Introduction:
Non-ferrous metallurgical industries produce a variety of gaseous solid and liquid wastes for disposal. India’s radical economic reforms and industrial policies since July 1991 have opened up the mining sectors for domestic and foreign private investors.
Several private copper smelters (Sterlite, Swil, Metdist, Indogulf), zinc smelter (Saraf alloy, Indo Zinc. Bharat Zinc) etc. are being planned. Because of possible enhanced use of sulphide mineral resource for base metals the issue of control of sulphur bearing off gases has assumed importance.
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Also environmental friendly disposal techniques for solid residues are being looked into afresh, since all metallurgical industries would produce such waste. Additionally, strict control on liquid effluents produced from various process plants would continue to merit attention prior to setting up of new processing plants. An attempt is made in this paper to highlight some environmental issues related to the above aspects.
Sulphur Bearing Process Stacks:
The flue gas from pyrometallurgical processes generally contains appreciable quantities of SO2. There are two strategies to follow for attaining desired level of smelter emission control – upgrading existing smelter operations and installation of a new smelter based on different, non-polluting technology. Where little new capacity is foreseen.
It would appear that resources should be concentrated on technology to upgrade existing smelters. Where upgrading could probably be accomplished with less capital outlay than replacement with new smelting furnace, new technology furnaces offer substantial advantages with respect to operating costs through enhanced productivity energy conservation and pollution control.
Recent years have seen the emergence of a number of processes which are essentially direct smelting routes. For base metals like lead, the blast furnace in tandem with a sintering/roasting machine has been the mainstay of lead smelting industry.
The sulphur dioxide content of the off gases are rather low (sometimes too low) to be processed conventionally. The various recent direct smelting rates contain higher percentage of SO2 (Kivcet: 30-45 vol%, QSL: 14-25%, Mitsubishi: 16% Flash: 25-30%, Isasmelt 25-30%) which can be conveniently handled. Typical gas volumes range from 100,000 m3 hr to 200,000 m3 hr.
A single exception is the TBRC process which produces dilute stream of SO2 because of dilution of ventilation gases during electric furnace operation. New technology direct smelting furnaces which are semi-continuous in nature offer substantial benefits in terms of capital and energy costs.
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However, significant improvements in sulphur containment over upgrading approaches can result only if fugitives associated with transfer of hot metal in open ladles are minimised. Thus, new continuous system process technology would appear to offer significant improvement in sulphur and particulate containment.
Typical sulphur containment scenarios for different smelting operations are given in Table 31.1.
Various alternative smelting approaches have led to formulation of SO2 treatment stream related strategies namely:
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(i) Conventional/strong SO2 stream control (metallurgical acid plant etc.).
(ii) Weak SO2 steam control (flue gas scrubbing).
Conventional Strong Stream Control Technology:
The sulphur by-product market is extremely important to the selection of an economic route to SO2 control for conventional process streams. Although technology exists for production of all the three marketable forms of sulphur (liquid sulphur dioxide, sulphuric acid and elemental sulphur).
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Each has its disadvantages. Liquid sulphurdioxide can be profitably marketed to nearby pulp/paper production areas. Sulphuric acid marketing is limited by shipping distance to market and hence shipping cost. For remote location of smelters from the acid market, there may be net income loss owing to high freight costs.
Single absorption sulphuric acid plants operate optimally at 7-9% SO2 in the feed with outlet gas varying 500-2500 ppm. Double absorption processes (Ugine Kuhlman (1) etc.) have higher absorption efficiency.
Traditional plants operate at about 4 to 8% SO2 feed gas and a minimum oxygen to sulphur dioxide ratio of 1:1. A normal approach to handling rich SO2 from the new smelting processes would be to dilute the incoming gas to about 10% SO2 and process the gas in conventional plants.
However, capital and operating costs are directly related to gas volume and a design which reduces significantly the gas volume and will have major effect on cost factors. In one of the approaches, the prevent overheating and loss of vanadium catalyst part of the incoming SO2 is diluted and the reaction is carried out in two stages by suitably mixing the product gas from first stage and feed gas.
