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

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.

The production of Ferrochrome is carried out in huge electrically heated arc 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 the process. 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.

The usage of fine chrome ore material, so called „fines" or„flour", is economically of interest, but 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 to reach the hottest zones for reduction. Most of this material will be blown out of the furnace with the gas stream. To get the fines in a suitable shape, pellets were formed out of the fines as disclosed in ZA2004-03429A. In Minerals Engineering (2012), 34, 55-62, a detailed description of the used binders and their properties and effects on the pellet strentgh and other poperties are given. The particles produced according to the process of ZA2004-03429A and in Minerals Engineering (2012), 34, 55-62, are made of chromite 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 around 18 weight-% of silicium.

There are some disadvantages of this process and the particles 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).

Thus, it was the object of the present invention to provide a process for preparing iron- and chrome-containing particles that avoids the disadvantages of the prior art and that leads to particles that can preferably be used in a process for the preparation of Ferrochrome that shows a reduced slag formation. Further, the particles should preferably be usable in a process for the preparation of Ferrochrome that is more energy-efficient. Further, it was an object of the present invention to provide iron- and chrome-containing particles that exhibit a sufficient stability to be stored and/or transported, e.g. transported to the process for the production of Ferrochrome.

The invention therefore provides a process for preparing iron- and chrome-containing particles comprising the steps a) producing ore particles by mixing

- in a first step chromite ore material, Chrome Ore Process Residue (COPR) and a reductant component, and

- in second step the mixture obtained after the first step with water, b) drying the ore particles obtained after step a), and optionally c) reducing the ore particles obtained after step b) to produce metallised particles. Step a)

Chromite ore material

Preferably, the chromite ore material used in step a) contains:

• chrome(lll)oxid (Cr ₂ 0 ₃ ): 26 to 54 weight-%, particulary preferably 40 to 45 weigh t-%, · aluminium oxide (Al ₂ 0 ₃ ): 10 to 30 weight-%, particulary preferably 13 to 18 weight-%,

• iron(ll) oxide (FeO): 12 to 36 weight-%, particulary preferably 20 to 28 weight-%, • magnesium oxide (MgO): 9 to 22 weight-%, particulary preferably 10 to 15 weigh t-%,

• calcium oxide (CaO): < 5 weight-%, particulary preferably < 1 weight-%, and · silicium oxide (Si0 ₂ ): 1 to 18 weight-%, particulary preferably 2 to 5 weight-%, whereas the weight-% refer to the weight of the chromite ore material.

The world wide largest chromite ores deposits are located in South Africa, Zimbabwe, Turkey and the Philippines and in some other countries. The chromite ore is divided in two categories: The metallurgical grade with≥ 45 weight-% of Cr ₂ 0 ₃ and the chemical grade with < 45% weight-% and≥40 weight-% of Cr ₂ 0 ₃ . The largest known deposit of chromite ore is found in Zimbabwe with over 300 mio. tons.

Chrome Ore Process Residue

Preferably, Chrome Ore Process Residue (COPR) contains metal oxides such as chromium(lll) oxide (Cr ₂ 0 ₃ ), aluminium oxide (Al ₂ 0 ₃ ), iron(lll) oxide (Fe ₂ 0 ₃ ), iron(ll) oxide (FeO), magnesium oxide (MgO), calcium oxide (CaO), silicon oxide (Si0 ₂ ), vanadium oxide (V ₂ 0 ₅ ), sodium oxide (Na ₂ 0) and sodium monochromate (Na ₂ Cr0 ₄ ).

The Cr(VI) is preferably present as sodium monochromate (Na ₂ Cr0 ₄ ) in the COPR.

The CaO content of the COPR is preferably less than 15% by weight, particularly preferably less than 10% by weight, most preferably less than 5% by weight.

COPR preferably contains:

• chrome(lll) oxide (Cr ₂ 0 ₃ ): 7 to 13 weight-%, preferably 7,5 to 12,5 weight-%,

• aluminium oxide (Al ₂ 0 ₃ ): 10 to 30 weight-%, preferably 18 to 24 weight-%, · iron(ll) oxide (FeO): 36 to 44 weight-%, preferably 37 to 42 weight-%,

• iron(lll) oxide (Fe ₂ 0 ₃ ): >0,5% weight-%, preferably >2 weight-%,

• magnesium oxide (MgO): 9 to 18 weight-%, preferably 10 to 17 weight-%,

• calcium oxide (CaO): < 10 weight-%, preferably < 5 weight-%, • silicium oxide (Si0 ₂ ): 0 to 3 weight-%, preferably 1 to 3 weight-%,

• vanadium oxide (V ₂ 0 ₅ ): < 1 weight-%, preferably < 0.5 weight-%,

• sodium oxide (Na ₂ 0): 0 to 5 weight-%, preferably 2 to 5 weight-%, and

• sodium monochromate (Na ₂ Cr0 ₄ ): 0 to 4,7 weight-%, preferably <0,0003 weight-%, whereas the weight-% refer to the weight of the COPR. Preferably, the Cr(VI) content of the COPR is 0,01 - 1 weight-%.

