WO2025141479A1

WO2025141479A1 - A method for treating liquid wastes from the titanium dioxide industry

Info

Publication number: WO2025141479A1
Application number: PCT/IB2024/063160
Authority: WO
Inventors: Maria De Los Angeles Garcia; Maria Fernanda Garcia
Original assignee: Ingar Di Fernando Horacio Garcia & C Sas; Omega Sas Di Mario Alberto Dell'omodarme & C; SolTrEco Bonifiche Srl
Current assignee: Ingar Di Fernando Horacio Garcia & C Sas; Omega Sas Di Mario Alberto Dell'omodarme & C; SolTrEco Bonifiche Srl
Priority date: 2023-12-29
Filing date: 2024-12-24
Publication date: 2025-07-03
Anticipated expiration: 2026-06-29

Abstract

In a modified sulphate process for making titanium dioxide, based on an acid attack of a titanium-containing mineral such as Ilmenite and/or titaniferous slags, a method for treating an acidic liquid waste (6,6a) of a titanium dioxide production process via sulphate includes two concentration stages (S10,S20) comprising respective steps of bubbling (S11,S21) a first and a second gas (11,22) into the acidic liquid waste (6,6a,16,16a), carried out in a first reactor (41,61) at respective first and second process temperatures of at least 90°C and 150°C, respectively, the second gas containing, in particular, carbon dioxide, so as to retain obtain solid metal sulphates (23) in the first reactor (61) and a gaseous stream (24) in which sulphur trioxide SO3 and vapours of H2SO4 are present, together with water and carbon dioxide, a selective adsorption phase (S22) of SO₃/H₂SO₄ into a water solution (26) progressively concentrating in a second reactor (91) connected to first reactor (61) via a feed duct (92) arranged to convey gas form the first to the second rector (61,91), up to a predetermined concentration, forming a concentrated sulphuric acid (28), in which a residual gaseous stream (27) after absorption can be further fractionated to recycle the carbon dioxide into the carbon dioxide-containing gas stream (22) fed to the effluent mass, while the solid metal sulphates are recovered once the acidic liquid waste mass is exhausted. A filtration step (S12) can further be provided of filtering the partially concentrated liquid acidic waste coming from first concentration stage so as to cause the solid metal sulphates (13) settled during the first bubbling step, mainly including Ferrous sulphate to be separated.

Description

TITLE

A METHOD FOR TREATING LIQUID WASTES FROM THE TITANIUM DIOXIDE INDUSTRY

DESCRIPTION

Scope of the invention

[0001] The present invention relates to a method for treating acidic liquid wastes from a titanium dioxide production “sulphate” process. More in general, the invention relates to a method for treating acidic liquid wastes containing metal sulphates.

Prior art - Technical problems

[0002] Titanium dioxide (TiO2) differs from most white pigments for its ability to reflect all the colours of the visible spectrum and to maintain its whiteness and hiding power in almost all media to which it is associated. For these reasons, TiO2 finds application in every technical area where pigments are used, e.g. in the production of paints, plastics, paper, inks and so forth, and is the most widely used white pigment in the world, with a total output almost exceeding the output of all other pigments combined together.

[0003] Most of the TiO2 is produced by the so-called sulphate process. The raw materials mainly used in this process are:

Ilmenite, a titanium-iron oxide mineral, currently indicated as FeTiOa and also containing other metals than Titanium, mainly Fe; titaniferous slag, i.e. TiO2-enriched Ilmenite, or mixtures thereof.

The sulphate process schematically comprises the following steps: mineral grinding; attacking the ground titanium-containing mineral with concentrated sulphuric acid; various treatments, including hydrolysis of the material from the previous steps by steam injection, thus forming: a TiO2 H2O-phase that is insoluble in acidic solutions, and a phase consisting of soluble sulphates of all the metals that are contained in the starting mineral; filtration, resulting in a liquid known as “hydrolysis filtrate” or “strong effluent”, which has an acidity, expressed as free H2SO4, normally set between 250 and 350 g/l, and which also contains Fe, 12-20 g/l; Mg, 4-10 g/l; Ti, 2-10 g/l; Al, 1 -?5 g/l, as well as smaller amounts of other metals, and a solid mainly consisting of TiC hydrate; purification operations, including e.g. washing the solid obtained from the filtration with water: in these operations solutions are obtained collectively known as “weak effluent”, including the same components as the strong effluent but at a lower concentration than in the strong effluent: in particular, the acidity, expressed as free H2SO4, is normally set between 70 and 80 g/l; drying/calcination of the washed solid, by which anhydrous TiC is obtained; TiO2 finishing operations; neutralisation of the strong effluent and the weak effluent by addition of lime (CaO) and/or calcium carbonate (CaCOa): normally, the strong and the weak effluents are mixed together before the treatment. In this step, large amounts of calcium sulphate dihydrate CaSO42H2O are obtained, also including the hydroxides of the metals that are present in the source mineral, mainly iron. For this reason, this material is often called “red chalk”, and is usually sent to landfill.

[0004] Fig. 1 is a simplified block diagram of a titanium dioxide production sulphate process, using Ilmenite and/or titaniferous slag as a raw material.

[0005] As well known, the landfilling of red chalk, at the rate of 7 - 9 tonnes per tonne of TiO2, is a serious environmental issue. Modifications of the sulphate process have been proposed to overcome this problem, as summarised below.

[0006] In a first modification, an attempt is made to use at least one part of the strong effluent to obtain useful products. In particular, ferrous sulphate, FeSC can be obtained through Fe-enrichment of the waste and subsequent crystallisation. However, FeSCU is a “poor” product, for which a narrow market space is available. Therefore, only a small reduction of the red chalk to be landfilled is possible this way. Other salts can also be produced, such as MgSO4, (NH4)2SO4, Al2(SC )3, and so on, but also in this case they would be hard to be sold, as various well-established and more convenient production processes are available to manufacture them.

[0007] A second modification consists in recovering the sulphuric acid by evaporative concentration of the acidic liquid waste. Typically, only the strong effluent is concerned, as the weak effluent would require too much energy for evaporating the large amount of water contained therein, and the related costs would be unsustainable. This acid recovery process includes a vacuum concentration step in which multiple evaporators are arranged in such a way to attain a 60-?70% wt. intermediate concentration. During this step, one part of the sulphates precipitates, and is separated from the liquid by a settling step and a subsequent filtration. A second evaporation step follows to increase the concentration to a final value of about 85% wt., if possible.

[0008] As the evaporation proceeds, the acidic liquid waste becomes more and more concentrated in sulphuric acid and metal sulphates, in particular, the amount of the latter depends upon the features of the starting mineral. As a consequence, the viscosity increases, and the liquid becomes more and more difficult to handle, in particular, more and more difficult to filter. More specifically, this results from the strong aggregation capacity of the metal sulphates have towards sulphuric acid molecules: a large amount of the free acidity is “trapped” in the solid phase. For this reason, a calcination of the solid matter strongly imbibed with sulphuric acid is required after concentration. Moreover, this sulphate process modification involves high energy costs. The SO2/SO3 emissions occurring during the calcination are also an issue. Moreover, the acid recovered by vacuum evaporation has a metal content larger than what is allowed by the commercial specifications.

[0009] Due to the problems described above, a different route to Titanium dioxide has been more recently proposed as an alternative to the modified sulphate process. This is the so-called “chloride process”, which is based on the following reactions:

TiO2 (raw) + 2CI2 (

TiCk + O2 — pure

In this process, raw TiC in the very expensive form of rutile is used. Moreover, the chlorine is operated in a closed-circuit mode and is used first to attack the raw TiO2, and then to be regenerated by oxidation at a high temperature, normally 650-1000°C. The chloride process yields much less sludge to be landfilled than the sulphate process, but such drawbacks as the high cost of the rutile raw material, the high energy consumption to maintain the oxidation temperatures, and the risks involved by the use of chlorine at high temperatures must be taken into account.

[0010] WO/2021/124224 relates to a process for removing water from a mixture, in particular from a liquid mixture, by feeding a treatment carbon dioxidecontaining gas to the mixture. In particular, the use of the process to concentrate a water solution of a mineral acid, e.g. sulphuric acid, is described. In this application, a dilute acid solution is put into a tank, in which a concentrated acid solution is left after a suitable treatment time at a given temperature. The document does not explain how to treat mineral acid solutions also containing impurities such as metallic salts.

[0011] US 5,229,087 relates to a modified sulphate process for manufacturing Titanium dioxide in which a “waste acid” corresponding to the above acidic liquid waste, after separation from the “hydrolizate”, i.e. the above TiO2 FkO-phase, is evaporated up to an H2SO4 concentration between 60% and 70% by weight, the sulphuric acid solution is separated from the solid metal sulphates formed due to the concentration and is further evaporated up to 70% to 80% H2SO4 by weight, after which SO3 (oleum) is absorbed into one portion of the 70.-80% solution to bring this portion to a concentration of 98-99% by weight, and then combining this 98-99% sulphuric acid with the remainder 70-80% sulphuric acid portion in such a proportion to obtain a final combined 88-93% H2SO4 solution for reuse in the acidic attack of the raw mineral.

