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chromium processing
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What is chromium processing?
Chromium processing involves preparing chromium ore for use in various products. Chromium (Cr) is a hard, brilliant metal resistant to corrosion, making it useful as a protective coating.
Chromium is consumed mostly as an addition to iron. It provides steel with corrosion resistance, improves hardenability, and increases high-temperature strength. Chromium is also used as a refractory material and in chemical compounds for tanning agents and pigments.
Pure chromium is produced through thermal reduction of (Cr_2O_3) with aluminum or by electrolysis of trivalent chromium solutions. Chromium surfaces are produced on other metals by electroplating and chromizing.
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Where is chromium found in nature?
Chromium is a relatively abundant element, but the free metal is never found in nature. Most chromium ores consist of the mineral chromite, or FeCr_2O_4, widely dispersed in natural deposits, typically contaminated with oxygen, magnesium, aluminum, and silica.
Chromite is commonly found as brittle masses in peridotites, serpentines, and other basic igneous and metamorphic rocks. Africa has a large share of the world's mineral resources, including chromium. Almost the entire world reserve of chromium is in Southern Africa and, to a much lesser extent, in western and northeastern Africa. The highest concentrations are found in Zimbabwe, and the largest deposits of chromite are in South Africa.
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How is chromium extracted from its ores?
Chromium extraction from its ores involves several methods depending on the desired purity. Chromite ore (FeCr₂O₄) is commonly used, with a chromium content ranging from 42 to 56 percent.
Methods of Extraction
- Ferrochromium Production Chromite is reduced with carbon in a furnace to produce ferrochromium, an alloy of iron and chromium.
- Pure Chromium Production
- Chromite is treated with molten alkali and oxygen, converting the chromium to alkali chromate, which is then dissolved in water and precipitated as sodium dichromate (Na₂Cr₂O₇). The dichromate is reduced with carbon to chromium sesquioxide (Cr₂O₃), which is then reduced with aluminum to yield chromium metal.
- Thermal reduction of Cr₂O₃ with aluminum, known as the aluminothermic process, roasts fine ore with soda and lime in air, creating a calcine containing sodium chromate. This is leached, reduced, and precipitated as Cr₂O₃, then mixed with aluminum powder and ignited, yielding 97 to 99 percent pure chromium.
- Electrolysis of trivalent chromium solutions involves leaching milled high-carbon ferrochromium with a mixture of reduced anolyte, chrome alum mother liquor, and sulfuric acid. After filtering out solids and iron, chromium is precipitated as ammonium chrome alum, dissolved in hot water, and fed into the cathode portion of an electrolytic cell, resulting in chromium plating on the cathode.
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What methods are used to refine chromium after extraction?
After extraction, chromium can be refined through several methods to achieve different levels of purity. Primary methods include the aluminothermic process and the electrolytic process.
The aluminothermic process involves roasting fine ore with soda and lime in air, creating a calcine containing sodium chromate. This is leached, reduced, and precipitated as (Cr_2O_3), which is then mixed with finely divided aluminum powder and ignited. This process yields chromium with a purity of 97 to 99 percent, depending on the starting materials.
The electrolytic process involves leaching milled high-carbon ferrochromium with a mixture of reduced anolyte, chrome alum mother liquor, and sulfuric acid. After filtering out solids and iron, the chromium is precipitated as ammonium chrome alum, dissolved in hot water, and fed into the cathode portion of an electrolytic cell. Chromium is then plated onto a stainless-steel cathode sheet. The oxygen content is reduced by combining it with carbon and heating, resulting in a metal that is more than 99.9 percent pure.
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What are some main uses of processed chromium?
Processed chromium serves multiple key functions across various industries:
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Steel Production Chromium is mostly used as a ferrochromium alloying agent in steel manufacturing, imparting corrosion resistance when present at levels between 10% and 26%. It improves hardenability, wear resistance, and high-temperature strength.
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Surface Treatment Chromium chemicals are utilized in metal-surface treatments and corrosion control, including chromium plating, chromizing, anodizing of aluminum, and treatment of zinc and magnesium.
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Pigments Chromium chemicals are used as pigments. Chrome oxide green is a stable green pigment used in roofing granules, cements, and plasters. Chromium yellow, essentially lead chromate, is a paint for wood and metal.
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Tanning Approximately 25% of chromium chemicals are used in the chrome tanning of leather, using chrome reagents in the form of basic chromic sulfates.
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How does chromium processing affect the environment?
Chromium processing, essential for creating metal alloys and surface treatments, poses environmental risks, particularly through the generation of hexavalent chromium. Hexavalent chromium is toxic and is created during various industrial processes, including leather tanning, chromium plating, and the production of certain pigments and coatings. Exposure can occur through inhalation, ingestion, or skin contact, leading to respiratory issues like asthma and even lung cancer.
