Technical field

The present invention relates, broadly, to the treatment of ore materials.

In particular, the present invention relates to a method for treating a titano-magnetite ore, i.e. an iron-containing titanium ore.

Background

Titanium dioxide is a white pigment which is used to a large extent in industry and is normally obtained by the processing of titanium ores such as, for example, ilmenite. Iron is the principal impurity of titanium ores and consequently the primary objective of the methods known in the art is to achieve the greatest degree of separation of titanium and iron at the lowest costs.

A method for processing vanadium-containing titano-magnetite ores with direct alloying of steel is disclosed in RU patent No. 2423530. The method comprises the reduction of vanadium-containing titanomagnetite ore materials with coal or coal-containing materials in a direct liquid-phase reduction device with the simultaneous obtaining of cast iron and hot reducing gases, the metallization of oxidized vanadium-containing pellets in a metallization device at feeding hot reducing gases from the direct liquid-phase reduction device, feeding of the liquid cast iron, metallizated pellets and scrap to the electric arc furnace, and melting with obtaining of vanadium alloyed steel. However, a disadvantage of the method disclosed in RU 2423530 is loss of titanium to slug.

Another method for processing vanadium-containing titano-magnetite ores is disclosed in WO2011/143689. The method includes processing a titanomagnetite concentrate with hydrochloric acid, followed by extraction separation of vanadium and iron as vanadium oxide and ferrous oxide in the commodity form. The disadvantage of the method is the presence of titanium both as unreacted residue and as product solution.

Still another method for processing titanium-containing raw materials is disclosed in RU patent No 2365647. The method comprises the fluorination of the raw material (titanomagnetite ore material) by means of sintering with ammonium fluoride, ammonium bifluoride or the mixture thereof at 110 - 240°C followed by heat treatment of the fluorinated bulk at 300 - 600°C with formation of sublimation products. The sublimation products are trapped by water obtaining an ammonium fluorotitanate solution which is processed with an ammonia aqueous solution causing the precipitation of hydrated titanium dioxide and the formation of ammonium fluoride solution. The precipitate is filtered from the solution of the fluoride solution and heat treated obtaining anhydrous titanium dioxide. The ammonium fluoride solution is instead directed to regeneration of the fluorinating agent and then, after separating titanium by sublimation, the residue is subjected to oxidative pyrohydrolysis at 300 - 650°C for 0.5 - 3 hours with formation of iron (III) oxide.

However, although the method disclosed in RU 2365647 may be satisfactory for application at an industrial level, it has the disadvantage that titanium dioxide may become contaminated by silicon compounds (mainly silicon dioxide) present in the starting raw ore materials. As a result, the titanium oxide thus obtained may have a purity non adequate for some industrial applications. In this case, it would be necessary to remove silicon compounds which however would be complex and involve high operation and equipment costs. This problem takes on considerable importance especially when the ore material has a relatively high content of silicon which is the case of raw ore materials extracted from certain natural sites of origin and of some titanomagnetite concentrates.

A further method for processing titanomagnetite raw material is disclosed in WO 2015/094008. The titanomagnetite feedstock undergoes to a fluorination reaction using a fluorinating agent. The mixture containing fluorinated compounds of Si, Ti and Fe is heated at a first temperature, at which silicon compounds sublimate. The solid residue of the first heat treatment is then heated at a higher temperature, to sublimate the titanium compounds. The two separate gas streams are then cooled down and the solid compounds obtained are dissolved in water and treated with an ammonia solution to precipitate silicon dioxide and titanium dioxide respectively. The hydrated oxides are then dried at high temperature to remove water and ammonium fluoride. The solid residue of the second heat treatment undergoes to a pyrohydrolysis reaction using water vapor at high temperature to recover iron oxides, which can then be reduced to metallic iron.

In this context, it should also be noted that silicon dioxide has per se a certain economic value so that its recovery from raw ore materials may be of significant interest.

The main object of the present invention is therefore to provide a method for processing titanomagnetite ore materials which allows to obtain separately silicon dioxide, titanium dioxide and preferably also iron oxides (which may be then converted to metallic iron) from a complex raw material containing them in an efficient and cost-effective way so as to be applicable at an industrial level.

