MINERAL SOURCES AND EXTRACTION METHODS FOR THE ELEMENTS
0
A Correlation Based on Elements' Positions in the Periodic Table RALPH HULME University College, Gold Coast, British West Africa
classification the relation of element source to type of element is at once apparent (Table 1). Thus, it is no longer necessary to burden a student's mind with the various mineral sources of calcium, for example. Reference to the general classification shows at once that carbbnate or sulfate minerals would be expected, just as sulfide and some oxide would be anticipated for copper. Inevitably there are some shortcomings. Thus, lead occurs as sulfate and carbonate as well as sulfide and oxide, while the diagonal relations of lithium and magnesium, and of beryllium and aluminium are not emphasized. Nevertheless, exceptions are more easily (1) Alumino-silicates and soluble salts remembered as such when there are clear guiding prin(Cl, COX,etc.) such as those a t Stassfnrt. ciples available. Lithophil (2) Insoluble carbonates and sulfates. I n passing, it is worth noting that many elements (crust) (3) Oxides and mixed oxides (oxyacid occur in nature as insoluble salts, these often being just salts). Siderophil the ones which are precipitated in group analysis. (4) Free in the earth. (core) Besides compounds of the desired element, a mineral (5) Free in the atmosphere. Atmophil generally contains quantities of waste material which (6) Sulfides, with some oxides. Thiophil must be removed prior t o extracting the element. The (intermediate zone) ore is usually crushed and the enriched mineral sep" , bv " washine. -, or bv froth flotation. When demarcation lines are drawu over the periodic arated2maeneticallv.
F R O M an examination of a number of elementary textbook in chemistry, it has become apparent that the periodic classification is seldom stressed as a means of correlating the common sources of the elements with their extraction procedure. These two features are each necessarily related to the nature of the element, which in turn depends upon the position it occupies in the periodic classification. It is, therefore, to be expected that the two features should be related to one another. Mineral sources may be variously classified' but we may distinguish the following broad types:
1
-
' Grssonr, D. T., Quart.Rev. Chem. Soe., London, 3,263 (1949).
BARNETT,E. DE B., AND C. L. WILSON,"Inorganic Chemistry," Longmans, Green and Co., London, 1953, pp. 92 IT.
TABLE 1 Principal Types of Mineral Source.
I
C
I
S P
K
Sc
Ti
V
Cr
Mn
Rh
Y
Zr
Nb
Mo
(T)
Ta. W (Fr) (1).
1 1 Ra
Ach
(2)
" And the lsnthsnons.
(31
Re
S
H
He
F
SP
C
A
F:"hg '1
1 I
And the actinons. For explanation of numbers refer to the text.
O
-
Fe
Ni
Co
Cu
AO
(4)
1
I
Zn
Ga
Ge
As
Se
Br
fir
Cd
In
Sn
Sh
Te
I
X
~g
TI
~h
Bi
PO
( ~ t ) ~n
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JOURNAL OF CHEMICAL EDUCATION
In certain cases chemical methods of slag formation or volatile compound formation may be used. Extraction methodsScau be grouped as follows: (I) Fractional distillation, for the naturally oecurring gaseous elements. (2) Lixiviation, with cyanide or molten lead for the free noble metals, or with superheated steam for sulfur, etc. (3) Reduction processes, which have been considered theoretically from the point of view of the freeenergy change involved in MX-+M4 Reduction may be by:
+
02,the tem(a) Heat alone, e. g., HgO- Hg perature required becoming greater the further away the element is from the noble metals. A variant of this method is the hot-wire thermal decomposition of halides which is especially valuable for elements of high melting point, incapable of being fused, e. g., boron, zirconium, etc. ( b ) The use of reducing agents, of varying power, suited to the particular case. Sulfides, which are generally more difficult to reduce than oxides, can sometimes be made to reduce themselves in a limited air supply: partly CuS dCuO;
CuS
+ 2Cu0
-
3Cu
+ SO*
Rut hydrogen, carbon, and calcium carbide are the more common agents used, sometimes at the temperatures attainable in an electric furnace, e. g.: V20s 5C 2V 5CO For basic elements, such as phosphorus, silica is added to the carbon used to reduce the oxysalt (phosphate).
+
-
+
'EPKRIAM, F., "Inorgmie Chemistry,'' 6th ed., Oliver & Boyd, London, 1954, pp. 163-234; Ann. Reu. Chem. Soc., London, 40, 68 (1943). 'ELLINGKAM, H. J. T., J . Soe. C h a . Ind., 63, 125 (1944). H. H. KELWGG, J. Metals, 188, 862 (1950); 191, 137(1951).
(c) Replacement, either in the dry way by Goldschmidt's thermite process (using aluminum primed with a magnesium/peroxide mixture), e . g., for cbromium; or in the wet way using an amalgam, e. g., sodium amalgam for the lanthanons, and zinc amalgam for indium and thallium. (4) Electrolysis, which is really a special case of reduction and which is tending to supercede older reduction methods where electricity is cheap. Depending upon which metal it is desired to prepare, fused hydroxides, chlorides, or mixed salts are generally used. The choice depends upon such factors as the diffusibility of the metal in its molten salt and the nature of the cathode deposit formed, e . g., alkali metals use hydroxides, while aluminum, niobium, etc., use double fluorides. (5) Oxidation, which may be chemical or, more usually, electrolytic, e. g., oxygen and the halogen family.
Boundary lines, summarizing these groupings, can be drawn over the periodic classification (Table 2). Dotted boundaries are used near those elements equally well prepared by methods lying on each side of the boundary. A comparison of Tables 1 and 2 makes clear the broadly similar classification of mineral sources and extraction method. Once prepared, the crude element requires purification, but the methods employed are more difficult to coordinate neatly on account of the variety of special ones which are in use. Distillation, oxidation of impurities with slag formation (acidic or basic), and electrolysis probably cover most elements. In conclusion, it may be of interest to make the correlations the result of a class assignment, for it is the writer's opinion that a student remembers nothing so well as that which he has discovered and concluded for himself. The compilation of data can be kept quite simple by confining the search to two or three textbooks, from which data for the main type(s) of
TAB= 2 Extraction Methods for the Elements
Na
Mg
A1
K
Ca
Sc
Rb
Sr
(Fr)
R&
A Ti
S Ab
(4) b
For explanation of numbers, refer to thetext. And the lanthanans. And the actinons.
(5) :
i
I
i
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VOLUME 33, NO. 3, MARCH. 1956
mineral source and the main type@) of extraction method can be gleaned. The information can be written in a space below each element in two duplicated periodic classifications, upon which the student may be eucouraged to draw in his own boundary lines. One neat way to emphasize the similar classification of source and extraction method is to record all the data for the latter on tracing paper, which, after boundary
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lines are drawn, can be placed over the corresponding mineral source diagram. Class discovery of correlations and classifications can be extended to cover numerous other elemental "properties," such as abundances, melting points, crystal structures, and hydride type, as well as to plot existence areas for such compounds as the metal carhonyls or nitrides.
