How sodium dichromate roasting kilns work
Sodium dichromate is made by roasting ground chromite ore with soda ash in a rotary kiln at 1,000 to 1,150°C in an oxidising atmosphere, converting the chromium to soluble sodium chromate. Older plants add lime as a diluent; modern no-lime plants recycle leached residue instead. The kiln is fired from the discharge end with natural gas, fuel oil or pulverised coal, with substantial excess air to supply the oxygen the reaction needs. The gas leaves the feed end at 600 to 800°C carrying dust of ore, soda ash and sodium chromate, which is a hexavalent chromium compound. It passes a dust chamber, a waste heat boiler or gas cooler, and an ESP or bag filter before the ID fan and stack. The roasted product is cooled and leached to give chromate solution.
Where the ID fan sits
- Rotary roasting kiln
- Dust chamber
- Waste heat boiler or cooler
- ESP or bag filter
- ID fan
- Stack
The fan sits after the ESP or bag filter, where the gas has been cooled to about 180 to 250°C. Placing it there keeps the bulk of the chromate dust, which is recovered and returned to the process, away from the fan. The suction covers the kiln, dust chamber, cooler and collector, typically 300 to 500 mmWC. The fan holds the kiln feed end at slight suction so chromate dust does not escape at the seals.
Gas at the ID fan
| Gas temperature, normal | 180 to 250°C after the dust collector |
| Maximum or upset | Up to 350°C if cooling fails |
| Dust at the fan | Low; fine sodium chromate and ore dust, toxic |
| Moisture and dew point | Normal combustion moisture; high excess air |
| Corrosive species | Alkaline and chromate dust; SO₂ with oil or coal firing |
| Density at the design temperature | About 0.72 kg/m³ at 220°C, 60% 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 sodium dichromate roasting kilns
| Blade form | Backward inclined, with wear strips on leading edges |
| Material of construction | Mild steel with wear-resistant plate; stainless steel where wetted |
| Drive and arrangement | Direct coupled, arrangement 8, with casing on sliding supports |
| Bearings and cooling | Cooling disc and heat shield |
| Flow control | Variable speed drive on kiln feed end draft |
| Also specified | Gas-tight casing, double shaft seal, and inspection door with sealed gasket |
What this ID fan has to cope with
Hexavalent chromium leakage
Any dust escaping from the fan casing or shaft exposes workers to a carcinogen. Gas-tight joints, a double shaft seal and maintenance procedures that avoid opening a dirty fan in service are essential.
Sticky dust and build-up
Soda ash and sodium chromate become sticky at high temperature and absorb moisture when cool. Dust passing a weak collector builds up on the impeller and throws it out of balance. A good collector and washable blade forms keep it clean.
Erosion from ore dust
Chromite ore is hard. Dust passing the collector erodes blade inlets and the inlet cone. Wear strips and liners at those points extend life. They also make repairs possible without replacing the whole impeller.
Energy and control
Roasting kilns run continuously at high excess air, so the gas volume is large compared with the fuel fired. Speed control on kiln draft saves power compared with damper control. Holding oxygen at the level the reaction needs, rather than above it, cuts both fan power and fuel. The waste heat boiler, where fitted, also benefits from a steady gas flow.
ID fan and FD fan in sodium dichromate roasting kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP or bag filter, before the stack | At the kiln discharge end, feeding the burner |
| What it handles | Kiln gas with chromate dust | Ambient air |
| Temperature at the fan | 180 to 250°C | Ambient |
| Main design concern | Containment of hexavalent chromium dust | Stable primary air and flame shape |
| Control | Kiln feed end draft | Measured air flow |
See also the Forced Draft (FD) Fan for sodium dichromate roasting kilns.
Questions buyers ask
Why must a chromate kiln ID fan be gas-tight?
The dust contains hexavalent chromium, a carcinogen. Leaks at the casing or shaft spread it into the workplace, so the fan is built with sealed joints and a double shaft seal.
Why is the gas volume high in a sodium dichromate kiln?
The roast is oxidising and needs substantial excess air to convert chromium to sodium chromate. The excess air increases the gas volume the ID fan handles. The fan must be sized for the highest excess air the process will run at.
What temperature does a chromate roasting kiln ID fan see?
After a waste heat boiler or cooler and the dust collector, typically 180 to 250°C, with an upset to about 350°C if cooling fails. The impeller should be checked for creep strength at the upset temperature.
Information needed to quote an ID fan for sodium dichromate roasting kilns
- 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.
- Dust collector. Collector type, outlet dust loading and chromate content.
- Containment. Shaft seal and leak tightness required by the plant's health and safety rules.
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.