Preferably, the Cr content in the COPR is 2 to 25 weight-%, particulary preferably 5 to 9 weight-%.

Preferably, the Fe content in the COPR is 28 to 35 weight-%, particulary preferably 29 to 34 weight-%.

Preferably, the Si content in the COPR is 0 to 1 ,5 weight-%, particulary preferably 0,4 to 1 ,0 weight-%.

The COPR used in step a) can be obtained as 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).

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 < 5% by weight).

Alternatively, the Cr(VI) content of the COPR is preferably < 0,0001 weight-%.

COPR with a Cr(VI) content of < 0,0001 weight-% is preferably obtained via a reduction process of COPR with a Cr(VI) content of 0,01 - 1 weight-% in that the reduction of Cr(VI) to Cr(lll) takes preferably place via polyethyleneglycole (PEG) or glycerole as disclosed in WO2014006196A1 or, alternatively, in an atmosphere containing less than 0,1 % by volume of an oxidizing gas as disclosed in WO2016074878A1 .

All the above mentioned weight-% refer to the weight of the COPR. Reductant component

The reductant component used in step a) is preferably selected from the group consisting of anthracite, char and bituminous coal, particulary preferably anthracite.

Anthracite contains preferably less than 1 weight-% of organic compounds. Such organic compounds preferably evaporate at temperatures above 70°C up to 1400°C in an inert atmosphere. These organic compounds are preferably saturated and unsaturated hydrocarbons.

The quantity of these organic compounds is determined by heating the reductant component in an inert atmosphere up to the target temperature and taking readings on the mass loss. This detected mass loss is subtracted from the mass loss found on heating the particles in a second step, in order to calculate the degree of reduction.

Preferably, 0,5 - 1 equivalents of the reductant component with reference to the to be reduced Fe- and Cr-ions contained in the chromite ore material and the COPR are used in step a), particulary preferably 0,7-0,9 equivalents. Thus, the Fe- and Cr-ions contained in the chromite ore material and the COPR used in step a) are at least partially reduced to iron and chrome metal in step a).

The process

There are different ways known to the skilled person for mixing the chromite ore material, COPR and a reductant component in the first step of step a). Preferably, the mixing is conducted by using a dry mill.

The components used in the first step of step a) are preferably milled. The milling can take place prior to the mixing in the first step of step a), during the mixing in the first step of step a) or after the mixing in the first step of step a).

Preferably, the chromite ore material, COPR and a reductant component are milled during mixing in the first step of step a).

Preferably, the mixture obtained after the mixing of the chromite ore material, COPR and a reductant component in the first step of step a) comprises

• 50 to 95 weight-%, particularly preferably 75 to 90 weight-% of chromite ore material,

• 1 to 15 weight-%, particularly preferably 2 to 5 weight-% of COPR, and

· 3 to 25 weight-%, particularly preferably 6 to 20 weight-% of the reductant component, whereas the weight-% refer to the weight of the mixture obtained after the mixing of chromite ore material, COPR and a reductant component in the first step of step a).

Preferably, the mixture obtained after the mixing of the chromite ore material, COPR and a reductant component in the first step of step a) provides a D90 of 50 to 100 μηη, particulary preferably of 65 to 85 μηη. According to the invention, a D90 of 50 μηη means that 90% by volume of the particles of the mixture have a particle size of 50 μηη.

The mixture obtained after the mixing of chromite ore material, COPR and a reductant component in a first step is mixed with water in a second step.

The weight ratio of water to the sum of the components chromite ore material, COPR and a reductant component is preferably between 1 :6 and 1 : >100, particulary preferably between 1 :8 and 1 : >100, most preferably 1 : >100.

The second step of step a) may take place in either a pan or drum pelletizing unit. Thereby, composite carbon containing (so called "green") ore particles are obtained.

Preferably, the ore particles obtained after step a) have a diameter of 8-20 mm, particulary preferably of 10-15 mm.

Preferably, the silicium content of the ore particles obtained after step a) is below 2,5 weight-%, particulary preferably below 2 weight-%, whereas the weight-% refer to the weight of the ore particles obtained after step a).

