime, as a soil conditioner, or as a popular decorative addition to rock gardens.

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Binder materials

Binder materials become solids from pulpy or liquid state due to chemical and physical processes and they clot the solid materials mixed in them together. Classification: • By origin

- natural (clay, bitumen) - artificial (cement, slaked lime, gypsum)

• By material quality - minerals (clay, slaked lime, cement) - organic (bitumen, glue, resin)

• By phase - liquid (sodium-silicate) - solid (cement)

• By bounding mechanism - hydraulic (cement)-underwater setting - non hydraulic (slaked lime, gypsum)-no underwater setting

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Cement

Cement Cement is the substance which holds concrete together, which means that it is extremely widely used in our society. Portland cement (the only type of cement in common use today) is manufactured in a four step process. Step 1 – Quarrying Limestone and a 'cement rock' such as clay or shale are quarried and brought to the cement works. These rocks contain lime (CaCO3), silica (SiO2), alumina (Al2O3) and ferrous oxide (Fe2O3) - the raw materials of cement manufacture. Step 2 - Raw material preparation (blending) To form a consistent product, it is essential that the same mixture of minerals is used every time. For this reason the exact composition of the limestone and clay is determined at this point, and other ingredients added if necessary. The rock is also ground into fine particles to increase the efficiency of the reaction. Step 3 – Clinkering The raw materials are then dried, heated and fed into a rotating kiln. Here the raw materials react at very high temperatures to form 3CaO•SiO2 (tricalcium silicate), 2CaO•SiO2 (dicalcium silicate), 3CaO•Al2O3 (tricalcium aluminate) and 4CaO•Al2O3•Fe2O3 (tetracalcium alumino-ferrate). 12

Dry process:

If the lime stone and clay are hard, then the dry process is used. in this process the lime stone is first broken into small pieces. it is then mixed with clay in the proper proportion (3:1) and finally pulverized to such a finesse that 90-95% passes through a 100 mesh sieve. Then the raw mix is fed to a rotary kiln.

Wet process:

If lime stone and clay are soft, the wet process is preferred. In this process, the clay is washed with water in wash mill to remove the foreign materials, organic matters etc. the powdered limestone is then mixed with the clay paste in the proper proportion (3:1) and the two ingredients are finely ground and homogenized. in this process, the slurry contains about 40% of water. Now the slurry can be fed to rotary Kiln.

Burning:

The dry pulverized raw mixture or slurry is introduced into a rotary kiln which consists of an inclined steel rotating cylinder. 150-200 feet long and 10 feet in diameter lined with fire bricks. The water evaporates at the upper ends of kiln by means of hot gases. The Kiln rotates on its axis at the rate of ½ to 1 revolution per minute. As the Kiln rotates the charge slowly moves down-wards due to the rotary motion of the Kiln. Now the charge is heated by blast of air charged with coal dust is admitted. this produces a temperature range of 1500 to 1700 0C in step wise process as:

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Rotary kiln

A rotary kiln is a pyroprocessing device used to raise materials to a high temperature (calcination) in a continuous process. Materials produced using rotary kilns include: Cement, lime, Refractories, Metakaolin, Titanium dioxide, Alumina, Vermiculite, Iron ore pellets They are also used for roasting a wide variety of sulfide ores prior to metal extraction.

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a. Drying zone:

In this zone temperature raises to maximum 750 0C so that entire moisture in the slurry gets evaporated. The clay is broken into Al2O3, SiO2 and Fe2O3.

Al2O3.2SiO2.Fe2O3.2H2O Al2O3 + 2SiO2 + Fe2O3 + 2H2O

b. Calcinations zone:

When the temperature raises at 1000 0C, the limestone is completely decomposed into CaO.

CaCO3 CaO + CO2

c. Reaction zone (Clinkering zone):

When the temperature reaches about 16000C, the mixture is partly fused and chemical combinations between lime, alumina, ferric oxide and silica, resulting in the formation of calcium aluminates and silicates occur.

• 2CaO + SiO2 2CaO.SiO2 (di calcium silicate)

• 3CaO + SiO2 3CaO.SiO2 (Tri calcium silicate)

• 2CaO + Al2O3 2CaO. Al2O3(di calcium Aluminate)

• 3CaO + Al2O3 3CaO. Al2O3(tri calcium Aluminate)

• 4CaO + Al2O3 + Fe2O3 4CaO. Al2O3 Fe2O3(Tetra calcium Aluminate 15

The resulting product is known as cement clinkers and as it comes out into the cooler. The clinkers are very hot (10000C. The clinkers have the appearance of small greenish black or grey colored Step 4 - Cement milling The 'clinker' that has now been produced will behave just like cement, but it is in particles up to 3 cm in diameter. These are ground down to a fine powder to turn the clinker into usefull cement. Cement production has several quite serious environmental hazards associated with it: dust and CO2 emissions and contaminated run-off water. Mixing of cement clinkers with gypsum: The cooled clinker is ground and almost 3% of gypsum is mixed with it in order to reduce the rate of setting. After the initial setting, Al2O3 which is a fast setting constituent of clinker reacts with gypsum to form the crystals of calcium sulpho aluminate.

