In this article we will discuss about the methods and procedures to treat and control wastewater from iron and steel industry.
ADVERTISEMENTS:
The iron and steel industry, as treated here, includes pig iron production, steel making, rolling operations and those finishing operations common in steel mills, i.e. cold reduction, tin plating and galvanising. Most steel firms operate iron ore mines, ore beneficiation plants, coal mines, coal cleaning plants and coke plants; many have fabricating plants or produce a variety of speciality steel products.
Iron and Steel Industry:
Manufacturing Operations:
Manufacturing operations of the iron and steel industry may be grouped as pig iron manufacture, steelmaking processes, rolling mill operations and finishing operations. A single mill is not likely to incorporate all of the many combinations and variations of these operations that are possible. Most mills specialise in the production of broad categories of steel products; in a large mill, however, the product list is long.
The manufacture of pig iron is accomplished in the blast furnace. Steel-making processes include pneumatic processes, open hearth processes and electric furnace processes. Rolling mill operations include rolling of blooms, slabs and billets; scarfing and other preparations of semi-finished steel; rolling of shapes, bars, strip and plates; wire drawing; tube drawing and pipe forming; and pickling or other oxide removal operations. Finishing operations include tin plating, galvanising, cold reduction and coating.
The blast furnace process consists essentially of charging iron ore, limestone and coke into the top of the furnace and blowing heated air into the bottom. Combustion of the coke provides the heat necessary to attain the temperatures at which the metallurgical reducing reactions take place.
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The incandescent carbon of the coke accounts for about 20 per cent of the reduction of the iron oxides; the carbon monoxide formed between the coke and the oxygen of the blast accounts for the remaining reduction accomplished. The function of the limestone is to form a slag, fluid at the furnace temperature, which combines with unwanted impurities in the ore.
Two tons of ore, 1 ton of coke, ½ ton of limestone and 3½ tons of air produce approximately 1 tonne of iron, ½ ton of slag and 5 tons of blast furnace gas containing the fines of the burden carried out by the blast; these fines are referred to as flue dust.
Characteristics of Steel Mill Wastes:
Wastes from the various operations in steelmaking vary widely in characteristics and in volume water pollutant in a typical steel mill complex are shown in Table 19.1. These wastes generally have physical and chemical effects on receiving streams different from the oxygen-consuming characteristics of municipal sewage and organic industrial wastes.
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Because the waste streams vary so widely and are usually separated by the distances between the several operations, composite effects are of little significance; treatment and disposal generally must be considered for the separate wastes.
The water requirements of steel plants vary widely, depending primarily upon the quantity and quality of the available supply. The use of as little as 1500 gal of water per ton of product has received much attention in one instance where recirculation is extensively practiced, due primarily to short supply.
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A figure of 65,000 gal per ton of product has also been widely quoted and has been valid in certain installations that have had practically unlimited water supply. The use of 30,000-40,000 gal of water per ton of product has been typical of many large plants; actual consumptive use of water, i.e. water withdrawn but not returned, is probably less than 1000 gal per ton of product.
A recent industry survey indicated a maximum water use of 49,000 gal per ton of product and an average use of 17,000 gal per ton. Water use in the various departments of a typical integrated mill is approximately as shown in Table 19.2. Most of the water required by a steel plant is used for indirect cooling and needs no treatment, provided it is not excessively hard; chlorination is often desirable to prevent slime formation.
The water used in Blast furnace gas washing and in hot mills for roll cooling and scale transport is not necessarily of high quality; it is usually used as pumped. In the various finishing operations such as cold reduction, stainless strip rolling, electrolytic tin lines and galvanising, purer water is required and treated water is often used.
The various waste-waters from a typical steel plant are considered here individually, roughly segregated according to the operations from which they result. It must be remembered, however descriptions of the various operations that no such clearcut segregation exists in actual practice. Indeed one of the major problems in installing waste treatment facilities in older mills is the segregation of waste streams from integrated operations.
The water used in washing Blast furnace flue gas contains from 1000 to 10,000 mg/l of suspended solids, depending upon the furnace burden, size of the furnace, operating methods employed and type of gas-washing equipment.
Following a ‘slip’ in the furnace, the concentration of solids in washer water may exceed 30,000 mg/l. The use of fine ore and high blast rates result in the highest concentrations of solids in the washer water; the top pressure used in the furnace is also an important factor.
The efficiencies of dry dust catchers and wet washers vary considerably and account for many of the differences found in various installations. Conventional wet washers use an average of about 3000 gpm of water; the newer venturi scrubbers use 600-1000.
The wash water from electrostatic precipitators adds little to the washer flow, but increases the concentration of the finest particles in the waste stream. Blast furnaces producing ferromanganese have a high percentage of semi-colloidal dust particles in the washer water.
The fume from pneumatic steelmaking processes, open hearth and electric furnaces and hot scarfing operations is often eliminated by electrostatic precipitators or venturi scrubbers which produce waterborne wastes. These suspensions are generally similar to gas-washer water, but the particles are much finer.
The flue dust particles in washer water are probably 50 per cent finer than 10 microns and approximate the composition of the furnace burden; the specific gravity is about 3.5 on the average. Effects on the receiving streams include objectionable colour and interference with aquatic life through formation of bottom deposits and impedance of light transmission; in extreme cases sludge banks are formed that interfere with navigation.
Gas-washer waters, especially from furnaces operating on ferromanganese, contain appreciable though highly variable concentrations of complex cyanides and may have a toxic effect on aquatic life.
