Roots Blowers for Heap-Leach and Bioleaching Aeration

Heap-leach and bioleaching aeration supplies air through ore beds to support the selected leach process, subject to pad and distribution design. A leaching solution is distributed over the surface, percolates through the ore and dissolves the target metal. The resulting pregnant leach solution is collected beneath the heap and transferred to downstream metal-recovery equipment.

Forced aeration is especially relevant to the treatment of sulfide ores and bio-assisted heap-leaching processes. Low-pressure air introduced through distribution pipes beneath the heap supplies oxygen for mineral-oxidation reactions and supports the activity of acidophilic microorganisms.

Roots blowers can provide the continuous low-pressure airflow required by appropriately designed heap-aeration systems. Their positive-displacement characteristic enables them to maintain a comparatively stable inlet volume as the resistance of the heap and distribution network changes. However, not every heap-leach operation requires forced aeration, and a blower should only be installed where metallurgical testing demonstrates an oxygen requirement and a measurable process benefit.

Difference Between Heap Leaching and Heap Bioleaching

Heap leaching is a broad term covering processes in which a chemical solution dissolves valuable components from stacked ore. Depending on the mineralogy, the lixiviant may be an acidic, alkaline, chloride-bearing or cyanide-based solution.

Heap bioleaching is a more specific process in which microorganisms facilitate or accelerate mineral oxidation. In sulfide-ore treatment, microorganisms can regenerate ferric iron and oxidize reduced sulfur species. These reactions help break down the sulfide mineral structure and make the target metal more accessible to the leaching solution.

A simplified microbial oxidation reaction is:

The regenerated ferric iron acts as an oxidant for certain sulfide minerals. Oxygen is therefore not simply added as a general process gas; it participates indirectly and directly in the biological and chemical oxidation mechanisms.

Forced aeration is most strongly associated with sulfide heaps, particularly copper-sulfide bioleaching and some bio-oxidation processes. Conventional oxide-ore heap leaching may rely on natural gas exchange and may not benefit from a forced-air system.

Why Oxygen Supply Matters

Microorganisms involved in heap bioleaching generally require oxygen as the terminal electron acceptor for oxidation reactions. If oxygen transport into the heap is lower than the biological and chemical oxygen demand, oxygen-depleted zones can develop.

Insufficient oxygen may result in:

• Reduced ferrous-iron oxidation
• Lower ferric-ion regeneration
• Slower sulfide-mineral oxidation
• Lower microbial activity
• Uneven reaction fronts
• Longer leaching cycles
• Reduced metal recovery
• Incomplete utilization of the stacked ore

Natural convection can introduce some air into a permeable heap, but it may not provide a sufficiently controlled or uniform oxygen supply. Forced aeration increases the oxygen available at the base of the heap and establishes an upward gas flow through the ore bed.

Nevertheless, oxygen availability is only one of the controlling factors. Metal recovery may still be limited by mineralogy, passivation, particle size, solution distribution, temperature, microbial population or diffusion through mineral reaction layers. Increasing airflow beyond the useful oxygen demand does not necessarily improve extraction.

Typical Heap-Aeration Arrangement

A forced-aeration system generally consists of:

• Atmospheric-air inlet
• Inlet filter
• Roots Blower or another low-pressure air mover
• Inlet and discharge silencers
• Main air header
• Flow-control valves
• Branch manifolds
• Perforated aeration laterals
• Non-return and isolation valves
• Pressure, flow and temperature instruments
• Leach-heap monitoring equipment

Aeration laterals are normally installed near the base of the heap before ore stacking. They may be manufactured from corrosion-resistant polymeric pipe and arranged in zones so that airflow can be controlled according to the leaching stage.

The blower draws relatively clean atmospheric air and discharges it into the aeration manifold. The air then travels through the laterals and enters the void spaces between the ore particles. It moves upward through the heap while the leach solution generally moves downward.

The blower should remain on the clean-air side. It should not normally handle acidic heap off-gas or entrained leach solution. If a process requires gas recirculation, the gas composition, acid mist, moisture, temperature and corrosion potential must be evaluated separately.

Oxygen Mass Balance

The oxygen introduced with dry atmospheric air can be estimated from:

Only part of this oxygen is consumed by the heap. Oxygen utilization can be expressed as:

If the required oxygen-consumption rate and expected utilization are known, preliminary airflow can be estimated as:

This calculation provides an oxygen-based airflow estimate. It must be checked against the minimum flow necessary for acceptable air distribution throughout the heap.

A design based only on total oxygen demand can be misleading if air bypasses reactive zones through preferential channels. Uniform distribution is often as important as total blower capacity.

Airflow per Unit Heap Area

Aeration requirements are frequently expressed relative to the plan area of the active leach zone:

Alternatively, a mine may define airflow per tonne of stacked ore:

These ratios are useful for scaling test results, but they should not be transferred blindly from one heap to another. Heap height, agglomeration, mineralogy, particle-size distribution and permeability can change the relationship between applied airflow and oxygen delivery.

Pressure Drop Through the Heap

The blower must overcome resistance in the inlet filter, piping, valves, manifolds, aeration laterals and ore bed.

The required blower pressure differential is:

The pressure drop through a uniform packed bed may be approximated using the Ergun equation:

Glossary

Bioleaching: Mineral recovery using microorganisms to promote chemical dissolution.

Heap leaching: Metal recovery by applying solution to a permeable ore pile.

Heap permeability: Capacity of stacked ore to transmit gas or leach solution.

Heap-irrigation uniformity: Consistency of liquid application across the leach-pad surface.

Irrigation rate: Liquid application flow per unit heap surface area.

Leachate: Solution draining from a heap after contact with ore.

Oxygen transfer: Movement of oxygen from supplied air into liquid or moist ore.

Pore-gas oxygen concentration: Oxygen fraction in gas occupying voids within the ore heap.