THERMALLY TREATED BRIQUETTES CONTAINING A "QUICK" CALCIUM-MAGNESIUM COMPOUND AND CALCIUM FERRITES, AND METHOD OF MANUFACTURE THEREOF

Abstract

Composition in the form of green or thermally treated briquettes comprising at least one quick calcium-magnesium compound and an iron-based compound, the method of production thereof and uses thereof.

Claims

1. Composition in the form of thermally treated briquettes, comprising a quick calcium-magnesium compound, preferably in the form of quicklime and an iron-based compound in the form of calcium ferrite, characterized in that said calcium ferrite forms a matrix in which particles of quick calcium-magnesium compound are dispersed.

2. Composition in the form of thermally treated briquettes according to claim 1, characterized in that said panicles of quick calcium-magnesium compound have a two-dimensional size under 63 m, observable by scanning electron microscopy coupled to energy dispersive analysis, in a section of said briquette.

3. Composition m the form of thermally treated briquettes according to claim 1, characterized in that it further comprises particles of quick calcium-magnesium compound of two-dimensional size above 63 m and under 5 mm, observable by scanning electron microscopy coupled to energy dispersive analysis, in a section of said briquette.

4. Composition in the form of thermally treated briquettes according to claim 3, in which said particles of quick calcium-magnesium compound of two-dimensional size above 63 m and under 5 mm. observable by scanning electron microscopy coupled to energy dispersive analysis, in a section of said briquette cover at least 20% of the area of said section.

5. Composition in (he form of thermally treated briquettes according to claim 3, in which said particles of quick calcium-magnesium compound of two-dimensional size above 63 m and under 5 mm, observable by scanning electron microscopy coupled to energy dispersive analysis, in a section of said briquette cover at most 60% of the surface area of said section.

6. Composition in the form of thermally treated briquettes according to claim 3, in which said particles of quick calcium-magnesium compound of two-dimensional size above 63 m and under 5 mm, observable by scanning electron microscopy coupled to energy dispersive analysis, in a section of said briquette cover less than 20%, preferably less than 10% of the surface area of said section.

7. Composition in the form of thermally treated briquettes according to claim 1, in which at least 70 wt %, preferably 80 wt %, more preferably 90 wt % of said iron-based compound is in the form of calcium ferrites.

8. Composition in the form of thermally treated briquettes according to claim 1, characterized in that at least 40 wt %, preferably 50 wt % of said calcium ferrites are in the form of monocalcium ferrite Ca.sub.2Fe.sub.2O.sub.5.

9. Composition in the form of thermally treated briquettes according to claim 1, characterized in that at least 40 wt %, preferably 50 wt % of said calcium ferrites are in the form of dicalcium ferrite Ca.sub.2Fe.sub.2O.sub.4.

10. Composition in the form of thermally treated briquettes according to claim 1, characterized in that it has a BET specific surface area greater than or equal to 0.4 m/g, preferably greater than or equal to 0.6 m.sup.2/g, more preferably greater than or equal to 0.8 m.sup.2/g.

11. Composition in the form of thermally treated briquettes according to claim 1, characterized in that it has porosity greater than or equal to 20%, preferably greater than or equal to 22%, more preferably greater than or equal to 24%.

12. Composition in the form of thermally treated briquettes according to claim 1, characterized in this said thermally treated briquettes have a shatter index below 10%, preferably below 8%, advantageously below 6%.

13. Method for manufacturing a composition in the form of thermally treated briquettes according to claim 1, comprising the steps of: mixing panicles of calcium-magnesium compound, preferably in the form of particles of quicklime with particles of an iron-based compound, preferably in the form of iron oxide so as to obtain a pulverulent homogeneous mixture; feeding a roller press with said pulverulent homogeneous mixture, compressing said pulverulent mixture in said roller press, with obtaining a calcium-magnesium composition in the form of green briquettes, and characterized in that the rollers of said roller press developing linear speeds at the periphery of the rollers between 10 and 100 cm/s, preferably between 20 and 80 cm/s, and linear pressures between 60 and 160 kN/cm, preferably between 80 and 140 kN/cm, and even more preferably between 80 and 120 kN/cm and in that said briquettes are thermally treated briquettes, the method further comprises a thermal treatment of said green briquettes at a temperature between 1050 C. and 1200 C. for a time between 5 and 25 minutes, preferably between 10 and 20 minutes said mixing step being carried out with a fraction of particles of calcium-magnesium compound having at least 30 wt % of the particles 90 m, which comprises at least 20 wt % of CaO equivalent relative to the total weight of said pulverulent homogeneous mixture, and with at least 20 wt % of iron particles having a d.sub.90 under 200 m, preferably under 150 m, more preferably under 130 m and even more preferably under 100 m.

14. Method according to claim 13, characterized in that it further comprises a step of recycling fines from said briquetting step and/or from said step of thermal treatment and a step of introducing these fines in said mixing step.

15. Method according to claim 13, further comprising pretreatment of the briquettes under modified atmosphere containing at least 2 vol % of CO.sub.2 and at most 10 vol % of CO.sub.2 relative to said modified atmosphere.

16. (canceled)

Description

[0136] Other features, details and advantages of the invention will become dear from the description given hereunder, which is non-limiting and refers to the examples and to the figures.

[0137] FIG. 1 is a graph of the BET specific surface area and of the BJH pore volume as a function of the content of Fe.sub.2O.sub.3 equivalent in the briquettes according to the present invention.

[0138] FIG. 2 is a graph of the shatter index (STI) as a function of the content of Fe.sub.2O.sub.3 equivalent in the thermally treated and green briquettes according to the present invention.

[0139] FIG. 3 is a graph of the percentage of Fe.sub.2O.sub.3 converted to calcium ferrites as a function of the content of Fe.sub.2O.sub.3 equivalent in the thermally treated briquettes according to the present invention.

[0140] FIG. 4 is a graph of the percentage of Fe.sub.2O.sub.3 converted to calcium ferrites as a function of the content of Fe.sub.2O.sub.3 equivalent in the thermally treated briquettes according to the present invention.

[0141] FIG. 5 shows photographs of sections of different briquettes of compositions according to the present invention.

[0142] The present invention relates to a method for briquetting fine particles of calcium-magnesium compounds and iron based compound, said iron-based compound having a very fine granulomere distribution characterized by median size d.sub.50 under 100 m, preferably under 50 m as well as a size d.sub.90 under 200 m, preferably under 150 m, preferably under 130 m, more preferably under 100 m.

[0143] The method of briquetting according to the invention comprises supplying a pulverulent mixture comprising at least one quick calcium-magnesium compound, said mixture comprising at least 40 wt % of CaO+MgO equivalent relative to the weight of said composition and having a Ca/Mg molar ratio greater than or equal to 1, preferably greater than or equal to 2. more particularly greater than or equal to 3 and an iron-based compound having a very fine granulometric distribution characterized by a median size d.sub.50 under 100 m, preferably 50 m as well as a size d.sub.90 under 200 m, preferably under 150 nm, preferably under 130 m, more preferably under 100 m; in which said quick calcium-magnesium compound comprising at least 40 wt % of CaO+MgO equivalent also comprises at least one fraction of particles of calcium-magnesium compound having a particle size 90 m, which further comprises at least 20 wt % of CaO equivalent relative to the weight of said pulverulent mixture and in which said iron-based compound is present at a content of at least 20 wt %, preferably at least 25 wt %, more preferably at least 30 wt %, in particular at least 35 wt % relative to the total weight of said pulverulent mixture.

[0144] Advantageously, according to the present invention, said fraction of particles of calcium-magnesium compound having a particle size 90 m contains at most 60 wt % of CaO equivalent relative to the weight of said pulverulent mixture.

[0145] According to the present invention, not only the formation of calcium ferrite is improved and gives a yield in conversion of iron oxide to calcium ferrite of about 90%. but also the balance between the formation of monocalcium ferrites and dicalcium ferrites can be controlled. In fact it has teen found to be beneficial, from an industrial standpoint, to be able to control the proportion of dicalcium ferrites relative to the proportion of monocalcium ferrites as necessary, and vice versa.

[0146] The approximately homogeneous mixture in which the iron-based compound is distributed homogeneously is fed to a roller press, sometimes also called a tangential press, for example a Komarek, Sahut Konreur, Hosokawa Bepex, or Kppern press.

[0147] In the roller press, the homogeneous pulverulent mixture is compressed, optionally in the presence of a binder or a lubricant, more particularly selected from the group consisting of binders of mineral origin such as cements, clays, silicates, binders of vegetable or animal origin, such as celluloses, starches, gums, alginates, pectin, glues, binders of synthetic origin, such as polymers, waxes, liquid lubricants such as mineral oils or silicones, solid lubricants such as talc, graphite, paraffins, stearates, in particular calcium stearate, magnesium stearate, and mixtures thereof, preferably calcium stearate and/or magnesium stearate, at a content between 0.1 and 1 wt %. preferably between 0.15 and 0.6 wt %. more preferably between 0.2 and 0.5 wt % relative to the total weight of said briquettes.

[0148] In use, the rollers of the roller press develop linear speeds at the periphery of the rollers between 10 and 100 cm/s, preferably between 20 and 80 cm/s, and linear pressures between 60 and 160 kN/cm, preferably between 80 and 140 kN/cm, and even more preferably between 80 and 120 kN/cm.

[0149] Assuming an angle of degree at which the linear pressure is applied on the surface of (he hoops, the surface pressure can be calculated, which is equal to the linear pressure divided by (D)/360, where D is the diameter of the hoops in cm. The surface pressure is between 300 and 500 MPa, preferably between 300 and 450 MPa, and more preferably between 350 and 450 MPa.

[0150] After compression, the calcium-magnesium composition is obtained in the form of green briquettes, which are collected.

[0151] In the method according to the present invention, the green briquettes collected are treated thermally at a temperature between 900 C. and 1200 C., preferably between 1050 C. and 1200 C. more preferably between 1100 C. and 1200 C. inclusive. The thermal treatment is preferably carried out for a predetermined time between 3 and 20 minutes, obtaining thermally treated briquettes in which said iron oxide has been converted to calcium ferrite and forms a matrix of calcium ferrite in which particles of calcium-magnesium compound are dispersed.

[0152] Said matrix is to be understood as being a continuous phase based on calcium ferrite in which particles of quick calcium-magnesium compound, in particular quicklime, are dispersed. A distinction is made between the case when said particles of quick calcium-magnesium compound are of small size so that they melt visibly in the matrix based on calcium ferrite, and the case when particles of quick calcium-magnesium compound are of larger size and appear as inclusions of quick calcium-magnesium compound in said matrix.

[0153] The aforesaid distinction is made concrete on the basis of a section of a briquette according to the invention, applying scanning electron microscopy coupled to energy dispersive analysis. This provides visualization in two dimensions (the surface of the section) of an object initially in three dimensions (briquette), but also of the particles that make up the briquette. Thus, the particles of calcium magnesium compound also appear in two dimensions on the section plane. As it is customary to liken particles in three dimensions to spheres and determine their size as the diameter of the equivalent sphere (three-dimensional size), in the present invention the cut surface of the particle is likened to an equivalent disk and its two-dimensional size to the equivalent diameter of this disk. More precisely, the two-dimensional sizes are calculated with a program that finds, for each particle of quick calcium-magnesium compound dispersed in the continuous matrix of calcium ferrite, the sum of the smallest and the largest dimension of its cut surface divided by two. This sum divided by two represents the diameter of the equivalent disk.

[0154] In this acceptation, it is considered that the panicles of quick calcium-magnesium compound melt or merge in said matrix (continuous phase) of calcium ferrite when said particles of quick calcium-magnesium compound have a two-dimensional size under 63 m, observable by scanning electron microscopy coupled to energy dispersive analysis, in a section of the briquette.

[0155] In one embodiment of the invention, the thermal treatment of the green briquettes is carried out in a rotary kiln at high temperature, optionally equipped with a preheater.

[0156] Alternatively, the thermal treatment is carried out in a horizontal kiln, for example a tunnel kiln, a through-type kiln, a car-type kiln, a roller kiln or a mesh band kiln. As a variant, any other type of conventional kiln may be used, provided it does not cause a change in the integrity of the compacts, for example through excessive attrition.

[0157] Cooling may either be performed conventionally in the downstream part of the kiln, or outside the kiln, for example in a vertical cooler in countercurrent for the cooling air or else in a fluidized-bed cooler with cooling air in the case of tempering.

[0158] In a particular embodiment, cooling at the end of the thermal treatment is carried out quickly, in less than 15 min, preferably in less than 10 min, in a fluidized bed with cooling air.

[0159] Semi quantitative analysis of the iron-based compounds (iron oxides Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, calcium ferrites CaFe.sub.2O.sub.4, Ca.sub.2Fe.sub.2O.sub.5) is performed based on an X-ray diffraction pattern by the Rietveid method.

[0160] This method consists of simulating a diffraction pattern starting from a crystallographic model of the sample, then adjusting the parameters of this model so that the simulated diffraction pattern Is as close as possible to the experimental diffraction pattern. At the end of semi-quantitative analysis, it is verified that the total amount of iron expressed in Fe.sub.2O.sub.3 equivalent does not differ by more than 10% relative to the values obtained by XRF. The total percentage of iron in the form of calcium ferrites is obtained by simple division (Fe in the ferrites divided by Fe in the total of the iron-based compounds).

[0161] In an advantageous embodiment of the method according to the present invention, said pulverulent mixture further comprises less than 10% of particles of quick calcium-magnesium compound having a particle size 90 m and 5 mm relative to the total weight of said pulverulent mixture.

[0162] Thus, the briquettes obtained by the method according to the present invention are of relatively homogeneous granulometry, i.e. the briquette, when it is cut open, has a granular composition in most of its volume. Thus, a continuous phase is observed, formed from calcium ferrite, calcium-magnesium compound, for example quicklime and optionally iron-based compound, such as iron oxide, depending on the green briquette's initial content of calcium-magnesium compound, of calcium component in the latter, and of iron-based compound.

[0163] In another advantageous embodiment of the method according to the present invention, said pulverulent mixture further comprises between 10% and 60% of particles of quick calcium-magnesium compound having a particle size 90 m and 5 mm relative to the total weight of said pulverulent mixture.

[0164] An advantageous alternative according to the invention is to provide inclusions of quick calcium-magnesium compounds, in particular of quicklime, dispersed in the continuous phase (matrix) of calcium ferrite, as described above. In fact, the quick calcium-magnesium compound is then available in situ at the place where the calcium ferrites have promoted slag formation, acting as flux to allow the quick calcium-magnesium compound to act immediately.

[0165] In this advantageous embodiment of the method, it was found that on cutting open a thermally treated briquette obtained according to the present invention, the cut surface was strewn with inclusions of calcium-magnesium compound and/or quicklime, making the latter available in the form of unreacted quicklime for forming calcium ferrites and thus remaining available for use in the form of quicklime, for example in iron and steel metallurgy, for example for forming slag. The content of these inclusions of calcium-magnesium compound may vary, as indicated below in the section relating to the thermally treated briquettes according to the present invention.

[0166] More particularly, in the method according to the present invention, said at least one iron based compound is present at a content greater than or equal to 20 wt % relative to the total weight of said pulverulent mixture.

