FI87237C - FOERFARANDE FOER REDUKTION AV KROMITMALM - Google Patents

Description

87237

The present invention relates to a process for the reduction of chromite ores and in particular to the reduction of these ores in the solid state.

Chromite is a solid spinel solution, the composition of which can be represented by the general formula (Fe, Mg) Cr, Al, Fe 2 O 2. The reduction of such a Spinell in the solid state using a carbonaceous material is complex and involves a high degree of metallization of the commonly present iron and a significantly lower reduction of chromium at a given time and temperature.

In the context of this description, the degree of metallization of iron or chromium after reduction is based on the ratio of the mass of iron or chromium present in the non-oxide form to the total mass of iron or chromium originally present in the ore in all its forms.

By way of example, in a mixture of finely divided solid chromite and carbon, the degree of metallization of iron is generally almost complete and of chromium just over 50% when the mixture is heated for two hours at 1300 ° C. This low reduction (or metallization) of chromium is presumably the result of the formation of stable magnesium, chromium, alumina-Spinel, which occurs during iron reduction and metallization. Apparently, the reduction rate increases with temperature, leading to improved metallization of chromium but at increased cost and bringing with it problems often associated with high temperatures, such as material softening and melting, which occur at temperatures above about 1500 ° C.

Such softening is an independent aspect of the process described herein by the Krupp process, in which the mixture of fine chromite, quartz and 35 carbon required for reduction is greatly heated to 1500-1600 ° C over a stoichiometric amount of 2,87237, whereby the chromium is more than 80% metallized. This process is described in more detail in Fried Krupp Gesellschaft1's South African Patent No. 84/0101 entitled "Process 5 for the Production of Ferrochromium".

Recent advances in solid state chromite reduction have shown that the reduction rate can be significantly increased by adding small amounts of sodium salts and fluorine slag to the mixture of chromium-10 and reducing agent. In more detail, the addition of 10% fluorine slag and the addition of 1% sodium salt result in an increase in chromium metallization from slightly below 50% to slightly below 100% when maintained at 1200 ° C for 90 minutes. The reduction under these conditions is so rapid that in practice the rate of reduction is determined by the rate of heat transfer to the pellets.

It is assumed that the components of the hitherto stable Spinell (see above) are soluble in the formed liquid calcium fluoride-sodium fluoride phase and that the non-reducing oxides precipitate as MgA 2 O takes place.

Although the rate of reduction of chromium can be increased up to 25 times, this method would not be economically viable due to the high cost of fluorine slag and sodium salts.

Thus, it is an object of the invention to provide a process for the reduction of chromite ores in a solid state, which process can provide a high degree of reduction of chromium oxide on an economically viable basis.

According to the invention, a process for the reduction of chromite ores in a solid state comprises heating finely divided chromite ore in a thorough mixture with a finely divided carbonaceous reducing agent and a flux, 3 87237 comprising at least one alkali metal aluminosilicate, quartz and calcium fluoride and the formation of a substantially liquid flux phase in contact with the carbonaceous reducing particles.

Furthermore, according to the invention, the carbonaceous reducing agent preferably comprises at least one of the following: coke, anthracite or vegetable, animal or bone char. Other carbon-10-containing reducing agents can also be used either alone or in combination with other forms of carbon-containing reducing agents. Even the use of sewage sludge is considered possible at least as part of a reducing agent.

Still further, according to the invention, the alkali metal aluminosilicate is a compound having a relatively low melting point when the alkali metal is potassium, sodium or calcium. Such a compound is preferably a naturally occurring compound in the form of feldspar, feldspar syenite or syenite. Feldspar can even be lithium-containing crude-20 feldspar, such as lepidolite.

Still further, according to the invention, calcium fluoride is a naturally occurring compound in the form of fluorine feldspar.

Still further, according to the invention, the alkali metal aluminosilicate and quartz are added in the form of a naturally occurring material, such as granite.

Still further, according to the invention, the carbonaceous reducing agent is present at least and preferably more than the stoichiometric amount necessary to effect the reduction of all chromium and iron oxides present in the chromite together with the associated small amounts of chromium and iron carbides and silicides at higher temperatures.

