THE ART AND SCIENCE O F RINSING

ABSTRACT

Proper design and operation of r inse systems following electroplating

Equations a r e given to define the pr imary rinsing

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and other metal finishing operations a r e essential fo r adequate rinsing and economical use of water. factors. r inse water permit substantial savings in water consumption. current r inse systems also permit additional substantial savings in water. Savings in water consumption with multiple r inse tanks a r e readily calcu- lated by means of simple equations.

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Conductivity measurements with auxiliary control of flow of the Counter-

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LIST O F SLIDES

1. 2. 3. 4. 5. 6. 7. 8. 9.

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22, 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.

AES Slide Title Slide Scope The Flowing Rinse Tank Four P r i m a r y Rinsing Fac tors Concentration in a Flowing Rinse Conversion Factors Equilibrium Equation The Rinsing Ratio Concentration in the Preceding Tank Concentration in the Rinse Volume of Drag-in Photograph of Conductivity Meter Typical Drag-out Curves Flow Regulation Loss in Concentration Due to Flow Controlled Flow Rate of Flow Photograph of Rinse Tank Controller Size of a Rinse Tank Typical Regional Water Hardness Contamination Levels in Types of Tanks Contamination Levels in P rocess Double Counter cur rent Rinse Double Wall Rinse Flow in Multiple Rinse Tanks Rinsing Equations Multiple Rinsiflg Flow Multiple Rinsing C onc ent r at ion Roots of R Reciprocal Powers of R The Effectivity

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Printed AES Press

Copyright 1973, American Electroplaters' S o c i e t y , Inc.

THE ART AND SCIENCE O F RINSING

Slide 1

This introduces "The Art and Science of Rinsing" presented a s one of a s e r i e s of educational lectures offered b y the American Electro- platers Society.

Slide 2

The author wishes to thank Beckman Instruments, Inc. for sponsoring this lecture, f o r providing information from their ex- perience with conductivity measurements and f o r preparation of the slides.

The author also wishes to express his appreciation to Dr. D. A. Swalheim for his advice and suggestions.

K-77

Slide 3

This p re sen ta t ion i s concerned wi th fou r bas i c r i n s i n g f a c t o r s , By e f f i c i e n t r i n s i n g w e mean us ing t h e minimum of w a t e r requi red t o r i n s e t h e p a r t so t h a t drag-out from t h e r i n s e w i l l no t i n t e r f e r e wi th t h e next process ing s t e p . This simply means t h a t t he water flow t o t h e r i n s e tank may vary by about 7,500 f o l d as you s h a l l see la te r .

Four primary r i n s i n g f a c t o r s are expressed by the simple equat ion o r modi f ica t ion of t h a t equat ion . The equat ion de f ines the accep tab le contamination l e v e l i n a flowing r i n s e tank and i t a p p l i e s t o r i n s i n g under equ i l ib r ium cond i t ions i n a c o n t r o l l e d r i n s e tank.

A form of t h e equat ion i s used t o determine t h e volume of drag- i n t h a t i n t roduces t h e contaminants t o t h e r i n s e .

The equat ion a p p l i e s b e s t when t h e r i n s e i s con t ro l l ed auto- m a t i c a l l y i n response t o a c t i v a t i o n of a c o n t r o l l e r by t h e contami- n a t i n g substances.

Modified forms of t h e b a s i c equat ion de f ine c h a r a c t e r i s t i c s of countercur ren t r i n s e s t h a t account f o r s u b s t a n t i a l w a t e r savings.

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Slide 4

The s i n g l e flowing r i n s e tank is commonly used t o d i l u t e chemi- cals c a r r i e d i n and t o flow them t o t h e sewer. Rinsing i s e f f e c t i v e when the w a t e r enters near t h e bottom of t h e tank, flows d iagonal ly a c r o s s t h e work and overflows on t h e oppos i te s i d e . p ropor t iona l t o t h e q u a n t i t y of s o l u t i o n dra ined i n t o t h e r i n s e tank. The water consumption can be reduced g r e a t l y by c o n t r o l l i n g t h e flow.