[0167] When the content of iron-based compound, more particularly of iron oxide with very fine granulomere distribution, is at least 20 wt % relative to the weight of the pulverulent mixture, but also when the level of CaO in the calcium-magnesium compound in the form of very fine particles (d.sub.30 21 90 m) is at least 20 wt %, not only the formation of calcium ferrite is improved and gives a yield in conversion of iron oxide to calcium ferrite of about 90%, but also the equilibrium between the formation of monocalcium ferrites and dicalcium ferrites is oriented towards formation of dicalcium ferrites, particularly when the contents of CaO equivalent and very fine Fe.sub.2O.sub.3 are balanced. In fact it proved beneficial, from an industrial standpoint, to be able to control the proportion of dicalcium ferrites relative to the proportion of monocalcium ferrites as necessary, and vice versa.

[0168] In a preferred embodiment of the method according to the present invention, the percentage by weight of quicklime in the fraction of quick calcium-magnesium compound having a particle size <90 m relative to the total of the percentage by weight of quicklime in the fraction of calcium-magnesium compound having a particle size <90 m and the percentage of Fe.sub.2O.sub.3 equivalent in said iron-based compound with very fine granulomere distribution is 30%. preferably 32%, more preferably 34%, especially preferably 36%.

[0169] In fact it was found, advantageously, that it was possible to influence and control the proportion of monocalcium ferrite and dicalcium ferrite during calcination of the briquettes by adjusting the percentage by weight of CaO equivalent having a particle size <90 m relative to the total of the percentage by weight of said particles of quicklime. When the percentage by weight of quicklime in the fraction of quick calcium-magnesium compound having a particle size <90 m relative to the total of the percentage by weight of quicklime in the fraction of calcium-magnesium compound having a particle size <90 m and the percentage of Fe.sub.2O.sub.3 equivalent of said iron-based compound with very fine granulomere distribution is 30%, preferably 32%, more preferably 34%, especially preferably 36%; calcination of the briquettes will rather promote the formation of dicalcium ferrite (Ca.sub.2Fe.sub.2O.sub.5).

[0170] This means that if:

[0171] P1 represents the percentage, in the pulverulent mixture intended for briquetting, of the particles of the quick calcium-magnesium compound whose size is under 90 m (fraction of calcium-magnesium compound having a particle size <90 m),

[0172] P2 represents the percentage, in the pulverulent mixture intended for briquetting, of the particles of the quick calcium-magnesium compound whose size is above 90 m,

[0173] P3: percentage of the iron-based compound (with very fine granulomere distribution) in the pulverulent mixture intended for briquetting,

[0174] C1 represents the percentage of CaO equivalent in the particles of quick calcium-magnesium compound whose size is under 90 m

[0175] C2 represents the percentage of CaO equivalent in the particles of quick calcium-magnesium compound whose size is above 90 m

[0176] C3 represents the percentage of Fe.sub.2O.sub.3 equivalent in the iron-based compound (with very fine granulomere distribution)

[0177] The weight ratio P1/(P1+P3) is a key parameter that must be controlled for forming either predominantly monocalcium ferrites or predominantly dicalcium ferrites, and more generally the weight ratio P1.C1/(P1.C1+P3.C3) is one of the possibilities for predominant formation of monocalcium ferrite or else predominant formation of dicalcium ferrite.

[0178] In such an instance, said thermal treatment is preferably thermal treatment at a temperature less than or equal to 1150% preferably less than or equal to 1100 C., more particularly greater than or equal to 900 C., preferably according to the relation (predetermined time)/(temperature of thermal treatment 1000 C.)>5.

[0179] The percentage P2 is a key parameter that must be controlled for forming briquettes with or without inclusions of quick calcium-magnesium compound having a two-dimensional size above 63 m.

[0180] In another embodiment, said iron-based compound comprises at least 50 wt %, preferably at least 60 wt %, more preferably at least 70 wt %, more preferably at least 80 wt % and in particular more than 95 wt % of iron oxide in the form of magnetite Fe.sub.3O.sub.4 relative to the total weight of the iron-based compound expressed In Fe.sub.2O.sub.3 equivalent.

[0181] In another preferred variant of the method according to the present invention, the percentage by weight of CaO equivalent having a particle size <90 m relative to the total of the percentage by weight of CaO equivalent having a particle we <90 m and of said iron-based compound is <40, preferably <38, more preferably <36% in order to influence the formation of monocalcium ferrites during calcination of the briquettes.

[0182] In such an instance, said thermal treatment is thermal treatment at a temperature less than or equal to 1150 C., preferably less than or equal to 1100 C. more particularly greater than or equal to 900 C., preferably according to the relation (predetermined time)/(temperature of thermal treatment 1000 C.) <5, which favours the formation of monocalcium ferrites even more.

[0183] More particularly, In this variant of the method according to the present invention, said iron-based compound comprises at least 50 wt %, preferably at least 60 wt %, more preferably at least 70 wt %. more preferably at least 80 wt %, and in particular more than 95 wt % of iron oxide in the form of haematite Fe.sub.2O.sub.3 relative to the total weight of the iron-based compound expressed in Fe.sub.2O.sub.3 equivalent.

[0184] At least 40%, preferably at least 50%, preferably at least 60% and more preferably at least 70% of the total iron is n the form of calcium ferrites.

[0185] Quantification of the calcium ferrites is performed by XRD/Rietveld analysts after grinding the briquettes, as for the green briquettes.

[0186] The thermally treated briquettes of the present invention have a shatter index (STl, i.e. percentage by weight of fines below 10 mm after 4 drops from 2 m) below 6%, regardless of the content of iron-based compounds.

