How to Control Water Pollution in Non-Ferrous Process Industry:
Water is used in several places in lead and zinc extraction, including the blast furnace, cooling of the sinter, and in the acid plant if one is connected to the works. This water can contain many pollutants including lead, cadmium, arsenic and cyanide, so that treatment before discharge is essential. However, the treatment of wastewaters is fairly standardised for different types of industrial plants.
In the treatment, water, usually in the form of sludge, is treated from three places, the blast furnace, the sinter plant and the acid plant. Sludge, from the acid plant, is neutralised by milk of lime, and is centrifuged into two streams.
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The 70% solids slurry goes to the cooling drum of the sinter plant and the decanted water—to a reactivator. Sludge from the blast furnace is thickened, further treated and sent to the cooling drum. The sinter plant sludge goes straight to the reactivator, is thickened, and some water is discarded, the rest being reused. The circuit is shown in the Fig. 31.1.
The final discharge has the following analysis:
While not particularly spectacular, this treatment may be sufficient for most cases.
Dust Control in Non-Ferrous Process Industry:
This is not a problem specific to the lead-zinc industry, but is highly important because of the toxic nature of lead. Here again the control techniques are fairly standardised. Dust collection hoods collect the dust by vacuum from over the sinter machine, furnaces and other equipment.
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The dust is then collected in a bag-house and reused. The storage areas are adequately ventilated to prevent build-up of hazardous dust. Apart from the dusts collected in the manner mentioned above, there is also a lot of particulate matter in the exit gases. This obviously has to be cleaned before discharge to the atmosphere.
Utilisation and Disposal of Slag Residues in Non-Ferrous Process Industry:
This is probably the most difficult problem as each slag is different and so has to have an individual solution. It is only fairly recently that the idea of environmentally-planned residue utilisation and disposal has gained any ground. Previously, and still to a large extent, residues from metallurgical industries have been dumped into huge piles.
These dumps are environmentally detrimental for several reasons:
(a) They are usually open so any dust associated with them will be blown around causing dust pollution.
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(b) Rain falling on them can leach out certain substances, including toxic ones, which can be carried into water supplies.
(c) They are unsightly to look at and take up valuable land, usually poisoning the land underneath and directly around them.
(d) There is a danger of collapse.
(e) They frequently contain valuable components.
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For these reasons it is important that the residue is treated in an environmentally consistent manner, and many processes have been and are being, devised to do this.
In one of the plant which produces lead by smelting and zinc by the electrolytic process. Slags from the lead are reduced to give copper-silver mattes, iron slags, and zinc and lead oxides.
These are all used in some way, giving valuable by-products, such as germanium for use in the semi-conductor industry. The final waste is disposed of as a liquid containing As, Sb, In, Ga, and F. This plant then obtains all possible valuable components from the residue before discard, thus lessening considerably the amount of waste which has to be disposed of.
Some countries use successfully slags from a lead manufacturing plant as a source of raw material in the production of Portland cement clinker.
The slag analysis was –
and small amount of Pb, Cu, As, and K2O.
Upto 20% of this slag was added to the normal raw materials with satisfactory results.
Yet another two-stage system has been tried in both laboratory and experimental industrial tests. These showed a high degree of copper extraction, recovery of zinc and lead in the slag, and extraction of iron for use as cast iron. The final slag from this process can be used for slag pumice. This method results in a total slag usage, and shows good promise of being economic.
One of the most successful processes for the recovery of valuable constituents is the slag fuming process. The process involves mixing finely-divided slag with coal and air to maintain strongly-reducing conditions. The volatile metals, such as zinc and lead, are “fumed” off at the high temperatures involved.
These are carried off in the gases, from which they can be re-oxidised, collected, and returned to the process. The Trail plant is particularly interesting as it is set in a beautiful natural environment, and so was designed to keep pollution to a minimum. The remaining slag, after all valuable metals have been removed, is returned to the mine as filling.
The slag fuming process is gaining widespread acceptance. The lead plant of the Broken Hill Associated Smelters at Port Pirie is currently treating its blast furnace slag for recovery of zinc by slag fuming. The company has a stockpile of six million tons of slag and this is being slowly reused, together with the slag currently being produced.
The Waelz process for treating leach residues is being used at the Nisso Smelting Company’s works at Aizu in Japan. The process is a combination of slag fuming and a complex processing operation. Leach residues are first sent to a slag fuming furnace where the volatile metals—zinc, lead and cadmium—are fumed off and recovered for further processing.
Nearly all of these three metals are removed in this process, being in part replaced by carbon. This slag is then crushed and magnetically separated to remove iron. The remainder is sent to a furnace where a copper matte (also containing silver and gold) is tapped off and the metals recovered. The remaining slag is water granulated and sold.
The important thing to notice from these processes is that the residue can be treated to obtain valuable products which can be sold economically. Frequently, a company is making considerable profit from what was once considered waste. Clearly situations like the one at Port Pirie, where slag was just dumped, even though it contained considerable percentages of zinc, should not be tolerated.
It is not always possible to use all products from a large, complex manufacturing operation; there will always be a certain amount of waste to discard. After processing, the remaining residue often contains high percentages of SiO2, CaO, Al2O3, and Fe oxides, which are all non-toxic and fairly inert, and so they can be disposed of easily in an environmentally consistent manner, without producing large dumps.
Such material, being inert, can be used as covering material in landfill operations, and as fill in mines. Some people are experimenting with slags of high CaO content and traces of phosphorous for use as fertilizers when combined with a nitrogenous fertiliser, particularly in acid soils.
What is often needed is a little imagination and a great deal of work in order to come up with acceptable methods for disposing of many of the unwanted materials of the metal processing industries.