FI88622C - Process for thermal reduction of agglomerated metallurgical materials - Google Patents

Description

1 88622

This invention relates to solid state thermal reduction of agglomerated metallurgical feedstocks, and more particularly to agglomerated metallurgical feedstocks. thia-agglomerating.

More specifically, but not exclusively, the invention relates to the reduction of chromium oxides, iron oxides such as hematite, and mixtures thereof, agglomerated feed materials containing manganese oxide, vanadium oxide, or any other carbon thermally reducible oxide, preferably using a carbonaceous reducing agent, in the form of vegetable or animal charcoal or other carbonaceous reducing agent 20, in a step commonly referred to as pre-reduction, prior to melting and separating the slag and metal in a suitable furnace.

In the solid state reduction of the agglomerated finely divided oxide material to be reduced, this agglomerated material, together with the carbonaceous reducing agent and any necessary fluxes and other additives, is exposed to the flue gases of the combustion process to heat and / or reduce them. Such flue gases contain varying proportions of carbon monoxide and carbon dioxide and may even contain free oxygen. The composition of the gas can seriously adversely affect the efficiency of the reduction reaction.

In particular, oxidation of the carbon reducing agent inside the pellets may occur as a result of the reaction with 2,386,2 carbon dioxide contained in the process gases, whereby carbon monoxide is formed in the so-called Boudouard reaction.

5 C (pellet) + CO 2 - 2 CO

(process gases) There are two consequences of this reaction. On the one hand, the removal of carbon from the pellets reduces the amount of potential-reducing agent available for pre-reduction and metallization of the chromium and iron sludge in chromite or iron oxide in the pellet, and on the other hand incompletely burned gas (carbon monoxide) is formed. free and thermochemical vessel for energy recovery or disposal with waste.

In addition, back oxidation of metallized ferrochrome may occur when conditions become less reducing in the pellet upon carbon oxidation. Oxidized ferrochrome is often identified in the microstructure by the presence of sesquioxide strips (Al 2 O 3 * Cr 2 O 3) alongside the remaining metal beads.

It is an object of the present invention to provide a method by which the harmful effects of such a Boudouard reaction between carbon gases and process gases in close contact with agglomerated material can be reduced at least to a measurable extent.

The invention relates to a process for the reduction of an agglomerated metallurgical feed material in a solid state, which material consists of a mixture of a comminuted material to be reduced and a comminuted solid reducing agent. The process is characterized in that the agglomerated units of material are coated with a layer of material selected 3 38622 so as to at least prevent contact between the components of the agglomerate and any component of the heating gases used to heat it which may react adversely with it. the material forming said layer is comminuted metal, alloy, metal carbide, alloy-carbide, or a mixture thereof, wherein said layer is substantially metallic in nature.

According to further features of the present invention, the reduction is a pre-reduction step carried out at a temperature of 1200 to 1500, preferably about 1300 ° C; the agglomerate is in any suitable form, such as pellets, briquettes and the like; the material to be reduced is a fine oxide of chromium, iron or both, in particular chromite-15 ore; the reducing agent is a carbonaceous reducing agent, in particular anthracite, coal, coke or animal or vegetable coal; the agglomerate contains any necessary melting additives; the material covering the agglomerate units is either purely metallic in nature or contains at least 50% by weight of metallic material among the oxide coating material; and heating is performed in process gases containing: 0-100% of a mixture of CO 2 and CO, wherein the ratio of CO 2: CO 0 is at least 1: 1 and optionally 3: 1 or more; 0 0-100% air, oxygen or a mixture thereof; the remainder consisting of other combustion products, nitrogen and impurities.

It should be noted that existing methods are somewhat limited in this regard. Vier-30 furnaces are generally used with a CO2: CO2 ratio of about 2: 1 when. . while the shaft furnace is operated with a CO2: CO2 ratio of about 1:10.

The process of this invention can in turn be used in a CO2: CO ratio of up to 3: 1 or even 100: 1. This allows the use of process gas containing large amounts of CO2 in the presence or slime of excess oxygen (excess air) of € 4 to 622, and allows complete combustion of the heating furnace while protecting the reducing agent.

The metal material is preferably ferrochrome, Kro-5 myrrh alloy or other iron alloy or a mixture of ferrochrome, iron-chromium alloy or other iron alloy and an oxide coating material such as andalusite. Many of these materials are conveniently available in suitable inexpensive forms; for example, ferrochrome waste 10 is generated as a normal product in ferrochrome smelting processes.

The material used to form the coating is usually finely divided and can be mixed with a suitable binder before forming 15 layers on the surface of the agglomerate units. Such a layer can be conveniently formed, for example, by pneumatically conveying the finely divided material and spraying it on the surface of the agglomerate, or simply by adding the coating material to a conventional pelletizer 20 together with preformed pellets, the agglomerate being in the form of pellets. Coatings can be formed by any convenient means, for example, by immersing the agglomerate in a slurry or by applying a dusty coating to a surface.

