Autogenous roasting of iron ore

Abstract: Iron ore concentrate is converted to magnetic gamma hematite in an autogenous roasting operation which is self-sustaining. The iron ore concentrate is preheated and contained magnetite is oxidized to hematite. Hematite is reduced to magnetite using carbon monoxide. After cooling, the magnetite is oxidatively exothermically converted to magnetic gamma hematite. The thermal energy resulting from the latter step is recycled to the preheating and reduction steps while thermal energy resulting from the cooling step also is recycled to those steps. The magnetic gamma hematite may be subjected to magnetic separation to produce a very low silica high purity iron oxide concentrate, which may be blended with high silica concentrate to provide a pellet feed for making blast furnace feed pellets.

Description: This invention relates to the roasting of iron ore and particularly to the thermal conversion of iron ore to gamma hematite by an autogenous roasting process. When iron ores are roasted at temperatures above about 815°C (1500°F), the magnetite mineral contained in the ore oxidizes rapidly enough to act as a significant source of heat for the process. The fuel value of magnetite burned in this way is about 3175 BTU/kg (7000 BTU/lb). When magnetite is burned, hematite is produced. Hematite, naturally-occurring or produced from magnetite, can be reduced to artificial magnetite, using hot carbon monoxide as reducing agent. When conditions are properly controlled, a small amount of heat is generated in the conversion process. Artificial magnetite can be burned by oxidation at low temperatures to produce magnetic gamma hematite. In this latter reaction, the exothermic heat produced is so substantial that the overall three-step process can be made self-sustaining. US-A-2693409 discloses a process for the thermal conversion of iron ore to magnetic gamma hematite having the features of the preamble of claim 1. According to a first aspect of the present invention, there is provided a process for the thermal conversion of iron ore to magnetic gamma hematite, comprising the steps of: (a) heating an iron ore concentrate feed; (b) reducing hematite contained in the concentrate produced in step (a) to magnetite; and (c) oxidising magnetite in the concentrate produced in step (a) to magnetic gamma hematite, heat from step (c) being employed in step (a) such that, after being brought up to operating temperature and steady operating conditions, the process is effected in an autogenous closed cycle of thermal energy which is self-sustaining, characterised in that, in step (a), the iron ore concentrate feed is heated to oxidise magnetite therein to hematite and in that , between steps (b) and (c), there is an additional step of cooling the concentrate to a lower temperature, the heat from the additional step also being employed in step (a) with the heat from step (c). According to a second aspect of the present invention, there is provided a process for forming pelletised iron ore concentrate for feed to a blast furnace, characterised by: (a) subjecting a portion of a first iron ore concentrate containing hematite and magnetite and having an iron content of at least 60 wt% and a silica content of at least 3 wt% to the process of any preceding claim to convert hematite and magnetite to magnetic gamma hematite; (b) magnetically concentrating the magnetic gamma hematite to form a second iron ore concentrate having an iron ore content greater than 99 % and containing less than 0.5 wt% silica; (c) blending the remainder of the first iron ore concentrate with the second iron ore concentrate to form a blended iron ore concentrate as pelletiser feed; and (d) pelletising the blended iron ore concentrate. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made by way of example only, to the accompanying drawings, in which: Figure 1 is a schematic illustration of one embodiment of the autogenous roast process in accordance with the invention; Figure 2 is a schematic illustration of another embodiment of the autogenous roast process in accordance with the invention; Figure 3 is a schematic illustration of a further embodiment of the autogenous roast process in accordance with the invention; Figure 4 is a sectional view showing details of the heating section of the apparatus of Figure 3; Figure 5 illustrates in graphical form the process cycle effected during an autogenous roast process in accordance with the invention; Figure 6 contains thermal expansion curves for various substances of interest; Figure 7 is a schematic representation of an alternative form of roaster provided in accordance with a further