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Source Reduction Measures Employed in the Metal Finishing Sector Report supplementing WRC Report TT 645/15: Natsurv 2 – Water and Wastewater Management in the Metal Finishing Industry by HA Ally, W Kamish and T van der Spuy i ACKNOWLEDGEMENTS The research in this report emanates from a project that was undertaken by the Water Research Commission, entitled: NATSURV 2: Water and Wastewater Management in the Metal Finishing Industry (Ed 2) The Steering Committee is thanked for contributing their knowledge and insights to the project and the content of this guide. The project team would like to extend their thanks to the following people: • All the metal finishing companies who gave of their time to complete the survey and participate in the site visits and interviews • The regulators who provided input into the development of the guide ii TABLE OF CONTENTS Page No 1. BACKGROUND .......................................................................................................... 1 1.1 1.1.1 INTRODUCTION ...................................................................................................... 1 Industry overview ...................................................................................................... 1 1.2 THE OVERALL OBJECTIVES OF THE PROJECT ................................................. 2 1.3 DESCRIPTION OF THE RESEARCH PRODUCTS................................................. 3 2. WASTEWATER TREATMENT IN THE METAL FINISHING SECTOR ..................... 4 2.1 3. WASTEWATER TREATMENT OF POTENTIAL RELEASES TO THE ENVIRONMENT ........................................................................................................................ 4 CLEANER PRODUCTION INITIATIVES FOR THE METAL FINISHING SECTOR .. 8 3.1 4. MANAGEMENT AND HOUSEKEEPING ................................................................. 8 BEST PRACTICES APPLICABLE TO JIG AND BARREL PLATING OPERATIONS10 4.1 REDUCING DRAG-OUT ........................................................................................ 10 4.2 IMPROVEMENTS IN RINSING .............................................................................. 11 4.3 CONTROLS ON RINSE WATER FLOWS:............................................................. 11 4.4 PREVENTING “BACK-MIXING” OR “SHORT-CIRCUITING” ................................ 12 4.5 MINOR PROCESS BATH IMPROVEMENTS ........................................................ 13 4.6 IMPROVE MAINTENANCE PROCEDURES ......................................................... 13 4.7 PROCESS CONTROL ............................................................................................ 14 4.8 PRODUCT STORAGE ........................................................................................... 14 4.9 SOLVENT AND ALKALINE DEGREASING SYSTEMS ......................................... 15 4.10 IMPROVE THE OPERATION OF CONVENTIONAL VAPOUR DEGREASERS ... 17 4.11 USE A LOW-EMISSION VAPOUR DEGREASER ................................................. 18 4.12 PLATING................................................................................................................. 18 4.13 CHEMICAL AND ELECTROCHEMICAL CONVERSION COATINGS (CHROMATING, PHOSPHATING, ANODISING, AND COLOURING) ................................... 19 4.14 ALTERNATIVE STRIPPING PROCESSES ........................................................... 20 5. DESIGN METHODOLOGY FOR COUNTERFLOW RINSING ................................. 21 5.1 THEORY OF COUNTERFLOW RINSING SYSTEMS ........................................... 21 5.2 DESIGN MODEL ASSUMPTIONS ......................................................................... 23 5.3 DESIGN MODEL BASIS ......................................................................................... 23 5.4 5.4.1 5.4.2 5.4.3 DESIGN MODEL OUTPUT .................................................................................... 24 Additional design considerations ............................................................................ 24 Equipment sizing..................................................................................................... 24 Rinse tank operation ............................................................................................... 24 iii 5.4.4 5.4.5 Process control ....................................................................................................... 24 Drag-out control ...................................................................................................... 24 5.5 5.5.1 5.5.2 5.5.3 5.5.4 5.5.5 CALCULATIONS AND FINDINGS ......................................................................... 25 Water consumption ................................................................................................. 25 Equipment sizing..................................................................................................... 25 Process operation ................................................................................................... 26 Process control automation .................................................................................... 26 Drag-out control ...................................................................................................... 26 5.6 SUMMARY OF THE DESIGN ................................................................................ 27 6. COSTING OF A CP INITIATIVE IN THE METAL FINISHING INDUSTRY .............. 28 6.1 6.1.1 7. THEORY OF THE COSTING APPROACH ............................................................ 28 Study estimate ........................................................................................................ 28 REFERENCES .......................................................................................................... 31 LIST OF FIGURES Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Design for preventing "back-mixing" in tanks ............................................... 12 Schematic drawing of an Eductor ................................................................. 15 Operation of Eductors................................................................................... 16 Diagram demonstrating the setup of an eductor array in a cleaning tank .... 16 Counterflow rinsing (Hyder Consulting and Hemsley 1999) ........................ 21 Counterflow rinsing with 3 stages ................................................................. 23 Air lift pump (Van Der Spuy, 2004) ............................................................... 26 Design and dimensions of a 5-stage rinsing unit.......................................... 28 LIST OF TABLES Table 1 Table 2 Table 3 Table 4 Typical treatment regimens for effluent treatment .......................................... 4 Possible treatment(s) specific to each constituent of concern ....................... 6 Typical components of an Environmental Management Plan (EMS) ............. 8 Metal concentrations in Rinse tanks 1 to 3................................................... 25 iv GLOSSARY Anode is an electrode through which an electric current flow into a polarised electric device EPA environmental protection agency ISO international standards organisation METSEP METals SEParation is the Johannesburg based company that recycles acids for industry. www.metsep.co.za mg/m3 milligrams per cubic metre (concentration) Micron 1 millionth of a metre MSDS materials safety and data sheet ppm parts per million (concentration) SABS South African Bureau of Standards Tensides is a substance which lowers the surface tension between two liquids and supports the formation of dispersions. They are also known as emulsifying agents UNEP United Nations Environment Program zamak often referred to as mazak, is an alloy of zinc aluminium magnesium and copper and is used extensively in die casting industry v 1. BACKGROUND 1.1 INTRODUCTION The last National Survey relating to the management of resources in the Metal Finishing Industry was completed in October 1987, 25 years ago. Since then there has been concerted effort to develop coating systems that rely on more environmentally friendly chemicals along with scientific evaluation of process lines that has resulted in significant improvements to layout and handling methods. Since the last NATSURV, a portion of the South African metal finishers have benefitted from financial and technical assistance from DANIDA (the Danish Foreign Aid agency) with the implementation of our own Resource Conservation program. This initiative, the CPMFI project [Cleaner Production in the Metal Finishing Industry] ran for a period of three and a half years, officially ending in 2004 and culminating in the establishment of SAMFA, the South African Metal Finishing Association. Ultimately eighteen companies rebuilt their plants with partial subsidies from DANIDA totalling 1.2 million rands. Together they achieved combined savings of around 3.8 million Rands in year one and on average recouped their investments through savings in an average period of 18 months. Scores of others made smaller but measurable improvements using their own resources. To quantify the extent to which the metal finishing industry in South Africa has changed since the last NATSURV conducted in 1987, it was again necessary to collate up-to-date information such that meaningful conclusions could be drawn. The major outcomes expected from this study are as follows: • A review of current state of the metal finishing sector which would provide the platform for further initiatives aimed at positively transforming the sector. • The development of a product that will allow more cost effective planning for the sector, resulting in better run factories with the potential to become more profitable. • An assessment of the extent of compliance with ISO 14000 and Chemicals Management Action Plans (CMAP) to provide an indication of the protective measures currently provided to workers in this sector. • To provide an indication of the varying degrees of training required for general worker safety awareness, the use of personal protective gear, correct operational procedures, improving the state of poor housekeeping and to rectify poor administration. 