The present invention relates to a method of processing an iron ore containing phosphorus, the iron ore having a gangue, in order to remove said phosphorus.

Phosphorus is an undesirable impurity in iron ore to be smelted for the production of pig iron or steel, as it negatively affects the strength of the product, possibly causing hot shortness and/or temper embrittlement. The removal of phosphorus during the high temperature iron or steel making process is costly and the ores with a high content of phosphorus are less desirable or can be marketed with high penalties.

Oolitic iron ores are widespread in the world. However, their high phosphorus content as well as their fine dissemination of silica and aluminum minerals is an obstacle to their intensive use.

This is particularly the case when the phosphorus is not represented as definite minerals but forms a solid solution into oolites structure. Then the usual methods for physical or physicochemical removal of phosphorus from the iron ore, such as flotation, magnetic separation or chemical methods such as acid leaching, fail. For example, in the oolitic iron ore from Lisakovsk, Kazakhstan, less than 1 % of the phosphorus is present as apatite, the rest forming a solid solution with goethite that remains relatively untouched during leaching, even after fine grinding.

Document CN-A-101 338 361 discloses a phosphorus reduction method, in particular applicable for dephosphorisation of an iron ore of hematite or limonite type. In the method reductive roasting, magnetic separation and filtration are used as a first stage to reduce phosphorus; in a second stage the iron ore concentrate with about 5 % moisture is acid leached by sulphuric acid or hydrochloric acid. As a result, the phosphorus is lowered from 0.84 wt% to 0.30 wt%.

Document AU-A-2009/251075 discloses a method for arsenic removal and phosphorous removal out of an iron ore comprising the steps of: crushing and grinding the ore, performing preliminary calcination, alkaline leaching, magnetic separation and filtering of both magnetic and non-magnetic fractions. The liquid phase is forwarded for extraction of P, As, V, etc. The invention is characterized in that the crushed ground iron ore is mixed with carbon containing reducing agents and NaCI and sludge of CaC0 ₃ and NaOH. The mixture is roasted in kiln and the obtained product is cooled in a water or alkaline aqueous solution. After leaching, the leached mixture (also called pulp) is submitted to magnetic separation and the iron ore concentrate is filtered while rinsing with nitric acid for removal of salts sedimented along the surfaces of the particles. Such methods have proven unsatisfactory to treat adequately some iron ores, in particular the limonitic ones.

An aim of the invention is to provide a method of processing an iron ore containing phosphorus, in particular a goethite oolitic iron ore, such that the obtained iron ore concentrate has more value for iron and steel producers, the cost for implementing the method being moderate.

To this end, the invention proposes a method of processing an iron ore containing phosphorus, the iron ore having a gangue, the method comprising the steps consisting in:

- mixing the iron ore with at least one alkaline additive able to react with the gangue in order to obtain a starting mixture, the alkaline additive being substantially devoid of calcium carbonate;

- roasting said starting mixture at a temperature between 600°C and 1000°C in order to obtain a roasted mixture;

- leaching said roasted mixture (with an aqueous solution having a pH equal to or above 7 in order to obtain a first leached mixture; and

- performing a first solid-solid separation of the first leached mixture into a first solid product and a first residual mixture, said first residual mixture containing liquor and slimes.

In other embodiments, the method comprises one or several of the following features, taken in isolation or any technical feasible combination:

- the alkaline additive is taken from the list consisting of NaOH, Na ₂ C0 ₃ , and their mixtures;

- the alkaline additive represents a weight fraction between 2% and 10%, preferably between 2% and 8%, with respect to said iron ore;

- during said mixing, the iron ore is also mixed with a reductive compound in order to obtain the starting mixture, the reductive compound being preferably taken from the list consisting of coal, coke, charcoal, and their mixtures;

- the reductive compound represents a weight fraction between 5% and 12% with respect to said iron ore;

- the duration of said roasting is more than 30 minutes;

- said roasted mixture is ground prior to said leaching in order to obtain a ground mixture;

- said leaching with the aqueous solution is carried out for at least 20 minutes, the aqueous solution being at a temperature between 60°C and 95°C;

- the method further comprises the steps consisting in: leaching the first solid product with an acidic solution in order to obtain a second leached mixture; and performing a second solid-solid separation of the second leached mixture into an iron ore concentrate and a second residual mixture, said second residual mixture containing liquor and slimes;

- said first solid-solid separation and/or said second solid-solid separation are carried out by magnetic separation;

- said first solid-solid separation and/or said second solid-solid separation are carried out using hydrocyclones, screw classifiers, spiral separators or other gravity separation pieces of equipment;

