2. Environmental impacts and protective measures
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A product undergoes numerous production stages in the course of the metalworking process. The environmental impacts of these stages affect the workplace and hence the people working there. They also affect the air, water and soil.
Due to their proximity to the point of origin, it is the workforce who are most seriously exposed to the production hazards. In highly industrialised countries this is the subject of comprehensive worker protection rules. The workplace hazards are listed below, taking as examples the most important and environmentally relevant machining processes. This is followed by a description of the wider environmental effects including the problems of waste disposal.
2.1 Potential hazards of selected operations
2.1.1 Metal cutting
Machining processes such as drilling, milling, turning, cutting, honing, grinding etc. make use of oils and oil preparations for lubricating and cooling tools and workpieces, to prevent overheating and possible melting of the workpiece and tool. Oils are dosed by spraying or pouring systems at rates of up to 100 litre/min. in order to dissipate heat. The spraying of moving and sometimes very hot tools and workpieces produces vapours containing droplets known as aerosols.
Metalworking techniques require appropriate coolants which must combine several different properties (non-foaming, corrosion-inhibiting, non-decomposing etc.).
Such a wide range of properties can only be achieved through the addition of varying quantities of chemical additives. These are added to the coolants in the form of non-water- miscible cutting oils or water-miscible concentrates.
More than 300 individual substances are used as coolant components. The following table divides these into substance groups by areas of application.
|Substance group||Reason for use||Examples|
|Mineral oil||Lubrication effect||Hydrocarbons with different boiling ranges; fatty oil; esters|
|Polar additives||Enhanced lubrication properties||Natural fats and oils of synthetic esters|
|EP additives||To prevent micro-welds between metal surfaces at high pressures and temperatures||Sulphurized fats and oils, compounds containing phosphorous, compounds containing chlorine|
|Anti-corrosion additives||To prevent rusting of metal surfaces||Alkano-amines, sulphonates, organic boron compounds, sodium nitrite|
|Anti-misting additives||To prevent breakdown of the oil and thus generate less mist||High molecular substances|
|Anti-ageing additives||To prevent reactions within the coolant||Organic sulphides, zinc dithiophosphates, aromatic amines|
|Solid lubricants||To improve lubrication||Graphites, molybdenum sulphides, ammonium molybdenum|
|Emulsifiers||To combine oils with water||Surfactants, petroleum sulphonates, alkali soaps, amine soaps|
|Foam-inhibitors||To prevent foaming||Silicon polymers, tributyl phosphate|
|Biocides||To prevent formation of bacteria/fungus||Formaldehyde, phenol, formaldehyde derivatives, cathon MW|
A significant increase in certain occupational diseases has occurred parallel with the introduction of the coolants which are now commonplace. According to scientific findings, diseases of the skin and respiratory tracts and cancer may occur.
Where coolant use is unavoidable, mist extraction as close as possible to the point of origin or encapsulation is necessary. Consistent use must be made of personal protection measures such as the wearing of protective clothing and the use of special skin protection substances. Factories should produce skin protection plans.
Bacteria which can have severe effects on health can occur due to the organic nature of coolants. Bacteria formation is promoted by warm/hot ambient temperatures. Anti-bacterial additives are introduced to counter this. Timely replacement of coolants avoids the need for high doses of anti-bacterial additives, which also represent a health hazard. However, this increases the total quantity of waste to be disposed of. Proper storage of "spent" coolants and subsequent separation of emulsified oils and greases, and also of metal compounds and other components, is imperative.
Safety data sheets informing of the danger of coolants and instructions for use should be displayed in the national language(s). It is important that staff are aware of the long-term dangers of coolants; a particular difficulty here being the often creamy, pleasant smelling and seemingly harmless nature of coolants.
No generally applicable limit values exist for coolants in the breathing air. The only guide is the relevant MAK values9) for the individual substances. The management should find out which are the most environment-friendly coolants and ensure that these are procured.
9) The term MAK (maximum allowable concentration) in Germany refers to the maximum possible concentration of a substance in the air of the workplace, in the form of gas, vapour or suspended matter.
