Analysing the feed to the processing plant or mill
Large natural variability in ore grade and quality is common in the minerals industry and although on-belt analysers have been used in the coal industry for many years, the adoption of on-belt analysis in the minerals processing industry has been slow. In this article, experts from AMC Consultants and Scantech explore the benefits and options available to operators considering adopting on-belt analysis.
Feed material from the mine provides a flow of tonnes at an expected average quality to the process operations. Mine designs influence the ore quality through planned and unplanned dilution, ore losses and over-break. Ore sources are blended through ore passes, crushers and stockpiles – or so we think. The process operators expect they will receive a blended feed that approximates the scheduled quality and quantity.
Figure 1 shows feed ore variability within a 24hr period based on two minute results of continuous, transmissive, full stream, elemental analysis where Prompt Gamma Neutron Activation Analysis (PGNAA) has been used on a 1,000t/hr run of mine feed flow. Looking at the large variability in feed grade, is it any surprise that the process plant performance is not quite what we expect?
Figure 1. Zinc-lead ore mill feed variability (average 34t/measurement)
The average ore grade (based on a 1 hour moving average and shown by the black lines) matches the daily average grade for each element for less than 20% of the day. Can processing performance be as efficient for the other 80% of the time? There is no doubt metal recovery will be adversely affected.
There are a number of ways we can control feed quality to a process; however, unless we apply measurement systems that provide timely feedback we are not likely to improve our performance. Many technologies are promoted for their real-time measurement capabilities. A comparison shows significant benefits for PGNAA.
This is considered the most representative elemental analysis technology for conveyed bulk material measurement:
- It is penetrative and is not affected by particle size, layering/segregation or dust, as neutrons and gamma rays can penetrate to a depth of 0.5m,
- It provides instantaneous measurement, within nanoseconds of neutron absorption,
- It is continuous, constantly measuring irrespective of belt speed.
The technique is proven with many successful installations, including more than 60 in the minerals sector. It is safe, with full shielding, requires no restrictions to access to the surrounding area and leaves no residual radiation in the conveyed material. Correlations between analyser and sample analysis from conveyed flows for the main elements of interest is very good (0.8 – 0.999). Measurement times of 2 to 5 minutes are suited to blending and some control applications. Faster measurement times, down to 30 seconds, are now possible in bulk ore sorting applications where selectivity needs to be optimized. Shorter times are expected in future. Figure 2 shows a typical Geoscan PGNAA on-belt analyser.
Figure 2. Typical Geoscan PGNAA on-belt analyser
Surface measurement technologies
Surface measurement technologies include Laser Induced Breakdown Spectroscopy (LIBS), X-ray fluorescence (XRF), Laser Induced Fluorescence (LIF), and Near InfraRed (NIR). These methods of measurement do not penetrate the material beyond the surface, and evidence has shown that although these techniques may be suitable for conveyed flows where the material is homogeneous, this is almost never the case on ROM material or crushed ore in mill feed. No amount of mixing will homogenize large rocks and fines to the extent that the surface measurement adequately represents the full volume. Correlations between surface analysis results and sampled laboratory results are usually not great. Typically, there are limited conditions under which surface analysis techniques are designed to operate, and these may include:
Limited range of particle sizes.
Slower belt speeds.
Absence of dust.
Consistent bed depth.
Other penetrative technologies
Two other penetrative technologies: Magnetic Resonance (MR) and XRay Transmission (XRT) have limited applications in conveyed flow. MR measures minerals (currently only chalcopyrite), and XRT measures relative molecular weight distribution through conveyed material. This means that neither are elemental analysis technologies. Both work as instantaneous and continuous measurement systems and XRT has been applied successfully in particle sorting.
Choosing a product
There are variations of PGNAA in the market, some use neutron generators rather than radionuclides, and no two suppliers have the same specifications. There are some basic differences to look out for. One is the source location – under the belt is protected and much safer than a source located above the belt. Source size is important as smaller sources, while requiring less shielding, don’t result in as many gamma rays being generated, resulting in poorer measurement performance.
Detector types and configurations should also be considered. High performance detection systems with state of the art data signal processing capability will measure more of the gamma rays for the same surface area. Calibration is everything – customized calibration for composition ranges for each conveyor, and site specific calibration, provide maximum measurement performance. Maintenance requirements should be considered, particularly where there is contact between the analyser and conveyor as that means wear components and higher operating cost. It may also mean more down time, something the industry is trying to avoid for the incremental improvements required to be more competitive.
How can the technology be applied?
Generating reliable feed data does not lead to better processing results if the data is not used in some way to improve the subsequent processing. There are several ways that this data could be applied to generate better outcomes during processing. Some examples could be:
Low grade or off-spec material could be diverted to a low-grade stockpile, preventing uneconomic treatment of this material.
Improved blending performance of the feed into the plant.
On-line process control, making processing adjustments to more effectively treat pockets of high or low-grade material.
Improved management of recoveries and process performance.
Benefits of improved management of the feed quality could include reduction of costs per tonne of product. Improved forward process control could increase recoveries, reduce cost and lead to increased revenue and better margins. Good understanding of the variability, and then using this understanding to improve the processing efficiency, should always provide both financial and operational benefits.