Extraction of Gold and Silver
Old extraction methods for recovering gold and silver were developed under the same considerations that modern techniques. Basically, the extraction is accomplished by mechanical and chemical means; the selection of the process depends largely upon the occurrence and condition of the ore. In general, it may be considered that gold and silver have been extracted by a combination of chemical and mechanical aspects. Another important consideration in the extraction of gold and silver from ores was their reduction to a size suitable for the removal of the individual particles of useful metal or mineral. It is clear that the size of the particles and their matrix are important factors in the economical processing of a given gold ore.
The mechanical treatment of gold ores may be considered as milling, while the chemical treatment was usually known as metallurgy. Nevertheless, there is no sharp line dividing the two processes and they have been probably more often used together as supplementary processes. Than separately, although in smelting there was often no connection, except in some cases a slight hand treatment, as sorting. As between mining and milling, the latter was probably of the most importance as a factor in the growth of the gold mineral industry. Means of winning ores from the earth naturally antedated the extraction of values therefrom, for which reason attention was first turned to improvements in mining the gold res. Another important factor was the condition of the gold in the ore, little or no attention was at first paid to those deposits in which gold occurred in free or native state. The extraction of values of such ores meant a simple reduction, but later the methods of extracting the ores from the ground were largely independent of the after-treatment and were the immediate object of improvement.
It is not surprising that gold mining reached a comparatively high state of perfection at an early date, while the advance made in extraction of values by slow and devious ways. Nevertheless, it was largely through the overcoming of apparently impossible conditions in extraction of gold that the remarkable and phenomenal growth in all its phases was due. For example, the reduction of working costs of the Comstock was considered remarkable and extraordinary. The economical processing of some low gold grade ores was important and drew the attention of operators and metallurgists until an optimum point was reached. Such a process was the tantamount to the discovery of new gold mine and was important factor in maintaining and increasing production of gold and silver. Essentially, the extraction was considered under the two general heads of milling and metallurgy, but was it was impossible to develop them exhaustively. The washing of gravel for gold dust and nuggets constitutes probably the first attempt at extraction of gold from their natural and crude surroundings. Further, this may be considered the first method employed in each new locality in which gold was discovered.
Where free gold occurred in the upper oxidized portions of veins, its recovery from the loosely associated matrix of weathered rock was a comparatively easy operation and did not involve extensive and costly appliances. Later, when considerable depth was reached and the free gold gave way to combined forms, as sulphides, in which the gold and silver occurred in smaller quantities and from which it was more difficult to recover, the problem of extraction was more complicated. Improved methods both mechanically and chemically were employed, thus elaborated equipment was necessary, which was planned as much with an idea of preventing loss as ensuring rapid and economical work.
- Old Milling Process for Gold Ores
- Old Amalgamation Process
- Sulphides and Amalgamation
- The Patio Amalgamation Process
- The Plate Amalgamation Process
- Barrel Amalgamation Process
- Pan Amalgamation
- Addition of Mercury in Amalgamation
- Amalgam Extraction
- Amalgam Cleaning-Up
- Gold Amalgam Retorting