CH376946A - Process for preparing coherent granules - Google Patents

Description

Process for the preparation of coherent granules The subject of the present invention is a process for the preparation of coherent granules intended, for example, for obtaining cast iron. The invention also relates to a coherent granule prepared by this process.

The process object of the invention for the preparation of coherent granules containing iron oxide and carbon in an amount greater than that which is necessary to completely reduce the iron oxide, in which a mixture of iron oxide and of finely divided carbon material in a proportion by weight of between 8:1 and 1:1 with 10 to 20% water is transformed in a rotating drum into granules which are heated to between 870 and 12500 C so that part of the said Iron oxide is reduced to iron, is characterized in that it is used as the finely divided carbonaceous material in the granules of non-coking coal and in that the granules are heated by igniting a bed of the latter and made therein pass a stream of air containing 16 to 26% oxygen, so that the coal is distilled by destruction and converted into a graphitic matrix which acts as a binding agent for the other constituents of the granules, while a part iron oxide is reduced to iron until 15 to 40% metallic iron is obtained in the granules, more than 6% carbon A still being contained in the granules.

The coherent granule prepared by the above process is characterized in that it contains 40 to 55% by weight of iron, of which 15 to 40% by weight of the total iron is in the elemental iron state, the consisting of iron oxide, the iron being in the state of separate particles and the carbon, in an amount constituting 6.5 to 32.5% by weight of the granule, being in the form of a graphitic matrix binding the other elements of the granule, which is compact, non-porous to the naked eye and porous under the microscope. The appended drawing represents, schematically and by way of example, an installation for implementing the claimed process.

fig. 1 is a schematic diagram of the successive processing operations; fig. 2 shows a variant of the heating installation which allows operation under different conditions.

Example <I>1</I> According to fig. 1, an iron ore charge (Benson's magnetite concentrates) is prepared in a mill 10 as a substantially dry powder. The composition and RTI ID="0001.0271" WI="13" HE="4" LX="1618" LY="1681"> grain size of the crushed ore are as follows

<I>Composition of ore</I> <U>Percentage (dry weight)</U> Total iron 61.6 Mn (dvalue) 1.0 SiO,, 7.3 A1.Ö, 3.7 Ca0 0, 16 Mg0 0.18 <B>T10:,</B> - <B>0.80</B> <I>Grain size distribution of crushed ore</I> Sieve mesh size, Percentage of <U>mm grains retained</U> <B>+0.25</B> 2.64 <B>-0.25</B> + 0.20 2.56 -0.20 -I- 0.149 9.24 -0.149 -f- 0.105 11.16 -0.105 +0.074 13.44 - 0.074 -1- 0.044 14.16 -0.044 46.8 Similarly, an crushes a coal from Book Cliff, said to be non-coking in the standards of commerce, in a crusher 11. The composition and the size of the grains of the crushed coal are as follows

<I>Composition of coal</I> Percentage (dry weight) Volatile matter 37.8 Fixed carbon 56.2 Ash 6.2 Sulfur 0.62 <I>Composition of coal ash</I> <U >Percentage (dry weight)</U> Ca0 0.48 Mg0 < 0.01 SiO. 1.7 A1203 2.7 <I>Grain size distribution of</I> <I>crushed hard coal</I> Sieve mesh size, Percentage of mm grains retained <B>+0.297</B > 0.0 - 0.297 -i- 0.210 0.08 -0.210 -I- 0.149 0.9 - 0.149 r- 0.105 18.25 -0.105 + 0.074 53.75 -0.074 27.02 Ort passes crushed ore particles and coal. grinders 10 and 11 in a mixer 12 in which they mix intimately. The throughput of the crushers 10 and 11 is adjusted so as to obtain the desired relative proportions of ore and hard coal, for example 60 parts fines ore and 40 parts fines hard coal by weight. A proportion of 10 to 20% water by weight and preferably about 15% is introduced into the mixer through a garden hose 50 so as to moisten the mixture and make it cohesive, but not to the 1st. wet. It has been found that with mixtures containing less than 10% water, granules of insufficient strength are obtained in the following processing operation. If the water content of the mixtures is greater than about 20%, the granules obtained are too wet and too soft for the next operation.

