Single Cylinder Dryer for Kaolin: Inlet and Outlet Temperature Guide

A kaolin dryer is sold on tonnage and quoted on temperature, and the two numbers in that quotation rarely match the mineral. The published sheet for a Single Cylinder Dryer lists a maximum inlet gas temperature of 700–800 °C and a final moisture of 3–10% adjustable. Both are machine limits. Neither tells you what to run on kaolin, because the constraint that matters is not in the steel — it is in the clay.

Kaolinite is a hydrated aluminium silicate. The free water on its surface leaves at around 100 °C, but the hydroxyl groups inside the lattice do not leave until 550–600 °C, where the structure collapses into metakaolin. That collapse is the whole point of calcination and exactly what a dryer must avoid: metakaolin has lost the plasticity that makes kaolin useful in ceramics, and its brightness and surface area have changed. A dryer that lets the material reach that window has quietly made a different, cheaper product.

So the operating question is not how hot the drum can go, but how hot the gas can be while the material stays cool. On a rotary drum that is decided by flow direction, by discharge temperature and by moisture — not by the burner setting. Here is what each of those does, with the arithmetic attached.

What the Sheet Actually Gives You

The range runs 27 models from Ф600×6000 to Ф3200×25000, with drum volumes of 1.7–201 m³ and drive power of 3–110 kW. Feed size is published as below 10 mm, inlet moisture up to 25%, final moisture 3–10% adjustable, gas at 700–800 °C. Six models that bracket the kaolin range:

How to read this table: every column except the last is published. The last is computed on 20% inlet to 2% outlet, where water removed per tonne of wet feed is (0.20 − 0.02) ÷ (1 − 0.02) = 0.1837 t. Divide drum volume by that column and the answer sits between 22 and 26 m³ per t/h of water across the whole range — in other words the published bands are built on roughly 0.04 t of water per m³ of drum per hour, at whatever moisture basis the factory used. That ratio is the quickest way to audit a quoted kaolin capacity. A number implying 0.08 t/(m³·h) is not a better machine; it is a different moisture basis or a different material.

The 550 °C Ceiling, and Why a 700 °C Drum Is Still Safe

Two numbers sit 450 degrees apart and both are true. The drum accepts gas at 700–800 °C; the clay begins losing structural water at 550–600 °C. They coexist because of the wet-bulb effect and flow direction, not because the clay tolerates heat.

In co-current operation the burner sits at the feed end. The hottest gas meets the wettest material, and the energy that would otherwise raise the clay's temperature is spent evaporating surface water instead. A wet particle in a 700 °C stream stays near 100 °C for as long as it is wet — the same reason a paper cup of water survives a campfire. By the time the material is dry, the gas around it has given up most of its heat and is closer to 120 °C. The temperature profile is the safety mechanism.

Run the same drum counter-current and that mechanism disappears. The driest material meets the hottest gas at the discharge end, with no water left to absorb the heat. On sand or slag nobody notices. On kaolin it is the fastest way to make metakaolin by accident, and no thermocouple reading at the inlet will warn you, because the inlet reading is fine. Co-current is not a preference here; it is the reason a 700 °C limit is compatible with a 550 °C mineral.

The operating rule that follows is simple: control the material, not the flame. Discharge product at 85–95 °C, well under the dehydroxylation onset, and the inlet temperature can sit anywhere inside the published limit that the gas train can hold.

Inlet Temperature Buys Gas Volume, Not Quality

If the inlet temperature does not improve the product, what does it change? The amount of gas the system must move. Heat carried per kilogram of flue gas is fixed by its specific heat and the drop from inlet to outlet, so a lower inlet temperature means more kilograms for the same gigajoules. At 65% thermal efficiency a tonne of water needs about 4.25 GJ, and that number is set by the physics of evaporation rather than by the burner.