Scale-bearing water originates in the various rolling mill operations and consists of the water used to dislodge scale and to cool the rolled product, plus the water used to transport scale through the flumes beneath the mill line.
The characteristics and quantities of scale-bearing water vary widely depending upon the particular rolling operations. The total iron loss in the form of scale averages about 2½ per cent, from the blooming mill through the final rolling operation.
Scale produced in the rolling of blooms and slabs in primary mills is relatively coarse material and most of it settles out of suspension readily. The scale particles from such mills are 90 per cent or more coarser than 200 mesh. Scale produced in a billet mill is considerably finer; 25 per cent of such particles may be finer than 200 mesh.
The water use in primary mills ranges from 2000 to 7000 gpm, depending upon the design of the mill and the rolling practices. The scale particles are mixtures of various iron oxides, with the higher oxides pre-dominating in scale from primary mills.
The specific gravity of mill scale is about 5.0, hence such particles, particularly the coarse material, tend to clog sewers and to deposit in receiving streams. These are, in general, the only effects of primary mill flume water.
The scale produced in finishing mill rolling operations has, in general, the same composition and specific gravity as that from primary mills. It differs, however, in particle size and quantity and in its effects. Considerably greater variation occurs among installations than in the case of primary mills.
Ten to 20 per cent of the scale particles from finishing mills is smaller than 200 mesh. The coarse particles are still relatively fine and the finest particles may be 5 microns or less in diameter. Water use in finishing mills ranges from 5000 gpm or less in bar mills and cold reduction mills to 25,000 gpm or more in the newest hot strip mills.
Finishing mill flume water may settle in the receiving streams and form bottom deposits or sludge banks and may increase the turbidity of the stream or impart an objectionable colour if the scale particles are extremely fine.
Spent pickling solutions and acid rinse waters differ widely in quantity, composition and concentration, depending upon the manner of pickling, production rate, type of steel being cleaned and the degree to which control over the operation is practiced.
Spent pickling solutions of various types may be produced in different, separated operations at a large mill. Acid rinse waters have the same relative proportions of iron salts and free acid as pickling solutions, but are much more dilute; 10-15 per cent of the acid used in pickling is discharged in rinse waters.
Spent pickling, solution discharges may inhibit bio-oxidation processes in streams and may be injurious to aquatic life if the quantity released in relation to the stream flow is sufficient to lower the pH of the stream significantly.
Sulphuric acid comprises about 90 per cent of the total of acids used in pickling steel. Spent sulphuric acid pickling solutions contain free acid, ferrous sulphate, undissolved scale and dirt and the various inhibitors and wetting agents, as well as dissolved trace metals.
The spent solutions from continuous strip picklers contain 5-9 per cent free acid and 13-16 per cent ferrous sulphate; from batch operations the spent solutions may contain 0.5-2.0 per cent free acid and 15-22 per cent ferrous sulphate.
Ten to 15 per cent of the acid used in pickling is discharged in the rinse water as highly diluted free and combined acid. Spent sulphate pickling solutions are discharged at 170°F-190°F and can amount to 1,00,000 gal per day in a large mill.
Hydrochloric, nitric, phosphoric and hydrofluoric acids are used in pickling stainless steels. These acids may be used alone, in various combinations, or in combination with sulphuric acid. Stainless steel pickling practices vary widely, in the industry.
A typical pickling operation for stainless steel plates consists of a 10 per cent sulphuric acid bath at 160°F, followed by a 10 per cent nitric acid, 4 per cent hydrofluoric acid bath at 150°F. A typical continuous pickling line for stainless steel strip consists of a 15 per cent hydrochloric acid bath at 160°F followed by a 4 per cent hydrofluoric acid, 10 per cent nitric acid bath at 170°F.
Phosphoric acid is often used in pickling when a phosphate coating is desired. Hydrochloric acid is being used increasingly for mild steels in the new vertical tower pickling installations. These spent solutions contain free acids and the various iron salts, as well as undissolved scale and dirt, inhibitors, wetting agents and trace metals. Compositions vary widely according to the specific operation and plant practice.
Rolling oils, lubricants and hydraulic oils are present in the effluents from many operations in a steel mill and occur as both free and emulsified oils. Volumes of the waste streams and the concentrations of oil vary widely according to operating practices and housekeeping methods.
Emulsified oil in an effluent can be esthetically objectionable and may add a significant BOD. Free oil is particularly objectionable in a stream because very small quantities can result in widespread surface films; larger quantities foul boats and docks and result in unsightly accumulations along stream banks. Severe oil pollution can have serious adverse effects on aquatic life, birds and land animals.
So-called soluble oils are present in the waste discharges from cold reduction mills, electrolytic tin lines and a variety of machine shop operations. Natural palm oil and synthetic proprietary substitutes are used in these operations and form stable emulsions when mixed with water at elevated temperatures, especially when kerosene and various detergent cleaning compounds are used.
Concentrations of soluble oils vary according to the degree of recirculation practices; volumes of the waste streams likewise vary widely. Typically, the effluent from once-through use in a cold reduction mill will contain 200 mg/l oil, 25 per cent of which is a stable emulsion.
Lubricating oils and hydraulic fluids are present in the effluents from all rolling operations and most other machine operations. These oils exist mostly as free, floating films and the quantities depend primarily upon machine maintenance and manual lubrication practices, i.e. upon housekeeping.
Other waste-waters from steel mills include alkaline cleaning solutions, water used in granulating slag and cooling water. Alkaline cleaning solutions are used to remove rolling oils prior to finishing operations. Caustic soda, soda ash, silicates and phosphates are common cleaning agents.
