Method for Manufacturing Briquettes Containing a Calcium-Magnesium Compound and an Iron-Based Compound, and Briquettes Obtained Thereby

Abstract

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

Claims

1: Method for manufacturing a calcium-magnesium composition in the form of briquettes, comprising the following steps: i. 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; ii. feeding a roller press with said homogeneous mixture, iii. compressing said pulverulent mixture in said roller press, with obtaining a calcium-magnesium composition in the form of green briquettes, and iv. collecting said green briquettes, characterized in that said pulverulent mixture comprises an iron-based compound present at a content of at least 3%, preferably at least 12%, preferably at least 20%, preferably at least 30%, more preferably at least 35 wt % of Fe.sub.2O.sub.3 equivalent relative to the weight of said composition, said iron-based compound having a very fine granulometric distribution characterized by a median size d.sub.50 below 100 m, preferably below 50 m as well as a size d.sub.90 below 200 m, preferably below 150 m, preferably below 130 m, more preferably below 100 m, wherein said at least one quick calcium-magnesium compound comprising at least 40 wt % of CaO+MgO equivalent comprises a fraction of particles of calcium-magnesium compound having a particle size 90 m having at least 20 weight % CaO equivalent with respect to the weight of said pulverulent mixture, and wherein said iron-based compound comprises at least 50 weight %, preferably at least 60 weight %, more preferably at least 70 weight %, more preferably at least 80 weight % and particularly more than 95 weight % of iron oxide under the form of hematite Fe.sub.2O.sub.3 with respect to the total weight of the iron-based compound and in that the rollers of the 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.

2: Method according to claim 1, in which said compression step is effected 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.

3: Method according to claim 1, further comprising a thermal treatment at a temperature lower or equal to 1150 C., preferably lower or equal to 1100 C., preferably higher or equal to 900 C., preferably according to the rule (predetermined duration)/(thermal treatment temperature1000 C.)>5.

4: Method according to claim 3, wherein said thermal treatment of green briquettes occur for a predetermined duration of between 3 and 20 minutes, preferably greater than or equal to 5 minutes and less than or equal to 15 minutes.

5: Method according to claim 1, in which said quick calcium-magnesium compound is quicklime.

6: Method according to claim 1, further comprising, before said supplying of a homogeneous pulverulent mixture, v. feeding a mixer with at least 40 wt % of CaO+MgO equivalent of a quick calcium-magnesium compound relative to the weight of said composition and with at least 3 wt %, preferably at least 12 wt %, more preferably at least 20 wt %, preferably at least 25 wt %, more preferably at least 30 wt %, more preferably at least 35 wt % of Fe.sub.2O.sub.3 equivalent of an iron-based compound relative to the weight of said composition, said iron-based compound having a very fine granulometric distribution characterized by a median size d.sub.50 below 100 m, preferably below 50 m as well as a size d.sub.90 below 200 m, preferably below 150 m, preferably below 130 m, more preferably below 100 m; said at least one quick calcium-magnesium compound comprising at least 40 wt % of CaO+MgO equivalent comprises a fraction of particles of calcium-magnesium compound having a particle size 90 m having at least 20 weight % CaO equivalent with respect to the weight of said pulverulent mixture, and wherein said iron-based compound comprises at least 50 weight %, preferably at least 60 weight %, more preferably at least 70 weight %, more preferably at least 80 weight % and particularly more than 95 weight % of iron oxide under the form of hematite Fe.sub.2O.sub.3 with respect to the total weight of the iron-based compound vi. mixing said quick calcium-magnesium compound with said iron-based compound for a predetermined length of time, sufficient to obtain an approximately homogeneous pulverulent mixture of said quick calcium-magnesium compound and of said iron-based compound.

7: Method according to claim 6, in which said binder or lubricant is added to the mixer, and in which said binder or lubricant is included in said homogeneous pulverulent mixture.

8: Method according to claim 1, in which said quick calcium-magnesium compound contains at least 10 wt % of quicklime in the form of ground particles.

9: Method according to claim 1, further comprising a pre-treatment step of the briquettes under modified atmosphere containing at least 2 vol % CO.sub.2 and at most 30 vol % CO.sub.2, preferably at most 20 vol % CO.sub.2, more preferably at most 15 vol % CO.sub.2, even more preferably at most 10 vol % CO.sub.2 with respect to the modified atmosphere.

10: Method according to claim 1, wherein said pulverulent mixture further comprises at least 10% of particles of quick calcium-magnesium compound having a particle size 90 m and 5 mm with respect to the weight of the pulverulent mixture.

11: Method according to claim 1, wherein 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 with respect to the weight of the pulverulent mixture.

12: Method according to claim 1, wherein said iron-based compound is present at an amount of at least 20 wt %, preferably at least 25 wt %, more preferably of at least 30 wt %, in particular of at least 35 wt % relative to the total weight of the pulverulent mixture.

13: Method according to claim 1, wherein the weight percentage of CaO equivalent in the fraction of quick calcium-magnesium compound having a particle size <90 m relative to the total of the weight percentage of quicklime in the fraction of calcium-magnesium compound having a particle size <90 m and the % of Fe.sub.2O.sub.3 equivalent of said iron-based compound having a very fine granulometric distribution is <40, preferably <38, more preferably <36% and higher than 20%, preferably higher than 22%, preferably 24%.

14-26. (canceled)

27: Composition in the form of green briquettes comprising at least one quick calcium-magnesium compound and an iron-based compound, characterized in that the composition comprises at least 40 wt % of CaO+MgO equivalent relative to the weight of said composition, said composition having a Ca/Mg molar ratio greater than or equal to 1, preferably greater than or equal to 2, more preferably greater than or equal to 3 and characterized in that said iron-based compound is present at a content of at least 3 wt %, preferably at least 12 wt %, more preferably at least 20 wt %, preferably at least 30 wt %, more preferably at least 35 wt % of Fe.sub.2O.sub.3 equivalent relative to the weight of said composition, said iron-based compound having a very fine granulometric distribution characterized by a median size d.sub.50 below 100 m, preferably below 50 m as well as a size d.sub.90 below 200 m, preferably below 150 m, preferably below 130 m, more preferably below 100 m, wherein said at least one quick calcium-magnesium compound comprising at least 40 wt % of CaO+MgO equivalent comprises a fraction of particles of calcium-magnesium compound having a particle size 90 m having at least 20 weight % CaO equivalent with respect to the weight of said composition; said iron-based compound comprises at least 50 weight %, preferably at least 60 weight %, more preferably at least 70 weight %, more preferably at least 80 weight % and particularly more than 95 weight % of iron oxide under the form of hematite Fe.sub.2O.sub.3 with respect to the total weight of the iron-based compound.

