Atom efficiency (by-products Mwt)
For soluble scavengers, low Mol. Wt. reagents are preferred. Solid scavengers need to be optimized with respect to weight used. Most solid scavengers are single use only and post processing will be incinerated during the metal recovery process, or sent to landfill disposal if not incinerated.
- Safety Concerns
Most metal operations are covered by normal plant operating procedures. Care needs to be exercised with S compounds that can generate toxic/flammable gases – H2S, CS2 If precious metals are recovered in the bulk (metallic ) state, these could be pyrophoric –especially if finely divided. If oxidants are to be added, then the usual safety precautions around mixing oxidants with organic solvents/flammable materials should be observed.
- Toxicity and environmental/aquatic impact
Most soluble reagents have minimal environmental impact. Higher Mol. Wt. phosphines can be persistent and bioaccummulative It is unlikely that solids would enter the aqueous environment.
- Cost, availability, & sustainable feedstocks
Most metal scavenging reagents are available in bulk and most soluble reagents /carbons are relatively cheap. Functionalized resins/polymers are available but are expensive reagents, and can add considerable cost and life cycle impact to a manufacturing process.
- Sustainable implications
Most base and precious metals need to be prevented from entering the environment and precious metals need to be recovered and reused to minimize life cycle impact.