Spent cleaning solutions contain saponified oils and dirt and have substantial residual alkalinity. Total volumes are small, ranging from 1000 to 10000 gal per week for individual operations; these quantities are usually dumped batchwise. The effects on the receiving stream are probably not adverse, especially if the volumes of acid wastes are relatively large, as is usual.
The quenching of blast furnace slag produces small quantities of water containing slag particles. Effluent from a slag pit may range from 100 to 200 gpm and is usually of a clear appearance. The highly abrasive nature of the suspended slag particles is the principal objection to such effluents; the bulk gravity of slag particles ranges from 0.8 to 1.5 because of expansion in the granulating process.
Cooling water discharges comprise the largest percentage of steel mill effluents and are usually 10°-15°F warmer than the water withdrawn from the source of supply. The rise in temperature is the only change in water used for indirect cooling and is usually not significant if the effluent is discharged into a reasonably large stream.
Discharged cooling water can have an adverse effect at certain plants where the receiving stream is small and supports a temperature-sensitive fish population. More often than not, the cooling water discharge is of better quality than the water withdrawn from the stream, because of the treatment used for corrosion control.
Disposal of Steel Mill Wastes:
Methods of waste disposal in the steel industry vary widely from plant to plant. The age of the mill is probably the most important single factor accounting for these variations. In older mills, space for large treatment facilities is often not available; the space required may well be of more potential value for production facilities than the total direct costs of a waste treatment plant.
Other factors influencing the variability in methods include the effluent standards that are applicable, the attitudes of management and the competitive position of the particular operations involved, as well as the characteristics of the waste streams.
Blast furnace gas-washer water is usually treated by plain gravity sedimentation in mechanically cleaned circular clarifiers or in simple rectangular sedimentation basins. Circular clarifiers are used almost exclusively in newer installations.
Atypical modern installation may consist of a 75 ft diameter clarifier, handling gas-washer water at 6.9 mgd from a blast furnace rated at 1200 tons per day. The effluent would contain approximately 80 mg/l suspended solids; the underflow of 180 tons per day of wet dust would be pumped to the sinter plant for additional thickening and filtration on leaf type vacuum filters.
Older installations might be typified by two 13 ft x 111 ft rectangular sedimentation basins handling gas-washer water at 3.5 mgd from a Blast furnace rated at 880 tons per day. The effluent concentration might average 250 mg/l suspended solids. Sludge would be dredged from the basins by clamshell buckets at the rate of about 22 tons of wet dust per day and hauled to the sinter plant in railroad cars.
The clarified gas-washer water may be recirculated either wholly or in part. Recirculation is practiced when supply conditions dictate water economy and usually requires secondary treatment such as chemical flocculation. The effluent from simple rectangular basins is not ordinarily suitable for recirculation without such extra treatment.
The use of separate clarifiers for each Blast furnace or pair of furnaces may result in the discharge of untreated wastes whenever clarifier operation is interrupted. A more satisfactory arrangement consists of collecting the gas-washer water from all Blast furnaces, with treatment in centrally located clarifiers. Interconnection of the clarifiers insures continuous treatment even if the operation of one clarifier is interrupted for an extended period.
Some plants operate the washer water clarifiers in series, the underflow of each being added to the influent of the next. A single line to the sinter plant and the agglomerating effect of added sludge are possible benefits of this scheme.
Where effluent requirements are stringent or where existing equipment is called upon to handle greater than design flows, chemical flocculation or the various polyelectrolytes may be used, usually in secondary treatment units.
Polyelectrolytes alone have usually not resulted, in plant practice, in the rather spectacular improvements indicated by laboratory experiments. With improved methods of determining and controlling optimum dosages and with probable price reductions, these materials will doubtless become more commonly used.
The design of circular clarifiers and rectangular sedimentation basins for gas-washer water requires specialised techniques; the conventional criteria for sanitary wastes are not satisfactory. The methods outlined here on scale-bearing waters are generally applicable for gas-washer water clarification.
Rolling mill flume water has long been partially clarified in small, simple sedimentation basins known as scale pits, in order to prevent sewer clogging. These pits are usually small in relation to the water flow and the deposited scale is cleaned out periodically with clamshell buckets. In newer mills, scale pits are larger and are designed with the objective of water pollution control; continuous mechanical cleaning is often incorporated.
A scale pit typical of older practice was 18 ft wide, 30 ft long and 8 ft deep, to handle flume water at the rate of 3500 gpm from the slab rolling section of a hot strip mill. Effluent concentration averaged 200 mg/l of suspended solids; there was no provision for removing oil. The pit effluent went directly to a river. Flume water from the finishing end of the mill contained only line scale, not likely to clog the sewer and went to the river untreated.
When the mill cited above was rebuilt, flume water treatment was improved to provide more effective pollution control. The scale pit was tripled in size and handles all water from the mill; oil is removed continuously through split-pipe skimmers.
The scale pit effluent goes to a 35 ft diameter clarifier for additional solids removal and oil separation and the clarifier effluent is returned to the mill for reuse. Little or no waste-water from this mill is now discharged.
Treatment following once-through use in newer mills usually consists of primary clarification in a scale pit and secondary clarification and oil removal in relatively larger rectangular sedimentation basins.
Scale pits are typically cleaned by dredging with clamshell buckets and secondary clarifiers are usually cleaned continuously by scrapers on endless chains. Many variations of this basic scheme are found in various rolling mills; in fact, few installations are identical.