3CaO. Al2O3 3(CaSO4.2H2O) + 2H2O 3CaO. Al2O3 3CaSO4.2H2O + 6H2O

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Gypsum Gypsum plaster has been used as building material for at least 4000 years. Currently it is used to make plaster boards, fibrous plaster, building decorations and moulds for many applications. Plaster is produced using the following process: Step 1 - Plaster of Paris Manufacture (Calcination) The gypsum (CaSO4.2H2O) is heated to remove 75% of its combined water, resulting in the formation of Plaster of Paris (CaSO4.½H2O). This reaction is called calcination.

CaSO4.2H2O → CaSO4.½H2O + 1½H2O Step 2 - Rehydration Dry plaster powder is mixed with excess water and any additives. It can then be cast in moulds, extruded, applied as a thick slurry to a surface or laminated between paper boards.. The additives are used to change the density of the plaster and, in the case of plaster board, to help the plaster to mechanically bond to the cardboard. The basic rehydration reaction is the reverse of calcination:

CaSO4.½H2O + 1½H2O → CaSO4.2H2O Step 3 - Setting In a manufacturing operation, excess water is added to ensure complete rehydration of plaster back to gypsum and to provide sufficient fluidity for manufacturing processes. The excess water is then removed either by simply leaving the plaster to dry by evaporation or by heating it to up to 250oC for up to 60 minutes. During this time the plaster solidifies so that it can be removed from a mould in one piece.

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Uses of plaster Present uses of plaster include the manufacture of: plaster boards - a layer of plaster sandwiched between two sheets of cardboard. Probably the best known example is Gib® board fibrous plaster - plaster with fibres (often made of glass) mixed into it to increase its strength. Fibrous plaster is usually cast into a mould then used in slabs.) IX-Materials-D-Plaster-2 plaster cornices - the decorative plaster projections used under the eaves and above doorways and windows in buildings plaster mouldings chalk plaster

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Slaked lime

Limestone is primarily calcium carbonate CaCO3, mined in open quarries

Quicklime CaO: CaCO3 CaO + CO2

Limestone is crushed and decomposed in a rotating kiln

slaked lime Ca(OH)2 from reacting CaO with H2O

Lime mostly used in steel making, some in chemicals manufacture, pollution control and H2O treatment

Glass Glass is a non-crystalline amorphous solid that is often transparent and has widespread practical, technological, and decorative usage in, for example, window panes, tableware, and optoelectronics. Glass is composed of an homogenous mixture of oxides in variable proportions. Main raw materials: silica, soda, limestone, feldspar, other additives, recycled glass.

General properties of glass • at room T°, it is very hard and fragile, • It is not porous, • It has a strong shininess characteristic, • It refracts light rays, • It is a bad heat conductor, • It does not dissolve in water and acids, even if concentrated, except for hydrofluoric acid, even

if disposes of -in small amounts, and particularly when hot -modifying ions from its surface. • It does dissolve in basic solutions; • It does not burn, does not calcine; • Under the effects of high heat it goes through various stages of viscosity, when white

incandescence it is fluid, when red it is soft and doughy. • It is in this last viscosity state that glass can be shaped. 19

Primary materials and additives GLAZER Silica (SiO2, silicone dioxide) is the most common moulder and therefore the most important primary material. About half of the earth's crust is made up of silica (silicates and quartz), the main component in rock and sand. FLUXES To lower the quartz fusion temperature (about 1700°C) a flux is added, generally sodium oxide, potassium or potassium carbonate (K2CO3). They being also produced industrially now. Soda (or potash), in addition to making the silica more meltable, has the property of lengthening the range of temperatures within which the glass solidifies (production range) and makes the glass “longer”. STABILIZERS They change some properties of the glass, for example the chemical resistance, thermal expansion, viscosity, etc. Stabilizers are: CaO, Al2O3, …

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THINNERS The vitrifiable mixture is still not complete. The molten is a viscous fluid in which there are numerous gassy bubbles formed by the decomposition of the carbonates or other origins. The following compounds help to remove the gas bubbles: As and Sb oxides, KNO3, NaNO3, NaCl, fluorides, sulphates. They help with the mass produciton. COLOURANTS Mostly oxides, with 3d or 4f electron structure.