28: Composition in the form of green briquettes according to claim 27, in which said calcium-magnesium compound is quicklime.

29: Composition in the form of green briquettes according to claim 27, in which said quick calcium-magnesium compound comprises: fine particles of calcium-magnesium compound selected from fine particles rejected in screening in the production of the pebbles of said quick calcium-magnesium compound, calcium-magnesium filter dust at a concentration from 0 wt % to 90 wt % relative to the total weight of said quick calcium-magnesium compound, and from 10 to 100 wt % of quicklime in the form of ground particles, relative to the total weight of said quick calcium-magnesium compound.

30: Composition in the form of green briquettes according to any one of claim 27, having a BET specific surface area greater than or equal to 1 m.sup.2/g, preferably greater than or equal to 1.2 m.sup.2/g, more preferably greater than or equal to 1.4 m.sup.2/g.

31: Composition in the form of green briquettes according to claim 27, having a porosity greater than or equal to 20%, preferably greater than or equal to 22%, more preferably greater than or equal to 24%.

32: Composition in the form of green briquettes according to claim 27, further comprising 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.

33: Composition in the form of green briquettes according to claim 27, further comprising at least 10% of particles of quick calcium-magnesium compound having a particle size 90 m and 5 mm relative to the total weight of the composition.

34: Composition in the form of green briquettes according to claim 27, further comprising 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 the composition.

35: Composition in the form of green briquettes according to claim 27, wherein the weight percentage of CaO equivalent in the fraction of quick calcium-magnesium compound having a particle size <90 m relative to the total of the weight percentage of quicklime in the fraction of calcium-magnesium compound having a particle size <90 m and the % of Fe.sub.2O.sub.3 equivalent of said iron-based compound having a very fine granulometric distribution is <40, preferably <38, more preferably <36% and higher than 20%, preferably higher than 22%, preferably 24%.

36-44. (canceled)

45: Composition in the form of thermally treated briquettes, comprising at least one iron-based compound, said composition 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 preferably greater than or equal to 3, characterized in that said iron-based compound is present at a content of at least 3 wt %, preferably at least 12 wt %, more preferably at least 20 wt %, preferably at least 30 wt %, more preferably at least 35 wt % of Fe.sub.2O.sub.3 equivalent relative to the weight of said composition, said iron-based compound comprising at least 60% of calcium ferrite, expressed by weight of Fe.sub.2O.sub.3 equivalent, relative to the total weight of said iron-based compound expressed by weight of Fe.sub.2O.sub.3 equivalent, at least 40 wt %, preferably 50 wt % of said calcium ferrites are under the form of monocalcium ferrite CaFe.sub.2O.sub.4.

46: Composition in the form of thermally treated briquettes according to claim 45, in which said iron-based compound comprises at least 70%, preferably at least 80%, and even more preferably at least 90 wt % of calcium ferrite relative to the total weight of said iron-based compound.

47: Composition in the form of thermally treated briquettes according to claim 45, having a BET specific surface area greater than or equal to 0.4 m.sup.2/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.

48: Composition in the form of thermally treated briquettes according to claim 45, having a porosity greater than or equal to 20%, preferably greater than or equal to 22%, more preferably greater than or equal to 24%.

49: Composition in the form of thermally treated briquettes according to claim 45, in which the thermally treated briquettes have a Shatter test index below 8%, preferably below 6%, preferably below 4%, and more preferably below 3%, in particular below 2%, said Shatter test index being the percentage by weight of fines under 10 mm generated after 4 drops from 2 m starting from 10 kg of product, the fines being quantified by sieving through a screen with square mesh of 10 mm after 4 drops from 2 m.

50: Composition in the form of thermally treated briquettes according to claim 45, characterized in that it further comprises particles of quick calcium-magnesium compound, preferably particles of quicklime having a 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 and covering at most 20% of the area of said section and preferably at most 10% of the area of said section.

51: Composition in the form of thermally treated briquettes according to claim 45, characterized in that it further comprises particles of quick calcium-magnesium compound, preferably particles of quicklime having a 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 and covering at least 20% of the area of said section and preferably at most 60% of the area of said section.

52-63. (canceled)

Description

[0329] Other features, details and advantages of the invention will become clear from the description given hereunder, which is non-limiting and refers to the examples and to the figures.

[0330] FIG. 1 is a graph of the BET specific surface area and of kerosene intrusion porosity as a function of the content of Fe.sub.2O.sub.3 equivalent in the briquettes according to the present invention.

[0331] FIG. 2 is a graph of the Shatter test 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.

[0332] 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.

[0333] FIG. 4 is a graph of the variation of the content of calcium ferrites expressed as 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.

[0334] FIG. 5 shows photographs of sections of the briquettes according to examples 9 to 16.

[0335] 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 granulometric distribution characterized by a median size d.sub.50 below 100 m, preferably below 50 m as well as a size d.sub.50 below 200 m, preferably below 150 m, preferably below 130 m, more preferably below 100 m.

[0336] The method of briquetting according to the invention comprises supplying an approximately homogeneous pulverulent mixture comprising at least 40 wt % of CaO+MgO equivalent of a quick calcium-magnesium compound and at least 3 wt %, preferably at least 12 wt %, more preferably at least 20 wt %, preferably at least 30 wt %, more preferably at least 35 wt % of an iron-based compound expressed in Fe.sub.2O.sub.3 equivalent relative to the weight of said composition, in which said quick calcium-magnesium compound comprising at least 40 wt % CaO+MgO equivalent further comprises at least a fraction of particles of calcium-magnesium compound having a particle size 90 m, which latter further comprises at least 20 wt % CaO equivalent with respect to the weight of the pulverulent mixture.

[0337] In a particular embodiment of the invention, said pulverulent mixture comprises at most 97 wt %, preferably at most 90 wt %, preferably at most 88%, in certain embodiments at most 60 wt % of CaO+MgO equivalent relative to the weight of said composition.

[0338] The homogeneous mixture in which the iron-based compound is uniformly distributed is fed into a roller press, also sometimes called a tangential press, for example a Komarek, Sahut Konreur, Hosokawa Bepex, or Koppern press. In the roller press, the approximately 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.

[0339] In operation, 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.