Still further, according to the invention, if feldspar is used, the ratio of feldspar to fluorine feldspar may vary from 0.5: 1 to 5: 1 (usually 1: 1 to 3: 1) and the ratio of feldspar to 4,87237 to quartz may vary from 0.5: 1 to 5: 1 (usually 1: 1 to 2: 1). The exact ratios will largely depend on the purpose to be achieved and the nature and composition of the starting materials. The flux mixture can be modified to minimize the decomposition of the hard-to-melt parts of the furnace while optimizing the metallization of the ore.

Still further, according to the invention, the alkali metal aluminosilicate, quartz and calcium fluoride together comprise between 5% and about 35% by weight of the mixture, preferably between 10% and 25% and most preferably about 15%.

Still further, according to the invention, the mixture is agglomerated before the reduction is carried out and the heating is carried out in a reducing atmosphere.

Technically, the mixture need not be agglomerated as long as it is and remains thoroughly mixed during the reduction process and as long as good contact of the internal particles is maintained. However, on the practical side, the homogeneity of the mixture is most easily maintained during the reduction by agglomerating it into pellets, briquettes or other agglomerated units.

Any suitable binder can be used for agglomeration, but when used alone or in combination with sodium silicate and bentonite binders, a good, strong agglomerate is best suited for the continuous process. The particular agglomerated composition also depends on whether curing of the agglomerate is necessary and practical before the agglomerated units can withstand the stresses applied to them during the process without breaking or collapsing.

It is also important that the different materials are sufficiently fine-grained to produce the best effect on the above-mentioned reduction process. In this regard, the materials are ground one by one prior to mixing so that the chromite is as fine as economically practical and preferably about 75% of it passes through a 75 micron screen.

In the figures, Figure 1 is a series of graphs depicting the reduction percentage as a function of time with four different Chromite mixtures, and a comparative experiment not using the invention; Fig. 2 is a series of graphs illustrating the effect of reduction at different temperatures; Fig. 3 is a series of graphs illustrating the effect of different compositions leading to different amounts of flux; Fig. 4 is a graph illustrating the individual metallization of iron and chromium during the reduction of typical chromite without the addition of a flux (prior art); Figure 5 is a graphical representation of individual composite metallizations using the present invention.

An experimental program was performed to verify the reduction behavior of four chromites from different sources when reduced by the method of this invention. These four chromites are labeled A, B, C, and D for convenience and their compositions are shown in Table 1.

25 TABLE 1

Average chemical analyzes of the four chromites tested

Composition by weight,%

Component Chromite Chromite Chromite Chromite 30 A B C D

Cr203 46.60 44.62 45.50 45.35

Fe 2 O 3 7.1 6.77 7.23 7.85

FeO 19.5 19.95 19.9 19.10

MgO 9.83 9.57 9.14 9.17 35 Al2 ° 3 13.8 14.53 14.50 14.60 6 87237 TABLE 1 (continued)

Component Chromite Chromite Chromite Chromite

A B C D

CaO 0.20 0.25 0.26 0.27 5 SiO 2 0.50 1.64 0.95 1.02

Ti02_0,53_0,54_0,57_0,52

Total components analyzed 98.06 97.87 98.02 97.88 10 Cr / Fe ratio_1.58_1.51_1.52_1.53

Samples of the four chromites were ground at approximately 75% to 75 pia and similarly mixed with a finely divided carbonaceous reducing agent in the form of anthracite or vegetable, charcoal or bone charcoal, the analyzes of which are shown in Table 2, in bentonite as a binder and melt in granite feldspar and quartz supplements required. The compositions of granite and fluorine feldspar are shown in Table 3 and the compositions of 20%, 20% and 30% flux additive mixtures are shown in Table 4.

TABLE 2

Chemical analysis of two reducing agents,% by weight Reducing agent Moisture Ash Volatile Non-volatile Sulfur substances carbon 25 Anthracite 1.3 12.9 8.0 77.8 1.4

Plant, wood or charcoal 1.6 17.8 3.4 77.3 0.4 TABLE 3 30 Chemical analysis of flux components, weight% Component Granite Fluorine slag

CaF2 87.6

SiO 2 71.86 5.9 Al 2 O 3 13.97 35 MgO 0.27 7 87237 TABLE 3 (continued)

Component Granite Fluorine Slate

CaO 1.23

Fe 2 O 3 1.32 5 Na 2 O 4.13 K „0 3.91 -Xj- - _96.69_93.5

In terms of mineral composition: granite approximately 75 percent feldspar 10 25 percent quartz TABLE 4

Individual component additions to different pellet batches Component Net component addition weight% 15 10% melt- 20% melt- 30% melting agent tusing agent