Summary of the invention

This problem is solved by a method for processing titanomagnetite ore materials (e.g. a titanomagnetite concentrate), the method comprising the steps of:

- reacting a titanomagnetite raw material with a fluorinating agent to obtain a fluorinated product,

- heat treating said fluorinated product, to obtain a sublimate product containing ammonium fluorotitanate compound(s), fluorosilicate compound(s) and excess of fluorinating agent, and a first solid residue,

- cooling down said sublimate product to a first de-sublimation temperature to obtain a first de-sublimated product containing ammonium fluorotitanate compound(s) and a first gaseous residue,

- cooling down said first gaseous residue to a second de-sublimation temperature lower than said first de-sublimation temperature to obtain a second de-sublimated product containing ammonium fluorosilicate compound(s), and a second gaseous residue.

According to an aspect of the invention, the heat treatment of the fluorinated product provides for the production of a sublimated product containing ammonium fluorotitanate compound(s) and fluorosilicate compound(s); in other words, fluorinated compounds such as hexafluorotitanate, hexafluorosilicate, and the excess of the fluorinating agent used to react the titanomagnetite raw material, are transferred to the gaseous phase, i.e., sublimated. Additionally, the heat treatment of the fluorinated product provides for the decomposition of complexes of non-volatile fluorides, which results in the formation of the first solid residue.

According to embodiments, the first de-sublimation temperature is not lower than 320°C and preferably ranges from 320°C to 350°C.

By cooling down the sublimated product to this first de-sublimation temperature, the de- sublimation of ammonium fluorotitanate compound(s) is obtained.

According to embodiments, the second de-sublimation temperature is lower than 320°C, and preferably ranges from 210°C to 230°C. By cooling down the first gaseous residue obtained after the de-sublimation of the ammonium fluorotitanate compound(s) to this second de-sublimation temperature, the de- sublimation of ammonium fluorosilicate compound(s) is obtained.

Advantageously, after that the fluorinated compounds, such as hexafluorotitanate, hexafluorosilicate and the excess of the fluorinating agent (e.g. ammonium fluoride) are transferred to the gaseous phase, said gaseous phase (i.e., the sublimate product) is stepwise cooled down to provide an effective and specific separation of Ti and Si compounds. In other words, Ti and Si compounds are selectively separated by a succession of de-sublimation steps. In particular, ammonium fluorotitanate compound(s) are separated in a first step of de-sublimation (namely, a first step of cooling of the gaseous phase), whilst ammonium fluorosilicate compound(s) are separated in a second step of de-sublimation (namely, a second step of cooling of the residual gaseous phase after the de-sublimation of ammonium fluorotitanate compounds). In other words, the temperature of the sublimate product (i.e., the gaseous phase obtained after the heat treating step) is initially lowered to a first de- sublimation temperature, so that ammonium fluorotitanate compound(s) can be separated as a first de-sublimated product; subsequently, the first gaseous residue (i.e. the residual part of the above mentioned gaseous phase) is further lowered to a second de-sublimation temperature, so that ammonium fluorosilicate compound(s) can be separated as a second de- sublimated product. Of course, the second de-sublimation temperature is lower than the first de-sublimation temperature.

In the description, the term "de-sublimation" or "de-sublimated" has the meaning of an inverted sublimation, i.e. the process of transition of a substance directly from the gas state to a solid state without passing through an intermediate liquid phase. Thus, a "de- sublimated" product is a solid product.

Such process is also known as frosting.

In a preferred embodiment of the invention, the method further comprises a step of subjecting said first solid residue to pyrohydrolysis with water vapor, to obtain a second solid residue containing iron oxide and a gaseous stream containing hydrogen fluoride. In this case, advantageously, it is possible to obtain high purity iron oxides (which may be then converted to metallic iron) from a complex raw material containing several different inorganic materials. In embodiments, the method according to the invention may further comprising a step of cooling down said second gaseous residue to collect said excess of fluorinating agent. In other words, the second gaseous residue (i.e. the residual part of the above mentioned gaseous phase after the de-sublimation of ammonium fluorosilicate compounds) may be further lowered to a temperature suitable for the recovering of the part of the fluorinating agent that has sublimated during the heat treatment. Of course, this temperature is lower than the second de-sublimation temperature.

Preferably, the temperature suitable for the recovering of the fluorinating agent ranges from 110° to 150°C.

