Butyl diglyme, BDG for short (also known as Dibutyl Carbitol, CAS number: 112-73-2) is a liquid solvent that has a great affinity for gold chloride. It doesn't mix with water so one of its uses is for liquid - liquid extraction of gold from solutions. The gold is released later when the BDG is contacted with a solution containing, for example, oxalic acid. The BDG is reused for more extractions. Some people prefer DEM for LLX.
Gold extracted with BDG can attain purity up to 99.999% according to reports.
Short description of the procedure
The extraction and separation is most easily done in a separatory funnel when doing small batches.
- Gold is turned into gold chloride via any of several methods, for example HCl + Cl or aqua regia
- The gold chloride containing liquid is mixed with DBG (by shaking the vessel) Do not use too much hydrochloric acid, it will cause some metal salts to go over in the DBG phase.
- Let the mixture stand for a while, the phases should separate within minutes
- Remove the barren solution
- Add 0.5M to 3M diluted HCl to the DBG and shake, This washes out some of the base metals carried over in the extraction step.
- Remove the wash acid.
- The gold is precipitated by adding the gold chloride loaded DBG to a solution with a gold specific reducer, commonly 5% oxalic acid solution at 70-80 C for 4-5 hours.
Practically, the BDG can be reused several times before the gold needs to be extracted. A gold chloride loading of 25-100 g/liter is a practical limit before the gold is precipitated. At 120 g/l the solution becomes so heavy it will sink below the water phase. With a gold chloride loading of 25g/l in the DBG the remaining acidic liquid will contain about 10 mg/l of gold chloride.
BDG is often described as a magic liquid that draws out gold chloride and nothing else. That is a gross simplification and anyone wanting to use BDG successfully needs to know more about the partition coefficient. It the relationship between a substance dissolved in BDG and dissolved in the water phase. The coefficient is dependant on the pH of the solution. That is why diluted hydrochloric acid is used to wash the BDG before precipitation. According to some sources the separation should be done with maximum of 3M HCl while other talks about 0.5M HCl being optimal. Higher HCl concentration will cause more contaminants to pass over with the gold and also larger losses of DBG.
In US patent 5,942,024 there is a nice graph of the partition coefficient for some metals. The graph van also be found in RF at this post.
There are several places where BDG can be bought. For example
- US 6,827,837 : Method for recovering trace elements from coal
- US 5,942,024 : Method of recovering gold powder
- GRF : BDG extraction paper and a tutorial
- GRF : Lazersteve gives a procedure for gold refining with BDG
- GRF : Description of BDG and pictures of precipitate
- GRF : Description of partition coefficient
- GRF : Russian paper on BDG extraction
- GRF : Comparision of DEM and BDG
- Gallica : Studies on the Chemistry of Gold, Victor Lenher, American chemical society. Journal of physical chemistry 1926, pg 126-129
- BASF webpage for BDG
- Alternative names