[0187] They are also characterized by a specific surface area greater than or equal to 0.4 m.sup.2/g. preferably 0.5 m.sup.2/g, preferably 0.6 m.sup.2/g.

[0188] The porosity is greater than or equal to 20%, preferably 22%. preferably 24%.

[0189] The thermally treated briquettes have an apparent density between 2.0 and 3.0 and preferably between 2.2 and 2.8.

[0190] The thermally treated briquettes have good resistance to ageing. Thus, when they are exposed to a humid atmosphere containing for example 5 to 15 g/m.sup.3 of absolute humidity, degradation of their mechanical properties (STI) only occurs beyond 4% of weight increase, preferably 4.5% of weight increase, and more preferably 5% of weight increase, following the reaction of hydration of quicklime CaO to slaked lime Ca(OH).sub.2.

EXAMPLES

Example 1: Preparation of quicklime fines from grinding and pilot preparation of the briquettes

[0191] The quicklime fines from grinding were prepared starting from a soft-burned lump lime produced in a parallel-flow regenerative kiln. Grinding is carried out in a hammer mill equipped with a 2-mm screen and a recycling loop for sizes above 2 mm. These quicklime fines from grinding contain 29% of particles with particle size under 90 m (d.sub.30 <90 m, 71% of particles above 90 m, 37% of particles above 500 m, 21% of particles above 1 mm and 1% of particles between 2 and 3 mm. The value of t.sub.60 from the water reactivity test is 0.9 min. The BET specific surface area (measured by nitrogen adsorption manometry after vacuum degassing at 190 C. for at least two hours and calculated by the multi-point BET method as described in standard ISO 9277:2010E) is 1.7 m.sup.2/g. These quicklime fines from grinding contain 95.7% of CaO and 0.8% of MgO by weight.

[0192] A Gericke GCM450 powder mixer is used, with a capacity of 10 dm.sup.3, equipped with standard paddles with a radius of 7 cm, rotating at 350 revolutions per minute (i.e. 2.6 m/s). This mixer is used in continuous mode for preparing a mixture consisting of:

[0193] quicklime fines, optionally quicklime fines from grinding,

[0194] iron oxide fines,

[0195] powdered calcium stearate.

[0196] The total flow rate of the powder is 300 kg/h and the residence time is 3.5 s.

[0197] The mixture obtained is very homogeneous. This signifies that the Fe content for different 10-g samples taken from the final mixture is always within 5% of the average value.

[0198] A tangential press is used, equipped with hoops with a diameter of 604 mm and a width of 145 mm for producing briquettes with a theoretical volume of 7.2 cm.sup.3 with the shape of a bar of soap (4 arrays of 67 pockets per hoop, i.e. 268 pockets per hoop), which can develop a linear pressure of up to 120 kN/cm.

[0199] Starting from 10 tonnes of the mixture, after feeding the tangential press, compaction is carried out at a speed of 12 revolutions per minute (or a linear speed of 38 cm/s) at a linear pressure of 120 kN/cm (or a calculated surface pressure of 455 MPa for an angle of 0.5 degree).

[0200] Some several tonnes of briquettes are obtained having an average volume of 8.4 cm.sup.3, an average weight of 21.4 g and an average density of 2.4. These briquettes have a length of about 36 mm, a width of about 26 mm and a thickness of about 15.8 mm. These briquettes develop a total mercury pore volume (determined by mercury intrusion porosimetry according to part 1 of standard ISO 15901-1:2005E, which consists of dividing the difference between the skeletal density measured at 30000 psia, and the apparent density measured at 0.51 psia, by the skeletal density).

[0201] The water reactivity of the briquettes is determined by adding a predetermined amount of these briquettes, previously ground in the form of fines with a size between 0 and 1 mm, to 600 ml of water at 20 C., so as to correspond to 150 g of quicklime.

[0202] A shatter test is performed, starting from 10 kg of these briquettes, performing 4 successive drops from 2 m. The amount of fines under 10 mm generated at the end of these 4 drops is weighed.

[0203] The granulometric distribution of the iron-based particles in the composition in the form of briquette is determined by scanning electron microscopy and X-ray mapping, coupled to image analysts.

[0204] The briquettes are also characterized by performing thermal treatment (hot charge/discharge) on several of these briquettes, at the end of which a powder with granulometry below 80 m is prepared. The latter is characterized by X-ray diffraction, and quantification of the phases is carried out by Rietveld analysis.

Examples 2 to 9

[0205] Green briquettes are prepared according to the invention with quicklime from grinding containing particles with sues between 0 and 2 mm, but having different granulometric profiles, and contents of iron oxide of the haematite type, expressed in Fe.sub.2O.sub.3 equivalent, ranging from 10% to 60%. The iron oxide used in these examples is characterized by a d.sub.10 of 0.5 m, d.sub.50 of 12.3 m and d.sub.50 of 35.7 m. In each example, the particles of quicklime from grinding with a size between 0 and 2 mm have at least 30% of particles that are under 90 m. The protocol for preparation is described in example 1.

[0206] Green briquettes with the same composition were treated thermally at 1100 C. or 1200 C. for 20 minutes to obtain thermally treated briquettes having different contents of quicklime and iron-based compounds. The composition of the briquettes and the thermal treatment carried out are presented in Table 1, For these green and thermally treated briquettes, several tests were carried out, and are described below, referring to FIGS. 1 to 4.