The amount of water needed is

Slide 5

Four f a c t o r s t h a t desc r ibe a r i n s i n g tank w i l l be d iscussed . These are presented as CT, CR, F and D and are def ined as fo l lows:

CT i s t h e concent ra t ion i n t h e preceding tank. It i s a l s o t h e concen t r a t ion of t h e s o l u t i o n dragged i n .

CR i s the concent ra t ion of t h e r i n s e a t equi l ibr ium.

F i s t h e f low through t h e r i n s e .

D i s t h e drag-in. It is t h e volume of t h e s o l u t i o n dragged i n t o t h e r i n s e .

Slide 6

The equat ion t h a t de f ines r i n s i n g e f f i c i e n c y a p p l i e s when the q u a n t i t y of chemicals in t roduced i n t o t h e r i n s e i s equal t o t h e q u a n t i t y

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flowed away. tration as racks of work a r e introduced to the rinse at equal intervals of time. but an even smaller amount is carr ied away before the second rack is introduced. librium, the amount of contaminant carr ied away is equal to the amount introduced. efficiency and the equilibrium applies.

This Figure shows the fluctuating increase in concen-

The first rack introduces a small amount of contaminant

Each rack increases the concentration until, a t equi-

At this level the rinse operates a t essentially 100 percent

Slide 7

Rinse concentrations a r e conveniently described a s par ts p e r In more familiar t e rms 1000 ppm equals one gram pe r million: ppm.

liter. so it is useful to visualize the concentrations of interest by keeping in mind that one ounce pe r gallon equals 7500 ppm. rinsing contamination of 1 / 10 ounce per gallon a s 750 ppm.

Of course 7-1/2 grams per l i ter equals one ounce per gallon,

O r visualize a common

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Slide 8

The equ i l ib r ium equat ion is: D x CT = F x CR

The drag-in t i m e s t h e concent ra t ion i n the tank equals t h e flow t i m e s t he concent ra t ion i n t h e r i n s e .

The c a l c u l a t i o n app l i ed t o one rack is:

0.04 x 40 = 2 x 0.8

1.60g = 1.60g

The las t express ion on the s l i d e i s very important . The express ion can a l s o be s t a t e d as fol lows:

Volume of Drag-in x Conc. = Flow x Conc. i n t h e Rinse

This simply means t h a t w e should use t h e minimum amount of rinse requ i r ed t o remove t h e sal ts dragged i n . Any useage t h i s amount i s wasted and u n j u s t i f i e d .

f low i n t h e of water beyond

S l i d e 9

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The equat ion can be w r i t t e n as:

These can be def ined as two equal r i n s i n g r a t i o s .

& = E D

When t h r e e of t h e f o u r f a c t o r s are known, t h e f o u r t h can be c a l c u l a t e d . This g ives u s a va lue f o r RC from which t h e flow can be r e l a t e d t o t h e drag-in. A l so , as w e s h a l l soon see, t h e s e va lues can be used i n o t h e r equat ions t o estimate t h e economy of m u l t i p l e r i n s i n g .

CT i s known and CR is e s t a b l i s h e d from experience.

Slide 10

.C, CONCENTRATION IN THE

PRECEDING TANK

1. Estimsting the nominal composition.

2. A complete chemical analysis.

The concen t r a t ion i n t h e preceding tank , CT, i s known from t h e The nominal composition is s u f f i c i e n t l y accu ra t e f o r ba th make-up.

r i n s i n g c a l c u l a t i o n s . t h e v a l i d i t y of t h e r i n s i n g equat ions. a complete chemical a n a l y s i s o r by r e l a t i n g t h e conduct iv i ty of t h e s o l u t i o n t o t h e conduct iv i ty of a s i m i l a r known s o l u t i o n .

