In a closed raw‑milling circuit, the baghouse or ESP isn’t just for emissions — it captures nearly 100% of the product stream and sends it back to the silo. When it’s tuned and maintained, throughput, energy, and raw‑mix stability follow.
Modern cement plants run raw mills as closed grinding circuits, where fine powder (“raw meal”) is separated from coarse grits and returned to the mill — and essentially all fines are captured by a dust collector. By design, this dust collector (a baghouse, i.e., a fabric filter, or an ESP, an electrostatic precipitator) must capture nearly 100% of the product stream to maximize material yield and process efficiency.
A case in point: replacing an ESP with a pulse‑jet baghouse at an Indonesian raw mill cut stack dust from ~35 mg/Nm³ to ~6 mg/Nm³ (researchgate.net). The ESP had required cooling the 400 °C mill exhaust down to ~100 °C (consuming large cooling water), whereas the baghouse tolerated the full 400 °C gas, sharply improving capture and lowering total thermal losses (researchgate.net) (researchgate.net). In the Indocement case the baghouse also drew about 70 kW less fan power than the ESP (saving ~0.24 ton CO₂/yr) (researchgate.net).
Indonesian regulations (Permen P.19/2017) cap cement‑mill dust at roughly 60–75 mg/Nm³ (id.scribd.com), demanding >99% capture efficiency. For context, Indonesian norms also limit raw‑mill dust ≲60 mg/Nm³, underscoring the need for >99% particle capture (id.scribd.com).
Closed‑circuit capture and yield
In practical terms, a high‑performance dust collector sequesters almost all fines. That prevents raw meal loss, avoids environmental fines, can cut auxiliary energy use, and stabilizes throughput. Field reports point to gains beyond emissions cuts: with cooling eliminated, baghouses can operate at the full 400 °C gas temperature that raw mills produce, improving capture and lowering thermal losses (researchgate.net) (researchgate.net), and in the cited Indocement case, trimmed about 70 kW of fan power, or ~0.24 ton CO₂/yr (researchgate.net).
Dust recycling to the homo silo
Capturing dust only pays if it is returned to the process. In a raw mill circuit, the filtered dust — which is the desired fine raw meal — is conveyed from the dust‑collector hopper back into the mixing or homogenizing system (a homo silo, i.e., a blending silo). As one process flow diagram puts it, “the filter dust is mixed with the RM product and both are directed to the homo silo” (researchgate.net).
If the collector is well sealed and operated, virtually no material is lost. Conversely, bag leaks or spillage cause material losses and upset the kiln feed chemistry. Poor dust capture can shift the measured Blaine (a fineness index) or loss‑on‑ignition of the raw mix, forcing extra adjustments. By contrast, >99.5% dust capture ensures the mill’s output — both via the classifier fines and baghouse fines — is fully utilized, stabilizing the raw‑mix quality seen by the kiln. Indonesian law effectively treats raw meal fines as product — dust must be recycled to avoid exceeding emission limits (id.scribd.com).
Pressure drop, airflow, and capacity
A well‑maintained dust collector directly affects the raw mill’s operational stability and energetics. Excess dust in the mill’s air path impedes airflow, so capturing it minimizes internal power consumption. One cement‑plant optimization reduced baghouse pressure drop (ΔP, i.e., the differential pressure across the filter) from ~9–14″ w.g. (inches of water gauge) down to 4″ (slideshare.net). That allowed the raw mill’s fan to deliver ~25% higher airflow — from ~33,000 to ~40,970 cfm (cubic feet per minute) — with the same fan power (slideshare.net). The same upgrade quadrupled bag life from ~1 yr to 4 yrs (slideshare.net).
Conversely, a plugged or leaky dust collector throttles the mill. If ΔP is too high, airflow falls: the mill cannot draw in full design gas flow, so drying capacity and grind rate drop (mills may run “full” and slow down). If ΔP is too low, it usually indicates bag tears or leaks — fines leak raw meal out to atmosphere instead of to the silo. As a cement‑plant training guide explains, excessive baghouse ΔP “can reduce the air flow” and bad cleaning “not only causes short bag life but very poor performance” (cementequipment.org) (airbestpractices.com).
In practice, plant operators log baghouse ΔP along with mill ΔP and airflow to ensure normal conditions. A well‑tuned raw mill might run with baghouse ΔP ≈2–4″ w.g.; if this creeps much higher (say >5″) or suddenly drops, it triggers inspection (cementequipment.org).
Maintenance routines and filter media
Because the dust collector works around the clock, maintenance is critical. Worn or damaged bags significantly degrade efficiency even before visible spikes in emissions. Operators typically replace bags when average ΔP “gets too high” or after upset events (e.g., separator blockages) (astcanada.ca). Leading practice now uses continuous monitoring: some plants fit electronic ΔP sensors and pulse counters to auto‑trigger cleaning only when needed, avoiding over‑cleaning or short cycling.
Upgrading to advanced media also helps. Use of pleated ePTFE membrane bags (expanded polytetrafluoroethylene) is reported to triple typical bag life (versus 16 oz polyester felt) and cut energy and emissions (astcanada.ca). Donaldson/AST data claim such filters can halve dust‑collector downtime (50% fewer bag replacements) while lowering fan power by reduced ΔP (astcanada.ca). Similar results showed that switching to larger‑area pleated bags gave 4× longer service life in real mills (slideshare.net).
In short, dust‑collector upkeep — proper pulse timing, ensuring adequate compressed‑air volume, and timely bag replacement — is itself an efficiency measure. Neglecting it is very costly: mis‑timed pulses or clogged lines lead to “short bag life… and very poor performance” (airbestpractices.com), meaning unscheduled shutdowns and lost production. Proactive care maintains steady airflow through the mill and returns all fines to the circuit — the combination that underpins stable throughput and consistent product quality (cementequipment.org) (astcanada.ca).
Sources and references
Field studies and industry reports: Purnomo et al. (Indocement), 2018 (researchgate.net) (researchgate.net); Tsamatsoulis, 2011 (researchgate.net); Infinity/Cement Equipment Portal (process training manual) (cementequipment.org); N. Asnani (CII), Bag Filter Optimization in Cement Industry (slideshare.net); Hank van Ormer (Donaldson), Managing Dust Collectors in Cement Production (astcanada.ca) (airbestpractices.com); Republik Indonesia, Permen LH No. P.19/2017 (industry cement emission standards) (id.scribd.com).
