Calculation of the Theoretic Reducing Powers of Various Organic and Inorganic Mineral Substances usually Occurring in Association with Ore-Deposits, Based upon the Weight of Oxygen Consumed.
The quantitative value, or amount of work accomplished in the formation of ore-deposits by the various reducing substances, is measured by the weight of oxygen with which they unite. This work of deoxidation may be termed the “duty” of the reducing agent.
In calculating this duty for the more common organic and inorganic minerals which occur in ore-bodies or in strata in which ores were formed, or into which they were introduced by the waters, either in the original deposition or in the secondary enrichment of the deposits, it has been found more convenient to make the values relative, assuming hydrogen, the most powerful deoxidizing agent, to have a value of 100.
Let R represent the relative reducing power, or duty, of any mineral substance.
Q be the weight of oxygen consumed by one part of the mineral.
S be the weight of the mineral required to unite with one part of oxygen.
P be the weight of hydrogen which combines with 100 parts of oxygen.
The value of Q may be determined in each case from the chemical reactions which take place.
Thus, for hydrogen,
2H + O = H2O,
2 + 16=18,
whence, by proportion, the weight of oxygen consumed by 1 part of hydrogen is determined.
2 : 16 :: 1 : Q= 8.00.
R has been assumed for hydrogen as 100.
In the case of carbon, the reaction is
C + O2 = CO2,
12 + 32 = 44,
whence, 12 : 32 :: 1 : Q = 2.6666.
The relative power, or duty, of carbon compared with hydrogen is
8 : 2.6666 :: 100 : R= 33.33.
For oxygen the value of P is determined from the weight of hydrogen that unites with one atom of oxygen:
O + 2H = H2O,
16 : 2 :: 100 : P=12.5.
P is a constant and always a minus quantity; the oxygen combined with a mineral substance diminishing its reducing power.
R, Q and S are determinable by the following formulas:
From the above it is seen that in assuming, for convenience, the value of R for hydrogen as 100, it was equivalent to multiplying the corresponding value of Q (= 8) by 12.5. Consequently the several values of R, being calculated for equal weights of the mineral substances, are in each instance the weight of oxygen which is consumed by 12.5 parts of the reducing agent. Thus, 12.5 parts of carbon consume 33.83 parts of oxygen, etc.
The “ duty ” of any compound substance is the sum of the reducing powers of the elements of which it is composed. Thus, for the hydrocarbons:
Let a = the percentage of carbon,
b = the percentage of hydrogen,
c = the percentage of oxygen.
Then R = (33.33 a + 100 b) – 12.5 c.
By a formula of this kind R can be calculated directly from the percentage-composition of any substance. Even in the complex metallic sulphides, arsenides, etc., R may be calculated from the composition by substituting in the formula the value of R for sulphur, arsenic, etc.
For carbon-monoxide the value of R may be calculated from the percentage-composition: carbon, 42.86; oxygen, 57.14.
CO + O = CO2
28 +16 = 44.
28 : 16 :: 1 : Q= 0.5714,
8 : 0.5714 :: 100 : R = 7.14.
Marsh-Gas,CH4.—Marsh-gas (methane) belongs to the paraffin series of volatile hydrocarbons; its composition is
By oxidation, marsh-gas forms carbonic acid and water, thus:
Petroleum.—American petroleum is in great part a mixture of hydrocarbon oils of the paraffin series, represented by the formula CnH2n+2. The heavier oils average, approximately, carbon, 85 per cent.; hydrogen, 15 per cent.; corresponding very nearly to the formula, C16H34. Assuming that the carbon is completely oxidized to carbonic acid, and the excess of hydrogen to water:
C16H34 + O49 = 16H2CO3 + H2O,
from which R = 43.36.
Bitumen.—With bitumen is included mineral-tar, maltha, and the solid oxygenated hydrocarbons, such as grahamite. Their composition, while variable, usually falls within the limits of the analyses No. 1 and No. 2 :
The value of R for analysis No. 1 = 39.92, and for analysis No. 2 = 40.67. Analysis No. 3, of grahamite (gilsonite), with but 10 per cent, of hydrogen, gives a value for R of 35.88. In the above analyses it is probable that some nitrogen and sulphur are included with the oxygen, so that the deduction made for oxygen is slightly too great.