Often the secondary clarification includes chemical treatment, typically with additions of lime, ferric sulphate and polyelectrolyte coagulating agents. Chemical treatment is most often used with circular clarifiers as the secondary basins.
When chemical treatment is used, the water is generally reused in the mill; it may be passed through cooling towers, especially in the warm weather months. The recovered scale is sintered for use in the Blast furnace or open hearth furnaces.
Mill scale is comparable to high grade iron ore and is thus a salvaged material of considerable value. Generally speaking, the recovery of mill scale from primary scale pits shows an economic return; more than 90 per cent of recovered scale is obtained from these pits. Scale removal from secondary pits must be justified on the basis of pollution control or as necessary for water reuse.
Steel Mill Waste-Water Sedimentation:
Research sponsored by the American Iron and Steel Institute has resulted in design procedures that are, applicable for steel mill sedimentation equipment, including scale pits, secondary basins for scale removal and gas-washer water clarifiers.
These procedures predict basin performance in terms of an empirical measure known as the Sedimentation Index (SI), expressed in minutes. The sedimentation index may be interpreted as the settling time, under specified laboratory conditions, that will result in sedimentation equal to that of a particular basin at a specified flow rate.
Values of SI are approximately 0.10 for simple scale pits, 1.0 for secondary mill scale basins, 10.0 for small gas-washer water clarifiers and 30.0 for large washer water clarifiers. This work has shown that there are optimum ratios of width and depth to length for rectangular basins and that large circular clarifiers have less volumetric efficiency than smaller clarifiers at comparable flow rates. Overflow rates and superficial linear velocities are not adequate criteria for the design of steel mill sedimentation equipment.
Rolling mill flume water and Blast furnace gas-washer water should be sampled with care when such samples are to be used as the basis of basin design for required effluent concentrations. Composite samples should be taken over periods of typical operation; samples should be randomised so as not to coincide with process cycles such as slab rollings or Blast furnace chargings.
Settling rate tests should be made soon after collection because many of these suspensions cannot be effectively reconstituted after settling has occurred. Existing installations similar to contemplated new installations can often be used as sample sources for design purposes, but differences in raw water quality due to location and season of the year should be borne in mind.
Few industrial wastes have received as much attention as spent pickling solutions in terms of research and process development effort. The recovery of by-products from waste treatment processes seems attractive, but has not proved economically sound. Relatively dilute solutions of cheap bulk chemicals are involved and the quantities are large in comparison with most possible markets for by-products.
Other steel mill waste-waters such as alkaline cleaning solutions, slag pit effluents and sanitary wastes present few special problems, but are important in planning pollution control comprehensively.
Alkaline cleaning solutions may be used as additional alkaline agents in spent pickling solution neutralisation, or may simply be diluted prior to discharge if the quantities are relatively small. Slag pit effluents are treated by rotary screening if discharge is to a navigable stream or a recreational stretch of a stream.
Sanitary wastes may be conventionally treated in a mill-operated facility or sent to a municipal sewage treatment plant. The greatest problem encountered with sanitary wastes is in segregating them from process waste streams, especially in older mills; the cost of sewer segregation is usually the greatest cost of treating these wastes.
Reuse of water in the steel industry will increase in the future. Some of this increased reuse will be for the purpose of conservation in localised situations of water shortages, as in circumstances where the low flow period reduces surface streams to a critical point or where the groundwater faces serious depletion.
The principal factor influencing reuse will probably be the increasing requirement for high effluent quality and the criterion for the extent of reuse will be economics. The completely closed process water system is, of course, the final answer in industrial waste-water treatment.
Even under conditions of abundant supply, complete recirculation can become economical when effluent quality requirements become sufficiently high. Such system will probably provide the solution to waste-water control problems in the steel industry increasingly in the future.
Nonferrous Metals:
Current usage divides all metals into three groups: iron and steel including alloy steels; ferroalloys; and nonferrous metals.
The subject matter of this article is confined to the processes and related operations intermediate between production of metal ores and the finished product. These intermediate processes are usually those involved in the extraction or refining of commercially pure metal from ores and the fabrication of the metal into usable shapes.
The four major nonferrous metals are aluminium, copper, zinc and lead.
The processes described are concerned with either the extraction of pure metal from the ore or fabrication of the metal. Extraction of pure metal includes a variety of purification methods, such as dissolving metal compounds by leaching, production of oxides, reduction of the oxides to metal by smelting and refining by electrolysis.
Smelters and refiners are primary or secondary, depending on whether they use natural ores or scrap as their principal source for metals. Fabrication of metals includes such operations as alloying, casting, extrusion, forging, rolling, wire-drawing and heat treating and provides sheets, wire, tubing and other industrial shapes.
Production of aluminium from bauxite ore includes an aqueous extraction of aluminium oxide (alumina), followed by electrolytic reduction of molten alumina. Almost 50 per cent of the aluminium produced is made into sheets, including plates and foil; about 25 per cent into extruded tubing; and about 20 per cent into castings. Much of the rest is made into rolled shapes.
Copper is extracted from sulphide concentrates and from ‘cement’ copper by smelting in a reverberatory furnace to produce anodes of about 98 per cent copper, followed by electrolytic refining using aqueous sulphate solutions.
Most of the ores, used are sulphides. Copper oxides are converted to ‘cement’ copper by leaching the ores with sulphuric acid and precipitating the copper with scrap iron. About 55 per cent of the copper is fabricated in wire mills and about 40 per cent in brass mills.
In the rolling and drawing of tubes and wires in several steps, the metal tends to become hard and annealing is required after every two or three steps. The oxide scale formed in annealing is removed by dipping the metal products in sulphuric acid baths, followed by rinsing in water.