A g r e a t e r accuracy i s des i r ed f o r checking CT is then determined by

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Slide 11

The concentration in the rinse, CR, is quantitatively evaluated b y conductimetric means, a s shown in this graph. additions of the solution in question a r e added to the r inse water and the conductivity is measured af ter each addition i s well mixed. ally provides a straight conductivity- concentration line that can be used as a reference to analyze r inse waters. to natural waters m a y reveal a break in the conductivity-concentration line. This indicates that the acid is neutralizing the bicarbonate in the water. It a l so indicates that the r inse is only distinctly acid above the break. Beyond this point the line reveals the normal conductivity in- crease. The curve serves as a mean of analyzing the alkaline-acid character is t ics of the r inse and also a s a standard to analyze for con- tamination levels in the rinse.

A se r i e s of known

This usu-

Small amounts of acid added

Alternatively the concentration in the r inse can be determined b y wet chemical means o r by any other desired analytical procedure.

CR and CT can be related by any single constituent that is soluble and stable both in the tank and the r inse since the r inse con- stituent that is dragged in, will define the ratio, Rc, just as well a s will the total concentration.

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Slide 12

To determine the drag-in:

Add a small amount of solution and take a conductivity reading; and then add a known amount of solution and take a second reading.

T h e s e two readings establish the conductivity increase f o r a known volume addition (the calibration volume).

Process a successive number of racks through a fresh r inse and take a conductivity reading af ter each rack is removed. mean line through the points expressing the conductivity increase versus the number of racks.

Plot a

From the above combined data read the mill i l i ters dragged in per rack.

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Slide 1 3

Conductivity measurements can be used to determine solution concentrations. at different concentrations can be found in chemical handbooks. Mixtures of salts and acids o r bases form more complex solutions, but the over a l l concentration of these mixtures may still be determined by measuring conductivity a s long a s the different components maintain a fixed rat io relationship to one another with only the amount of water varying. Generally the relationship between mixture concentration and conductivity is established by the use of a laboratory conductivity bridge. Several samples of different concentrations a r e made up, and the con- ductivity of these is measured a t some fixed temperature. A plot of concentration vs. conductivity can then be used to determine concen- tration f rom conductivity at that particular temperature. Once these relationships have been established concentrations of a wide variety of mater ia ls can be determined quickly and inexpensively.

The conductivity of single component water solutions

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Slide 14

Typical drag-out curves , t h a t were e s t a b l i s h e d by conduct iv i ty measurements, are shown he re . By e x t r a p o l a t i o n t h e drag-out a t ze ro t i m e i s 10 t o 15 m i l l i l i t e r s per square f o o t . It i s g r e a t e s t wi th h o r i z o n t a l , so lu t ion - re t a in ing s u r f a c e s and least wi th v e r t i c a l s h e e t . And t h e drag-in t o t h e r i n s e i s g r e a t l y reduced by al lowing the s o l u t i o n t o d r a i n back i n t o t h e preceding tank. A d r a i n t i m e of 15 seconds re- duced t h e drag-in t o 3 t o 7 m i l l i l i t e r s pe r square f o o t . S tudies of t h i s type are e a s i l y made wi th conduct iv i ty equipment and they r e a d i l y re- veal t h e sav ings i n drag-over t h a t r e s u l t from a s h o r t de lay i n t h e d r a i n i n g t i m e .

A l i g h t fog r i n s e , ac tua t ed j u s t as t h e rack leaves t h e preceding tank , w i l l r i n s e drag-out d i r e c t l y back t o t h e tank and reduce t h e drag- i n t o t h e r i n s e . A spray r i n s e , ac tua t ed as t h e rack l eaves t h e r i n s e , w i l l reduce t h e drag-in t o a second r i n s e when double r i n s i n g is used. The curves r e a d i l y r e v e a l t h e reduct ion of drag-out t h a t r e s u l t s from a s h o r t de lay i n d r a i n i n g t i m e .

The s i g n i f i c a n c e of spray and fog r i n s e s i n reducing drag-out l o s s e s can no t be overemphasized.

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Slide 15

2. Restricted

REGULATION

4. M8WUrOd

A great amount of water is wasted by allowing uncontrolled flow through r inse tanks. throttled by a meaningful twist of the valve.

When water i s allowed to run freely it should be

If the temptation is too great to le t the water flow, a permanent res t r ic t ion is recommeded in the water line.