[0207] FIG. 1 is a graph showing:

[0208] the variation of the BET specific surface area (SSA) as a function of the content of iron-based compound expressed in Fe.sub.2O.sub.3 equivalent, for green briquettes;

[0209] the variation of porosity as a function of the content of iron-based compound expressed in Fe.sub.2O.sub.3 equivalent, for green briquettes;

[0210] the variation of the BET specific surface area (SSA) as a function of the content of iron-based compound expressed in Fe.sub.2O.sub.3 equivalent, for thermally treated briquettes that have undergone thermal treatment (TT) of 1100 C. for 20 minutes; and

[0211] the variation of porosity as a function of the content of iron-based compound expressed in Fe.sub.2O.sub.3 equivalent, for thermally treated briquettes that have undergone thermal treatment (TTA) of 1100 C. for 20 minutes.

[0212] As can be seen, these variations of porosity and specific surface area show a slight linear decrease with the content of iron-based compound for the green and the thermally treated briquettes. The thermally treated briquettes have a lower specific surface area than that of the green briquettes, whereas they have higher porosity for identical contents of iron-based compound.

[0213] FIG. 2 is a graph showing:

[0214] the variation of the shatter index for green briquettes, as a function of the contents of iron-based compound expressed in Fe.sub.2O.sub.3 equivalent; and

[0215] the variation of the shatter index for thermally treated briquettes treated thermally at a temperature (TT) of 1100 C. for 20 minutes, as a function of the contents of iron-based compound expressed in Fe.sub.2O.sub.3 equivalent.

[0216] As can be seen, the shatter Indices are below 20% for green briquettes having contents of iron-based compound expressed in Fe.sub.2O.sub.3 equivalent below 40%, whereas for the thermally treated briquettes, all the shatter tests are below 10%, or even 6%.

[0217] FIG. 3 is a graph showing the variation of the yield of iron-based compound (iron oxide) converted to calcium ferrite, at a function of the iron oxide content expressed in Fe.sub.2O.sub.3 equivalent, as well as the amounts of iron oxide converted to monocalcium ferrite and dicalcium ferrite. The thermal treatment is carried out in a static bed for 20 min at 1100 C. in a tunnel kiln on 100 mm of thickness of briquettes.

[0218] As can be seen, the yield in conversion to calcium ferrite begins to decrease for contents of iron oxide expressed in Fe.sub.2O.sub.3 equivalent above 40%. The percentage of monocalcium ferrites passes through a maximum for contents erf iron oxide of 40%. The percentage formation of dicalcium ferrites decreases with the iron oxide content.

[0219] FIG. 4 shows the variation of the content of calcium ferrites expressed in Fe.sub.2O.sub.3 equivalent in the thermally treated briquettes as a function of the iron oxide content expressed in Fe.sub.2O.sub.3 equivalent in the green briquettes before thermal treatment

[0220] As can be seen, the contents of calcium ferrites in the thermally treated briquettes increase with the iron oxide content in the green briquettes However, this variation passes through a maximum at 50% in content of calcium ferrites for contents of iron oxide in the green briquettes in the range between 40 and 45%. and then decreases at contents of calcium ferrites of about 40% for contents of iron oxide in the green briquettes of 60%.

[0221] It is nevertheless possible to push the yield in conversion of iron oxide to calcium ferrites beyond 90% and obtain contents of calcium ferrites in the thermally treated briquettes beyond 50%, even beyond 70% for example by increasing the temperature of the thermal treatment to 1200 C. or by optimizing the grinding of the quicklime so as to increase the proportion of quicklime particles under 90 m, or a combination of the two. Several examples were carried out and measured, and are presented in Table 1.

TABLE-US-00001 TABLE 1 % of % of % of calcium CaFe.sub.2O.sub.4 Ca.sub.2Fe.sub.2O.sub.5 % ferrites in the by by conversion thermally weight in weight in % Fe.sub.2O.sub.3 T thermal to calcium treated calcium calcium Examples equivalent treatment Type of CaO ferrites briquette ferrites ferrites Ex. 2 20% 1200 C. CaO < 2 mm, with 30% < 90 m 95% 31% 7 93 Ex. 3 30% 1200 C. CaO < 2 mm, with 30% < 90 m 98% 47% 22.5 77.5 Ex. 4 40% 1200 C. CaO < 2 mm, with 30% < 90 m 98% 58% 55.3 44.7 Ex. 5 50% 1200 C. CaO < 2 mm, with 30% < 90 m 97% 74% 39.4 60.6 Ex. 6 50% 1100 C. 50% of (CaO < 2 mm, with 90% 65% 69.9 30.1 30% < 90 m) + 50% of CaO < 90 m Ex. 7 50% 1100 C. 100% of CaO < 90 m 96% 73% 47.2 52.8 Ex. 8 50% 1200 C. 50% of (CaO < 2 mm, with 99% 76% 43.9 56.1 30% < 90 m) + 50% of CaO < 90 m Ex. 9 50% 1100 C. CaO < 2 mm, with 30% < 90 m 61% 43% 82.6 17.4

[0222] As can be seen in Table 1, it is possible to optimize (he various parameters of percentage of iron oxide, temperature of the thermal treatment, granulometry of the quicklime, so as to obtain yields in conversion of iron oxide to calcium ferrite above 70%, preferably above 80%, more preferably above 90% with at least 40 wt % of calcium ferrites in the form of monocalcium ferrites.