Preferably, the ore particles obtained after step a) do not crack when dropped from a height of up to 0,2 m, particulary preferably of up to 0,4 m, most preferably of up to 0,5 m, on a steel plate.

Step b)

The drying of the ore particles obtained after step a) may be conducted in different ways known to the skilled person. The ore particles 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.

Preferably, the drying is effected by heating the ore particles under atmospheric conditions. Particulary 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, particulary preferably 6 to 30 hours. The drying can be performed in an oven.

Optionally, the ore particles can be sintered in step b).

The ore particles obtained after step b) provide at least a sufficient mechanical stability to be stored and transported. Preferably, the ore particles obtained after step b) do not crack when dropped from a height of up to 0,2 m, particulary preferably of up to 0,4 m, most preferably of up to 0,5 m, on a steel plate.

Step c)

The ore particles obtained after step b) can optionally be further reduced in a reduction unit.

The reduction unit is preferably a rotary kiln, a muffle furnace or a tube furnace.

The nature and configuration of the unit are 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 metallisation and commensurately limited degrees of carbon burn-off.

In the reduction unit, the ore particles obtained after step b) are exposed to 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.

Preferably, the inert atmosphere contains less than 0,1 vol-% of oxygen. Particulary preferably, the inert atmosphere is argon.

After heating, the ore particles can be cooled down, preferably to a temperature of 18 to 25°C.

Energy supply to the reduction unit operation can be achieved via controlled, oxygen assisted combustion of a carbonaceous fuel source, which may be a bituminous coal or fuel oil, or combination thereof. Control of the relative supply of oxygen enrichment in relation to the fuel type and, in turn, the control of these components relative to the hot particle feed rate are a key component in achieving optimum metallisation that is only achieved under localized, but highly reducing conditions while sustaining good throughput rates that implies high thermal draught, sustained by optimum combustion conditions pertaining to the generation and effusion of chemical based energy from the combustion operation. Alternatively, the furnace can be heated indirectly using electrical power or gas with an inert atmosphere as described above inside the reduction unit.

The metallised particles obtained after step c) are discharged, either via direct hot transfer to the smelting furnace or via controlled cooling of the metallised product, to yield cool, metallised particles.

The reduction facility is preferably operated in a manner so as to ensure that the preheated particles are maintained at a temperature of preferably 1350 to 1450°C for a period of between 2 and 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 metallisation.

The term "metallised particles" means that the degree of metallisation in the metallised particles obtained after step c) is above 65%, preferably above 75%, particulary 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).

The degree of metallisation can be determined via thermographimetric 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 metallisation 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 metallisation can be calculated with ease.

Preferably, the metallised particles obtained after step c) contain chrome as chrome(lll)oxide (Cr ₂ 0 ₃ ) and as chrome metal, iron as iron(ll) oxide (FeO) and iron(lll) oxide (Fe ₂ 0 ₃ ) and as iron metal, and silicium as silicium oxide (Si0 ₂ ).

The ratio of iron (11,111) to iron metal in the metallised particles obtained after step c) is preferably <1 :2, particulary preferably <1 :10.

The ratio of chrome(lll) to chrome metal in the metallised particles obtained after step c) is preferably <1 :2, particulary preferably <1 :10.

The ratio of iron (11 , 111 ) and chrome(lll) to iron and chrome metal in the iron- and chrome- containing particles is preferably <1 :2, particulary preferably <1 :10. In the metallised particles obtained after step c) the Cr(VI) content is preferably <0,0001 weight-%. The metallised particles obtained after step c) are particulary preferably free of Cr(VI).

The metallised particles obtained after step c) provide increased mechanical stability compared to those obtained after step b).

The metallised particles can be further stored or transported, e.g. to an electric submerged arc furnace for the preparation of Ferrochrome.

Preferably, the metallised particles obtained after step c) do not crack when dropped from a height of up 0,8 m, particulary preferably of up to 1 ,2 m, most preferably of up to 1 ,5 m, on a steel plate.

By the use of the metallised particles obtained after step c) the electrical energy consumption for the complete reduction to iron metal and chrome metal in the arc funace is reduced.

Iron- and chrome-containing particles

The invention further provides iron- and chrome-containing particles that contain:

• chrome: 22 to 26 weight-%, preferably 23 to 25 weight-%,

• iron: 13 to 19% weight-%, preferably 14 to 18 weight-%, and

• silicium: 0,5 to 2 weight-%, preferably 0,5 to 1 weight-%, whereas the weight-% refer to the weight of the iron- and chrome- containing particles.