The flow should be checked occasionally by allowing it to over- flow into a bucket o r by observing the time for an empty tank to f i l l .

i

A knowledge of the flow rate can be related to the rinsing character is t ics and the flow can conveniently be observed by the use of a flow me te r in the inlet line. An interesting and revealing com- parison is made by relating the flow of all the rinse tanks to the water bill. r inse tanks has been found to account for the major consumption of water.

Even in large plants, with multiple water usage, the flow through

When water costs a r e significant, they will pay for the convenience of automatic control.

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Slide 16

The equat ion t h a t de f ines r i n s i n g e f f i c i e n c y as d iscussed ear l ie r a p p l i e s when t h e q u a n t i t y of chemicals introduced i n t o t h e r i n s e i s equal t o t h e q u a n t i t y flowing away. When a r i n s e tank i s allowed t o run , with- ou t process ing work, the e f f i c i e n c y decreases r a p i d l y . The o r d i n a t e on t h i s graph can be read as concen t r a t ion o r as r i n s i n g e f f i c i e n c y . I f 100 i s t h e maximum d e s i r e d concent ra t ion , i t w i l l drop t o 50 percent i n seven minutes. It w i l l then demand t w i c e as much water t o remove t h e same q u a n t i t y of salts as i t d i d a t t h e s ta r t . A t 10 minutes, t he t o t a l f low is one t ank volume and t h e concen t r a t ion i s only 37 percen t .

This curve, which a p p l i e s t o a flowing r i n s e a f t e r t h e work has been removed, w i l l fol low a s imple exponent ia l equat ion wi th in t h e l i m i t s of experimental e r r o r .

This is an important f a c t . It shows t h a t t h e f r e s h incoming water mixes completely wi th t h e s o l u t i o n i n the r i n s i n g tank. It i s a l s o t r u e t h a t t he drag-in mixes very w e l l wi th t h e s o l u t i o n i n t h e r i n s i n g tank.

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Slide 17

The r inse tank controller consists of a measuring unit, a conduc- tivity cell and a solenoid operated valve. r inse tank in response to the $hanging resistance fed to the measuring bridge by the conductivity cell. The conductivity input turns water on and off and signals the response with colored lights.

The control unit monitors the

When an automatic r inse tank controller is used, the r inse tank functions theoretically and the basic equation applies. To repeat:

D t imes CT equals F t imes CR: The drag-in t imes the concen- t ra t ion in the preceding tank equals the flow times the concentration in the rinse.

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Slide 18

This figure shows the volume of flow, expressed a s tank volumes, necessary to rinse away the drag-in a t different contamination levels. At a range of 90 to 100 par ts pe r million, o r 90 to 100 percent of maximum concentration, it takes one-tenth of a tank volume to rinse f rom the maxi- mum to the minimum level: Or it takes approximately a 10 percent flow to reduce the concentration by 10 percent. If the range i s 10 to 20 par t s p e r million, then a reduction in concentration of 50 percent is required which will take seven-tenths of a tank volume to reduce the concentration by 50 percent.

These considerations of equilibrium conditions reveal that the

In practice these con- Even an automatic plating machine will skip an

equilibrium is only maintained when racks a r e introduced a t regular intervals and the drag-in is equal on each rack. ditions a r e very rare. occasional rack and present racks that a r e unequally loaded. r inse control will overcome irregular processing efficiency because water will not flow when contaminants a r e not introduced. of an automatically controlled tank will approximate the ideal equilibrium of this Figure.

Automatic

So the behavior

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Slide 19

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Some important f e a t u r e s of a p l a t i n g p l a n t r i n s e tank c o n t r o l l e r are embodied by the instrument shown on t h e s l i d e . They inc lude :

An ins t rument case which provides both a means of mounting t h e instrument s ecu re ly t o a p ipe , w a l l , o r pane l as w e l l as a f f o r d i n g p r o t e c t i o n a g a i n s t p l a t i n g p l a n t environments. A l l opeinings should be gasketed and p rov i s ion made f o r s e a l i n g t h e l e a d s f o r t h e conduct iv i ty ce l l , so lenoid valve, and power l i n e .

