The worldwide push toward cleaner energy is increasing demand for critical minerals, particularly copper. At the same time, meeting this demand is becoming more difficult due to tightening ESG regulations, more complex ore bodies, and declining ore grades.
Bulk ore sorting is one solution that is increasingly being used to help improve processing efficiency.
As a preconcentration approach, bulk ore sorting helps miners maintain or increase head grade when processing diverse ore bodies. By separating below-grade or waste material from material that can be blended, it allows operators to maintain a consistent and economically viable feed stream.
Effective bulk ore sorting can reduce the scale of new processing plants, extend productive mine life, and help lower the amount of energy, water, and tailings associated with producing useful material.
A high degree of ore body heterogeneity is essential for effective bulk ore sorting.
For assets that meet this requirement, sorting efficiency depends on precise and accurate measurement of the ore leaving the mine so that waste can be reliably distinguished from processable material. As a result, the performance of the analyzer used for this measurement is critical.
Thermo Scientific™ CB Omni™ Agile Online Elemental Cross-Belt Analyzer. Image Credit: Thermo Fisher Scientific – Analytical Instruments & Solutions
Using PGNAA for Bulk Ore Sorting
Bulk ore sorting depends on sensing technology that is capable of:
- Differentiating processable ore from waste. In other words, differentiating ore that is above or below the required head grade.
- Representatively analyzing bulk material
Prompt Gamma Neutron Activation Analysis (PGNAA) is an apt technology for these needs.
PGNAA is a non-destructive method that determines elemental composition by analyzing the gamma ray ‘signatures’ that are emitted when an atomic nucleus, excited by thermal neutron capture, returns to its ground state (Figure 1).
The neutrons employed in PGNAA are highly penetrating, and the resulting gamma rays have high transmission through the sample.
Figure 2 compares PGNAA’s high penetrability with other analytical techniques by plotting penetrability (x axis) against the relative detection capabilities (y axis) of alternative methods.
PGNAA is a unique method in that it combines deep material penetration with enough sensitivity for bulk stream analysis, which facilitates the precise characterization of heterogeneous ore across the material’s whole cross-section.
In addition, PGNAA is very fast, providing instantaneous measurement that is essential for responsive control. Continuous sorting based on such measurements enables miners to divert small increments of feed, rather than truckloads or shovel loads, thus improving the accuracy of the sorting process.
Figure 1. Schematic illustrating the principle of measurement of PGNAA. Image Credit: Thermo Fisher Scientific – Analytical Instruments & Solutions
Figure 2. Performance characteristics (detection (y-axis) and penetration (x-axis)) for a range of analytical techniques highlight the unique suitability of PGNAA for bulk ore sorting. Image Credit: Thermo Fisher Scientific – Analytical Instruments & Solutions
Analyzer Performance
Both accuracy and precision must be evaluated in order to determine whether a PGNAA system is suitable for bulk ore sorting (Figure 3).
Accuracy is a measure of agreement with a ‘true’ or accepted reference value and is foremost affected by systematic errors. Precision is defined by the magnitude of random errors, and measures agreement with other similar measurements
The Thermo Scientific™ CB Omni Agile™ Elemental Cross-Belt Analyzer is engineered, configured, and calibrated to minimize both random and systematic errors, providing excellent accuracy and precision for each application.
Figure 3. PGNAA systems may vary with respect to the accuracy or precision of measurements, or both. Image Credit: Thermo Fisher Scientific – Analytical Instruments & Solutions
The uniformity of measurements is the primary factor that influences the accuracy of online PGNAA systems (see Figure 4). While inherently heterogeneous, the incoming ore may be prone to segregation. It is critical, therefore, to ensure that measurements are consistent across the belt, in a similar way to representative sampling. This is a prerequisite for any analytical measurement.
The CB Omni Agile Analyzer provides versatility in configuring both the neutron source ((isotope (Californium 252, 252Cf) or electrical neutron generator)) and the detector volume to reduce systematic error and optimize measurement uniformity.
A strict factory calibration process, spanning the entire range of expected ore compositions and belt loading conditions, further guarantees accuracy.
Precision is primarily determined by the number of detected neutron interactions, making both detection capability and neutron output critical. The CB Omni Agile Analyzer enables improved neutron flux and increased measurement precision by supporting up to 78 μg of 252Cf – more than any other PGNAA system currently available.
Additionally, it employs larger volume NaI scintillation crystals (as much as 12 times larger than commercial alternatives) for better detection. This combination of high neutron generation and effective detection of the resulting gamma rays means the CB Omni Agile Analyzer delivers market-leading precision for a given mass loading and measurement duration.
Figure 4. Uniform measurement is crucial for accurate PGNAA. Image Credit: Thermo Fisher Scientific – Analytical Instruments & Solutions
Comparing Performance
Figure 5 displays the direct correlation between bulk ore sorting efficiency and performance characteristics. The right-hand side displays a simulated dataset from a PGNAA system equipped with a single detector and a 50 μg 252Cf source; the left-hand side shows comparable data for a CB Omni Agile Analyzer with four detectors and a 78 μg 252Cf source.
Red data points indicate misclassified material: either waste processed as ore (false positives) or ore discarded as waste (false negatives). These misclassifications significantly impact resource recovery and process efficiency.
Processing ore with copper that is below the specified cut-off value worsens process efficiency, while underestimating copper content means that valuable ore goes to waste. Both of these impacts directly affect the bottom line.
By lessening the chance of inaccurate ore characterization, the CB Omni Agile Analyzer minimizes head grade variability, which in turn facilitates more predictable, controllable, and stable downstream mineral separation processes. This reduces the process capacity spent processing waste and below-cutoff material and the loss of valuable material.
Figure 5. Measurement accuracy and precision directly impact the efficiency of bulk ore sorting. Image Credit: Thermo Fisher Scientific – Analytical Instruments & Solutions
Conclusion
Bulk ore sorting is a valuable pre-concentration approach for miners looking to increase output, maximize asset value, and reduce environmental impact.
Success depends on sensing technology that can deliver representative and reliable bulk feed flow characterization, while clearly distinguishing below-grade material from ore that can be processed profitably.
Using the powerful PGNAA method, the CB Omni Agile Analyzer provides instantaneous elemental analysis, supporting high-efficiency bulk ore sorting.
Its high precision and accuracy help minimize both false acceptances and false rejections, improving asset utilization while reducing the processing of sub-economic material.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Analytical Instruments & Solutions.
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