How precious metal refining furnaces work
A precious metal refinery recovers gold, silver and platinum group metals from doré, scrap jewellery, electronic scrap, catalysts and sweeps. Feed is melted in induction or gas-fired crucible furnaces at 1,000 to 1,200°C, sweeps and filter cakes are incinerated, and metals are refined by chlorination, electrolysis or wet chemical routes with aqua regia, nitric acid and hydrochloric acid. These steps release combustion gas and metal fume from furnaces and incinerators, and acid fume containing NOx, HCl and chlorine from chemical refining. Furnace and incinerator gas is cooled and passes a bag filter, then often a wet scrubber; acid fume goes to packed scrubbers. ID fans draw each system.
Where the FD fan sits
- Inlet filter
- FD (combustion air) fan
- Air control valve
- Burner
- Furnace or incinerator
The blower is placed beside the furnace or incinerator and supplies the burner through a control valve. On a sweep incinerator it may supply both burner air and secondary air for burning the organic content of the feed. Pressure required is typically 150 to 500 mmWC. Clean air from outside the furnace room is preferred. The air line is kept short.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Clean air; refinery room air may contain fume |
| Pressure at the fan outlet | Typically 150 to 500 mmWC |
| Turndown | Batch melting and incineration with frequent changes |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
FD Fan selection for precious metal refining furnaces
| Blade form | Backward curved or radial blade, small direct-driven blower |
| Material of construction | Mild steel; stainless steel where room air carries acid fume |
| Drive and arrangement | Direct mounted |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | Modulating valve or variable speed drive with burner ratio |
| Also specified | Inlet filter and inlet duct from clean air |
What this FD fan has to cope with
Acid fume in room air
Refinery buildings that house nitric, hydrochloric and aqua regia processes often carry acid fume in the room air, even with good extraction. A blower drawing this air corrodes, loses balance and flow, and its bearings fail early. Ducting the inlet from clean outside air, or building the blower in stainless steel, prevents this.
Incinerator air for complete burn-out
Sweeps and filter cakes contain organic matter that must be fully burned to release the metals. The blower must supply enough secondary air to burn organics without blowing fine metal dust out of the hearth. Staged air, measured flow and a controlled burn-out cycle help achieve both aims, so that the ash leaving the incinerator is clean enough for efficient metal recovery.
Batch firing range
Melting and incineration are batch processes with high and low firing. A blower with stable low-flow performance and modulating control gives steady firing. Oversized blowers throttled by a damper give coarse control at low fire, which leads to uneven melting and fuel waste. Matching the blower to the burner and using ratio control keep the flame steady through each heat.
Energy and control
Fuel-fired melting and incineration in a refinery run in batches. Modulating control on the blower avoids wasted air and fuel. Converting melting to induction removes the combustion air duty but leaves the fume extraction duty unchanged. Small blowers on fuel-fired units are often oversized, and matching them to the actual burner duty, or adding speed control, reduces both power use and noise in the furnace room.
ID fan and FD fan in precious metal refining furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After filters and scrubbers, before the stack | Beside fuel-fired furnaces and incinerators |
| What it handles | Cleaned furnace gas or scrubbed acid fume | Ambient air |
| Temperature at the fan | 40 to 65°C, saturated | Ambient |
| Main design concern | Metal recovery and acid corrosion | Batch firing and clean inlet air |
| Control | Hood and scrubber suction | Burner ratio |
See also the Induced Draft (ID) Fan for precious metal refining furnaces.
Questions buyers ask
Does an induction furnace in a gold refinery need an FD fan?
No. Induction melting has no burner, so there is no combustion air fan. The furnace still needs fume extraction, which is handled by the ID fan system with a filter. Many refineries have moved to induction melting for this reason, leaving only incinerators and cupellation furnaces with combustion air blowers.
What does the FD fan do on a sweep incinerator?
It supplies burner air and, often, secondary air for burning the organic content of the sweeps and filter cakes. The air must burn the organics completely without carrying fine metal dust out of the hearth. Ash quality and metal recovery depend on getting this air right, so the blower is controlled from the incinerator temperature.
What pressure does a precious metal furnace blower need?
Typically 150 to 500 mmWC, depending on the burner and furnace type. The burner maker's data gives the required air pressure at full firing. We select the blower for that pressure plus the valve and line losses, with a modest margin, and can confirm the duty from the existing blower nameplate where one is being replaced.
Information needed to quote an FD fan for precious metal refining furnaces
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Fuel-fired units. Furnaces and incinerators fired by gas or oil, and their burner data.
- Incinerator air. Secondary air requirement for sweep incineration.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.