In far ancient times, the only source of gold was relatively pure elemental gold that was found in the form of nuggets and powder. Some thousands of years ago, however, it was discovered that gold could be extracted from ore by a process known as mercury amalgamation. This process was based upon the fact that gold particles wetted by mercury adhere to each other and to mercury coated copper plates. For many centuries, this process was the only method used for extracting from ores. While the percentage of gold recovered by a mercury amalgamation process varies with the type of ore, it is a relatively inefficient process leading to a considerable loss of gold.
The amalgamation process remained dominant until the 1890's, when cyanide processes gained favor due to the dangers of mercury amalgamation (i.e. slow death by mercury poisoning), the relative inefficiency of the amalgamation process, and the scarcity and high cost of the required mercury. The cyanide processes was first used in South Africa--the largest producer of gold in the world --and is still the main gold extraction process used to this day.
The cyanide process was a vast improvement over the amalgamation process in terms of safety, cost, and efficiency. In this process, the gold in finely ground ore is dissolved by treating it with a very dilute solution of sodium cyanide or the less expensive calcium cyanide plus lime and oxygen from air. The mixture is held for some hours in large tanks equipped with agitators. The chemical reaction yields a water solution of gold cyanide and sodium cyanoaurite. This solution of gold is treated to remove oxygen, and is then clarified and mixed with zinc dust to precipitate the gold and the other metals, such as silver and copper, that were dissolved by the cyanide. The precipitate is then treated with dilute sulfuric acid to dissolve residual zinc plus most of the copper. The residue is washed, dried, and melted with fluxes (materials used to promote fusion of the gold and silver and to dissolve the remaining copper). The operation may be repeated to flux off more base metal. The remaining gold and silver alloy, called dore, is then cast into molds for assay.
In FIG. 1, a prior art cyanide process 10 for extracting gold from ore begins with a step 12 of mining the ore. Next, in a step 14, the ore is ground to a sand-like consistency. In a step 16, a dilute mixture of NaCN, lime, and oxygen is added to the finally ground ore and is agitated in a large tank. This produces a water solution of gold cyanide and sodium cyanoaorite. The solution may also include silver and gold cyanides. Next, in a step 18, the oxygen is removed and a zinc dust is added to cause a precipitation of the gold from the solution. Along with the gold, silver and copper are also precipitated. In a step 20, H.sub.2 SO.sub.4 is added to dissolve the zinc and copper. This produces an alloy known as dore, which is essentially an alloy of gold and silver. Dore can be up to about 96% pure gold, with the majority of the remainder being silver. Next, a purification process such as a Wohlwill process 22 or a Miller process 24 is used to create essentially pure gold. Both of these processes are well known to those skilled in the art. The Wohlwill process can create 99.95% pure gold, while the Miller process can create 99.5% gold purity. The Wohlwill process is an electrolytic process wherein essentially pure gold coats a cathode, and wherein impurities such as silver form chlorides and remain near the anode. Typically, Pt and Pd also dissolve in the electrolyte. In the Miller process, silver and other metals are converted to chlorides by passing chlorine though molten dore, and then are poured off or volatized. The Miller process creates a purity of only about 99.5%, since it is stopped before the gold converts into a chloride.
Cyanide processes have been well developed over the past century. However, these processes have a number of recognized deficiencies. For one, the use of cyanide is extremely hazardous and the resulting effluents are damaging to the environment. Further, it will be noted that the cyanide processes involve a large number of process steps, including a series of separations, alloying steps and final purification steps. These processes also involve the use of a number of chemicals, some of which are quite expensive, and a considerable expenditure of energy in the large number of process steps. The cyanides processes are therefore considered too expensive for use with low grade ores, and limit potential production due to the slowness and cost of the total process.
The standard gold extraction processes principally use dangerous and expensive chemicals like the cyanides, and have many steps resulting in a complex process. They also require extensive purification processes after extraction. It is therefore desirable to create a process that uses more benign chemicals and uses simpler processes, thus having the potential to reduce the overall cost of extracting and refining of gold.
In U.S. Pat. No. 5,221,421 of Leibovitz et al., a controlled etching process for performing fine-geometry conductive gold circuit lines on a substrate is disclosed for use in the electronics industry. Briefly, the disclosed invention is concerned with the production of fine geometry electronic circuitry by controlling the gold content in the etchants. This requires reducing the dissolved gold content in the etchants when the gold content begins to rise. Failure to reduce the gold content in the etchants will affect process control. The reduction in gold content in the etchants is achieved by recovering a dissolved gold complex compound (AuI.KI.sub.3) from the etchant, thereby restoring the etchant for continuous etching of the fine geometry gold circuit lines. The recovered complex is further converted to AuI and subsequently to Au. Leibovitz et al. also propose to reduce the gold content in the etchants by removing gold electrolytically.
Therefore, the Leibovitz et al. process teaches the removal of gold from a etchant solution to permit the etchant solution to be reused. However, this process as disclosed for providing fine electronic circuitry is not be suitable for the mass production of gold from gold ores in that it is a slow, controlled process used to maintain etchant purity and not a fast, bulk process for economically producing large quantities of gold. Therefore, this slow controlled process used in the electronics industry would not appear to be applicable to a gold mining industry. This is due, in part, to the fact that for the Leibovitz et al. process to provide a commercially viable gold extraction and refinement method, its processes would need to be different and would need to be accelerated by at least an order of magnitude or more to be economically viable. In addition, substantially changes would have to be made to the Leibovitz et al. process in order to provide the ability for continuous or semi-continuous ore processing and extraction of gold from the liquid chemicals.