- the acidic solution is a solution of sulfuric acid with a concentration of sulfuric acid between 1 wt% and 15 wt% with respect to the solution;

- at least a fraction of said second residual mixture is recovered in order to produce an acidic solution, said acidic solution being used for said leaching with the acidic solution;

- at least a fraction of the first residual mixture is recovered in order to produce an alkaline solution, said alkaline solution being used as part of the alkaline additive in said mixing;

- the iron ore to be processed comprises between 45 wt% and 60 wt% of iron, and more than 0.6 wt% of phosphorous;

- the iron ore concentrate comprises: an iron content equal to or above 60 wt%, preferably equal to or above 65 wt%, and a phosphorus content equal to or below 0.2 wt%, preferably equal to or below 0.1 wt%.

By "wt%", it is meant a weight fraction expressed in %.

The invention and its advantages will be better understood on reading the following description given solely by way of example and with reference to the appended drawing, in which the figure illustrates a process.

With reference to the figure, a method 1 according to the invention is described. The method 1 aims at processing an iron ore 3 in order to obtain an iron ore concentrate 5.

The iron ore 3 contains phosphorus and has a gangue. For example, the iron ore 3 comprises between 45 wt% and 60 wt% of iron, and more than 0.6 wt% of phosphorous.

In a variant, the method 1 can be applied to iron ores having a phosphorus content lower than 0.6 wt%, for example a phosphorus content between 0.2 wt% and 0.6 wt%.

The iron ore 3 advantageously contains less than 8 wt% of alumina, for example less than 6 wt% of alumina.

In particular embodiments, the iron ore 3 contains less than 4 wt% of alumina, advantageously less than 2 wt% of alumina. In this example, the iron ore 3 is a pre-concentrate obtained from a limonitic oolitic iron ore coming from Lisakovsk, Kazakhstan. The composition of the iron ore 3 is provided in the below table.

Pre-concentration of the ore in order to obtain the iron ore 3 is for example performed by gravity separation and/or magnetic separation in order to eliminate part of the gangue minerals. Thanks to the pre-concentration, the iron content in the iron ore 3 is for example about 48-55 wt%, whereas it is only 35-36 wt% in the original Lisakovsk iron ore.

Pre-concentration is optional and helps eliminating part of the gangue of the iron ore. As a variant of method 1 , the iron ore 3 is not subject to a pre-concentration.

The method 1 comprises a step A of mixing the iron ore 3 with at least one alkaline additive 7 in order to obtain a starting mixture 9, a step B of roasting the starting mixture 9 in order to obtain a roasted mixture 1 1 , a step C of leaching the roasted mixture 1 1 with an aqueous solution in order to obtain a first leached mixture 22, and a step C1 of performing a first solid-solid separation of the first leached mixture 22 into a first solid product 23 and a first residual mixture 25.

The method 1 also comprises a set B' of optional steps between said steps A and C, and a set C of complementary steps after said step C1 in order to obtain the iron ore concentrate 5 and tailings 15 from the first solid product 23 and the first residual mixture 25.

By "tailings" it is meant by-products of the method 1.

In step A, the alkaline additive 7 is able to react with the gangue, for example according to the following global reactions:

Si0 ₂ + 2NaOH => Na ₂ Si0 ₃ + H ₂ 0

Al ₂ 0 ₃ + 2NaOH => 2NaAI0 ₂ + H ₂ 0

P ₂ 0 ₅ + 6NaOH => 2Na ₃ P0 ₄ + 3H ₂ 0

The alkaline additive 7 is substantially devoid of calcium carbonate. For example the calcium carbonate content in the alkaline additive 7 is such that it adds less than 1 wt%, preferably less than 0.1 wt% of calcium carbonate into the starting mixture 9. According to a particular embodiment, the alkaline additive 7 is NaOH with industrial grade.

Calcium, for example introduced in the form of calcium hydroxide, may form salts with phosphorus, such as apatite Ca ₅ (P04) ₃ (OH,CI,F), that are not soluble in water and that, with sulphuric acid, form gypsum which remains inside the iron ore concentrate 5, increasing its sulphur content. The alkaline additive 7 is advantageously taken from the list consisting of NaOH, KOH, Na ₂ C0 ₃ , K ₂ C0 ₃ , and their mixtures. NaOH, Na ₂ C0 ₃ and their mixtures are preferred, as they are usually less expensive. For example, the alkaline additive 7 is NaOH. The alkaline additive 7 represents a weight fraction between 2% and 10% with respect to the iron ore 3, preferably between 2% and 8%, for example about 5%. According to a particular embodiment, the alkaline additive 7 represents a weight fraction of about 7%.

