Gold Processing5 min read
Barren carbon comes out of the strip looking usable and still fails in the next adsorption tank. Organic foulants, calcium scale, over-temperature damage, and clinker tell…
Author: Wayne Ncube
Barren carbon comes out of the strip looking usable and still fails in the next adsorption tank. Organic foulants, calcium scale, over-temperature damage, and clinker tell different stories — and each maps to a different zone of the rotary reactivation kiln. This teaching case walks through a thermal profile and how to read a bad batch.
The rotating drum is the mental model: carbon moves axially while the shell turns, trays or flights lift and shower the bed, burners and zones create the profile, and the discharge drops into quench water.
Reactivation temperature profile
Four thermal stages along the tube; the last metre holds the highest temperature for residue burn-off.
| Stage | Approx. temperature | What happens to foulant |
|---|---|---|
| Drying | <200 °C (inlet ~60 °C, significant moisture out) | Water leaves; no useful organic removal yet |
| Vaporization | 200–500 °C | Lower-boiling organics (type I class) leave |
| Pyrolysis | 500–700 °C | Heavier material cracks; deposits (type II/III pathways) form or break down |
| Residue removal | >750 °C | Steam-assisted burn-off of stubborn residue; last section of tube |
Illustrative operating points for a teaching case:
Confirm every number against your kiln datasheet and SOP; these are teaching values.
Five-zone teaching examples (IA, IB, IIA, IIB, III) let the profile ramp, hold, and finish. A single “750 °C at the exit” can hide:
Steam or controlled atmosphere keeps oxygen low enough to pyrolyse rather than burn the carbon matrix — excess air is a carbon-loss and dust problem.
| Foulant class | Typical source | Primary treatment |
|---|---|---|
| Inorganic (Ca, Mg, silica, salts) | Lime, hardness, elution chemistry | Acid wash (e.g. dilute HCl teaching case, ~3% to pH 2–3, then rinse) |
| Type I organics | Light oils, solvents, plant fluids | Early kiln vaporization |
| Type II / III organics, polymers, heavy grease | Reagents, lubricants, ore organics | Higher-temperature pyrolysis / residue stages |
Acid wash does not replace the kiln; the kiln does not replace acid wash.
| Signature | Likely mechanism | Response direction |
|---|---|---|
| Red-hot agglomerates, fused lumps | Local overheating, ash fusion, metal contamination + heat | Check zones, foreign material, feed consistency; avoid treating as “more time” fix |
| Long black deposits, reduced ID | Incomplete pyrolysis products deposited along tube | Profile, atmosphere, feed rate, carbon condition before strip |
| High exit temperature, low activity | Overburn / excessive severity | Shorten peak exposure, verify zone control |
| Low exit temperature, foul carbon | Under-burn | Profile, burner, fouling load |
Always correlate with elution performance before the kiln — a dirty strip chemistry can feed dirtier carbon into regeneration.
An illustrative design places the discharge chute so the carbon exits under a water seal on the order of 200–300 mm. Purpose:
A dry discharge or shallow seal undoes hours of careful profile control in the last metre of the process.
One illustrative duty-tracking case uses kiln operation near ~740 °C and diesel burner consumption in a wide operating band (order of ~11–30 L/h depending on load). Treat these as teaching values, not a universal fuel standard, and verify the kiln datasheet and site procedure.
Per batch or campaign record:
700–750 °C
illustrative reactivation final temperature
5 zones
IA · IB · IIA · IIB · III profile control
200–300 mm
illustrative discharge quench seal depth
1# Illustrative axial residence check (teaching values, not a design package)2tube_length_m12.03linear_speed_m_per_h0.55# example solids velocity from feed/rpm practice4residence_htube_length_mlinear_speed_m_per_h5printf"approx axial residence = {residence_h:.1f} h"6print"Peak temperature time is much shorter than total residence."
Fuel-fired kilns, hot carbon, steam, and quench water involve burn, heat, and confined-equipment hazards. This article is educational; only competent operators under site procedures run or clear a kiln.
Further reading in this series: