Sensor-based ore sorting

 Exploring gold ores becomes more challenging every year. Mines increasingly deliver lower grades and complex mineralogy, forcing mills to process more material and thus compounding the expanding ratio between these tonnages and the ounces of ore produced.

 It is of size 40 mm visible through XRT .

It is clear that pre-concentration with sensor-based sorting provides a number of bottom-line benefits to users including increasing cash values per tonne, reducing environmental impact in terms of resource inputs (energy, water, wear and tear of equipment, process reagents…) as well as reductions in the volume of tailings produced.

Sensor-based ore sorting can be used to significantly upgrade ROM ores prior to feeding them to the concentrator. With less barren material being treated, there is a knock-on effect which produces further savings in a range of ancillary mining activities from haulage costs through to a reduced tailings footprint. All these factors contribute to a lower mining cost per tonne. The primary aim is to increase mine recovery by reducing the cut-off grade and processing the extra material without compromising current mine production. This creates new mill capacity by rejecting waste material without significant capital expenditure.

Adopting sensor-based ore sorting technology promotes a pre-concentration of ore in the early stages of the production process. This pre-concentration is achieved by deploying X-ray-transmission (XRT) and/or LASER sensor analysis to identify viable material alongside an air ejection process to separate the ore from sub-economic waste.

DUAL-PHASE SENSOR-BASED ORE SORTING FOR EVEN BETTER RESULTS

Gold ores are typically amenable to sensor-based ore sorting to some degree, and this is usually accomplished with XRT or laser sensors, or a combination of both. A two-step sorting flow-sheet consisting of an XRT rougher plus a laser scavenger (or alternatively, a laser rougher and an XRT scavenger) has already proved an optimal recovery method for a number of deposits. In quartz-type gold deposits, as well as purely with the quartz itself, the gold can also be associated with pyrites both within the quartz vein and outside (the alteration halo effect) – albeit often in lower concentrations. Ore recovery of over 90% and mass removal of 30 – 60% has been regularly experienced using these two-step particle sorting flow-sheets . 

Although the focus is not on gold detection itself, the correlations between detectable characteristics and gold content need to be present, well-understood and reliable in order to discern how gold deportation into product and waste fractions can best be achieved.      

Sulphide associated gold

Gold ores are frequently amenable to some degree of particle sorting when sulphide or quartz is associated with the gold. As regards the complex combination of sulphide associated with gold, the pre-concentration of the ore in the early stages of the production process via sensor-based ore sorting is best achieved by X-ray-transmission (XRT).

XRT sorting technology is based on a planar projection of data derived from the X-ray attenuation of a flowing particle stream. Where metals are present in ores, the elements with higher atomic numbers will be identified, but gold itself will not be recognized by the XRT sensor because it occurs in such low concentrations. However, the metals which occur in higher concentrations, such as Fe and Cu, can be present as sulphide mineral phases. And if these sulphide mineral phases are associated with gold, and are also found in sufficient size and concentration, they can be recovered with XRT sorting technology.

Particle sorting of gold ores, using XRT and/or laser sensing, is generally an effective method. For instance, a two-stage sorting flow-sheet utilizing an XRT rougher and a laser scavenger (or this same combination in reverse) has already delivered an optimized recovery for a range of deposits. It should be noted that, even for quartz-type gold deposits, the gold is typically associated with pyrites inside the quartz vein and in a ‘halo effect’ around the edge, as well as in the quartz itself – though mostly in smaller concentrations. Two-step particle sorting flow-sheets using these types of configuration have regularly accomplished ore recoveries of more than 90% and mass removal of between 30 – 60%. 

Quartz associated gold

Whenever sulphide or quartz is associated with gold, the resultant gold ores are typically amenable to a certain degree of particle sorting. And in the case of the complex blend of quartz associated with gold, the pre-concentration of the ore by means of sensor-based ore sorting during the early stages of the production process can be effected by harnessing the scattering effect of a LASER beam.

The principle of LASER sorting is based on the known diffraction of LASER light in transparent minerals such as quartz. This LASER sensor technique will not identify the gold itself, because it is present in such low concentrations. However, if the quartz is of sufficient size and concentration and has gold associated with it, then the ore can be recovered using laser sorting technology.

Experience shows that gold ores are commonly amenable to some degree of particle sorting, generally using laser- or XRT sensors, or a combination of these techniques. A dual-phase sorting flow-sheet comprising a laser rougher and an XRT scavenger (or also vice versa) has already demonstrated optimal recovery for a number of deposits. In the case of quartz-type gold deposits, gold ore can be associated with pyrites both within the quartz vein and on the periphery (the alteration halo effect), as well as purely with the quartz material itself,– though generally in lesser concentrations. Dual-phase particle sorting flow-sheets have regularly achieved ore recovery in excess of 90% and mass removal in the region of 30 – 60% it.