Economical decisions in Grade Control are made by balancing between mining cut-off grade level (i.e break even cost level) and ore losses. Inaccuracy in sampling for Grade Control leads to economic losses by replacing ore with waste rock in mill and concentrator (i.e. waste rock dilution) and by loosing valuable ore to waste rock in mining. Further, separating clean Non-Acid Forming (NAF) and Potentially Acid Forming (PAF) sulphide waste rock in mining are to be also to be considered in Grade Control. The PAF waste needs to be stored in a separate and isolated waste areas to prevent Acid Rock Drainage.
Image 1. All ores are more heterogenous (brown is ore, grey is waste) than expected
We have found out that all ores are more heterogenous than mines consider today. The heterogeneity is revealed by dense sampling and analysis from drill cores or drill cuttings (chips) i.e. taking samples in dense sampling grid and in short hole depth lengths (<100cm) per sample.
Blast bench composite samples from drill holes for Grade Control are at best just on estimate of a large average value, which the plant control engineer and metallurgist can see in ore grade fluctuations in the concentrator.
Root causes of the above are direct consequence from limited information of the ore grade on benches based on sparse but accurate drill core or R/C hole sampling and analysis information typically from drilling grid of 100m-by-100m or at about 10m-by-20m scale at best. The sampling and analysis density at depth varies from 1m to 3 meters. It is a known fact that this information is insufficient to describe extend of ore and waste variability in the blasting bench.
Image 2. PitViper (Epiroc) drill rig finishing next bench and a loading machine behind the swollen blasted muck pile.
Blast hole drilling provides a much better option for more frequent sampling and analysis than drill core and R/C drilling. Blast holes drilled at 7m-by-10m or down to 3m-by-4m drilling pattern are commonly used depending on the used hole diameter. The added benefit is that the blast holes are being drilled for that purpose only, and there is no need to drill separate R/C or core drilling holes with higher cost. However, with blast holes poor of representativeness of samples may spoil the accurate analysis received from the mine laboratory. –If your sample is wrong accurate analysis will not help you!
Conventional sampling from the drill cuttings heaps or cones next to the holes are sometimes dramatically biased, which is explained as follows.
The Challenge of blast hole sampling
Blast hole drilling is done by systematic drill plan to produce holes for explosives for blasting ore and waste rock. Mine Grade Control commonly uses samples, which are collected from the drill chips cones(piles) around the drill holes for laboratory analysis.
Image 3. Drilled and charged blast bench with containing hundreds of holes each having up to one ton of explosives – no entry.
Here the challenge is how to get representative samples from these cones of chips, which contain up to 5000kg of chips having a mixture of grades of ore and waste.
Why samples taken from drill cones tend to be bias? Download the white paper in this article and learn more.
Next week, we discuss our method, equipment and results for blast hole sampling.