ACID AND WATER USE IN GALVANIZING The UK galvanizing industry uses large quantities of acid for pickling, and many companies also use significant quantities of water for rinsing. The purchase and disposal costs for both acid and water are considerable. The data and advice in this Guide can help you to optimise both acid and water consumption so you can: EG46 GUIDE save money be prepared for rising disposal costs and more stringent legislation perform better than your competitors improve your environmental image ENVIRONMENTAL PERFORMANCE GUIDE: The benchmark for profitable environmental improvement
THE SURVEY A confidential questionnaire was sent to operators in the UK metal finishing industry to gather data on acid and water use in the general metal finishing industry and in the galvanizing industry 2. Data were collated and analysed to enable an industry profile to be built, and to give statistical information on resource use. GALVANIZING INDUSTRY PROFILE The profile from the respondents can be summarised as follows: The majority of sites have more than 4 employees. Over 65% of sites have an annual turnover of more than 2 million. Under half the companies had readily available data relating acid consumption to production. Most operators serve more than one sector of industry. Over 8% serve the general engineering sector, with architectural/construction being the next most common market for goods. ACID DATA All the galvanizing sites which responded to the survey use hydrochloric acid in their pickling processes and measure production throughput in tonnes. Various concentrations of hydrochloric acid are purchased with 28% being by far the most common. To enable comparisons of consumption to be made, this Guide uses 28% hydrochloric acid throughout. If you use a different concentration you can easily convert your acid volume to its 28% equivalent. For example 1 litres of 36% hydrochloric acid are equivalent to: 1 x 36 = 128.6 litres of 28% 28 hydrochloric Specific Acid Consumption Related to Production Data were collected on the basis of litres of 28% hydrochloric acid per tonne of product galvanized (Fig 1). 5% of respondents use less than the equivalent of 24 litres of 28% hydrochloric acid per tonne galvanized. Specific Acid Consumption Related to Quantity of Production The data in Fig 2 show that acid consumption per unit of production is lower in sites with a high production Fig 1 Specific acid consumption: litres per tonne of steel galvanized Percentage of respondents 3 25 2 15 1 5 1 2 3 4 5 6 7 8 9 1 11 Hydrochloric acid (litres/tonne) Fig 2 Specific acid consumption related to annual product treated Hydrochloric acid (litres/tonne) 45 4 35 3 25 2 15 1 5 1 2 3 4 5 6 7 8 9 1 Annual product treated (thousands of tonnes) throughput. This trend may occur because sites with a high throughput treat larger items with relatively smaller surface area per tonne, or it may be that these sites invest more time and money in acid conservation because of their high annual acid use. Acid Costs Average annual acid consumption increases in line with site turnover (Fig 3). Data show that around half of the respondents each use more than 5 litres of hydrochloric acid annually, which can result in acid purchase and disposal costs in excess of 7 /year. Acid Sludge Production Hydrochloric acid is normally disposed of when its iron content exceeds around 12%. Data from the survey show that the ratio of acid sludge to acid consumption tends to be higher in the sites that use most acid each year (Fig 4). In the future, rising costs and stricter control of waste disposal may force sites to consider free acid recovery techniques and reduce their amount of acid waste. 5% of respondents generate less than.1 tonne sludge/1 litres hydrochloric acid. 2 This Guide contains information relating to acid and water use in the galvanizing industry only. Details of acid and water use in the general metal finishing industry are covered in EG44 and EG45, respectively. Copies of these related publications can be obtained, free of charge, through the Environmental Helpline on 8 585794.
