Performance Reviews - Treatment of Anodising, Etching and Plating Effluents
A manufacturing site which carried out considerable amounts of aluminium etching and chrome anodising together with lesser amounts of other electroplating and surface finishing activities had developed compliance problems within its effluent discharge. RPA carried out a thorough review of the chrome reduction and other relevant aspects of the treatment plant, the effluent collection arrangements and the effluent sources themselves. This review was based upon both the current effluent profiles and the range of likely future profiles.
RPA identified that the current problems could be overcome by a combination of minor plant changes and operator re-training. The designs for these changes were developed such that both the current and the predicted future requirements would be overcome at the same time. The necessary capital investment had a pay back of less than six months. RPA also carried out the operator re-training and issued amended operating and maintenance manuals.
The same facility also commissioned RPA to carry out a detailed mass balance for all financially and environmentally significant components around its component plating, pickling and rinse water recovery Line. This study highlighted rinse inefficiencies which affected product quality created unnecessarily high operating costs and reagent usage. Relatively simple changes to the operating and control concepts, particularly within the rinse water recovery equipment overcame the quality problems and created significant savings as regards both production costs and environmental load.
Steel Mill Rolling, Surface Cleaning, Plating and Painting Effluents
As part of the Environmental Master Plan work for a large Iron and Steel Facility in South Africa, one of the urgent short term measures was to overcome the operating difficulties at the effluent treatment plant for the hazardous effluents. The plant was complex and it suffered from the effects of surges in the flows of individual input streams (causing occasional short duration exceedances in the output quality criteria). Also, the discharge criteria were about to be tightened to a level which were not achievable by the process routes that were being used, even when all aspects were working ideally.
In essence, all of the different treatment processes had been set up to treat each effluent on its own. The processes look no account of what could be achieved by using one effluent to assist in the treatment of another. For example, some of the acidic effluents contained sufficient Fe2+ to enable all of the Cr6+ to be treated without the need for a separate pre-treatment process for all the chrome effluents. Similarly, if all the acidic effluents were pH neutralised together, the resultant aqueous phase contained enough dissolved calcium to precipitate the fluorides in the other effluent streams, whilst keeping the final sulphate concentration within limits.
RPA developed a way of re-arranging pipe work and controls such that all these process improvements could be achieved using only the existing equipment and buffer storage arrangements. This scheme enabled almost half of the processing stages to be shut down, over half of the previous reagent consumption to be avoided and for the ultimate production of sludge residues from all the treatment processes to be reduced to less than 25%. At the same time, the new quality criteria for the effluents were able to be met and the frequency and magnitude of non-compliances was greatly reduced.
RPA then looked at the causes of the remaining non-compliances and together with the relevant site staff, evolved and implemented a sequence of upgrades and changes at the different effluent sources and transfer arrangements as well as within the remaining treatment equipment, such that all the remaining causes of quality exceedances were overcome.
Further issues within this plant have been associated with ground instability beneath the large clarifiers and with occasional slugs of oil getting past the processes upstream of these clarifiers. A novel alternative to the normal notched overflow weirs for the clarifier resolved the problems associated with both of these issues. This development greatly reduced the suspended solids content of the final effluent and eliminated the consequences of the occasional oil contamination issues.
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