The thickness of the coating layer is preferably at most about 1.5 mm and most preferably at most about 1 mm. In any case, the amount of coating material is usually at most 50% by weight of the total weight of the coated pellet, and is usually of the order of 15-20%, preferably about 10%.

In the case of preformed agglomerates containing chromite and a carbonaceous reducing agent (such as anthracite) and fluxes, studies to date have shown that the preferred coating materials are finely divided ferrochromes mixed with a suitable binder, or mixtures thereof with an oxide material, namely refractory oxides and andallucite, to which a binder, preferably bentonite clay, has been incorporated.

5 Other quinox materials could be used.

Such materials include sheet-like alumina, portland cement, spent aluminosilicate brick, and refractory aluminosilicate cements.

Experiments to date have shown that the coating material mixture must be carefully selected because some materials perform better than others and some are difficult to form a uniform layer on agglomerate units free of excessive cracking or peeling or other adverse phenomena during agglomerate-15 treatment, such as melting.

However, in the case of purely metallic coatings, such as ferrochrome, there are generally no such difficulties.

To demonstrate the operation of the present invention, the results of a few selected experiments are presented below with reference to the accompanying drawings.

In the drawings, Figures 1 and 2 are both curves illustrating the metallization of iron (Fe) and chromium (Cr) separately when a pelletized chromite feed material without a coating of the present invention is treated for 180 min at 1300 ° C in a gas atmosphere consisting of carbon dioxide and carbon monoxide in varying proportions; Fig. 3 is a similar graph showing the coated pellets of the present invention being heated in the presence of the carbon monoxide and carbon dioxide portions of Fig. 2; and Figure 4 is a graph depicting the metallization (%) of pellets containing a mixture of hematite and anthracite coated with ferrochrome as a function of temperature when heated for 180 min in a 100% CO 2 atmosphere.

Initially, experiments were performed with two slightly different compositions containing chromite ore and a fine anthracite reducing agent in which the particle size of the chromite, anthracite and fluxes was such that about 75% passed through a 74 μm (i.e. 200 mesh) sieve. These two compositions are detailed in Tau-10, Chapter 1, and have been found to contain granite and fluorine slag as fluxes, which have been found to promote chromite reduction and are described in detail in our Patent No. 87/5774, entitled "Method for Promoting Chromite Reduction." .

However, these pellets do not necessarily have to contain fluxes for the efficient operation of the coatings.

The compositions were mixed by rotating them in plastic drums with light balls. The pellets were then formed on a plate pelletizer by adding a specified amount of water, followed by air drying for 2-12 hours and drying in an oven at 110 ° C for about 12 hours.

7 88622

table 1

Pellet compositions (in weight percentages and proportions in compositions)

Composition 1 Composition 2 5% (parts)% (parts)

Moinoyl chromite 65.4 (100) 56.6 (100)

Grinaker anthracite 19.6 (30) 28.3 (50)

Granite 9.8) 8.5)

Fluorine feldspar 3.3 J (20) 2.8 j (20) - * - 0 Bentonite 1.9 3.8 + moisture (%) 10.1 11.8

Note. Figures in parentheses Indicate the proportion of anthracite to total chromite in the fluxes (100) The uncoated pellets were then isothermally heated in various gas mixtures of carbon monoxide and carbon dioxide at 1300 ° C, and the process was monitored with an on-line data logger. The different gas mixtures used are as follows: · ... CO (% by volume) 100 95 85 75 50 25 0 'CO2 (% by volume) 0 5 15 25 50 75 100 and the results are shown graphically in Figures 1 and 2 for for two pellet compositions after heating for 180 min in the aforementioned gas atmospheres. It has been found that in both cases, metallization of both iron and chromium was extremely low in gas compositions containing less than about 60-70% carbon monoxide (i.e., more than about 30-40% carbon dioxide) compared to the case where a carbon monoxide content of 100% was used.

The pellets of composition 1 described above were then coated with two different materials to test the operation of this invention. The following materials were investigated as coating layers: (i) 98 wt% pure fine ferrochrome (75% <74 μm) and 2 wt% bentonite binder; and (ii) 56 wt% pure fine FeCl 2, 38 wt% andalusite and 6 wt% bentonite clay binder.

All coated pellets were made from the same batch of pellets of conventional composition 1, which were classified to a grain size of approximately 9 mm to minimize variations in coating thickness.

The coating materials were premixed when practicable and gradually added to the graded preformed pellets on a rotating pelletizing plate. A small amount of water (or sodium silicate) was added to rewet the surfaces of the pellets before adding the coating material.

