WATER & EFFLUENT TREATMENT
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 > 1 µ S / cm conductivity which is equivalent to about 0.5mg / l total dissolved solids ( TDS ) whilst RO permeate is typically about 5 µ S / 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 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
42 NOVEMBER 2021 twitter : @ surfaceworldmag