Advantageously, the recovered fluorinating agent may be recycled in the fluorination step. In embodiments, the method according to the invention may further comprise the steps of:

- dissolving said first de-sublimated product in water and treating the resulting solution with an aqueous ammonia solution to obtain the precipitation of hydrated titanium dioxide and an ammonium fluoride solution.

- separating the hydrated titanium dioxide precipitate from the ammonium fluoride solution, and

- drying said hydrated titanium dioxide precipitate.

In embodiments, the method according to the invention may further comprise the steps of:

- dissolving said second de-sublimated product in water and treating the resulting solution with an ammonia aqueous solution to obtain the precipitation of hydrated silicon dioxide and an ammonium fluoride solution.

- separating the hydrated silicon dioxide precipitate from the ammonium fluoride solution, and

- drying said hydrated silicon dioxide precipitate.

Advantageously, the ammonium fluoride solution separated from the hydrated titanium dioxide and/or the ammonium fluoride solution separated from the hydrated silicon dioxide may be subjected to appropriate treatments to obtain a fluorinating agent (e.g. ammonium fluoride) and an ammonia aqueous solution to be recycled in the fluorination step and in the treatment of the dissolved fluorinated product respectively.

In embodiments, the method according to the invention may further comprise the step of reacting the second solid residue containing iron oxides with a reducing agent to obtain metallic iron. According to embodiments, the heat treatment of the fluorinated product, i.e. the product obtained by fluorinating the titanomagnetite raw material, may be carried out by heating the fluorinated product to a first temperature not exceeding 320°C, and preferably ranging from 190°C to 210°C), followed by a separate second heating to a temperature ranging from 650°C to 700°C.

In this way, the complete separation of all volatile components present as fluorinated compounds in the fluorinated product may be obtained in a fast and effective way.

Advantageously, it has been observed that stepwise cooling of the gaseous phase (i.e., the stepwise de-sublimation of the sublimate product) allows a substantial and selective separation of Ti and Si compounds, initially present in the feedstock, in the form of ammonium fluorotitanates and ammonium fluorosilicates into separate solid products, namely de-sublimated products.

As a result, high purity titanium dioxide and silicon dioxide can be obtained from the gaseous phase (i.e., the sublimate product) containing ammonium fluorotitanates and ammonium fluorosilicates by following a succession of de-sublimation (i.e. cooling) steps according to the method of the invention as specified above.

In fact, it has been observed that, starting from a sublimated product containing different fluorinated compounds, ammonium fluorotitanate compound(s) and fluorosilicate compound(s) can be effectively separated with high selectivity by adjusting the temperature of the sublimated product, i.e. the gaseous phase, which can be lowered to selected "de- sublimation temperatures", i.e. temperatures that are selected to specifically separate, by de- sublimation, predetermined compounds.

Moreover, the above advantages are achieved without introducing significant complications to the existing plants for processing titanomagnetite raw materials so that the method of the invention is cost-effective and easily implementable at an industrial level.

Detailed description of the invention

The titanomagnetite raw material to be processed according to the method of the invention is normally an extract coming from appropriate ore extraction sites (e.g. ilmenite ores) and contains at least Ti, Si and Fe compounds. The extract may be subjected to appropriate pre- treatments before processing with the method of the invention. The starting material may also be a titanomagnetite concentrate obtained for example by dual wet magnetic separation methods. In the present description, the method of the invention will be illustrated making reference to titanomagnetite concentrate as starting material.

According to the invention, the titanomagnetite concentrate is mixed with a fluorinating agent and the mixture is heated at a temperature preferably comprised between 110°C and 240°C and reacted under stirring to obtain a fluorinated product. The fluorinating agent is preferably chosen among ammonium fluoride, ammonium bifluoride and mixtures thereof. The fluorinating agent is used in a stoichiometric amount or in stoichiometric excess up to 50% with reference to the fluorination reactions of reactive titanomagnetite concentrate components. Typical amounts of fluorinating agent may be comprised between 240% and 350%) by weight on the weight of the titanomagnetite concentrate.