Chromite 70.1 65.33 61.25

Carbon 21.0 19.60 18.37 20 Granite 5.25 9.80 13.78

Fluorine feldspar 1.75 3.27 4.59

Bentonite 2.00 2.00 2.00

After thorough mixing, the mixtures were fed to pelle-25 and the pellets were air dried under forced ventilation for 24-36 hours and then brought to thermal equilibrium. An even smaller amount of moisture (1-2%) was evaporated in the initial blowing cycle when the sample temperatures reached above 120 ° C in the cool zone of the furnace under a nitrogen-air atmosphere. After the mass stabilized, the sample was raised to the hot zone of temperature equilibrium and reacted. The reaction temperatures used were 1200 ° C, 1300 ° C and 1400 ° C. The device was monitored by a device that recorded data on the line.

35 The data collected by the data acquisition device were processed 8 87237 separately to determine the actual mass of each run, making corrections to the measured sample mass loss figures due to loss of volatility of the carbonaceous reducing agent and possibly boiling off of the melt 5 and other mass loss. The final corrected mass loss associated with chromite reduction was then calculated as the percentage reduction of both chromium and iron per time.

After removal from thermal equilibrium, the reduced sample was examined in detail for pellet shape and agglomeration characteristics. Portions of the sample were then taken for microscopic examination and chemical analysis.

To determine the actual degree of metallization, the samples were extracted for 18-24 hours in 50% HCl solution and the resulting extract liquid and residue were analyzed to determine the proportion of soluble (metallized) chromium and iron. The mass balances obtained in this way were always found to be closely consistent with those expected from mass loss.

20 The results of the experiments can be summarized as follows:

Evidence of a significant improvement in the net reduction rate (i.e., combined iron and chromium) that was achievable with the addition of flux was observed. Figure 25 1 shows the percentage reduction per time for three ores reacted at 1300 ° C using 20% flux. At least 80% reduction of chromium and iron is achieved for 120 minutes after heating at this temperature. Also shown graphically is a similar reduction in solid state to similar pelletized materials, but without the addition of flux-forming ingredients. The improvement made by the invention will be quite clearly described.

The significant effect of temperature on the reaction kinetics using chromite A as an example is shown in Figure 2. This curve also shows the reduced scattering at the reduction rate with increasing temperature.

It will be readily understood that the standard reduction curve 5 is a combination comprising individual iron and chromium reduction curves, and to further determine the effect of various parameters, an analysis was performed based on the individual metallization rates of iron and chromium.

According to this, in order to determine the individual metallization rates 10, iron-chromium metallization envelopes had to be observed for the different sets of conditions tested. These curves show that temperature has the most significant effect on the shape of the metallization envelope (see Figures 4 and 5). In the narrow range of the tested compositions, the ore type was not found to have a significant effect. Accordingly, one set of metallization curves was found to be applicable to all four ores at 1200 ° C and 1300 ° C, while a slightly narrower set of curves was found to be applicable to all ores at 1400 ° C.

Examining the effect of different parameters on the metallization rates of iron and chromium, it appears that the chromium reduction rates of the four ores show a clear sequence A-D-B-C, supported by results with pelletized material showing best behavior of A and D with melt-25 additions of 20% to 30%. In each case, the larger difference in reduction rate may be mainly due to differences in the reduction rate of chromium ore.

A specific comparison of variables such as the type of reducing agent and the effect of melt addition was made using the time required to reach a certain degree of metallization of either iron or chromium as shown in Table 5, to illustrate the difference between these different factors.

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n 87237

An example of the curves obtained is shown in Figure 3, in which the reduction percentage of chromium in chromite A is plotted as a function of time using 10%, 20% and 30% flux at 1300 ° C.

The results shown in Table 5 show that raising the flux additive above 20% results in a small, relatively insignificant increase in the reduction rate, while reducing the flux additive from 20% to 10% has a more significant effect on the overall reduction rate. This decrease does not appear to decrease with increasing temperature, which is observed to occur for other effects, such as reduction types. However, the most significant effect of changing the flux additive was on the physical properties of the pellet, which effectively set the upper limit of the flux additive at about 20%. The amount of liquid phase produced above this level adversely affected the physical properties of the more digestible pellets, indicating the possibility of poor behavior under load, i. pellet-20 road adhesion.

