Electroplating Rinse Practice
and Evaporator Sizing

This Technical information Sheet is intended to be a practical guide to rinsing as applied to an evaporative recovery system. However, most of the information contained herein is applicable to any rinsing situation, whether or not a recovery system is being used.

Rinsing Principles

Rinsing is nothing more than a dilution of the chemicals adhering to the work surface. Two criteria that must be met in carrying out this dilution are: speed and effectiveness.

Traditionally, this has been accomplished by using large volumes of water at high flow rates. However, with the advent of pollution abatement and water conservation, a third criterion comes into play: efficiency. Rinse-water volumes and flow rates must now be drastically reduced without compromising rinse quality.

Where permissible, simple techniques such as increasing dwell time over the plating bath, or the use of "air knives" or spray rinsing over hot baths can effectively reduce bath drag-out volume. When this can't be done, an efficient rinsing step is essential when planning a recovery system.

Following are the important principles that must be considered in order to optimize rinsing efficiency.

Rinse Ratio

The term "rinse ratio" is a means of quantifying the amount of rinse water available relative to a particular work load.

For example, assume that a particular item being plated results in a drag-out rate, DO, of 1 gal./hr. If the flow rate of water, W, in the rinse tankis 100 gal./hr., the rinse ratio, R, would be 100:1.

  R =  W/DO,  or   (100 gal./hr.)/(1 gal.hr.)

If the work load changes, and the drag-out rate increases to 2 gal./hr., the rinse ratio at the same rinse-water flow rate would now be

	(100 gal./hr.)/(2 gal./hr. , or 50:1

The challenge in designing a rinse system is to determine the optimum rinse ratio, particularly when an evaporative recovery system is being considered. The lower the rinse ratio, the smaller the evaporator and the lower the operating costs.

However, if the rinse ratio is too low, rinsing quality may suffer. One of the key aspects of determining the optimum rinse ratio is how effective the mixing is in each rinse tank. As fresh water dilutes some of the chemicals on the work surfaces, it must be removed and replaced quickly with more fresh water.

One method of achieving this mixing is the use of a high water flow rate in the rinse tank. This is why it is typical to see water flow rates as high as 300 gal./hr., or even higher. The plater may therefore be falsely led to believe that high flow rates are essential to good rinsing.

What is essential is that good mixing occur in each rinse tank, particularly at the work surface.

Air Agitation

Air agitation is a means of achieving good mixing in a rinse system. It is, in most respects, better than high water flow rates. Air agitation should be as turbulent as possible without overflowing the tank or dislodging the work from the racks.

The source of the air should be a low-pressure blower (compressed air may contain oil), and it should be dispersed with a properly designed air spider, allowing uniform, thorough agitation of the rinse water. A single hose or pipe in the corner of the rinse tank does not constitute good air agitation. The air must break over the surface of the work in order to be effective.

Mechanical Mixing

In cases where air agitation is not permissible, such as with gravity-contact racks or where the work surface area is large and flat, mechanical mixing using electric- or air-driven stirrers may be used. However, this form of mixing is less efficient than air agitation, and as a result, rinse ratios will have to be somewhat higher.

Countercurrent Mixing

Countercurrent mixing is the single most important principle of good rinsing practice. The use of true countercurrent rinse water flow, as opposed to cascade flow which allows rinse water to overflow from one tank across the top of the preceding rinse tank, greatly lowers the required rinse ratio and drastically reduces rinse-water consumption.

It is absolutely essential if efficient evaporative recovery Is being considered.

Countercurrent rinsing is simply the use of two or more rinse tanks connected in series. Fresh water enters the tank furthest from the plating tank and flows into each preceding tank until it exits from the rinse tank closest to the plating tank.

The result is that the work exits the rinse system from the tank having the cleanest water. When an evaporative recovery system is used, the rinse water leaving the rinse tank nearest the plating bath has the highest concentration of plating solution and is the feed stream to the evaporator. The water distillate from the evaporator is returned to the rinse tank furthest from the plating tank.

The most important design criterion for a countercurrent rinse system is good mixing. This is achieved by having the inlet and outlet of each rinse tank located as far apart as is practically possible. Usually, the inlet is at the bottom of one side and the outlet at the top of the opposite side.

The outlet should also have an outflow weir so as to minimize short-circuiting of the water flow path.

The size of the rinse tank is essentially unimportant, as long as the work can be easily accommodated. Tank size only affects the length of time it takes to achieve equilibrium. It will have little effect on the quality of rinsing.

Spray or mist type rinse systems, while mechanically and hydraulically more complex than flowing rinse systems, can also benefit from the economies of the "pump forward" or countercurrent flow principle.

Evaporator Sizing

Proper sizing of an evaporator depends on a number of factors relative to a particular situation. If one has adhered to the above principles, and if the rinse system has three or more countercurrent rinse tanks, the evaporator can generally be sized to approximately 15 times the drag-out rate from the plating tank.

In most installations, a rinse ratio of at least 15:1 has been found to provide an optimum balance between economic operation, good mixing, and effective rinsing. The following formulas should help you determine the evaporator capacity for your particular application and rinse-tank arrangement.

Calculations

The concept of rinse ratio has already been explained as the ratio of the rinse-water flow rate to the drag-out rate. The water flow becomes the design capacity of the evaporator in an evaporative recovery system. In order to determine if a given rise ratio is adequate, the concentrations in each rinse tank can be calculated for various rinse ratios by using the formula

	Ci = Co/Ri

C_i = Target concentration of the principal metal ion in rinse tank i in a counter-current system;
C_o = Concentration of principal metal ion in the plating tank; and
R_i = Rinse ratio raised to the power of i, as used in C_i.

For example, assume that you wish to calculate the hex chrome concentration in the third rinse tank of a 3-tank counter-current rinse system for a chrome plating operation. The plating bath concentration is 30 oz./gal. of Cr0₃.

If we use a rinse ratio of 15:1, the calculation then becomes

C3  = (30 oz./gal.)/153 = 0.0089 oz./gal.  CrO3

To convert oz./gal. of Cr0₃ to ppm of hexavalent chromium, multiply the above answer by the factor 7500 (which converts oz./gal. to ppm) and divide by 2 (the percentage of chrome in a given amount of CrO₃, is 50%). Thus,

	(0.0089 x 7500)/2 = 33 ppm hexavalent chrome

For most decorative chrome operations, the final rinse should contain no more than 10 ppm of hexavalent chrome. For this example, therefore, the proper procedure would be to recalculate using a slightly higher rinse ratio or a fourth countercurrent rinse tank. Either of these strategies will reduce the chrome concentration in the final rinse tank.

The total quantity of chrome that can be theoretically captured with an evaporative recovery system is calculated using the formula

	%capture =  (1 - 1/Ri ) x 100

With the assumptions of the preceding example,

	%capture =  (1 - 1/153) x 100 = 99.97%

Therefore, in this example, 99.97% of the chrome bath usually lost by drag-out is recovered, and it becomes a relatively simple mathematical process to work through various combinations of rinse ratios, numbers of rinse tanks, plating bath concentrations, etc., to arrive at the required evaporator capacity for a given drag-out rate and rinse-tank configuration.

The forgoing calculations are based on 100% effective rinsing in each stage of a countercurrent rinse system.

This Process Profile supersedes all previous issues.

QVF Process Systems pursues a policy of continuous product improvement. We therefore reserve the right to alter any product or process as described and illustrated.