The contents of the mixer 12 are fed at a regulated rate to the upper end of a granulating drum 13, for example 1.22 m in diameter and 2.44 m in length, rotating around a shaft inclined slightly downwards towards its exit end, for example at an angle of 70 from the horizontal and rotating at a peripheral speed of about 67 m per minute. A sprinkler pipe 51 directs water spray into the pelletizing drum near its upper or loading end in a proportion of about 2% by weight of the ore fines mixture and of coal which arrives there so as to form granules. As the drum rotates, small agglomerated granules form at the start and roll on top of each other on the rest of the mixture coming from the mixer 12 and gradually grow in size forming balls or compact spheres of increasing size which roll towards the lower end. drum outlet 13.

The granules rolling out of the drum 13 pass over a double screen 14 in which the upper perforated surface 142 retains the granules with a diameter greater than 15.9 mm and passes them into a retaining channel 15. The perforated surface 142 lower 14b of the RTI sieve ID="0002.0264" WI="10" HE="4" LX="1242" LY="888"> retains granules with a diameter greater than 9.5 mm but less than 15.9 mm and allows smaller particles and fines of the mixture to pass onto a conveyor 16. 0.30 m, an inclination of 21° from the horizontal and have square mesh with an opening of 19.1 X 19.1 mm in the upper screen.

The fines which fall on the conveyor 16 arrive in a retaining channel 17, from where a conveyor 18 returns them to the loading end of the granulating drum 13, in which they can grow into granules of the desired size. The oversized granules retained by the upper screen of the double screen 14 and arriving in the retaining channel 15 are transported by a conveyor 19 to the loading end of the mixer 12, in which they are crushed and distributed. load of the pellet drum 13 is being prepared. The granules of the desired size from the double screen 14 fall on a conveyor 20 which brings them into a hopper 21.

When the granulating drum 13 and the double screen 14 are in operation, granules of the desired size of 9.5 to 15.9 mm are slowly formed within 20 to 30 minutes after the start of the granulating operation. During this time, a large proportion of the mixture of fines arriving in the pelletizing drum 13 is recirculated through the double screen 14 under the action of the conveyors 16 and 18 and the retaining channel 17. After 30 to 45 minutes of operation, granules of the desired size are easily formed in a continuous manner with a throughput of 1.5 tons per hour.

The granules which arrive in the hopper 21 are sufficiently coherent and self-reducing and their resistance allows them to withstand the pressure and heating conditions which exist during the first heating operation. The granules exit continuously from the hopper 21 onto a moving grate 22 in a layer 76 to 127 mm thick. As the moving grate 22 advances the pellets, an igniter 23 ignites them and they advance in continuous motion through a heating zone represented schematically by the envelope of a wind box 24. heat to a temperature of 1090 to 12500 C (that is to say a temperature below the slagging temperature of the mixture, at which it will be noted that no flux has been added according to this example) under the effect of a stream of oxygen-containing gas being drawn into the shroud 24 by an exhaust fan 25 with a flow rate of about 1200 cm 2 em 2 of the grid area. The result is that the granules become dehydrated, then the slurry undergoes a destructive distillation and at the same time the carbon contained in the various granules causes a preliminary reduction, for example of 10 to<B><I>50%,< /I></B> of iron oxide, so that the mass forms a smooth granule on carbonization which has a graphitic bonding matrix of unexpected strength and a particularly high carbon content. The heating and blowing time on the moving grate between the time the green pellets ignite and the time the carbon bound pellets emerge is approximately 8 to 12 minutes.

The char-bonded granules leaving the moving grate 22 fall continuously onto a 6.35 mm mesh screen 26. The lines of carbonized granules which pass through the screen 26 fall onto a conveyor 27, then arrive on a conveyor 28 which brings them to the loading end of the mixer 12, in which they are incorporated. to the feed of the pellet drum 13. The charred and oversized pellets leaving the sieve 26 cool in the presence of air in a few seconds to the temperature of dark red, and can be stored or passed through a hopper. 29, which makes them arrive regularly in a melting furnace 30.

