Patent Application
20160244864
This invention pertains to the production of chromium iron alloys directly from chromite ore.
Chromium is an irreplaceable ingredient in all grades of stainless steels. It is the ingredient that renders steel “stainless”. It is present in alloys in amounts from 12% to about 35% Cr, with generally the more Cr the more corrosion resistant. It is also a key ingredient in the high end “super alloys” used for turbines and jet engines. Chromite ores are the only source of chromium. The majority of chromite ores are processed into an intermediate product called high carbon ferro-chrome, or charge chrome, an alloy containing greater than 50% Cr, about 6-8% C, varying amounts of Fi (0-4%, depending on the process used), with the balance Fe.
This material is the feed stock for the Argon Oxygen Decarburiser (AOD) process, which is a modified steel converter and the first step in producing a low carbon melt of Cr and Fe to which other alloying elements such as Ni are added before the liquid steel is cast into plates and then rolled into sheet which is the bulk of the stainless steel market, and the feed for the myriad of stainless products such as pipes, tanks, containers, flanges, valves etc. required for industry and domestic consumers. Prior to the development of the AOD process, and its various derivatives and hybrids, stainless steel was very expensive to produce because the intermediate low carbon product required a tricky and time consuming decarburisation step using chromite ores.
The production of stainless and low alloy steels containing chromium has rapidly expanded, particularly in Asia. The source of the chromium in the stainless steel is partly from the recycling of scrap, but this is limited by the availability of such materials, particularly in developing countries. Chromium in stainless steels is not open to substitution by other metals. It is essential for the corrosion and heat resistance of the material. The short fall in the chromium additions required during the steel making process is met by the addition of alloys of chromium and iron, collectively known as “ferro chrome”. These alloys are produced by the smelting of chromite ores, using solid carbonaceous reductants in a Submerged Electric Arc Furnace, (SAF). This process is extremely energy and carbon intensive. Existing plants using “best world practices” consume between three and four megawatt hours (MWH) of electricity and two hundred to three hundred kilograms of carbon per ton of ferro chrome alloy produced. Comprehensive gas cleaning systems are required to meet clean air standards. Large quantities of slag are produced and placed in long term storage in above ground dumps.
A small amount of metallic Cr is produced by reacting chemical grade chromic oxide with metallic aluminum, analogous to the common thermite reaction between iron oxide and aluminum to produce molten iron. Production of low carbon FeCr alloy by aluminoghermic reduction directly from chromite ores has not generally been practiced because of a generally unfavorable energy requirement, especially with low grade ores.
There are no commercially viable deposits of chromite ore in the USA and all ferro chrome used in the production of steel is imported, typically from South Africa and Kazakhstan. Recent discoveries of very large deposits of such ores have been made in Canada in a geographic area known as the “Ring of Fire” (ROF).
The development of huge deposits of natural shale gas in the USA and Canada has led to a decrease in the long term cost of natural gas and the prospect of stable pricing for many years to come. The present invention exploits the availability of the Ring of Fire chromite and low cost natural gas.
Large quantities of Directly Reduced Iron (DRI) are currently produced in many countries using existing processes. The present invention uses a modification of this basic and well established technology to produce a chromium iron alloy by using natural gas to reduce both oxides of chromium and iron contained within the ROF chromite ore, the morphology of which has been shown in testing to facilitate the progress of the reduction reactions.
The present invention provides a process for producing chromium iron alloy suitable for steel making directly from chromite ore wherein the fines of chromite ore with additions of carbon finds, an accelerant and a binder are agglomerated and dried, and thereafter the agglomerates are fed into a reaction vessel with natural gas as a reducing agent at elevated temperatures adequate for reduction for thereby producing a chromium iron alloy suitable for steel making.
The accelerant is an alkaline in the form of an oxide, hydroxide or carbonate, such as sodium hydroxide or potassium hydroxide. Sodium hydroxide has been found during testing to be more effective than other alkaline chemicals in enabling the reactions required to rapidly reduce the chrome and iron oxides from the chromite ore concentrates.
The accelerant is included in an amount sufficient for the stoichiometric formation of sodium silicate of silica encapsulating the chromite fines plus an additional amount to enable the combination of sodium with the chrome oxide in the chromite. The accelerant is included in each agglomerate in the approximate range of 2% to 15% by weight. However, the range of accelerant inclusion by weight depends upon a number of variables, one of which is the silica content of the ore concentrate and the second is the chrome oxide content.
Carbon is included in the amount sufficient for reduction of the reduceable metal oxides of chromium and iron contained in the agglomerate, for example a carbon inclusion in each agglomerate in the approximate range of 15% to 25% by weight.
