WO2019213723A1

WO2019213723A1 - Process for production of titanium dioxide pigment by means of hydrochloric acid digestion of titanium contaning raw materials in the presence of a fluorine based substance

Info

Publication number: WO2019213723A1
Application number: PCT/BR2018/050155
Authority: WO
Inventors: José Osael Gonçalves De FARIAS
Original assignee: Mineração Floresta S/A
Priority date: 2018-05-11
Filing date: 2018-05-11
Publication date: 2019-11-14
Also published as: BR112020022626A2; CN112166090A

Abstract

A process for production of pigment grade titanium dioxide is disclosed, including: (a) acid digestion under atmospheric pressure of a titanium raw material using a fluoride containing substance and in the presence of metallic iron; (b) solid/liquid separation; (c) precipitation of ferrous chloride; (d) centrifugation; (e) thermal hydrolysis of the resulting titanium using boric acid as a hydrolysis aid; (f) washing and acid treatment of titanyl hydroxide in the presence of small amounts of hydrochloric acid and powdered aluminium; (g) filtration, followed by a second step of washing; (h) calcination in air of washed titanyl hydroxide; (i) milling, surface treatment in the presence of small amounts of silica and alumina of calcined material; (j) micronizing of the surface treated material. The product obtained by applying such process presents very high brightness and opacity, as well as excellent coverage properties. As a result, it serves as an ideal pigment to be used in the production of paints, plastics, rubber and paper. The outlined process stands out as a promising alternative to existing processes of titanium dioxide manufacture.

Description

"PROCESS FOR PRODUCTION OF TITANIUM DIOXIDE PIGMENT BY MEANS OF HYDROCHLORIC ACID DIGESTION OF TITANIUM CONTANING RAW MATERIALS IN THE PRESENCE OF A FLUORINE BASED SUBSTANCE"

Field of invention

The present invention relates to a novel technology for production of titanium dioxide pigment. In particular, a new process for production of TiCp pigment based upon HC1 digestion of ilmenite or any other titanium containing material in the presence of a fluorine based substance is disclosed .

State of the art

Titanium is a transition metal belonging to the IVB group of Periodic Table of the Elements with atomic number 22. It has a silver color, low density, and high strength. Titanium is quite resistant to corrosion in sea water, aqua regia, and chlorine. Alloys of titanium are extensively employed in the aerospace industry and also used in medical, chemical and military applications, as well as in sporting goods. Due to these outstanding properties, the titanium metal industry is an economically important activity in the USA, Russia, Japan, China and Great Britain .

However, the main industrial use of titanium materials nowadays is the manufacture of titanium dioxide (TiCg) pigment, which accounts for about 95% of titanium raw materials presently produced in many parts of the world. Indeed, the titanium pigment industry is a very important inorganic chemistry business, surpassed only by those of ammonia and phosphoric acid in terms of gross revenue.

Titanium dioxide is a very bright compound that possesses an extremely high refractive index, even greater than that of diamond. As a result very fine powdered TiCq is extensively used because of its efficiency in scattering visible light, and enhancing whiteness, brightness, and high opacity of products such as paints, coatings, plastics, papers, inks, foods, medicines (i.e. pills and tablets), as well as most toothpastes. Moreover, the ability of TiCg to absorb UV light energy can significantly improve weather resistance and durability of polymer products. In recent years, titanium dioxide pigment has also been employed in cosmetics, such as in skin care products and sunscreen lotions, as it protects the skin from ultraviolet radiation due to its property of absorbing ultraviolet light.

Presently, two processes are employed for manufacturing TiCg pigment in industrial scale: chloride and sulphate. The chloride process is based on the carbochlorination of high grade ores and concentrates at a temperature of about 1000²C using petroleum or metallurgical coke as reducing agent. As a result, a mixture of volatile chlorides are formed, from which titanium tetrachoride (TiCl₄₎ is separated by selective distillation. T1CI4 is later reacted at high temperature with oxygen causing titanium dioxide to be formed and chlorine to be regenerated.

