Does All Extracted Material Really Need to Be Transported to the Surface?

Does All Extracted Material Really Need to Be Transported to the Surface?

In conventional underground mining, the general sequence is straightforward:

Ore extraction → Underground transportation → Hoisting → Surface processing → Tailings disposal

But as mines become deeper, ore grades decrease, and material handling and processing costs increase, an important question becomes increasingly relevant:

Does all extracted material really need to be transported to the surface before it is processed? The answer may be no. This is where the concept of Mine-to-Mill Integration and, more specifically, Underground Mineral Processing becomes important.

From Mine-to-Mill Integration to Underground Processing

In many mining operations, mining and mineral processing are still treated as relatively independent activities. The philosophy of Mine-to-Mill Integration aims to connect these processes and optimize the entire production chain rather than optimizing each operation independently. This integration can occur at different levels, including organizational, human and technical integration. From a technical perspective, one of the important opportunities is to bring selected mineral processing steps closer to the mining operation. This does not necessarily mean moving the entire concentrator underground.

Instead, selected processing stages can be integrated into the underground production system where they can reduce the amount of material that needs to be transported, hoisted and processed. The underlying principle is simple:

Move less mass.

Why Does Mass Movement Matter More in Deep Underground Mines?

In a conventional underground operation, material extracted from the production faces is generally transported to the surface processing plant. The entire mass of run-of-mine material therefore enters the transportation and hoisting system, even though a significant fraction may ultimately become waste or tailings. As mining depth increases, the cost and complexity of material transportation and hoisting become increasingly important. This creates an important opportunity:

Can barren or low-value material be rejected before it reaches the surface?

If the answer is yes, the mine can potentially reduce the amount of material moving through the entire downstream system. This is the basic principle behind integrating waste rejection and other processing steps underground.

Two Approaches to Underground Processing

There are two principal approaches to underground mineral processing.

1. Stationary Underground Processing

A stationary underground processing plant is designed as a long-term installation. The plant remains in a fixed location for the life of the mine, or at least for the life of a major mining level. Depending on the orebody, throughput and selected beneficiation processes, several processing stages can potentially be installed underground. Crushing and screening are relatively common examples. More advanced processing and separation technologies can also be considered where geological, spatial and operational conditions allow.

However, stationary underground processing requires significant infrastructure, including:

• Power supply
• Water
• Compressed air
• Dust control
• Ventilation
• Material handling systems
• Additional excavation and ground support

The deeper the installation, the more important excavation stability, equipment transportation and underground logistics become. Large equipment may also need to be disassembled before being transported through shafts or ramps.

2. Semi-Mobile Underground Processing

The second approach is semi-mobile underground processing. Here, selected processing stages are moved closer to the mining workings and can follow the production areas when required. This provides greater flexibility compared with a stationary plant. One particularly important application is early waste rejection. Instead of transporting the entire ROM stream to the surface concentrator, barren or low-value particles can potentially be separated underground before the remaining material continues through the mine-to-mill system. This changes the material flow. And more importantly, it can change the economics of the entire system.

Figure A: Modular semi-mobile senor-based sorting processing plant

 

The Real Value: Reject Waste Before Expensive Processing

Not every tonne extracted from a mine has the same value. If barren material can be identified and rejected before fine processing, that material does not need to pass through all subsequent processing stages. This is particularly important because comminution is one of the major energy-consuming stages of mineral processing. Grinding material that ultimately becomes waste creates cost without creating value.

Therefore, early waste rejection can potentially reduce:

• Material handling and hoisting
• Underground and surface transportation
• Crushing and grinding requirements
• Energy consumption
• Fine waste generation
• Tailings handling
• Backfill handling

At the same time, the average grade of the material reaching the surface concentrator can increase. This creates an important interaction between the mine, underground processing and the existing surface processing plant.

In general, the cost of rejecting waste can be significantly lower than the cost of milling that same material. The reduction in mass also means that less material needs to be transported and processed, potentially reducing operating costs per tonne of valuable product.

 

But There Is an Important Trade-Off

It would be too simplistic to say:

“Reject as much waste as possible underground.”

The downstream processing plant does not operate independently from changes occurring upstream. When waste rejection is introduced, the grade, throughput, particle-size distribution and mineral composition of the concentrator feed can change. These changes can affect process stability and recovery. For example, reducing mass flow can create problems if the existing processing plant is designed to operate within a particular throughput range.

Similarly, changes in feed characteristics can affect:

• Grinding and classification circuits
• Slurry pump control
• Flotation reagent dosing
• Automation
• Process stability

Autogenous and semi-autogenous grinding circuits introduce another important consideration. If coarse particles are rejected upstream, the mill may no longer receive sufficient coarse material for effective operation. In the worst case, additional coarse material may need to be obtained elsewhere, or rejected coarse particles may need to be transported back underground to the processing plant. In such cases, part of the economic benefit generated by waste rejection could be lost. Therefore, the existing comminution plant may need to be evaluated in terms of both capacity and technological requirements. This is why underground processing should not be evaluated as an isolated piece of equipment.

It should be evaluated as part of the entire Mine-to-Mill system.

 

 

Impact on Mineral Processing

The introduction of a separation technology between the extraction operation and the surface concentrator changes the characteristics of the concentrator feed. Several important properties can be affected:

Feed Grade → Particle-Size Distribution → Mineral Composition → Throughput

The surface infrastructure can also be affected. If less waste reaches the surface, smaller tailings ponds and waste dumps may be required. There can also be reductions in dust and noise emissions because less material, and potentially higher-grade material, is transported and processed at the surface. But the most important issue is the interaction between feed grade, recovery and process efficiency. This is where Figure B becomes particularly relevant.

