Phill,
The volume of solution I evaporated varied, but as much as a couple liters was not unusual. From the perspective of volume, I limited the gold content to something less than 18 troy ounces in a four liter beaker, if that helps. I never handled volumes greater than that, choosing, instead, to handle several beakers, assuming a large lot was at hand. The greatest amount of gold I precipitated at any one time would be in the vicinity of 75 ounces, ±, and split in four units. I did that on a regular basis, when I'd re-refine my gold.
In such a case, I'd evaporate until the volume was reduced to less than a liter, at which time it was filtered in preparation for precipitation. Please bear in mind, when I evaporated gold solutions, my common practice was to include a button of gold, with the purpose of consuming any free nitric instead of evaporating to eliminate the excess. I evaporated beyond the point of expulsion of the nitric to eliminate free HCl as well, so the solution wouldn't destroy the filter paper. I generally evaporated until the gold chloride had a deep red color, at which time I'd take it up with water, to achieve the desired liter of solution. That insured it would filter without destroying the paper.
As far as distance from flame---it really makes little difference. If you had your solution a great distance from the flame, you might have the flame set higher. A ready answer isn't forthcoming due in part to the various types of vessels one might use. If a Coors evaporating dish is employed, the surface area is much greater than it might be if a small Griffin beaker was used. The critical thing is to keep the solution from evaporating too fast. If it achieves a temperature at which it can boil, you are guaranteed to lose gold in the vapors. That's a tough lesson to learn. (Don't ask!)
I used a three burner gas hot plate for all of my heating. Think of the common gas stove and you'd have a pretty good idea of the distance between the flame and the beakers. The big difference is I ALWAYS used an asbestos pad between the burner and the beaker. Beakers can deal with heat, but they do not do well when heated unevenly, so a thin layer of asbestos prevents cracking the beaker. Needless to say, today, asbestos isn't readily available, so a different product would be required, but I advise always having something between the beaker and flame.
Time, for me, was a few hours, again, depending on the volume involved. I generally evaporated while I had other batches in various stages of progress, parting the silver and base metal, or even dissolving the resulting gold. I never timed any of my operations, so I am unable to provide any meaningful comments.
The best way to get familiar with evaporation is to pursue the activity. You won't lose any values, even if you carry the process too far-----not as long as you don't toss anything, and you don't allow your solution to boil. Keep close watch on the column of vapors that come off. If you see the slightest indication of color, you are evaporating too fast. Cut the heat. When a solution is evaporating at an acceptable rate, you often think nothing is happening. Place a cold object (not metal) above the solution to see if anything condenses on the surface. If it does, you are likely running at an acceptable temperature. Do remember, as you eliminate all the water, the boiling point will rise---as will the temperature of the solution, so be careful when introducing any solutions. You risk cracking your vessel, or having a violent eruption of steam. Adding sulfuric acid (to expel lead) when the solution is highly concentrated and hot can be an interesting, and dangerous, experience.
Harold