1.1.1 Industry overview It is often necessary to use metals or metal alloys which have certain surface properties, e.g. resistance to corrosion, hardiness, high temperature tolerance, etc. Generally, no one metal will possess all the necessary properties. For example, steel has many good properties. It is inexpensive, abundant and a strong material that can easily be worked into all manner of shapes and forms. However, its major flaw is poor resistance to corrosion, causing it to rust severely in damp atmospheres. It needs to be coated with another material to provide corrosion resistance. The term metal finishing refers to a range of techniques that treat metal surfaces for its intended purpose. Metal Finishing enables the engineer to combine the good mechanical properties of one metal with the desirable surface properties of another. 1 Electroplating thus offers the engineer a wide choice in combining various metals in determining a specific objective. Currently, metal finishing operations take place in two broadly defined divisions: 1. The jobbing shop sector (or commercial finishers): This sector includes businesses that offer metal finishing services to companies that elect not to install an in-house finishing capability to deal with their finishing requirements. 2. In-house finishing operations: It is possible that this sector has experienced growth since the last NATSURV, particularly amongst OEM (original equipment manufacturers) and second tier automotive component manufacturers (SAMFA, 2013). Some of the reasons for this growth are: • • • Inventory control is less complicated as parts are not transported to off-site locations; The manufacturer is able to concentrate his resources on dealing with specific component configurations and finishes; and The manufacturer is better able to manage finishing quality control, with no third party involvement Even with a national survey, an estimate of the number of metal finishing installations in South Africa is difficult to quantify as a significant percentage of the work is performed in-house as an essential part of an overall manufacturing activity. The remainder of the metal finishing work is undertaken on a contract basis by specialist companies that include some large and dominant companies along with a greater number of smaller operators. Not all of the latter necessarily adhere to desirable industry standards, local and national legislation. 1.2 THE OVERALL OBJECTIVES OF THE PROJECT The project objectives as formulated in the Agreement with the Water Research Commission were to: 1. Provide a general overview of the metal finishing industry in South Africa, its changes since 1980 and its projected change. 2. Evaluate and document the generic industry processes 3. Determine the water consumption and specific water intake 4. Determine the wastewater generation and typical pollutant loads 5. Determine local electricity, water and effluent prices and by-laws within which these industries function. 6. Critically evaluate the water (inclusive of wastewater) management processes adopted and provide recommendations 7. Evaluate the industry adoption of the following concepts: cleaner production, water pinch, energy pinch, life cycle assessments, water footprints, and ISO 14 000 to name a few. 8. Provide recommendations for best practice The major findings obtained in meeting the abovementioned objectives are described in this report in the traditional format of previous NATSURV documents. Other information which 2 could support this document, but cannot necessarily be included within the structure of the NATSURV document have been completed as supporting documents and will be discussed further in the ensuing sections. 1.3 DESCRIPTION OF THE RESEARCH PRODUCTS In the original proposal one major research product and two supporting documents were envisaged for this Project: • NATSURV 2 : Water and Wastewater Management in the Metal Finishing Industry (Ed 2) • An overview of the status quo of the metal finishing sector in South Africa • An overview of source reduction methods which are or can be employed in the metal finishing sector. The project yielded three deliverables in the form of reports: • Ally, S.H., Kamish, W. and van der Spuy, A. (2015). NATSURV 2 : Water and Wastewater Management in the Metal Finishing Industry (Ed 2). WRC Report No. K5/2224, Water Research Commission, Pretoria. • Ally, S.H., Kamish, W. and van der Spuy, A. (2015). Status Quo of the Metal Finishing Sector in South Africa. WRC Report No. K5/2224, Water Research Commission, Pretoria. • Ally, S.H., Kamish, W. and van der Spuy, A. (2015). Source Reduction Measures Employed in the Metal Finishing Sector. WRC Report No. K5/2224, Water Research Commission, Pretoria. (This report) The major findings from the survey on industries in the metal finishing sector are described in this report. The updated NATSURV 2: Water and Wastewater Management in the Metal Finishing Industry (Ed 2) can provide support to professionals in the plating industry, officials of the DWS as well as practitioners who supply support services in the field of cleaner production in the metal finishing sector. The supporting documents provide a more detailed account of the status quo and the extent of implementation of cleaner production (CP) technologies in the metal finishing sector. Once again, the documents can be used by the sector as well as those providing support services to the sector as a point of departure for what is practically possible in term of CP implementation. 3 2. WASTEWATER TREATMENT IN THE METAL FINISHING SECTOR 2.1 WASTEWATER TREATMENT OF POTENTIAL RELEASES TO THE ENVIRONMENT Constituents of concern which are potentially released during metal finishing processes include metals, organic material, cyanides, hypochlorite, chlorine, AOX, surfactants, complexing agents, acids/alkalis, ions, solvents, dusts and general wastes. The emission of pollutants in wastewaters and the production of waste are considered more significant than emissions to air. Since most surface treatments by metal deposition are water-based, significant quantities of water is used as well as copious amounts of wastewater is generated. The most effective method for preventing pollutants entering the water environment is in the minimisation of the loss of materials, where these materials are lost by drag-out into rinse waters. Furthermore, wastewater treatment serves to remove the constituents of concern before discharge into the surrounding water bodies. These treatments include neutralisation and precipitation, oxidation and reduction, filtration, absorption techniques, crystallisation, atmospheric evaporation, vacuum evaporation, electrolysis, ion exchange, electodeionisation, acid sorption, membrane filtration, reverse osmosis, diffusion dialysis, membrane electrolysis and electrodialysis. These common techniques are listed in Table 1 below. Table 1 Typical treatment regimens for effluent treatment Treatment type Oxidation Description of treatment • • • • Reduction • • Neutralisation and precipitation • • • Filtration • • • • Absorption techniques • Crystallisation This is specifically for cyanides Cyanides are oxidised to cyanates or completely destroyed using sodium hypochlorite (NaOCl) or calcium hypochlorite [Ca(OCl)2] The process is usually batch processing Wastewaters containing hexavalent chrome are treated with sodium bisulphite or sodium metabisulphite This produces a trivalent chrome compound which is treated to produce chromic hydroxide which precipitates out as sludge Caustic soda is usually used to produce nickel hydroxide, copper hydroxide and sodium sulphate. The hydroxides precipitate out and form sludge The operation is at pH 8.5 to 9.5 so that other metals present also react and precipitate out Sand filters are used for cleaning raw water or polishing effluents Belt filters or filter presses are used with higher solids applications such as wastewater sludges, often in conjunction with coagulants The filter medium with the filtrate is usually disposed of as a waste Activated carbon is used to adsorb unwanted organic substances formed from breakdown products in a solution Activated carbon will also remove a portion of the useful organic chemical additives (e.g. brighteners), which will need replacing The absorbent material along with the retentate and filter medium is usually disposed of as a waste, although precious metals may be recovered Various evaporation and cooling systems are used to bring solutions to a super-saturation point where solid crystals form and can be separated from solution 4 Treatment type Description of treatment • Atmospheric evaporation • Atmospheric evaporation occurs when solution are heated. It reduces the volume of process solutions and allows drag-out to be returned of fresh chemicals to be added to the process solution Evaporators are often used with a condenser to recover distilled water Vacuum evaporation Reduced pressure and elevated temperature combine to separate constituents with relatively high volatility from constituents with lower volatility Electrolysis – plating out Transition metals can be removed from wastewater streams by plating out on high surface area electrodes in metal recovery cells Electrolysis – oxidation It is possible to oxidise both unwanted organic by-products and metals in solutions, such as trivalent chromium to hexavalent chromium Ion exchange – resin Ions in solution are selectively removed by exchanging positions with resin-functional groups The direct ion exchange treatment of wastewater provides a means of concentrating multivalent cations for subsequent treatment on column regeneration or by plating out Water from ion exchange can be recycled (Dahl) Electrodeionisation Ions are removed using conventional ion exchange resins • Acid (resin) sorption • • Ion exchange – liquid/liquid • • • • • Membrane filtration • • • Acid sorption is configured similarly to ion exchange. Resins are designed to selectively adsorb mineral acids while excluding metal salts (adsorption phase) Purified acid is recovered for re-use when the resin is regenerate with water (desorption phase) Ionic contaminants are removed from process solutions into immiscible primary liquid extraction solutions Secondary liquid extraction solutions are sued to remove the contaminants and to regenerate the primary extraction solution Water from ion exchange can be recycled (Dahl) Membrane filtration can be used for the purification and recirculation of oily water (Dahl) Various types of membrane filtration exist that are dependent on the pore size Microfiltration is a membrane filtration technology that uses low applied pressures with pore sizes in the range of 0.02 to 10 microns, to separate relatively large particles in the macromolecular and micro particle size range Ultrafiltration passes ions and rejects macromolecules of 0.005 to 0.1 microns and removes organics from process solutions Nanofiltration is used for larger size rejection reverse osmosis (rejects molecule larger than 0.001 to 0.008 microns) – for partial desalination of rinse water, removal of aluminium from pickling baths and concentration of chromating chemical in rinse water Purified water can be re-used for the degreasing bath or rinsing 5 Treatment type Description of treatment • Reverse osmosis • • • • Diffusion dialysis • • Membrane electrolysis • • Electrodialysis • Reverse osmosis is effectively a filtration of ions through a semi-permeable membrane at high pressure which desalinates chemically treated wastewater It provides an alternative means of concentrating metal impurities for subsequent removal This approach can be capital intensive and any solids or organics have to be removed prior to treatment The chemicals from zinc-, chrome- and copper plating baths can be re-circulated Diffusion dialysis is a membrane separation process that typically uses an anionic exchange membrane to transport acid anions and protons from waste acid solutions into deionised water streams The anions and protons are treated in wastewater treatments plants and the acid is recovered Membrane electrolysis used one or more ion-selective membranes to separate electrolyte solutions within an electrolysis cell The membranes are ion-permeable and selective Anions and cations are removed from solutions with an applied electric field in cells with alternation anion- and cation-permeable membranes Various acids from electrolyte solutions can be recycled The possible treatment(s), specific to each constituent of concern can now be listed (Table 2). Table 2 Possible treatment(s) specific to each constituent of concern Constituent of concern Immiscible organics: Non-halogenated (oils, greases, solvents) and halogenated (oils, degreasing solvents, paint solvents) Possible treatment • Reduced to solubility limit by physical separation (e.g. flotation) or by liquid/liquid phase separation Followed by either air stripping (activated carbon) or oxidation to carbon dioxide (using UV irradiation and hydrogen peroxide addition) Soluble organics increase the difficulty in removing metals by flocculation Concentration may be reduced by oxidation (by UV irradiation and hydrogen peroxide addition) Dissolved organics increase COD; biological treatment may be necessary pH adjustment is usually required pH may be partially neutralised by mixing with other streams May be removed by settling or filtration Filtration uses a filter- or belt press to produce a cake manageable as a solid • • Soluble organics: Wetting agents, brighteners, organic ions and ligands • Acids and alkalis • • Particulate material: Metal hydroxides, carbonates, powders, dusts, film residues, metallic particles • • • • Metals For soluble anions the capture of precious metals for re-use, e.g. platinum, gold, silver, rhodium and ruthenium may be achieved by electrochemical recovery or ion exchange In some cases it may be necessary to reduce the oxidation state of the metal ion as the higher oxidation state may not be readily flocculated and precipitated by pH change • 6 Constituent of concern Possible treatment • Complexing agents: Sequestering and chelating agents Nitrogenous materials: Ammonia, Nitrites Cyanides Sulphide Fluorides Phosphate compounds Other salts Multivalent ions are most conveniently removed by precipitation and pH adjustment • May be removed by precipitation with the predecessor of activated carbon process • Microbiological oxidation is also a possibility for removal of complexing agents • Ammonia can be removed by steam stripping or by oxidation to nitrogen with sodium hypochlorite • Nitrites can also be oxidised with sodium hypochlorite • Note that AOX may be formed when using hypochlorite solutions Cyanide from degreasing may be oxidised by using hypochlorite or chlorine gas at high pH When in excess sulphide can be precipitated out as elemental sulphur on oxidation with hydrogen peroxide or iron (III) salts. Is readily precipitated out as calcium fluoride at a pH above 7 Precipitated out as calcium hydroxide phosphate • Other ions such as Cl-, SO42-, K+, Na+ and Ca+ • Sulphate can be readily precipitated as calcium sulphate • Ion exchange, reverse osmosis or evaporation to remove other ions Sludge waste is produced when effluent from metal finishing operations is treated to create insoluble metal compounds like metal hydroxides or carbonates. These precipitate to create a watery sludge which needs to be dewatered by various methods such as filtering through a filter press, belt press or centrifuge to produce a cake manageable as a solid. When operating at pressures above 15 bar the final cake can have 15 to 35% solids. The filter can then be dried further to lower the water content. Some waste solutions can be disposed of as liquid- or hazardous wastes, or can be recovered or recycled. Examples of wastes that have the potential to be recovered or recycled include autocatalytic plating, spent etchants and sludge from anodising. Further steps to abate potential releases to the environment include cleaner production initiatives, which aim to minimise the use of water and material discharged from the processes. It is important to note, however, that the minimisation of water usage can increase the concentration of dissolved salt and various metals, which increases the solubility of metals in the wastewater. Further, it can become difficult to maintain a stable pH in the narrow margins required to minimise the solubility of individual metals when dealing with a mixture. This suggests that when dealing with a mixture of metals it will become increasingly difficult to optimise every parameter. 7 3. CLEANER PRODUCTION INITIATIVES FOR THE METAL FINISHING SECTOR 3.1 MANAGEMENT AND HOUSEKEEPING The most practical way of improving plant operation to reduce the wastewater generated is by improving management and housekeeping. The best environmental performance is usually achieved by the installation of the best technology and its operation in the most effective and efficient manner. An Environmental Management System (EMS) is a tool that operators can use to address design, construction, maintenance, operation and decommissioning issues. Environmental management systems typically ensure the continuous improvement of the environmental performance of the installation. Components of the EMS typically include: • • • • • • • • • • A definition of the environmental policy Planning and establishing objectives and targets Implementation and operation of procedures Checking and corrective action Management review Preparation of a regular environmental statement Validation by certification body or external EMS verifier Design considerations for end-of-life plant decommissioning Development of cleaner technologies and Benchmarking These factors are summarised in Table 3. Furthermore, if workpieces are treated incorrectly, in terms of incorrect specification or faulty application of the correct specification, the outcome will be significant amounts of metal stripping or scrapping of the workpiece. Implementation of a Quality Management System (QMS) can reduce the reworking and scrap. Avoiding rework minimises losses in raw material, energy and water inputs, as well as minimising wastewater treatment and the generation of sludge and liquid acid wastes. Table 3 Typical components of an Environmental Management Plan (EMS) Components of EMS Description • Definition of environmental policy • • Planning Implementation and operation of procedures • Top management are responsible for defining an environmental policy that includes a commitment pollution prevention and control and to comply with all relevant applicable environmental legislation The policy must provide a framework for setting and reviewing environmental objectives and targets, must be documented and available to all interested parties Procedures to identify the environmental aspects of the installation and the legal and other requirements to which the organisation subscribes Establishing and reviewing documented environmental objectives and targets, and establishing and regularly updating an environmental management programme Effective environmental management must ensure that procedures are known in areas of • Structure and responsibility 8 Components of EMS Checking and corrective action Description • • • • • • • Training, awareness and competence Communication Employee involvement Documentation Efficient process control Maintenance programmes and Emergency preparedness and response • Establishing and maintaining documented procedures to monitor and measure key characteristics of operation and activities Establishing and maintaining procedures for defining responsibility and authority for handling and investigating non-conformance with permit conditions Establishing records, EMS audits and periodic evaluation