[0340] Assuming an angle of degree at which the linear pressure is applied on the surface of the hoops, the surface pressure can be calculated, which is equal to the linear pressure divided by (.Math..Math.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.

[0341] After compression, the calcium-magnesium composition is obtained in the form of green briquettes, which are collected.

[0342] In a preferred embodiment of 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 carried out preferably for a predetermined time of between 3 and 20 minutes, obtaining thermally treated briquettes in which said iron oxide is converted to calcium ferrite, i.e. thermally treated briquettes comprising a quick calcium-magnesium compound and a calcium ferrite compound present at a content of at least 3%, preferably at least 12%, more preferably at least 20%, preferably at least 30%, more preferably at least 35% of Fe.sub.2O.sub.3 equivalent.

[0343] In one embodiment of the invention, said thermal treatment of the green briquettes is carried out in a rotary kiln at high temperature. Preferably, the rotary kiln is used for thermal treatment of briquettes whose iron oxide content is below 40%.

[0344] 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.

[0345] 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 quenching.

[0346] 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.

[0347] In a preferred embodiment according to the present invention, the method comprises, before said supplying of a homogeneous pulverulent mixture,

[0348] i. feeding a powder mixer with at least 40 wt % of CaO+MgO equivalent of a quick calcium-magnesium compound and with at least 3%, preferably at least 12%, more preferably at least 20%, preferably at least 30%, more preferably at least 35% of an iron-based compound expressed in Fe.sub.2O.sub.3 equivalent having a very fine granulometric distribution characterized by a median size d.sub.50 below 100 m, preferably below 50 m as well as a size d.sub.90 below 200 m, preferably below 150 m, preferably below 130 m, more preferably below 100 m; said quick calcium-magnesium compound comprising at least 40 wt % CaO+MgO equivalent further comprises at least a fraction of particles of calcium-magnesium compound having a particle size 90 m, which latter further comprises at least 20 wt % CaO equivalent with respect to the weight of the pulverulent mixture.

[0349] and

[0350] ii. mixing said quick calcium-magnesium compound with said iron-based compound for a predetermined length of time, sufficient to obtain an approximately homogeneous pulverulent mixture of said quick calcium-magnesium compound and of said iron-based compound.

[0351] In a variant of the invention, the calcium-magnesium compound comprises at least 10 wt % of ground quicklime particles, preferably at least 20 wt %, more particularly at least 30 wt % and at most 100 wt % relative to the total weight of said calcium-magnesium compound.

[0352] The green briquettes are based on quicklimes (optionally dolomitic) and ultrafine particles of iron oxide. They are characterized by an iron content by weight of at least 3 wt %, preferably at least 12 wt %, more preferably at least 20 wt %, preferably at least 30 wt %, more preferably at least 35 wt % expressed in Fe.sub.2O.sub.3 equivalent. The green briquettes are also characterized by a content by weight of calcium and magnesium of at least 40 wt %, expressed in CaO and MgO equivalent. Chemical analysis is performed by X-ray fluorescence spectrometry (XRF) according to standard EN 15309.

[0353] Semiquantitative chemical analysis by XRF for determining the relative concentration by weight of the elements whose atomic mass is between 16 (oxygen) and 228 (uranium) is carried out starting from the samples ground to 80 m and formed into pellets. The sample is excited by a high-energy source (primary X-rays), and on recovering its original state of excitation, the sample emits secondary X-rays, characteristic of the chemical elements making up the sample.

[0354] The samples are put in a PANalytical/MagiX Pro PW2540 apparatus, operating in wavelength dispersion mode. Measurement is performed with a power of 50 kV and 80 mA, with a Duplex detector.

[0355] The analysis results give the calcium, magnesium and iron content and these measurements are reported in weight of CaO and MgO equivalent, and weight of Fe.sub.2O.sub.3 equivalent.

[0356] Semiquantitative 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 carried out based on an X-ray diffraction pattern by the Rietveld method.

[0357] This method consists of simulating a diffraction pattern using 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 semiquantitative 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 percentage of total iron in the form of calcium ferrites is obtained by simple division (Fe in the ferrites divided by Fe in all of the iron-based compounds).

[0358] The green briquettes are also characterized by a BET specific surface area greater than or equal to 1 m.sup.2/g, preferably 1.2 m.sup.2/g, preferably 1.4 m.sup.2/g.

[0359] The porosity of the green briquettes is greater than or equal to 20%, preferably 22%, preferably 24%.

[0360] The green briquettes have an apparent density between 2.0 and 3.0 and preferably between 2.2 and 2.8.

[0361] The 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 1.5% of weight increase, preferably 2% of weight increase, and more preferably 2.5% of weight increase, following the reaction of hydration of quicklime CaO to slaked lime Ca(OH).sub.2.

[0362] The thermally treated briquettes comprise a calcium-magnesium compound, for example quicklimes (dolomitic) and an iron-based compound, containing ultrafine particles of iron oxide and calcium ferrites CaFe.sub.2O.sub.4 and/or Ca.sub.2Fe.sub.2O.sub.5.

[0363] The thermally treated briquettes are characterized by an iron content by weight of at least 3 wt %, preferably at least 12 wt %, more preferably at least 20 wt %, preferably at least 30 wt %, more preferably at least 35 wt % expressed in Fe.sub.2O.sub.3 equivalent. They are also characterized by a content by weight of calcium and magnesium of at least 40 wt % expressed in CaO and MgO equivalent. Chemical analysis is carried out by XRF, as mentioned above.

[0364] At least 40%, preferably at least 50%, preferably at least 60% and more preferably at least 70% of the total iron is in the form of calcium ferrites.

[0365] Quantification of the calcium ferrites is performed by XRD/Rietveld analysis after grinding the briquettes, as for the green briquettes.

[0366] The thermally treated briquettes of the present invention have a Shatter test index (STI, 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.

[0367] 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.

[0368] The porosity is greater than or equal to 20%, preferably 22%, preferably 24%.

[0369] The thermally treated briquettes have an apparent density between 2.0 and 3.0 and preferably between 2.2 and 2.8.

[0370] 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 1Briquettes of Quicklime and Iron Oxide

[0371] Quicklime fines from grinding were prepared from a soft-burned lump lime produced in a parallel-flow regenerative kiln. Grinding is performed 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 having a particle size lower than 90 m (d.sub.3090 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 of 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 multipoint 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.