A l l c o n t r o l s should be loca ted where they are e a s i l y a c c e s s i b l e but i n such a way t h a t they are tamperproof and cannot be c a s u a l l y re- ad jus t ed by p l a n t personnel .

S igna l l i g h t s should be easy t o see a t a d i s t a n c e as an i n d i c a t i o n of ins t rument ope ra t ion . Green l i g h t f o r r i n s e concent ra t ion below t h e contamination l e v e l . Red l i g h t f o r r i n s e concen t r a t ion above t h e con- taminat ion l e v e l .

Automatic temperature compensation provides c l o s e r c o n t r o l when r i n s e tank w a t e r temperature v a r i e s . The conduct iv i ty of water inc reases wi th i n c r e a s i n g water temperature . I f t h e r e are wide swings i n temp- e r a t u r e i n t h e r i n s e tank water, a c o n t r o l l e r without automatic temp- e r a t u r e compensation may c o n t r o l a t a h ighe r o r lower contamination level than i n i t i a l l y set a t .

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Slide 20

The size of the rinse tank does not influence the rinsing ratio. It does not change the influence of the concentration in the preceding tank on the concentration in the rinse (the ratio defined a s RC - the ratio based on concentration). Nor does it change the influence of the volume of drag-in on the flow (the ratio defined a s RV - the ratio based on volumes). However, at a se t flow, a small tank will wash away contaminants fas ter and it will respond more frequently to drag- in within a controlled range.

It i s recommended that rinse tanks be just large enough to ac- commodate the largest rack of work plus any sensing equipment.

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Slide 21

Water supplies that a r e fed by sur face waters or snow fields are quite soft , whereas waters in lakes, deep wells and ar id regions a r e harder .

Typical regional water hardness values a r e shown in this Table and, of course, the extremes can be lower and much higher than these. The salts that cause water hardness a r e generally not as objectionable to plating tanks a s a r e equivalent concentrations of processing solutions. Work being processed will generally tolerate water with a hardness of 250 ppm plus the contamination levels that will be shown next. An exception to this generalization is the hard, water that promotes spotting on finished work that i s to be dried.

Local waters can be incompatible with plating quality and al- When lowable contamination levels can be exceeded by such waters.

this is true, the waters a r e treated o r deionized to an acceptable level. In some instances the treated waters a r e reprocessed and used again.

The column headed I'micromhos" is the electrolytic conductivitv of the water in micromhos/cm at 25OC. Refer to ASTM- Test Method

'

D- 1 125.

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Slide 22

Hexavalent chromium stains a r e avoided by holding the rinse a t a very low contamination level. In a single rinse this requires large amounts of water so this is done by double rinsing o r countercurrent rinsing. chromium is reduced to the trivalent state by use ot reducing substances in the rinse, a s in U. S. Patent 3,552,993. This i s a form of chemical rinsing, similar to the Lancy integrated waste treatment system, that destroys unacceptable forms in a circulating rinse and avoids problems with high water consumption at low contamination levels.

Much la rger contamination levels can be tolerated if the

Rinses containing metal salts a r e held at low levels to avoid con- tamination of plating baths with objectionable metals. Low metal con- centrations that a r e dragged into the bath will continuously codeposit without harm. Higher concentrations will cause dark, rough o r brittle deposits. nant in the r inse with the quality of the plated work.

Thus a relationship is established by correlating the contami-

Final r inses also a r e held quite low to maintain good appearance , and avoid corrosion that can be promoted by residual salts.

Some intermediate r inses between process steps can be operated a s high a s 2 to 4 grams p e r l i ter (2000 to 4000 ppm).

Holding rinses, that retain drag-out o r keep a surface wet fo r further processing, can be operated in the range of 5 to 15 grams per l i ter (one to two ounces pe r gallon).

Here we see that the spectrum of the contamination levels covers a range of 2000 t imes and, of course, the water demand in a single con- trolled r inse can have this same range.