The sulphate process consists in the reaction of ilmenite and/or titania slag with concentrated sulphuric acid at 180-220²C, followed by thermal hydrolysis of the resulting sulphate solution and calcination of Ti0(0H) 2 to yield solid TiCg.

In the final stages of both processes small amounts of modifying agents such as silica and alumina are added in order to improve the surface properties of the final pigment .

Description of prior art

Even though the sulphate and chloride are the sole processes used in industrial scale nowadays, several promising alternatives are known, of which hydrochloric acid digestion of the titanium raw material is an outstanding one.

Indeed, many patents and academic research work on the chemical breakdown of ilmenite or titania slag with solutions of HC1 of varying concentration in order to produce pigment grade titanium dioxide are described in the literature. Such process routes are often referred as hydrochloric acid digestion.

However, several drawbacks have prevented such process routes from reaching industrial scale utilization. The main process inconvenients are the need of use of a high concentration of HC1 or the expensive unit operations that are used downstream of digestion, such as solvent extraction, that are necessary for obtaining high purity TiCq, which is a mandatory step in order to obtain an adequate pigment by hydrometallurgical based processes.

Along these lines, in US patent 3,597,190 a process for the HC1 digestion of ilmenites in co-current reactors is disclosed. In this patent it is imperative to use a hydrochloric acid concentration of at least 35% (w/w) in order to prevent premature hydrolysis of titanium chlorides and oxychlorides formed during digestion.

Another patent, US 6,375,923, teaches a process for recovery of pigment grade TiCg which includes the following operations: leaching of ore - mainly ilmenite - with concentrated HC1, reduction of all solubilized iron to Fe(II), cooling of the leaching liquor to bring about crystallization and separation of FeCl2 thus formed, two successive stages of solvent extraction using an organophosphorus compound as extractant in the first step and an amine as extractant in the second stage, followed by the usual steps of hydrolysis, calcination and micronizing of titanium dioxide. The great number of unit operations, as well as the high cost of the organic compounds in the solvent extraction steps are considered to be the main disadvantages of such process.

More recently, in the paper "A new method for production of titanium dioxide pigment, Hydrometallurgy 131-132, pp 107- 113 (2013)", by Middlemas and co-workers, a process for production of TiCq pigment from titaniferous slag is outlined. Such process comprises the following steps: calcination of slag in air at 500²C in the presence of sodium hydroxide (NaOH) , washing of the calcined product with water, leaching of the washed product with hydrochloric acid, liquid-liquid extraction of the leaching liquor using an amine as extractor, followed by hydrolysis, calcination and micronizing of the final titanium dioxide product. In addition to the elevated price of both NaOH and the extractant employed in the liquid-liquid extraction step, the energy intensive operation of calcination at 500²C remain as intrinsic handicaps of this process.

As shown in the continuation of this text, it will be demonstrated that the technology disclosed in the present patent report make possible to overcome the disadvantages of the above mentioned processes.

Summary of the invention

In previous work by the authors of this same invention, said work being outlined in Brazilian patent request PI- 102015006835-2, presently under evaluation in Brazil, a process for production of pigment grade TiCq based upon HC1 digestion of ilmenite under atmospheric pressure is disclosed. In this process digestion is carried out in the presence of small amounts of the fluoride ion (F^_) , such ion being supplied by aqueous solutions of fluorosilicic acid (H2S1F6) . The presence of fluoride during digestion is a key point for the success of this process and provides an important aspect for overcoming the major drawbacks of prior art processes of HC1 digestion of titanium ores and concentrates .

Fluorosilicic, or hexafluorosilicic acid is an attractive source of the fluoride ion, since it is generated in large quantities in the manufacture of phosphoric acid. To date, no major industrial applications have been identified for this substance and, as a result, huge amounts of aqueous solutions of H2S1F6 are discarded every year in the fertilizer industry. Therefore, it can be sourced at almost no cost. However, the use of such substance within the framework of the present invention requires some considerations. The main issue of concern is the fact that this substance is only available in aqueous solutions and, as a result, it is inevitable that it leads to a certain dilution of the chloridic digestion liquor, which is undesirable in downstream operations.

