The recirculation process - Concentration control and stock losses - Part 3 -

- Part 3 - Concentration control, material losses and summary / continued from Galvanotechnik 11/2024

The recycling process consists of a coating process followed by a recycling rinse. The use of soluble anodes results in deficit or surplus operation, depending on whether the metal concentration is below or above the desired target value. Mathematical models of continuous and discontinuous feedback processes are presented. The models are used to demonstrate process engineering methods for controlling the concentration. Possibilities for minimizing material loss are also discussed. It is made clear that in surplus operation, reducing the current yield difference is the only way to reduce metal losses. Last episode of a three-part series.

6 Concentration control during recirculation

6.1 Lowering the concentration

Discarding from the coating process

The direct way to reduce the metal concentration in the coating process is to partially discard the process solution. If the concentration c0(t) is to be lowered to a new value c0,set, the volume to be discarded is

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Here V0 is the volume of the coating process before discarding. If the original volume is replenished with metal-free solution after discarding, the desired concentration c0,set is obtained.

Discarding from the recirculation sink

Discarding from the recirculation rinse and replacing it with fresh water also leads to a reduction in the metal concentration in the coating process. However, this does not take effect immediately, but only when the evaporation losses of the coating process are subsequently compensated for via the recirculation rinse. The effect also gradually diminishes again, as the recirculation rinse concentrates again until the stationary concentration is reached again. If the stationary concentration in the coating process is to be reduced by discarding from the recirculation rinse, it must therefore be regularly discarded from there.
Alternatively, the solution can also be continuously discarded from the recirculation rinse. In this case, there is a continuous volume flow into the wastewater treatment system in addition to the return volume flow. The deficit resulting from the two volume flows is replaced by fresh water. The corresponding structure is shown in Figure 13. The fresh water volume flow V.ri1 required to reduce the process concentration to a desired target value c0,set is derived from the material balance described in section 2.1:

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It should be noted here that the concentration in the separation process cannot be lowered arbitrarily by discarding from the recirculation sink, but at most to the "natural concentration" c*0 according to equation (11).

Throttling the recirculation

How the throttling of the recirculation can be used to reduce the metal concentration in the coating process was described in section 2.2. If the maximum possible volume flow corresponding to the evaporation is recirculated, the "natural recirculated concentration" c*0.rt is set according to equation (12). However, if the recirculation is reduced below the evaporation rate and the resulting volume deficit in the deposition process is supplemented with fresh water, the metal concentration in the coating process is reduced. The continuous recirculation is reduced to a value according to equation (13) to adjust the target concentration. Even with discontinuous recirculation, the metal concentration in the coating process can be reduced by throttling, see section 4.3.

Reduction of the current yield difference

As an alternative to discarding and throttling the recirculation, the metal concentration occurring in the deposition process can also be lowered by reducing the current yield difference. To do this, either the cathodic current yield must be increased and/or the anodic current yield must be reduced. An increase in the cathodic current yield can be achieved by changing the electrolyte composition. Improved hydrodynamics also contribute to this. The anodic current yield can also be reduced by changing the electrolyte composition. Changes to the anode surface area and thus the anodic current density can also have a corresponding effect. Last but not least, a partial replacement of the soluble anodes with insoluble anodes leads to a reduction in the total anodic current yield.

Figure 14 illustrates the reduction in concentration during the recycling process. The naturally recycled concentration c*0,rt is set when recirculation corresponds to evaporation. The desired target concentration c0,set is lower. In order to lower the steady-state concentration accordingly, the recirculation volume flow can be throttled, see blue curve Δη1 in Figure 14. Alternatively, the current yield difference can be reduced with unthrottled recirculation, green curve Δη2.

Figure 14: Recirculation process in excess operation

6.2 Increasing the concentration

If a desired target concentration is not reached at maximum recirculation (i.e. when the entire evaporation deficit from the recirculation sink is balanced out), measures to increase the metal concentration are necessary.

Dosing of metal salt

An easy measure to carry out is the dosing of metal salt. Section 2.3 shows how high the mass flow rate must be when dosing dry salt or highly concentrated salt solution in the steady state (equation 18). In addition, the formula for calculating the dosing volume flow when dosing with metal salt solution of limited concentration is given (equation 20).