Optionally, the mixture of the iron ore 3 and the alkaline additive 7 is dried during step A.

Optionally, at least one reductive compound 17 able to reduce the phosphorus of the iron ore 3 is mixed with the iron ore 3 during step A in order to obtain the starting mixture 9. The reductive compound 17 advantageously contains carbon in a non oxidized form. For example, the reductive compound 17 is taken from the list consisting of coal, coke, charcoal, and their mixtures. For example, the reductive compound 17 is coke.

The reductive compound 17 allows reducing goethite of the iron ore 3 into maghemite or magnetite, and makes magnetic separations easier. Besides, phosphorus is more prone to be associated with goethite than with hematite, maghemite and magnetite than in goethite.

One practical way to perform step A is for example as follows. The iron ore 3 is mixed with the alkaline additive 7, with the exception of NaOH which is in the form of flakes. The NaOH is dissolved in water to obtain an alkaline solution. The alkaline solution is mixed with the iron ore 3, and this mixture is then dried. The sample is sieved and the oversized particles disaggregated and mixed again. This technique enables each particle to be coated with NaOH and provides an even distribution of NaOH. The reductive compound 17 is advantageously introduced as powder after said drying of the mixture of the iron ore 3 and the alkaline solution. The reductive compound 17 represents a weight fraction between 5% and 12% with respect to the iron ore 3.

In step B, roasting is performed at a temperature between 600°C and 1000°C, for example at approximately 900°C. Roasting is performed for a given duration, advantageously more than 30 minutes, preferably more than 35 minutes, more preferably over 40 minutes.

In a particular embodiment, the duration of roasting is over 45 minutes, preferably over an hour.

During said roasting, phosphorus is driven out from the crystal lattice of hematite, maghemite or magnetite into cracks formed into the oolites during the process of dehydroxylation. Then phosphorus is more sensitive to acid leaching. During step B, in parallel with the reduction of iron oxides contained in the starting mixture 9, phosphorus oxides are reduced for example according to the following reactions:

P ₂ 0 ₅ +3C => 2PO + 3CO

P ₂ 0 ₅ + C => 2P0 ₂ + CO

P ₂ 0 ₅ + 5C => 2P + 5CO

The phosphorus and its oxides having second and fourth valence are more volatile than the fifth valence oxides and evaporate. This explains a lower phosphorus content in the roasted mixture 1 1 versus the starting mixture 9.

The set B' of optional steps comprises a step B1 of cooling the roasted mixture 1 1 in order to obtain a cooled mixture 19, a step B2 of grinding the cooled mixture 19 in order to obtain a ground mixture 21 , and a step B3 of recycling at least a fraction of the reductive compound 17 still present in the ground mixture 21 in order to obtain a mixture 13 with less residual reductive compound 17.

Steps B1 , B2 and B3 may be performed independently from each other and in any order.

Step B1 advantageously includes a two-stage cooling, for example air and then water cooling. Part of the heat released in step B1 can be utilized in step C, for example to heat the mixture 13.

In step B1 , the cooled mixture 19 is substantially at ambient temperature. By

"ambient temperature" it is meant for example approximately 20°C.

In step B2, the particle size distribution of the ground mixture 21 can be adjusted in a quite wide range. Advantageously the ground mixture 21 contains at least 30 wt%, preferably at least 40 wt%, of particles with a size below 0.040 mm. The measurement of particle size is for example performed using the laser particle sizer "Mastersizer 2000".

Step B2 advantageously includes a classification substep in order to return oversized particles to the grinding step.

As step B2 is optional, there may be no grinding step in a variant of the method 1. In step B3, the recycling of the reductive compound 17 is for example carried out by flotation. The obtained froth is filtered and returned to step A according to a well known technique.

In step C, the mixture 13, or the roasted mixture 1 1 in case optional steps B1 , B2 and B3 are not performed, is leached with an aqueous solution having a pH equal to or above 7, for example water or a NaOH solution. The sodium silicate formed during roasting is partly dissolved in water, giving Na ⁺ ions and a pH raise from 7 to 9 - 9.5. Step C is advantageously carried out for at least 20 minutes, for example for approximately 30 minutes. The aqueous solution in contact with the mixture 13 is advantageously at a temperature between 60°C and 95°C, for example approximately 80°C.

Advantageously, as a result of steps B and C, at least 40% in weight of the phosphorus initially contained in the iron ore 3 is removed from the first leached mixture 22.

