PROCESS FOR PREPARING OPTIMIZED CALCINED, IRON- AND CHROME-CONTAINING PELLETS

Description

[0001] The invention relates to a process for preparing calcined, iron- and chrome-containing pellets, to calcined, iron- and chrome-containing pellets and their use for the preparation of Ferrochrome as well as to a process for the preparation of Ferrochrome using the calcined, iron- and chrome-containing pellets.

[0002] A widely used iron- and chrome-containing alloy is the so called Ferrochrome. Ferrochrome is used in the stainless-steel industry to increase the resistivity of steel against water and air to prevent the formation of rust. The iron and chrome source for the Ferrochrome production is usually chromite, a chrome ore, which is found in some parts of the world, like in South Africa.

[0003] The production of Ferrochrome is usually carried out in huge electrically heated arc furnaces or blast furnaces at high temperatures, using a carbon based reductant which can be part of the electrode or which is mixed in the chrome ore, or both. The consumption of electricity is significant and determines the cost effectiveness of a process using electrically heated furnaces. The process results in a liquid, molten alloy which is casted in casts, and a layer of partially molten residue floating on top of the liquid metal, the slag.

[0004] The usage of fine chrome ore material, so called "fines" or "flour", is economically of interest, because it decreases the necessary dwell time of the ore in the melt during reduction process significantly, but is practically difficult. In the arc furnaces a strong stream of hot gas is formed which results in an upstream. The particles of the fines are too small to be dropped into the furnace: they would not reach the hottest zones (melt) for reduction. Most of this material will be blown out of the furnace with the off-gas stream. In order to get the fines in a suitable shape, pellets were formed out of the fines as disclosed in ZA 2004-03429 A. In Minerals Engineering (2012), 34, 55-62, a detailed description of the used binders and their properties and effects on the pellet strength and other properties are given.

[0005] The pellets produced according to the process of ZA 2004-03429 A and according to Minerals Engineering (2012), 34, 55-62, are made of chrome ore, a carbonaceous reductant and a so-called unconverted or non-activated binding agent. The binding agent is a mainly silica based clay, like Bentonite. Bentonite comprises ca. 24 wt% of silicon.

[0006] Besides inorganic binders organic binders (organic binder additives) were tested and disclosed, e.g. Carboxy-methylcellulose. The usage is limited to strength improvement in low temperature regimes as "wet green pellets" at room temperature as discussed in O. Sivrikaya, A. I. Arol (2013) Method to improve preheated and fired strengths of haematite pellets using boron compounds with organic binders, Ironmaking & Steelmaking, 40:1, 1-8, DOI: 10.1179/1743281212Y.0000000016.

[0007] There are some disadvantages of using inorganic, clay-based binders and the pellets produced according to this process: the binding agent Bentonite contains neither iron nor chrome and thus, cannot contribute to the formation of Ferrochrome afterwards. Instead, the silica contained in Bentonite increases the amount of useless and costly slag. Slag is not only an undesired by-product in the process for the production of Ferrochrome but furthermore its formation requires additional electrical energy (e.g. for heating and for the reduced throughput of the furnace). Another disadvantage of the process is the effect of Bentonite on the density of the wet pellets fed into a tunnel furnace for pre-calcining step. The Bentonite is absorbing water and decreases the density of the pellets, which leads to a lower throughput of material through the tunnel furnace, as this is limited by volume per time unit, but the producer get paid by mass of pellets produced. On the other hand, the pore volume of the pellets is important, as the reducing gases formed in the arc furnace process need to reach the oxides in the pellets. So, a higher Chrome content and an increased density of the raw, dried pellets is desired with only small changes in the porosity of the indurated, calcined pellets. This results in lower slag production, higher throughput through the (tunnel-) furnace.

[0008] WO 2018/172284 A1 discloses a method for producing pre-reduced calcined, iron- and chrome-containing pellets, wherein chromite ore material and COPR are mixed with large amounts of a reductant component and subsequently reduced at temperatures of 1250 °C to 1600 °C in an inert or reducing atmosphere. However, while the process according to said process yields calcined pellets with a higher cold crush strength compared to pellets obtained by using bentonite binder, they have a lower density than the latter.

[0009] DWARAPUDI ET AL. "Development of Cold Bonded Chromite Pellets for Ferrochrome Production in Submerged Arc Furnace, ISIJ INTERNATIONAL,, vol. 53. no. 1, 1 January 2013 (2013-01-01), pages 9-17" relates to the development of cold bonded chromite pellets for ferrochrome production in submerged arc furnace from chromite fines using suitable binder that induce less gangue into the pellets but cures quickly. In this article, different binders were studied through laboratory pelletizing experiments for their stability for cold bonding the pellets. As a result, a composite binder comprising dextrin and bentonite, was found to be suitable and pellets from the same were tested for their low and high temperature behavior.

[0010] CN 106 086 402 A discloses a method for producing ferrochrome. The method comprises the steps of grinding ferrochrome ore and reducing agents separately into particles more than 70% of which has a particle size of 200 mesh; mixing; and forming to obtain pellets. The method uses a binder which is one or more selected from bentonite and starch solution.

[0011] Furthermore, the yet unpublished patent application EP 18196811.6 describes the usage of COPR as binder in the absence of reductant. However, said use results in calcined pellets with a higher density compared to pellets obtained by using bentonite binder, but at the same time, they suffer of a lower cold crush strength compared to the latter.

