Method for manufacturing briquettes containing a calcium-magnesium compound and an iron-based compound, and briquettes thus obtained

Abstract

A method for manufacturing green or thermally treated briquettes which are made up of at least one quick calcium-magnesium compound that is an iron-based compound. The method includes the steps of supplying a homogeneous pulverulent mixture to a roller press, the press having pockets where the pulverulent mixture is compressed to form the green briquettes. The rollers of the roller press develop linear speeds at the periphery of the rollers between 10 and 100 cm/s and linear pressures between 60 and 160 kN/cm. The method can also include a thermal treatment of the green briquettes to produce fired briquettes containing calcium ferrite, the briquettes having a Shatter Test Index less than 8%, and a porosity value greater than or equal to 30%.

Claims

1. Method for manufacturing a calcium-magnesium composition in the form of briquettes, comprising the following steps: i. supplying a homogeneous 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; ii. feeding a roller press with said homogeneous pulverulent mixture, said roller press being equipped with rollers provided with pockets, iii. compressing said homogeneous pulverulent mixture in said roller press while obtaining a calcium-magnesium composition in the form of green briquettes, and iv. collecting said green briquettes said method being characterized in that said homogeneous pulverulent mixture further comprises an iron-based compound present at a content of at least 20 wt % of Fe.sub.2O.sub.3 equivalent relative to the weight of said composition, said iron-based compound having a granulometric distribution characterized by a median size d.sub.50 below 100 m as well as a size d.sub.90 below 200 m, in that said quick calcium-magnesium compound is quicklime, said quicklime having a value of t.sub.60 below 10 min, in that the rollers of the roller press develop linear speeds at the periphery of the rollers between 10 and 100 cm/s and linear pressures between 60 and 160 kN/cm, and in that the method further comprises a thermal treatment of said collected green briquettes at a temperature between 900 C. and 1200 C. for a predetermined duration of between 3 and 20 minutes, while obtaining fired briquettes containing calcium ferrite and having simultaneously a Shatter Test Index less than 8%, and a porosity value greater than or equal to 30%.

2. Method according to claim 1, in which said compression step is effected in the presence of a binder or a lubricant at a content between 0.1 and 1 wt % relative to the total weight of said green briquettes.

3. 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 quicklime having a value of t.sub.60 below 10 min relative to the weight of said composition and with at least 20 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 granulometric distribution characterized by a median size d.sub.50 below 100 m as well as a size d.sub.90 below 200 m; and vi. mixing said quicklime with said iron-based compound for a predetermined length of time, sufficient to obtain an approximately homogeneous pulverulent mixture of said quicklime and of said iron-based compound.

4. Method according to claim 3, in which a binder or lubricant is added to the mixer, and in which said binder or lubricant is included in said homogeneous pulverulent mixture.

5. 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.

6. Method according to claim 1, further comprising a pre-treatment step of the collected green briquettes under modified atmosphere containing at least 2 vol % CO.sub.2 and at most 30 vol % CO.sub.2 with respect to the modified atmosphere.

7. Method for manufacturing a calcium-magnesium composition in the form of fired briquettes, comprising the following steps: i. supplying a homogeneous 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, ii. feeding a roller press with said homogeneous pulverulent mixture, said roller press being equipped with rollers provided with pockets, iii. compressing said homogeneous pulverulent mixture in said roller press, while obtaining a calcium-magnesium composition in the form of green briquettes, and iv. collecting said green briquettes, said method being characterized in that said homogeneous pulverulent mixture further comprises an iron-based compound present at a content of at least 20 wt % of Fe.sub.2O.sub.3 equivalent relative to the weight of said composition, said iron-based compound having a granulometric distribution characterized by a median size d.sub.50 below 100 m as well as a size d.sub.90 below 200 m, in that said quick calcium-magnesium compound is burned dolomite, said burned dolomite having a value of t.sub.70 below 10 min, in that the rollers of the roller press develop linear speeds at the periphery of the rollers between 10 and 100 cm/s and linear pressures between 60 and 160 kN/cm, and in that the method further comprises a thermal treatment of said collected green briquettes at a temperature between 900 C. and 1200 C. for a predetermined duration of between 3 and 20 minutes, while obtaining fired briquettes containing calcium ferrite and having simultaneously a Shatter Test Index less than 8%, and a porosity value greater than or equal to 30%.

8. Method according to claim 7, in which said compression step is effected in the presence of a binder or a lubricant at a content between 0.1 and 1 wt % relative to the total weight of said green briquettes.

9. Method according to claim 7, further comprising, before said supplying of a homogeneous pulverulent mixture, i. feeding a mixer with at least 40 wt % of burned dolomite having a value of t.sub.70 below 10 min relative to the weight of said composition and with at least 20 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 granulometric distribution characterized by a median size d.sub.50 below 100 m as well as a size d.sub.90 below 200 m, and ii. mixing said burned dolomite with said iron-based compound for a predetermined length of time, sufficient to obtain an approximately homogeneous pulverulent mixture of said burned dolomite and of said iron-based compound.