In order to overcome such negative dilution aspect, further work was undertaken and it was discovered that by introducing an operation of evaporation/crystallization of ferrous chloride (FeCl₂.4H₂0) in order to lower the amount of iron dissolved in the digestion liquor, a major improvement with respect to what is described in Brazilian patent request PI-102015006835-2 is achieved.

In another embodiment of the present invention it was ascertained that solid bi-fluorides can also be employed as a source of fluoride during digestion with similar results to that of fluorosilicic acid. Preferred such compounds are the bi-fluorides of lithium (LiHF2) , sodium (NaHF2) and potassium (KHF₂₎ , as well as ammonium bi-fluoride (NH₄HF₂₎ . Solid bi-fluorides are very soluble in chloridic solutions and present the inherent advantage of avoiding dilution of the digestion liquor. Thus, if a solid bi-fluoride is used during digestion it is not necessary to introduce the aforementioned operation of evaporation/crystallization of FeCl₂.4H₂0 generated during digestion, which is a non- negligible benefit. However, under nowadays market situation, bi-fluorides are much more expensive than fluorosilicic acid and, therefore, its use can only be determined on an economic basis.

For both process options, that is, with either H₂S1F₆ or bi-fluorides as source of F-, the amount of fluoride - expressed as HF - to be added in the digestion step is calculated as a function of the weight of titanium ore to be digested.

Another key issue of the present invention is that, in both process alternatives, namely using H₂S1F₆ or bi-fluorides in the digestion step, small amounts of boric acid (H₃BO₃) are added to the chloridic liquor during the operation of hydrolysis in which titanyl hydroxide - Ti0(0H)₂ - is precipitated .

Brief description of drawings

The present invention will be described below with refer ence to the attached drawings, in which:

FIG. 1 illustrates a flowsheet of the process for production of pigment grade TiCq using fluorosilicic acid as a source of fluoride during hydrochloric acid digestion of the titanium ore;

FIG. 2 represents a flowsheet of the process for production of pigment grade Ti0₂ with the use ammonium bi-fluoride as a source of fluoride during hydrochloric acid digestion of the titanium ore.

Detailed description of the invention

The present invention, described in detail in the following paragraphs, is characterized by unique combinations of unit operations known in the state of the art of mineral processing and chemical processes, the result of which is the production of pigment grade titanium dioxide.

The first process step is the digestion of the titanium containing ore in hydrochloric acid. The preferred ore is ilmenite (FeTiCq) , although this process can successfully be applied to any titanium ore, such as rutile, anatase or leucoxene, and also to titania slags. In order to ensure a high rate of dissolution of the Ti values of the raw material, the ore particle size must be secured at 100% minus 60 pm, which can easily be obtained by conventional techniques, such as ball milling.

Digestion must be performed at a minimum temperatuire of 80²C and the HC1 concentration must lie within the range of 15% to 25% (w/w) . Digestion is normally carried out in multiple stages, either in a co-current or in a counter- current basis, preferably counter-currently . The total ore residence time in the multiple reactors must be at least 4 hours. A typical arrangement is obtained using 4 reactors with a 1 hour per stage residence time, with intense agitation of the slurry in each stage.

Titanium ores normally contain some ferric iron - Fe(III) - as an impurity, which must be reduced to Fe(II) during digestion. Although any reducing agent can be used to fulfill such requirement, within the scope of the present invention powdered metallic iron in powdered form was chosen as the preferred reductant. The amount of iron to be added is a function of the Fe20₃ content of the ore; in practical terms it was found to range from 1% to 5% of the mass of ore.

As stated previously, a key issue of the present invention is the use of a fluoride containing substance as an additive during HC1 digestion of the ore. This is done either with the addition of an aqueous solution of H2S1F6 (concentration of 20 to 25 wt%), as indicated in FIG. 1 or by a solid bi-fluoride, as shown in FIG. 2. For both alternatives it was established that the optimum quantity of fluoride, calculated as HF, must lie in the range of 0.5 to 2.0 wt% of ore, preferably around 1%.