The resistance and the quakte of the granules bonded by carbonization which arrive in the hopper 29 allow them to withstand the pressure and load conditions existing in a melting furnace, in which the height of the load is for example 2.44 m. The granules arriving at high temperature, during a continuous operation, continue to reduce. The quality of the bound granules can be defined by 1. the yield index, 2. the degree of reduction and 3. their state of reducibility.

The term yield index is a number which corresponds to the percentage of char-bound granules which are retained on a 4.76 mm mesh sieve, i.e. the yield index represents the number of carbonization-bonded granules from the moving grate which can be charged and processed in the melting furnace without excessive loss of fines in the form of flying dust.

The expression degree of reduction is a number which corresponds to the total percentage of iron reduced to the metallic state during the First Heating Operation and represents the ratio between the metallic iron of the granules bonded by carbonization and the total iron. of these granules, expressed as a percentage by weight.

The expression state of reducibility is a number which corresponds to the percentage of the initial iron in the granules, which by final reduction gives metallic iron under the action of the carbon remaining in the granules. The state of reducibility is calculated by the following formula

Note that the value of the state of reducibility may be greater than 100, indicating that the initial mixture contained an excess. coal, which was retained during the First Heating Operation and which is available for carburizing and heating. An example of the composition of carbonization granules thus prepared is as follows:

Total iron in carbon bonded granules <B>56%</B> Metallic iron in these granules. . . . 17% Carbon in these pellets. . . . . . <B><I>15%</I></B> Degree of reduction . . . . . . . . <B>30%</B> Yield index . . . . . . . <B>75%</B> State of reducibility . . . . . . . . 124% The fact that it is possible to heat in a stream of blown air containing oxygen raw pellets containing up to 40% non-coking coal to form pellets resistant and cohesive to high residual carbon content is an unexpected phenomenon, as it is contrary to all known principles. It has been well established and generally accepted that when the fuel content of an initial mixture of iron ore and fuel exceeds about 12 to 15% and this mixture is subjected to a treatment of ordinary sinter, the fuel continues to burn until it is almost completely consumed, so that whatever the initial fuel content of the mixture, the residual fuel content after heating is approximately less than 1%. By specially regulating the heating operation, as will be seen in detail later, strong and coherent bonded granules with a high content of residual carbon are obtained.

On examining a granule bonded by carbonization with the naked eye, it is found to have the appearance of a dense, resistant mass, RTI ID="0003.0537" WI="16" HE="4" LX="1367" LY ="2640"> coherent, amorphous carbon. very similar to that of charcoal, with no visible indication of the presence of molten material on the surface or on a broken fragment. It is observed under the microscope and in particular with high magnification that the structure of the granule is not homogeneous but is formed by four main elements: 1. A graphitic carbon matrix analogous to an epoch. The porosity of this carbon matrix is microscopic and the diameter of its largest pores is of the order of 0.0635 mm.

2. Very fine particles of metallic iron between 0.00025 and 0.00125 mm in size often forming a rim around larger grains of iron oxide, about 0.075 mm in diameter.

3. Grains of iron oxide dispersed in the carbon matrix which, with a magnification of less than 500 diameters and illuminated under ordinary light, appear homogeneous, whereas under polarized light and with higher magnification , the oxide grains have a sharply defined duplex structure with extremely fine veins of metallic iron intersecting in the oxide.