10. Method according to claim 9, in which a binder or lubricant is added to the mixer, and in which said binder or lubricant is included in said homogeneous pulverulent mixture.

11. Method according to claim 7, further comprising a pre-treatment step of the collected green briquettes under modified atmosphere containing at least 2 vol % CO.sub.2 and at most 30 vol % CO.sub.2 with respect to the modified atmosphere.

Description

(1) 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.

(2) FIG. 1 shows the correlation between the shatter test index and the compressive force on different samples of briquettes of calcium-magnesium compound and optionally of iron-based compound.

(3) FIG. 2 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.

(4) FIG. 3 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.

(5) 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.

(6) FIG. 5 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.

(7) 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.90 below 200 m, preferably below 150 m, preferably below 130 m, more preferably below 100 m.

(8) 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 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.

(9) 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.

(10) 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 Kppern 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.

(11) 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.

(12) Assuming an angle of X 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.

(13) After compression, the calcium-magnesium composition is obtained in the form of green briquettes, which are collected.

(14) 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 12%, more preferably at least 20%, preferably at least 30%, more preferably at least 35% of Fe.sub.2O.sub.3 equivalent.

(15) 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%.

(16) 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.

(17) 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.

(18) 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.

(19) In a preferred embodiment according to the present invention, the method comprises, before said supplying of a homogeneous pulverulent mixture,

(20) i. feeding a powder mixer with at least 40 wt % of CaO+MgO equivalent of a quick calcium-magnesium compound and with 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 do below 200 m, preferably below 150 nm, preferably below 130 m, more preferably below 100 m; and

(21) 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.

(22) 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.

(23) 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 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.

(24) 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.

(25) 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.

(26) 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.

(27) 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.3) is carried out based on an X-ray diffraction pattern by the Rietveld method.

(28) 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).

(29) 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.

(30) The porosity of the green briquettes is greater than or equal to 20%, preferably 22%, preferably 24%.

(31) The green briquettes have an apparent density between 2.0 and 3.0 and preferably between 2.2 and 2.8.

(32) 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.

(33) 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 CaFe.sub.2O.sub.3.

(34) The thermally treated briquettes are characterized by an iron content by weight of 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.

(35) 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.

(36) Quantification of the calcium ferrites is performed by XRD/Rietveld analysis after grinding the briquettes, as for the green briquettes.

(37) 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.

(38) 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.

(39) The porosity is greater than or equal to 20%, preferably 22% preferably 24%.

(40) The thermally treated briquettes have an apparent density between 2.0 and 3.0 and preferably between 2.2 and 2.8.

(41) 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

(42) 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 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 to 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.

(43) 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 die 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.

(44) 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). This mixer is used in continuous mode for preparing a mixture consisting of: 89.75 wt % of said quicklime fines from grinding, 10 wt % of said iron oxide fines, 0.25 wt % of powdered calcium stearate.

(45) The total flow rate of powder is 300 kg/h and the residence time is 3.5 s.

(46) 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.

(47) 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.

(48) 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).

(49) 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%.

(50) 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.50 is 1 min.

(51) 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.

(52) 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.

(53) 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

(54) 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.90 of 102 m. These iron oxide fines contain 67.1% of Fe.

(55) The mixture, prepared by the method in example 1, consists of: 89.75 wt % of said quicklime fines from grinding, 10 wt % of said iron oxide fines, 0.25 wt % of powdered calcium stearate.

(56) 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%.

(57) 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.

(58) 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.

(59) The volume fraction of iron oxide at the surface of the iron oxide particles in the composition in briquette form is 63%.

(60) 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 s 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

(61) 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.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.90 of 35.7 m. These iron oxide fines contain 64.6% of Fe.

(62) The mixture, prepared by the method in example 1, consists of: 89.75 wt % of said quicklime fines from grinding, 10 wt % of said iron oxide fines, 0.25 wt % of powdered calcium stearate.

(63) 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.

(64) 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.

(65) The volume fraction of iron oxide at the surface of the iron oxide particles in the composition in briquette form is 88%.

(66) 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

(67) 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.

(68) 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%.

(69) 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.

(70) 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.

(71) The results are presented in Table 1.

(72) 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.

(73) 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.

(74) 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

(75) 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.

(76) 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%.

(77) 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.

(78) 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.

(79) 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.7O.sub.4 or Ca.sub.2Fe.sub.2O.sub.5, and 9% is still in the form of Fe.sub.2O.sub.3.

(80) 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

(81) 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 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 to 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.

(82) The mixture, prepared by the method in example 1, consists of: 44.75 wt % of said quicklime fines from grinding, 5-45 wt % of said screened quicklime fines, 10 wt % of said iron oxide fines, 0.25 wt % of powdered calcium stearate.