[0012] Organic binders are expected to have no large impact on the cold crush strength after firing, as they burn off at low temperatures below 500 °C.

[0013] Hence, it was the object of the present invention to avoid the disadvantages of the prior art to provide a process for preparing calcined, iron- and chrome-containing pellets suitable for use for the preparation of ferrochrome that combines a reduced slag formation and leads to calcined pellets having a high cold crush as well as a high density.

[0014] It was surprisingly found, that this object can be achieved by the process according to the claims of the present patent application.

[0015] The invention therefore provides a process for preparing calcined, iron- and chrome-containing pellets comprising the steps:

a) mixing chrome ore material, COPR and saccharide binder and forming the obtained mixture into pellets,

b) optionally drying the pellets obtained by step a), and

c) calcining the pellets,

wherein the pellets obtained by step a) comprise less than 3 wt% of carbonaceous reductant based on the amount of chrome ore material, COPR, saccharide binder and carbonaceous reductant,

wherein the COPR contains

• chrome(III) oxide: 7 to 13 wt%,
• aluminum oxide: 10 to 30 wt%,
• iron(II) oxide: 36 to 44 wt%,
• iron(III) oxide: > 0.5% wt%,
• magnesium oxide: 9 to 18 wt%,
• calcium oxide: < 10 wt%,
• silicon oxide: 0 to 3 wt%,
• vanadium oxide: < 1 wt%,
• sodium oxide: 0 to 5 wt%, and
• sodium monochromate: 0 to 4.7 wt%,

wherein the calcining in step c) is carried out at oven temperatures of 1250°C to 1600°C, for periods of 1 minute to 8 hours, preferably of 1300°C to 1500°C for periods of 5 minutes to 5 hours.

Chrome ore material

[0016] Preferably, the chrome ore material used in step a) contains:

• chrome(III)oxid (Cr₂O₃): 26 to 54 wt%, particularly preferably 40 to 45 wt%,
• aluminum oxide (Al₂O₃): 10 to 30 wt%, particularly preferably 13 to 18 wt%,
• iron(II) oxide (FeO): 12 to 36 wt%, particularly preferably 20 to 28 wt%,
• magnesium oxide (MgO): 9 to 22 wt%, particularly preferably 10 to 15 wt%,
• calcium oxide (CaO): < 5 wt%, particularly preferably < 1 wt%, and
• silicon oxide (SiO₂): 1 to 18 wt%, particularly preferably 2 to 5 wt%.

[0017] The worldwide largest chrome ores deposits are located in South Africa, Zimbabwe, Turkey and the Philippines and in some other countries. The chrome ore is divided into two categories: The metallurgical grade with ≥ 45 wt% of Cr₂O₃ and the chemical grade with < 45 wt% and ≥ 40 wt% of Cr₂O₃. The largest known deposit of chrome ore is found in Zimbabwe with over 300 million tons.

Chrome Ore Process Residue

[0018] Chrome Ore Process Residue (COPR) sometimes also named chromite ore processing residue is known to person skilled in the art as a waste stream comprising chrome and other metal oxides from the industrial production of chromate. The Chrome Ore Process Residue (COPR) used in step a) is preferably a by-product from the sodium monochromate production process. Therein, chrome ore is mixed with soda ash and heated to a temperature of about 1200 °C under oxidizing condition. The reaction mixture is leached with water, and the dried solid residue is the so-called Chrome Ore Process Residue (COPR).

[0019] Preferably, the COPR is obtained in the process for producing sodium monochromate from chromite via an oxidative alkaline digestion with sodium carbonate (no lime process, CaO content of < 5wt%).

[0020] Preferably, COPR contains metal oxides such as chromium(III) oxide (Cr₂O₃), aluminum oxide (Al₂O₃), iron(III) oxide (Fe₂O₃), iron(II) oxide (FeO), magnesium oxide (MgO), calcium oxide (CaO), silicon oxide (SiO₂), vanadium oxide (V₂O₅), sodium oxide (Na₂O) and sodium monochromate (Na₂CrO₄).

[0021] The Cr(VI) is preferably present as sodium monochromate (Na₂CrO₄) in the COPR.

[0022] The CaO content of the COPR is preferably less than 15wt%, particularly preferably less than 10wt%, most preferably less than 5wt%.

[0023] COPR preferably contains:

• chrome(lll) oxide (Cr₂O₃): 7 to 13 wt%, preferably 7.5 to 12.5 wt%,
• aluminum oxide (Al₂O₃): 10 to 30 wt%, preferably 18 to 24 wt%,
• iron(II) oxide (FeO): 36 to 44 wt%, preferably 37 to 42 wt%,
• iron(III) oxide (Fe₂O₃): > 0.5% wt%, preferably > 2 wt%,
• magnesium oxide (MgO): 9 to 18 wt%, preferably 10 to 17 wt%,
• calcium oxide (CaO): < 10 wt%, preferably < 5 wt%,
• silicon oxide (SiO₂): 0 to 3 wt%, preferably 1 to 3 wt%,
• vanadium oxide (V₂O₅): < 1 wt%, preferably < 0.5 wt%,
• sodium oxide (Na₂O): 0 to 5 wt%, preferably 2 to 5 wt%, and
• sodium monochromate (Na₂CrO₄): 0 to 4.7 wt%, preferably < 0.0003 wt%.

[0024] Preferably, the Cr(VI) content of the COPR is 0.01 to 1 wt%.