of legal compliance • • • Management review • Reviewing the EMS at regular intervals to ensure its suitability, adequacy and effectiveness Ensure that necessary information is collected to carry out evaluation and documentation of the review Preparation of a regular environmental statement This statement compares the results achieved by the installation to the environmental objectives and targets Validation by certification body or external EMS verifier Having the EMS validated by an accredited certification body or external EMS verifier to enhance the credibility of the system Design considerations for end-of-life plant decommissioning Giving consideration the environmental impact from the eventual decommissioning of a unit Development of cleaner technologies Giving consideration to the development of cleaner technologies that incorporate techniques at the earliest possible design stage to be more effective and cheaper Benchmarking Carrying out systematic and regular comparisons with the sector on a national or regional scale Other management techniques include a reduction in reworking by process specification and quality control which minimises wastewater treatment and the generation of sludge and liquid acid wastes. A theoretical optimisation of the process line improves the consumption of water, energy and conservation of raw materials, as well as minimising emissions to water. Real time process control systems that collect data and react to maintain predetermined process values improve plant efficiency and product quality as well as lowering the emissions. 9 4. BEST PRACTICES APPLICABLE TO JIG AND BARREL PLATING OPERATIONS 4.1 REDUCING DRAG-OUT • Better positioning of work-pieces on racks: parts should be placed on the rack so that the largest surface or plane is nearly vertical and the longer dimensions are horizontal. The lower edge should be inclined, so that run-off will occur at a corner rather than the entire edge. Avoid positioning parts directly over one another where possible. • Optimise the design of racks: The rack should be positioned/inclined so that horizontal surfaces on the rack are minimised. • Better operation of barrels: Barrel holes should be kept clear to permit maximum drainage. Make sure barrel doors face upwards during withdrawal (usually less holes on doors). Intermittently rotate the barrel during draining and hold for 10 seconds in each position of rotation • Slower work-piece withdrawal from tanks. The faster the work-piece is withdrawn from the process bath, the thicker the film on the work-piece surface and the greater the drag-out volume. The effect is so significant that most of the time allowed for draining a rack could instead be used for withdrawal only. This is obviously easier to achieve with electrically operated systems than manual systems. • Drainage time. Allow as much time as is feasible to drain above the process tank, except in the case of processes that require a rapid cessation of a reaction on the surface, such as passivation. Some guide values for minimum withdrawal and drainage times are: • Choice of barrel: Use barrels with as high a proportion of the surface covered in perforations as is possible. The size of the holes in a barrel are important. If one is plating fine pins, the holes have to be small. This also means above-tank drainage will be slow. It also reduces plating speed. Using the same barrel for M20 bolts and nuts is inadvisable because such small holes are unnecessary. • Spray rinses can be installed over heated process tanks that will remove drag-out and make up for evaporative losses. These rinses can be automated to only be in operation when the jig is being lifted, or for manual systems a switch with a timer could be used. • Choice of solution chemistry. The higher the concentration of the solution, the larger the amount of drag-out (in terms of volume and concentration). Experiment to establish if it is possible to achieve the desired results with a less concentrated solution. • Wetting agents can also reduce drag-out. Ensure that the level of wetting agent in the solution is at the optimum level. 10 • 4.2 Operating temperature. A higher operating temperature will reduce drag-out as viscosity will be lower and create space for the effective use of dragout solution. This has to be off-set against increased energy costs. IMPROVEMENTS IN RINSING • Where there is no space or capital available for additional rinse tanks, a simple but very effective improvement in rinsing is to dip twice in the same rinse tank. • Use drag out tanks effectively (also referred to as static rinse tanks.) These are rinse tanks without a flow of water. Water from the dragout tank can be added back into the process tank to make up for evaporative losses. They must be used in combination with running rinses. For hard chrome plating solutions in particular and other concentrated heated solutions like acid nickel plating, two, or even three dragout tanks can be used. • Use the drag out tank in both directions. Use the dragout tank as the pre-dip before parts go into the process bath as well as when they come out of the process bath. This is because clean rinse water clinging to parts will be carried into the dragout tank from the proceeding rinse, diluting the drag out slightly. Then at the next step dragout solution will be carried into the process tank, effectively replenishing it. After the process tank the jig (or barrel) is returned to the dragout tank, increasing its concentration. This will help stabilise the concentration of chemicals in the dragout tank at approximately 50% of the bath concentration. This method is particularly suitable for ambient process tanks as it also prevents the volume from growing, stopping the return of solution to it. This system is sometimes referred to as an eco-rinse. If preferred an additional tank can be included prior to the process tank, and coupled to the dragout tank in such a way that the dragout is circulated through this “drag-in” tank, effectively equalising the strength of the “drag-in” and dragout tank. This eliminates the necessity for the jig to have to overshoot the process tank to be immersed in the dragout tank before returning to the process tank. We cannot refer to this “drag-in” tank as a pre-dip or conditioning step which still has to be included in the line. As an example, for nickel plating a weak solution of sulphuric acid, a so-called “sour dip” has to be present before the plating process tank to condition the surface and combat any slight oxidation that may develop as the jig travels towards the process tank. Creating a “drag-in” tank rather than simply overshooting the process tank, immersing into the dragout tank and then back-tracking to the process tank, will add the cost of one extra tank to the line (including the cost of extra space, overhead lift equipment, plumbing, etc.). 4.3 CONTROLS ON RINSE WATER FLOWS: • Flow restrictors on flow-through rinses. These simple mechanical flow control valves give constant flow independent of pressure and are very cheap. This eliminates variations in water flow rate arising from water line pressure changes or operators adjusting valves in error. • Control valves on rinse water flows with timers delivering a fixed amount of water triggered by programming or a switch. 11 • Conductivity control in the flow-through rinse tank. This will deliver fresh rinse water only when a preset level of conductivity, indicating contamination, has been exceeded. • Counterflow rinsing. If there is room for more than one rinse tank set up a counterflow rinse system, i.e. have the fresh water entering the last tank that the work-pieces go through. Let the overflow from this last tank be the feed for the earlier rinse tank. A two-stage countercurrent rinse system will drastically reduce the amount of water needed to clean a part to the same degree of cleanliness – water reduction can be 90% or more. All new plant builds should incorporate counterflow rinsing. 4.4 PREVENTING “BACK-MIXING” OR “SHORT-CIRCUITING” The diagram below demonstrates a design that prevents the phenomenon of “back-mixing” or “shortcircuiting” in a counter flow rinse system. When the barrel is immersed in Tank 1 is should not displace enough water to rise up and cascade into Tank 2, as this will work against the counter current principle. Water should naturally flow from Tank 2, the cleanest water, to Tank 1, the first port of call, with the most contaminated water. If water from Tank 1 contaminates the water in Tank 2, then the system is compromised and will not deliver the theoretical results. Flow of Water Immersed Plating Barrel Baffle 1 2 Rinse Water In Flow of Work Figure 1 Design for preventing "back-mixing" in tanks • Closed loop counterflow rinsing. With a multiple counter-current rinse tanks it may be possible to reduce rinse water flow such that the rinse water can be used to make up for evaporative losses. • Spray rinses. Spray rinses can be installed over rinse tanks. These act like a second rinse step. These rinses can be automated to be activated only when the jig is being lifted. For manual systems a switch with a timer could be used. It is also possible to design a process tank to incorporate a spray manifold length wise along the top of the tank on both sides. This may be activated to direct a fog spray onto the workpieces, rinsing dragout directly back into the process solution as soon as the jig moves past it on removal. The system is ideal for hot process solutions, and for parts that do not have recessed or internal surfaces that the spray cannot access. It is not easy to retrofit as a bit of extra freeboard is needed at the top of the tank, and this also affects the distance that the carrier device has to lift and lower at that station. 12 • An overall master control valve on the main water supply to the finishing process with a switch easily accessible to the operator for stopping water loss during breaks, over nights and weekends. This avoids having to change settings on individual valves on tanks. • Air agitation. This keeps the rinse tank stirred and also has a scrubbing effect in removing process solution from parts. • Photo sensor on an automated line. This can detect when dripping stops. 