Summary of the invention

[0012] It is therefore an object of the present invention to provide a method for treating an acidic liquid waste from a titanium dioxide production sulphate process, in particular for treating the so-called “strong effluent”, that overcomes the above-mentioned typical drawbacks of this process and of the modifications thereof. [0013] More in detail, It is an object of the invention to provide such a treatment method that makes it possible to recover commercially-pure sulphuric acid, i.e. a sulphuric acid with a metal content that meets the most common commercial specifications or, as an alternative, a sulphuric acid that that has a concentration high enough to attack new titanium-containing mineral, according to the user’s requirements.

[0014] It is also an object of the invention to provide a such a treatment method that makes it possible to recover such sulphuric acid at a yield larger than in the case of a conventional concentration process, in particular a multiple-effect evaporation process.

[0015] It is a particular object of the invention to provide such a method that makes it possible to recover metal sulphates at an industrially acceptable degree of purity, at least comparable with what is possible by conventional processes.

[0016] It is an object of the present intention to provide a method for treating an acidic liquid waste containing sulphuric acid, soluble metal sulphates and water, that allows the advantages indicated above.

[0017] These and other purposes are achieved by a process for making titanium dioxide as defined by independent claims 1 and 14, in which innovative treatment of acidic liquid wastes is provided. Preferable embodiments and modifications of the process are defined by the dependent claims.

[0018] Reference is made to a titanium dioxide production “sulphate” process, comprising the conventional steps of: prearranging a ground titanium-containing mineral, e.g. Ilmenite; prearranging an attack solution containing sulphuric acid, for instance set between 93%-? 98%; mixing the ground titanium-containing mineral with the attack solution and injection of water vapour, so as to obtain a two-phase mixture of: an acidic liquid waste comprising water, sulphuric acid and soluble metal sulphates, and solid hydrated titanium dioxide; separating the solid hydrated titanium dioxide from said acidic liquid waste containing sulphuric acid, the soluble metal sulphates and the water; calcining the solid hydrated titanium dioxide so as to obtain anhydrous titanium dioxide; wherein, according to a first aspect of the invention, steps are further provided of: providing a first reactor arranged to receive the acidic liquid waste; providing a second reactor; providing a feed duct arranged to convey a gaseous stream from the first reactor to the second reactor; the process further comprising, still according to the invention, an acidic liquid waste treatment including: a first concentration stage of the acidic liquid waste, comprising a first bubbling step of bubbling a first gas into the acidic liquid waste, so as to obtain: a partially concentrated acidic waste having an intermediate sulphuric acid concentration; first solid metal sulphates in the partially concentrated acidic waste; a first gaseous stream containing the first gas and water vapour, wherein the first concentration stage is carried out at a first process temperature between 70°C and 100°C in particular between 70°C and 90°C; a second concentration stage of the partially concentrated acidic liquid waste comprising: a second bubbling step of bubbling a C02-containing second gas into the acidic liquid waste within the first reactor, obtaining solid metal sulphates precipitating from the acidic liquid waste, the solid metal sulphates settling within the first reactor, and further obtaining, in the first reactor, a second gaseous stream containing CO2, water and sulphuric vapours, i.e. a mixture of SO3 and H2SO4 vapours, hereinafter indicated as (SO3/H2SC )vap, wherein the second bubbling step is carried out at a second process temperature above 150°C; conveying this gaseous stream from the first reactor to the second reactor selectively absorbing the sulphuric vapours contained in the second gaseous stream into a water solution within the second reactor, so as to form a sulphuric acid solution progressively concentrating in sulphuric acid within the second reactor and obtaining a substantially SOa-free third gaseous stream containing CO2 and water vapour up to obtaining a final concentrated sulphuric acid; recovering a concentrated sulphuric acid from the second reactor; recovering the solid metal sulphates from the first reactor.

[0019] The steps of prearranging a ground titanium-containing mineral, prearranging an attack solution containing sulphuric acid, mixing the ground titanium- containing mineral with the attack solution and injection of water vapour followed by hydrolysis and two-phase mixture formation, separating the solid hydrated titanium dioxide and calcining the latter so as to obtain anhydrous TiO2 are not described further here, as these steps are present in the conventional sulphate process for making titanium dioxide TiC and are therefore well known to a person skilled in the art.

[0020] During the first concentration stage, the first gas is bubbled into the acidic liquid waste causing a strong agitation and working as a “carrier”. In other words, the first gas carries away the vapour phase that is in equilibrium with the liquid, thus drawing additional water molecules from the liquid phase to the vapour phase in an attempt to restore a liquid-vapour equilibrium condition between the liquid phase and the gas phase. As the H2SO4 concentration of the sulphuric acid solution increases, metal sulphate hydrates precipitate. The latter can be separated in a possible subsequent filtration step after the first concentration stage, as explained more in detail hereinafter.

[0021] During the second concentration stage, the carbon dioxide of the second gas, upon coming into contact with the water, reacts with the latter forming carbonic acid H2CO3., An equilibrium is then established between the CO2 and water, on the one hand, and carbonic acid, on the other hand, according to the reaction:

The CO2 exceeding the stoichiometric amount according to this reaction acts as a “carrier”, in other words, it carries away the vapour phase that is in equilibrium with the liquid, thus drawing additional water molecules from the liquid phase to the vapour phase that is in contact with the liquid in an attempt to restore a liquidvapour equilibrium condition between the liquid phase and the gas phase. This causes a progressive decrease of the water content of the liquid phase, which means a progressive increase of the sulphuric acid concentration.

[0022] The carrier or dragging effect of the gas causes a strong turbulence in the treated mass. From a fluid-dynamic point of view, this is the main difference of the concentration system operating according to the invention with respect to a conventional evaporation concentration system, e.g. one in which a multipleeffect evaporator is involved. This is the case, in particular, of the lower-pressure effects of such an evaporator, where the evaporation of water from the mass is not accompanied by particularly turbulent conditions.

[0023] Advantageously, a heating device can be arranged along the extension of the feed duct, in order to maintain the inside of the same at a temperature at which the water of gaseous stream remains in the vapour state, while the mixture (SO3/H2SC )vap condensates, for instance, at a temperature of at least 100°C. This way, substantially all of the sulphuric matter contained in the second gaseous stream as the latter is transferred from the first reactor to the second reactor, where the H2SO4 concentration of acid water solution in the second reactor progressively increases as bubbling step progresses. However, such a heating device may not be necessary to maintain the inside of the feed duct at that the above-mentioned temperature.

[0024] The above H2CO3 equilibrium-related effect and the carrier effect cooperates therefore to maximize the amount of water and sulphuric vapours that leave the first reactor, while the progressively precipitating ferrous sulphates remain back in the first reactor, from which they can be removed at the end of the second concentration stage. At the same time, the sulphuric vapours containing SO3 and H2SO4 as such, along with excess CO2 and extracted water vapour flow along the feed duct and reach the second reactor at such a temperature that in the second reactor sulphuric vapours form a progressively concentrating high purity H2SO4 water solution that is substantially free from metal sulphates and possible other solidi impurities, which are left back in the first reactor, while excess CO2 and one part of water vapour flow beyond the second reactor and are con- venably treated for recover or disposal, as described hereinafter.

[0025] Briefly, the C02-containing second gas flow makes it possible to maximize the amount of water and sulphuric vapours separated from the solid impurities that are left in the first reactor, and cooperates with the serial arrangement of the first and second reactors, in improving H2SO4 recovery efficiency, thus increasing the yield in recovered sulphuric acid and, above all, in maximizing the purity of the recovered sulphuric acid, at a concentration that can be as high as 98%, according to the user’s requirements, and in any case containing only very small amounts of metal sulphates.

[0026] As shown by the examples below, a treatment, according to the invention, of the acidic liquid waste allows a better liquid-solid separation than the conventional vacuum concentration method. The latter is much less efficient, because the solid matter that is formed retain a significant portion of the free acidity. Therefore, in order to achieve an acceptable yield, the solid must subsequently be calcined. In the calcination, the H2SO4 decomposes forming an SO2/SO3 mixture that can be reduced to SO2 alone to be used as in a conventional H2SO4 manufacture process, therefore this option preferably requires the presence of a plant exploiting such manufacture process.

[0027] The relatively high sulphuric acid concentration in the liquid waste subjected to the second concentration stage means that almost all water molecules are bound to the sulphuric acid, forming a sort of complex that can be represented as H2SO4XH2O. Concerning the second process temperature, it has been observed that at least 150°C are required for CO2 to operate as described above. It is believed that such a process temperature is sufficient to “swell” this complex, i.e. to increase the distance and thus decrease the attraction forces between the H2SO4 molecules and the H2O molecules, so that carbon dioxide can operate as a carrier. In particular, the second process temperature can be set between 170°C and 190°C.