[0025] Preferably, the Cr content in the COPR is 2 to 25 wt%, particularly preferably 5 to 9 wt%.

[0026] Preferably, the Fe content in the COPR is 28 to 35 wt%, particularly preferably 29 to 34 wt%.

[0027] Preferably, the Si content in the COPR is 0 to 1.5 wt%, particularly preferably 0.4 to 1.0 wt%.

[0028] In an alternative embodiment, the Cr(VI) content of the COPR is preferably < 0.0001 wt%.

[0029] COPR with a Cr(VI) content of < 0.0001 wt% is preferably obtained via a reduction process of COPR with a Cr(VI) content of 0.01 to 1 wt% in that the reduction of Cr(VI) to Cr(III) takes preferably place via polyethylene glycol (PEG) or glycerol as disclosed in WO 2014/006196 A1 or, alternatively, in an atmosphere containing less than 0.1 % by volume of an oxidizing gas as disclosed in WO 2016/074878 A1.

Saccharide binder

[0030] The saccharide binder to be used in the present invention can be selected from monosaccharides, disaccharides, oligosaccharides, and polysaccharides or mixtures thereof, preferably from disaccharides, oligosaccharides, and polysaccharides or mixtures thereof, more preferably from oligosaccharides, and polysaccharides or mixtures thereof, and most preferably from polysaccharides and mixtures thereof.

[0031] Examples of monosaccharides are glucose, galactose, fructose and xylose, examples of disaccharides are sucrose, lactose, maltose and trehalose, examples for oligosaccharides include maltodextrins, raffinose, stachyose and fructo-oligosaccharides, examples of polysaccharides are amylose, amylopectin, starch, modified starches, arabinoxylans, chitin glycogen, cellulose, hemicellulose, pectins, hydrocolloids, agarose, dextran and guaran.

[0032] A particularly preferred embodiment of the present invention the saccharide binder comprises guaran, also called guar gum.

[0033] The amount of saccharide binder in the pellets obtained by step a) is typically below 5 wt%, preferably below 4 wt%, and most preferably in the range of 1 wt% to 3 wt% based on COPR.

[0034] In further preferable embodiments, the pellets obtained by step a) comprise the saccharide binder in an amount of 0.003 wt% to 1wt% based on the amount of chrome ore material, COPR, saccharide binder and, if present, carbonaceous reductant.

Carbonaceous reductant

[0035] In a preferred embodiment of the present invention, the pellets obtained by step a) comprise less than 3 wt%, preferably less than 2 wt%, more preferably less than 1 wt%, and most preferably 0 wt% of carbonaceous reductant based on the amount of chrome ore material, COPR, saccharide binder and carbonaceous reductant.

[0036] The term carbonaceous reductant refers to all organic substances, which are capable of reducing the Fe- or Cr oxides in the chrome ore material under the calcining conditions of step c). In particular, said expression refers to the compounds anthracite, char, coke and bituminous coal. For the sake of clarity it is understood in the context of the present invention that the term carbonaceous reductant does not include saccharide binder.

Pellets

[0037] The term pellets as used in the present invention refers to particular or granular material with regular or irregular shape, preferably with a spheric shape.

[0038] Preferably, the term pellets refers to particles or granules having a volume equivalent to that of a sphere with a diameter of 4-30 mm, more preferably 8-20 mm, most preferably of 10-15 mm.

The process

Step a)

[0039] There are different ways known to the skilled person for mixing the chrome ore material, COPR and an optional carbonaceous reductant before water addition in step a).

[0040] Preferably, the mixing is conducted by using a dry ball mill.

[0041] The solid components used in step a) are preferably milled. The milling can take place prior to the mixing in step a), during the mixing in step a) or after the mixing in step a).

[0042] Preferably, the chrome ore material, COPR, saccharide binder and optionally carbonaceous reductant are milled during mixing in step a).

[0043] In a preferred embodiment, the pellets obtained by step a), comprise:

• 82 to 98.9 wt%, particularly preferably 93 to 98.9 wt% of chrome ore material,
• 0.1 to 15 wt%, particularly preferably 0.1 to 5 wt% of COPR,
• 0.003 to 1 wt%, preferably 0.01 to 0.5 wt%, and particularly preferably 0.02 to 0.2 wt%, of saccharide binder, and
• 0 to < 3 wt%, preferably 0 to 2 wt%, particularly preferably 0 to 1 wt% of carbonaceous reductant,

based on the amount of chrome ore material, COPR, saccharide binder and optional carbonaceous reductant in the pellets.

[0044] Preferably, the mixture obtained after the mixing of the chrome ore material, COPR and optional carbonaceous reductant in step a) provides a particle size distribution (d90) of 50 to 100 µm, particularly preferably of 65 to 85 µm. According to the invention, a d90 of 50 µm means that 90% by volume of the pellets of the mixture have a particle size of 50 µm and below.

[0045] Preferably, the mixture obtained after the mixing of chrome ore material, COPR and an optional carbonaceous reductant is further mixed with water. Thereby, the pelletization takes place. Alternatively, pelletization can be effected by pressing the mixture into the desired form.

[0046] The weight ratio of water to the sum of the components chrome ore material, COPR and a carbonaceous reductant is preferably between 1:6 and 1: >100, particularly preferably between 1:8 and 1: >100, most preferably 1:125.

[0047] The pelletization may take place in either a pan or drum pelletizing unit. Thereby, composite carbon containing (so-called "green") pellets are obtained.