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Overview of Glass Manufacture

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Glass is produced in a two step process, and then shaped to make it suitable for a variety of applications. Step 1 - Batch mixing The mixture of ingredients to make up the glass (silica, Na2CO3, CaCO3 and recycled glass, together with small quantities of various other minor ingredients) are mixed in a rotary mixer to ensure an even mix of ingredients and fed into the furnace. Step 2 - Batch melting The mixture is heated to 1500-1550oC, where the ingredients melt, various chemical reactions take place and CO2 and SO3 are evolved. Shaping plate glass The molten glass is cooled to 1000oC in a drawing canal, and then drawn up a tower (the drawing tower) where it is pressed into the desired width and thickness, and cools to 280oC. Individual plates of glass are snapped off at the top of the tower and further cooled before being put into storage ready for sale. Molding glass containers Here molten glass is channeled off in forehearths (heated channels) where it is slowly cooled to tempertaures of 1100 - 1150oC to increase its viscosity. Precisely weighed slugs of glass are cut off, molded with compressed air, cooled slowly in annealing lehrs (special ovens) and coated with a special spray to prevent scratching.

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Batch Melting Processes

1. Release of gases

• Gases expand to large volumes: CaCO3 → CaO + CO2↑

• One mole limestone: ∼37 cm3 CaO 22,400 cm3 CO2

• Gases produce a considerable stirring/homogenization effect

2. Formation of liquid phases

• Direct melting of batch components

• Melting of eutectic mixtures

• Eutectic: (Greek: easily melted): lowest point on the solid-liquid diagram

• freezing point depression

• Melting of Cullet (Note: liquid phase reactions are much faster than solid-state reactions; addition

of cullet, fluxes accelerate various batch reactions

3. Volatilization of melt components

• Oxide (liquid) → Oxide (gas)

• Alkali oxides (Li<Na<K<Rb<Cs

• Pb, B, P, halides have relatively high vapor pressures

• Glass composition can vary from that expected from the batch- losses

reduced at lower temperatures.

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4. Fining Reactions: removing bubbles

• Gases trapped in interstices between particles

• Fine sands (or batch components): lots of bubbles

• Gases created by batch decomposition reactions

• Gases created by refractory corrosion reactions, etc.

• Bubbles created by precipitation from melts with supersaturated gases

a) Removal of bubbles by buoyancy effects

b) Removal of bubbles using fining agents: chemical method

• Releasing large quantities of gas to sweep away small bubbles

• Remove oxygen from bubbles, reducing them below critical size where surface tension

eliminates the smallest

• Arsenic, antimony oxides are efficient fining agents

As, Sb are toxic, so other temperature dependent fining reactions are

often used (SO32-, CeO2)

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Bricks Materials Required: clay, sand, fuel (varies), and manpower. Clay is extracted/mined from clay quarries. Soil must contain from 25 to 50% clay and silt.

Brick Production

• The clay soils are mixed with water for plasticity and sand to keep them from sticking to the brick molds

• The material is then extruded through a large opening making a sheet that can be cut into bricks

• Through new Stiff Extrusion technology, the bricks contain much less moisture, shortening the drying and heating processes and reducing the wastage of bricks through handling

• After being cut, bricks are stacked and dried, either by open air or by the excess heat from the brick kilns

• Once they have dried sufficiently to remove moisture, the bricks are heat treated

• It is through this heating that the alkalis in the clay and the oxides from iron and other metals join with the alumina and silica in the clay to give the bricks their desired hardness

• A kiln is an oven or furnace where bricks are fired or heat treated to develop their hardness

• Brick production is a very energy intensive process and large amounts of fuel must be burned to acquire the correct hardness in the bricks

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Refractories

• Firebricks for furnaces and ovens. Have high Silicon or Aluminium oxide content.

• Brick products are used in the manufacturing plant for iron and steel, non-ferrous metals, glass, cements, ceramics, energy conversion, petroleum, and chemical industries.

• Used to provide thermal protection of other materials in very high temperature applications, such as steel making (Tm=1500°C), metal foundry operations, etc.

• They are usually composed of alumina (Tm=2050°C) and silica along with other oxides: MgO (Tm=2850°C), Fe2O3, TiO2, etc., and have intrinsic porosity typically greater than 10% by volume.

• Specialized refractories, (those already mentioned) and BeO, ZrO2, mullite, SiC, and graphite with low porosity are also used.