[0028] In this connection, it is worthwhile to note that SO3 is formed and leaves the acidic liquid waste being concentred at process temperatures, such as those mentioned above, that are lower than the boiling point of sulphuric acid at the same concentration, at least in a final part of the second concentration stage. This suggests that the interaction between CO2 and sulphuric acid involves resonance phenomena of H2SO4 molecules, to such an extent to cause the decomposition of said molecules to SO3 and H2O. [0029] The bubbling steps, and in particular the second bubbling step, can be discontinued by a user when desired concentration and/or recovery rate are attained as the step proceeds.

[0030] Preferably, the first gas fed to the acidic liquid waste in the first step of bubbling a gas stream is air. As an alternative, the further gas can be carbon dioxide or an air I carbon dioxide mixture, or any technically acceptable gas.

[0031] According to a second aspect of the invention, after the TiC manufacture conventional steps, steps are further provided of: providing a first reactor arranged to receive the acidic liquid waste; providing a second reactor; providing a feed duct arranged to convey a gaseous stream from the first reactor to the second reactor; the process further including, still according to the second aspect of the invention, an acidic liquid waste treatment including: a first concentration stage of the acidic liquid waste, comprising a first bubbling step of bubbling a first gas into the acidic liquid waste, so as to obtain: a partially concentrated acidic waste having an intermediate sulphuric acid concentration; first solid metal sulphates in the partially concentrated acidic waste; a first gaseous stream containing the first gas and water vapour, wherein the first concentration stage is carried out at a first process temperature between 70°C and 100°C in particular between 70°C and 90°C; a second concentration stage of the partially concentrated acidic liquid waste comprising: a second bubbling step of bubbling a second gas into the acidic liquid waste within the first reactor, obtaining solid metal sulphates precipitating from the acidic liquid waste, the solid metal sulphates settling within the first reactor, and further obtaining, in the first reactor, a second gaseous stream containing water and sulphuric vapours, i.e. a mixture of SO3 and H2SO4 vapours, hereinafter indicated as (SO3/H2SC )vap, wherein the second bubbling step is carried out at a second process temperature above 190°C; conveying this gaseous stream from the first reactor to the second reactor selectively absorbing the sulphuric vapours contained in the second gaseous stream into a water solution within the second reactor so as to form a sulphuric acid solution progressively concentrating in sulphuric acid within the second reactor and obtaining a substantially SOa-free third gaseous stream containing water vapour up to obtaining a final concentrated sulphuric acid; recovering a concentrated sulphuric acid from the second reactor; recovering the solid metal sulphates from the first reactor.

[0032] During both the first and the second concentration stages, the first or second gas acts as a “carrier” as explained with reference to the process according to the first aspect of the invention. In the first concentration stage, as the H2SO4 concentration of the sulphuric acid solution increases, metal sulphate hydrates precipitate and can optionally be separated in a possible subsequent filtration step before the second concentration stage, as explained more in detail hereinafter. In the second concentration stage, the metal sulphate precipitate and settle in the first reactor, from which they have to be recovered after this concentration stage, while the sulphuric vapours containing SO3 and H2SO4 leave the first reactor and are collected in the second reactor as a sulphuric acid water solution, like in the process according to the first aspect of the invention, until they reach a predetermined final sulphuric acid concentration.

[0033] Also in this case, the use of a carrier gas cooperates with the reactor arrangement to allow high sulphuric acid recovery yield and, above all, high purity of the recovered sulphuric acid, even if in the second concentration stage only the carrier effect operates.

[0034] The second process temperature is higher than in the process according to the first aspect, because only the carrier effect of the gas is exploited and the above-explained effect of CO2 in connection with the equilibrium reaction [1 ] is absent. Preferably, the second process temperature is set between 21 CPC and 230°C. The process temperature is supposed to operate as explained in the case of the first aspect, and is, at least in a final part of the second concentration stage, lower than the boiling point of sulphuric acid at the same concentration. [0035] The first gas fed in the first bubbling step and the second gas fed in the second bubbling step can be selected independently form one another, and each of them is preferably air or, as an alternative, any technically acceptable gas or gas mixture.

[0036] In both first and second aspect of the invention, the first concentration stage can be carried out within the same first reactor where the second concentration stage is performed. As an alternative, the first concentration stage can be carried out within a further reactor upstream of the first reactor, and an outlet duct is arranged to convey the partially concentrated acidic waste from the further reactor into the first reactor.

[0037] As anticipated, in both aspects of the invention, after the first concentration stage and before the second concentration stage, a step can be provided of filtering the first solid metal sulphates from the partially concentrated acidic waste obtained by the first concentration stage, in which case a filtered partially concentrated acidic waste is obtained. In this case, the partially concentrated acidic liquid waste as discharged by the first reactor is indicated as a raw partially concentrated acidic waste. However, this filtration step is not necessary to obtain the desired purity of the concentrated sulphuric acid, but is useful to obtain a higher purity of the recovered solid metal sulphates, as discussed below.

[0038] If the acidic liquid waste is filtered after the first concentration stage, the solid that is separated contains sulphates of the metals that are present in the highest concentration in the titanium-containing mineral. In the case of Ilmenite, which contains iron, the solid separated in this concentration step is Ferrous sulphate acceptably pure for different industrial uses. For instance, this Ferrous sulphate can be used for reducing the hexavalent chromium present in the cement to trivalent chromium. The purity of the solid sulphates globally recovered by the process is one advantage of the filtration step. On the other hand, the solid withdrawn from the liquid acidic waste by the filtration is accompanied by a relatively important amount of liquid sulphuric acid that must be removed to obtain a dry sulphate, and that would be otherwise present in the final concentrated sulphuric acid, if the filtration were not carried out. This means that the intermediate filtration between the first concentration stage, besides improving the purity of the recovered sulphate, may reduce the recovered sulphuric acid yield. [0039] The convenience of the intermediate filtration can be therefore assessed as a trade-off between the importance of a commercial use of the metal sulphates and the extent of the recovered sulphuric yield reduction.

[0040] Filtration is generically defined as a usual/liquid separation operation that takes place in an apparatus with a wall that is pervious to the liquid phase and impervious to the solid phase, such as a filter of various types, e.g. a filter press, or a centrifuge of various, types, the term filtration thus including, beside true filtration, similar separation techniques such as centrifugation.

[0041] The first concentration stage of the acidic liquid waste and the possible subsequent filtration step of the first metal sulphates from the raw partially concentrated acidic waste can be performed either in batch or continuous mode, both modes being easily implemented by the person skilled in the art on the basis of this description.

[0042] In the former case, the first bubbling step is performed after a step of feeding including loading a predetermined amount of the acidic liquid waste into a treatment vessel, and is continued until this acidic liquid waste reaches a predetermined intermediate sulphuric acid concentration, after which a discharge phase is carried out of a partially concentrated acidic waste from said vessel.

[0043] In particular, the continuous mode, with its well-known advantages, can be implemented without particular difficulty due to the nature of the raw partially concentrated acidic waste obtained from the first concentration stage according to the present invention. Indeed, as shown by the experiments carried out by the inventors, described below, the raw partially concentrated acidic waste can be more easily filtered and handled than the corresponding raw partially concentrated acidic waste after a first stage of vacuum-assisted concentration.

[0044] In particular, the process can further include the steps of: condensing the water from the first gaseous stream, thus obtaining an incondensable gas stream containing the first gas, e.g. air; recirculating the non-condensable gas stream to the first bubbling step. In this case, the further gas is operated in a closed-circuit mode.

[0045] Preferably, the intermediate sulphuric acid concentration, at which the first concentration stage is stopped, is set between 60 and 70 per cent by weight, beyond which the carrier gas-assisted evaporation process taking place during the first bubbling step would stop. In particular, the intermediate sulphuric acid concentration is about 65% by weight.

[0046] Advantageously, before the second concentration stage, a step can be provided of pre-heating the acidic liquid waste feed, preferably, up to a the second process temperature.

[0047] Preferably, the final sulphuric acid concentration is between 90 and 95 per cent. Although the concentration stage could be extended to a final concentration of 98-98.5%, which is the concentration of commercial sulphuric acid, it can be advantageous to limit the recovery of sulphuric acid to a lower concentration, for example a concentration set between 90% and 95%, in particular between 90% and 92%. This is the case, in particular, if a sulphuric acid production plant is present in the vicinity of the acidic liquid waste treatment unit, provided that, as it is the case with the present invention, the recovered sulphuric acid is substantially metal-free and, in particular, has an iron ion concentration lower than 10 ppm. Besides allowing to save resources for the concentration stage and shorten the same, such a final concentration value makes it possible to absorb the SO3 produced by the sulphuric acid production plant without the need of adding distilled water to adjust the concentration to the commercial value.

[0048] On the other hand, in the case of a conventional concentration process by vacuum evaporation, if an 85% concentrated sulphuric acid is recovered, it would be necessary to mix the latter with oleum in a specific plant section, or to calcine solids and absorbing the SO3 produced this way into the recovered acid, up to the desired final concentration but, in any case, a product would be obtained with a metal content well above the values allowed by the commercial specifications.

