Rotary kiln or Slep furnace: choosing between 1 and 3 t/h

What actually differs

A rotary kiln is a single inclined tube. A Slep furnace stacks several hearths in one vertical shell, with rabble arms sweeping material from hearth to hearth. Both hold material at 850–950 °C in steam; the difference is how the material moves and how evenly it sees temperature.

In a rotary kiln, material tumbles through a moving bed, so residence time is a distribution rather than a number — some carbon exits early, some late. In a Slep furnace, every particle follows the same path down the hearths, so residence time is tight and repeatable.

When Slep wins

Two situations. The first is specialty carbon where a ±20 mg/g spread across a batch is commercially unacceptable — catalyst supports, medical-grade, or high-end gold recovery where the buyer audits consistency. The second is mixed or fine feed, where a rotary kiln's tumbling bed segregates particles by size and the fine fraction over-activates.

In both cases the extra capital and maintenance is buying consistency, and consistency has a price in the market. If your buyer does not pay for it, you are buying engineering you cannot sell.

When rotary wins

For 80% of the plants we build, at 1–3 t/h, on screened shell char going into water treatment, rotary is the better financial decision: one machine instead of three, one lining instead of three, and a maintenance regime a single fitter can handle.

The honest version of the argument is this — a rotary kiln at 3 t/h produces carbon at 950–1,000 mg/g with a ±25 mg/g spread. If that is inside your specification, buy it and put the savings into a waste-heat boiler, which will improve your margin more than tighter activation ever will.

Footprint, building and cranes

A rotary line at 1 t/h needs roughly 24 × 10 m for the activation section and 9 m clear under the hook — numbers from our standard utility schedule. A Slep furnace stacks vertically: floor area per unit is well under half of a rotary section, but building height typically exceeds 12 m and the crane has to serve a taller, narrower envelope. For three or four Slep units at 3 t/h, add aisle space for arm extraction and hearth handling — the floor advantage narrows faster than the brochure suggests.

The practical test: if your building is long, low and already standing, rotary fits it. If you are building new on expensive ground and can justify the height, the Slep footprint starts to matter.

Five questions before you sign

• Does your buyer pay for consistency, or only for iodine value? Get this in writing from your own sales side.
• What is the real particle-size distribution of your feed, measured at the gate, not on the specification sheet?
• How many units can your maintenance crew actually serve — one kiln, or four furnaces with arms and seals?
• Is the building existing or new, and what clear height and crane coverage does it offer?
• Where does the next capacity step come from — a second rotary, or a shift in product mix that would justify Slep?

Commissioning and the road to nameplate

A rotary line erects and commissions in 15–25 days on a prepared site, and reaches stable iodine value within the first two weeks of continuous feed — the kiln's behaviour is visible from the control room, and corrections are temperature and speed changes. A Slep furnace needs the same erection window plus a longer shake-down: rabble arms and hearth seals bed in on site, and the first months are about finding the machine's real residence time, not the catalogue one.

Plan the ramp accordingly. A rotary line typically sells specification product inside its first production month; a Slep installation is a consistency investment whose payoff starts after the machine has been run in. Neither is a flaw — but only one of them suits a buyer whose cash flow starts on day thirty.