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Firing temperature and firing time 28

The manufacturing process of fine ceramics

This consists of four basic stages: shaping, drying, firing and glazing. Sometimes the glaze is applied before firing (once-firing), and sometimes the item is fired, then the glaze is applied and then the item is refired (twice-firing). Both methods are used and their chemistry is very similar. Step 1 – Shaping The materials are thoroughly mixed with water and the excess squeezed out to give a workable clay mix. The material is either shaped by casting in enclosed moulds which open in half to release the formed article, or shaped by "jiggering". This involves laying a sheet of mix onto an open mould, which spins, and then pressing a profile into the sheet to give the article the required shape. All the equipment is fully or partly automatic. Operations like putting handles on cups are done by hand. Step 2 – Drying Before the ware can be fired to high temperatures it must first be dried to remove water. Water is added to increase the plasticity of the clay; this water is still present in the body after it has been formed, and can be removed only slowly as it must migrate through the spaces between the particles of clay, silica and feldspar to evaporate from the surface. During the drying period the body will shrink by a significant amount. Shrinkage stops when the particles come into contact. However, if drying is not uniform, stresses can build to the extent that the body warps or possibly cracks.

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The manufacturing process of fine ceramics

Castware is in the mould for 0.5 - 1 hour, where some drying occurs, and then air-dried for 1 - 4 days. Jiggered-ware is dried at a little above ambient temperature for a little over an hour in a "mangle drier" and then air-dried for 1 - 5 days. Step 3 - Firing process Once drying is complete the body is ready for firing. All unglazed articles and many glazed ones are fired using the "once-firing" method. However, a small number of articles are fired twice in a method whereby the glaze is applied after the first, biscuit, firing and is fixed on by a second, glost, firing. In this method the dried articles pass through the first, biscuit (or "bisque"), firing at a slow rate. For hollow-ware, such as cups, the total time from cold through the kiln and back to cold is about 26 hours, while for other articles it is 44 hours, although modern kiln design is able to significantly decrease both these times. After this the glaze is applied and the ware is fired again. The maximum temperature in both kilns is 1170oC. The firing process uses a kiln featuring great energy consumption, and natural raw materials are used to produce ceramics. It is essential to have a correct understanding on the following thermal changes (drying, dehydration, decomposition, combination, inversion, vitrification) before working out the firing curve.

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The manufacturing process of fine ceramics

(1) From room temperature to 900°C

The green body is dried in the first phase. Normally drying is completed before firing, but it may contain 1 to 2 percent moisture before entering the kiln. This moisture is evaporated before the temperature reaches 200°C. Then, when the temperature is between 300 and 500OC, organic substances contained in the material are carbonized or combusted. The strength of the material is reduced in this period. The water of crystallization of the clay mineral contained in the material is subjected to hydration and decomposition at 500 to 700°C. Since this reaction is an endothermic reaction, heat is absorbed into the gray body, and temperature does not rise. This requires supply of necessary heat in sufficient amount. The organic substance carbonized at 300 to 500°C is subjected to oxidation from about 800°C and so-called soot removal is carried out. To remove soot completely during this period, it is necessary to take time to supply sufficient amount of air. Firing-starts partly at the end of the oxidation period, the strength is increased slightly over that of the gray body. If left cooled, the biscuit ware (unglazed earthen ware) will be produced. This is provided with glazing and glost firing. In the case of pottery, there is no dehydration of carbonized clay mineral of the organic substance or oxidation of carbon up to 900°C; therefore, it is not necessary to pay particular attention to the heat curve.

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Steps of firing

Active sintering takes place during this period, and the gray body goes on shrinking considerably. Therefore, temperature must be raised uniformly while sufficient attention is paid to avoid uneven shrinkage and deformation. At this stage of temperature, reduction and sintering may be performed depending on the type of the firing goods. The temperature of the sintered pottery and refractories is raised to the specified value, then heating is terminated. When the gray body is large, temperature differences occur. To prevent this, the product must be kept for some time at the maximum temperature zone. (3) Cooling When cooling has started after the maximum temperature is exceeded, the gray body is vitrified to the maximum density, and the glaze is molten to be vitreous. The key point in the cooling process is to cool glass inversion point of the cristobalite at about 573°C and about 250°C gradually when quartz (SiO2) is included in the gray body. At other temperature ranges it is not affected by the cooling speed, so the speed should be increased maximally to reduce the firing period.