embodiment of the invention; and Figure 8 is a schematic representation of a magnetic concentrator. The autogenous roasting process of the first aspect of the invention needs initial thermal energy to start it, but once started and operating temperature and steady state conditions have been established, the thermal energy generation achieved within the process enables a self-sustaining process to be provided. The richer the iron ore feed to the process is in iron content, the easier are establishment and control of .. the reactions. The initial thermal energy to start the process may be provided by electric elements, microwave energy, or coke or fuel furnaces. A feed iron content (acid soluble iron) of more than about 40%, usually more than about 50%, in the iron ore concentrate usually is required for an effective process. The mixed metamorphosed magnetite/hematite iron ores of the Labrador Trough (Canada) are particularly useful feeds for the process. High purity concentrates (i.e. 99%+) have been produced from the spiral concentrates of past and present operating mines by using the autogenous roast process of the first aspect of invention, followed by magnetic concentration of the product. The violent shattering of mineral particles by an approximately 10% increase in volume accompanying the conversion of porous artificial magnetite to magnetic gamma hematite is a basic reason for the excellent results obtained by magnetically concentrating the roasted product, as described in more detail below. It has been found difficult to control the process in shaft furnace and high temperature kiln equipment. A new approach, using a three stage rotary cooler to utilize the exothermic heat generated, and to control the violent oxidation of the artificial magnetite to magnetic gamma hematite forms an aspect of the invention (see Figures 2 and 7). The autogenous roasting of iron ores in accordance with the first aspect of the present invention requires three distinct operations, as illustrated schematically in Figure 1. The first operation (Step 1 - Figure 1) involves preheating the iron ore and reducing the hematite content to artificial magnetite, preferably at less than about 750°C with a reducing gas rich in carbon monoxide, in accordance with the equation: Hematite (Fe 2 O 3 ) + CO → Magnetite (Fe 3 O 4 ) + CO 2 For iron ores with relatively low contents of magnetite compared to hematite, any magnetite present in the ore fed to the first operation is not affected by this reduction step, provided that the temperature used is not above about 750°C. At higher temperatures, magnetite shrinks enough to become a denser, less reactive material, which is undesirable. For ores containing higher ratios of magnetite, it may be desirable to use a preheating unit (see Figure 2). The artificial magnetite produced by this first operation is porous and reactive. When the carbon monoxide content of the hot gas used to effect the reduction is over about 65% by volume, a small amount of heat is generated by the reduction reaction, sufficient to sustain the reaction. Generally, the gas ratio of CO:CO 2 is at least about 60:40 by volume. The hot mixture of natural and artificially-reduced magnetite must then be cooled, preferably to less than about 400°C (Step 2 - Figure 1) in an inert or reducing gas atmosphere, to prepare the mixture for the final oxidation step. The heat recovered from this cooling step is used to help maintain the temperature in the first reduction step. Following such cooling operation, preferably by supplying cold air at a carefully controlled rate, all of the magnetite present in the cooled mass is oxidised to magnetic gamma hematite. The temperature at this stage is preferably below 400°C, more preferably about 350°C. The artificial magnetite-is very porous and so reactive that efficient cooling must be supplied to keep the oxidation reaction temperature below about 400°C. The reaction involved (Step 3 - Figure 1) is represented by the equation: Magnetite (Fe 3 O 4 ) + Air (O 2 ) → Gamma Hematite (Fe 3 O 4 ) + Nitrogen (N 2 ) The heated gas resulting from this cooling step is used to help maintain the temperature in the first reduction step. The process of the first aspect of the invention may be carried out in separate rotating coolers for each step, as illustrated in and described below with reference to Figure 2 for high magnetite ratio ores. Alternatively, a single unit can be used, with provision for separating the different atmospheres, and recycling the hot gases to the first preheat and reduction steps, as illustrated in and described below with reference to Figures 3 and 4. A rotary cooler is an externally heated