Grade, Recovery and the Operating Point

Figure B: Relationship between grade and recovery and the impact on the current operating point

 

The figure illustrates the position of the current operating point within the grade–recovery relationship and shows how different changes in the material stream or separation performance can shift this point. The four directions shown around the current operating point represent four different effects.

Waste Rejection → Upward Movement

Rejecting barren material from the ore stream increases the grade of the material reporting to the processing plant. In this sense, waste rejection provides a mechanism for controlling and potentially increasing the feed grade while reducing the mass that needs to be processed.

This is one of the fundamental advantages of integrating separation closer to the mining operation. The objective is therefore not simply to move less material to the surface. It is to deliver a higher-value material stream to the downstream processing plant.

Dilution → Downward Movement

The opposite effect occurs when additional waste or low-value material enters the ore stream. Dilution reduces feed grade and increases the amount of material that must pass through the downstream processing stages. This is particularly important in underground mining, where dilution can increase the mass that must be transported, hoisted and processed without providing a corresponding increase in valuable mineral production. Therefore, waste rejection and dilution represent opposite directions in terms of their effect on the feed grade.

Losses → Movement to the Left

Waste rejection must also be controlled carefully. If valuable minerals are rejected together with the barren fraction, valuable mineral losses increase and recovery decreases. Therefore, the objective is not simply to maximize waste rejection. The objective is:

Reject as much barren material as possible while minimizing the loss of valuable minerals.

This is a critical point in the design and evaluation of any underground separation system. A system that rejects a large amount of waste but also rejects a significant amount of valuable mineral may not provide the expected economic benefit.

Higher Efficiency → Movement to the Right

Improving the efficiency of the separation process can move the operating point toward higher recovery. This highlights an important distinction. Increasing feed grade through waste rejection and increasing recovery through improved separation efficiency are not the same thing. Waste rejection primarily changes the characteristics of the material stream entering the downstream plant.

Improved separation efficiency, on the other hand, changes how effectively valuable minerals are recovered from that material. Both can affect the overall value recovered from the mining operation, but through different mechanisms.

Why Does the Operating Point Matter?

Once waste rejection is introduced between the mine and the surface concentrator, the processing plant receives a different feed stream.

The new stream may have:

• Higher grade
• Lower mass flow
• Different particle-size distribution
• Different mineral composition
• Different impurity characteristics

Therefore, the existing processing plant may no longer operate under exactly the same conditions for which it was originally designed. A higher-grade feed can be highly beneficial. The same concentrator capacity can potentially process a more valuable feed stream, while less barren material is sent through the comminution and downstream processing circuits. This can increase the effective productivity of the overall mining system. However, changing the feed conditions can also create new constraints. For example, a reduction in throughput may negatively affect the economics and stability of a processing plant that is designed to operate continuously at a specific capacity.

Many mineral processing units require relatively stable material flow and feed characteristics to operate effectively. If these conditions are not maintained, separation efficiency can decrease. Buffer storage may therefore be required to maintain a continuous and controlled material flow. The operating schedule of the processing plant may also need to be adjusted.

 

The Key Question: Where Does the New Operating Point End Up?

This leads to a more important question than simply asking how much waste can be rejected underground: Where does the new operating point of the entire Mine-to-Mill system end up?

The objective is to find a balance between:

Higher Feed Grade + Lower Mass Flow + Minimum Valuable Mineral Losses + Stable Processing Conditions

rather than maximizing one parameter independently. This is the fundamental implication of Figure B. The economic value of underground waste rejection cannot be determined simply by calculating the tonnes of waste removed underground. The real question is:

What is the net effect of changing the feed stream on recovery, processing capacity, process stability and overall valuable mineral production?

A higher feed grade can reduce the amount of barren material entering the plant and reduce the amount of material requiring comminution. However, if the changed feed characteristics move the processing plant away from its effective operating conditions, a reduction in recovery or process stability can offset part of the expected benefit.

Therefore, underground processing requires a system-level evaluation.

From “Moving Ore” to “Moving Value”

The traditional approach is largely based on moving material:

Mine → Surface → Processing Plant → Tailings

An integrated underground approach aims to change this philosophy toward:

Mine → Selective Processing → Reject Waste → Transport Valuable Material

The objective is not necessarily to process everything underground. In some mines, the appropriate solution may be a stationary underground crushing plant. In others, a semi-mobile waste-rejection system may provide greater value. In more advanced concepts, a combination of stationary and semi-mobile processing could move the operation toward the concept of an “Invisible Mine”—a scenario in which the required processing steps are implemented underground and waste fractions can potentially be placed underground as backfill, while only the final valuable product is transported to the surface.

However, the appropriate solution depends on:

• Orebody characteristics
• Mining method
• Mining depth
• Rock conditions
• Processing characteristics
• Infrastructure
• Material handling requirements
• Overall project economics

The key is to optimize the whole system, not one individual process.

The Question for Future Underground Mines

Perhaps the question for future underground mines should no longer be:

“How do we transport more ore to the surface?”

Instead, it may be:

“How much of the extracted material actually needs to reach the surface?”

This shift—from moving ore to moving value—is at the heart of Mine-to-Mill Integration and the growing potential of Underground Mineral Processing. And ultimately, the success of this approach depends not only on how much waste we can reject underground, but on where that decision places the entire mine-to-mill system on the Grade–Recovery relationship.