By virtue of the fluorination step, the output product is a mixture of fluorinated compounds including ammonium fluorinated compounds of at least Fe, Si and Ti obtained according to the following reactions:

4FeTi03+26 H4HF2+02=4( H4)3TiF7+4( H4) ₃ FeF6+2 H3+14H20

CaO+ H ₄ HF ₂ =CaF ₂ + NH ₃ +H ₂ 0

Al20 ₃ +4 H4HF2=2 H ₄ A1F4+2 H ₃ +3H20

MgO+2 H4HF2= H4MgF ₃ + H ₃ +HF+H20

Mn02+3 H ₄ HF2=(NH ₄ )2MnF6+ H ₃ +2H20

Si02+4 H ₄ HF2=(NH ₄ ) ₃ SiF7+ H ₃ +HF +2H ₂ 0

V20 ₅ +4 H ₄ HF2=2 H4VOF4+2 H ₃ +3H20

Ti02+4 H ₄ HF2=(NH4) ₃ TiF7+ H ₃ +HF+2H20

The process gases also formed during the reaction and mainly containing ammonia and water are recovered and subjected to further treatments, as explained hereinafter, for appropriate recycle.

According to a preferred embodiment of the invention, the fluorinated product obtained from the fluorination step is subjected to heat treatment at a temperature of about 650 - 700 °C, resulting in the formation of a gaseous phase, i.e. a sublimate product, and a solid residue, i.e. a "first" solid residue.

The sublimate product (i.e., the gaseous phase) so obtained is cooled down according to a stepwise procedure, to provide a "separated" de-sublimation of ammonium fluorotitanate compounds and ammonium fluorosilicate compounds. Preferably, in a first de-sublimation step, ammonium fluorotitanate compound(s), e.g. ammonium hexafluorititanate, are collected as a first de-sublimated product by cooling down the sublimate product to a preferred first de-sublimation temperature of about 320 - 350 °C. This first de-sublimated products is, generally, mainly constituted by ( H4)2TiF6 and impurities of ammonium fluoride. Said first de-sublimated product is then dissolved in water and treated with an ammonia aqueous solution to precipitate hydrated titanium dioxide.

In a second de-sublimation step, the "first" gaseous residue obtained in the first de- sublimation step is cooled to a second de-sublimation temperature, preferably of about 210 - 230 °C, so that the ammonium fluorosilicate compound(s), e.g. ammonium hexafluorosilicate, are collected as a second de-sublimated product. This second de- sublimated product is in general mainly constituted by ( H4)2SiF6 and impurities of ammonium fluoride. Said second de-sublimated product is then dissolved in water and treated with an ammonia aqueous solution to precipitate hydrated silicon dioxide.

According to a preferred embodiment, the temperature of the remaining part of the gaseous phase (namely, the "second" gaseous residue obtained in the second de-sublimation step) is further decreased to about 1 10 - 150 °C to collect ammonium fluorides.

This fraction of ammonium fluorides may be recycled for the use as fluorinating agent in the fluorination step of the titanomagnetite feedstock.

During the heat treatment of the fluorinated product, multiple reactions can occur:

( H4)3TiF7=(NH4)2TiF6(gas)+ H ₄ F(gas),

( H4)3FeF6=FeF3(soiid)+3 H ₄ F(gas),

H4AlF4=AlF3(soiid)+ H ₄ F(gas),

H4MgF3=MgF2(solid)+ H4F(gas),

( H4)2MnF6=MnF4(soiid)+2NH ₄ F(gas),

( H ₄ ) ₃ SiF7=(NH4)2SiF6(gas)+ H ₄ F(gas),

According to embodiments of the invention, the sublimate product (gaseous) obtained from the heat treatment is initially de-sublimated at a first de-sublimation temperature to provide a first de-sublimated (solid) product, which is then dissolved in water. An ammonia aqueous solution is then added to the resulting solution, allowing precipitation of hydrated titanium dioxide in an ammonium fluoride solution, according to the following reaction:

( H ₄ ) ₂ TiF ₆ + 4 H ₃ + 4H ₂ 0 = Ti0 ₂ xnH ₂ 0| + 6 H4F

The ammonia aqueous solution has a concentration preferably comprised between 5 % and 25 % by weight on the weight of the ammonia solution. In particular, a preferred ammonia aqueous solution may have a concentration of 25%. Typical amounts of ammonia aqueous solution may be comprised between 145% and 245% by weight on the weight of ammonium fluorotitanate compounds, such as ( H ₄ ) ₂ TiF6, contained in the first de-sublimated product, in the case of use of a 25 % ammonia solution.

