Optimizing Air Purification: Lessons Learned and Best Solutions for Gold Mining Company

In this case, we want to describe common mistakes that occur during the integration of dust and/or gas cleaning equipment into the production process. Even when a well-developed project is in place—one that accounts for dust loads, cleaning efficiency, and other factors—it sometimes happens that such a project is not fully trusted. Instead, alternative solutions are implemented based on incorrect assumptions and expectations.

As a result, the cleaning system does not operate as originally intended, leading to the need to investigate root causes and make adjustments.

This issue is particularly common when determining the air volume capacity of the system. In most cases, the specified values are underestimated, but occasionally they are overestimated—examples of both scenarios will be provided. Another frequent mistake involves selecting the wrong filtration method. For instance, using a cartridge dust collector instead of a cyclone and baghouse can cause the filter to "suffocate" from dust buildup and stop functioning properly.

All these mistakes lead to one consequence — financial losses.

Calculation of Equipment Parameters for the Flotation Concentrate Drying Unit

According to the project, 0.64 tons per hour of flotation concentrate with a moisture content of 20% should be dried to a residual moisture content of no more than 5% to prevent product freezing in winter and reduce transportation costs.

Moisture to be removed from the flotation concentrate: 120.0 kg/h
Assuming the intensity of the drying drum volume per moisture equals 0.120 m³/kg (by analogy with clay), the necessary internal volume of the drum for the flotation concentrate, excluding the filling with partitions (8-10%), will be 2.4 m³.

The customer has a drying unit "Drum Dryer BN1-01".
Geometric volume of the drum: 3.14 m³
Thus, the drum volume is sufficient to remove the calculated amount of moisture.
As fuel, we use diesel fuel.
Assuming the combustion of diesel fuel is carried out in a diesel burner L 400 (customer's decision), which provides the calculated combustion parameters.

From the fuel combustion calculation:

• Excess air coefficient: 1.2
• Theoretical air consumption 11,13 nm³/kg
• Actual amount of dry air 13,14 nm³/kg
• Amount of atmospheric air (with a moisture content of 10 g/kg) - 13,4 nm³/kg

Amount and composition of complete combustion products:


Heat Content of Combustion Products – 3152,7 kJ/Nm³
The drying of the concentrate is carried out with gases at a temperature of 500°C, for which the combustion products are diluted with atmospheric air at 20°C.
The amount of added air is determined by solving the heat balance equation where efficiency of the furnace - 45 nm³/kg
Total amount of air for combustion and dilution of flue gases 58,7 nm³/kg
Overall air consumption coefficient: 5,23
Moisture content of flue gases, diluted with atmospheric air 34,4 g/kg of dry gas
Moisture content of gases at the final point equals 205 g/kg dry gas.

Calculation of drying flotation concentrate
Dry gas consumption for the theoretical drying process 703,4 kg of dry gas/h
Heat consumption for material heating 46080 kJ/h
Heat content losses will be equal 95,9 kJ/ kg dry gas
Actual gas consumption for drying will be equal 918,8 kg dry gas/h
Heat consumption for drying 478740.8 kJ/h
Considering the furnace efficiency of 0.9, the required heat for drying is 531,934.2 kJ/h
Diesel fuel consumption 12 kg/h
Amount of air needed for combustion 160.8 nm³/h
Amount of air needed for diluting flue gases 526.4 nm³/h
Amount of gases exiting the drying drum 1070.4 kg/h
Density of outgoing gases 0.8286 kg/m³
Actual volume of moist gases leaving the drying drum 1291.8 m³/h
Gas velocity at the drum outlet 0.57 m/s
Drum filling coefficient is assumed to be 0.2.
Gas velocity at the drum outlet below the permissible limit - 2 to 3 m/s, so excessive dust carryover is not expected. Dust carryover from the drying oven is assumed to be 5% of the amount of dried material.
Dust removal from gases is carried out in a cyclone and a baghouse.
Cyclone with a diameter of 300 mm resistance coefficient 180.
Due to suction, the mass of gases before the cyclones increases by 10%, the temperature decreases by 10°C = 1420.99 m³/h
Resistance of one cyclone is assumed Δp = 70 mm water column.

Cyclone performance will be:
Installation of a group of 2 cyclones with a diameter of 300 mm is assumed.
Installation of a Baghouse with a capacity of up to 1920 m³/h with a filtering surface of 20 m² is assumed.
Pressure drop in the group of cyclones is 700 Pa, in the baghouse 1500 Pa, in the gas ducts 200 Pa. Total pressure drop 2400 Pa.
Efficiency of gas cleaning in the cyclone is 80%, in the baghouse – 95%. The total efficiency of dust removal will be 99.0%
Material losses and dust emissions with outgoing gases will be 0.09 g/s.

IMPLEMENTATION OF A MULTI-STAGE AIR CLEANING SYSTEM

• Cyclone and Baghouse Installation: To enhance air cleaning efficiency and reduce the load on the Venturi scrubber, Torch-Air recommends installing a cyclone and baghouse as the first filtration stages.
• Cyclone: Diameter 300 mm, efficiency 80%, pressure drop 700 Pa
• Baghouse Filter: Capacity 1920 m³/h, filtration area 20 m², efficiency 95%, pressure drop 1500 Pa
• Total dust removal efficiency: 99% (combined cyclone + baghouse)

SELECTION OF AN OPTIMIZED VENTURI SCRUBBER
Torch-Air proposed a Venturi scrubber designed for handling high dust loads, fine particles (75 microns and smaller), and combustion byproducts.
Wet Scrubber specifications:

• Airflow Capacity: Up to 2600 m³/h
• Gas Temperature: ≤ 150°C
• Cleaning Efficiency: ≥ 85%
• Construction Material: Stainless steel (recommended), carbon steel (alternative).
• Discharge System: Rotary valve + water discharge (condensate removal from the water curtain).
• Additional Option: Protective lining for increased resistance to wear and corrosion.

Material Considerations for Longevity
Torch-Air strongly recommended using stainless steel for the scrubber due to the presence of sulfur and abrasive particles, which could cause rapid deterioration of carbon steel.
Alternative options: Carbon steel (less durable but more cost-effective).

Performance Optimization through System Adjustments
Torch-Air suggested adjustable settings for the Venturi scrubber to optimize pressure drop and water flow rate, allowing for better adaptability to fluctuating dust loads.
Automated monitoring of differential pressure and gas flow for proactive maintenance.

The scrubber was installed without the initial stages, such as the cyclone and baghouse. The reasoning was that the cyclone and baghouse supplier was delayed in shipping, and to avoid losing time and ensure on-time project completion, it was decided to use the available equipment in stock. Unfortunately, that was only the scrubber.

The scrubber experienced rapid wear because the abrasive wear and sulfur impact were high, while the scrubber was made of carbon steel without additional protection.

The client requested a stainless steel scrubber and ultimately decided to install a cyclone and baghouse before the scrubber. In the drawing, the undamaged carbon steel scrubber components are marked in green.

It is evident that the primary wear occurred in the Venturi tube. This is not surprising, as the Venturi tube, due to its fundamental positioning and the effect it creates inside, bears the highest load.