The case of Hinkley, California, highlighted in the film Erin Brockovich, demonstrates the dangers of water contamination from chromium processing, resulting in severe health consequences for residents. Although remediation efforts have been made, concerns about water contamination persist, prompting stricter regulations.
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chromium processing, preparation of the ore for use in various products.
Chromium (Cr) is a brilliant, hard, refractory metal that melts at 1,857 °C (3,375 °F) and boils at 2,672 °C (4,842 °F). In the pure state it is resistant to ordinary corrosion, resulting in its application as an electroplated protective coating for other metals. It dissolves in nonoxidizing mineral acids but not in aqua regia or nitric acid, which passivate the metal.
Because chromium and chromium-rich alloys are brittle at room temperature, they have limited application. By far the largest consumption is as an alloying addition to iron. In amounts varying from 10 to 26 percent, chromium imparts corrosion resistance to steel; it is also used to improve hardenability, wear-resistance, and high-temperature strength.
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As the mineral chromite, chromium is employed extensively as a refractory material. Other chromium chemicals are used as pigments and tanning agents.
History
Chromium is unusual among metals in that its ores and chemical compounds were used extensively long before the pure metal was prepared. As early as 1800, they were used to make pigments and chemicals for leather tanning, and in 1879 they were successfully used as refractories for the lining of steelmaking furnaces.
Chromium metal was discovered by the French chemist Louis-Nicolas Vauquelin in 1797; the following year he isolated the metal by the carbon reduction of crocoite, or red lead, a chromate mineral whose brilliant hue inspired Vauquelin to give the metal its current name (from Greek chrōmos, “colour”). Iron containing chromium was first produced in the mid-19th century, and the first use of chromium as an alloying agent in the manufacture of steel took place in France in the 1860s. In 1893 Henri Moissan smelted chromium ore and carbon in an electric furnace and produced ferrochromium; this has remained the basis of the modern commercial method of producing the alloy even while that method has continuously evolved under the influence of changing markets, technology, and raw materials. In 1898 Hans Goldschmidt, a German chemist, produced pure chromium by the aluminothermic reduction of chromium oxide; the silicothermic process for producing low-carbon ferrochromium was developed in 1907. Chromium metal was produced by electrolysis in 1854, but this method did not find wide commercial acceptance until a century later.
Ores
Although chromium occurs in many minerals, the only ore exploited commercially is chromite. This spinel mineral is ideally composed of ferrous oxide and chromic oxide with the chemical composition FeO · Cr2O3, but it is often found in nature with magnesia (MgO) substituting for FeO and alumina (A12O3) or ferric oxide (Fe2O3) substituting for Cr2O3. Other minerals such as silica (SiO2) are also present.
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By the early 21st century, South Africa, India, Kazakhstan, and Turkey had become the world’s leading producers of chromite. The bulk of chromite reserves are found in stratiform deposits (thin, even layers covering a broad area), but podiform deposits (scattered pod-shaped formations of varying size) are also important.
Mining and concentrating
Chromite deposits are mined by both underground and surface techniques. Much of the ore is rich enough to be used directly: for production of ferrochromium, a rich, lumpy ore containing more than 46 percent Cr2O3 and having a chromium-iron ratio greater than 2:1 is preferred, but ores with a lower ratio and as little as 40 percent Cr2O3 are also used. (Ores high in alumina are preferred for processing into refractory brick.) As finely divided ores, which do not smelt efficiently, come under greater exploitation, a number of processes are employed to agglomerate them for more satisfactory use in furnaces. Fines can be blended with fluxes and coke (the principal source of carbon) and then preheated or “prereduced” before being charged into an electric smelting furnace.
Extraction and refining
If carbon and Cr2O3 are combined in a molar ratio of 3:1 and subjected to increasing temperature, a number of oxidation-reduction reactions will ensue that will produce first a series of chromium carbides and finally, at 2,080 °C (3,775 °F), pure chromium and carbon monoxide. (This will take place at 1 atmosphere, or about 100 kilopascals, of pressure, but reducing the pressure will lower all of the reaction temperatures.) This theoretical reaction does not account for the presence, in commercial practice, of impurities in the metal and slag that may alter reaction temperatures and cause undesirable reactions of their own. For this reason, while a ferrochromium of very low carbon content (less than 0.1 percent) can in principle be produced in a single stage of smelting, in practice not all carbon is eliminated owing to the presence of magnesia, alumina, and silica in the ore and the use of silica as a flux to lower the melting point of the slag. In practice, therefore, the primary product is usually a high-carbon ferrochromium that can subsequently be refined to a low-carbon product. If pure chromium is desired, iron must be removed from the ore or from an intermediate ferrochromium product by hydrometallurgical techniques (see below).