[0049] Advantageously, the process also includes a step of recycling at least one portion of the final concentrated sulphuric acid recovered from the acidic liquid waste into the attack solution used to attack the ground titanium-containing mineral.

[0050] Advantageously, the process comprises the steps of: selectively condensing water from the third gaseous stream, so as to obtain a non-condensable gas stream; recycling this non-condensable gas stream to the second step of bubbling second gas into the acidic liquid waste to be concentrated.

This is particularly advantageous in the first aspect of the invention, in which case the non-condensable gas stream contains CO2, which is therefore operated in closed-circuit mode during the second concentration stage, in order to limit the CO2 consumption to a small make-up amount only.

[0051] Referring back to the conventional sulphateTiC manufacture process, the step of separating the solid hydrated titanium dioxide from the acidic liquid waste can comprise a step of filtering an amount of solid hydrated titanium dioxide from a more concentrated acidic liquid waste, having a sulphuric acid concentration higher than a predetermined concentration value, in particular exceeding 200 g/l, and typically set between 250 g/l and 350 g/l, which is the above-mentioned "strong effluent", or "hydrolysis filtrate", of the sulphate process, and the at least one concentration stage is performed on that more concentrated effluent only.

[0052] The composition of the effluent of the sulphate process for manufacturing titanium dioxide is strongly dependent on the starting mineral. A typical composition of a strong effluent in the case of a process using Ilmenite and titan- iferous slag is given in Table 1 , in which the composition of the corresponding weak effluent is also given for comparison.

- Table 1 -

Typical composition of TiO? production effluents via sulphate, in grams/litre

[0053] In other words, in this embodiment of the invention, the more concentrated acidic liquid waste, i.e., the "strong effluent" of a TiC sulphate process, is maintained separate from the less concentrated acidic liquid waste and is sent to the concentration treatment of the present invention, while the less concentrated acidic liquid waste, i.e., the " weak effluent" is treated otherwise, for instance, by neutralization with lime or calcium hydroxide the, in order to contain the process energy costs.

[0054] In a third aspect of the invention, a process for treating an acidic liquid waste containing sulphuric acid, soluble metal sulphates and water, includes the features of the treatment according to the first or to the second aspect of the invention, regardless the provenience of said acidic liquid waste. In other words, the acidic liquid waste can be produced by an industrial process different from the sulphate TiO2 manufacture process.

Brief description of the figures

[0055] The invention will be illustrated below with the following description of its embodiments, by way of example and not of limitation, with reference to the accompanying drawings, in which:

Fig. 1 is a block diagram of a sulphate process for making pure titanium dioxide, according to the prior art;

Figs. 2 and 3, individually combined with Fig. 4, form block diagrams of modifications of the sulphate processes for making pure titanium dioxide, in which acidic liquid waste concentration treatments are performed according to two different embodiments (Fig. 2 and Fig. 3) of the invention; in particular, in Fig. 2, a treatment of the strong effluent and at least one part of the weak effluent is carried out according to the invention; in Fig. 3, a treatment of the strong effluent only is carried out according to the invention;

Fig. 4 is a block diagram of the treatment of the acidic liquid waste of Fig. 2 of Fig. 3;

Figs. 5 and 6 are simplified diagrams of plants to actuate the first concentration stage, according to different embodiments of the invention; Figs. 7 and 8 are simplified diagrams of plants to actuate the second concentration stage, according to different embodiments of the invention.

Fig. 9 is a simplified diagram of a plant to actuate the first and the second concentration stage in a same reactor, still according to different embodiments of the invention.

Detailed description of embodiments of the invention

[0056] With reference to Figs. 2-4, a treatment 100 (Fig. 4) is described of an acidic liquid waste 6a/b or 6a (Figs. 2 and 3, respectively) from a sulphate process for making titanium dioxide.

[0057] Conventionally, as shown in Figs. 1 - 3, a starting mineral 1 , including metals other than titanium, after grinding, is attacked with an attack solution 2 containing sulphuric acid. As well known, this attack comprises a step S3 of mixing mineral 1 with attack solution 2, a step S4 of treating with water vapour 3 the obtained mixture, in order to cause hydrolysis S5 of mineral 1 . A two-phase mixture 4 is obtained, in which a liquid phase comprises sulphuric acid, water and soluble sulphates Mex(SC ) y Of metals Me that are present in the mineral, and a solid phase consists of solid hydrated titanium dioxide TiC H2O.

[0058] A subsequent separation and purification steps S6 of solid hydrated titanium dioxide 5a result in acidic liquid wastes 6a, 6b that contain sulphuric acid at different concentrations, along with soluble metal sulphates and water. In particular, a more concentrated acidic liquid waste, or "strong effluent" 6a, results primarily from a filtration operation S6a of two-phase mixture 4, in which the sulphuric acid concentration is normally between 250 g/l and 350 g/l. Another less concentrated acidic liquid waste in sulphuric acid, or "weak effluent" 6b, results from multiple and different purification operations S6b of hydrated titanium dioxide 5a as obtained from filtration S6a. Purification operations S6b are generically referred to as "washing", and are well known to a person skilled in the art.

[0059] Hydrated titanium dioxide 5b, as obtained from separation and purification steps S6, is subjected to a calcination step S7 by which anhydrous titanium dioxide TiO2 7 is obtained. [0060] Steps of prearranging S1 a ground titanium-containing mineral 1 , prearranging S2 sulfuric acid-containing attack solution 2, mixing S3 ground titanium- containing mineral 1 with attack solution 2 by injection S4 of water vapour, hydrolysis S5, separating S6 solid hydrated titanium dioxide 5a, 5b, calcining S7 the latter to obtain anhydrous TiO2 7 are not described further herein, because they are a part of the conventional sulphate process (Fig.1 ) for making titanium dioxide TiO2 and are well known to the person skilled in the art.

[0061] Fig. 4 schematically shows a treatment S100, according to the invention, of strong effluent 6a only (Fig. 3), or of a mixture 6 (Fig. 2) of strong effluent 6a and at least one part of weak effluent 6b. Weak effluent 6b, or the remaining part thereof, respectively, can be subjected to a different treatment 6c. Typically, this can be a conventional neutralisation treatment with lime or calcium carbonate, which is also well known to the person skilled in the art.

[0062] Treatment 100 basically includes a first concentration stage S10, in which acidic liquid waste 6 or 6a is turned into a partially concentrated acidic liquid waste 16, and a second concentration stage S20, in which a final concentrated sulphuric acid 28 and solid metal sulphates 23 are recovered from partially concentrated acidic liquid waste 16 or 16a.

[0063] Optionally, treatment 100 can also include a filtration step S12 between first concentration stage 10 and second concentration stage 20, in which a first amount of solid metal sulphates is recovered from partially concentrated acidic liquid waste 16.

[0064] First concentration stage S10 comprises a first step S1 1 of bubbling a first gas 12 into acidic liquid waste 6 or 6a.

[0065] Similarly, second concentration stage S20 includes a second step S21 of bubbling a second gas 22 into raw partially concentrated acidic liquid waste 16 or filtered partially concentrated acidic liquid waste 16a, according to whether filtration step S12 is carried out or not.

[0066] First and second bubbling steps 1 1 ,21 can take place in a same reactor, as shown in Fig. 9, in which case a reactor 61 is arranged to receive acidic liquid waste 6 or 6a (Fig. 2) from a titanium dioxide production unit, in particular strong liquid waste 6a (Fig. 3). Reactor 61 is also arranged to receive first gas 12 or second gas 22 according to whether it operates first bubbling step 1 1 or second bubbling step 21 , respectively.

[0067] Otherwise, First and second bubbling steps 1 1 ,21 can be carried out in two distinct reactors, as shown in Figs. 5-8, in which case a further reactor 41 is arranged to receive acidic liquid waste 6 or 6a and first gas 12, while reactor 61 is arranged to receive partially concentrated acidic liquid waste 16 or 16a from further reactor 41 , as well as second gas 22.

[0068] During first bubbling step S1 1 , acidic liquid waste 6 or 6a is maintained at a first process temperature set between 70°C and 100°C, in particular between 70°C and 90°C.

[0069] With reference to Figs. 5 and 6, further reactor 41 includes a gas distributor 48 arranged to receive first gas 12 and to disperse it into a bottom portion of further reactor 41 , as well as a heating member 43, such as a jacket. A heating circuit is arranged to receive a heating fluid 1 1 like saturated steam or diathermic oil from a heating circuit, and a temperature regulating device 44 comprising a temperature sensor 45, and further including a temperature controller 46 and a regulation organ such as a valve 47, these components being represented conventionally and mounted in a manner that is well known to the person skilled in the art.

[0070] Further reactor 41 can be advantageously associated with a feeder tank 40 that is arranged to receive liquid waste and is hydraulically connected to further reactor 41 via a feed pipe, along which a pump 42 is possibly installed.