[0048] Preferably, the silicon content of the pellets obtained by step a) is below 2.5 wt%, particularly preferably below 2 wt%.

[0049] Preferably, the pellets obtained by step a) do not crack when dropped from a height of up to 0.2 m, particularly preferably of up to 0.4 m, most preferably of up to 0.5 m, on a steel plate. The green wet pellets show, after drying in air, a density higher than pellets produced with a binder state-of-the-art.

Step b)

[0050] The pellets can be pre-dried under ambient conditions, preferably at a temperature of 18 to 30 °C for 4 to 40 hours, preferably for 12 to 30 hours, but this is optional.

[0051] The optional drying is done by heating the pellets under atmospheric conditions, preferably. Particularly preferably, the drying takes place at a temperature above 70 °C, most preferably above 100 °C. The time for the drying is preferably 2 to 50 hours, particularly preferably 6 to 30 hours, and can be performed in an oven.

Step c)

[0052] The calcining of the pellets obtained by step a) or step b), if step b) is performed, may be conducted in different ways known to the skilled person. This calcining process can be performed either under ambient gas atmosphere or under an atmosphere with reduced oxygen level compared to ambient atmosphere.

[0053] The heating unit is preferably a rotary kiln, a muffle furnace, a tube furnace or a tunnel furnace, preferably a belt tunnel furnace.

[0054] In the heating unit, the wet or optionally dry, pellets obtained by step a) or step b) are calcined at oven temperatures of 1250 °C to 1600 °C for periods of 1 minute to 8 hours, preferably of 1300 °C to 1500 °C for periods of 5 minutes to 5 hours.

[0055] Preferably, the inert atmosphere contains less than 0.1 vol-% of oxygen. Particularly preferably, the inert atmosphere is argon.

[0056] In an alternative embodiment of step c) relating to the production of metallized chrome- and iron-containing particles, the heating unit is operated as a reduction unit i.e. the heating unit is operated such as to maintain sufficient reducing conditions in the immediate environment of the particle bed so as to achieve high degrees of both iron and chromium metallization and commensurately limited degrees of carbon burn-off. In such case, the calcination is preferably carried out at oven temperatures of 1250 °C to 1600 °C for periods of 1 to 8 hours, preferably of 1300 °C to 1500 °C for periods of 2 to 5 hours, in an inert atmosphere.

[0057] Preferably, the inert atmosphere contains less than 0.1 vol-% of oxygen. Particularly preferably, the inert atmosphere is argon.

[0058] The reduction facility is preferably operated in a manner so as to ensure that the preheated pellets are maintained at a temperature of preferably 1350 °C to 1450 °C for a period of 2 to 5 hours, through the use of an oxygen enhanced coal combustion device. Oxygen support of the combustion device is considered a vital part in ensuring adequate flame geometry and energy release, while enabling the maintenance of a suitably reducing environment in the particle bed to achieve the requisite degree of metallization.

[0059] The term "metallized particles" means that at least a part of the Fe- and Cr-ions contained in the chromite ore material and the COPR as used in step a) are reduced to chromium and iron in the oxidation state 0. Preferably, the degree of metallization in the metallized particles obtained by step c) is above 65%, particularly preferably above 75%, and most preferably above 85% of the weight of the Fe- and Cr-ions contained in the chromite ore material and the COPR as used in step a).

[0060] The degree of metallization can be determined via thermogravimetric analysis that is known to the skilled person. The loss of weight at those temperatures is directly correlated with the loss of bound oxygen consumed by the reductant component, and therewith the degree of metallization to Fe and Cr, and the loss of the reductant component itself. After the subtraction of organic compounds in the reductant component, which is determined in a separate measurement with pure reductant component, the degree of metallization can be calculated with ease.

[0061] By the use of the calcined pellets obtained by step c) the electrical energy consumption for the complete reduction to iron metal and chrome metal in the arc furnace is reduced.

[0062] In both processes the calcined pellets obtained by step c) are discharged, either via direct hot transfer to the smelting furnace or via controlled cooling of the calcined product, to yield cool, mechanically stable pellets.

[0063] In the calcined pellets obtained by step c) the Cr(VI) content is preferably < 0.0001 wt%.

[0064] The calcined pellets obtained by step c) provide increased mechanical stability compared to those obtained by step a).

[0065] The calcined pellets can be further stored or transported, e.g. to an electric submerged arc furnace for the preparation of Ferrochrome.

[0066] The calcined pellets obtained by step c) have an average cold crushing strength of at least 130 kgf/pellet, preferably at least 160 kgf/pellet, and most preferably at least 200 kgf/pellet. This value is determined in consideration of DIN EN 993-5 (2018) by placing a pellet between two steel plates arranged in parallel. With an hydraulic system, the plates are constantly moved towards each other and the pellet in the gap is squeezed. The applied force is measured continuously. The measurement is stopped, as soon as the applied force decreases while the plates are still moving towards each other (pellet has cracked). The maximum force measured in the described setup is calculated as an applied weight in kgf. 1 kgf are equivalent to 9.806650 N. In the present examples, the cold crushing strength is given the as average of 100 pellets of same size.