Fig 3 Average acid consumption related to turnover Hydrochloric acid (thousands of litres) 7 6 5 4 3 2 1 to 1 1 to 1.9 2 to 2.9 3 to 4.9 5 to 9.9 1+ Turnover ( million) Fig 4 Ratio of acid sludge production to annual acid consumption Average sludge ratio (tonnes sludge/1 litres hydrochloric acid) 1.5 1..5 98 84 7 56 42 28 14 WATER DATA Compared with the general metal finishing industry, the survey found that water consumption among galvanizers is generally low, with one-quarter of sites using no water for rinsing after pickling. (Galvanizing can proceed without any rinsing; ferrous chloride resulting from pickling can act as a flux for galvanizing, although it can increase running costs because of the heavy transfer of iron salts directly into the zinc.) Data show that 5% of galvanizers use less than 25 litres of water per tonne of product, compared with 2 litres in the general metal finishing industry. Hydrochloric acid costs ( ) (based on 14 per 1 litres) 1 2 3 4 5 6 7 8 9 1 Hydrochloric acid consumption (1 litres) Fig 5 Water consumption and its cost related to turnover Average water consumption (m 3 /year) 12 1 8 6 4 2 to 1 1 to 1.9 2 to 4.9 5 to 9.9 1+ Turnover ( million) 144 12 96 72 48 24 Average water costs ( /year) (water cost based on 1.2 /m 3 ) Water Consumption Related to Site Size and Turnover Data from the survey show that average water consumption appears to increase in line with turnover, except for the largest sites. This probably indicates that the largest sites use water more efficiently, possibly because they invest more in water conservation methods (Fig 5). Effluent Sludge Production Increasing costs for effluent treatment and disposal of effluent sludge have made effluent control a key area for saving money. The costs can be high, reaching 9 pence/m 3 in some industrial areas. Trade effluent charges tend to be based on the volume of fresh water supplied, as well as the extent to which the effluent is contaminated, so reducing the consumption of fresh water has a direct bearing on effluent costs. Sludge in the effluent (as slurry or filter cake) is normally sent to landfill, an increasingly expensive option. The survey showed that effluent sludge production is not directly proportional to water consumption. REVIEWING PERFORMANCE Monitoring Measuring is vital to any review of performance. Unless you know how much acid and water you are using, you cannot manage these resources. Continued monitoring will reveal when savings occur, thus encouraging you to carry on. The acid and water consumption table (Table 1) at the top of the next page allows you to calculate your specific acid consumption and specific water consumption. You can see how your specific acid consumption compares with the rest of the industry by finding where your performance fits on Fig 1. All the columns to the left of your position indicate those companies with a better specific acid consumption than yours. If you do your acid monitoring over a year, you will be able to use the acid economy diagram (Fig 6) to gauge how well you are performing against others in the industry. If you do your monitoring over any other period, you will have to adjust A and B in Table 1 to reflect annual values before you use Fig 6. To use Fig 6, locate the value of A on the vertical axis and B on the horizontal axis, to see which band you are in.
Table 1 Acid and water consumption table - the first step in reviewing your performance Acid used in a suitable time period, say one month (or one year) Metal processed in the same time period Current specific acid consumption Water used in a suitable time period, say one month (or one year) Metal processed in the same time period Current specific water consumption Fig 6 Acid economy diagram Hydrochloric acid consumption (litres/year) 1 2 (137 tonnes) 9 6 3 Worst 25%........ litres of 28% A hydrochloric acid........ tonnes B........ litres/tonne 26 346 Worse than average Better than average Best 25% Average 5 (57 tonnes) A B........ litres of water C........ tonnes D........ litres/tonne C D Considerations when Developing a Savings Strategy The following list covers some of the techniques used in the industry for acid and water conservation, which may help you to establish a savings strategy. Good Housekeeping Surprising savings can come from simple good housekeeping. Encouraging good practice will not only lead to savings in acid and water consumption, but also in other areas, like energy. Good Practice Guide (GG26), Saving Money Through Waste Minimisation: Reducing Water Use, outlines a systematic approach to reducing the costs associated with water use and wastewater disposal. This publication is available free of charge through the Environmental Helpline. Rinse Water Use Do water costs justify rinsing the product after pickling, or could savings be made by galvanizing without rinsing? If using rinse water is economic, have water conservation measures been considered? Acid Recovery Plant Do production level and acid use make it economic to install an acid recovery plant? Companies using more than 2 litres of hydrochloric acid each year may find it economic to install plant. Acid Conservation Measures Check that drag-out and bath chemistry are adequately controlled. Consider automating procedures for tighter control, if necessary. Even if your performance matches the best 25%, there may still be areas for improvement and you will need to be vigilant to maintain your current position. Commitment To make savings, the whole company needs to be committed to any savings strategy. Involve all operators in establishing the strategy, instead of just presenting them with your ideas and extra work. Make sure that everyone is aware of the amount of acid and water being used and what they cost. Invite suggestions for making savings, perhaps offering an incentive for the best idea. Set realistic targets for savings. 12 5 25 37 5 5 Galvanizing production (tonnes/year) It is important to review your savings strategy at regular intervals, to ensure that the options selected are still the best for your operation. Previously discarded ideas may become more attractive with changes in procedure, production, material costs and legislation. Keep everyone up-to-date on progress being made. Recognise the contribution of operators in achieving savings.