The slurry from digestion is transferred to a solid/liquid separation step, from which a Ti rich liquor containing 80- 120 g/L T1O2 is recovered. The solid residue generated in the digestion step is normally discarded, but it can also be granulated - not shown in FIG.l or FIG. 2 - and used as an additive for soil remediation due to its high S1O2 content .

The Ti rich liquor from digestion is either subjected to a step of evaporation/ crystallization (FIG. 1) or simply cooled down to a temperature of about 15²C, which turns out to be the crystallization step shown in FIG. 2. Both operations bring about the precipitation of solid ferrous chloride tetrahydrate (FeCl2.4H2O) , which is separated from the liquor by means of centrifugation.

The titanium rich liquor thus recovered is the feed to the following step of thermal hydrolysis. In this operation the liquid is heated to a minimum temperature of 75²C for a period of at least 3 hours, which results in the precipitation of titanyl hydroxide - TiO(OH)2. In the technology described in the aforementioned patent request PI-102015006835-2 it was necessary to add a small amount of magnesium oxide (MgO) as an auxiliary agent to hydrolysis. Within the scope of the present invention, it has been unexpectedly discovered that by introducing the evaporation/crystallization operation, the use of MgO is no longer needed.

Another fundamental and innovative aspect of the present invention is the addition of boric acid, or orthoboric acid (H₃BO₃) , which is done in the operation of hydrolysis. The main function of boron in this step is that it combines with the fluoride contained in the liquor resulting in the formation of tetrafluoroboric acid (HBF₄) which has a high vapour pressure under hydrolysis conditions. As a result, essentially all fluoride is removed from the system as HBF₄ which distills off the system. The efficiency of hydrolysis, measured as the amount of titanium precipitated, is greatly increased by the use of boric acid in this step. In addition, by using boric acid as an additive in this operation, the duration of hydrolysis is substantially reduced. The amount of H₃BO₃ is a function of the mass of titanium - calculated as TiCg - contained in the liquor from centrifugation. A typical value for the quantity of boric acid is in the range of 1% to 10% of the TiCg in the liquor, preferably from 2% to 5%.

The remaining operations are almost identical to the final steps of the sulphate process of pigment manufacture, all of which are state of the art techniques. In sequence, those operations are: primary filtration and washing, acid treatment at 80²C in the presence of small amounts of HC1 and powdered aluminium, secondary filtration and washing, calcination during which limited quantities of potassium chloride (KC1) and zinc oxide (ZnO) are incorporated, milling, surface treatment with addition of small amounts of SiCg and AI₂O₃ and, finally, micronizing. Liquid phases recovered in those operations are delivered to acid regeneration in order to recover the HC1 values needed in the initial process stage of digestion. Techniques inherent to HC1 regeneration are all based on the pyrohydrolysis of the chloride containing liquor and are state of the art processes. Thus, a detailed description of such processes is outside the scope of this invention.

The temperature of calcination depends upon the type of titanium pigment desired. Typical values of such parameter are 800²C for anatase type pigment, whereas 900²C is employed for rutile type pigment. Such temperatures are well established within the state of the art of titanium pigment manufacture.

The following examples are given with the objective of illustrating the process of the present invention. However, it is to be understood that said examples are given merely for purposes of illustration and that the nature and scope of such process are not necessarily limited thereto.

Example 1

The main operations of this example are depicted in FIG. 1. A sample of ilmenite concentrate weighing 25.0 kg and with the chemical composition indicated in the table below was digested in 97.0 L of hydrochloric acid (25 wt% HC1) for a period of 4 hours, during which the system temperature varied between 90² and 100²C.

Ilmenite chemical composition (wt% )

In this step 3.0 L of a solution of 25% (w/w) H2S1F6 and 0.85 kg of powdered iron were used as additives. Digestion lasted for 3 hours and was undertaken in a special alloyed steel, HC1 resistant, cylindrical shaped reactor fitted with stirring and heating devices.

