Modern dry‑process plants pipe 300–350°C exhaust from the kiln or preheater through the raw mill to strip moisture fast — but only if temperature and flow are tuned to a tight window. Operators guard an 80–100°C outlet and ~300–320°C inlet to hit <1% product moisture and avoid shutdown‑level risks.
In today’s kilns, the hottest stream in the plant doubles as a drying medium. A portion of the kiln or multi‑stage preheater exhaust — often at 300–350°C — is ducted through the raw mill to evaporate moisture in limestone, clay and other raw feed, which commonly runs several percent and can hit 5–10% before grinding (cementindusneed.com; cementequipment.org). Stable kiln operation wants the finished raw meal below 1% moisture, and plants routinely target <1% at the separator discharge (pdfcoffee.com; cementequipment.org).
Miss that target and the penalties stack up: wet milling chokes classifiers, saps throughput, and hikes specific energy (cementequipment.org). The fix is counterintuitive: add heat and velocity. Those hot gases deliver both the enthalpy to evaporate water and the “sweep” to carry fines to the separator (cementequipment.org; cementindusneed.com).
Air‑swept designs and moisture limits
Plants lean on air‑swept milling (gas carries material through the mill) to match drying load. Ball mills with dedicated drying compartments operate at ~2.5–3.5 m/s gas velocity and can handle feed moistures up to ~8%. Fully air‑swept ball mills run ~5–6 m/s and stretch to ~12–14% moisture (scribd.com). Vertical roller mills (VRMs, a vertical grinding technology with an integrated classifier) push further — up to ~20% feed moisture — while consuming ~30% less grinding power (cementindusneed.com).
Those performance figures assume hot‑gas inlets typically up to ~300–320°C, a boundary many plants formalize as a hard cap to protect bearings and liners (scribd.com; pdfcoffee.com). Industry sources plainly note “raw mill gas inlet temperatures up to 320°C,” and cite plants that set the inlet limit at ~320°C (pdfcoffee.com; cementindusneed.com).
Two setpoints that run the show
Operators live by a pair of temperatures: mill outlet and mill inlet. The outlet — measured after grinding and before the separator — is typically controlled at ~80–100°C, an empirical band that yields <1% raw meal moisture (pdfcoffee.com). If outlet temperature rises above this band (indicating insufficient gas flow or lower gas humidity), the product will be wetter, increasing power draw and risk of mill plugging. If it falls too low, the mill draws extra hot gas to restore temperature.
The inlet (the hot‑gas stream) is constrained for hardware safety. Bearing and lubricant limits typically keep the inlet around 300–320°C, with specific cases noting a raw mill inlet limit at ~320°C (“Circle Cement”) to protect the mill inlet trunnion and other components (pdfcoffee.com; pdfcoffee.com). Many mills also manage the outlet to 80–100°C explicitly “to ensure <1% product moisture” (pdfcoffee.com).
Overheating risks and gas chemistry
Letting temperatures creep past design brings layered risks: overheated bearings, seals and liners; possible “flash calcination” of fine lime; and safety hazards from combustible species in kiln exhaust. Trade references flag that drawing kiln gas — with its high CO/CO₂, low O₂ and combustibles — into the raw mill can create an explosion hazard if it mixes with other fuels (cementequipment.org). Plants temper very hot gas with ambient air or evaporative cooling to avoid condensing acids or exceeding dust reactivity limits (pdfcoffee.com; cheresources.com).
Drying capacity equals flow times enthalpy
The math is simple: moisture removal depends on the mass flow and temperature of gas. Maintaining high flow improves both heat transfer and fines “sweep.” One plant reported ~150,000 m³/h of air at 90°C — about 1.53 kg gas per kg raw meal — for optimum sweeping (pdfcoffee.com). VRMs often admit 50–70% high‑temperature gas into the grinding zone, with the balance as cooler ambient air above the table to manage both drying and separation (patents.google.com).
Control uses two knobs: the hot‑gas damper or fan speed (how much kiln gas is drawn through the mill) and any bypass/bleed. If product moisture rises and outlet temperature drops, more hot gas is admitted to pull the outlet back toward ~85–90°C. If the outlet climbs (product over‑drying or risk of heating), hot‑gas extraction is reduced or tempered; some lines recycle gas or bleed ambient air to conserve flow (pdfcoffee.com; pdfcoffee.com). Modern plants run automated PID (proportional–integral–derivative) loops on inlet/outlet temperatures with cascade control to dampers/fans.
Off‑nominal modes and evaporative cooling
When the kiln is down or at low load, mills are often bypassed to prevent overheating. In bypass, all kiln gas goes to stack or heat recovery. One engineering case quantified the jump in available waste heat: roughly ~4 MW during normal milling versus ~20 MW when all gas bypasses the mill (researchgate.net). To keep temperatures manageable in bypass, plants cool the exhaust via evaporative spray towers or by adding ambient tempering air. Adding cold air raises total flow — increasing fan power — and can de‑rate kiln performance, whereas water spray lowers temperature without extra flow (pdfcoffee.com).