The amount of coating material to be added was chosen so that the thickness of the coating was about 1 mm. The pellets were then dried at 110 ° C for about 12 hours. In the case of layer (i) consisting essentially of ferrochrome alone, the coating accounted for about 30% by weight of the dry coated pellets, while for the alloy coating layer (ii), the coating accounted for about 36% of the dry coated pellets.

The coated pellets were then heated in the same manner as the uncoated pellets at 1300 ° C, and the compositions were determined after heating for 180 min. The coated pellets were also heated in air.

Suitable coating layers were obtained from each coating material, namely 30 ferrochrome and a mixture of ferrochrome and andalusite, in terms of their physical integrity and metallization of chromium and iron, as shown in Figure 3. (The percentage of metal bonding of Cr and Fe was calculated as follows, taking into account the Fe and Cr content of the coating): 9 88622

Metallization and mass balance calculations (CL X FM / ΙΛ) - (IM X B)

Metal alloy of Cr - x 100 (E)

hours (%) IM x A

5 (FL X FM / LM) - (IM X D)

Metallization of Fe = --—— x 100 (F)

tunin (%) IM X C

(CLxFM / LM) - (IMXB) + (FLxFM / LM) - (IMxD)

Total metal = ---- Xl 00 ringing (%) (IM X A) + (IM X C)

CR X RM X 0.01 X FM / LM

^ ..... = - X100 (G)

Cr yield 3 aan- tm v a

At 15% (%) x A

IR X RM X 0.01 X FM / LM

"^ = - X100 (H)

Fe yield in distribution (%) 20

Deviation from the mass balance of the closed loop: ACr (%) = [E + G] - 100 AFe (%) = [F + H] - 100 25 "... where CL = mass of Cr (metal) dissolved in the extraction solution (g) FL = mass (g) of Fe (metal) dissolved in the extraction solution FM = final mass of the sample 30 LM = mass of the sample used for extraction '* IM = initial mass of the sample (g);: RM = mass of the residue (g) CR = mass of the residue Cr- content (%) IR = Fe content of the residue (%) 35 A = mass of Cr from chromite of the base pellet / g of coated pellet (g) 10 88622 B = mass of Cr from FeCr or alloy coating / g of coated pellet (g) C = mass of Fe from the base pellet chromite / g of coated pellet (g) 5 D = mass of Fe from FeCr or alloy coating / g of coated pellet (g)

It should be noted that in this method it was assumed that only a negligible part of the FeCr coating oxidizes and remains in the residue. This assumption is justified in an atmosphere containing up to 100% 10 CO2 by SEM (scanning electron microscopy) analysis, which showed that only the outer layer of FeCr material up to 50-100 μm thick is oxidized to a thickness of 1 mm from the coating.

It has been found that in the case of a ferrochrome coating, about 90% of the iron and 73% of the chromium are metallized in an atmosphere containing up to 100% carbon dioxide. Compared to the results obtained with uncoated pellets, the metallization of chromium is only slightly less than in an atmosphere containing 10% carbon monoxide. The metallization of iron was almost independent of the composition of the gas.

Experiments have further shown that reducing the nominal particle size of both the base pellets and the coating from that used (75% <74 μm) does not significantly increase the achievable final degree of metallization.

In the case of a coating made of a mixture of ferrochrome and andalusite, it has been found that the degree of metallization depended to some extent more on the composition of the gas than in the case of ferrochrome-coated pellets. The metallization of iron, as in the case of ferrochrome-coated pellets, is largely completely independent of the ratio between carbon monoxide and carbon dioxide.

Thus, it has been found that extremely advantageous results can be obtained using this invention, but the coating material must be carefully selected.

In general, experiments performed to date show that metal coatings are more effective than coatings based on mixtures of metals and oxides in preventing carbon dioxide from entering the pellet core. Ferrochrome-coated pellets appear to rapidly form a chromium oxide-based outer film, which - although only 25-50 μm thick - plays an important role in protecting the pellets from atmospheric carbon dioxide. Once the protective film is formed, metallization can continue inside the pellet using 100% carbon dioxide gas compositions or even in the presence of elemental oxygen. However, carbon monoxide gas, which is a product in the carbothermal reduction of chromite ore, can escape through this protective layer and may return to CCl4 with the introduction of air as a source of additional oxygen, increasing the thermal energy available to the process (i.e. reducing the need to import this energy from another source).

Additional experiments have shown that ferrochrome coatings are effective even in the presence of free oxygen in the furnace atmosphere. When the ferrochrome-coated pellets of Composition 1 were heated in air at 1300 ° C for 2 hours, 92%: 25% metallization of iron and 70% chromium was achieved. Results obtained after 180 min and 120 min (dashed line) - shown in Figure 3 on the left.

