THE ART AND SCIENCE OF RINSING ABSTRACT

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2 THE ART AND SCIENCE OF RINSING ABSTRACT Proper design and operation of rinse systems following electroplating and other metal finishing operations are essential for adequate rinsing and economical use of water. Equations are given to define the primary rinsing factors. Conductivity measurements with auxiliary control of flow of the rinse water permit substantial savings in water consumption. Countercurrent rinse systems also permit additional substantial savings in water. Savings in water consumption with multiple rinse tanks are readily calculated by means of simple equations. LIST OF SLIDES , AES Slide Title Slide Scope The Flowing Rinse Tank Four Primary Rinsing Factors 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 Process 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 AI1 rights reserved. Printed in the United Statesof America. This publtcation may not be reproduced. stored in a retrieval system. or transmitted in whole or in part. in any form or by any means. electronic. mechanical. photocopying, recording. or otherwise. without the prior written permission of AES Louisiana Ave.. Winter Park. FL Printed AES Press Copyright 1973, American Electroplaters' Society, Inc.

3 THE ART AND SCIENCE OF RINSING Slide 1 This introduces "The Art and Science of Rinsing" presented as one of a series of educational lectures offered by the American Electroplaters Society. Slide 2 The author wishes to thank Beckman Instruments, Inc. for sponsoring this lecture, for providing information from their experience with conductivity measurements and for preparation of the slides. The author also wishes to express his appreciation to Dr. D. A. Swalheim for his advice and suggestions. K-77

4 Slide 3 This presentation is concerned with four basic rinsing factors, By efficient rinsing we mean using the minimum of water required to rinse the part so that drag-out from the rinse will not interfere with the next processing step. This simply means that the water flow to the rinse tank may vary by about 7,500 fold as you shall see later. Four primary rinsing factors are expressed by the simple equation or modification of that equation. The equation defines the acceptable contamination level in a flowing rinse tank and it applies to rinsing under equilibrium conditions in a controlled rinse tank. A form of the equation is used to determine the volume of dragin that introduces the contaminants to the rinse. The equation applies best when the rinse is controlled automatically in response to activation of a controller by the contaminating substances. Modified forms of the basic equation define characteristics of countercurrent rinses that account for substantial water savings. - 2-

5 Slide 4 The single flowing rinse tank is commonly used to dilute chemicals carried in and to flow them to the sewer. Rinsing is effective when the water enters near the bottom of the tank, flows diagonally across the work and overflows on the opposite side. The amount of water needed is proportional to the quantity of solution drained into the rinse tank. The water consumption can be reduced greatly by controlling the flow.

6 Slide 5 Four factors that describe a rinsing tank will be discussed. These are presented as CT, CR, F and D and are defined as follows: CT is the concentration in the preceding tank. concentration of the solution dragged in. It is also the CR is the concentration of the rinse at equilibrium. F is the flow through the rinse. D is the drag-in. into the rinse. It is the volume of the solution dragged Slide 6 The equation that defines rinsing efficiency applies when the quantity of chemicals introduced into the rinse is equal to the quantity -4-

7 flowed away. This Figure shows the fluctuating increase in concentration as racks of work are introduced to the rinse at equal intervals of time. The first rack introduces a small amount of contaminant but an even smaller amount is carried away before the second rack is introduced. Each rack increases the concentration until, at equilibrium, the amount of contaminant carried away is equal to the amount introduced. At this level the rinse operates at essentially 100 percent efficiency and the equilibrium applies. Slide 7 Rinse concentrations are conveniently described as parts per million: ppm. In more familiar terms 1000 ppm equals one gram per liter. Of course 7-1/2 grams per liter equals one ounce per gallon, so it is useful to visualize the concentrations of interest by keeping in mind that one ounce per gallon equals 7500 ppm. Or visualize a common rinsing contamination of 1 / 10 ounce per gallon as 750 ppm. -5-

8 Slide 8 The equilibrium equation is: D x CT = F x CR The drag-in times the concentration in the tank equals the flow times the concentration in the rinse. The calculation applied to one rack is: 0.04 x 40 = 2 x g = 1.60g The last expression on the slide is very important. can also be stated as follows: The expression Volume of Drag-in x Conc. = Flow x Conc. in the Rinse This simply means that we should use the minimum amount of rinse required to remove the salts dragged in. Any useage this amount is wasted and unjustified. flow in the of water beyond Slide 9-6-