Increasing the recirculation

One way to prevent the addition of metal salt is to increase the recirculation by increasing the natural evaporation. This can be achieved in particular by increasing the temperature in the separation process. Figure 15 shows the situation on the blue curve Δη1. With recirculation corresponding to evaporation, the naturally recirculated concentration c*0,rt is achieved, although a higher target concentration c0,set is desired. If it is possible to increase the evaporation and thus the recirculation to a higher value according to equation (13), the target concentration is set in the separation process.
However, operation at a higher temperature requires an increased use of heating energy. In addition, an increase in temperature is not possible for every electroplating process for process engineering reasons or from the point of view of a high-quality coating. The use of additional concentrator technology can be useful here. For example, evaporators or vaporizers can be used to remove water from the solution to be recycled and thus return a more concentrated solution.

Figure 15: Recirculation process in deficit operation

Increasing the current yield difference

One way to increase the concentration with unchanged recirculation is to increase the current yield difference. To do this, either the cathodic current yield must be reduced and/or the anodic current yield must be increased. It is often not possible to increase the anodic current yield, as this is usually already close to 100 %. On the other hand, the cathodic current yield can be reduced by changing the electrolyte composition. Figure 15 shows the increase in the metal concentration by increasing the current yield difference with the orange curve Δη2.

Discontinuous instead of continuous recirculation

There is a special process engineering option for increasing the metal concentration in the recirculation process if the unthrottled recirculation takes place continuously in the actual state. Alternatively, if the entire volume of the recirculation sink is recirculated discontinuously, this results in a higher process concentration. This can be seen by comparing the steady-state concentration c*0,rt with continuous recirculation according to equation (12) and the mean concentration with discontinuous recirculation c-0 according to equation (38). The quotient of both concentrations is

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The increase in concentration resulting from this equation with discontinuous recirculation compared to continuous recirculation is shown in Figure 16. If, for example, the recirculation (corresponding to evaporation) is as high as the carryover, the process concentration can be increased by approx. 36 % by switching from continuous to discontinuous recirculation. If the evaporation is twice as large as the carryover, the concentration can even be increased by 56 %. Overall, Figure 16 shows that an increase in concentration is achieved by discontinuous recirculation, especially with high evaporation.

Figure 16: Increased concentration with discontinuous recirculation

7 Material losses in the recirculation process

7.1 Operating situations

As the previous sections have shown, different operating situations must be distinguished in the recirculation process. If the carryover rate, the evaporation, the electrolysis current and the difference between the cathodic and anodic current yield are given, the natural-feedback concentration c*0,rt is stationary for the electrodeposition process according to equation (12). Depending on whether the desired target concentration c0,set is lower or higher than this concentration, the operating situations shown in Table 3 result.

7.2 Deficit operation

The direct measure to reduce substance losses in deficit operation is to reduce the drag-out from the recirculation sink. 

By reducing the drag-out, the concentration in the recirculation rinse is raised and the naturally recirculated metal concentration in the separation process increases as a result. The difference between this concentration and the target concentration is reduced and therefore less metal salt needs to be added.
Increasing the recirculation (e.g. by raising the temperature) also has a positive effect. This also leads to an increase in the naturally recycled concentration. The deficit to be compensated for by metal salt is reduced accordingly. This not only saves salt to be dosed, but also minimizes the same amount of material losses, as the salt required to increase the concentration ends up completely in the wastewater treatment. The transition from continuous to discontinuous recirculation described in section 6.2 also increases recirculation; here too, the dosing of metal salt and the associated metal loss is reduced.
A special case, however, is the increase in the current yield difference. This is generally achieved by reducing the cathodic current yield. Although this leads to a reduction in the dosage of metal salt, the metal losses remain unchanged with otherwise unchanged conditions in the recirculation process (drag-out, recirculation). In addition, the electrolysis current may have to be increased in order to compensate for the effect of the reduced current yield on the layer thickness. This increases the use of electrical energy.

