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 FD fan sits
- Inlet filter
- FD fan
- Primary air duct
- Discharge end burner
- Rotary roasting kiln
The fan stands at the discharge end beside the burner. It supplies primary air to the burner and, with coal firing, conveying air. Secondary and excess air enter through the kiln hood or product cooler, drawn by the ID fan. Outlet pressure is typically 400 to 800 mmWC for the burner. A separate cooler fan supplies cooling air to the product cooler.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Filtered air; chromate dust in the kiln area |
| Pressure at the fan outlet | Typically 400 to 800 mmWC |
| Excess air | High; oxidising roast |
| Turndown | Moderate, following kiln feed |
| 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 sodium dichromate roasting kilns
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, arrangement 4 or 8 |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on air flow |
| Also specified | Inlet filter and silencer, inlet ducted from a clean area |
What this FD fan has to cope with
Chromate dust at the inlet
Dust in the kiln area includes hexavalent chromium. An unfiltered inlet carries it onto the impeller and into the burner, and maintenance staff are exposed when cleaning the fan. A ducted inlet from a clean area and a filter avoid this.
Oxygen supply for the roast
Too little air leaves chromium unconverted and lowers yield. The total air, primary plus secondary, must be measured and controlled, not set by damper positions. Oxygen measurement at the kiln inlet is the usual check.
Primary air drift
The ratio of primary air to total air changes the flame length and so where the roasting zone sits in the kiln. If primary air drifts as filters load or the fan heats, the zone moves. Measured flow and speed control hold it steady.
Energy and control
The combustion air fan runs continuously. Speed control on measured air flow saves power and keeps primary air steady. Most air enters through the hood, so the larger saving comes from holding total excess air at the level the roast needs, which also reduces the ID fan volume. The fan runs at a steady duty for most of the year.
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 Induced Draft (ID) Fan for sodium dichromate roasting kilns.
Questions buyers ask
Is the FD fan on a chromate kiln the combustion air fan?
Yes. It supplies primary air to the discharge end burner, and conveying air with coal firing. Most of the excess air for the roast enters through the hood or cooler.
Why does a sodium dichromate kiln need high excess air?
The roast converts chromium in the ore to sodium chromate, which needs oxygen. Excess air supplies it; too little lowers conversion and yield. The air must still be controlled, because more than needed just adds gas volume.
Should the chromate kiln FD fan inlet be filtered?
Yes. Dust in the kiln area includes hexavalent chromium. A filtered inlet from a clean area protects the fan, the burner and maintenance staff. Maintenance staff then handle a clean fan.
Information needed to quote an FD fan for sodium dichromate roasting kilns
- 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.
- Burner and fuel. Burner type, fuel and primary air pressure and flow at full firing.
- Air distribution. Split between primary air and air entering through the hood or cooler.
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.