or cooled high temperature metal alloy tube. Process temperatures are relatively low at about 700°C maximum. Alloys resistant to oxidation, carburization and sulphur, at about 700°C, such as Monel metal and Fahralloy (35 Cr/15 Ni), are suitable as materials of construction. In embodiments using rotary coolers, external electric heating of the reduction gas keeps gas volume and velocity low. Only reaction gases are located within the cooler. The lifters shown in Figure 4 give excellent contact of gases with the fine concentrate charge within the rotary coolers. Referring more specifically to the drawings, Figure 1 illustrates schematically one embodiment of the autogenous roast process 10 in accordance with the invention. As seen therein, a concentrate feed containing magnetite and hematite is fed by line 12 to a first step oxidation-reduction reactor 14 wherein the concentrate feed is initially preheated by hot air recycled by line 16 and by line 18 while the magnetite content of the concentrate feed is converted to hematite. The thermal energy generated by this oxidation along with that recycled is sufficient to maintain the succeeding reduction operation. An exhaust air stream is vented from the reactor 14 by line 20. The heated concentrate then is reduced with carbon monoxide fed to the reactor 14 by line 22 to convert hematite to magnetite. The reduced concentrate, in which the iron values comprise magnetite, is forwarded by line 24 to a cooling chamber 26, wherein the hot concentrate is cooled to a lower temperature in a neutral or reducing gas atmosphere. An ambient temperature air stream cools the outside of the cooling chamber 26. Hot air resulting from the cooling operation is forwarded by line 18 to the reactor 14. The cooled concentrate is forwarded by line 30 to a third step oxidation reactor 32 wherein the magnetite is oxidized to gamma hematite and cooled by ambient air fed by line 34. Nitrogen remaining after removal of oxygen from the air in the oxidation step is forwarded by line 16 to the cooling chamber 26 and to the first stage reactor 14. The product gamma hematite concentrate is removed by line 36 from the third stage reactor 32. Typical operating temperatures for the various stages and gas streams are given in Figure 1. In Figure 2, there is shown an alternative autogenous roasting procedure for high magnetite ores in which rotary coolers 1, 2 and 3 are employed at various stages of operation. The operations which are effected are the same as those described above with respect to Figure 1. Figure 3 illustrates a further autogenous roasting procedure. In this case, an integrated structure 100 is provided in which the operations are effected in contiguous regions of the roaster. The roaster is equipped with electric heating elements to provide the initial energy to bring the system up to the required autogenous roasting temperature. Figure 4 is a sectional view of the first stage of the roaster 100 of Figure 3, showing a rotating metal tube 102 in which the procedures are effected along with lifters 104 having an effect similar to that obtained in a fluidized bed. To illustrate in more detail the process cycle employed in the process of the first aspect of the invention, the sequence of events now is described with reference to Figure 5 as a specific illustration of the process of the invention. As a mixed magnetite hematite spiral concentrate is heated in air, the contained magnetite is oxidized to hematite. This reaction provides a significant source of heat to the process. Magnetite starts to oxidize at a significant rate at about 650°C. The material, which is now all hematite, is contacted by a mixed carbon monoxide/carbon dioxide gas, provided by reformers or by burning of coke. Reduction of the hematite to artificial magnetite at less than 700°C results in a porous very reactive magnetite structure. This magnetite then is cooled in a neutral or reducing atmosphere to less than 400°C. Oxidation of the artificial magnetite provides a significant amount of heat to the whole process, allowing it to become autogenous, requiring no external heat when this stage is reached. In Figure 5, a curve (shown by the dashed line) has been superimposed showing the stages at which shattering of mixed grains and phase changes in quartz contribute to a mechanical shattering of the mineral grains. Conversion of the artificial magnetite to magnetic gamma hematite is accompanied by a 10.6% increase in volume which gives rise to very effective shattering. Heating iron ore concentrate grains shatters some grains containing minerals having different thermal expansion rates. Quartz is a common constituent of mixed