Of the four main channels through which our supply is derived, the simplest and most ancient method for recovering gold is by washing sands and gravels which contain it in solid particles. This method is used in all parts of the world where gold occurs in loose material in sufficient quantity; it is called placer mining.
If gold occurs in solid rock, as it does in many places, the rock must be crushed to free the gold which can be recovered by washing over copper plates amalgamated with mercury to catch and retain the particles of gold as they are washed over. This type of recovery is called 'milling and aml-gamation.
We have already indicated that much gold is recovered in the smelting of copper and lead ores; such metal might be called by-product gold, for it is recovered incidentally in the working up of these other metals.
Another highly important method for recovering gold wherever it occurs in minute particles, as it usually does in solid rock, is by treating the very finely crushed rock with a dilute solution of sodium cyanide or potassium cyanide. Fine gold is dissolved easily by such a solution, and the gold can be recovered by treating the solution with a more electropositive metal, such as zinc or aluminum, when the gold will be precipitated out and can be filtered off, dried, and melted into bullion.
Placer mining can be carried out with the very simplest outfit, such as a gold pan, a rocker with riffles, or a sluice box whose bottom is suitably roughened to collect the heavy particles of gold which are inclined to stop wherever they can find lodgment. Mercury usually is sprinkled on during the operation to assist in the recovery by catching the particles of gold and by drawing them within the heavy globules of the extremely heavy liquid.
An extension of the simpler placer-mining idea is carried out by hydraulicking with water under pressure, so that loosely cemented gravel banks can be worked just the same as ordinary gravels.
Another extension is by thawing frozen gravels with steam points, so that the dirt can be subjected to common sluicing methods.
A final refinement of placer mining is to use dredges to dig through whole banks of gravel whenever they occur under water. These dredges will float in rivers, lakes, or artificial ponds, as they dig up the gravel with continuous digging equipment at one end, wash the gravel on board the boat, and finally discharge the refuse at the other end to make new ground. Fig. 47 shows one of these huge dredges at work. The digging ladder is at the right, the discharge behind at the left. The gold recovered will be put in shape on board to be handled by the United States mint.
The simpler methods of milling rock by dropping stamps on the chunks and of recovering the gold by amalgamation has received much refinement in recent years.
Other types of grinding machines, concentrating machines, and accessory apparatus have been introduced to advantage. Cyaniding accompanies amalgamation in some cases.
Fig. 48. Suction Filter lor Cyanids Pulp - Movable Type Courtesy of the Moore Filter Company.
The full recovery of gold from a crushed ore usually demands very fine grinding. The plants have been meeting the requirements and now have tube and ball mills of continually improving duty.
If the cyanide solution has not been introduced in the grinding it will be added immediately after. Its best effect always is produced in the presence of enough aeration and agitation to allow fully the reaction indicated in the following, which is the common equation for the chemistry of the process.
Special tanks for this purpose have been devised which do the work quickly. Other chemicals may have to be added at this time, for any acidity is to be neutralized with lime, and some ores require further oxygen carriers.
When the gold finally is in solution, the solid matter of the pulp may be separated by decantation or by filtration; both operations are extensively used.
Fig. 48 indicates to what refinement the filtering devices have advanced. These huge filter leaves are immersed in the pulp, and the solution is sucked through until a cake has formed, after which the whole row of leaves is transferred to another tank for washing, then to another for discharge of the barren pulp.
The cyanide solution is run through zinc boxes, or is charged with zinc dust and then filter pressed; either of the processes gives the metal to be dried, melted with fluxes, and finally sent to the mint for parting.
Electrolytic gold refining is used extensively in this country and abroad. The electrolyte is a hydrochloric-acid solution of gold chloride, and the transfer is made from anode to cathode, as in all such processes. This is by far the most precise method for separating gold from the other noble metals, which remain either as slimes or in solution.
This process is adapted to recovering silver, if certain modifications are introduced. We have already .learned that silver is refined in exactly the same way. Thus, in describing copper and lead smelting, placer mining, and cyaniding, we have covered the metallurgy of silver as now practiced.