Regarding the different variants of the sulphuric acid process, it has to be remembered that the amount of SO2 emitted is inversely proportional to the conversion efficiency, as shown below:
Plants having single absorption process are required to reduce SO2 emission for instance by absorption in sodium sulphite/bisulphite solution. The following standards for sulphur dioxide emission are stipulated.
Current technology for production of elemental sulphur from SO2 requires a reducing agent and is energy intensive. In its present form, it may not be economically attractive unless natural gas is available at negligible costs.
Regarding production of liquid sulphur dioxide, it is to be mentioned that economical drying of SO2 bearing gases may require use of concentrated H2SO4 and it is desirable to the SO2 content of the tail gas of liquid SO2 plant is not negligible and must be processed further e.g., in an acid plant.
Where there is no utilisation possibility for sulphuric acid or liquid SO2 the sulphur content of the gases can be fixed as gypsum, a solid waste product which can be theoretically stored. However, more than 3 tons of limestone is required to scrub 1 ton of the flue gas sulphur producing more than 4 tons of waste. The longer stability of dumped gypsum is an open question. Gypsum produced may be marketable only under particularly local conditions (e.g. Japan).
Weak Stream Control Technology:
The problem of sulphur dioxide control in the non-ferrous industry is mainly associated with the weak sulphur dioxide streams produced during smelting. In lead smelting about 85% of the sulphur in the concentrate is liberated as SO2 in the sintering step. Half of the remaining sulphur reports to SO2 and half to sulphates. In copper smelting major sources of weak SO2 off gas are the reverbatory furnace, multiple roaster and fugitive emissions.
An example of uncontrolled emissions of SO2 from primary smelters is the reverbatory furnace giving rise to weak SO2 emissions. Approaches to weak SO2 stream control consists of various operating measures such as oxygen enrichment use of concentration/neutralisation systems etc.
More and more smelters are weighing the economic advantages and disadvantages of returning converter slag to reverbatories. Considerable success in increasing SO2 concentration were obtained at the Onahama (3) (Green charge) and other smelters like and Naoshima (Calcine charge) through various process modifications contributing to higher SO2 content of the off gases.
The furnaces were tightly constructed and kept sealed for minimising air infiltration. As most of the concentrates were shipped to Japan from foreign sources, endeavours were made for receiving highest possible grades. Preheating of secondary air contributed to more rapid smelting and consequent faster release of combined sulphur.
A few flue gas desulphurisation systems have been operating on the off gases from reverbatories in Japan. One is a magnesium oxide concentration system which absorbs SO2 from the weak stream and regenerates it as a concentration in the 10-13% SO2 level. This route is backed up by a few years of operating experience. The second is the non-regenerative lime/limestone system which produces gypsum as a saleable product.
Mitsubishi Metals Company, Japan has carried out extensive work on the lime/limestone gypsum SO2 control system. It was found that operating conditions of the adsorbents were critical to scale formation and required proper range for pH. Concentration temperature, L/G etc. Bench scale testing at SO2 concentrations of 20,000 ppm was done. The pH of the lime/limestone slurry system was closely controlled.
The pregnant slurry was passed to oxidising towers where calcium sulphide was oxidised to calcium sulphate with compressed air. The slurry was thickened and centrifuged and gypsum product was separated. Another full-scale lime slurry scrubbing system operating on metallurgical gases containing 0.3-0.75 SO2 is the Duval lime scrubbing systems.
The development and installation of the Magnesium oxide SO2 concentration system at the Onahama smelter dates back to the mid-seventies. Typical upscaling ratio were 1:400 (lab scale) and 1:20 (pilot scale). In this system after gas conditioning, Magnesium hydroxide slurry is used as scrubbing liquid.
The absorbant is regenerated by passing the pregnant slurry through steam dryer and then claciner in presence of coke. The MgO from the calcine is slaked to magnesium hydroxide for further use. Off gases typically contain about 100 ppm of SO2.