[0071] As a result of first gas-bubbling step S11 , a strong turbulence takes place in acidic liquid waste 6 or 6a in which first gas 12 acts as a carrier, in other words, first gas 12 carries away the vapour phase that is in contact with the liquid acidic liquid waste 6 or 6a, thus drawing additional water molecules from the liquid phase to the vapour phase in an attempt to restore a liquid-vapour equilibrium condition.

[0072] A gaseous stream 14 containing the carrier gas and water vapour is then formed. Simultaneously with first gas-bubbling step S1 1 , first concentration stage S10 can further comprise a step S16 of condensing the water contained in the gaseous stream 14 and recovering a condensate 1 1 b. The latter can be used for further energy recovery in a manner known to the person skilled in the art. The remaining non-condensable stream 19 containing the gas used in first gas-bub- bling step S1 1 can optionally be released into the atmosphere, in particular, if first gas 12 is air. Otherwise, non-condensable stream 19 can be recycled to and bubbled into acidic liquid waste 6 or 6a. This way, the external input of air 12 is limited to a make-up amount 12a to compensate for air losses.

[0073] To this purpose, as shown in Fig. 6, a condenser 51 can be provided, for example a shell-and-tube heat exchanger 51 . In this case, a shell-side chamber of condenser 51 is preferably in pneumatic communication with further reactor 41 , and is arranged to receive gaseous stream 14 containing the carrier gas and water vapour therefrom, condense water as condensate 1 1 b into a condensate tank, not shown, and allow non-condensable gas stream 19 to pass through. On the other hand, an inner-tube side chamber of condenser 51 is arranged to receive a cooling fluid 53.

[0074] As first gas-bubbling step S1 1 proceeds, acidic liquid waste 6 or 6a becomes progressively more concentrated. First gas-bubbling step S1 1 is discontinued when a certain intermediate H2SO4 concentration is attained. In particular, a raw preliminarily or partially concentrated acidic waste 16 has an H2SO4 concentration set between 60% and 70%, more in particular the intermediate concentration is about 65%.

[0075] Moreover, as first gas-bubbling step S1 1 proceeds in further reactor 41 , a precipitation also takes place therein of solid hydrated sulphates Mex(SO4)yzH₂O 13 of the metals Me other than titanium that were present in the starting mineral 1 . An outlet duct 17 is provided to discharge partially concentrated acidic waste 16 and to convey it into first reactor 61 to carry out second concentration stage 20, in particular, through feeder tank 60 (Figs. 7 and 8).

[0076] As anticipated, according to one embodiment of the invention, a filtration step S12 can be provided to filter solid hydrated sulphates 13 away from raw partially concentrated acidic waste 16, thereby obtaining a filtered partially concentrated acidic waste 16a. [0077] To this purpose, as shown in Fig. 6, along outlet duct 17 of further reactor 41 , a filter 50 is arranged to receive raw partially concentrated acidic waste 16 from further reactor 41 and is configured to retain solid hydrated sulphates 13 and to allow filtered partially concentrated acidic waste 16a to pass through. A pump 49 is preferably arranged along outlet duct 17 upstream of filter 50.

[0078] Still with reference to Fig. 4, second concentration stage S20 includes a second step S21 of bubbling a second gas 22 into raw or filtered partially concentrated acidic liquid waste 16 or 16a, according to whether filtration step S12 is carried out or not after first bubbling step 1 1 .

[0079] According to a first aspect of the invention, second gas 22 is a CO2- containing second gas, i.e. it contains carbon dioxide at a given concentration, or is substantially pure carbon dioxide. In this case, during second gas-bubbling step S21 , partially concentrated acidic liquid waste 16 or 16a being further concentrated is maintained at a second process temperature above 150°C, preferably between 170°C and 190°C. In any case, this process temperature Is lower than the boiling point of the sulphuric acid mixture having the same concentration as acidic liquid waste 6 or 6a.

[0080] According to a second aspect of the invention, second gas 22 can be any technically acceptable gas, in particular it can be air. In this case, during second gas-bubbling step S21 , partially concentrated acidic liquid waste 16 or 16a being further concentrated is maintained at a second process temperature above 190°C, preferably between 210°C and 230°C. Also in this case, at least in a final part of the second concentration stage, this process temperature is lower than the boiling point of the sulphuric acid mixture having the same concentration as acidic liquid waste 16 or 16a.

[0081] In order to perform the second concentration stage, as shown in Figs. 7 and 8, first vessel or first reactor 61 can include a gas distributor 68 arranged to receive C02-containing gas 22 and to bubble and disperse it into a bottom portion of first reactor 61 , Reactor 61 can also include a heating member 63, such as a jacket, arranged to receive a heating fluid 1 1 like saturated steam or diathermic oil from a heating circuit, and a temperature regulating device 74 comprising a temperature sensor 75, and further including a temperature controller 76 and a regulation organ such as a valve 77, these components being represented conventionally and mounted in a manner that is well known to the person skilled in the art.

[0082] First reactor 61 is advantageously associated with a feeder tank 60 arranged to receive partially concentrated acidic liquid waste 16 or 16a from further reactor 41. Feed tank 60 is hydraulically connected to first reactor 61 via a feed pipe 69, along which a pump 62 is possibly installed.

[0083] As shown in Fig. 8, a heat exchange device such as a shell-and-tube heat exchanger 71 can also be provided along feed pipe 69. Typically, heat exchanger 71 comprises a first chamber, in this case the inner-tube side chamber, that is in hydraulic communication with an upstream portion and with a downstream portion of feed pipe 69, and is therefore arranged to receive and convey acidic liquid waste 6 or 6a to first reactor 61 . Heat exchanger 71 further comprises a second chamber, in this case the shell-side chamber, that is arranged to receive a heating fluid, for example the same as heating fluid 1 1 fed to heating member 63 of first reactor 61 . A temperature regulation device 64 can also be associated with heat exchanger 71 for controlling the temperature of acidic liquid waste 6 or 6a fed to first reactor 61 . For example, temperature regulation device 64 can include a temperature sensor 65, a temperature controller 66 and a regulation organ such as a valve 67, these components being represented conventionally and mounted in a manner that is well known to the person skilled in the art.

[0084] In the first aspect of the invention, as a result of C02-containing second gas bubbling step S21 , upon coming into contact with the water of partially concentrated acidic liquid waste 16 or 16a, the CO2 reacts with the same to form carbonic acid H2CO3, according to the equilibrium reaction:

CO2 (g) + H₂ O (l)^ H2CO3 (aq).

The amount of CO2 exceeding the stoichiometric amount carries away the vapour phase that is in equilibrium with the liquid 16 or 16a, drawing additional water molecules from the liquid phase to the vapour phase that is in contact with the liquid 16 or 16a in an attempt to restore a liquid-vapour equilibrium condition.

[0085] In the above-mentioned process conditions, a decomposition of sulphuric acid also takes place to an extent, thus yielding a certain amount of gaseous sulphur trioxide SO3. Therefore, a gaseous stream 24 is formed containing CO2, water vapour and (SO3/H2SC )vap, a mixture of SO3 and H2SO4 vapours (Fig. 4).

[0086] Besides second gas bubbling step S21 , the concentration stage S20 also comprises a simultaneous step S22 of selectively condensing and absorbing the (SO3/H2SC )vap contained in gaseous stream 24 into a water solution. Hence, an aqueous sulphuric acid solution 26 is formed, the concentration of which progressively increases as concentration stage S20 progresses, as explained hereinafter. Upon reaching a predetermined final H2SO4 concentration, second gas bubbling step S21 is stopped and aqueous sulphuric acid solution 26 is withdrawn as a recovered concentrated sulphuric acid 28.

[0087] To this purpose, as shown in Figs. 7 and 8, a second vessel or second reactor 91 can be provided having a feed duct 92 arranged to convey said gaseous stream 24 from first reactor 61 to second reactor 91 . An own open-end portion of said feed duct (92) can be arranged within the bottom part of second reactor 91 and have the opposite end portion pneumatically connected to a vent outlet at the top of first reactor 61 , so as to convey gaseous stream 24 from first reactor 61 to the bottom part of second reactor 91 . A jacket 90 or an equivalent heating device is preferably arranged along the extension of feed duct 92, in order to maintain the inside of the same at a temperature of at least 100°C, i.e. at such a temperature at which the water of gaseous stream 24 remains in the vapour state, while the mixture (SO3/H2SC )vap condensates. Therefore, substantially all of the sulphuric matter contained in gaseous stream 24 as is transferred from first reactor 61 to second reactor 91 where the H2SO4 concentration of acid water solution 26 in second reactor 91 progressively increases as bubbling step S21 progresses. At the same time, the water vapour of gaseous stream 24 flows across second reactor 91 and further into a condenser 81 , as described hereinafter, preferably under the action of an aspiration device 72. At the end of second gas bubbling step S21 , a sulphuric acid will be left within second reactor 91 with a concentration of about 90-92% by weight, with an iron content lower than 10 ppm.

