Chromium plays an essential but hidden role in human life. In many ways, chromium contributes to a clean, efficient, and healthy life style. Many consumer products use chromium and many others are made possible by industrial use of chromium.
This document tells what chromium is used for and how it is used. Consumer and industrial uses are listed followed by information on processes and background information on chromium in the environment.
CHROMIUM IN THE HUMAN ENVIRONMENT
• Consumer uses and products
Where does the consumer find chromium in daily life? The nearest place is inside the human body. One of the most important aspects of the quality of life is health. Chromium is essential for good health as a required trace mineral. Without a sufficient supply of chromium in the diet, health is compromised. Insufficient chromium results in glucose intolerance, a symptom similar to that experienced by people with diabetes. Good dietary sources of chromium are organ meats, mushrooms, wheat germ, and broccoli. Chromium finds its way into beauty aids as chromium-bearing pigments.
Chromium also provides a hygienically cleanable material—that is, a material that does not harbor disease-causing bacteria. That material is properly finished stainless steel. Chromium makes stainless steel stainless by providing a protective oxide coating. The useful attribute of stainless steel, the protective oxide coating that prevents rust, is a hygienically cleanable surface. Stainless steel is found in kitchens and dining rooms where food is prepared and served. Its most popular use is in flatware; however, cooking ware, utensils, and kitchen sinks are also popular uses. Commercial kitchens use counter tops, shelves, and drawers made of stainless steel. Food-processing equipment in factories and commercial kitchens use stainless steel for parts that come in contact with food. Chromium also contributes to health in the form of medical and dental tools.
A second aspect of quality of life is mobility. Chromium plays a role in transportation. Human beings walk on chromium, at least those who wear shoes do, because chromium chemicals are used to process leather. On the U.S. roadway, chromium pigments are essential to the yellow lines indicating traffic lanes or the edge of the road. In automobiles, besides being used in chrome decorations (such as ornaments, trim, and hubcaps) the more important applications of chromium are in the engineering alloys. Chromium is useful in engineering applications where temperatures are high, i.e., in post-combustion gas processing and in selected engine parts. The catalytic converter, used today in many parts of the world to reduce unhealthy exhaust emissions, is housed in stainless steel. Exhaust pipes are commonly made of stainless steel. In buses and passenger trains, stainless steel reduces vehicle weight and maintenance costs. In airplane jet engines, chromium in engineering alloys called superalloys permits them to operate in a high-temperature, high-stress, chemically oxidizing environment. Trucks, buses, trains, and planes benefit from the use of chromium materials.
A third quality of life is the living environment. Stainless steel and other chromium containing alloys are used in architectural applications. Stainless steel is used on building exteriors as a facing material and in building interior areas such as elevators and doorways. Street furniture can use chromium. Lampposts are constructed of stainless steel, and wood is treated with chromium-containing chemicals.
Chromium protects the environment and contributes to the permanence and beauty of architectural objects. Stainless steel products can be enhanced with a variety of surface finishes including the traditional bright, silver-tone finish. One of the newer uses of stainless steel is in environmental applications where it is used in the piping or ductwork of process plants that clean industrial fluid streams of toxic materials. Fortunately, chromium is in abundant supply. At the current rate of consumption, there are hundreds of years of chromium natural resources in the ground.
Chromium in stainless steel is recycled when stainless steel is reused. Recycling chromium by reusing stainless steel reduces the amount of resources used to meet human needs.• Industrial uses and products
Industry uses chromium in three forms, ferrochromium, chromium chemicals, and chromite-containing refractories to make industrial products, which, in turn may end up in consumer products. As used here, industrial products are those consumed by industry; consumer products, by the general public.
Materials are used by industry in two ways, as feed materials and as process materials. Feed materials are materials that, when consumed, end up as part of the product. For example, ferrochromium is mixed with iron to make stainless steel. The iron and chromium in the ferrochromium become part of the stainless steel. Process materials are those that are consumed as part of a production process yet do not become part of the product. For example, chromic acid and sodium dichromate are used as surface-finishing chemicals. They are consumed in the process of treating aluminum, magnesium, and steel alloys; however, they are not part of the alloy products.