Calcined, iron- and chrome-containing pellets and their use

[0067] The present invention further provides calcined, iron- and chrome-containing pellets that contain:

• chrome: 25 to 36 wt%, preferably 28 to 33 wt%,
• iron: 14 to 24% wt%, preferably 15 to 21 wt%, and
• silicon: 0.4 to 2 wt%, preferably 0.4 to 1 wt%,

having a density of > 3.35 g/cm³, preferably > 3.40 g/cm, more preferably > 3.41 g/cm³.

[0068] The calcined iron- and chrome-containing pellets have an average cold crushing strength of at least 130 kgf/pellet, preferably at least 160 kgf/pellet, and most preferably at least 200 kgf/pellet.

[0069] Preferably, the calcined, iron- and chrome-containing pellets contain chrome as chrome(III)oxide (Cr₂O₃) and as chrome metal, iron as iron(II) oxide (FeO) and iron(III) oxide (Fe₂O₃) and as iron metal, and silicon as silicon oxide (SiO₂).

[0070] The ratio of iron metal to iron(II,III) in the calcined, iron- and chrome-containing pellets is preferably < 1:2, particularly preferably < 1:10.

[0071] The ratio of chrome metal to chrome(III) in the calcined, iron- and chrome-containing pellets is preferably < 1:2, particularly preferably < 1:10.

[0072] The ratio of iron and chrome metal to iron(II,III) and chrome(III) in the calcined, iron- and chrome-containing pellets is preferably < 1:2, particularly preferably < 1:10.

[0073] In the calcined, iron- and chrome-containing pellets the Cr(VI) content is preferably < 0.0001 wt%. The calcined, iron- and chrome-containing pellets are particularly preferably free of Cr(VI).

[0074] Preferably, the calcined, iron- and chrome-containing pellets have a diameter of 6 to 13 mm.

[0075] Preferably, the calcined, iron- and chrome-containing pellets according to the invention are obtained by the process for preparing calcined, iron- and chrome-containing pellets according to the invention.

[0076] The invention further provides the use of calcined, iron- and chrome-containing pellets according to the invention for the preparation of Ferrochrome.

[0077] The invention further provides a process for the preparation of Ferrochrome, wherein the calcined, iron- and chrome-containing pellets according to the invention are fed into an electric arc furnace and molten under reduction of iron- and chrome oxides.

[0078] The invention will be described in more detail in the following non-limiting example.

Example A:

[0079] 99.2 parts by weight of chrome ore material (milled in a dry ball mill process down to d90=82µm, type: UG2 chemical grade, origin: Sibanya mine in Waterval Rustenburg, South Africa, moisture 8.7 % by weight), was mixed intensively with 0.8 parts by weight of COPR comprising less than 0.7 ppm of Cr(VI), obtained according to the procedure described in WO 2014/006196 and with 0.00247 parts of guar gum.

[0080] The material was placed in a pelletization disc and water was sprayed on the surface while the disc was turning to produce small pellets of about 3 mm, which were screened out and used as seed pellets for the actual pelletizing process.

[0081] After the pelletizing process pellets with a diameter of above 11,2 mm were screened out (with 7 wt% of water) and dried. The density of the dried pellets was determined using the displaced volume method.

[0082] Hereafter, the pellets were calcined in a chamber furnace. The temperature was increased rapidly (within few minutes) to 1400 °C and hold for 10 minutes. The pellets were then left to cool down to ambient temperature. The density of the indurated pellets was determined on 200 pellets by the volume of displacement method. The cold compression strength (CCS) was determined using 200 indurated pellets produced as above.

Example B (comparative example according to EP 18196811.6)

[0083] 99.2 parts by weight of chrome ore material (milled in a dry ball mill process down to d90=82µm, type: UG2 chemical grade, origin: Sibanya mine in Waterval Rustenburg, South Africa, moisture 8.7 wt%), was mixed intensively with 0.8 parts by weight of COPR, received according to procedure described in WO 2014/006196.

[0084] The material was placed in a pelletization disc and water was sprayed on the surface while the disc was turning to produce small pellets of about 3 mm, which were screened out and used as seed pellets for the actual pelletizing process.

[0085] After the pelletizing process pellets with a diameter of above 11,2 mm were screened out and dried. The density of the dried pellets was determined using the displaced volume method.

[0086] Hereafter, the pellets were calcined in a chamber furnace. The temperature was increased rapidly (within few minutes) to 1400 °C and hold for 10 minutes. The pellets were then left to cool down to ambient temperature. The density of the indurated pellets was determined on 200 pellets by the volume of displacement method. The cold compression strength (CCS) was determined using 200 indurated pellets produced as above.

Example C (comparative example according to ZA 2004-03429 A)

[0087] 99.2 parts by weight of chrome ore material (milled in a dry ball mill process to d90=82µm, type: UG2 chemical grade, origin: Sibanya mine in Waterval Rustenburg, South Africa, moisture 8.7 wt%), was mixed intensively with 0.8 parts by weight of Bentonite MB100S (Supplier: LKAB Sweden, 52 wt% SiO₂, 3 wt% Na₂O, 1 wt% K₂O, 0.4 wt% S, containing 77% Montmorillonite and 6.4 wt% CaO and 10 wt% water).

[0088] The material was placed in a pelletization disc and water was sprayed on the surface while the disc was turning to produce small pellets of about 3 mm, which were screened out and used as seed pellets for the actual pelletizing process.

[0089] After the pelletizing process pellets with a diameter of above 11,2 mm were screened out and dried. The density of the dried pellets was determined using the displaced volume method.

