Gunnar Nordberg
Occurrence and Uses
Iron is second in abundance amongst the metals and is fourth amongst the elements, surpassed only by oxygen, silicon and aluminium. The most common iron ores are: haematite, or red iron ore (Fe2O3), which is 70% iron; limonite, or brown iron ore (FeO(OH)·nH2O), containing 42% iron; magnetite, or magnetic iron ore (Fe3O4), which has a high iron content; siderite, or spathic iron ore (FeCO3); pyrite (FeS2), the most common sulphide mineral; and pyrrhotite, or magnetic pyrite (FeS). Iron is used in the manufacture of iron and steel castings, and it is alloyed with other metals to form steels. Iron is also used to increase the density of oil-well drilling fluids.
Alloys and Compounds
Iron itself is not particularly strong, but its strength is greatly increased when it is alloyed with carbon and rapidly cooled to produce steel. Its presence in steel accounts for its importance as an industrial metal. Certain characteristics of steel—that is, whether it is soft, mild, medium or hard—are largely determined by the carbon content, which may vary from 0.10 to 1.15%. About 20 other elements are used in varied combinations and proportions in the production of steel alloys with many different qualities—hardness, ductility, corrosion resistance and so on. The most important of these are manganese (ferromanganese and spiegeleisen), silicon (ferrosilicon) and chromium, which is discussed below.
The most important industrial iron compounds are the oxides and the carbonate, which constitute the principal ores from which the metal is obtained. Of lesser industrial importance are cyanides, nitrides, nitrates, phosphides, phosphates and iron carbonyl.
Hazards
Industrial dangers are present during the mining, transportation and preparation of the ores, during the production and use of the metal and alloys in iron and steel works and in foundries, and during the manufacture and use of certain compounds. Inhalation of iron dust or fumes occurs in iron-ore mining; arc welding; metal grinding, polishing and working; and in boiler scaling. If inhaled, iron is a local irritant to the lung and gastrointestinal tract. Reports indicate that long-term exposure to a mixture of iron and other metallic dusts may impair pulmonary function.
Accidents are liable to occur during the mining, transportation and preparation of the ores because of the heavy cutting, conveying, crushing and sieving machinery that is used for this purpose. Injuries may also arise from the handling of explosives used in the mining operations.
Inhaling dust containing silica or iron oxide can lead to pneumoconiosis, but there are no definite conclusions as to the role of iron oxide particles in the development of lung cancer in humans. Based on animal experiments, it is suspected that iron oxide dust may serve as a “co-carcinogenic” substance, thus enhancing the development of cancer when combined simultaneously with exposure to carcinogenic substances.
Mortality studies of haematite miners have shown an increased risk of lung cancer, generally among smokers, in several mining areas such as Cumberland, Lorraine, Kiruna and Krivoi Rog. Epidemiological studies of iron and steel foundry workers have typically noted risks of lung cancer elevated by 1.5- to 2.5-fold. The International Agency for Research on Cancer (IARC) classifies iron and steel founding as a carcinogenic process for humans. The specific chemical agents involved (e.g., polynuclear aromatic hydrocarbons, silica, metal fumes) have not been identified. An increased incidence of lung cancer has also been reported, but less significantly, among metal grinders. The conclusions for lung cancer among welders are controversial.
In experimental studies, ferric oxide has not been found to be carcinogenic; however, the experiments were not carried out with haematite. The presence of radon in the atmosphere of haematite mines has been suggested to be an important carcinogenic factor.
Serious accidents can occur in iron processing. Burns can occur in the course of work with molten metal, as described elsewhere in this Encyclopaedia. Finely divided freshly reduced iron powder is pyrophoric and ignites on exposure to air at normal temperatures. Fires and dust explosions have occurred in ducts and separators of dust-extraction plants, associated with grinding and polishing wheels and finishing belts, when sparks from the grinding operation have ignited the fine steel dust in the extraction plant.
The dangerous properties of the remaining iron compounds are usually due to the radical with which the iron is associated. Thus ferric arsenate (FeAsO4) and ferric arsenite (FeAsO3·Fe2O3) possess the poisonous properties of arsenical compounds. Iron carbonyl (FeCO5) is one of the more dangerous of the metal carbonyls, having both toxic and flammable properties. Carbonyls are discussed in more detail elsewhere in this chapter.
Ferrous sulphide (FeS), in addition to its natural occurrence as pyrite, is occasionally formed unintentionally when materials containing sulphur are treated in iron and steel vessels, such as in petroleum refineries. If the plant is opened and the deposit of ferrous sulphide is exposed to the air, its exothermic oxidation may raise the temperature of the deposit to the ignition temperature of gases and vapours in the vicinity. A fine water spray should be directed on such deposits until flammable vapours have been removed by purging. Similar problems may occur in pyrite mines, where the air temperature is increased by a continuous slow oxidation of the ore.
Safety and health measures
The precautions for the prevention of mechanical accidents include the fencing and remote control of machinery, the design of plant (which, in modern steel-making, includes computerized control) and the safety training of workers.
The danger arising from toxic and flammable gases, vapours and dusts is countered by local exhaust and general ventilation coupled with the various forms of remote control. Protective clothing and eye protection should be provided to safeguard the worker from the effects of hot and corrosive substances, and heat.
It is especially important that the ducting at grinding and polishing machines and at finishing belts be maintained at regular intervals to keep up the efficiency of the exhaust ventilation as well as to reduce the risk of explosion.
