Molten Metal Hazards

Molten metal work is any process in which metals are melted, poured and molded.

Hazards associated with molten metal include:

• Heat stress from exposure of persons to heat and infrared and ultra-violet radiation generated by molten metal work.

• Exposure to airborne hazardous substances – dusts, fumes, gases and vapours.

• Noise and vibration generated by mold making machines, grinding and impact tools used to release and dress the work.

• Mechanical handling of heavy equipment such as molds, ladles, scrap and products.

• Manual handling of heavy equipment such as molds, ladles, scrap, tools and products.

• Danger of explosions due to presence of water or sealed containment in the scrap being added to the furnace or water in the molds.

• Physical injury and severe first, second and third-degree burns from molten metal splash, grinding equipment etc.

• Trips and falls.

• Vibration induced injury from use of pneumatic tools.

Hazards and Their Prevention

Prevention of health risks and accidents in the metallurgical industry is primarily an education and technical question. Medical examinations are secondary and have only a complementary role in the prevention of health risks. A harmonious exchange of information and collaboration between the planning, line, safety and occupational health departments within the company give the most efficient result in the prevention of health risks.

The following are some of the specific hazards and precautions that are found in smelting and refining.

Injuries

The smelting and refining industry has a higher rate of injuries than most other industries with injuries commonly the result of: splattering and spills of molten metal and slag resulting in burns; gas explosions and explosions from contact of molten metal with water; collisions with moving industrial plant, travelling cranes and other mobile equipment; falls of heavy objects; falls from a height; and slipping and tripping injuries from obstruction of floors and passageways.

Precautions include: adequate training; appropriate personal protective clothing/equipment; good storage, housekeeping and equipment maintenance; traffic rules for moving equipment (including defined routes and an effective signal and warning system); and a fall protection program.

Heat

Heat stress illnesses such as heat stroke are a common hazard, primarily due to infrared radiation from furnaces and molten metal. This is especially a problem when strenuous work must be done in hot environments.

Prevention of heat illnesses can involve water screens or air curtains in front of furnaces, spot cooling, enclosed air-conditioned booths, primary level heat protective clothing and air-cooled suits, allowing sufficient time for acclimatisation, work breaks in cool areas and an adequate supply of beverages for frequent drinking.

Chemical Hazards

Exposure to a wide variety of hazardous dusts, fumes, gases and other chemicals can occur during smelting and refining operations. Crushing and grinding ore in particular can result in high exposures to silica and toxic metal dusts (for example, containing lead, arsenic and cadmium). There can also be dust exposures during furnace maintenance operations. During smelting operations, metal fumes can pose a major problem.

Dust and emissions can be controlled by enclosure, automation processes, local and dilution exhaust ventilation, wetting down of materials, reduced handling of materials and other process changes. Where these are not adequate, respiratory protection will be required. Many smelting operations involve the production of large amounts of sulphur dioxide from sulphide ores and carbon monoxide from combustion processes. In these operations, dilution and local exhaust ventilation (LEV) are essential.

Sulphuric acid is produced as a by-product of smelting operations and is used in electrolytic refining and leaching of metals. Exposure can occur both to the liquid and to sulphuric acid mists. For these operations, skin and eye protection and LEV is required.

The smelting and refining of some metals can have special hazards. Examples include nickel carbonyl in nickel refining, fluorides in aluminium smelting, arsenic in copper and lead smelting and refining and mercury and cyanide exposures during gold refining. These processes require their own special precautions.

Pollution and Environmental Protection

Emissions of irritant and corrosive gases such as sulphur dioxide, hydrogen sulphide and hydrogen chloride may contribute to air pollution and cause corrosion of metals and concrete within the plant and in the surrounding environment. Particulate emissions may contain non-specific particulates, fluorides, lead, arsenic, cadmium and many other toxic metals. Waste water effluent may contain a variety of toxic metals, sulphuric acid and other impurities. Solid wastes can be contaminated with arsenic, lead, iron sulphides, silica and other pollutants.

Smelter management should include evaluation and control of emissions from the plant. This is specialised work which should be carried out only by personnel thoroughly familiar with the chemical properties and toxicities of the material discharged from the plant processes. The physical state of the material, the temperature at which it leaves the process, other materials in the gas stream and other factors must all be considered when planning measures to control air pollution. It is also desirable to maintain a weather station to keep meteorological records and to be prepared to reduce output when weather conditions are unfavourable for dispersal of stack effluents.

Other Hazards

• Glare and infrared radiation from furnaces and molten metal can cause eye damage including cataracts. Proper eyewear protection and face shields should always be worn. High levels of infrared radiation may also cause skin burns unless adequate protective clothing is worn.