This is where utilities matter: evaporative cooling relies on consistent spray delivery and plant‑standard support systems, including metering hardware such as dosing pumps and service‑side water‑treatment ancillaries. Regardless, the process imperative is thermal: conditioning towers are tuned to avoid dropping below the sulfuric acid dew point — typically ~125°C — to prevent corrosion and sticky deposits (pdfcoffee.com; cheresources.com).
Split‑gas control in vertical mills
Patent literature and vendor practice highlight multi‑stream gas control. Loesche’s VRM configuration feeds hot kiln gas at the mill bottom and fresh ambient air higher up; adjusting swirl‑damper angles tunes the fraction of hot versus cold gas to hit a target classifier outlet temperature (patents.google.com; patents.google.com). One scheme uses ~70% carrier gas as hot gas (kiln gas at roughly 230°C inlet) and ~30% ambient air; varying this split sustains “simultaneous grinding and drying” at peak throughput without water injection (patents.google.com; patents.google.com).
Many plants still keep it simple: cascade feedback on the hot‑gas damper/fan to hold inlet around 300–320°C and outlet near 80–100°C. Some fine‑tune with tempering air at the exit or by recirculating cooled mill gas back to the inlet. Wherever the setpoints drift, “overheating is vigorously avoided” — for example, at “Circle Cement,” outlet ~100°C and inlet max 320°C with automatic cutback if limits approach (pdfcoffee.com; pdfcoffee.com).
Moisture, energy and throughput outcomes
With control tight, raw mills routinely deliver residual moisture <0.5–1% (pdfcoffee.com; cementequipment.org). Stabilizing temperatures smooths operation and reduces shutdowns by limiting “gluing” and classifier fouling. Higher gas temperature (within safe limits) increases drying capacity and can lift throughput before wet‑grinding limits kick in; the gains must offset higher fan energy and any kiln‑efficiency impacts. Vendor analyses argue that hot‑gas control in the mill can “maximize throughput with minimum energy,” because synchronized drying and grinding keeps mill power optimal (patents.google.com).
The penalty for inadequate drying is quantifiable: removing 1% extra moisture costs about ~2.45 MJ/kg H₂O evaporated (latent heat), plus fan work. Case reports link improved temperature/flow control to ~1.5–5 kWh/t raw savings and, in extreme cases when mills are better air‑swept and controlled, throughput jumps of 10–30% (pdfcoffee.com; scribd.com). (Such numbers come from mill teddy data: e.g. air‑swept mills vs under‑swept ball mills yield such kWh/t differences scribd.com.)
Reliability and corrosion constraints
Thermal discipline extends equipment life: keeping bearings below ~100°C (a bearing‑critical threshold) matters, and avoiding wide swings prevents thermal shock to linings and seals. On the chemistry side, stable control minimizes dust deposition and guards against condensing acids; plants avoid cooling below the sulfuric acid dew point — typically ~125°C — to prevent corrosion and sticky deposits (cheresources.com).
Policy context and plant practice
There is no specific Indonesian regulation on raw mill temperatures per se, but strict particulate and acid dewpoint limits (e.g., Ministry of Environment emission standards) make gas dew point and downstream temperature control de facto compliance parameters. Overheating the mill can elevate acid carryover via water condensation or raise dust load, so maintaining gas above the ~125°C sulfuric acid dew point is recommended to avoid corrosion (cheresources.com; pdfcoffee.com). Industry roadmaps emphasize energy efficiency and emission control; Indonesian producers cite continuous improvement under “Menteri Perindustrian Roadmap 2012” and GRK regulations (asi.or.id).
Bottom line: precise heat and flow
The operating recipe is consistent across plants and references: cap the raw mill inlet around 300–320°C; control the outlet at ~80–100°C to secure <1% product moisture; and vary hot‑gas flow to stay on target (pdfcoffee.com; scribd.com; patents.google.com; pdfcoffee.com). Too little gas or too cool and the mill wets out; too much heat and components, safety and emissions are at risk. Tight loops — and the option to temper with air or evaporative cooling — make that balancing act routine.
Sources: industry handbooks and case studies document outlet 80–100°C for <1% moisture (pdfcoffee.com); ball‑mill moisture limits up to 12–14% with ~300°C gas (scribd.com); multi‑stream gas control (mixing ~230°C kiln gas with cool air) to meet setpoints (patents.google.com); waste‑heat availability shift from ~1.5–4 MW to ~20 MW when bypassing the mill (researchgate.net). Together they underline the central control challenge: deliver enough hot gas to dry aggressively — and never so much that the mill overheats.