9 The equation can be written as: These can be defined as two equal rinsing ratios. &=E D When three of the four factors are known, the fourth can be calculated. CT is known and CR is established from experience. This gives us a value for RC from which the flow can be related to the drag-in. Also, as we shall soon see, these values can be used in other equations to estimate the economy of multiple rinsing. Slide 10.C, CONCENTRATION IN THE PRECEDING TANK 1. Estimsting the nominal composition. 2. A complete chemical analysis. The concentration in the preceding tank, CT, is known from the bath make-up. The nominal composition is sufficiently accurate for rinsing calculations. A greater accuracy is desired for checking the validity of the rinsing equations. CT is then determined by a complete chemical analysis or by relating the conductivity of the solution to the conductivity of a similar known solution. -7-

10 Slide 11 The concentration in the rinse, CR, is quantitatively evaluated by conductimetric means, as shown in this graph. A series of known additions of the solution in question are added to the rinse water and the conductivity is measured after each addition is well mixed. This usually provides a straight conductivity- concentration line that can be used as a reference to analyze rinse waters. Small amounts of acid added to natural waters may reveal a break in the conductivity-concentration line. This indicates that the acid is neutralizing the bicarbonate in the water. It also indicates that the rinse is only distinctly acid above the break. Beyond this point the line reveals the normal conductivity increase. The curve serves as a mean of analyzing the alkaline-acid characteristics of the rinse and also as a standard to analyze for contamination levels in the rinse. Alternatively the concentration in the rinse can be determined by wet chemical means or 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 rinse since the rinse constituent that is dragged in, will define the ratio, Rc, just as well as will the total concentration. -8-

11 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. These two readings establish the conductivity increase for a known volume addition (the calibration volume). Process a successive number of racks through a fresh rinse and take a conductivity reading after each rack is removed. Plot a mean line through the points expressing the conductivity increase versus the number of racks. From the above combined data read the milliliters dragged in per rack. -9-

12 Slide 13 Conductivity measurements can be used to determine solution concentrations. The conductivity of single component water solutions at different concentrations can be found in chemical handbooks. Mixtures of salts and acids or bases form more complex solutions, but the over all concentration of these mixtures may still be determined by measuring conductivity as long as the different components maintain a fixed ratio 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 are made up, and the conductivity of these is measured at some fixed temperature. A plot of concentration vs. conductivity can then be used to determine concentration from conductivity at that particular temperature. Once these relationships have been established concentrations of a wide variety of materials can be determined quickly and inexpensively

13 Slide 14 Typical drag-out curves, that were established by conductivity measurements, are shown here. By extrapolation the drag-out at zero time is 10 to 15 milliliters per square foot. It is greatest with horizontal, solution-retaining surfaces and least with vertical sheet. And the drag-in to the rinse is greatly reduced by allowing the solution to drain back into the preceding tank. A drain time of 15 seconds reduced the drag-in to 3 to 7 milliliters per square foot. Studies of this type are easily made with conductivity equipment and they readily reveal the savings in drag-over that result from a short delay in the draining time. A light fog rinse, actuated just as the rack leaves the preceding tank, will rinse drag-out directly back to the tank and reduce the dragin to the rinse. A spray rinse, actuated as the rack leaves the rinse, will reduce the drag-in to a second rinse when double rinsing is used. The curves readily reveal the reduction of drag-out that results from a short delay in draining time. The significance of spray and fog rinses in reducing drag-out losses can not be overemphasized. -11-

14 Slide Restricted REGULATION 4. M8WUrOd A great amount of water is wasted by allowing uncontrolled flow through rinse tanks. When water is allowed to run freely it should be throttled by a meaningful twist of the valve. If the temptation is too great to let the water flow, a permanent restriction is recommeded in the water line. The flow should be checked occasionally by allowing it to overflow into a bucket or by observing the time for an empty tank to fill. A knowledge of the flow rate can be related to the rinsing characteristics and the flow can conveniently be observed by the use of a flow meter in the inlet line. An interesting and revealing comparison is made by relating the flow of all the rinse tanks to the water bill. Even in large plants, with multiple water usage, the flow through rinse tanks has been found to account for the major consumption of water. i When water costs are significant, they will pay for the convenience of automatic control

15 Slide 16 The equation that defines rinsing efficiency as discussed earlier applies when the quantity of chemicals introduced into the rinse is equal to the quantity flowing away. When a rinse tank is allowed to run, without processing work, the efficiency decreases rapidly. The ordinate on this graph can be read as concentration or as rinsing efficiency. If 100 is the maximum desired concentration, it will drop to 50 percent in seven minutes. It will then demand twice as much water to remove the same quantity of salts as it did at the start. At 10 minutes, the total flow is one tank volume and the concentration is only 37 percent. This curve, which applies to a flowing rinse after the work has been removed, will follow a simple exponential equation within the limits of experimental error. This is an important fact. It shows that the fresh incoming water mixes completely with the solution in the rinsing tank. It is also true that the drag-in mixes very well with the solution in the rinsing tank. -13-

