Iron Production

The main volume of produced iron is in forms of cast iron and steel. The main iron metallurgical principles did not changed during millenniums. The furnace-charge consists of iron oxide ore, coke and flux such as dolomite CaCO₃ MgCO₃, while a blast of heated air is forced into the furnace at the bottom. In the furnace, the coke is burnt in oxygen of the air blast to produce carbon monoxide. The carbon monoxide reduces the iron oxide (III) ore to molten iron, becoming carbon dioxide in the process:

2Fe₂O₃ + 3CO = 4Fe + 3CO₂

The steel manufacturing is based on melting of cast iron in the presence of oxidizers, during which the carbon concentration falls until 1.5-2%, as a result FeO is formed and involved in redox reaction, in which the impurities are oxidated and iron is reduced.

The highly pure iron (containing less than 0,001% contaminations), prepared by carbonyl complex: distillation and chemical decomposition of purified iron pentacarbonyl Fe(CO)₅ which in its turn is the product of commercial iron treatment under pressure, at 150-200°C by carbon monoxide. The carbonyl iron, which usually has the appearance of grey powder, is the product of this process.

1. One of the earliest methods of obtaining a pure iron consists in reducing ferrous chloride by heating in an atmosphere of hydrogen. The iron is deposited in the form of cubic crystals of microscopic size.
2. Reduction of iron oxide, carbonate, or oxalate in a current of hydrogen results in the formation of a very pure iron. The reaction is fairly rapid at 600° C., but higher temperatures are more efficient, whilst at lower temperatures the product is usually contaminated with ferrous oxide.
3. Iron may be reduced from its salts by means of zinc. Thus at high temperatures ferrous chloride is reduced by zinc vapour, whilst aqueous solutions of ferrous salts are reduced by metallic zinc at their boiling-points, yielding finely divided iron.
4. An exceptionally pure form of reduced iron has been obtained by Lambert and Thomson by reduction of pure, colourless crystals of ferric nitrate. The crystals were first converted into oxide or basic nitrate by ignition in an iridium boat. The whole was then introduced into a silica tube and heated in an electric resistance furnace to just above 1000° C. in a current of pure hydrogen gas, obtained by the electrolysis of barium hydroxide solution.
5. Electrolytic Iron. - Iron may be obtained in a high state of purity by electrolysing a mixed solution containing 20 per cent, of ferrous sulphate (calculated as FeSO₄.7H₂O) and 5 per cent, of magnesium sulphate (MgSO₄.7H₂O). Some 25 grams of sodium hydrogen carbonate are added to every 6 litres of the solution, whereby a skin of ferric hydroxide forms on the surface of the bath and protects the liquid from oxidation. A precipitate settles to the bottom and is allowed to remain undisturbed. An anode of wrought iron is employed, the cathode being made of copper, thinly silvered and iodised, and maintained in rotation. The best results are obtained with a current density of 0.3 ampere per sq. decimetre of cathode, and a temperature of 15° to 18° C. The bath is kept continuously at work, and 20 to 25 grams of carbonate are added every three or four days.

Both the magnesium sulphate and the sodium hydrogen carbonate appear to be essential constituents of the bath, and the good quality of the metal is attributed to the small concentration of hydrogen ions which prevents the deposited iron from containing occluded hydrogen.

Numerous attempts have been made to prepare iron by the electrolysis of ferrous chloride, but with this salt an elevated temperature is essential for good results, namely, 60° to 70° C. The current density at the cathode should not exceed 0.4 ampere per sq. decimetre, and the quality of the deposit is improved by rotation of the cathode.

According to Noyes, the minimum potential required for the electrolysis of an aqueous solution of a ferrous salt at 20° C. is 0.66 volt, when electrodes of electrolytic iron are employed. This value falls by 0.007 volt per degree rise in temperature up to 110° C., when it attains a minimum. Further rise in temperature necessitates an increased voltage.

Hicks and O'Shea recommend the electrolysis of a 5 per cent, solution of ferrous chloride to which ammonium chloride has been added in sufficient quantity to establish the ratio

FeCl₂ to NH₄Cl = 1 to 2.

