EOW May 2007

english

Result : A specific throughput of 1 is equal to a specific concentration of 0.63.

Example :

In estimating the efficiency and water consumption during rinsing operations, stationary values can usually be expected.

Carry-over........................................V X Rinsing factor................................ c 2 /c 0 = 1:200 divided in two stages of approx...1:14 each (14x14 = 196) Process bath concentration:......................... c 0 Rinsing bath 1 0 /14 = 0.0714 c 0 Water consumption..........0.1 x 14 = 1.4 l/m 2 = 0.1 l/m 2 Concentration...............c 1 = c

This is corresponding to 12.6%.

2. Rinsing methods 2.1. Drag-out rinse with no overflow A drag-out rinse is a rinse operated without an overflow. It consists of one tank and is dumped after a certain time. These operation conditions result in an enrichment of the chemicals carried over from the process bath, in an extreme case amounting to the concentration of the previous bath. The drag-out rinse is used: After pickling baths with high iron content, to prevent precipitation of iron (III)-Hydroxide on the wire or tube surface; To relieve the following overflow rinses; After process baths with high evaporation and drag-out losses. A typical procedure is to allow the concentration in the drag-out rinse to reach about 20% of the previous bath and to counteract the losses there with the drag-out rinse. • • •

2.2. Overflow rinse A rinsing bath being fed continuously or intermittently with water is called an overflow rinse. Apart from the period following make-up, the concentration in the overflow rinse remains mainly constant. The concentration and hence the rinsing factor is set by the overflow. For the overflow rinse the following formula applies approximately in the stationary state:

Rinsing bath 2

Concentration..............c 2 1 /14 = 0.005 x c 0 Water consumption..........0.1 x 14 = 1.4 l/m 2 = c

V

= V

x c

/c

Total water consumption:...................2.8 l/m 2

Z

X

0

in which c 0 is the concentration of the previous bath and c that of the overflow rinse. Example :

The water consumption is thus reduced from 20 l/m 2 with a single-stage rinse to around 3 l/m 2 with a two-stage rinse.

2.4. Cascade rinsing The term cascade rinsing is applied when in a multi-stage rinse the overflow from the subsequent rinse is used as the influx water feed . For an n-stage cascade rinse the following formula approximately applies:

Carry-over.........................................V X Rinsing factor..................................c/c 0

= 0.1l/m 2 = 1:200

Water consumption V Z

= 0.1 x 200 = 20l/m 2

2.3. Multi-stage rinse The water consumption can be drastically reduced by rinsing in several stages with a constant rinsing factor.

In this way the chemicals are recycled.

V

= V

x n

co/cn

Z

X

2.1.1. Calculation The enrichment is calculated as follows:

V

fresh water feed

V

fresh water feed

c

= c

0 x ( 1 - e Vx x F/V )

Z1

Z2

1

c 1 = concentration in the drag-out rinse as a function of the throughput c 0 = concentration of the previous bath V x = carry-over volume per workpiece surface V = volume of the rinsing bath F = work piece surface throughput D = specific surface throughput Example :

V x = 0.1l/m 2 carry-over V = 2,000 l bath volume c 0 = 20% concentration F = 20,000m 2 throughput

Figure 2 : Diagram showing the principle function

s

V z fresh water feed

By insertion the following is obtained:

C 1 = 20 x ( 1 – e –0.1x20000/2000 ) Result : The concentration in the drag out rinse is 12.6%. Alternatively from the diagram with the specific through-put (D):

D = 0.1 x 20,000/2,000 =1

s Figure 3 : Diagram showing the difference to a multi-stage rinse

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EuroWire – May 2007