In step C1 , the first leached mixture 22 is separated in order to recover the first solid product 23 in the form of particles. A Low Intensity Magnetic Separation, known as "LIMS", is used in the example described here.

Thanks to the reductive compound 17, the magnetic properties of iron minerals are changed from paramagnetic to ferromagnetic during said roasting. A magnetic separation allows separating gangue minerals having diamagnetic properties from the iron ore roasted into a fraction having ferromagnetic properties.

In case no reductive compound 17 is used, goethite is transformed during roasting into hematite, which has a lower magnetic susceptibility. Hence a Wet High Intensity Magnetic Separation, knows as "WHIMS", or a three-stage decanting, may be carried out to perform the solid-solid separation of step C1.

In case a magnetic separation is performed, the first residual mixture 25 is the obtained non magnetic fraction. In case a three-stage decanting is performed, the first residual mixture 25 is the obtained supernatant. The first residual mixture 25 comprises liquor and slimes.

The set C of complementary steps comprises a step C2 of leaching the first solid product 23 with an acidic solution 27 in order to obtain a second leached mixture 29, and a step C3 consisting of a second solid-solid separation of the second leached mixture 29 into a second product 31 and a second residual mixture 33 of liquor and slimes.

Advantageously, the set C of complementary steps further comprises independent optional steps, such as a step R1 of recycling the first residual mixture 25 in order to produce an alkaline additive 35 to be used in step A, a step R2 of recycling the second residual mixture 33 in order to produce an acidic solution 37 to be used in step C2, and a step C4 of filtration of the second product 31 in order to obtain the iron ore concentrate 5.

In step C2, the acidic solution is advantageously a solution of sulfuric acid with a concentration of sulfuric acid between 1 wt% and 15 wt%, preferably between 5 wt% and 15 wt%, with respect to the solution, for example approximately 10 wt%.

Advantageously, the acidic solution is at ambient temperature when added. Then the reaction with the first solid product 23 being exothermic, the temperature raises, for example up to 32°C - 36°C. The leaching is advantageously carried out for approximately 5 to 15 minutes.

Thanks to step C2, sodium compounds that are not soluble in water during step C are attacked by the acidic solution, leading to a disaggregation of the particles of the first solid product 23. Step C2 is advantageously carried out in a continuously stirred chemical reactor.

In step C3, the second leached mixture 29 is for example magnetically separated.

As an alternative to the magnetic separations of steps C1 and C3, three-stage decanting may be performed, or a separation using hydrocyclones, screw classifiers, spiral separators or other gravity separation pieces of equipment.

Step R1 for example includes treating the first residual mixture 25 in order to produce the alkaline additive 35 for mixing with the iron ore 3 in step A.

Step R2 for example includes treating the second residual mixture 33 in at least one electrolytic cell in order to produce the acidic solution 37 and using it in step C2 for leaching the first solid product 23.

Typically the iron ore concentrate 5 comprises:

- an iron content equal to or above 60 wt%, preferably equal to or above 65 wt%; and

- a phosphorus content equal to or below 0.2 wt%, preferably equal to or below 0.1 wt%.

Thanks to the above described steps, in particular steps A, B, C and C1 , the method 1 provides an iron ore concentrate 5 in which the phosphorus content is decreased and the iron content is increased. As a consequence, the iron ore concentrate 5 has more value for iron and steel producers.

Also, the iron recovery being good and the method 1 involving affordable steps, the cost for implementing the method 1 is moderate. The increase in specific cost of the iron ore concentrate 5 versus the iron ore 3 due to the implementation of the method 1 is small compared with the value increase for iron and steel producers, making the method 1 profitable.

Another advantage of the invention is that step C2 of acid leaching may be performed at atmospheric pressure without decreasing the efficiency of dephosphorisation.

Tests

The following tests were carried out at laboratory scale.

For each test, unless otherwise told, an initial sample of the iron ore 3, an oolitic pre- concentrate from Lisakovsky GOK, Kazakhstan, was submitted to: a step A of mixing the initial sample with additives to produce a starting mixture, a step B of roasting the starting mixture at 900°C for an hour to produce a roasted mixture,

a step B1 of cooling the roasted mixture using ambient air,

- a step C of leaching the cooled roasted mixture with water or NaOH at 90°C for test 1 and 65°C for tests 2 to 1 1 for 30 minutes to produce a first leached mixture, a step C1 of solid-solid separation of the first leached mixture to produce a first solid product,

a step C2 of leaching the first solid product with a solution of sulphuric acid at 10wt% with respect to the solution in order to obtain a second leached mixture, a step C3 of solid-solid separation of the second leached mixture to produce a second product, and

a step C4 of washing out of salts from the second solid product with hot water to obtain an iron ore concentrate.