Bituminous Coal.—The average composition of bituminous coal may be stated as falling within the limits of the analyses 1 and 2:
The duty calculated from the composition in the above analyses is, for coal No. 1, R = 29.13; and for coal No. 2, R = 35.71.
Lignite.—The composition of lignite is extremely variable and is much affected by the amount of decomposition it has undergone. Assuming that the analyses given below represent the ordinary limits of composition, the value of R for lignite No. 1 is 19.50, and for No. 2, 28.83.
Native Humus Acid.—Dana gives the composition of humus acid from Bohemian brown-coal as C46H46O25, which corresponds to:
Or, calculated from the composition,
Sulphur.—Sulphur in ore-deposits may oxidize under certain conditions to sulphurous acid, but usually sulphuric acid is formed.
Combined sulphur may, for convenience in calculating the duty of sulphides, be regarded as oxidizing to SO4; as if all the oxygen combined with the sulphur and none with the base, giving the values R = 25.00; Q = 2.00; S = 0.50.
Sulphuretted Hydrogen.—The complete oxidation of sulphuretted hydrogen forms sulphuric acid, H2S + O4 = H2SO4, from which R = 23.53. When oxygen is deficient, water is formed, with separation of sulphur. The reaction then is:
H2S + O = S + H2O; whence, R = 5.88.
Pyrite and Marcasite.—Three distinct reactions may occur in the oxidation of pyrite and marcasite (FeS2):
(1) With liberation of sulphur, and formation of ferrous-sulphate—
FeS2 + O4= S + FeSO4; whence, R = 6.67; Q = 0.533; S= 1.876.
(2) With formation of ferrous sulphate and free sulphuric acid, one atom of sulphur may be regarded as oxidizing to SO3, the other to SO4—
FeS2 + O7 + H2O = FeSO4 + H2SO4; whence, R = 11.67; Q= 0.933; S = 1.072.
(3) When the oxidation of pyrite takes place with excess of air, ferrous sulphate is first formed, and by a complicated series of reactions, with further absorption of oxygen, the final result is the formation of limonite and sulphuric acid. The equation may be written:
4FeS2 + O30 +11H2O = 2Fe2O3,3H2O — 8H2SO4,
Giving R= 12.50; Q= 1.00; S= 1.00.
In the substitution of blende and galena for pyrite in secondary deposition, the reactions corresponding to equation (1) may be expressed:
FeS2 + ZnSO4 = ZnS + FeSO4 + S.
FeS2 + PbSO4 = PbS + FeSO4 + S.
Pyrite also deoxidizes sulphuric acid, with the formation of sulphuretted hydrogen and sulphur :
FeS2 + H2SO4 = H2S + S + FeSO4.
The reduction of zinc- and lead-sulphate and lead-carbonate by pyrite is shown by the following equations, corresponding to equation (2):
FeS2 + ZnSO4 + H2O + O3 = ZnS + FeSO4 + H2SO4.
FeS2 + PbSO4 + H2O + O3 = PbS + FeSO4 + H2SO4.
FeS2 + PbCO3 + H2O + O3 = PbS + FeSO4 + H2CO3.
In the presence of an excess of air, reactions, corresponding to equation (3), take place in the reduction of these soluble salts of zinc and lead, as follows:
4FeS2 + ZnSO4 + 11H2O + O26 = ZnS + 2Fe2O3,3H2O + 8H2SO4.
4FeS2 + PbSO4 + 11H2O + O26 = PbS + 2Fe2O3,3H2O + 8H2SO4.
4FeS2 +PbCO3 + 11H2O + O26 =PbS + 2Fe2O3,3H2O + H2CO3 + 7H2SO4.
Arsenic and Antimony.—The values of R for arsenic and antimony are obtained from the following equations :
2As + O5 = As2O5; whence, R = 6.67.
2Sb + O3= Sb2O3; whence, R = 2.50.
Arsenopyrite.—The composition of arsenopyrite, FeAsS, is as follows:
The value of R may be calculated either from the reaction, the arsenic oxidizing to As2O5, or from the composition, as follows:
2FeAsS + O13 = As2O5 + 2FeSO4.
