Steam as an Oxidising Agent

When a current of steam is made to pass over many substances, whether metals or non-metals, oxidation frequently takes place, particularly at elevated temperatures. Thus, at temperatures above 100° C. sulphur is both oxidised and reduced:

2H₂O + 3S = 2H₂S + SO₂.

With phosphorus at 250° C:

3H₂O + 2P = PH₃ + H₃PO₃.

When steam is allowed to impinge upon incandescent coke, hydrogen is liberated and oxides of carbon are formed. At relatively low temperatures, such as 500° to 600° C., the main products are hydrogen and carbon dioxide. Thus:

(i) C + 2H₂O ⇔ CO₂ + 2H₂ - 18,900 calories.

At 1000° C. and upwards a mixture of hydrogen and carbon monoxide - the so-called water-gas - is formed, the two gases being present in equal volumes. Thus:

(ii) C + H₂O ⇔ CO + H₂ - 29,100 calories.

At temperatures intermediate between the foregoing a mixture of hydrogen and oxides of carbon is obtained, the percentage of carbon monoxide increasing with the temperature, that of the carbon dioxide decreasing.

From the equation

(iii) CO + H₂O ⇔ CO₂ + H₂ + 10,200 calories

it is evident that any variation in the pressure on the system as a whole would be without any influence upon the equilibrium, inasmuch as no change in volume is introduced by any movement of the equilibrium from right to left or in the reverse direction. Hence the value for K in the expression



is independent of the pressure.

Since an increase in temperature tends to shift the state of equilibrium represented in equation (iii) from right to left, it follows that the value for K will rise. This has been experimentally confirmed for temperatures between 786° and 1405° C., the results being as follow:

Temperature, ° c.
786	0.81
886	1.19
986	1.54
1086	1.95
1205	2.10
1405	2.49

In the commercial preparation of hydrogen, since carbon dioxide is more easily removed than the monoxide, the aim will clearly be to work at a low temperature and thus reduce the fraction to a minimum. For the production of water-gas, on the other hand, with a maximum combustible efficiency, the percentage of carbon dioxide must be reduced to a minimum, and high temperatures are essential.

The reaction between steam and carbon is facilitated by the presence of certain inorganic salts, such as the carbonates of sodium and potassium, which function as catalysts.

Steam reacts slowly with silicon at red heat, hydrogen being evolved, whilst the residue consists of silica.

When electrolytic iron foil is heated in steam to about 330° C., tarnishing begins to take place. At 400° C. a small but measurable quantity of hydrogen is formed, and the velocity of the reaction increases rapidly with further rise of temperature. The reaction appears to take place in three stages, involving

1. Dissociation of the steam, H₂O ⇔ H₂ + O.
2. Formation of ferrous oxide, Fe + O ⇔ FeO.
3. Oxidation to ferroso-ferric oxide, 3FeO + O ⇔ Fe₃O₄.

For ordinary iron shavings, the lowest temperature at which hydrogen is evolved is about 300° C., and the optimum yield is obtained at 800° C.

If the reaction is allowed to take place in an enclosed space, it does not proceed to completion. Equilibrium is set up, and the reaction obeys the law of Mass Action. The initial and final stages of the equilibrium may be represented as follows:

3Fe + 4H₂O ⇔ Fe₃O₄ + 4H₂.

Designating the pressure of water-vapour as p₁ when equilibrium has been reached, and the hydrogen pressure as p₂, Preuner obtained the following mean values for the ratio p₁/p₂:

Temperature, ° C.	p1/p2
900	0.69
1025	0.78
1150	0.86

The superficial oxidation of iron with steam is used technically as a means of protecting the metal against corrosion. This is the principle of the Bower-Barff process.

When steam is passed over molybdenum at high temperatures, hydrogen and molybdenum dioxide are formed. The reaction is reversible, and has been studied from the equilibrium point of view over the temperature range 700° to 1100° C.

2H₂O + Mo ⇔ MoO₂ + 2H₂.

It is found that the values for the equilibrium constant, namely,

obtained by the oxidation of the metal agrees closely with that from reduction of the dioxide in hydrogen. Steam has no action on copper or cuprous oxide, but cupric oxide dissociates in steam yielding cuprous oxide. Magnesium readily ignites in steam yielding magnesium oxide and free hydrogen. At red heat nickel slowly decomposes steam, and cobalt becomes superficially oxidised. Tin decomposes steam at red heat yielding the dioxide, SNO₂.

Metallic sulphides are decomposed by steam at high temperatures. At incipient red heat ferrous sulphide yields magnetic oxide as follows:

3FeS + 4H₂O = Fe₃O₄ + 3H₂S + H₂.

At higher temperatures sulphur dioxide and sulphur are produced. Lead sulphide at bright red heat yields the free metal:

3PbS + 2H₂O = 3Pb + 2H₂S + SO₂
2H₂S + SO₂ = 2H₂O + 3S,

and, probably, to a small extent,

PbS + 2SO₂ = PbSO₄ + 2S,

for a little lead sulphate is generally produced simultaneously. At white heat cuprous sulphide is converted into metallic copper:

Cu₂S + 2H₂O = 2Cu + SO₂ + 2H₂.