Flotation and subsequent processes (refractory ore processing)

Gold ores and their metallurgical treatment

Only a few minerals are important as gold carriers. Native gold is by far the most important, followed by gold tellurides. Other gold compounds are economically irrelevant. There is a series of minerals that can carry gold as a physical constituent such as pyrite and arsenopyrite.

The mineralogy determines the recovery process of gold. Two types of gold ores can be distinguished: Non refractory ores and refractory ores. Non refractory gold ores permit a straightforward recovery by a relatively simple conventional technology. Refractory gold ores, however, require rather sophisticated recovery techniques.

Gravity concentration

Gravity concentration is the oldest technique for gold recovery. It makes use of the high specific gravity of gold that amounts to 19.3. The devices developed and used for gravity concentration are innumerable. The most common ones are: Sluice boxes, rocker boxes, shaking tables, jigs, spirals, centrifugal concentrators, and dry washers. The limits of gravity concentration are mainly determined by the size and shape of gold particles: Particles of less than 30 microns size can hardly be recovered by gravity and tiny flakes do not settle as fast as rounded grains. The flow sheet of a gravity process is rather simple. It generally consists of a conditioning and sizing of the feed material followed by one ore two stages of gravimetric gold recovery.

Amalgamation

The amalgamation process is based on the low surface tension between gold and mercury, which permits gold particles to be wetted and enclosed exceedingly well by the liquid metal forming an amalgam. The physical properties of an amalgam permit its recovery by elutriation (desliming), cycloning or another gravity process. Filter pressing removes surplus mercury. Heating of the filter cake evaporates the mercury, and the remaining material is a gold concentrate that can be smelted down to a gold bullion.

Flotation and subsequent processes (refractory ore processing)

The flotation process is described in detail in chapter 3.3. Native gold, gold tellurides, and sulphides that contain finely disseminated gold can be floated and recovered in a concentrate. In gold metallurgy flotation is often used as a pre-enrichment process for refractory ores. Subsequent stages of gold recovery may require oxidation and leaching steps. Regrinding of an auriferous sulphide concentrate can create access for a leaching solution and permit the gold to be recovered. If this is not the case, the auriferous sulphides have to be chemically destroyed to liberate the gold. This can be achieved by roasting or pressure oxidation that transform leaching resistant gold carriers into porous oxides or oxidic residues that can be leached. A third alternative to degrade gold carriers is biooxidation by means of Thiobacillus and Leptospirillum bacteria.