Maximising Pump Efficiency in Black Liquor Evaporation 

Pulp and paper mills are constantly under pressure to improve efficiency, reduce costs and meet stringent environmental regulations. One of the most challenging aspects of the process is handling black liquor, the highly alkaline byproduct of wood pulping. Managing this abrasive, corrosive liquid efficiently is crucial to maintaining smooth operations and minimising downtime.

Black liquor evaporation plays a key role in recovering valuable chemicals and preparing the liquor for combustion, but it can present significant operational challenges. From extreme temperatures to high solid concentrations, mills need robust solutions to keep the process running reliably.

The Challenge: Managing Abrasive and Corrosive Conditions

The black liquor process begins with weak black liquor—an alkaline solution with a pH of around 12—collected from the washing process. This liquor contains a mix of organic material, including lignin and salts and needs to be concentrated for recovery.

Through a series of steam-heated evaporators, the black liquor's solids content increases from 13-17% to around 50%. To reach the required concentration for combustion in the recovery boiler, it is further processed to 60-75% solids in a final concentrator. Throughout this process, pumps must handle highly abrasive solids at temperatures ranging from 55˚C to 135˚C under pressures of up to 5 bar (75 psi).

Inefficient or unreliable sealing in these conditions leads to process dilution, rotating equipment wear and costly downtime. Mills need sealing solutions that prevent failures, extend equipment life and reduce maintenance costs.

The Solution: Advanced Sealing Technologies

Double mechanical seals are almost always required to ensure long-lasting and reliable operation in black liquor evaporation. These seals prevent process dilution, protect against abrasive particles and avoid dry running in vacuum conditions. John Crane offers engineered sealing solutions designed specifically for the harsh conditions of black liquor processing, including:

1. Engineered Fitted Seals

Safeseal Type SE2 and JCS2P integrated double mechanical seal: Designed for high-demand applications, these double-balanced seals provide superior performance and reliability.

2. Standard Cartridge Seals

• Type SB2 and Type SB2A: A dual cartridge mechanical seal with a double-balanced design, ideal for use with environmentally hazardous and abrasive fluids, most often used in pumps in pulp mills, minerals processing and other slurry duties up to 40% solids by weight and a solids hardness of 8 Mohs.

• Type 5620USP: This Upstream Pumping (USP) seal is part of the universal cartridge series, a modular seal family that includes interchangeable elastomer bellows, metal bellows and elastomer O-ring pusher seal designs. With a seal face constructed of SiC and carbon, the Type 6520USP ensures non-contact sealing, reducing wear and improving reliability in liquids with suspended solids. The USP effect prevents harmful particles from reaching the seal faces by maintaining a clean barrier fluid, which enhances the mechanical seal’s lifespan and reduces maintenance needs.