ISDUSTRIAL .4ND ENGISEERING CHEMISTRY
1100
carbonic acids and are capable of salt formation as well as of base exchange ( 2 ) . In Table I1 the amounts of the ash Constituent. of the German brown coals are compared with the average concentrations of the corresponding elements in both the earth’s crust and the various forms of organic life as estimated by Vernadsky (10). I n case an ash constituent exceeds that of the organisms, a storing effect is obtained; if the amount of an element in the ash is midway between the values for the earth’s crust and for the organisms, a simple averaging ha, taken place; if an ash constituent represents the lowest of the three values, a loss during coal formation is probable. Calcium, iron, magnesium, and manganese are capable of forming insoluble humates, and the accumulation of these four elements in the ash may be traced back to this fact. The concentration of the elements silicium, aluminum, and sodium is apparently equalized. Potassium and phosphorus, both of the utmost importance in agriculture, seem to be lost by leaching. Some of the elements present in traces may have played a role in building up the extremely abundant forests of the brown coal period, and they may still play an
VOL. 27, NO. 9
important role in industrial processes; these two possibilities are open to further experimental work and consideration.
Literature Cited (1) Classen, d.,Handbuch der analytischen Chemie, Vol. I, S t u t t gart, Ferdinand E n k e , 1922. (2) Fuchs, W.,“Die Chemie der Kohle,” Berlin, Julius Springer, 1931.
( 3 ) Fuchs, IT., Gagarin, R., and K o t h n y , H., Biochern. Z., 259,85 (19331,
(4) Geilmann, W., and Briinger, K., 2. a n o r g . allocm. C‘hetn., 196, 312 (1931). (5) Gerlach, IT., and Schweitzer, E., “Die chemische Emissionsspektralanalyse,” Leipzig, Theodor Steinkopff, 1930. (6) Heyne, G . , and Moers, K., 2. anorg. allgena. Cheni., 196,143 (1931). (7) Papish, J., and Holt, D. A, J. Phys. Chem., 32, 142 (1928). (8) Schleicher, A., 2. Elektrochem., 39,2(1933). (9) Schleicher, A., and Clerrnont, J., 2. anal. Chem., 86,191 (1931). (IO) Ternadsky, W., “Geochemie,” Leipzig, Akademische Verlagsgesellschaft, 1930.
RECEIVEDApril 8, 1935. Presented before the Division of Gas and Fuel Chemistry a t the 89th hleeting of the American Chemical Society, New Y’ork, N. Y., April 22 t o 26, 1935.
Rare Elements in Coal Ashes V. M. GOLDSCHMIDT Mineralogical Institute, University of Gottingen, Germany
C
OAL ashes contain many elements in small amounts
which are not commonly reported in ash analyses t u t which may affect the behavior of coal in use. The high percentage of boron as borates or borosilicates in the ashes of many coals may influence the softening temperature and clinkering characteristics of the ash and so affect the combuction of the coal. Certain catalytically active metals, such a s vanadium, nickel, cobalt, molybdenum, tin, acd/or g e r r a nium which may be present in the mineral matter of coal, may be of importance in determining the nature of the products of distillation or hydrogenation of such coals. The high percentage of arsenic in a number of ashes, which may be concentrated in the flue dust and soot, may be an important source of pollution near industrial centers. The obEerved concentrations of certain elements in the ashes of coal tars, especially zinc, germanium, arsenic, silver, and cadmium. may be of importance in the subsequent industrial utilization of the tar or the products prepared from it. Carbon rods for spectral analysis prepared from coal may be rendered unsatisfactory by the presence of many elements in detectable amounts; only after much work was done by Bauer and Harmann on this problem was success achieved in eliminating even the last spectroscopical traces of these impurities. I n only a few cases studied in the past may the percentage of rare elements found in the ashes of coals be sufficient to warrant industrial recovery; one such case is the abundance of germanium in some coals. Rare metals and metalloids, which occur in the earth’s crust in amount less than 0.05 per cent, have been observed in the ashes of coal in many cases. Nearly 50 years ago Jensch (4) published data on the percentages of zinc, cadmium, and lead in coals from Upper Silesia. Forty years ago Jorissen (6) found in flue dust, derived from the burning of Belgian coal, the elements copper, zinc, tin, molybdenum, and lead. The occurrence of vanadium, sometimes in large amounts, in ashes of bitumen and other hydrocarbons
is well known. Sickel in amounts up to one per cent has been observed in ashes from British coals ( 7 ) .