Upon completion of this operation and after a step of solid/liquid separation by filtration, 103.5 L of a liquor containing 100.8 g/L of TiCq and 80.6 g/L of Fe (total), as well as 3.1 kg of a solid residue with 10.4% TiCq and 19.9% Fe (total) were recovered. This represents a digestion yield of 97.0% for titanium and 93.1% for iron. After filtration, the solid residue was discarded and the liquor was fed to a batch, steam heated, vacuum evaporator/crystallizer. After 2 hours of operation, a slurry made of 5.5 kg of precipitated ferrous chloride (FeCl₂.4H₂0) and 107.3 L of a liquor containing 96.8 g/L TiCg and 39.7 g/L Fe (total) were recovered. Separation of these two phases was subsequently accomplished by centrifugation .

In the next step, 23.7 kg of 3.0 bar pressure, saturated steam, were injected in the liquor, which heated the liquid phase to 80²C, temperature in which the system was kept for about 3 hours in order to promote the hydrolysis of essentially all dissolved titanium as TiO(OH)2. In the beginning of this operation 0.24 kg of solid H₃BO₃ as well as a slurry made up of 0.07 kg of TiCg pigment and 2.3 L of a weak HC1 containing solution, the later being the so- called primary seeding, were incorporated to the system. Upon hydrolysis completion and after a first stage of filtration and washing, a slurry made of 10.1 kg of the hydrolyzed product and 9.2 L of a solution containing 3.8 g/L of TiCg and 38.9 g/L of Fe (total) were recovered, which translates into a hydrolysis efficiency of 96% based upon the mass of TiO(OH) 2 precipitated. Also recovered in this operation were 96.3 L of a liquor almost depleted of titanium and containing the remaining values of iron and other impurities, all as chlorides, pertaining to the ore that were dissolved during the HC1 digestion step. Although such operation was not performed within the scope of this example, treatment of this liquor by pyrohydrolysis , that is, hydrochloric acid regeneration, shall be carried out on industrial scale in order to recover its HC1 values.

In the next stage, the incoming slurry was subjected to washing with 177 L of water, followed by acid treatment with 6.4 L of 25% HC1, 0.02 kg of aluminium powder and 10.3 L of water, said treatment being done at 80²C for a period of 1 hour. After a second stage of filtration and washing, 20.0 L of a chloride solution containing 1.0 g/L of Fe (total) and 20.1 kg of a 50% slurry were recovered, in which the liquid phase contains 1.4 g/L Fe (total) as chloride and very little free HC1. Again, such chloride containing solution must be forwarded to HC1 regeneration. In the present example, such operation was not carried out.

The 50% slurry fed the remaining stages of the process. Initially, 0.45 kg of Ti0₂ pigment and 5.6 L of a weak HC1 containing solution were added to such slurry, followed by mixing with 0.11 kg of ZnO and 0.02 kg of KC1. The resulting slurry fed the combined operation of drying/calcination, which was performed at a maximum temperature of 900²C for a period of 1 hour in an electrically heated, rotating horizontal furnace.

The unloaded solid from calcination was ground in a Raymond mill and, then, was subjected to surface treatment, in which the calcined product was mixed with 0.10 kg of aluminium hydrate, A1 (OH) ₃, and 0.11 kg of silica (Si0₂₎ . Finally, after micronization in a jet mill of the surface treated material so as to obtain a material with a particle size distribution suitable for application as a pigment, 10.4 kg of a product with the chemical composition shown in the following table were obtained.

Ti0₂ pigment chemical composition

(wt% )

96.6 1.0 1.1 1.1 0.1

This material, which has a particle size distribution between 0.1 and 1.0 pm, represents the final product of the process, that is, the TiCq based pigment.

Example 2

The main steps of this example are illustrated in FIG. 2. A 20.0 kg sample of an ilmenite concentrate with the chemical composition shown in the following table was digested in 77.6 L of hydrochloric acid (25 wt% HC1) for a period of 4 hours, during which the temperature varied between 80^s and 100²C.

Ilmenite chemical composition (wt% )

In this operation, 0.29 kg of potassium bi-fluoride (KHF₂) and 0.68 kg of powdered scrap iron were added to the system. Such operation was carried out in a cylindrical shaped, alloyed steel, HC1 resistant reactor possessing stirring and heating devices.

