Overview. The process engineer’s focus is usually on the washing efficiency

The process engineer’s focus is usually on the washing efficiency. Among the targets

set for a washing application may be a recovery rate or, more frequently, a

carry-over. The carry-over denotes the amount of a substance which escapes with

the pulp leaving the washing system. Frequently, the carry-over is expressed as

chemical oxygen demand (COD) or saltcake loss. The latter has been used traditionally

because it was easy to analyze, and provided an indication of the recovery

rate of inorganic chemicals. Whilst interest in the recovery of chemicals persists

for economical reasons, the focus has been shifting towards organic compounds

as closed cycles and environmental aspects gain increasing importance in process

design.

The efficiency of a pulp washing system is judged by how much of the solute it

removes and the amount of wash water required to accomplish this solute

removal. While a number of schemes and nomenclatures have been developed to

express the performance of washers and washing systems [19], we will focus here

on the most used and most practical approaches.

Careful readers of the previous sections will suspect that the theoretical calculation

of the washing efficiency without practical input is virtually hopeless. Fortunately,

hands-on experience is available for all types of commercial washing equipment,

which can be used for an estimation of the washing efficiency to be

expected under a given set of conditions.

In this subsection, we will explore briefly the wash yield and displacement ratio,

before examining more deeply today’s most relevant measure for washing efficiency,

the Norden E factor.

5.5.2