• High noise levels from crushing and grinding ore, gas discharge blowers and high-power electric furnaces can cause hearing loss. If the source of the noise cannot be enclosed or isolated, then hearing protectors should be worn. A hearing conversation program including audiometric testing and training should be instituted.

• Electrical hazards can occur during electrolytic processes. Precautions include proper electrical maintenance with lockout/tagout procedures, insulated gloves, protective clothing and tools and ground fault circuit interrupters where needed.

• Manual lifting and handling can cause back and upper extremity injuries. Mechanical lifting aids and proper training in lifting methods can reduce this problem.     

Molten Alloys and Melting Points

The melting point of a substance is the temperature at which it changes state from solid to liquid at atmospheric pressure: at the melting point, the solid and liquid phases exist in equilibrium. A substance's melting point depends on pressure and is usually specified at standard pressure in reference materials. The melting point is also referred to as liquefaction point, solidus or liquidus.

An alloy is defined by the Oxford English Dictionary as "a metal made by combining two or more metallic elements, especially to give greater strength or resistance to corrosion: for example, an alloy of nickel, bronze or zinc". A similar definition by the Merriam-Webster Dictionary defines an alloy as "a substance composed of two or more metals or of a metal and a non-metal intimately united usually by being fused together and dissolving in each other when molten: the state of union of the components".

PR97® is ArcFlashWear's textile of choice for protection against molten metal. PR97® is recognised throughout the world as a leading FR textile for secondary protection in the molten metal industry due to its superior protective properties: uncompromising safety, natural comfort, cost effectiveness and proven performance. Designed specifically as a very high performance hot materials safety textile, PR97® provides workers with unparalleled safety and performance.

*Cryolite is an uncommon mineral of very limited natural distribution. Mostly considered a one locality mineral, although there are a few other minor localities, it is only found in large quantities on the west coast of Greenland. It was used as a solvent of the aluminium rich ore, bauxite, which is a combination of aluminium oxides such as gibbsite, boehmite and diaspore. It is very difficult to remove atoms of aluminium from atoms of oxygen which is necessary in order to produce aluminium metal. Cryolite made an excellent flux to make the process less expensive. Now it is too rare to be used for this purpose and Sodium Aluminium Fluoride (Na₃AlF₆) is produced artificially to fill the void.

Aluminium Smelting and Re-melting

Molten aluminium is typically handled at 700° C to 788° C (1300° F to 1450° F) to avoid premature solidification. Contact with molten aluminium can cause severe burns and create a serious fire hazard. Mixing water or other contaminants with molten aluminium can cause explosions. Explosions can also occur in the aluminium scrap re-melting process due to moisture and contamination in scrap. These explosions range widely in violence and can result in injury or death as well as destruction of equipment and plant facilities.

And Why So Dangerous?

Prior to the collapse of the Twin Towers at the World Trade Centre, powerful explosions were heard within the buildings. The explosions were later attributed to large amounts of molten aluminium being mixed with water and other contaminants. Expert opinion believes that when the airplanes entered the towers and lodged presumably near the centre of the buildings, the airplane hulls became trapped inside an insulating layer of building debris. The opinion further assumes that the hulls, rather than the buildings, absorbed most of the heat from the burning jet fuel. The intense heat melted the aluminium in the hulls which then flowed downward through the building gaps and stairways. The molten aluminium reacted with water from the sprinkler systems on the floors below and other contaminants, such as plasterboard and rust, resulting in violent explosions. It is believed the force generated by these explosions is the reason why the towers collapsed and why so quickly. To date this opinion remains unproven due to the relevant authorities refusing to release any of the remaining World Trade Centre debris for further testing. 

The aluminium industry has reported more than 250 aluminium/water explosions since 1980. Alcoa Aluminium carried out an experiment under controlled conditions, in which 20 kilograms of aluminium smelt was allowed to react with 20 kilograms of water, to which some rust was added. The explosion destroyed the entire laboratory and left a crater 30 metres in diameter.

The following video (1:23) demonstrates how a thermal explosion can occur when water contacts molten aluminium.

Molten Metal Splash - The Purpose of Secondary Protective Clothing

Secondary protective clothing in the hot metal industry is defined as "protective clothing for continuous wear during work activities in designated workplace locations in which intermittent exposure to molten substance splash, radiant heat and flame source are possible."

The essence of secondary protective clothing in this industry rests in two critial factors:

• The textile must be flame resistant so that it will not ignite and continue to burn when the heat source is removed.

• In the specific instance of exposure to molten metal (ferrous and those mentioned above), the textile must demonstrate the ability to shed molten metal from its surface without sticking.