IS THE course of an investigation on the percentages Q and distribution of the different elements in minerals, rocks, and ores the author observed that germanium, an (1)
element previously considered to be very rare, iq concentrated in considerable amounts in the ashes of many coals, even up to more than one per cent of germanium dioxide. Systematic investigation revealed that many rare elements are sometimes concentrated in the mineral matter of coal. The analyses have been carried out partly by chemical means and in most cases by the methods of quantitative optical or x-ray spectral analyses ( 3 ) This laboratory has developed the technic of quantitative chemical analysis by means of optical spectra; the majority of chemical elements have been observed to date (see, for instance, citation 6 ) . Concentration of rare elements, as compared with the average percentage in the rocks of the earth’s crust, has been observed in the ashes of many coals but not in all coals. Vsually the enrichment of rare elements in their mineral matter is the more marked, the less the total amount of mineral matter in the coal. It is remarkable that the rare elements which are found concentrated in the ashes of coals may have very different chemical properties; for instance, we find beryllium, strontium, barium; boron; scandium, yttrium, lanthanum, and the lanthanides (elements of atomic numbers 57-71) ; zirconium; vanadium, cobalt, nickel, molybdenum, uranium; copper, zinc, gallium, germanium, arsenic, antimony, cadmium, tin, iodine, lead, bismuth; silver, gold, rhodium, palladium, platinum (not yet tested or incomplete data, among others, thorium, indium, thallium, selenium, tellurium). In many cases all these elements, or most of them, have been concentrated in the same ashes. Besides the rare elements, some common elements may
also be concentrated in the ashes of coal, as compared with their average abundance in the crust of the earth-for example, calcium, aluminum, silicon, iron, manganese, phosphorus, and sulfur (as in the minerals calcite, quartz, pyrite). I n such ashes in which rare elements have been concentrated, we usually find an enrichment also of aluminum, and a deficiency of iron and magnesium. Table I gives data for a number of elements commonly concentrated in coal ashes in order to illustrate their degree of enrichment, as compared with their average percentage, ab far as is known by reliable data, in the common rocks of the earth's crust. The data refer to percentages and are representative only for such ashes, which show the phenomenon of enrichment. IN ASHES OF COAL AKD TABLEI. RAREELEMENTS
IS THE
E.4RTH'S CRUST
Element Be B
SO
co Xi
Zn Ga Ge A3 1Zr
M0 Sb Sn Pb ~~~
Bi
Ag
.lu Rh
Pd Pt
Max. Percentage
A\.. Percentage of "Rich" Percentage in -4shes Earth's Crust 0.03 0.06
0.1 0.3 0.04 0.15 0.5 1 0.04 1.1 0.5 0.08 0.5 0.05
0.006
0.03 0.07
....
0.01 0.05 0.05 0.01
....
0.02 0.02 0.02
0.1
0.05 0.1
.. . .. .. ,
0.003
O.OOO5-0,001 0.0002 0.00002-0.00005 . . . . 0.000002 .. , 0 00002 . . . 0. 00007 ....
Factor of Enrirhment -417.0;
"Rich' Ashes
Max.
100-500 1000 7&130 0.004 40 80 0.01 50 0.02 0.001-0.0015 30-40 0.0004-0.0007 1600-2500 1600 0.0005 50 0.001 25 0.02 30 0.0015
0.0002-0.001 0.0003
0.00034.0006
...
7-10 70-120 100 10
...
13
...
......
10
0.005 0.0016
,
......
