Two major factors affecting the profitability of a placer mining venture are the relative efficiency of the mining equipment, and the rate and percentage of recovery obtained while conducting mining operations. If either the rate of recovery or the percentage of recovery is increased, then the immediate profits also increase. When considering the total amount of precious metals in the deposit, however, the rate and percentage of recovery must be balanced.
If, for instance, an operation processing 20 cubic yards of placer gravel per day produces one ounce of gold, and different methods or equipment could produce one ounce of gold per day while processing 15 cubic yards of gravel, then it becomes obvious that the second option would produce the larger long-term profit, while cutting losses by 25 percent. While this is an extreme example, it does show the relationship between quantity and quality.
Many placer mining operations are being conducted under the assumption that the equipment used in the processing of gravels is constantly operating with the advertised efficiency, regardless of the type or quantity of material going through it. Quite often this is not the case. The type of equipment and methods of operation must be selected and adapted to the particular placer deposit and the size of the operation involved. Running small samples through the equipment can be very misleading, as the relative efficiency between light and heavy loading of the equipment varies considerably.
The best method for determining the maximum efficiency of a particular piece of equipment is to constantly sample the tailings or waste material while operating under various loads, conditions, and periods of time. This testing should be done before any full-scale operations are begun, especially with equipment that requires periodic “clean-up” – riffle or sluice type recovery systems, for example.
Sluices and reverse spiral helix equipment are noted for “packing” with heavy black sand material after being operated for certain periods of time. The time intervals between “clean-up” or clearing out of heavy materials in the equipment depends on the quantity of heavy black sands that are present in the material and the volume of gravel per unit of time.
In areas where large quantities of black sand are encountered, it is very difficult to separate gold and other precious metals from the concentrates, even in the gold pan. If a riffle-type recovery system is used, “packing” of the riffles occurs in a relatively short period of time, and cleanup must be performed often or a high percentage of values will be lost. When riffles are filled with a high specific gravity black sand, three types of gold particles will not be recovered and will wash right through the entire system with the tailings. These particles are: (1) flat or “flake” gold particles, (2) “float” gold (extremely thin or small), and (3) “flour gold” (extremely fine particles). It is surprising how large some of the flat or “flake” particles are that “skip” over the packed riffles and out of the system. If any appreciable gold is found below the third or fourth riffle, cleanup must be performed more often, the equipment is not set up properly, or the method of operation is incorrect.
The same problem is encountered when using reverse spiral helix equipment in heavy black sand areas. As the heavy particles of black sand collect in the equipment, the gold, unless very heavy, does not have a chance to settle low enough in the material to be picked up by the grooves. Especially large, flat flakes will remain suspended within the heavy materials, and with the accumulation of additional heavy material, be forced out of the system into the tailings.
This is true in the gold panning process as well, but a fourth type of particle is also involved. Very rough or jagged gold particles, flake gold and float gold will not separate satisfactorily in a gold pan filled with a large amount of heavy material unless the panning is done in a certain manner. Surprisingly, most of the finer gold will separate because it moves between the heavy particles and reaches the bottom of the pan. Concealment of gold particles in a gold pan by heavy black sands is called “masking,” and unless it is detected immediately, it could be misleading.
The best panning method to eliminate masking by black sand is one in which smaller quantities of material are panned at a time, and material panned carefully to the point where there is only black sand remaining. Then, by slowly washing away the black sands, tap the pan steadily with the flat of one hand on the side where the black sand is located, close to the center of the pile. The gold particles, being heavier, will “walk” out from the heavy material towards the area you are tapping.
Use of this method is very important when panning down concentrates, as masking of values can sometimes amount to fifty or sixty percent, and you would never see it. To check the pan recovery, re-pan the waste material several times. If you still find gold by re-panning, tap harder or pan less concentrate each time.
Another way that values are lost is when a magnet is used to extract black sand. When the concentrate is heavy, or the gold particles are flat or rough, the particles remain suspended in the material. When the magnet is used, it picks up any suspended gold along with the magnetic sand. This is because the lines of force on the magnet pick up magnetic material under the gold particles, and lift them out of the pan. This can be proven by panning the magnetic sand after it is extracted. In a heavy concentrate, use of a magnet can remove 75 percent of the gold. Also important is the fact that platinum is slightly magnetic, and is often alloyed with iron. Use of a magnet would remove most of the platinum group alloys as well. There are instances reported of platinum group alloys with 11 to 15 percent of iron by volume.
All waste material and tailings should be checked regularly for loss of values occurring during processing. Magnetic separation should be used only under controlled conditions, and the removed material checked for values. Any time concentrates are separated by panning, the discarded portion should be re-checked. If losses are recurring in the recovery system, they can generally be corrected by a slight adaption of either the method or the equipment.