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Semiconductors exhibit transitionary behaviors between conductors, like pure metals, and insulators, like halite (NaCl). They can accept electrons or allow the transferral of electron holes through their lattice in electrochemically active systems and become more conductive at higher temperatures, unlike metals for which the converse is true.
Ore Minerals Exhibit Semiconductor Properties
Semiconductor properties are exhibited by numerous ore minerals including oxides and sulfides. The composition and crystal structure of semiconductors will affect the efficacy with which charge transfers can occur under a certain set of conditions.
The semiconductor properties of the metal-sulfide ore minerals were discovered by Karl Ferdinand Braun in the late 19th century. He noted that both chalcopyrite (Cu,Fe)S2 and galena (PbS), important ore minerals of copper and lead, respectively, exhibited asymmetrical conduction, i.e. they would conduct electricity in one direction but not in another when in contact with metal charge carriers. This would lead to the development of wireless communication devices like the “catwhisker” in the late 19th and early 20th centuries.
“n-Type” and “p-Type” Semiconductors
Two major types of semiconductor exist; these are “n-type” and “p-type”. A material containing more free electrons than electron holes and therefore having a negative surface charge is an n-type semiconductor while a material containing more electron holes than free electrons and therefore having a positive surface charge is a p-type semiconductor. Impurities in a crystal phase will determine which type of semiconductor an ore mineral and will also govern its conductive ability. Natural pyrites (FeS2) are known to incorporate a range of metals and semi-metals as substitutes for Fe. Resultantly conductivities of natural pyrites have been reported to vary by over four orders of magnitude depending on the substituting species and generation of solid solutions, relative stoichiometry within the mineral and thermal excitation across the energy bandgap.
While conductivity has limited application in mineral identification it is sensitive to the mineral’s geochemistry and hence can supplement other analytical techniques or determine contaminants in a phase. This would be of particular interest when identifying penalty elements, such as toxic arsenic, in sulfide ore. Additionally, the mineral-fluid interface in an ore processing scenario would be of great importance as interactions between either an n-type or p-type semiconductor would influence how it would behave during froth flotation, for example.
Froth Flotation Technique
Froth flotation is a commonly used beneficiation technique designed to separate minerals of particular economic interest from gangue phases. Froth fluids comprise numerous reagents including modifiers, that change mineral characteristics, activators that attach to mineral surfaces, depressants that prevent flotation, collectors that gather the minerals by attaching to air bubbles in addition to frothers that generate bubbles. These reagents, including carbamates, xanthates, and thiols, often rely on ionic properties to perform their role and their interactions with ore minerals will, therefore, be governed by the intrinsic properties of the minerals.
Understanding semiconductor properties based on mineral composition is, therefore, a vital step in terms of determining how an ore and its constituent minerals might behave during beneficiation. It would aid in determining which reagents could be efficiently used to gather the greatest proportion of desired metals, whether repeated steps would be needed during the process to remove gangue phases and would help determine the overall cost of the procedure.
Sources and Further Reading
- Lee TH (2007) The (Pre-)History of the Integrated Circuit: A Random Walk. IEEE Solid-State Circuits Newsletter. 12, 16-22.
- Lotter NO, Bradshaw DJ, Barnes AR (2016) Classification of the Major Copper Sulphides into semiconductor types, and associated flotation characteristics. Minerals Engineering, 96-97, 177-184.
- Maree W, Kloppers L, Hangone G, Oyekola O (2017) The effects of mixtures of potassium amyl xanthate (PAX) and isopropyl ethyl thionocarbamate (IPETC) collectors on grade and recovery in the froth flotation of a nickel sulfide ore. South African Journal of Chemical Engineering 24, 116-121.
- Salazar-Palma M, Sarkar TK, Sengputa D (2009) A brief chronology of the origin and developments of wireless communication and supporting electronics. Applied Electromagnetics Conference (AEMC), IEEE, 2009
- Shuey R (1975) Developments in Econnomic Geology, 4; Semiconducting Ore Minerals, 1st Ed.