4.5 MINOR PROCESS BATH IMPROVEMENTS Some relatively inexpensive modifications can be carried out on existing process baths: 4.6 • Drain boards. These can be installed between tanks and angled to allow drips to return to the process bath when parts are transferred between tanks. They also help to keep the area in and around the tanks clean. • Drip trays. These can be inserted either manually or automatically under the parts suspended from the jig to prevent contamination of other process baths when the jig is being moved. IMPROVE MAINTENANCE PROCEDURES Routine removal of sludge, soils and other contaminants from baths should be carried out. Most plating systems benefit from continuous filtration which prevents sludge build up. However, in the event that this option is considered too costly, then periodic de-sludging and filtration should be carried out. Oil can be removed from process baths and degreasing solvent or solutions by skimmers, gravity separators and centrifuges. Ion exchangers can be used in some applications to remove various contaminants (for example, removal of copper/zinc/nickel contaminants from trivalent chromium plating baths). Certain salt contaminants can be removed by cold crystallisation – the bath is cooled down and the salts precipitate out (such as sodium carbonate from plating baths that use caustic). Such a process can be incorporated into a normal shut-down when baths will cool anyway. Carbon filtration of organic contaminants can be achieved by either adding carbon to the process bath and removing it in the filter or else having a carbon filter installed in the same line as the solids filter (for example, removal of organic contaminants which can cause dullness in the finish from a nickel plating bath). Minimising drag-out means that more systematic methods for regular removal of contaminant build up will be required. This is because drag-out does assist in the removal of contaminants (along with the valuable process solution). Maintenance of racks/barrels to prevent products of corrosion entering the process baths: Check barrels regularly to see that holes are not being closed off through the tumbling action during processing. Any holes in polypropylene barrels that are starting to close off can be drilled back to normal size. 13 Periodic checking of jig insulation (if present) for cracks which would trap process solution. 4.7 PROCESS CONTROL The process solution should be regularly checked to see that it is maintained at the desired strength. The correction of solution strength by making small and frequent additions is much more effective than making a few large additions. An accurate log of all checks and additions should be kept. Make use of chemical suppliers to advise on addition rates and which parameters should be used for control purposes. A simple Twaddel or Baumé hydrometer test will easily identify whether a solution has become severely weakened as a result of solution loss or depletion through lack of anodes. The test will work on many solutions including acid copper, acid nickel, decorative and hard chrome solutions, etc. It does not work on solutions that build up carbonates or that become loaded with dissolved metal. Conductivity probes may assist in maintaining a constant concentration in the case of some process solutions. Use a deioniser on your water supply for additions to process baths as well as for rinse water used in static baths and in closed loop rinses. This will help to reduce contaminant build-up. Standard deioniser packages can be fairly easily incorporated into your water supply. It should be noted that deionised water is not recommended for the rinsing step directly after nickel and before chrome plating as it can have the effect of passivating the nickel surface, hampering chrome receptivity. 4.8 PRODUCT STORAGE Try to store products prior to processing away from the humid and acidic conditions of processing areas for as long as possible. This will reduce corrosion formation. Corrosion on products might need additional cleaning before they are processed. 14 4.9 SOLVENT AND ALKALINE DEGREASING SYSTEMS • Reduce or Eliminate the Need for Cleaning The subsequent processes a work-piece is to be subjected to will have a bearing on what degree of cleaning is required. Careful storage may reduce cleaning requirements. Protective packaging may help reduce the amount of cleaning required. This does need to be considered against the additional resource use as a result of such packaging. Reusable packaging or coverings may be applicable. • Use Alternative Cleaning Processes If you operate a degreaser or use solvent to clean parts by cold soaking, the need to use these processes in every case should always be assessed. In many cases they are used all the time just because they are there. Consider the following alternatives for cleaning: • Carry out some initial manual cleaning with dry rags or paper for larger parts prior to use of the degreaser. This will extend the degreaser’s working life (although it will generate waste soiled rags/paper). Mechanical cleaning such as power wire brushing or shot blasting may be applicable. Use water-based solutions. There are a wide variety of cleaners available for different types of soiling, different types of substrate and for the different types of follow on processes. Work-pieces are soaked in the solution. The solutions do require regular replenishment. Exhausted solutions will need to be either treated or properly disposed. Specialised equipment is available for using such solutions but is not always necessary. Some such equipment would be relatively simple and inexpensive. Some have integral brushes and heating to assist the cleaning process. Use Eductors in the Alkaline Cleaning Tank Eductor nozzles use the venturi principle to amplify and direct solution flow from a pump to the required area of the tank. For 1 litre of solution pumped through an eductor at required pressure, discharge flow from nozzle will be 5 litres. Therefore it becomes economically viable to have a smaller pump perform agitation! Figure 2 15 Schematic drawing of an Eductor Figure 3 Operation of Eductors 1. Vigorous solution agitation focussed around, across or directly at work and sweeping the tank floor 2. More efficient cleaning, lower rejects without air 3. More uniform temperature 4. Better heat transfer, possibility to lower temperature and reduce heating costs 5. Improved cleaning efficiency of brass sand castings, eliminating roughness rejects after plating 6. Ability to lower cleaner temperature from 80°to 60° whilst maintaining cleaning efficiency. Achieving very positive heat savings up to 40%. Figure 4 • Diagram demonstrating the setup of an eductor array in a cleaning tank Biological Cleaning Systems. Use a biological degreasing system. Specialised equipment is needed. Uses microorganisms to break down the oil and grease on the surface of work-pieces. These are closed systems so only very small amounts of sludge need occasional disposal. Only very small amounts of cleaning chemicals need to be added occasionally. It has the advantage of there being no oil or solvent for disposal. The chemistry may not be suitable for all types of oil. Such systems are commercially available. 16 • Ultrasonic Cleaning Ultrasonic cleaning can be used to enhance cleaning systems, but specialised equipment is needed. Usually this is considered to be an expensive option for anything but the smallest of operations such as manufacturing jewellers, or manufacturers of costume jewellery, fashion trim and the like. An ultrasonic cleaning unit is tank that has an attached transducer capable of generating sound waves. High frequency sound waves are transmitted through the cleaning solution, causing formation and collapse of small vapour bubbles at the solid surface, called micro-cavitation. This agitation assists in the removal of soiling, especially in hard to reach areas. Hence the efficiency of the cleaning process is enhanced. Tanks are usually operated with aqueous and other non-volatile media. 4.10 IMPROVE THE OPERATION OF CONVENTIONAL VAPOUR DEGREASERS The use of solvent degreasing is strongly discouraged in the modern environmentally conscious world. If it is considered that it is absolutely necessary to use a solvent degreasing system, then there are a number of operational procedures that should be observed to minimise the loss of vapour from conventional vapour degreasers. When inserting or withdrawing components avoid speeds greater than 3 metres per minute to minimise the amount of solvent vapour lost. The use of a power-operated hoist is recommended for all degreasing plants to control the speed of entry and exit of the workpieces. Avoid heavy loads that will result in the collapse of the vapour blanket and infiltration of air into the unit. This solvent saturated air will then be expelled when the vapour layer re-establishes. Ensure that the parts have reached the temperature of the vapour before removal so that condensation has ceased. Look to see that there is no liquid on the parts. Rim ventilation should be high enough to protect operators, but excessive extraction results in unnecessary solvent consumption. An extraction rate of 640-915 m3/hr per m2 of bath surface is recommended. For any degreaser with a specific rim vent slot design, extract fan specification and ductwork configuration, there will be a specific rim vent slot velocity. Some users may find it easier to check this measurement rather than the total volume of air extracted. The degreaser supplier should be contacted for the appropriate figure. To ensure the degreaser functions efficiently, the solvent temperature should be maintained at a level adequate for vapour production. Improvements that can be made to conventional vapour degreasers