How barium and strontium reduction kilns work
Barium and strontium chemicals start with the reduction of barite or celestite, the natural sulphates, by coal or petroleum coke in a direct-fired rotary kiln at 1,000 to 1,200°C. The product, black ash, is barium or strontium sulphide, which is leached with water and used to make lithopone, barium carbonate, blanc fixe and strontium carbonate. The kiln is fired from the discharge end by oil, gas or pulverised coal, with the burner run near stoichiometric so the product is not reoxidised. The gas leaves the feed end at 400 to 600°C carrying CO₂, some CO, SO₂, coke and ore dust. It passes a dust chamber, is cooled by a waste heat boiler, cooler or dilution air, and is cleaned in a bag filter or scrubber before the ID fan.
Where the FD fan sits
- Inlet filter
- FD fan
- Primary air duct
- Discharge end burner
- Rotary reduction kiln
The fan stands at the discharge end of the kiln, beside the burner platform. It supplies primary air to the burner and, with coal firing, conveying air for the pulverised coal. Secondary air enters through the kiln hood or the product cooler, drawn by kiln draft. Outlet pressure is typically 400 to 800 mmWC for the burner. The fan is kept cold and away from the hood radiation.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Filtered air; ore and coal dust in the kiln area |
| Pressure at the fan outlet | Typically 400 to 800 mmWC |
| Air flow accuracy | Near stoichiometric firing; excess air reoxidises the product |
| 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 barium and strontium reduction kilns
| Blade form | Backward curved, with a steep curve for stable flow |
| 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 with flow measurement |
| Also specified | Inlet filter and silencer |
What this FD fan has to cope with
Excess air and product loss
Air beyond stoichiometric reoxidises barium sulphide and lowers yield. The fan must give an accurate, stable flow, measured and controlled, rather than relying on damper positions. A flow meter in the primary air duct gives the reference.
Coal and ore dust at the inlet
Dust from the kiln area enters an unfiltered inlet and coats the impeller. An inlet filter or a ducted inlet from a clean area prevents imbalance. The filter should be checked weekly in a dusty kiln house.
Burner flame shape
Flame shape sets the reduction zone length. A fan that cannot hold pressure across the burner range shortens or lengthens the flame. Stable fan pressure keeps the zone where it belongs.
Energy and control
The combustion air fan runs continuously while the kiln fires. Speed control on measured air flow saves power and gives the accurate air the process needs. Low excess air improves yield and saves fuel, which matters more than the fan power itself. A standby fan avoids a stop that would cool and stall the kiln.
ID fan and FD fan in barium and strontium reduction kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack or scrubber | At the kiln discharge end, feeding the burner |
| What it handles | Kiln gas with SO₂ and fine dust | Ambient air |
| Temperature at the fan | 160 to 220°C | Ambient |
| Main design concern | Fine draft control and acid dew point | Accurate air near stoichiometric |
| Control | Kiln feed end draft | Measured air flow |
See also the Induced Draft (ID) Fan for barium and strontium reduction kilns.
Questions buyers ask
Is the FD fan on a barium kiln the combustion air fan?
Yes. It supplies primary air to the discharge end burner, and on coal firing the conveying air for pulverised coal. Secondary air enters through the hood. The fan handles ambient air only and never sees kiln gas.
Why is air flow accuracy important in a barium reduction kiln?
Barium sulphide reoxidises with excess air. The burner must run close to stoichiometric, so the air must be measured and controlled accurately. Primary air flow is measured and the fan speed set to hold it.
What pressure does a reduction kiln burner fan need?
Typically 400 to 800 mmWC, depending on burner type and fuel, enough for flame shape plus duct losses. Pulverised coal burners with conveying air sit at the higher end of this range.
Information needed to quote an FD fan for barium and strontium reduction 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.
- Coal conveying. Whether the fan also supplies pulverised coal transport air.
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.