DISCUSSION
CHAPTER 7 DISCUSSION
7.2 CHARACTERISATION OF RINSE WATER SOLUTIONS
7.2.4 NICKEL PLATING RINSE SYSTEM
Tank solutions 10, 11, 12 and 13 make up the rinse system used after nickel plating. Tank 10 solution is a static rinse drag-out located immediately after the plating solution. Aliquots of this tank solution are periodically used to top up the nickel plating solution of Tank 9 (see Table 3.1 Section 3.3.2)
Tanks 11, 12 and 13 are counter current flowing rinses used after the drag-out tank after the nickel plating solution. The main reason for using a counter current flowing rinse is to save water and improve the efficiency of rinsing. In the system of rinsing, Tank 11 is the "dirtiest"
rinse tank and Tank 13 the "cleanest" rinse solution. This is because workpieces are dipped first in Tank 11 and process chemicals are washed off the workpieces' surfaces into this tank. It also receives the solution exiting from Tanks 12 and 13 which is not a fresh water supply.
Water exiting Tank 11 is piped to the central effluent plant.
It is important to rinse the workpieces well after nickel plating in order to remove from the surface of the workpieces any chemicals which might be detrimental to the quality of the chromium plating and to prevent contamination of the chromium plating solution from dragged-in solutions. Iron, zinc, lead and copper concentrations are very low in Tank 13 (less than or equal to 0.1 mg/L) and not likely to present a problem. This is reflected in the limited
chemical treatment carried out on the chromium plating solution. Chlorides and sulfates from the previous tank solution originate from the hydrochloric and sulfuric acid based nickel electrolyte in the plating solution. The concentration of nickel in the plating solution is measured by the suppliers. It is expressed as the total metal, nickel chloride and nickel sulfate concentration. The level of chlorides and sulfates can be established from the values when the nickel is in excess. The levels of chlorides and sulfates in the final rinse have been calculated based on the dilution factor calculated for nickel levels in Tank 9 and Tank 13 and the specified (optimum) sulfate and chloride concentration in the process solution (Tank 9). The sulfate levels in Tank 13 were estimated to be 21 mg/L on 9/06/03 and 25 mg/L on 20/06/03.
The chloride levels in Tank 13 were estimated to be 4.6 mg/L on 9/06/03 and 54 mg/L on 20/06/03.
The analyses results show that nickel and sodium are present in the highest concentration in all four solutions. The dissolved nickel species originates from the soluble nickel anode in the acidic nickel plating solution which has been washed off the workpieces into the drag-out. The concentration of sodium is mainly from the brighteners, wetting agents and fume suppressants used in the nickel plating solution. Obviously iron and zinc levels would be expected to be low since progressively less of the substrate is exposed to the solution during plating. The nickel levels are higher in all the rinse solutions than those measured in Tank 8 solutions. The concentration of the other metals are higher in the drag-out, and in many cases in Tanks 11 to
13 solution also, than those found in Tank 8. This is consistent with the drag-out rinse being static and so a buildup of metal ions is observed over time.
A comparison of the results for the Tank 10 solution over the two sampling days shows a higher concentration of all metal ions on 20/06/03. The most significant change was observed for nickel, sodium, iron and zinc. The concentration change of the metals went up by a factor of 8.7, 7.9, 5000 and 18 respectively. The concentration of nickel in the plating solution reported by Orlik chemical suppliers ranged from 78 to 87 g/L from 03/06/03 to 24/06/03 (see Table 6.11 in Chapter 6). The recommended concentration is 82.5 g/L. The measured concentration of nickel in the drag-out from the analysis result was 0.744 g/L on 9/06/03 and 6.44 g/L on 20/06/03. The low concentration of the drag-out compared to the plating solution
was due to the dilution of the dragged-in chemicals by the rinse water in the drag-out tank.
About 70 L of mains water is added to the drag-out tank daily when 70 L of the drag-out is transferred to the nickel plating tank. It would be expected that the nickel levels would be highest in a sample of the drag-out solution taken just before transfer and lowest in a sample just after transfer. However assuming the concentration measured on 20/06/03 was prior to top-
up i.e. addition of drag-out to process solution and of water to replace drag-out, the concentration expected immediately after top-up would be 4.12 g/L. This is 5.5 times higher than the value recorded on the 9/06/03. This suggests that the selected top-up had not taken place at the correct time or other factors, probably the number of workpieces going down the line had contributed to lower the nickel level in the bath than expected soon after top-up.
The analyses results of the samples of the three counter current rinse tanks of the two experimental days shows a decrease in concentration of the metals from Tanks 11 to 13. When the concentrations of the metals were compared over the two sampling days, higher concentrations were observed for nickel, iron, sodium, zinc and copper on 20/06/03. This is expected based on the concentrations in the drag-out tanks. For example in Tank 11 (the most concentrated), the concentration increased for these metals by a factor of 3, 368, 1.3, 4.6 and 7 respectively. Surprisingly the concentration of iron was observed to increase from Tank 11 to 13 in the samples taken on 9/06/03. During sampling there were around 23 pieces of metal dropped in Tank 13, 11 pieces of metal in Tank 12 and six pieces of metal in Tank 11. It is likely these metals had dissolved and caused deviation from the expected trend in the concentrations.
Over the monitoring period, however it is interesting to note that on both days the concentration of nickel and sodium in Tank 13 was virtually the same. The higher concentration of nickel and sodium in the drag-out on the 20/06/03 means the dilution factor cannot be the same throughout the rinse system on both days. The dilution factor between Tanks 11 and 12 solutions is similar on both days (1.5 and 1.7) whereas it is considerably greater between drag-out and Tank 11 (31 and 89) and between Tanks 12 and 13 (2.8 and 6).
Cushnie's acceptable "rinse criteria for permissible levels of contamination" for rinsing following bright plating ranges from 5 to 40 mg/L.128 Mohler quotes an equivalent value of 40 mg/L for this parameter.166 The values recorded on both sampling dates in all the nickel rinse tanks exceed this value. However during this monitoring period the filtration equipment on the nickel plating solution was not working. This may lead to higher than expected total dissolved solids levels.