WASTE MINIMISATION AND THE POWDER COATING INDUSTRY
4) Improved Housekeeping
3.6 Waste Minimisation Opportunities and Options in the Powder Coating Industry
A great number of waste minimisation opportunities have been recognized in the metal finishing industry. This has led to many options being devised and implemented in order to reduce the volume of water used and of effluent wasted, the amount of process and waste treatment chemicals used and the amount of solid waste stored and/or sent offsite for disposal. These have been widely published in the literature for electroplating and to a lesser extent for powder coating. The literature has been surveyed and the important waste minimisation options are presented here. Points 1 to 4 discuss the waste minimisation options found for the wet process while point 5 looks at the waste minimisation options applicable to the dry process.
1) Chemicals lost in the pretreatment process, due to drag-out and production, result in the chemical contamination of proceeding baths and depletion of the process solutions. This means that the process solutions need to be replenished regularly with raw materials while rinse tanks need to be re-charged with clean fresh water.
Drag-out can be reduced in the following ways:
• lowering the viscosity of the process solution by maximizing the bath operating temperatures and minimising the chemical (raw material) content of the solution
• using wetting agents in the bath to reduce the surface tension of the solution and increase the flow rate of the drag-out on the surface of the workpiece
• placing drainage boards between process and rinse tanks so that drag-out can be re-routed into the process tank from which it has been drawn
• using drag-out (static rinse) tanks after process baths to capture drag-out and prevent it entering subsequent process solutions
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• extending the drip time that the workpiece is over the solution (after its withdrawl) in order to give maximum drainage of drag-out back into the process solution. This can be achieved by withdrawing the workpiece slowly from the immersion tank or by slowing down the speed of the workpiece as it moves through a spray system
• using air knives or spray rinses above process tanks to 'power' rinse any excess process solution off the workpiece surface and back into the process solution
• improving the position of the workpiece on the conveyor system i.e securing it at an angle or adding drainage holes, to allow the process solution to flow more easily off the workpiece surface.77'167,168 However, the latter is rarely used because the workpiece may not usually be approved of by the client.
2) Process solutions in the metal finishing industry can last for years if correctly managed. The life span of process solutions can be extended by proper maintenance treatment and monitoring. Measurements of parameters, such as pH, conductivity and chemical composition, are used to determine the minimum amount of chemical that needs to be added to the bath in order to top it back up to the specified concentration level. This ensures that raw materials are not overused and therefore wasted.77
3) The life span of a process solution is shortened as a result of sludge build-up in the tank. Sludge also incurs high disposal costs, especially if needs to be treated prior to disposal. The volume of sludge formed can be reduced by the following methods:
• using treatment chemicals that reduce the generation of sludge such as caustic soda167 to remove hard water ions that contribute to sludge formation
• using appropriate chemical pre-treatment systems, to coat the metal surface, which are known to be low heavy metal sludge producers. This is especially important in spray operations where sludge tends to block the spray nozzles.
• Pre-treating the mains water or using deionized water to make up the process solution in order to reduce natural contaminants which contribute to the build-up of sludge138
• dewatering the sludge, using a filter press or centrifuge, to reduce the water content of the sludge169170
• reducing drag-out from process solutions to reduce the likelihood of chemicals precipitating out and forming sludge167'171
• routinely filtering and purifying process solutions to physically remove the sludge.
4) A primary waste reduction option for a wet process like the phosphating process is the reduction of water use. Water use in the surface finishing industry can be kept to a minimum by implementing a number of different practices given below:
• using a concurrent rinse system to significantly reduce the volume of water required for immersion rinsing77
• using flow restrictors on operations that employ a continuous supply of water. These are placed directly in the rinse water inlet to restrict the water flow rate in the system to an acceptable level77'171
• using immersion and spray rinsing systems in combination rather than as separate operations to make a more economical use of water157
• closing water off to areas of the plant which do not require water continuously so stopping water running constantly into the drain
• treating rinse water to remove contamination and reusing it for rinsing or in other areas of the plant to clean floors and containers172
• reusing used rinse to dilute process chemicals or directly for other purposes. It needs to be ensured that the quality of the rinse water is suitable, so that any impurities present do not react with the process solution157
• keeping the temperature of the rinse bath low to ensure that water is not lost unnecessarily by evaporation
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• installing conductivity meters to automatically control the amount of water added to the rinse tank during operation. These meters can be use to automatically control the volume of the rinse water added to the tank
• using discharge from the acid rinse stream to neutralize the alkali effluent stream if both alkali and acid cleaning systems are used in the process.
This has been shown to reduce the amount of clean water required by 50%.157
• using alternative, cheaper water supply sources, to mains water, such as borehole water. This reduces the water costs rather than the volume of water used
• checking that nozzles have not been knocked and become misdirected, where there is a large throughput of big workpieces
• checking pipes, valves, tanks and connection for damage in order to prevent water leaks
• segregating waste streams thus making it easier to obtain a level of water quality suitable for water reuse of effluent discharge77 This can also effluent reduce the discharge volume and treatment.
5) The application of powder to the workpiece can generate solid waste. Powder loss has been described in terms of transfer efficiency. This is a measure of the actual mass of the sprayed powder which reaches the workpiece surface (see Equation 3.8).
_ ~~ . Amount of powder used for coating », . Transfer efficiency = Equation 3.8
The total amount of powder used
High transfer efficiencies can be obtained by adopting the following practices:
• reusing the powder that does not adhere to the workpiece173
• selecting a suitable application method to maximise transfer efficiency.
Fluidized beds have been quoted as giving a 100% transfer efficiency compared to spray guns which give 65%105 efficiency. However, fluidized
beds are mostly suitable for large workpieces but the parts to be coated must be pre-heated.174 The latter requires additional time, space and energy79
• maintaining good spray techniques. This includes factors such as speed of delivery by the gun and distance of the gun from the workpiece. Maintaining a steady gun will ensure that a uniform thickness of the coating will be obtained. Since spray losses increase with distance, the gun should be held at between 15 and 20 cm from the workpiece173
• selecting a suitable coating material to meet the product specification. For example powders have been found to be environmentally friendly, as they produce no VOCs in comparison to liquid finishes64
• capturing and reusing oversprayed powder. This is the excess powder which gets transferred to the workpiece and also becomes deposited on the spraybooth floor. This is collected by an air cleaner system (in the spray booth's local exhaust ventilation system) such as a cyclone. The excess powder deposited on the workpiece is considered to be wasted powder and has
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been found to result in streaks in the coating. Using cyclones has been found to be the cheapest method of capturing powder. They have few moving parts, a wide choice of materials for construction, can be easily cleaned174 and have low maintenance costs.175 In addition, they have recovery efficiencies of between 85 and 95%.
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