[0372] The iron oxide fines are obtained from grinding an iron ore of the magnetite type, Fe.sub.3O.sub.4, passing through a 125-m sieve and characterized in Coulter laser granulometry by a d.sub.10 of 8 m, a d.sub.50 of 52 m and a d.sub.90 of 126 m. These iron oxide fines contain 66.4% of Fe.

[0373] A Gericke GCM450 powder mixer is used, with a capacity of 10 dm.sup.3, equipped with standard paddles with radius of 7 cm, rotating at 350 revolutions per minute (i.e. 2.6 m/s).

[0374] This mixer is used in continuous mode for preparing a mixture consisting of: [0375] 89.75 wt % of said quicklime fines from grinding, [0376] 10 wt % of said iron oxide fines, [0377] 0.25 wt % of powdered calcium stearate.

[0378] The total flow rate of powder is 300 kg/h and the residence time is 3.5 s.

[0379] The mixture obtained is very homogeneous. This signifies that the Fe content for different 10 g samples taken from the final mixture is always plus or minus 5% of the mean value.

[0380] A tangential press is used, equipped with hoops with a diameter of 604 mm and width of 145 mm for producing briquettes with a theoretical volume of 7.2 cm.sup.3 in the shape of a bar of soap (4 arrays of 67 pockets per hoop, or 268 pockets per hoop), capable of developing a linear pressure of up to 120 kN/cm.

[0381] Starting with 10 tonnes of the mixture, the tangential press is supplied and compaction is performed at a speed of 12 revolutions per minute (i.e. 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).

[0382] Nearly 8.5 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 BET specific surface area of 1.6 m.sup.2/g and have 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) by 26%.

[0383] The water reactivity of the briquettes is determined by adding 166.7 g of these briquettes, previously ground to fines with a size between 0 and 1 mm, to 600 ml of water at 20 C. The 166.7 g of briquettes corresponds to 150 g of quicklime. The value of t.sub.60 is 1 min.

[0384] A Shatter test is carried out with 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. A Shatter test index of 4.6% is obtained.

[0385] The granulometric distribution of the iron-based particles in the composition in briquette form is determined by scanning electron microscopy and X-ray mapping, coupled to image analysis. The results are presented in Table 1. The volume fraction of iron oxide at the surface of the iron oxide particles is 54%. The iron oxide powder therefore contains 54% of active iron oxide.

[0386] The briquettes are also characterized by carrying out a thermal treatment of 10 min at 1100 C. (hot charge/discharge) on 3 of these briquettes, at the end of which a powder with granulometry under 80 m is prepared. The latter is characterized by X-ray diffraction, and phase quantification is performed by Rietveld analysis. 52% of the total iron is in the form of calcium ferrites CaFe.sub.2O.sub.4 or Ca.sub.2Fe.sub.2O.sub.5, and 48% is still in the form of Fe.sub.2O.sub.3.

Example 2Briquettes of Quicklime and Iron Oxide

[0387] The quicklime fines from grinding are those from example 1. The iron oxide fines are obtained from grinding an iron ore of the magnetite type, Fe.sub.3O.sub.4, passing through a 150 m sieve and characterized in Coulter laser granulometry by a d.sub.10 of 9 m, a d.sub.50 of 37 m and a d.sub.50 of 102 m. These iron oxide fines contain 67.1% of Fe.

[0388] The mixture, prepared by the method in example 1, consists of: [0389] 89.75 wt % of said quicklime fines from grinding, [0390] 10 wt % of said iron oxide fines, [0391] 0.25 wt % of powdered calcium stearate.

[0392] The briquettes are produced from this mixture by the method in example 1. 8.6 tonnes of briquettes are obtained having an average volume of 8.4 cm.sup.3, an average weight of 20.3 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 15.6 mm. These briquettes develop a BET specific surface area of 1.6 m.sup.2/g and have a total mercury pore volume of 26%.

[0393] The water reactivity of the briquettes is determined by adding 166.7 g of these briquettes, previously ground to fines with a size between 0 and 1 mm, to 600 mL of water at 20 C. The 166.7 g of briquettes corresponds to 150 g of quicklime. The value of t.sub.60 is 0.9 min.

[0394] A Shatter test is carried out with 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. A Shatter test index of 4.5% is obtained.

[0395] The volume fraction of iron oxide at the surface of the iron oxide particles in the composition in briquette form is 63%.

[0396] The briquettes are also characterized by carrying out a thermal treatment of 10 min at 1100 C. (hot charge/discharge) on 3 of these briquettes, at the end of which a powder with granulometry under 80 m is prepared. The latter is characterized by X-ray diffraction, and phase quantification is performed by Rietveld analysis. 61% of the total iron is in the form of calcium ferrites CaFe.sub.2O.sub.4 or Ca.sub.2Fe.sub.2O.sub.5, and 39% is still in the form of Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4.

Example 3Briquettes of Quicklime and Iron Oxide

[0397] The quicklime fines from grinding are those from example 1. The iron oxide fines are obtained from grinding an iron ore of the hematite type, Fe.sub.2O.sub.3, passing through a 150 m sieve and characterized in Coulter laser granulometry by a d.sub.10 of 0.5 m, a d.sub.50 of 12.3 m and a d.sub.50 of 35.7 m. These iron oxide fines contain 64.6% of Fe.

[0398] The mixture, prepared by the method in example 1, consists of: [0399] 89.75 wt % of said quicklime fines from grinding, [0400] 10 wt % of said iron oxide fines, [0401] 0.25 wt % of powdered calcium stearate.

[0402] The briquettes are produced from this mixture by the method in example 1. 8.3 tonnes of briquettes are obtained having an average volume of 8.5 cm.sup.3, an average weight of 20.1 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 15.7 mm. These briquettes develop a BET specific surface area of 1.7 m.sup.2/g and have a total mercury pore volume of 26%. The water reactivity of the briquettes is determined by adding 166.7 g of these briquettes, previously ground to fines with a size between 0 and 1 mm, to 600 mL of water at 20 C. The 166.7 g of briquettes corresponds to 150 g of quicklime. The value of t.sub.60 is 0.9 min.

[0403] A Shatter test is carried out with 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. A Shatter test index of 3.7% is obtained.

[0404] The volume fraction of iron oxide at the surface of the iron oxide particles in the composition in briquette form is 88%.

[0405] The briquettes are also characterized by carrying out a thermal treatment of 10 min at 1100 C. (hot charge/discharge) on 3 of these briquettes, at the end of which a powder with granulometry under 80 m is prepared. The latter is characterized by X-ray diffraction, and phase quantification is performed by Rietveld analysis. 84% of the total iron is in the form of calcium ferrites CaFe.sub.2O.sub.4 or Ca.sub.2Fe.sub.2O.sub.5, and 16% is still in the form of Fe.sub.2O.sub.3.

