1. The Kraft process: architecture and pumping points
The Kraft process (also called sulfate) is the dominant method of industrial-scale pulp production worldwide, responsible for approximately 90% of manufactured pulp. The principle is chemical: wood chips (in Brazil, overwhelmingly eucalyptus) are cooked in a digester with an alkaline solution of sodium hydroxide and sodium sulfide (white liquor), at temperatures of 150 to 170 °C and pressures of 7 to 10 bar, for periods of 1 to 3 hours.
The lignin binding the cellulose fibers dissolves, releasing the fibers as pulp. The resulting solution — now laden with dissolved lignin, organic products and consumed soda — is called black liquor, and passes through a chemical recovery chain that regenerates the original soda and, along the way, generates thermal energy for the mill itself.
From a pumping standpoint, this chemical chain is the first source of complexity: each of the three liquors (white, black, green) has distinct properties — pH, temperature, suspended solids, abrasiveness — and requires a pump with specific materials. The second source of complexity is the pulp stock itself, which traverses the mill at varying concentrations (called consistency) from 1% to 15% and above.
The rule of thumb is that conventional centrifugal pumps handle consistency up to 4% well (technically up to 6% with special pumps); above this, high-consistency pumps with special impellers are required. The third source is the paper machine, with its own circuit of white water, broke, showers and stock preparation — all high-flow, water-abundant points.
2. White liquor: high alkalinity and material recommendation
White liquor is the active cooking solution, composed essentially of sodium hydroxide (NaOH, 90 to 120 g/L as Na₂O) and sodium sulfide (Na₂S, 25 to 40 g/L), with extremely alkaline pH (pH 13 to 14). Operating temperature is between 70 and 95 °C in transfer lines and 150 to 170 °C inside the digester.
This combination — high alkalinity and high temperature — is aggressive especially for carbon materials and low-nickel alloys. The standard recommendation for FBCN in white liquor service is AISI 316L austenitic stainless steel casing and impeller (CF8M in cast form) or, in critical high-temperature high-speed applications, duplex 2205, which combines corrosion resistance with greater mechanical toughness.
White liquor sealing is a critical point. The solution tends to crystallize when exposed to air or local dehydration at the seal face, forming white sodium carbonate deposits that cause leakage and mechanical damage. The recommendation is pressurized double mechanical seal (Plan 53B or 54), with barrier fluid in industrial treated water or diluted alkaline solution, barrier pressure 1 to 2 bar above process pressure, and continuous monitoring of barrier reservoir pressure and temperature.
Seal faces should be silicon carbide against silicon carbide, or silicon against carbon for lower-demand applications. Elastomer: EPDM or PTFE — FKM (Viton) does not resist alkaline white liquor for long periods.
3. Black liquor: high viscosity, abrasion and chemical recovery
Black liquor is the digester effluent after cooking, containing dissolved lignin, organic compounds and residual soda. When leaving the digester it is diluted (weak black liquor, about 15% solids), but passes through a multi-effect evaporation chain until reaching burning concentration (strong black liquor, 70 to 80% solids) that feeds the recovery boiler.
This chain is critical from a pumping standpoint because it covers a huge spectrum of conditions: weak liquor at 90-100 °C, moderately viscous, relatively easy to pump with 316L FBCN; and strong liquor at 120-130 °C, extremely viscous (500 to 2,000 cP), abrasive due to partial crystallization of inorganic salts, and with abrasive particulate that wears impellers in a few months.
For weak black liquor, standard FBCN in CF8M 316L with rotation up to 1,750 rpm is sufficient. For medium black liquor (40 to 55% solids), the recommendation is FBCN with reduced rotation to 1,450 rpm and double seal with face water flush.
For strong black liquor (above 60% solids), speed is limited to 1,150 rpm, material is duplex 2205 or superaustenitic, and the seal must be double-face with external flush and cooling jacket or, alternatively, gear pump (FBE or FBEI) to leverage PD technology tolerance for viscous fluids. The final choice depends on exact temperature, solids content and operating regime (batch or continuous).
4. Pulp stock: consistency and centrifugal pump limits
Pulp stock consistency — percentage of fibers by dry weight relative to the total suspension — is the most important parameter in pulp and paper pump selection. Stock at 1% is hydraulic, behaving like thick water, and any standard FBCN handles it without difficulty. At 2% to 3%, it is still served by conventional closed-impeller FBCN.
At 4%, NPSH problems begin to appear due to flow resistance, and the recommendation is FBCN with semi-open impeller and moderate rotation. Above 5%, stock stops being hydraulic and takes on viscoelastic behavior — a standard FBCN starts to churn stock instead of moving it, and the solution is a dedicated pulp pump with back-swept "vortex" or "recessed impeller" style vanes.
