Alternative Technologies for Chromium-Metal Production
The geologic sources of chromium are essentially mixtures of spinels of the ideal form RO·R2O3 (e.g., chromite is FeO·Cr2O3), although a more accurate representation of an ore source would be (Mg,Fe)Cr2O4 with silica and alumina gangue materials. The refining processes for chromium metal have been thermodynamically defined by their spinel origins. The two processes of electrolytic deposition, discussed in the previous chapter, and aluminothermic reduction have evolved over time as more efficient than such other processes as carbon and silicon reduction (Sully, 1954). Both the electrolytic and the aluminothermic processes have been successfully improved over time to result in lower levels of impurities so that the requirements of high-purity applications have continued to be met, even though purity specifications have become increasingly stringent over time. Improvements in the efficiencies and economics of the processes are probably still possible, but no obvious quantum-jump increase in the purity obtainable appears likely. This chapter discusses the strengths, weaknesses, uses, limits, and possible improvements to alternative processes for producing chromium metal.
PREPROCESSING OF CHROMITE ORE
Chromite ore is processed in preparation for aluminum reduction in a number of different ways, two of which are briefly described below.
In the first method, chromite ore (Na2CO3) and a carrier (e.g., limestone) are roasted to produce sodium chromate. This product is then leached, filtered, and acidified to form the dichromate. The dichromate is filtered, crystallized out of solution, and then reduced with a reducing agent (e.g., sulfur or carbon) to produce chromic oxide.
In the second method, the sodium chromate liquor can be treated to reduce impurities (e.g., silicon) and then treated with alkaline hydroxides or ammonium hydroxide to produce a hydrated chrome oxide. This product is then
heated in a rotary furnace to produce chromic oxide. This method can produce a high grade of aluminothermic chromium.
The by-products of these two methods have environmental liabilities involving the chromium (VI) compounds. The second method has historically produced a purer grade of oxide and chromium metal and may still be in use in some parts of the world today. Improvements in treating the environmentally important waste products are possible with increased costs. Potential future liabilities may promote such considerations.
ALUMINUM REDUCTION METHOD FOR PRODUCING CHROMIUM METAL
The aluminothermic process is a batch process usually carried out in a steel cylindrical container with a basic or neutral refractory lining. The lining may be rammed MgO with an organic binder or alumina from the slag produced in the process. The process can be run in either an autogenous fashion or carefully preheated with good premixing and then ignited with barium peroxide. The reaction then proceeds exothennically as follows:
Cr2O3 + 2Al → 2Cr + Al2O3.
The end-product is a cake composed of metal and slag. The final purity can be adjusted within limits for adaptation to the intended use. Generally, if the product is to be used in aluminum alloys, the aluminum content can remain high. Silicon can be added if lower aluminum is desired. If low content of both aluminum and silicon is required, however, the reaction can be run with a less than stoichiometric amount of aluminum, resulting in a lower yield of metal and residual unreduced chromium oxide. Method one of the previous section has been used to produce chromium metal low in sulfur when the initial ore is low in sulfur. The foregoing general process description is basically relevant to current operations, but there appears to be detailed process variations by the manufacturers to meet specific objectives relating to end usage.
OTHER METHODS FOR PRODUCING CHROMIUM METAL
The dichromate can be reduced by a number of different methods besides the aluminum reduction process.
The ore, (Mg,Fe)Cr2O4, containing 15-65 percent Cr2O3 must first be chlorinated by heating to 600 °C:
Cr2O3 + Cl2 → CrCl3 + Cr oxychlorides.
Heating the reaction to above 1300 °C then causes the decomposition to metal:
2CrCl3 + heat → 2Cr + 3Cl2.
The volatilization of the chlorides are such that complete separation from the iron chlorides is difficult, however. This process is also possible using iodine. The oxide is treated with iodine to form the iodide, followed by thermal decomposition by heating. However, these processes require heating to high temperatures, whereas the reduction with aluminum is self-propagating.
Reduction by hydrogen is possible and produces the acceptable by-product water:
Cr2O3 + 3H2 → 2Cr + 3H2O.
The reduction of the oxide with hydrogen requires extremely high temperatures, however, for relatively low yields. These constraints make the process economically uncompetitive.
Carbon and silicon have also been used as reducing agents, but heat must again be supplied to the process. The availability of aluminum with its high-energy investment, the stability of Al2O3 as a reaction product, and the autogenous nature of the process have made the aluminothermic process the second major engineering method of choice for the reduction of chromium metal.
A major improvement to the aluminothermic process involves the refinement of the degassing systems, such as the double-degassed briquette (DDB) treatment of Delachaux. The DDB process shows that chromium-metal products and powder can be produced with lower iron, carbon, oxygen, or sulfur by degassing the purest sections of the chromium-metal buttons with
possible additions. Unlike chromium metal produced electrolytically, however, the entire button produced by the aluminothermic method cannot be refined to aerospace quality; only roughly one-fifth of the metal is suitable for double degassing to attain higher grades. Thus, to increase the quantity of high-purity chromium metal produced, an aluminothermic producer must also increase the quantity of less pure material manufactured. Alternate uses for this material would have to be found in the marketplace.
