Gold processing 101

Flowsheet selection drivers

Metallurgical processing of metalliferous ores in general and gold ores in particular consists essentially of three stages:-

Bashing
Mashing
Hashing

In the context of flowsheet selection, the Bashing and Mashing stages are relatively straightforward. There are a number of other considerations when it comes to the Hashing stage.

Stages 1 and 2: Bashing and Mashing

Traditionally, the process selection choice was between a conventional, well-tried, three-stage crushing circuit followed by ball-milling, or single-stage crushing followed by a Semi-Autogenous (SAG) mill and ball mill. The latter is preferred for wet sticky ores to minimize transfer point chute blockages etc, and can offer savings in both capital costs and long term operating and maintenance costs. However the SAG route is more power-intensive and, for very hard ores, comes with some process risk in predicting performance.

More recently, a hybrid solution has become available in the form of high pressure grinding rolls (HPGR’s), which can replace the third stage of crushing and also the SAG mill. Now that initial wear issues have largely been overcome, they offer significant advantages over a SAG mill route where power costs are high and the ore is very hard. They can be attractive too in a heap leach where the micro-cracking induced by the high pressure has been demonstrated in many cases to improve heap leach recovery.

Table 1 Bashing and Mashing stages

Table 1

Stage 3: Hashing

The hashing stage (corresponding to metal extraction and recovery stages) is a little more complex for gold ores in particular, as the optimal process flowsheet selection choice is heavily dependent on a good understanding of two fundamental geometallurgical parameters, the gold mineralogical associations, and the gold particle size and liberation characteristics. These are summarized in Table 2, where the processing options that correspond to the various combinations of mineral associations and liberation are shown along with some examples.

Table 2 Hashing stage (click to download larger image)

Table 2 Hashing Stage

Some other key drivers in flowsheet selection at the Hashing stage are:-

Supergene weathering
This is common in tropical environments (e.g. West Africa) and typically oxidises gold-bearing sulphides down to 50-100 m, transforming commonly refractory gold in sulphides to “free-milling” gold behaving in a similar fashion to gold associated with quartz.

Gold : Sulphur Ratio (GSR)
Refractory ores are typically treated by flotation and the resulting flotation concentrate may be sold directly to a smelter (common, for example, in China) or subjected to downstream processing by pressure oxidation or bio-leach.

An ore containing 1% S will produce a mass pull of approximately 3% by weight to a flotation concentrate. If this ore also contains 1 g/t Au (for GSR =1), and 95% recovery to concentrate is achieved then 0.95g will be recovered and with a concentration ratio of 33 (3% to concentrate) this corresponds to 32 g/t Au in concentrate. At current gold prices this is worth about $400. Both smelter treatment charges and oxidation or bio-leach costs are around $200/t of concentrate so a minimum GSR for effective downstream processing is 0.5, but clearly this is a function of gold price.

A lower GSR can be tolerated if the flotation concentrate is amenable to direct cyanide leaching without the costly oxidation stage to release the gold from the sulphides.

Gold/Silver base metal concentrate deportment
Smelters typically pay >95% (Au) and 90% (Ag) in copper and lead concentrates, but will only pay 60% -70% (maximum, depending on degree of Pb/Zn smelter integration) for gold and silver in zinc concentrates.

Cost drivers are also important in flowsheet selection, and are summarised in Table 3.

Table 3 Gold processing typical cost drivers (click to download larger image)

Table 3

It can be seen that the key cost elements are power, cyanide and grinding steel, plus, for refractory ores, the costs associated with pressure oxidation or bio-leach. It should also be noted that, where cyanide destruction is required (increasingly the norm), cyanide detox unit costs are usually of a similar order of magnitude to the cyanide unit cost.

In summary, and of particular relevance to project screening, an early appreciation of gold mineralogical associations and liberation can provide considerable insight into metallurgical process flowsheet selection and processing costs.