The agglomerates may be efficiently dried with furnace off gas and then charged to the reaction vessel having a temperature range of between 750° and 1,150° C.
The agglomerates are preferably formed as pellets, and in one embodiment, may be swept into the reaction vessel having an elevated temperature in the range of 750° C. to 1,150° C. by reformed natural gas. The fines of chromite ore and carbon for making up the pellet agglomerate are preferably in the range of 50 and 250 microns in size, and the binder is preferably selected as bentonite or an organic alternative such as corn starch, which is included in the amount of 0.5% to 1.5% of the pellet mass.
The reaction vessel in one embodiment includes a vertical moving bed process and the natural gas reducing agent is selected as natural gas or reformed natural gas. In alternative embodiments the reaction vessel includes a static bed patch process or a moving belt process, and the natural gas reducing agent is selected as reformed natural gas.
Other objects and advantages appear hereinafter in the following description and claims. The accompanying drawings show, for the purpose of exemplification, without limiting the scope of the present invention or the appended claims, certain practical embodiments of the present invention wherein:
The process for producing chromium iron alloys suitable for steel making directly from chromite ore in accordance with the teachings of the present invention is carried out as described and outlined in the afore-described Summary of the Invention, preferably utilizing ROF chromite ore.
Extensive laboratory work has been completed which demonstrates the validity of the present invention. Samples of chromite ore concentrates from a deposit within the ROF have been successfully reduced in accordance with the teachings of the present invention to a highly metallised chromium iron alloy suitable for steel making. The temperature required for the reduction of chromium is much higher than that for the reduction of iron alone. In order to enable the reduction process to proceed at an acceptable rate at lower temperatures an accelerator is used. The chromite ore concentrate is supplied as fines and needs to be agglomerated prior to the reduction stage. This may be accomplished by using a disc pelletiser or other suitable agglomerating equipment commonly available for the production of iron ore pellets. It has been shown that carbon is a required additive to the chromite to facilitate reduction. The pellet composition is therefore principally of chromite, carbon fines and an accelerator, typically an alkaline salt, and an addition of a binder, such as bentonite or an organic alternative, completes the pellet composition. The pellets are dried using offgas prior to entering the reduction reactor.
The inventive process outlined above has been shown to produce metalization levels of chromium and iron of 80% or more. Higher metalization rates for both chromium and iron can be expected with process development. The resulting pellets of reduced chromite are suitable for stainless and alloy steel making, either as batch or continuously charged components of the steel making charge. Substantial cost advantages are expected when compared to the usage of conventionally produced ferro chrome alloys.
The carbon content of the reduced chromite is intended to be considerably lower than the ferro alloys produced in a SAF. This will result in significant process advantages for the steelmaker and therefore lower the cost of production. The reduced chromite pellets can form part of the charge of a conventional SAF furnace producing ferro chrome, with significant cost benefits.
Large quantities of Directly Reduced Iron (DRI) are currently produced in many countries using several existing processes. The current invention uses modifications of this basic and well established technology for direct reduction of iron to produce a chromium iron alloy by using reformed natural gas to heat and reduce both oxides of chromium and iron contained within the ROF chromite ore, the morphology of which has been shown in testing to facilitate the progress of the reduction reactions. The reduction of chromium and iron oxides in the chromite ore by carbon monoxide normally requires temperatures in excess of 1,350° Celsius. The present invention utilizes a controlled addition of an accelerant to reduce the temperature required for reduction to occur in the range from 750° to 1,100° Celsius. This lower temperature requirement reduces the energy required for the reduction process to around 1/5th of that needed in the conventional SAF process of the prior art.
In the laboratory studies, chromite used for the development work was sourced from the Black Horse deposit located within the Ring of Fire region of Northern Ontario Canada. As received chromite concentrate chemistry is shown Table 1, and the ore chemistry in elemental form is shown in Table 2.
Experimental results establish than when a suitable catalyst or accelerant is used, then the reduction reactions have been shown to occur much more quickly and at significantly lower temperatures. This is shown in a comparison of Table 3 with Table 4, Table 3 showing the time required to achieve a given percentage reduction at temperature when un-catalysed, and Table 4 showing the time required to achieve a given percentage reduction at temperature when catalysed.
The substantial reduction and reaction times demonstrated in these experiments result in very large increases in specific throughput at a given temperature. As an example, at an operating temperature of 1,200° C. at atmospheric pressure, the catalyst system of the present invention utilizing an accelerant has been shown to have a six fold increase in specific output than a similar sized reactor without the benefit of the accelerant. This in turn results in a much lower capital expenditure for a given output.
