SELENIUM 



575 



together in biological processes. Selenium replaces 

 sulfur in amino acids; it is taken up by plants and 

 concentrated by some species to a level which can 

 produce toxic effects on animals that eat the plants 

 (Lakin, 1972) . 



The substitution of selenium for sulfur in bio- 

 logical processes and its reduction to selenides by 

 anaerobic microorganisms often leads to the presence 

 of selenium in pyrite contained in black shales and 

 coal. Selenium is found in pyrite in some unweath- 

 ered sedimentary rocks and may be found in con- 

 centrations many times its crustal abundance in 

 ores rich in organic matter. Highly organic shales 

 may contain as much as 1,500 ppm selenium in 

 selected samples and as much as 20 ppm over ex- 

 panses of thousands of miles. Phosphorites may con- 

 tain as much as 300 ppm selenium and are almost 

 without exception enriched above crustal abundance. 

 Brimstones may contain as much as 20 ppm selenium. 

 Coals of the United States average 3.3 ppm (Pillay 

 and others, 1969). The selenium content of soils 

 varies widely, from less than 0.1 ppm to as much as 

 1,200 ppm. 



High concentrations of selenium, to about 1 per- 

 cent, occur in or with sandstone-type uranium 

 deposits and other similar metal deposits in sedi- 

 mentary rocks. The selenium is often found in the 

 form of elemental selenium and sometimes as selen- 

 ide minerals. 



Relatively high concentrations (in hundreds of 

 parts per million) occur in tuffs, or tuffaceous rocks, 

 and concentrations of several percent are known in 

 mercury and antimony deposits. 



CRUSTAL ABUNDANCE 



The abundance of selenium in the earth's crust is 

 still poorly known owing to lack of adequate data. 

 Sindeeva (1964) gave a crustal abundance of 0.14 

 ppm selenium for rocks of the U.S.S.R. on the basis 

 of the analysis of 18 samples. Brunfelt and Steinnes 

 (1967) found an average of 0.04 ppm selenium in 

 eight standard rock samples provided by the U.S. 

 Geological Survey. Wells (1967) reported an average 

 of 0.6 ppm selenium in 20 volcanic ash samples from 

 New Zealand and 0.4 ppm in lava flows. 



Goldschmidt and Strock (1935) estimated that the 

 sulfur-selenium ratio in sulfides is about 6,000 and 

 that the crustal abundance of sulfur is 520 ppm. 

 Dividing 520 by 6,000 gave a figure of 0.09 ppm for 

 the estimated crustal abundance of selenium. This 

 figure was accepted until 1961 when Turekian and 

 Wedepohl (1961) revised the crustal abundance of 

 sulfur downward to 300 ppm and, by using the Gold- 



schmidt and Strock ratio, obtained an estimated 

 crustal abundance for selenium of 0.05 ppm. 



RESOURCES 



Selenium is obtained primarily as a byproduct of 

 copper refining ; 90 percent of the production in the 

 United States is derived from the anode mud de- 

 posited during electrolytic refining of copper. Blister 

 copper contains an average of 0.05 percent selenium, 

 or 1 pound of selenium per ton of copper, but actual 

 recovery of selenium ranges from 0.4 to 0.75 pounds 

 per ton of copper. Thus the production of selenium in 

 the United States, and in most of the world, is direct- 

 ly related to the production of copper. In situ leach- 

 ing processes for the recovery of copper do not 

 result in selenium recovery, and the increasing use 

 of these processes could reduce the supply of seleni- 

 um materially. Other countries with major poten- 

 tial resources of selenium are those with major po- 

 tential resources of copper: Chile, U.S.S.R., Zambia, 

 Peru, Zaire, Canada, and Mexico (Ageton, 1970 ; see 

 also "Copper" chapter) . Minor amounts of selenium 

 are recovered from lead and pyrite ores. 



Resources of selenium estimated to be potentially 

 available in the identified resources of copper, lead, 

 pyrite, and coal in the United States and in the rest 

 of the world are shown in table 118. 



Table 118. — Selenium resources, in thousands of pounds, po- 

 tentially available in identified resources ' of copper ores, 

 lead ores, pyrite, and coal of the United States and the 

 rest of the world 



United Other 



Source = States countries 



Copper ores' 52,000 169,000 



Lead ores 880 2,350 



Pyrite 560 7,840 



Coal 4,740,000 23,760,000 



1 Identified resources: Specific, identified mineral deposits that may or 

 may not be evaluated as to extent and grade, and whose contained min- 

 erals may or may not bo profitably recoverable with existing technology 

 and economic conditions. 



= Identified resources from i-espective chapters of this volume, except 

 for pyrite which is from Lewis (1970). 



■'' Assuming 65 percent recovery of selinum. 



In electrolytic refining of blister copper, about 65 

 percent of the contained selenium is recovered from 

 the slime residue. Using the ratio of 0.65 pound of 

 selenium per ton of copper produced, we estimate a 

 total of 52 million pounds of selenium available in 

 identified copper resources of the United States, and 

 169 million pounds available in identified copper 

 resources of the rest of the world. 



Less than 10 percent of the present production of 

 selenium is recovered from lead ores. If it is assumed 

 that galena contains an average of 10 ppm selenium 