2.1.2 Cleaning and degreasing of workpieces
For subsequent surface treatment, adhesive or thermal bonding etc., workpieces have to be freed of substances such as oils, fats, resin, wax, cellulose, rubber or plastics. Solvents are widely used for this purpose. Workpieces can be degreased and cleaned by various methods, for instance by cold, hot and/or vapour degreasing or combined processes.
Cold cleaning frequently involves the use at room temperature in open baths of solvent mixtures whose precise composition is not known to the user. Mixed with air, the vapours of these solvents or solvent mixtures can be explosive. Most solvents represent a health hazard for man.
Solvents are classified as organic compounds such as hydrocarbons, halogenated hydrocarbons, ethers (diethyl-ether, tetrohydrofuran, dioxan), ketones (acetones, methylethylketone) and organic alkalis (sodium hydroxide solution, ammonia) and acids (hydrochloric acid, nitric acid, sulphuric acid).
The most important halogenated hydrocarbons are chlorinated hydrocarbons (CHCs), such as tri-, tetra-, perchloroethylene, dichloromethane, tetrachloroethane etc.10) On account of their grease-dissolving properties and high volatility, CHCs are used in almost every branch of metal working as cleaning agents in cold cleaners and in hot degreasing. The high volatility ensures quick drying after cleaning, but also means it is necessary to monitor solvent concentrations in the workplace. Through skin contact and inhalation, CHCs can damage mucous membranes, central nervous system, liver, kidneys and lungs.
10) The best known are CFCs (chlorofluorocarbons) used in other application e.g. as refrigerants. CFCs are partially responsible for the destruction of the vital ozone layer in the atmosphere. CFCs and carbon tetrachlorides and certain other chlorinated hydrocarbons are banned in Germany under the CFC halogen prohibition directive of 6 May 1991 and the chloro-aliphatic compounds directive.
In addition, most solvents are inflammable and represent a particular pollution hazard for water.
Alternative processes use alkaline aqueous solutions (with surfactants and other washing components in varying concentrations) or water (high-pressure cleaning).
Apart from the need for worker protection, it should be remembered that practically all solvents seriously pollute the environment. Particular problems in this regard include damage due to solvent evaporation, soil and groundwater pollution and the difficulties of disposing of used solvents and solvent sludges.
Foremost among modern methods of alleviating the problems of disposal are primary measures to prevent wastewater occurring in the first place, rather than subsequently treating highly contaminated bath and flushing water before it enters the drainage system. Membrane filtration and ion exchange processes can be used to regenerate process baths and extend their useful life. Similarly, flushing water can be used several times over with continuous dirt and oil separation (recycling via ion exchangers, emulsion cracking and cascade flushing techniques). Resulting wastewater quantities and pollutant loads are reduced. One might also attempt to process and reuse solvents in a closed solvent circuit. This technique is rarely successful in the case of reprocessing surfactants, so the improvement of their biodegradability is an important factor. Management should optimize selection of solvents based on technical and environmental factors11).
11) Only wastewater experts can definitively optimise the choice of solvents. Information is available in: Dagmar Minkwitz, "Ersatzstoffe für Halogenkohlenwasserstoffe bei der Entfettung und Reinigung in industriellen Prozessen" (serial publication of the Bundesanstalt für Arbeitsschutz (German Federal Institute for Occupational Safety and Health) GA 38) Dortmund, Bremerhaven 1991 (Wirtschaftsverlag NW) ISBN-3-89429-086-2. See also "Zeitschrift Oberflächentechnik, Bezugsquellennachweis für die Oberflächentechnik mit Trendübersichten und Tabellen, Munich, 4th edition 1991) (Seibt Verlag), ISBN 3-922948-70-7.
