How copper anode and refining furnaces work
An anode furnace, usually a tilting rotary vessel, takes molten blister copper from the converter, or melts scrap in a refining furnace, and fire-refines it at about 1,150 to 1,250°C. The cycle has three stages: oxidation, where air is blown through tuyeres to remove sulphur and impurities; reduction or poling, where natural gas, LPG, diesel or ammonia lowers the oxygen content; and casting into anodes. The burner holds bath temperature throughout. Off-gas leaves the mouth at 1,100 to 1,250°C and goes through a post-combustion chamber, where unburnt reductant is burned, then a water-jacketed duct or evaporative cooler, a bag filter and the ID fan, often followed by an SO₂ scrubber or a link to the acid plant gas system.
Where the ID fan sits
- Furnace mouth and hood
- Post-combustion chamber
- Water-jacketed duct or evaporative cooler
- Bag filter
- ID fan
- SO₂ scrubber
- Stack
The fan sits after the bag filter, so it handles gas that has been cooled to suit the filter bags and stripped of most copper oxide, arsenic and lead fume. Its suction covers the hood, the post-combustion chamber, the cooler and the filter, typically 300 to 500 mmWC. Placing it on the clean side protects the impeller from sticky metal fume, but it also means the fan is the first equipment to meet acid condensate if the gas cools too far.
Gas at the ID fan
| Gas temperature, normal | 120 to 180°C after the bag filter |
| Maximum or upset | Up to 250°C if the cooler sprays fail, limited by the bag protection damper |
| Dust at the fan | Fine copper, lead, zinc and arsenic oxides; usually below 20 mg/Nm³ after the bag filter |
| Moisture and dew point | Water vapour from cooler sprays and combustion; SO₃ raises the acid dew point to roughly 120 to 150°C |
| Corrosive species | SO₂ and SO₃ during oxidation; traces of chlorides from scrap charges |
| Gas volume swing | Highest during oxidation and poling, lowest during casting and holding |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% 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 copper anode and refining furnaces
| Blade form | Backward curved, on the clean side of the bag filter |
| Material of construction | Mild steel casing with an acid-resistant internal coating, or SS 316 liners where the gas runs near its dew point; impeller in high strength steel or SS 316 |
| Drive and arrangement | Direct coupled, arrangement 8, casing on sliding supports |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc; no water cooling needed at this temperature |
| Flow control | Variable speed drive on hood suction, with a fast inlet damper for stage changes |
| Also specified | Casing drain at the lowest point, insulated casing to stay above the dew point, and an inspection door |
What this ID fan has to cope with
Acid condensation at low load
During casting and holding the gas flow drops, the cooler overshoots, and the gas reaching the fan can fall below its acid dew point. Sulphuric acid then attacks the casing and the back plate. Insulating the casing, controlling cooler outlet temperature rather than spray rate, and using SS 316 at the cold spots stop the damage.
Fugitive fume during poling
Reductant injection produces a sudden surge of combustible gas and black smoke at the furnace mouth. If the fan cannot follow the surge quickly, fume escapes from the hood into the converter aisle and the crane cab. A fan sized for the poling peak, with a drive that ramps fast on a hood pressure signal, keeps the capture complete.
Bag failure and metal fume at the fan
A burst bag lets fine copper, lead and arsenic oxides through to the clean side. They coat the blades unevenly in a thin, hard layer and vibration rises within days. Broken bag detection on the filter outlet, a vibration trip on the fan bearings and an access door large enough to clean the wheel limit the damage and the downtime.
Post-combustion upsets
If the post-combustion chamber does not burn the reductant fully, unburnt hydrocarbons and soot pass into the cooler and filter, where they can ignite or blind the bags. The fan duty must include the air admitted for post-combustion, so that oxygen is always available, and the fan should not be throttled to save power during poling.
Energy and control
An anode furnace works in batches of several hours, with oxidation and poling at high gas flow and long casting periods at low flow. A fixed-speed fan throttled by a damper spends most of the cycle wasting power. A variable speed drive linked to hood suction, with speed set points for each stage of the cycle, typically gives large savings and also helps keep the gas warm enough to stay above the acid dew point.
ID fan and FD fan in copper anode and refining furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the scrubber or stack | Beside the furnace, feeding the burner |
| What it handles | Cooled off-gas with SO₂ and metal fume traces | Clean ambient air |
| Temperature at the fan | 120 to 180°C | Ambient |
| Main design concern | Acid dew point and stage-dependent volume | Stable burner air across the cycle |
| Control | Hood suction | Air header pressure and burner ratio |
See also the Forced Draft (FD) Fan for copper anode and refining furnaces.
Questions buyers ask
What temperature does the ID fan on a copper anode furnace see?
After the cooler and bag filter the gas is usually 120 to 180°C. The furnace gas starts above 1,100°C, but it is cooled in a water-jacketed duct, an evaporative cooler or by dilution air before the filter, and the fan sits after the filter. The impeller is checked for the upset temperature if the cooler fails.
Why does the ID fan casing corrode on a copper refining furnace?
The off-gas carries SO₂ and some SO₃ from the oxidation stage. When the gas cools below its acid dew point, typically during low-flow casting periods or after a stop, sulphuric acid condenses on the casing and back plate. Insulation, stable cooler control and stainless steel at the cold spots prevent most of it.
Should the anode furnace fan be sized for oxidation or poling?
For whichever stage gives the highest gas volume at the fan inlet, including post-combustion air, cooler spray water and air leakage at the hood. On many furnaces that is poling with natural gas or diesel. The fan then runs at reduced speed for the rest of the cycle on a variable speed drive.
Information needed to quote an ID fan for copper anode and 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.
- Refining cycle. Duration and gas volume of the oxidation, poling, casting and holding stages, and the reductant used.
- Gas cooling and treatment. Type of cooler, filter design temperature, and whether the gas goes to a scrubber or an acid plant.
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.