Example 4Thermally Treated Briquettes of Quicklime and Iron Oxide

[0406] Starting from 1 tonne of briquettes from example 1, arranged in boxes in such a way that the thickness of the bed of briquettes is 100 mm, a thermal treatment of 20 min at 1100 C. is carried out, with ramps of temperature rise and fall of 50 C. per minute.

[0407] Briquettes are obtained having an average volume of 8.2 cm.sup.3, an average weight of 19 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 15.5 mm. These briquettes develop a BET specific surface area of 1.2 m.sup.2/g and have a total mercury pore volume of 27%.

[0408] A Shatter test is carried out with 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. A Shatter test index of 1.6% is obtained.

[0409] The granulometric distribution of the iron-based particles in the composition in briquette form is determined by scanning electron microscopy and X-ray mapping, coupled to image analysis.

[0410] The results are presented in Table 1.

[0411] The volume fraction of iron oxide at the surface of the iron oxide particles is 43%. The iron oxide powder therefore contains 43% of active iron oxide.

[0412] Starting from 30 of these thermally treated briquettes, a powder is prepared with granulometry under 80 m. The latter is characterized by X-ray diffraction, and phase quantification is performed by Rietveld analysis. 54% of the total iron is in the form of calcium ferrites CaFe.sub.2O.sub.4 or Ca.sub.2Fe.sub.2O.sub.5, and 46% is still in the form of Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4.

[0413] The water reactivity of the briquettes is determined by adding 174.2 g of these briquettes, previously ground to fines with a size between 0 and 1 mm, to 600 mL of water at 20 C. The 174.2 g of briquettes corresponds to 150 g of free quicklime (i.e. not in the form of calcium ferrites). The value of t.sub.60 is 4.7 min.

Example 5Thermally Treated Briquettes of Quicklime and Iron Oxide

[0414] Starting from 1 tonne of briquettes from example 3, arranged in boxes in such a way that the thickness of the bed of briquettes is 100 mm, a thermal treatment of 20 min at 1100 C. is carried out, with ramps of temperature rise and fall of 50 C. per minute.

[0415] Briquettes are obtained having an average volume of 8.5 cm.sup.3, an average weight of 20.0 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 15.7 mm. These briquettes develop a BET specific surface area of 0.9 m.sup.2/g and have a total mercury pore volume of 27%.

[0416] A Shatter test is carried out with 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. A Shatter test index of 1.4% is obtained.

[0417] The volume fraction of iron oxide at the surface of the iron oxide particles is 84%. The iron oxide powder therefore contains 84% of active iron oxide.

[0418] Starting from 30 of these thermally treated briquettes, a powder is prepared with granulometry under 80 m. The latter is characterized by X-ray diffraction, and phase quantification is performed by Rietveld analysis. 91% of the total iron is in the form of calcium ferrites CaFe.sub.2O.sub.4 or Ca.sub.2Fe.sub.2O.sub.5, and 9% is still in the form of Fe.sub.2O.sub.3.

[0419] The water reactivity of the briquettes is determined by adding 179.4 g of these briquettes, previously ground to fines with a size between 0 and 1 mm, to 600 mL of water at 20 C. The 179.4 g of briquettes corresponds to 150 g of free quicklime (i.e. not in the form of calcium ferrites). The value of t.sub.60 is 3.8 min.

Example 6Briquettes of Quicklime and Iron Oxide

[0420] The quicklime fines from grinding are those from example 1. The screened quicklime fines were recovered after the ungraded material at the outlet of a rotary kiln equipped with a preheater was screened through a 3 mm screen. These screened quicklime fines contain 26% of particles having a particle size lower than 90 m, 74% of particles above 90 m, 60% of particles above 500 m, 47% of particles above 1 mm and 18% of particles between 2 and 3 mm. The value of t.sub.60 in the water reactivity test is 4 min. The BET specific surface area is 1.2 m.sup.2/g. These screened quicklime fines contain 97.1% of CaO and 0.7% of MgO by weight. The iron oxide fines are those from example 3.

[0421] The mixture, prepared by the method in example 1, consists of: [0422] 44.75 wt % of said quicklime fines from grinding, [0423] 45 wt % of said screened quicklime fines, [0424] 10 wt % of said iron oxide fines, [0425] 0.25 wt % of powdered calcium stearate.

[0426] The briquettes are produced from this mixture by the method in example 1. 8.6 tonnes of briquettes are obtained having an average volume of 8.6 cm.sup.3, an average weight of 20.3 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 15.7 mm. These briquettes develop a BET specific surface area of 1.4 m.sup.2/g and have a total mercury pore volume of 26%. The water reactivity of the briquettes is determined by adding 166.7 g of these briquettes, previously ground to fines with a size between 0 and 1 mm, to 600 mL of water at 20 C. The 166.7 g of briquettes corresponds to 150 g of quicklime. The value of t.sub.60 is 1.6 min.

[0427] A Shatter test is carried out with 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. A Shatter test index of 4.4% is obtained.

[0428] The volume fraction of iron oxide at the surface of the iron oxide particles in the composition in briquette form is 86%.

[0429] The briquettes are also characterized by carrying out a thermal treatment of 10 min at 1100 C. (hot charge/discharge) on 3 of these briquettes, at the end of which a powder with granulometry under 80 m is prepared. The latter is characterized by X-ray diffraction, and phase quantification is performed by Rietveld analysis. 83% of the total iron is in the form of calcium ferrites CaFe.sub.2O.sub.4 or Ca.sub.2Fe.sub.2O.sub.5, and 17% is still in the form of Fe.sub.2O.sub.3.

Example 7Briquettes of Quicklime and Iron Oxide

[0430] The screened quicklime fines are those from example 6. The iron oxide fines are those from example 3.

[0431] The mixture, prepared by the method in example 1, consists of: [0432] 89.75 wt % of said screened quicklime fines, [0433] 10 wt % of said iron oxide fines, [0434] 0.25 wt % of powdered calcium stearate.

[0435] The briquettes are produced from this mixture by the method in example 1. 8.1 tonnes of briquettes are obtained having an average volume of 8.5 cm.sup.3, an average weight of 20.0 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 15.6 mm. These briquettes develop a BET specific surface area of 1.3 m.sup.2/g and have a total mercury pore volume of 26%. The water reactivity of the briquettes is determined by adding 166.7 g of these briquettes, previously ground to fines with a size between 0 and 1 mm, to 600 mL of water at 20 C. The 166.7 g of briquettes corresponds to 150 g of quicklime. The value of t.sub.60 is 3.7 min.