16 Slide 17 The rinse tank controller consists of a measuring unit, a conductivity cell and a solenoid operated valve. The control unit monitors the rinse 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. When an automatic rinse tank controller is used, the rinse tank functions theoretically and the basic equation applies. To repeat: D times CT equals F times CR: The drag-in times the concentration in the preceding tank equals the flow times the concentration in the rinse. -14-

17 Slide 18 This figure shows the volume of flow, expressed as tank volumes, necessary to rinse away the drag-in at different contamination levels. At a range of 90 to 100 parts per million, or 90 to 100 percent of maximum concentration, it takes one-tenth of a tank volume to rinse from the maximum to the minimum level: Or it takes approximately a 10 percent flow to reduce the concentration by 10 percent. If the range is 10 to 20 parts per 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 equilibrium is only maintained when racks are introduced at regular intervals and the drag-in is equal on each rack. In practice these conditions are very rare. Even an automatic plating machine will skip an occasional rack and present racks that are unequally loaded. Automatic rinse control will overcome irregular processing efficiency because water will not flow when contaminants are not introduced. So the behavior of an automatically controlled tank will approximate the ideal equilibrium of this Figure. -15-

18 Slide 19 Some important features of a plating plant rinse tank controller are embodied by the instrument shown on the slide. They include: An instrument case which provides both a means of mounting the instrument securely to a pipe, wall, or panel as well as affording protection against plating plant environments. All opeinings should be gasketed and provision made for sealing the leads for the conductivity cell, solenoid valve, and power line. All controls should be located where they are easily accessible but in such a way that they are tamperproof and cannot be casually readjusted by plant personnel. Signal lights should be easy to see at a distance as an indication of instrument operation. Green light for rinse concentration below the contamination level. Red light for rinse concentration above the contamination level. Automatic temperature compensation provides closer control when rinse tank water temperature varies. The conductivity of water increases with increasing water temperature. If there are wide swings in temperature in the rinse tank water, a controller without automatic temperature compensation may control at a higher or lower contamination level than initially set at. -16-

19 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 as RC - the ratio based on concentration). Nor does it change the influence of the volume of drag-in on the flow (the ratio defined as RV - the ratio based on volumes). However, at a set flow, a small tank will wash away contaminants faster and it will respond more frequently to dragin within a controlled range. It is recommended that rinse tanks be just large enough to accommodate the largest rack of work plus any sensing equipment

20 Slide 21 Water supplies that are fed by surface waters or snow fields are quite soft, whereas waters in lakes, deep wells and arid regions are harder. Typical regional water hardness values are shown in this Table and, of course, the extremes can be lower and much higher than these. The salts that cause water hardness are generally not as objectionable to plating tanks as are 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 is to be dried. Local waters can be incompatible with plating quality and allowable contamination levels can be exceeded by such waters. When this is true, the waters are treated or deionized to an acceptable level. In some instances the treated waters are 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

21 Slide 22 Hexavalent chromium stains are avoided by holding the rinse at a very low contamination level. In a single rinse this requires large amounts of water so this is done by double rinsing or countercurrent rinsing. Much larger contamination levels can be tolerated if the chromium is reduced to the trivalent state by use ot reducing substances in the rinse, as in U. S. Patent 3,552,993. This is 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. Rinses containing metal salts are held at low levels to avoid contamination of plating baths with objectionable metals. Low metal concentrations that are dragged into the bath will continuously codeposit without harm. Higher concentrations will cause dark, rough or brittle deposits. Thus a relationship is established by correlating the contaminant in the rinse with the quality of the plated work. Final rinses also are held quite low to maintain good appearance, and avoid corrosion that can be promoted by residual salts. Some intermediate rinses between process steps can be operated as high as 2 to 4 grams per liter (2000 to 4000 ppm). Holding rinses, that retain drag-out or keep a surface wet for further processing, can be operated in the range of 5 to 15 grams per liter (one to two ounces per gallon). Here we see that the spectrum of the contamination levels covers a range of 2000 times and, of course, the water demand in a single controlled rinse can have this same range. -19-