Six different grades of double-degassed chromium metal are available from Delachaux with purity levels of 99.81 to 99.89 percent. Iron, silicon, and oxygen are still major impurities in the range of 200 to 1,000 parts per million by weight in these materials, but carbon and sulfur are present in the range of 200 parts per million or less. Consumers of chromium metal currently utilize the higher-purity grades with in-house processes to produce products critically dependent on purity. The details of Delachaux's process, its chemical additions for specific purity objectives, and its possible variations are currently not public.
ACCEPTANCE OF ALUMINOTHERMIC CHROMIUM METAL
As stated in Chapter 2, the double-degassed aluminothermic chromium metal has now been certified for many aerospace applications. Recent developments in the processing and secondary treatment of aluminothermic material has led the committee to conclude that there is currently little difference in purity between the chromium products produced by the aluminothermic and electrolytic processes. However, as also stated in Chapter 2, while aluminothermic material of equivalent quality to the electrolytic material may now be available, changes in such critical applications as rotor-grade materials must be mutually acceptable to producer and user and would have to be subject to extensive qualification studies involving production and testing of alloys, test specimens, final cast and forged products, and possibly the engines containing them.
Overall, the aluminothermic chromium-metal industry does not currently appear to be producing at full capacity. Aluminothermic chromium metal is currently produced in the United Kingdom (London and Scandinavian Metallurgical Corporation, Ltd (LSM)), Germany (Gesellschaft für Elektrometallurgie (GfE)), China, and Russia. The LSM high-grade chromium metal appears nearly comparable to that of Delachaux, with slightly higher (parts per million) iron and oxygen contents and, to a lesser extent, sulfur, silicon, aluminum, nitrogen, and phosphorus. The Delachaux DDB grades are somewhat higher in overall
purity, and special grades can provide reduced levels of specific impurities. The Chinese and Russian products are not as reliably specifiable at this time.
Additional sources of aluminothermic chromium metal could also be quickly redirected or instigated. As stated in Chapter 1, there are no aluminothermic chromium-metal production plants currently extant in the United States, but aluminothermic production plants do exist that produce other high-purity metals. These plants could produce chromium metal if required. Also, since producers have indicated that the aluminothermic process is not as capital-equipment intensive as the electrolytic process, the construction of new facilities appears to be a minor factor should the need arise. As stated in Chapter 3, neither the electrolytic nor the aluminothermic process offers a significant advantage over the other in terms of environmental impact.
The degassing operations are harder to initiate quickly, however, because of the relatively large vacuum furnaces needed. The committee believes that suchvvacuum-degassing furnaces could be constructed within two years, if the need were sufficiently great.
POSSIBLE IMPROVEMENTS IN THE PROCESSING TECHNIQUES
The technique of electroslag remelting, utilizing the products of the above processes, could provide a level of purity higher than either high-grade aluminothermic or electrolytic chromium metal. The objective would be to obtain chromium metal in bulk metallurgical form. The process could address the current impurities of iron, silicon, carbon, oxygen, sulfur, and other impurities that, at the parts-per-million level, are still relatively high. The obvious difficulty is to obtain appropriate consumable electrodes from the above sources to run the process. Processes utilizing powder metallurgy, the chloride route, or chemical vapor deposition methods could be adapted to making electrodes. One must still remember that only a small fraction of the chromium-metal market would be involved, and economic factors may very well override the technical considerations in any such venture.
Current processes for producing high-purity chromium metal can provide powder as a possible source material for producing electrodes for an electroslag remelt process. The vapor pressure of chromium even at intermediate temperatures is high enough to promote vapor-phase sintering as a reasonably economic process. The powder might be used in a way similar to the current production of briquettes. A product relatively low in carbon, nitrogen, oxygen, and sulfur is currently produced. The sintering operation might be examined for adaptation
to producing electrodes suitable for electroslag remelting. Slags high in fluorite, CaF2, could be examined with the view to producing bulk metallurgical chromium with lower carbon, nitrogen, oxygen, and especially sulfur contents.
The committee considers it possible that a process based on induction plasma technology could use a chromium powder to produce satisfactory electrodes for remelt processing. Induction plasma methods, as opposed to arc plasma methods, can have inherently lower process-induced contamination. The powder could be melted directly with the plasma or injected through the plasma at a suitably arranged target that permits rapid solidification. This may be a feasible method to explore even with the high vapor pressure of chromium at its melting point (8-10 mm at about 1700 °C). The electrodes could then be processed in the electroslag remelt process with the object of obtaining ultra-high purity from elements that promote inclusion contents in later processing.
All of the above processes are energy-intensive and unattractive economically unless the need for the product is great enough.