[0090] Hereafter, the pellets were calcined in a chamber furnace. The temperature was increased rapidly (within few minutes) to 1400 °C and hold for 10 minutes. The pellets were then left to cool down to ambient temperature. The averaged density of the pellets was determined on 200 pellets by volumetric displacement. The cold compression strength (CCS) was determined using 200 indurated pellets produced as above.

[0091] The porosity was calculated based on mass (m_Pellets), volume (V_Pellets) and density (ρ_Pellets) of the pellets, by the following formula:

Results:

[0092] 

	Example A	Example B (comparative)	Example C (comparative)
Average density of dried pellets (g/cm3)	3.24	3.30	3.09
Average density of calcined pellets (g/cm3)	3.41	3.45	3.34
Average Cold Crush Strength (kgf/pellet)	> 200	> 120	> 200

Claims

1. A process for preparing calcined, iron- and chrome-containing pellets comprising the steps:

a) mixing chrome ore material, Chrome Ore Process Residue (COPR) and saccharide binder and forming the obtained mixture into pellets,

b) optionally drying the pellets obtained by step a), and

c) calcining the pellets,

wherein the pellets obtained by step a) comprise less than 3 wt% of carbonaceous reductant based on the amount of chrome ore material, COPR, saccharide binder and carbonaceous reductant,

wherein the COPR contains

• chrome(lll) oxide: 7 to 13 wt%,

• aluminum oxide: 10 to 30 wt%,

• iron(II) oxide: 36 to 44 wt%,

• iron(III) oxide: > 0.5% wt%,

• magnesium oxide: 9 to 18 wt%,

• calcium oxide: < 10 wt%,

• silicon oxide: 0 to 3 wt%,

• vanadium oxide: < 1 wt%,

• sodium oxide: 0 to 5 wt%, and

• sodium monochromate: 0 to 4.7 wt%,

wherein the calcining in step c) is carried out at oven temperatures of 1250 °C to 1600 °C for periods of 1 minute to 8 hours.

2. A process according to claim 1, wherein the calcining in step c) is carried out at oven temperatures of 1300 °C to 1500 °C for periods of 5 minutes to 5 hours.

3. A process according to claim 1, wherein the pellets obtained by step a) comprise less than 2 wt% of carbonaceous reductant based on the amount of chrome ore material, COPR, saccharide binder and carbonaceous reductant.

4. A process according to claim 1, wherein the pellets obtained by step a) comprise less than 1 wt% of carbonaceous reductant based on the amount of chrome ore material, COPR, saccharide binder and carbonaceous reductant.

5. A process according to any of claims 1 to 4, wherein the pellets obtained by step a) comprise 0.003 to 1 wt% of saccharide binder based on the amount of chrome ore material, COPR, saccharide binder and, if present, carbonaceous reductant.

6. A process according to any of claims 1 to 4, wherein the pellets obtained by step a) comprise 0.01 to 0.5 wt% of saccharide binder based on the amount of chrome ore material, COPR, saccharide binder and, if present, carbonaceous reductant.

7. A process according to any of claims 1 to 4, wherein the pellets obtained by step a) comprise 0.05 to 0.2 wt% of saccharide binder based on the amount of chrome ore material, COPR, saccharide binder and, if present, carbonaceous reductant.

8. A process according to any of claims 1 to 4, wherein the saccharide binder is selected from glucose, galactose, fructose and xylose, sucrose, lactose, maltose, trehalose, maltodextrins, raffinose, stachyose, fructo-oligosaccharides, amylose, amylopectin, starch, modified starches, arabinoxylans, chitin glycogen, cellulose, hemicellulose, pectins, hydrocolloids, agarose, dextran, guaran and mixtures thereof.

9. A process according to any of claims 1 to 4, wherein the saccharide binder comprises guaran.

10. A process according to any one of claims 1 to 4, wherein during the calcining step c) a reduction of at least a part of the iron- and chrome oxides in the pellets into their metallic form is achieved.

11. Calcined, iron- and chrome-containing pellets prepared by a process according to any one of claims 1 to 10 containing:

• chrome: 25 to 36 wt%,

• iron: 14 to 24% wt%, and

• silicon: 0.4 to 2 wt%,

having a density of > 3.35 g/cm³ and an average cold crushing strength of at least 130 kgf/pellet.

12. Calcined, iron- and chrome-containing pellets according to claim 11 having a density of > 3.40 g/cm³ and an average cold crushing strength of at least 160 kgf/pellet.

13. Calcined, iron- and chrome-containing pellets according to claim 11 having a density of > 3.41 g/cm³ and an average cold crushing strength of at least 200 kgf/pellet

14. Use of calcined, iron- and chrome-containing pellets according to any of claims 11 to 13 for the preparation of Ferrochrome.

15. Process for the preparation of Ferrochrome, wherein the calcined, iron- and chrome-containing pellets according to any one of claims 11 to 13 are fed into an electric arc furnace and molten under reduction of iron- and chrome oxides.