[0088] Second reactor 91 is also provided with a heating member 93 such as a jacket 93, arranged to receive a heating fluid 21 like saturated steam or diathermic oil from a heating circuit, and further provided with a temperature regulating device 84 comprising a temperature sensor 85, a temperature controller 86 and a regulation organ such as a valve 87, these components being represented conventionally and mounted in a manner that is well known to the person skilled in the art.

[0089] Temperature control device 84 is configured to maintain second reactor 91 at a temperature slightly above the temperature of first reactor 61 , in particular, between 180°C and 200^sC, a temperature which contributes to allow water vapour to flow further into and condensate within condenser 81 .

[0090] Second reactor 91 is also preferably equipped with an inlet duct, not shown, to preliminary feed an amount of a liquid such as water or of a sulphuric acid water solution, to form a predetermined liquid head 25 in the bottom part of second reactor 91 so that selective condensation/absorption step S22 of (SO3/H2SC )vap can be easily started and brought to a steady state.

[0091] The concentration of recovered concentrated sulphuric acid 28 can be set between 90% and 95%, in particular between 90% and 92%, or it can be about 98%, which is the concentration at which sulphuric acid is usually commercially available. This concentration value is advantageous if a sulphuric acid production plant is present in the vicinity. Treatment 100 according to the present invention makes it possible to recover a substantially metal-free concentrated sulphuric acid 28, for instance, with a Fe content not exceeding 10 ppm, which is the maximum Fe content of sulphuric acid as made by a production plant.

[0092] Therefore, concentrated sulphuric acid 28 recovered by the treatment according to the present invention meets the common commercial specifications of sulphuric acid. As an alternative, or in addition, recovered concentrated sulphuric acid 28 can be used at least in part to carry out steps S3-S4 of attacking raw titanium-containing mineral 1 (Figs. 2 and 3).

[0093] After step S22 of selectively condensing and absorbing the (SO3/H2SC )vap contained in gaseous stream 24 into a water solution, gaseous stream 24 is transformed into a substantially SOs-free gaseous stream 27 (Fig. 4) that contains water vapour and, in the first aspect of the invention, carbon dioxide CO₂. [0094] Therefore, a condensation step S26 can be provided from which a condensate 19a can be recovered and possibly used for further energy recovery in a manner known to the person skilled in the art. A stream 29 of non-condensable gas is also produced. In the first aspect of the invention, non-condensable gas 29 contains a CO2 amount that can optionally be recycled into gas 22, as indicated by the dotted line in Fig. 4, and bubbled into partially concentrated acidic liquid waste 16 or 16a. This way, only a small amount 22a of fresh make-up water is required so as to compensate for unavoidable water losses.

[0095] To this purpose, as shown in Figs. 7 and 8, a condenser 81 can be provided, for example a shell and tube heat exchanger 81 . In this case, a shellside chamber of condenser 81 is preferably in pneumatic communication with second reactor 91 , and is arranged to receive substantially SOa-free gaseous stream 27 therefrom, condense water as condensate 19a into a condensate tank, not shown, and allow non-condensable gas stream 29 to pass through. On the other hand, an inner-tube side chamber of condenser 81 is arranged to receive a cooling fluid 73.

[0096] In the process according to the first aspect of the invention, as shown in Fig. 8, the shell-side chamber of condenser 81 can also be arranged to convey back non-condensable gas stream 29 into first reactor 61 , through a duct along which a fan 72 is mounted, in order to establish the above-mentioned CO2 closed- circuit operation mode.

[0097] As step S21 of bubbling second gas 22 into partially concentrated acidic liquid waste 16 or 16a proceeds in first reactor 61 , a precipitation also takes place therein of solid hydrated sulphates Mex(SC )y ZH2O 23 of the metals Me other than titanium that were present in the starting mineral 1 . At the end of second bubbling step S21 , only solid hydrated sulphates 23 settled in first reactor 61 remains therein. First reactor 61 is advantageously provided with an inlet 18 for a solubilisation water that is intended to solubilise solid hydrates sulphates 23, in order to make it easier to discharge them.

[0098] Fig. 9 is a simplified diagram of a plant to actuate both first and second concentration stages S10, S20, i.e. bubbling steps S11 , S21 in a same reactor 61 , according to the first or to the second aspect of the invention. A detailed description of Fig. 9 is omitted, as it can be easily deducted from the description of Figs. 5-8 above. The main differences with respect to Figs. 7 and 8 are: a) reactor 61 is arranged to receive acidic liquid waste 6 or 6a directly from a titanium dioxide production unit; b) reactor 61 is arranged to receive both first gas 12 and second gas 22, according to whether first or second gas-bubbling step is performed, respectively; c) heating member 63 and temperature regulating device 74 are configure to set both first and second process temperature in first reactor 61.

[0099] The above description also applies to the process for treating any acidic liquid waste 6 containing sulphuric acid, soluble metal sulphates and water, regardless the provenience of waste 6, according toa third aspect of the invention.

EXAMPLES

[0100] Laboratory tests were carried out to verify the performances of the treatment, according to the invention, of a typical strong effluent of a sulphate process for manufacturing titanium dioxide. More in detail, an example 1 includes a laboratory test reproducing the conditions of first concentration stage 10 of the treatment according to the invention, carried out by bubbling air into a given amount of a strong effluent. On the other hand, an example 2 includes a laboratory test reproducing the conditions of second concentration stage 20 of the treatment according to the invention, carried out by bubbling carbon dioxide into the partially concentrated acidic waste solution obtained from example 1 , after filtering away the solids formed during first concentration stage 10.

[0101] Comparative concentration laboratory tests were also carried out in which another sample of the same strong effluent treated in the tests of Examples 1 and 2 was concentrated by vacuum evaporation, under the typical operating conditions of a conventional vacuum evaporative concentration process. In order to compare both first concentration stage 10 and second concentration stage 20 of the treatment according to the invention to a respective corresponding stage of the conventional evaporative process, in Comparative Example 1 the vacuum evaporation conditions were maintained during a time equal to the time during which the air bubbling was maintained in the test of Example 1 . [0102] The composition of the strong effluent used for both the concentration treatment examples according to the invention and the comparative vacuum evaporation examples is shown in Table 2.

- Table 2 - Composition of the strong effluent used in both Example 1,2 and Comparative examples 1,2 (grams/litre)

EXAMPLE 1 (Concentration by air bubbling)

[0103] In order to carry out the first-stage concentration test according to the invention, a laboratory apparatus was used, substantially corresponding to the diagram in Fig. 6, comprising a first flask as further reactor 41 , heated by means of a heating mantle and equipped with a heated column and a condenser along its vent duct, a second flask arranged to collect the water condensed by the condenser and a recycling line for the air leaving the condenser. The test of Example 1 was carried out under the operating conditions summarized in Table 3. - Table 3 -

Operating conditions of Example 1 (first concentration stage)

[0104] More in detail, 1280 g of 320 g/l H2SO4 effluent was introduced into the first flask or reactor flask, inside of which air was bubbled at a pressure of 0.2 to 0.3 bar. The temperature of the mass in the first flask was maintained at 88^SC by the heating mantle, which was controlled by a temperature probe immersed in the flask.

[0105] After 90 minutes of treatment, 704 g of condensed water had collected in the second flask, while 576 g of residual effluent remained in the first flask, which was decanted into a beaker and left there during 24 hours in order to promote crystallisation and decantation of the metal sulphates. After 24 hours, the liquid was filtered, yielding 134 g of solid material containing 40% wt. H2SO4 and 395 g of liquid containing 63% wt. H2SO4, which means an acid recovery of about 78%.

[0106] After filtration, no further crystallisation of salts was observed in the filtered liquid.

EXAMPLE 2 (concentration by CO2 bubbling)

[0107] In order to carry out the second-stage concentration test according to the invention, a laboratory apparatus was used, substantially corresponding to the diagram of Fig. 8, comprising a first container as first reactor 61 , consisting of a flask heated by means of a heating mantle and equipped with an agitator and with a heated column, a second container as second reactor 91 , consisting of a pear-shaped bottle heated by means of a heating plate and arranged to receive the sulphur trioxide condensed from the vapours released by the first container, also containing water and carbon dioxide, and equipped with a heated column and a condenser along its vent duct, and further comprising a third container, consisting of a flask arranged to collect the water condensed by the condenser associated to the second container, and a recycling line for the CO2 leaving the condenser on the vent line of the third container. For the test of Example 2 the partially concentrated acidic waste obtained from the test of Example 1 was used after filtration, and was treated under the operating conditions summarized in Table 4.

- Table 4 -

Operating conditions of Example 2 (second concentration stage)

[0108] The temperature was controlled and maintained at a predetermined value by means of the heating mantle and temperature probe. The distilled acid, after passing through the heated column of the first container, collected in the second container. The temperature inside the second container is controlled via the heating plate controlled by a temperature probe. The temperature of the heated development column of the second container was kept constant at 90°C. The condensed water vapour was collected in the third container.

