Let me start out by saying that a process which is good for one feedstock is not necessarily good for another. When I evaluate a process, the first thing I consider is what equipment is available for the refining. This can eliminate some processes immediately. No one likes to spend more money than they have to and no refiner likes to turn away work because they are not set up to process it. For this reason I favor versatile equipment. Once a refinery starts to see a consistent quantity of a certain material then it is fine to set up to maximize efficiency and yield with dedicated setups.
I would never build a refinery without a process hood where individual lots of varied material could be processed if required.
Now the material at hand. The preliminary assays indicate 99.5% precious metals to start with. Wow, pinch me! What an ideal feedstock to start with! You have not indicated if there are any traces of Platinum Group metals in the material so I have assumed not. This becomes important because the flow chart Lou provided and his descriptions include a resin system to catch these values before waste treatment. In your original post you stated 0.04% other metals. Even if that was all platinum, in a 10 kilo lot you are talking about $4.50 worth of platinum (less recovery cost). But even at that level, your refinery is set up to process 100 Kg a day and $45 a day can add up to real money over time. Plus lots of higher PGM value can sneak in. Anyway, it is good to track its presence. A good fire assayer, and some good fresh stannous chloride can determine its presence qualitatively and allow you to separate your waste streams, one holding tank set aside for PGM trace recovery, and another for Gold and silver traces.
This is a resin system which solutions are pumped through to recover values before waste treatment.
If you have been reading about refining on this forum you have surely noticed that in the best case you want to clean up your material as much as possible before using aqua regia, the reason being you do not want to affect your final purity by dragging down other metals in solution. That is why some members suggested inquartation for this material. In reality, the quantity of base metals you will have in solution will be exceptionally low for this material almost as if you had done an inquartation with the exception being an inquartation and treatment will have removed the silver as well. In this case the cost of acids and extra handling does not justify inquartation.
So what does this mean about the refineries in the Mid East where your methods came from? We do not know what the purity of the material they are adding to the aqua regia is. If they pre treat by inquartation or other processes before aqua regia then the relatively high purity of the material going into aqua regia will allow for the chemistry you have outlined. I am not saying I like the method, I am saying it can work (obviously it does). But the reason it works is how clean the material is going into the aqua regia. The picture you posted of your digestion vessel full of beautiful red acid tells me your feedstock has very little base metals without even looking at the preliminary assay. (That could be a centerfold in a refining magazine!)
Let's for a moment say you were refining karat gold in your system. No pretreatment, no inquartation, just straight melted and granulated karat. In the industry, karat gold can be refined on a large scale by melting all of the metal for a particular lot together and making sure the cumulative silver content is under 9%. With that silver content or less, a straight aqua regia dissolve will give you a good dissolve and leave you with silver chloride as an insoluble. The acid however will be a green color, not even close to the pretty red acid you have shown us. If you were to proceed to neutralize to pH 7 the excess nitric with soda ash as you described, you would be bringing down metals as hydroxides which would mix with your gold dropped with metabisulfite. In short you would have a mess.
The soda ash can have an unintended consequence adversely affecting your gold purity. Remember your primary goal is to eliminate excess nitric acid from the dissolve process. You can add soda ash and have problems with some of the other metals dissolved in the acid, or you can add sulfamic acid and not cause any unwanted precipitation save the dropping of the lead as lead sulfate. In the karat gold refining scenario, the sulfamic choice will allow you to drop only the gold leaving the base metals in solution. Note you add the sulfamic before filtering out the silver chlorides.
Continuing on in your process following the Mid East method. After filtering out your precipitated gold, you continue to add Caustic soda to recover any other materials. Here we call that waste treatment. The problem being you have not attempted to cement out remaining values, you have dropped all of the metals as hydroxides and co-mingled your values with any base metals. Not to mention the fact that any Platinum will remain in solution and go out with your waste water.
On to waste treatment. Classic waste treatment is done after you have recovered all of the values by cementation or resin column. The remaining metals will be base metals and we raise the pH to drop them as metal hydroxides. The base metal hydroxides are collected in a filter press and the water from the filter press is neutralized and checked for metals to meet discharge standards and if it passes, discharged to sewer. The photo you attached shows the filter press feeding off the tank where normally the pH is raised with good agitation so the hydroxides remain suspended in solution and can be pumped through the press. I do not see any mixing motors on the tank. Is this where you raise the pH or does the liquid come into these tanks alkaline as per the Mid East recipe?
This is what a typical mixing tank feeding a filter press would look like.
And this is the hydroxides that come out. These particular hydroxides are very high in coppe
r due to the cementation process used. They are dried and shipped to a copper smelter who pays on copper and any precious metals contained.
I notice your filter press, and the one in the photo I attached are both European design. The difference is American made presses pass the liquid through the plates and discharge out the end of the press into a pipe all using the power of the double diaphragm pump to move the liquid. European models allow each plate to drain into a trough which has to be collected and transferred to the next process. I have used both and both work well.
3) The filters used - We have been using 2 layers of Polypropylene filter cloths - first layer is of 1-2 microns and the layer below this is 4-5 microns. (We are not 100% sure of the microns as this is India and we learned a lot of things after starting this setup that you will get all kinds of stuff but guarantee of the quality is given by very few).
The paper of the most coarse porosity, in this case the 4-5 micron, should be the first layer to contact the liquid being filtered, followed by the least coarse 1-2 micron. Your goal is to filter out the larger particles first and allow the smaller particles to pass and be collected by the finer paper. It will not clog as fast if you do coarser first.
2) The Gold Bars - Sir (4metals) When we complete the refining and get the pure gold in hand we do an assay test. For the last batch we had got the purity of 999.22. We generally get a purity of 999 in all the batches. But the local market where we sell our bars require the product to be of purity 995.0. So for this we do the following conversion:
995.80 gms Pure Gold of purity 999.22 + 4.20 gms Silver = 1000 gms weight of Gold Kilobar of purity 995.0
In this case we sell the gold at the price of 995.
Excellent, no sense giving gold away!
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