Ansprüche

1. Verfahren zur Herstellung von kalzinierten, eisen- und chromhaltigen Pellets, umfassend die folgenden Schritte:

a) Mischen von Chromerz-Material, Chromerz-Verarbeitungsrückstände (COPR) und Saccharid-Bindemittel und Formen der erhaltenen Mischung zu Pellets,

b) gegebenenfalls Trocknen der in Schritt a) erhaltenen Pellets und

c) Kalzinierung der Pellets,

wobei die in Schritt a) erhaltenen Pellets weniger als 3 Gew.-% kohlenstoffhaltiges Reduktionsmittel, bezogen auf die Menge an Chromerz-Material, COPR, Saccharid-Bindemittel und kohlenstoffhaltigem Reduktionsmittel, enthalten,

wobei die COPR Folgendes enthalten

• Chrom(III)oxid: 7 bis 13 Gew.-%,

• Aluminiumoxid: 10 bis 30 Gew.-%,

• Eisen(II)oxid: 36 bis 44 Gew.-%,

• Eisen(III)oxid: > 0,5 Gew.-%,

• Magnesiumoxid: 9 bis 18 Gew.-%,

• Calciumoxid: < 10 Gew.-%,

• Siliziumoxid: 0 bis 3 Gew.-%,

• Vanadiumoxid: < 1 Gew.-%,

• Natriumoxid: 0 bis 5 Gew.-%, und

• Natriummonochromat: 0 bis 4,7 Gew.-%,

wobei die Kalzinierung in Schritt c) bei Ofentemperaturen von 1.250 °C bis 1.600 °C über einen Zeitraum von 1 Minute bis 8 Stunden durchgeführt wird.

2. Verfahren nach Anspruch 1, wobei die Kalzinierung im Schritt c) bei Ofentemperaturen von 1.300 °C bis 1.500 °C über einen Zeitraum von 5 Minuten bis 5 Stunden durchgeführt wird.

3. Verfahren nach Anspruch 1, wobei die im Schritt a) erhaltenen Pellets weniger als 2 Gew.-% kohlenstoffhaltiges Reduktionsmittel enthalten, bezogen auf die Menge an Chromerz-Material, COPR, Saccharid-Bindemittel und kohlenstoffhaltigem Reduktionsmittel.

4. Verfahren nach Anspruch 1, wobei die im Schritt a) erhaltenen Pellets weniger als 1 Gew.-% kohlenstoffhaltiges Reduktionsmittel enthalten, bezogen auf die Menge an Chromerz-Material, COPR, Saccharid-Bindemittel und kohlenstoffhaltigem Reduktionsmittel.

5. Verfahren nach einem der Ansprüche 1 bis 4, wobei die im Schritt a) erhaltenen Pellets 0,003 bis 1 Gew.-% Saccharid-Bindemittel, bezogen auf die Menge an Chromerz-Material, COPR, Saccharid-Bindemittel und, falls vorhanden, kohlenstoffhaltigem Reduktionsmittel, enthalten.

6. Verfahren nach einem der Ansprüche 1 bis 4, wobei die im Schritt a) erhaltenen Pellets 0,01 bis 0,5 Gew.-% Saccharid-Bindemittel, bezogen auf die Menge an Chromerz-Material, COPR, Saccharid-Bindemittel und, falls vorhanden, kohlenstoffhaltigem Reduktionsmittel, enthalten.

7. Verfahren nach einem der Ansprüche 1 bis 4, wobei die im Schritt a) erhaltenen Pellets 0,05 bis 0,2 Gew.-% Saccharid-Bindemittel, bezogen auf die Menge an Chromerz-Material, COPR, Saccharid-Bindemittel und, falls vorhanden, kohlenstoffhaltigem Reduktionsmittel, enthalten.

8. Verfahren nach einem der Ansprüche 1 bis 4, wobei das Saccharid-Bindemittel ausgewählt ist aus Glucose, Galactose, Fructose und Xylose, Saccharose, Lactose, Maltose, Trehalose, Maltodextrinen, Raffinose, Stachyose, Fructo-Oligosaccharide, Amylose, Amylopektin, Stärke, modifizierte Stärken, Arabinoxylane, Chitin-Glykogen, Cellulose, Hemicellulose, Pektine, Hydrokolloide, Agarose, Dextran, Guaran und Mischungen davon.

9. Verfahren nach einem der Ansprüche 1 bis 4, wobei das Saccharid-Bindemittel Guaran enthält.

10. Verfahren nach einem der Ansprüche 1 bis 4, wobei während des Kalzinierungsschritts c) eine Reduktion mindestens eines Teils der Eisen- und Chromoxide in den Pellets in ihre metallische Form erreicht wird.

11. Kalzinierte, eisen- und chromhaltige Pellets, hergestellt nach einem Verfahren nach einem der Ansprüche 1 bis 10, enthaltend:

• Chrom: 25 bis 36 Gew.-%,

• Eisen: 14 bis 24 Gew.-%, und

• Silizium: 0,4 bis 2 Gew.-%,

mit einer Dichte von > 3,35 g/cm³ und einer durchschnittlichen Kaltstauchfestigkeit von mindestens 130 kgf/Pellet.

12. Kalzinierte, eisen- und chromhaltige Pellets nach Anspruch 11, mit einer Dichte von > 3,40 g/cm³ und einer durchschnittlichen Kaltstauchfestigkeit von mindestens 160 kgf/Pellet.

13. Kalzinierte, eisen- und chromhaltige Pellets nach Anspruch 11, mit einer Dichte von > 3,41 g/cm³ und einer durchschnittlichen Kaltstauchfestigkeit von mindestens 200 kgf/Pellet.

14. Verwendung von kalzinierten, eisen- und chromhaltigen Pellets nach einem der Ansprüche 11 bis 13 zur Herstellung von Ferrochrom.