For each test, analysis samples were taken. Their composition was determined using a Philips XRF spectrometer.

Results are provided in wt% in the below table. The table provides the composition of the initial sample (feed) and, for each test, the specificities of the test and the composition of the roasted mixture and of the iron ore concentrate.

^* LIMS - Low Intensity Magnetic Separation ^** 3SD -three-stage decanting

Table: Tests results

The table reads as follows.

In test 1 , the sample is mixed with 5 wt% of NaOH with respect to the sample. After drying and remixing, 8 wt% of coke is added, the 8 wt% being calculated on the dry basis of the sample. After roasting to magnetite, the sample is cooled down to ambient temperature and ground in a ball mill so that 40 wt% of the particles have a size below 0.040 mm. The un-burnt coke is separated by flotation with diesel oil as collector and 4- methyl-2-pentanol as frother. The pulp leaving the flotation cell is heated at 90°C to be leached for 30 minutes and then separated by Low Intensity Magnetic Separation (LIMS) inside a unit having an induction of 0.34T in close vicinity of the poles of the magnets. The magnetic fraction is leached with a solution of sulphuric acid at 10 wt% with respect to the solution for 5 minutes at ambient temperature. After leaching the sample is separated by LIMS once again. The magnetic fraction is filtered on a filter paper and washed with distilled water in order to obtain the iron ore concentrate.

In test 2, no alkaline additive is added and no grinding is performed. In step C, sodium hydroxide is used in a quantity equal to the quantity of alkaline additive used in step A of test 1 . So step C is carried out in alkaline conditions, pH = 1 1.4, and for 30 minutes at 65°C. For step C1 , a three-stage decanting is carried out as follows: after leaching the pulp is left for 3 minutes and then decanted, repulped with distilled water, stirred for 3 minutes and decanted again. The procedure is tripled for extensive removal of slimes. The residue is leached with acid following the same procedure as in test 1 , except that step C2 duration is 15 minutes instead of 5 minutes. Step C3 is a three-stage decanting.

Due to use of a reductive compound (coke) in tests 1 to 5, roasting is performed to magnetite. Due to the absence of a reductive compound in tests 6 to 1 1 , roasting is performed to hematite.

In test 6, no alkaline additive is added and no grinding is performed. In step C, sodium hydroxide is used in a quantity equal to the quantity of alkaline additive used in step A of test 1.

In all tests except test 2 and test 6, step C is performed using water, not a sodium hydroxide solution.

Tests 1 to 5 show that the best dephosphorisation is achieved with test 1 , showing that roasting with NaOH is more efficient than with Na ₂ C0 ₃ . The iron ore concentrate obtained in test 1 has the highest iron content - 65.90 wt% and the lowest phosphorus content - down to 0.05 wt% thanks to a higher rate of mineral liberation. Test 2 shows that without an alkaline additive in roasting, dephosphorisation is not efficient. Test 5 shows that earth alkaline additives are not as efficient.

The use of coke as a reducing compound results in a transformation of goethite into magnetite and similar results could be obtained with coal.

Roasting in a reductive environment brings a high contrast between the magnetic susceptibility of iron minerals and those of the gangue. It facilitates the liberation and removal of phosphorus. The alkaline compounds formed during roasting cause a release of heat during the acid leach, in step C2, and the temperature increases from 23°C to 32°C - 36°C. The worse dephosphorisation results are obtained in test 2 without alkaline additive and in test 5 with the addition of lime.

Comparison between the tests 1 and 3 proves the benefit of grinding.

Tests 6 to 10 aim at answering whether roasting to hematite in a more economic regime could bring similar levels of dephosphorisation.

The results from tests 7 to 10 indicate that the phosphorus content can be lowered down to 0.07 - 0.10 wt% with 5 to 10 wt% sodium hydroxide, and to 0.12 wt% with 5 wt% sodium carbonate as alkaline additive. Partial dissolution of sodium silicates is obtained by aqueous leach and solid-solid separation of non-dissolved material by magnetic separation or three-stage decanting. The three-stage decanting process could be replaced by Wet High Intensity Magnetic Separation (WHIMS) when roasting leads to hematite, but the recovery of iron then drops down because of the lower separation efficiency for fine particles.

Comparison with test 6 shows that the introduction of alkaline additive is mandatory. Test 1 1 is similar with test 10, except that the solid-solid separation steps C1 and C3 are not performed. Test 1 1 shows the importance of solid-solid separations.