[0223] In example 4, thermally treated briquettes having a yield in conversion to calcium ferrite of 98% and containing 55.3 wt % of monocalcium ferrite relative to the amount of calcium ferrites are produced after thermal treatment at 1200 C. for 20 minutes on green briquettes containing about 40 wt % of haematite and 60 wt % of quicklime having a d.sub.97 equal to 2 mm and a d.sub.30 equal to 90 m (i.e. 30% of particles under 90 m), except for the presence of 0.25 wt % of calcium stearate, relative to the total weight of the green briquettes.

[0224] In example 6, thermally treated briquettes having a yield in conversion to calcium ferrite of 90% and containing 69.9 wt % of monocalcium ferrite relative to the amount of calcium ferrites are produced after thermal treatment at 1100C. for 20 minutes on green briquettes containing about 50 wt % of haematite and 25 wt % of quicklime having a d.sub.100 equal to 2 mm and d.sub.30 equal to 90 m (i.e. 30% of particles under 90 m) and 25 wt % of quicklime having a d.sub.30 equal to 90 m, except for the presence of 0.25 wt % of calcium stearate, relative to the total weight of the green briquettes.

[0225] In example 7, thermally treated briquettes having a yield in conversion to calcium ferrite of 96% and containing 47.2 wt % of monocalcium ferrite relative to the amount of calcium ferrites are produced after thermal treatment at 1100 C. for 20 minutes on green briquettes containing about 50 wt % of haematite and 50 wt % of quicklime having a dix equal to 90 m. The yield of monocalcium ferrite can be increased by lowering the temperature of the thermal treatment to 1100 C., except for the presence of 0.25 wt % of calcium stearate, relative to the total weight of the green briquettes.

[0226] In example 8, thermally treated briquettes having a yield in conversion to calcium ferrite of 99% and containing 43.9 wt % of monocalcium ferrite relative to the amount of calcium ferrites are produced after thermal treatment at 1200 C. for 20 minutes on green briquettes containing about 50 wt % of haematite and 25 wt % of quicklime having a d.sub.97 equal to 7 mm and a d.sub.30 equal to 90 m (i.e. 30% of particles under 90 m) and 25 wt % of Quicklime having a d.sub.97 equal to 90 m, except for the presence of 0.25 wt % of calcium stearate, relative to the total weight of the green briquettes.

[0227] In example 9, thermally treated briquettes having a yield in conversion to calcium ferrite of 61% and containing 82.6 wt % of monocalcium ferrite relative to the amount of calcium ferrites are produced after thermal treatment at 1100 C. for 20 minutes on green briquettes containing about 50 wt % of haematite and 50 wt % of quicklime having a d.sub.97 equal to 2 mm and a d.sub.30 equal to 90 m (i.e. 30% of particles under 90 m). The yield of monocalcium ferrite ran be increased by increasing the amount by weight of quicklime having a d.sub.100 equal to 90 m, except for the presence of 0.25 wt % of calcium stearate, relative to the total weight of the green briquettes.

[0228] It may be advantageous in a metal refining process to have an amount of monocalcium fertile above 40 wt %. as monocalcium ferrite has a lower melting point than dicalcium ferrite. and this may accelerate dissolution of the briquettes in the slag.

[0229] It is also possible to optimize the various parameters of percentage of iron oxide, temperature of the thermal treatment granulometry of the quicklime, so as to obtain yields in conversion of iron oxide to calcium ferrite above 70%, preferably above 80%, more preferably above 90% with at least 40 wt % of calcium ferrites in the form of dicalcium ferrites. Although, as in example 7, it is possible to obtain, at 1100 C. for 20 minutes, 52.8% of dicalcium ferrites relative to the amount of calcium ferrites, most of the other examples show that the formation of at least 40% of dicalcium ferrites relative to the amount of calcium ferrites is promoted when the briquettes are submitted to a thermal treatment of 1200 C. for 20 minutes.

[0230] It may be advantageous to optimize the process parameters so as to obtain at least 40% of dicalcium ferrites relative to the amount by weight of calcium ferrites, in order to obtain a larger amount of dicalcium ferrites with a higher melting point relative to the melting point of monocalcium ferrite and thus minimize the risk of melting of the briquettes in the furnace.

[0231] FIG. 5 shows photographs of the sections of the briquettes from examples 2 to 9. The textures of the thermally treated briquettes from examples 2 to 9 were analysed by scanning electron microscopy coupled to energy dispersive analysis, by preparing a section of these briquettes, by encapsulating these briquettes in a resin, and by polishing the surface of the section. These analyses make it possible to construct a map of the distribution of each element in a section of the briquettes. Using image analysis software, it is possible to combine the maps obtained for each element and measure the size distribution and the relative coverage of each element.