Experiments on the reduction of iron oxide have been carried out at temperatures of 800 to 1000 ° C by coating hematite and anthracite-based pellets which do not contain:. ferrochrome and heating in an atmosphere containing 10% CCl 2. The composition of the pellets is shown in Table 2, and the metallization of iron (Fe) in Figure 4. The latter shows that 85% of Fe 35 metallized after 180 min treatment at 12 88622 800 ° C, while 100% Fe metallized after 180 min treatment at 1000 ° C C.

Table 2

FeCr-coated hematite pellet composition 5 Hematite 71.6 1) The whole material was classified as 75% 74yum

Anthracite 26.4

Bentonite 2.0 2) Addition of coating = 30 m-% of dry coated pellets 10 Coating

The present invention thus provides a very efficient means for use in the solid state reduction of pelletized feed materials mixed with a solid reducing agent, thereby preventing, at least to a measurable extent, the contact between such 4 * solid reducing agent and the gases used to heat it. in 20 reduction processes performed in the state.

In addition, the usable free energy contained in hot oxidising gases and the chemical energy contained in combustible gases, which have hitherto not been able to be used to heat and reduce oxide materials, can now be exploited and complete combustion of fuels (gaseous, liquid or solid) in fully oxidized form, in which case all available combustion energy is utilized. Even the presence of free oxygen in the combustion gases should not lead to a significant reduction in reduction when protected by such metal or alloy coatings.

An additional advantage of such coatings is that they can prevent the pellets from sticking and improve the physical strength of the agglomerate (e.g., a six-fold improvement in result when using a conventional drop test).

i

Claims (19)

A method for reducing agglomerated metallurgical feedstock in solid state, which consists of a mixture of a finely divided material to be reduced, and a finely divided solid reducing agent, characterized in that the agglomerated material units are coated with a material layer which is selected to see that it at least prevents contact between the components of the agglomerate and any such component in the heating gases used for heating the same, which may react adversely with the same, the material forming said layer , is a finely divided metal, an alloy, a metal carbide, an alloy carbide or a mixture thereof, said layer being inherently metallic. 2. A process according to claim 1, characterized in that the reduction is a pre-reduction step and the heating is carried out at a temperature of 1200-100 ° C. Process according to claim 2, characterized in that the heating is carried out at a temperature of about 1300 ° C. 4. A process according to any one of claims 1-3, characterized in that the heating is carried out in process gases containing: 0-100% of a mixture of CO; and CO, where the ratio CO,: CO is at least 1: 1 and optionally 3: 1 or even. major; 0-100% air, oxygen or a mixture thereof; the remaining part consisting of other combustion products, nitrogen and pollutants. Process according to claim 4, characterized in that the process gases are obtained by substantially completely combusting a heating fuel, possibly in the presence of an excess of air (oxygen excess). Process according to any of the preceding claims, characterized in that the agglomerated material units are pellets, briquettes or the like. Process according to any of the preceding claims, characterized in that the material to be reduced is formed together with the finely divided solid reducing agent and any other constituents, such as fluxing agents, agglomerated units which are dried, after which the formed agglomerated units are coated with a layer and the coating layer is dried prior to reduction. 15 Process according to any one of the preceding claims, characterized in that the finely divided material to be reduced is finely divided chromium or iron oxide or a mixture thereof and the finely divided solid reducing agent is a carbonaceous reducing agent. . 9. A process according to claim 8, characterized in that the material to be reduced is comminuted chromite ore and the comminuted reducing agent is comminuted coal, anthracite, coke or vegetable or animal koi. Process according to any of the preceding claims, characterized in that the coating layer is substantially completely metallic. 11. A process according to claim 10, characterized in that the coating layer consists substantially entirely of ferrochrome, a chromium-iron alloy or other ferrochromous alloy and a suitable binder. Process according to any of claims 1-9, characterized in that the coating layer 35 contains at least 50 m% (of the dry substance) metal mixed with oxide coating material. ie 88622 13. A method according to claim 12, characterized in that the ox coating material is andalusite. Process according to any of the preceding claims, characterized in that the finely divided material to be reduced and the solid reducing agent have such a particle size that about 75% passes a 74 µm screen. Process according to any of the preceding claims, characterized in that the material to be reduced and its solid reducing agent are mixed with flux. Method according to any of the preceding claims, characterized in that the thickness of the coating layer is not more than 1.5 mm. 17. A method according to claim 16, characterized in that the thickness of the coating layer is about 1 mm. 18. Process according to any of the preceding claims, characterized in that the proportion of material forming the coating is less than 50 m% of the coated agglomerate. Process according to claim 18, characterized in that the proportion of material forming the coating is less than 30 m% by weight of the coated agglomerate. in