DISCUSSION
CHAPTER 7 DISCUSSION
7.2 CHARACTERISATION OF RINSE WATER SOLUTIONS
7.2.6 THE COMPOSITION OF THE MDCPL RINSE SYSTEMS
the drag-out solution from Tank 18. Analyses results for Tank 19 show that the concentration of nickel, chromium, sodium and zinc increased by a factor of 29, 58.5, 4.4, 15.2 respectively between each sampling day. Tank 19 is changed (dumped) every two days. Buildup of these metals in this tank can increase the concentration of the metals in the tank. The increase in concentration of chromium was highest of all metals. During sampling the colour of Tank 19 solution was yellow. This could be due to drag-in of chemicals mainly the brass passivating solution by brass plated workpieces. The decrease in concentration of copper is due to its low concentration in Tank 18. Thus the dragged-in copper will be low.
Comparison of the total dissolved solids recorded in the chromium rinse solutions show values above Cushnie's acceptable range of values on both dates of sampling periods. The normal range for adequate rinsing after bright plating is given as 5 to 40 mg/L.128
calculated by multiplying the conductivity (uS/cm) by a factor between 0.55 and O.9.75 For the least contaminated rinse solutions in Tanks 8, 13 and 18 this factor was found to be about 0.6 in each case. This suggests that the quoted value (200 uS/cm) for the contaminant limit value of conductivity required for good rinse water quality may not have been set quite correctly.164 However conductivity relates closely to the sodium and zinc levels in the acid and alkaline cleaner rinses (Tank 2), sodium levels in the electrolytic alkaline cleaners rinse (Tank 5), nickel levels in the nickel drag-out and chromium levels in the chromium drag-out.
The cleanest rinse tank in the MDCPL is Tank 13. Except for Ni, which has an average concentration of 6.4 mg/L, the average concentration of the other metal ions (Fe, Cr, Zn, Cu and Pb) in this tank, sent to the central effluent treatment plant, were below both the contaminant limit and the effluent discharge limit values given in Table 7.9. The highest average concentrations recorded in this tank were for chromium and zinc (0.48 and 0.061 mg/L respectively). The roughly estimated concentration of SO^- and CI" levels in Tank 13 (see page 129 in Chapter 7) showed that the concentration of these species was below both the effluent discharge limit and the contaminant limit value on both sampling dates. Similarly the pH was within the recommended range while the TDS value was much lower than the effluent discharge limit and the contaminant limit values (see Table 6.6). As mentioned previously the conductivity was less than the effluent discharge limit value but greater than the contaminant limit value.
7 J CHEMICALS USED AND WASTED
Many waste minimisation analyses techniques such as Scoping Audits, Mass Balancing and True Cost of Waste determinations can be used to quantify waste streams and to identify and prioritise them, with varying degrees of accuracy, as waste minimisation opportunities. This prioritising of potential waste problems for waste minimisation solutions (options or measures) is most frequently based on the expense they incur to the company. The cost of waste includes for example unused raw materials, the treatment and disposal costs of the waste and the utilities used in making the waste. The workpiece is a raw material. In a job shop it may not be viewed as a direct cost to the electroplating company. This study considers the chemical and other material waste but excludes energy beyond the Scoping Audit analysis.
In a Scoping Audit (see Section 7.3.1) the source and sink for the waste does not have to be established as long as the levels of inputs to and/or outputs leaving the process are known. This means little is known about what actually happens to the wasted raw materials during processing i.e. how much of a particular raw material becomes what kind of waste and where it accumulates in or leaves the process. New data from the chemical monitoring of the rinse water allow some materials lost in the wastewater to be measured directly (see Equations 7.5 and 7.6). The movement of this waste in the rinse water waste streams can therefore be tracked, quantified and costed as wasted raw materials in the waste stream. The waste present in the wastewater leaving the line contains unused plating chemicals and substrate. The real cost of wastewater takes into account the cost of the loss of these chemicals. The concentrations of the elements nickel and chromium in the wastewater have been used to determine raw material wastage. The mass of the constituent elements in the wastewater, which originate from the raw materials, can be calculated from the volume of exiting rinse water and the constituents' measured concentrations (see Equation 7.5).
Mass in the wastewater = Measured concentration x Volume of rinse water Equation 7.5 This can be expressed as a mass of that raw material if the composition of the raw material is known.
, . . . Mass of the constituent elements in the wastewater .__
Mass of rawmaterial lost = xlOO Equation 7.6
% composition
Existing data have been used to estimate the total wastage indirectly (see Section 7.3.2).
Material balance calculations based on raw material usage have been carried out to estimate the nickel and chromium losses in the plating process. The losses from other process solutions like the soak cleaners, electrolytic cleaners and acid cleaners have been calculated based on these loss values obtained for chromium. The lack of compositional data on the formulation of the cleaners, together with the multi-use of these pretreatment rinses, made this the best available means of estimating losses to the wastewater stream.