In cement raw grinding, dust collectors don’t just clean air — they recover product, stabilize the circuit, and protect margins. Plants capturing 99%+ of fines recycle 5–10% of kiln feed, cut emissions, and unlock higher mill throughput.
In the raw milling circuit, baghouses (fabric filters) and electrostatic precipitators, or ESPs (electrostatic particle capture using ionization), “collect product as much as they clean gas” (www.cementequipment.org). Fine raw meal — the “dust” — represents about 5–10% of kiln feed, a fraction that would be lost without capture (www.cementequipment.org). Modern fabric filters routinely exceed 99% particle‑capture efficiency (asi.or.id), which means nearly all milled fines are returned rather than emitted.
The stakes are visible in the stack. Replacing an ESP with a pulse‑jet baghouse (pulse‑jet uses compressed‑air pulses to clean the filter) at an Indonesian plant, PT Indocement in Cirebon, cut measured dust from about ~30 mg/Nm³ to ~6 mg/Nm³ (mg/Nm³ = milligrams per normal cubic meter; “normal” standardizes temperature and pressure) — an 80% reduction and a recovery of what otherwise would have been raw material loss (www.researchgate.net).
This is about more than compliance; it’s throughput. With high‑temperature bag filters, plants can run hotter gas (up to ~400 °C) through the mill, increasing drying capacity and milling rate (www.researchgate.net). By contrast, EP systems have required cooling raw‑mill exhaust from 400 °C to ~100 °C, which has limited throughput and raised power draw (www.researchgate.net). One upgrade saved ~0.24 tonnes CO₂‑equivalent/year in electricity for a single plant (www.researchgate.net).
High capture and mass balance discipline
Efficient dust capture enables a “closed‑loop” raw milling circuit: milled material and entrained fines stay in the mill–dryer system instead of being vented. Industry practice reconciles this in mass balances; typical feed‑to‑clinker ratios are ~1.65–1.75, reflecting extra material (dust and fuel ash) collected and recycled back as part of the raw feed (www.cementequipment.org).
The quality benefit is real. Fines tend to be more reactive (e.g., higher silica ratio), so recirculating them in a controlled fashion prevents feed composition swings. Engineers describe the “least negative option” as sending the raw‑mill filter catch to the blending silo (a homogenization vessel for averaging raw meal properties over time) (www.cementequipment.org).
How plants recycle captured dust
In practice, pulse‑jet baghouses and cyclones (inertial separators that spin out particles) drop collected fines into screws or hoppers that feed back into the raw mill or to the preheater inlet. Case descriptions show the filter dust mixed directly with ground raw meal and sent to the homogenization silo — making the “filter dust” part of the kiln feed (www.researchgate.net). In fact, closed‑circuit recycling means “the dust’s chemical composition constitutes [the control system’s] input” for raw mix blending (www.researchgate.net).
When the raw mill is offline, the kiln exhaust can send unchanged (kiln feed) dust to the collector, altering dust chemistry. Plants handle this by routing collector dust to a buffer silo when the mill is stopped, then blending it back carefully; the dust catch “must still be compensated,” with experts advising return to the blending silo or storage for controlled feed (www.cementequipment.org).
Yield, costs, and the numbers
Recycling significantly boosts yield per unit raw feed. With a kiln feed/clinker ratio of ~1.7, every tonne of clinker effectively requires only ~0.85 t of virgin feed and ~0.85 t of recovered dust/fuel. Without efficient scrubbing, that recovered portion shrinks, forcing more purchased raw or lowering output. Plants report that good dust scrubbing can reduce raw material costs by up to 5–10% of throughput — a large effect in high‑volume operations.
The “virtually all material” premise is the point: virtually all material ground by the mill — coarse and ultrafine — ultimately goes to the kiln when capture exceeds 99% (asi.or.id). That keeps the feed stream stable and the chemistry uniform (www.cementequipment.org).
Operation, ΔP, and maintenance discipline
The dust collection system’s reliability directly affects mill stability. A clogged or poorly operating filter raises differential pressure (ΔP, the pressure drop across the filter) and chokes airflow, slowing or stalling pneumatic transport and classifier function. “Proper operation of dust collectors is critical to minimizing cost and maximizing system effectiveness” (www.airbestpractices.com).
In practice, this means ensuring pulse timing and air pressure are adequate so dust cakes are removed completely. If not, the collector “does not remove dust effectively” and filter media wear accelerates (www.airbestpractices.com). Upgrades can be decisive: replacing conventional bags with pleated, PTFE‑coated (polytetrafluoroethylene) bags expanded media area by 3.4×, cut the raw‑mill transporter ΔP dramatically, and extended filter life 5‑fold (from ~6 months to over 30 months) while holding emissions under 10 mg/Nm³ (casestudies.nordic-air-filtration.com).
The maintenance link to uptime shows up in broader metrics. A recent Indonesian case study put a raw mill’s overall equipment effectiveness, or OEE (combined measure of availability, performance, and quality), at only ~57% due to breakdowns and stoppages — underscoring that “maintenance is essential” to uphold efficiency and productivity (www.mdpi.com; www.mdpi.com). Unchecked ΔP growth forces unplanned shutdowns for bag changes, disrupting the grinding cycle.
Compliance, capture rates, and profit
Strict Indonesian regulations (MOEF Reg.19/2017) cap particulate emissions, effectively penalizing poor filtration (asi.or.id). Executives note that avoiding dust stack emissions also protects profit, since dust exit equals material loss. In summary, a well‑operated dust collection system — often achieving 99%+ removal (asi.or.id) — is central to a high‑efficiency raw milling loop: it recovers valuable raw meal, stabilizes the feed stream, and prevents disruptions that would cut throughput or increase costs (www.cementequipment.org; www.airbestpractices.com).