The major waste-waters in the copper and brass industry are these rinses; they contain a considerable amount of dissolved copper, zinc, chromium and sulphuric acid. The acid or ‘pickle’ baths, although they are dumped only infrequently, provide waste-waters containing the same toxic compounds; spent liquor wastes may be considered related to rinse-water wastes, but are of higher concentration and much lower volume.
Oil-bearing waste-waters are formed from lubrication, similar to those formed in the aluminium industry. These are frequently discharged into municipal sewers or rivers with little or no treatment.
Characteristics and disposal of this type of waste are discussed under aluminium. Other wastes of the copper and brass industry are the solid scrap, almost all of which is recovered for reuse and zinc fume from the electrolytic melting furnaces, most of which is discharged in stack gases without treatment.
Pickle Rinse Waters from Fabrication:
Rinse water and acid bath dumps are discussed together because both contain the same noxious compounds—copper, zinc, chromium and acid—and are related in other ways. Of these two wastes, rinse waters contain the larger mass of contaminant—90 per cent of the total in one study. Although acid bath dumps are more concentrated, the flow rates of the relatively dilute rinse waters are large, averaging 200-1000 gal/ton of product.
Rinse water concentrations vary with time and with the individual plant, but some not at typical values are indicative. Pickle baths are batch vessels of about 1000 gal capacity, filled with a 5-10 per cent sulphuric acid solution.
During the time they are used for pickling, the acid content becomes depleted and the metal content accumulates. When spent, the pickle liquor is discarded and a new batch of acid is prepared. Dumping cycles vary, but are frequently once a month.
Bright dip baths, used to remove stains on the finished tube or wire, operate similarly to pickle baths except that 3-8 per cent sodium dichromate is added to fresh batches and dumping cycles are usually every week or every few days.
Typical compositions of pickle baths contain in mg/l: 80,000 sulphuric acid, 10,000 copper and 10,000 zinc, with maximum values 2-4 times these concentrations. Bright dip baths, when dumped, have similar copper content, somewhat lower acid and zinc content and substantial chromium content (20,000 mg/l).
Discharge of these wastes without treatment is toxic to aquatic life and harmful to sewers and sew age plants; dilution is seldom adequate. Acidity of water below a pH of 6 is often lethal to the aquatic life that forms food for fish. The presence of copper, zinc or chromium above 2 mg/l is lethal to fish; furthermore, natural purification of a stream is inhibited.
In a Japanese study, the lethal concentration for salmon was reported as 0.05 mg/l copper or 0.6 mg/l zinc. Permissive metal concentrations are usually set between 0.02 and 1.0 mg/l. These metal sulphate wastes are acidic and corrosive, so they reduce the life of municipal sewers and corrode sewage treatment plant equipment. These wastes also interfere somewhat with the biological treatment of sewage and are not completely removed by municipal sewage treatment processes.
Pickle wash waters from the copper industry have most of the undesirable qualities possessed by steel industry pickle wastes and copper, zinc and chromium are considerably more toxic than iron. The most objectionable liquid wastes in the production and fabrication of copper are the iron sulphate solutions from leaching of oxide ores and the pickle rinse waters from wire and brass mills. Another, but minor, waste-water is the process water effluent containing entrained solid.
Zinc and lead wastes are similar to wastes from the copper industry; the major ores are sulphides and the primary production of metal is by smelting and electrolysis, except that most of the zinc is refined by distillation. Ores of zinc and lead often occur in the same deposit, sometimes with copper deposits.
In the production of magnesium hydroxide, the water effluent contains fewer salts than the influent sea water used as raw material. Furthermore, magnesium salts are not toxic in the usual concentrations found in water. Little waste-water is formed during production of the metal by electrolysis; recovery of the metal and of the chlorine gas evolved is almost complete.
Gold:
Gold is extracted from gold ores by cyanidation, amalgamation and rest from placers or base metal ores, but only which produces a potential waste-water hazard. This hazard is due to the high toxicity of cyanide and arises not only from the sodium and calcium cyanides used to form complexes with gold, but also from the cyanide sometimes used in the flotation circuits of mineral beneficiation plants.
After the gold cyanide complexes are split or electrolysed to remove the gold, the cyanides are frequently recycled, thus minimising waste discharge. Waste cyanide solutions can be oxidised to nitrogen and carbon dioxide for electroplating wastes.
Thus, the major potentially objectionable waste-waters arising from primary production and fabrication of nonferrous metals are the mud slurries from bauxite, fluorine solutions from aluminium refining, oil- bearing wastes from lubrication in fabrication, iron sulphate solutions from copper oxide leaching, pickle washings from brass mills, entrained solids from many operations and cyanides from gold extraction. Of all these, pickle wash waters from brass mills are probably the most objectionable in quantity and toxicity.
From a long-range view, the best solution to waste problems is waste elimination by process changes. Wastes may be eliminated in existing plants by reuse of the noxious material or by recovery of by-products for sale; in new plants, choice of an alternate process may eliminate production of the undesirable material.
Examples of these approaches in the nonferrous metal industries have been mentioned. Even where these approaches are not used, process changes to minimise waste formation have reduced treatment costs and decreased the amount of waste discharged to the surroundings. Treatment and disposal should be considered only where process alternatives are not available or where a temporary expedient is needed.
Where wastes are noxious materials that can be converted to harmless form by chemical reactions, treatment may involve such decomposition. In the nonferrous industries, however, noxious materials usually persist through treatment and ultimate disposal becomes significant.
