Recycling

Actually almost all kind of tungsten products, if available in large enough amounts and economically feasible, are recycled today (Ref 11, 39). Even unsorted low grade scraps containing enough tungsten can be used as a raw material for the chemical APT production process or can be used for alloying in the steel industry (Ref 40). Nevertheless, any options or possibility to use sorted tungsten scrap directly is of course preferably applied for economic reasons (Ref 40). Typical examples include:

• Sorted production scrap of pure tungsten can simply be crushed in high energy mills (planetary mills) and can be directly reused for applications with lower quality demands.

• Turning chips of heavy metal, obtained during the shaping operations, can be converted into reusable powder by oxidation and subsequent reduction. Too large solid pieces can be machined into smaller parts to enable further recycling. The oxidation step leads to a total disintegration of the solid chips into oxide powder containing WO3 and tungstates of iron, nickel, or cobalt. Subsequent hydrogen reduction leads to a metal powder, whereby all components (W/Fe/Co/Ni) of the original heavy metal are still present (Ref 40).

• Composite scrap mixtures, such as W/Cu, are difficult to oxidize and are therefore not suitable for chemical recycling as described above. Also the recycling via the steel industry cannot be recommended due to the detrimental effect of copper in steel. A possible alternative is salt melt digestion with NaNO2, NaNO3, and Na2CO3 mixtures. However, this method suffers from environmental problems due to the NOx formation (Ref 40). An alternative would be the electrochemical recovery of tungsten (Ref 41).

• W/Th recycling has specific problems due to the radioactive radiation. Recycling in the steel industry has become almost impossible during the last years. Chemical recycling in the conventional process leads to insufficient separation of thorium and is therefore also not applicable. The only way to recycle this material is still under development and based on the electrolytic dissolution of tungsten in ammonia (Ref 42). The difficulty is to run the process in a way to prevent dissolution of any of the radioactive fission products.

0 0

Post a comment