FB Bombas covers all process points up to 4% consistency with the standard FBCN line and semi-open impeller variants. Above this, especially in thick-stock preparation stages (5% to 14%), the Brazilian market traditionally uses imported Sulzer Ahlstar A-series or Andritz Stock Pump pumps — FB Bombas does not compete in this specific very-high-consistency range, keeping focus on the range where its technology is competitive and mature.
For consistencies between 4% and 6% with moderate flow, there are hybrid solutions with modified FBCN that may be evaluated case by case, but honest scope is: 0 to 4% consistency is FB, above 6% is not.
| Consistency (%) | Typical application | Technology | FB scope |
|---|---|---|---|
| 1 to 2 | White water, showers, effluent | Standard FBCN | Yes |
| 2 to 4 | Post-wash brown stock | FBCN closed or semi-open impeller | Yes |
| 4 to 6 | Stock in storage towers | Modified FBCN or pulp pump | Case by case |
| 6 to 15 | Thick stock preparation | Dedicated pulp pump (Sulzer Ahlstar, Andritz) | No |
5. ECF/TCF bleaching: aggressive chemistry and duplex materials
The bleaching plant is the most corrosive section of the Kraft process.
Brazilian market pulp is predominantly produced by the ECF (Elemental Chlorine Free) process, using chlorine dioxide (ClO₂) and hydrogen peroxide (H₂O₂) in a sequence of D0-E0-D1-E1-D2 stages, each with specific chemistry: D0 uses ClO₂ in acid medium, E0 uses NaOH + O₂ in alkaline medium, D1 repeats D0 chemistry at different concentration, E1 repeats E0 chemistry, and so on.
Temperature varies from 60 to 90 °C, pH oscillates from 2 to 12 within the same plant, and chlorides at moderate concentration are always present. This combination makes bleaching the most demanding service in the mill in terms of corrosion.
For FBCN in bleaching service, 316L is insufficient in ClO₂ stages due to chloride pitting risk. The standard recommendation is duplex 2205 (UNS S32205), which combines chlorinated-medium corrosion resistance with adequate mechanical toughness. In D0 and D1 chlorine dioxide stages at high temperature (>80 °C), it may be necessary to migrate to super duplex 2507 or superaustenitic 254 SMO. Sealing is also critical: Plan 53B double seal with compatible barrier fluid, solid silicon carbide faces, PTFE or Kalrez elastomer.
The system needs active seal face cooling because high localized temperature exponentially increases the corrosion rate in chlorinated medium.
6. Paper machine and thermal oil dryers
The paper machine integrates the mill's final pumping points. The white water circuit recirculates filtrate from wire sections and head box, with large flows (500 to 3,000 m³/h) in standard FBCN with moderate-pH-compatible material. Wire and felt cleaning showers demand elevated pressure (10 to 20 bar), typically handled by multi-stage FBCN or single-stage FBCN with increased rotation.
Broke (paper rejected at cutting or shutdowns) is pumped back to the stock preparation circuit and falls under the same consistency logic — FB up to 4%, dedicated pump above that.
The drying section of the paper machine is where the FBOT line comes in. In modern machines, dryer cylinders are heated by thermal oil (as an alternative to saturated steam) at temperatures of 260 to 300 °C, enabling greater energy efficiency and more precise temperature control than steam. The thermal oil circuit is closed, with boiler, expansion tank, heat exchangers on the cylinders and circulation pumps.
FBOT is the appropriate pump for this application: designed for high-temperature fluid, natural-convection-cooled seal chamber, oversized bearings, materials compatible with Therminol/Dowtherm/Marlotherm. Typical flows in paper machines range from 150 to 800 m³/h at pressures of 5 to 12 bar.
7. Brazilian pulp and paper market: context and operators
Brazil is the world's second-largest producer of short-fiber (eucalyptus) pulp and is among the top ten global paper producers. Current installed capacity is approximately 25 million tons per year of pulp, with growth expected to 30 million by 2028 considering projects in progress.
The Brazilian pulp and paper sector concentrates tens of millions of tons per year of capacity distributed among major national operators.
The most recent greenfield projects adopted international energy efficiency standards, including electric cogeneration from black liquor burning in the recovery boiler — a synergy with the power plant cluster on the FB site.
From a pump standpoint, the Brazilian pulp and paper market is well divided: specialized high-consistency pumps and critical process pumps traditionally come from European manufacturers (Sulzer, Andritz, KSB), while auxiliaries, utilities, effluents, partial bleaching, paper machine and thermal oil are served by national manufacturers.
FB Bombas positions itself as the dominant national supplier in this second group, with more than four decades of supply history to all major sector operators, competitive advantage in delivery time (12 to 20 weeks vs 40 to 60 weeks for imported equipment) and local technical support for commissioning, maintenance and retrofit.