[0436] A Shatter test is carried out with 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. A Shatter test index of 11.6% is obtained.

[0437] The volume fraction of iron oxide at the surface of the iron oxide particles in the composition in briquette form is 87%.

[0438] The briquettes are also characterized by carrying out a thermal treatment of 10 min at 1100 C. (hot charge/discharge) on 3 of these briquettes, at the end of which a powder with granulometry under 80 m is prepared. The latter is characterized by X-ray diffraction, and phase quantification is performed by Rietveld analysis. 81% of the total iron is in the form of calcium ferrites CaFe.sub.2O.sub.4 or Ca.sub.2Fe.sub.2O.sub.5, and 19% is still in the form of Fe.sub.2O.sub.3.

Example 8Briquettes of Dolomitic Quicklime and Iron Oxide

[0439] The quicklime fines from grinding are those from example 1. The fines of burned dolomite from grinding were prepared starting from a burned lump dolomite produced in a parallel-flow regenerative kiln. Grinding was carried out in a hammer mill. These fines of burned dolomite from grinding contain 91% of particles above 90 m, 44% of particles above 500 m, 31% of particles above 1 mm and 17% of particles above 2 mm and 8% of particles between 3 and 5 mm. The value of t.sub.70 of the water reactivity test is 3.1 min. The BET specific surface area is 2.8 m.sup.2/g. These fines of burned dolomite from grinding contain 58.5% of CaO and 38.4% of MgO by weight. The iron oxide fines are those from example 3.

[0440] The mixture, prepared by the method in example 1, consists of: [0441] 64.75 wt % of said quicklime fines from grinding, [0442] 25 wt % of said fines of burned dolomite from grinding, [0443] 10 wt % of said iron oxide fines, [0444] 0.25 wt % of powdered calcium stearate.

[0445] The briquettes are produced from this mixture by the method in example 1. 8.3 tonnes of briquettes are obtained having an average volume of 8.4 cm.sup.3, an average weight of 19.9 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 15.5 mm. These briquettes develop a BET specific surface area of 2.1 m.sup.2/g and have a total mercury pore volume of 25%.

[0446] A Shatter test is carried out with 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. A Shatter test index of 5.3% is obtained.

[0447] The volume fraction of iron oxide at the surface of the iron oxide particles in the composition in briquette form is 86%.

[0448] The briquettes are also characterized by carrying out a thermal treatment of 10 min at 1100 C. (hot charge/discharge) on 3 of these briquettes, at the end of which a powder with granulometry under 80 m is prepared. The latter is characterized by X-ray diffraction, and phase quantification is performed by Rietveld analysis. 84% of the total iron is in the form of calcium ferrites CaFe.sub.2O.sub.4 or Ca.sub.2Fe.sub.2O.sub.5, and 16% is still in the form of Fe.sub.2O.sub.3.

Comparative Example 1Briquettes of Quicklime and Iron Oxide of Low Activity

[0449] The quicklime fines from grinding are those from example 1. The iron oxide fines are obtained from grinding an iron ore of the magnetite type, Fe.sub.3O.sub.4, passing through a 250 m sieve but not passing through a 125 m sieve, characterized in Coulter laser granulometry by a d.sub.10 of 140 m, a d.sub.50 of 227 m and a d.sub.50 of 318 m. These iron oxide fines contain about 67% of Fe.

[0450] The mixture, prepared by the method in example 1, consists of: [0451] 89.75 wt % of said quicklime fines from grinding, [0452] 10 wt % of said iron oxide fines, [0453] 0.25 wt % of powdered calcium stearate.

[0454] The briquettes are produced from this mixture by the method in example 1. 8.2 tonnes of briquettes are obtained having an average volume of 8.5 cm.sup.3, an average weight of 20.5 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 BET specific surface area of 1.6 m.sup.2/g and have a total mercury pore volume of 26%.

[0455] The water reactivity of the briquettes is determined by adding 166.7 g of these briquettes, previously ground to fines with a size between 0 and 1 mm, to 600 mL of water at 20 C. The 166.7 g of briquettes corresponds to 150 g of quicklime. The value of t.sub.60 is 1.0 min.

[0456] A Shatter test is carried out with 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. A Shatter test index of 4.9% is obtained.

[0457] The granulometric distribution of the iron-based particles in the composition in briquette form is determined by scanning electron microscopy and X-ray mapping, coupled to image analysis. The results are presented in Table 1. The volume fraction of iron oxide at the surface of the iron oxide particles in the composition in briquette form is 24%.

[0458] The briquettes are also characterized by carrying out a thermal treatment of 10 min at 1100 C. (hot charge/discharge) on 3 of these briquettes, at the end of which a powder with granulometry under 80 m is prepared. The latter is characterized by X-ray diffraction, and phase quantification is performed by Rietveld analysis. 16% of the total iron is in the form of calcium ferrites CaFe.sub.2O.sub.4 or Ca.sub.2Fe.sub.2O.sub.5, and 84% is still in the form of Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4.

Comparative Example 2Briquettes of Quicklime and Iron Oxide of Low Activity

[0459] The quicklime fines from grinding are those from example 1. The iron oxide fines are obtained from grinding an iron ore of the magnetite type, Fe.sub.3O.sub.4, passing through a 500 m sieve but not passing through a 2501 m sieve, characterized in Coulter laser granulometry by a d.sub.10 of 282 m, a d.sub.50 of 417 m and a d.sub.90 of 663 m. These iron oxide fines contain about 67% of Fe.

[0460] The mixture, prepared by the method in example 1, consists of: [0461] 89.75 wt % of said quicklime fines from grinding, [0462] 10 wt % of said iron oxide fines, [0463] 0.25 wt % of powdered calcium stearate.

[0464] The briquettes are produced from this mixture by the method in example 1. 8.5 tonnes of briquettes are obtained having an average volume of 8.4 cm.sup.3, an average weight of 20.3 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 15.7 mm. These briquettes develop a BET specific surface area of 1.6 m.sup.2/g and have a total mercury pore volume of 26%.

[0465] The water reactivity of the briquettes is determined by adding 166.7 g of these briquettes, previously ground to fines with a size between 0 and 1 mm, to 600 mL of water at 20 C. The 166.7 g of briquettes corresponds to 150 g of quicklime. The value of t.sub.60 is 0.9 min.

