106 



UNITED STATES MINERAL RESOURCES 



mums of zinc and lead are less. In addition, cadmium 

 used in pigments requires a low content of thallium, 

 and cadmium Solders cannot contain even trace 

 amounts of aluminum. 



The major use of cadmium is in the electroplating 

 of fabricated steel products to give them superior 

 corrosion resistance. Such electroplating uses about 

 50 percent of the available metal. The metal is also 

 used in low-melting-point alloys. The chief use of 

 cadmium compounds is as pigments; cadmium sul- 

 fide, cadmium sulfoselenide, and lithopone generally 

 account for the color range of yellow, orange, and 

 red in corrosion-resistant paints. Cadmium stearate 

 is used as a stabilizer in the production of some of 

 the vinyl plastics. Cadmium phosphors are used wide- 

 ly in black-and-white television and as coatings for 

 fluorescent tubes; these phosphors are also increas- 

 ingly being used in color television for blue and green 

 components. Cadmium nitrate is one of the chief 

 components of the nickel-cadmium battery. Despite 

 a variety of substitutes, cadmium is sufficiently 

 superior to warrant continued use, and therefore 

 most available supplies will be consumed. (See 

 Heindl, 1970.) 



EXPLOITATION 



The industrial use of cadmium dates from the 

 1870's, although the metal was first detected as an 

 impurity in zinc ores as early as 1817. All commer- 

 cial cadmium used in the latter part of the 19th cen- 

 tury was produced from the ores of the Upper Silesi- 

 an zinc-lead district, at that time in Germany but 

 after World War I in Poland. The cumulative Ger- 

 man, and hence total world production, in the 19th 

 century amounted to about 115 mt (metric tons). 

 The United States first began to extract cadmium 

 from zinc ores of the Joplin district, Missouri, in 

 1906 and since then has produced more than 150,000 

 mt of this metal. Total world production of cadmium 

 thus far in the 20th century has exceeded 320,000 

 mt. The annual world production rate was SVo, 13 V2, 

 3,150, and 16,000 mt in 1882, 1900, 1935, and 1970, 

 respectively. In comparison, the U.S. annual produc- 

 tion rate was 6, 112, 1,800, and 4,300 mt in 1907, 

 1917, 1935, and 1970, respectively. 



Because cadmium is recovered as a byproduct dur- 

 ing the smelting and refining of zinc, its production 

 is clearly a function of zinc production. (See chapter 

 on "Zinc") However, because cadmium is recovered 

 at the smelter, world production more closely paral- 

 lels zinc smelter production than mine production. 

 Comparison of zinc production data with cadmium 

 production data shows that the world has doubled its 

 use of cadmium in the last 15 years, whereas its con- 



sumption of zinc has required about 20 years to 

 double. This somewhat greater increase in the rate 

 of cadmium production relative to that of zinc is due 

 to the improvement of zinc-refining techniques and 

 to the development of more efficient recovery meth- 

 ods. At the same time, some major zinc production 

 has been shifted to ores somewhat richer in cadmi- 

 um, as for example those of the Mississippi Valley 

 type. These relations are clearly seen in shift of the 

 Zn:Cd ratio of production data from values of more 

 than 80,000 in the 1880's (the first decade of cadmi- 

 um production) down to an average of about 525 for 

 the 1930's, 340 for the 1940's, 350 for the 1950's, and 

 310 for the 1960's. From the ratio for the 1960's 

 (310) and on the assumption that the average zinc- 

 bearing concentrate contains 55 percent zinc and 

 that about 75 percent of cadmium in the concentrate 

 is recovered, it is estimated that world zinc concen- 

 trates average about 0.24 percent cadmium. 



GEOLOGIC ENVIRONMENT 



GEOCHEMISTRY 



Cadmium is the chemical element with atomic 

 number 48. Cadmium has an atomic weight of 112.41, 

 and eight stable isotopes in proportions as follows : 



Isotope Percent 



Cd™ 1.22 



Cd™ .88 



Cd"° 12.39 



Cd'" 12.75 



Cd'^ 24.07 



Cd'" 12.26 



Cd"* 28.86 



Cd"' 7.58 



Cadmium has a valence of + 2 and an ionic radius 

 of 0.97 A. It belongs to Group lib of the periodic 

 system, along with zinc and mercury. Thus, cad- 

 mium is chemically very similar to these elements, 

 a similarity that accounts for its relative concen- 

 tration in zinc minerals despite a much larger ionic 

 radius (0.97 A versus 0.74 A). However, cadmium is 

 relatively more sulfophile than zinc and hence is 

 relatively more concentrated in the later stages of 

 magmatic differentiation and associated hydrother- 

 mal ores and even more so in the low-temperature 

 reducing sedimentary environments. This relation- 

 ship is clearly shown by the Zn : Cd ratios of various 

 geologic materials given in table 20. 



MINERALS 



There are relatively few independent cadmium 

 minerals. The native metal is unknown, and natural 

 cadmium oxide (CdO) has been found only in Sar- 

 dinia, as a coating on hemimorphite. Cadmium sul- 

 fide (CdS) occurs both as hawleyite, the cadmium 