Preferably, the iron- and chrome-containing particles contain chrome as chrome(lll)oxide (Cr ₂ 0 ₃ ) and as chrome metal, iron as iron(ll) oxide (FeO) and iron(lll) oxide (Fe ₂ 0 ₃ ) and as iron metal, and silicium as silicium oxide (Si0 ₂ ).

The ratio of iron (11 , 111 ) to iron metal in the iron- and chrome-containing particles is preferably <1 :2, particulary preferably <1 : 10.

The ratio of chrome(lll) to chrome metal in the iron- and chrome-containing particles is preferably <1 :2, particulary preferably <1 :10.

The ratio of iron (11 , 111 ) and chrome(lll) to iron and chrome metal in the iron- and chrome- containing particles is preferably <1 :2, particulary preferably <1 :10. In the iron- and chrome-containing particles the Cr(VI) content is preferably <0,0001 weight-%. The iron- and chrome-containing particles are particulary preferably free of Cr(VI).

Preferably, the iron- and chrome-containing particles have a diameter of 6-13 mm. Generally, more than 80%, preferably more than 90%, more preferably more than 95 % and most preferably more than 98 % of the iron- and chrome-containing particles do not crack when dropped from a height of 0.5 m on a steel plate.

Typically, more than 80%, preferably more than 90%, more preferably more than 95 % and most preferably more than 98 % of the iron- and chrome-containing particles do not crack when dropped from a height of 1.2 m on a steel plate.

Usually, more than 80%, preferably more than 90%, more preferably more than 95 % and most preferably more than 98 % of the iron- and chrome-containing particles do not crack when dropped from a height of 1.5 m on a steel plate.

Preferably, the iron- and chrome-containing particles do not crack when dropped from a height of up to 0,5 m, particulary preferably of up to 1 ,2 m, most preferably of up to 1 ,5 m, on a steel plate.

Preferably, the iron- and chrome-containing particles according to the invention are obtained by the process for preparing iron- and chrome-containing particles according to the invention. The invention further provides the use of iron- and chrome-containing particles according to the invention for the preparation of Ferrochrome.

The invention further provides a process for the preparation of Ferrochrome by using iron- and chrome-containing particles according to the invention. Preferably, the process comprises the step of smelting the iron- and chrome-containing particles according to the invention in the presence of a carbon based reductant.

The invention will be described in more detail in the following non-limiting example. Example 1 :

A mixture of 82,2 weight-% of chromite ore material, 15,3 weight-% of anthracite and 2,5 weight-% of COPR (Cr(VI) content < 0,0001 weight-%) was milled down to a D90 of 75 μηη (90 per cent by volume have a particle size of below 75 μηη), whereas the weight-% refer to the sum of the components chromite ore material, COPR and reductant component.

The received material was filled in a rotating drum and sprayed with water (weight ratio received material : water = 10 : 0,98) while the drum was turning. The particles were pre- dried at 25°C in air for 24 hours. After this, the particles were dried in an oven for 24 hours at 105°C .

These ore particles were then reduced. The reduction was conducted in a muffle furnace at 1400°C (heating rate of 10°C/min) for a period of four hours under inert atmosphere using argon, before cooling down a temperature of 23 to 25°C.

Comparative Example according to ZA2004-03429A: A mixture of 82,2 weight-% of chromite ore material, 15,3 weight-% of anthracite and 2,5 weight-% of Bentonite was milled down to a D90 of 75 μηη (90 per cent by volume have a particle size of below 75 μηη), whereas the weight-% refer to the sum of the components chromite ore material, Bentonite and reductant component.

The received material was filled in a rotating drum and sprayed with water (weight ratio received material : water = 10 : 0,98) while the drum was turning. The particles were pre- dried at 25°C in air for 24 hours. After this, the particles were dried in an oven for 24 hours at 105°C.

These ore particles were then reduced. The reduction was conducted in a muffle furnace at 1400°C (heating rate of 10°C/min) for a period of four hours under inert atmosphere using argon, before cooling down to a temperature of 23 to 25°C.

Tests of mechanical stability

Drop tests were conducted by dropping ten selected particles obtained according to example 1 or the comparative example, respectively, each three times at each height interval (0,5 and 1 ,5 meters) and counting the number of drops from that height before the particle cracks. Should all particles not crack after three drops at the initial height (i.e. 0,5 m), the next height was evaluated. The particles were dropped on a steel plate. Results are summarized in table 1 . Table 1

The results show that particles obtained after different process steps according to the process of the invention provide at least the same mechanical strenght as those of the comparative example according to ZA2004-03429A. Further, the particles according to the invention contain a higher amount of Cr and Fe and a lower amount of Si and thus, can advantageously be used in a process for the preparation of Ferrochrome.