Fig 3 Average acid consumption related to turnover Fig 4 Ratio of acid sludge production to annual acid consumption Water Consumption Related to Site Size and Turnover Data from the survey show that average water consumption appears to increase in line with turnover, except for the largest sites. This probably indicates that the largest sites use water more efficiently, possibly because they invest more in water conservation methods (Fig 5). Effluent Sludge Production Increasing costs for effluent treatment and disposal of effluent sludge have made effluent control a key area for saving money. The costs can be high, reaching 9 pence/m 3 in some industrial areas. Trade effluent charges tend to be based on the volume of fresh water supplied, as well as the extent to which the effluent is contaminated, so reducing the consumption of fresh water has a direct bearing on effluent costs. Sludge in the effluent (as slurry or filter cake) is normally sent to landfill, an increasingly expensive option. The survey showed that effluent sludge production is not directly proportional to water consumption. REVIEWING PERFORMANCE WATER DATA Compared with the general metal finishing industry, the survey found that water consumption among galvanizers is generally low, with one-quarter of sites using no water for rinsing after pickling. (Galvanizing can proceed without any rinsing; ferrous chloride resulting from pickling can act as a flux for galvanizing, although it can increase running costs because of the heavy transfer of iron salts directly into the zinc.) Data show that 5% of galvanizers use less than 25 litres of water per tonne of product, compared with 2 litres in the general metal finishing industry. Fig 5 Water consumption and its cost related to turnover Monitoring Measuring is vital to any review of performance. Unless you know how much acid and water you are using, you cannot manage these resources. Continued monitoring will reveal when savings occur, thus encouraging you to carry on. The acid and water consumption table (Table 1) at the top of the next page allows you to calculate your specific acid consumption and specific water consumption. You can see how your specific acid consumption compares with the rest of the industry by finding where your performance fits on Fig 1. All the columns to the left of your position indicate those companies with a better specific acid consumption than yours. If you do your acid monitoring over a year, you will be able to use the acid economy diagram (Fig 6) to gauge how well you are performing against others in the industry. If you do your monitoring over any other period, you will have to adjust A and B in Table 1 to reflect annual values before you use Fig 6. To use Fig 6, locate the value of A on the vertical axis and B on the horizontal axis, to see which band you are in.
Action Plan This Guide was produced by the Environmental Technology Best Practice Programme. Prepared with assistance from McLellan and Partners Ltd. For more information about the Environmental Technology Best Practice Programme and how its free services can help you, please phone the ENVIRONMENTAL HELPLINE 8 585794 http://www.etsu.com/etbpp/ THE ENVIRONMENTAL TECHNOLOGY BEST PRACTICE PROGRAMME IS A JOINT DTI AND DOE INITIATIVE MANAGED BY AEA TECHNOLOGY PLC THROUGH ETSU AND THE NATIONAL ENVIRONMENTAL TECHNOLOGY CENTRE Crown copyright. First printed 1996. This material may be freely reproduced except for sale or advertising purposes. Paper made from 1% post-consumer waste.