Nitric acid and baking soda, fist gives sodium nitrate and a carbonic acid:
HNO3 + NaHCO3 --> NaNO3 + H2CO3
Then:
NaNO3 + H2CO3 --> NaNO3 + H2O + CO3 gas
So we see here you also formed a nitrate salt in your powders.
This is not considering other acidic metal salts in the mix or other reactions, which would be involved.
As Geo stated you would drop most all of the metals from solution from adjusting the pH, and you can also form other salts, and in the evaporation, you will also concentrate, to further precipitate metal salts.
When you say this is what this what your material looks like after a short time under the torch, and you show a wet gray powder in the filter in the picture.
It sounds to me like you were working towards incineration, but a short time under the torch to me does not sound like you had a completed the process of incineration, unless you got these powders to glow red hot, keeping them crushed and stirred to oxidize them, keeping them glowing red hot, until no smoke of acid or carbon based material leaves the powders, and allowing the powders to get hot enough to glow red hot and long enough to oxidize completely the base metals in the powders, which can take time at this high red heat, to drive off acids from the salts as gas (many times noticeable by a color change in the powders as the powders oxidize).
So for some reason I feel you may have torched the powders a little, but I wonder if you actually gave them a good roast or incineration, which would actually improve what your working with, and oxidize these powders.
First make sure you did incinerate them well, if you did not I would redo that process, getting them glowing red hot, and keeping them red hot about a half hour...
It looks like you filtered these powders (which would help to wash out some of the acid salts or soluble metals.
At this point (since I do not believe these to be roasted well), I would put filter and powders in a ceramic casserole dish on the hot plate solid burner, slowly raise heat to dry the powders, raise heat as they are dried keeping powders stirred and crushed to powders, a Pyrex pestle works good to crush the powder, raise heat but (keep heat low enough to prevent forming gas bubbles that will pop and splash powders or syrup out of dish), continue till powders give off no gases, then when powders completely dried and crushed with hot plate on high heat from below use the torch from above (keeping powders crushed and stirred) make the powders glow red hot and keep them red hot and stirred for long enough time to drive off all acids or other salts and oxidize the base metals in oxygen (or air) stirring them well to get good exposure, glowing red hot for 30 minutes or longer at a glowing red heat.
Turn down hot plate to cool powders, spray the dish with a mist from a water bottle to wash sides of the dish down add water to cover the powders give a boil in water, lower heat and let powders settle, remove the water wash after powders settle well put keep water hot, decant wash water from dish with suction bulb tool and pipette, if this water is highly colored with metals repeat water boil wash and decant till water comes off fairly clear.
Leaving powders in the dish cover with HCL, raise heat a little at a time till at a slow boil, keeping powders stirred well and crushing out any lumps, give this a good long boil to dissolve some of the metal base oxides, as metal chlorides, and converting some like silver or lead to chlorides as well, lower heat add just a small amount of water (we do not want to dilute this acid too much, but we also do not want to pick up silver later if left too concentrated) let powders settle well but keep it as warm as you can without the heat stirring the powders, after it has settled well decant this acid wash with your suction bulb tools while still hot to pick up lead chloride, tin chlorides and other base metal chlorides, the this liquid is decanted to a cooling and settling jar (you may see lead crystals form in this jar as solution is cooled, there also may be traces of value carried over to this jar,
(So I normally save these to process later when I collect a jar full of these powders).
If this HCl wash is highly colored with metals I repeat the HCl wash.
Again powders stay in the dish all the time.
Then I give boiling hot water washes until these washes come clear, and I no longer am picking up metals like lead, following the techniques described above.
This helps to remove many of the trouble some metals, like tin, lead, and base metals oxides as chlorides.
From here I will process a couple of different ways depending how much, and what I have left in these powders.
Much depends what I am dealing with, and what I wish to do with it or how I plan to process it.
If much copper or other base metal I may go to an acid peroxide process to remove more base metals (but leaving my more valuable powders) being careful not to dissolve gold or remove the fluffy silver when I decant.
Or for a material with higher grade powders of gold and silver and possible PGM I may just dissolve silver and palladium and copper and so on in nitric acid, letting powders settle well before decanting these metals to a jar, settled and filtered before cementing values out this solution with a copper metal buss bar, then the powders left in the dish would go to aqua regia to put gold into solution.
Or if the material was just gold which was in high content to base metal and silver I may just go for the gold, using HCl/NaClO bleach (easier to deal with) or using aqua regia
I normally keep powders in the dish on the hot plate from beginning of the process till the end of the process, as much as possible, decanting solutions with suction tools as the metals dissolve into solution, depending on what these decanted solutions are is how they are processed, most of the time they are decanted to settling jars, let sit for a long period of time till solution is clear, then solution is decanted and filtered before being further processed.
The remaining powders in my dish after all of this may contain sand, or maybe even some PGM or other undissolved valuable metal, just because it did not dissolve I do not consider it invaluable, depending on what I am working on these insoluble powders may go to a plastic coffee can with other material to be incinerated and processed again later, the scrap in this bucket may contain many other things like IC chips, ceramic capacitors bits of filter scrap and other material which may hold some value.
The goal is to incinerate the troublesome metal powders driving off all previous acids or salts to oxidize the metals that will oxidize, then remove these troublesome metals in washes, also removing the base metals, and then put only the valuable metals in solution, and to process each of these solutions to recover them for values for further refining.
The Idea is also a one pot heating vessel continued reaction and process reaction from beginning to end, keeping powders in the pot as much as possible, and just removing dissolved liquids to remove troublesome metals and base metals in washes,(although several jars are used) keeping from moving powder from place to place and losing values in each move, or in filters and so on, use settling as much as possible, and filtering only fairly pure liquids, processing the powders in each step to remove unwanted metals and keeping the valuable metals in the pot until you can dissolve the values in a fairly pure condition, to recover the values in the purest condition possible.
Since your powders I believe came from electronic scrap you should choose a process to deal with what these contain.
Also keep in mind any time a powder has come from, or contains salts of chloride or which has previously seen a reaction with HCl and you wish to use nitric later, and your goal is not to put the gold into solution, but to remove base metals with the nitric, these powders will need to be incinerated again to remove the chlorides before using nitric acid, (this can be done in the same ceramic casserole dish). But if your goal is to dissolve gold you can move on to the next step.
Keeping in mind and using what you have learned from Hokes book (how metals react in solution).