Electron microscopic analysis of the reaction products using 20% flux revealed that some of the magnesium oxide and alumina in the chromite spinel (20% to 30%) remained insoluble after a reaction time of 1-2 hours.

Accordingly, it seems possible that the amount of melt phase could be reduced without significantly changing the reduction rate.

If the inert nitrogen ring was removed from the reducing samples, then allowing a slight increase in the oxygen content, both the highly reduced and relatively little reduced pallets were encapsulated in a ceramic glass layer 0.5 to 1 mm thick. Examination of this outer glass layer showed that it consisted of three phases of 35, with a low MgO content of slag α2 87237 containing slag oxide containing more than 60% recrystallized sesquioxide. Cr 2 Oj) strips and highly reduced recrystallized spinel (Mg (Cr.Ai) 2 ° 3 'containing approximately 6% Cr 2 O 3).

It is important to note that the experiments were performed in the absence of heating gases which are evolved during combustion, and the operation of such gases must be taken into account when performing the reduction in the presence of such gases, such as 10 ° C '.

The experiments described show the very advantageous results of the use of melting agents as defined in the reduction of chromite ores in the solid state. The ingredients of the fluxes are freely available as naturally occurring substances. It should be noted that depending on the composition of these materials, it may be necessary to supplement them with other substances. Thus, while granites normally provide the necessary feldspar and quartz, substances such as nepheline syenite can make quartz additions necessary.

It is an object of this patent that a melting agent as defined be used to improve the rate and extent of reduction in an apparatus in which material (preferably in agglomerated form) is treated under controlled atmospheric and temperature conditions to ensure the optimum amount of reduction. Such devices include a rolling oven, a rolling oven, sometimes called a pancake-type oven, and a vertical shaft oven. Such a device is preferably one in which the heat transfer rate to the reducing material is proportional to the reduction rate allowed by the use of a melting agent.

Figure 1: Curves showing the reduction percentage of different ores using fluxes compared to a typical ore without flux.

35 Figure 2: Curves showing the effect of temperature on the reduction of i3 i3 87237 type A ore using a flux.

Figure 3: Effect of flux additive modification at type B ore at 1300 ° C as a reducing agent on vegetable, animal or bone char.

Figure 4: Single metallization of iron and chromium during typical chromium reduction without addition of melt.

Figure 5: Reduction of chromite ore with 10% NaF-Ca 2 with the addition of flux.

Claims (12)

A process for reducing solid state chromite ores, characterized in that it comprises heating finely divided chromite ores in intimate admixture with a finely divided carbonaceous reducing agent and a flux comprising at least one alkali metal aluminum silicate, quartz and calcium fluoride. perature, and for a time selected to effect a substantially liquid flux phase in contact with nearby solid chromite particles and carbonaceous reducing agent particles. Process according to Claim 1, characterized in that the alkali metal aluminum silicate 15 is a naturally occurring compound selected from feldspar, feldspar syenite or syenite. 3. A process according to claim 2, characterized in that alkali metal aluminum silicate and quartz are added as a naturally occurring granite mixture. 4. A process according to any of the preceding patent claims, characterized in that calcium fluoride is added as naturally occurring fluorospate. Process according to any of the preceding patent claims, characterized in that the mixture contains feldspar and fluorospate, the ratio of feldspar to fluorospate being in the range 0.5: 1 - 5: 1. 6. A process according to claim 5, characterized in that the ratio of feldspar to 30 fluorospatter is in the range 1: 1 to 3: 1 and the ratio of feldspar to quartz is in the range 1: 1 to 2: 1. Process according to any of the preceding patent claims, characterized in that the combined mass of alkali metal aluminum silicate, quartz and calcium fluoride is between 5% and about 35%, preferably I; Between 10 and 25% of the total mass of the intimate mixture. Process according to any of the preceding patent claims, characterized in that the carbonaceous reducing agent is present in at least the stoichiometric amount necessary to effect reduction of all chromium and iron oxides present in the chromite together with the associated the emergence of a small amount of chromium and iron carbides and silicides. 10 9. A process according to any of the preceding claims, characterized in that the intimate mixture is agglomerated before being heated. Process according to claim 1, characterized in that the mixture is formed into carbon sinks or briquettes. Method according to any of the preceding patent claims, characterized in that the fineness is such that about 75% passes through a 75 µm screen. Process according to any of the preceding claims, characterized in that the heating is carried out at temperatures between 1200 ° C and 1500 ° C, preferably at about 1300 ° C.