The hydrated titanium dioxide may be separated from the ammonium fluoride solution by any conventional means, e.g. by filtration. The solid hydrated titanium dioxide may be dried in a conventional manner, e.g. by calcination at temperatures comprised between 500°C and 900°C, preferably 600°C, for a time sufficient to remove water. The remaining ammonium fluoride solution is also recovered and preferably subjected to further treatments, as explained hereinafter, for appropriate recycle.

According to a preferred embodiment of the present invention, the first gaseous residue obtained after the cooling down of the sublimated product to a first de-sublimation temperature, and the formation of the first de-sublimated product (i.e., the remaining gaseous phase after the first cooling treatment), is further cooled to a second de-sublimation temperature of about 210 - 230 °C, thus obtaining a second de-sublimated (solid) product, generally mainly constituted by ammonium hexafluorosilicate.

According to embodiments, the second de-sublimated product is dissolved in water. An ammonia aqueous solution is then added to provide for the precipitation of hydrated silicon dioxide in an ammonium fluoride solution, according to the following reaction:

( H ₄ ) ₂ SiF ₆ + 4 H ₃ + 4H ₂ 0 = Si0 ₂ xnH ₂ 0| + 6 H ₄ F

The ammonia aqueous solution has a concentration preferably comprised between 5% and 25% by weight on the weight of the ammonia solution. In particular, a preferred ammonia aqueous solution may have a concentration of 25%. Typical amounts of ammonia aqueous solution may be comprised between 160 % and 260 % by weight on the weight of ammonium fluorosilicate compounds, such as ( H ₄ ) ₂ SiF6, contained in the second de-sublimated product, in the case of use of a 25 % ammonia solution. The hydrated silicon dioxide may be separated from the ammonium fluoride solution by any conventional means, e.g. by filtration. The solid hydrated silicon dioxide may be dried in a conventional manner, e.g. by calcination at temperatures comprised between 500°C and 700°C, preferably 600°C, for a time sufficient to remove water. The remaining ammonium fluoride solution is also recovered and preferably subjected to further treatments, as explained hereinafter, for appropriate recycle.

As above mentioned, the heat treatment of the fluorinated product provides for a sublimate product (i.e. a gaseous phase) and a first solid residue.

In a preferred embodiment, the first solid residue obtained from said heat treatment is subjected to pyrohydrolysis with water vapor. This process allows conversion of iron fluorides (and possibly other metallic fluorides present in the first solid residue) to iron oxides according to the following reactions:

CaF ₂ +H ₂ 0=CaO+2HF

2A1F ₃ +3H ₂ 0=A1 ₂ 0 ₃ +6HF

MgF ₂ +H ₂ 0=MgO+2HF

MnF ₄ +2H ₂ 0= Mn0 ₂ +4HF

2VOF ₃ +3H ₂ 0=V ₂ 0 ₅ +6HF

The pyrohydrolysis is carried out at a high temperature, preferably comprised between 400°C and 700°C, more preferably at 650°C, preferably up to the termination of emanation of gaseous stream consisting essentially of hydrogen fluoride. Such a gaseous stream is advantageously washed with an ammonia aqueous solution so as to dissolve hydrogen fluoride and obtain an ammonium fluoride solution. The ammonium fluoride solution so obtained may be preferably subjected to further treatments, as explained hereinafter, for appropriate recycle. The solid residue obtained after pyrohydrolysis, i.e. the second solid residue, mainly contains iron oxides and may be advantageously subjected to a reduction step with an appropriate reducing agent to obtain metallic iron. In other words, the second solid residue containing iron oxides may be reacted with a reducing agent to obtain metallic iron. Such a reduction step can be carried out by mixing said second solid residue containing iron oxides with said reducing agent, and heating the mixture of the second solid residue containing iron oxides and the reducing agent at a temperature comprised between 1600°C and 2000°C to produce a melt and maintaining such a temperature for a time comprise between 2 hours and 8 hours. According to embodiments, appropriate reducing agents may be coal or any coal-containing material. The reduction step may be performed by any conventional method known in the art, e.g. by electro smelting.

Further features and advantages of the method for processing titanomagnetite raw materials according to the present invention shall become clearer from the following description of a preferred embodiment thereof, given for indicating and not limiting purposes with reference to the attached drawing.