[0109] After 90 minutes of treatment, 243 g of a substantially clear 91 % wt. sulphuric acid water solution was collected in the second container, so as to obtain a sulphuric acid recovery of approximately 69%, with a negligible metal content. In particular, the iron content in the recovered sulphuric acid solution was lower than 10 ppm, well below common commercial specifications. In the third container, 94 g of water were collected. EXAMPLE 3 (no filtration between 1 st and 2nd concentration stages)

[0110] Tests in the same conditions as examples 1 and 2 were carried out without filtering he partially concentrated liquid acidic waste from the first concentration test. In this case, an increase of about 25% of the recovery of sulphuric acid was possible, since no partially concentrated acidic liquid waste was withdrawn from the process as imbibition liquid of the first solid metal sulphates, which were absent in this example.

COMPARATIVE EXAMPLE I (Evaporation concentration, 1 st stage)

[0111] In order to carry out the vacuum evaporation concentration test, to be compared with Example 1 according to the invention, a conventional evaporation equipment was used, including a first container heated by means of a heating mantle and provided with an agitator and a condenser along its vent duct, a second container in the form of a dispenser to collect the water condensed by the condenser, and a vacuum pump pneumatically connected with the vent duct of the second container. The test of Comparative example I was carried out under the operating conditions summarized in Table 5.

- Table 5 -

Operating conditions of Comparative Example I (vacuum evaporation concentration, first stage)

[0112] During the test, the pressure was progressively reduced until a maximum vacuum of 530 mmHg was attained. As the effluent became more concentrated, solids metal sulphates crystallised due to solubility, while the condensed water collected in the second container. After 90’, 533 g of water were collected, while 696 g of concentrated effluent were left in the first container. This liquid was transferred into a tall beaker, where it remained for 24 h, but no apparent solid settling was observed. A filtration step was then carried out in a with porosity 4 glass filter (10-16 p), after which the concentrated effluent had a clear appearance. However, after 3 hours, suspended crystallised salts had appeared, therefore the liquid it was left at rest for further 24 hours to promote complete crystallisation, before a second filtration, after which the filtrate remained indefinitely clear. At the end of the test, 380g of 53% H2SO4 were obtained, which means a recovery of about 63%.

[0113] The comparison of Example 1 and Comparative Example I (first step) shows some advantages of the treatment according to the invention with respect the vacuum evaporation concentration technique.

By the vacuum evaporation concentration technique, no substantial settling of solids was observed after 90' treatment. On the contrary, two well-separated liquid and solid phases were formed by the technique according to the invention.

In the vacuum evaporation concentration technique, the filtration of the concentrated effluent is slower and more difficult than in the technique according to the invention. In particular, in the conventional evaporation technique it was necessary to filter the concentrated effluent twice, due to intermediate crystallisation of the solid metal sulphates.

By the vacuum evaporation concentration technique, a larger volume of solids with a larger specific content of acid was obtained (250 g of solid matter containing 45% wt. H2SO4) than by the technique according to the invention (134 g of solid matter at 40% wt. H2SO4). The H2SO4 recovery efficiency of the conventional evaporation technique is therefore lower that the H2SO4 recovery efficiency of technique of the invention.

The absolute H2SO4 recovery is significantly higher by the technique according to the invention (78%) than by the vacuum evaporation concentration technique (63%).

COMPARATIVE EXAMPLE II (Evaporation concentration, 2nd stage)

[0114] The test of Comparative example II, to be compared with Example 2 according to the invention, was carried out by the same equipment used for Comparative Example I, under the operating conditions summarized in Table 6. - Table 6 -

Operating conditions of Comparative Example II (vacuum evaporation concentration, second stage)

[0115] After 30', it was no longer possible to continue with the concentration text due to the almost complete solidification of the 254 g of concentrated effluent left in the first container, while 1 15 g of water had collected in the second container. The concentrated effluent was filtered, obtaining 1 15 g of filtered salts and 98 g of 89% sulphuric acid, which means a recovery of 27.3%, compared to 69% obtained according to the invention, Example 2.

[0116] An overall comparison of the two techniques, considering both Examples 1 and 2, on the one hand, and both Comparative examples I and II, on the other hand, shows the following advantages of the invention over the conventional vacuum evaporation concentration technique.

In the vacuum evaporation concentration technique: a final filtration is required to separate the solid matter from the concentrated acid. The latter contains a large amount of metals that give it a dark green coloration; a large amount of sulphuric acid, about 50% wt., remain in the filtered solid matter.

On the contrary, in the gas and CO2 bubbling-based process according to the invention: the concentrated acid is directly collected as a substantially pure acid, i.e. free of solids, which remain in first reactor 61 and can be washed away therefrom by simple water solubilization; the iron content in the concentrated acid is lower than 10 ppm, which meets the requirement of the common specifications for commercial sulphuric acid; not only a more concentrated sulphuric acid is recovered: more important, the amount of recovered acid is far larger than the amount obtained by vacuum evaporation concentration, as can be seen from the final recovery rates of the two techniques (69% instead of 27,3 %).

[0117] The following table summarises and compares the results of first- and second-stage concentration tests of Examples 1 and 2, according to the invention, on the one hand, and of first and second step concentration tests of Comparative examples I and II, according to a conventional vacuum evaporation technique, on the other hand.

- Table 7 -

Comparison of the performances of the concentration treatment according to the invention and a conventional vacuum concentration method

[0118] The foregoing description of specific embodiments and examples is capable of showing the invention from a conceptual point of view in such a way that others, using the known technique, will be able to modify and/or adapt in various applications such specific embodiments without further research and without departing from the inventive concept, and, therefore, it is understood that such adaptations and modifications will be considered as equivalents of the specific embodiments. The means and materials for realising the various functions described can be of various kinds without departing from the scope of the invention. It is understood that the expressions or terminology used are purely descriptive and therefore not limiting.

Claims

1. A process for making titanium dioxide comprising the steps of: prearranging (S1 ) a ground titanium-containing mineral (1 ); prearranging (S2) an attack solution (2) containing sulphuric acid; mixing (S3) said ground titanium-containing mineral (1 ) with said attack solution (2) and injection (S4) of water vapour (3), so as to obtain a two-phase mixture (4) of: an acidic liquid waste comprising water, sulphuric acid and soluble metal sulphates, and solid hydrated titanium dioxide (5a, 5b); separating (S6,S6a,S6b) said solid hydrated titanium dioxide (5a, 5b) from said acidic liquid waste (6, 6a, 6b) containing said sulphuric acid, said soluble metal sulphates and said water; calcining (S7) said solid hydrated titanium dioxide (5a, 5b) so as to obtain anhydrous titanium dioxide (7); characterised in that steps are further provided of: providing a first reactor (61 ) arranged to receive said acidic liquid waste (6,6a); providing a second reactor (91 ); providing a feed duct (92) arranged to convey a gaseous stream from said first reactor (61 ) to said second reactor (91 ); and in that a treatment (S100) of said acidic liquid waste (6,6a) is provided including: a first concentration stage (S10) of said acidic liquid waste (6,6a), comprising a first bubbling step (S11 ) of bubbling a first gas (12) into said acidic liquid waste (6,6a), so as to obtain: a partially concentrated acidic waste (16) having an intermediate sulphuric acid concentration; first solid metal sulphates (13) in said partially concentrated acidic waste (16); a first gaseous stream (14) containing said first gas and water vapour, wherein said first concentration stage (S10) is carried out at a first process temperature between 70°C and 100°C; a second concentration stage (S20) of said partially concentrated acidic liquid waste (16,16a), comprising: a second bubbling step (S21 ) of bubbling a carbon dioxide-con- taining second gas (22) into said acidic liquid waste (6,6a) within said first reactor (61 ), obtaining solid metal sulphates (23) precipitating from said acidic liquid waste (6,6a), said solid metal sulphates (23) settling within said first reactor (61 ), and further obtaining, in said first reactor (61 ), a second gaseous stream (24) containing carbon dioxide, water and sulphuric vapours, wherein said second bubbling step (S21 ) is carried out at a second process temperature above 150°C; conveying said second gaseous stream (24) from said first reactor (61 ) to said second reactor (91 ); selectively absorbing (S22) said sulphuric vapours contained in said second gaseous stream (24) into a water solution within said second reactor (91 ), so as to form a sulphuric acid solution (26) which progressively concentrates in sulphuric acid within said second reactor (91 ) and obtaining a substantially SOa-free third gaseous stream (27) containing carbon dioxide and water vapour up to obtaining a final concentrated sulphuric acid (28); recovering (S23) a final concentrated sulphuric acid (28) from said second reactor (91 ) recovering (S25) said solid metal sulphates (23) from said first reactor (61 ).

2. The process according to claim 1 , wherein said first concentration stage (S10) is carried out in a reactor selected between: said first reactor (61 ); a further reactor (41 ), wherein an outlet duct (17) is arranged to convey said partially concentrated acidic waste (16,16a) from said further reactor (41 ) into said first reactor (61 ).

3. The process according to claim 1 , wherein said partially concentrated acidic liquid waste (16) is a raw partially concentrated acidic liquid waste (16) and, after said first concentration stage (S10) and before said second concentration stage (S20), a filtration step (S12) is provided of filtering said first solid metal sulphates (13) from said raw partially concentrated acidic waste (16), thus obtaining a filtered partially concentrated acidic waste (16a).

