Roots Blowers for Pneumatic Conveying in Mining and Mineral Processing

Pneumatic conveying in mining and mineral processing moves dry bulk solids through enclosed pipelines between process stages. Compared with mechanical conveyors, a pneumatic system can follow complex routes, serve multiple destinations and significantly reduce material spillage. Its enclosed construction is particularly valuable when handling dusty, toxic, hygroscopic or high-purity products.

Roots blowers are well suited to many low- and medium-pressure pneumatic conveying duties because they deliver an approximately constant volumetric airflow over their operating range. However, the blower must be selected as part of the complete conveying system. Material properties, pipeline geometry, feeder leakage, altitude, filtration and operating pressure all influence the required airflow and blower capacity.

Materials Commonly Conveyed in Mining Operations

Pneumatic conveying can be applied to many dry materials encountered in mining and mineral processing, including:

• Lime and hydrated lime
• Limestone powder
• Alumina
• Silica and mineral sands
• Bentonite
• Soda ash
• Pulverized coal
• Dry mineral concentrates
• Cement and backfill binders
• Flotation reagents and powdered additives
• Filter dust and collected fines
• Dry tailings
• Fly ash used in mine backfilling
• Activated carbon
• Fine ores and micronized minerals

Not every material is suitable for every type of pneumatic conveying system. Particle size distribution, abrasiveness, moisture content, cohesiveness, friability and bulk density must be evaluated before the conveying method and air mover are selected.

Highly abrasive minerals can cause rapid wear in bends, feeders and receiving equipment. Cohesive powders may deposit inside the pipeline, while fragile materials can degrade if conveyed at excessive velocity. Hygroscopic products may agglomerate if the conveying air contains too much moisture.

Positive-Pressure Pneumatic Conveying

In a positive-pressure system, the blower introduces clean air at the beginning of the pipeline. A rotary valve, screw feeder, pressure vessel or another suitable feeding device introduces the mineral product into the pressurized airflow.

The material-air mixture then travels toward a receiving silo, process vessel or separation unit. A filter or cyclone separates the solids from the conveying air before the air is discharged.

Positive-pressure systems are commonly selected when:

• Material must be transported over a relatively long distance.
• One source supplies one or more receiving points.
• The system must operate above atmospheric pressure.
• A controlled, continuous material feed is required.
• Dust must remain inside a closed pipeline.

A roots blower used in this arrangement remains on the clean-air side of the system. Mineral particles must never pass through its rotor chamber. An inlet filter protects the blower from atmospheric dust, while a non-return valve helps prevent material or process air from flowing backwards after shutdown.

Vacuum Pneumatic Conveying

In a vacuum system, the air mover is positioned downstream of the receiving and filtration equipment. Atmospheric air enters at the material pickup point and draws the product through the pipeline.

• Vacuum conveying is useful when:
• Material must be collected from several pickup points.
• Dust leakage into the workplace must be minimized.
• Open piles, hoppers, railcars or process equipment must be unloaded.
• The system handles a hazardous powder that should remain under negative pressure.
• Flexible suction connections are required.

A vacuum roots blower can create the pressure differential required for suitable mineral-powder duties. However, effective separation is essential. A primary receiver and a correctly sized secondary safety filter should be installed upstream of the blower so that abrasive particles cannot enter the machine.

Vacuum capability decreases as the required absolute suction pressure becomes lower. Deep-vacuum duties may require a different vacuum technology or a staged system rather than a conventional roots blower.

Combined Vacuum and Positive-Pressure Systems

Some mining plants use a combined system in which material is collected under vacuum, separated in an intermediate receiver and subsequently conveyed under positive pressure. This arrangement can provide flexible pickup while allowing material to be delivered over a longer distance.

The vacuum and pressure sections must be evaluated separately. Each section may have different airflow, pressure and filtration requirements. A combined system should not be sized by adding the two pressure differentials without analysing the actual process arrangement.

Dilute-Phase and Dense-Phase Conveying

Dilute-Phase Conveying

In dilute-phase conveying, particles are suspended in a relatively high-velocity gas stream. The solids loading ratio is usually lower than in dense-phase transport, and the material generally travels as a dispersed suspension.

Roots blowers are frequently used for dilute-phase mineral conveying because these systems often require substantial airflow at a moderate pressure differential.

Dilute-phase conveying is generally appropriate for:

• Fine, dry and free-flowing powders
• Moderate conveying capacities
• Continuous operation
• Materials that can tolerate relatively high velocity
• Systems in which pipeline cleanliness is important

Its main disadvantages are increased pipeline wear, particle degradation and higher filter loading when unsuitable velocities are used.

Dense-Phase Conveying

Dense-phase conveying uses lower gas velocity and a higher concentration of solids. Depending on the product and system design, material may travel as plugs, dunes or a moving bed.

This method can reduce wear and product degradation, but it requires materials with suitable air-retention or permeability characteristics. Dense-phase systems may also require substantially higher pressure than a conventional roots blower can provide.

A roots blower can serve certain low-pressure dense-phase or fluidized conveying systems, but it should not automatically be specified for every dense-phase application. High-pressure pressure-vessel systems commonly require a screw compressor or another compressor technology capable of the necessary discharge pressure.

Conveying-Air Velocity

The superficial conveying-air velocity is calculated from the actual volumetric flow at the pipeline condition:

The required minimum velocity depends on the material, particle size, particle density, pipeline orientation and solids loading. It should be established from reliable conveying tests, previous plant data or a validated design model.

An excessive safety margin should not be added indiscriminately. Higher velocity increases pressure loss, power consumption, particle impact energy and wear. Conversely, insufficient velocity can cause saltation, material deposition, unstable conveying and complete pipeline blockage.

Glossary

Bend erosion: Material wear caused by particle impacts at pipeline direction changes.

Bend radius: Centerline radius of a pipe bend in the conveying route.

Conveying-velocity margin: Difference between operating gas velocity and the validated minimum conveying velocity.

Dense-phase conveying: Transport at lower gas velocity and higher solids concentration.

Dilute-phase conveying: Transport at higher gas velocity and lower solids concentration.

Ore concentrate: Mineral-rich product separated from gangue during processing.

Pickup velocity: Gas speed needed to entrain solids at the pipeline feed point.

Solids loading ratio: Solids mass flow divided by conveying-gas mass flow.