Orvi - per the things I highlighted above - I have processed MANY kilos of the sintered tungsten type contact points - the "binder" metal in them that you are questioning should be nothing other then silver in them
So they are a "matrix" (not an alloy) of ultra fine tungsten & silver metal powders that as you say have been "sintered" (a processed of mixing the metal powders together & then applying HIGH heat & pressure to form the "sold" points)
They generally come in two types - 60/40 W/Ag or 70/30 W/Ag with the 60/40 being the more common
Having done MANY kilos of them the one & only place I have ever seen them is in "hand thrown" circuit breakers like in household electrical panels (which have very small points) as well as larger to VERY large industrial hand thrown circuit breakers (which of course have larger points in them)
They should always be processed separate from other types of points & that is because they are difficult the get "all' the silver leached out of them because as you pointed out (for the most part) the tungsten does not dissolve in nitric
Therefore - to get the nitric to leach the silver all the way out to the core (center) of the matrix you need to literally BOIL them in the nitric - the BOILING action of the nitric provides the kinetic energy needed for the nitric to "penetrate" the matrix for leaching the silver out all the way to the core of the point(s)
Depending on the size of the points &/or the size of the batch it can take anywhere form 24 hours to 48 hours of BOILING them in the nitric to insure all the silver is leached out to the core of the matrix
So because they should be processed separate from other points there are two ways to identify them
1) if they come out of hand thrown circuit breakers you can bet/count on them being the W/Ag type contact points
2) if they are de-soldered from the bus bar they are brazed to they will have some sort of "waffle" pattern on the bottom side of the point(s) - when I say "some sort" of waffle pattern I mean a rough/raised pattern on the bottom side surface of the point(s) - it can be straight lines - or raised dots - or actual squares that look like a waffle
And one more note - though for the most part tungsten will not dissolve in nitric because W/Ag points are made by sintering ultra fine metal powders & due to the BOILING in nitric "some" tungsten will actually dissolve - you will see it as a DARK blue/black ink if/when you dilute the solution - that is the result of colloidal tungsten precipitating out of solution upon dilution (somewhat like silver dissolved in AR precipitating out as silver chloride when you dilute the AR that has "some" silver dissolved in it)
For what it is worth
Kurt
Yup, I know what you are talking about.
And also that "waffle" pattern is very common. However I also encountered some sintered tungsten, which only shown W on XRF. There certainly wasn´t any silver in some, that is why I wrote about this experience.
There can be a significant difference in typical breaker materials compositions between US products and late-Soviet era or post-Soviet goods in former USSR countries
Here, the material for practically 90% of all points for high current switching devices (breakers, switches, emergency switches, relays...) was AgC or AgCd. Later in time (90´s and later), sintered points started to be more common (mainly magnetic Ag ones, sintered with W/Ni/C whatever).
As older buildings were reconstructed or demolished, significant influx of this scrap came to the refiners - as many people were "instructed" by us and set this type of escrap aside in local scrapyards and such
Also, many industrial premises and production facilities are refurbished with new electrical lines and equipment, resulting in quite a bit of this "breaker/switch" escrap to be available. Regular "Joe´s" are after copper wires and usually leave the breakers be
Some dismantle the large ones, but usually only take copper from them, leaving silver+brass aside.
I know that tungsten holds relatively well in nitric, but it it etched in AR. Due to high silver content, iodine/iodide won´t be the best way to process these filaments - despite the fact that tungsten is virtually intact by this leaching mixture. Otherwise, this will be the first method I will recommend
So, we need somebody to take pure solid piece of tungsten and simmer it in 50 % nitric
to tell us what happened.
But if I was him, I will take few hundred grams (if he supposedly have 80+ pounds) and leach in nitric. This should be straightforward and quick. And result should be obvious. Reduce everything down with ascorbic acid + base and you have metal powder ready to be melted within hour or two. Since there would be only metals of interest you can overshoot base quite a bit without any issues - it will just speed up the reduction.