The citrate type absorbents have featured in process routes which have been considered for metallurgical process off gases by USBM and Flakt Boliden. The absorbent developed is a brine of citric acid and sodium citrate which improved flexibility of the system over cold water.
The double alkali process in which sodium sulphate is the active absorbent and is regenerated with lime, has the advantages that the scrubber solution is clear. L/G requirement is lesser etc. However, lime is to be used for regeneration and sodium carbonate is used as additive to make up sodium losses.
In a variant of the process (Wellman Lord) the loaded absorbent is sent to a evaporator/crystalliser for regeneration of Na2SO3 crystals which are dissolved and recycled back. The processing cost is strongly dependent on the cost of low pressure steam.
The recovery of sulphur dioxide from dilute waste gases by aqueous ammonia solutions has been practised since long. The absorber product could be thermally regenerated but contamination of SO2 with ammonia Vapours occurs. Acidulation with sulphuric acid followed by stripping of the tails solution leads to recycle SO2 gas and ammonium sulphate product.
Thus, this route is linked with marketability of ammonium sulphate by-product. With a view to obviating these difficulties acidulation with ammonium bi-sulphate has been practiced leading to the formation of ammonium sulphate which is thermally (at 350°C) regenerated producing ammonium bi-sulphate.
In the citrate based process version developed by USBM the pregnant solution is reduced by H2S generated by reaction of CH4 and recycled sulphur. The use of this system has not been satisfactorily demonstrated as an integrated system.
Sulphur dioxide reduction with carbon has been reported but there are no major efforts to use these processes on reverbatory furnace off-gases, perhaps because relatively high concentration of input SO2 has been used.
Amine based processes (DMA/Xylidine process) have been reported where regeneration procedure involves separation of SO2 from DMA/Xylidine by scrubbing with water. Again, no major uses have been reported perhaps because the process economics is not favourable at very lean concentrations.
Selection of SO2 Control Procedure:
As brought out above SO2 streams from non-ferrous smelters can be controlled by a variety of systems.
Selection of a proper plant for regenerable systems is based on many factors such as market considerations, concentration of SO2 stream etc. For example elemental sulphur/liquid SO2 plants are favoured by high SO2 concentrations whereas sulphuric acid plants are designed for SO2 concentrations of 4-10%.
Although capital investment for a sulphuric acid plant for dilute gas streams (2.0% SO2) may increase 3 -4 fold as compared to a plant handling 8% SO2 for a 10,000 kg/hr SO2 feed stream, operating costs increase by much larger factor.
The capital cost for regenerable and non regenerable systems for gas volume of 200,000 m3/hr may vary between 40-80 Mio USD, whereas operating costs are 10-20 Mio USD.
Capital and operating costs decrease with lowering of SO2 concentration levels. Thus, there appears to be a point at which it would be more economical to use regenerable/non-regenerable process as compared to a sulphuric acid process.
Depending on the gas volume to be handled and the gas concentration for a particular smeiter off-gas application it should be possible to select the control system.
Solid Residue:
A metallurgical plant using an ore will invariably produce solid residue after extraction of metal values from the ore. The residue from pyro metallurgical plants may be disposed off either by trucking or conveying based on techno-economics and feasibility.
However, pipeline conveying of the residue slurry is also practised for certain applications where residue quantities are large. In the larger mineral beneficiation and hydrometallurgical plants slurry transportation is generally adopted since the residue/tailings are already moist.
The tailings disposed off in dry form are dumped into valleys or previously mined out areas for back-filling. After filling up of an area it could be covered with top soil and vegetated for rehabilitation/restoration of land.
The tailings in the form of slurry are conventionally discharged into a tailings pond. The supernatant is reused in the process and/or recycled for re-pulping the tailings. Large tailing ponds are generally located in valleys surrounded by hills such that by constructing one or two dams will provide the requisite hold up for tailings.
