Experiments made by the author on the oxidation of petroleum show that the heavy hydrocarbon oils unite very slowly with oxygen, when first exposed to its action, even at temperatures as high as 150° C.; but after the action is once started, by the combination of even a little oxygen with the hydrocarbon, the further oxidation then proceeds with constantly increasing energy. By aspirating a current of air for ten days through heavy petroleum oil, at 140° to 155° C., there were formed solid hydrocarbons, resembling certain natural asphalts. Very little water was formed; the oil “ cracking ” and the hydrogen being removed in the form of light naphthas and non-condensable gases, containing a greater percentage of hydrogen than the original oil, and leaving oxidized hydrocarbons, with less hydrogen and a greater proportion of carbon. The asphalts made in the above experiments absorbed oxygen from the air at ordinary temperatures; the rapidity of the absorption being increased if the asphalt was in a fine powder. Dana refers to these results as the manufacture of grahamite from petroleum.

The oxides of lead, zinc and manganese, and certain salts of the metals, notably manganese-borate, are powerful driers of the vegetable oils, such as linseed oil. Even in solution, many salts of the metals have a drying action if agitated with the oil. In the case of the vegetable oils, these metallic oxides and salts appear to exert a catalytic action in accelerating the combination of oxygen with the hydrocarbon.

Experiments by the writer indicate that oxides of lead and manganese have a similar action in promoting the union of oxygen with petroleum ; and that the asphalts produced retain 2 per cent, or more of lead-oxide, even after treatment with boiling acetic acid and purification by solution in chloroform or ether.

From this point of view, it is probable that the oxidation of bitumen, in effecting the re-formation of the sulphides, is accelerated, and the intensity of the reducing action increased, by the catalytic influence of ferrous and ferric sulphates and by the various sulphates, carbonates, chlorides and oxides of lead, zinc, copper and manganese, which are always present in greater or less amount in the ore-deposits. With petroleum having a paraffin base, the union with oxygen would be extremely slow, were not the chemical activity stimulated in some way. Petroleum with an asphalt base, owing to the heavy hydrocarbons in the oil being combined with oxygen, and approaching fluid bitumen or maltha in composition, would probably absorb oxygen more rapidly.

That the natural asphalts and bitumens, when wet, are more subject to the action of oxygen than when dry, has been observed in the wear of asphalt pavements, which rapidly disintegrate in spots where surface-water accumulates.

Chemical Reactions that Take Place

In the reduction of metallic sulphates to sulphides, by carbon, the action in each case is deoxidation, with formation of carbonic acid, according to the following reactions:

ZnSO4 + 2H2O + 2C = ZnS + 2H2CO3.
PbSO4 + 2H2O + 2C = PbS + 2H2CO3.

The complete reduction of ferrous sulphate to pyrite or to marcasite requires that free sulphuric acid be present:

2FeSO4 + 2H2SO4 + 5H2O + 7C = 2FeS2 + 7H2CO3.

The carbonates of lead, zinc and iron, in the presence of alkaline sulphates, are reduced by carbon to the corresponding sulphides, galena, blende and pyrite:

PbCO3 + Na2SO4 + 2H2O + 2C = PbS + Na2CO3 + 2H2CO3.
ZnCO3 + CaSO4 + 2H2O + 2C = ZnS + CaCO3 + 2H2CO3.
2FeCO3 + 4CaSO4 + 5H2O + 7C = 2FeS2 + 4CaCO3 + 5H2CO3.

Chalcopyritc, CuFeS2, is formed by the double reduction of cupric sulphate and ferrous sulphate:

CuSO4 + FeSO4 + 4H2O + 4C = CuFeS2 + 4H2CO3.

The reactions which take place where the hydrocarbons form the reducing agents are more complex. The hydrocarbons at first lose hydrogen and gain oxygen, until disintegration occurs; then they rapidly oxidize to carbonic acid and water. With bitumen and coal, it is probable that practically all the carbon is finally converted into carbonic acid, and the hydrogen into water. In the oxidation of petroleum some “ cracking ” may occur, and a portion of the hydrogen and carbon may escape in the form of light hydrocarbon gases, as did take place in the experiments described above.

Oxygen seems to be needed to complete the reaction in the reduction by petroleum of the paraffin series:

12ZnSO4 + C16H34 + O = 12ZnS + 16H2CO3 + H2O.

Where the petroleum is partly oxidized, ferric sulphate forms pyrite in the presence of an excess of free sulphuric acid. The reaction may be written as follows:

3Fe2(SO4)3 + 3H2SO4 + C16H30O2 = 6FeS2 + 16H2CO3 + 2H2O.

To illustrate the partial reduction of ferric sulphate to pyrite, with formation of ferrous sulphate, the formula of an oxidized hydrocarbon nearly corresponding to humus acid is assumed, as in the following equation:

4Fe2(SO4)3 + 8H2O + C16H16O8 = 4FeS2 + 4FeSO4 + 16H2CO3.

The complete reduction of ferrous sulphate to pyrite or to marcasite by an oxygenated hydrocarbon, such as gilsonite, C13 H20O, also requires the presence of free sulphuric acid:

5FeSO4 + C13H20O + 6H2SO4 + 3H2O = 5FeS2 + 13H2CO3 + 5H2O.