7.3 Surplus operation

In surplus operation, the possibility of minimizing material loss is fundamentally different. The steady-state concentration in the electroplating process is higher than the desired target concentration. The measures listed in the previous section for deficit operation are therefore counterproductive here. Both the reduction of carryover and the increase in recirculation would cause a further increase in the concentration. In excess operation, on the other hand, it must be ensured that the excess metal released by the current yield difference is discharged from the recirculation process. As shown in Figure 12 (see Galvanotechnik 11/2024, p. 1425), there are three ways to do this. If waste from the process or from the recirculation rinse is used to remove the excess metal, this is selected so that the desired target concentration is achieved. Alternatively, the throttling of the recirculation system described in section 2.2 can be used to set the target concentration without any additional waste.
However, regardless of the metal discharge path, the metal losses are always the same. The amount of metal loss cannot be changed by selecting the discharge route; it is only possible to influence the concentration range in which the metal-containing wastewater is produced:

- Discharge from the coating process → highly concentrated wastewater
- Discharge from the return rinse → medium concentrated wastewater
- Carryover from the recirculation rinse → Low-concentration waste water from the subsequent rinsing cascade

If the metal losses in surplus operation are to be reduced, the only direct way is to reduce the current yield difference. On the one hand, this can be achieved by increasing the cathodic current yield. There are various options for this, such as improving the hydrodynamics or a targeted change in the electrolyte composition. On the other hand, the anodic current yield can be reduced. This can be achieved, for example, by increasing the anodic current density (reduction in surface area). However, excessive passivation of the anodes must be avoided. Another way to reduce the total anodic current yield is to make some of the anodes insoluble. If the insoluble anodes are fed by separate rectifiers, the current ratio soluble to insoluble can be used to control the current yield difference and thus the metal concentration in the galvanic process.

7.4 Ideal operation

Ideal operation is when the process concentration is set to the desired target value under the given conditions in the feedback process. No measures to increase or decrease the concentration are then necessary. However, it will rarely be the case that the metal concentration adjusts to a precisely specified target value. However, it is possible to check within certain limits whether the naturally occurring concentration is still acceptable from the point of view of quality-compliant separation. For example, in the event of a deficit, the operation of a reduced metal concentration can make the dosing of metal salt unnecessary. In the case of a surplus, allowing a higher steady-state metal concentration avoids process interventions to reduce the concentration. However, the material losses remain unchanged as long as the current yield difference does not change.

8 Discussion

8.1 Special cases

In the present work, the classic recirculation process was described, in which the recirculation sink is single and the deficit caused by the recirculation is filled with fresh water. The formulas presented here were developed for different operating regimes of this process structure. In practice, other modified structures of the recirculation process are used which are not discussed here:

Recirculation flushing cascades

In recirculation flushing cascades, several recirculation flushing stages are used, which are operated as a countercurrent cascade. This allows a high degree of recirculation to be achieved; however, this can also lead to a problematically high concentration of impurities in the process.

Flow-through recirculation sink

A classic flow-through rinsing cascade is used, whereby water is recirculated from the first rinse into the electroplating process and the remaining (larger) proportion of water is fed into the waste water treatment system. With this type of recirculation, however, only a low degree of recirculation (typically 10 %) is achieved. A compromise is to throttle the cascade water flow through the recirculation sink; this can increase the recirculation effect and at the same time achieve a higher rinsing effect compared to the recirculation stand sink.

Recirculation sink with pre-soaking

Pre-immersion in the recirculation bowl can effectively increase the recirculation effect [8]. A pure pre-soak stand sink already achieves a recirculation rate of 50 %. If the evaporation losses in the process are also compensated via this sink, significantly higher levels of recirculation can be achieved.
No analytical solutions are available for the structures mentioned here. However, it is possible to calculate the steady-state concentrations using the general flushing model presented in [9].

8.2 Limits of the model

The model equations given in this paper are based on various assumptions. If these are not fulfilled, the calculated values are inaccurate or the formulas are not applicable. The following limitations must be observed:

Tow-in and tow-out are not the same:

If the drag-in and drag-out in the coating process and in the recirculation rinse differ, the concentration ratios change. In particular, the recirculation possibility is reduced if the carry-out in the coating process is lower than the carry-in.

Incomplete mixing:

In rinsing practice, the concentration of carryover on the fabric is usually higher than the concentration in the rinsing stage. This effect is not considered here in the model equations. However, the effect should be manageable in a single recirculation rinse if the rinse is not too short, as incomplete mixing occurs particularly in low-concentration rinsing stages [10]. However, the concentration in the recirculation sink is still relatively high.