The rate of reduction of the chromite has been shown to be affected by the following variables:
1. Particle size of the ore.
2. Particle size of the reductant.
3. Reactivity of the solid reductant.
4. Temperature.
5. Presence of accelerants.
The process variations which are available are based on the use of a carbon containing pellet of around 12 mm in diameter produced on a disc pelletizer or a smaller pea sized product made in a standard industrial agglomerator. The feed for these operations is typically comprised of around 80% chromite concentrate, 17% carbon powder as a partial reductant, up to 1.5% of bentonite or other suitable organic binder and accelerant.
Full scale plant configurations capable of processing the agglomerates or pellets to the metallized product can utilize reaction vessels of different types to perform the process of the present invention. The following is a description of some, but not an exclusive summary, of the different types of reaction vessels which may be utilized in the process of the present invention.
Referring to
A second reaction vessel which may be utilized in the process of the present invention is a high temperature natural gas fired rotary kiln preceded in series by a lower temperature kiln of similar design using the off gasses from the hotter kiln to preheat the pellet feed.
A third type of reaction vessel which may be utilized in the process of the present invention is illustrated in
A forth type of reaction vessel which may be utilized in the process of the present invention is a moving metal conveyor belt which passes through a sealed muffle furnace as illustrated in
The vertical moving bed reactor is flexible and the very latest installations can use either natural gas or reformed natural gas. However, most existing DRI plants have gas reformers. Natural gas is basically methane, CH4, whereas steam reformed natural gas is primarily H2 plus CO. The static bed batch process and belt options require reformed gas. The reformed gas has free hydrogen plus carbon monoxide and hydrogen is a much more effective reducing gas than is carbon monoxide.
The existing or prior art processes used to produce chromium iron alloys from chromite use large quantities of electricity and carbon containing reductants. The Submerged Arc Furnace or SAF is the standard method for producing ferro chrome alloys at this time. This process is energy inefficient and produces large quantities of off gas which need to be captured, cleaned and eventually emitted to the atmosphere. Substantial quantities of carbon dioxide are also discharged. This process produces a liquid metal as the chrome iron alloy and a large quantity of chrome containing slag with no beneficial use. This has to be land filled. By comparison, the natural gas base solid state process described hereinbefore emits no off gasses to the atmosphere. The water produced is condensed to liquid water with a level of purity close to that of potable water. Carbon dioxide is the only other gas produced as a byproduct of the reduction reactions. This is collected, compressed and sold to industrial users.
The overall energy consumption for the gas based process of the present invention is estimated to be approximately 1/3 of the SAF process and this is shown in the equivalent fossil fuel requirements for the existing processes and that projected for the present invention in the chart of
Also the land footprint is much lower for the gas based process of the present invention than for the SAF process, and no provision is required for the landfill of slag.
The process outlined hereinbefore has been shown to produce metalization levels of chromium and iron of 80% or more. Higher metallization rates for both chromium and iron can be expected with process development. The resulting pellets of reduced chromite are directly suitable for stainless and alloy steel making, either as batch or continually charged components of the steel making charge.
The reduced chrome iron alloy can easily be separated from the unreduced gangue compounds by established industrial processes using the differences in density or magnetic properties, thus producing a highly desirable metallic component of a steel making charge, particularly to an Argon Oxygen Decarburisation vessel. The unreduced gangue may be used as an inert filler or in the production of building brick or block and as a sand substitute on roofing shingles.
Substantial cost advantages are experienced when compared to the usage of conventionally produced ferro chrome alloys. The carbon content of the reduced chromite is considerably lower than the ferro alloys produced in an SAF, which are normally saturated with carbon. This will result in significant process advantages for the steel maker and therefore lower the cost of production. The reduced chromite pellets can form part of the charge of the conventional SAF furnace producing ferro chrome, also with significant cost benefits.
The process of the present invention clearly demonstrates the following advantages.
1. The need for the installation of a capital intensive smelting step is eliminated.
2. An intermediate process which upgrades the ore to a saleable intermediate product is viable.
3. The process of the present invention has lower capital requirements than that of charge chrome smelting.
4, The process of the present invention effectively utilizes the substantial cost and environmental benefits of natural gas for energy.
5. The need for subsidized electrical energy is eliminated.
6. The operating costs for the process of the present invention are significantly lower than those involving smelting as a primary method of upgrading.
7. Pollution is greatly reduced.
This application claims the benefit of U.S. Provisional Patent Application No. 61/893,400, filed on 21 Oct. 2013, for PRODUCTION OF CHROMIUM IRON ALLOYS DIRECTLY FROM CHROMITE ORE, the entire contents of which are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US14/54644 | 9/9/2014 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61893400 | Oct 2013 | US |
