Pure water for a perfect powder coat

Powder coating is going green. In some cases this is the result of legislation which requires existing process cleaning solutions to be replaced with more environmentally friendly ones. Increasingly, however, it’s coating quality that is the driver. Some coating specifications now demand salt spray testing for more than 1000 hours, and that means that surface preparation is absolutely critical. One of the proven ways of achieving high quality surface preparation is the use of pure water. Mains water contains a wide range of dissolved salts. These are harmless in drinking water but can promote corrosion and poor adhesion of powders to metal surfaces.

Using pure water increases the life expectancies of cleaning tanks and sprays and is a proven way of making both the cleaning and protective barrier chemical more effective. This means longer periods between tank drain downs and cleans, extending process operating time and less maintenance. It also means less wastewater generation and lower operating costs. A more environmentally and sustainable process. All this and improved product quality too.

There are two main processes to produce pure water: ion exchange demineralisation and reverse osmosis (RO). Ion exchange uses a chemically active synthetic resin to remove dissolved ions from water, in exchange for hydrogen in the case of cations and, hydroxyl in the case of anions. The resulting hydrogen and hydroxyl ions form water molecules. On the other hand, RO passes the water through a semi-permeable membrane which effectively filters out the dissolved ions. Selecting the best process depends on the quality of the mains water supply and the water purity required.

Ion exchange demineralisation is a batch process. The ions removed from solution accumulate on the resins and then have to be removed by regeneration. This uses hydrochloric acid for the cation resin and caustic soda for the anion. The regeneration process isn’t 100% efficient, so some of the chemical – typically about 20% - is wasted, and the regenerated resins also have to be rinsed prior to returning to service. This generates a waste stream with volume equivalent to about 5 - 15% of the plant throughput depending on the raw water quality. RO is a continuous process that uses no chemicals but depends on a high operating pressure generated by an electrically driven pump. This makes it appear more environmentally friendly than ion exchange. The ions removed from the raw water are concentrated into a waste stream that is typically 25% of the throughput suggesting that it is less sustainable. If the raw water is hard, RO can be fouled by scaling and this is normally prevented by softening upstream of the membranes, but this requires salt and produces more wastewater.

A simple two-stage ion exchange demineralisation system can produce purified water of >1µS/cm conductivity which is equivalent to about 0.5mg/l total dissolved solids (TDS) whilst RO permeate is typically about 5µS/cm (about 2mg/l TDS).

RO has a higher capital cost than ion exchange but operating costs for both processes depend on the system flow rate and the TDS of the raw water. The chemical consumption in ion exchange is directly proportional to the raw water TDS whilst the power consumption of RO remains fairly constant, so RO becomes more economic for higher TDS raw waters. The actual cut-off point will depend on the raw water composition, the costs of chemicals and the anticipated lifetime of the plant.

Using pure water in all stages of surface preparation will not only improve the adhesion of the coating but will also extend chemical life and reduce waste. Not only sustainable but also cost effective.

Telephone or visit adeptpwt.co.uk for more information on water purification for surface finishing.
ADEPT Pure Water, 36 Huxley Close, Park Farm South, Wellingborough, Northants NN8 6AB

Telephone: +44 (0) 1933 677181
Email: enquiries@adeptpwt.co.uk
Visit: www.adeptpwt.co.uk