...
0 0000005
50-100 40-100
.....
. .
......
'4
...
70
0,00001
......
30-150 200 10-20 8 7
...
20
...
. . . .
...
...
The following data serve to illustrate how the concentration of one of the rare elements, germanium, in different bamples of ash from the same coal seam (seam 13, level IV, of the mine Consolidiert Rudolph, Kopprich near Neurode in Silesia, Germany), is inversely dependent on the total percentage of mineral matter: Total ash, 70 GeO?inash,($
1.6
0.2
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INDUSTRIAL AND ENGINEERING CHEMISTRY
SEPTEMBER, 1935
3.6 0.1
5 5 0.01
14.1 0.01
27.5 0.001-0.01
Generally the ash containing remarkably high amounts of rare elements is not more than 3 per cent by weight of coal, or even about 1 per cent by weight of the coal. Some of the most interesting cases have been found in ashes from the famous coal seams of Hartley, near Newcastle-on-Tyne in England (main seam, yard seam). These ashes show the highest percentage of germanium, up to 1.6 per cent of germanium dioxide. However, in some cases ashes of very pure coals have shown only lo^ percentages of the rare elements. I n banded coals the ashes from vitrain and clarain usually show higher percentages of rare elements than the ashes from durain. Some enrichment of the same groups of rare elements has also been observed in the ashes of lignites and even peats. THREE main stages of processes must be considered by 5 which the rare elements have become enriched in the concentration during life of plants; (2) ashes of coals: (1)
concentration during decay of vegetable matter, especially in humus soils; and (3) concentration during mineralization of fossil fuels. Examples of a process of the first type are the concentration of calcium, magnesium, potassium, sulfur, and phosphorus among the mineral substances of the living matter
of plants which take part in the vital processes; or of manganese which is concentrated, for instance, in the mineral matter of the leaves of trees; or of boron which can be shown to be concentrated often to the factor 500 in the mineral matter of plants, as compared with the percentage of boron in the soil ( 2 ) . Generally the elements concentrated in ashes of coals are not the same as those concentrated in the ashes of plants. This difference has been known for a long time t o be due to the solubility of the compounds noininally found in plant ashes, which may be easily leached by circulating water from the decaying vegetable matter. By. such leaching, proportional enrichment takes place of other substances, which either form insoluble, or sparingly soluble, hydroxides, or which are precipitated by organic constituents as insoluble compounds. It is in this manner that enrichment in decaying vegetable matter is effected (stage 2 ) . Besides aluminum and silicon, there are enriched most of the elements which also are found in coal ashes, while sodium, potassium, calcium, magnesium, iron, manganese, sulfur, and phosphorus are more or less efficiently removed. I n addition, a direct enrichment can take place if the rare constituents of percolating waters are precipitated by components of the organic matter; this direct enrichment may continue even during the process of coalification, as in the formation of pyrite from circulating solutions of iron compounds reacting with sulfur compounds in coal beds. A similar case is the enrichment of vanadium in some bituminous substances.
e
PROCESSES as classified in stages 1 and 2 are not only to be recognized in the constituents of ashes of fohsil fuels, but can be recognized as taking place on a large scale in forests and their humus soils a t the present time. A very great number of rare elements are concentrated among the mineral substances of the humus soils, and generally these are just the same rare elements as are Concentrated in the ashes of coals. The mineral content of fresh leaves, decaying leaves from previous years, and the humus soil of the old beech and oak forest of Sababurg, near the Weser River, have been compared with that of the subsoil which is formed by a weathered Triassic sandstone (Buntsandstein). The data demonstrate an enrichment of aluminum, scandium, yttrium, beryllium, barium, vanadium, cobalt, nickel, zinc, gallium, germanium, arsenic, cadmium, tin, lead, as well as silver and gold in the mineral matter of the humus. The great enrichment of boron and manganese, which is found in the ashes of fresh leaves, is gradually diminished through decay. Typical data are shown in Table 11. TABLE 11.