that will reduce solvent emissions are as follows: Locate the degreaser away from drafts or use baffles to prevent upset. Proper location can reduce solvent loss by up to 30%. Features that create air currents that can disturb the vapours include doors, windows, heating and ventilation systems and busy passages. Install support frames within the condensation zone to allow work that is mounted on jigs to be supported while degreasing is in progress. This enables the lifting device to be raised and the lid closed over the work during the degreasing process. Install covers to eliminate drafts and reduce diffusion. These lids should be fitted below the rim ventilation slot, otherwise this can allow the extraction system to pump the system dry, and should be 17 fitted at the top of the freeboard zone. A roller or a slide design should be used rather than lift-out panels as this is less likely to disturb the vapour. Double-door systems can be used, which are more expensive to install, but reductions in solvent consumption of up to 80% have been claimed. Such systems can have timed interlocks. The use of baskets having an area less than 50% of the degreaser opening will limit vapour dragout due to the piston effect. Increase freeboard height. A freeboard ratio (freeboard height divided by the width of the tank) of at least 0.75:1 and preferably 1:1 is recommended. Install fixed pipework, connected to the sump, for topping up with new solvent, rather than manually pouring new solvent into the degreaser from drums or buckets. Install refrigerated coils, which will condense solvent from the air leaving the degreaser and return it to the unit. A rapid-response temperature sensor installed immediately below the condensation zone acts as an energy saving device. The sensor cuts the energy input to the sump in response to the vapour temperature. The cooling effect of the new load being placed in the unit reactivates the main heating system. 4.11 USE A LOW-EMISSION VAPOUR DEGREASER The Low-Emission Vapour Degreaser, by contrast to conventional degreasers, is a completely sealed unit. It incorporates both refrigerated condensation and carbon adsorption/desorption to trap and regenerate solvent. Alternative solvents that are less hazardous are available which could be used instead of chlorinated solvents and aromatic hydrocarbons. These include terpenes, aliphatic hydrocarbons, alcohols, esters and glycol ethers. 4.12 • PLATING Electrolytic Metal Recovery Electrolytic recovery of metals from static or closed loop rinse systems can be used for precious metals as well as nickel, copper, zinc, and chromium. Commercially produced packages are available in different sizes. Electrolytic recovery is usually used as one step in a treatment system as recovery from very dilute solutions cannot be achieved using this method. A heated solution is desirable to overcome electrode polarisation and low diffusion rates. Mechanical mixing is also necessary to improve plating, this is achieved by a moving or rotating cathode or a high solution velocity over fixed cathodes. In some cases a high surface area is used where the metal is deposited onto a fibrous or filamentous substrate. This can be sold as a low volume residue or the deposited metal can be stripped chemically or electrochemically so that the end result is a concentrated solution of the metal which is recovered for reuse. 18 If inert anodes are used the metal can be reused as anode material. These anodes can be reused several times as the inert base metal will not be dissolved during plating. The process also has the advantage of destroying any cyanide in the solution in parallel with the electrolytic recovery of the metal. • Alternative Plating Processes The use of alternative coatings that will provide the same degree of protection, decoration, etc. should always be assessed. Developments in the area of alternative plating processes were accelerated by Directive 2002/95/EC on the restriction of the use of certain hazardous substances in electrical and electronic equipment (ROHS Directive). This Directive required the removal of lead, mercury, cadmium, and hexavalent chromium in such equipment, from 1 July 2006, except for certain applications where there are no feasible alternatives (for example cadmium plating). • Non Cyanide Plating Baths There are several alternatives available in relation to the replacement of cyanide plating baths: • Acid zinc plating in place of cyanide zinc plating. Has the added benefit of reduced energy requirements. Needs good process control and work-pieces must be very clean for this process. Alkali cyanide-free zinc plating. Has disadvantage of a higher energy requirement. As for acid zinc plating, this process needs good control and well pre-cleaned work-pieces. It has been reported to achieve better metal distribution than cyanide plating. Alternative to Hexavalent Chromium Plating Baths Earlier problems with trivalent chromium as an alternative to hexavalent chromium plating such as colour differences and variations have been overcome with newer products. Such processes require a higher degree of control and some modifications will be needed to equipment. Product quality is reportedly better than with hexavalent chromium plating. Trivalent chromium cannot replace hard chromium processes. There are very few, if any, operational decorative trivalent chromium systems in RSA at the time of the drafting of this report. Despite the claims of manufacturers it would seem that industry operators have decided that the system is more expensive to install and operate as it requires far more sophisticated equipment and controls. 4.13 CHEMICAL AND ELECTROCHEMICAL CONVERSION COATINGS (CHROMATING, PHOSPHATING, ANODISING, AND COLOURING) Trivalent chromium conversion coatings are available as an alternative to hexavalent chromium conversion coatings. These solutions can be used in existing equipment with only minor modifications, so they are relatively easy to retrofit. There are chrome-free conversion coatings available but workpieces may need additional coating to provide corrosion protection. 19 4.14 ALTERNATIVE STRIPPING PROCESSES There are alternatives available for cyanide-free nickel stripping solutions. Tank lining may needed. be During the course of the survey several companies in the metal finishing sector were visited to establish the current practices in terms of chemicals management regarding handling, processing, storage and transportation with particular attention being focused on water usage and wastewater practices. The sector types that were visited included: • Electroplaters • Powder coaters • Chemical conversion The information presented in the ensuing sections was abstracted by emailing the selected companies with the questionnaire as well as conducting interviews with the appropriate staff members of the various companies. 20 5. DESIGN METHODOLOGY FOR COUNTERFLOW RINSING Counterflow rinsing is renowned as a CP technique that reduces the consumption of water used by any electroplating facility. The design methodology used for counterflow rinsing unit is discussed in the ensuing sections which includes: • • • • • 5.1 An evaluation of the mathematical theory used in counterflow rinsing techniques; The list of the assumptions made for the design; The list of model inputs and motivation for its chosen quantities; The model outputs to be calculated; and Other process parameters which may affect the design. THEORY OF COUNTERFLOW RINSING SYSTEMS Rinsing takes place by the thinning of the liquid layer on the surface of an object. The more times the liquid layer is thinned the greater the cleanness of the object’s surface. Rinsing can be carried out in one or more stages by dipping, flushing or a combination of both. A method which significantly reduces the volume of water required for rinsing is the counterflow rinsing method (Hyder Consulting and Hemsley 1999). In counterflow rinsing the water flows between the rinsing stages in a direction counter to that of the object flow, which can be either continuous or discontinuous. In electroplating the workpiece can therefore be dipped into the first tank (Rinse A) and then the second and so forth until it exits the final rinse tank (Rinse D), while fresh water flows from the final rinse tank and is let out of the first rinse tank (see Figure 5). The overflow from Rinse A has the highest metal concentration and can either be sent to the wastewater treatment plant or be recycled to the process tank if this is practical. Figure 5 Counterflow rinsing (Hyder Consulting and Hemsley 1999) The water required to effectively rinse workpieces can be improved by extending the rinsing process with more rinsing tanks/stages. The freshwater consumption in counterflow rinsing is calculated from the following formula. Qn = D ⋅ n C0 Cn = D⋅n F Equation 1 Where, • n is the number of rinsing stages; 21 • • • • • Qn is the consumption of fresh rinsing water (volume/time) when using counter-current rinsing with n stages; D is the drag-out from the cleaning vat (volume/time); Co is the concentration of the constituent in the plating bath (mass/volume); Cn is the concentration of the constituent in the nth rinsing tank (mass/volume); and F is the dilution factor (dimensionless). It is seen from Equation 1 that the amount of freshwater consumption (Qn) will decrease for a decrease in drag-out (D), an increase in the number of rinsing stages (n), and a decrease in the dilution factor (F). The dilution factor is dependent on the metal concentration that can be tolerated in a rinse without having adverse effects on the rinsing quality (Van Der Spuy, 2004) while the drag-out is dependent on the workload and dripping time of workpieces over the process tank. Since the dilution factor has a limit that varies dependent on which process tank it serves, the freshwater consumption can only be minimized by employing methods which reduce drag-out and by increasing the number of rinsing stages. The use of a five-stage counterflow