The principal products of the chromium metallurgical, chemical, and refractory industries are all industrial products:
Metallurgical
In the metallurgical industry, the principal product is ferrochromium, a mixture of chromium and iron with varying amounts of other chemical constituents. Ferrochromium is further classified by its carbon content as high-carbon, medium-carbon, or low-carbon ferrochromium. By far, the largest production of ferrochromium is the high-carbon variety. It is the major source of chromium for the production of steel alloys. Low-carbon ferrochromium is also used as a source of chromium by the steel industry and is used as a source of chromium for nonferrous alloys that permit some iron content. Charge chrome is one of the chromium industry’s most important products. Charge chrome is a high-carbon ferrochromium with chromium content in the range of 50% to 55%.
Chromium metal is also a metallurgical industry product. It is produced by one of two processes: electrodeposition or aluminothermic reduction. Chromium metal produced by these processes is essentially the same differing only in amount of various trace chemical constituents. The largest production of chromium metal is by the aluminothermic reduction process. Chromium metal is a major source of chromium for nonferrous alloys that exclude iron, such as certain superalloys.
Chemical
In the chemical industry, the principal product is sodium dichromate. Sodium dichromate is used both as a feed material for the production of other chromium chemicals and as a process material for the surface treatment of metals. The use of chromite by the chemical industry is substantially smaller than that of the metallurgical industry. Chemicals made from sodium dichromate and which, along with sodium dichromate, are industrial products include chromic acid, chromic oxide, and potassium dichromate. The uses of chromium chemicals are quite diverse and include metal finishing, leather tanning, pigment production, and wood preservation.
Refractory
In the refractory industry, the principal products are shaped and unshaped refractories. Refractories are used to line metallurgical furnaces that produce steel or copper alloys. They are also used in the heat exchange section of glass making furnaces and in cement kilns. These refractories are consumed in the process of making steel, copper, glass, and cement, but they are not part of those products.
• Metallurgical industry use process
In the metallurgical industry, chromite ore is blended with carbon-rich material and with fluxes (material that promotes fusing of metals and otherwise aids physical and chemical processing), fed into an electric-arc furnace, and smelted. The smelting process uses electrical energy, and lots of it, to melt the feedstock and raise the melt to a temperature at which the mixture will chemically react. The net result of chemical reaction is that carbon combines with oxygen from the ore to form carbon monoxide and carbon dioxide, gases that evolve from the melted mixture leaving an iron-chromium rich melt (ferrochromium) covered by a slag containing other residual materials. Once enough ferrochromium has been produced, the furnace is opened, permitting the ferrochromium and an approximately equal amount of slag to flow out and be separated.
There are variations on this process. For example, a variety of kinds of carbonaceous material are used in the feedstock. These materials affect the level of trace elements contained in the ferrochromium product. The feed material may be mixed and processed into briquettes or pellets so that the feed materials form a porous bed through which gases formed during smelting exit the furnace. The feed material may be heated and, perhaps, partially chemically reacted before entering the smelting furnace. These process variations are used to control product quality or to improve ferrochromium production efficiency by improving heat recovery as well as improving energy source efficiency, and reducing smelting time.
There are also variations in post-smelting practices. When the hot, liquid ferrochromium is taken out of the furnace, the product could be cast into large blocks that are subsequently broken into smaller pieces, cast into ingots of commercially useable size, or processed into granules of commercially useable size and more easily handled than ingots. It is also possible, when the ferrochromium producer and steel producer are near each other, to transfer hot, liquid ferrochromium from the smelter furnace to the steel-producing furnace.
