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 ID fan sits
- Rotary reduction kiln
- Dust chamber
- Gas cooler or waste heat boiler
- Bag filter
- ID fan
- Stack or SO₂ scrubber
The fan sits after the bag filter, where the gas has been cooled to about 160 to 220°C. Placing it there keeps the abrasive barite and coke dust off the impeller. The fan must hold the kiln feed end at a slight suction, typically −2 to −5 mmWC, because too much suction pulls air into the kiln and reoxidises the sulphide. Total suction covers the kiln, dust chamber, cooler and filter, typically 250 to 400 mmWC.
Gas at the ID fan
| Gas temperature, normal | 160 to 220°C after the bag filter |
| Maximum or upset | Up to 300°C if cooling fails |
| Dust at the fan | Low; fine barite, coke and barium sulphide dust, high on a bag failure |
| Moisture and dew point | Moderate moisture; SO₂ and some SO₃ raise the acid dew point |
| Corrosive species | SO₂, traces of H₂S and CO in a reducing kiln |
| Density at the design temperature | About 0.77 kg/m³ at 190°C, 64% 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 barium and strontium reduction kilns
| Blade form | Backward inclined, with wear protection on leading edges |
| Material of construction | Mild steel with wear-resistant plate on blade inlets and the inlet cone |
| Drive and arrangement | Direct coupled or belt driven, arrangement 8 or 9 |
| Bearings and cooling | Cooling disc for the upset temperature |
| Flow control | Variable speed drive on kiln feed end draft |
| Also specified | Gas-tight casing and shaft seal, insulated casing and inspection door |
What this ID fan has to cope with
Reoxidation from excess draft
Barium sulphide reoxidises to sulphate if air enters the kiln. A fan that pulls too much suction draws air in at the seals and lowers yield. Fine draft control with speed control holds the small suction the process needs.
Acid corrosion of the casing
SO₂ and SO₃ in the kiln gas condense as acid where the fan or ducts run cold, especially at start-up and during kiln stops. Insulation and keeping the gas above the acid dew point prevent casing corrosion.
Toxic dust leakage
Soluble barium compounds are toxic. Leaks at the fan casing or shaft spread dust that people can inhale or ingest. Gas-tight joints and a sealed shaft are needed.
Energy and control
Reduction kilns run continuously with throughput set by ore and fuel. The ID fan duty is steady, but draft must be held tightly. Speed control saves power compared with damper throttling and gives the fine control needed, while low air ingress improves both yield and fuel use. Fuel savings from lower air ingress usually exceed the power saving.
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 Forced Draft (FD) Fan for barium and strontium reduction kilns.
Questions buyers ask
Why is kiln draft control critical in barium sulphide production?
Barium sulphide reoxidises if air enters the kiln. The ID fan must hold a small, steady suction at the feed end, so that gas does not escape but air is not drawn in at the seals.
What dust does a barium kiln ID fan handle?
Fine barite, coke and barium sulphide dust. Most is removed by the dust chamber and bag filter, so the fan sees little, but a bag failure raises the load sharply.
What protects a barium reduction kiln ID fan from corrosion?
Keeping the gas above its acid dew point with insulation and a minimum fan inlet temperature, since the kiln gas carries SO₂ and some SO₃. Wear plate at the blade inlet handles the residual dust.
Information needed to quote an ID fan for barium and strontium reduction 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.
- Kiln gas. SO₂, CO and dust content at the fan, and the cooling method between kiln and filter.
- Draft. Required kiln feed end draft and present control method.
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.