[0232] It has thus been shown for the briquettes from examples 2 to 9 that calcium ferrite forms a matrix (or continuous phase) in which particles of quicklime (discontinuous phase) are dispersed. A calcium ferrite matrix can be obtained after thermal treatment for 20 minutes at temperatures between 900 C. and 1200 C., preferably between 1050 and 1200 C., of green briquettes containing at least 20 wt % of particles of calcium-magnesium compound, preferably in the form of quicklime and at least 20 wt % of iron oxide having a d.sub.90 under 200 m, preferably under 150 m, more preferably under 100 m and a d.sub.50 below 50. The two-dimensional sizes of the particles of lime dispersed in the matrix are calculated by a program that finds the average of the smallest and largest dimension of each particle of quicklime in the calcium ferrite matrix. The particles are classified in a first group of particles whose two-dimensional size is under 63 m and above the limit of detection of the measuring equipment, and a second group of particles whose two-dimensional size is above 63 m. Table 2 below shows, for the briquettes from examples 2 to 9, the relative coverage of the calcium ferrite matrix, of the particles of quicklime under 63 m and of the particles of quicklime above 63 m in the cut section from each briquette.

TABLE-US-00002 TABLE 2 Ex 2 Ex 3 Ex 4 Ex 5 Ex 6 Ex 7 Ex 8 Ex 9 Matrix 41 50 52 72 70 83 80 54 (% surface coverage) CaO < 63 m 2 3 2 4 8 11 4 4 (% surface coverage) CaO > 63 m 56 47 46 24 22 6 17 42 (% surface coverage)

[0233] The percentages of surface coverage of the particles of quicklime above 63 m are less than 25% for thermally treated briquettes having contents of calcium ferrites above 60 wt % of the composition.

Example 10

[0234] Green briquettes were prepared with 38.85 wt % of iron oxide in the form of magnetite Fe.sub.3O.sub.4 having a d.sub.97 of 150 m and with 60.9 wt % of quicklime having a d.sub.97 below 2 mm and a d.sub.+below 90 m as well as 0.25 wt % of calcium stearate, relative to the weight of the briquette. Thermal treatment was carried out on a static bed of three layers of briquettes for 20 mm at 1100 C. in order to obtain thermally treated briquettes and the percentage by weight of iron converted to monocalcium ferrite is 8% whereas the percentage of iron converted to dicalcium ferrite is 82%.

Example 11

[0235] Green briquettes were prepared with 39.9 wt % of iron oxide In the form of haematite Fe.sub.2O.sub.3 characterized by a d.sub.10 of 0.5 m, d.sub.50 of 12.3 m and d.sub.90 of 35.7 m and with 59.85 wt % of quicklime having a d.sub.97 below 2 mm and a d.sub.30 below 90 m and 0.25 wt % of calcium stearate relative to the weight of the briquette. The green briquettes obtained were treated thermally in the same conditions as in example 17 in order to obtain thermally treated briquettes, in this case, the percentage of iron converted to monocalcium ferrite is 65 wt % and the percentage of iron converted to dicalcium ferrite is 24 wt %.

Examples 12 to 28-Protreatment under modified atmosphere containing CO.SUB.2 .corresponding respectively to tests 1 to 17 in Table 3.

[0236] In the following examples, compressive strength tests were performed on the briquettes using a Pharmatron Multitest 50, or* of the plates of which is equipped with a point. The presence of a point reduces the force necessary to cause rupture of the briquettes relative to a compressive strength test carried out without the point. 10 green briquettes containing 59.85 wt % of quicklime similar to that used in example 1, 39.9% of Fe.sub.2O.sub.3 from example 11 and 0.25% of calcium stearate were characterized by this compressive strength test. The average value is 33 kg-force.

[0237] Several ore-treatment tests were carried out, varying the parameters as indicated in Table 4, each time charging 10 new green briquettes in an 11-litre electric muffle furnace. All these pre-treatments were carried out between 20 and 450 C. under a flow of 10 litres per minute of a gas mixture formed from H.sub.2O and CO.sub.2. The ramps of temperature rise are between 2 and 10 C./min.

[0238] The concentrations by volume of H.sub.2O in the gas are between 3.9 and 20.1%. The concentrations by volume of CO.sub.2 in the gas are between 0.9 and 9.1%.

[0239] At the end of the pre-treatment, for each test, the 10 briquettes were characterized by compressive strength testing. In addition, all 10 pre-treated briquettes were analysed to determine the weight gains relating to hydration dm(H.sub.2O)/m and to carbonation dm(CO.sub.2)/m. All of the results are presented in Table 3.

[0240] As can be seen, beyond 2 vol % of CO.sub.2 in the gas forming the modified atmosphere, the pre-treatment leads to consolidation of the briquettes. Conversely, below 2 vol % of CO.sub.2, the briquettes become less cohesive.

Comparative Example 4

[0241] The shatter indices were compared with the compressive force for several samples of green briquettes to establish the correlation between the shatter index and the compressive force. The green briquettes tested comprised quicklime with particle sire between 0 and 3 mm with different contexts of iron oxide, from 0 to 60 wt % and different contents of lubricant, ranging from 0.125 to 0.5 wt %, relative to the total weight of the briquettes. The parameters of the briquetting process were also altered to ensure that the population was large enough for establishing the correlation.

[0242] A compressive force of greater than 144 kg. corresponding to 317.5 pounds, is required for briquettes having a shatter index below 10%.

[0243] Of course, the present invention is not in any way limited to the embodiments described above, and a great many modifications may be made to it while remaining within the scope of the appended claims.