Ferroalloys
A ferroalloy is an alloy of iron with an element other than carbon. These metallic mixtures are used as a vehicle for introducing specific elements into the manufacture of steel in order to produce steels with specific properties. The element may alloy with the steel by solution or it may neutralize harmful impurities.
Alloys have unique properties dependent on the concentration of their elements. These properties vary directly in relation to the concentration of the individual components and depend, in part, on the presence of trace quantities of other elements. Although the biological effect of each element in the alloy may be used as a guide, there is sufficient evidence for the modification of action by the mixture of elements to warrant extreme caution in making critical decisions based on extrapolation of effect from the single element.
The ferroalloys constitute a wide and diverse list of alloys with many different mixtures within each class of alloy. The trade generally limits the number of types of ferroalloy available in any one class but metallurgical developments can result in frequent additions or changes. Some of the more common ferroalloys are as follows:
- ferroboron—16.2% boron
- ferrochromium—60 to 70% chromium, that may also contain silicon and manganese
- ferromanganese—78 to 90% manganese; 1.25 to 7% silicon
- ferromolybdenum—55 to 75% molybdenum; 1.5% silicon
- ferrophosphorus—18 to 25% phosphorus
- ferrosilicon—5 to 90% silica
- ferrotitanium—14 to 45% titanium; 4 to 13% silicon
- ferrotungsten—70 to 80% tungsten
- ferrovanadium—30 to 40% vanadium; 13% silicon; 1.5% aluminium.
Hazards
Although certain ferroalloys do have non-metallurgical uses, the main sources of hazardous exposure are encountered in the manufacture of these alloys and in their use during steel production. Some ferroalloys are produced and used in fine particulate form; airborne dust constitutes a potential toxicity hazard as well as a fire and explosion hazard. In addition, occupational exposure to the fumes of certain alloys has been associated with serious health problems.
Ferroboron. Airborne dust produced during the cleaning of this alloy may cause irritation of the nose and throat, which is due, possibly, to the presence of a boron oxide film on the alloy surface. Some animal studies (dogs exposed to atmospheric ferroboron concentrations of 57 mg/m3 for 23 weeks) found no adverse effects.
Ferrochromium. One study in Norway on the overall mortality and the incidence of cancer in workers producing ferrochromium has shown an increased incidence of lung cancer in causal relationship with the exposure to hexavalent chromium around the furnaces. Perforation of the nasal septum was also found in a few workers. Another study concludes that excess mortality due to lung cancer in steel-manufacturing workers is associated with exposure to polycyclic aromatic hydrocarbons (PAHs) during ferrochromium production. Yet another study investigating the association between occupational exposure to fumes and lung cancer found that ferrochromium workers demonstrated excess cases of both lung and prostate cancer.
Ferromanganese may be produced by reducing manganese ores in an electric furnace with coke and adding dolomite and limestone as flux. Transportation, storage, sorting and crushing of the ores produce managanese dust in concentrations which can be hazardous. The pathological effects resulting from exposure to dust, from both the ore and the alloy, are virtually indistinguishable from those described in the article “Manganese” in this chapter. Both acute and chronic intoxications have been observed. Ferromanganese alloys containing very high proportions of manganese will react with moisture to produce manganese carbide, which, when combined with moisture, releases hydrogen, creating a fire and explosion hazard.
Ferrosilicon production can result in both aerosols and dusts of ferrosilicon. Animal studies indicate that ferrosilicon dust can cause thickening of the alveolar walls with the occasional disappearance of the alveolar structure. The raw materials used in alloy production may also contain free silica, although in relatively low concentrations. There is some disagreement as to whether classical silicosis may be a potential hazard in ferrosilicon production. There is no doubt, however, that chronic pulmonary disease, whatever its classification, can result from excessive exposure to the dust or aerosols encountered in ferrosilicon plants.
Ferrovanadium. Atmospheric contamination with dust and fumes is also a hazard in ferrovanadium production. Under normal conditions, the aerosols will not produce acute intoxication but may cause bronchitis and a pulmonary interstitial proliferative process. The vanadium in the ferrovanadium alloy has been reported to be appreciably more toxic than free vanadium as a result of its greater solubility in biological fluids.
Leaded steel is used for automobile sheet steel in order to increase malleability. It contains approximately 0.35% lead. Whenever the leaded steel is subject to high temperature, as in welding, there is always the danger of generating lead fumes.
Safety and health measures
Control of fumes, dust and aerosols during the manufacture and use of ferroalloys is essential. Good dust control is required in the transport and handling of the ores and alloys. Ore piles should be wetted down to reduce dust formation. In addition to these basic dust-control measures, special precautions are needed in the handling of specific ferroalloys.
Ferrosilicon reacts with moisture to produce phosphine and arsine; consequently this material should not be loaded in damp weather, and special precautions should be taken to ensure that it remains dry during storage and transport. Whenever ferrosilicon is being shipped or handled in quantities of any importance, notices should be posted warning workers of the hazard, and detection and analysis procedures should be implemented at frequent intervals to check for the presence of phosphine and arsine in the air. Good dust and aerosol control is required for respiratory protection. Suitable respiratory protective equipment should be available for emergencies.
Workers engaged in the production and use of ferroalloys should receive careful medical supervision. Their working environment should be monitored continuously or periodically, depending on the degree of risk. The toxic effects of the various ferroalloys are sufficiently divergent from those of the pure metals to warrant a more intense level of medical supervision until more data have been obtained. Where ferroalloys give rise to dust, fumes and aerosols, workers should receive periodic chest x-ray examinations for early detection of respiratory changes. Lung function testing and monitoring of metal concentrations in the blood and/or urine of exposed workers may also be required.