(2) From 900°C to the maximum temperature

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Steps of firing

Glazes

Chemically resistent, glassy coating Raw materials: Boric acid, borax, feldspar, soda, saltpetre, quartz, cand , chrysolite, barium-carbonate, clay, kaolin Coluring pigments Opaque additives (metal-oxides, Sb2O3, TiO2, SnO2, CeO2, ZnO, etc.) After melting the raw materials together – grinding To put on the workpiece: with plunge int o wet suspension or with dry disperion Melting on the workpiece in two layers: base layer, cover layer

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Chlor-alkali industry

• Electrolysis of brine to produce:

– Chlorine (anode)

– NaOH

– H2 (cathode)

• Energy is about 50% of production costs

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The uses of chlorine

• Chlorine has been manufactured industrially for more than 100 years. During this time, the industry's firm commitment to the best safety, health and environmental practices has ensured continuous improvement.

• There are three methods to produce Chlorine:

– The membrane cell process

– The diaphragm cell process

– The mercury cell process

Electrolysis of NaCl brine

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The mercury cell process

• In the mercury cell process, negative electrode is made of flowing mercury. • Sodium is above hydrogen in the electrochemical series, sodium is preferentially

discharged as it forms an alloy (known as an amalgam) with the mercury • Na+(ag) + e- +Hg(l) Na/Hg(l) • The mercury flows out of the electrolysis cell into a separate chamber Reacts

with water to produce hydrogen and sodium hydroxide solution • The mercury is recycled back into the electrolytic cell. • Na/Hg(l) +H2O(l) Na+(ag) + OH-(ag) +1/2 H2(g) +Hg(l)

• The cell is made of PVC-lined steel and the positive electorde where is chlorine is

formed is made of graphite. • 2Cl-(ag) Cl2(g) + 2e-

• As the brine is usually re-circulated, solid salt is required to maintain the saturation of the salt water. The brine is first de-chlorinated and then purified by a precipitation-filtration process.

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Mercury cell NaCl electrolisis A) Hg cell: a) Hg in; b) Anodes; c) final compartment; d) washing compartment B) Horizontal decomposer: e) Hidrogen gas cooler; f) Grafite plates; g) Hg pump C) Vertical decomposer: e) Hidrogen gas ; g) Hg pump; h) Hg distributor;

i) Seal gripping springs

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The diaphragm cell process

• In the diaphragm cell process – The positive electrode( made of titanium) and negative(

made of steel) electrodes are separated by a permeable diaphragm.

– Hydrogen is formed at negative electrode • 2H2O(l) + 2e- H2(g) +2OH-(ag)

– Chlorine is formed at positive electrode • 2Cl-(ag) Cl2(g) +2e-

• The diaphragm is made of asbestos – Sodium chloride solution can flow between the electrodes – Chlorine and hydrogen gas can’t flow through ( preventing

the OH- ions flowing towards the positive electrode)

• The Sodium hydroxide solution formed accumulates in the cathode compartment and is piped off.

• The Resulting solution contains about – 10% sodium hydroxide – 15% unused sodium chloride by mass

• The solution is concentrated by evaporation and the sodium

chloride crystallizes out leaving a 50% solution of sodium hydroxide.

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The membrane cell process

• The anode and the cathode are separated by an ion-exchange membrane. Only sodium ions and a little water pass through the membrane.

• The brine is de-chlorinated and re-circulated. Solid salt is usually needed to re-saturate the brine. After purification by precipitation-filtration, the brine is further purified with an ion exchanger.

• The caustic solution(NaOH) leaves the cell with about 30% concentration and, at a later stage in the process, is usually concentrated to 50%. The chlorine gas contains some oxygen and must often be purified by liquefaction and evaporation.

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[1] 2Cl- ==> Cl2 + 2e- (anodic reaction)

[2] 2Na+ + 2Hg + 2e- ==> 2Na (In Hg) (cathodic reaction)

[3] 2Cl- + 2Na+ + 2Hg ==> Cl2 + 2Na (In Hg) (brutto reaction)

[4] 2Na (Hg-ban) + 2H2O ==> H2 +2NaOH + Hg (decomposition reaction)

[5] 2NaCl + 2H2O ==> Cl2 +2NaOH + H2 (unified process reaction)

1] 2Cl- ==> Cl2 + 2e- (anodic reaction)

[6] 2H2O + 2e- ==> 2OH- + H2 (cathodic reaction)

[7] 2Cl- + 2H2O ==> Cl2 + H2 + 2OH- (brutto ionic reaction)

[5] 2NaCl + 2H2O ==> Cl2 +2NaOH + H2 (brutto reaction)

[8] Cl2 + 2NaOH ==> NaOCl + NaCl + H2O (side reaction)

[9] 3NaOCl ==> NaClO3 + 2NaCl (side reaction)

Electrochemical reactions in the mercury cell process

Elektrochemical reactionin the diaphragma and membrane cell processes

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Sodium Hydroxide (Caustic) Evaporation Process 48

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