How to read this table: calculated on 4.25 GJ per tonne of water, 1.05 kJ/(kg·K) for the gas, and a fixed 110 °C outlet. The outlet volume is the number that sizes the fan and the bag filter, and it is why a "gentler" low-temperature drying recipe is not automatically cheaper to build — dropping from 700 °C to 400 °C doubles the gas-side equipment for the same water load. Fuel per tonne of water stays at 4.25 GJ either way; only the capital cost of moving the gas changes.

This is the trade a kaolin buyer is actually making when they ask for a lower inlet temperature. Below about 500 °C the drum gets longer or wider for the same duty, the fan power climbs, and the collector grows. If the reason for a low inlet temperature is fear of overheating the clay, the cheaper answer is co-current flow with a controlled discharge temperature.

Outlet Temperature Is a Dew Point Decision

The outlet setting is not about the clay at all; it is about the gas leaving the drum. Evaporating 3 t of water into roughly 21 t of gas gives about 0.14 kg of water per kg of dry gas, and the dew point of that mixture is around 59 °C. Any surface below that temperature will collect water — the duct bends, the filter bags, the fan impeller.

Hold the outlet at 100–110 °C and the margin is 40 degrees, which absorbs a cold morning, a damp feed and a little inleakage without condensing. Drop it to 85 °C to "save heat" and the margin shrinks to 26 degrees, which is where caked bags and corroded ductwork come from. The saving is real but small: every 10 °C of unnecessary outlet temperature is heat that went up the stack, as the fuel arithmetic in what one ton of water costs to evaporate sets out. The mistake is buying that saving with a wet filter.

Two practical consequences. First, the bag filter must be rated for the outlet temperature you actually run, not the one in the proposal. Second, the outlet thermocouple belongs upstream of the filter, not in the stack, or it will read a number that has already been diluted by inleakage.

A Working Recipe for a Kaolin Line

The setpoints below are the ones a kaolin duty can be commissioned against. They are a starting recipe, to be trimmed against the first week of samples — kaolin varies too much between deposits for any table to be final.

How to read this table: the moisture limits, feed size and gas ceiling are published figures. The three temperatures are the operating band this article argues for, derived from the 550–600 °C dehydroxylation onset and a 59 °C computed dew point, not quoted from a factory sheet.

Run the numbers once on a real duty and the recipe stops being abstract. Fifteen tonnes per hour of wet kaolin at 20% down to 2% removes 2.76 t of water per hour and leaves 12.2 t/h of product. That duty falls inside the Ф2200×18000 band of 12.8–16.2 t/h, where the drum does 0.040 t of water per m³ per hour — squarely inside the 22–26 m³ per t/h ratio the published range implies. At a 650 °C inlet the gas flow works out at about 20.7 t/h, and the fan and filter are sized on roughly 11,000 m³/h at the outlet.

Frequently Asked Questions

Q1: Can I run the drum at 800 °C on kaolin?
A: The shell will take it, and in co-current flow the wet feed is protected by evaporation. The risk is the dry end. Cap the discharge product at 85–95 °C and never operate counter-current on a brightness-sensitive clay, and a 700–800 °C inlet is defensible. Without those two controls, the same setting is a calciner running by accident.

Q2: What outlet moisture should kaolin leave the dryer at?
A: 1–2% for a product going to a mill. Downstream grinding equipment publishes a feed humidity limit of 6% or below, so 1–2% leaves margin for re-wetting in storage without paying to evaporate water nobody asked to remove.

Q3: Does a hotter inlet save fuel?
A: No. Fuel per tonne of water is set by thermal efficiency — 2.76 GJ of theoretical minimum becomes about 4.25 GJ at 65% — and the inlet temperature does not change that number. A hotter inlet shrinks the gas volume, duct and fan, which is a capital saving, not a fuel one.

Q4: How do I check a quoted kaolin capacity before I buy?
A: Convert the quote to water removed per hour at your own moisture figures, then divide drum volume by it. The published range implies 22–26 m³ of drum per tonne of water per hour. A quote well outside that band is not necessarily wrong, but the supplier should be able to explain the moisture basis and the material it refers to.

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