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POLLUTION CONTROL IN FOUNDRIES
Dr. M K Jain & Suresh Kumar
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AIR POLLUTION
Air pollution occurs due to increased industrialisation and
urbanisation in addition to natural causes like cyclones, volcanic
eruptions. As a result of rapid development of industries, air pollution by waste
material, smoke and gases formed by industrial processes has
become a problem of vital importance.
On our planet, nearly 2 Billion tonnesof coal and 1 Billion tonnes
of petroleum are burnt every year, which are accompanied by theliberation of about 250 million tonnesof Sulphur Dioxide.
Heavy metals like As, Cd, Hg, Pb, Antimony and Bismuth etc, are
found alarming in fly ash fall outs (upto 50 to 500 Tonnes/Km2/Month)
from the 500 MW TPS stack at a distance one to four kms.
Airborne Ni-dust and vapours causes cancer of lungs and sinus.Exposure to dust or fumes of tin causes Pneumoconiosis / Stannosis.
Arsenic is poisonous in most chemical forms, and same is true with
Cd, Cu, Hg, and Pb.
Bismuth compounds are absorbed through skin and may cause
kidney damage.
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It is therefore necessary to identify and accurately quantify various pollutants in the air
to ascertain the air quality we breath.
The major pollutants which pollutes the air are:-
SUSPENDED PARTICULATE MATTER (SPM) are a complete group of substances,
liquids or solids, dispersed in the atmosphere, which range in diameter from a fraction ofa micron to several hundred microns.
The most significant fraction of suspended particulate matter is the respirable size, i.e.,
larger than about 0.1 microns and smaller than about 5-10 microns.
The measuring method may be either based on gravimetric principle or on
photometric characteristics of the particulate matter. The gravimetric method involve
collection of suspended particulate matter often for a 24 hours period. The mass of
particulate matter is determined by weighing filters before and after sampling.
For the photometric methods, particles are collected on filters for periods one to 24
hours. In these methods, transmittance or reflectance of the deposited matter is
measured. The collected dust may also be analysed to find out its chemical composition.
Ambient air is drawn under a fixed-area gabled roof through a filter by means of a
heavy duty turbine blower at a constant flow rate ranging between 1.1 and 1.7 cubic
meters per minute (m3/minute).
Suspended particles having diameters (Stokes equivalent diameter) between 0.1 and
100 um are removed from this air stream by filtration on a glass fibre filter.
The mass concentration (ug/m3) of suspended particulates in the air is determined by
measuring the mass of the collected particulates and dividing the volume of the airsampled.
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(i) Iodine Pentoxide Method
The reaction between carbon monoxide and iodine pentoxide at 135 to 150C to yield
carbon dioxide and iodine vapour. The concentration of carbon monoxide in the air
sample may be determined by analysis of the amount of either carbon dioxide or
iodine produced.
5CO + I2O5= 5 CO2+ I2It is necessary to pass the air sample through an elaborate purification train
consisting of activated carbon, an efficient drying agent, a chromic acid wash tower
and some solid absorbent to remove carbon dioxide.
The dry air is then passed through a U-tube containing iodine pentoxide. Free iodine
is liberated according to above reaction if carbon monoxide is present. The iodine
liberated in the reaction is absorbed in potassium iodide solution and titrated withstandard sodium thiosulphate solution.
(ii)Indicator Tube Method.
Carbon monoxide reduces yellow silicomolybdate to lower oxides. The colour
changes from yellowish green to green and finally deep blue depending on the extent
reduction which again under identical conditions depends on concentration of carbonmonoxide in air.
Hydrogen sulphide, unsaturated hydrocarbons, moisture,etc, interfere but they may
be removed by passing the sample through various absorbents provided with the
indicator tubes.
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(iii)Non-Dispersive Infrared Absorption Method
Samples containing carbon monoxide in the range of 0 to 100 ppm are analysed ona non-dispersive infrared absorption gas analyser, namely, an electro-optical
spectrophotometer with no spectral dispersion component. It may consist of a single
or double source of infrared energy and one or more infrared detectors separated by
an optical cell or cells through one or more which the sample flows, whereby the
specific spectral absorption of the component of interest is determined.