Treatment of such waste- waters often involves separation of the deleterious substances from water by chemical reaction and phase change to gas or solid form, usually designed so that more concentrated mixtures of the noxious material are produced.
Less frequently, treatment may involve concentration to another liquid phase; examples of these concentration treatment methods include ion-exchange and the separation of immiscible liquids. On the other hand, disposal of these materials involves the permanent or semi-permanent relocation of the waste.
Frequently called ‘ultimate disposal’, this includes dispersion of wastes by dilution and storage of solid materials or slurries on land. Therefore, for these persistent materials, treatment in itself is not adequate, but must be followed by some form of disposal.
Waste disposal may be accomplished, in one or more steps, with or without treatment—either of which may be satisfactory or unsatisfactory, depending upon such conditions as nature of the surroundings and concentration and amount of waste.
An example of unsatisfactory treatment, indicative of some parts of the nonferrous metal industry today, occurs when a gaseous waste is converted into a noxious liquid waste, as in the scrubbing of fluoride gases in the aluminium industry.
This treatment is unsatisfactory at many locations because it is incomplete; satisfactory disposal in these cases requires subsequent treatment and disposal of the new form of the waste. Before considering disposal practices for specific wastes, three aspects common to most of the nonferrous metal industry should be examined: effect of surrounding, plant size and waste concentration.
The primary production of these metals often occurs in arid, sparsely populated areas, whereas much of the secondary production and fabrication is located in or near heavily populated industrial centers where water is often more plentiful.
Much of the primary aluminium, however, is produced in sparsely settled areas having plentiful water. A popular method of treatment and disposal in sparsely populated areas is lagooning, because of low land values. This method, however, is often considered too expensive within heavily populated industrial areas, even though smaller plants are the rule.
Partly because of the more favourable attitude on the part of large companies, which usually operate larger facilities, it is generally true that treatment of wastes from primary production is better accepted than adequate treatment for wastes at small fabrication plants.
It also is generally true that waste disposal in these industries is more of a problem in industrial centers where water is plentiful, but where treatment costs are higher and the accumulation of wastes from several industries is more likely to occur.
These, of course, are generalisations for which notable exceptions occur and which will probably be changed in time. Waste-waters contaminated by metal ions in large concentrations offer the best possibility of economic extraction of the metals for reuse or sale as by-products; treatment costs are often low, as in precipitation of iron sulphate by evaporation in the copper industry.
Dilute solutions, because of their large water volume, are so costly to treat that the practice of discharge of these solutions to large waterways is widespread. This dilution method is possible only if sufficiently abundant water flows are near and only as long as it is condoned by the public and the government.
Where valuable metals are involved, as in electrolytic copper wastes, it is usually economical to concentrate the solutions and to recover the metals. If dilute solutions cannot be stored and reused, the alternative is expensive treatment; dilute wastes arising from the washing of pickled metal are a current example of this problem. Other rinse waters and cooling waters used in fabrication also pose the problems of dilute solutions.
The treatment method most frequently used in the nonferrous metal industries is sedimentation of solids; it is used for entrained particulate matter as well as for precipitates formed by evaporation and by treatment with alkaline chemicals. Disposal methods most frequently used for waste-waters of this industry are discharged into rivers and oceans and discharge of solids onto land areas.
Copper:
Iron Sulphate Solutions from Leaching:
Leaching wastes are formed in the iron launders used to extract copper from oxide ore and to recover copper from tailings of sulphide ore, low grade ore and mine waters. Because these operations are located mainly in the arid rocky area, treatment of this waste is necessary to prevent making streams unpotable and unfit for agricultural or recreational use.
Most of the ferrous sulphate wastes are treated with lime to make them alkaline. The solutions are then transferred to lagoons where soluble ferrous hydroxide oxidises to ferric hydroxide, which precipitates, aided somewhat by water evaporation. In this way, iron is disposed of by land storage and clean effluents are produced.
At some locations, the effluents are reused. The copper sulphate leach solution is electrolysed to deposit copper at the cathodes and the resultant regenerated sulphuric acid liquid is recycled. In this way, production of iron sulphate waste is avoided.
Integrated Waste Treatment for Primary Production:
Where more than one waste-water is produced, the possibility of combining them in some way should be examined. An example of integrated treatment, using leaching wastes and ore tailings, is used by various companies.
The essential feature of this joint treatment is a combination of the alkaline tailing waste with the acidic leaching waste to form a neutral iron-free effluent for discharge. Two tailing waste sources are used: sand and slime sediment formed from lagooning tailing wastes in previous years and waste-water from currently used tailing disposal lagoons; both contain residual lime.
The leaching waste-water, having a pH of 4, first passes through the old tailings lagoon where it is partially neutralised by contact with the residual lime. By thus passing over the area, it controls dusting which otherwise presents an air pollution problem.
After collection, the water flows through a ditch where clear water from the active tailing lagoon, of pH 11, is added to produce a pH of about 7. At times of high leach flow or cold weather, milk of lime is added to complete neutralisation.
This combined stream flows into a 400 acre settling pond which permits oxidisation of the ferrous hydroxide to ferric hydroxide. In settling, ferric hydroxide carries down with it any other solids suspended in the stream.
Pickle Rinse Waters from Fabrication:
At plants where pickle bath rinse waters and dumps are segregated from other wastes, rinse waters have been treated successfully by neutralisation and precipitation. A good example of segregated wastes treated in this way, where rinse waters contain 10-20 mg/l each of copper and zinc and have a pH of about 2.5.
