Surface Treatment / Finishing
There is a vast array of Surface Treatments which are carried out to a wide variety of materials for decorative or engineering purposes. The Surface Treatment might involve plating on top of a substrate material or altering the surface of the substrate material, typically to confer some corrosion resistance advantage. An example of where a process offers both corrosion resistance and decorative enhancement is anodising.
Further examples of Surface Treatments which alter the nature of the substrate material include:-
- Chromating.
- Phosphating.
- Galvanising.
Many different metals are plated on a variety of substrates, for both decorative and engineering purposes. Examples of metals plated include zinc, copper, chromium, cadmium, silver gold and rhodium. Sometimes a number of plating processes are employed to give the desired end finish, for example, electroless nickel plating prior decorative plating of chromium on plastic items.
To ensure a satisfactory finish a number of pre-treatment processes are often employed, for example cleaning and acid pickling. The surface treatment finish might also require a further stage such as passivation post chromium plating or hot demineralised water seal post anodising of aluminium.
With such a variety of processes available and often undertaken on a single site, the waste streams produced are themselves often varied. Some plating solutions such as Cd and Zn may contain cyanides which require removal, along with the metal, prior to discharge to drain. Cyanide destruction can be achieved through chemical oxidation at high pH by the addition of sodium hypochlorite.
Chromium present in plating solutions is present in hexavalent form and requires reducing to the trivalent form before it can be precipitated as a hydroxide. Chromium reduction takes place under acidic conditions through addition of a strong reducing agent such as sodium bisulphite.
Both chromium reduction and cyanide oxidation can be controlled automatically within a reaction tank by redox potential. Redox potential is measured in millivolts (mV) and is the measurement of the potential of a solution to carry out reduction and oxidation.
Where the level of contaminants is relatively high in a rinse water stream, conventional treatment methods such as hydroxide precipitation of heavy metals, flocculation and settlement are typically used for treatment of rinse waters and process concentrates. Additional treatment stages such as cyanide oxidation and chromium reduction might also be used and further polishing filtration such as multi-media filtration might be employed post settlement. Filter pressing of metal hydroxide sludge is also common to reduce volume of waste for off-site disposal.
Demineralised water is often favoured for rinsing post finishing processes and guarantees a high quality finish free of stains. It can have other benefits as well, including ensuring silica free water for rinsing post anodising.
Increasing water costs, reduction in discharge consent levels and in some cases the need to prevent discharge at all has led many surface finishing companies to consider water recovery within their finishing areas. First it is necessary to consider the mass balance of the various contaminants so that a suitable plant can be adequately sized. Often there is a need to segregate waste streams that may be hazardous or difficult to treat if combined.
Ion exchange in the form of a demineraliser is most often used for recirculation of the rinse water. Many contaminants present in the rinse water are fouling to an ion exchange resin and may be difficult to remove through regeneration. EWS (UK) Ltd demineralisers for water recovery are designed to accommodate for these compounds, offering excellent inter-regeneration capacity and the facility for additional maintenance associated with this type of application.
On exhaustion of the demineraliser resins, highlighted by a detectable rise in conductivity, a regeneration is required. The resultant regenerant waste is high in contaminants and requires additional treatment before discharge to drain. This is typically carried out on a batch basis and may include cyanide oxidation or chromium reduction stages.
Many surface treatment companies are now looking to evaporation to reduce the volume of regenerant waste before off-site disposal. Vacuum evaporation applies a negative pressure to the boiling chamber allowing water to boil and lower temperatures. This type of approach is often referred to as “zero discharge” and has the major benefit of not having to operate and police a discharge to drain.
Other technologies are often employed within the finishing area to remove a specific contaminant at point of source. For example, electro-winning cells can be used for plating out metals from dragouts, thereby reducing the levels carried forward to other rinse stages. An example of this might be on a static rinse post cadmium plating where the operation of an electro-winning cell also facilitates the electrolytic oxidation of cyanide.
Ion exchange polishing columns are also used to remove metals at point of source or before final discharge to drain. This type of system often employs highly selective resins which can facilitate removal of metals even under difficult operating conditions.
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