[0466] A Shatter test is carried out with 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. A Shatter test index of 4.8% is obtained.

[0467] The granulometric distribution of the iron-based particles in the composition in briquette form is determined by scanning electron microscopy and X-ray mapping, coupled to image analysis. The results are presented in Table 1. The volume fraction of iron oxide at the surface of the iron oxide particles in the composition in briquette form is 10%.

[0468] The briquettes are also characterized by carrying out a thermal treatment of 10 min at 1100 C. (hot charge/discharge) on 3 of these briquettes, at the end of which a powder with granulometry under 80 m is prepared. The latter is characterized by X-ray diffraction, and phase quantification is performed by Rietveld analysis. 11% of the total iron is in the form of calcium ferrites CaFe.sub.2O.sub.4 or Ca.sub.2Fe.sub.2O.sub.5, and 89% is still in the form of Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4.

Comparative Example 3Thermally Treated Briquettes of Quicklime and Iron Oxide of Low Activity

[0469] Starting from 1 tonne of briquettes from comparative example 2, arranged in boxes in such a way that the thickness of the bed of briquettes is 100 mm, thermal treatment is carried out for 2 hours at 1200 C., with ramps of temperature rise and fall of 50 C. per minute.

[0470] Briquettes are obtained having an average volume of 7.2 cm.sup.3, an average weight of 20.1 g and an average density of 2.4. These briquettes have a thickness of about 15.4 mm. These briquettes develop a BET specific surface area of 0.4 m.sup.2/g and have a total mercury pore volume of 23%.

[0471] A Shatter test is carried out with 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. A Shatter test index of 1.5% is obtained.

[0472] The volume fraction of iron oxide at the surface of the iron oxide particles is 9%. The iron oxide powder therefore contains 9% of active iron oxide.

[0473] Starting from 30 of these thermally treated briquettes, a powder is prepared with granulometry under 80 m. The latter is characterized by X-ray diffraction, and phase quantification is performed by Rietveld analysis. 16% of the total iron is in the form of calcium ferrites CaFe.sub.2O.sub.4 or Ca.sub.2Fe.sub.2O.sub.5, and 84% is still in the form of Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4.

[0474] The water reactivity of the briquettes is determined by adding 169.0 g of these briquettes, previously ground to fines with a size between 0 and 1 mm, to 600 mL of water at 20 C. The 169.0 g of briquettes corresponds to 150 g of free quicklime (i.e. not in the form of calcium ferrites). The value of t.sub.60 is 13 min.

TABLE-US-00001 TABLE 1 Granulometric distribution (expressed in percentage surface area of a section of the briquettes) determined by scanning electron microscopy and X-ray mapping, coupled to image analysis, of the iron-based particles in the briquettes Ex 1 Ex 4 CE 1 CE 2 >2 mm 0.0 0.0 0.0 0.0 <1-2 mm> 0.0 0.0 0.0 0.0 <0.5-1 mm> 0.0 0.0 0.0 0.0 <315-500 m> 0.0 0.0 0.0 0.0 <250-315 m> 0.0 0.0 0.0 15.6 <200-250 m> 0.0 0.0 0.0 7.9 <160-200 m> 0.0 0.0 0.0 48.4 <125-160 m> 0.0 0.0 0.0 20.0 <90-125 m> 0.0 0.0 0.0 7.7 <80-90 m> 0.0 0.0 40.4 0.0 <63-80 m> 0.0 0.0 36.0 0.0 <50-63 m> 0.0 0.0 11.9 0.2 <45-50 m> 0.0 13.4 0.0 0.0 <40-45 m> 0.0 0.0 10.1 0.1 <32-40 m> 44.6 23.4 0.0 0.0 <20-32 m> 37.8 37.4 1.2 0.0 <10-20 m> 12.9 21.1 0.4 0.0 <5-10 m> 3.8 3.7 0.0 0.0 <2-5 m> 0.7 0.9 0.0 0.0 <1-2 m> 0.0 0.0 0.0 0.0 <1 m 0.0 0.0 0.0 0.0

Examples 9 to 16

[0475] Green briquettes are prepared according to the invention with ground quicklime containing particles with sizes between 0 and 2 mm, but having different granulometric profiles and contents of iron oxide of the hematite type expressed in Fe.sub.2O.sub.3 equivalent ranging from 10% to 60 wt %. 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 ground quicklime with size between 0 and 2 mm have at least 30% of particles that are under 90 m. Each green briquette contains also 0.25 wt % calcium stearate as lubricant.

[0476] Green briquettes of identical composition were treated thermally at 1100 C. or at 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 treatments carried out are presented in Table 2. Several tests were carried out on these green and thermally treated briquettes, and are described below with the aid of FIGS. 1 to 4.

[0477] FIG. 1 is a graph showing: [0478] the variation of the BET specific surface area as a function of the content of iron-based compound expressed in Fe.sub.2O.sub.3 equivalent, for green briquettes; [0479] the variation of the porosity as a function of the content of iron-based compound expressed in Fe.sub.2O.sub.3 equivalent, for green briquettes; [0480] the variation of the BET specific surface area 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 of 1100 C. for 20 minutes; and [0481] the variation of the 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 of 1100 C. for 20 minutes.

[0482] 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 thermally treated briquettes. The thermally treated briquettes have a lower specific surface area than the green briquettes, whereas they have higher porosity for identical contents of iron-based compound.

[0483] FIG. 2 is a graph showing: [0484] the variation of the Shatter test index for green briquettes, as a function of the contents of iron-based compound expressed in Fe.sub.2O.sub.3 equivalent; and [0485] the variation of the Shatter test index for thermally treated briquettes that have undergone thermal treatment at a temperature of 1100 C. for 20 minutes, as a function of the contents of iron-based compound expressed in Fe.sub.2O.sub.3 equivalent.

[0486] 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%.

[0487] FIG. 3 is a graph showing the variation of the yield of iron-based compound (iron oxide) converted to calcium ferrite, as a function of the iron oxide content expressed in Fe.sub.2O.sub.3 equivalent as well as the amount of iron oxide converted in monocalcium ferrite and dicalcium ferrite. The thermal treatment is done in static bed during 20 min at 1100 C. in a tunnel furnace on a 100 mm thickness of briquettes.

[0488] 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 in monocalcium ferrites go through a maximum for amount of iron oxide of 40%. The percentage of the formation of dicalcium ferrites reduced with the iron oxide content.