ENRICHMENT O F ELEMENTS DURING OAKAKD BEECHHUMUS
B203 MnO % % Mineral s o i 1 (sand) 0 0007 0 O+ .Ish from fresh oakleaves 0.5-1.0 2.00" .Ish from oak humus 0.02 0.24 .Ish from beech humus 0.003 0.14 Ash from fresh beech leaves; year, 0.77 per cent MnO.
Xi0 %
Ge02 %
0.002
0 0005
As205
%
DECAYO F .ig
%
Au
4%
. . . . . . .
0.005 0.0005
. . . . . . . .
0.01
..
0.007
0.0001
. . .
0 05
0 0005 0,00002 in nesthered leaves from the previous
0.01
0.007
The general result of these analyses is to show a rather uniform process of concentration which affects a great number of elements, apparently independently of the differences of chemical properties. I n the water and moisture of the subsoil the different elements or their compounds as present in the subsoil are dissolved in accordance with their solubility relations. This process of solution may give a preference to the rare constituents, as the amount dissolved is dependent only on the properties of the solvent and not on
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IIC’DUSTRIAL AND ENGINEERING CHEMISTRY
the presence of an excess of undissolved matter. But of course the solution will contain especially the strong bases and the acids of the soil, such as ?;a, K, Mg, Ca, Mn-&, Fe -&,C1, SO4, BOs, COa, and S O s . The solution is taken up through the roots of plants, and its water is evaporated a t the place of greatest transpiration (the leaves) in which we find therefore the greatest percentage of ash, left behind by the process of evaporation. The bulk of this ash is composed of easily soluble substances, such as salts of the alkaline metals, calcium and magnesium, mostly as carbonates, sulfates, phosphates, and also borates. The composition is therefore very unlike the usual coal ash, as has been demonqtrated already by many observers. When these soluble salts in the mineral matter of decaying vegetable substance. are removed by percolating waters, the relative abundance of many rarer elements in the remaining mineral matter is increased. The entire process of concentration of rare elements therefore involves extraction of the rare conqtituents, in $0 far as they are soluble, from a great volume of subsoil, filtration of this solution through the roots of plants, and transportation of them to the place of evaporation in the leaves. These stages are followed by a process of fractional extraction in the presence of the organic matter of humus soils, which removes the more soluble substances, such as the alkaline metals, magnesium, calcium, and the anions of strong acidI., S a c h r . Ges. Wiss. Gottingen, Math.-physik Klasse, 1930, 398. (1) Goldschmidt, V. M., and Peters, C., Ibid., 1932, 528. ( 3 ) Goldschmidt, V. M., and Peters, C., Ibid., 1933, 141, 371; S a c h r . Ges. Wiss. Gdttingen, Math.-physik. Klasse, Fachgruppe I V , 1, 11 (1934); Thilo, E., 2. anorg. allgem. Chem., 218, 201 (1934): Geol. Foren. Forh., 56, 385 (1934). Goldschmidt, V. M,, (4) Jensch, E., Chem. Ind., 10, 54 (1887). (5) Jorissen, A,, Ann. soc. gBoZ. Belg., 23, 101 (1895-6). (6) Mannkopff, R., and Peters, C., Z. Physik, 70, 444 (1931); GoldSchmidt, V. M.,and Peters, C., X a c h r . Ges. Wiss.Gottingen, Math.-physik. Klasse, 1932, 377; Goldschmidt, V. M., Rauer, H., and TVitte, H., Sachr. Ges. Wiss. Gottingen, Math.-physik. Klasse, Fachgruppe I V , 1, 39 (1934); Rauer, H., 2. anorg. allgem. Chem., 221, 209 (1935). ( 7 ) M o t t , R. 1., and Wheeler, R. V., Fuel, 6 , 416 (1927). RECEIVED April 8, 1935.
Presented before the Division of Gas and Fuel Chemistry a t the 89th Meeting of the American Chemical Society, S e n York, S . Y., April 22 to 26, 1935.