To even out large fluctuations in flow rate that occur (0-800 gpm), these dilute wastes are first collected in an equalisation lagoon; based on the average flow rate of 400 gpm, this lagoon has an 8-hour capacity.
A steady 400 gpm flow-from the equalisation lagoon is pumped to a 1000 gal mixing tank where slaked lime is added to neutralise the acid and to raise the pH to 11-12. The spent pickle liquor, when dumped, is sent to a separate storage tank from which it is pumped slowly, over several days, to the mixing tank where it is combined with rinse waters.
These strong wastes are dumped when the mill is shut down, using the same pump and parts of the piping used for rinse waters. Provision is made for a pre-treatment reduction to the trivalent form of the hexavalent chromium in dichromate pickle liquors; this is done by addition of sodium bisulphite and acid to the liquors in the storage tank.
The high pH liquid from the mixing tank is separated in a clariflocculator, composed of a flocculator in the center of an annular clarifier; at average flowrates, these two parts have detention times of 20 and 170 minutes, respectively. The slurry underflow is discharged to one of two sludge lagoons, of 250 day combined capacity; top water from the sludge lagoon is returned to the equalisation lagoon and compacted sludge is removed periodically.
The clear effluent of pH 11-12 and containing 1-2 mg/l each of copper and zinc is diluted fourfold by mixing with other process water effluent and discharged to the nearby river. These treatment facilities have for several years produced a final effluent containing acceptable levels of copper, zinc and pH.
Attempts to reduce the size of treatment facilities by process changes in the mills themselves have been successful. In one approach, by changes in cycle time and drain angles, more of the pickle liquor is allowed to drain back into the acid bath before rinsing.
This improvement in rinse procedure results in a decrease in the amount of acid waste produced and a reduction in the flow rate of rinse water required, both of which allowed construction of a smaller waste treatment plant than would otherwise have been possible.
The other approach also involved rinse procedures; countercurrent rinses were used and the rinse water flow rate was controlled by pH. As a result, the wastes produced are fairly consistent in concentration, so smaller variations in lime addition are required; fluctuations in flow are decreased the equalising lagoon.
Metal Fabricating Plants:
Generally the waste-waters originating in metal fabricating plants are similar regardless of whether they originate from small shops or large production facilities. For the most part the waste-waters from such plants contain small amounts of metal particles, free and soluble oils, and various cleaning compounds used in cleaning either the product or the shop itself.
Some plants have waste-waters from air pollution control devices for painting operations. These residues are treated in a manner similar to free oils. Normally, the wastes are slightly acid or alkaline, and they are usually opaque, milk-coloured, and contain some free (non-soluble) oil.
The simplest form of treating such wastes is by means of gravity skimming tanks. In many cases such skimming will be sufficient to permit discharge of the industrial waste to a sewer. Optimum gravity skimming tanks are generally designed along standards set forth by the American Petroleum Institute which relate tank dimensions to particle size, oil rise rates and detention time as well as to the nature of the type of oil to be skimmed.
Such tanks operate best when supplied with a limited, uniform influent flow rate. As a result it is often wise to optimise the skimming tank design and then utilise large holding or equalising tanks upstream of the actual skimming tanks to permit pumps to deliver the waste-water to the gravity skimmers at a fixed design rate.
The gravity skimming will remove free oils. The metal particles will settle in the bottom of the tank and can be removed manually at infrequent intervals or automatically by means of drag conveyors if such metal particles are deposited in significant quantities.
If the metal fabricating plant waste-water contains considerable soluble oils, generally used as a machining coolant, further treatment will be required in addition to gravity skimming. Usually wastewaters from a metal fabricating plant will be contaminated with soluble oils, since even if soluble oils are not used in plant processes, they may well develop because of the mixing of free oils and metal cleaners or emulsifying agents when brought together in the waste-water collection system.
Under these conditions a basic decision must be made when considering such waste treatment – whether or not the qualities and quantities of the waste-waters are such that a batch system is in order of whether a constant flow system utilising continuous skimming and other oil removal methods would be preferred.
Generally speaking, small quantities of less than 2000 gpd can best be handled by batch type systems, with larger quantities being treated by a continuous system. However, in the event there is a possibility of toxic contaminants such as plating waste-waters, a batch treatment system becomes essential.
A batch type system more readily permits testing and additional treatment to be applied to the wastewaters if this becomes necessary. Certain cleaning compounds can mix with the soluble oil waste and convert it into a jellylike material which will defy treatment by usual methods. Such complications can be accommodated only by a batch system.
These treatment systems consist of parallel batch collection and skimming tanks. When sufficient waste-waters have been accumulated in one batch tank, the free oil is skimmed and conveyed to a waste oil collection tank.
Miscellaneous grit and metal particles can be allowed to accumulate in the batch tanks until manual cleaning is required. The remaining waste-waters containing soluble oils are then cracked in either the batch tank or a separate retention tank to break the oil emulsion.
This is usually done by means of adding acid or alum until the pH has been lowered sufficiently to cause the emulsion to break down. The free oil is then skimmed or decanted in a separator and the remaining waters are neutralised by means of caustic soda or lime.
If sodium hydroxide (caustic soda) is utilised the clarified waste-water may be high in dissolved solids consisting mainly of the soluble sodium salts resulting from neutralisation. Frequently such waste cannot be discharged directly to a stream, but it is usually acceptable in municipal sewer systems.
If lime is used for neutralisation the waste-waters will contain few dissolved solids because of the relative insolubility of the resulting calcium salts. However, neutralisation with lime will precipitate a sludge consisting of the calcium salts of the acid. This presents the problem of disposal of the resulting sludge.