A recent technique on tailings disposal called “Thickened Tailings Disposal (TTD)” system comprises of thickening the tailings to a suitable high solids consistency. Pumping it to disposal site and its disposal above ground.
The slurry consistency is high enough to form a sloped deposit on ground and the water run-off including rain water is collected in a holding pond. This water is recycled to plant and can also be used for sprinkling in case dust emission is experienced in the disposal area.
The overall objective of the mud stacking operation is to spread the mud slurry in layers on suitably graded terrain and allow it to dry to at least 70% solids. At this solids concentration, the mud has permanently gained sufficient shearing strength or greater to support mechanised equipment and vehicles on the surface and to prevent breach of the containment dykes.
There is some further gain in shrinkage to be obtained by continuing drying and some of the mud will achieve this solids concentration during the dry months. However, because of the twin goals of suppressing dust generation and maximising pond water evaporation it is considered more desirable to keep the maturing mud surface moist at the expense of consistently achieving 70% solids.
The tailings disposal area will be divided into a few zones across the width of the entire drying area. Each zone will be subdivided into drying beds and fed by spigots/standpipes.
The drying beds will be graded to a slope that approximates to the natural angle of repose of the trailings slurry.
Upon discharge the mud will flow down the slope leaving a deposited layer of slurry the thickness of which will be dependent on the closeness of match between the angle of repose of the mud and the gradient of the land surface.
Typically a target thickness of about 100-200 mm is aimed. When a drying bed has been completely converted with a layer of mud the discharge point is switched to another drying bed that has just completed a drying cycle.
Rain which falls on the mud surface will flow down the slope and collect at the toe areas from where it will be decanted by gravity to a large holding pond. Water that is not evaporated from the system is pumped back to the plant for treatment.
The advantages of this system over conventional wet ponding are:
(i) More storage capacity per unit area of land due to high consolidation and stack heights upto 25- 30 metres at 3-5% slope.
(ii) Small perimeter dykes are adequate instead of high dams necessary for the conventional system.
(iii) Much less hold up of water thus safer.
(iv) Lower extent of seepage of tailings water into the soil.
(v) Partial and systematic rehabilitation/restoration of filled up disposal areas is possible.
The disposal of silicate slags generally does not cause problems because their siliceous matric prevents solubilisation of toxic compounds such as cyanides or heavy metals.
According to EPA regulations under the Resource Conservative and Recovery Act (RCPA) a solid waste is to be listed as hazardous waste if exhibits ignitability, corrosivity, reactivity, and/or EP toxicity.
Metallurgical plant rejects will not exhibit any properties of ignitability/reactivity. Corrosivity applies primarily to liquid wastes and should not be a problem if adequate waste management practices are used during washing of the rejects. The only applicable hazardous waste criterion is the EP toxicity.
Briefly, this test consists of agitating for 24 hours a minimum sample weight of 100 gm of filtered material in 1600 ml of distilled water (16:1 water/solid ratio) to which a maximum of 400 ml of 0.5 N acetic acid is added to maintain a pH of 5.0.
After filtering the solution on 0.45 µm filter the resulting extract (liquid portion) is not to exceed 100 times the US National Drinking Water standard for concentrations of eight metals (Ag, As, Ba, Cd, Cr, Hg, Pb, Se).
EP toxicity limits in micrograms per millilitre are as follows –
Ag-5, As-5, Ba-100, Cd-1, Cr-5, Hg-0.2, Pb-5 & Se-1.
A second leachate test proposed by ASTM consists of contacting a minimum of 300g of dried material with distilled deionised water (4:1 water/solids ratio) for 48 hours and filtering the hynid portion of 0.45 µm filter.
Pollution Control Board in India have brought out a rule regarding hazardous waste management and handling as per which production units generating wastes exceeding quantity stated in a specified schedule are required to submit applications for authorisation as a regulatory measure.
Eighteen categories of hazardous wastes have been identified under the schedule and it includes cyanides waste, metal finishing waste, waste containing water simple compounds of copper, lead, zinc.
