Extraction and mutual separation of rare earths from used polishes by chemical vapor transport
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I.
INTRODUCTION
IN recent years, rare earth elements have found applications in permanent magnets, phosphors, and hydrogen storage alloys, and their demand has been increasing every year. However, rare earth concentrates or ores occur in only limited countries and are currently imported by several countries. For a good supply of rare earths, sludge, scrap, and wastes are noted as new sources. Development of recovery processes of rare earths from them is expected. Used rare earth oxide polish is one of the wastes that can be recycled. Significant amounts of rare earth oxide mixtures are used as polishing agents for glass substrates.[1,2] The Japanese glass industry uses 2000 tons per year of the polishes, and a large portion of them is thrown away after their polishing lifetime. An efficient recovery process of rare earths from used polishes is needed in terms of recycling of the waste. Wet processes, which have been industrially used for extraction of rare earths from ores and concentrates, are proposed as recovery processes of rare earths from used polishes. However, these processes require a number of complicated treatments, such as acidolysis, alkali fusion, precipitation, and calcination, and a large quantity of waste water is discharged in these processes. Therefore, more efficient and environmentally harmless separation processes are required. On the other hand, chlorination and carbochlorination processes have been found to be effective for recovering valuable metals from scrap.[3,4,5] In these processes, valuable metals are converted to their corresponding chlorides and then separated based on the difference in volatility between the metal chlorides. It is difficult to apply this process to extract and separate rare earths, because rare earth chlorides are less volatile and can be hardly separated from other less volatile metal chlorides, especially from alkaline earth chlorides. Chlorination extraction of rare earths requires considerably high temperatures and long reaction times. In addition to that, rare earth elements have similar chemical properties to each other, and their mutual sepa-
ration is quite difficult. Rare earth chlorides also have very similar volatility. Mutual separation of rare earths is not expected in the chlorination processes. Application of the chlorination processes to rare earth separation requires an increase in apparent vapor pressure of rare earth chlorides. It has been reported that rare earth chlorides form vapor complexes with alkali chlorides and aluminum chloride:[6,7] RCl3 (s, l) 1 KCl (s, l) 5 KRCl4 (g) RCl3 (s, l) 1 n /2 Al2Cl6 (g) 5 RAln Cl313n (g) (n 5 1 to 4)
[1]
[2]
where R denotes rare earth elements. The apparent vapor pressure of rare earth chlorides is enhanced by formation of vapor complexes. In the presence of 1 atm of Al2Cl6 (g), volatility enhancement of NdCl3 at 600 K was calculated to be about 1013.[7] Rare earth chlorides, which have low volatility, are transported chemically from higher temperature to lower temperature. We have carried out extract
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