DISCUSSION
CHAPTER 7 DISCUSSION
7.2 CHARACTERISATION OF RINSE WATER SOLUTIONS
7.3.2 MASS BALANCE ANALYSIS
The measured concentration values used in Mass Balancing are averaged over the start and the end of a 9 day monitoring period over which detailed output data were collected (see Table 6.4 in Chapter 6). The concentrations were averaged because the concentration levels were assumed to be steady state values. This is indicated by the lack of regular (e.g. weekly) dumping of the flowing rinse water because of the accumulation of metals to levels which adversely affect plating. A daily rate of raw material usage was obtained using the data in
Table 6.2. The consumption after the 9 day monitoring period was based on this figure using Equation 7.11.
Average mass of chemicals used in a given time period
The monitoring results for the nickel and chromium plating lines on the 9/06/03 and 20/06/03 show that these metals lost to the rinse system from the plating solution largely end up in the drag-out solutions. From there they are re-used in the plating solution and dragged-out into the flowing rinse system. It was not possible to include chemical treatment of portions of the drag- out for disposal in this study because it is infrequent and not documented.
The final (exiting) rinse water from Tank 2, 5 and 11 contains an average of 13, 2.1 and 48 mg/L Ni respectively which is sent to the effluent treatment plant. Tanks 16 and 19 are static. The solution in Tank 16 is dumped every 3 days. The volume of the tank is 0.214 m3 (see Table 3.1 in Chapter 3). In 9 days the volume of solution discharged is estimated to be 0.64 m3 and the average concentration to be 41.7 mg/L. Tank 19 has a volume of 0.931 m3. The contents are dumped every 2 days. In 9 days the volume of solution discharged will be 4.12 m3 and the average concentration will be 3.47 mg/L. Tables 7.14 and 7.15 show the nickel losses calculated using existing and new (monitoring) data respectively.
Table 7.14 Estimation of Ni waste using existing data
From input data
From output data
Mass of Ni anodes used/day Mass of Ni anodes used/9 days Total area plated/9 days Density of nickel134 Thickness of Ni plate147 Mass of Ni plated/9 days
Mass of Ni lost = Ni anodes used/9 days - Mass of Ni plate/9 days Cost of Ni lost
. , „ _ , MassofNi lost/9days . . .
% Ni lost = — xl 00 MassofNi used/9 days
17.6 kg 158 kg 966 m5 8.9 g W
10 urn 86 kg 72 kg R4869
45.6
Average mass of
chemicals used/day X Time period/day Equation 7.11
Table 7.15 Estimation of Ni waste using new data
Average concentration of Ni in exiting rinse water (mg/L) Volume of exiting rinse water (m3) Mass of Ni lost in rinse/9 days (g) Cost of Ni lost in rinse/9 days (R) Estimated mass of Ni lost in rinse (kg) Estimated mass of Ni lost in the second Ni line (kg)
Total cost of Ni lost as drag-out (R)
Tank 2 13 26.4
343 23.2
Tank 5 2.1 48 101 6.82
Tank 11 48 48 2.30x10J
156
Tank 16 41.7 0.64 26.7 1.80
Tank 19 3.47 4.12 12.9 0.87 2.8
3.0 392
A comparison of the results of Table 7.14 and 7.15 suggests that very little of the total nickel losses (72 kg) can be ascribed to the drag-out (about 6 kg) over the 9 day period. If the plating thickness were at the lower end of acceptability, 8 urn, the nickel losses would still be high (69 kg). The calculated surface area plated is a very rough estimate which could conceivably be an over-estimate. However, even at half of the value quoted in Table 7.14 the losses would be 43 kg. Anode purity, the mass of the anode stubs and of the suspended nickel removed during filtration or capture in anode bags, over-plating, stripping of nickel with chromium in the chromium stripping bath (Tank 20), dissolving of nickel in the acid dipping tank (Tank H) and dripping of the dragged plating solution onto the floor due to insufficient drain time of the solution from the workpieces when transferred to the drag-out tank and to the flowing rinse system (see Figure 3.2 in Chapter 3) represent some other possible losses not quantified by this study. These results also show that the wetting agent and/or work practices are helping to reduce drag-out from the nickel plating solution. It could be argued that the results obtained on the trial dates (31/03/03 and 11/04/03) are more typical for the system. This means drag-out losses would be higher than those determined here. However job shop operations have an irregular throughput of workpieces which are reliant on the clients' orders and it is felt that the monitored situation reflects such fluctuations in production rate.
The main form of Cr(VI) purchased for use in the plating solution is simple C1O3 (see Table 6.2). Lumina 34 is used infrequently in the chromium plating solution as a source of Cr(VI).
The 8 month consumption rate (see Table 6.2) is therefore taken as being the same as that for the full year for Lumina 34. This means an average of 0.27 kg of Lumina 34 is used per day and 2.5 kg over 9 days. Lumina 34 contains C1O3 and organic acids. The percentage
composition of C1O3 in Lumina 34 is assumed to be 100% from the information given in the MSDS. It will in fact be slightly less than this due to the presence of the catalyst(s). This means there are an estimated 1.3 kg of Cr(VI) present in the plating solution originating from Lumina 34.
The exiting rinse water leaving Tanks 2, 5, 16, 17 and 19 (and going to the effluent treatment plant) in the chromium plating line contains an average of 2.35, 0.87, 1574, 24.3, and 38.7 mg/L Cr respectively. Tables 7.16 and 7.17 show the figures for the mass of Cr lost to the effluent streams based on existing and new data respectively.
Table 7.16 Estimation of Cr waste using existing data
From input data
From output data
Mass of Cr03 used/day Mass of C1O3 used/9 days Mass of Cr used/9 days Total area plated/9 days Density of Cr134
Thickness of Cr plate147 Mass of Cr plated/9 days
Mass of Cr lost = mass of Cr used/9 days - mass of Cr plated/9 days Cost of Cr lost
. , „ , MassofCr lost/9days . _ .
% Cr lost = — x 100 Mass of Cr used/9 days
0.85 kg 10.2 kg 5.3 kg 648m' 7.2 g W
0.4 urn 1.9 kg
3.4 R90
64
Table 7.17 Estimation of Cr waste using new data
Concentration of Cr in exiting rinse water (mg/L)
Volume of exiting rinse water (m5) Mass of Cr lost in rinse water/9 days (g) Cost of CrC»3 lost in rinse/9 days Total mass of Cr lost in rinse (kg) Total cost of Cr lost as drag-out (R)
Tank 2 2.35 26.4 62.0 0.96
Tank 5 0.87
48 41.8 0.65
Tank 16 1574 0.64 1007 0.016
Tank 17 24.3 38.4 933 14.5
Tank 19 38.7 4.12 159 2.47 2.2
34.2
The results from Tables 7.16 and 7.17 indicate that drag-out (2.2 kg) is responsible for about two third of the Cr losses (3.4 kg). This implies that the release of Cr(VI)-containing acid mist during plating is likely to be significant and the foam blanket (Fumetrol 140) is not stopping all Cr(VI) emissions to the atmosphere. Drag-out control appears to need some improvement. A hoist is used to lift workpieces in and out of the chromium plating solution and hold them
above this solution to allow for good drainage. However perhaps insufficient drain time is allowed and so little of this viscous plating solution is actually returned to the tank. Further, when the line is busy the drain time is likely to become further compromised.
The average Cr losses are around 36 times less than the average Ni losses. This is consistent with much less Cr being used than Ni over the 9 day period.