4. The process according to claim 1 , wherein said first gas (12) is air.

5. The process according to claim 1 , wherein said intermediate sulphuric acid concentration is set between 60% by weight and 70% by weight.

6. The process according to claim 1 , wherein said intermediate sulphuric acid concentration is about 65% by weight.

7. The process according to claim 1 , wherein said second process temperature is set between 170°C and 190°C.

8. The process according to claim 1 , wherein the final sulphuric acid concentration is between 90% by weight and 95% by weight.

9. The process according to claim 1 , further comprising a step of recycling at least one portion of said final concentrated sulphuric acid (28) into said attack solution (2) used to attack said ground titanium-containing mineral (1 ).

10. The process according to claim 1 , further comprising the steps of: selectively condensing (S26) water from said third gaseous stream (27), so as to obtain a non-condensable gas stream (29) containing said carbon dioxide; recycling (S27) said non-condensable gas stream (29) to said second bubbling step (S21 ).

11. The process according to claim 1 , wherein a step is provided of providing a heating device (90) along the extension of said feed duct (92) configured in such a way to maintain said feed duct (92) at a temperature at which the water of gaseous stream is in the vapour state and said sulphuric vapours can condensate, in particular, at a temperature of at least 100°C.

12. The process according to claim 1 , wherein said step (S6,S6a,S6b) of separating said solid hydrated titanium dioxide (5a, 5b) from said acidic liquid waste (6,6a) comprises a step (S6a) of filtering a first solid hydrated titanium dioxide (5a) from a more concentrated acidic liquid waste (6a) having a sulphuric acid concentration exceeding a predetermined value, and said treatment (S100) is performed only on said more concentrated acidic liquid waste (6a).

13. The process according to claim 13, wherein said predetermined value for said sulphuric acid concentration is 200 g/L

14. A process for making titanium dioxide comprising the steps of: prearranging (S1 ) a ground titanium-containing mineral (1 ); prearranging (S2) an attack solution (2) containing sulphuric acid; mixing (S3) said ground titanium-containing mineral (1 ) with said attack solution (2) and injection (S4) of water vapour (3), so as to obtain a two-phase mixture (4) of: an acidic liquid waste liquid comprising water, sulphuric acid and soluble metal sulphates, and solid hydrated titanium dioxide (5a, 5b); separating (S6,S6a,S6b) said solid hydrated titanium dioxide (5a, 5b) from said acidic liquid waste (6, 6a, 6b) containing said sulphuric acid, said soluble metal sulphates and said water; calcining (S7) said solid hydrated titanium dioxide (5a, 5b) so as to obtain anhydrous titanium dioxide (7); characterised in that steps are further provided of: providing a first reactor (61 ) arranged to receive said acidic liquid waste (6,6a); providing a second reactor (91 ); providing a feed duct (92) arranged to convey a gaseous stream from said first reactor (61 ) to said second reactor (91 ); and in that a treatment of said acidic liquid waste (6,6a) is provided including: a first concentration stage (S10) of said acidic liquid waste (6,6a), comprising a first bubbling step (S11 ) of bubbling a first gas (12) into said acidic liquid waste (6,6a), so as to obtain: a partially concentrated acidic waste (16) having an intermediate sulphuric acid concentration; first solid metal sulphates (13) in said partially concentrated acidic waste (16); a first gaseous stream (14) containing said first gas and water vapour, wherein said first concentration stage (S10) is carried out at a first process temperature between 70°C and 100°C; a second concentration stage (S20) of said partially concentrated acidic liquid waste (16,16a) comprising: a second bubbling step (S21 ) of bubbling a second gas (22) into said acidic liquid waste (6,6a) within said first reactor (61 ), obtaining solid metal sulphates (23) precipitating from said acidic liquid waste (6,6a), said solid metal sulphates (23) settling within said first reactor (61 ), and further obtaining, in said first reactor (61 ), a second gaseous stream (24) containing sulphur trioxide and water, wherein said second bubbling step (S21 ) is carried out at a second process temperature above 190°C; conveying said second gaseous stream (24) from said first reactor (61 ) to said second reactor (91 ); selectively absorbing (S22) said sulphur trioxide contained in said second gaseous stream (24) into a water solution within said second reactor (91 ) so as to form a sulphuric acid solution (26) which progressively concentrates in sulphuric acid within said second reactor (91 ) and obtaining a substantially SOa-free third gaseous stream (27) containing water vapour up to obtaining a final concentrated sulphuric acid (28); recovering (S23) said final concentrated sulphuric acid (28) from said second reactor (91 ); recovering (S25) said solid metal sulphates (23) from said first reactor (61 ).

15. The process according to claim 14, wherein said first concentration stage (S10) is carried out in a reactor selected between: said first reactor (61 ); a further reactor (41 ), wherein an outlet duct (17) is arranged to convey said partially concentrated acidic waste (16,16a) from said further reactor (41 ) into said first reactor (61 ).

16. The process according to claim 14, wherein said partially concentrated acidic liquid waste (16) is a raw partially concentrated acidic liquid waste (16) and, after said first concentration stage (S10) and before said second concentration stage (S20), a filtration step (S12) is provided of filtering said first solid metal sulphates (13) from said raw partially concentrated acidic waste (16), thus obtaining a filtered partially concentrated acidic waste (16a).

17. The process according to claim 14, wherein said first gas (12) and/or said second gas (22) are/is air.

18. The process according to claim 14, wherein said intermediate sulphuric acid concentration is set between 60% by weight and 70% by weight.

19. The process according to claim 14, wherein said intermediate sulphuric acid concentration is about 65% by weight.

20. The process according to claim 14, wherein said second process temperature is set between 210°C and 230°C.

21. The process according to claim 14, wherein the final sulphuric acid concentration is between 90% by weight and 95% by weight.

22. The process according to claim 14, further comprising a step of recycling at least one portion of said final concentrated sulphuric acid (28) into said attack solution (2) used to attack said ground titanium-containing mineral (1 ).

23. The process according to claim 14, further comprising the steps of: selectively condensing (S26) water from said third gaseous stream (27), so as to obtain a non-condensable gas stream (29); recycling (S27) said non-condensable gas stream (29) to said second bubbling step (S21 ).

24. The process according to claim 14, wherein a step is provided of providing a heating device (90) along the extension of said feed duct (92) configured in such a way to maintain said feed duct (92) at a temperature at which the water of gaseous stream is in the vapour state and said sulphuric vapours can condensate, in particular, at a temperature of at least 100°C.

25. The process according to claim 14, wherein said step (S6,S6a,S6b) of separating said solid hydrated titanium dioxide (5a, 5b) from said acidic liquid waste (6,6a) comprises a step (S6a) of filtering a first solid hydrated titanium dioxide (5a) from a more concentrated acidic liquid waste (6a) having a sulphuric acid concentration exceeding a predetermined value, and said treatment (S100) is performed only on said more concentrated acidic liquid waste (6a).

26. The process according to claim 25, wherein said predetermined value for said sulphuric acid concentration is 200 g/L

27. A process for treating an acidic liquid waste (6) containing sulphuric acid, soluble metal sulphates and water, including: a first concentration stage (S10) of said acidic liquid waste (6), comprising a first bubbling step (S11 ) of bubbling a first gas (12) into said acidic liquid waste (6), so as to obtain: a partially concentrated acidic waste (16) having an intermediate sulphuric acid concentration; first solid metal sulphates (13) in said partially concentrated acidic waste (16); a first gaseous stream (14) containing said first gas and water vapour, wherein said first concentration stage (S10) is carried out at a first process temperature between 70°C and 100°C; a second concentration stage (S20) of said partially concentrated acidic liquid waste (16,16a) comprising: a second bubbling step (S21 ) of bubbling a second gas (22) into said acidic liquid waste (6) within said first reactor (61 ), obtaining solid metal sulphates (23) precipitating from said acidic liquid waste (6), said solid metal sulphates (23) settling within said first reactor (61 ), and further obtaining in said first reactor (61 ) a second gaseous stream (24) containing sulphur trioxide and water, wherein said second bubbling step (S21 ) is carried out at a second process temperature above 150°C; conveying said second gaseous stream (24) from said first reactor (61 ) to said second reactor (91 ); selectively absorbing (S22) said sulphur trioxide contained in said second gaseous stream (24) into a water solution within said second reactor (91 ) so as to form a sulphuric acid solution (26) which progressively concentrates in sulphuric acid within said second reactor (91 ) and obtaining a substantially SOa-free third gaseous stream (27) containing water vapour up to obtaining a final concentrated sulphuric acid (28); recovering (S23) said final concentrated sulphuric acid (28) from said second reactor (91 ); recovering (S25) said solid metal sulphates (23) from first reactor (61 ).

28. The process according to claim 27, wherein said second gas (22) is a carbon dioxide-containing second gas, and said second and third gaseous streams (24,27) further contain carbon dioxide.