Chromium, nickel, selenium, barium and antimony, mercury, arsenic, thillium bearing wastes, and sludges arriving from treatment of wastewater containing heavy metal toxic organic etc.
Liquid Emissions:
Generally the process liquid effluents released from metallurgical industry are manageable in quantity as well as quality. The process liquid effluent comprises of water with small quantities of dissolved solids.
Extensive recycle of effluent is practiced with a view to achieving zero effluent discharge and to minimise intake of fresh water. Mineral benefication plants require water for maintaining desired pulp densities of solids along with small dosage of additive.
After dewatering the solids water is generally recycled in the process. However, the impurity level does not build up to appreciable values since the solid residue (tailings) carries a substantial portion of water to the tailing pond.
The runoff water from the ponds is reused in the process. Special additives such as surfactants remain attached to the solid residue and some of these get degraded due to exposure to sun over long duration in the pond.
In alumina refinery alkaline effluents are generated as a result of use of caustic soda in process. The plants are designed to retain even the rain water collected in the first few minutes and either reuses it in the process or disposal after treatment.
Multiple effect evaporation are provided in the plant for maintaining process water balance and provisions are kept for disposing off the least alkaline water (after treatment) in case such a need arises.
Hydrometallurgical processes utilising acidic medium also follow the same approach. The choice of acids is generally limited to use of H2SO4 or HCl which are relatively safer from environmental consideration. Typical effluent quantities from a 150,000 TPA copper plant using electro-refining and anode-casting are shown in Table 31.2.
Recent Trends in Waste Management Technology:
1. Sulphur Bearing Gases:
In 1993, Paques BV and Hoogovens Technical Services Energy and Environment collaboratively brought out a biological flue-gas-desulphurisation technique which may well find several applications in the near future.
In the absorption section sulphur dioxide is absorbed in a washing liquid and the sulphite containing solution is treated in an anaerobic reactor forming dissolved hydrogen sulphide.
In a second bio-reactor the hydrogen sulphide is oxidised to elemental sulphur through partial oxidation. It is claimed that combination of scrubber technology and bio-technology creates a reliable high performance system and the operating data on actual systems are scanty. However, the potential use of this system for weak SO2 stream control needs to be highlighted.
2. Biological Treatment of Liquid Effluents:
New approaches have recently emerged for bio-treatment of liquid effluents namely metal precipitation through use of micro-organisms including activated sludge and biosorption.
Microbiological treatment of industrial wastewater for metal recovery utilises various heterotrophic micro-organisms and addition of organic substances for reduction/oxidation of metals or to precipitate these in the form of sulphides. Activated sludge has been to extract heavy metals from wastewater and efficient removal of mercury has been reported.
Sulphate reducing bacterial can be used to purify mine water. For example, effluent from a tailings pond settler containing 0.08 mg/l copper and 12.0 mg/l sulphides was treated with in a column (desulphovibrio desulphuricans) and calcium lactate (carbon source) and copper concentration dropped to 0.02 mg/l after a few weeks-time.
The effluents of some non-ferrous metallurgical plants contain trivalent arsenic in the form of arsenites. Although pyrolusite serves as a chemical oxidant some cations like Fe, Cu and Zn get absorbed and diminish activity of pyrolusite requiring frequent regeneration of pyrolusite columns.
Pseudomonas putida immobilised on a suitable substrate precipitates the arsenite after oxidation. Bacterial expolysaccharides actively accumulate metals. Various organisms (Zoogloea remigar, Acinetobacter calcoacetius etc.) when cultured produces substantial amounts of polysaccharides and these bind metals like copper, cadmium, uranium etc.
Denitrifying bacteria immobilised on coal particles can be used for removing uranium and nitrates from industrial wastewater. An integrated large scale process has been implemented in the United States to treat effluents of gold mining/milling operation using rotating disc biological contacting units for simultaneous degration of cyanide ammonia etc. Living microbial systems (algae ponds) have been used for uranium and molybdenum removal and base metal contaminates from effluents.