Electrolytic chromium metal is produced by an electrodeposition process. Ferrochromium fines are first dissolved; then chromium metal is deposited onto cathodes from the solution. The metal is deposited in the form of a plate several millimeters thick, which is broken up to produce a product called flake. The purity of this metal may be enhanced by crushing, briquetting, and vacuum degassing. Aluminothermic chromium metal is produced by the exothermic reaction of chromic oxide, a chemical industry product, with aluminum powder. The reaction of aluminum with chromic oxide leaves chromium metal in the form of a large pellet, which is crushed. The purity of aluminothermic chromium metal can be enhanced by crushing, briquetting, and vacuum processing. Vacuum processing reduces the level of selected impurities that consumers of this material wish to avoid.
• Chemical industry use process
The chemical industry feeds dried, crushed, and ground chromite ore mixed with soda ash (a source of sodium carbonate) into a rotary kiln furnace. The feed materials are heated as they flow downward through the inclined, rotating, cylindrical furnace. These materials react chemically to produce sodium chromate, which is converted to sodium dichromate from which many other chromium chemicals are produced.
• Refractory industry use process
The refractory industry crushes chromite ore and blends it with magnesia to make shaped and unshaped refractory products. Shaped refractories are bricks that are assembled where they are used such as in a furnace. Unshaped refractories are like plaster or grouting. They too are used in furnaces. Chromite ores high in combined chromic oxide (Cr2O3) and alumina (Al2O3) and low in silica (SiO2) are generally recognized to be desirable for the refractory industry.
CHROMIUM IN THE NATURAL ENVIRONMENT
• Background
Chromite is the name of the mineral and of the ore mined for its chromium content. Chromite mineral is composed principally of chromium (Cr), oxygen (O), and iron (Fe) in proportions of one iron atom to two chromium atoms to four oxygen atoms, represented in chemical notation as FeCr2O4. Another way of representing chromite in chemical notation is FeO•Cr2O3. The Cr2O3 part of this formula is called chromic oxide and provides the most common measure of ore grade. In trade, chromite ore is typically classified by its chromic oxide content.
Chromite and other minerals originally formed when magma, liquid rock, cooled inside the Earth’s crust. These minerals may have changed, ultimately forming the rocks that we see around us. Ore deposits are formed by different processes, including cooling and precipitation directly from magma. Chromite is a solid solution of variable chemical composition. Chromite ore is always a combination of the mineral chromite and associated minerals. As a result, commercial ore has a Cr2O3 content ranging from about 40% to over 55% and has a chromium-to-iron ratio in the range of 1.4 to 3.6.
• Chromite ore mining
The economics of chromite ore recovery depend on the occurrence of the mineral and its chemical composition. Occurrence, for example, could range from small grains of chromite mineral imbedded in a large volume of host rock to large volumes of relatively pure chromite ore. Chemical composition of the ore depends on both the composition of the chromite mineral and the type of host rock. Mining usually results in the extraction of both the desired mineral and some of the host rock with which it is mixed, referred to as gangue. Beneficiation is the physical process by which the quality of chromite ore is enhanced. For example, ore can be sorted by size. Each resulting size fraction may have different chemical characteristics. By removing fractions with low chromite mineral content and retaining fractions with high chromite mineral content, the mineral content of the ore is concentrated. Generally, less beneficiation is required when more discriminating mining methods, such as hand picking, are used. More beneficiation is required when less discriminating mining methods, such as blasting, are used. Sometimes ore is beneficiated at the mine site, and sometimes there are centrally located concentrators (also called beneficiation plants) taking crude ore from mines in a wide area and beneficiating them.
• Other sources of information about chromium
DeYoung, J.H., Jr., Lee, M.P., and Lipin, B.R., 1984, International Strategic Minerals Inventory summary report—Chromium: U.S. Geological Survey Circular 930–B, 41 p.
National Research Council, 1989, Recommended dietary allowances, (10th ed.): Washington, D.C., National Academy Press, 284 p.