The following precautions should be taken where degreasing is carried out with organic solvents:
- do not use substances which are unknown;
- use enclosed equipment where possible;
- ensure effective ventilation and aeration of work rooms;
- ensure good extraction at the workplace;
- avoid skin contact;
- use protective equipment;
- as solvents are heavier than air, they force the breathing air out of trenches, cellars, containers and depressions in the ground; suffocation can be avoided by means of floor openings and ventilation;
- use only non-combustible washing vessels with self-closing covers for cleaning small parts with inflammable solvents;
- keep only quantities of flammable solvents at the workplace as are required for the work and store in suitable containers with effectively sealed covers;
- avoid electrostatic charges;
- in operating manuals indicate the solvents used, limitations on use and safety precautions, instruct personnel;
- secure and lock installations when not in use;
- avoid hand-spraying of degreasing agents with spray guns;
- avoid blow-drying with compressed air of surfaces which have been treated with chlorinated solvents;
- with open degreasing equipment, note the quantities of solvent displaced on immersion of the workpiece and dimension the system accordingly;
- workpieces should leave the system free of solvents.
Most spray paints and brush paints contain considerable quantities of hydrocarbon and chlorinated hydrocarbon solvents (spray paints as much as 90%, normally 50 - 70%) which evaporate on spraying and drying. Paints also contain finely dispersed pigments. Some of these are highly toxic. Depending on the application, paints may have to satisfy a wide range of quality requirements. Available paint systems are accordingly diverse.
There are three possible ways of avoiding solvent emissions from painting installations; these can be used separately or in combination:
- use of low-solvent paints
"High solids" paints, water soluble paints and dispersion paints have been developed for this purpose. A further alternative is solvent-free powder paint, for which new applications are constantly being found.
- use of high-performance application methods
Solvent emissions depend not only on the paint formulation but also on the application method. An important evaluation criterion is the application efficiency factor, which is defined as the ratio of paint remaining on the product to the total quantity of paint used. Lower efficiency means higher paint consumption and thus higher solvent emission. Application efficiency is primarily determined by the process and by the form of the parts to be painted.
The following application efficiency values can be taken as guidelines for the painting of large surface areas by various methods:
- compressed air spraying 65%
- "airless" spraying 80%
- powder painting 98% (with recycling of spraying loss)
- electrostatic spraying 95%
- dipping, flooding 90%
- rolling, pouring approx. 100%
- brush, roller application 98%
The choice of application method depends on certain quality requirements, e.g. coating thickness, surface roughness etc. and is hence closely related to the purpose of the painted object.
The various levels of waste gas resulting from the different methods can be greatly reduced by enclosing the application zone and additionally by air circulation. This will reduce the outlay on waste gas cleaning.
- collection and cleaning of waste gas (with solvent recycling).
To obtain different surface properties (surface refining), workpieces are electroplated with chromium, zinc, tin, copper, cadmium, lead or brass. For this the selected metallic coating is deposited from an electrolyte solution in an electro-chemical process. To enable the metal coatings to be applied, workpieces must be cleaned and degreased.
Where cold cleaning and degreasing are carried out, the hazards from cold cleaners must be taken into account (see 2.1.2). The boil-off technique is also used for rough cleaning. Strong alkalines such as sodium or potassium hydroxide solutions are used for this purpose. These alkalines can damage eyes, skin and respiratory tracts if splashed or given off as mist or dust. An electrolytic process is often used for subsequent fine cleaning. Electrolytes are often alkaline solutions (5% sodium hydroxide solution) or cyanidic salts. Apart from the dangers posed by the boil-off technique, extraction ventilation is necessary in view of the large quantities of hydrogen produced, so as to avoid reaching the explosibility limit of the air-hydrogen mixture. Safety in the workplace is increased by installing gas warning devices.
Pickling degreasers and pickles are used to remove oxidation layers and casting or rolling scale from metal surfaces. These are acids (sodium hydroxide solution for aluminium) such as sulphuric, hydrochloric, phosphoric, hydrofluoric or nitric acid which attack and dissolve the workpiece surface. The main health hazards are skin diseases; dangerous vapours and gases can be inhaled in the case of inadequate extraction. Especially dangerous are nitrous gases which can occur when using nitric acids, also fluorine compounds from hydrofluoric acid and hydrogen chloride from hydrochloric acid.
