Accurate chemical analysis has long been an integral part of any mining operation throughout the whole process. Whether it is about early exploration steps and mine development, ore sorting, blending and beneficiation during the extraction phase, or waste management – chemical analyses are an essential, unavoidable step in the process. Historically, a few methods were utilized by the mining industry. Wet chemistry, even though still regarded as a golden standard, is moving to the backstage, mainly due to the very long feedback loops. Handling of expensive and dangerous acids is another important factor in reducing the footprint of wet chemistry in the analytical methods suite utilized by mining companies.
Today bulk X-ray fluorescence (XRF) analyses are by far the main technique used in day-to-day operation to closely monitor the chemical composition at the mine, during the beneficiation process and for the quality check of the end product. Another method, gradually gaining popularity is on-line neutron analyses, offering cross-belt chemical analyses of an ore with extremely short feedback loops.
Can chemical analyses alone ensure the process efficiency and maximize the recovery rates?
The answer, of course, is NO. Another equally important parameter affecting every step in any ore-to-metal process is mineralogy. Whilst both, chemistry and mineralogy define the ore grade, the recovery rate is directly determined by the mineralogy.
Different minerals have different material properties and therefore require different treatment during ore beneficiation.
Failure to account for the mineralogy of an ore deposit will reduce the recovery rate and can result in tens of millions loss during mine operation.
Let’s take a classical example of copper ore sulfides and oxides. Copper sulfides are best separated via flotation, however, the recovery of the oxidized part of the ore during this process is very poor. Heap leaching, on another hand, does provide a higher recovery rate for the copper oxides.
Another well-known example of “beneficiation strategy determined by mineralogy” is Ni recovery from different modifications of pyrrhotite, known as mpo and hpo pyrrhotite. The latter is known to have lower magnetic susceptibility and be more reactive, does causing issues during magnetic separation and requiring different treatment during flotation. These are just a few out of many industry-specific examples.
A “common challenge” for any mining operation is the presence of soft minerals (such as clays) and/or hard minerals (such as quartz). Soft minerals will cause machinery blockage, reduce flotation efficiency or increase the acid consumption during leaching. Hard minerals will complicate the crashing and milling processes, speed-up mill deterioration. Accurate monitoring of raw material mineralogy before the comminution and ore beneficiation steps allows us to account for these “common challenges” and put effective counteractive measures in place.
Good understanding and control over the mineralogy of a mine operation not only helps to set-up an overall strategy for a run-of-mine post-processing, it also allows to optimize beneficiation steps such as:
- Selection of the correct reagents for separation
- Usage of the optimal amount of reagents
- Definition of the best blending ratio to ensure consistent product quality
- Determination of mineralogical domains to efficiently mine the ore body.
How mineralogy can be accessed?
Ore mineralogy can be accessed by various methods. The main three methods are: microscopy, x-ray diffraction (XRD) and near-infrared spectroscopy (NIR). Each of these techniques has its benefits and drawbacks.
NIR is an easy-to-use technique capable of identification and quantification of most minerals. The quantitative mineralogical analyses using NIR requires a statistical model using a set of reference samples (~100 or above) with the mineralogy quantified by a primary technique, like XRD. Therefore, NIR quantification lacks the accuracy and flexibility of XRD. However, a major benefit of NIR is design simplicity. This makes NIR very portable and easy to implement for online analyses. Therefore, NIR is very convenient for exploration work in the field as well as for monitoring mineralogy on a conveyor belt.
XRD and microscopy do provide more accurate quantitative results. However, historically, both, XRD and microscopy, were regarded as lab-based, infrastructure-demanding techniques requiring an expert operator. To a large extent this is still valid for microscopy technique today. Microscopy does have its place, but typically is used in a lab environment as an R&D tool.
Modern XRD equipment can be very compact, rigid in design, and suitable for use by inexperienced personal directly at the mine side (eg container lab) or next to the process line, with or without automated sample feed, with the direct fast process feedback (5-10 min) to an operator and/or LIMS. These make XRD the most suitable sensor for the fast and accurate mineralogy check at every step of mine operation, starting from exploration and mine development to the process control during ore beneficiation and quality check of a final product.
Written by: Dr. Olga Narygina – Malvern Panalytical (www.malvernpanalytical.com)