High-carbon ferrochromium
Most ores smelted with coke in an electric furnace produce metals that are saturated with carbon. For ferrochromium, the saturation point is approximately 9 percent, but actual carbon content varies with the condition of the ore and the composition of the slag. For example, with a lumpy, refractory ore and a slag containing approximately equal amounts of magnesia, alumina, and silica, a ferrochromium is produced that contains 4 to 6 percent carbon and less than 1.5 percent silicon. This is a result of high temperatures generated by a viscous slag, of a slowly reacting bulky ore, and, possibly, of refining of the molten metal by undissolved ore in the slag. When the rate of the reducing reaction is increased by using fine ore, or when the slag is made less viscous by adding lime to the melt, the carbon level of the ferrochromium approaches saturation.
Adding more silica to the charge produces what is called charge ferrochromium, a carbon-saturated alloy with an increased silicon content. Some South African ores produce charge ferrochromium containing 52–54 percent chromium, 6–7 percent carbon, and 2–4 percent silicon; ores from Zimbabwe with a higher chromium-iron ratio yield a product that is 63–67 percent chromium, 5–7 percent carbon, and 3–6 percent silicon.
During the smelting of high-carbon or charge ferrochromium, slag and metal are tapped from the furnace into a ladle and separated by decanting or skimming. The recovery of chromium from the ore varies: in a good operation 90 percent is recovered in the molten metal; of the 10 percent remaining in the slag, some is in metallic form and can be recovered by mineral processing techniques. The smelting of charge ferrochromium consumes 4,000 kilowatt-hours of electric power per ton of alloy produced, compared with 4,600 kilowatt-hours per ton for high-carbon ferrochromium.
Ferrochrome silicon
If silica is added to the charge until its weight equals that of the ore, the smelting processes will yield what is known as ferrochrome silicon. Containing 38–42 percent silicon and less than 0.1 percent carbon, this alloy is used as a deoxidizer in the production of stainless steel and as an intermediate material in the production of low-carbon ferrochromium. Because of the greater energy required to reduce silica to silicon, the smelting of ferrochrome silicon consumes 7,600 kilowatt-hours per ton of product.
Low-carbon ferrochromium
When chromite and lime are melted in an open electric-arc furnace and then contacted with ferrochrome silicon, a low-carbon (0.05 percent) ferrochromium can be obtained. In an alternate process, high-carbon ferrochromium is oxidized and then blended with additional high-carbon ferrochromium. The briquetted mixture is placed in a large vacuum furnace, which is heated by graphite resistors to 1,400 °C (2,550 °F) at a reduced pressure of 30 pascals. The carbon is removed from the alloy (going off as carbon monoxide) to a level as low as 0.01 percent.
Chromium metal
Pure chromium is produced either by the thermal reduction of Cr2O3 with aluminum or by the electrolysis of trivalent chromium solutions.
The aluminothermic process begins with the roasting of fine ore, soda, and lime in air at 1,100 °C (2,000 °F). This creates a calcine containing sodium chromate, which is leached from the insoluble gangue and then reduced and precipitated as Cr2O3. The Cr2O3 is blended with finely divided aluminum powder, charged to a refractory-lined container, and ignited. The combustion quickly generates temperatures in excess of 2,000 °C (3,600 °F), giving a clean separation of chromium from the slag. The purity of the metal, varying from 97 to 99 percent chromium, depends on the starting materials.
In the electrolytic process, milled high-carbon ferrochromium is leached by a mixture of reduced anolyte (electrolytic solution recycled from the anode side of the smelting cell), a chrome alum mother liquor based on a solution of ammonium sulfate recycled from a later stage in the process, and sulfuric acid. The resultant slurry is cooled, and silica and other undissolved solids are filtered from the solution. The iron forms crude ferrous ammonium sulfate crystals, which also are filtered out. The mother liquor is then clarified in a filter press, and about 80 percent of the chromium is stripped by precipitation as ammonium chrome alum. The mother liquor is sent back to the leach circuit while the chrome alum crystals are dissolved in hot water and fed into the cathode portion of an electrolytic cell. The cell is divided by a diaphragm in order to prevent the chromic and sulfuric acids formed at the anode from mixing with the catholyte (cathode electrolyte). With the passage of electric current from a lead anode to a thin stainless-steel cathode sheet, chromium is plated onto the cathode and, every 72 hours, is stripped from the sheet, sealed in stainless steel cans, and heated to 420 °C (790 °F) to remove water and hydrogen. This electrolytic chromium contains 0.5 percent oxygen, which is too high for some applications; combining it with carbon and heating the briquettes to 1,400 °C (2,550 °F) at 13 pascals lowers the oxygen content to 0.02 percent, resulting in a metal more than 99.9 percent pure.