4. Chains or groupings of very small, irregularly shaped patches of ore gangue on which there is no evidence of melting.

A very remarkable characteristic of the structure of the Us granules by carbonization and which appears by microscopic examination under polarized light consists in the fact that the carbon matrix is formed by graphitic carbon in very fine crystals instead of amorphous carbon. . The product generally obtained from the breeze temperature carbonization of hard coal consists of <B>100%</B> amorphous carbon, while coke, which is the product of carbonization of high temperature hard coal, contains at most about 10 to 20 percent graphitic carbon, the balance being amorphous carbon. It is admitted, from the experience acquired in the manufacture of graphite electrodes, that the transformation of amorphous carbon into graphitic carbon requires a heating cycle of very long duration (4 to 6 days) at a very high temperature. 61 high, about 1980°C. 1205 C in a stream of air to completely transform the amorphous carbon into graphitic carbon with very fine crystals and noting that the graphite obtained by heating to very RTI ID="0004.0275" WI="10" HE="4 " LX="1025" LY="2334"> long life at high temperature has a large crystal grain structure. The reasons for this phenomenon are not known. One could admit that it is explained by considering that the particles of coal are extremely fine, that they are in intimate contact with the very fine particles of iron oxide (which can exert a catalytic action) and that the heating speed is extremely fast. This product is distinguished from charcoal and coke by its apparent density, that of metallurgical coke being about 0.432 because it contains about 81% total voids, of which 45% is formed by space. which separates the particles and<B>36%</B> by the voids inside the particles. The carbonization-bonded granules have 74% total voids, about 29% are within the particles, and their bulk density is about 1200 kg/m3. The large cavities visible in the coke pieces do not exist in the char-bonded granules.

The various operations of the process described can be carried out continuously or by intermittent loads and independently with several granulation installations and several heating installations which cause the bound granules to arrive in a single melting furnace. The bonded granules can be cooled and shipped to a different plant for reduction in a melting furnace. They therefore constitute a solid and resistant manufactured object which, separated from the fines, can be shipped by truck and, because of its cohesion, can be loaded into furnaces with the addition of flux and fuel, in sufficient quantity to melt the gangue. . It will be noted that the granules thus bonded by carbonization are self-reducing and sufficiently coherent because they contain a quantity of carbon sufficient to reduce the iron to the metallic state and to carburize the metal thus formed; or else, according to fig. 1, the char-bonded pellets can retain the heat they contain when leaving the moving grate if they are loaded directly from the moving grate exit into the melting furnace. Another solution is to discharge the smooth, hot pellets from the moving grate into a separate holding furnace where the burnt gases from the melting furnace continue to heat them for a sufficient time to increase the RTIID="0004.0550". " WI="8" HE="4" LX="1331" LY="1859"> degree of reluctance.

An important advantage of the written process consists in the possibility of carrying out the heating operations under the operating conditions indicated above, at individual speeds which are the most advantageous for the installation and the materials chosen and of placing them in train and to stop them separately at will. For example, with a blowing time of 15 minutes or less (generally less than 10 minutes, as the following examples show) on the moving screen to perform the Char Bonding operation, and with a residence time of 30 minutes in the melting furnace, the plant can be completely shut down in less than an hour from the moment Fon has interrupted the granulation operation in the rotating drum, and it can be completely restarted after the same time. . Ort can make similar stops at the corresponding times needed when the Charcoal Bonding and Fusion operations occur at different points.

<I>Example 2</I> To prepare the bound, sufficiently coherent granules of Example 1, they are passed from the hopper 21 onto the moving screen 22 at a rate chosen so as to form a uniform layer of 75 to 125 mm thick, then the pellets are ignited; or, as shown in fig. 2., a drying device 54 can be interposed between the hopper 21 and the ignition device 23 and cause hot gas to arrive there from a chamber (not shown), by means of a suction fan 55. The hot gases coming from the chamber 24 can be used. It is not possible to dehydrate the granules beforehand before they arrive in the hopper 21 because in the dry and non-carbonized state their resistance is insufficient. The preliminary dehydration must be carried out in such a way as to avoid mechanical handling of the dehydrated, non-carbonized granules, before the first heating or the operation of destructive distillation and thermal bonding. Pre-drying can be done by passing hot air or combustion products through the layer of raw granules of the moving grate. The prior dehydration has the effect of reducing the time necessary for heating the granules, once ignited. The table below indicates, by way of example, the effect exerted by prior dehydration on the duration of heating after ignition. These results were obtained on grids of the type with intermittent loads.

Composition of bound granules Size Duration of State of N of Type of granules, heating, Total iron Metallic iron Carbon Reducibility index test granules mm minutes /o in o/o /o yield % 109 green 19 to 22 30 53, 2 17.5 17.5 66 148 110 sec 19 to 22 12 54.9 <B>1</B>5.7 15.1 78 12.5 112 sec 22. to 25 13 52.2 25.2 18.2 77 170 <I>Example 3</I> The quality of the granules bound by carbonization is determined by the three indices of example 1 and this quality is obtained by adjusting the variable factors of: 1. the composition of the initial mixture, 2. the size of the granules, 3. the flow of the wind during the first heating operation, 4. the blowing time during the first heating operation, 5. the final temperature of the grate, 6. the temperature of the wind during the first heating operation, 7. the oxygen content of the wind during the first heating operation and 8. the thickness of the layer of granules on the moving grate during the first heating operation.