(iv)Gas Chromatography Method
A sample of air containing carbon monoxide is injected into the gaschromatography where it is carried from one end of the column to the other. During
its movement, the constituents of the sample undergo distribution at different rates
and ultimately get separated from one another. The separated constituents emerge
from the end of the column one after the other and are detected by suitable meanswhose response is related to the amount of a specific component leaving the
column.
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POLLUTION IN FOUNDRIES
Molten metals are cast into objects of desired shapes
in foundries.
The main production steps include:(i) Preparation of raw materials,
(ii) Metal melting,
(iii) Preparation of molds,
(iv) Casting and(v) Finishing (which includes fettling and tumbling)
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Castings of iron, steel, light metals, and heavy metals are made in units
that may be independent or part of a production line. Auto manufacturing
facilities usually have foundries within their production facilities or as
ancillaries.
Emissions of particulate matter (PM) from the melting and treatment ofmolten metal, as well as from mold manufacture, shakeout, cleaning and
after-treatment, is generally of greatest concern. Particulate matter may
contain metals that may be toxic. Major pollutants present in the air
emissions include particulates of the order of 1,000 mg/Nm3. Generally,
foundries produce 10 kg of dust per ton of molten metal, with a range of 5
30 kg/t, depending on scrap quality and other related factors.
However, induction furnaces (with emissions of 3 kg/t of molten metal) and
flame ovens tend to have lower air emissions than cupolas and electric arc
furnaces (EAF).
Oil mists are released from the lubrication of metals. Oil and suspended
solids are released into process effluents, and treatment is warranted before
their discharge.
Wet scrubbers release wastewaters that may contain metals. Wastewater
from tumbling may contain metals and surfactants. Odor and alcohol vapor
(from surface treatment of alcohol-based blacking) and emissions of other
volatile organic compounds (VOCs) are also of concern.
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Sand molding creates large quantities of waste sand. Other wastes include
slag (300500 kg/t of metal), collected particulate matter, sludge from
separators used in wastewater treatment, and spent oils and chemicals.
Discarded refractory lining is another waste produced.
The primary hazardous components of collected dust are zinc, lead, andcadmium, but its composition can vary greatly depending on scrap composition
and furnace additives. (Nickel and chromium are present when stainless steel
scrap is used.)
Cooling waters (upto 20 m3/t), may contain oil and some chemicals for the
control of algae and corrosion.Foundries can generate up to 20 m3of wastewater per metric ton of molten
metal when cooling water, scrubber water, and process water are not regulated.
Untreated wastewaters may contain high levels of total suspended solids,
copper (0.9 mg/l), lead (2.5 mg/l), total chromium (2.5 mg/l), hexavalent
chromium, nickel (0.25 mg/l), and oil and grease. The characteristics of thewastewater will depend on the type of metal and the quality of scrap used as
feed to the process. Solid wastes (excluding dust) are generated at a rate of
300500 kg/t of molten metal. Sludge and scale may contain heavy metals
such as chromium, lead, and nickel.
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Air Pollution control Systems (APCS)
Different air pollution control systems, which may be
adopted in foundries are:
(1) Furnace Fume Extraction & Filtration System (FES)(2) Scrubber (Wet Type) & Bag Filter (Dry Type)
(3) Dust Collection system for Knock Out Plant
(4) Dust Collection system for Sand Reclamation Plant
(5) Dust Collection system for Shot Blasting & SandBlasting
(6) Dust Collection System for Thermal Sand
Reclamation Plants with air to air heat exchangers
(7) Sand Unloading Stations with Dust Collection
System
(8) Dust Collection Systems for Material Handling Points
viz., Bucket Elevator, Belt Conveyer, Screens, MixingPlants
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(9) FD Centrifugal Blowers for Cupola Furnaces
(10) Pneumatic Sand Transporters, Wear Resistance
Bend
(11) Vent Dust Collectors for Storage Silo(12) Welding Fume Extractors / Central Fume Extraction
for Weld Shop
(13) Fettling Booth with Dust Collector