rinse has been reported as an ideal situation design if a closed loop rinse system is being considered but a cost benefit analysis show that for most requirements a three stage rinse is most adequate and is therefore considered for the design of this CP initiative (Van Der Spuy 2004). It is also very plausible that not all facilities have the available space or capital for a 5-stage unit. The required freshwater flow for a 5-stage configuration is expected to be low enough to keep up with the drag-out and evaporative losses in the process tank so that the concentrated rinse water (in Rinse A) may be recycled to the process tank. A closed loop system is therefore formed, achieving zero discharge. This system is most applicable to chrome and nickel plating installations where there is considerable evaporation. A counterflow rinse with 3 stages is shown in Figure 6. Concentrations C1, C2, and C are the mixed concentrations in Rinse tanks 1, 2 and 3 respectively. The mass conservation principle applied for metal concentrations in the rinse tanks is as follows: • A workpiece coated with a solution of the original process solution (C0) mixes with the overflow from Rinse tank 2 (C2) in Rinse tank 1 to achieve a final concentration,C1 An similar approach would be applicable for determining the drag-out and tank concentrations in the subsequent tanks. Furthermore, since drag-out (D) and the flow of rinse water (Q3 for 3 stages) is assumed to be constant over all the rinse tanks, the flow rate of rinse water between rinse tanks is determined as Q3+D. 22 Co, D C1, D C2, D Rinse tank 1 (C1) Process tank C1, Q3+D Figure 6 Rinse tank 2 (C2) Rinse tank 3 (C3) C3, Q3+D C2, Q3+D Counterflow rinsing with 3 stages C0 With a predetermined value of the dilution factor F, C3 can be calculated as concentrations can then be solved with simple mass balances: Mass balance of metal over Rinse tank 3: Q3+C2D=C3x(Q3+D) Equation 2 Mass balance of metal over Rinse tank 2: C1D+C3x(Q3+D)=C2x(Q3+D)+C2D Equation 3 Mass balance of metal over Rinse tank 1: CoD+C2x(Q3+D)=C1x(Q3+D)+C1D Equation 4 5.2 F . All other metal DESIGN MODEL ASSUMPTIONS The design of this counterflow rinsing unit is based on the following assumptions: • • • • • • 5.3 The counterflow rinsing unit is intended for rinsing of workpieces that have just been immersed in an electroplating bath Only one electroplating solution enters the rinsing tanks; The flows of drag-out across each tank are equivalent; The overflows of rinsing water from each tank are equivalent; Perfect mixing occurs in each rinsing tank ensuring that the concentration of solutions leaving the tank are equivalent; The flow rate of the overflow from the first rinsing tank does not significantly disturb the process tank volume because it is compensated for by the sum of the evaporation- and dragout rate. DESIGN MODEL BASIS Based on the presented theory, a few parameters that affect the process are required to be predetermined before the three stage counterflow rinsing unit can be designed. These include determining the drag-out rate (D), metal concentration of the process tank (C0), and dilution factor (F). 23 Q3 For this design, a drag-out rate of 1 ℓ/h has been assumed as a rough estimation based on a Danish study (Dahl, 1997). For zinc plating, the metal concentration of the process tank has been assigned a value of C0 = 55x103 mg/ℓ. This is the maximum zinc concentration that can be present in the process solution when an acid zinc electroplating method is used (IPPC, 2006). Finally, the required dilution factor is estimated at F = 1000, which is at the more concentrated end of the proposed range for rinsing processes implemented after electroplating baths (Van Der Spuy, 2004). This means that the concentration required to effectively rinse the workpiece in the third tank (C3) must be equivalent to one thousandth of the metal concentration in the process tank, i.e. 55 mg/ℓ. 5.4 DESIGN MODEL OUTPUT The design model inputs can be used to determine the required freshwater flowrate (Q3) with Equation 1. The final concentration of the rinse exiting Rinse tank 1 (C1) can then be calculated using Equations 2 to 4. 5.4.1 Additional design considerations In addition to the aforementioned parameters there are also other considerations for the unit design, namely equipment sizing, rinse tank operation, process control, drag-out control, and power requirements. 5.4.2 Equipment sizing Recommendations on the size of a rinsing tank should be based on the maximum expected size of the workload. In most cases, the counterflow rinsing unit will consist of a single tank divided into a series of compartments. 5.4.3 Rinse tank operation If there is no way to monitor or control the flow of water through a rinsing system it would not make any meaningful difference whether one operated a single static rinse or a three stage counterflow rinse (Van Der Spuy, 2004). An effective system will therefore need to be used to ensure that the rinse water is transported from the one tank to the next at a controlled flow rate. This can be achieved by implementing a gravity feed whereby tanks are installed at different levels to provide the required head. However, this is not always practical, for example, when auto lines are used to transport workpieces with fixed travel on lifts. In that case, pumps can be used to circulate the water. Furthermore, proper mixing of the drag-out and rinse water should be ensured. 5.4.4 Process control To ensure efficient control of the rinse water flow, the flow rate should depend on and be adjusted according to parameters that are monitored in the rinse tank. It is impractical to manage that which is not measured. 5.4.5 Drag-out control The drag-out rate is dependent on the workload and dripping time of workpieces over the process tank from which it has emerged. However, where electroplating plants are manually operated, the drag-out rate can fluctuate if the workpiece is not drained for a consistent length of time. It is therefore 24 imperative that consistent and effective drag-out reduction methods are implemented in conjunction with a counterflow rinsing unit so that the unit may operate efficiently. 5.5 CALCULATIONS AND FINDINGS 5.5.1 Water consumption Based formulas defined in previous sections the freshwater consumption rate (Q3) was calculated as 10 ℓ/h ignoring evaporation. If a single stage static rinse tank was used, 1000 ℓ/h freshwater would be required for this design. Using a three stage counterflow rinsing unit therefore enables a 99% water saving when rinsing workpieces that have been electroplated. By solving Equations 2 to 4, the metal concentration rinse tank 2 can be determined. The results are shown in Table 4. Table 4 Metal concentrations in Rinse tanks 1 to 3 Tank number Concentration (mg/l) Rinse tank concentrations of of Zn Zn (mg/l) Process tank 55000 Tank 1 5887 10.7% of Co Tank 2 541 0.98% of Co Tank 3 55 0.1% of Co The first rinse tank therefore has a zinc concentration of 5887 mg/ℓ, which represents 10.7% of the initial zinc concentration in the process tank. Ideally, this rinse water should be concentrated before adding back to the process tank thereby not diluting it. 5.5.2 Equipment sizing On a line where components are transported either manually or automatically over the process line tanks in sequence, the rinse tanks have to be sized to accommodate the same size load as the main plating tank. As these tanks do not have to include anodes or heaters, they can be a bit narrower to save on materials. For three stage counterflow rinses, 3 similarly sized tanks are required. These can be individual tanks or a large tank divided into compartments. On totally manual lines where individual smaller jigs are removed from the plating bath and the operator physically transports them to rinses, one or two at a time, these tanks can be a lot smaller than the process tank. Nonetheless, 3 equally sized tanks are required, usually comprised of one tank divided into three compartments. The same applies on manual barrel plating lines where components are ejected from the barrels and transported to rinse stations in perforated baskets Sizing will depend on the volume the jigs (or baskets) will occupy allowing for adequate space all around for convenience. Usually, the material of construction dictates sizes as polypropylene sheets (and other materials) are manufactured in standard sizes such as 1.2 x 2.4 m or 3 x 1.5m and divisions of these sizes will make up panels of the tanks. It can also be understood that provided that rinse tanks are sized appropriately to adequately contain the volume of components that is expected, no specific size is dictated. What is important is the dilution factor and that is determined by the flow rate. . This explains why the cost of installing a counterflow rinse system is much more expensive on automatic and semi-automatic lines. Of necessity the tanks are much bigger than those used on the 25 more antiquated totally manual lines where operators physically move jigs and baskets in and out of rinses. 5.5.3 Process operation If it was preferred not to design the system so that the water cascades (effectively runs downhill) then it would be possible to position all the tank overflows at the same height and rely on an airlift pump to transport rinse water. This pump has no moving parts and thus suited to the potentially corrosive environment nor is it possible to accidentally empty a tank through a leak or siphon action, and it is inexpensive (Van Der Spuy, 2004). The mechanism of the airlift pump is illustrated in Figure 7. Figure 7 Air lift pump (Van Der Spuy, 2004) To ensure proper mixing, the use of air agitation must be implemented. This will help remove any plating solution clinging to the surface of the workpiece as well as mix the contents of the tank. Air can be introduced into the tank by an aerator placed diagonally across the bottom of the tank (Water conservation for electroplaters, http://www.p2pays.org/ref/01/00050.htm). 