22 Slide 23 A number of studies have been made that resulted in recommendations for contamination levels. Extreme and intermediate contamination levels are shown here, ranging from one to 7500 parts per million. The figures shown on the slide are intended merely as a guide. You will note that the rinse following alkaline cleaning prior to alkaline stannate tin can tolerate a relatively high concentration of cleaner drag-in. This seems logical since the alkaline tin bath also contains sodium hydroxide. Now let's examine the third type of process i.e., pickling followed by acid plating. Let's assume the acid plating bath is a bright acid copper sulfate bath operated with about 50 mg/l of chloride. It is obvious that the limit of 375 mg/l of hydrochloric acid allowed in the rinse is much too high if the pickle consisted of hydrochloric acid. This limit would apply if a sulfuric acid pickle were used because drag-in of sulfuric acid would not be harmful in the bright acid copper bath. With the current emphasis on recovery and recycle, the allowable limit of 375 mg/l in the rinse following pickling in hydrochloric acid prior to chromium plating is also probably too high. The concentration levels, such as 38, 15 and 1 mg/l, are maintained at low levels because the steps following rinsing are more sensitive to the contaminants from the tank preceding the rinse. It is readily apparent from this slide that the contamination levels in the rinse can vary about 7,500-fold and, of course, the water demand in a single controlled rinse can also vary by the same amount. Slide 24 Double rinsing substantially reduces the amount of water required to rinse to a desired concentration. The water enters the last rinse in line and flows into the first rinse while the work is processed in the countercurrent direction. -20-

23 in line and flows into the first rinse 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 first rinse, the water rises in the tank and floods or splashes into the last rinse. This unduly contaminates the last rinse and reduces the rinsing efficiency. Also, the water flows into and out of the first rinse at the surface and does not rinse as effectively as 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 less water than one tank, they use twice as much water as an efficient double countercurrent rinse. Slide 25 A double-wall countercurrent rinse overcomes the problems with the cascade countercurrent rinse of the previous slide. The double wall between the two rinses prevents splashing over the wall. It also baffles the back-flow caused by the introduction of bulky work. And it directs the flow diagonally across the first rinse. -21-

24 Slide 26 An additional tank at a rinsing station provides a processing advantage. Work that is processed through two tanks is assured of more consistent rinsing because as a minimum it must be completely immersed and removed at least twice. On the other hand, two tanks take up twice the floor space and they increase the processing time. A second tank markedly increases water savings and additional tanks further increase the savings. The second tank often saves dollars, the third is occasionally economical and a fourth is very unlikely to be worth while. This slide shows the reduction of flow that can be expected with addition of two and three tanks at various rinsing ratios. Rinsing ratios are generally in the range of 100 to 1,000 so the second tank will reduce the water demand by 90 to 97 percent. The addition of a third tank will reduce the demand by 95 to 99 percent. It is readily seen why the third tank is seldom economical. However, a look at the remaining flow, rather than the reduction, is worth consideration. At a rinsing ratio of 100 the remaining flow in two tanks is 10 percent and in three tanks it is 5 percent; or a saving of 50 percent 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

25 -EZ- ax%=d : 10

26 - 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 series 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 times 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 rinse decreases as the reciprocal of the square of the rinsing ratio. Let the rinsing ratio equal 10 and the number of rinse tanks in series equal 2. Then the reciprocal of R equals one-tenth and the reciprocal of R squared equals one-hundredth. The concentration of the first rinse tank in the series comes to equilibrium at one tenth the con

27 centration in the preceding tank. The second rinse tank in the series comes to equilibrium at 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 examining tables of the roots of R. The roots of R compare reduction in flow as tanks are added to the countercurrent series. When RC equals 100, the flow reduces in the ratio of 100 to 10 to 4.6 for one, two and three tanks in series. When 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 are added to a series. So, the larger the value of Rc, the less the need for additional tanks. -25-

28 Slide 31 The reciprocal powers of R show the reduction in each successive tank in a series of countercurrent tanks. When RV equals 10, the reductions in the first, second and third tanks in the series are one-tenth, one-hundredth and one-thousandth of the concentration in the tank preceding the first 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 it takes three rinse tanks to reduce to this same concentration. Slide 32 The final slide defines the effectivity of rinsing. Theoretically r the ratic? of the two R-'s lislially?.'j divided by RC equals ene. In nyap+.ipequals 1.00 t and this is defined as the practical value of E, the effectivity. So E equals the flow times the concentration in the rinse divided by the drag-in times the concentration in the tank The value of CT is fixed and if processing is steady D is reasonably consistent as an average value. When the rinse 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 examination of the four primary rinsing factors will suggest an explanation

29 At low values of R and low flows in multiple rinsing E may deviate from t indicating a need to revise the rinsing theory. The value of E can provide an empirical constant to describe practice in terms of corrected rinsing equations when deviations from theory exist. References (1) J. B. Mohler, Plating and Surface Finishing, 66 pg. 42, October