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S l i d e 23

S l i d e 24

A number of s t u d i e s have been made t h a t r e s u l t e d i n recommendations f o r contamina- t i o n levels . from one t o 7500 p a r t s p e r m i l l i o n . The f i g u r e s shown on t h e s l i d e are intended mere- l y as a guide. You w i l l n o t e t h a t t h e r i n s e fol lowing a l k a l i n e c leaning p r i o r t o a l - k a l i n e s t a n n a t e t i n can t o l e r a t e a r e l a t i v e l y high concent ra t ion of c l e a n e r drag-in. This seems l o g i c a l s i n c e t h e a l k a l i n e t i n ba th a l s o con ta ins sodium hydroxide.

Extreme and in t e rmed ia t e contamination levels are shown h e r e , ranging

Now le t ' s examine t h e t h i r d type of process i . e . , p i c k l i n g followed by a c i d p l a t - L e t ' s assume t h e a c i d p l a t i n g ba th i s a b r i g h t a c i d copper s u l f a t e ba th opera ted ing.

wi th about 50 mg/L of ch lo r ide . c h l o r i c a c i d allowed i n t h e r i n s e is much too high i f t h e p i c k l e cons i s t ed of hydro- c h l o r i c a c i d . This l i m i t would apply i f a s u l f u r i c a c i d p i c k l e were used because drag-in of s u l f u r i c a c i d would no t be harmful i n t h e b r i g h t ac id copper ba th .

It i s obvious t h a t t h e l i m i t of 375 mg/L of hydro-

With t h e c u r r e n t emphasis on recovery and r ecyc le , t h e al lowable l i m i t of 375 mg/L i n t h e r i n s e fo l lowing p i c k l i n g i n hydrochlor ic a c i d p r i o r t o chromium p l a t i n g is a l s o probably too h igh .

The concen t r a t ion l e v e l s , such as 38, 15 and 1 mg/L, are maintained a t low l e v e l s because t h e s t e p s fol lowing r i n s i n g are more s e n s i t i v e t o the contaminants from t h e tank preceding t h e r i n s e . It i s r e a d i l y apparent from t h i s s l i d e t h a t t h e contamina- t i o n levels i n t h e r i n s e can vary about 7,500-fold and, of course , t h e water demand i n a s i n g l e c o n t r o l l e d r i n s e can a l s o vary by t h e same amount.

Double r i n s i n g s u b s t a n t i a l l y reduces t h e amount of w a t e r requi red t o r i n s e t o a des i r ed concent ra t ion . The w a t e r e n t e r s t h e l a s t r i n s e i n l i n e and flows i n t o t h e f i r s t r i n s e whi le t h e work i s processed i n t h e countercur ren t d i r e c t i o n .

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in line and flows into the f i r s t r inse while the work is processed in the counter cur rent direction.

,

A double rinse with an overflow dam is shown in this Figure. When bulky work is rapidly introduced into the f i r s t rinse, the water r i s e s in the tank and floods or splashes into the las t rinse. This un- duly contaminates the las t r inse and reduces the rinsing efficiency. Also, the water flows into and out of the first rinse a t the surface and does not r inse a s effectively a s the diagonal flow of the second rinse. These problems can be avoided by the use of two separate rinse tanks. Although two such tanks use much l e s s water than one tank, they use twice a s much water a s an efficient double countercurrent rinse.

Slide 25

A double-wall countercurrent rinse overcomes the problems with The double wall

It also baffles And it directs

the cascade countercurrent r inse of the previous slide. between the two rinses prevents splashing over the wall. the back-flow caused by the introduction of bulky work. the flow diagonally across the f i r s t rinse.

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Slide 26

_-

An additional tank at a rinsing station provides a processing Work that i s processed through two tanks is assured of advantage.

more consistent rinsing because a s a minimum it must be completely immersed and removed a t least twice. take up twice the f loor space and they increase the processing time.

On the other hand, two tanks

A second tank markedly increases water savings and additional tanks further increase the savings. dollars, the third is occasionally economical and a fourth i s very un- likely to be worth while.