A commercial process AMT-BIOCLAIM has been developed using a proprietary granulated non-living biosorbent. Although biotechnological processes for removal of metals is an alternative to conventional methods the full potential for use of biological systems is yet to be realised for diverse applications.
Some environmental issues for the non-ferrous metal producing industries have been briefly discussed. With the large demand-supply gap scenario of several base metals sulphide smelting procedures operating on various types of sulphide ores would be commonly requiring careful consideration and assessment of sulphur control procedures. Production of sulphuric acid need not be the only option – rather the process to be adopted would be more related to the actual smelting route, gas volume handled etc.
Non-ferrous industries would also produce solid wastes existing waste handling procedures are getting modified because of the requirement to recirculate maximum quantity of process water leading to the concept of dry stacking of tailing.
Conventional treatment procedures for liquid effluents are reviewed and although biological techniques appear promising their full potential for treatment of liquid effluents is yet to be realised. It is suggested that the above issues be properly considered prior to the planning and implementation of new environment control approaches.
Waste Management in the Lead-Zinc Industry:
The metals industries generate enormous amounts of waste material; due partly to the large scale of the operations, partly to the amount of material that often has to be handled to gain valuable metals, and partly to the variety of products obtained. Because of this the pollution control problems cover a wide range.
A major problem of the load-zinc industry is the toxicity of lead itself. Lead accumulates in the body and damages the central nervous system, including the brain, and it has been suggested by some workers that the lead blood levels of our urban population is rising, and in some locations exceeds the danger level. While not yet proven, this is a matter of some concern.
The problem of sulphur dioxide in lead-zinc processing, although not considered in this paper, is also a major one.
Brief Description of the Production of Lead and Zinc:
Three processes will be considered as they account for nearly all of the lead and zinc production:
(i) The Lead Blast Furnace:
Lead ore, coke and a flux are charged into a furnace; air is blown through the charge and the lead oxide is reduced to lead, which is tapped periodically. The flux combines with the remaining material to form a slag which is also tapped periodically; the lead then goes through a number of refining processes to remove copper, silver, gold, cadmium, iron and zinc. The slag varies considerably from place to place but is mainly composed of varying amounts of silica, iron oxide, calcium oxide, and small residual amounts of lead and zinc.
(ii) Zinc Production by the Electrolytic Process:
The zinc sulphide ore is roasted to produce zinc oxide, giving off large quantities of sulphur dioxide. The oxide is then dissolved in sulphuric acid to produce a zinc sulphate solution, which, after purification, is used as the electrolyte in a cell with a zinc cathode and an inert anode. Zinc is deposited at the cathode and oxygen is given off at the anode, leaving sulphuric acid in the bath. The latter is bled-off and reused as more zinc sulphate solution is added.
(iii) A Combined Process using the Imperial Smelting Furnace:
A combined lead-zinc ore is fed into a blast furnace which operates at a temperature above the boiling point of zinc, but below that of lead. Lead metal collects at the bottom and is tapped off as usual.
The zinc vapour comes off at the top of the furnace where it is shock-cooled to a liquid by molten lead droplets. The resulting alloy is cooled so that it separates into two phases—a lead-rich phase which is re-circulated to cool more zinc, and a zinc-rich phase which is further refined, often electrolytically.
The pollution problem from these processes is fourfold:
(a) Burning sulphides inevitably produce large quantities of sulphur dioxide, which can be used to produce sulphuric acid.
(b) Large amounts of water are used for cooling purposes, and for washing or cleaning. This water is used in two ways, either as indirect cooling, where it does not actually touch the products and so remains clean, or as direct cooling or washing where the water becomes heavily polluted and has to be treated before discharge.
(c) Blast furnaces are notorious for the production of dusts—a serious problem in the lead industry—which must be controlled for health reasons.
(d) Slag disposal, probably the most difficult of all the problems, because of its varied composition. In what follows, the last three pollution problems will be considered.