Brief description of the drawing

Figure 1 is a block diagram which schematically represents an embodiment of the method for processing titanomagnetite raw materials according to the present invention.

Detailed description of a preferred embodiment

With reference to the embodiment of the method schematically represented in figure 1, the processing of the titanomagnetite raw material starts with a mixing step 1, wherein a titanomagnetite raw material, such as titanomagnetite concentrate, and a fluorinating agent, e.g. ammonium fluoride, are mixed. The mixing of the titanomagnetite concentrate and the fluorinating agent may be carried out in a mixing unit, such as for example a mixing screw. The resulting mixture is subjected to a fluorination step 2, wherein the fluorination reactions occur, providing for a fluorinated product, as a mixture of fluorinated compounds, including ammonium fluorinated compounds of at least Fe, Si and Ti, and also obtaining a gaseous stream mainly containing ammonia, water, hydrogen fluoride and dusts.

For example, the mixture obtained in the mixing step 1 may be sent to a fluorination reactor, e.g. a rotary drum furnace, wherein the fluorination reactions take place, thus obtaining a fluorinated product.

The gaseous stream obtained in the fluorination step 2 is recovered and processed for appropriate recycle. In particular, for example, it may be sent to a dust collector (e.g. a baffle for dusts) to undergo a dust-collecting step 3. During said dust-collecting step 3 dusts are separated from the remaining gas components of said gaseous stream. Dusts are recovered, e.g. from the dust collector, whereas the gaseous stream deprived from dusts is subjected to a heat-exchanging step 4. For example, the gaseous stream deprived from dusts obtained in the dust-collecting step 3 may be sent to a heat exchanger, where it is cooled. The cooled gaseous stream obtained in the heat exchanging step 4 undergoes a first absorption step 5. In the first absorption step 5, the gaseous stream may be washed with liquid water, thus obtaining an aqueous solution containing ammonia and ammonium fluoride which is recycled as explained below. Air is vented from the absorption unit where the adsorption step 5 has taken place.

Turning now to the fluorination step 2, the fluorinated product obtained in the fluorination step 2 is subsequently heat-treated in a heat-treating step 6. Said heat-treating step 6 may be carried out in a sublimation unit, e.g. a furnace. In the heat-treating step 6, the fluorinated product is subjected to a heat treatment at a preferred temperature of about 650-700°C obtaining a sublimate product in gaseous phase containing ammonium fluorotitanate compunds (in particular (NFL^TiFe), ammonium fluorosilicate compounds (in particular ( H4)2SiF6), ammonia and hydrogen fluoride, and obtaining also a first solid residue containing iron fluorides and possibly other metallic fluorides (e.g. A1F ₃ , MgF ₂ ). The gaseous phase (the sublimate product) obtained in the heat-treating step 6 undergoes a first de-sublimation step 7. In the first de-sublimation step 7, the sublimate product is cooled down to a first de-sublimation temperature, not lower than 320°C, and preferably ranging from 320 to 350°C, obtaining a first de-sublimated solid product containing ammonium fluorotitanate compounds (in particular ( FL^TiFe) and impurities of ammonium fluoride. For example, when the sublimation step is carried out in a sublimation unit, the sublimation product may be sent to a first de-sublimation unit where it is cooled to a first de-sublimation temperature, obtaining a first de-sublimated solid product containing ammonium fluorotitanate compounds (in particular ( FL^TiFe) and impurities of ammonium fluoride, and a first gaseous residue. The solid first de-sublimated product obtained in the first de- sublimation step 7 is then subjected to a first dissolution step 8. In the first dissolution step 8, said solid first de-sublimated product is mixed with water, obtaining an aqueous solution containing ammonium fluorotitanate compounds (in particular ( FL^TiFe). For example, the first dissolution step 8 may be carried out in a first dissolution unit.