Page, B.J., and Loar, G.W., 1993, Chromium compounds in v. 6 of Kirk-Othmer Encyclopedia of Chemical Technology (4th ed.): New York, John Wiley & Sons, p. 263–311.
Papp, J.F., 1994, Chromite, in Industrial minerals and rocks (6th ed.): Littleton, CO, Society of Mining, Metallurgy, and Exploration, Inc., p. 209-228.
Thayer, T.P., 1973, Chromium, in Brobst, D.A., and Pratt, W.P., eds., United States mineral resources: U.S. Geological Survey Professional Paper 820, p. 111-121.
U.S. Geological Survey (World Wide Web site - http://minerals.usgs.gov/minerals/).
Westbrook, J.H., 1993, Chromium and chromium alloys in v. 6 of Kirk-Othmer Encyclopedia of Chemical Technology (4th ed.): New York, John Wiley & Sons, p. 228–263.
• List of figures
Figure 1. The chromium life cycle process from mining to commercial products.
Figure 2. World map indicating chromite ore production by country.
Figure 3. World map indicating ferrochromium production by country.
Figure 4. World map indicating stainless steel production by country.
• Appendix I
Chromium terminology Because of the international nature of the chromium industry, there are often different words used for the same thing. For example, the element was originally named chrome in French after the Greek word “”, pronounced khrma meaning color, and translated as “chromium” in English, “chrom” in German, “cromo” in Italian and Spanish,
(khrom) in Russian, and “chroom” in Dutch. There is a similar variety of names for chromium materials. In English, the original French name for chromium is still used for some products developed early in the history of chromium use. For example, one chromium-containing pigment is chrome yellow.
When chromite is mined, the commercial product is called chromite ore. Chromite ore is also called chrome ore and chromium ore. In general, ore is a material economically recoverable by mining. In this case, the material is chromite mineral, hence, chromite ore. The term chromite may be used to describe either a mineral or an ore.
Ferrochromium, also called ferrochrome, ferro chrome, and ferro-chrome, is an iron-chromium alloy used by industry to supply chromium units to metal alloys used in fabrication. Variations in chemical content are indicated by the terms high-carbon, low-carbon, and charge-grade. The name charge chrome is often used for charge-grade ferrochromium. Ferrochromium contains only small amounts of silicon. When greater amounts of silicon are present, it is called ferrochromium-silicon. Ferrosiliconchromium, ferro-silico-chromium, ferrochromiumsilicon, chromium silicide, silico-chrome, and silico chrome are other names for ferrochromium-silicon. Chromium in its pure form and under natural physical conditions is a metal. It is called chromium metal or chrome metal.
In trade, countries classify chromite ore as chromite ore and concentrate. In international commerce, only chromite ore ready for industrial use by a metallurgical smelter, a chemical plant, or a refractory plant is traded.
Chromium materials can be categorized by the valence state of chromium in that material. Chromium has valence state 0 when it appears as metal, metallic chromium. There are two other valence states in which chromium is commonly found, +3 and +6. Chromium compounds that have chromium valence state +3 are called trivalent chromium compounds; +6, hexavalent chromium compounds. Other valence states occur but are much less common. Some examples of 0 valence, chromium(0), metallic chromium are chromium metal, chromium ferroalloys like ferrochromium and ferrochromium-silicon, and chromium in ferrous and nonferrous alloys such as stainless steel, and nickel and cobalt alloys. Some examples of trivalent chromium, chromium(III), are chromite ore, chromic oxide also called chromium sesquioxide, and chromium sulfate. Some examples of hexavalent chromium, chromium(VI), compounds chromic acid, and many chemical compounds that contain chromate in their name such as sodium dichromate and potassium dichromate. (A common name for chromium trioxide, a hexavalent chromium compound, is chromic acid. In this case the common name does not imply the valence state.)
Chromium-bearing pigments include both trivalent and hexavalent chromium compounds. They usually include some form of oxide or chromate. Some chromium-bearing pigments include chrome yellow, chrome green, zinc chromate, and chromic oxide green.