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 ID fan sits
- Furnace, incinerator or reactor hoods
- Cooling or quench
- Bag filter
- Wet scrubber
- ID fan
- Stack
On the furnace and incinerator line the fan sits after the bag filter and, where acid gases are present, after the wet scrubber, so it handles saturated air. On acid fume lines it sits after the packed scrubber. Suction is typically 200 to 400 mmWC. Placing the fan downstream of all collection avoids losing metal-bearing dust in the fan and keeps it in the recovery stream.
Gas at the ID fan
| Air temperature, normal | 40 to 65°C after the scrubber, saturated |
| Maximum or upset | Up to 120°C if the scrubber water supply fails |
| Dust at the fan | Traces of metal-bearing fume after the filter and scrubber |
| Moisture and dew point | Saturated after the scrubber; droplet carry-over possible |
| Corrosive species | NOx, HCl, chlorine and acid mist |
| Density at the design temperature | About 1.08 kg/m³ at 55°C, 90% 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. |
ID Fan selection for precious metal refining furnaces
| Blade form | Backward curved or radial, in corrosion-resistant construction |
| Material of construction | FRP, polypropylene or rubber-lined steel for acid fume; SS 316 or higher alloy for mixed duties |
| Drive and arrangement | Belt driven or direct coupled, with the motor outside the gas stream |
| Bearings and cooling | Bearings outside the gas stream; acid-resistant shaft seal |
| Flow control | Variable speed drive on hood or scrubber suction |
| Also specified | Casing drain returning to the recovery system, access doors and mist eliminator before the fan |
What this ID fan has to cope with
Loss of metal through the stack
Fine precious metal fume that passes the filter is lost value. Correct filter design, gas cooling to the right temperature and a fan on the clean side keep the losses low, and any deposit in the fan is recovered when it is cleaned. Regular cleaning also recovers metal.
Acid fume corrosion
NOx, HCl and chlorine from chemical refining attack steel quickly. FRP, polypropylene or rubber-lined fans, with mist eliminators to stop acid droplets, resist this attack. The material is chosen from the actual acids, their concentrations and the temperature, since plastics lose strength with heat and some lose resistance to strong oxidisers. Stainless steel is not suitable for wet chloride and chlorine duty.
Scrubber water failure
If scrubber water fails, hot or acidic gas reaches the fan. Temperature interlocks and materials able to withstand the upset protect a plastic or lined fan. Plastic fans in particular lose strength quickly above their temperature limit. A low water flow alarm, a high temperature trip that stops the hot process, and an emergency water supply to the quench protect the fan and scrubber.
Energy and control
Refineries often run batch processes, with furnaces and reactors in use for part of each shift. Variable speed drives and dampers on individual hoods cut fan power when processes are idle, while keeping hoods under suction for safety. Scrubbers add significant pressure drop, so the fan should be matched to the actual system resistance.
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 Forced Draft (FD) Fan for precious metal refining furnaces.
Questions buyers ask
What type of fan is used for acid fume in a gold refinery?
Fans in FRP, polypropylene or rubber-lined steel are used for fume containing nitric acid, hydrochloric acid, NOx and chlorine. They are placed after the scrubber and usually fitted with a mist eliminator to stop acid droplets reaching the impeller. The material is selected from the actual acid mix and temperature.
How is precious metal recovered from the extraction system?
Most fume is captured in the bag filter or scrubber. Deposits in the ducts and fan, and the casing drain, are collected and returned to the refinery's recovery stream. A fan placed after collection keeps losses small. Fan deposits are worth collecting carefully at each cleaning.
Can one ID fan serve furnaces and acid fume?
It is usually better to keep them separate. Furnace and incinerator gas needs a bag filter for dust and metal recovery, while acid fume needs a wet scrubber and corrosion-resistant fan. Mixing them complicates both treatment and materials. Separate systems also let each fan be sized correctly.
Information needed to quote an ID fan for precious metal refining furnaces
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Processes. Furnaces, incinerators and chemical processes on each extraction line.
- Gas composition. Acids, fume and temperatures at each hood and at the fan.
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.