Any ferric chloride present is reduced to ferrous by shaking with reduced iron. A thin copper plate serves as cathode, is cleaned with dilute nitric acid, rubbed with cotton-wool and sand, and finally washed with potassium cyanide and then with water. Swedish iron constitutes the anode, and is placed in a porous cell to prevent the spongy carbon, which normally separates from the metal, from reaching the cathode. The sulphur in the metal passes into solution as sulphate, and requires removal at intervals with the anode liquor. A current of 0.08 to 0.2 ampere per sq. decimetre of cathode surface is recommended, the voltage being 0.7. The iron content of the bath should not fall below the equivalent of about 4 per cent, of ferrous chloride.

Skrabal obtained a very pure iron by electrolysis of a solution of ferrous ammonium oxalate, the metal being deposited on a cathode of platinum foil. The electrode thus prepared was now used as anode in an acidified solution of ferrous sulphate, an E.M.F. of 0.4 volt being employed, the cathode again consisting of platinum foil. The metal thus obtained was white and crystalline; it dissolved slowly in warmed, dilute sulphuric acid, leaving no residue, and evolving a pure, odourless hydrogen. To prevent oxidation of the bath, the electrolysis was carried out in an atmosphere of carbon dioxide, and the cathode separated from the anode by means of a porous diaphragm. Excellent results were also obtained with ferrous ammonium sulphate. With this salt a concentration of 70 grams of the hexahydrate (NH₄)₂SO₄.FeSO₄.6H₂O per litre is recommended, with a maximum cathode current density of 0.5 ampere per sq. decimetre, working at a temperature of 15° to 18° C., or not less than 26 grams per litre, with a cathodic current density of 0.2 to 0.65 ampere per sq. decimetre, the surface of the bath being protected from oxidation by a layer of solid paraffin, a stirrer being employed, and the wrought-iron anodes enclosed in linen bags; or a saturated solution, with a current density of 1 ampere at 30° C. Acidified solutions of ferrous sulphate have been used under various conditions, the best results being obtained, according to Pfaff, with a current density of 2 amperes per sq. decimetre at the cathode, a temperature of 70° C., and a concentration of at least two equivalents of ferrous sulphate per litre and 0.01 equivalent of sulphuric acid.

Electrolytic iron deposited from solution at ordinary pressures and temperatures is apt to be admixed with ferric hydroxide, and to contain hydrogen and carbon. The last-named element is commonly derived from the oxalates or tartrates in the baths, when these are used, and as the result of transfer from the anodes. The carbon may be free, combined as carbide, or present as occluded carbon monoxide or dioxide.

Electrolytic iron is frequently brittle, a property that is usually attributed to occluded hydrogen, but there are probably other auxiliary causes.

Iron has also been obtained from solutions of its salts in organic solvents by electrolysis between platinum electrodes. A solution of ferric chloride in methyl chloride conducts electricity well, and may be used for the purpose. Produced in this way, however, the metal is particularly liable to be contaminated with carbon.

Deposits of iron are frequently applied to engraved copper plates to harden their surfaces and thus increase their life for printing purposes. Various solutions are recommended for this purpose. A simple one yielding good results consists of

Ferrous ammonium sulphate	1 lb. or 100 grams.
Water	1 gallon or 1 litre.

This solution must be perfectly neutral for use. Another useful mixture contains ferrous chloride and ammonium chloride in molecular proportions to the extent of 50 to 60 grams per litre, or

FeCl2.2NH4Cl	½ lb., or 50 grams.
Water	½ lb., or 50 grams.

This works well with a current density of 0.15 to 0.17 ampere per sq. decimetre, the initial density being 0.2 ampere until a thin deposit has been obtained on the cathode. Voltage 0.7. The anodes should consist of pure Swedish charcoal iron. These become covered with a black carbonaceous slime after a time, and require cleaning. It is desirable that the anodic area should somewhat exceed that of the cathode.

It is frequently desirable to protect iron by coating it superficially with copper. The following bath is recommended for the electro-deposition of this metal upon iron -

Copper sulphate	60 grams.
Sodium hydroxide	50 grams.
Sodium potassium tartrate	159 grams.
Water	1000 grams.

with a cathode density of 0.1 to 0.5 ampere per sq. decimetre and an anode density not exceeding 1.04 amperes. As this bath evolves no dangerous fumes, it is preferable to cyanide baths.