Gold refining, for that matter all precious metals, require refining to convert them in to purest or nearly pure form before it can be subjected further manufacturing in to either carat gold jewllery for adornment or into investment grade bars or coins with minimum acceptable fineness of 995 in case of large bars and 999 or 999.5 or 999.9 in case of smaller bars of 1 kg and denominations.
There are number of processes used for refining but two of them, either an electrolytic or Aquaregia based acid refining process are the most prevalent. Both processes have their advantages and disadvantages but both ensure a quality of refined gold that is free from PGM (platinum group metals) and other base metal impurities.
The refining process emits unwanted and hazardous gases as well as chemical waste solution that need to be neutralized before letting off in the environment or recycled for further use in the recovery process.
All the refining systems are now equipped with not only acid fume neutralization scrubbers but also effluent treatment plants (ETP) for treatment of solid and liquid wastes.
The main feedstock for the refinery is either scrap gold (mainly jewellery) or gold Dore (mined gold). Any good refinery will ensure that the raw material procured or coming to refinery is from responsible sources and by legitimate means.