The second tank often saves

This slide shows the reduction of flow that can be expected with addition of two and three tanks a t various rinsing ratios. ing ratios a r e generally in the range of 100 to 1,000 so the second tank will reduce the water demand by 90 to 97 percent. of a third tank will reduce the demand by 95 to 99 percent. readily seen why the third tank i s seldom economical. However, a look at the remaining flow, rather than the reduction, i s worth con- sideration. At a rinsing ratio of 100 the remaining flow in two tanks is 1 0 percent and in three tanks it is 5 percent; o r a saving of 50 per - cent for the third tank over the second. With low rinsing ratios and large scale automatic operations the third tank can be economical. The= too consider that when I: is large the w-ater deiiiand is large and with a big scale operation a remainder savings of a few percent over the first tank becomes a substantial savings when based on the second tank.

Rins-

The addition It is

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-EZ-

ax%=d

: 10

Slide 28

--

Equation 4, applied to a double countercurrent rinse, shows that the flow decreases as the square root of the rinsing ratio:

Let the rinsing ratio equal 100 and the number of tanks in a ser ies equal 2. Then the square root of the rinsing ratio equals 10.

The flow for one rinse tank is 100 times the drag-in and the flow for two countercurrent rinse tanks is 10 t imes the drag-in or a reduction in flow of 90 percent.

Slide 29

Equation 5 applied to a double countercurrent rinse shows that the concentration in the r inse decreases a s the reciprocal of the square of the rinsing ratio.

Let the rinsing ratio equal 10 and the number of rinse tanks in Then the reciprocal of R equals one-tenth and the r e -

The concentration of the ser ies equal 2. ciprocal of R squared equals one-hundredth. f i r s t rinse tank in the ser ies comes to equilibrium a t one tenth the con-

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centration in the preceding tank. The second rinse tank in the ser ies comes to equilibrium a t one-tenth the concentration in the first rinse tank or one-hundredth the concentration in the tank preceding the first rinse.

Slide 30

The response of multiple rinse tanks can be anticipated by ex- amining tables of the roots of R.

The roots of R compare reduction in flow a s tanks a r e added to the countercurrent series. the ratio of 100 to 10 to 4.6 for one, two and three tanks in series. RC equals 1000, the ratio of reduction will be 100 to 3. 2 to one. When RC is large, the reduction in flow is substantial as tanks a r e added to a series. So, the larger the value of Rc, the l e s s the need for additional tanks.

When RC equals 100, the flow reduces in When

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Slide 31

The reciprocal powers of R show the reduction in each successive tank in a se r ies of countercurrent tanks. ductions in the first, second and third tanks in the ser ies a r e one-tenth, one-hundredth and one-thousandth of the concentration in the tank preced- ing the f i r s t rinse. Notice that when RV equals 1000 that two rinse tanks reduce the concentration to 1 x 10-6 of the concentration of the tank preceding the first rinse and when RV equals 100 i t takes three rinse tanks to reduce to this same concentration.

When RV equals 10, the r e -

Slide 32

The final slide defines the effectivity of rinsing. Theoretically ?.'J divided by RC equals ene. In nyap+. ip- r - - - -- - - the ratic? of the two R-'s lislially equals 1.00 t 0. 10 and this i s defined a s the practical value of E, the ef- fectivity. So E equals the flow times the concentration in the rinse divided by the drag-in t imes the concentration in the tank and if processing is steady D i s reasonably consistent a s an average value. When the r inse concentration is controlled, a high value of E indicates a high flow and inefficient rinsing. This may be due to splash over in a countercurrent rinse. Whatever the cause, a calculation of E and exami- nation of the four pr imary rinsing factors will suggest an explanation.

The value of CT i s fixed

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At low values of R and low flows in multiple rinsing E may deviate f r o m 1.0 - t 0. 10 indicating a need to revise the rinsing theory. of E can provide an empirical constant to describe practice in t e r m s of corrected rinsing equations when deviations f rom theory exist.

The value ~

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References

(1) J. B. Mohler, P l a t i n g and Surface F in ish ing , 66 pg . 42, October 1979

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