Cyanides are used for cleaning in salt baths (fluorides), pickling (removal of thin surface films), with chemical and electrolytic polishing or burnishing, and also with surface coating and thermo-chemical hardening processes. These can cause hydrocyanic poisoning as well as skin diseases when solutions containing cyanide come into contact with acids. Therefore baths containing acids and cyanide must be covered and separated by partitions. Containers and equipment are to be clearly marked to prevent carry-over of substances which can mutually react. In all cases one should check to determine whether cyanide can be replaced by substances less hazardous to health.
The actual electroplating of the workpiece can be done in countless different process variants and stages. Materials posing just about every conceivable danger can be used in electroplating. The dangerous properties result both from the main components of the bath fluid and from different additives such as emulsifiers, foam inhibitors and wetting agents.
Strong aerosols can occur during bath filling and further preparation. Dangerous substances may enter the breathing air due to the production of gas (hydrogen) during the electrolytic process.
The main hazards with coatings are skin complaints and in particular allergies due to nickel and chromates. If consumed, both nickel and chromates can be carcinogenic. Nickel in fluid particle form is subject to a TRK value12) 4 of 0.05 mg/m³ breathing air.
12) TRK value: German technical directive on concentration of carcinogenic substances
Welding is the joining of materials using heat and/or force, with or without the use of welding fillers (anti-oxidation substances).
The individual processes most commonly used are gas welding, arc welding and inert gas shielded welding.
Polluting factors in welding workshops are:
- chemicals in the generated gases, vapours and dusts
- high temperatures (approx. 3,200°C - 10,000°C)
- radiation (ultraviolet radiation): eye damage, severe inflammation of unprotected skin;
infrared radiation: can penetrate the vitreous body of the eye, reaching the retina and causing cataracts)
- noise (up to 110 dB(A))
Diverse hazards occur depending on the fuels, inert gases, filler materials, workpiece coatings etc. in use. The following table summarizes the pollutants occurring with the different welding methods. The carcinogenic and mutagenic elements chrome and nickel are especially relevant. Certain hazardous elements are detectable in welding fumes in concentrations of over 1% and can lead to health damage. Clinical and epidemiological investigations indicate a frequent occurrence amongst welders of chronic bronchitis and increased impairment of the respiratory tracts.
Pollutants occurring in various welding processes include:
|Pollutant||Causes||Welding process||MAK *
|Lead||PbO||Welding of lead or lead-coated workpieces||all||0.1|
|Chromium||Cr2/ 3||Welding with alloyed electrodes, Cr Ni steel||all|
|Carbon monoxide||CO||Welding with basic coated electrodes, gas flame||all||30|
|Carbon dioxide||CO2||Gas welding with coated electrodes, inert gas||all||5000|
|Copper||CuO||Welding of copper, copper-coated workpieces||all||0.1|
|Manganese||Mn O||Welding of workpieces containing Mn, all electrodes||all||5|
|Nickel||NiO||Welding of Cr Ni steel, alloyed electrodes||all|
|Nitrogen||NO2||Welding in confined spaces, trenches, tanks||all||9|
|Zinc||ZnO||Welding of zinc, galvanized workpieces, zinc paint||all||5|
|Aluminium||Al2O3||Welding of aluminium, almost all types of electrodes||Arc welding||-|
|Iron||Fe2O3||Welding of steels, all electrodes||Plasma arc welding||8|
|Fluorides||F||Welding with basic and alloyed electrodes||Arc welding||2.5|
|Calcium||CaO||Welding with coated electrodes||Arc welding||5|
|Sodium||Na2 OH||Welding with coated electrodes||Arc welding||2|
|Oxygen (ozone)||O3||Strong UV radiation||Plasma arc welding||0.2|
|Titanium||TiO2||Welding with coated electrodes||Arc welding||8|
|Vanadium||V2O3||Welding workpieces containing vanadium||Arc welding||0.5|
* MAK: maximum allowable concentration
The welding of metals with anti-corrosion coatings may also have adverse toxicological consequences. Pollutants may be released depending on the type of coating.
Alkyl resins: acrolein, butyric acid
Phenolic resins: phenols, formaldehyde
Polyurethane: isocyanates, hydrogen cyanide
Epoxy resins: phenols, formaldehyde, hydrogen cyanide.