(83) 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 too is 1.6 min.

(84) 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.

(85) The volume fraction of iron oxide at the surface of the iron oxide particles in the composition in briquette form is 86%.

(86) 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

(87) The screened quicklime fines are those from example 6. The iron oxide fines are those from example 3.

(88) The mixture, prepared by the method in example 1, consists of: 89.75 wt % of said screened quicklime fines, 10 wt % of said iron oxide fines, 0.25 wt % of powdered calcium stearate.

(89) 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.

(90) 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.

(91) The volume fraction of iron oxide at the surface of the iron oxide particles in the composition in briquette form is 87%.

(92) 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% s still in the form of Fe.sub.2O.sub.3.

Example 8Briquettes of Dolomitic Quicklime and Iron Oxide

(93) 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 to 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.

(94) The mixture, prepared by the method in example 1, consists of: 64.75 wt % of said quicklime fines from grinding, 25 wt % of said fines of burned dolomite from grinding, 10 wt % of said iron oxide fines, 0.25 wt % of powdered calcium stearate.

(95) 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%.

(96) 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.

(97) The volume fraction of iron oxide at the surface of the iron oxide particles in the composition in briquette form is 86%.

(98) 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

(99) 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.90 of 318 m. These iron oxide fines contain about 67% of Fe.

(100) The mixture, prepared by the method in example 1, consists of: 89.75 wt % of said quicklime fines from grinding, 10 wt % of said iron oxide fines, 0.25 wt % of powdered calcium stearate.

(101) 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%.

(102) 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 too is 1.0 min.

(103) 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.

(104) 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%.

(105) 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

(106) 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 250 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.

(107) The mixture, prepared by the method in example 1, consists of: 89.75 wt % of said quicklime fines from grinding, 10 wt % of said iron oxide fines, 0.25 wt % of powdered calcium stearate.

(108) 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.a, 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 SET specific surface area of 1.6 m.sup.2/g and have a total mercury pore volume of 26%.

(109) 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.

(110) 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.

(111) 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%.

(112) 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

(113) 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.

(114) 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 SET specific surface area of 0.4 m.sup.2/g and have a total mercury pore volume of 23%.

(115) 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.

(116) 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.

(117) 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 CaFe.sub.2O.sub.5, and 84% is still in the form of Fe.sub.2O.sub.3, or Fe.sub.3O.sub.4.

(118) 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 to is 13 min.

(119) 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

(120) 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 expressed in Fe.sub.1O.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.90 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.

(121) 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.

(122) FIG. 2 is a graph showing: 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; 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; 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 the variation of the porosity as a function of the content of iron-based compound expressed in Fe.sub.2O.sub.5 equivalent, for thermally treated briquettes that have undergone thermal treatment of 1100 C. for 20 minutes.

(123) 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.

(124) FIG. 3 is a graph showing: 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 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.

(125) 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.5 equivalent below 40%, whereas for the thermally treated briquettes, all the shatter tests are below 10%, or even 6%.

(126) FIG. 4 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.

(127) 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%.

(128) FIG. 5 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.

(129) 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%.

(130) 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 proportion 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.

(131) TABLE-US-00002 TABLE 2 % of calcium ferrites in Thermal % conversion the thermally % Fe.sub.2O.sub.3 treatment to calcium treated Examples equivalent temperature Type of CaO ferrites briquette Ex. 9 20% 1200 C. CaO < 2 mm, with 95% 31% 30% < 90 m Ex. 10 30% 1200 C. CaO < 2 mm, with 98% 47% 30% < 90 m Ex. 11 40% 1200 C. CaO < 2 mm, with 98% 58% 30% < 90 m Ex. 12 50% 1200 C. CaO < 2 mm, with 97% 74% 30% < 90 m Ex. 13 50% 1100 C. 50% of (CaO < 2 mm, 90% 65% with 30% < 90 m) + 50% of CaO < 90 m Ex. 14 50% 1100 C. 100% of (CaO < 90 m) 96% 73% Ex. 15 50% 1200 C. 50% of (CaO < 2 mm, 99% 76% with 30% < 90 m) + 50% of CaO < 90 m Ex. 16 50% 1100 C. CaO < 2 mm, with 61% 43% 30% < 90 m

Comparative Example 4

(132) The shatter indices were compared with the compressive force on several samples of green briquettes to establish the correlation between the shatter test index and the compressive force. The green briquettes tested comprised quicklime whose particle size was 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 modified to ensure that the population for establishing the correlation was large enough.

(133) As can be seen in FIG. 1, a compressive force above 144 kg, corresponding to 317.5 pounds, is required for briquettes having a shatter test index below 10%.

(134) Of course, the present invention is not in any way limited to the embodiments described above and many modifications may be made while remaining within the scope of the appended claims.