Cleaning up Concentrates containing Gold and Platinum

Hematite particles forming ferro-fluid with a rare earth magnet beneath a sheet of glass.

One of the problems in the recovery of gold,silver and platinum from concentrates of stream heavies is the removal of iron oxides that make up the majority of heavies. Most of the other heavies are composed of silicates and in rare cases oxides of various elements. In the case of iron oxide there are two types present in most heavies. The most easily removed is magnetite because it is attracted to an ordinary magnet with the best being an Alnico magnet. This will quickly remove the magnetite that is the easiest to remove. Hematite is the second iron oxide in heavies that is not attracted to an ordinary magnet, but is attracted to a rare earth magnet that can be ordered on the internet. The remaining heavies composed of mineral silicates and oxides being much less dense and having a far lower specific gravity then gold, silver or platinum that is left behind in the pan.

The iron oxides can be removed in two stages; the first is removing magnetite. There are commercial magnetic separators available, but for the small amount usually encountered an ordinary magnet will suffice. A small amount of concentrate that has been thoroughly dried is placed on a sheet of paper and the magnet is passed over the concentrate just above its surface, but high enough not to touch the concentrate. The magnetite present will stick to the magnet so you will have to periodically remove it from the magnet.

The hematite is removed in the second step by using a small rare earth magnet by repeating step 1 using the more powerful magnet the remaining concentrate containing heavies, gold, silver and platinum group metals. This concentrate is poured into a separate container for further work in the process.

Although geologists know ways of separating heavies using dense fluids, but they are all highly toxic and shouldn’t be used by an untrained layman. The best way to remove them is with a small ten inch gold pan immersed in water using conventional panning methods. Its tedious work, but the pan full of concentrate can be worked down until the only thing left in the pan is gold, silver and platinum group metals (PGM).

The remainder in the pan is gold, silver and PGMs that are saved for further processing; because of the length of the whole process we are going to divide it into several articles we will publish in the near future. Watch for them!

What’s that in the bottom of your gold pan?

Iron oxide that has been removed from gold concentrate with a magnet.
Photo by Peter Kuiper

Gold isn’t the only thing that ends up in your gold pan, although with a density of 19.1 it is one of the minerals that might be staring up at you.  Platinum or platinum group metals (PGM) having a density almost matching gold is another.  The rest of the odds and sods found with the rest of the black sands can range from a to z with zircon the last.  They can be divided into several different classes with iron oxides being one of the most abundant,

Iron Oxides:

Magnetite:  This is mixed with hematite on about a 50-50 ratio.  Magnetite can be removed with a common magnet.  This mineral is opaque black and is a member of the spinel group of minerals.

Hematite:  This is the second most abundant mineral found in the bottom of a gold pan. Unlike magnetite is can’t be removed with an ordinary magnet, but is attracted to a rare earth magnet.  Both magnets are used in the process or removing iron oxides from gold concentrate.

Iron bearing sulfides:

Pyrite: this is also called “fool’s gold” because it resembles gold but can be recognized by its rotten egg smell when tested with acid. Pyrite is brittle whereas gold is malleable.

Chalcopyrite:  The same tests apply as to pyrite except chalcopyrite is somewhat redder then pyrite.

Arsenopyrite: this occurs as silver colored cubic crystals that look like pyrite.  The simplest test is made by striking one of the crystals with a hammer when it emots a garlic smell.

Corundum:

This is aluminum oxide with a hardness of 9 on the Moh’s Scale that includes rubies and sapphires.

Diamond:

This is the hardest mineral of all with a hardness of 10 on the Moh’s Scale.  It occurs as a bypyramidal crystal.   

Uranium/Thorium:

There are several minerals present in stream heavies that contain these elements the most common are pitchblende, uranite, columbite/tantalite and allenite.

Tungsten:

The two common minerals of this element are wolframite and scheelite/  Both of these minerals fluoresce under ultraviolet light in a bluish/white glow.

PGMs:

Platinum group metals occur with gold on about a 5 to 1 ratio with more gold.  They look like steel and like gold are quite malleable.  They also leave a silver streak on a touchstone and are not affected by any acid except aqua regia a mixture made from 1 part of nitric acid and three parts of hydrochloric acid.

A zircon crystal
Photo by Rob Lavinsky

Zircons

These are the ore of zirconium that has many uses in modern technology.

Many of these minerals themselves are ores for their various elements.  In many places they are actually mined from beach sands for their metal content in places like Australia of the Southeast United States using specialized equipment mainly the spiral classifier.

Generally, any mineral with a hardness of 7 on the Moh’s Scale and denser then 3 will wind up in the bottom of your gold pan.  We did not include complete descriptions of these minerals because there are more then adequate descriptions available at www.mindat.org. :