Pulp and Paper Mill Sample Probes — Black Liquor, White Liquor, and Bleach Plant
Sample probes for kraft pulp mills: black liquor, white liquor, green liquor, bleach plant chlorine dioxide, and recovery boiler sampling. Material selection for caustic and oxidizing chloride service.
Overview
Pulp and paper mills cycle through nearly every aggressive process chemistry: hot caustic (white liquor), oxidizing chlorides (bleach plant), reducing sulfides (green liquor), and high-temperature combustion gas (recovery boiler). Sample probes must be matched to the specific stage of the kraft cycle.
Stage-by-Stage
| Stage | Service | Material |
| Wood prep, chip wash | Water + bark | 316L |
| Digester | Hot white liquor (NaOH + Na2S) | 316L or duplex |
| Brown stock washer | Weak black liquor | 316L |
| Evaporators | Concentrating black liquor | 316L; duplex above 50% solids |
| Recovery boiler | Smelt, hot flue gas | Inconel 625 |
| Causticizing (green liquor) | Na2S + Na2CO3 | 316L |
| White liquor clarifier | NaOH + Na2S | 316L |
| Bleach plant — D stage (ClO2) | ClO2 + Cl- | Hastelloy C276 |
| Bleach plant — E stage (NaOH) | NaOH | 316L or duplex |
| Bleach plant — P stage (peroxide) | H2O2 | 316L |
The Bleach Plant Problem
Chlorine dioxide bleach (ClO2) at 50-70 °C is one of the most aggressive industrial chloride environments. Free Cl⁻ at concentrations of several thousand ppm combined with the strong oxidizing action of ClO2 destroys 316L by chloride pitting within weeks.
The standard answer is Hastelloy C276, which combines the chloride resistance of 16% Mo + 4% W with the oxidation resistance of 16% Cr. For the most severe service, C22 or titanium grade 7 is specified.
Recovery Boiler
The recovery boiler burns concentrated black liquor and recovers sodium sulfate as smelt. Sample probes in the flue gas path see 1500-2000 °F combustion products with sulfur and sodium compounds. Inconel 625 is the standard material.
Configurator Defaults
When the user selects "pulp and paper" in the SPA Configurator:
1. The material recommender asks which stage of the kraft cycle
2. Bleach plant defaults to Hastelloy C276
3. Recovery boiler defaults to Inconel 625
4. Caustic service defaults to 316L with high-temperature derating