The composition of the initial mixture depends partly on the proportions of ore and coal, which can be for example 60 parts of ore for 40 parts of coal, 70 parts of ore for 30 parts of coal, etc., and of which some examples are given below.

Composition of the bound granules Composition of the initial mixture N Total iron Metallic iron Carbon Index of Condition of the test by weight, % in o/o of total iron % yield reducibility % <U>ore/coal</U> 10 60- 40 51.0 30.4 26.3 - 240 16 50-50 46.8 23.5 32.5 - 292 The composition of the initial mixture also depends in part on the nature of the iron ore chosen. For example, certain direct ship ores, ore concentrates and fly dusts are feedstocks that work well in the process described. Satisfactory bonded granules have, for example, been manufactured with a Mesabi hematite ore. The composition of this ore and the grain size distribution of the crushed ore are as follows

Ore Composition <U>Percentage</U> Fe 50.2 Mn 0.6 Si0_, 14.1 A1.0j 2.0 CaO 0.57 Mg0 0.30 Moisture 8 to 12

<I>Ore grain size distribution</I> <I>crushed</I> Sieve mesh size, Percentage of mm grains retained +0.210 1.1 -0.210 + 0.149 2.3 -0.149 <B ><I>+0.105</I></B> 8.0 -0.105 + 0.074 11.3 -0.074 77.3 The carbonaceous material of the initial mixture may also consist of lignite or other lower quality fuel.

Granules of 9 to 15 mm were chosen in the test of Example 1, but it has been found that larger granules are very advantageous in certain types of melting furnaces. For example, with larger granules, a higher gas permeability of the melting furnace charge is obtained. Examples of larger granules which have given satisfactory results in this operation are given below, but larger granules can still be chosen in certain melting furnaces.

Composition of bonded granules N Diameter Total iron Metallic iron Carbon Index of State of the <U>e</U>test of the granules in mm % in o/<U>o</U> of total iron <U>0 /a</U> r<U>endeme</U>n<U>t</U> reducibility % 110 19 X 22.2. Le blowing rate during the first heating operation refers to the quantity of air (also referring to the air modified by the addition of oxygen or by the products of combustion) forced back through the pellets once ignited. By appropriately adjusting the value of the other heating variables referred to in detail later, the blowing rate can be varied as follows to obtain carbon bonded granules of acceptable quality.

Composition of bound granules Wind flow <B><I>NI</I></B> cmd/min. per cm@ of total Iron Metallic iron Carbon Index of State of of the test grid surface /o in 11/o of total iron % yield reducibility o/o 128 1950 54.4 18.5 17.4 72 146 131 2400 54.9 19.4 14.5 80 128 148 3030 54.9 19.0 11.8 78 110 The blowing time during the first heating operation is a function of the speed of the forward motion of the continuous grate , and by appropriately adjusting the other heating variables, this duration can be varied to meet production conditions. The final grate temperature is an extremely useful measure of the quality of bound pellets. It is preferably between 870 and 1093 This has an influence on the quality of the granules bonded by carbonization, as indicated in the table below.

Composition of bonded granules NO Final temperature Total iron Metallic iron Carbon Index of State of the grid test, C <U> /o</U> in % of total iron <U> /o</U> r< U>e</U>n<U>deme</U>nt reducibility <B>01 0</B> 225 870 52.0 12.1 16.1 78 132 2.27 1093 52.9 15.5 15, 8 81 134 The temperature of the wind during the first heating operation is that at which the air current or the modified air current penetrates the layer of pellets once ignited, and by raising the temperature of the wind one can generally decrease the blowing time that is required to obtain char-bonded granules of acceptable quality, as shown in the table below.