5.5.4 Process control automation The freshwater flow can be adjusted accordingly by monitoring conductivity readings. Sensors in the tank relay the conductivity reading to a control limit (in this study, a conductivity relating to 55 mg/ℓ zinc), which opens the valve allowing freshwater in until the acceptable limits are reached. 5.5.5 Drag-out control To maintain a constant drag-out volume the dripping time for all workpieces should be identical. On automatic and semi-automatic plants, when the carrier lifts the components out of solution, it must wait for the appropriate time (10 to 30 seconds) to allow for dripping before moving on. On totally manual plants where operators lift and lower components into the process tanks, the process tanks should be equipped with a rail directly overhead from which operators can temporarily suspend the jigs to allow 26 for drip off. Alternatively, a ledge of perforated material can be provided at the edge of the tank on which operators can rest a jig to allow for drainage back to the process tank. 5.6 SUMMARY OF THE DESIGN Based on the aforementioned discussion it can be concluded that the installation of a 3-stage counterflow would result in a water consumption reduction of 990 l/h from the initial single stage rinse setup of 1000l/h. The loss of plating solution is also prevented by returning concentrated rinse water from a dragout tank to the plating bath so that the running rinse system deals with substantially lower concentrations of process solution than would be the case if no dragout tank was in place.. Savings can therefore be realised in terms of water costs, effluent treatment costs and the cost of plating solution. 27 6. COSTING OF A CP INITIATIVE IN THE METAL FINISHING INDUSTRY The objective of this part of the survey was to present a study estimate of installing the prioritized cleaner production initiative, a counterflow rinsing unit, for the metal finishing industry. 6.1 THEORY OF THE COSTING APPROACH 6.1.1 Study estimate A diagram of a proposed dual dragout/three stage counterflow rinse was submitted to a respected plant builder in RSA for a quotation. The most important factor in rinse tank design is to ensure that the rinse water is completely mixed. A rinse tank must include a feed water distribution line, air agitation, and a flow control valve. The flow control valve can be controlled by measuring the conductivity in the rinse water. The major equipment that was identified for the 3-stage counterflow unit was therefore the tanks, aerators, flow control valves, and the conductivity control system. The contractor is highly experienced and full aware of all these requirements. Tanks The material chosen for the rinse tanks was polypropylene. At the contractors suggestion, a material thickness of 10 mm was identified as being adequate taking into account operating temperature and density of the solutions. Appropriate ribbing would be added to reinforce the tanks. The unit designed included two dragout tanks followed by a three stage counterflow system, and would be ideal for a system like acid nickel plating, but would be equally effective for any other type of plating as well. Figure 8 Design and dimensions of a 5-stage rinsing unit Each compartment therefore has the dimensions of 1 m x 1 m x 1.2 m. compartment was raised to 1.2 m to allow for a 20% disengaging space. The height of each Price: Such a tank with reinforcing and valves with cascade overflow and down pipes in the next tank to ensure proper counterflow will cost about R60 000 for 5 stations. Aerators Air agitation mixes the contents of the tank and helps to remove any plating solution clinging to the surface of the work piece. Air can be introduced into the tank by an aerator placed diagonally across the bottom of the tank. A suitable blower to aerate all 5 tanks was quoted at R9000 with an additional R500 per tank for the air sparge in each. Total for Aerators – R11,500 Flow control valves A flow control valve is used to restrict the fresh water feed rate to an optimum level. The conductivity control system uses a conductivity probe to measure the level of dissolved solids in the rinse tank. 28 When the level exceeds a predetermined maximum value, the controller will open the fresh water feed valve. When the level reaches a pre-set minimum value, the water flow is stopped. This equipment was quoted at an additional R16,000. Total cost from study estimate The study estimate conducted therefore revealed that the total cost for the dual dragout 3-stage counterflow rinsing unit amounts to R87 500. [R60 000 + R11 500 + R16 000] This is estimated to be the price that would have to be paid for a well-constructed item of plant of this nature in RSA today. Payback period The payback period was determined by estimating how long it would take the electroplater to pay off the capital costs incurred to implement a rinse system that incorporates a dual dragout followed by a 3-stage counterflow rinse. Capital costs would be paid off due to savings in water and metal solution. In designing the 3-stage counterflow rinse it was reported that water savings of 990 ℓ/h would be achieved when compared to a single-stage static rinse. However, in these calculations the impact of an effective dragout recovery system has not been factored into the equation. It is known that up to 95% of metal would be trapped in the dragout tanks before the workpieces move on into the three stage counterflow system. Depending on how well the dragout system is managed, the concentration of the dragout can obviously be maintained at levels of a fraction of the strength of the process tank itself. This means that workpieces entering the three stage counterflow arrive there with very much lower levels of the process solution clinging to them than would be the case if they entered the rinse system directly from the process tank. There are several variables at play here. The amount of dragout that can be returned to the process tank will depend on: • • • • The temperature and evaporation rate of the solution in the process tank How much volume is being dragged into the process tank when parts are introduced for plating Whether the pre-dip and the dragout tank can be the same tank – i.e. the drag in equals the drag out. How well the parts are drained over the process tank before moving into the dragout tank The latter consideration is of some importance, because good jig design and excellent drainage ensure that the dragout does not become concentrated too quickly, so too reducing the load on the dragout tank and ultimately the rinse system. All of the above translates to the fact that the flow rate can in effect be much lower than the calculation that follows, resulting in even bigger savings on water. It also means that the operator may consider working with two stage rinse tanks, rather than three, saving money on plant design without compromising on rinse quality. This explains why there are not more three stage rinse units in use. A plant working a 40 hour week and saving 990 l/h would reduce their water consumption by 39.6 kl per week. At City of Cape Town rates of R22.13 per kl this represents a saving of R876 per week. [Rate is comprised of Cost of Water R12.51/kℓ and Sewage Surcharge R9.62/kℓ combined = R22.13] Thus if the price of an effective dragout and counterflow rinse system is R87,500 then the system would pay for itself in 100 weeks based on water savings alone. [1.92 years] 29 Savings on Process Chemicals Savings on process chemicals depends very much on the system under consideration. We will consider two processes here. Nickel Plating Solution – Standard Watts formulation and brighteners An acid nickel plating solution costs about R35.00 per litre [March 2015] to make up inclusive of proprietary specialised additives. Thus if 1 litre per hour of process solution was being dragged out of the tank, and not recovered, then in a 40 hour week solution to the value of R1 400 would be lost. If this could be restricted to 10% of that figure by the correct use of a dual drag out system, then savings of R1 260 per week would be made on the price of the process solution. Adding the saving on process chemicals to the savings on water [R876 + R1 260 = R2 136] the payback period reduces to 41 weeks Because an acid nickel plating system typically operates at 60-70oC the level of the operating solution will drop due to evaporation making way for the return of dragout solution. Thus it is more feasible to operate a dragout recovery system on heated process solutions. In addition, the value of the nickel solution is 7 times higher than that of the acid zinc solution, further incentivising the operator to consider this CP option. Acid Zinc Plating Solution An acid zinc plating solution costs about R5.00 per litre [March 2015] to make up inclusive of proprietary specialised additives. Thus if 1 litre per hour of process solution was being dragged out of the tank, and not recovered, then in a 40 hour week solution to the value of R200 would be lost. If this could be restricted to 10% of that figure by the correct use of a dual drag out system, then savings of R180 per week would be made on the price of the process solution. Adding the saving on process chemicals to the savings on water [R876 +R180 = R1056] the payback period reduces to 83 weeks [1.6 years] Acid Zinc plating solutions, depending on whether rack or barrel plating and on the particular chemistry traditionally operate in a temperature band between 20-45oC. A plant operating at the lower temperature levels will not experience much evaporation, and if over-tank dripping is optimised there won’t be room for return of dragout. If one wanted to make use of the dragout solution, it would be necessary to concentrate this using one or another technology. Dragout concentration devices have proposed, but with potential savings of only R200 per week, in this scenario, operators are not inclined to spend money on a fairly expensive device. 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