Waste Water in the Non-Ferrous Metals Industries. The Scope for Recycling
Substantial quantities of water are used in the non-ferrous metals industries for rinsing work after it has been pickled to remove oxide scales and for scrubbing gases to remove contaminating dusts and fumes. Such contaminated waters are usually treated with time or other alkali to neutralise acid and precipitate metal hydroxides before they are discharged. This procedure has changed little over the past decade, although the separation of the hydroxide tloc by flotation rather than settling has made some progress. Some authorities are now proposmg effiuent standards which will be difficult to meet by the conventional practice. Furthermore there are increasing difficulties in disposing of the metal hydroxide sludge which arises. Although this contains valuable metals, recovery is difficult. An analysis of the situation suggests that with the present high cost of water and taking into account the charges frequently levied for accepting treated effiuent water, emphasis should be placed on recycling rather than merely treating effiuents to a standard acceptable for discharge. Despite the high capital cost of the plant, ion-exchange now look,s economic for treating pickte rinse waters, since the water can then be recirculated. Reverse osmosis is another possibility and one large plant is said to be under construction. Taking account of the value of the recycled water, such processes now appear more economic than conventional time precipitation. With suitable changes in piekling practice to allow the pickles tobe regenerated there is the distinct possibility of a complete closed-loop in which rinse water is recycled and metals are recovered electrolytically. In order to assess the magnitude of the use of water in the non-ferrous metals industry and to determine the scope for economies, a survey was made of the water usage in five wrought aluminium plants and eleven copper and copper alloy manufacturing plants in the United Kingdom. With the assistance of the International Wrought Copper Council, this survey was extended to thirty-three copper manufacturing plants in ten other European countries. A summary1 of the water usage in each of the plants manufacturing copper and copper alloy products is given in Table 1. It was deduced from this survey that about 95 m3 of water is required to produce 1 tonne of copper or copper alloy semi-manufactured product i.e. plate, sheet, strip, rod, wire tube etc. This is somewhat more than the amount of water used in the steel industry (50 m3/tonne), rather less than is used in the paper industry (155 m3/tonne) or the synthetic fibres industry, (133 m3/tonne) and comparable to the wallboard industry (98 m3/tonne).2 Analysis of the returns reveals that a large proportion of the water is used for cooling purposes. In fact, in the aluminium semi-fabricating industry this accounts for over 95% .of the total requirement. The returns from the U.K. copper industry showed that about 75% of the water is used for cooling, and although the proportion positively identified as for cooling was less in other countries, the use of a substantial proportion of the water was not identified and, might weil be for cooling. This cooling water is used for extracting heat from moulds in which metal is cast, for cooling furnaces, for cooling rolls, compressors and other ancillary equipment. For the most part, this water is contaminated only with heat and it is therefore possible to recirculate the water for re-use after the heat has been removed. It is in this direction that there is the greatest scope for water economy. Cooling water, treated if necessary to prevent scale formation and corrosion, may be recirculated almost indefinitely if a suitable heat exchanger is included in the circuit. At the time at which this survey was made, about five years ago, the proportion of water recirculated showed great variation from firm . to firm, approaching 100% in a few cases down to virtually nil in many instances. It is believed that increasing water costs together with the difficulty of abstracting more water from wells, lakes, rivers and canals to meet the demands of increasing production, have considerably changed the situation and that a much greater proportion of cooling water is now recirculated, thus reducing the demand on water resources. CONTAMINATED WASTE WATER In the copper semi-fabricating industry, there is a substantial demand for water for rinsing work after it has been acid pickled. This demand is almost totally absent in the aluminium industry where the oxide film formed when the metal is heated for manipulation in various ways is proteelive and remains ~thin and scarcely visible. Water used for pickle-rinsing is contaminated with the pickle Iiquor itseH, in the case of copper and its alloys mainly sulphuric acid sometimes assisted by an oxidising agent such as sodium bichromate, and with metal salts. It may be seen that rinse water accounts on average for about 20% of the water consumption in the copper semifabricating industry. In individual firms it can rise to over 90% of the water demand depending on the nature of the product. Reliable figures are difficult to obtain, but the water consumed for pickle-rinsing in the U.K. copper and copper alloy industry is probably of the order of 10M m3/annum. The demand for water for rinsing purposes in the metal finishing industry of the U.K. has been estimated as between 50 M m3 and 150M m3 per annum. Because of its acidity and more particularly because of contamination with heavy metals such as copper, zinc, nicke!, Iead etc. this rinse water is unacceptable for discharge to public sewers or inland waten! without costly preliminary treatment.