- ▸The carbon matrix itself is alkali-resistant: NaOH attacks graphite mildly at ordinary concentrations — the real variables are the impregnant and temperature
- ▸Furan-resin impregnation is the workhorse for caustic service: broad acid/alkali coverage, porosity below 2% after impregnation, usable to roughly 200 °C
- ▸When 150–200 °C stacks on 50%+ concentration, resin can hydrolyze slowly; evaluate metal routes such as antimony
- ▸Graphite against reaction-bonded silicon carbide remains the first-choice pair; watch the cobalt binder of tungsten carbide in hot alkali
- ▸Black liquor combines fibers with alkali, so flush and material must be decided together: solids decide the flush scheme, while concentration and temperature decide the soft ring's impregnation
Cooking, washing and chemical recovery in pulp and paper all sit in NaOH — from dilute liquor to over 50% concentration, from ambient to 150–200 °C. The wrong first question is "is graphite alkali-resistant": the matrix handles alkali well; what decides life is how the impregnant behaves in caustic. This article lays out the impregnation decision path across concentration and temperature.
I. The Corrosion Logic of Caustic Service
1.1 The Matrix: Alkali Resistance Is the Baseline
Carbon graphite is chemically stable against NaOH; at ordinary concentrations and temperatures matrix corrosion is negligible. Contrast this with strong oxidizers (nitric acid, chromic acid), which graphite does not tolerate — caustic is not off-limits, it is home ground.
1.2 The Impregnant Is the Variable
Media enter the ring through pores, so the typical failure mode is "impregnant first": resin slowly hydrolyzes and swells under alkali plus heat, pores reconnect, and leakage follows. That is why a ring that performs well in cold caustic leaks within months on a 180 °C black-liquor pump.
II. Choosing Impregnation by Concentration-Temperature Zones
2.1 Ambient–80 °C, Low-to-Medium Concentration: Furan Resin
Cooking-liquor transfer and mid/low-temperature washing duties: furan resin impregnation is the mainstay — broad acid/alkali coverage, low cost, porosity below 2% after impregnation, and usable to about 200 °C. In this zone furan is generally sufficient.
2.2 80–150 °C: Furan Still Works, Watch Concentration
As temperature rises, resin hydrolysis accelerates. When 50%+ concentration stacks on 150 °C, start evaluating the transition to metal impregnation. Solids-laden black liquor also brings particle wear — choose a fine-grain, high-density matrix for the soft ring.
2.3 150–200 °C and Above: Metal Routes
Antimony-impregnated graphite, rated near 400 °C in hot oils, is also more stable than resin in hot alkali and is the candidate for the high-temperature band. Note its higher cost and slightly increased brittleness, and weigh against actual temperature and concentration.
III. Face Pair and System Configuration
1.Face pair: impregnated graphite (soft) against reaction-bonded silicon carbide (hard) is the caustic first choice; cobalt-bound tungsten carbide is attacked in hot alkali — use with caution or switch to nickel-bound
2.Flush: black liquor carries fibers, so solids content drives the choice of external flush or double seals, with a clean caustic or water flush compatible with the process
3.Face parameters: caustic has low viscosity and modest lubricity; set the balance ratio at 0.75–0.85 to cut face loading and wear
IV. Summary
The selection order for caustic service: fix the impregnation by concentration-temperature (furan first, metal routes for high-temperature/high-concentration), then the face pair and flush by solids, then check the hard-face binder. Huahao Sealing produces carbon graphite rings with furan, resin, antimony, babbitt and PTFE impregnations, supports made-to-drawing orders, and can advise duty-specific schemes for black-liquor and recovery pumps by actual concentration and temperature.