Hi,
Apology for my English. Please feel free to correct me if I'm wrong. I'm willing to learn.
I have received gold from mines. The weight is 37 Kgs and I want to refine this mining gold.
The Contents are:
Gold - 83%
Silver - 15%
Others - 2%
I am using aqua regia method. Now in aqua regia process, I'm facing a problem which is a layer of insoluble silver chloride protecting the gold from aqua regia. How can I dissolve the gold by using aqua regia? I will not opt for the option of using inquarting process because I have to use a lot of silver to form a 25% of gold content.
Thank you for your help.
You are gonna refine 37 kg of gold and yet do not have resources to buy at least few kg´s of silver ? That is strange. But allright, there can be various and unexpected circumstances. I wouldn´t ever let anyone refine mine 37 kg of gold, if that one seek help with task thoroughly solved in 15th century. But who am I to judge.
---------------------
There are these two pathways, both well known. In AR, you can sequentially leach gold in AR and then silver chloride with ammonia. But it is sequential process (more time) and non negligible quantity of ammonia is needed to dissolve AgCl. You can use optional solubilizing reagents such as thiosulfate or cyanide, but ammonia is with least ammount of hassle (except of the smell). Dissolution of this alloy in AR must also be very slow.
On the other hand, inquartation with silver is very effective and straightforward. You can cycle the silver as long as you want, and it will always add up. You can choose copper instead of silver, but you will consume high ammounts of nitric in the process.
This problem was thoroughly discussed here in "Lost gold thread" where the retention of gold in AgCl was one of the major issues.
I don´t like dissolving gold-silver alloys in AR as it is always somehow messy process. Hard to monitor, possibly incomplete, adding steps to the path and fractioning silver to both solids and also solution. Moreover, gold solution is saturated with AgCl, so it is obviously harder to refine pure gold from it, altough if done as it should be there are little to no problems.
Hard to say what is your goal - reducing time of processing, reducing cost of processing, reducing necessary labor in order to accomplish the task...
Cheapest option is to macerate in AR (that is probably why you are trying to do it this way). It will work, but slow. Others gave you suggestions like enlarging surface area - this speeds dissolution, but does not eliminate locking problem completely. For inquartation with copper you will need to add nearly 100 kg. And then spend more than 300 L of nitric for the task (just rough assumptions, real numbers will differ). With silver, you will cut the nitric consumption to 1/3 and use roughly 100 L. Depending on your purity goal, you won´t necessarily need to dissolve the gold from the inquart, just melt it and sell it as it is (or electrorefine it for high purity). To dissolve these 37 kg of alloy you have you would roughly need 150 L of HCL and 40-50 L HNO3, with this number being possibly higher since lots of AR will be wasted due to prolonged boiling and active reagents effervescence loss (nitrosyl chloride and chlorine). Then you will need the sulfite to drop the gold. And ammonia to free that last bit of locked gold particles.
You need to do the math, there is no magic in this chemistry. Agricola summarized it in De Re Metallica Libri in 16th century. Not much changed from his times in this particular refining task. Copper needed for inquart is around 700-1000 USD. Nitric for inquart with copper is around 500 USD if bought by the drum at my local supplier (bulk price is lower). Depending on treatment method you need to factor the price of hydroxide (if you are going to convert waste to hydroxides) or iron needed (negligible). If you look at it, these numbers are ridiculous compared to the value you are refining.
PS: You always minimize losses of metal in the process if you do not dissolve it into the solution in the first place.