15. Verfahren zur Herstellung von Ferrochrom, bei dem die kalzinierten, eisen- und chromhaltigen Pellets nach einem der Ansprüche 11 bis 13 in einen Elektrolichtbogenofen eingebracht und unter Reduktion von Eisen- und Chromoxiden geschmolzen werden.

Revendications

1. Procédé de préparation de boulettes calcinées contenant du fer et du chrome comprenant les étapes consistant à :

a) mélanger un matériau de minerai de chrome, un résidu de traitement de minerai de chrome (COPR) et un liant à base de saccharide et former le mélange obtenu en boulettes,

b) éventuellement sécher les boulettes obtenues à l'étape a), et

c) calciner les boulettes,

en ce que les boulettes obtenues à l'étape a) comprennent moins de 3 % en poids de réducteur carboné sur la base de la quantité de matériau de minerai de chrome, de COPR, de liant saccharide et de réducteur carboné,

en ce que le COPR contient

• de l'oxyde de chrome (III) : 7 à 13 % en poids,

• de l'oxyde d'aluminium : 10 à 30 % en poids,

• de l'oxyde de fer (II) : 36 à 44 % en poids,

• de l'oxyde de fer (III) : > 0,5% % en poids,

• de l'oxyde de magnésium : 9 à 18 % en poids,

• de l'oxyde de calcium : < 10 % en poids,

• de l'oxyde de silicium : 0 à 3 % en poids,

• de l'oxyde de vanadium : < 1 % en poids,

• de l'oxyde de sodium : 0 à 5 % en poids, et

• du chromate de sodium : 0 à 4,7 % en poids,

en ce que la calcination à l'étape c) est effectuée à des températures de four comprises entre 1250 °C et 1600 °C pendant des périodes allant de 1 minute à 8 heures.

2. Procédé selon la revendication 1, en ce que la calcination à l'étape c) est effectuée à des températures de four comprises entre 1300 °C et 1500 °C pendant des périodes allant de 5 minutes à 5 heures.

3. Procédé selon la revendication 1, en ce que les boulettes obtenues à l'étape a) comprennent moins de 2 % en poids de réducteur carboné sur la base de la quantité de matériau de minerai de chrome, de COPR, de liant saccharide et de réducteur carboné.

4. Procédé selon la revendication 1, en ce que les boulettes obtenues à l'étape a) comprennent moins de 1 % en poids de réducteur carboné sur la base de la quantité de matériau de minerai de chrome, de COPR, de liant saccharide et de réducteur carboné.

5. Procédé selon l'une des revendications 1 à 4, en ce que les boulettes obtenues à l'étape a) comprennent 0,003 à 1 % en poids de liant saccharide sur la base de la quantité de matériau de minerai de chrome, de COPR, de liant saccharide et, si présent, de réducteur carboné.

6. Procédé selon l'une des revendications 1 à 4, en ce que les boulettes obtenues à l'étape a) comprennent 0,01 à 0,5 % en poids de liant saccharide sur la base de la quantité de matériau de minerai de chrome, de COPR, de liant saccharide et, si présent, de réducteur carboné.

7. Procédé selon l'une des revendications 1 à 4, en ce que les boulettes obtenues à l'étape a) comprennent 0,05 à 0,2 % en poids de liant saccharide sur la base de la quantité de matériau de minerai de chrome, de COPR, de liant saccharide et, si présent, de réducteur carboné.

8. Procédé selon l'une des revendications 1 à 4, en ce que le liant saccharide est sélectionné parmi le glucose, galactose, fructose et xylose, le sucrose, lactose, maltose, tréhalose, les maltodextrines, le raffinose, le stachyose, les fructo-oligosaccharides, l'amylose, l'amylopectine, l'amidon, les amidons modifiés, les arabinoxylanes, la chitine et glycogène, la cellulose, l'hémicellulose, les pectines, les hydrocolloïdes, l'agarose, le dextrane, la gomme de guar et leurs mélanges.

9. Procédé selon l'une des revendications 1 à 4, en ce que le liant saccharide comprend de la gomme de guar.

10. Procédé selon l'une des revendications 1 à 4, en ce que pendant l'étape c) de calcination une réduction d'au moins une partie des oxydes de fer et de chrome dans les boulettes sous leur forme métallique est réalisée.

11. Boulettes calcinées contenant du fer et du chrome préparées par un procédé selon l'une des revendications 1 à 10 contenant :

• du chrome : 25 à 36 % en poids,

• du fer : 14 à 24 % en poids, et

• du silicium : 0,4 à 2 % en poids,

ayant une masse volumique > 3,35 g/cm³ et une résistance moyenne à la compression à froid d'au moins 130 kgf/boulette.

12. Boulettes calcinées contenant du fer et du chrome selon la revendication 11 ayant une masse volumique > 3,40 g/cm³ et une résistance moyenne à la compression à froid d'au moins 160 kgf/boulette.

13. Boulettes calcinées contenant du fer et du chrome selon la revendication 11 ayant une masse volumique > 3,41 g/cm³ et une résistance moyenne à la compression à froid d'au moins 200 kgf/boulette.

14. Utilisation de boulettes calcinées contenant du fer et du chrome selon l'une des revendications 11 à 13 pour la préparation de Ferrochrome.

15. Procédé pour la préparation de Ferrochrome, en ce que les boulettes calcinées contenant du fer et du chrome selon l'une des revendications 11 à 13 sont chargées dans un four à arc électrique et fondues sous réduction d'oxydes de fer et de chrome.