At the end of this step and after solid/liquid separation by filtration, 82.8 L of a liquor containing 101.8 g/L of TiCq and 81.5 g/L of Fe(total), as well as 2.5 kg of a solid residue with 10.6% TiCq and 19.5% Fe(total) was recovered. This represents a digestion yield of 97.0% for titanium and 92.6% for iron.

After filtration, the solid residue was discarded and the liquor was cooled down to about 15²C, resulting in a precipitation of 4.4 kg of FeCl₂.4H₂0, such compound being subsequently separated from the liquor by means of centrifugation, as well as 80.5 L of a liquor having the following concentrations of TiCq and Fe (total), respectively 103.2 g/L and 42.5 g/L.

Following, 6.3 kg of 3.0 bar pressure saturated steam were injected in this liquor, which heated the liquid phase to 80²C, temperature in which the system was kept for about 3 hours in order to promote the hydrolysis of essentially all dissolved titanium as Ti0(0H)2. Also added to the liquor in this step were a slurry made up of 0.06 kg of TiCq pigment and 1.9 L of a weak HC1 containing solution, that is, the primary seeding, and also 0.16 of solid boric acid (H3BO3) , in order to improve hydrolysis efficiency. At the end of hydrolysis and after a primary stage of filtration and washing, a slurry made of 8.1 kg of the hydrolyzed product and 7.0 L of a solution containing 4.0 g/L of TiCq and 41.0 g/L de Fe (total) were recovered, which represents a hydrolysis yield of 96.5% calculated based on the amount of titanyl hydroxide formed during hydrolysis. Also recovered in this operation were 77.0 L of a liquor almost depleted of titanium and containing the remaining values of iron and other impurities, both as chlorides, initially contained in the ore which were attacked during the HC1 digestion step. Even though such step was not a part of the operations undertaken within the scope of this example, treatment of this liquor by pyrohydrolysis , that is, hydrochloric acid regeneration, shall be carried out on industrial scale in order to recover its HC1 values.

In the next stage, the incoming slurry was subjected to washing with 125 L of water, followed by acid treatment with 7.4 L of 25% HC1 and 0.02 kg of aluminium powder, said treatment being carried out at 80²C for a period of 1 hour. After the so-called secondary filtration and washing, 15.9 L of a chloride solution containing 1.0 g/L of Fe (total) and 16.1 kg of a 50% slurry were recovered, in which the liquid phase contains 3.1 g/1 Fe (total) as chloride and a very small amount free HC1. As previously, such chloride rich liquor must be forwarded to HC1 regeneration. In the present example, such operation was not carried out.

The 50% slurry became the feed to the remaining stages of the process. Firstly, the slurry was mixed with 0.40 kg of Ti0₂ pigment and 4.6 L of a weak HC1 containing solution, which was the secondary seeding step. Following, 0.09 kg of zinc oxide (ZnO) and 0.02 kg of potassium chloride (KC1) were combined with the seeded material and the resulting slurry became the feed to the combined operation of drying and calcination. Such operation was carried out for a period of 1 hour in an electrically heated, rotating horizontal furnace. Maximum temperature of this operation was 900²C.

The material discharged from calcination was ground in a Raymond mill and, then, was subjected to the so-called surface treatment, in which the calcined product was mixed with 0.08 kg of aluminium hydrate, A1 (OH) ₃, and 0.09 kg of silica (Si0₂) . After micronizing the surface treated material in a jet mill so as to obtain a material with a particle size distribution suitable for pigmentary applications, 8.3 kg of a product with the chemical composition indicated in the following table were obtained.

Ti0₂ pigment chemical composition

(wt% )

96.2 1.1 1.0 0.9 0.2

Such material, having a particle size distribution in the range of 0.1 to 1.0 pm, is the titanium dioxide pigment and represents the final product of the outlined process.

The products obtained in the afore described examples, present very high brightness and opacity, as well as excellent coverage properties. As a result, they serve as an ideal pigment to be used in the production of paints, plastics, rubber and paper.

