Open and Closed Operation of Bleaching Stages

The drawback with liquor circulation is the accumulation of material in the liquor

cycles. There are cases, where such accumulation is inappropriate from a process

perspective, for example when transition metals build up in a totally chlorine-free

bleaching sequence. In elementary chlorine-free sequences, liquor circulation can

lead to an increase in the concentration levels of corrosive chloride ions. Depending

on the liquor management, such stages may also be affected which are not

charged with chlorine-containing chemicals and which, therefore, were not originally

designed for such conditions.

The normal practice of limiting the accumulation of undesirable matter is to

sewer either a part or the totality of a liquor, rather than to recirculate it. The stage

is then said to be operated partly or totally open, as opposed to closed operation

where all the filtrate is used elsewhere in the process (Fig. 7.143).

In an open stage, the concentrations of dissolved liquor constituents depend

primarily on the pulp consistency delivered by the washer preceding this stage. If

the feed pulp comes from a press at high consistency, a respectable amount of

intra-stage circulation is needed to dilute the pulp before medium-consistency

bleaching. This circulation brings the filtrate concentrations up to approximately

double the values observed in a stage fed from a washer with medium-consistency

discharge. At the same time, the effluent volume from a high-consistency fed

stage is about half that from a medium-consistency fed stage (Fig. 7.144).

Whilst there is a near-linear dependency of the filtrate concentrations on the

feed consistency to the stage, the effects of partial closure are highly nonlinear.

Partial closure occurs when filtrate which is otherwise sent to the sewer is utilized

elsewhere in the bleaching process. As the volume of filtrate sent to the sewer is

gradually reduced, the initial change in concentrations is rather low, especially in

bleaching systems using medium-consistency washing equipment. Note that a

reduction in effluent load (e.g., COD load) by 50% is reached only after 65–75% of

the filtrate volume is circulated back to the process (Figs. 7.145 and 7.146). In

these figures, a zero degree of closure represents an open stage (as per Fig. 7.142a)

and 100% closure means operation (according to Fig. 7.143c).

When a given bleaching stage is closed and concentrations rise, the carry-over

of dissolved matter to the subsequent stage increases in proportion to the filtrate

concentration. This fact must be carefully considered, since the amount of carryover

may affect the performance of the subsequent bleaching stage.

890 7Pulp Bleaching

Pump,

Mixer,

Дата добавления: 2015-10-21