Sucho said:
Guys.I run this type of material very often.the yield runs from 4 to cca 20 g / kg depending on manufacturer, year etc.most common yields are between 7 and 9 grams per kg.
They are kovar based and its horrible to dissolve it in nitric.it takes ages and needs a lot of heating.
I prefer straight AR way.kovar dissolves in AR easily.the only thing you have to achieve is to have all metals in highest ox. State.it is visible on loaded AR colour.if not, you will loose lot of gold that is metallic colloidal because of cementing on undissolved kovar and few other reactions.
Precipitation is without problems.dissolution is always vigorous, be careful.
In the 70s, I worked for a refiner that literally ran tons of these in nitric. He used a large steam-jacketed 304 stainless kettle, if I remember right. He could do a big batch in a day.
When I have run these or other kovar items, such as gold-lidded CPU packages, in nitric, I ran them in heavy duty stainless containers (about 5 gallon size) with a lot of heat. I put about a 3"-5" layer of parts in the container, covered them about a half inch over the parts with 50/50 nitric and heated the solution quite hot. As they dissolved, I added concentrated nitric in increments to drive them faster. I stirred occasionally. When the solution level was about 2" above the parts, I stopped adding nitric and let them work hot until the reaction was almost nil. I have noticed that, when dissolving most anything, the dissolving slows down when the solution gets too deep - the parts are down there and the acid is up here. I usually stop at 2" over the parts. I then diluted the solution back to about 50/50, stirred it, let it cool to where I could safely handle it, poured off most of the solution, and started over with fresh 50/50 nitric with subsequent concentrated nitric additions. After doing this fresh nitric thing 2 or more times, depending on the amount of parts I started with, all the kovar was gone. Several batches of fresh acid on a single batch of parts really speeds things up. I would guess it takes about 4 to 5 hours if you keep at it. After pouring off the last nitric, I rinsed a couple of times, transferred the gold/garbage residue to 4 liter beakers and used aqua regia.
I have used this "starting with lots of material and using several batches of fresh acid approach" on quite a few other things - the direct dissolving of karat gold as an example. It does speed things up and you can start with a lot more material in a container.
If you don't want to go through all this until all of the kovar is dissolved, try to get, say, 80-90% of the kovar dissolved in nitric, dilute to at least 50/50, let it cool, and pour the solution off carefully. Then use the aqua regia in big beakers. You'll end up with a lot less gold solution that way.
A guy I knew bought gold CPU packages by the drum load. He used fairly warm aqua regia diluted with about 40% water, to tame the AR down a bit, and used SO2 gas to drop the gold. There was a lot of nitric left and it took a lot of SO2. Also, with the weak AR, the kovar went fairly fast, but it took forever to get the gold braze under the chip and he seemed to always leave some behind. You need full-strength AR to get that gold with decent speed. Even then, it takes quite awhile.
Admittedly, AR is faster. I like to use nitric first because I end up with a much smaller amount of material to dissolve in AR and I also end up with a lot less gold solution to deal with. Also, when using AR to start, what sort of container do you put it in when you have a large batch? About the only thing would be glass or, maybe, titanium. A big steam-jacketed, glass-lined Pfaudler would be perfect, but not too many people have one of those. Most people would be limited to 4 liter beakers, which wouldn't hold much if you didn't want it to foam over. If you first get rid of all, or most, of the base metals with nitric, a beaker will hold quite a bit of gold residue.
Either a gallon of 70% nitric or a gallon of aqua regia will dissolve about 2 pounds of kovar.
fficial