iron ore concentrate grains. Phase inversion of quartz at 572°C gives a volume expansion differential of about 4% compared to magnetite. At the conversion temperature of magnetite to gamma hematite, such mixed grains of iron ore and silica are shattered, producing popping sounds. The large differential expansion when magnetite is converted to gamma hematite is a basic reason for the success of superconcentration by magnetic concentration following the autogenous roasting method (see Figure 6). A sensitive directional microphone with noise filter can pick up and record the "pop rate" within the rotary coolers. Pop rate recorders on the first reduction stage and the third oxidation stage (see Figure 5) can provide assistance in process control. If the pop rate changest temperature or gas rate can be automatically controlled to achieve the desired rate. An overall heat balance has been calculated for an initial spiral concentrate at 65% iron and a ratio of 60% magnetite/40% hematite, roasted at 800°C (1500°F) as shown in the following Table I: The heat available for the process, arising from the noted operations, exceeds the heat requirements of the process, so that the process can be self-sustaining with respect to heat requirements, if heat losses are less than about 25%. The relatively coarse high purity product produced by this procedure may be used in a direct steel-making process as described in the applicant's EP-A-0551217. Briefly, the high purity product is laid down on a gas-permeable bed through which reducing gas is blown at high temperature to produce a porous hot steel cake, which can be hot rolled at one pass to make steel sheet. One useful application of the first aspect of the present invention is the production of low silica concentrates from operating iron mines, such as those in the Labrador Trough. The producing deposits mine iron ore generally containing less than about 40% iron. This material usually is ground to less than 10 mesh particle size, concentrated and then fine ground and pelletized to form pellets suitable for blast furnace feed. Pellet specifications for blast furnace feed generally include a maximum silica content of 6 wt% and an iron content of over 65 wt%, i.e. about 92% of the purity of 100% iron oxide containing about 70% iron and 30% oxygen. Silica is required in the blast furnace to promote slag formation to dissolve and remove other impurities. Recent studies have indicated that decreasing the silica content of pellets below about 3 wt% leads to a significant increase in blast furnace production. The autogenous roast procedure of the first aspect of the present invention enables high purity concentrates above 99% purity and less than 0.5% silica, to be made from the current 92% pure iron concentrates containing about 6% silica. The resulting low silica concentrate can be blended with concentrate containing about 6 wt% silica to obtain a blend containing a desired lower silica content, preferably below about 3 wt% silica. By operating in this way, it is unnecessary to upgrade all the current 6% silica concentrate to produce a 3% silica pellet. This procedure may be used to form a blend of desired lower silica content from a concentrate containing any higher silica content, generally at least about 3 wt%. For example, blending 100 tons of 0.5% silica high purity (99%+) concentrate formed by the autogenous roasting process of the invention with 80 tons of 6% silica standard concentrate produces 180 tons of 2.9% silica pellet feed. Using the autogenous roasting procedure of the invention, approximately 110 tons of standard concentrate are required to make 100 tons of 0.5% silica high purity concentrate. Accordingly, about 60% of the standard pellet feed concentrate may be autogenously roasted by the process of the invention and magnetically concentrated to form the 99%+ purity blending material, while the remaining 40% of the standard concentrate is blended with the high purity material to make the low silica pellet feed. In current spiral concentrate flow sheets, rougher spirals reject a low iron tailing, resulting in a high iron recovery, medium iron content first concentrate at between 45 and 50% iron, which then is a suitable feed for an autogenous roast of some of the product, leading to an overall higher iron recovery for the flowsheet. EXAMPLE This Example illustrates the practical utility of the processes of the present invention in producing very low silica concentrates from concentrates from operating iron mines in the Labrador Trough. A standard iron concentrate from a Labrador Trough iron mine was processed as described below. The iron concentrate contained both magnetite