I had some time today so I prepared this.
The cost of refining a lot of karat gold with silver at 12%. The average size of these lots is 4.5 kg
The first part of the process involves separating the copper and nitric soluble metals, along with the silver from the inquarted alloy. Fortunately, this can be easily done in a stainless-steel vessel which are available larger and at a price much more reasonable than glass. The stainless steel can be heated to heat the acid on either an electric or gas hot plate. And a stainless pot can take a lot more dings than glass.
Melt all of the bars together and add the material you are inquarting with. Eventually this will all be silver. Stir the melt well and pour the metal in a thin stream into a tank of water with a piece of wood sticking out of the water at an angle so you can pour the stream of metal to the point where the wood meets the water. This will help you have smaller granules of alloy which react quicker. These shotted granules are what we will part in the nitric acid.
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Because of the high silver in the assay this alloy will require inquartation with silver or copper. Given the size of the lot, the extra nitric required for this process will result in an excessive use of nitric acid if it is inquarted with copper. As this process will be to attain the highest purity of gold when processing the gold remaining after inquartation, to assist in this the inquarted alloy needs to be parted twice.
The first parting is done with 1-part nitric acid and 2 parts distilled water. When the reaction is complete the evolution of the red nitric fumes will stop. Based on the quantity of nitric acid added the dissolving out of the should be complete and the metal remaining undissolved should be primarily gold. I always stir the reaction about halfway through with a glass rod to be sure it is well mixed. When the reaction appears to be complete, give it another stir to be sure.
Decant the solution and rinse the gold mud sitting on the bottom with distilled water, swirl the water and gold mud and allow the gold to settle. The gold is heavy, and it behaves well by staying in the pot if you are careful. Save the liquors you have just removed from the first parting process. These will later be filtered, and the silver will be recovered.
Now the second parting solution can be added. This parting liquor is 1 part nitric to 1 part distilled water. Add ¼ of the volume of the acid you added the first time as the volume of the metal will be much less and the purpose of this leach is to clean up any remaining silver of base metals, so your gold mud approaches a purity of .995. Heat this solution and let the gold mud sit in the hot parting acid for about 45 minutes. This acid will have leached out a very small amount of silver and copper and once you learn the process you can filter this acid and re-use it as a first parting acid to save acid costs. The rinse water you add should bring the nitric ratio close enough to the 2:1 of the first acid and save on nitric. Only start doing this once you are comfortable doing the process using new acids each tie until you are comfortable. After the swirling rinse like the first time, decant the liquid for recycling for a second use. The gold mud, which looks just like used coffee grinds at this point should be rinsed through a filter where it is rinsed well with hot distilled water.
I have gone through the calculation to determine the quantities of acid to do the steps above here for you to see.
Starting weight 4.5 kilograms
Gold assay 0.625. gold content 144.625 oz
Silver assay 12%. Silver content 17.361 oz
Remaining metals (Cu, Zn possibly nickel). 100% - (.625 + .12) = .255 which equals 36.892 oz other metals (we assume copper)
Good numbers to remember.
129 ml of 68% nitric acid dissolves 1 ounce of copper
38 ml of 68% nitric acid dissolves 1 ounce of silver
Easy to see why silver is preferred to inquart.
The alloy we start with has 90.422 ounces of gold in it. To make that one quarter of the total inquarted alloy multiply it by 3 and that’s how much we need to dilute the gold with. But there is already copper and silver in the alloy, so we need some subtraction.
Total metal needed to have gold at 25% 271.266 ounces.
But we already have 17.361 ounces of Ag in the alloy and 36.893 ounces of what we assume is copper (and other nitric soluble metals)
So 271.266 ounces minus 17.361 ounces minus 36.893 ounces means we need to add 217.013 ounces of silver
Now to calculate the amount of nitric acid to part this.
We added 217.013 ounces and there were 17.361 ounces of silver in the alloy so we need to add enough nitric to dissolve 234.374 ounces
From the good number to remember above that means we will need
234.374 x 38 = 8906.193 ml of 68% nitric acid.
And…..
36.893 x 129 = 4759.084 ml of nitric to dissolve the copper.
To do the first parting of this lot, Inquarted with silver, requires 13.665 liters of nitric acid. This quantity of nitric acid is diluted with 2 parts of distilled water so your total acid for this parting will be 40.995 liters.
Using the cost numbers you gave for nitric this first parting will cost you (in acid)
EUR 8.86 for the cheaper acid
EUR 40.996 for the better grade acid
The second parting we need the strength of the acid to be 1:1 with distilled water but we only need enough of the acid to cover the gold mud with acid by a little bit. I assume ¼ of the original parting acid quantity. The second parting should cost you in acid;
EUR 2.215 for the cheaper acid
EUR 6.645 for the better grade acid
Two things to consider. The cheaper nitric is cheaper for a reason. Is it clear or discolored. And does it have chlorides in it?
If it has chlorides and can dissolve gold it is a problem, and you cannot use it. If it is just discolored, it is probably from some dissolved metals and it should be OK with rinsing. If it is diluted, you will need more acid.
The method to convert the silver nitrate are covered elsewhere on the forum. As are silver cells if you want to set up a small cell. A silver cell will be more important if you want to go with straight aqua regia because you will hang up gold in the chlorides. And you will want to recover it quickly.
The gold mud is dissolved in a mixture or 4 parts Hydrochloric Acid to 1 part of Nitric acid. Add the gold mud to an appropriately sized glass vessel and add the HCl. For the lot size listed use the rule of thumb that says for every 7.5 ounces of gold to dissolve use 1 liter of aqua regia. Do not add all of the nitric at first, add about 1/3 of the nitric to start. (This example with 90.422 ounces of gold will require 12 liters of aqua regia or 2.41 liters of nitric and 9.6 liters of hydrochloric.). The gold mud will dissolve quickly so go slow with small additions of nitric acid as the reaction slows. This way you will not add too much and you won’t need to neutralize any excess free nitric acid.
The gold solution needs to be filtered and the gold dropped with either sodium metabisulfite or preferably sulfur dioxide gas. Add ice to the solution before you filter it and this will drop out any silver chloride so the filter catches it. The ice will make your gold purer. If your alloys have any lead, add a few CC of sulfuric acid to the digestion to keep it out of your precipitate.
As far as labor is concerned for the lots you are discussing I would leave an hour for melting and inquarting and the silver parting process should be done in 3 hours. The gold digestion, filtration and dropping should be done in under 4 hours. Due largely to the small particle size of the gold mud after parting process.
Since you will not be dealing with gold hanging up in the parting process your losses will be minimal and with the exception of any values in the exhaust (minimal) you will not lose more than 0.0625% in this process. You will find the assay between you and your refiner may vary by as much as .5% unless you run proof assays as most refiners use a standard proof factor of .995 when it should be closer to .998
As with all refining, technique and experience matters, any losses experienced when starting up will get smaller with experience.
You can plug in the labor cost and Hydrochloric Acid cost to get pretty close to your total cost. You also need metabisulfite or Sulfur Dioxide but that isn't very costly.
And sreetips have your back with nice and informative AR gold refining videos on YouTube. Quite not your scale, but for example, 4metals have broad experience with scaling up such operations. Together we can work it out 
