Jmk88 said:
I believe if it’s in the form of gold chloride (III) you would indeed lose your gold by burning the papers.
Although that "can" be true it most certainly is not true in the absolute - ashing (which is the proper term for burning) filter papers to recover the "traces" of gold chloride that is "always" left in filters from filtering AR is "common" practice
Can you loose your gold when ashing filters - Absolutely - if not "done right" you can/will loose most if not all of it up in smoke
On the other hand - if done right the "vast" majority can be recovered (like 90 - 95% - & even as high as 98 - 99%) & for what it is worth the 5 - 10% lost during the ashing of the filters can be "captured" for future recovery
Note; - this holds true for silver nitrate &/or other metal salts trapped in filters
The question of loosing the values - or recovering the values is answered in the conditions of how you go about preforming the ashing of the filters
In chemistry we have two reactions - oxidation - or reduction --- gold chloride &/or silver nitrate are metals in a state of oxidation --- so when we ash filter papers we want to cause reduction of the metal oxides in order to recover those "trace" values trapped in the paper
The reduction of the metal oxides is called calcination which is a reduction of the oxides by way of heat - at "controlled" temp
Example; - you can actually reduce gold chloride (&/or silver nitrate) by evaporating a solution down to its sold crystal salts - then continue to apply heat to a point that the chloride &/or nitrate salts will reduce to the metals of gold &/or silver
In "fact" that is a common method used for reducing PGMs once they have been precipitated from solution as PGM salts
So the question is the conditions under which you ash the filters - temp plays a roll as well as whether done in the presence of oxygen - or the lack of oxygen
I you burn a single paper in the open air with a torch most if not ALL the gold chloride will go up in smoke because there is no control of the temp & the oxygen prevents the gold from reducing
Crumple the filter up into a "tight" wad - put it in a melting dish - put a torch to it & MOST of the gold will go up in smoke (because of the above) however "some" gold - in the "center" of the wad will reduce to gold because the oxygen can't get to the center of the wad
In other words a "flat" filter burned in open air exposes the entire "surface" of the filter to oxygen -where as a waded paper reduces the surface exposed to oxygen
So what does that all mean - it means when ashing papers it needs to be done in a lack of oxygen
So - I ash my filters in one of my "vision ware" pots - with a SS lid on it that has three or four 1/2 inch holes drilled in the SS lid (you want "some' smoke to escape but NO oxygen to entire (with out the holes in the lid - pressure can build up & blow the lid off)
Now then - I talked about temp as well - the "proper" (ideal) temp to cause calcination (reduction) of the chlorides/nitrates is 380 C - or 717 F - so temp control is important - the best way to do that is to do it in an electric furnace where you can "set" the temp - or you can do it on a hot plate but you should have a hand held laser temp reader
One more note to make when ashing filter papers (& I am not sure why this is true) but loses "will" be less if the papers are damp rather then dry --- I "think" its because the steam - at the beginning of heating starves the oxygen in the covered pot
I also talked about surface area (exposed to oxygen) there for you are best served doing more papers rather the less papers with the papers being wadded up
That said - you can over load the vessel with filters (&/or pack them in to tight) so that they don't completely ash to the middle of the vessel - that part is a bit of a learning curve --- the point being if you do only a couple/few filters at a time you "may" only achieve 90 - 95% recovery - by doing more filters at a time there should be no reason not to achieve 98 - 99% recovery
Once this part of the ashing is done the papers/filters will be mostly carbon - however you have now accomplished the task of calcination of the metals (the chlorides &/or nitrates have been driven off under controlled temp with no oxygen) so you can now complete the ashing by "incineration" --- meaning high enough heat in the presence of oxygen so as to turn the carbon to true ash with out worry of losing values
In other words - once the chlorides &/or nitrates have been driven off in a controlled manner - there is no longer a worry of losing values at higher temps
You can now ether recover the metals from the ash by leaching the ash or smelting the ash --- personally I prefer smelting with an added collector metal (copper or silver) - but that's because I do "a lot" of other smelting - so I just add the ashed filters to those smelts - in other words I "collect" my values in one larger batch rather then in several smaller batches
Bottom line - yes you can ash your filters with VERY little to next to NO lose - IF you do it right
Like EVERYTHING else in refining - there is a learning curve to it & can certainly be done with "little" to no lose
Kurt