The solution obtained in the first dissolution step 8 is then processed for separating Ti as titanium dioxide and other valuable products to be recycled in the process. In particular, the solution obtained in the first dissolution step 8 is subjected to a first precipitation step 9. In the first precipitation step 9, the solution obtained in the first dissolution step 8 is mixed with a portion of the aqueous solution containing ammonia and ammonium fluoride obtained in the first absorption step 5, as above mentioned, which causes the precipitation of titanium dioxide, i.e., hydrated titanium dioxide. As a result, a dispersion of solid titanium dioxide in an ammonium fluoride solution is obtained, which is then subjected to a first separation step 10. In the first separation step 10, solid hydrated titanium dioxide is separated, e.g. by filtration, from the ammonium fluoride solution, and then subjected to a first drying step 11. In the first drying step 11, titanium dioxide is calcinated to obtain a dry titanium dioxide finished product. The above mentioned first dissolution step 8, first precipitation step 9, first separation step 10 and first drying step 11, may be respectively carried out into a first dissolution unit, first precipitation unit, first separation unit (e.g. a first filtration unit) and first calcination unit. The ammonium fluoride solution separated in the first separation step 10 is mixed with gaseous water obtained in the first drying step 11, and the mixture is subjected to an evaporation step 12, preferably carried out in an evaporation unit, obtaining gaseous water and a liquid dispersion. The gaseous water is then subjected to a condensation step 13, preferably carried out in a condenser, wherein it is condensed, and wherein the condensate (liquid water) is recovered. The liquid dispersion obtained during the evaporation step 12is instead subjected to a filtration step 14, obtaining solid ammonium fluoride and liquid water.

Turning now to the first de-sublimation step 7, the residual gaseous phase, i.e. the first gaseous residue obtained after that the sublimate product has been cooled to a first de- sublimation temperature (thus obtaining the first de-sublimated solid product), contains mainly ammonium fluorosilicate compounds (in particular ( FL^SiFe). This first gaseous residue is subjected to a second de- sublimation step 15 wherein it is further cooled down to a second de-sublimation temperature, lower than 320°C, and preferably comprised between 210°C and 230°C, thus obtaining a second de-sublimated solid product containing ammonium fluorosilicate compounds (in particular ( FL^SiFe) and impurities of ammonium fluoride, and a second gaseous residue. The second de-sublimated solid product obtained in the second de-sublimation step 15 is then subjected to a second dissolution step

16. In the second dissolution step 16, said second de-sublimated solid product is mixed with water, obtaining an aqueous solution containing ammonium fluorosilicate compounds (in particular ( FL^SiFe).

The solution obtained in the second dissolution step 16 is then processed for separating Si as silicon dioxide and other valuable products to be recycled in the process. In particular, the solution obtained in the second dissolution step 16 is subjected to a second precipitation step

17. In the second precipitation step 17, the solution obtained in the second dissolution step 16 is mixed with a portion of the aqueous solution containing ammonia and ammonium fluoride obtained in the first absorption step 5, as above mentioned, which causes the precipitation of silicon dioxide, i.e. hydrated silicon dioxide. As a result, a dispersion of solid hydrated silicon dioxide in an ammonium fluoride aqueous solution is obtained, which is subsequently subjected to a second separation step 18. In the second separation step 18, solid hydrated silicon dioxide is separated from the ammonium fluoride solution and then subjected to a second drying step 19. In the second drying step 19, silicon dioxide is calcinated to obtain a dry silicon dioxide finished product.

The above mentioned second dissolution step 16, second precipitation step 17, second separation step 18 and second drying step 19, may be respectively carried out into a second dissolution unit, second precipitation unit, second separation unit (e.g. a second filtration unit) and second calcination unit.

Turning now to the second de-sublimation step 15, the remaining part of the gaseous phase, i.e. the second gaseous residue obtained after that the first gaseous residue has been cooled to a second de-sublimation temperature (obtaining the second de-sublimated solid product), is subjected to a cooling step 20, wherein it is cooled down to a temperature of about 110 - 150 °C to collect ammonium fluorides, i.e. the excess of the fluorinating agent that sublimated during the heat treating step 6.

The ammonium fluorides so obtained may be recycled as fluorinating agent in the fluorination step 2.

Turning now to the heat treating step 6, the first solid residue containing iron fluorides and possibly other metallic fluorides (e.g. AIF3, MgF ₂ ) obtained in this heat treating step 6 is subjected to a pyrohydrolysis step 21, with water vapor. , For example, the pyrohydrolysis step 21 may be carried out in a pyrohydrolysis unit, e.g. a furnace. In this case, the water vapour is also introduced into the pyrohydrolysis unit. In the pyrohydrolysis step 21, the first solid residue is subjected to pyrohydrolysis as known in the art, obtaining a second solid residue, containing mainly iron oxides and possibly aluminum oxide (AI2O3) and magnesium fluoride (MgF ₂ ), and a gaseous stream containing water and hydrogen fluoride (HF). The second solid residue is recovered and may be subjected to a reduction step 16 for obtaining metallic iron. The gaseous stream obtained in the pyrohydrolysis step 21 is instead subjected to a second absorption step 23 wherein it is contacted by a portion of the ammonia and ammonium fluoride aqueous solution obtained in the first absorption step 5 above mentioned. As a result, an ammonium fluoride aqueous solution is obtained.