Although the inert gases carbon dioxide, argon and helium are not toxic, in poorly ventilated rooms they can displace the breathing air and under extreme conditions cause suffocation. Ozone may be produced during arc welding. Even low concentrations (0.1 parts per million (ppm)) of ozone can cause irritation to the eyes and upper respiratory tracts and in the event of exposure to 5-10 ppm over several minutes, pulmonary oedema.
At high temperatures nitrogen oxides are formed and emitted from the nitrogen and oxygen in the air on the periphery of the welding flame. Nitrogen oxides are highly toxic and, after a relatively long asymptomatic period, can lead to radical lung changes, pulmonary oedema and death. If the workpiece has been degreased with solvents containing chlorine and not properly dried, phosgene may be produced during welding. Phosgene is highly toxic and can also cause pulmonary oedema after a long asymptomatic period.
Since the welding of plastics is not yet widespread in many countries, it is not dealt with here. It must be pointed out however that the hazards for man and the environment are also considerable with the welding of plastics. Protective measures and special disposal procedures are necessary to guard against the release of solvents and other waste gases containing pollutants.
Soldering is the thermal joining of two materials using a material (solder) whose melting point is below that of the workpiece.
If the solder melts above 450°C the process is termed "hard soldering or brazing" and at lower temperatures "soft soldering". Apart from additional hazards due to the base material binders, the hazards involved in soldering are mainly associated with the flux and the solder.
The composition of a flux depends on the base material, the solder and the intended use. More than 300 different types of flux are currently available, all of which contain aggressive chemicals. Soldering paste usually contains colophonium, talc and salmiac, while soldering fluid contains zinc chloride or tin chloride. Chlorine and chlorine compounds cause irritation of the respiratory tracts and the skin and, in high concentrations, lung damage. Fluxes also frequently contain fluorine compounds (irritation of the respiratory tracts, burns). Fluxes often contain substances responsible for allergies. These are mainly colophonium and hydrazine. Hydrazine is additionally classed as carcinogenic.
Tin-based solders containing lead are used for soft soldering and silver solder containing cadmium for hard soldering. Flux vapours carry metal particles which can be inhaled.
Environmental protection measures to combat the emission of liberated gases and component substances of solders and fluxes include the installation of extractor systems with downstream separator filters (cyclone method). This method may also be used to contain the environmental impact of the production stage discussed in the next section.
Grinding is the cutting of a workpiece with a geometrically undefined cutting process.
Grinding processes are characterised by high temperatures, workpiece removal and abrasive wear. In addition to noise, the health hazards from grinding are principally emissions from abraded dust or particles from the abrasive tool, workpiece and any coating, and - in the case of wet grinding - from coolants. There is an attendant risk of health disorders especially affecting the skin and respiratory tracts. Additives in coolants and the metal dusts produced (e.g. from chromium, cobalt, nickel or beryllium) can result in allergies. These metals may also be carcinogenic. The following table shows the potential pollutant sources when grinding metals.
Potential pollutant sources in the grinding of metals
Grinding tool Formation of superfine dust with
- abrasive material - profiling and dressing of containing zircon grinding discs
- lead chloride, antimony - tool grinding sulphide in separating - fettling, due to adhering cutting in stationary operation mould residues
- additives in grinding - manual grinding, usually belts containing fluorine carried out without extraction ventilation
Coolant - coarse grinding additives, with respect to - use of magnesite binders toxicity, carcinogenicity and mutual reactivity
Material containing Combustion and pyrolysis
- more than 80% %/wt. products which can occur nickel (e.g. depositing with the thermal decomposition of materials) rubber or synthetic resin
- less than 80% %/wt. nickel (e.g. high grade corrosion-resistant steel)
- Lead (e.g. in automatic Accumulation of heavy metals and steel) superfine particles in coolant
- Cobalt (e.g. hard metal, co- due to inadequate filtration alloys) or overuse
- Beryllium (e.g. Ni Be alloys)
Atomization of coolant and thus also of additives, reaction products, dissolved heavy metals and superfine, non-separated particles.
Protective measures include environment-oriented selection of grinding tools, coolant and - where possible - materials, extraction of the abraded materials and personal breathing and hearing protection.
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