• Dissolved in the carrier metal and separated during refining of the carrier metal: Te, Se, and precious metals in Cu are concentrated in anode slimes during electro-winning of Cu.

• Collected in the slag or residue: Mo during Cu smelting, Ge in Zn hy-drometallurgical process.

• Volatized and thus present in dust and off-gas: Se and Re during copper smelting, In during zinc smelting, and Ge during Zn ore roasting.

The recovery efficiency of the minor by-product metals depends on the presence of the appropriate recovery processes (see Appendix 1). In many cases such separation processes for by-product metals are either not installed at all or are rather ineffi cient. This offers a signifi cant potential for increasing the production of many minor metals without starting new exploration or mining. Coupled elements have much higher recoveries as the mining and smelting processes are aimed at the recovery of these elements. For example, the dissolution efficiency of dissolving tantalum from its ore is over 99% (Ullmann 2002). Additional losses occur during the final production of the minor metals, but these are small compared to their losses during smelting/refi ning of the major metal. For high-tech applications, such as PV and electronics, the purity of the minor metal obtained after production is insufficient and further refining steps to obtain high purity metal are necessary.6

To summarize, recovery of minor metals is accomplished using highly complex and interconnected processes.7 Most importantly, the supply of by-product minor metals is not driven by its own demand but is governed primarily by demand for the carrier metal. The difference between the minor metal supply to the market and the amount present in the ore concentrate can be significant, due to the losses along the different steps in the process.

Improvement Opportunities Primary Production

Several opportunities exist to increase the yield of minor metals and would improve the resource efficiency and supply security:

• Invest to enable the recovery of minor metals by installing available technology (i.e., scrubber to capture Re) or conduct research to develop new technologies.

In the case of Si, a metallurgical grade is first obtained (96-99% pure), which is then refined to 99.9999% (6N) pure solar grade or 9N-11N pure electronic grade Si (Flynn and Bradford 2006). Similar requirements exist for Ge (9N-13N), but requirements for Se and Te are less stringent.

The flow sheets discussed in Appendix 1 assume that all the minor metals will be recovered. This is not always the case. Some of the minor metals are removed because they disturb the primary process and then it depends fully on the economics and technical possibilities if the minor element(s) is recovered.

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