Composition of the bonded granules Temperature Duration Metallic iron Wind, blowing State of N Total iron in ,/o Carbon Reducibility index the test <B>OC</B> minutes /o of total iron /o yield % 244 538 7 49.4 13.6 19.3 80 164 147 799 6.5 54.9 15.7 15.5 72 127 The oxygen content of the wind can be increased during the first heating operation by adding oxygen to the draft or reduce it by adding combustion products. The results obtained are given in the table below.

Composition of bound granules NO Wind oxygen, Total iron Metallic iron Carbon State index of <U>of</U> the <U>test</U> 1% <U>in volume /o in</U> % of <U>tot iron</U>a<U>l % return</U>ment r<U>eductib</U>ility % 153 4.2 55.0 12.1 9.1 76 80 151 6.2 55.0 17.1 10.0 78 95 <B><I>1</I></B>50 11.8 55.0 16.2 10.0 80 93 234 26.0' 53 .8 15.8 18.7 65 151 <B>*</B> Under these conditions, granules bonded by carbonization of acceptable quality are obtained. The thickness of the layer of granules on the moving grid influences the quality of the product and preferably does not exceed 150 mm for granules of 9 to 15 mm and can be greater for larger granules.

During the first heating operation, it is preferable after having ignited the granules from above, to pass a stream of air (or modified air) through the layer from top to bottom, i.e. i.e. in the direction in which the combustion progresses. , <I>Example 4</I> In the process for preparing the granules described by way of example in Example 1, they are formed in the green state from the initial mixture by the movement of a granulating drum which rotates slowly around a substantially horizontal axis and into which an initial mixture of fine ore and carbonaceous material is charged. Ordinary and common carbonaceous binders, such as starch, pitch, tar, molasses, lignin products, etc., can also be incorporated into the initial mixture to increase the strength of the green granules or facilitate their formation, but these additives are not necessary according to the invention.

<I>Example 5</I> Similarly, the process described in Example 1 consists of a continuous operation, but satisfactory results have also been obtained by operating with intermittent loads and this process also forms part of the invention. For example, aggregates of green pellets have been successfully heated on both moving grates and stationary grates with intermittent loads to obtain carbonization-bound pellets of acceptable quality. <I>Example 6</I> Instead of forming the initial mixture with 40 parts of non-coking coal and 60 parts of rai ore, it is possible to form clusters of granules similar to bunches of grapes with 16 parts of coal 24 parts of non-coking coal and 60 parts of ore, that is to say that 40% of the coal in the mixture consists of coking coal.

One can also form clusters of bound granules by projecting onto the layer of green or dehydrated granules on the moving grid iron ore powder.

They were also obtained by preparing them in the green state by the process of Example 1, but of a slightly smaller size, by passing these green granules into a second granulating drum in which iron ore is fed powder and additional water, and allowing the iron ore to accumulate on the green pellets as a film representing approximately 20% of the total weight of the finished green pellet.

The masses in clusters of granules linked by carbonization are interesting in the process described because when they are loaded into the melting furnace, the permeability of the mass to the passage of gases from the chimney becomes greater than that of the separated granules or other aggregates.

The size of the granules arriving in the melting furnace can be between 6.0 and 31.7 mm. The size limits of the granules are determined during the continuous forming, primary heating and melting operations so that at the lower limit they are large enough and therefore heavy enough in relation to their peripheral surface and to their section. so that the wind at the chosen rate does not lift them and carry them down the kiln chimney, and that at the upper limit they are small enough that the heating during the dehydration and carbonization operations does not cause explosion under the effect of a sudden release of steam or gas under pressure in the various granules, and that the heat is transmitted in an appropriate manner during the melting operation so as to lower the load regularly. The size limits of 9 to 15 mm, 16 to 22. mm and 22 to 25 mm preferably chosen in the preceding examples are determined according to the possibility of adjusting the flow of the wind and the conditions between the aforementioned limits. during the carbonization and melting operations, without resulting in any appreciable loss in the chimney or irregularity in the descent of the load. The proportions by volume of the granules between the limits given are about 1:5 for small granules and 1:1.5 for large ones, when the bound granules are supported by little or no coke , and it will be noted that the particles are less likely to settle or descend irregularly when they are substantially of the same size. In general, the layer of larger granules may be thicker during the first heating operation, the wind flow may be greater and the final grate temperature higher, and the amount of carrier coke in the merging operation can be less. If the granules are smaller, the necessary dehydration time is shorter before they undergo the first heating operation.