and hematite and analyzed 66.07% Fe and 5.03% SiO 2 . The complete analysis of the concentrate is given below. An externally-heated rotary kiln alloy metal tube, 20 cm (8 inches) in diameter and 3 m (10 feet) long, was operated in batch mode using 11.4 kg (25 lb) samples using a mixed carbon monoxide and carbon dioxide gas stream for concentrate reduction and an argon gas stream for cooling. The samples were subjected to a cycle of operations, as follows: (a) oxidation of magnetite in the concentrate to hematite during heat up of the kiln to 650°C, (b) reduction of hematite to artificial magnetite by carbon monoxide at 650°C, (c) cooling of the reduced product in argon to 350°C, and (d) oxidation of the artificial magnetite to gamma hematite at 350°C. The resulting product then was subjected to magnetic separation (see Figure 8), which resulted in a high purity gamma hematite concentrate having a very low silica content and a tailings fraction rich in silica. The overall iron recovery in the concentrate from the feed was 92.52% and concentrate weight was 85.4 wt% of the initial feed to the rotary kiln. The analysis of the initial concentrate, final concentrate and tailings stream is set forth in the following Table II: Concentrate (wt%) Tailings (wt%) Initial Final Fe 66.07 71.45 34.6 SiO 2 5.03 0.45 52.4 Al 2 O 3 0.32 CaO 0.025 MgO 0.023 TiO 2 0.13 MnO 0.028 P 2 O 5 0.030 Na 2 O 0.004 K 2 O 0.013 Fe 3 O 4 1.03 Moisture 2.26

Claims: A process for the thermal conversion of iron ore to magnetic gamma hematite, comprising the steps of: (a) heating an iron ore concentrate feed; (b) reducing hematite contained in the concentrate produced in step (a) to magnetite; and (c) oxidising magnetite in the concentrate produced in step (a) to magnetic gamma hematite, heat from step (c) being employed in step (a) such that, after being brought up to operating temperature and steady operating conditions, the process is effected in an autogenous closed cycle of thermal energy which is self-sustaining, characterised in that , in step (a), the iron ore concentrate feed is heated to oxidise magnetite therein to hematite and in that , between steps (b) and (c), there is an additional step of cooling the concentrate to a lower temperature, the heat from the additional step also being employed in step (a) with the heat from step (c). A process as claimed in claim 1, wherein step (b) is carried out at a maximum temperature of 750°C using carbon monoxide, the lower temperature in the additional step is 400 °C, and step (c) is carried out at a temperature below 400 °C. A process as claimed in claim 2, wherein the carbon monoxide is employed in a gas mixture with carbon dioxide, the mixture having an initial volume ratio of CO:CO 2 of at least 60:40. A process as claimed in claim 1, 2 or 3, wherein the additional step is effected at least partially by conductance and radiation from a metal shell of a rotary cooler. A process as claimed in any preceding claim, wherein steps (a) and (c) are controlled by monitoring the rate of production of an audible sound caused by the shattering of particles of concentrate. A process as claimed in any preceding claim, further comprising cooling the magnetic gamma hematite to ambient temperature at least partially by conductance and radiation from a metal shell of a rotary cooler. A process as claimed in any preceding claim, further comprising magnetically concentrating the magnetic gamma hematite to produce a highly purified (>99%) iron oxide concentrate. A process for forming pelletised iron ore concentrate for feed to a blast furnace, characterised by: (a) subjecting a portion of a first iron ore concentrate containing hematite and magnetite and having an iron content of at least 60 wt% to the process of any preceding claim to convert hematite and magnetite to magnetic gamma hematite; (b) magnetically concentrating the magnetic gamma hematite to form a second iron ore concentrate having an iron ore content greater than 99 % and containing less than 0.5 wt% silica; (c) blending the remainder of the first iron ore concentrate with the second iron ore concentrate to form a blended iron ore concentrate as pelletiser feed; and (d) pelletising the blended iron ore concentrate. A process as claimed in claim 8, wherein the first iron ore concentrate has a silica content in the range of from 5 to 6 wt%, the blended iron ore concentrate produced in step (c) having a silica concentrate having a silica content below 3 wt%.

Inventors: CAVANAGH PATRICK E, CAVANAGH, PATRICK E.
Applicants: VIRGIN METALS CANADA LTD, VIRGIN METALS (CANADA) LIMITED
Application year: 1993
Main classification: C22B1
IPC codes: C22B1, C01G49