Claims

What is claimed is:

1 - A process for production of titanium dioxide pigment by means of hydrochloric acid digestion of titanium containing raw materials in the presence of a fluorine based substance, such process comprising the following sequence of operations:

(a) digestion of the titanium containing raw material in hydrochloric acid in the presence of a fluoride containing substance;

(b) separation of the resulting titanium rich liquor and the solid residue formed during the HC1 attack of the ore by filtration;

(c) evaporation/crystallization or simply cooling of the digestion liquor in order to promote ferrous chloride precipitation;

(d) separation of ferrous chloride and digestion liquor by centrifugation,

(e) thermal hydrolysis of the resulting titanium rich solution bringing about precipitation of titanyl hydroxide;

(f) recovery of hydrolyzed solid product by a first set of filtration and washing step;

(g) acid treatment of the resulting slurry in the presence of a small amount of a hydrochloric acid and aluminium powder;

(h) followed by a second filtration and washing step;

(i) calcination of the resulting slurry in the presence of limited amounts of zinc oxide (ZnO) and potassium chloride (KC1) ;

(j) and, finally, grinding and surface treatment of the calcined product in the presence of a small amount of silica (SiCq) and alumina (AI2O3)

2 - The process outlined in claim 1 wherein the raw material contains predominantly one or any mixture of the following materials: ilmenite, titania slag, rutile, anatase, leucoxene or titaniferous magnetite.

3 - The process outlined in claim 1 wherein the hydrochloric acid digestion step is carried out in the temperature range of 80²C to 105²C, preferably between 90²C and 100²C.

4 - The process outlined in claim 1 wherein the concentration of hydrochloric acid (HC1) employed in the digestion step lies in the range of 15 to 35% (w/w), preferably between 20 and 25% (w/w) HC1.

5 - The process outlined in claim 1 wherein the hydrochloric acid digestion step is undertaken at atmospheric pressure.

6 - The process outlined in claim 1 wherein the fluoride containing substance employed in the operation of hydrochloric acid digestion is an aqueous solution of fluorosilicic acid (fkSiFg) .

7 - The process outlined in claims 1 and 6 wherein the concentration of fluorosilicic acid (fkSiFg) remains within the extent of 15 to 30 weight percent, preferably between 20% to 25% (w/w) H2S1F6.

8 - The process outlined in claims 1 , 6 and 7 wherein the mass of fluorosilicic acid (fkSiFg) which is added in the digestion operation is calculated in terms of its equivalent in hydrofluoric acid (HF) and is fixed in the range of 0.5% to 10.0% of the mass of titanium raw material, preferably between 1 and 5% of sad mass.

9 - The process outlined in claim 1 wherein the fluoride contaning substance employed in the operation of hydrochloric acid digestion is a metallic bi-fluoride in the solid state.

10 - The process outlined in claim 8 wherein the fluoride contaning substance is one of the following compounds: bi-fluoride of lithium (LiHF2) , bi-fluoride of sodium (NaHF2) , bi-fluoride of potassium (KHF2) , or ammonium bi-fluoride (NH4HF2) . 11 - The process outlined in claims 1, 9 and 10 wherein the solid bi-fluorides used in the step of hydrochloric acid digestion can be employed individually or in any of their mixtures, regardless of their proportions.

12 - The process outlined in claims 1, 9, 10 and 11 wherein the mass of bi-fluoride that is added in the digestion step is calculated in terms of its equivalent in hydrofluoric acid (HF) and is fixed in the range of 0.5% to 10.0% of the mass of titanium raw material, preferably between 1 and 5% of sad mass.

13 - The process outlined in claims 1, 6, 7, 8, 9, 10, 11 and 12 wherein boric acid (H₃BO₃) is used as an additive in the operation of thermal hydrolysis of the titanium rich liquor resulting from hydrochloric acid digestion .

14 - The process outlined in claim 13 wherein boric acid (H₃BO₃) is utilized in the solid state.

15 - The process outlined in claims 1, 13 and 14 wherein the mass of boric acid (H₃BO₃) used in the hydrolysis step lies within the range of 0.5 to 20.0% of mass of titanium contained in said liquor, expressed as titanium dioxide (TiCq) , preferably between 1.0% and 10.0%.