The bonded granules which come from the first heating treatment are characterized by a surprising resistance and a low reduction in weight by abrasion when the granules are rubbed on each other. Microscopic examination of sections and portions of the granules is found to contain the iron ore and reduced iron as separate particles, which are only slightly bound or not bound at all by the molten elements of the ash. ; they are mainly and clearly linked together by a graphitic carbon matrix. The bonded granules are therefore distinctly different from the masses bonded by sintering or by vitrification, which comprise connection bridges formed by elements of molten slag; but it should be noted that the granules can be obtained without the source of this carbon consisting of coking coal. It should also be noted that these bonded granules contain a quantity of carbon sufficient to reduce and carburize the existing iron oxide (for example a state of reducibility comprised between 100 and 170 and a carbon percentage comprised between 10 and 25 %) with a proportion of reduced iron between 9.2 and 31.4% (representing a reduction of 15 to 40% of total iron). In some cases the resistance, indicated by the yield index, increases with the proportion of metallic iron contained in the particles, although the particles are independent of each other.

Claims (2)

CLAIMS I. Process for the preparation of coherent granules containing iron oxide and carbon in an amount greater than that which is necessary to completely reduce the iron oxide, in which a mixture of iron oxide and carbonaceous material finely divided in proportion by weight between 8: 1 and 1: 1 with 10 to 20% water is transformed in a rotating drum into granules which are heated between 870 and 1250e C so that a part of said iron oxide is reduced in iron, characterized in that non-coking coal is used as the finely divided carbonaceous material in the granules and in that the granules are heated by igniting a bed of the latter and passing through it a stream of con holding 16 to 2.6% oxygen, so that the coal is destructively distilled and converted into a graphitic matrix which acts as a binding agent for the other constituents of the granules, while part of the iron oxide is reduced in iron until 15 to 40% metallic iron is obtained in the granules, more than 6% carbon still being contained in the granules. He. Coherent granule prepared by the process according to claim 1, characterized in that it contains 40 to 55% by weight of iron, of which 15 to 40% by weight of the total iron is in the state of elemental iron, the remainder consisting of iron oxide, the iron being in the state of separate particles and the carbon, in an amount constituting 6.5% to 32.5% by weight of the granule, being in the form of a graphitic matrix binding the other elements of the granule, which is compact, non-porous to the naked eye and porous under the microscope. SUB-CLAIMS 1. Method according to claim I, characterized in that the granules undergo a first heating dehydrating them then are ignited and subjected to the action of the air stream for a period not exceeding 15 minutes after the inflammation of the layer of granules. 2. Method according to claim 1, characterized in that the iron oxide and the non-coking coal are mixed in a proportion by weight of between 85: 15 and 50: 50 in the presence of 10 to 20% water, shakes the mixture on a rotating surface in the presence of a proportion of irrigation water of about 2% so as to form green granules and to make them grow, the shaken mass is removed from this surface and the particles with a size of less than 6 mm which are returned to the rotating surface, the particles of a size greater than 31 mm are separated from the mass, they are fragmented and are returned to the rotating surface to form, a moving layer on the other motionless green granules in the layer whose thickness is between 75 and 125 mm, the layer is ignited at the upper part while it advances supporting the granules, the current is passed through it of air containing 16 to 26% oxygen with a flow rate of between 750 and 3030 cms per minute and per cm<U>"</U> of surface area of the diaper, so as to pass the air from top to low through the seam to cause it to burn and heat it at a rate sufficient to drive the water out of it to a vapor state in not more than 15 minutes and to carbonize the coal without bursting the granules, while reducing iron from the oxide of the granules in the metallic state with a content of about 15 to 40% by weight of the total iron and by causing the layer to take a final temperature of 870 to 1250 C so as to form a graphitic carbon matrix in each granule, then the air is stopped and the bound granules are discharged from the layer.