Additional metal losses:

It is pointed out in [2] that considerable metal losses can occur due to filtration and cleaning. The formulas also do not take into account losses that occur when anode metal is lost as a solid, in particular due to metal particles that detach from the anode.

Insoluble anodes:

If only insoluble anodes are used, many of the formulas given in the paper can no longer be meaningfully applied. In accordance with the operating situations introduced in section 7.1, insoluble anodes are always operated in deficit mode. The deposited and carried over metal must be introduced by salt dosing or chemical metal dissolution (e.g. zinc dissolution compartment). 

Alloy deposition:

The model equations were derived for the deposition of a single metal. The model equations must be suitably adapted for alloy deposition with soluble anodes.

Electroless processes:

Galvanic processes were considered. The formulas given are not applicable to external-current-free recycling processes. However, it is also true for processes without external current that the measures discussed for increasing recirculation can reduce material losses.

Stationary models:

In the model equations derived for continuously operated recirculation processes (Sections 2 and 3), constant process conditions were assumed or average values were used in the calculations. In practice, fluctuations in the assumed process variables occur. This leads to fluctuations in the calculated variables that are not represented in the stationary models. In the discontinuous models (Section 4), mean values and concentration ranges were calculated. Although dynamic processes were thus described, a stationary view was also taken here, as the state of a stable oscillation was described. Deviations occur here due to the averaging of the carryover concentration from the process. However, these can often be neglected if the width of the concentration fluctuation is small.

Validation of the models:

The theoretically derived models could not be consistently validated with practical measurements for the range of process variants described. However, all model calculations and in particular the calculation of discontinuous operation were verified using simulation models based on the solution of differential equations. Models of electroplating processes [11] realized in MATLAB/Simulink were used for this purpose.

Model parameters:

The parameters used in the model equations must be provided. In particular, the current yield difference must be known as the central parameter in the feedback process. Experimental methods based on differential weighing have long been used to determine the current yield [12]. In some cases, it is also possible to calculate the current yield from consumption data of anode material and process chemistry. In addition, an online measurement technique has been available for several years that can be used to measure current yields in industrial electroplating processes [13].

9 Summary

The feedback process discussed here consists of an electroplating process operated with soluble anodes and a subsequent feedback rinse. Mathematical models were developed to calculate the steady-state metal concentrations in continuously recirculated processes. Furthermore, formulas were derived with which mean values and fluctuation ranges can be calculated when discontinuous recirculation takes place at longer intervals.
If the evaporation losses in the galvanic process are fully compensated from the recirculation rinse, a "naturally recirculated concentration" is achieved in the deposition process under constant operating conditions. If this metal concentration is below the desired target concentration, the recirculation process is in deficit mode. If the concentration is above the target value, the process is in surplus operation.

It has been shown how the concentration in the separation process can be controlled. In excess operation, the concentration can be lowered by discarding from the coating process or from the recirculation rinse. Alternatively, a non-discarding concentration reduction was proposed, in which the recirculation is throttled in a targeted manner and the volume deficit arising in the deposition process is filled with water. In deficit operation, the concentration can be increased by dosing metal salt. Salt dosing costs can be reduced if the recirculation can be increased. This can be achieved by increasing evaporation (temperature increase) or water can be removed from the process using concentrators (evaporators, evaporators). It is worth noting that recirculation can also be increased by switching from continuous to discontinuous recirculation.
The final focus of this work was the topic of minimizing material losses. The reduction of material losses is the actual goal of recirculation. In deficit operation, minimization is not sufficient. Measures to increase recirculation save on metal salt dosing and reduce the necessary wastewater treatment.
The situation is fundamentally different in surplus operation. Increased recirculation or a reduction in carryover causes the concentration to rise further above the target value, making wastewater treatment necessary. In this case, all of the metal introduced into the electrolyte by the current yield difference must be removed from the process! Consequently, in excess operation, reducing the current yield difference is the only way to reduce material losses. This can be achieved by suitable electrochemical measures, but also by partially replacing the soluble anodes with insoluble anodes.

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