The ammonium fluoride aqueous solution obtained in the absorption step 23, the gaseous phases (essentially water) obtained in the second drying step 19 and the solution separated from the second separation step 18 may be mixed together and the resulting mixture may be recycled into the evaporation step 12 (e.g. in the evaporation unit) for processing as explained above. Also the solution separated in the first separation step 10 and the gaseous phase (essentially water) obtained in the first drying step 1 lmay be mixed together, and the resulting mixture may be recycled into the evaporation step 12 for processing as explained above.

In other words, the evaporation unit wherein the evaporation step is carried out may also receive the mixture of the solution separated in the first separation step 10 and of the gaseous phase (essentially water) obtained in the first drying step 11 for processing as explained above.

Examples

Example 1

50 g of a titanomagnetite concentrate obtained from the Zhambyl ore deposit of Tymlay (Kazakhstan) and containing 21,9% FeO, 46,0 % Fe ₂ 0 ₃ , 26,0% FeTi0 ₃ , 1,7% MgO, 1,6% A1 ₂ 0 ₃ , 2,8%) Si0 ₂ was mixed with 100 g of ammonium fluoride and the resulting mixture was heated with constant stirring at 200°C, maintaining this temperature up to the termination of emanation of gaseous reaction products. A fluorinated product containing a mixture of fluorides was obtained. Such product was heated to 680°C, maintaining this temperature up to the termination of emanation of gaseous products, so obtaining a sublimate (gaseous) product and a first solid residue.

The sublimate product was cooled to a first de-sublimation temperature of 350°C to produce a solid product (i.e. a first de-sublimated product) and a gaseous residue. The first de- sublimated solid product was dissolved in water. Then, 16,51 of a 25%> ammonia aqueous solution were added causing the formation of a titanium dioxide precipitate in the solution. Such solid hydrated titanium dioxide precipitate , was separated by filtration and calcinated at 600°C for 2 hours. 6,64g of titanium dioxide product were obtained after calcination which correspond to a theoretical yield of 97%>. The gaseous residue was further cooled to a second de-sublimation temperature of 220°C, to produce a solid product (second de-sublimated product) which was dissolved in water. Then, 4, 15g of a 25% ammonia aqueous solution were added causing the formation of a silicon dioxide precipitate in the solution. Such solid precipitate - hydrated silicon dioxide, was separated by filtration and calcinated at 600°C for 2 hours. 1,36 g of silicon dioxide product were obtained after calcination which correspond to a theoretical yield of 98%. The remaining mixture of fluorides, i.e. the first solid residue, was processed with water vapor at 650 °C up to the termination of emanation of hydrogen fluoride. The resulting solid residue was allowed to cool, then mixed with the 39,4.g of coal and the mixture was melted to reduce iron oxides to metallic iron. At the end of the reduction step, 12,53 g of metallic iron (Fe) were obtained which correspond to a theoretical yield of 87%.

Example 2

The process of example 1 was repeated with the difference that 80 g of ammonium hydrodifluoride was used as a fluorinating agent in place of ammonium fluoride. The yield of titanium dioxide is of 6,7g (97% of the theoretical value), the yield of silicon dioxide is of 1,34 g (96%) of the theoretical value) and the yield of iron is of 13, 11 g (92% of the theoretical value).

Example 3

The process of example 1 was repeated with the difference that a slag from the smelting of iron from titanomagnetite concentrates containing 1,8 % FeO, 66,7 % Ti0 ₂ , 5,8 % MgO, 11 % AI2O3, 14,7 % Si0 ₂ ; was used as raw material for processing and that 250 g of ammonium fluoride were used. The yield of titanium dioxide is of 64,5 g (96,7% of theoretical value), the yield of silicon dioxide is of 14,4g (98% of theoretical value) and the yield of iron is of 0,37 g (89% of theoretical value).