Enargite.—For enargite, Cu3AsS4, or 3Cu2S, As2S5, the composition and the computations are as follows:
3Cu2S, As2S5 + 2H2O + O35 = As2O5 + 6CuSO4 + 2H2SO4.
One-fourth of the sulphur, or 8.15 per cent., is oxidized to SO3, and the remainder (24.45 per cent.) to SO4.
Stibnite.—For stibnite, Sb2S3, the composition and the computations are as follows :
Dana gives the product of the oxidation of stibnite as valentinite, Sb2O3.
Pyrrhotite.—Similarly for pyrrhotite, Fe11S12:
Fe11S12 + O47 + H2O = 11FeSO4 + H2SO4,
Giving R = 9.40; Q = 0.752.
Ferrous Sulphate.—The reactions in the case of ferrous sulphate, FeSO4 are as follows :
In the oxidation of pyrite and marcasite, a mixture of ferrous sulphate and free sulphuric acid is first formed. Weed gives the products of the further absorption of oxygen as
H2SO4, Fe(OH)3 and Fe2(SO4)3.
The reaction that takes place may be written:
6FeSO4 + 2H2SO4 + 4H2O + O3 = 2(H2SO4, Fe(OH)3) + 2(Fe2(SO4)3) + H2O.
Six parts of ferrous sulphate absorb 3 parts of oxygen; or, by reduction, 2 parts of ferrous sulphate absorb 1 part of oxygen.
304 : 16 :: 1 : Q = 0.05263.
8: 0.05263 :: 100 : R = 0.66.
S= 19.1.
The following table of comparative reducing powers gives the quantitative value, or gross amount, of the work done by each of the deoxidizing agents. It is necessary, however, to supplement these theoretic results by observations in the field, especially of ore-deposits undergoing decomposition and reformation ; and also by experimental research in the laboratory, in order to estimate accurately in each particular instance the chemical energy, or velocity with which the action takes place.
Fortunately, with respect to the greater number of the more important reducing substances commonly occurring in ore- deposits, the gravimetric power, or duty, and the chemical energy run nearly parallel; so that one may be taken as the measure of the other.
The results of these calculations confirm the observations made in the zinc- and lead-mines of the Joplin, Mo., region, in the investigation of the secondary formation of the ores, of the relative order of the reducing powers of the principal deoxidizing agents, viz.: 1, bitumen ; 2, bituminous coal and carbonaceous shales; 3, marcasite and pyrite; 4, blende; 5, galena.
Ferrous sulphate appears to be exceptional; notwithstanding the extreme low duty (0.66), and notwithstanding that, informing ferric sulphate, 19.1 parts of the salt combine with only one part of oxygen, yet in the zone of oxidation, where the chemical activities have full play in the breaking-up of an ore- body, it fulfils a special mission, at once oxidizing and reducing. Its chemical energy is such that it reduces cuprous oxide to the metallic state; a change which none of the other deoxidizing agents usually found in ore-bodies are able to accomplish. Further, its field of operation is the zone of oxidation and that border-land where the zone of oxidation merges into the zone of reduction.
This low quantitative-value is in many instances more than offset by the large amount of ferrous and ferric sulphates continuously supplied by the progressive oxidation of the pyrite in the ore-deposits. The mine-waters lixiviating the decomposing ore, although the volume of the flow may be considerable, are frequently strongly acid from the free sulphuric acid and iron sulphates held in solution.
In conclusion, the hydrocarbons, combining the highest quantitative deoxidizing power with an intense chemical activity, are the most powerful of all reducing agents. The action of the solid oxygenated hydrocarbons, bitumen, bituminous coal, and the lignitic matter finely disseminated in shales, is greatly accelerated by the facility with which these carbon- compounds, when in powder, are carried by the circulating waters into every part of the ore-bodies.
Table
The relative reducing power, B, or duty, of equal weights of the organic and inorganic mineral substances usually occurring in the ore-bodies, or in the wall-rock, or introduced with the circulating waters ; hydrogen being assumed as 100.