This sludge can be collected and stored in a holding tank for ultimate disposal. Not infrequently it is wise to dry this sludge on vacuum filters, since with its high water content it can present a difficult problem with respect to storage and/or disposal of the precipitate.
Removal of most of the water will reduce the volume of the precipitate or sludge to a more easily handled quantity. While lime neutralisation of acid waste-waters is less expensive than caustic soda neutralisation, the cost of sludge handling or drying may far outweigh the savings achieved by the use of lime. The volume and concentrations of waste-waters must be known and the resulting sludge volumes calculated before a comparative economic study can be made.
Several new proprietary compounds have recently been marketed which will crack oil emulsions. Such chemicals usually will not require subsequent neutralisation with possible resulting sludge complications.
Perhaps the most successful of all metal fabricating plant continuous waste treatment systems are those involving air flotation. In this process the soluble oil contaminated waste-waters are collected in large hold of equalising tanks.
This permits one-shift operation of the air flotation unit at fixed input rates. Operation for three shifts would permit smaller hold tanks to be used but might not provide the quantitative or qualitative equalising of the waste-waters.
After collection the waste-waters are conveyed by transfer pumps to the retention tank. Acid, alkali and soda ash are added as indicated, and the waste is detained in the retention tank under air pressure long enough to absorb quantities of air.
Upon release through a pressure regulator valve into the air flotation unit the cracked soluble oil is floated to the top surface along with the minute floe quantities and swept into a sludge hold tank by scrappers or flight conveyors. The relatively clear effluent can then be disposed of as indicated and the heavy oil filled floe disposed of by incineration or tank wagon disposal methods.
The effluent may require further filtration or sedimentation in detention ponds to meet stream requirements. The less stringent requirements for discharges to sanitary sewers will usually obviate needs for filters or detention ponds.
Incineration of resulting precipitates or sludge is an excellent method of disposing of this waste product. Incinerators are available which develop temperatures high enough to burn the residues completely and which will present few air pollution hazards. In the event toxic material such as the metal salts of plating baths is in the sludge, this method is most desirable.
The oil content of the sludge fed into such a device is generally high enough to be self-sustaining in combustion: however, it is always necessary to provide supplementary fuel such as gas or oil to permit the incinerator to be heated initially to a high enough temperature to destroy the waste product when it is first introduced into the incinerator. If the water content of the sludge is high or the oil content is low, continuous use of supplementary fuel may be required.
The other means of continuously removing contaminants from metal fabricating plant waste-water is by means of automating a batch treatment system, e.g. settling out the metal particles, skimming the free oils, chemically cracking the soluble oils and again skimming, neutralising the now oil free waters using caustic soda or lime, and dumping the clarified waters into a sewer or stream.
This amounts to a continuous flowing, instrumented version of the batch system. This would require pH sensing devices in the batch tanks to regulate the acid pump. Level switches in the batch tanks would be needed to activate the pump to the retention tank and the alum pump.
Also, automatic monitoring and recording of the final effluent properties would be required. There are many points in this type of system which must be instrumented to achieve the required automatic control using pH and level sensing devices, and it may therefore be difficult to achieve reliability.
The clinging of residual oils and/or sludge to the electrodes of pH sensing devices and the possibilities of unexplained materials finding their way into the system and fouling the chemical treatment and sensing devices almost preclude a trouble free automatic system.
As a result, for most plants the most practical approach to an automatic system is the air flotation system. Several waste-water treatment equipment manufacturers construct such devices, and in general most of them are satisfactory.
However, care must be taken not to overload an air flotation machine. A slight overload of the machine will cause substantial reductions in the pollutant removal efficiencies. A 10 per cent hydraulic overload may cause as much as 50 per cent reduction in oil removal efficiencies and result in carryover of oil bearing floe with the clarified effluent.
All automatic or batch systems must be examined to be sure that valves, piping materials, tanks, coatings and linings are suitable for the oils, pH variations and chemicals used in the waste treatment process. Such considerations will be important to the cost of the facilities.
As in all industrial waste treatment problems, the question of anticipating waste quantities and strengths in conjunction with the design of a new metal fabricating waste-water treatment facility is much more difficult than measuring the known quantities of properties of a waste that occurs in existing facilities.
As a result, the waste-water treatment facility for a new proposed metal fabricating plant must be designed much more conservatively than one for an existing plant in which the effluent quantities and properties can be measured.
A possible solution, if it can be achieved, is to go into production on a new facility and, once waste quantities are determined, design the waste-water treatment equipment. This takes cooperation on the part of state and regulatory agencies and may require tank wagon disposal or scavenger service for the first few months of operation.
Since waste-water treatment is usually an overhead cost, all possible means to defray expenses must be examined. In the case of metal fabricating plant waste-waters, several possibilities exist. The recovered waste oil may be reused or at least sold for road oil, and the treated waste-water may be reused to satisfy plant process, cooling, or other non-potable water demands.
Carried to the ultimate end, such reuse may virtually eliminate plant waste-water discharges, thereby negating the involvement with waste-water regulatory agencies.
Thus, to make greater reuse of water possible, more stringent requirements on quality of the effluent to the nation’s waterways will be enforced on cities and industries. Some relief will be accomplished by better water planning, for instance, by storing of water from periods of high precipitation.
Research will be required to develop improved methods of treating sanitary wastes. Economical means are required to remove a higher percentage of pollutants. However, if the techniques now available were applied, a vast improvement in the nation’s waterways would result.