(I'd like to have these black sand concentrates in my hot little hands!)
I've touched on the subject of fine gold recovery from placer concentrates a number of times here in Bedrock Dreams but I've never gone into any great depth on the subject. Depending on where you're performing your small-scale mining activities, those heavy black sand concentrates you're dealing with will contain various amounts of extremely fine gold. If you're committed to recovering every little speck of the placer gold in those black sands then you need to realize just how important classification is to that recovery process.
First off, let's start with the basics. Concentrates are those materials left in your gold pan or equipment sluice box (portable sluice box, dredge, trommel, highbanker, etc.) or dry washer riffle tray after you've run a certain amount of gold-bearing material for a given period of time. The moment of truth gold-wise is when you perform a clean up (or clean out) of those materials. For most of us the usual step is to pan those concentrates out and remove the visible gold from them. I've talked a lot recently about the heavy black sands that constitute the bulk of any miner's clean up so I won't go into describing their constituents again here.
(Black sand concentrates sitting in a sluice box.)
As most of you already know, those black sand "left-overs" often contain very fine or microscopic gold particles that are not even visible to the naked eye. Recovering these fine particles is a bitch, to put things simply and directly. There are any number of methods for recovering this fine gold from concentrates, including:
- Mercury amalgamation;
- Borax smelting/refining; and
- Using a range of fine gold recovery equipment (mini-sluice, spiral wheel, Blue Bowl, shaker table, etc).
(Mercury amalgam...the guy holding this is either desperate or an idiot.)
Nix on Mercury
All of these methods work well in grabbing fine gold and re-concentrating it. The old timers used the mercury amalgamation method almost exclusively, but without fully realizing its dangers...both to their health and the extreme environmental pollution involved in its widespread use. So nix on mercury. It isn't a viable or safe method for you to use and I recommend you drop it from your mind completely. Even if you completely ignore my admonitions here on mercury amalgamation use and plunge ahead with it anyway, you can rest damn well assured that the Enviro-Nazis will be on your ass in a heartbeat. The use of common borax (yep, like Boraxo soap) has proven to be a very efficient means of recovering fine gold from black sand concentrates without the dangers of mercury, but the boraxo approach is more suited to refining and smelting those fine gold particles AFTER they've been re-concentrated through some sort of fine gold recovery process. Regardless of the fine gold recovery process you use, the most important factor in gaining full recovery efficiency is classification. In other words, how well you reduce those concentrates down BEFORE you actually use one or more of the methods I've already described.
(Yep, even humble borax can be used in the fine gold recovery process.)
As Long as it Does the Job
What's classification? It's simply the reducing down of those black sand concentrates using a series of screens or "classifiers." In my 37 years at this small-scale gold mining gig I've seen all sorts of classifiers used to complete this process and I myself have used a number of them. I'm talking here about everything from those size-numbered or "scaled" store-bought classifying screens right on down to flour sifters and window screens. It really doesn't matter what you use as long as it does the job for you. Many small-scale guys and gals use a strict "winnowing" method to classify concentrates wherein they start with a larger classifier and then work their way down step-by-step to the smallest screen. This method will work but it's not always necessary or even the best way to approach things, truth be told. I tend to screen my bulk black sand concentrates through a Number 4 screen first and then a Number 8. That seems to work best for me especially if I'm going to re-concentrate that material through a mini-sluice, Blue Bowl, or spiral gold wheel. But what works best for me is not necessarily what'll work best for you. The important factor here is that you classify those black sands.
(A spiral gold wheel doing its thing with concentrates.)
It's All About Fine Gold Recovery
Why is classification so important in fine gold recovery? Simply because the more you "fine tune" those concentrates beforehand the more efficiently that fine gold recovery process you use is going to be. And that's the name of the game here. Fine gold recovery. If you want to get every last bit of microscopic gold out of those black sands then classify them down first. It's a field call in terms of how far down those concentrates need to be screened and sometimes it takes a bit of trial and error to hit on the best classification process for any given gold locale because each area's black sands will be different in coarseness and size as well as in the amount of microscopic gold values they contain. Getting the drift here? Now I know many of you old salts out there already have a line on this, but most newbies don't and even a few journeyman-types are not solid on the importance of classification in fine gold recovery from concentrates.
(Some of that fine gold I've talking about.)
Of course, none of this is mining rocket science. In fact, almost everything we do in small-scale gold mining is based on good ol' common sense. There ain't nothing better than common sense when it comes to getting the job done. And classification is pure, 100% common sense. So use your head OK?
Best to all of you. Keep coming back.
(c) Jim Rocha (2016)
Questions? E-mail me at email@example.com