Sorting machines have been used successfully for decades in the food production sector, in waste segregation, and for high speed tasks such as mail classifying. In most applications, optical sensors are used to detect differences in colour between the “accept” and “reject” objects – a simple example would be the separation of ripe (red) from unripe (green) tomatoes.
Most sorting machines work on the principle of finding a correlation between a measurable variable (such as colour) with an immeasurable variable (such as ripeness). If the logic prevails that all ripe tomatoes are red and all unripe tomatoes are green, and sensors can detect red from green and cause a mechanical device to accept the red and reject the green, the sorting system would work efficiently.
In the mineral processing field, similar logic exists, and optical sorting technologies have been available for over twenty years. In the last five years, significant improvements in sensor technology, ejection methods and software have allowed target minerals to be separated from waste based on a number of criteria used either singularly or in tandem.
The principle application in the mining industry for ore sorting is as a pre-concentration technology. For example, if a mineralised waste pile contained one million tonnes at an average grade of 0.5g/t, it would be unviable to reprocess. However, if the same pile was crushed and ore sorted, the tonnage may be reduced to 200,000t but the head grade would typically be increased to 1.5g/t. There would be minor losses of gold in the rejects, but the overall process would now be viable.
An often-repeated complaint from ore sorting users is that laboratory testwork results are significantly better than operational results, leading to unfulfilled expectations and disappointment within the customer base. In field trials lasting several months, we recognised that the key enemy of ore sorters operating in the field were dust and discolouration of rocks. In conjunction with vendors and key stake holders we helped rectify this critical element which improved overall performance substantially.