[0489] 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.

[0490] 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% content of calcium ferrites for contents of iron oxide in the green briquettes in the range from 40 to 45%, and then decreases to contents of calcium ferrites of about 40% for contents of iron oxide in the green briquettes of 60%.

[0491] Nevertheless, it is 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 grinding of the quicklime so as to increase the profraction of quicklime particles smaller than 90 m, or a combination of the two. Several examples were undertaken and the measurement results are presented in Table 2.

TABLE-US-00002 TABLE 2 % of calcium ferrites in the Thermal % conversion to thermally % of CaFe.sub.2O.sub.4 % of Ca.sub.2Fe.sub.2O.sub.5 % Fe.sub.2O.sub.3 treatment calcium treated in weight of in weight of Examples equivalent temperature Type of CaO ferrites briquette calcium ferrite calcium ferrite Ex. 9 20% 1200 C. CaO < 2 mm, with 30% < 90 m 95% 31% 7 93 Ex. 10 30% 1200 C. CaO < 2 mm, with 30% < 90 m 98% 47% 22.5 77.5 Ex. 11 40% 1200 C. CaO < 2 mm, with 30% < 90 m 98% 58% 55.3 44.7 Ex. 12 50% 1200 C. CaO < 2 mm, with 30% < 90 m 97% 74% 39.4 60.6 Ex. 13 50% 1100 C. 50% of (CaO < 2 mm, with 30% < 90 m) + 90% 65% 69.9 30.1 50% of CaO < 90 m Ex. 14 50% 1100 C. 100% of CaO < 90 m 96% 73% 47.2 52.8 Ex. 15 50% 1200 C. 50% of (CaO < 2 mm, with 30% < 90 m) + 99% 76% 43.9 56.1 50% of CaO < 90 m Ex. 16 50% 1100 C. CaO < 2 mm, with 30% < 90 m 61% 43%

[0492] As can be seen in Table 2, it is 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 monocalcium ferrites.

[0493] In example 11, 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 hematite and 60 wt % of quicklime having a d.sub.97 equal to 2 mm and 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.

[0494] In example 13, 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 1100 C. for 20 minutes on green briquettes containing about 50 wt % of hematite and 25 wt % of quicklime having a d.sub.97 equal to 2 mm and a d.sub.30 equal to 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.

[0495] In example 14, 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 hematite and 50 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.

[0496] In example 15, 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 hematite and 25 wt % of quicklime having a d.sub.97 equal to 2 mm and 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. The yield of monocalcium ferrite can be increased by reducing the thermal treatment temperature to 1100 C.

[0497] In example 16, 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 hematite and 50 wt % of quicklime having a d.sub.97 equal to 2 mm and a d.sub.300 equal to 90 m, except for the presence of 0.25 wt % of calcium stearate, relative to the total weight of the green briquettes. The yield of monocalcium ferrite can be increased by increasing the amount by weight of quicklime having a d.sub.97 equal to 90 m.

[0498] It may be advantageous in a metal refining process to have an amount of monocalcium ferrite 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.

[0499] 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 14, 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.

[0500] It may be advantageous to subject green briquettes to a thermal treatment at 1200 C. to maximize the conversion yield of iron oxide to calcium ferrites.

[0501] FIG. 5 shows photographs of the sections of the briquettes from examples 9 to 16. The textures of the thermally treated briquettes from examples 9 to 16 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.

[0502] It has thus been shown for the briquettes from examples 9 to 16 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 1501 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 631 m. Table 3 below shows, for the briquettes from examples 9 to 16, the relative coverage of the calcium ferrite matrix, of the particles of quicklime under 63 m and of the particles of quicklime above 63 am in the cut section from each briquette.

TABLE-US-00003 TABLE 3 Ex 9 Ex 10 Ex 11 Ex 12 Ex 13 Ex 14 Ex 15 Ex 16 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)

[0503] 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 17

[0504] 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 a d.sub.50 of 40 m with 60.9 wt % of quicklime having a d.sub.97 below 2 mm and a d.sub.30 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 min 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 18

[0505] green briquettes were prepared with 39.9 wt % of iron oxide in the form of hematite 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 19 to 35Pre-Treatment Under Modified Atmosphere Containing CO.SUB.2 .Corresponding Respectively to Tests 1 to 17 in Table 4

[0506] In the following examples, compressive strength tests were performed on the briquettes using a Pharmatron Multitest 50, one 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.

[0507] 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.

[0508] Several pre-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 N.sub.2, H.sub.2O and CO.sub.2. The ramps of temperature rise are between 2 and 10 C./min.

[0509] 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%.

[0510] 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 4.

[0511] 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.

TABLE-US-00004 TABLE 4 Characterization of the thermally pre-treated briquettes Thermal pre-treatment variation in T ( C./min) H2O (% vol) CO2 (% vol) dm (CO2)/m (%) dm (H2O)/m (%) crush test (kg-force) the crush test (%) Essai 1 3.0 6.0 2.0 0.74 0.73 55 67% Essai 2 9.0 6.0 2.0 0.43 0.44 50 52% Essai 3 3.0 18.0 2.0 0.95 1.67 43 29% Essai 4 9.0 18.0 2.0 0.42 1.03 33 1% Essai 5 3.0 6.0 8.0 2.23 0.20 60 82% Essai 6 9.0 6.0 8.0 1.26 0.24 49 48% Essai 7 3.0 18.0 8.0 2.51 0.90 51 55% Essai 8 9.0 18.0 8.0 1.08 0.87 44 33% Essai 9 1.9 12.0 5.0 3.29 0.59 60 83% Essai 10 10.1 12.0 5.0 0.77 0.69 46 40% Essai 11 6.0 3.9 5.0 1.08 0.24 49 47% Essai 12 6.0 20.1 5.0 1.21 1.07 49 49% Essai 13 6.0 12.0 0.9 0.13 1.32 9 74% Essai 14 6.0 12.0 9.1 1.82 0.46 60 81% Essai 15 6.0 12.0 5.0 1.03 0.64 45 36% Essai 16 6.0 12.0 5.0 1.11 0.51 49 48% Essai 17 6.0 12.0 5.0 1.25 0.68 57 74% Legend: Essai = test

Comparative Example 4

[0512] 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 size between 0 and 3 mm with different contents 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.

[0513] A compressive force of greater than 144 kg, corresponding to 317.5 pounds, is required for briquettes having a Shatter index below 10%.

[0514] 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.